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) poly silicon is important for micromechanical processes involving surface m icromachining. Residual stress in polysilicon thin films has been stud ied 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 pre ssures. The stress in such as-deposited undoped polysilicon thin films is usually compressive, but very low stress values or even tensile st ress can be obtained in some cases by doping and/or annealing the film s. The stress in thin films is greatly influenced by the film structur e, 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. De position temperatures between 580 and 630-degrees-C are used. Some waf ers are doped by ion implantation, and some wafers are annealed under various conditions. Stress values are obtained by using polysilicon mi cromechanical structures, such as cantilevers and doubly-supported bea ms, fabricated by surface micromachining. Compressive stress is obtain ed in all cases, and processing conditions that produce very low stres s values are reported.