Sol-gel silicon dioxide films

Abstract

(Received November 5, 1990; revised July 1, 1991; accepted September 4, 1991 ) Abstract The influence of various preparations conditions such as sol composition, application and heat treatment conditions on the properties of sol gel silicon dioxide thin films has been studied. !. Introduction The preparation of silicon dioxide thin films from metal alkoxides as precursors has been known for more than 50 years. The first experiments concerning the prepara- tion of optical coatings by this method were carried out in Germany [1] and also in the U.S.S.R. [2, 3]. During the last decade this method has become a part of a new intensively advancing sol-gel technology [4]. This tech- nology is based on conversion to gels after chemical reactions of hydrolysis and polycondensation of sols, which are usually the solutions of metal alkoxides. Various types of materials and structures can be obtained as final products of this technology: gels, powders, ceramics, glasses, films etc. Recent investigations in this field allowed new information to be obtained on the fundamental physicochemical processes which are the basis of sol-gel technology and the determination of the main advantages and limitations of this method [5]. In microelectronics sol-gel films were used mainly as a diffusion source [6, 7]. During the last few years some new problems in microelectronics as well as the develop- ment of sol-gel technology gave a strong push to this method of thin film fabrication. Thus sol-gel silicon dioxide films (spin-on glass) are used as follows: for planarization of interlevel dielectrics in the multilevel metallization in very-large-scale integration [8, 9], for the taper etching of thermal oxide layers [10, 11], for the processes of device fabrication on AraB v materials [12, 13], for bonding silicon wafers [14, 15], for fabrica- tion of various types of sensors [16, 17] etc. The microstructure as well as the physical properties of silica films are significantly governed by the precursor polymer structure before deposition and the relative rates of condensation and evaporation during spinning. Owing to the brief time span of the film deposition process, the structures of the resulting films and bulk gels, prepared from the same precursors, may have significant distinc- tions [ 18]. The lack of systematic studies of the effect of the processing parameters on the properties of sol-gel films causes difficulty in designing optimum processes for broad industrial exploitation of this technology in mi- croelectronics and other fields. This paper is concerned with the study of some conditions that can influence the properties of sol-gel silicon dioxide thin films. 2. Experimental details Silica precursor solutions were prepared by mixing alkoxide with absolute alcohol, water and HC1. After stirring, all solutions were kept for 24 h at 60 'C. Application of films was performed by spinning in specially made equipment in a closed chamber in which gas with a fixed moisture content was supplied. We used mainly nitrogen in the course of the application and heat treatment processes. In order to obtain films with the necessary thickness, multiple applications with annealing of each layer for 20 min were used. Silicon wafers with a diameter of 60-100 mm purified by a standard mi- croelectronics cleaning process were used as substrates. The films' thickness and the refractive index were measured by multiangle ellipsometry at 632.8 nm [19]. The percent change in thickness of the films on drying and heat treatment