wbdiff update

Mary Tang mtang at stanford.edu
Thu Nov 19 17:35:45 PST 2009

Dear wbdiff users:

Thanks for your patience as we work to resolve resistivity and drying 
issues at this station.  A rebuilt spin-rinse-dryer (SRD) was installed 
yesterday -- it's got completely new seals, hoses and valves and has 
been electropolished to a lovely finish.  We soon found, however, that 
cleaned wafers still did not rinse to resistivity. 

Uli and Jim ran a number of experiments and found that the problem to be 
the teflon boats.  The boats themselves are clean as ever -- when empty, 
they rinse to resistivity with a minute to spare after acid processing.  
But over time, grooves have formed in the wafers slots where trace 
amounts of acid get trapped by wafers.  As acid is slowly released, the 
resistivity remains low...   The solution is new teflon boats (you can 
see this easily -- the base of the wafer slots are shiny in new boats 
and dull and grooved in the old ones.)

Nancy and Uli managed to find and decontaminate 5 unused/new teflon 
boats.  All the old boats at wbdiff have been removed.  Five boats may 
not be quite enough to keep everyone happy when the station is busy, so 
we ask all wbdiff users to remove your wafers and return the boats as 
soon as you are done processing.  In the meantime, we have ordered more 
teflon cassettes.

As for the drying problems, the rebuilt SRD has a newer controller which 
allows for a second, low speed dry cycle.  The rationale is that slowing 
the spin allows better drying where the wafers contact the boats.  This 
is recommended by the manufacturer and is standard industry practice.  
Several tests on this rebuilt SRD show that a full load of 25 wafers is 
consistently dry after processing.

The SRD setup is now as follows:

1.  Rinse runs for 180 seconds at 400 rpm.  The resistivity setpoint is 
16 MOhms.  When both resistivity is met and time is out, the SRD will 
proceed to the first dry step.  With the new boats, resistivity should 
be met well before 180 seconds is reached.

2. First dry runs for 240 seconds at 1600 rpm.  The HEAT light should 
come on.  This indicates that the N2 is being heated and the heating 
blankets around the drum are working.  If the light is not on during the 
dry cycle, please report on Coral.

3.  Second dry runs for 200 seconds at 400 rpm.  The HEAT light should 
remain on in this cycle.

This makes for a longer drying time, but will assure consistently dry 
wafers and cassettes.  Thanks again for your patience and please do let 
us know how this works for you.

Your SNF Staff

Mary X. Tang, Ph.D.
Stanford Nanofabrication Facility
Allen Room 136, Mail Code 4070
Stanford, CA  94305
mtang at stanford.edu

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