Control of stress in highly doped polysilicon multi-layer diaphragm structure

Polysilicon films can be used as stress regulating (or compensating) films to achieve zero resultant stress or low resultant tensile stress in a multi-layer diaphragm structure. Influence of LPCVD deposition condition, substrate, film thickness, crystallized degree and pre-annealing on residual stress in LPCVD polysilicon films was studied. The polysilicon deposited on PSG substrate shows the lowest residual stress. The relationship between crystallized degree of polysilicon films and the film thickness was investigated with the aid of Raman Scattering Spectrometry. The residual stress shows a significant dependence on the film thickness because the crystallized degree raises with the film thickness. The test results show that: (1) for a thinner film (0.20 μm), even if a higher deposition temperature is used (630°C), its crystallized degree is still quite low and a quite higher residual tensile stress results in the film; and (2) for a thicker film (4 μm), even if an amorphous deposition temperature (580°C) is used, significant crystallization will still occur in as-deposited films and a residual tensile stress results in the films. The stress control test of highly boron doped polysilicon-oxide diaphragm structure was carried out. The result shows that the property and magnitude of the stresses in a highly boron doped polysilicon-oxide diaphragm can be arbitrarily changed in a certain range by varying the holding time of final annealing.