ULTRASONIC FORCE MICROSCOPY FOR NANOMETER RESOLUTION SUBSURFACE IMAGING

We present a novel method for nanometer resolution subsurface imaging. When a sample of atomic force microscope (AFM) is vertically vibrated at ultrasonic frequencies much higher than the cantilever resonance, the tip cannot vibrate but it is cyclically indented into the sample. By modulating the amplitude of ultrasonic vibration, subsurface features are imaged from the cantilever deflection vibration at the modulation frequency. By adding low-frequency lateral vibration to the ultrasonic vibration, subsurface features with different shear rigidity are imaged from the torsional vibration of cantilever. Thus controlling the direction of vibration forces, we can discriminate subsurface features of different elastic properties.