Microtechnology - Dry etching

Contrary to wet etching, that requires the immersion of the target in a gas, or more commonly in a liquid, dry etching consists in using plasma (partially ionized gases) to etch the target.
It is a more precise and controllable form of etching from the microtechnology point of view in a lot of cases, but also a more expensive one.
This page intends to introduce the principles and provides example of dry etching technics.

This page contains several parts:

What is a plasma?

Dry etching can be associated with the use of a plasma. Before talking about the etching technics, we need to be clear about what a plasma is.

Gases are made of molecules with no linking force between them (by opposition to liquids, and even more to solids!). A gas naturally has a small ionization rate, this means a small quantity of ionized molecules compared to the total quantity of molecules in a given volume. These ions are created and ions recombines very fast inside the gas.
A plasma can be seen as a highly ionized gas, so that the global «cloud» has a neutral electric charge, but at a local scale, you can see electric charges for a not so short time.
This definition is not very academic, but it describes quite well what a plasma is. A ionized gas, so ionized that it local charges are large enough with a long enough lifetime to produce effects!

Difference between a gas and a plasma, plasma have far more ionized molecules
Schematics of a gas and its comparizon with plasma

There is several ways of producing a plasma from a gas. In every cases, it is a question of energy brought to the molecules. If you excite molecules with a high energy, the link between its components will break up, and ions will separate.
In microtechnology, plasmas are made either by increasing temperature, or by using electromagnetic excitation. In microtechnology's dry etching, this is the second method that is used.

Plasma is considered as a fourth state of matter. As the ionization is all but natural for the molecules forming the gas, it is chemically highly reactive. This makes plasma very interesting in microtechnology processing!
In the following sections, we will see the different ways plasma are used for dry etching.

Plama Etching - PE

As its name tells, Plasma Etching, that we will note more commonly PE, is done by ionizing a gas mix inside a chamber to obtain ions that will react with the target material.

PE chamber, in which ions etch a material layer on a wafer through a mask. Top electrode is connected to R.F. signal, wafer support to the ground.
Schematics of the principle of Plasma Etching (PE)

This video shows the complete plasma etching process (Xvid format).

The ionization of the gases is done by R.F. excitation emitted by an electrode at the top of the chamber. The target wafer is placed on an electrode connected to the ground. The pressure inside the chamber is in the range of a few mTorr to a few hundreds of mTorr. The random movement of the ions inside the chamber makes them reach the target and a chemical reaction etch the material.
Plasma etching makes highly isotropic etching.

The process depends, of course, on the chamber's geometry, that is not controllable. What the operator has to set as parameters are: gas flow, pressure and R.F. power.
As for every etching technics, the gases nature determine the efficiency and selectivity of the etching. This is the first point to choose when calibrating an etching recipe. An equilibrium between R.F. power and gas flow controls of the ionization rate of the gases, while a higher pressure increases the probability of ions to get in contact with the target material.
Also note that the gases flows control the rate at which products are evacuated from the chamber.

RIE is a variation of PE. The principle of etching device is almost the same, but this time, the chamber top electrode is connected to the ground and the wafer is placed on the excitation electrode. This makes a large difference...

Reactive Ion Etching - RIE

In RIE, the products are accelerated in the direction of the wafer. This has consequencies on the etching process, especially if you want a more anisotropic etching!

RIE chamber, in which top electrode is connected to the ground and wafer support to the R.F. signal.
Schematics of RIE chamber

Since the wafer is connected to the R.F. signal instead of the ground in PE case, electrons are statistically more often in contact with the target than positive ions (they are far lighter!). Electrons are highly reactive species, so they are easily adsorbed by the target material, polarizing it negatively. In the same time, the loss of electrons in the plasma makes a global positive charge in the ions «cloud».
This produces an acceleration of the ions toward the wafer!

RIE effect: electrons are adsorbed by the wafer, creating an electric field between the wafer and the ions cloud
Schematics of the RIE effect

The first noticable effect is the apparition of another etching phenomena due to the ions velocity: physical etching! The layer molecules are stripped away by impact of ions on the wafer. Physical etching is characterized, of course, by a very low selectivity, but is highly anisotropic!

The second effect is that contrary to plasma etching, where ions move randomly above the target, in reactive ion etching, they have a higher probability to have a movement in the direction given by the electric field. This makes a tendancy to anisotropic etching.

Reactive ion etching: physical and vertically oriented chemical etching
Schematics of the physical and vertically directed chemical etching in RIE

RIE offers the possibility to set up anisotropic etching recipes. This depends on the type of material to etch. If it has a poor capacity to adsorb electrons and polarize, RIE produces the same effects as PE (not much difference). On the contrary, a material with a high capacity to adsorb electrons allows the calibration of highly anisotropic recipes.
Dielectric materials have a poor electron adsorption capacity, semiconductors a higher one. So, anisotropic etching recipes concern mainly silicon, polysilicon, composite materials, etc.
The difficulty of the calibration is an equilibrium to find so that the plasma provides a lot of electrons and ions, the electrons are adsorbed to the surface, ions etch the surface, but not too fast so that electrons have a long time enough to be adsorbed again (or you get an isotropic etching again!). Sometimes, the chemical reaction products must play a role in the process to enhance anisotropy.
Getting a high etching rate during an anisotropic etching is a not so easy task, and calibrating a recipe can need a lot of tries.

Compared to PE, RIE has a lot of advantages in term of anisotropy. Another kind of etching devices, more evolved, have been developed to get even higher anisotropy in etching process.

Inductively Coupled Plasma - ICP
& Deep Reactive Ion Etching - DRIE

Deep Reactive Ion Etching is an extension of Reactive Ion Etching to indicate that the etching is anisotropic and can be used to etch pattern very deeply in the substrate (as its name tells...)
In this technology, plasma is generated by a magnetic field, and densified by a second magnetic field generator. The objective is to reach a high ionization rate in the gases to enhance the RIE effect.
This is where we talk about Inductively Coupled Plasma, noted ICP.

Ion Beam Etching - IBE

Also called Ion Beam Milling, Ion Beam Etching is the extreme case of physical etching. It consists in using very high energy and a neutral gas (Argon) to make a pure physical etching of wafers.

Most of time, this is useful to etch metals and other materials that have tendancies to not react very well to chemical etching.
IBE is very efficient, and can etch almost anything. The counterpart is that physical etching is always done by ion bombardment, and this process generates heat, even more with a high energy. So IBE is very likely to burn a polymer mask, and etch any other material! The difficulty is to find an equilibrium between etching time, and temperature management.

IBE is rather anisotropic, but the problem is that you etch a large quantity of pure material, that has not reacted chemically, and is not volatile!
So, to avoid etched material molecules to redeposit on the target substrate, it is tilted, so that molecules have a tendancy to fall rather that redeposit!

IBE chamber with tilted target in front of the generator, so that etched molecules don't redeposit on the target
Schematics of an IBE chamber during process

O2 plasma stripper

Completely opposed to ICP, another use of plasma etching can be seen through the study of the O2 plasma stripper.
Here, plasma is generated by microwaves, and the goal is to avoid any physical etching while keeping an efficient chemical etching.

Examples of recipes and reactions