Protecting frontside features during backside etch for the P5000 etcher

[C. Richter]

Dear Specmat,

I am working on a second round of wafers where I need to dryetch 
features on the frontside and backsides of wafers on the P5000. The 
wafers are DSP silicon with 3 microns of thermal oxide and about a half 
micron of amorphous silicon on both sides. On the frontside, I have 
submicron features etched into the oxide that will eventually be sealed 
with another wafer by fusion bonding. I am ready to start etching the 
backsides, however I would really want to minimize scratching on the 
frontsides. My previous batch of wafers I processed cannot be used since 
there are both fine and major scratches on most of the wafers.

Is there a way to protect these frontside features so I can etch the 
backsides? What would the consequences be if there was thick resist 
(3-4um) applied over these features that was carbonized by baking it at 
about 200C for about 90min in the smaller Blue-M oven in the litho room? 
The resist should be fully crosslinked and not likely to be removed with 
acetone....I can test this first to make sure the resist isn't soluable. 
Any other ideas? I'd appreciate any help on this.

Best regards,

Claudia