Determination of Strain Distributions in Aluminum Thin Films as a Function of Temperature by the use of Synchrotron Graz
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DETERMINATION OF STRAIN DISTRIBUTIONS IN ALUMINUM THIN FILMS AS A FUNCTION OF TEMPERATURE BY THE USE OF SYNCHROTRON GRAZING INCIDENCE X-RAY SCATTERING RAMNATH VENKATRAMAN*, PAUL R. BESSER*, SEAN BRENNAN** AND JOHN C. BRAVMAN* *Dept. of Materials Science and Engineering, Stanford University CA 94305 Synchrotron Radiation Laboratory. Menlo Park, CA 94025
"**Stanford
ABSTRACT We have measured elastic strain distributions with depth as a function of temperature in Al thin films of various thicknesses on oxidized silicon using synchrotron grazing incidence X-ray scattering (GIXS). Disregarding minor surface relaxation effects that depend on the film thickness, it is shown that there are no gross strain gradients in these films in the range of temperatures (between room temperature and 400*C) considered. We also observe X-ray line broadening effects, suggesting an accumulation of dislocations on cooling the films, and their annealing out as the films are reheated. INTRODUCTION The variation of stress with thermal cycling in Al thin films on Si has been extensively studied and documented [1-5]. The stress is commonly obtained using substrate curvature techniques which give the average value of the stress in the film. Due to the variety of deformation processes that could take place in Al thin films at elevated temperatures, it could be hypothesized that there may be large strain gradients through the film thickness at those temperatures. For example, consider an Al thin film on a Si substrate that has been heated to an elevated temperature and has significant equibiaxial stresses induced in it. From several studies, the grain structure is known to be columnar. The tractions on the grain boundaries can be relaxed by grain boundary diffusion, a process that involves transport of atoms between the grain boundaries and the surface of the film. Formation of annealing hillocks, for example, has been suggested to occur by this process. However, as pointed out by Jackson and Li [6], this would not relieve stresses throughout the film since the film is rigidly bonded to the substrate. A local relaxation at the grain boundaries would then result in a large strain gradient extending from the film-substrate interface to the top of the film. Doerner and Brennan [7], using grazing incidence X-ray scattering (GIXS), showed that in aluminum films of thicknesses 0.251gm and 0.6gtm cooled to room temperature the surface is relaxed with respect to the bulk. The experiments that are described here are an extension of that investigation and were performed with the aim of determining the magnitude of strain gradients at elevated temperatures. A brief description of the GIXS technique is provided along with the materials used and the experimental strategy. Results of strain distribution as a function of depth at different temperatures are presented. Also presented are some qualitative observations on line broadening effects seen on thermal cycling. MATER IALS AND EXPERIMENTAL TECHNIQUE The films used in these experiments were: i) Three thickn
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