Firing stability of tube furnace-annealed n-type poly-Si on oxide junctions

Stability of the passivation quality of poly-Si on oxide junctions against the conventional mainstream high-temperature screen-print firing processes is highly desirable and also expected since the poly-Si on oxide preparation occurs at higher temperatures and for longer durations than firing. We measure recombination current densities (J(0)) and interface state densities (D-it) of symmetrical samples with n-type poly-Si contacts before and after firing. Samples without a capping dielectric layer show a significant deterioration of the passivation quality during firing. The D-it values are (3 +/- 0.2) x 10(11) and (8 +/- 2) x 10(11) eV/cm(2) when fired at 620 degrees C and 900 degrees C, respectively. The activation energy in an Arrhenius fit of D-it versus the firing temperature is 0.30 +/- 0.03 eV. This indicates that thermally induced desorption of hydrogen from Si-H bonds at the poly-Si/SiOx interface is not the root cause of depassivation. Postfiring annealing at 425 degrees C can improve the passivation again. Samples with SiNx capping layers show an increase in J(0) up to about 100 fA/cm(2) by firing, which can be attributed to blistering and is not reversed by annealing at 425 degrees C. On the other hand, blistering does not occur in poly-Si samples capped with AlOx layers or AlOx/SiNy stacks, and J(0) values of 2-5 fA/cm(2) can be achieved after firing. Those findings suggest that a combination of two effects might be the root cause of the increase in J(0) and D-it: thermal stress at the SiOz interface during firing and blistering. Blistering is presumed to occur when the hydrogen concentration in the capping layers exceeds a certain level.