I've seen this as well. There is some kind of outgassing from the LPCVD
films. The poly seals it in, then blisters during the 1000 C annealing step.
The soluiton is to anneal the LTO and PSG before poly deposition at the same
or higher temperature and time as you'll use for the poly (e.g., 1050 C for
1 hour).
    --Kirt Williams

----- Original Message -----
From: "mrutyu swamy" 
To: 
Sent: Monday, September 24, 2007 2:47 AM
Subject: [mems-talk] Blister problem in PSG film


> Hi all,
>
>          Has anyone encountered with the problem of formation of blisters
> inside the PSG film (LPCVD ~1.5um thick) which is deposited on top of
> LPCVD SiO2 (~0.5um thick) which inturn is deposited on top of LPCVD
> nitride and/or patterned LPCVD P-doped PolySi-1 (0.5um thick)...??
>
>  P.S: These blisters start to appear only when the LPCVD P-doped PolySi-2
> (~2um thick) [which is deposited on top of the above mentioned patterned
> PSG layer], is annealed @ ~1000 oC.
>  Possibility of surface contamination is expected to be the least, since,
> cleaning @ each stage of the above mentioned process is thoroughly
> performed as per that of standard CMOS line. Also the pattern of
> blistering from Die to Die across the whole wafer is very similar &
> consistant.