STS2 Update -- Matching Network and Pulse Generator

Jim McVittie mcvittie at cis.Stanford.EDU
Thu Jul 29 21:37:47 PDT 2010

STS2 Users,

I was hoping to be able to tell you that I had found a specific problem
with the STS2 matching network, which would solve all the problems we have
had with it. But I was not able to find such a cure. My conclusion is that
the matching network is of an marginal design. Until we get a better long
term solution, we have to be careful how we setup recipes and how we make
changes to them. The tool is up and I am cuurently working with students
to get their previous recipes back up and running.  Here are the near term
solutions for the matching problem.

1. If you want to use STS2, I need to modify your recipe so it work well
with matching network.  Please send me the name of the recipe you want me
to look at. It can take me a hour to modify a recipe if it is new, so I
need time to do these modifications. I usually need to run a recipe
several times to check that it matching correctly. Please do not run
unmodified recipes on the tool. You can tell if I have modified a recipe
by looking at the version number on the 1st page of the recipe. The
modified recipes have a "m1" version number. Note I have no problem taking
users off qualified user list if they abuse the tool by not following

2. How are the reciped being modified?

a. The new tolerance recipe (dated June 3) is added. These tighter
tolerances cause the process to fault if such things as the reflected
power goes too high.

b. Addition of a zero power etch/dep step before the main etch/dep step.  
This step is needed to position the load and tune capacitors so they go in
the correct direct during stablization step. The matching unit should not
need this step but it has a bad hysteresis problem which makes this step

c. Setting of specific pre-set load and tune valves on the matching recipe
page. These values have to be tested under plasma conditions.

d. Checking that the pulse mode box is checked on the low frequency
generator page.

Low frequency duty cycle setting --- if you are using one of the low
frequency (LF) SOI recipes, you may or may not know that the LF (380 KHz)  
rf is pulse modulated at a frequency around 40 Hz and at a duty cycle
between 33 and 20%.  In particular, our system was suppose to have been
setup some years ago by STS so the LF was on for 5 ms and off for 20ms ( a
20% duty cycle). In June STS told me that this pulse rate should be
checked with a sensor on the rf line and not simply by looking at the
pulse rate coming from the pulse generator, I found the pulse width coming
from the pulse generator was indeed 5ms but the pulse width on the rf line
was only 3 ms.  The rf generator introduces about a 2 ms delay on turn-on
but no delay on turn-off. This means that we have been operating at a duty
cycle of only 12%.

This difference (12% instead of 20%) appears to be important. I increased
the duty cycle to 20% and it appears to be causing a significant decease
in resist selectivity for one user who previously had a selectivity of 400
and now it is down to 300 and below, however, all the data is not in on
this one.  A year or so ago, a user developed a SOI recipe and found that
he had to turn off the electromagnet because of a glass problems.  
Increasing the duly cycle to 20% would probably allow the electromagnet to
be kept on for this recipe.

There is a question about whether we should keep the duty cycle at 12% or
increase it to the currently STS recommanded 20% to 25%. Alternatively, We
could keep the pulse generator in the lab next to thed tool and allow
users to change it for best process results.

I also need to tell you about the failure of the pulse generator. Sometime
between mid-June and mid-July the pulse generator failed. The failure was
such that the duly cycle went to 100% (LF full on during the etch step). I
have a temporary unit with an oscilloscope in place. It will be a few
weeks we get replacement pulse generator.

	Thanks, Jim 

James (Jim) P. McVittie, Ph.D.	        Sr. Research Scientist 
Paul G. Allen Building                  Electrical Engineering
Stanford Nanofabrication Facility       jmcvittie at
Stanford University             	Office: (650) 725-3640
Rm. 336X, 330 Serra Mall		Lab: (650) 721-6834
Stanford, CA 94305-4075			Fax: (650) 723-4659 

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