Hexagonal phase in tensile low-pressure CVD poly-Si film

Abstract

Polycrystalline silicon (poly-Si) films have found many applications in integrated circuits, and in actuators and sensors. Important considerations in the processing of these devices are structural stability and repeatable mechanical properties of the films. The texture and stress state of the films depend strongly on the microstructures and morphology of the films. Tensile films, which are preferred to compressive films in devices whose lateral dimensions of clamped structures are not to be restricted by compressive buckling, are characterized by equiaxial grain morphology, while compressive films are characterized by columnar grain growth during deposition.A diamond hexagonal (d.h.) Si structure with lattice parameters of a = 3.8Å and c = 6.28Å was first reported in 1963. More recently, transformation from the diamond cubic (d.c.) Si to the hexagonal phase has been observed in Si under hydrostatic pressure at temperatures between 350 C and 700 C, in heavily ion-implanted silicon, and in as-grown, implanted, and annealed CVD Si thin films, in addition to the known high pressure Si phases.