Hi Li. Zhang I etched more than 30 um down to Silicon with a recipe that was similar what you have used. But I found no grassing. Anyhow my hard mask was 500nm SiO2. and my recipe was as given below SF6 468sccm C4F8 346sccm O2 146sccm and etching for 3 seconds and passivation for 2 seconds. Best regards Renil --- On Sat, 20/2/10, Li. Zhangwrote: From: Li. Zhang Subject: Re: [mems-talk] DRIE Grass Cleaning To: mems-talk@memsnet.org Date: Saturday, 20 February, 2010, 10:49 PM Hey Renil, The hard mask is sputtered Al with thickness of 1500A and the the depth etched down to silicon is about 10um. I applied an etching cycle of 3s and a passivation cycle of 1s. The flow rate of SF6 = 200sccm and C4F8 = 100sccm. I've heard lowing the passivation flow rate might help with grass problem, but not quite sure. My Best, Li. Zhang