Nate: I have a few suggestions that might help. You don't mention why the profile is not meeting your requirements but based on the information you provided, you are probably seeing an undercut profile. Also, you don't provide any parameters for the conditions that you have tried but presumably you are running at a few hundred millitorr and at or near room temperature, or higher, for the system that you described. In order to get a straight vertical profile, with the combination of SF6/O2, you might consider trying to operate at the lowest pressure and lowest temperature that you can safely achieve in your system. The safe operating ranges are typically provided by the manufacturer. The use of backside helium cooling will also help to keep the wafers cooler during processing and should be set to 5-10 Torr is possible. If an electrostatic clamp is used, you might find that you need to optimize the clamping voltage setpoint to get optimal clamping and cooling. If you are processing small pieces of wafers, then you could investigate bonding of the pieces to a larger substrate using eutectic alloys or vacuum grease, if allowable in your system. Once you have determined the operating limits in temperature and pressure, you might consider starting with an oxygen rich process, say 10:1/O2:SF6 to determine if you can obtain sufficient passivation of your sidewall to produce a non-undercut profile. If you can achieve a non-undercut, angled profile at 10:1, then with further reduction in the O2/SF6 ratio you should be able to find a mix that produces the vertical profile angle. If you do see an undercut at 10:1, then you may want to try increasing the O2/SF6 ratio further. As you introduce more O2 to the mix, your silicon and SiN etch rates will drop but this is a tradeoff that you may be forced to make. If you are unable to get a positive profile, then you could try reducing the bias power level. This will also result in further reduction in etch rate but may provide the profile performance that you are targeting. If you cannot find a set of conditions that produces the non-undercut profile by reducing the pressure, temperature, and power, then it is unlikely that this chemistry will work for you. Regards, Robert Ditizio -----Original Message----- From: mems-talk-bounces+rditizio=tegal.com@memsnet.org [mailto:mems-talk-bounces+rditizio=tegal.com@memsnet.org] On Behalf Of Nate Lawrence Sent: Monday, August 23, 2010 7:55 AM To: mems-talk@memsnet.org Subject: [mems-talk] Anisotropic RIE Etching of Si and SiN Hi, I'm trying to find a good RIE recipe for anisotropically etching Si (crystalline and poly-Si) and SiN while maintaining straight and as smooth as possible sidewall profile. I'm using a metal etch mask (have tried both Cr and Ni) and would like some selectivity over SiO2, though high selectivity is not crucial. I would like to be able to etch vertically as much as possible, my current goal is 2um. I have a plasma therm RIE system with the following gases Channel1 Ar, CH4, He Channel2 O2, H2 Channel3 SF6, CBrF3, CHF3, CF4 Channel4 Cl2 All combinations of gases I have tried with O2 seem to give poor sidewall profile. My latest attempt was CF4/H2 with the hope of forming some polymer passivation on the sidewalls though this the etch rate of crystalline Si seems too slow in this process and my mask is not holding up to as deep an etch as I want (also no selectivity to SiO2). If anyone has any suggestions I would greatly appreciate it! I have read that etching with Cl or Br may improve the process and give selectivity to SiO2 though I have no experience myself with those gases. Thanks! Nate Lawrence PhD Candidate Department of Electrical and Computer Engineering, Boston University