Try taking a look at this paper.  While most of it is dedicated to dry
etching, there is information and data on Citric acid: Hydrogen Peroxide
selective etching.

http://www.mse.ufl.edu/~spear/recent_theses/david_hays_sel_etching_of_compou
nd_semi.pdf

I've done this kind of selective etching in the past so I have a couple of
suggestions:

(1)  When you initially mix the solution it will get cold (colder than room
temperature).  Let it stabilize to room temperature.

(2)  Don't agitate the solution while doing the etch.  This is mostly for
repeatability.  This seemed to result in a clean, smooth etch without
residue.

(3) Once you etch the GaAs, and reveal the AlAs surface AND you intend to
etch the AlAs, quickly rinse the citric acid:hydrogen peroxide solution and
etch the AlAs.  The longer the AlAs is exposed to air the more it will
oxidize and result in rough or incomplete etch.

Good Luck!

************************************
Philip D. Floyd, Ph.D.
Iridigm Display Corporation
2415 Third Street, Suite 235
San Francisco, CA 94107
ph:  (415) 626-8800 x138
fax: (415) 626-9775
email: [email protected]
web:  http://www.iridigm.com/
*************************************
############################################################################
##############################################
This email communication may contain CONFIDENTIAL INFORMATION of IRIDIGM
DISPLAY CORPORATION and is intended only for the use of the intended
recipients identified above. If you are not the intended recipient of this
communication, you must not use, disclose, distribute, copy or print this
email. If you have received this communication in error, please immediately
notify the sender by reply email, delete the communication and destroy all
copies.
############################################################################
##############################################


-----Original Message-----
From: Sai Raghav Parasa [mailto:[email protected]]
Sent: Sunday, December 14, 2003 5:06 PM
To: General MEMS discussion
Cc: [email protected]
Subject: Re: [mems-talk] High Selectivity for GaAs over AlAs for Wet
Chemical Etching

Hi,

Can you send me the useful references in PDF format, as I do not have access
to the Compendex. Can you tell me how to take the access in Compendex? What
is the website for that?

Bye,
Parasa,
University of Kassel.

William Lanford-Crick  wrote:
Hi,
We have worked on this a lot in our group, using CitricaAcid:Hydrogen
Peroxide in the appropriate concentration. The ratio will tune which
concentration of Al in AlGaAs you can and cannot etch. See the references
below. If you don't have access, I can send you 1 or 2 in PDF format.

-Bill

4 records found in Compendex for: ((((adesida) WN AU) AND ((citric acid) WN
AB)) AND ((selective) WN AB)), 1990-2004
Select all on page | Select range:to | Clear all on page | Clear all
selections

1. Comparative study of wet and dry selective etching processes for
GaAs/AlGaAs/InGaAs pseudomorphic MODFETs
Tong, M. (Univ of Illinois); Ballegeer, D.G.; Ketterson, A.; Roan, E.J.;
Cheng, K.Y.; Adesida, I. Source: Journal of Electronic Materials, v 21, n 1,
Jan, 1992, p 9-15
Database: Compendex
Abstract / Links | Detailed Record / Links


2. Process for enhancement/depletion-mode GaAs/InGaAs/AlGaAs pseudomorphic
MODFETs using selective wet gate recessing
Tong, M. (Univ of Illinois); Nummila, K.; Seo, J.-W.; Ketterson, A.;
Adesida, I. Source: Electronics Letters, v 28, n 17, Aug 13, 1992, p
1633-1634
Database: Compendex
Abstract / Links | Detailed Record / Links


3. Selective wet etching characteristics of lattice-matched
InGaAs/InAlAs/InP
Tong, M. (Univ of Illinois); Nummila, K.; Ketterson, A.A.; Adesida, I.;
Aina, L.; Mattingly, M. Source: Journal of the Electrochemical Society, v
139, n 10, Oct, 1992, p L91-L93
Database: Compendex
Abstract / Links | Detailed Record / Links


4. InAlAs/InGaAs/InP MODFET's with uniform threshold voltage obtained by
selective wet gate recess
Tong, M.; Nummila, K.; Ketterson, Andrew; Adesida, Ilesanmi; Caneau, C.;
Bhat, Rajaram Source: IEEE Electron Device Letters, v 13, n 10, Oct, 1992, p
525-527
Database: Compendex
Abstract / Links | Detailed Record / Links




---- Original message ----
>Date: Thu, 11 Dec 2003 16:58:10 -0800 (PST)
>From: Sai Raghav Parasa

>Subject: [mems-talk] High Selectivity for GaAs over AlAs for Wet Chemical
Etching
>To: [email protected]
>
>Hi all,
>
>Can you give 3 best etchants which have high selectivity for GaAs over AlAs
for Wet Chemical Etching?
>
>The etchant should best etch GaAs but should not attack AlAs.
>
>Thanks,
>Bye,
>Parasa,
>University of Kassel, Germany.
>
>
>---------------------------------
>Do you Yahoo!?
>New Yahoo! Photos - easier uploading and sharing
>_______________________________________________
>[email protected] mailing list: to unsubscribe or change your list
>options, visit http://mail.mems-exchange.org/mailman/listinfo/mems-talk
>Hosted by the MEMS Exchange, providers of MEMS processing services.
>Visit us at http://www.memsnet.org/

_______________________________________________
[email protected] mailing list: to unsubscribe or change your list
options, visit http://mail.mems-exchange.org/mailman/listinfo/mems-talk
Hosted by the MEMS Exchange, providers of MEMS processing services.
Visit us at http://www.memsnet.org/

---------------------------------
Do you Yahoo!?
New Yahoo! Photos - easier uploading and sharing
_______________________________________________
[email protected] mailing list: to unsubscribe or change your list
options, visit http://mail.mems-exchange.org/mailman/listinfo/mems-talk
Hosted by the MEMS Exchange, providers of MEMS processing services.
Visit us at http://www.memsnet.org/