Hi Michael, I've tried miture of BOE(6:1) and glycerol 5:3. The evaporated aluminium still seems to be attacked, and the etch rate is very non-linear. My guess is that the solution doesn't mix very well. poly, you might want to try Silox Vapox from Transene. They claim it's an optimized oxide etchant for a metalized wafer. You can check their website. Please let me know the result. Yenchun Lee ----- Original Message ----- From: "ZICKAR Michaël"To: "polly" ; "General MEMS discussion" Sent: Friday, October 07, 2005 12:17 AM Subject: RE: [mems-talk] how to etch SiO2 without etching underlying Aluminum Hi polly, BHF:glycerol 2:1 etches aluminium with a selectivity of 0.006 towards thermal SiO2, HF vapor phase etching etches aluminium with selectivity of 0.002. Check out the publications and application on idonus.com.