I would like to use silicone rubber in the lab

Mary Tang mtang at snf.stanford.edu
Wed Feb 2 12:11:34 PST 2005

Hi Yves --

We discussed your request in yesterday's SpecMat meeting.  I think this 
is generally OK.  But we were also wondering if you had a chance to 
explore some other tried-and-true solutions.  First, there is a nifty 
little holder designed for protection of wafers during KOH etching.  Uli 
has one of these or she can tell you where to purchase one.  Second, 
some people have used silicone elastomer (Sylgard 184/PDMS as opposed to 
the RTV your are using) for KOH etching.  It's a little nicer, because 
you don't have acetic acid in it.  I believe it has been used up to 80C, 
but don't know the results and would suggest you try this on test wafers 
before doing your devices, if you are interested.  The third question 
was which labware you intended to use...  If you are using a general-use 
beaker and a gold-contaminated refluxer, then it's OK, because these are 
not "clean" anyway. 

In summary, please let us know if you have explored or will consider 
these other solutions -- if these don't work for you for some reason, 
please do let us know why, and then we can approve your RTV use (sorry, 
we would rather not approve "just in case" because we would end up with 
too many approved, but not-used, chemicals in the lab.)



Yves Acremann wrote:

> One of the tricky processing steps is KOH backside etching of wafers 
> with metal structures on the frontside. The main problem is
> the protection of the front side during the etch process. We currently 
> use black wax for that purpose. We had some problems with
> this in the past where the black wax layer failed during very long 
> etches (up to 40 hours for 1.2mm thick Si wafers).
> We would like to try to put two wafers front to front and use silicone 
> rubber to seal the edge. In addition, we plan to have black wax on the
> front side.
> We did some tests with silicone rubber and its ability to withstand 
> KOH in our lab at SLAC and this method looks promising.
> We used the following product for our tests (copied from the home 
> depot catalog :-)) ):
> "GE 2.8 oz. Clear Silicone Household Glue
> Model GE280 3TG
> 100% silicone. Permanently flexible. Adheres to tile, porcelain, 
> glass, fiberglass, marble, wood, steel, aluminum, brick, mortar, 
> concrete, and most plastics. Lifetime satisfaction guarantee. Won't 
> dry out, crack, chip or peel. Easy to use squeeze tube. Can be used as 
> glue, sealant, caulk or gasketing, and for many patch and repair 
> applications around the house and shop."
> For our real wafers, we would like to test this method in the 
> cleanroom. As similar materials are likely used during construction of 
> the wet benches, I hope it will be possible to get approval for
> this material. Of corse we only deal with gold contaminated wafers and 
> this is our last processing step (followed by solvent cleaning and 
> sawing the wafer). We also will NOT heat the
> wafer to a temperature higher than 75C (in KOH) and if necessary, we 
> can do the KOH etch in our lab at SLAC.
> I did not find the MSDS that corresponds directly to this product, but 
> there is a link containing more information about it:
> http://householdproducts.nlm.nih.gov/cgi-bin/household/brands?tbl=brands&id=7011007 
> Sincerely
> Yves Acremann

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

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