Dave, of course you could also use electrochemical etching with aqueous
Ammoniumbifluorid (ABF) solution or NaF solution, then your etch process is
under full electrical control and it should be easy to etch homogeneously if
you have a typical P diffusion profile with high P concentration at the
surface. Just you need an electrochemical setup to accomplish this.

Best regards, Thomas
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-----Original Message-----
From: Dave Goldstein [mailto:[email protected]]
Sent: Friday, January 18, 2008 6:30 PM
To: General MEMS discussion
Subject: Re: [mems-talk] How to uniformly etch ~0.05 - 0.3um off a 3" Si
wafer?

Thanks for all your suggestions. My way of telling how much materials have
been removed is by measuring the sheet rho on the Si. Remember that I have a
diffusion at the surface? I am suspecting that at the very top Si surface
there are lotta lattice damage by phosphorus precipitates etc so that is why
I try to remove a small amount of material at the top surface.

I have tried very diluted CP etch (HNA=1:10:9) but it always leaves a
white-ish cloud Si surface. I don't understand why.
For the room temperature KOH, I leave it in there for 5 minutes but it
doesn't seem to etch anything. I will do more trials on this. the KOH seems
pretty promising.