Hi Beverly,

thanks for you description, however I need to put a light source into the setup
because I want to check the membrane thickness optically.

But I followed the Shivaliks recommendation, AMMT seems to have a ready device
which has LEDs to built into the chuck.

Again, thanks for your help,

Chris

________________________________
From: Beverley Eyre 
To: Chris P. Park 
Sent: Tuesday, March 24, 2009 5:10:47 AM
Subject: Re: [mems-talk] KOH - backside etch - frontside protection

Chris P. Park wrote:

Hi Chris,
I've done this quite a bit. I had to made thin membranes for a large area
particle detector used in solar research for NASA. They had to be very uniform
and very smooth.

I built a special wafer holder for the process. I'll see whether I can describe
it without a picture. First, it was an open cylinder made of delrin with the
inner diamter of 90mm and an outer diameter of 120mm. The height was around 6cm.
An identical piece of delrin, except it was only 3mm high was also made. We cut
grooves into one end of both pieces so that an o-ring fit, and created a
clamping piece so that, when clamped, the seam was water-tight.

So, the idea was that we would put a 100mm wafer inside the o-ring with the side
to be etched facing up, and then clamp it. We would put the whole thing into a
large dish of water so that the water would touch the bottom of the wafer. We
would pour KOH into the top part, which had the wafer as the bottom. So the
wafer would have KOH on the top side and water on the bottom side. Then we would
heat the water to just below boiling. The water would heat the wafer which would
heat the KOH and it would etch the top only. If any KOH leaked it would be
diluted by the water and so wouldn't hurt the bottom side.

Sounds complicated, but once we built the holder, we would just clamp it on, put
it in water, pour KOH, and it worked really well. If you need instructions on
how to affect the uniformity or smoothness, I have info on that also.