Try plasma dry etch.

> ----------
> From:         Yagang Li[SMTP:yagang.li@materials.oxford.ac.uk]
> Reply To:     Yagang Li;mems-cc@ISI.EDU
> Sent:         Wednesday, February 11, 1998 6:14 PM
> To:   MEMS@ISI.EDU
> Subject:      etchant for SiO2 without etching alumina
>
> Dear colleagues,
>
> My friend gived me this email address because he thought it might
> be a forum site for discussing etching-related issues. Anyway,
> I like to have a goahead.
>
> My problem is to find an etchant for SiO2 which does not substantially
> etch alumina Al2O3. Is somebody there who happens to know the
> formula of this sort of etchant. Your suggestion and help would be
> much appreciated.
>
>
> Cheers.
>
>
> Yagang Li
>
> Department of Materials
> Oxford University
>
> Yagang.LI@materials.ox.ac.uk
>
>