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 develope d that employs a thermally driven bimetallic cantilever to sense surfa ce topography. Oscillations are induced in a composite cantilever, com prising a Si3N4 layer and Au overcoat, by local heating with a resisti ve wire heater placed in close proximity to the cantilever. Cantilever bending occurs upon heating due to the difference in thermal expansio n coefficients of the Si3N4 and Au layers. The magnitude of this bendi ng is a function of the heat input, the cantilever geometry, the frequ ency 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 w as typically fixed near the cantilever's resonance frequency, which wa s in the range of 15-50 kHz in the air and 2-15 kHz in solution for th e cantilevers used here. Simultaneous cantilever deflection and amplit ude measurements during sample approach indicate that the cantilever f ree amplitude is damped upon surface contact. While imaging, a fixed d amping of the cantilever oscillation is used as a feedback signal to m aintain a constant tip-sample separation. Images with this ac imaging mode were obtained in both air and liquid environments. Results show a n improvement in image quality in the ac mode over the corresponding c ontact images, which is the result of a decrease in lateral forces wit h 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 s light modification to a commercially available contact-mode microscope . (C) 1997 American Institute of Physics.