Control of stresses in highly doped multilayer polysilicon structures used in MEMS applications

Abstract

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 Spectrum. The residual stress shows a significant dependence on the film thickness because crystallized degree raises with the film thickness increase. The test results show that (1) for a thinner film (0.20 micrometer), even if to use a higher deposition temperature (630 degrees Celsius), its crystallized degree is still quite low and a quite higher residual tensile stress is resulted in the film. (2) for a thick film (4 micrometer), even if to use a lower deposition temperature (580 degrees Celsius), a significant crystallization still will occur in as-deposited films and a residual tensile stress is resulted in the films. A pre-annealing step before polysilicon boron doping is brought into the fabrication process of multi-layer diaphragm structure. It can be used as a method to control stress in highly doped polysilicon 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 highly boron doped polysilicon-oxide diaphragm can be arbitrarily changed in certain range by varying the holding time of final annealing.