I think you'll find that most or all of the silicon dioxide etchants (e.g.,
pad etch)
that claim not to contain hydrofluoric acid (HF) really do.
They are not mixed with HF directly, but
instead contain ammonium fluoride (NH4F), which dissociates into NH4+ and F-
ions,
and an acid such as nitric or acetic acid, which also dissociates,
generating H+ ions.
Some of the H+ and F- combine to form HF.
HF is a weak acid, which to chemists means that
in equilibrium only some of it dissociates; H+, F-, and HF all are present.
For weaker concentrations the combined HF is what acutally does the etching
of SiO2
(see the description and references in Williams and Muller,
"Etch Rates for Micromachining Processing,"
J. Microelectromechanical Sys., vol. 5, no. 4, Dec. 1996, pp. 256-269).
Thus, whatever wet silicon dioxide etchant you work with probably still has
HF in it.
--Kirt Williams Agilent Technologies
> -----Original Message-----
> From: Luiz Marangoni [mailto:[email protected]]
> Sent: Sunday, December 09, 2001 2:10 PM
> To: MEMS Talk List
> Subject: [mems-talk] More safety Porous Silicon process
>
>
> Hello group,
>
> I4m beginning a phD research about porous silicon. The first
> step of my
> study is to develop a reactor to get macroporous silicon by anodical
> etching in a reliable way. A preliminary bibliographic seek about this
> item revealed that the more commonly used etchant is the
> hydrofluoric acid
> (HF). I think all you know how dangerous is to work with HF, a highly
> corrosive compound. My inquiry to the group is if someone know other
> etchant that enables a more safety and a less hazardous process to get
> porous silicon. (I found something about ammonium fluoride...).
> I4m looking for people that have interest in form a research
> group about
> this subject. Someone have interest?
>
> Thank you
>
>
> Luiz Marangoni
> MEMS Enginneering Phd. Student
> State University of Campinas - UNICAMP - Brazil
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