Technical Support for Revisions of Subpart I: Electronics Manufacturing
Table of Contents
1	Introduction	3
2	Default Emission Factors	4
2.1	Introduction	4
2.2	Overview of Emission Factor Calculation Methods	4
2.2.1	Dominant-input-gas method	5
2.2.2	All-input-gas method	5
2.2.3	Modified all-input-gas methods ("Skim-off-the-top" methods)	5
2.2.4	Approaches based on emission factors from single-gas recipes	6
<IP_ADDRESS> Reference-emission-factor method	6
2.2.5	Multi-gas method	7
2.2.6	Criteria for Evaluating Emission Factor Calculation Methods	8
<IP_ADDRESS> Mathematical vs. physical influences on calculated emission factors	8
2.3	Description of Emission Factor Data and Proposed Defaults	11
3	Destruction and Removal Efficiency and Abatement Systems	24
3.1	Summary of Revisions	24
3.2	Description of Destruction or Removal Efficiency Data and Development of Proposed Default DREs	24
3.3	CF4 from Emissions Control systems	27
4	Revisions to the Stack Testing Method	31
4.1	Revisions to simplify the stack test method	31
4.2	Gamma Weighting Factor	32
4.3	Revisions to improve by-Product estimates	39
5	APPENDIX	42
5.1	Additional Scenarios for Emission Factor Calculation Methods	42
5.2	Emission Factor Tables	48

Introduction
The U.S. Environmental Protection Agency (EPA) is proposing to amend specific provisions in the Greenhouse Gas Reporting Rule (40 CFR part 98) to improve the quality and consistency of the data collected under the rule, streamline implementation, and to clarify or propose minor updates to certain provisions that have been the subject of questions from reporting entities. As part of these revisions, the EPA is proposing amendments to the reporting requirements for Electronics Manufacturing (40 CFR part 98, subpart I; hereafter referred to as "subpart I"). This technical support document provides background and technical supporting information for the proposed revisions to subpart I.  Default Emission Factors
Introduction
This section reviews and evaluates (1) the range of methods that can be used to calculate process-specific input gas and by-product gas emission factors based on measurements for individual processes (Section 2.2), and (2) the methods and data used to calculate the revised default input gas and by-product gas emission factors for each process type and subtype in the proposed Technical Revisions rule (Section 2.3).
Overview of Emission Factor Calculation Methods
In electronics manufacturing, fluorinated GHGs are used to etch patterns into devices and to clean wafers and chemical vapor deposition chambers. During these processes, the F-GHGs are dissociated into fluorine atoms and other reactive species by plasma or thermal energy, and the fluorine atoms and reactive species perform the etching or cleaning. However, not all the mass of the F-GHGs fed into the process is consumed (utilized) by the process. Some fraction of each F-GHG fed into the process (input gas) is emitted from it without being dissociated. Another fraction of each F-GHG is transformed into other F-GHGs (by-products) during the process, and these by-products are also emitted.
To account for these process dynamics, emission factors for the electronics industry are established for both input gases ("input gas emission factors" or "1-U factors," where U represents the fraction of the input gas utilized) and for by-products ("by-product gas emission factors" or "by-product formation rates").  Both input gas and by-product emission factors are indexed to the quantities (masses) of input gases fed into the process and are based on process-level measurements comparing the identities and masses of the input gases to the identities and masses of the F-GHGs emitted from each process.
When there is only one F-GHG input gas (i.e., for a single-gas recipe), the fraction of the gas utilized by the process is clear, and the formation rates of by-products from that gas are similarly clear. However, when there are multiple F-GHG input gases (i.e., for a multi-gas recipe), estimating input gas and by-product emission factors is complicated in two ways. First, when an F-GHG is both an input gas and a likely by-product of the other input gases, it is not clear what fractions of the emitted F-GHG consist of the unreacted input gas vs. by-products generated by the other input gases.  Second, when an F-GHG is not an input gas but is generated as a by-product of the input gases, it is not clear what fractions of that by-product are generated by the various input gases.
Different conventions (or methods) can be used to manage the uncertainty associated with the calculation of input gas and by-product emission factors for multi-gas recipes. Section 2 of this TSD reviews several methods for calculating input gas (1-U) and by-product emission factors (BEF) for multi-gas recipes and evaluates their pros and cons.
Dominant-input-gas method
The dominant-input-gas method is one of the two methods historically used to calculate 1-U and BEF factors. The dominant input gas method attributes all emissions of each input gas to the 1-U factor for that gas; that is, it assumes that none of that gas is generated as a by-product by the other input gases. This results in relatively large 1-U factors.  By-products that are not also input gases are attributed in their entirety to the input gas that is used in the largest quantity. This results in relatively large BEFs for the dominant input gas, and no BEFs for the other input gases.
All-input-gas method
The other method historically used to calculate 1-U and BEF factors is the all-input-gas method. Like the dominant-input-gas method, the all-input-gas method attributes all emissions of each input gas to the 1-U factor for that gas. However, by-products that are not also input gases are attributed to all input gases in proportion to the quantity of each input gas used in the process.  This results in equal BEFs for each by-product across all input gases.
One issue that has arisen with these two approaches is that they may result in 1-U factors greater than one, which violates conservation of mass and clearly does not fully represent the physical processes giving rise to the emissions. This is most likely to happen when an input gas such as CF4 makes up a relatively small share of the total input gas mass and is also generated in significant quantities as a by-product of the other input gases.  Data from single gas processes indicate that most gas/process combinations produce CF4 and C2F6 as a by-product and often in significant quantities.  These approaches may also result in the total mass emitted from an input gas being greater than 1 (i.e. the input gas emission factor plus all of its by-product emission factors is greater than 1), which also violates conservation of mass.  This may occur even if the 1-U factor is less than 1.
The following calculation methods address these issues in different ways.
Modified all-input-gas methods ("Skim-off-the-top" methods)
The subpart I stack testing provisions use a variant of the all-input-gas method that avoids 1-U factors greater than one. This method attributes emissions of each input gas to that input gas until the mass emitted equals the mass fed into the process, that is, until the 1-U factor equals one. Remaining emissions of the input gas are then assigned to the other input gases as a by-product in proportion to the quantity of each input gas used in the process. By-products that are not also input gases are also attributed to all input gases in proportion to the quantity of each input gas used in the process (i.e., they are treated the same way as in the all-input-gas method).
This "skim-off-the-top" approach avoids violating conservation of mass in the resulting 1-U, but it still does not represent the physical reality of the process, since an input gas is only fed into the process because some fraction of it is utilized, that is, converted into fluorine atoms to etch devices or clean CVD chambers.  Another version of the modified all-input-gas method sets the maximum 1-U factor at 0.8 rather than 1, recognizing that some fraction of the input gas is utilized by the process and possibly converted into by-products.  This modification better reflects the likely processes occuring in the process; however, it would result in a fixed value for the input gas emission factor when that gas is emitted in large quantities as a by-product.
Approaches based on emission factors from single-gas recipes
Other approaches that could provide a more realistic result begin with the input gas utilizations (1-U factors) and/or by-product generation factors (BEFs) observed based on single gas recipes (and for by-products, potentially also on multi-gas gas recipes where the by-product of interest is not also an input gas).  For example, if the average 1-U factor for an input gas based on single gas recipes was 0.7, then emissions of that F-GHG would be attributed to its use as an input gas until the 1-U factor reached 0.7. Remaining emissions of the input gas would then be assigned to the other input gases as a by-product in proportion to the quantity of each input gas used in the process.  Alternatively, the BEFs based on single gas recipes could be used as the starting point. In this case, emissions of the input gas F-GHG would be attributed to the other input gases as by-products based on the average BEFs observed for those other gases in single-gas recipes. Remaining emissions of the F-GHG would be assigned to its 1-U factor.
An issue that arises with either of these approaches is that they assume that either the 1-U factor or the BEFs remain constant in all processes. That is, where the 1-U factor is assumed to be equal to the 1-U factor observed in single-gas recipes, only the BEFs can differ from those observed in single-gas recipes. This may be acceptable if moving from single- to multiple-gas recipes is more likely to change BEFs than 1-U factors (i.e., based on knowledge of plasma processes). Where the BEFs are used as the starting point, only the 1-U factor can differ from that observed in single-gas recipes.  This may be acceptable if moving from single- to multiple-gas recipes is more likely to have a significant impact on 1-U factors than BEFs.
    <IP_ADDRESS> Reference-emission-factor method
However, if it is not known whether 1-U or BEFs are more likely to change in moving from single- to multiple-gas recipes, a better approach is to apply both the 1-U and the BEF factors that are observed in single gas recipes, and then to adjust both of these based on the ratio between the emissions actually observed in the multi-gas process and the emissions predicted based on the single gas 1-U and BEFs. This method can be termed the reference-emission-factor method, since it is based on pre-existing, normative 1-U and BEF factors. For example, if one kg of CF4 and five kg of C3F8 were fed into a process, and if the 1-U and BEF factors for CF4 based on single gas recipes were 0.7 and 0.1 respectively for that process type, then the predicted emissions of CF4 would be 1.2 kg (0.7 kg + 0.5 kg). If the actual emissions of CF4 from the process were 1.5 kg of CF4, then the single gas 1-U and BEF would be adjusted by multiplying them by 1.5/1.2 = 1.25, resulting in a new 1-U of 0.875 and a new BEF for CF4 from C3F8 of 0.125. This approach uses all of the information available on utilization and by-product generation rates from single-gas recipes while avoiding assumptions about which of these is changing in the multi-gas recipe.
Multi-gas method
This method assumes that all the F-GHGs emerging from the process are equally attributable to all the input gases, e.g. if CF4 and CHF3 are input gases, both CF4 and CHF3 are contribute equally to CF4 emissions. This assumes that that CHF3 is as likely to dissociated and recombine to form CF4 as CF4 is to not dissociate in the process (or dissociate and re-form CF4).  The mass of each emitted F-GHG is divided by the total mass of all the input gases, resulting in equal emission factors for that emitted F-GHG for all the input gases. For example, if 1 kg each of C2F6, C3F8, and CHF3 are used as input gases, and emissions of 0.5 kg CF4, 0.6 kg C2F6, 0.3 C3F8, and 0.3 CHF3 are observed, then the 1-U factors would be 0.2kg/kg for C2F6, 0.1 kg/kg for C3F8, and 0.1 kg/kg for C3F8. The BEFs for each emitted input gas would be the same, assigned to each input gas. The BEF for CF4 for each input gas would be 0.17.
Table 1 summarizes the 1-U and BEF calculation methods.
             Table 2.1: Summary of 1-U and BEF Calculation Methods
                                    Method
                                      1-U
                   BEF for By-Products That Are Input Gases
                 BEF for By-Products That are Not Input Gases
Dominant-Input-Gas
Mass of each input F-GHG emitted/Mass of that input F-GHG fed into process
None
Mass of BP emitted/Mass of dominant input gas fed into process
All-Input-Gas
Mass of each input F-GHG emitted/Mass of that input F-GHG fed into process
None
Mass of BP emitted/Total mass of all input gases fed into process
Modified-All-Input-Gas ("Skim-off-the-top")
Mass of each input F-GHG emitted/Mass of that input F-GHG fed into process, with a maximum value of 0.8.
None, unless the mass of an input F-GHG emitted exceeds 0.8 x the mass fed into the process. In that case, BEF = (Mass of that F-GHG emitted  -  0.8 x mass of that F-GHG fed into the process)/Total mass of all input gases fed into process
Mass of BP emitted/Total mass of all input gases fed into process
Reference-Emission-Factor
(Mass of each input F-GHG emitted/Mass of that F-GHG expected based on reference 1-U and BEF values) x Reference 1-U
(Mass of each input F-GHG emitted/Mass of that F-GHG expected based on reference 1-U and BEF values) x Reference BEF
Mass of BP emitted/Total mass of all input gases fed into process
Multi-gas
Mass of each input F-GHG emitted/Total mass of all input gases fed into process
Mass of each input F-GHG emitted/Total mass of all input gases fed into process
Mass of BP emitted/Total mass of all input gases fed into process

