Measurement of Strains at Si-SiO2 Interface

When Si is thermally oxidized, the SiO2 resulting is in a state of compression on the surface. This paper reports an experimental determination of the magnitude of this stress and its dependence on the temperature of oxidation. Samples of (111)- and (100)-oriented Si were oxidized at several temperatures between 875°–1200°C to thicknesses of 2000 Å-20 000 Å. Stress determination was made by two techniques: (a) the thin Si sample was used as a beam, and the amount of bowing under the strain exerted by oxide left on one surface was measured; (b) the unsupported SiO2 window was used as a balloon, and strain was measured as a function of air pressure inflating the balloon. Results of these measurements are in agreement with the stress expected from thermal mismatch of Si and SiO2. For an oxidation carried out at 1200°C the measured stress is 3.1×109 dyn/cm2. The data from the SiO2 balloons showed that the strain grown in the oxide was greater than 10−3. In addition, determination was made of Young's modulus from the stress-strain curves of the unsupported films. The value of 6.6×1011 dyn/cm2 obtained compares favorably with the value 7±1.5 1011 dyn/cm2 found for silicate glasses. Some implications of this stress are briefly discussed. Dislocation generation at borders of holes cut into oxide, bowing and slip processes of slices, impurity segregation, and precipitation are all possible results of this mismatch between the two materials.