Determination of the effect of processing steps on the CMOS compatibilityof a surface micromachined pressure sensor

Abstract

A surface micromachining process for the fabrication of apressure sensitive field effect transistor (FET), compatible withcomplementary metal oxide semiconductor (CMOS) processing, has beenestablished in which residual membrane stress can be tuned withoutchanging the underlying CMOS operation. The residual membrane stress mustbe controlled at a low tensile value for optimum operation of the pressureFET. Controlling this residual stress involves the development ofsuitable processing conditions, and the effect of alteration from standardCMOS processing on the underlying circuitry must be evaluated. Theeffects of different processing conditions for a surface micromachinedpolysilicon pressure sensing membrane on the CMOS characteristics arepresented. Variations in the polysilicon sensor layer deposition andimplant conditions in the surface micromachining process and back-end CMOSinterlayer dielectric reflow were examined as these strongly influence theresidual stress in the membrane. The electrical characteristics fordevices that had only CMOS processing did not vary significantly from theelectrical characteristics of devices that had the pressure sensor surfacemicromachining layer processing.