It is very unlikely that you are seeing a redeposition of Si however
I have read many reports of people who find a teflon like polymer
deposited on their wafer after a XeF2 etch.

This is most commonly the result of too much moisture in the chamber
and I have found a 200 degC dehydration bake for 5min prior to
putting the wafer in the chamber tends to prevent this problem from
occurring.  Actually this might prevent the etching of Tungsten as
well since I have found several instances (Gold and Si3N4 in
specific) that some materials who were dehydrated prior to etching
did not etch, but materials that were not dehydrated did.

Hope this helps,

Nicolas "Nik" Duarte
PhD Candidate under Dr Srinivas Tadigadapa
Penn State University, Electrical Engineering Department

>
>On 5/24/07, kris  wrote:
>>
>>Hello,
>>
>>In continuation to my previous posting (XeF2 attack on
>>Tungsten), I have another question.
>>
>>Does XeF2 etch on Si leaves any residue??
>>
>>I am trying to etch Si wafer which is bonded to
>>another wafer. Both the wafers has Au patterned on
>>them and are Au-Au thermocompression bonded. The top
>>wafer (which is etched in XeF2) has the Tungsten
>>adhesion layer between the gold and silicon wafer.
>>
>>After the complete removal of the top silicon wafer
>>(STS DRIE and XeF2 etches), I can clearly see that the
>>Tunsten is etched away completely which confirms the
>>answers from the previous posting.
>>
>>Also, I can find some residue left on the bottom
>>wafer. I have performed the EDAX on the material that
>>was scattered on the entire Au and SiO2 layers on the
>>bottom wafer. The content of Si is more than any other
>>material.
>>
>>I was wondering if the Si etched in the XACTIX got
>>redeposited as a thin layer on the bottom wafer?? Or
>>is it the thin native or virgin oxide of the bare
>>silicon top wafer that was etched??