Advances in the understanding of low molecular weight silicon formation and implications for control by AMC filters

Abstract

1. ABSTRACT Trimethylsilanol (TMS) is a low molecular weight / low boilin g point silicon-containing, airborne contaminant that has received increased interest over the past few years as an important cause for contamination of optical surfaces in lithography equipment. TMS is not captured well by carbon-based filters, and hexame thyldisiloxane (HMDSO), even though captured well, can be converted to TMS when using acidic filter media comm only used for ammonia remova l. TMS and HMDSO co-exist in a chemical equilibrium, which is affected by the acidity and moisture of their environment. This publication shows that HMDSO is converted to TMS by acidic media at concentrations typically found in cleanroom environments. This is contrary to published results that show a re-combination of TMS to HMDSO on acid media. We also demonstrate that, based on its conversion to TMS, HMDSO is not a suitable test compound for hybrid chemical filter performance, as the apparent lifetime/capacity of the filter can be substantially skewed towards larger numbers when conversion to TMS is involved. We show lifetime test results with toluene and HMDSO on acidic and non-acidic filter media. Appropriately designed, asymmetric hybrid chemical filters significantly minimize or eliminate the conversion of HMDSO to TMS, thereby reducing the risk to scanner optical el ements. Similarly, such filters can also prevent or reduce acid-sensitive reactions of other AMC wh en passing through filter systems. Keywords: TMS, HMDSO, AMC, chemical filter, filter performance, silicon, silo xanes, photolithography, optics hazing. 2. INTRODUCTION Silicon containing hydrocarbons are a clas s of airborne molecular contamination (AMC) causing persistent degradation of UV exposure tool optical surfaces