Hi Sunny,

 Time: The total etch time for our structures was 3 hours and I did the
etching in 1hr. increments. That is, after one hour of etching I
measured the etch rate and refreshed the solution with 0.0017 g/(l of 5%
w/w TMAH) of AP and 0.025% v/v of 25% w/w TMAH. In my case 1g of AP and
~15ml of 25% w/w TMAH.

  Depth and Rate: The etch rate of the new solution (freshly mixed) was
50-35 microns/hr. at 80 deg. C and temperature compensated pH=11.1 . The
large variation in the etch rate comes from the strong dependance of the
rate on the length of time between mixing and using the solution.
Usually I use the solution within 1/2 hr. after mixing.
   The refreshed solution gives an etch rate of only about 15
microns/hr. This is actually nice since the lower etch rates make
smoother etch features.
   The total etch depth was ~75 microns.

  Tricks/Pitfalls of MOSIS die etching:
 Mounting the die for etching was a bit tricky. We eventually used Super
Glue (cyanoacrylate) to adhere the die glass slides. You might find
oxygen plasma cleaning useful to improve adhesion.
 Of course we use a reflux condenser in order to keep the TMAH
concentration as stable as possible but we usually use a band of
electrical tape around the lid to keep a good seal.
  Stirring was performed with a 1.5" Teflon stir bar at 200 rpm.
  I noticed also that even with stirring we seemed to have a pretty big
temperature gradient as a function of depth in the solution. I found
about a 5 degree difference between the top 0.5" of solution and the
bottom 1" of solution. Of course it is at the bottom where the die are
etched.
  The last thing to keep in mind is that pH goes down as temp.
increases.

Hope this helps,
   Mike


>>> [email protected] 05/30/02 10:21 AM >>>
Chakry,


I have a question for Michael D Martin.

Would you tell me how long you etched the MOSIS chip?

In other word, how deep did you etch the silicon?

What is the longest TMAH etching that you performed without any damage
on
MOSIS chip?

Please, give me all the details.

Thanks.



__________________________________________________________
Il-seok  Son (Sunny)
Dept. of Electrical and Computer Engineering
University of Wisconsin - Madison, Sonic MEMS
________________________________________________________


----- Original Message -----
From: "chakravarthy.S.Yamarthy" <~!B*+R^&>To:
<~!B*+R^&>Sent: Tuesday, May 28, 2002 2:10 PM
Subject: Re: [mems-talk] TMAH Etching


> Dear Sunny,
>
> Thanks alot for ur suggestions.
>
> 40% SA is silicic acid in water.we could able to get
> the chemical from alfa aesar company.u can get that
> chemical directly from that company.
>
> the mixture consists of
>
> 10%TMAH solution(710gms)+40% sio2(100gms)+DI
> water(110gms).......MIXTURE (I)
>
> The above mixture was heated heated for 80 degree
> centigrade and stirred continously.after the mixture
> appears to be void of white precipitate i.e,completely
> clear
>
> Ammonium peroxy Disulphate(APODS)(1.25gms)+10ml of DI
> water was prepared separately.
>
> The above APODS mixture along with 20 ml of 10% TMAH
> was added to MIXTURE(I) at regular intervals and
> silicon chips were etched.The Aluminum bond pads were
> attacked.
>
> APODS was added to reduce the hillocks.
>
> SA was added to maintain the PH of the entire mixture.
>
> Can u suggest me which chemical am i need to add  to
> the mixture to maintain the proper PH.
>
> Thanks
> Chakry(AMSTC,AUBURN-AL)
> --- il-seok Son  wrote:
> > Dear Chakry,
> >
> > pH is a good indicator for aluminum passivatoin in
> > doped TMAH.
> > You have to keep the pH at 12.
> > If the pH is higher than 12, such as 12.3, you can
> > literally see the
> > aluminum disappearing.
> > If the pH is lower than 12, the etching rate of
> > silicon is decreased.
> > You can find this information from many articles
> > about TMAH.
> > You might need to put more chemical that you  are
> > currently using in order
> > to drop the pH.
> >
> > Some of your descriptions are not clear to me.
> > Would you describe more in detail about your recipe?
> > What does  "40% SA aqueous solution" mean?
> > Is this silicic acid?
> > Also, how did you make 40% SA solution?
> > Would you tell me how many gram of what chemical you
> > put in a liter of water
> > or TMAH solution?
> > What is the resource for this recipe?
> > Is there any specific reason for using 10% TMAH
> > solution?
> > I cannot tell much about it based on your
> > description.
> >
> >
> >
> __________________________________________________________
> > Il-seok  Son (Sunny)
> > Dept. of Electrical and Computer Engineering
> > University of Wisconsin - Madison, SonicMEMS
> >
> __________________________________________________________
> >
> > ----- Original Message -----
> > From: "chakravarthy.S.Yamarthy"
> > <~!B*+R^&>> > To:
<~!B*+R^&>> > Sent: Sunday, May 26, 2002 3:57 PM
> > Subject: [mems-talk] TMAH Etching
> >
> >
> > > Dear friends,
> > >
> > > i am trying to do TMAH etching of silicon under
> > the
> > > cantilever beam  made of (silicon
> > > dioxide+polysilicon).
> > >
> > > We have used the solution of TMAH(10% w/w in
> > > water)+SA(40% aqueous solution)+Ammonium peroxy
> > > disulphate(APODS about 1.25gms).
> > >
> > > we have heated the entire solution @ 80 degree
> > > centigrade and chips were etched in this solution.
> > >
> > > But,we could see that Al bond pads,which were
> > opened
> > > thru silicon dioxide passivation layer(9000
> > Angs)were
> > > attacked by the etching solution.
> > >
> > > Can anyone of u please suggest me any good etching
> > > technique for silicon that should not attack
> > Aluminum.
> > >
> > > thanks
> > > Chakry
> > >
> > > =====
> > > To Know That We Know What We Know,And That We Do
> > Not Know,What We Do Not
> > > Know:That Is The True Knowledge.
> > > Confucius
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>
> =====
> To Know That We Know What We Know,And That We Do Not Know,What We Do
Not
> Know:That Is The True Knowledge.
> Confucius
> Yahoo! - Official partner of 2002 FIFA World Cup
> http://fifaworldcup.yahoo.com
> _______________________________________________
> [email protected] mailing list: to unsubscribe or change your list
> options, visit
http://mail.mems-exchange.org/mailman/listinfo/mems-talk
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