Polysilicon integrated microsystems: technologies and applications

Abstract

Co-fabrication of polysilicon microstructures with CMOS electronics enables monolithic inertial sensors to be fabricated. Correlations are well established between the deposition, doping, and annealing conditions of LPCVD polysilicon and its mechanical properties, such as residual strain, strain gradient, and Young's modulus. Surface passivations for alleviating stiction have been demonstrated recently that greatly reduce the tendency for polysilicon microstructures to adhere to adjacent surfaces when dried after release by wet etching and rinsing, or when brought into contact due to mechanical shock during use. Sigma-delta control strategies are attractive for linearizing closed-loop sensors and are well suited to implementation in CMOS. Basic design principles for sense elements and electromechanical actuation in a single structural layer of polysilicon have emerged rapidly in the past several years. Monolithic polysilicon integrated sensors for the X, Y, and Z components of linear acceleration, angular rate, and angular acceleration have been demonstrated using the BiMEMS process of Analog Devices, Inc. Mechanical suspensions, electrostatic actuators and capacitive pickoffs, and interface- and control-circuit building blocks are all portable to alternative integrated technologies that share the basic characteristics of thin, laminar suspended microstructures.