I have worked on membranes 10-100 um thick, KOH-etched mostly, 4.5 mm or so square. 29 um is too thick for boron etch stop, via either diffusion or ion implantation. SOI is probably the best bet, even with the anticipated stress issue. You could actually compensate for the oxidation-related stresses, using appropriate implantation, per Brian's description. (B is tensile stress, As is compressive stress, if the concentration is high enough.) If, however, the stress is truly a problem, then you need to use other means. Timed etch gives good results, but be aware that if 29 um is your target thickness, then you will end up with variation of thickness across the membrane of 29 +/- 3 um or +/- 5 um, depending upon the quality of your starting material, and the control of KOH stoichiometry and temperature during the etch. I have heard that timed etch is much easier to control with EDP. However, EDP is very tough stuff to work with, from an MSDS perspective, and not everyone is willing to take the risk. I have also heard electrochemical etch of n-type doped silicon can be tightly controlled. --- Albert K. Henning, PhD Director of MEMS Technology NanoInk, Inc. 215 E. Hacienda Avenue Campbell, CA 95008 408-379-9069 ext 101 ahenning@nanoink.net -----Original Message----- From: Brian Stahl [mailto:bstahl@mrl.ucsb.edu] Sent: Monday, December 21, 2009 12:25 PM To: General MEMS discussion Subject: Re: [mems-talk] silicon membranes Morten is right - heavily boron-doped silicon etches much slower than undoped or lightly-doped silicon in anisotropic wet etchants like KOH and TMAH, and makes a nice etch-stop. There is a significant amount of literature out there on this subject. Keep in mind that because boron is a substitutional impurity and has a smaller covalent radius than silicon, boron doping creates a residual tensile stress in the silicon. The more significant issue is creating an etch-stop at your target thickness of 29µm. I would say that thermally diffusing boron from a solid or gaseous source would be prohibitively time consuming (email me if you'd like a detailed explanation). I can't speak to the feasibility of ion implantation because I don't know what kind of penetration depths can be achieved with ion beams. I fabricated ~5um thick membranes as part of my MS thesis research. I doped one side of the wafer with boron to create an etch stop, and I measured the etch progress by periodically removing the wafers from the etchant (refluxing 25wt% TMAH @ 90°C) and measuring the etch depth with a stylus profilometer. This method, or something similar (without the etch-stop), could work for your application provided that you know the initial thickness of your wafer with an accuracy of +/- 1µm or so. Please contact me if you'd like to discuss this further. Good luck, -- Brian C. Stahl