I need to uniformly remove a thin layer of Si off a 3" Si wafer. No
structures, just the same material. No RIE. How to do it?

I've tried the HNA solution but it doesn't look very uniform and some
white-ish looking cloud was left on the wafer.

I have also tried using NaOH but it looked very bad after dipping the wafer
in there for couple of minutes. And doesn't look very uniform.

Right now I am trying using Piranha (H2SO4:H2O2=3:1) to chemically grow a
thin oxide and then etch off in Si. I am thinking that each cycle should
consume 2nm of Si then if I am doing this 30 times then I am removing 60nm
of material. But I have no way to verify this.

What do you suggest to use to remove a thin layer off a Si wafer in a
uniform fashion?

Dave