3.7 Micromachining the AFM tip and cantilever
3.7.1 The machined-at-once tip and cantilever
Just as in conventional manufacturing, micro engineering is cheapest to do if as few different materials as possible are used and if the number of separate processes involved is minimised. Therefore, the idea of making the cantilever and the probe out of the same material and in the same process step is a very attractive one.
When silicon nitride is deposited onto a silicon surface, it produces a thin film that coats the whole of the material to an equal thickness. We have already seen how it is possible to adjust the process parameters to tune the materials properties of the film. Silicon nitride's high Young's modulus relative to its density makes it a good choice for the AFM cantilever, as does its extreme resistance to chemical attack. This last property also makes it ideally suited to a process in which it can be left behind as the only material to survive a chemical dissolution.
The anisotropic etching of silicon can produce deep rectangular-pyramidal pits in the surface of the wafer. The faces of these pits are defined by the densely packed crystallographic planes in the silicon. They are therefore very repeatable from pit to pit and from wafer to wafer, and the apex of the pit is (at least in theory) atomically sharp. Figure 13 shows a process sequence in which these etch pits are used to define the tips of silicon nitride AFM probes – the silicon nitride being a low-pressure chemical-vapour deposition (LPCVD) coating of controlled thickness and composition. During the same process sequence, the cantilever on which the tip is mounted is also first defined as an etched region of the silicon wafer. The resulting structure, made entirely of silicon nitride, is extremely light and has a hollow pyramidal tip. If a dry-etch process was used to define the shape of the cantilever, any shape (in plan view) is possible, giving lots of scope for tailoring its torsional and lateral stiffness and its resonant frequency to suit the specific application. This simple and effective way of making AFM probe assemblies has been widely adopted.
However, the process does not provide the sharpest tips it is possible to make, and the rather shallow angle of the tip makes it unsuitable for use on surfaces whose features are likely to be steeper than this.
It is also possible to make the probes entirely from silicon. One advantage of doing this is that it allows tips with a narrower angle to be made. This process often, but not always, involves a combination of anisotropic and isotropic etching steps. Because single-crystal silicon cannot easily be produced in sub-micrometre thicknesses, silicon cantilevers are thicker – and hence stiffer – than silicon nitride ones. The anisotropic etching that is used in their manufacture leaves them with a trapezoidal cross-sectional shape. Figure 10 is an example of an all-silicon AFM probe: it is typical of its kind also in the simple diving-board shape of the cantilever (more complex or angled shapes are not easily achieved with anisotropic etching).