PECVD carbon is usually highly stressed
Shay

-----Original Message-----
From: mems-talk-bounces@memsnet.org [mailto:mems-talk-bounces@memsnet.org]
On Behalf Of Mehmet Yilmaz
Sent: Tuesday, May 19, 2009 10:10 PM
To: mems-talk@memsnet.org
Subject: [mems-talk] SOI wafers, Burried OXide (BOX) layer,and after release
issues

Hello Zhijian,

First of all, I am sorry, I could not see your e-mail although I checked my
e-mails regularly.

Yes, I was concerned about the stress issues at the silicon/oxide interface.
But, later I started thinking that this might not be the only reason for the
stress issues.

I am getting my SOI wafers already annealed before I do any processing on
them.

After getting my SOI wafers, then, I am depositing a stack of  Cr (10nm) /
Au (100nm) / Cr (25nm) / Carbon (150nm) / Oxide (5nm) layers.
Cr, Au, and Oxide are e-beam evaporated, while Carbon is PECVD deposited.
First layer on the Si substrate is 10nm Cr as adhesion layer, and last layer
is 5nm Oxide.

I started suspecting that the main reason for the stress issues is that
stack of layers. (In case you wonder, I did not do any stress measurements
after depositing all those layers)

Do you have any suggestions about that multi-layer structure?
In case you wonder how my release  process steps are, I am writing them
below:

Before I do the release, I have only Cr (10nm) / Au (100nm) / Cr (25nm) as
thin layers. So, I am removing Carbon (150nm) / Oxide (5nm) before the
release process is performed.

Then, I am using CR14 to etch Chromium. So, I have only Cr (10nm) / Au
(100nm) on my substrate.

Then, I am using Isotropic Silicon Etchant (HNO3 : H2O : BOE(30:1) = 126
: 60 : 5)  for a very short time (1 minute) just to get rid of the sharp
edges caused during BOSCH process.

After that, I am using BOE(6:1) for ~100 minutes to etch the BOX layer oxide
and release my structures.

Then, I am using a Critical Point Dryer tool to make sure that I do not have
any stiction issues.

The only time I check whether my structures are released is after the
Critical Point Dryer process is finished. So, I am checking my structures
only in the end of all the release steps.

Next time, I will check my structures after every wet etch step, but I do
not expect to see any problems because the structures are still fixed to BOX
layer.

I think, whatever happens, happens in the BOE etch step.

What do you think? Do you have further suggestions?
I am looking forward to your reply.

Best regards,

Mehmet