Comment pquest 2004-04-21 21:45:37: external fiber optic
mcvittie at snf.stanford.edu
Thu Apr 22 09:55:53 PDT 2004
Up until now on the PQuest, you have been restricted to using wafers
which had fairly reflective
backsides. The reason for this restriction was that the wafer sensor in
the load lock was
operating in the reflection mode. The particular wafer sensor on the
tool has both a red LED
source and a photo transistor. In the reflection mode, a single optical
fiber is split and connected to both the LED and the phototransistor.
The output of this fiber was placed under the wafer in the loadlock. It
only sensed a wafer if enough reflected light off the wafer backside
made it back to the fiber and onto the phototransistor.
In looking into this sensor, I found that it can also be operated in a
transmission mode if a
second fiber is added. After a lot of web searching, I located a fiber
made for this application.
Last night, Cesar temporarily installed the second fiber and switched
the sensor to transmission mode. It appears to work great in that it
senses all wafers. We can go back to the reflection mode in a few
minutes if needed. After a few weeks if there are no problems, we will
make the transmission mode fiber installation permanent.
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