Stress charaterization of thin film materials and fabrication of FPW device

Abstract

This thesis with the stress characteristics of thin films in chapter 1 and the fabrication of flexural plate wave device in chapter 2. In chapter 1, polysilicon is most frequently used as structural layers in the field of MEMS based on surface micromachining technology and it should have the low stress for the fabrication of free standing microstructures. We suggested a symmetrical stacking method for the preparation of a thick film. In order to minimize the stress and the stress gradient of multi-stacked polysilicon films for the microactuator applications, we investigated the stress behavior in terms of phosphorus distribution and the polysilicon/polysilicon interface at which phosphorus and oxygen atoms were piled up. The phosphorus dopant enhanced the stress relaxation during annealing and also introduced the compressive stress in polysilicon films. The thin oxidized layer formed at the interfaces had the major contribution to formation of the stress gradient of multi-stacked films. The influence of the interface on the stress could be minimized by using the symmetrical stacking of polysilicon films, so that it resulted in the low stress gradient of $-0.15 MPa ㎛^{-1}$/ for 6.5 ㎛ thick film with the stress of -7.6 MPa. The micromachined resonator showed quality factor of 270 and maximum vibrating amplitude of 5 mm under 15 V DC bias and 0.05 V input AC signal in a vacuum chamber at 1 torr. Using the symmetrical stacking process, the thick structural layer with a low stress could be fabricated in the conventional LPCVD equipment which is being used in semiconductor IC technology without any modification. Therefore, this proposed method could be useful in the fabrication of surface micromachined structures. We examined the stress in single crystalline silicon theoretically and experimentally with phosphorus doping level and heat treatment. The dopant atom plays an important role in the formation of stress profile, since dopants located in substitutional sites ...