An optical instrument for overall stress and local stress relaxation analysis in thin metal films

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Abstract
An optical instrument for simultaneous overall stress and local stress relaxation measurements in thin metal films on substrates has been designed, developed, and tested. With this instrument it is possible to study overall stress and stress induced topographic changes in real time, since there is no need for hold times during the measurement. In comparison to x-ray diffraction technique and x-ray instrumentation for stress and stress relaxation measurements, our optical technique yields superior sensitivity, high measurement speed, and a moderate cost. A common reason for stress-induced topographic changes in a metal film can be traced to thermal expansion/contraction; the instrument in the present configuration has been combined with a heating system for the analysis of effects of thermal treatments. But it can just as well be configured for in situ real time monitoring of film stress and defect formation throughout a vacuum deposition process. The physical principle of the optical instrument is based on the fact that the overall film stress bends the substrate when subject to heating, and that the surface is slightly roughened when hillocks, the evidence of local stress relaxation, are being formed under high compressive stress in the film. The spherical deformation of the substrate is detected by sensing the deflection of a laser beam. By designing the deflection system in a way such that the beam hits the bending surface twice, an improved performance is obtained. The instrument yields a precision in change in substrate curvature of approximately 3.5×10−5 m−1 or an equivalent radius of curvature of 40 km. Without any prior knowledge of the initial substrate curvature, the change in curvature could be determined with an accuracy of about 4%. The stress in a thin film can be calculated from the measured change in substrate curvature. The local stress relaxation is measured indirectly by the light scattering obtained from the roughened metal film, using the illuminated laser spot on the sample from the beam deflection system as scatter source. The direct measurement of the scattered light yields a superior sensitivity relative to measuring losses in the specular reflectance, and provides the means to detect single, submicron-sized relaxed regions (hillocks) within an observation area of 1 mm2. The area sensitivity to relaxation features appearing as hillocks or voids in optical quality films is, therefore, measured in the parts per million range. The scattered light is collected by a partial integrated scattering system, with a dynamic range covering the scattering from supersmooth surfaces to rough surfaces. The simultaneous use of beam deflection and light scattering techniques can be quite valuable in correlation studies between overall stress and local stress relaxation in thin metal films during environmental changes, where it is not possible to perform these types of studies in sequence due to irreversible microstructural changes that occur in the film. The design of the optical system made the instrument relatively compact, easy to align, and most important of all, it combines the two well-known techniques of laser beam deflection and total integrated scattering, that have previously always been used separately, into one instrument. Instrument details and an example of combined measurements of a thermally treated aluminum film on a silicon substrate are given.