Criteria for Evaluating Emission Factor Calculation Methods
This TSD evaluates each emission factor calculation method based primarily on two criteria:
(1) the extent to which the calculation method is likely to yield 1-U and BEF factors that accurately predict emissions regardless of the proportions in which input gases are used; and
(2) the extent to which the calculation method is likely to yield 1-U and BEF factors that are comparable to the factors calculated historically. This is important for understanding the magnitude and direction of trends that may occur in emission factors over time as production technologies change.
Note that in some cases, these goals may not be completely compatible. For example, if the emission factor calculation methods used historically resulted in inaccurate emission factors under some circumstances, then it may be appropriate to use a calculation method that yields different (accurate) emission factors under those circumstances, even though the new emission factors will not be fully comparable to the previously measured emission factors.
<IP_ADDRESS> Mathematical vs. physical influences on calculated emission factors
Calculated emission factors for the same input gas may vary among electronics manufacturing processes for both physical and mathematical reasons. They can vary physically due to different process conditions, including, for example, differences in surface chemistry, plasma power, temperature, pressure, flow rate, and changing identities and/or proportions of other input gases (with which the input gas or its by-products may react and which may include gases that are not GHGs). As discussed further below, they can also vary mathematically due to changing proportions of input gases even if the actual, physical 1-U and BEF factors for each input gas do not change.
Any emission factor calculation method that accounts for all the F-GHG masses fed into and emitted from a process will result in 1-U and BEF factors that accurately predict emissions under the conditions of that process, even if an emitted F-GHG is attributed to one or more of the input gases incorrectly. As long as the input gases are used in constant proportions under the same process conditions, multiplying the 1-U and BEF factors by the input gas masses will result in accurate emissions estimates.
However, emission factors are generally not applied only to process conditions identical to those under which they were measured. Instead, they are applied to processes that may vary either slightly or significantly from those under which they were measured. (In the latter case, they are often averaged with emission factors measured for other processes of the same process type or subtype.)  Notably, emission factors are applied to processes where the proportions of input gases may differ significantly from those used in the process for which the emission factors were originally calculated.
For a multi-gas recipe, we cannot know with certainty how 1-U and BEF factors vary physically as input gas proportions or other process conditions change. However, we can still evaluate the robustness of the different calculation methods by (1) comparing calculated multi-gas 1-U and BEF values to 1-U and BEF values based on single-gas recipes, and (2) modeling how calculated multi-gas 1-U and BEF values vary mathematically as the proportions of the input gases change, making the simplifying assumption that the 1-U and BEF factors are actually constant.
Regarding the first evaluation approach, it is important to note that the actual, physical 1-U and BEF values for particular input gases in a multi-gas recipe are not expected to be identical to the 1-U and BEF values in a single-gas recipe, even if the other process conditions are similar. Changing the identities and proportions of input gases is likely to have some impact on 1-U and BEF values. However, if the 1-U and BEF values yielded by a calculation method are much higher or lower than the range of values in single-gas recipes for that input gas, or in single-gas recipes for input gases generally, this indicates a need for further evaluation of the calculation method.
Similarly, regarding the second evaluation approach, it is important to note that 1-U and BEF values are not expected to be constant as gas proportions change. This is only a simplifying assumption used to clarify how the mathematics of changing input gas proportions can affect the results of certain calculation methods. Using this assumption, we can show that certain calculation methods described above yield emission factors that are strongly affected by changing proportions of input gases even when the actual emission rates do not change (see Figure 2.1). These emission factors will yield inaccurate results when applied to processes whose gas proportions differ significantly from those of the process for which the factors were measured.
Figure 2.1 shows how the 1-U and BEF values yielded by the emission factor calculation methods described above compare to hypothetical constant CF4 1-U and BEF values where the process uses shifting proportions of two input gases: CF4 and CHF3. (The results of the Dominant Gas Method are not shown separately here because they are identical to those for the All-Input-Gas Method for the scenario shown here, i.e., where the emitted gases are the same as the input gases.)
Figure 2.1 CF4 1-U and BEF values as a function of shares of CF4 and CHF3 input gases by method
(Assumes constant EFs and BEFs, the 1-U for CF4 is 0.75 the 300-mm reference value and the BEF for CF4 from CHF3 is the same as the reference value)

