The ultrasonic/shear-force microscope: Integrating ultrasonic sensing into a near-field scanning optical microscope

Abstract

An ultrasonic transducer is incorporated into a near-field scanning optical microscope ͑NSOM͒ to augment its versatility to characterize the properties of layers adsorbed to a sample's surface. Working under typical NSOM operation conditions, the ultrasonic transducer-attached underneath the sample-demonstrates sufficient sensitivity to monitor the waves generated by the tapered NSOM probe that oscillates in the proximity of, and parallel to, the sample's top surface. This capability makes the newly integrated ultrasonic/shear-force microscope a valuable diagnostic tool in the study of sliding friction and surface phenomena in general. Here, it is used to concurrently and independently monitor the effects that probe-sample interactions exert on the probe ͑that is attached to a piezoelectric tuning fork͒ and on the sample ͑that is attached to the ultrasonic transducer͒. The signal from the tuning fork ͑TF͒ constitutes the so called "shear-force" signal, widely used in NSOM as a feedback to control the probe's vertical position but whose working mechanism is not yet well understood. Tests involving repeated vertical z motion of the probe towards and away from the sample's surface reveal that the TF and ultrasonic ͑US͒ signals have distinct z dependence. Additionally, where the TF signal showed abrupt changes during the approach, the US changed accordingly. A shift in the probe's resonance frequency that depends on the probe-sample distance is also observed through both the TF and the US responses. Within the sensitivity of the apparatus, ultrasonic signals were detected only at probe-sample distances where the probe's resonance frequency had shifted significantly. These measured signals are consistent with a probe entering and leaving a viscoelastic fluid-like film above the sample. The film acts as the medium where waves are generated and coupled to the ultrasonic sensor located beneath the sample. To our knowledge, this is the first reported use of ultrasonic detection for detailed monitoring of the distance dependence of probe-sample interactions, and provides direct evidence of sound as an energy dissipation channel in wear-free friction. This newly integrated ultrasonic/shear-force microscope, which can be implemented with any functionalized proximal probe ͑including aperture and apertureless NSOM͒, can become a valuable metrology tool in surface science and technology.

Acousto characterization of fluid-like mesoscopic films under shear

“…The WGAS signal instead correlates directly with the QTF mechanical motion; their frequency responses in fact superimpose each other. [11] Thus, when exact information of the QTF's mechanical motion state is needed (this would be relevant in friction phenomena studies), the use of the WGAS technique would be preferred. Figure 4 shows the existent correlation among the QTF, WGAS and SANM signal as the probe (tapered bare fiber probe) approaches an uncoated mica sample.…”

“…A gold coated (via sputtering) mica sheet was placed on top of the SANM acoustic sensor (SE40-Q, Dunegan Engineering Consultants, Inc.) The WGAS sensor is an acoustic transducer (3 mm diameter sensitive area SE 25-P 42, from DECI) positioned around the perimeter of the microscope's frame, which plays also the role of an acoustic cavity. The exact location of the sensor is determined by the different acoustic nodes where the WGAS signal reaches a maximum (such locations vary, depending on the QTF operating frequency) [11]. The probe-sample distance is controlled by either i) moving the probe with a linear piezoelectric stage (OP65, Mad City Labs, equipped with strain gauge sensory feedback control to overcome piezoelectric effects) not shown in the figure, or ii) moving the sample with the z-stage of a XYZ scanner (Tritor-100, Piezosystem Jena) equipped with capacitance feedback sensory to overcome piezoelectric hysteresis) as shown in the figure.…”

“…As shown in Fig.1, to implement SANM an acoustic transducer is attached to the bottom of the flat solid substrate [10], while WGAS is implemented by simply attaching an acoustic transducer at the perimeter of the microscope frame (the exact position around the frame's perimeter optimized until attaining a maximum response.) [11]. The application of a thin layer of vacuum grease between the sample and the acoustic transducer improves the acoustic coupling.…”

“…Given their now apparent important role as the source of shear-forces, herein we present preliminary results of a planned series of systematic measurements aimed at testing the dynamics of mesoscopic fluid-like films using a variety of metrology tools. The latter includes the conventional tuning-fork based scanning probe microscopy [9] and the newer acoustic-based probing technique that our laboratory has developed recently, namely Shear-force/Acoustic Nearfield Microscopy (SANM) [10] and Whispering-Gallery Acoustic Sensing (WGAS) [11].…”

Scanning near-field acoustic microscope and its application

“…Because the probe-sample interaction is perpendicular to the vibration of tip, it is called shear-force which can shift the probe's resonance frequency and vibrating amplitude. This periodical shear-force, affected by the probe's vibration, excites the local area of sample's surface to vibrate and generate an ultrasonic wave which can transmit the sample and be detected by the ultrasonic transducer attached underneath the sample [12,13]. The so-called "shear-force" signal gotten from the tuning fork is now widely used in SFAM as a feedback to control the probe's vertical position.…”