High Throughput, Low Cost Deposition of Alumina Passivation Layers by Spatial Atomic Layer Deposition
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High Throughput, Low Cost Deposition of Alumina Passivation Layers by Spatial Atomic Layer Deposition. Ad Vermeer1, F. Roozeboom2,3, P. Poodt2, Roger Gortzen1 1 SolayTec B.V., Dillenburgstraat 9G, 5652AM Eindhoven, The Netherlands 2 TNO, De Rondom 1, 5612AP Eindhoven, The Netherlands 3. Eindhoven University of Technology, Eindhoven, The Netherlands. ABSTRACT Atomic Layer Deposition (ALD) is a gas phase deposition technique for depositing very high quality thin films with an unsurpassed conformality. The main drawback of ALD however is the very low deposition rate (~ 1 nm/min). Recently, record deposition rates for alumina of up to 1 nm/s were reached using spatial ALD, while maintaining the typical assets regarding film quality as obtained by conventional, slow ALD [1]. This allows for ALD at high throughput numbers. One interesting application is passivation of crystalline silicon solar cells. Applying a thin alumina layer is reported to increase solar cell efficiency and enables the use of thinner wafers, thus reducing the main cost factor [2]. In this paper we report on the latest progress made by SoLayTec that delivered a working prototype of a system realizing full area single sided deposition of alumina on 156 x 156 mm2, mono- and multi crystalline silicon wafers for solar cell applications. The alumina layers showed excellent passivation. Based on this concept, a high-throughput ALD deposition tool is being developed targeting throughput numbers of up to 3000 wafers/hr, making ALD ready for mass production. This will bring on new opportunities in other applications. INTRODUCTION Atomic Layer Deposition (ALD) is a deposition technique capable of producing ultrathin conformal films with superior control of the thickness and composition of the films at the atomic level [3]. In conventional ALD, the deposition reaction is divided in two time-sequenced selflimiting half-reactions, each one being separated by purge steps. In the case of Al2O3, one deposition cycle includes a dose of an aluminum precursor (mostly trimethyl aluminum, TMA), followed by a purge step to remove excess precursor and reactants, a subsequent oxidation step by dosing H2O, O2 or O3 and, finally, another purge step. Conventional thermal ALD usually takes place at elevated temperatures and at low reactor pressure. One or more cycle steps may also be facilitated by e.g. a plasma (Plasma Enhanced ALD) [4]. The layer growth during such a cycle, or Growth Per Cycle (GPC), is typically ~0.12 nm/cycle [5]. Thus to obtain thicker films, the cycles have to be repeated many times. As each cycle step can take up to several seconds, the overall deposition rates are of the order of a few nanometers per minute. One way to speed up the process is by batch processing [6], but this is not always compatible with industrial needs. Recently, an ALD concept was presented based on the spatial separation of the half-reactions, rather than temporal, combined with gas-bearing technology [7,8]. In this concept, illustrated schematically in Fig. 1a, the reactor is divid
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