Fast etching of amorphous and microcrystalline silicon by hot-filament generated atomic hydrogen

A hot tungsten wire effectively dissociates H-2 into atomic hydrogen and thereby facilitates etching and hydrogenation of silicon. Hot filament generated atomic hydrogen etches amorphous silicon (a-Si:H) at a rate of up to 27 Angstrom/s and microcrystalline (mu c) Si at rates up to 20 Angstrom/s. A large laminar gas now is the key to high etch rates. It provides for a fast transport of the etch products out of the reaction zone and thereby avoids redeposition. The etch rate increases with pressure and with H-2 gas now. Likewise, the etch rate rises with the filament temperature and saturates at a filament temperature of approximately 2150 degrees C when approaching the maximum H-2 dissociation probability. The decrease of the etch rate at higher substrate temperatures is attributed to the loss of the surface coverage by atomic hydrogen. The etch selectivity between a-Si:H and mu c-Si drops at elevated substrate temperatures. Boron doping decreases the etch rates both for a-Si:H and mu c-Si, whereas phosphorous doping does not significantly affect it. This etch selectivity is caused by a catalytic effect of BH3 on the surface hindering the formation of the main etch product silane. Even for highest etch rates no surface roughening of a-Si:H occurs, however, a bond structure modification of the near surfaces arises, an effect which results in the formation of a nanocrystalline surface layer. The increase of the mu c-Si etch rate close to the film substrate interface characterizes the film thickness at which the coalescence of the microcrystalline nuclei starts.