amtetcher late night problems .....
shott at stanford.edu
Mon Apr 14 13:44:13 PDT 2008
SNF Lab Members:
As a number of your know only too well, the amtetcher often fails to
pump down late at night .... typically close to midnight and later.
While we do not yet have a fix for this problem, we now have some
reasonably convincing data and a hypothesis as to what is triggering
I apologize in advance for the length of this email .... but if you've
been plagued by these problems, you may wish to read on.
As you are likely aware, the energy costs of a cleanroom are very high
and are dominated by the energy required to heat, cool, and move all of
the air that makes a cleanroom clean. At normal fan speed there are 7
to 10 air changes per minute in there. To save energy, the fan speed
has been reduced starting at 11 p.m. and returning to full speed at 6
a.m. When did that start? I don't know but it has been going on for at
least several months. Note: Facilities Operations (FacOps) runs and
manages the HVAC system in SNF as well as virtually all of the other
While one would not expect this reduction in fan speed to result in
changes in temperature or humidity, we now have data in hand that shows
that the temperature near amtetcher drops dramatically by 2-3 degrees C
between 11 p.m. and midnight and stays pretty low overnight until the
fan speed turns back up at 6 a.m. It also appears as if the relative
humidity in that aisle increases by 7-8 percentage points between 11
p.m. and midnight. I have posted these plots outside the cleanroom
entrance .... or, if you are curious, send me email and I'll send you
the Excel file.
Our hypothesis is that significantly lower temperatures and somewhat
higher humidity cause the bell jar and the hexode to absorb more water
vapor when the bell jar is up and that additional moisture is just
enough to keep the system from pumping down below 0.5 mTorr which is
close to "as low as it goes" for that pumping package.
How can we fix this? At this point, we don't yet have a detailed plan
.... we need to look at a few things. Those include:
1. Working with FacOps to understand why the temperature varies so
dramatically and work to eliminate the dramatic temperature drop. Note:
simply getting the in-lab temperature to better track the temperature
set point may not be what we want because the actual set point is very
close to the low overnight temperature that causes problems. While
simply keeping the fan speed at the daytime setting may seem attractive,
that is certainly not the "green" approach in this case.
2. See if we can either increase the temperature or the heat capacity of
the hot loop (that is the liquid flow that heats the bell jar all the
time and the hexode when the bell jar is open) to better resist moisture
3. Check to make sure that the pumping system is working optimally.
4. Make sure that the ionization gauge is properly calibrated ... even
being off by 0.1-0.5 mTorr could greatly exacerbate this problem.
We will keep you posted as we work to resolve this issue, but we wanted
to let you know that we believe that we have finally identified what is
triggering this strange "time of day" failure.
Let me know if you have any questions,
More information about the amtetcher