[mems-talk] Lateral undercut in copper wet etch problem

[wizardz at citiz.net]

Now I have to etch 1um thick sputtered copper layer on silicon wafer.
To my knowledge, adding a little H2SO4 and H2O2 to DI water makes an easy copper etchant.
But I discovered severe lateral undercut . The undercut is typically
lager than 10um after removal of 
 1um thick copper layer.
Judging from the color change during etching, the etch process has poor
uniformity in speed through the wafer. I also tried diluted etchant to
lower the etch speed for better control,the etch then works slowly but
stilll undercutst.
I think in this case, the lateral etch of 2 or 3um is reasonalbe.
I've searched earlier post and found no good answer to this.Or some document provide speciallized equipment to controal this process,
Can anyone give me some suggest easy etchant to use? Does FeCl3
solution or other work better? 
Thank you!

Z.Z. Wu
SIMIT, Chinese Academy of Sciences