Process optimization of free-standing polysilicon microstructures

Abstract

Polycrystalline silicon (polysilicon) microstructures can be fabricated on silicon substrates by etching an underlying oxide layer (sacrificial layer). The maximum free-standing length of surface micromechanical structures is very sensitive to process details, such as the polysilicon deposition parameters, the doping process, the anneal conditions and the underetching and rinse procedures. For undoped and phosphorus doped polysilicon microstructures the value of strain was determined by buckling criteria. To avoid sticking of the polysilicon microstructures to the underlying surface after removal of the sacrificial layer a photoresist support layer and a subsequent oxygen plasma strip process were applied and optimized. For each polysilicon and sacrificial layer thickness the proper photoresist/acetone mixture as well as the dry underetching procedure was determined.