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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 °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.

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A surface micromachining process for the fabrication of a pressure sensitive field effect transistor (FET), compatible with complementary metal oxide semiconductor (CMOS) processing, has been established in which residual membrane stress can be tuned without changing the underlying CMOS operation. The residual membrane stress must be controlled at a low tensile value for optimum operation of the pressure FET. Controlling this residual stress involves the development of suitable processing conditions, and the effect of alteration from standard CMOS processing on the underlying circuitry must be evaluated. The effects of different processing conditions for a surface micromachined polysilicon pressure sensing membrane on the CMOS characteristics are presented. Variations in the polysilicon sensor layer deposition and implant conditions in the surface micromachining process and back-end CMOS interlayer dielectric reflow were examined as these strongly influence the residual stress in the membrane. The electrical characteristics for devices that had only CMOS processing did not vary significantly from the electrical characteristics of devices that had the pressure sensor surface micromachining layer processing.