Residual lattice strain in thin silicon-on-insulator bonded wafers: Thermal behavior and formation mechanisms

The residual lattice strain in annealed bonded silicon-on-insulator (SOI) wafers was investigated as a function of the temperature and duration of the heat treatment. Measurements were made in a temperature range from 900 to 1050 °C and annealing times were from 6 to 30 h. In as-received SOI wafers, a tensile strain parallel to the surface of the order of 10−4 was observed. For specimens annealed at greater than 950 °C, an abrupt change from tensile to compressive strain was observed after 12–15 h annealing, and then the strain approached a saturation value with increasing annealing time. However, at 900 °C no such change to compressive strain was observed even after long annealing times. In view of this, the onset of this strain behavior is consistent with viscous flow of the SiO2. Although it might be expected that the strain in the SOI layer would be greater for thinner layers, the observed strain was found to decrease with decreasing SOI layer thickness. There was no significant variation in the radius of curvature of samples with different SOI layer thicknesses. The radius of curvature is determined mainly by the Si substrate and the buried oxide, and the thermal behavior of these two layers depends upon the heat treatment conditions, particularly the time and the temperature. Taking into account the strain formation mechanism, these results suggest the possibility of plastic deformation of the SiO2 at the SOI–SiO2 and the SiO2–Si substrate interfaces during heat treatment.