Hallo Ed,

thank you for your info. Yes, I use the polymer (not polyimide but nanoimprint-
polymer) as ´sacrificial layer´, so its layer is underneath the SiNx (or any
dielectric), and Al is deposited on the SiNx and then the polymer should be
removed after that.

I have compared 2 samples for 3h undercuting using an asher:1) with Al, 2)
without Al.
The structure we investigated _to check the undercuting process_ looks like a
cantilever ca.100µm in lenght, 10µm in width. All of them in sample no. 2 are
peeled off. But the cantilevers in sample no.1 seemed stuck to the substrate.

I will try to undercut the 1st-sample much longer. or maybe we need to use other
metals to replace Al?

regards,

Onny

> From: esebesta@tx.rr.com
> To: mems-talk@memsnet.org
> Date: Thu, 29 Jan 2009 10:12:55 -0600
> Subject: [mems-talk] hard ashing polymer (if Al deposited): possible
isotropic etch by ICP?
>
> I am assuming that you are having trouble removing the polymer that is
> under patterned material.
>
> One thing to look at is to see if you can get a solvent to remove the
> polymer, such as NMP.
>
> Oxygen plasma can be used quite well to get under dielectric layers as
> long as there isn't some unreasonable geometry. What you need to do is
> have a plamsa that is isotropic in nature or downstream from a plasma
> with a lot of free radical oxygen or other reactive species.
>
> However, what I am surprised is that you take 3 hours to remove 2,000
> angstroms polymer. What polymer is this, polyimide? This suggests
> perhaps your problem.
>
> The lateral undercut of organics is going to be less than the rate of
> removal in the open areas. So if you  take 3 hours to remove 2,000
> angstroms, then to undercut a very small feature such as 4 microns,
> would take, (20000 A/(2000/3hr) or 30 hours at least. Since you are
> making MEMS devices I suspect you need to do more lateral etching than 2
> microns.
>
> It sounds like you might be using a barrel asher. I would go to a diode
> plasma etcher set up. I don't think you need an inductively coupled
> plasma.
>
> So I would work to make your oxygen plasma more intense and isotropic
> increasing power and pressure. Higher temperature increases reactivity
> of substrates with ozone and reactive species. Gap might need to be
> adjusted. Wider gap I think is less isotropic, if I remember correctly.
>
> Be CAREFUL that you don't cure your polymer with too high a temperature
> which would make removal worse.
>
> Ed -- Independent Process Engineer