I tried to do selectivity etch  P+(bulk) and to stop on the P-(EPI) for image
sensor thinning process, I do etch rate expirments while I measure delta Silicon
thickness from both sided, I get Etch rate for P+ 1mic/min and for EPI
0.5mic/min, according t litterature EPI ER should be very less. These exprments
I do in lab while I pu one peace of silicon on cup of HF:Nitric:Acetic 1:3:8.
Maybe I should do here electrochemical etching ? to get good selectivity.