I agree with the posts below. This is how I would proceed. If you can tolerate a variation in thickness in the 5 um range then time etch starting with thin wafers, good temperature and condensation control, and expect a variation across the wafer. You should get some parts. I would pick KOH and water at as high of a concentration (20-40 percent) that the mask can stand. Better thickness control with higher concentration but slower etch rates of silicon and worse selectivity to oxide. If you want tight thickness control then use SOI wafers and balance out the stress. You can use thinner oxides at the SOI interface or match the stress using deposited films or implants. This technique has been used in high volume. If you want tighter thickness control and don't want to use SOI wafers then you have several choices but they will all take some effort. 1) Electrochemical etching of P-type wafers with N-type epi or N diffusion regions can repeatedly produce diaphragm thickness variations of less than 1 um. Of course this depends on the etchant temperature and concentration control. Lots of literature available. This technique has been used in high volume. 2) You can produce 29 um thick diaphragms with a boron doped etch stop. You first must use a boron doped layer or an epitaxially grown layer and then grow a lighter doped epi layer to the desired thickness. You will get stress dislocations in the epi until you reach a certain thickness but 29 um should be fine. You can also use stress compensated boron doped regions as an etch stop which will allow you to grow thinner epi regions without stress dislocations but you would need to order special wafers from epi vendors. You then would need to pick the proper etchant and conditions to stop on the boron layer. There a few choices and they can all work. Lots of literature available. This technique has been used in high volume. EDP has the best boron selectivity. 3) You can create a cavity in one substrate and then bond and thin another wafer over the cavity. You still need to have an etch stop and you have to do bonding. This has also been done in high volume. These are probably the most common ways but there are several more techniques that have been used. I would use SOI wafers and balance the stress. I would stay away from EDP unless you need a high volume process using a boron etch stop. TMAH works well in most instances but I would only use it if my mask can't take the KOH. Dan Chilcott ITT -----Original Message----- From: mems-talk-bounces@memsnet.org [mailto:mems-talk-bounces@memsnet.org] On Behalf Of Albert Henning Sent: Tuesday, December 22, 2009 4:19 PM To: General MEMS discussion Subject: Re: [mems-talk] silicon membranes 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