Rapid communication Influence of ultrasonic surface acoustic waves on local friction studied by lateral force microscopy

We studied dynamic friction phenomena introduced by ultrasonic surface acoustic waves using a scanning force microscope in the lateral force mode and a scanning acoustic force microscope. An effect of friction reduction was found when applying surface acoustic waves to the micromechanical tip-sample contact. Employing standing acoustic wave fields, the wave amplitude dependent friction variation can be visualized within a microscopic area. At higher wave amplitudes, a regime was found where friction vanishes completely. This behavior is explained by the mechanical diode effect, where the tip’s rest position is shifted away from the surface in response to ultrasonic waves. PACS: 07.79. v; 07.79.Sp; 68.35 G One of the major technical problems with electron microscopes in the 1970s was the precision needed for the sample positioning. Friction especially made it difficult to keep the sample’s position after small movements due to reversible elastic forces. Using common lubricants, on the other hand, had the disadvantage of causing the stage to slip due to vibrations. Ultrasound was found to be a helpful way to overcome these problems, since it allows friction to be controlled by varying the amplitude of the waves [1]. The study of friction phenomena has also a number of other practical impacts, e.g. for the understanding of the shear force distance control in near-field scanning optical microscopes [2], or the optimization of ultrasonic motors [3]. Recently, ultrasound-induced lubricity was reported in microscopic contacts [4]. Several effects, however, may account for the friction reduction, such as the viscoelastic properties of the water layer on the surface of the sample or the material deformations when the tip slips over surface structures, i.e. an effect of surface roughness. In this paper, we report on measurements of the influence of surface acoustic waves (SAWs) on friction in a microscopic contact. SAWs offer a superior source of very defined surface oscillations with well known polarizations. Using the scanning acoustic force microscope, SAWs of all possible polarizations have been characterized in detail [5, 6]. The scanning force microscope employed in these measurements was a commercial multimode AFM system (Park M5) capable of the simultaneous acquisition of topography and lateral force microscopy signal. The deflection of the cantilever due to the forces acting on the AFM tip while scanning is measured with a standard beam-bounce technique. A laser spot focused to the very end of the cantilever is reflected towards a four-segmented photo detector thus delivering a signal proportional to the deviation from its rest position in the vertical and lateral direction. The lateral direction yields the lateral force microscopy (LFM) signal which reflects the torsion of the cantilever due to friction or topographical features [7]. The cantilever is twisted around its rest position by forces counteracting the movement of the tip. The pristine LFM trace exhibits a pronounced hysteresis effect between the forward and backward scan directions. The LFM mode is commonly applied as a tool for the visualization of variations in local friction [8]. In order to study the influence of surface acoustic waves (SAWs) on the tip-surface contact, special interdigital transducers (IDTs) have been designed and fabricated. The IDTs were deposited on GaAs(001) substrates with [011] propagation direction and on ST-cut quartz with propagation along the crystal X-axis by a photolithographic process. In these configurations generalized Rayleigh-type SAW modes can be excited with high efficiency. These acoustic modes are confined within a wavelength to the surface of a solid and propagate along specific crystalline directions. They are predominantly polarized in the sagittal plane that is formed by the SAW propagation direction and the vector normal to the surface, i.e. the surface oscillates in a very defined way with a large vertical and a smaller lateral oscillation component. Three types of devices have been fabricated. The first two are splitfinger 198 MHz (GaAs) and 562 MHz (quartz) delay lines, corresponding to 14.4 μm and 5.6 μm SAW wavelengths, respectively. The other type consists of single-finger IDTs at 540 MHz (5.2 μm wavelength) on GaAs with particular short electrodes, allowing the entire SAW beam to be covered by one AFM scan. However, the electrical performance of these devices is less satisfactory. For the measurements c.w. or amplitude modulated r.f. power was applied to the IDTs with-