EFFECT OF TRACE AMOUNTS OF OXYGEN ON THE HCL ETCHING OF SILICON

The introduction of small amounts of oxygen (or water) during the gas phase etching of silicon with HCl at high temperatures (1400 K) has a pronounced effect on the etch rate. When at fixed HCl vapour pressure and temperature the oxygen concentration is increased from 1 to 20 ppm the etch rate increases due to the parallel etching of silicon according to Si+2HCl→SiCl2+H2 and Si+ 1 2O2→SiO. Going to higher O2 concentrations the total etch rate passes a maximum and beyond 40 ppm O2 - depending on T and carrier gas - the total etch rate drops to zero. This is due to the formation of an oxide skin at the silicon surface, which layer physically hinders the etching of silicon with HCl. In argon this oxide layer has a closed structure causing an abrupt drop of the total etch rate within a very narrow range of oxygen input concentrations. In hydrogen the oxide layer has an open structure so that the HCl etching is only partially interfered with and the total etch rate only very gradually goes to zero with increasing O2 input concentration. The observed formation of SiO2 on Si in H2 and Ar systems with and without HCl agrees with thermodynamical calculations on the formation of a second solid phase of SiO2. This demonstrates that near-equilibrium conditions are present during the etching of silicon in the system studied. © 1979.