P.D. has a good point about native oxide stopping XeF2. A little oxide
completely halts XeF2 etching of silicon.
If an HF dip is an issue, you can instead expose the surface to vaporous
HF+H2O by holding it over a plastic beaker of 49% HF for 30 s.
Keep the time short enough to avoid condensation.
See papers online for further description and etch rates.

And try the etching at room temperature.

   --Kirt Williams

----- Original Message -----
From: "P.D. Floyd" 
To: "General MEMS discussion" 
Sent: Friday, May 18, 2007 12:37 PM
Subject: Re: [mems-talk] XeF2 Etching Silicon


> Your problem is likely not the size of the hole. The pressure of 4 Torr
> should be adequate.
>
>  There may be other problems, though
>  (1) The resist may nto be cleared from the etch holes
>
>  (2) The Si could have a significant native oxide that is slowing
> initiation of the etch.  You might consider doing a BOE or BHF dip to
> minimize the native oxide before etching.
>
>  (3) If the *wafer* is at 75-95C this could slow the etch rate, since the
> etch rate is partially governed by adsorption of XeF2 at the Si surface.
> You might try keeping the wafer at a lower temperature.