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) w afers 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 degrees 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 degrees C, an abrupt change from tensile to compressive strain was observed after 12-15 h anneal ing, and then the strain approached a saturation value with increasing anne aling time. However, at 900 degrees 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 cu rvature of samples with different SOI layer thicknesses. The radius of curv ature is determined mainly by the Si substrate and the buried oxide, and th e thermal behavior of these two layers depends upon the heat treatment cond itions, particularly the time and the temperature. Taking into account the strain formation mechanism, these results suggest the possibility of plasti c deformation of the SiO2 at the SOI-SiO2 and the SiO2-Si substrate interfa ces during heat treatment. (C) 2000 American Institute of Physics. [S0021-8 979(00)01502-4].