As can be seen from Figure 2.1, no method estimates the posited CF4 1-U and BEF perfectly, but some methods perform better than others. Under the modelled scenario, which assumes that the CF4 1-U of the process is 0.75 x the reference CF4 1-U, the Reference Emission Factor Method slightly underestimates the CF4 BEF from CHF3 for all CF4/CHF3 proportions. That method slightly overestimates the CF4 1-U when CF4 makes up a small proportion of the total input gas mass, but the calculated 1-U value swiftly approaches the 1-U of the process as the CF4 proportion grows. The All-Input-Gas Method underestimates the CF4 BEF from CHF3 for all CF4/CHF3 proportions, equating it to zero. This is expected because this method attributes all CF4 emissions to the 1-U factor.  The All-Input-Gas Method estimates the CF4 1-U factor relatively accurately when CF4 makes up most of the input gas mass, but as the proportion of CF4 declines, it begins to significantly overestimate the 1-U factor. In fact, the 1-U factor rises hyperbolically as the entire mass of CF4 generated as a by-product from the growing mass of the CHF3 input gas is attributed to (and divided by) a smaller and smaller mass of CF4 input gas.  The modified All-Input-Gas method mitigates but does not eliminate these under- and overestimates.  Finally, the Multi-gas Method yields 1-U and BEF values that rise together from the posited BEF value (when CF4 makes up a small proportion of the mass of the input gases) to the posited 1-U value (when CF4 makes up a large proportion of the mass of the input gases). In this case, the accuracy of the 1-U and BEF values are inversely related; as the accuracy of one increases, the accuracy of the other decreases, and at least one is always inaccurate. The size of the error depends on the difference between the 1-U and BEF values. The error disappears when the 1-U and BEF values are actually identical, but this appears to be unlikely based on measurements of processes using single input gases.
Additional scenarios are presented in the Appendix (see EFMethod_Compare_300mm.xlsx for calculations). Most show patterns similar to that seen in Figure 2.1. The Reference Emission Factor Method often (though not always) estimates 1-U and BEF values more accurately than the other methods, but even when it is less accurate than another method, the size of the error is relatively modest. The Reference Emission Factor Method does not result in the large under- or overestimates of 1-U or BEF values that characterize the other methods under certain circumstances.
Regarding the consistency of the modified and new methods with the methods historically used, Figure 2.1 and the figures in the Appendix show that the Modified All-Input-Gas Method is generally (though not always) most consistent with the All-Input-Gas Method. The Reference Emission Factor Method is the next most consistent with the All-Input-Gas Method. As discussed above, the All-Input-Gas Method overestimates 1-U values, with the overestimate becoming large as the share of CF4 in the mass of input gas declines. Where the posited 1-U value is smaller than the reference value and the posited BEF is larger than the reference value, the Reference Emission Factor Method also overestimates 1-U values, with the overestimate growing as the share of CF4 in the mass of input gas declines. In general, this overestimate is smaller than the overestimate of the All-Input-Gas Method. Where the posited 1-U value is larger than the reference value and the posited BEF is smaller than the reference value, the Reference Emission Factor underestimates 1-U values, with the underestimate growing as the share of CF4 in the mass of input gas declines. In contrast, the Multi-Gas Method always underestimates 1-U values, usually by a larger percentage than the Reference Emission Factor does even when the posited 1-U value is larger than the reference value and the posited BEF is smaller than the reference value.
Description of Emission Factor Data and Proposed Defaults

The proposed emission factors for Tables I-3, I-4, I-8, I-11, and I-12 were calculated from measured data submitted by U.S. semiconductor manufacturers as part of the 2020 technology assessment report, 2017 technology assessment report, and data collected in previous years. The total data set contains 4,358 input gas and by-product gas emission factors across all commonly used gas and process type combinations, with 1,506 of these data points newly available via the 2020 and 2017 triennial technology assessment reports. All the data submitted via the 2020 and 2017 triennial technology assessment reports were applicable to the 300-mm wafer size. All data sets were reviewed for errors, including, but not limited to, transcription errors and violations of the fluorine balance. Calculated emission factors (1-U or BEFs) greater than 1.00 (a total of 18 data points) were excluded from the calculation of the proposed default emission factors (EFs). Input gas and by-product gas emission factors were also analyzed for each test to see whether the fluorine balance was violated. This resulted in the exclusion of 40 data points from the calculation of the proposed default emission factors. There were also a small number of transcription or other errors, including duplicate rows of data, that were corrected or excluded prior to calculating the proposed emission factors. Transcription and other errors resulted in the exclusion of 33 data points. A single reported by-product value for SF6 was also excluded from the calculation, as there was no source of sulfur. Four emission factors for NF3 in remote plasma cleaning (RPC) processes that were previously excluded were re-included in the data set.  These data points had previously been identified as outliers. Although no explanation had been provided for their relatively high values, it was believed to be possible that they reflected data from older or poorly maintained systems.  However, more recent data submitted via the triennial Technology Assessment Report also contained a measured emission factor greater than 0.1 for NF3 used in a remote plasma cleaning process, indicating that NF3 processes currently operating may sometimes have emission factors outside the typical range.

Submitted data were also reviewed for methodological consistency with previously submitted data. Data submitted to the EPA prior to 2017 used either the dominant gas or all-input gas convention. However, many of the data points for etching submitted as part of the 2020 and 2017 technology assessment reports used an alternative convention that had not been previously used. As described above, this new "multi-gas" convention differed in how it assigned emissions of input gases. Instead of assigning measured emissions of an input gas entirely to the input gas, emissions of an input gas were assigned to all input F-GHGs used in the process by dividing the measured mass emitted of a specific input gas by the total mass of all input F-GHGs and assigning this emission factor to each input F-GHG as either the 1-U factor or the by-product factor. In other words, all input F-GHGs were considered to equally contribute to the emissions of each input gas. This method was inconsistent with the methods of the previous data set, and the alternative method often resulted in large increases to the reported BEFs for carbon-containing F-GHGs and concurrently large decreases to reported 1-Us for carbon-containing F-GHGs when compared to the historical data (see tab "multi-gas compare 2013" in "Data Sets supporting Revised Emission Factors.xlsx" for a comparison of EFs using the "multi-gas" convention compared to the original 2013 emission factors). The 1-U values calculated for carbon-containing F-GHGs using the "multi-gas" convention were 24-69% lower on average (depending on the input gas) than the 1-U values provided to EPA in or before 2013.  Conversely, by-product emission factors calculated using the "multi-gas" convention were much higher than the by-product emission factors previously provided for most carbon-containing input gases (for BC2F6 this effect did not occur as C2F6 was not used as an input gas in the Triennial data sets).  This change appeared to be largely a result of the change in methodology and it was determined by the EPA that mixing the two methods by including both in the average was not appropriate.  It was not possible, however, based on the data received, to fully assess the effect of the new methodology as the data were not directly comparable to previously submitted data. The data points that were affected by the change in convention were excluded from the calculation of the proposed default EFs, resulting in the exclusion of 199 data points. This left a total of 3,951 data points in the combined data set that were included in the calculations of the proposed default emission factors in Tables I-3, I-4 and I-8.

                                   Table 2.2
       Comparison of Available Emission Factor Data from US Industry[a]
                     Total Number of Data Points Collected
                                     4358
Number of data points that were part of a set that violated a fluorine balance
                                      40
Number of data points where the emission factor was greater than 1.00
                                      18
Number of data points removed due to transcription errors
                                      33
Number of data points where SF6 was reported as a by-product with no Sulfur source
                                       1
Number of data points using incomparable method
                                      319
                 TOTAL Data Points Used to Develop Revised EFs
                                     3951
[a]Data points are counted by input gas/emission factor/process-type combination.  Each test may include multiple emission factors (e.g. a test using input gases of CF4 and C2F6 with a by-product of CHF3, would result in 4 EF data points: a (1-U) for CF4, a (1-U) for C2F6, a BCHF3 from CF4 and a BCHF3 for C2F6). Table does not include the data points used to calculated BF2 from NF3 in RPC processes.

