Laser inspection for surface acoustic wave performance in SAW device

Won Kweon Jang*, Jun Oh Park

Division of Electronic, Computer and Communication Engineering, Hanseo University

Abstract

Unlike the electric method capable of checking only product defect, the real time optical metrology is suggested for measuring and visualizing vibration with respect to position of surface acoustic wave in RF device. The measuring limits and conditions for surface acoustic wave are given, and the interference and diffraction due to RF signal are analyzed by optical interpretation. A single mode laser and a 105MHz-center-frequency repeater filter were employed for experiments and theoretical analysis. In this paper, the optical metrology providing visual energy distribution and real time inspection for surface acoustic wave is proposed for development of high quality multi-service and multi-frequency RF module.

keyword : surface acoustic wave, visualization, interference, diffraction, real time, RF device

SAW device has usually been applied to wireless communication as resonator, filter, delay-line for single frequency service, and dependent on the electric inspection method in mass production of small kinds. The most important thing in SAW technology has been how clear in frequency answering. However, it turns into the visualizing technology for effective performance of surface acoustic wave and noise effect caused by neighboring module as the multi-service with multi-frequency get the priority in period of 2.5~4 generation. Now the visualization technology for better module structure and finding out the local problem which could not be caught in design step become more recognized than before.

In this study, the laser interference and diffraction has been tried for visualizing vibration of surface acoustic wave formed on SAW device. The difference in optical signals between RF input signal was permitted and not-permitted was analyzed in theory and experiments, which provided the information of local energy distribution caused by surface acoustic wave.

The sensitivity of detector is the most important factor in laser added measurement of surface acoustic wave. Scuby and Drain suggested the minimum detectable surface vibration amplitude minA as following,

2/1

min 41

÷÷ø

öççè

æ D=

PfhA

hnw (1)

In this study, w is 0.73 mm, the energy of input photon, hn is 3.14´10-19 J, the quantum efficiency of optical detector, h is about 0.5, the laser power, P is 5 mW, calculated Amin of fD´ -151075.2 m×Hz. The optical detector we used (Newport, 818-BB-20) has the bandwidth, Df of 1.5GHz. Therefore, the minimum vibration amplitude detectable computed to 0.106 nm. Actual vibration amplitude formed on the surface of piezoelectric materials is usually 0.1~10 nm. Consequently, our experimental conditions were sufficient for measurement of surface acoustic wave between IDT.

In optical methods for detecting fine vibration, interference, diffraction, reflection, and deflection has been tried with instruments. In this study, the interference and diffraction methods with single mode laser were used for measurement of surface acoustic wave formed on SAW device.

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Figure 1 describes a comparison of interferences that obtained with and without RF signals.

Fig. 1 Comparison of interferences that obtained with and without RF signals.

Figure 2 is a experimental results of diffraction. Fig. 2(a) is for the 0th order reflected beam and (b) is for the 1st order diffracted beam. The difference could not be found in 0th order reflected beams between optical intensity distributions whether or not RF signal biased. However, the 1st order diffracted beams showed differences in distribution and position as shown in fig. 2(b). It is believed that the difference was caused by gradient of surface acoustic wave. The reason why the 0th order could not show the difference was that most probe beam light experienced reflection and pointed the same angle of incident angle by the law of reflection. As a result, the effect of deflection was much smaller than reflection in 0th order.

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Fig. 2 When the probe laser beam was incident obliquely to the surface of SAW device, (a) the detected light distribution at the angle of reflection and (b) diffraction. As a RF filter has narrow spaced long periodic electrodes, and the surface acoustic wave propagates to perpendicular direction, the optical methods using interference and diffraction are very appropriate and practical for visualizing effects caused by neighboring module operating with other frequency. Consequently, the optical methods we proposed in this paper make real time visualizing technique feasible in multi-band module design and inspection.

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