As other people have said, 10 microns is a rather thick oxide.  If the oxide
was formed by ion implanted oxygen (it would be much thinner), you tend to
get a lumpy mixture of silicon and oxide which is difficult to wet etch.
One way to check would be to etch with something that will attack both
silicon and oxide (for example, there's an isotropic silicon wet etch which
does this.)

David Nemeth
Senior Engineer
Sophia Wireless, Inc.
14225-C Sullyfield Circle
Chantilly, VA 20151

-----Original Message-----
From: mems-talk-bounces@memsnet.org
[mailto:mems-talk-bounces@memsnet.org]On Behalf Of Julie Verstraeten
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