High vacuum chemical vapour deposition of oxides: A review of technique development and precursor selection
Thin films of oxide materials are widely used for various types of applications. The selection of an appropriate deposition method depends on the aimed material and application. We review here a high vacuum chemical vapour deposition (HV-CVD) method, which can be considered as a hybrid technique between classical low pressure chemical vapour deposition (LP-CVD) and molecular beam epitaxy (MBE). The principal features of HV-CVD are summarized and its main differences from other techniques analysed. The evolution of the design of precursor delivery systems from simple pressure reduction to a multiple effusion sources system has enabled the versatility of the HV-CVD method. Full wafer scale deposition, application of controlled precursor flux gradients and, based on it, combinatorial process optimisation are three main features of this development. In this contribution a comprehensive overview of oxide materials, which have been deposited by HV-CVD, and types of precursors reported in the literature is presented and analysed. Mostly metal-alkoxides, metal-beta-diketonates and metal-alkyls have been utilized in HV-CVD processes. In our laboratory the following oxide materials have been deposited on full wafer substrates in HV-CVD reactors using alkoxide precursors or derivatives: TiO2, TiO2-SiO2, Al2O3, Nb2O5, Nb2O5-HfO2, LiNbO3. The deposition chemistry and the efficiency of the process vary strongly depending on the precursor type. Due to the reduced/absent intermolecular collision events in the gas phase in HV-CVD as compared to LP-CVD, substantial differences in the physics and chemistry of the deposition processes are observed. Efficient precursor decomposition with more than 95% efficiency and deposition rates up to 500 nm/h have been observed for certain alkoxide precursors, whereas the presence of strong oxidizers (O-3 or O-2 plasma) seems to be indispensable in order to obtain an oxide deposit using beta-diketonate precursors. Here, slower deposition rates in the order of tens of nm/h are achieved. The main concern of the applicability of the technique for new oxide materials is the availability of precursors satisfying the requirements for easy precursor delivery, chemical stability in the delivery system, and efficiency of the absorption and decomposition on the substrate in high vacuum. (C) 2013 Elsevier B.V. All rights reserved.