You can use poly-silicon as hard mask.  Structure the poly by using normal
photo-resist and then etch the Glass in 50%HF solution at 22°C (etch rate about
6µm per minute).... it will not be a perfect isotropic etch but not bad at all.

Nimo

--- On Tue, 11/3/09, Nathan McCorkle  wrote:

From: Nathan McCorkle 
Subject: [mems-talk] Looking for a way to do deep etch in glass (25uM)
To: "mems-talk" 
Date: Tuesday, November 3, 2009, 1:52 PM

I am currently working with lexan (polycarbonate) and am experiencing
optical blurring when I use O2 plasma etch for too long, as well as
warping. I have decreased power and time to alleviate this problem,
but it takes many more runs to achieve the 20 - 30 micron etches that
I desire.

I asked an advisor about switching to glass, maybe a quartz or
borosilicate, and he said that I would need a gold hard mask to get
around having mask lift off during etch. I feel like there should be
some way to get a nice isotropic etch with a glass, using the current
lab equipment that we have.

Any ideas?

Nathan McCorkle