Characterization of Collector Optic Material Samples Before and After Exposure in Laser Produced Plasma and Discharge Produced Plasma EUV Sources

August 16, 2006

J. Microlithography, Microfabrication, and Microsystems, 5(3), 033006(1-11) (2006).

Qiu, H., Alman, D. A., Thompson, K. C., Spencer, J. B., Antonsen, E. L., Jurczyk, B. E., Ruzic, D. N., Spila, T. P.

The University of Illinois at Urbana-Champaign (UIUC) and several national laboratories are collaborating on an effort to characterize Xe plasma source exposure effects on extreme ultraviolet (EUV) collector optics. A series of mirror samples provided by SEMATECH were exposed for 10 million shots in an Xtreme Technologies XTS 13-35 commercial EUV discharge produced plasma (DPP) source at UIUC and 500,000 shots at the high-power TRW laser produced plasma (LPP) source at Sandia National Laboratory, Livermore (SNLL). Results for both pre- and post-exposure material characterization are presented for samples exposed in both facilities. Surface analysis performed at the Center for Microanalysis of Materials at UIUC investigates mirror degradation mechanisms by measuring changes in surface roughness, texture, and grain sizes as well as analysis of implantation of energetic ions, Xe diffusion, and mixing of multilayers. Materials characterization on samples removed after varying exposure times in the XTS source identify the onset of different degradation mechanisms within each sample over 1 million to 10 million shots. Results for DPP-exposed samples for 10 million shots in the XCEED (Xtreme Commercial EUV Emission Diagnostic) experiment show that samples are eroded and that the surface is roughened with little change to the texture. Atomic force microscopy (AFM) results show an increase in roughness by a factor of 2 to 6 times, with two exceptions. This is confirmed by x-ray reflectivity (XRR) data, which shows similar roughening characteristics and also confirms the smoothening of two samples. Scanning electron microscopy (SEM) pictures showed that erosion is from 5 to 54  nm, depending on the sample material and angle of incidence for debris ions. Finally, microanalysis of the exposed samples indicates that electrode material is implanted at varying depths in the samples. The erosion mechanism is explored using a spherical energy sector analyzer (ESA) to measure fast ion species and their energy spectra. Energy spectra for ions derived from various chamber sources are measured as a function of the argon flow rate and angle from the centerline of the pinch. Results show creation of high-energy ions (up to E=13  keV). Species noted include ions of Xe, Ar (a buffer gas), and various materials from the electrodes and debris tool. The bulk of fast ion ejection from the pinch includes Xe+, which maximizes at ~8  keV, followed by Xe2+, which maximizes at ~5  keV. Data from samples analysis and ESA measurements combined indicate mechanism and effect for debris-optic interactions and detail the effectiveness of the current debris mitigation schemes.