by Debra Vogler, Senior Technical Editor
The advent of 193nm (ArF) lithography brought with it a different kind of issue: contaminated scanner optics caused by the formation of silicon-containing organic compounds, called refractory organics (aka low-molecular-weight silicon-containing organic species [LMWS]). These compounds arise as the energetic beamline ionizes chemicals that are present in the vicinity of the beamline, or creates free radicals — both of which are highly reactive and can combine with other molecules that can form larger molecules that condense on the optics. Over time, these larger molecules build up and cause lens contamination that results in flare (i.e., scattered light) and incorrect printing.
One of the biggest challenges in the fab with respect to keeping scanner optics clean, according to David Ruede, VP and GM of Entegris’ gas microcontamination business unit, is contending with silicon containing species such as HMDS, an adhesion promoter that keeps photoresist from peeling. “Track manufacturers have done a really good job of making sure that HMDS vapors aren’t being given off from the track tools into the cleanroom,” said Ruede, Still, very low levels of process chemicals entering the ambient from silicon-containing organics, “even at the sub-parts-per-billion level, can accumulate on and contaminate lens surfaces over time,” he noted.
In particular, HMDS compounds are very photoactive, especially when they get in the beamline at 193nm — they will react and form ions and can combine with other things, and these could end up on the lens as SiO2 deposits, Ruede explained.
Lens elements exposed to major contamination problems must be taken off and replaced because they are not cleanable in the field. “Most of the silicon-containing organics are very large and very sticky, so we can get them with our normal condensable organic filtration mechanism,” he said. Yet it’s the smaller LMWS that need to be prevented from reaching the optics — and a technique for accomplishing this goal was presented at the SPIE Advanced Lithography Symposium. 
According to Ruede, a new absorbent material used in Entegris’ filter keeps the “sticky” organic HMDSO in its condensable form, preventing it from rearranging into TMS, which then would propagate through the filter and ultimately reach the optics. (Figure 1 illustrates the chemical compounds HMDS, TMS, and HMDSO.)
Part of the development process included working for almost two years on the kinetics and the analytical capability to measure TMS. Ruede explained that there are no commercial sources for reagent-grade TMS, and it’s not easy to measure it — even analytical/measurement tools can affect the results/reading of TMS in the cleanroom. Also, TMS has a very low boiling point, so it can pass quickly through the filter, whereas HMDSO is very condensable, he noted.
Ruede says the company now has an analytical service for both fabs and scanner manufacturers that baselines their cleanrooms for acids, bases, and low-molecular-weight silicon-containing compounds, and provides the improved filter set to enable additional protection against LMWS. — D.V.
1. A. Grayfer, O. P. Kishkovich, F. V. Belanger, P. Cate, D. Ruede, “New Filter Media Development for Effective Control of Trimethylsilanol (TMS) and Related Low Molecular Weight Silicon Containing Organic Species in the Photobay Ambient,” #6518-157, poster session, SPIE Advanced Lithography Symposium, Technical Abstract Summary Digest, p. 81, 2007.
FIGURE CAPTION: Chemical compounds of a) HMDS, b) HMDSO, and c) TMS. (Source: Entegris)