Nam, We would use a jig of some sort. I have tried to etch through using a number of masking layers in the past, i.e. resists, metals, oxides. They never seem to work. As in one of the previous posts I would use a teflon or peek jig. I think you can just buy them now so you will not have to make your own. I don't recall the vendor for such a product though. Good luck, Mike >>> "Le Cao Hoai Nam"08/27/06 3:46 AM >>> Mike, Thanks a lot for your prompt reply. Just one little question, how do you protect back side of the Si wafer when you etch for long time? If I am not wrong, from the etching set-up image, you etch the whole wafer, right? Best regards, Nam