The proposed default EFs for Tables I-3, I-4 and Table I-8 were calculated using a simple arithmetic mean of all non-erroneous EF data that used the all-input gas convention, used the dominant gas convention, or were from single-gas processes. The accuracy of using a simple mean depends on whether the identities and frequencies of the process conditions represented in the dataset for an F-GHG and process-type combination are similar to the identities and frequencies of the process conditions in the field for that combination.  Unfortunately, no information is known about the frequency of different process conditions (including gas combinations) used by the industry.  By-product emission factors were also retained for gases that were not also input gases.  For Table I-4, this includes data that were submitted through 2013 and data that were submitted in the 2017 and 2020 technology assessment reports. Using the arithmetic mean for the 2013, 2017, and 2020 data appears to be appropriate because the technology assessment reports reported no major changes to semiconductor production technology, the new EF data falls within the range seen for the previously submitted EF data for most of the default factors, and the impact of the new EF data on the emissions calculated for each input gas and process type is generally, though not always, small (i.e., less than +- 10 percent for most commonly used input gases). Therefore, it is assumed in most cases that the proposed default emission factors for F-GHGs reflect increased and/or improved data rather than changes in actual emission or utilization rates. This means that for each wafer size (<200 mm and 300 mm), the proposed emission factors are generally likely to represent emission rates over all the years of the GHGRP. However, for a few gas and process type combinations for the 300-mm wafer size, the differences between the averages calculated based on the new and previously submitted data are more significant and could have an appreciable impact on the overall calculated CO2e emissions.  This was the case for the 1-U values for CF4, CHF3, NF3, and SF6 for the etch process type; for the by-product emission factors for C2F6 from CF4, for c-C4F8 from CF4, and for c-C4F8 from CH2F2 for the etch process type; for the 1-U value for NF3 from the in-situ plasma process type; and for the by-product emission factor for CF4 from NF3 for the remote plasma clean process type.

To assess the impact of the 2017 and 2020 data on the calculated default emission factors, EPA first compared the average emission factors calculated based only on the 2017 and 2020 data to the average emission factors calculated based only on the 2013 corrected data (i.e., the 2013 data without 1-U values greater than one and without data from tests that violate the fluorine balance). (Both sets of emission factors, along with the proposed default emission factors, are shown in the Appendix. They are also available, along with other analyses, in a separate file that shows the calculations and that is titled "Triennial_only_vs_corrected_2013_data.xlsx.") Table 2.3 shows the ratio between the emission factors based only the 2017 and 2020 data and the emission factors based only on the corrected 2013 data. Input gas/by-product combinations for which data did not exist in 2013 but for which data were available in 2017 and/or 2020 are marked "New." Blank cells indicate that there were no new data points in 2017 or 2020. (In several instances, data were not available from 2013 either, e.g., because particular input gas/by-product combinations have not been observed.)

Table 2.3: Ratio of EFs Based Only on 2017&2020 Data/EFs Based Only on 2013 Corrected Data
                                 (300-mm Fabs)

CF4
C2F6
CHF3
CH2F2
CH3F
C3F8
C4F8
NF3
SF6
C4F6
C5F8
                                    Etching
                                     1-Ui
                                                                           0.93
                                                                              -
                                                                           0.44
                                                                           0.79
                                                                           0.71
                                                                              -
                                                                           0.77
                                                                           1.16
                                                                           0.83
                                                                           0.20
                                                                              -
                                     BCF4
                                                                              -
                                                                              -
                                                                           1.11
                                                                           1.86
                                                                           0.30
                                                                              -
                                                                           0.97
                                                                          0.001
                                                                           0.15
                                                                           0.95
                                                                              -
                                     BC2F6
                                                                           0.34
                                                                              -
                                                                           0.81
                                                                           0.59
                                                                          16.23
                                                                              -
                                                                           0.65
                                                                              -
                                                                           0.07
                                                                           0.32
                                                                              -
                                     BC4F6
                                                                            New
                                                                              -
                                                                         162.20
                                                                              -
                                                                            New
                                                                              -
                                                                           0.13
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                     BC4F8
                                                                           9.35
                                                                              -
                                                                           4.19
                                                                            New
                                                                            New
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                            New
                                                                              -
                                     BC3F8
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                     BC5F8
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                     BCHF3
                                                                           1.23
                                                                              -
                                                                              -
                                                                           1.40
                                                                           4.17
                                                                              -
                                                                           1.28
                                                                          0.003
                                                                           1.23
                                                                           0.46
                                                                              -
                                    BCH2F2
                                                                            New
                                                                              -
                                                                           0.55
                                                                              -
                                                                           3.32
                                                                              -
                                                                           1.71
                                                                           0.31
                                                                           0.03
                                                                              -
                                                                              -
                                     BCH3F
                                                                           0.75
                                                                              -
                                                                           3.53
                                                                           0.17
                                                                              -
                                                                              -
                                                                            New
                                                                              -
                                                                            New
                                                                            New
                                                                              -
                               CHAMBER CLEANING
                            In situ plasma cleaning
                                     1-Ui
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                           0.18
                                                                              -
                                                                              -
                                                                              -
                                     BCF4
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                     BC2F6
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                     BC3F8
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                            Remote plasma cleaning
                                     1-Ui
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                           1.02
                                                                              -
                                                                              -
                                                                              -
                                     BCF4
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                           0.20
                                                                              -
                                                                              -
                                                                              -
                                     BC2F6
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                     BC3F8
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                     BCHF3
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                            New
                                                                              -
                                                                              -
                                                                              -
                                    BCH2F2
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                            New
                                                                              -
                                                                              -
                                                                              -
                                     BCH3F
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                            New
                                                                              -
                                                                              -
                                                                              -
                           In situ thermal cleaning
                                     1-Ui
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                     BCF4
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                     BC2F6
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                     BC3F8
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -

As can be seen in Table 2.3, the ratios between the emission factors based on the old and new data can be quite large in some cases and quite small in others. In many cases, very large or very small ratios result from small numbers of data points in one or both datasets; for example, the by-product emission factor for C4F6 from CHF3 in etch processes is supported by one datapoint in the 2013 dataset and three datapoints in the 2017 and 2020 datasets, and the by-product emission factor for CHF3 from NF3 in etch processes is supported by 13 datapoints in the 2013 dataset and one datapoint in the 2017 and 2020 datasets.  The actual impact of the new data on each proposed updated emission factor depends not only on the ratio but on the relative number of datapoints in the old and new datasets for that factor. The impact of each proposed emission factor on total emissions depends in turn on the overall magnitude of the factor (i.e., tenths vs. hundredths vs. thousandths) and on the quantity of the applicable input gas typically used for the applicable process type.

Table 2.4 shows the change in calculated emissions that would result from using the proposed emission factors (including the 2013, 2017, and 2020 data) rather than the emission factors based only on the corrected 2013 dataset. The number in each cell represents the change in emissions for that combination of process type, input gas, and by-product gas (if applicable) as a fraction of the total calculated emissions for all process types and input gases based only on the corrected 2013 EF dataset.  Gas usage patterns were estimated based on data reported by semiconductor manufacturers to the GHGRP for 2013.  (Calculations are shown in the workbook titled "Analysis_fractional_change_in_emissions.xlsx.") The total change in emissions, summed across all process types and input gases, is -6.3 percent.

Table 2.4: Fractional Change in Total Estimated Emissions (mtCO2e) from 300-mm Fabs Based on Proposed Default Emission Factors vs. Emission Factors Based Only on Corrected 2013 Dataset, by Factor

                                   Input Gas

CF4
C2F6
CHF3
CH2F2
CH3F
C3F8
C4F8
NF3
SF6
C4F6
C5F8
                                     Etch
1-Ui
                                                                        (0.004)
                                                                              -
                                                                        (0.008)
                                                                       (0.0000)
                                                                       (0.0000)
                                                                              -
                                                                       (0.0002)
                                                                          0.007
                                                                        (0.005)
                                                                       (0.0000)
                                                                              -
BCF4
                                                                              -
                                                                              -
                                                                         0.0002
                                                                         0.0003
                                                                       (0.0001)
                                                                              -
                                                                      (0.00001)
                                                                        (0.001)
                                                                       (0.0004)
                                                                       (0.0000)
                                                                              -
BC2F6
                                                                        (0.013)
                                                                              -
                                                                        (0.001)
                                                                       (0.0004)
                                                                         0.0001
                                                                              -
                                                                       (0.0004)
                                                                              -
                                                                       (0.0003)
                                                                       (0.0007)
                                                                              -
BC4F6
                                                                         0.0000
                                                                              -
                                                                         0.0000
                                                                              -
                                                                         0.0000
                                                                              -
                                                                       (0.0000)
                                                                              -
                                                                              -
                                                                              -
                                                                              -
BC4F8
                                                                          0.002
                                                                              -
                                                                         0.0002
                                                                          0.002
                                                                         0.0001
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                         0.0003
                                                                              -
BC3F8
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
BCHF3
                                                                          0.001
                                                                              -
                                                                              -
                                                                         0.0003
                                                                         0.0002
                                                                              -
                                                                         0.0002
                                                                        (0.001)
                                                                         0.0000
                                                                       (0.0001)
                                                                              -
BCH2F2
                                                                         0.0002
                                                                              -
                                                                      (0.00001)
                                                                              -
                                                                         0.0000
                                                                              -
                                                                         0.0000
                                                                        (0.000)
                                                                       (0.0000)
                                                                              -
                                                                              -
BCH3F
                                                                        (0.000)
                                                                              -
                                                                          0.000
                                                                        (0.000)
                                                                              -
                                                                              -
                                                                         0.0000
                                                                              -
                                                                         0.0000
                                                                         0.0000
                                                                              -
                               CHAMBER CLEANING
                                In situ plasma
1-Ui
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                        (0.015)
                                                                              -
                                                                              -
                                                                              -
BCF4
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
BC2F6
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
BC3F8
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                 Remote plasma
1-Ui
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                         0.0003
                                                                              -
                                                                              -
                                                                              -
BCF4
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                        (0.029)
                                                                              -
                                                                              -
                                                                              -
BC2F6
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
BC3F8
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
BCHF3
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                         0.0003
                                                                              -
                                                                              -
                                                                              -
BCH2F2
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                         0.0002
                                                                              -
                                                                              -
                                                                              -
BCH3F
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                         0.0001
                                                                              -
                                                                              -
                                                                              -
                                In situ thermal
1-Ui
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
BCF4
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
BC2F6
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
BC3F8
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -
                                                                              -

