In Situ Measurement of Stresses in Thin Films
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IN SITU MEASUREMENT OF STRESSES IN THIN FILMS
REINHARD ABERMANN University of Innsbruck, Institute of Physical Chemistry, A-6020 Innsbruck, Austria. ABSTRACT: In this paper the results of experiments are summarized in which the internal stress (i.e. force per unit width) of thin films was measured in situ under UHV conditions with a bending beam apparatus. It is demonstrated that characteristic types of stress vs. thickness curves can be correlated with different growth modes (i.e. columnar grain growth and island growth) caused by differences in the adatom mobility of the deposited material. With a selection of thin film systems it is then shown that stress measurements can be used to study the effect of gas incorporation, of gas diffusion from the substrate, of reaction with the substrate and of the substrate temperature on the growth and structure of a thin film. Finally it will be demonstrated that stress measurements can even be used to study gas adsorption on vapor deposited films.
1. Introduction: It has been recognized for some time that internal stresses in thin films, built up either during or after their deposition, may significantly affect their properties. In extreme cases these stresses may even reach the tensile strength of the film material. Since thin films play an important role in electronic devices and other technological applications, the internal stresses may influence their reliability and lifetime. Thus the knowledge of the magnitude of these internal stresses is of considerable technological importance and may lead to improvements in the operation of thin film devices. Our interest in the investigation of thin film stress was initiated by the assumption that the film stress should be closely related to the film microstructure. We were then hoping to use stress measurements for in situ studies of the microstructure and the growth mode of thin films and their dependence on various deposition parameters. The main aim of this paper is, to demonstrate that stress measurements can indeed be used for such investigations. Using representative thin film systems, deposited under UHV conditions, it will be shown that
Mat. Res. Soc. Symp. Proc. Vol. 239. @1992 Materials Research Society
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specific stress vs. thickness curves can be correlated with specific growth modes (i.e. columnar grain growth and island growth) of the film. The influence of a number of deposition parameters on the growth of a particular film is then studied by investigating the changes in the stress curves due to the variation of a specific deposition parameter (film material, gas ambient, substrate properties, etc.). In the last part of the paper it will be demonstrated that the sensitivity of the apperatus used is sufficiently high, to study the changes in the macroscopic stress in vapor deposited films induced by chemisorption and surface reaction.
2. Experimental: The film stress (i.e. force per unit width of the bending beam) was measured continuously during the film deposition with a method based on the cantilever bea
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