A method for controlling residual film stress in LPCVD polysilicon films for surface micromachined MEMS

As the micro-electro-mechanical machine (MEMS) industry is maturing, an increased array of product applications and devices are being introduced. As these devices are being developed new processes are required to control and attain the desired levels of polysilicon stress. In this work the relationship between polysilicon blanket residual stress and dopant concentration and anneal conditions are investigated. It was found that as the sheet resistance increased, the magnitude of the stress increased several orders of magnitude. Annealing the wafers for increased durations and multiple cycles lowered the level of stress observed while reducing the response to dopant concentration. The response was reduced by fifty percent for a time increase from 20 to 180 minutes. It is suggested that specific levels of stress are best attained with modifications to the dopant concentration for the required thermal cycles. The characterization performed allows for reduced learning cycles and cost in the development of new MEMS process flows to achieve first pass success for device specific requirements.