Greetings,
Yes we had many similar problems.
What we experienced was that the etch is self limiting.What happens is that
As you keep etching the sides etch also erode creating an inverse trangular
configuratio the bottom of the triangle is so small it no longer can etch.
There is a chemial exchange that no longer can work at the exit.
We decided to develop a microblasting technique that works,howeve it
needs tuning for each application.
Kind Regards
Walter
www.elume.com.


----- Original Message -----
From: "Julie Verstraeten" 
To: "General MEMS discussion" 
Sent: Tuesday, May 02, 2006 8:28 AM
Subject: [mems-talk] Problem in etching buried oxide


>
> Hi,
>
>
> I need to etch holes through SOI wafer (for front/back alignment). The
> holes
> have a diameter of 200 microns.
>
> The procedure is as follow:
>
> - first of all I deep etch (DRIE) the holes through the device layer (30
> microns
> thick) until I reach the buried oxide,
>
> - then I wet etch the oxide layer (10 microns thick), in the bottom of the
> etched holes (BOE),
>
> - at last I etch the holes (DRIE) through the handle layer (400 microns
> thick).
>
>
> The problem is that the buried oxide doesn't etch well.
>
> In the first 1 hour nothing happens as if there was a protective layer
> above the
> oxide. The etching of the handle layer however finishes by an etching
> period
> and there's no C4F8 plasma during this period to avoid passivating the
> buried
> oxide.
>
> After 1 hour it begins to etch (2-3 microns in two hours - BOE at
> 30°C, the
> liquid is magnetically agitated).
>
> After these 3 hours etching I started again 2 hours and nothing was
> etched.
>
> I don't know the nature of the buried oxide.
>
> Has anyone ever had the problem?
>
>
> Thank you,
>
> Julie
>
>
>