Non-contact atomic force microscope (ncafm) operation in uhv using radiation pressure excitation of cantilever

2016
Uysallı, Yiğit
In this work a new method for excitation of Non-Contact Atomic Force Microscope (NCAFM) cantilevers by means of radiation pressure was developed and applied for NCAFM imaging for the first time in the world. Piezo excitation is the most common method of cantilever excitation in AFM devices. However, it has few drawbacks, sometimes spurious resonance peaks and non-ideal Lorentzian curves can be observed. Radiation pressure has earlier been used for calibration of AFM cantilevers but has never been used for imaging. A fiber optic interferometer with a single 1310 nm laser was used both for measuring the deflection of the cantilever and for the application of a radiation force onto the cantilever at its resonance frequency using a custom NCAFM which operates in p~2x10-10mbar. The laser power was modulated sinusoidally with constant amplitude at the cantilever‘s resonance frequency utilizing a digital Phase Lock Loop (PLL). For 750μW laser power, the force exerted by radiation pressure is calculated to be 5 pN. The nominal stiffness of ncAFM mode cantilevers are ~40N/m, which results in 0.125 pm displacement due to the radiation power. DC and AC laser current is adjusted to control the DC and AC radiation force on the cantilever. The UHV chamber is pumped down to 2.5x10-10 mbar to increase the Q of the cantilever to ~10,000 and hence to enhance oscillation amplitudes to ~1-2nm. The radiation pressure excitation also enabled the measurement of the cantilever stiffness directly which is in the range of 40-60 N/m. The excitation method was applied during NCAFM imaging and for the first time in the world a NCAFM surface topography image utilizing radiation pressure was taken.