jprovine at stanford.edu
Fri Sep 17 08:52:56 PDT 2010
thank you, mary. is anyone else able to weigh in?
i agree very much with mary about the need to stretch the contamination
policy from its current state asap. however, as is the case with the
savannah ald there isn't much stretching happening with new tools because
users invariably want to be able to take wafers to other tools. if those
other tools are mired in the old/current system then the new tools get
funneled toward it as well.
so, toward this line, let's look into receiving txrf data for films
deposited in the savannah and compare that to control wafers to determine
cleanliness. we can also compare to wafers coming from other classified
machines and after coating different substrates (like the W w/ <20ppm K in
question). what was the process for the spectrum ald to be classified as
if we can make the ald more like litho (what goes in is what comes out), it
would at least not show up as a new cleanliness issue. this maybe too
aggressive, but if it can bounce back and forth between clean and semiclean
that would be a huge step forward.
On Tue, Sep 14, 2010 at 10:41 PM, Mary Tang <mtang at stanford.edu> wrote:
> 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
> 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.
> 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?
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