You need to lower the oxygen flux (oxygen as it prevents from creation of a
PTFE layer aloso etch PMMA).
You will have an optimum before the creation of PTFE layer that will prevent
from any ething (neither PMMA, neither Silicon).
The working window depends on ion flux (depending on power, pressure...)

Best regards

Frank TORREGROSA
R&D Manager
ION BEAM SERVICES (IBS)

----- Original Message -----
From: "Karan deep" 
To: 
Sent: Sunday, October 09, 2005 1:03 AM
Subject: [mems-talk] Silicon Etching with Negative PMMA as mask


> Hi Everybody
> I am exposing PMMA as a negative resist.I am trying to etch Silicon by
using
> PMMA as mask.Gases used are Oxygen(30 SCCM) and CF4(6 SCCM) ..I tried it
for
> 5 min and 30 min.Unfortunately whole PMMA gets removed and I don't have
> anything left on Silicon wafer..Can anybody please suggest me the
> appropriate gases , power , etch time and gas pressure
>
> Thank You
> Karan
>
>