The process type, input gas, and (if applicable) by-product combinations with the largest changes are enclosed in bold lines. These correspond to the 1-U and BEF values listed in the paragraph below Table 2.2 above. These changes account for 75 percent of the increases and 94 percent of the decreases in emissions between the emission factors based on the two datasets.

For the process type, input gas, and (if applicable) by-product combinations with the largest changes, EPA performed statistical analyses to determine whether the differences between the means of the data submitted through 2013 and the data submitted more recently were significant. (The sole exception was the BEF for C4F8 from CH2F2, for which no data exist in the 2013 dataset.) These results are summarized in Table 2.5 below.

Table 2.5: Summary of Statistical Analysis of Difference Between Means of 2013 and 2017/2020 Data for Key Emission Factors (300-mm Wafer Size)

                                    Etching
Input Gas
                                      CF4
                                     CHF3
                                      NF3
                                      SF6
1-U
                                Not significant
                                  Significant
                                Not significant
                                Not significant
BEF (C2F6)
                                 Significant
                                      N/A
                                      N/A
                                      N/A
BEF (C4F8)
                                Not significant
                                      N/A
                                      N/A
                                      N/A
                            In Situ Plasma Cleaning
Input Gas
                                      NF3
1-U
2017/2020 dataset includes only one data point, which is lower than any of the eight datapoints from the 2013 dataset.
                            Remote Plasma Cleaning
Input Gas
                                      CF4
                                     CHF3
                                      NF3
                                      SF6
BEF (CF4)
                                      N/A
                                      N/A
                                Not significant
                                      N/A

There are several factors that probably contributed to the change in emission factors that are not related to a change in process conditions.  One contributing factor is that there were more by-product emission factors reported in the two most recent data sets than previously reported.  In the data sets submitted in 2013 or earlier, only by-product emission factors for CF4, C2F6, CHF3 and CH2F2 were commonly reported.  In the early and mid-2000s, only a few by-products were thought to be significant and thus, it is assumed that only a few by-products were measured, calculated and reported.  In the IPCC Good Practice Guidance and Uncertainty Management in National Greenhouse Gas Inventories (2000), "the expert group determined that the only by-product emission of significance was that of CF4".  By 2006, more by-products were known to be significant (the 2006 IPCC Guidelines for National Greenhouse Gas Inventories included by-product emission factors for C2F6, CHF3 and C3F8), but by-product emission factors for C4F8, CH3F and C4F6 were not yet included.  However, in data sets submitted to the EPA in 2017 and 2020, BEFs for C4F8, CH3F and C4F6 were commonly reported for processes that did not use these gases as input gases.  The reporting of additional by-products increased the total estimated emissions per ton of input gas for many input gases.

Another factor that may be affecting the calculated emission factors in the proposed rule is the difference in the total and variety of input gas combinations included from the 2017 and 2020 data sets.   As most of the submitted emission factors and by-product emission factors from the 2007 and 2020 data sets used the "multi-gas" convention, only 1-Us and BEFs from the following types of tests were included: single gas processes, multi-gas processes where only one gas contained carbon and BEFs from multi-gas processes where the by-product was not also an input gas.  In the data submitted in 2013 and earlier, the 1-U values calculated for several input gases (e.g. CF4 and CHF3) tend to be significantly higher for multiple input gas combinations that have more than one carbon containing gas.  (As discussed in Section 2.2, this is likely partially a consequence of assigning all emissions of those input gases to their 1-U values even when some of the emissions were likely generated as by-products of other carbon-containing input gases.) As a result, when only single gas and multiple gas processes with only one carbon containing gas are included, the average 1-U value is expected to decrease.   In summary, the limited group of emission factor data retained from the triennial reports is not fully representative of the industry's overall emissions.  Table 2.6 below shows the average emission factor reported for CHF3 by gas combination type for data submitted in 2013 or earlier.

Table 2.6:  Average Emission Factors for CHF3 Etch for 300-mm Fabs by Gas Combination (Data submitted in 2013 or earlier)
Gas Combination
Single Gas
CHF3 with NF3
CHF3 with SF6
CHF3 with other carbon containing gases
CHF3 (1-U)
0.37
0.20
0.38
0.51

This hypothesis is supported by histograms for the complete set of 2013 CHF3 1-U values, for the complete set of 2017/2018 CHF3 1-U values, and for modified sets of these values that exclude tests with carbon-containing input gases, shown in Figure 2.2.

            Figure 2.2: Histograms of Four Sets of CHF3 1-U Values

The histograms for the full 2013 and 2017/2020 datasets show that the higher CHF3 1-U average seen in the full 2013 dataset is the result of (1) the higher proportion of values between 0.2 and 0.4 in the 2013 dataset, and (2) the longer tail of values above 0.4 in the 2013 dataset. Removing the tests with other carbon-containing input gases results in a shorter, thinner tail for the 2013 dataset, reducing the impact of the second of these two influences. (The removal has little impact on the 2017/2020 dataset because only one value was from a test using other carbon-containing input gases.) Most of the remaining tail above 0.4 is attributable to single gas tests in 2013 data set.    Removing the single gas tests results in both data sets having results in the same narrow emission factor range with the exception of one data point in each set above 0.4.  This is reflected in the averages calculated before and after the removal of the tests with other carbon-containing input gases and single gas tests, shown in Table 2.4:

Table 2.4: Average 1-U Values for CHF3 for 300 mm Fabs Calculated Including and Excluding Tests with Other Carbon-Containing Input Gases

                                 2013 Dataset
                               2017/2020 Dataset
All tests including tests with other carbon-containing input gases and single gas tests
                                     0.42
                                     0.18
Multi-gas Tests excluding tests with other carbon-containing input gases
                                     0.29
                                     0.19

The differences between the two data sets when comparing only multigas tests excluding carbon-containing input gases was not statistically significant.

As noted in Table 2.5, the difference in the BEF for C2F6 from the use of CF4 was also statistically significant.

        Table 2.5: Average EF values for BC2F6 from CF4 for 300 mm fabs

                                 2013 Dataset
                               2017/2020 Dataset
Average
                                     0.080
                                     0.027
Average for only single gas tests
                                     0.044
                                     0.034
Average for multi-gas tests
                                     0.115
                                     0.025

The most significant difference between the two data sets for the etch process type appears to be from the BC2F6 values reported for multi-gas tests.  The source of the difference between the two data sets could not be identified by the data available.  Processes may have changed since 2013 in a way that has led to lower by-product emissions of C2F6.

There was also a significant change in the calculated by-product emission factor for CF4 from NF3 usage in remote plasma clean processes (from 0.05 when only averaging the 2013 300 mm data to a proposed value of 0.037, when including the data from the Triennial Reports).  The data contains one outlier value; however, upon review by the data submitter, only corrections to the corresponding (1-U) were identified.

