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I. INTRODUCTION The deposition of silicon oxide (SiO2) and silicon nitride (Si3N4) films by photochemical vapor deposition has been extensively investigated1 as a low temperature, damage-free, and selective area process for their application in VLSI technology as interlayer insulation or passivation, thin film transistors, opto-electronics devices, and so on. On the other hand, silicon oxynitride films are known to be an excellent material with many advantageous properties over SiO2 and Si3N4 films. For example, it is known that the mechanical stress is lower in SiON films, and the film properties such as thermal expansion coefficient and refractive index could be changed by controlling the ratio of oxygen to nitrogen in films. Much work has been reported on the deposition of SiON films by low-pressure chemical vapor deposition (LPCVD),2'3 thermal nitridation of thermally oxidized silicon,4"7 and plasma assisted chemical vapor deposition.8"10 As for the deposition of SiON films by photochemical vapor deposition, a few works have been reported using excitation light from a low pressure Hg lamp" or an ArF excimer laser12 in a gas mixture of silane, ammonia, and nitrous oxide. The photoCVD process is attractive not only because of the points mentioned above, but also because there is a possibility that the composition of the film could be controlled by the spectra of the excitation light, making use of the difference in absorption spectra of source gases.13 In this paper, we report the photo-CVD of silicon oxynitride films using a direct photolysis of a mixture of disilane, ammonia, and nitrogen dioxide excited by a deuterium lamp. As a silicon source gas, disilane is preferred rather than monosilane because the absorption cross section is much higher than that of monosilane in the vacuum ultraviolet (VUV) region.15 As an oxygen source gas, nitrous oxide is often used in SiON film deposition. In this experiment, however, nitrogen dioxide is preferred rather than nitrous oxide, because a profile of an absorption spectrum of nitrogen dioxide is quite different from that of nitrogen source gas, ammonia, in the wavelength region 882

http://journals.cambridge.org

J. Mater. Res., Vol. 4, No. 4, Jul/Aug 1989

Downloaded: 18 Mar 2015

of 160—200 nm, while that of nitrous oxide has a similar tendency to that of ammonia.16 The difference in absorption spectra is considered to be advantageous for control of film composition (oxygen-to-nitrogen ratio) by the excitation light spectrum. Although the investigation of various film properties is important for the characterization of deposited films, we restricted our investigation, in this paper, to the character of the deposition process of SiON films. The deposition rate and composition of the films are described in connection with NO2 flow rate and the excitation light spectram. II. EXPERIMENTAL A schematic diagram of a photo-CVD apparatus in this experiment is shown in Fig. 1. The excitation light emitted from a 30-W deuterium lamp (Hamamatsu Photonics L2581) was collected by a l