H loss mechanism during anneal of silicon nitride: Chemical dissociation

Remote plasma enhanced chemical vapor deposited silicon nitride $({\mathrm{Si}}_x{\mathrm{N}}_y{\mathrm{H}}_z\mathrm{),}$ produced at high ammonia to silane flow rates (ammonia rich) shows a reduction of hydrogen during rapid thermal anneal at temperatures that exceed the deposition temperature. This H release could be either due to a “slow” atomic diffusion of the covalent bonded atoms between bonding sites, or to a “fast” molecular diffusion of hydrogen containing molecules (e.g., ${\mathrm{H}}_{\mathrm{2}}{\mathrm{, NH}}_{\mathrm{3}}{\mathrm{, SiH}}_{\mathrm{4}}\mathrm{),}$ which dissociate from the network before they diffuse. In order to determine the dominant mechanism, layers of deuterated and hydrogenated silicon nitride on top of a crystalline Si substrate were annealed and the development of the NH- and ND-area densities were measured with Fourier transform infrared spectroscopy. Comparison of theoretical models with the measurements showed that chemical dissociation and subsequent rapid diffusion are the dominant processes. These results were confirmed by secondary ion mass spectroscopy. The experiments indicate that the H reduction in silicon nitride antireflection coatings of solar cells is mostly due to H migration out of the system and not into the Si area and make the hypothesis that postdeposition annealing of solar cell antireflection coatings can cause H-related bulk passivation of the underlying $\mathrm{c-}\mathrm{Si}$ therefore questionable.