ac-mode atomic force microscope imaging in air and solutions with a thermally driven bimetallic cantilever probe

An ac imaging mode for atomic force microscopy ~AFM! has been developed that employs a thermally driven bimetallic cantilever to sense surface topography. Oscillations are induced in a composite cantilever, comprising a Si3N4 layer and Au overcoat, by local heating with a resistive wire heater placed in close proximity to the cantilever. Cantilever bending occurs upon heating due to the difference in thermal expansion coefficients of the Si3N4 and Au layers. The magnitude of this bending is a function of the heat input, the cantilever geometry, the frequency of the excitation, and the thermal properties of the surrounding medium. A commercially available contact mode AFM has been modified to perform ac mode imaging by driving the cantilever with a peak-to-peak amplitude of 5–15 nm using resistive heating. The heating frequency was typically fixed near the cantilever’s resonance frequency, which was in the range of 15–50 kHz in the air and 2–15 kHz in solution for the cantilevers used here. Simultaneous cantilever deflection and amplitude measurements during sample approach indicate that the cantilever free amplitude is damped upon surface contact. While imaging, a fixed damping of the cantilever oscillation is used as a feedback signal to maintain a constant tip-sample separation. Images with this ac imaging mode were obtained in both air and liquid environments. Results show an improvement in image quality in the ac mode over the corresponding contact images, which is the result of a decrease in lateral forces with an oscillating tip. This provides a simple and robust method for ac-mode AFM imaging in air and solutions that can be achieved with only slight modification to a commercially available contact-mode microscope. © 1997 American Institute of Physics. @S0034-6748~97!04605-4#