Investigation of the effect of processing steps on stress in a polysilicon structural membrane

Stress control in low-pressure chemical vapour deposition (LPCVD) polysilicon is important for micromechanical processes involving surface micromachining. This paper outlines work determining the effect of processing steps on the stress in blanket films of sensor polysilicon after deposition, implant and anneal processing. The measured stress in the blanket films was then correlated with interferometry images of the microfabricated pressure sensor devices after each processing step. There were small reductions in the measured tensile stress, in the blanket films, for a number of deposition steps. However, a reflow of the borophosphosilicate glass (BPSG) interlayer dielectric, at 950 degrees C, caused the most significant change in stress, resulting in a transition from tensile to compressive for polysilicon deposited at higher temperatures. The layer in contact with the polysilicon during processing influenced the final measured stress in the polysilicon blanket film. For devices with tetraethylorthosilicate oxide (TEOS) in contact with the polysilicon membrane during processing, the blanket film was tensile after sealing and compressive after processing. When the layer in contact with the polysilicon blanket film during processing was LPCVD polysilicon or LPCVD nitride, the final measured stress was tensile. The measured blanket film values corresponded well to the interferometry images of the fabricated pressure sensor devices. For a TEOS layer in contact with the polysilicon membrane during processing, the devices exhibited a deflected, tensile membrane device after sealing and a buckled, compressive polysilicon membrane device after processing. When the layer in contact with the polysilicon membrane during processing was LPCVD polysilicon or LPCVD nitride, the patterned devices were deflected, exhibiting tensile post-sealing characteristics. At the end of processing, these membranes remained deflected.