Spatially-separated atomic layer deposition of Al₂O₃, a new option for high-throughput Si solar cell passivation

A next generation material for Si surface passivation is atomic layer deposited (ALD) Al ₂O ₃. However, conventional time-resolved ALD is limited by its low deposition rate. Initially, a high-deposition-rate prototype ALD reactor based on the spatially-separated ALD principle has been developed, with Al ₂O ₃ deposition rates up to 1.2 nm/s. Later, the spatial ALD technique has been transferred to an actual in-line process development tool (PDT) for commercial high-throughput ALD of Al ₂O ₃, resulting in a deposition rate of 30 nm/min. The passivation quality and uniformity of the spatially-separated ALD Al ₂O ₃ films are evaluated on p- and n-type Si, applying quasi-steady-state photo-conductance, carrier density imaging and infrared lifetime mapping. In all cases, a spatial ALD Al ₂O ₃ layer of only 10 nm reached an excellent passivation quality and uniformity, comparable to reference wafers passivated by equivalent temporal plasma-assisted or thermal ALD Al ₂O ₃. Effective surface recombination velocities as low as 1.1 or 2.9 cm/s were obtained after annealing at 350°C or firing, respectively. Using spatial ALD Al ₂O ₃ passivated local Al back surface field p-type Si solar cells, the sufficient passivation of this high-throughput Al ₂O ₃ layer is evaluated: an average gain in open circuit voltage as compared to SiO _x rear passivated i-PERC cells is obtained.