Development of Scanning Near-field Acoustic Microscopy using Piezoelectric Cantilever

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The sensitivity of the piezoelectric cantilever was checked by atomic force microscopy (AFM) imaging of atomic step structures of sapphire substrate which was prepared by thermal baking at 1400℃. The step structures with a height of about 2nm were clearly observed so that the sensitivity of the cantilever was high enough for AFM.Local acoustic and mechanical properties of ferroelectric polymer thin films were investigated by dynamic mode AFM. Since the grain in the films is composed of either amorphous structures or complicated structures made up tangled polymer chains, it is almost impossible to obtain clear images on a nanometer scale with an ordinary AFM. In our research the tip was brought closer to the sample surface to increase the tip-surface interaction. Thus viscoelastic properties were obtained due to some acoustic and mechanical coupling between the tip and the sample. As a result sub-grain structures with the size of some 10nm were found.We explored another possibility for obtaining acoustic and mechanical properties of a sample by measuring dissipation of the vibration energy of the PZT cantilever in the dynamic mode, which could give higher resolution in connection with non-contact AFM. In this mode the shift of the resonance frequency and the change in the vibration amplitude can be used as topographic and dissipation information, respectively. Frequency detection electronics consisted of a circuit for frequency shift detection and a self-excitation circuit where the dissipation was measured. We successfully developed a new frequency detection electronics which adopted a voltage controlled oscillator using a quartz crystal (VCXO) in order to increase the frequency sensitivity. The sensitivity obtained was 0.01Hz(1kHz bandwidth), which was greatly improved.