Design and Fabrication of a Nanocantilever for High-Speed Force Microscopy

Abstract

The atomic force microscope (AFM) has become an important tool in many fields ranging from materials science to biology. The central component of the AFM is a probe consisting of a soft cantilever to which a sharp tip is attached. By scanning the probe over the surface of a sample and measuring small deflections of the cantilever, atomic resolution images can be obtained for both conducting and non-conducting samples. Unfortunately, the scan speed of conventional AFM is limited such that several minutes are required to obtain a high-quality image. If the scan speed of the AFM could be increased to allow for dynamic imaging, it could be used for many new applications in materials science, life science and process control.

Much of the current work toward high-speed AFM has involved improvements to scanners and electronics. Innovative scanner design and control has resulted in operational frequencies up to 1 MHz while specialized electronics has pushed the feedback bandwidth up to 100 MHz. To realize the full potential of these systems, a cantilever with a resonance frequency much greater than 100 MHz is required. Unfortunately, current microfabrication techniques used to produce AFM cantilevers limits the fundamental resonance frequency to several MHz.

The purpose of this project was to miniaturize a cantilever into the nanometer regime allowing for increased resonance frequencies. Three modeling methods were used to design a 200 MHz silicon nitride cantilever suitable for integration into an atomic resolution, frequency-modulation AFM. A process was developed to fabricate the cantilever coupled to an atomic point contact (APC) displacement detector. The cantilever mask and APC electrodes were defined through electron-beam lithography and double-angle evaporation. The cantilever pattern was transferred to the nitride layer through focused ion beam milling; a subsequent wet etch into the underlying Si substrate suspended the structure. Then, using an active feedback system, electromigration was used to form the APC at 77 K and 10E-6 Torr. Progress was also made toward measuring cantilever motion with the APC displacement detector through microwave reflectometry.