Hi Scott, I can tolerate upto 1mil of undercut. And I think about 5microns per minute is "fast" enough. Thanks for your reply. Deepa Scott McWilliamswrote: Hi Deepa, it may help us respond if you can give an indication of how much mask undercut you can tolerate. Many chemicals etch GaAs "fast" and the degree of anisotropy can depend on the crystal planes. Also, can you define what is acceptably fast?