A multiple-scan focusing method for RF BAW device observation by laser probe system

This paper proposes a focusing method based on multiple scanning of a device surface to be observed, which is particularly-suitable for a fast-mechanical-scanning and phase-sensitive laser probe for radio frequency (RF) surface and bulk acoustic wave (SAW/BAW) devices. When high spatial resolution is required for the observation, one needs an objective lens of large magnifying power with extremely shallow focal depth. Accordingly, an uneven surface and tiny inclination of a BAW device cause acquired images a considerable defocus, by which it is difficult to obtain precise and reliable field quantities related to acoustic waves in the device. The proposed method is shown to be most effective in avoiding this sort of defocus and able to focus the entire surface. By this method, for example, we could clearly observe power leakage in an FBAR structure.

I. Introduction

A variety of laser probes have been applied to the visualization of propagating surface and bulk acoustic waves (SAW/BAW) [1]–[2]. The authors have recently developed a laser probe system based on Sagnac interferometer for the diagnosis of RF SAW/BAW devices [3]. This phase-sensitive laser probe possesses an advantage of its fast-mechanical scanning speed, and enables the scanning of a device under test (DUT) to be completed in several minutes.