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New Approaches to Titania and Silica CVD by Michael L

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New Approaches to Titania and Silica CVD by Michael L New Approaches to Titania and Silica CVD by Michael L. Hitchman and Sergei E. Alexandrov he companion article by Mark transmission and distribution systems exists in three crystalline forms— Allendorf1 highlights the and for other precision optics. The coat- anatase, rutile, and brookite—with rutile importance of on-line CVD ings of interest are usually oxides other having the highest refractive index of the coating for flat glass manufac- than SnO2, such as TiO2 and SiO2. This three (2.61 – 2.90). Rutile is therefore the ture. Two smaller, but fast article considers some aspects of the most interesting for many optical appli- growing, markets are those of CVD of these materials. In particular, it cations. It is also the most thermody- Tprecision optics and telecommunica- describes a novel process for the deposi- namically stable form at all tempera- tions. Both these markets are driven by tion of the rutile form of titania at low tures, but interestingly it is the anatase the rapidly developing requirement for temperatures and new approaches to form which grows preferentially at the high throughputs. Current deposition thin film formation of silica by plasma lower temperatures that are often methods rely on PVD techniques that are enhanced CVD (PECVD) without the required for film formation on thermally augmented with cumbersome tooling to need for the use of a vacuum. sensitive substrates. So, for example, move the complex geometries and there- TiO2 films produced by a range of tech- by achieve time-averaged deposition Thin Film Growth of Titania niques from low technology processes over whole surface areas. Such tech- (such as sol gel deposition and thermal niques limit throughput and usually give Titanium dioxide (TiO2) thin films are oxidation), through PVD methods (such a non-uniform deposit on parts of the used in a wide variety of applications as reactive sputtering and ion assisted coated sample, necessitating special because of their outstanding physical deposition), to a variety of CVD systems, alignment within an optic system and and chemical properties. In particular, are invariably either amorphous or crys- thereby incurring significant costs. The the high refractive index of the material talline anatase at temperatures below non-line-of-sight attribute of CVD makes and its excellent transmittance in the vis- 400°C.2,3 Reported temperatures for the the technology ideally suited for coating ible and near-IR spectral regions has led anatase to rutile transition range from complex lens and fiber geometries that to extensive applications for antireflec- 700 to 1100°C4 and while the direct CVD are used in optical telecommunication tion coatings and waveguides. Titania growth of rutile has also been described, FIG. 1. XRD patterns for (a) SnO2 coated glass and TiO2 films on SnO2 coated glass at (b) 257°C; (c) 268°C; and (d) 300°C. 40 The Electrochemical Society Interface • Summer 2001 it is usually for deposition temperatures These applications usually require butoxide, and water led to the unex- in excess of 500°C.5 Therefore while anatase, particularly with a controlled pected discovery of a low temperature rutile is energetically the most favored degree of crystallinity. So a number of CVD rutile process.6,7 There is nothing form of titania, the fact that anatase is CVD precursors were investigated for out of the ordinary about the type of formed at lower temperatures implies their influence on crystalline quality. deposition system required, with stan- that there is a higher kinetic activation The most widely reported precursors are dard gas handling facilities and a con- barrier for rutile than for anatase. This is titanium halides (e.g. TiCl4, TiCl3, TiI4) ventional hot-walled reactor with a typ- unfortunate because many optical appli- and titanium tetra-alkoxides (e.g. ethox- ical operating pressure of about 1 Torr. cations for TiO2 require low thermal ide, iso-propoxide and the tert-butox- What is remarkable is the transition budget processing to avoid the introduc- ide). The halides have the disadvantage from amorphous material to anatase to tion of stress or damage to temperature of producing hydrogen halide gases as rutile as the substrate temperature sensitive substrates. by-products from the hydrolysis process, increases from 200°C through 250°C to so the alkoxides are an attractive alterna- 300°C. The XRD patterns in Fig. 1 show A Serendipitous Discovery tive. Because an alkoxide contains four this variation for TiO2 deposited on tin oxygens, there is, in principle, enough oxide coated glass; this is a preferred A recent serendipitous discovery has oxygen to form TiO2 without additional substrate for photoelectrochemical led to the first low temperature CVD for- oxygen being needed. Often, though, applications. Of