Dear Colleagues,

 I tried to etching PSG, and found some strange phenomena.

 The structure is

 2um (+)Photoresist pattern (hard baked)/2um LPCVD PSG/0.4
 um n+polysilicon/ 0.5 um Si3N4/ Silicon wafer substrate.

  First, I used RIE to etching PSG for 15 mins, the pattern looks
fine. Then I used Buffered HF etching 30 secs, a layer of dark spots all
over the exposed area ( no Photoresist), these are not dirty, it seems
some chemical reaction occured.

 I searched MEMS-Talk archive. In sept. 2002 there was one essage very
similar to my case, while they use 49%HF. But I did not found the
confirmed answer.

 What is the layer formed?  and how to aovoid it, any papar can be
referred ?

 Anyhelp is highly appreciated.
 Best Regards!

-------------------------------
Yuyan Wang
EE. U. of Minnesota
email:  [email protected]