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, substr ate, film thickness, crystallized degree and pre-annealing on residual stre ss in LPCVD polysilicon films was studied. The polysilicon deposited on PSG substrate shows the lowest residual stress. The relationship between cryst allized 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 degr ee raises with the film thickness. The test results show that: (1) for a th inner film (0.20 I Lm), even if a higher deposition temperature is used (63 0 degreesC), 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 mum), even if an amorphous deposition temperature (580 degreesC) is used, s ignificant crystallization will still occur in as-deposited films and a res idual tensile stress results in the films. The stress control test of highl y boron doped polysilicon-oxide diaphragm structure was carried out. The re sult shows that the property and magnitude of the stresses in a highly boro n doped polysilicon-oxide diaphragm can be arbitrarily changed in a certain range by varying the holding time of final annealing. (C) 2001 Elsevier Sc ience B.V. All rights reserved.