H loss mechanism during anneal of silicon nitride: Chemical dissociation

Remote plasma enhanced chemical vapor deposited silicon nitride (SixNyHz), produced at high ammonia to silane flow rates (ammonia rich) shows a reduct ion of hydrogen during rapid thermal anneal at temperatures that exceed the deposition temperature. This H release could be either due to a "slow" ato mic diffusion of the covalent bonded atoms between bonding sites, or to a " fast" molecular diffusion of hydrogen containing molecules (e.g., H-2, NH3, SiH4), which dissociate from the network before they diffuse. In order to determine the dominant mechanism, layers of deuterated and hydrogenated sil icon nitride on top of a crystalline Si substrate were annealed and the dev elopment of the NH- and ND-area densities were measured with Fourier transf orm infrared spectroscopy. Comparison of theoretical models with the measur ements 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 nit ride antireflection coatings of solar cells is mostly due to H migration ou t of the system and not into the Si area and make the hypothesis that postd eposition annealing of solar cell antireflection coatings can cause H-relat ed bulk passivation of the underlying c-Si therefore questionable. (C) 2000 American Institute of Physics. [S0021-8979(00)09423-8].