cleanliness/semi-cleanliness specifications

Mary Tang mtang at stanford.edu
Tue Sep 14 22:41:14 PDT 2010


  Hi J --

Excellent question, which is basically "how good is good?"  I'm no 
expert and will defer to others on this committee who have more 
experience than I.  However, I have tried to learn and have had long 
discussions, mostly with Jim McV and Baylor Triplett about this.  Baylor 
once showed us a lot of data from Intel, which spent a lot of time 
trying to figure this out.  Basically, the data showed that you can 
think of contamination as a defect density -- then "good" or "bad" is 
based on the area and complexity of your device.  Counterintuitively, 
this means if you are making only a few nanoscale transistors (a few 
masks, devices a few microns in size) contamination is much less of a 
problem than if you are making conventional CMOS chip.

So, "good" is relative.  If you are making detectors, it's a different 
answer than if you are making an EE412 chip or some nanotransistors.

The criterion that I believe we've been using is a sort of "do no harm" 
kind of approach.  Basically, if contaminant appears at or below the 
detection level of the Evans TXRF system, then it's not considered a 
problem.  That said, it is also important to consider the possible 
contaminants.  At Evans, typically the sources used for TXRF are 
molybdenum and tungsten.  The W source has better sensitivity to lower 
atomic weight (like Fe) whereas the Mo source is better for higher 
atomic weights (like Au.)  When in doubt, people run both -- although in 
many cases, it's a matter of looking at the possible sources of 
contaminants and then looking for them.  So, if it's a metal deposition 
system where a lot of gold is deposited, Au should be one of the 
elements covered.  For the Evans systems, the typical limit of detection 
of K is around 10^10 atoms per cm2.  If your W has 20 ppm K and silicon 
is about 10^16 atoms/cm2, this is about 10^10 which strikes me as being 
at or around the limit of detection.  This may very well be how they can 
promise <20 ppm of alkali metals.  Please double-check these 
back-of-the-envelope numbers with Evans or other SpecMat members.

As for contamination, Jim always says "look for the mechanism of 
transfer."  The best mechanism of transfer is shared chemicals at a wet 
station.  Unless contamination is really gross, hard surfaces like wafer 
chucks or quartzware do not typically transfer contamination very 
effectively.  I suspect ALD will not have quite the range of concerns 
for contamination transfer that other stations are scrutinized for.

We've talked about stretching our contamination policy for years, but 
have faced an uphill battle with the lab community on established 
tools.  It should be a lot easier to establish different rules with new 
tools and a new community of users.  With ALD, there is a lot of 
opportunity to stretch these contamination definitions.

Sorry, I don't think I answered your question but had a good rant.

M

On 9/14/2010 9:51 PM, J Provine wrote:
> hi specmat,
> what are the numbers associated with clean and semi-clean status in 
> the snf.  ie what density of  trace contaminants are allowed?  i tried 
> to find this information on the snf website but to no success.  i 
> remember some of these numbers being presented at a clean-tamination 
> meeting last year.
>
> for instance, two questions have arisen recently concerning the savannah:
> 1) clean wafer goes in...how can we verify that it is clean or 
> semi-clean coming out?  what number do we need to hit with TXRF or 
> other methods.
>
> 2) a user can purchase W with "low alkali content" from a company. 
>  they promise <20ppm of alkalis in the W filament (most of the 
> contamination is likely K).  is that semi-clean?
>
> j




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