Thermal strain in lead thin films V: Strain relaxation above room temperature

Abstract

Strain relaxation in lead films on heating to 350 K was studied by both an X-ray diffraction technique and transmission electron microscopy. The films were deposited onto oxidized silicon wafers at room or liquid nitrogen temperature and ranged from 0.2 to 10 ωm in thickness. Concurrently with heating, whisker and grain growth were observed. The compressive strain at 350 K induced by the thermal mismatch between the films and the substrates did not reach the maximum value which was calculated on the basis of a biaxial strain model using reported thermal expansion coefficients of lead and silicon. Two strain relaxation processes were observed. The primary relaxation occurred during heating and is believed to be due to a combination of grain growth, hillock and whisker growth, and dislocation glide. During subsequent isothermal annealing a slower, secondary relaxation was observed. The relaxation rates associated with this slower relaxation were measured by the X-ray diffraction technique and were analyzed using the Nabarro-Herring diffusion creep model. The diffusion coefficients D thus obtained were found to be very close to the values of D obtained by extrapolating lattice diffusion coefficients determined for bulk lead at higher temperatures. Upon cooling from 350 to 300 K, tensile strains were observed in the films. Again, primary and secondary relaxation processes were observed and dislocations also appeared. The primary relaxation mechanism in this case is believed to be dislocation glide. For the secondary relaxation process, D values as small as 10-18 cm2 s-1 were determined at 300 K again using the diffusion creep model. The effects of film thicknesses, grain sizes and SiO coatings on the relaxation processes were also studied. © 1980.