I have read about this mix that etch P+ 1E18 and stop on P- (1E14).  I tried
this
while etching a silicon wafer with EPI. I took silicon and I put it in the mix
for 5
min. The result was that wafer etched on both sides with same thickness. Maybe
from P+ etched more about 30%, but I did not get any selectivity between P- and
P+.  Can somebody explain this please.