PQuest Flowmeter Problem Update

Jim McVittie mcvittie at snf.stanford.edu
Wed Sep 8 13:03:29 PDT 2004


CL2 flow meter/controller on the PQuest has three problems:

1. The control valve does not fully close at low flow settings. This
means that for a given line pressure, there is a flow for which we can
not go below. Normally, this is not much of a problem since we usually
do not operation flow meters at the bottom of their range. However, the
Pquest is the exception. Most of the GaAs etch processes use Cl2 flows
below 3 sccm. The flow is kept this low so that the processes can be run
at 2 mT of pressure. Note that the Cl2 flow meter has a maximum flow of
43 sccm of Cl2.

To get Cl2 flows down to 1.0 sccm from our flow meter, we have to
operate at line pressures at or below 10 psi. At these low pressures, we
are sensitive to the Cl2 flows to the other etcher on the Cl2 line. In
particular, both the Lam and P5000 etchers share this line. When either
the Lam or P5000 start drawing Cl2, the Cl2 line pressure does drop a
bit but it does appear to be able to deliver the required Cl2 to all the
tools.

As expected when the Cl2 pressure dropped, the minimum Cl2 flow from the
Pquest Cl2 flow controller also drops. This is not a major problem, if
the flow setting is above the minimum flow. However, if the flow setting
is below the minimum flow, the flow will also drop until the flow
setting is achieved.  This last point is presently an issue with the
Pquest because of the offset problem described below.

2. On the Pquest to adjust for errors in the D to A converter, there is
an adjustable offset for all the flow meters. Unfortunately, all the
offsets on the Pquest were set incorrectly some time in the past. I
think a former technician got confused with the setup procedure. For the
CL2, the correct offset error is 1.8 sccm low. This means that a setting
of 1.0 sccm actually called for a flow of -.0.8 sccm. Currently to get a
flow of 1.0 sccm of Cl2, you need to set the flow to 2.8 sccm. We plan
to correct all the offset errors later this week. For now for all the
flow meters, you need to adjust your flow settings to give desired
readout flows that you want. Except for a small error for the Cl2 flow
meter, all the read outs for the flow meters on the Pquest are accurate.


3. Finally, there is a gain adjustment error the Cl2 flow readout. Using
the rate of pressure rise procedure for measuring flow rate. I have
determined that the chamber volume is close to 75 liters and as stated
above the flow read outs are accurate except for a 13% high error for
the Cl2 flow. The correct Cl2 flow is 87% of the readout number. I
expect this error has been there for sometime. For now, I am not going
correct this error.  You will be informed when we make is correction.

    Thanks,  Jim McVittie
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