Hi,

As I understood you etched the device layer Si down to BOX layer.
Now there is the most difficult etching in a whole fabrication flow of your
device.

That, what you put as the protecting layer depend on such issue like:
1. further processes or in fact their lack;
2. what type of a  machine do you use for dry etching;
3. what type of process exactly to etch: standard Bosch for ex.?
4. how deep do you want to etch?
5. and what size are openinng window to etch, also how many percenatge of
your wafer they cover?
6. what kind of materials other than Si do you have put on Si or you are
willing to?

It is complex issue and from my experience I know, that SOI to etch are not
so gratefull substrates.

I used DRIE Alcatel machine standard Bosh process with Al mask.
I usually put the special tape that might be used in a vacuum environment.
It works, but it is not always guarantee your wafer will have sufficient
cooling during the etching process. You should very carefully to adhere the
tape in such way to the substrate to not expose its glue. If you don't teh
tape will burn immidiately.

 Another method is to cover with PR, but you need to suite the process
parameters so that to not burn PR.
Or to deposit thin layer of the other  metal layer than Al, that can be used
and is slectively etched to Al (Cr?).

If you are going performed etching from teh back side through the whole
handla layer to BOX, it is even convinient to strenght the buried oxide from
the device layer.
Anyway

Good luck!

Karolina

--- Li Zhang writes:

I'm handling with SOI wafer. I've completed depositing Al as hard masks on
both front-side and backside for DRIE etching. Currently I performed DRIE
etching for the front-side layer. The silicon on front-side is removed and
silicon oxide layer is then exposed.

Now I need to perform a long time DRIE etching for backside layer (around
1.5 hours). But I cannot find a good way to protect the front side structure
which is 10um thick. The material I plan to use is photoresist with
thickness of 1.3um. I'm not sure whether it can well protect the front side
structure.

Do you have any suggestion for front side protection?