You are completely etching through the  photoresist.  Basically your
1813 is just lasting for the 4.5 um etch and then you are just
thinning the sample.  To get anything deeper you will need a thicker
resist (or a hard mask).

To be honest though, you can't expect any good aspect ratio at 60 um
with a normal rie. You need a deep rie with the Bosch process (or a
cryo process).  If you switch to a DRIE you could getaway with a 1827
but probably not 1813.  We have found that with the Bosch process 1827
can last up to 100um safely.

As for your white chalky film, that could be anything from resist
residue to silicon grass from edge effects in your RIE.

-Nicolas "Nik" Duarte
Ph.D candiate at the Pennsylvania State University in Electrical
Engineering

On Aug 30, 2007, at 3:58 PM, Joseph Grogan 
wrote:

> Hi All,
>
> We have a very old Plasmalab RIE in our facility that I tried using
> to etch a Si wafer patterned with 1813 resist. I used the following
> parameters:
>
> 02 flow rate: 5 sccm
> SF6 flow rate: 20 sccm
> pressure: 20 mT
> power: 150 W
> time: 1 hr
>
> I had hoped to get a depth in the neighborhood of 60um or more with
> these settings (based on literature such as Williams' etch rates
> paper). Unfortunately, I only got around 4.5 um deep, and that was
> only in the center 2" of my 4" wafer. The outer 2" of my wafer was
> coated in an odd white chalky film that smudged off a little if I
> wiped it.
>
> Does anyone have a guess as to what my problem is?