Hi Nam Le,

If you are doing through wafter DRIE, it would not matter since you
will not have a bottom surface. But for the etched depth of 100um, you
will get more or less grass effect. Try to lower the passivation gas
flow to 50sccm, and check the profile. It would probably be helpful to
weaken the grass effect while may be harmful for the verticality.

You can also get some further informations about how to control the
etching profile and grass effect from following references:

1.Fei Wang, Xinxin Li, Nanxiang Guo, Yuelin Wang, and Songlin Feng, A
silicon cantilever probe card with tip-to-pad electric feed-through
and automatic isolation of the metal coating, Journal of
Micromechanics and Microengineering, 2006(16): 1215-1220.

2. A.A. Ayón, R. Braff, C.C. Lin, H.H. Sawin, and M.A. Schmidt,
Characterization of a time multiplexed inductively coupled plasma
etcher, Journal of the Electrochemical Society, 1999(146): 339-349.

Hope it helps, Good luck!

Yours,
Fei

2008/12/10, Le Cao Hoai Nam :
> 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

--
Best regards,
Yours sincerely
Fei Wang
______________
Postdoctoral researcher, Dr
MIC - Department of Micro and Nanotechnology
Technical University of Denmark (DTU)
Building 344, 1st floor, Room no. 130
DK-2800, Kgs. Lyngby
Denmark
Tel:  +45 4525 6311
Fax:  +45 4588 7762
Email: [email protected]
       http://www.nanotech.dtu.dk