Force microscopy for the investigation of high-frequency surface acoustic wave devices

Abstract

Surface acoustic wave (SAW) devices are of great importance in mobile communication and signal processing applications. For their optimization second-order effects, like diffraction or mass loading, have to be studied. However, meeting today's demands of GHz operation new ways of wave field mapping have to be developed, since common methods, like laser optical or electron microscope probing, are resolution limited to the micron range. Scanning acoustic force microscopy (SAFM) allows the detection of the high-frequency surface oscillations having sub-Å amplitudes with the force microscope's typical lateral resolution. The key for measuring the high mechanical frequencies is the nonlinear force curve. Another approach is the force microscope mapping of rearranged fine particles, revealing the nodes and antinodes of the standing wave field. We present measurements in the near field of, and within, acoustic devices fabricated on piezoelectric substrates, such as LiNbO 3 and quartz, and being operated at frequencies around 600 MHz. By employing SAFM, the local influence of the electrodes on the wave field, leading to undesired performance losses, was investigated.The investigation of surface acoustic waves (SAWs) goes back to Lord Rayleigh [1]. He discovered the existence of waves on the plane free surface of an infinite, homogeneous, isotropic and elastic solid based on the theory of elasticity. The modes show a wavelike behavior in directions parallel to the surface and decay perpendicular to it. In the field of microacoustics SAWs are of interest in the frequency range from some MHz to some GHz, thus being a part of the field of ultrasonics. Due to their localization in the vicinity of the surface, SAWs provide a suitable tool for thin film nondestructive evaluation purposes. Additionally, SAWs are widely used in the field of signal processing. This comes from the fact that the relatively slow sound velocities of some thousand m/s compared with those of electromagnetic waves lead to small wavelengths making signal processing devices small, and therefore, rugged and reliable. Moreover, the surface localization allows the design of a whole variety of devices since the waves can be easily manipulated, and, on the other hand, the integration in lithographic production processes is possible. The excitation of SAWs can be performed on piezoelectrics by a pair of conducting electrodes that produce spatially nonuniform and oscillating electric fields acting as a source of varying local stresses. In this way acoustic waves can be launched. A transducer like this is reciprocal, i.e. it can excite and receive acoustic waves. In order to increase the efficiency, many sources have to be arranged such that the waves they produce interfere coherently. This increase of efficiency is at the cost of larger bandwidth. However, first the invention of the interdigital transducer (IDT) [2], where the electric field changes its sign from electrode to electrode, gave the push towards industrial application.With the demand of GHz SAW devic...

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