The failure of the previous month did not discourage me too much. Reagents cooling in the fridge was failed so I put them into a freezer. All reagents, except water, have a melting point of about -30 degrees C. The result - great! The mixture had cooled to a temperature slightly below zero. Texturization took place in the same reaction mixture during slowly heating in temps: 0, 5, 10, 15, 20, 25. The best result I reached for the wafer etched in solution HF:HNO3:H2O = 8:1:1 in 5 C. Surface was smooth, uniform and black.
In the case of nanoporous layer etching I found that I used too strong solution, 3M KOH instead of a 1M. The experiment should be repeated.