oxide deposition process

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Dear MEMS community,
I need urgent help in fixing a process problem which I face from time to =
time.
I use a 6" wafer Plasmatherm 790 PECVD system. Recently, I am =
encountering tiny showerhead=20
marks uniformly all over the wafers every time I deposit PECVD oxide =
using SiH4/N2O at 350C.
I always find that the first wafer is clean. But whenever I deposit =
after a 30min plasma clean, I always
notice a uniform showerhead marks on the wafer. Sometime I notice on the =
first wafer also! I have already conformed that there is no leak in the =
gas lines. I have always been using the standard deposition and plasma =
clean recipe from a long time. Only recently, I am encountering this =
problem. Does anyone have any useful hints to offer? The gases I use =
are: CF4, N2O, SiH4, NH3 Helium and N2. Any suggestion or help would be =
highly appreciated.

PS: I have already written several mails to Plasmatherm. Unfortunately I =
have not received any response from them so far.

Best Regards
Ranganathan
IME, Singapore.

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Dear MEMS community,
I need urgent help in fixing a process =
problem=20
which I face from time to time.
I use a 6" wafer Plasmatherm 790 PECVD =
system.=20
Recently, I am encountering tiny showerhead 
marks uniformly all over the wafers =
every time I=20
deposit PECVD oxide using SiH4/N2O at 350C.
I always find that the first wafer =
is clean.=20
But whenever I deposit after a 30min plasma clean, I always
notice a uniform showerhead marks on =
the wafer.=20
Sometime I notice on the first wafer also! I have already conformed =
that=20
there is no leak in the gas lines. I have always been using the =
standard=20
deposition and plasma clean recipe from a long time. Only =
recently, I=20
am encountering this problem. Does anyone have any useful hints to =
offer? The=20
gases I use are: CF4, N2O, SiH4, NH3 Helium and N2. Any suggestion =
or help=20
would be highly appreciated.
 
PS: I have already written several =
mails to=20
Plasmatherm. Unfortunately I have not received any response from them so =

far.
 
Best Regards
Ranganathan
IME, =
Singapore.

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