TMAH can be made to etch silicon
but not etch aluminum by appropriate "doping" with silicon and controlling
the pH.
A formulation is TMAH (10%) + 0.66 mol/l Si + (NH4)HPO4 to set pH =11.5, 80
deg C,
giving a (100) etch rate of 0.7 um/min.
This is from the paper
Osamu Tabata, "pH-Controlled TMAH Etchants for Silicon Micromachining,"
Sensors and Actuators A, vol. 53, 1996, pp. 335-336.

        --Kirt Williams Agilent Technologies

> -----Original Message-----
> From: BERAUER,FRANK (HP-Singapore,ex7) [mailto:frank_berauer@hp.com]
> Sent: Wednesday, December 12, 2001 7:15 PM
> To: 'mems-talk@memsnet.org'
> Subject: RE: [mems-talk] How to protect Aluminium thin film from KOH
> solut ion
>
>
> Michael's statement requires a slight correction: Pure TMAH attacks
> Al very rapidly, it is doped (with 0.5% Silicon) TMAH that doesn't.
> I have this from literature, but cannot find the reference at the
> moment.
>
> Greetings,
>       Frank Berauer
>       Senior R&D Engineer
>       Hewlett-Packard Singapore
>
>
> -----Original Message-----
> From: Michael Pedersen [mailto:pedersen@mems-exchange.org]
> Sent: 12 December 2001 23:32
> To: mems-talk@memsnet.org
> Subject: Re: [mems-talk] How to protect Aluminium thin film from KOH
> solution
>
>
> Hi Soumen,
> You may want to consider using TMAH for the silicon etching. This
> etchant is known to have limited effect on aluminum.
>
> -Mike Pedersen
>
> Soumen Das wrote:
>
> > Hi all
> > We are working on fabrication of various MEMS devices.We
> are facing some
> > problem regarding how to protect the aluminium layer while doing the
> silicon
> > etching in KOH solution. Is there any chemical (just like
> photoresist)
> which
> > can be covered on aluminium layer and also can be patterned
> so that while
> > dipping the silicon wafer in KOH solution the chemical
> layer will not be
> > attacked by KOH solution and it can be used as protecting layer for
> underneath
> > material.
> > Any suggestions will be highly appreciated.
> > Bye
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