MOSFET performance degradation induced by a trench isolation oxide step

Abstract

The effects of an oxide step in shallow trench isolation (STI) on MOSFET performance are investigated. As the isolation oxide step, which is raised above the active area, is increased, the drive current of a narrow-width MOSFET is reduced. The reduction in the drive current is attributed to the increased poly depletion in the gate, which is induced by a thicker polysilicon at a narrow active area. The thicker polysilicon is a consequence of conformal polysilicon deposition across the isolation oxide step. The isolation oxide step is also shown to widen the spread of the threshold voltage, V-th, distribution. Tight control of the STI step height and a high doping density in the polysilicon gate are suggested for minimizing the STI-induced performance degradation in advanced ULSI technology.