jprovine at stanford.edu
Mon Sep 27 06:19:03 PDT 2010
based on a discussion with mary and the comparitive purity of SCT (close to
1000ppm) and gryphon targets to this <20ppm contaminated W, i am going to
allow it in the savannah ald and continue to classify the system and the
wafers coming out of it as semi-clean.
On Fri, Sep 17, 2010 at 8:52 AM, J Provine <jprovine at stanford.edu> wrote:
> 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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