XCEED – XTREME Commercial EUV Exposure Diagnostic

February 26, 2009

An extreme ultraviolet light source is investigated at the University of Illinois. The source is a z-pinch plasma using Xenon gas with a short pulse width (~1 ms). As the plasma compresses, high energy photons in the extreme ultraviolet range are released and available for EUV lithography. The light emission is followed by ejection of multiply charged ions (+8-+10 Xe) which can significantly damage nearby mirror surfaces. After the fast ions pass, a slower moving cloud of ions and neutrals including Xenon and electrode material spread from the source causing continued coating and sputtering of the mirror surfaces.

The XCEED experiment serves to characterize the ejecta from a commercial extreme ultraviolet light source, investigate the damage mechanisms that affect nearby optics lifetime, and evaluate debris mitigation techniques that may be used to increase the lifetime of these mirror optics.

Characterization of the ejecta is performed through several diagnostics. A spherical sector energy analyzer (ESA) is used to diagnose fast ion species by energy-to-charge ratio using ion time of flight (ITOF) analysis. This instrument is capable of characterizing up to 15 keV ions emitted from the source. For neutral characterization, a set of Burle microchannel plates is placed directly in line with the pinch debris and a deflecting potential is used to divert ions from the detector. Signals from particle incidence on a Faraday Cup are also observed.

The effects of particle flux on mirror samples is investigated through exposure experiments and surface analysis. Samples are placed at normal and grazing incidences to the incoming particles and exposed for varying timeframes to the source. Photodiodes measure reflectivity degradation over time. The samples are removed and examined using Scanning Electron Microscopy (SEM), Atomic Force Microscopy, and other methods to analyze surface quality and particle deposition.

Recent modifications of the XCEED system also allow for the investigation of laser assisted discharge produced plasmas (LADPP) with a solid Sn electrode as the EUV fuel source. The laser ablates Sn atoms into the pinch produced plasma, where is it becomes highly ionized and emits a spectrum of light. A small portion of this light is transmitted in the EUV spectrum. The shift to Sn is ongoing within industry due to the increase in EUV emission conversion efficiency seen by Sn over Li and Xe. Laser ablation is carried out in the modified XCEED system using an Nd:YAG laser (100 Hz, 325 mJ/pulse, ~10ns pulse, capable of frequency doubling to 532nm and 266nm).