Laser-probe analysis of modern electroacoustic microwave devices

Abstract

In this thesis, devices based on surface acoustic waves (SAWs) and microelectromechanical systems (MEMS) are investigated using laser-probe measurements. The type of SAW and MEMS devices studied here are typically used, for example, in mobile phones as radio frequency components, such as filters and reference oscillators. The dissertation focuses on measuring and analyzing vibration fields in the devices using the scanning Michelson laser interferometer developed at TKK. The interferometer as such detects out-of-plane vibrations, i.e., vibrations perpendicular to the surface of the device, with minimum detectable vibration amplitude reaching down to 0.3 pm for a detection bandwidth of 1 kHz. The spatial resolution in the scanning plane of the interferometer is better than 1 µm and the detectable vibration frequencies range from 1 MHz to 2.5 GHz. In the thesis, a novel detection method to measure also in-plane vibrations is presented and applied to the analysis of a square-plate MEMS resonator. In the first part of the thesis, operation of novel SAW devices, such as a longitudinal leaky SAW (LLSAW) resonator, a double resonance filter and a piston-mode filter with transverse mode suppression are thoroughly analyzed by combining the interferometric measurement results with numerical simulations and data from electrical measurements. The measurements of the vibration fields in the LLSAW resonator reveal side radiation of Rayleigh waves, which constitutes an acoustic loss mechanism for the resonator. The analysis of the piston-mode filter indicates that one of the parameters for the transverse mode suppression has been underestimated in the filter design. In the second part of the thesis, vibration modes in a square-plate MEMS resonator are analyzed. Comparing the laser-probe measurements with simulations, it can be concluded that the main in-plane vibration mode is companioned by a parasitic out-of-plane mode, that may affect the quality factor of the main mode. The laser-probe measurements enable studying directly the vibrations in SAW and MEMS devices and hence provide new information on the physics and operation of these devices, not obtainable from the electrical measurements or simulations alone. The measurements provide support for the development of new SAW and MEMS components satisfying stringent specifications.