In a pure-hydrogen glow discharge plasma, the etch rate of silicon increases with increasing temperature up to about >= 1100 angstrom/s at 60-80 degrees C and, upon a further increase of the temperature, etch rate strongly decreases, showing Arrhenius-like dependence with negative apparent activation energy of -1.5 kcal/mol. When the Si sample is at the floating potential, oxygen impurities of >= 10 at. ppm strongly decrease the etch rate. At more than 70 ppm of oxygen, the etching stops. Oxygen adsorbed on the Si surface can be removed by ion bombardment when negative potential is applied to the Si sample and the Si is then etched chemically by H atoms. The etching by atomic hydrogen is isotropic in an oxygen-free system. A controllable addition of a few ppm of oxygen in combination with negative bias of the Si sample results in highly anisotropic etching with thin oxide acting as side-wall passivation. (c) 2008 American Vacuum Society.