Hot phosphoric acid can etch silicon nitride very effectively. But you
cannot have PR on the wafers during that etch. So you have to deposit or
grow a thin (~ 50 nm should be enough, depending on the selectivity)
SiO2 layer on top of the nitride, which you can etch in BOE, then strip
the PR. The oxide will mask the nitride against the phosphoric during
the etch.

The etch selectivity of the oxide/nitride is controlled by the ratio of
water in phosphoric acid and the temperature. Commercial systems are
available.

Take all necessary precautions when handling acids, especially heated
ones.

Regards,
Pierre x286
-----Original Message-----
From: mems-talk-bounces@memsnet.org
[mailto:mems-talk-bounces@memsnet.org] On Behalf Of lrabieir@purdue.edu
Sent: October 25, 2004 20:40
To: General MEMS discussion
Subject: [mems-talk] thich Si3N4 layer

Hi,

I have a 3000A layer of Si3N4 on a 1000A layer of thermal oxide which
itself is
on doped poly silicon, I need to etch some areas of Si3N4 and then some
ares of
oxide under that from the opening, I wonder if anybody can suggest a
proper
etchant for this thickness of silicon nitride?
I wonder if anybody can help me?

Thanks,


Laleh Rabieirad
PhD Student
ECE Department
Purdue University
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