Response Summary: stsetch systematic problems
mcvittie at snf.stanford.edu
Mon Dec 5 11:36:27 PST 2005
Last year, when the STS process engineer was here from the UK. I had a brief
disccussion with him about our problem with byproduct buildup in our vacuum
line for STS1. He said on the older systems most users had gone to adding O2
to their processes to prevent the problem. On the new sydsem to eliminate the
problem everything is heated until the water cooled trap.
Matt Hopcroft wrote:
> I received a number of helpful and interesting replies to my
> email last week about the sts. Thanks to all who replied! Here is a
> summary of the answers:
> Executive Summary:
> The chamber pressure has been increasing over time due to machine
> aging. Increased pressure causes reduced sidewall polymerization, leading
> to sidewall blowouts. Conversely, increased pressure can contribute to
> micromasking ("grass") and imcomplete etching. Doing a chamber clean
> before etching is recommended.
> 1) Other people have noticed an increase in problems within a 12-18 month
> 2) A likely issue is changing pressure in the chamber during the
> etch. The chamber pressure has apparently increased since the etch
> recipes were first developed. Increased chamber pressure can cause reduced
> polymerization during the passivation step and lead to sidewall blowouts.
> 3) Two maintenance issues should be considered: the condition of the
> vacuum foreline (which evacuates gas from the etch chamber) and the APC
> valve (which regulates the chamber pressure). These both affect the
> pressure in the chamber. users reported better results immediately
> after the foreline had been cleaned and with decreased APC settings (for
> decreased pressure).
> 4) The "footing" and "mousebite" pictures that I showed are not good
> examples of either. They are more accurately described as sidewall etching
> due polymer breakdown related to point 2).
> 5) The incomplete etching problems are most likely lithography issues
> specific to our design and exacerbated by point 2).
> Possible Remedies:
> - Running a chamber clean (O2 clean) and then seasoning the chamber with a
> dummy wafer before etching device wafers.
> - Modifying recipes to increase the CF4 polymerization levels and reduce
> etch pressure.
> - Monitor and record the actual chamber pressures during your etch observe
> how they correlate with your results.
> -Matt Hopcroft
> hopcroft at snf.stanford.edu
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