Low Amplitude High Frequency Atomic Force Microscopy

Abstract

Heterodyne laser Doppler interferometry was used to implement a high frequency atomic force microscope (AFM) that can accommodate a cantilever with a natural frequency of up to 200 MHz. Since Doppler interferometry measures velocity, the signal level increases proportionally with frequency for a given amplitude of oscillation, or the noise floor in terms of displacement decreases with 1/f. A noise level of 0.5 fm/√Hz was attained around 2 MHz, giving a large signal to noise margin for high frequency cantilevers and higher vibration mode detection. We have so far confirmed the following: (i) atomic resolution of Si(111) 7×7 with an amplitude of drive as low as 0.03 nm at 1.6 MHz, (ii) true atomic resolution dynamic lateral force microscopy, (iii) lateral force gradient detection at the atomic level, (iv) manipulation of Si atoms at room temperature, (v) vibration measurement of Si nanowires, tungsten oxide whiskers and graphene cantilevers above 100 MHz, (vi) atomic resolution imaging with the 2nd and 3rd mode of deflection and (v) true atomic resolution imaging in water using deflection or torsion with a few 10 pm amplitude of drive.