Report on Recent PlasmaQuest Problem

Vijit Sabnis vsabnis at snowmass.Stanford.EDU
Tue Jun 18 12:22:45 PDT 2002


This is an update and follow up to Dr. McVittie's email
based observations from my work yesterday.

I apologize for the long email, but I hope that current
users read this carefully and communicate their comments 
and concerns with all of us.  

1.  Poor RF tuning and lack of DC bias voltage:

Problems:  These problems are flaky, but since I have been 
using the machine heavily in the last couple of weeks, I have
noticed them occurring more and more frequently.
RF tuning can be difficult to achieve.  Manual
tuning is required.  Even when the RF power is matched 
appropriately, the DC bias voltage reads 0 or a few volts, 
but at some random point can start reading the correct voltage. 

I hooked up the oscilloscope with high voltage probe to
the RF feed to the cathode.  I was able to confirm that
when the computer screen and RF power supply display 0 V
or a very low DC bias that there is indeed no DC bias
measured on the scope.  This means that there isn't a 
problem with the voltage readout on the computer screen or
RF power supply display, but that there is a real problem
with the plasma or RF being supplied to the chamber.  Based on this
finding we can eliminate any potential problems with the loose 
connector that we found last Friday.  At some point
during my investigation the DC bias suddenly was displayed
correctly on the computer screen and RF power supply.  I then
confirmed the presence of the bias on the oscilloscope.  
I found the same results as last Friday - for a displayed
voltage of -46 V, the DC bias read on the scope was about
-80 to -90 V.  The voltage read on the computer screen and
RF power supply is about a factor of 2 smaller than what
it should be, indicating a calibration problem or the
presence of a voltage divider in the readout circuitry.  

Some other interesting observations:
My recipe uses 35W of RF power.  When the DC bias reads 0 V
(or something close to that), the scope displays a 
sine wave with a 400 V peak-to-peak amplitude.  When
the DC bias is correct, the sine wave shifts down by
this bias and the amplitude of the sine wave drops to
100 V peak-to-peak.  

I intermittently heard strange noises coming out of
the RF matching box.  

When the DC bias reads properly, there is no problem
with your etching.  I did not have time to investigate
what happens to the etch rate when the DC bias reads 
close to zero, but based on the above results I think 
you will get poor results.  

My guess is that something is wrong with the RF
matching network and possibly the RF power supply.
However, someone more qualified should confirm this.  
I think this concern should be taken seriously and
acted on immediately by the SNF staff.

2.  Loading Integrity

I have experienced three wafer crashes in the last couple 
of weeks.  This may be due to the new pins that have been
installed into the machine.  Please also double check that
your wafers sit properly in the loading arm platen.  I found
that even certain prime wafers can be slighty too large and
hence do not sit properly in the load arm.  

3.  Loading - software problem

Sometimes the loading and unloading functions become
out of phase with proper pin positions, which end
up confusing the machine about the wafer locations.
Please pay special attention to this since you
can easily crash your wafer into the pins or worse
yet crash the load arm into the clamp.  Should you find
yourself in this position I have found that cycling the 
load/unload functions along with system reset will 
eventually get the system back into the appropriate state.
Using the prompt option in your recipe sometimes works.

4.  The load arm sometimes wobbles too much during loading and

5.  The module located just below the keyboard (MC-200
Motor Controller) is making weird noises from time to
time.  The fan may need to be replaced.

6.  Many of us use low Cl2 flow rates (ie. < 3 sccm).  
I really don't believe the Cl2 flow rate displayed on
the screen.  I think this particular flow sensor (better 
yet, all of them) needs to be recalibrated.

My overall conclusion is that the tool is pretty unreliable
right now and has several issues which need to be resolved.
YOu can still get your etches to work, but you need to be
particularly careful about the RF matching/bias and loading.
I hope that appropriate action will be taken immediately
by the SNF staff so that I and others do not have to spend
more time stuggling with the tool.  

- vijit

Vijit Sabnis
vsabnis at
Stanford University
Department of Electrical Engineering
Solid State and Photonics Laboratory
CISX B113-23
650/725-2774 office
    723-4659 fax

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