The Direct Photo-CVD of Silicon Dioxide from Si 2 H 6 and N 2 O 3
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THE DIRECT PHOTO-CVD OF SILICON DIOXIDE FROM Si 2H6 AND N2 0 3 YASHRAJ K. BHATNAGAR AND W. I. MILNE Department of Engineering, University of Cambridge, Trumpington Street, Cambridge CB2 lPZ, England. ABSTRACT The direct photo-CVD of silicon dioxide on c-Si substrates has been achieved using a new combination of Si 2H6 and N2 0 3 gases, with an external deuterium lamp as the VUV source. The variation of the deposition rate and the refractive index with process parameters is reported. FTIR studies on these oxides show that the oxide is very nearly stoichiometric with no Si-N bonds and only trace amounts of Si-H bonds. C-V, I-V and breakdown measurements made on these films are reported. a-Si:H thin film transistors have also been fabricated using this oxide as the gate dielectric. INTRODUCTION The direct photo-chemical vapour deposition (photo-CVD) of SiO 2 has been reported by other workers from SiI- 4 or Si 2 H6 diluted with an inert gas and mixed with oxidants such as 02, N 20 or N0 2 [1-6]. In experiments using pure disilane, we found that even at N2 0/Si2 H6 ratios as high as 150 to 200, the oxide films had very high refractive indices indicating oxygen deficiency in the films. Infrared absorption spectroscopy of these films revealed absorption bands associated with Si-N, Si-H and O-H bonds, in addition to the Si-O peaks. When pure disilane was mixed with 02, we found that gas phase reactions predominate even at low total pressures, resulting in profuse powder formation. When NO 2 was used as the oxidant, there was little powder formation, but the surface reactions were spontaneous and we observed high deposition rates without photo-excitation even at room temperature. These films were visibly porous. Therefore, an oxidant had to be found which would react with disilane only upon photo-excitation and have enough oxygen to oxidise the disilane more completely than N20. This paper reports on the direct photo-CVD of Si0 2 from Si 2H6 and N2 0 3 mixtures. At room temperature, N20 3 is largely dissociated into NO and NO 2 and in the light of further experiments, we believe that NO and its photo-dissociated fragments play the dominant role in the deposition process. EXPERIMENTAL The deposition chamber used in the direct photo-CVD of SiO 2 has been described elsewhere [7]. No deposition was observed without irradiation and it was found that successive depositions could be done without the need for cleaning the lamp window. The film thickness and refractive index were measured using a Gaertner model Ll 17 ellipsometer. IR absorption spectra of the 1 films were measured on a Perkin-Elmer 983 FTIR spectrometer to a resolution of 2 cm- . Laser induced ion mass spectroscopy (LIMA) was carried out at the department of Materials Science and Metallurgy of the University of Cambridge. Although it is not a quantitative technique, LIMA has a sensitivity of the order of 20 ppm for most elements. Aluminium dots 1mm in diameter were evaporated onto the samples for electrical measurements. The capacitance-voltage characteristics of t
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