Response Summary: stsetch systematic problems

Matt Hopcroft hopcroft at snf.stanford.edu
Sat Dec 3 15:13:21 PST 2005


Hello,
	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.


Summary:

1) Other people have noticed an increase in problems within a 12-18 month
timeframe.

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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