Fast Etching of Amorphous and Microcrystalline Silicon by Hot-Filament Generated Atomic Hydrogen

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Abstract
A hot tungsten wire effectively dissociates H2 into atomic hydrogen and thereby facilitates etching and hydrognation of silicon. Hot filament generated atomic hydrogen etches amorphous silicon (a-Si:H) at a rate of up to 27 Å/s and microcrystalline (μc) Si at rates up to 20 Å/s. A large laminar gas flow 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 H2 gas flow. Likewise, the etch rate rises with the filament temperature and saturates at a filament temperature of approximately 2150°C when approaching the maximum H2 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 μc-Si drops at elevated substrate temperatures. Boron doping decreases the etch rates both for a-Si:H and μc-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 μc-Si etch rate close to the film substrate interface characterizes the film thickness at which the coalescence of the microcrystalline nuclei starts.