[mems-talk] Removal of polyimide sacrificial layer

[Samadhan B. Patil]

Hi Suzanne,

To remove a sacrificial layer using asher, pressure in the chamber should be 
such that the mean free path (of the species in a plasma) is less than the 
thickness of the sacrificial layer.

I dont know much about the "Tegal 903E plasma etcher" but you will need to 
find a compramise between too high pressure and the pressure just enough to 
give less mean free path. Rough calculations should help you to decide this 
pressure.

     Regarding warpage of the bridge, it can be because of the residual 
stress in the structural layer and may arise because of  a difference in the 
thermal expansion co-efficient of polyimide and the structural layer (or 
deposition temperature of the structural layer).

regards,

Samadhan


----- Original Message ----- 
From: "Paradis, Suzanne" <Suzanne.Paradis at drdc-rddc.gc.ca>
To: <mems-talk at memsnet.org>
Sent: Monday, July 06, 2009 8:27 PM
Subject: Removal of polyimide sacrificial layer

> Good afternoon,
>
> I want to use  polyimide as a sacrificial layer (4 microns gap) to do a
> suspended bridge.  My concern is that I do not have a asher, I have a
> Tegal 903E plasma etcher which is meant to run at high pressure (1.5 to
> 4 torr).  I ran a sample at 1 torr and 40 SCCM O2 for 2 hours. ( I
> understand that this is still too high as I read in the literature that
> much lower pressure is recommended for suspended material).  Anyway, the
> polyimide started to be etch in a wavy manner but the portion of the
> suspended layer that started to be free was..warping.  I stopped the
> process .  I tried with 275mt (even though I am out of range of the
> instrument) and it took forever to remove most polyimide but the freed
> layer was very warped.
>
> Is there any hope that I can remove the polyimide with this instrument
> or should I forget about it?
>
> Thank you very much
>
> Suzanne Paradis, ing. M.Sc.