STRESS IN LOW-PRESSURE CHEMICAL-VAPOR-DEPOSITION POLYCRYSTALLINE SILICON THIN-FILMS DEPOSITED BELOW 0.1 TORR

Stress control in low pressure chemical vapour deposition (LPCVD) polysilicon is important for micromechanical processes involving surface micromachining. Residual stress in polysilicon thin films has been studied for a number of years for deposition temperatures in the range 570 to 650-degrees-C, and in the 200 to 500 mTorr range of deposition pressures. The stress in such as-deposited undoped polysilicon thin films is usually compressive, but very low stress values or even tensile stress can be obtained in some cases by doping and/or annealing the films. The stress in thin films is greatly influenced by the film structure, and this depends on both the temperature and pressure of deposition. In this work the stress in polysilicon thin films deposited at lower pressures (80 mTorr) is studied and related to the film structure. Deposition temperatures between 580 and 630-degrees-C are used. Some wafers are doped by ion implantation, and some wafers are annealed under various conditions. Stress values are obtained by using polysilicon micromechanical structures, such as cantilevers and doubly-supported beams, fabricated by surface micromachining. Compressive stress is obtained in all cases, and processing conditions that produce very low stress values are reported.