There's probably some residue of the C4F8 polymer still on the oxide after
the DRIE etch. Try an O2 plasma etch or a wet sulfuric acid clean to get rid
of it. Be aggressive about it too. If you can elevate the temp on the O2
plasma, get it up to 150C or 175C, or, the wet way, use a fresh solution of
sulfuric/peroxide with good agitation and elevated temperature (~ 110C).

Mike

x1927
[email protected]

-----Original Message-----
From: Julie Verstraeten [mailto:[email protected]]
Sent: Tuesday, May 02, 2006 11:28 AM
To: General MEMS discussion
Subject: [mems-talk] Problem in etching buried oxide


Hi,


I need to etch holes through SOI wafer (for front/back alignment). The holes
have a diameter of 200 microns.

The procedure is as follow:

- first of all I deep etch (DRIE) the holes through the device layer (30
microns
thick) until I reach the buried oxide,

- then I wet etch the oxide layer (10 microns thick), in the bottom of the
etched holes (BOE),

- at last I etch the holes (DRIE) through the handle layer (400 microns
thick).


The problem is that the buried oxide doesn't etch well.

In the first 1 hour nothing happens as if there was a protective layer above
the
oxide. The etching of the handle layer however finishes by an etching period
and there's no C4F8 plasma during this period to avoid passivating the
buried
oxide.

After 1 hour it begins to etch (2-3 microns in two hours - BOE at 30°C,
the
liquid is magnetically agitated).

After these 3 hours etching I started again 2 hours and nothing was etched.

I don't know the nature of the buried oxide.

Has anyone ever had the problem?


Thank you,

Julie