Acid etching of silicon?

Hi guys,

Would any of the following mixtures etch silicon wafers, and if so, does it etch isotropically or anisotropically?

1) Dilute HF

2) Aqua regia (3:1 HCl:HNO3)

3) Piranha (1:1 H2SO4:H2O2)

I know that 1) and 3) shouldn't because they're a part of the standard RCA clean process, but I want to know if there are any situations in which they would.

-Vijay-