Frequency modulation detection high vacuum scanning force microscope with a self-oscillating piezoelectric cantilever

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Abstract
This article attempts to describe a novel high vacuum scanning force microscope (HV-SFM) using a self-oscillating piezoelectric cantilever in frequency modulation(FM) mode. Since no external deflection sensor or external vibrator is needed, the new HV-SFM is very simple and easy to handle in vacuum conditions in comparison with conventional systems using optical sensors.FM detection is used to detect the force gradient acting on the end of the piezoelectric cantilever because it gives higher response speed in vacuum conditions compared to the commonly used slope detection. The unimorph cantilever consists of a 1.0 μm thick Pb(Ti,Zr)O 3 (PZT) layer on a SiO 2 elastic base, which becomes self-oscillating when an ac voltage is applied to the piezoelectric layer. The 160 μm long piezoelectric cantilever has successfully been oscillated at its natural resonance frequency of 117 kHz by connecting its piezoelectric layer directly into the FM loop, which applies positive feedback to the piezoelectriccharge current caused by cantilever vibration. The shift in oscillation frequency of the FM loop, or the shift of cantilever resonance frequency due to force gradient acting on the cantilever end, is measured as a SFM feedback signal. At a frequency shift of 80 Hz, the force gradient and the force acting on the cantilever tip were calculated as 0.012 N/m and 0.02 nN, respectively, from an obtained force curve. At that set point, high-resolution images of a goldfilm have been obtained. The constructed FM detection noncontact SFM with a self-oscillating PZT cantilever has proven to be stable and easy to handle in vacuum conditions.