Ultrashort cantilever probes for high-speed atomic force microscopy

Abstract

Since its invention in 1986, atomic force microscopy (AFM) has become the most widely used scanning-probe imaging technique.1 The microscope ‘maps’ the topography of the sample by scanning its surface with a small tip integrated near the free end of a flexible cantilever. In contrast to other microscope techniques, it is possible to image the surface of many materials—including electrically isolating ones, such as ceramics and glasses—with a resolution in both the vertical and horizontal direction of below 1nm. Figure 1 shows a typical example of an AFM measurement in vacuum, depicting a cerium oxide surface in true atomic resolution. The inherent mechanical characteristics of AFM requires serial acquisition of data, which limits the speed for obtaining high-resolution images. Using commercially available microscopes, a typical image of 256 256 pixels resolution can be obtained in 1–10min. This is significantly slower than the time resolution required, for example, to resolve biological processes such as the movement of motor proteins in cells.2 Enhancing imaging speed demands improved scanner technology and control electronics. Specifically, several limiting factors with respect to the cantilever probe must be addressed to reduce resolution time. First, the measurement bandwidth of the local interaction between the probe tip and sample, as well as the velocity at which the tip moves, must be increased. Additionally, the tip must be able to follow the sample’s topography at greater speed.3 Ideally, cantilever probes will function at resonance frequencies in the megahertz region with low force constants (approximately a few nano-Newtons per nanometer). Here, we report our progress in fabricating cantilever probes for highspeed imaging in AFM.4 Figure 1. Atomic force micrograph of a cerium oxide surface with true atomic resolution, imaged using a conventional atomic force microscope in noncontact mode.