Full Paper Atomic Layer Deposition of SrTiO 3 Thin Films from a Novel Strontium Precursor± Strontium-bis(tri-isopropylcyclopentadienyl)** By Marko Vehkamäki, Timo Hänninen, Mikko Ritala,* Markku Leskelä, Timo Sajavaara, Eero Rauhala, and Juhani Keinonen Strontium titanate thin films were grown by atomic layer deposition (ALD) at 250±325 C from the novel strontium com- pound, strontium bis(tri-isopropyl cyclopentadienyl), titanium tetraisopropoxide, and water. Though completely self-limiting, deposition of strontium could not be achieved because of some minor decomposition of the strontium compound. This de- composition was slow enough to ensure that good control of film stoichiometry was obtained by controlling either the (Sr-O)/ (Ti-O) pulsing ratio, or the strontium precursor exposure time. The films were polycrystalline and strongly oriented in the (100) direction. After annealing at 500 C in air, the films with the optimal composition were found to have measured permit- tivity values of around 180. Keywords: Atomic layer deposition, Dielectric, Strontium bis(tri-isopropylcyclopentadienyl), Strontium titanate, Thin films 1. Introduction Strontium is a constituent of many important thin film materials, such as high permittivity dielectrics SrTiO 3 and Ba x Sr 1±x TiO 3 , [1±4] ferroelectric SrBi 2 Ta 2 O 9 , [4±7] high-T c superconductor Bi 2 Sr 2 Ca n±1 Cu n O 5+(2n±1)+d , [8] and thin film electroluminescent display phosphors SrS:Ce and SrS:Cu. [9,10] Excellent conformality is required from many of the above mentioned thin films, especially from the high permittivity dielectrics and ferroelectrics, which show great promise for use in future generation dynamic random access memory (DRAM) and non-volatile ferroelectric memory, respectively. The demand for ever-increasing memory capacities calls for a corresponding shrinkage of the memory cell dimen- sions. The maintenance of the required cell capacitance in the DRAMs has only been made possible by the design of three-dimensional cells with high aspect ratios. Since physi- cal vapor deposition methods will not be able to meet the strict conformality requirements, CVD has increasingly been the method of choice, particularly for depositing SrTiO 3 and Ba x Sr 1±x TiO 3 thin films. However, even with CVD, there are problems in simultaneously achieving good material properties and strict conformality. A potential alternative is ALD, also known as atomic layer epitaxy (ALE), [11±14] which, through its self-limiting film deposition mechanism, ensures perfect conformality and large area uniformity. [15] The development of both CVD and ALD processes for materials containing strontium, or any other alkaline earth metal, has suffered from the limited number of volatile compounds of these metals. [8,16±19] In practice, the choice of alkaline earth metal precursors has been limited to the b-diketonate compounds. Though successful results have been obtained in CVD, these compounds suffer from their relatively low volatility and limited thermal stability. The numerous efforts made to avoid these problems include tai- loring the b-diketonate ligand with different kinds of sub- stituents, synthesis of adduct stabilized b-diketonate deriv- atives, [8,16±19] in-situ synthesis of the b-diketonates inside the ALD reactor for immediate use in the growth pro- cess, [20±22] and, perhaps most importantly, development of various kinds of liquid delivery methods, [16,18,23±26] but the major breakthrough remains to be made. In addition, whereas the b-diketonates react rather well with H 2 S in the ALD growth of alkaline earth metal sulfide films, [20±22,27±29] the ALD growth of the corresponding oxides is problemat- ic since the b-diketonates do not react with the most com- mon and convenient oxygen sources (water or molecular oxygen) at temperatures low enough to prevent thermal decomposition and the subsequent destruction of the self- limiting growth mechanism. Therefore, even if high quality epitaxial SrTiO 3 and BaTiO 3 films and their superlattices have been deposited from the b-diketonates by the process Chem. Vap. Deposition 2001, 7, No. 2 Ó WILEY-VCH Verlag GmbH, D-69469 Weinheim,2001 0948-1907/01/0203-0075 $ 17.50+.50/0 75 ± [*] Dr. M. Ritala, M. Vehkamäki, T. Hänninen, Prof. M. Leskelä Department of Chemistry, University of Helsinki, PO Box 55, FIN-00014 Helsinki (Finland) E-mail: Mikko.Ritala@helsinki.fi T. Sajavaara, Dr. E. Rauhala, Prof. J. Keinonen Accelerator Laboratory, University of Helsinki PO Box 43, FIN-00014 Helsinki (Finland) [**] Facilities provided by the Unit of Electron Microscopy at University of Helsinki were used for the SEM and EDX characterization. Dr. Esko Kauppinen, Dr. Unto Tapper, and Mr. Petri Ahonen at VTT Chemical Technology, Espoo, Finland, are thanked for access to the Leo DSM 982 SEM. This work was supported in part by the Academy of Finland (projects 43329 and 44215) and the Finnish National Tech- nology Agency (TEKES) (project 40560/99).