Boron drive in question
edmyers at stanford.edu
Tue Dec 14 08:42:20 PST 2004
Mary and Nancy,
I was looking over some diffusion information I've got. There are a couple
of items I want to point out. I'm concerned about reaching the 1E20 /cm2 5
um deep. The solid solubility level of B in Si is 5E20 /cm2 at
1100C. What does this imply. Diffusion models are based on a couple of
assumptions. Either there is a fixed amount of dopant incorporated in to
the surface (fixed total impurity) or there is a constant source of dopant
available to the surface (constant surface concentration).
The dope and drive process Erhan is suggesting falls in to the fixed total
impurity model. I don't believe this will work for him. As the dopant is
driven in, the surface concentration drops. Once it drops below 1E20 / cm2
he will never get his desired result. Therefor, he will need to
continuously replenish the surface with dopant. This means something like
90% (I have not done the math) of his thermal cycle will have to be in the
Drive in is about 2X quicker if it is done in an oxygen ambient. The B
segregation coefficient between the oxide and the silicon helps push the B
in to the silicon. Depending on his structure, maybe he can also oxidized
during the drive in to help reduce the total time.
At 06:59 AM 12/14/2004, Mary Tang wrote:
>Hi Nancy --
>Ed, Erhan, and I had a chat about this on Friday.... Just from a
>contamination standpoint, I think we're concerned that heavily doped boron
>wafers would outgas and contaminate tylans 1 and 2. Preferably, the
>pre-dep furnace itself would be a better place to do the drive, if the
>time/temp are acceptable.
>It's also seems that Erhan is trying several different things and is not
>certain this is what he wants to do -- he basically wants a boron stop for
>KOH etching and there are a couple of different ways he can do this. His
>criterion is a process that will eventually be compatible with a CMOS fab,
>since they (General MEMS) hope to transfer their processes to such a
>lab. It would seem that the criterion that we impose on allowing
>processes in our lab might be a good natural "test" of this.
>So, in short, do you think the pre-dep furnace could do a 24 hour, 1100 C
>drive-in on an occasional basis without damaging the tube or other
>processes that are run there?
>Nancy Latta wrote:
>>I have a request from a student to drive-in boron, after a pre-dep, to
>>5um. By his calculation, this should take about 24 hrs at 1100C. He has
>>proposed using tylan1 or 2 or the pre-dep furnace itself.
>>Do you have any thoughts on other options?
>>With the Dec shutdown coming up, a timely answer would be appreciated.
>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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