Study Of ZnO Thin Film Piezoelectric Micro Force Sensor/Actuator

A ZnO thin film piezoelectric micro force Sensor/actuator system chip (SOC) has two primary benefits. First, the piezoelectric micro force sensor/actuator provides with micromation, high frequency response, wide frequency application ranges and high sensitivity. A second advantage is that the piezoelectric micro force sensor/actuator could have the capabilities of the mutli-function integration, arraying and intelligentizing owing to the integration of CMOS circuits. However, there are a host of problems when ZnO piezoelectric thin film is applied in micromaching process. The studies of ZnO thin film piezoelectric micro force sensor/actuator and the integration with CMOS electronics are still in initiate stage. In this thesis, ZnO thin film piezoelectric micro force sensor/actuators are systematically investigated in terms of the preparation, characterization and performance improvement of the films, and the design, fabrication integration with CMOS electronics and testing of the micro force sensor/actuator.The preparations of ZnO thin films by Sol-Gel method were studied. The influences of process parameters (preheating temperature and annealing temperature) of Sol-Gel technique on the microstructures and electric characteristics were investigated, and the optimal process parameters were achieved. It was found that the resistive of pure ZnO thin film prepared by Sol-Gel method was low, which limited the ZnO film to be act as piezoelectric thin film. Aiming to improve performances of ZnO thin film and realize the integrated fabrication with CMOS electronics, the Mn doping experiments were performed. The resistances of at.1% Mn-doped ZnO film (1 micro-meter thickness) increased to the level of 10¹⁰Ω, which made the ZnO films prepared by Sol-Gel satisfy the high resistive requirement as piezoelectric film. Furthermore, the doping of at.1% Mn (CH₃COO)₂ decreased the annealing temperature to form preferred c-axis orientation, which made it possible to integrated fabrication of ZnO thin films prepared by Sol-Gel with CMOS electronics.Two kinds of sensor/actuators were designed. Firstly, a conventional piezoelectric micro force sensor/actuator based micro-cantilever structure was designed. The force-charges coupling mode of device were deduced. Then, the model was as a design tool for investigating the optimal film thickness of piezoelectric multimorphs to improving the sensitivity of micro force sensors. A novel ZnO thin film piezoelectric micro force sensor based on bulk acoustic resonator with micro-cantilever structure, which was integrated with actuating functions, was proposed to measure static force and simply the fabricating process. The relationship of resonator frequrency shift and force on the device was deduced.Aiming to improve the IC-compatibility between the micromachining process of piezoelectric micro cantilever and post-CMOS integrated method, a modified micromachining procedure of piezoelectric micro device was proposed, and the key processes were studied. Two kinds of ZnO thin film piezoelectric micro force sensors/actuators were fabricated successfully. By using the silicon substrate as strcture layer of micro cantilever, using ICP to release micro cantilever and using lift-off method to prepare Ti/Pt bottom electode, the problems of K⁺ pollution, etch compatibility and temperature compatibility were solved. The bubble problem happened to bottom electrode in preparing ZnO by Sol-Gel technique was solved.The measurements of the two kinds of piezoelectric micro force sensors/actuators were carried out. The piezoelectric constant d₃₁ of ZnO piezoelectric film prepared by Sol-Gel method is 0.4366pC/N, which is measured by the first time and an original contribution. The force sensitivity and actuating force of conventional piezoelectric micro force sensors/actuators are 28.6fC/μN and 0.042μN/V, respectively. The force sensitivity of bulk acoustic resonator micro force sensor is about 8.285kHz/μN.