Report on Recent PlasmaQuest Problem

Jim McVittie mcvittie at cis.Stanford.EDU
Mon Jun 17 12:15:10 PDT 2002

PlasmaQuest Problems

Here is a list of the problems reported during the week of June11 and what 
I think needs to be done. 

Poor RF tuning - (reported by Vijit Sabnis and others) It appears that the 
matching network controller needs to be adjusted. I suspect that the gain 
needs to be increased. Vitit and I tried to do these adjustment on Friday 
but were not successful. I have another matching network of the same type. 
I will try to work out the adjustment procedure off line before another 
attempt. In the mean time, users will have to continue to first do a rough 
manual tune before switching over to auto tune. 

Bias Voltage - (reported by Vijit Sabnis) The bias voltage read out is 
intermittent. We found that the bias voltage cable connector, which 
supplies dc voltage for the buffer amp for the DC output, was loose. We 
were able to fine one of the 2 missing screws on the floor under the 
connector. After putting in this screw, we got a stable DC reading for the 
rest of our time on the system. Note that Vitit did report poor etch 
results, which correlated with a zero bias voltage read out. I am hoping 
that this problem was due to chamber poisoning (see below) and not to a 
real plasma problem. We will have to keep an eye on this issue.

Bias Voltage Calibration error - While investigating the DC bias problem, 
I hooked up an O-scope onto the RF feed to the cathode. For the process 
conditions that we were using, we found that actual DC bias voltage was 80 
V while the DC readout gave a value of only 43.5V.  Although I need to 
verify the O-scoped calibration, I suspect that the DC readout is where 
the error is. More work is needed on this issue. I will let you know as we 
find out more. No recalibrations will be done without telling the users 
how to translate between old and new reading.

Low etch rate - (reported by Daniel Weaver and Yi Dong) - The etch rate of 
GaAs has dropped by 30 %. I suspect that this problem was caused by F 
poisoning of the chamber walls. I believe there was a long nitride etch 
using F based chemistry done in the chamber before the GaAs run.
If this is indeed the case, the solution is to keep minimize the use of F 
based chemistry in the chamber and to use longer seasoning steps after the 
use of F chemistry. I have a new optical spectrometer that may be useful 
in determining  proper seasoning times. Friday evening after Vijit and I 
had been running the std GaAs etch conditions for at least a hour, Daniel 
was able to etch his GaAs sample with good results. This is consistent 
with the argument that the chamber needed a longer seasoning. So the 
problem seems to be gone for now.  

Dummy wafers turning blue - (reported by Yi Dong) - About the time that Yi 
and Daniel were seeing the low etch, they also observed the dummy wafer 
coming out blue after their seasoning step. I was not able to see the 
wafer in question. I suspect that they may have been seeing what is called 
"black silicon". Occasionally, we see this problem in other Cl based etch 
tools in the lab. It is due to micro-masking on the Si surface most likely 
from oxide and etch products on the wafer surface.  Note that in the P5000 
where we see "black silicon" more often the wafers come up with a light 
blue color.  In the P5000 etcher we get around this problem by using dummy 
wafers which have an oxide pattern on them. Since this problem for the PQ 
has not been reported to me in the past, I suspect that the chamber 
poisoning that was causing the reduced etch rated also played a role in 
this problem.

Loading Problem - (reported by Vijit Sabnis) - The loading system in the 
PQ is not of the best design and has been heavily modified by us over the 
years.  Because users have recently started to run the system at higher 
cathode temperatures, we recently changed from teflon load pins to a 
higher temperature polymer. Perhaps, the loading system needs some 
adjustments with these new pins. I will ask the maintenance tech about 
this issue.
James P. McVittie	                Senior Research Scientist
Allen Center for Integrated Systems     jmcvittie at
Stanford University             	Tel: (650) 725-3640	
Rm. 336, 330 Serra Mall			Fax: (650) 723-4659
Stanford, CA 94305-4075	

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