Atomic Layer Deposition of LaF 3 Thin Films using La(thd) 3 and TiF 4 as Precursors** By Tero Pilvi, * Esa Puukilainen, Kai Arstila, Markku Leskela¨, and Mikko Ritala Lanthanum fluoride is a vacuum ultraviolet (VUV) transparent material which is widely used in optical applications. An atomic layer deposition (ALD) process is developed for depositing lanthanum fluoride thin films for the first time. LaF 3 films are grown at 225–350 -C using La(thd) 3 and TiF 4 as precursors. The crystallinity, morphology, composition, thicknesses, and refractive indices of the films are analyzed by X-ray diffraction/reflection (XRD/XRR), atomic force microscopy (AFM), scanning electron microscopy (SEM), time-of-flight elastic recoil detection analysis (TOF-ERDA), and UV-vis spectrophotometry. Electrical properties, such as permittivity and leakage current density, are also studied. An exceptionally high growth rate of about 5 A ˚ per cycle is achieved at 225–250 -C. The films are polycrystalline, the refractive indices vary between 1.57 and 1.61, and the permittivity is 12.3. The impurities detected in the LaF 3 film are Ti, C, O, and H. The level of all of these tends to decrease with increase in the deposition temperature, and is only 3.5 at.-% at 350 -C. Keywords: Atomic layer deposition (ALD), Lanthanum fluoride (LaF 3 ), Optical materials, Thin films 1. Introduction LaF 3 is a large band-gap (about 10.3 eV) [1] material having a hexagonal crystal structure with a refractive index of 1.61. [2] Lanthanum fluoride is a promising vacuum ultraviolet (VUV) transparent material similar to the other large band-gap fluorides such as GdF 3 and MgF 2 . LaF 3 thin films exhibit extremely good moisture resistance, therefore they are very useful, e.g., for protecting optical compo- nents. [3] Due to its high band-gap and refractive index, higher than those of the other VUV transparent films, LaF 3 is a useful material for VUV optics, particularly for constructing a high-low refractive index pair with, e.g., MgF 2 [4–13] as the low refractive index material. AlF 3 has also been used together with LaF 3 in optical multilayer structures. [8] These multilayers have been used for a variety of optical applications, e.g., for the high reflective UV mirrors for ArF excimer laser irradiation, and for narrow band-pass filters. [4] Another application, a Pt/LaF 3 gas sensor based on Pt and LaF 3 layers acting as the chemically sensitive components, has been used for the detection of fluorine, hydrogen fluoride, fluorocarbons, and oxygen. [14,15] The most often used techniques for depositing LaF 3 thin films have been physical vapor deposition (PVD) methods, e.g., electron-beam evaporation (EBE), [4,11,16–22] ion-assisted deposition, [5,7,13,23] thermal evaporation, [3,6–8,12,16,18,24–34] ion- beam sputtering, [7,18,28] radio frequency magnetron sputter- ing, [35,36] and molecular beam epitaxy. [37–41] Only a few CVD methods have been used for depositing LaF 3 thin films, i.e., pyrolysis of a single source precursor La(hfa) 3 diglyme complex (hfa ¼ hexafluoroacetylacetonate), [42] or by using HF or NF 3 as a fluorinating agent. [43] Atomic layer deposition (ALD), a special variant of the CVD method, is based on the film growth through alternate saturative surface reactions. ALD has many good benefits compared to the others, e.g., accurate thickness control, excellent step coverage, and uniformity as well as high reproducibility. [44] ALD could be the method-of-choice to produce optical multilayers, where precise thickness control is vital. A problem related to the deposition of fluoride films with ALD, or even with CVD, [45] has been a lack of a feasible fluorine source. Over ten years ago CaF 2 , ZnF 2 , and SrF 2 films were fabricated with ALD using HF as the fluorine source, obtained by thermally decomposing NH 4 F. [46] Nevertheless, HF is not a perfect choice for ALD because it etches glass and silicates, for example. Hence there is a clear need for a more viable fluoride precursor. Recently we demonstrated that TiF 4 can be used as a novel fluoride precursor, e.g., for depositing CaF 2 [47] and MgF 2 [48] by ALD. TiF 4 is a solid at room temperature and can thus be relatively safely handled and removed from the reactor exhaust gases. TiF 4 fulfils also the main requirements for an ALD precursor, i.e., being thermally stable and highly reactive as well as having sufficient volatility. [44] TiF 4 has DOI: 10.1002/cvde.200706681 Full Paper [*] T. Pilvi, Dr. E. Puukilainen, Prof. M. Leskela ¨, Prof. M. Ritala University of Helsinki, Department of Chemistry P.O. Box 55, FI-00014 University of Helsinki (Finland) E-mail: tero.pilvi@helsinki.fi Dr. K. Arstila [+] Accelerator Laboratory P.O. Box 43, FI-00014 University of Helsinki (Finland) [ + ] Present address: IMEC Kapeldreef 75, 3001 Leuven (Belgium). [**] We would like to thank Dr Martin Bischoff for conducting part of the optical measurements. Chem. Vap. Deposition 2008, 14, 85–91 ß 2008 WILEY-VCH Verlag GmbH & Co. KGaA,Weinheim 85