Comparison of low damage sputter deposition techniques to enable the application of very thin a-Si passivation films

Sputtering on passivating layers of silicon solar cells is often reported as a process that can harm the c-Si surface passivation quality. The loss of minority carrier lifetime and decrease in iV_oc are related to a "sputter damage" of the passivation layer or more probably to the interface to the silicon wafer as it is known that the thickness of the passivating layer plays an important role. The "damage" is often attributed to fast particles generated at the cathode and directed to the substrate. Damage by UV radiation might be another reason. However, sputtering can be done in different set-ups with different discharge excitation forms where direct current sputtering is the most industrially applicable. Within this work, different sputter setups were investigated using the same a-Si:H(i) passivated lifetime samples with a very thin passivation layer of only 8 nm. The change in passivating properties directly after sputtering and after an annealing step were quantified. The investigated sputter setups were industrial size tube magnetron sputtering with a single tube (direct current discharge) as well as with a double tube (alternating current), a special magnetron setup based on the facing target principle developed in particular for low damage processes (direct current), and gas flow sputtering (direct current). The discharge power plays a role in all three setups. However, clear differences in the "damage" can be seen for the different methods. The rotatable magnetron sputtering yields already a good iV_oc of more than 715 mV for the thin 8 nm thick a-Si:H film, the best values for the gas flow sputtering are close to 730 mV and the sputtering with the facing targets cathode results in values even higher than 730 mV. The excellent suitability of the latter two approaches to provide low damage during sputter deposition on ultra-thin a-Si films is substantiated by the fact that iV_oc after annealing is even higher compared to the unsputtered references.