Research Of Piezoelectric Composite Film Structure Based On AlN Thin Film

Microwave resonator is used in a vast range of radio frequency(RF) front-end devices, such as transmission and receiving filters, clock oscillators, and tuned amplifiers. In this sense, the miniaturization and integration of microwave resonator are the key issues of the development of RF front-end devices and systems. The size of the microwave resonators is usually directly related to the wavelength of the wave length at resonator frequency. Because the propagation wavelength of acoustic waves is around four or five times lower than that of electromagnetic waves, the piezoelectric composite film structure based on bulk acoustic resonance can solve the problem of RF front-end devices about miniaturization. In addition, this structure is compatible with standard CMOS technologies, which meet the requirement of integration of RF front-end devices and systems. Therefore, the research of piezoelectric composite film structure for microwave resonator has become an issue in research now.The piezoelectric composite film structure is constituted by “metal electrode / piezoelectric film / metal electrode” composite film which floats on silicon substrate. Because this is hard to realize high quality piezolelctric thin film and non-cracking air-gap structure, the domestic research on this air-gap piezoelectric composite film structure is at a very early stage right now. This thesis tries to solve the problems in the preparation of acoustic resonance structure and relevant material, such as planarization of sacrificial layer, etching of thin film and releasing of sacrificial layer. The solution in composite structure features simpler to implement, solid structure, low damage to substrate and compatible with standard IC process. These characteristics benefit the miniaturization and integration of RF communication front-end system.This thesis focused on the growth mechanism of membrane materials, the key factors which affected crystalline qualities of AlN films and the implementation of composite structure.Firstly, the basic design theory, ways and materials of piezoelectric composite film structure were introduced. This part derived the acoustic model and electrical impedance expressions of piezoelectric composite film structure, then discussed the influences of material thickness, dielectric constant, dielectric loss and the propagation velocity of sound on the material output performance. The material and structure parameters of piezoelectric composite film structure were optimized, which were the basis for the design of structural composites.Secondly, the growth theory and experiments of high-performanced piezoelectric thin films were researched. This part studied the structural relaxation, optimal adsorption sit and state density change of(002) oriented AlN films on Mo electrodes. The theoretical growth model was calculated in this part. Then the optimal growth parameters for highly c-axis AlN films depositing on Mo electrodes were researched. The structural properties were tested in this thesis.Finally, based on the high performanced piezoelectric films, this thesis studied the graphical method of structural composites with piezoelectric films. The evaporated SiO2 was chosed as the sacrificial layers, Mo was chosed as the electrode materials, and AlN was chosed as the piezoelectric materials. The sacrificial material was released by wet etching of hydrofluoric acid. At last, the cavity structure of structural composites was realized successfully. The hard points in the graphical process: planarization of sacrificial layer, the etching of AlN films and releasing of sacrificial layer were discussed in this part.