3.7.4 The carbon-nanotube tip
A way of escaping the issues affecting process compatibility that arise from the use of techniques such as oxidation sharpening is simply to assemble the probe from separate parts – and this has been successfully done using carbon nanotubes. Single-walled carbon nanotubes can have diameters as small as 0.4 nm, but more typically they are of the order of 1 to 2 nm. This represents a great improvement on the radii of curvature achieved with oxidation sharpening. One might have thought that it would be difficult to get the nanotube in position on the probe tip, but happily, electrostatic forces tend to attract the nanotubes to the tip. Aligning the tube along the axis of the tip is more fiddly, and requires the use of micromanipulation. Once in position, the nanotube can be attached to the tip by means of electron-beam deposition, in which an electron beam, such as is produced in a scanning electron microscope, is focused on a stationary spot on the substrate. Carbon, always present in vacuum systems, is deposited at the focus of the electron beam, and begins to pile up on itself. This heap of carbon atoms forms a very effective bonding material that will permanently bond the nanotube to the tip. In addition to the tiny radius of curvature of this tip, it is also a tip with a small included angle, and this enables steeply rising steps to be imaged without the result simply showing the shape of the AFM tip. Figure 17 is a good illustration of the improvements in resolution and step-profile imaging that can be obtained with this type of tip. Notice how in Figure 17(b), taken with a simple silicon nitride probe tip, the pillars appear much less steep than they do in Figure 17(a). This is just the result of the tip's shape: as it strikes the edge of the feature, the feature begins to interact with the flanks of the tip rather than the end, and the result is that the image reveals more about the shape of the tip than it does about that of the feature. In this particular example, we also see the benefit of the smaller radius of curvature of the carbon nanotube tip, in the much greater detail that is visible on the tops of the pillars.