Re: Why vinyl gloves are bad for your wafers

The same thing should go for use of silicones on "clean" vacuum wands.

The attached photo shows a user's wafer that came out of a furnace that
was loaded with a "clean" vacuum wand. When the user complained,
the staff response was "Try cleaning your wafers better". But on further
testing and investigation, it was from sealant (silicone ?) applied to
the "clean" vacuum wand, presumably to "fix" it instead of replacing it!

On Fri, 2 Oct 2009, Mary Tang wrote:

> Dear labmembers:
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> It has come to our attention that some labmembers are touching wafers with gloved hands
> instead of using tweezers or vacuum wands at "clean" stations, such as furnaces and wet
> benches. Although there may be special circumstances where manual handling is warranted,
> this violates all known standards of good clean room practices. This is no-gloves-on-wafers
> policy as was stated firmly in this labmembers' note from 2001:
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> http://snf.stanford.edu/cgi-bin/ezmlm-cgi?mss:147:ebffjnejjanhflmfgmjp
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> In this lab, vinyl gloves from the bag are "clean" for using wafer handling tools. The gloves
> we use are certified Class 1 so don't shed particles and are metal ion-free. So, these are good
> for handling cassettes, handles, wands, tweezers and other wafer handling tools for which
> we want to avoid cross-contact with other surfaces. Certainly, as anyone can see from the
> acid-etched buttons at any wet station, we are not as diligent as we should be with our glove
> hygiene. So, we encourage frequent glove changes in order to ensure that contaminants
> don't travel from one surface to another.
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> However, vinyl is polyvinyl chloride, or PVC, which is a hard, brittle material. To make it
> flexible, plasticizers are added -- up to 35-45% by weight in gloves. Plasticizers are low molecular
> weight (<400), oily materials. DEHP or DOP is the most common in cleanroom vinyl gloves.
> It is widely recognized in the cleanroom industry that vinyl gloves will leave a trace plasticizer
> residue on contact with surfaces (see page 270 of Cleanroom Technology: Fundamentals of
> Design, Testing, and Operation, available on Google books at
> http://books.google.com/books?id=-ufEtmr1sBgC&printsec=frontcover&source=gbs_navlinks_s#v=onepage&q=&f=false <http://books.google.com/books?id=-ufEtmr1sBgC&printsec=frontcover&source=gbs_navlinks_s#v=onepage&q=&f=false>)
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> Here is a simple demonstration. A vinyl glove was pressed firmly down on 1/2 of a clean
> silicon wafer and then removed. Then, contact angle measurements were made. On the
> bare silicon half, contact angles were too low to measure (<5 degrees), indicating very
> good wetting with the native surface oxide. On the other half, the average contact angle
> was 19 degrees (stdev=2 degrees), indicating a significant change in the surface energy.
> The wafer was then run three times through lampoly, using recipe #1 (60" main etch, silicon
> to resist selectivity ~ 3.6.) As can be seen in this photo, glove residue masks the etch.
> (The small, aligned dots show where the contact angle was measured.)
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> Granted, this was a simple and crude demonstration. For a more rigorous approach, see
> the attached paper describing effects of DOP plasticizer on electronic device performance.
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> Please understand that there is generally a good technical rationale behind our lab policies
> -- similar no-gloves-touching-wafers policies for CMOS-clean stations exist at the Berkeley,
> MIT, and Cornell cleanrooms. We trust that these should be convincing reasons as to
> why we should not use gloved hands to touch wafers.
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> Your SNF staff
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