Invited Paper TF-MoA1
Atmospheric ALD of Al₂O₃ for a High Throughput c-Si Solar Cell Passivation

Monday, October 31, 2011, 2:00 pm, Room 107

Surface passivation of advanced solar cells becomes more important with decreasing silicon wafer thickness (increased surface to bulk ratio). The International Technology Roadmap for Photovoltaics predicts that the thickness of wafers processed in mass production will be 100 μm by 2020. By 2012 new processes with lower Cost of Ownership (CoO) and better passivation properties will be needed [1].

It is widely accepted that Al₂O₃ layer has excellent surface passivation capabilities [2]. Solar cells with Al₂O₃ layers < 10 nm demonstrate the best results: higher efficiency and more stable structures when annealed at high temperatures [3,4]. ALD is an ideal technique for the deposition of thin layers. However, conventional ALD equipment can not be used in the PV industry because of the low throughput and associated high CoO. A suitable way to reduce the tool cost per cell is to increase the throughput of the system. The ITRPV requires a minimum throughput of production tools from 3600 wph in 2012 to 7200 wph in 2020.

We developed an ALD tool which meets these throughput requirements and is capable to process at a cost of 3-4 cent per wafer. In this tool we implemented a new approach: atmospheric, spatial ALD. Silicon wafers are transported in a linear track passing areas with TMA and H₂O precursor gas curtains. N₂ purges in between these curtains separate the different precursors thereby avoiding deposition on the track walls. The wafers are transported on a gas bearing (levitate) inside a narrow track. The Levitrack system operates in-line and has a length of ~10 m for the deposition of 10 nm of Al₂O₃. At the conference Levitrack tool details and process results will be presented. Some items are:

· Passivation quality: effective lifetime of 6.1 ms in mono-silicon 10 Ω cm at injection level of 3E15 cm^-3.

· Layer characteristics: 1σ uniformity <3% on 156x156 mm wafers, intrinsic layer charge of -4E12 cm^-2

· Tool options: deposition on both or one side of the wafer (N₂ flow protects the back side of the wafer against Al₂O₃ deposition)

· Tool automation (automated wafer loading/unloading, 100 wafer cassettes and use inline, control of process, safety, and other parameters)

· Tool reliability (reliable performance at marathon runs with throughput of ~4000 wph)

· Current solar cell results (efficiency obtained for p-type crystalline silicon cell is 16.6% and for n-type is 18.3%. Improvement of 1-2% is expected to be achieved by optimizing local BSF below metal contacts and contact/shunt resistances)

1. ITRPV.net, 2^nd edition, 2011

2. S.Chunduri, Photon Int., p.146, 2011

3. P.Brand, 37th IEEE PV Conference, Seattle, 2011

4. I.G.Romijn, 25th. EU PVSEC, 2010