Thanks to all who have contriubuted to this topic.

Yes I'd love to use a thicker resist such as AZ9260 however I do not
have this available to me. The thickest resist we have is AZ4562.
Normally if I spin AZ4562 on a 3 inch wafer at low spin speeds (e.g.
1500 rpm) I can obtain a resist thickness of ~ 10 microns. I know I
create other problems for myself with such a low spin speed
(non-uniformity, more edge bead etc.) however these are not show
stopping problems. Unfortunately my substrate is a CMOS chip of 5 mm by
5 mm and if I try to spin AZ4562 at 1500 rpm the resist essentially does
not spin and I got a very non-uniform coating. The resist does spin
nicely at spin speeds greater than 4000 rpm. So essentially I have 6.2
microns of photoresist to play with and have to etch 10 microns SiO2. Up
until now I cannot get the selectivity required for my process (~2:1). I
can achieve 1.5:1 however the etch rate is incredibly slow (~20 nm/min).
Note I only have RIE processes available to me for etching SiO2 (there
is also one ICP process for deep silicon etching).

I contacted Surface Technology Systems (STS) for details about their
Advanced Oxide Etch (AOE). This is an ICP based process and from the
paper released by STS using this process selectivities of up to 12:1 can
be achieved using photoresist. The gas chemistry is C4F8/H2 by the way.
I was also told if a metal mask is used selectivities of up to 200:1 are
possible. I will try and convince my line manager we should try this AOE
however it is all dependent on cost!

Thanks to Morten as well for sharing his process.

James


Xiaoguang Liu wrote:
> Hi Daniel
>
> Have you considered a thicker resist like the AZ9260. I know with
> careful lithography you should be able to get 10x10um feature for
> about 20um thick resist.
>
> Best
> Leo
>

Dr. James Paul Grant
Postdoctoral Research Associate
Microsystems Technology Group
76 Oakfield Avenue Room 3
University of Glasgow
Glasgow
Scotland
G12 8LS

Telephone: +44(0)141 330 3374