Hi all, I am doing a silicon cryo-etch process. My processes before the final cryo-etch step are as follows: 1- start with a silicon wafer that has about 500 nm thermal oxide layer on its surface. 2- lithography steps using positive resist(HPR 504) 3- RIE to remove sio2 4- Branson etching 5- final step cryo-etching I get my wafer covered with a black gunck in the middle of it at the end of cryo step. In my wafer I don't have too many objects or features and a lot of it is bare silicon to be etched. I suspect this is the source of my problem. Does anybody have an idea how to solve this issue? I greatly appreciate your suggestions and comments. Thank you Syd