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One of the limiting factors in fabrication of surface micromachined structures is the residual stress formed in the film during deposition. In order to fabricate the microstructure using the polysilicon layers deposited in a conventional LPCVD furnace, we used the multi-stacked polysilicon films and reported a method of stress control in that films. In the multi-stacked polysilicon film there exist the polysilicon/polysilicon interfaces, at which oxidized layers are formed during film stacking and dopant atoms are segregated. These facts made the multi-stacked film difficult to be used as structural layers for microstructure fabrication. In order to control the stress profile, we investigated the effects of dopant distribution and oxidized layers on the stress profile in the multi-stacked film using micromachined test structures. The stress profile could be modified considerably by multi-steps doping process and the residual stress was reduced to 15 MPa for 5 micrometer thick film. The contribution of the oxidized layer to the stress profile was also studied extensively and we could reduce the effect of the oxidized layer by the symmetrical stacking of films. Using the simple model, the dopant-induced stress profile was calculated theoretically from the dopant concentration profile and it suggested an improved method for estimating the stress profile of doped polysilicon films. Using the method developed in this study, the microstructure made of the multi-stacked polysilicon film was successfully fabricated with a low stress gradient of 0.5 MPa/micrometer. The conventional LPCVD equipment without any modification can fabricate the polysilicon structural layer for the microstructure fabrication by the multi-stacking process, which offered the convenient method of stress control.
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