Study of atomic force microscope based on digital-demodulating and self-oscillating frequency modulation

A frequency modulation atomic force microscope (AFM) using a tuning fork as the force sensor is developed. Utilizing the piezoelectricity and ultra high quality factor characteristics of tuning forks, a positive feedback loop is used to oscillate a tuning fork at its resonant frequency. An automatic gain control loop is used to keep the oscillating amplitude of the tuning fork constant. By a simple and accurate method, the quantitative amplitude of the tuning fork can be easily calibrated using the current amplitude through the fork. A digital phase lock loop is designed based on a field programmable gate array chip. It demodulates the real time resonant frequency of the tuning fork by digital method. The frequency modulation mode AFM is accomplished with a homemade AFM controller. Reliable images of various samples are achieved.