Rapid formation of TiO2 films by a conventional CVD method

Abstract

TiO2 is attractive as a dielectric material in integrated electronic applications [1], semiconductor devices in photo-catalytic reactions [2], gas sensors [3], and so on. The authors have already reported on the deposition of various ceramic thin films such as ZnO [4], MgO [5], Y203 [6] and ZrO2 [6] by a conventional chemical vapour deposition (CVD) method. Following this, a rapid formation of TiO2 films by the CVD process will be described in this report. Assuming that a homogeneous CVD reaction occurs in the gas phase, it is claimed that powder formation is negligible at low temperatures and predominates over film formation at higher temperatures [7]. This reaction model can explain a large body of experimental evidence and indicates the existence of an upper limit on the growth rate of films [7]. Several papers on the formation of TiO2 films by the CVD method have reported generally low growth rates of 0.1 to 7 .0nmsec l (e.g. 7 n m s e c t [8]). Recently, Komiyama et al. [9] succeeded in the rapid formation of porous and amorphous TiO2 films at around 30 nmsec ~ by the CVD method associated with thermophoretic precipitation [9]. However, the rapid formation of TiO2 films can be realized by the conventional CVD method without the adoption of the thermophoretic effect. This report demonstrates a rapid formation of TiO 2 films, and refers to some factors affecting the growth rate of films such as substrate temperatures, distance between the substrate and the nozzle head, vaporizing temperature, flow rate of gases and water vapour content. Concomitantly, the activation energy for the formation of TiO 2 films will be estimated. Fig. 1 shows the assembly used for the formation of TiO2 films. Dried N 2 gas (6 cm 3 sec 1) passed through CaC12 and P205 columns was bubbled through a flask containing titanium tetraisopropoxide, Ti(OC3H7)4, (TTI) held at 80 to 130 ° C. In addition, 02 gas (3 to 12 cm 3 secl ) was bubbled, if necessary, though a flask containing water held at room temperature ( ~ 68 ° C), then introduced into the reagent-containing N 2 gas near the top of a nozzle with an outlet diameter of 3 ram. The distance between the nozzle and the substrate was 5 to 25 ram. The nozzle and other parts of the apparatus were warmed by ribbon-type heaters in order to avoid the condensation of TTI. The mixed gas stream was injected with a linear velocity of 127cmsec i on to the crown glass substrate, which rested on a hotplate maintained at the desired temperature (300 to 500 ° C). The reaction conditions are summarized in Table I. The thickness of the TiO2 films obtained was measured by a roughness meter with a diamond probe. Adhesive TiO2 films were obtained above ca. 300 ° C substrate temperature (T~). Fig. 2 shows the relationship between Ts and the growth rate of the TiO2 film under the conditions of a vaporizing temperature (T~) of 80 and 100 °C. The growth rate increases with increasing T~, and beyond 100 nm sec i. X-ray diffraction patterns of the TiO2 films obtained above T~ = 400°C indicated the preferred orientation of (1 0 1), (20 0), (2 1 1) and (220) planes ofanatase TiO2