Hi, You can use Sulfuric/Hydrogen Peroxide, like 98:2 very little peroxide, use a hard mask. Or use a resist mask and use PA-11 which is Water:Hydrogen Peroxide:Ammonium Hydroxide use a ratio of 80:10:10. If you need to keep certain features like MESA's then adjust the pH using the NH4OH and you can make great mesa structures. To get it going faster add more H2O2 and NH4OH. Rick -----Original Message----- From: mems-talk-bounces@memsnet.org [mailto:mems-talk-bounces@memsnet.org] On Behalf Of deepa sree Sent: Wednesday, August 23, 2006 3:04 PM To: mems-talk@memsnet.org Subject: [mems-talk] Deep GaAs etching Hi I need to etch 3-4 mils of GaAs. I need to chemically etch it due to lack of facilities. Can anyone suggest a suitable masking layer and a fast chemical etchant that does this?