course, SnO2 has a mation of rutile with growth of this crys- further O2 is used to minimize carbon rutile structure itself and it could be talline form on a number of different incorporation into the deposited layers. argued that this is influencing the crys- 6,7 substrate surfaces at 300°C. This sur- An alternative oxygen source is H2O, tallization of the CVD layer. However, prising result came about during a study which has the additional feature of low- as the Raman spectra in Fig. 2 show, of CVD titania growth at low tempera- ering the reaction temperature. This rutile can also be grown on such dis- tures for photocatalytic applications. combination of an alkoxide, the tert- parate materials as steel, aluminum, sapphire and glass, as well as silicon (not shown). On other substrates such as copper and magnesia, though, the CVD layers grown at 300°C are anatase (Fig. 2). This phenomenon of low tem- perature CVD rutile growth appears to be unique to the combination of titani- um tetra-t-butoxide and water since investigations under similar conditions with the ethoxide, iso-propoxide, n- butoxide and sec-butoxide all yield either anatase or anatase/rutile mix- tures. The reasons behind this unex- pected result are still not clear. The rup- ture and formation of Ti-O bonds is known to be the rate determining step in the anatase-rutile phase transforma- tion and can be promoted by impuri- ties.8 One could speculate that this is perhaps facilitated in the deposition process from the t-butoxide by more carbon-containing impurities being present in the layer than when other precursors are used; β-hydrogen elimi- nation with the resulting formation of stable hydrocarbons on the surface is likely to be more complex and have a higher activation energy for a t-butox- ide than for less branched alkoxides. However, further studies on this point are needed. The role of the substrate is also puzzling because the materials that promote rutile growth have a range of different structures; e.g. an hcp octahe- dral structure for sapphire and possibly for native oxides on iron and alu- minum, while silicon is tetrahedral. There seems little likelihood of any epi- taxial effect with those structures, although one must remember that sur- face reconstruction could alter the sur- face energies significantly. Glass clearly -1 FIG. 2. Raman spectra of LPCVD TiO2 films deposited on different substrates. Expected frequencies/cm : has no well-defined structure, but it Anatase: 194, 397, 515, 637. Rutile: 144, 235, 448, 612. may be able to catalyse the anatase to The Electrochemical Society Interface • Summer 2001 41 rutile transition.9 Whatever the reasons duced downstream from the discharge downstream. The highest registered for the low temperature growth of rutile generation region. The results of a com- deposition rate of 300 ± 25 nm min-1 was from titanium tetra-t-butoxide and parative analysis13 of the main features achieved with a TEOS partial pressure of water vapor, two issues are clear. First, of direct and remote PECVD show that 0.2 Torr and an RF power of 400 W. It was while theoretical considerations are of the remote technique provides less inter- shown that the properties of the deposit- value and importance for understand- dependence for many process parame- ed films were comparable to those of ing and predicting CVD chemistry, ters and so there can be better control. thermally grown SiO2 films. there are still unexpected discoveries to For example, growth of SiO2 films by A similar idea based on the use of an be made. Second, low temperature remote PECVD has been shown to yield RF plasma torch has been developed for rutile growth has considerable potential layers with remarkably low concentra- the enhancement of CVD growth of sili- for the production of high refractive tions of contaminants because of this con oxide films using tetramethoxysilane index coatings on thermally sensitive better control of the plasma phase com- (TMOS) as a precursor.15 The reactive cold substrates. We are currently exploiting position.12 Therefore, the technique of plasma was generated between coaxial the new process for such applications. remote PECVD is a very promising one electrodes with an RF source of 13.56 for thin film technology. MHz and an applied RF power of 80-100 Thin Film Growth of Silica However, the use of low-pressure W. With this cold plasma torch, silicon PECVD, whether it is direct or remote, oxide films were deposited on various Like titania, thin films of silicon for very large quantities of optical mate- substrates with growth rates higher than oxide have interesting physical and rials is not a particularly appropriate 600 nm min-1. The carbon content in the chemical properties that make them technique because of the high cost of deposited films was rather high, though, attractive for a wide range of applica- volume vacuum equipment. Because of being in the range 1-7 atomic percent.
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