MEASUREMENT OF THE THERMAL STRESS IN SPECULARLY REFLECTING THIN FILMS USING REAL-TIME HOLOGRAPHIC INTERFEROMETRY

Abstract Thin film coatings of different materials on various substrates deposited by different techniques, have many industrial applications under widely different environmental conditions. It is well known that thin film coatings suffer from internal stresses and thermal stresses. The latter is caused by the mismatch of the coefficients of expansion of the thin film material and the substrate. Data for thermal stress of thin films is scanty and hence its measurement assumes significance. In this paper a method of measurement of thermal stress of thin films using real-time holographic interferometry is described, with particular emphasis on specularly reflecting thin films. A few thin film-substrate systems have been used for the measurements. For the copper film on brass substrate and silver on glass theoretical and experimental results are found to agree reasonably well at different temperatures. For the cases of aluminium film and magnesium fluoride film on glass there is a wide discrepancy. However it is pointed out that experimental values are closer to reality.