Ultrasonic force microscopy in waveguide mode up to 100 MHz

We present experimental results that extend the frequency range of ultrasonic force microscopy (UFM) to 100 MHz, operating in a mode in which the cantilever base is vibrated. In this "waveguide-UFM" mode flexural ultrasonic vibrations are launched down the cantilever as in a waveguide, without exciting any particular cantilever resonance. The nonlinearity of the tip-sample force-distance curve allows the conversion of a modulated ultrasonic frequency into a low frequency vibration of the cantilever, detected using an optical lever at the modulation frequency in an conventional atomic force microscope. Experiments were performed on a sample of InAs self-assembled quantum dots on a GaAs substrate. The dots, of order 10-100 nm in diameter, were clearly resolved up to operating frequencies /spl sim/100 MHz, demonstrating the difference in elastic properties. Images were also obtained for a polycrystalline chromium film structure deposited on a silicon substrate.