Table 2.5: Average BEF values for BCF4 from NF3 in Remote Plasma Clean for 300 mm fabs

                                 2013 Dataset
                               2017/2020 Dataset
Average
                                     0.050
                                     0.010
Average, with outlier excluded
                                     0.024
                                     0.010

The emission factor (1-U) for N2O used in CVD processes for 300 mm wafer sizes in Table I-8 was calculated from the 17 data points received where the process type and wafer size were known.  No data was available for N2O used in other processes or for other wafer sizes.  Thus, all other emission factors for N2O used for semiconductor manufacturing remained as 1 in Table I-8.  For N2O used in CVD for LCD manufacturing, we are proposing to adopt the value published by the IPCC in the 2019 Refinement.  As LCD are not currently manufactured in the US, no US data is currently available.
Emission factors in Tables I-11 and I-12 were calculated using a gas consumption (by process type) weighted average of the Tables I-3 and I-4 emission factors for each wafer size.   Where an emission factor is listed as NA in the proposed Tables I-3 and I-4 and no emissions for that gas are expected, that emission factor is treated as zero when calculating the average.  Where an emission factor is listed as NA and emissions of the gas are expected (e.g. the gas is used as input gas based on consumption data but no emission factor data is available) in Tables I-3 and I-4, 0.8 for the 1-U, 0.15 for BCF4, and 0.05 for BC2F6 are used when calculating the average.  Gas consumption by process type at an industry level was estimated from emissions from unabated fabs reported to the US EPA's Greenhouse Gas Reporting Program in 2013.  One facility was removed from the 300 mm consumption weighting calculation due to a likely error in wafer size reported.
Table 2.2 shows the gas consumption weighting by process type and wafer size used to develop the Table I-11 and I-12 emission factors.

                                   Table 2.2
               GAS/PROCESS TYPE WEIGHTING FACTORS BY WAFER SIZE
Input Gas
CVD In Situ Plasma Weight
CVD Remote Plasma Weight
CVD In-Situ Thermal Weight
Wafer Clean and Etch Weight
<=200mm wafer size
C2F6
                                                                           0.96
                                                                           0.00
                                                                           0.00
                                                                           0.04
C2HF5
                                                                           0.00
                                                                           0.00
                                                                           0.00
                                                                           1.00
C3F8
                                                                           1.00
                                                                             NW
                                                                           0.00
                                                                           0.00
C4F6
                                                                           0.00
                                                                           0.00
                                                                           0.00
                                                                           1.00
C5F8
                                                                           0.00
                                                                           0.00
                                                                           0.00
                                                                           1.00
C4F8
                                                                          0.50*
                                                                           0.00
                                                                           0.00
                                                                          0.50*
CF4
                                                                           0.34
                                                                           0.00
                                                                           0.00
                                                                           0.66
CH2F2
                                                                           0.00
                                                                           0.00
                                                                           0.00
                                                                           1.00
CH3F
                                                                           0.00
                                                                           0.00
                                                                           0.00
                                                                           1.00
CHF3
                                                                           0.00
                                                                           0.00
                                                                           0.00
                                                                           1.00
NF3
                                                                           0.76
                                                                             NW
                                                                           0.00
                                                                           0.24
SF6
                                                                           0.09
                                                                           0.00
                                                                           0.00
                                                                           0.91
C4F8O
                                                                        1.00†
                                                                           0.00
                                                                           0.00
                                                                           0.00
300 mm wafer size
C2F6
                                                                           0.00
                                                                           0.00
                                                                           0.00
                                                                           1.00
C3F8
                                                                           0.00
                                                                             NW
                                                                           0.00
                                                                        1.00†
C4F6
                                                                           0.00
                                                                           0.00
                                                                           0.00
                                                                           1.00
C5F8
                                                                           0.00
                                                                           0.00
                                                                           0.00
                                                                           1.00
C4F8
                                                                           0.00
                                                                           0.00
                                                                           0.00
                                                                           1.00
CF4
                                                                           0.00
                                                                           0.00
                                                                           0.00
                                                                           1.00
CH2F2
                                                                           0.00
                                                                           0.00
                                                                           0.00
                                                                           1.00
CH3F
                                                                           0.00
                                                                           0.00
                                                                           0.00
                                                                           1.00
CHF3
                                                                           0.00
                                                                           0.00
                                                                           0.00
                                                                           1.00
NF3
                                                                           0.55
                                                                             NW
                                                                           0.00
                                                                           0.45
SF6
                                                                           0.00
                                                                           0.00
                                                                           0.00
                                                                           1.00
Sources:

EPA (2018) Envirofacts. Greenhouse Gas Reporting Program (GHGRP), Subpart I: Electronics Manufacturing. Available online at: <http://www.epa.gov/enviro/facts/ghg/search.html>. Accessed in October 2018.
Tier 2a NF3 and C2F6 Emission Factor Calculations.  November 8, 2012.  Available under U.S. EPA docket ID: EPA-HQ-OAR-2011-0028-90.

†No emission data was available.  Gas consumption weighting was assigned based on where an emission factor existed in Tier 2c (e.g. C4F8O for 200 mm or smaller wafer sizes was assigned a weighting factor of one for CVD in-situ plasma and zero for the other process types, as CVD in-situ plasma was the only process type that has emission factor data for 200 mm or smaller wafer sizes).
*Average of 2009 and 2013 consumption weighting factors.
NW means no weight assigned. For NF3 and C2F8, no weight is assigned to CVD remote plasma, as this gas/process type combination has its own emission factor in all Tier 2 methods and is not included in the gas weighting by process type.

Destruction and Removal Efficiency and Abatement Systems
Summary of Revisions
The EPA is proposing to make several revisions related to default destruction or removal efficiency (DRE) values and abatement systems, including the following:
1) revising the default destruction or removal efficiency (DRE) value for semiconductor
manufacturing facilities to establish new default DREs by gas to reflect new data,
2) removing the distinction in DRE by process type,
3) expanding the provisions required to claim the default for a particular abatement system and gas,
4)  adding a calculation method for calculating emissions of CF4 generated by abatement systems not certified to not produce CF4

This document describes the development of items 1, 2, and 4.  This document was developed with information multiple sources: (1) Ch. 3 Vol. 6 Electronics Industry Emissions in the 2019 Refinement to the 2006 IPCC Guidelines for National Greenhouse Gas Inventories and (2) data provided in UPDATED-04-02-2018-Triennial Report Data for EPA.xlsx, all available in the docket EPA-HQ-OAR-2019-0424.
Description of Destruction or Removal Efficiency Data and Development of Proposed Default DREs
Currently, Table I-16 to subpart I lists default DREs by gas and process type. The DREs currently in Table I-16 were developed based on data submitted to the EPA, using analysis of variance (ANOVA) to set conservatively low DREs based on the expected variability of fab-average DREs across fabs. A conservative approach was used because the DRE performance of abatement equipment can vary depending on the specific design of each manufacturer, which is often proprietary, and the actual conditions in the fab. For the combined carbon-based etch gases, the default DRE for combined gases was calculated similarly to the default DRE for individual gases, with the exception that a fixed number of DRE counts, fab counts, and abatement systems per fab were assumed for each gas so that the variance components for fabs and devices were the same for each gas.   Where data was unavailable for some gas and chamber cleaning process combinations, DRE values were set to a conservative value of 60 percent.

