Low Temperature Si Oxidation with Excimer Lamp Sources
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INTRODUCTION
The oxidation of silicon is one of the most important steps in the fabrication of Si based integrated circuits and has traditionally required high temperatures, typically about 1000 'C. Low temperature oxidation of Si would be highly desirable for reduced thermal budget processing in device fabrication in order to produce ultra-thin films with low defect densities, high breakdown fields, excellent interfacial properties and high reliability, but unfortunately purely thermally driven reactions are much too slow to be of use. In recent years, various methods to enhance low temperature growth of thin high quality silicon dioxide (SiO 2) films have been investigated using different approaches, involving plasmas [1], ion beams [2], and visible [3] and ultraviolet (UV) [4] radiation. UV and vacuum UV (VUV) radiation, has been shown to increase the oxidation rates, in some cases by more than an order of magnitude over the thermal rate, by photoexciting the silicon and/or directly dissociating the 02 bond (5.1 eV) and liberating atomic species [5,6]. These can either react directly with the silicon or, through collisional processes, form 03, which is also much more reactive than its allotropic cousin. Recently, a new generation of excimer UV lamps capable of producing high power radiation over large areas and extended availability of wavelengths from the near UV (354 nm) to the deep UV (126 nm) has been developed [7,8]. Since the excimer molecules readily dissociate there is no self-absorption of the emitted radiation. Thus the sources can emit high UV intensities very efficiently (efficiencies between 7-15 % are readily achievable [9]). The development of these novel lamps offers enormous potential for materials processing. Several possibilities of applications of the UV sources have already been shown and include material deposition, polymer etching, and surface modification [10-12]. These have been also used to photo-deposit good quality silicon dioxide and silicon nitride films at low temperature by irradiating silane, nitrous oxide, and ammonia mixtures [ 10,13]. We have previously shown that irradiation from a low pressure mercury (Hg) lamp can significantly enhance the low temperature oxidation of Si [14-15] and the growth of dielectrics on silicon-germanium (SiGe) strained layers [6,16 ] which are of particular interest because of their potential optoelectronic applications. This work has been extended to the direct photooxidation of silicon using a newly developed Xe excimer UV lamp [17]. In this paper, we present data concerning the direct photo-oxidation of silicon at 250'C using 172 nm radiation 343 Mat. Res. Soc. Symp. Proc. Vol. 470 01997 Materials Research Society
from a Xe excimer lamp. The films grown were characterised using ellipsometry, Fourier transform infrared spectroscopy (FTIR), capacitance-voltage (C-V) and current-voltage (I-V) techniques. The current conduction mechanisms within the films are discussed and a simple model explaining the reduction in the leakage current in the film
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