Formation of low-stress multilayered thick polysilicon films for fabrication of microsystems

The effects of impurity doping and heat treatments on the characteristics of thick polysilicon films were studied for development of the structural materials in the microelectromechanical system (MEMS). In this study, 8-15 layers of 6.5-12 mum thickness polysilicon films were deposited to have a symmetrical structure using low-pressure chemical vapor deposition (LPCVD) with a novel stacking method. We have measured the physical and structural characteristics using micromachined test patterns to verify the minimal stress and stress gradient in the polysilicon layers, according to the film stacking, doping, and thermal treatment methods. The multilayer film revealed the complex orientation composed of (100), (220) and (311) grains after annealing and showed a higher doping concentration in the interfaces. The multilayer polysilicon films with thickness of 6.5 mum showed that the higher doping concentration induced a higher compressive stress of 70 MPa since phosphorus gave rise to a compressive stress in a polysilicon film. However, the doping method for the most uniform distribution of phosphorus induced the lowest stress gradient among all samples. A polysilicon microresonator with thickness of 6.5 mum were manufactured by the symmetrical stacking and optimum doping method in which the dopant concentration was lowered and annealing at 1000 degreesC. The film had a low stress of 7.6 MPa and a low stress gradient of -0.15 MPa/mum and revealed good slopes of sidewalls after dry etching. The fabricated test structure for a microgyroscope showed that the driving resonant frequency and the sensitivity was measured as 9,175 Hz and 5 mV-sec/deg, under the condition of a static angular velocity.