New data used to develop the DRE default values were provided by US fabs through the 2017 Triennial Report (UPDATED-04-02-2018-Triennial Report Data for EPA.xlsx) and 2020 Triennial Report (032020-Triennial Report Data for EPA.xlsx).  This data set was combined with data previously submitted to the EPA by US Fabs.  The DRE data sets submitted to the EPA by the U.S. electronics manufacturing industry contains 1,160 data points for DREs of F-GHGs and 26 data points for DREs of N2O.  Following the same criteria as in earlier analysis,[4] EPA is proposing to remove data from the calculation of default DREs from systems that were not properly installed, operated, and/or maintained.  Data points are also proposed to be excluded from the calculation of default DRE if the data points were reported as corresponding to emissions control systems that were not certified to abate that particular gas, to avoid biasing the data low; subpart I currently only allows facilities to claim abatement for systems that are certified to abate the particular gas.  Additional data points were removed from the calculation of the proposed DRE if an accurate DRE could not be measured due to detection limits.  Negative DREs, corresponding to 26 data points, were also excluded from the calculation for all F-GHGs.  A calculated negative DRE value for a particular gas would indicate that the mass of that gas emitted at the outlet of an emissions control system is higher than the mass of that compound at the inlet of the emissions control system. This phenomenon can occur if that gas is formed within the emissions control system (resulting in a higher mass of that gas at the outlet of the emissions control system than at its inlet), but a reported negative DRE value may also result from experimental errors (e.g. due to incorrect measurements of concentrations or dilution factors).  All but five of the negative DREs submitted for F-GHGs were on emissions control devices that were not designed to abate that particular gas and thus already excluded from this analysis.  For the five cases where the system was designed to abate the gas, the testers noted either low gas inlet, inherent noise in the measurement (e.g. due to low inlet), or detection limit constraints.  For the data points from devices not certified to abate the input gas, the negative DREs were reported only for CF4 from remote plasma cleaning processes and COF2 from etch processes (13 and 8 data points, respectively).  Results from literature (S.N. Li, et al., "FTIR spectrometers measure scrubber abatement efficiencies," Solid State Technology, Vol. 45 (2002); Gray, Fraser, and Afroza Banu, "Influence of CH4-F2 mixing on CF4 by-product formation in the combustive abatement of F2," Research Disclosure Sept 2018) indicate that CF4 can be formed within some abatement systems that use hydrocarbon fuels by reaction between the fuel and fluorinated species (e.g., F2) emitted from a NF3 remote plasma chamber clean.  However, since not all abatement systems form CF4, these data points are excluded from this analysis and this formation of CF4 is accounted for separately in the proposed rule.  Lastly, data points were excluded if the gas being measured was noted as being less than 2% of the total inlet fluoride to the abatement system during testing, as the error in measuring DREs for such low inlet concentrations may have biased the DREs low.  Low total inlet fluoride concentration (less than 2%) was noted for 9 data points.   58 of the data points corresponded to gases (C2F4 and COF2) that are not known to be input gases or significant by-products.  Table 3.1 summarizes the DRE data from all fluorinated GHGs that have emission factors in Table I-3 or I-4.

Table 3.1 Summary and Analysis of All DRE Measurement Data Submitted to EPA
Gas
                        Number of Data Points Available
               Number of Data Points Meeting Inclusion Criteria
                                   Mean (%)
                            Standard Deviation (%)
                      2021 Proposed Rule (All processes)

                               Current Etch DRE

                               Current Clean DRE
NF3
                                      431
                                      407
                                     95.5
                                      7.9
                                      96
                                      96
                                      88
CF4
                                      287
                                      247
                                     87.5
                                     15.8
                                      87
                                      75
                                      60
CHF3
                                      183
                                      169
                                     97.1
                                      0.9
                                      97
                                      97
                                      60
SF6
                                      98
                                      93
                                     94.8
                                      8.0
                                      95
                                      97
                                      60
C2F6
                                      89
                                      63
                                     97.9
                                      3.4
                                      98
                                      97
                                      60
c-C4F8
                                      57
                                      51
                                     93.3
                                     15.1
                                      93
                                      97
                                      60
CH2F2
                                      74
                                      63
                                     98.3
                                      3.5
                                      98
                                      97
                                      60
C4F6
                                      31
                                      28
                                     95.3
                                      7.9
                                      95
                                      97
                                      60
CH3F
                                      43
                                      29
                                     98.3
                                      3.5
                                      98
                                      97
                                      60
C3F8
                                       1
                                       0
                                      NA
                                      NA
                                      98
                                      97
                                      60
C5F8
                                       1
                                       1
                                     96.6
                                      NA
                                      97
                                      60
                                      60
C2HF5
                                      NA
                                       0
                                      NA
                                      NA
                                      97
                                      60
                                      60
C4F8O
                                      NA
                                       0
                                      NA
                                      NA
                                      93
                                      60
                                      60

Data was submitted for both etch and clean process types for some input gases (see Table 3.2 below).  For the gases where there was sufficient data, the difference in the DREs by process type were determined not to be statistically significant (see Combined DRE Data Sets.xlsx).  Thus, the EPA is proposing to assign chemical-specific DREs to all commonly used F-GHGs for the semiconductor manufacturing sub-sector without distinguishing between process types.
Table 3.2 Summary and Analysis of All DRE Measurement Data Submitted to EPA

                                Clean processes
                                Etch Processes
Gas
                        Number of Data Points Available
               Number of Data Points Meeting Inclusion Criteria
                                   Mean (%)
                            Standard Deviation (%)
                        Number of Data Points Available
               Number of Data Points Meeting Inclusion Criteria
                                   Mean (%)
                            Standard Deviation (%)
NF3
                                      314
                                      296
                                     95.1
                                      8.3
                                      117
                                      111
                                     96.6
                                      6.5
CF4
                                      19
                                       1
                                     89.8
                                      N/A
                                      268
                                      246
                                     87.5
                                     15.8
CHF3
                                      10
                                       2
                                     99.5
                                      0.1
                                      173
                                      167
                                     97.1
                                     16.7
C2F6
                                      14
                                       4
                                     96.4
                                      2.1
                                      75
                                      59
                                     98.1
                                      3.4

For several gases, C4F8O, C5F8, C2HF5, and C3F8, insufficient data was available to derive default DRE values.  Instead, EPA is proposing to assign these DREs by analogy with gases with similar chemical structures.  Table 3.2 above includes proposed DREs for C2HF5, C5F8, and C4F8O based on their similarity to CHF3 (for C2HF5 and C5F8) and c-C4F8 (for C4F8O).  This results in C2HF5 and C5F8 being assigned a default DRE of 97 percent. Based on its similarity to c-C4F8, C4F8O is assigned a default DRE of 93 percent. The EPA is proposing to base the DRE for C3F8 on the DREs for a chemically similar F-GHG for which the EPA does have data. C3F8 is expected to be no more difficult to abate than C2F6. Thus, the EPA is proposing to apply the DRE value for C2F6 (98 percent) to C3F8.

In the case of N2O, 16 out of the 21 data points corresponded to abatement systems that were not certified to abate N2O.  Of the 16 data points corresponding to uncertified abatement systems, one was also identified as being detection-limited and 7 data points corresponded to negative DRE values. Negative N2O DREs are likely resulting from the formation of N2O within abatement systems.  The average of the 5 data points corresponding to abatement systems certified to abate N2O was 52.4%.   The notes to 4 of the 5 data points indicates that the systems being tested were certified to abate N2O at 50%, indicating, on average, similar performance to the certified value.  When uncertified values were included, excluding negative values, detection limited values and instances where the test indicated a problem with the krypton, the average DRE increased to 63.7%, indicating that many systems may achieve values greater than 50% but may not be certified.  Thus, the DRE of 60% in the current rule was retained, as some systems may be certified at a value of 60%.
CF4 from Emissions Control systems

