Dear MEMS researchers,

I am doing a through-wafer DRIE of 300um thick Si wafer for my optical
devices. The etching patterns are 50um in width and few millimeters in
length. The mask for the DRIE is either 300nm evaporated Al or 1.2um
thermally-grown SiO2. Our goal is to achieve as smooth side-wall as
possible throughout the depth of the etched trenches. However, I have
experienced some problems with Si grass at the bottom of the trench.
Initially, we thought the problem was due to Al sputtered and
redeposited to the trench's bed and Al served as local masks leading
to Si grass. However, when we turned to use SiO2 mask, the problem
still persisted. In both cases, I could not see the scalloping after
etching through the wafer. However, if I etched only under 100um then
I could see some scalloping. I am wondering if somebody has seen this
problem before and if so could you kindly advise us your experience on
how to solve the problem.

Following are the etching parameters we have used:

System: MUC-21 ASE from Sumitomo and STS
Etching cycle (E): 4 s
Passivation cycle (P): 3 s
Gases: For etching cycle SF6 = 130 sccm, O2 = 20 sccm; For passivation
cycle: C4F8 = 85 sccm
Pressure: For etching cycle: 5.3 pa; For passivation cycle: 3.3 pa
Coil power: For etching cycle: 600 W; For passivation cycle: 600 W
Platen HF: For etching cycle: 18 W; For passivation cycle: 0 W
Platen LF: For etching cycle: 0 W; For passivation cycle: 0 W
Bias voltage: Not sure but around 4 V

I placed a .pdf file containing the SEM images for both Al and SiO2
masks, in case you might want to take a look. Here is the link:
http://lechnam.googlepages.com/Sigrassinthrough-waferDRIE.pdf

Thank you very much.

Best regards,

Nam Le, graduate student
Ritsumeikan University, Japan