Compliant substrates: A comparative study of the relaxation mechanisms of strained films bonded to high and low viscosity oxides

Relaxation of compressively strained heteroepitaxial Si0.7Ge0.3 films bonde d to high and low viscosity glass compliant layers was investigated. These structures were formed by transferring Si0.7Ge0.3 films to Si substrates co vered with thermal SiO2 and borophosphorosilicate glass (BPSG) films. Relax ation was studied through thermal annealing experiments. For the low viscos ity BPSG, relaxation was observed near 800 degrees C and was accompanied by buckling of the Si0.7Ge0.3 film. At this temperature, no change in the Si0 .7Ge0.3 film bonded to thermal SiO2 was observed, and through this comparis on relaxation on BPSG is interpreted as the result of viscous flow of the g lass. Finally, film buckling was successfully avoided by patterning the str ained films into small areas prior to annealing, and is an indication that film expansion must be considered for elastic strain relaxation on complian t media.