As discussed in the 2019 Refinement, a direct reaction between molecular fluorine (F2) and hydrocarbons (e.g. CH - 4) can occur in hydrocarbon-fueled combustion abatement systems to form CF4.[,] Generation of CF4 is possible during abatement of remote plasma cleaning (RPC) processes that use NF3 or F2 as the cleaning gas.  During these processes, large amounts of F2 can be formed as a by-product of the decomposition of the cleaning gas into atomic fluorine (F radicals and excited species), and subsequent recombination of the unreacted atomic fluorine into F2. The extent to which CF4 is actually generated under these conditions depends on the design of the hydrocarbon-fueled combustion abatement system.
When NF3 is used as a cleaning gas during the RPC process, the vast majority of the NF3 molecules (around ~98.3% based on the current default values for NF3 in remote plasma clean processes in Tables I-3 and I-4) are decomposed within the remote plasma tool to form fluorine and nitrogen radicals, ions, and excited species. While some of the fluorinated species remove the solid residues deposited on chamber walls and form gaseous by-products such as SiF4, HF, COF2, and CF4, some fluorine atoms also recombine to form F2. Based on confidential data received from Edwards, Ltd., a manufacturer of abatement equipment, the proposed by-product emission factor for F2 (BF2,NF3) from NF3 used in remote plasma clean processes is 0.5. Thus, for every kilogram of NF3 consumed during the RPC process, an average 0.5 kg of F2 enters the abatement system from the remote plasma tool and is available for potential reaction with CH4 (or other hydrocarbons) to form CF4. This data from Edwards was also used to update the 2019 Refinement. The proposed by-product formation factor is reasonable considering that emission factor data for NF3 from RPC processes submitted to the EPA show that emissions of NF3 and CF4 accounted for less than 20 percent of the total mass of fluorine in the NF3 used in the process in all but one of the 123 processes measured, and generally significantly less, leaving at least 80 percent of the fluorine available to form F2, SiF4, and HF. (The last two compounds are formed when fluorine combines with solid residue on the chamber walls, but the EPA's emission factor data do not include specific information on the quantities of those compounds -- or of the fluorine -- formed in the process.)
The proposed Equation I-9 would be used to estimate the emissions of CF4 from hydrocarbon-fuel based emissions control systems that are not certified not to generate CF4. (In this section III.E.1.d, all references to "uncertified hydrocarbon-fuel based emission control systems" refer to hydrocarbon-fuel based emissions control systems that are not certified not to generate CF4.) The equation calculates CF4 emissions by accounting for the mass of the fluorine entering uncertified hydrocarbon-fuel based emissions control systems (the product of the consumption of the input gas, the emission factor for fluorine from the input gas, ai, where ai is the ratio of the number of tools with uncertified abatement devices for the gas-process combination to the total number of process tools for the gas-process combination and the average uptime of abatement tools that use the gas-process combination) and multiplying that mass by a CF4 emission factor, ABCF4,F2. The proposed default emission factor for this reaction (ABCF4,F2) is 0.116.
EABCF4 = jCF2,j::1 - UF2,j::aF2,j::UTF2, j::ABCF4,F2+CNF3,RPC∙BF2,NF3∙aNF3,RPC∙UTNF3,RPC∙ABCF4,F2  (Eq. I-9)
Where:
EABCF4 = emissions of CF4 from hydrocarbon-fuel-based emissions control systems when direct reaction between hydrocarbon fuel and F2 is not certified not to occur by the emissions control system manufacturer or electronics manufacturer, kg
CF2,j = Amount of F2 consumed for process type or sub-type j, as calculated in Equation I-13 of this subpart, on a fab basis (kg).
UF2,j = Process utilization rate for F2 for process type or sub-type j (expressed as a decimal fraction).
aF2,j = Within process sub-type or process type j, fraction of F2 used in process tools with hydrocarbon-fuel-based abatement systems that are not certified not to form CF4, on a fab basis, where the numerator is the number of tools that are equipped with hydrocarbon-fuel-based emissions control systems that are not certified not to form CF4 that use F2 in process type j and the denominator is the total number of tools in the fab that use F2 in process type j (expressed as a decimal fraction).
UTF2,j = The average uptime factor of all abatement systems connected to process tools in the fab using F2 in process sub-type or process type j (expressed as a decimal fraction).
ABCF4,F2   = mass fraction of F2 in process exhaust gas that is converted into CF4 by direct reaction with hydrocarbon fuel in a combustion abatement system. The default value of ABCF4,F2 = 0.116.
CNF3,RPC  = Amount of NF3 consumed in remote plasma cleaning processes, as calculated in Equation I-13 of this subpart, on a fab basis (kg)
BF2, NF3 	= By-product formation rate of F2 created as a by-product per amount of NF3 (kg) consumed in remote plasma cleaning processes (kg).
aNF3,RPC = Within remote plasma cleaning processes, fraction of NF3 used in process tools with hydrocarbon-fuel-based abatement systems that are not certified not to form CF4, where the numerator is the number of tools running remote plasma cleaning processes that are equipped with hydrocarbon-fuel-based emissions control systems that are not certified not to form CF4 that use NF3 and the denominator is the total number of tools that run remote plasma clean processes in the fab that use NF3 (expressed as decimal fraction).
UTNF3,RPC,F2 = The average uptime factor of all abatement systems connected to process tools in the fab emitting by-product gas F2, formed from input gas NF3 in remote plasma cleaning processes, on a fab basis (expressed as a decimal fraction). For this equation, UT NF3,RPC,F2 is assumed to be equal to UT NF3,RPC as calculated in Equation I-15 of this subpart.
j = Process type  or sub-type.
Similarly, when F2 is used as a cleaning gas during the RPC process or direct F2 cleaning of diffusion furnaces, incomplete decomposition of the cleaning gas and the recombination of some fluorine atoms into molecular fluorine also occurs. Thus, not all available fluorine species react to clean the chamber walls and some F2 enters the abatement system, potentially reacting with CH4 (or other hydrocarbons) to form CF4. In the case of F2-based cleaning, the formation of CF4 is described by the first term of Equation I-9 (CF2,j::(1-UF2,j)::aF2,j::UTF2,j::ABCF4,F2). As in the case of NF3-based cleaning, a value of ABCF4,F2 = 0.116 should be used to estimate EABCF4,F2.  No data from processes that use F2 as an input gas are currently available for the 1-U of F2; however, data from NF3-using processes (where most of the NF3 is dissociated into atomic fluorine during the process) indicate that the 1-U value for F2 may be near 0.7.  As for other gas and process combinations where no data is available (listed as "NA" in Tables I-3 and I-4), a 1-U of 0.8 would be used for F2 in Equation I-9 for all process types, consistent with the 2019 Refinement.
The default factor for conversion of F2 into CF4 within abatement systems (ABCF4,F2=0.116) was estimated for the 2019 Refinement based on a series of experiments simulating use of different combustion technologies at different flow rates of CH4 fuel gas addition.[7,8]
The authors of the 2019 Refinement calculated ABCF4,F2 using the following equation:
ABCF4,F2=1kgF2::MCF4MF2 ::0.05=0.116
Where:
      ABCF4,F2 = Mass fraction of F2 emitted from the RPC process that is converted into CF4 by direct reaction with hydrocarbon fuel in a combustion emissions control system.
      MCF4 = Molar mass of CF4 (88 g/mol).
      MF2 = Molar mass of F2 (38 g/mol).
      0.05 = Molar conversion rate of F2 into CF4.

Revisions to the Stack Testing Method
The stack testing method provides reporters an alternative to using default emission factors to estimate annual F-GHG emissions.  The EPA is proposing several revisions to the stack testing method to for subpart I, including adding new equations and a table of default weighting factors to calculate the fraction of fluorinated input gases and by-products exhausted from tools with abatement systems; revising equations that calculate the weighted average DREs for individual fluorinated greenhouse gases (F-GHGs) across process types; requiring that all stacks be tested if the stack test method is used; and updating a set of equations that will more accurately account for emissions when pre-control emissions of a F-GHG approach or exceed the consumption of that gas during the test period. These revisions, in combination with the DRE revisions described above, are expected to have the following benefits:
 Improve accuracy of emission estimates calculated using the stack test method
 Decrease the burden associated with using the stack test method by removing the need to develop an apportioning model when using the stack test method
Revisions to simplify the stack test method
In order to simplify the stack testing method and remove the need to develop an apportioning model while retaining accuracy, several modifications to the existing stack testing method are necessary.

First, new Equations I-24C and I-24D and a table of default "gamma" weighting factors (new Table I-18) are proposed to be added to calculate the fraction of fluorinated input gases exhausted from tools with abatement systems, ai,f, for use in Equations I-19A through I-19C and I-21, and the fraction of by-products exhausted from tools with abatement systems, ak,i,f, for use in Equations I-20 and I-22. The development of the gamma weighting factors is discussed in section 4.1.  Second, we are proposing to revise Equations I-24A and I-24B, which calculate the weighted average DREs for individual F-GHGs across process types in each fab.  Third, we are proposing at 40 CFR 98.93(i)(3) to require that all stacks be tested if the stack test method is used.

These changes to the stack test method would remove the requirements to apportion gas consumption to different process types, to manufacturing tools equipped versus not equipped with abatement systems, and to tested versus untested stacks. Currently, the fractions of fluorinated input gases and by-product gases exhausted from manufacturing tools with abatement systems, used in equations 1-19a through I-22, must be estimated by apportioning gas consumption to these tools. The proposed Equations I-24C and I-24D would add the option to calculate the fraction of each input gas "i" and by-product gas "k" exhausted from tools with abatement systems based on the number of tools that are equipped versus not equipped with abatement systems, along with weighting factors that account for the different per-tool emission rates that apply to different process types. Facilities would continue to have the option to apportion gas consumption to tools with and without abatement systems by using paragraph 98.93(e). They would also have the option to apportion gas consumption to the different process types and sub-types, calculating ai,f and ak,i,f based on the numbers of tools with and without abatement systems within each process type or sub-type. Weighting factors are discussed in section 4.2.