Modern AFM cantilevers are manufactured using silicon and silicon nitride photolithographic micro machining techniques. This has allowed probes to be fabricated in wafers, reducing cost and improving reproducibility. Using this approach, sharp tips can be integrated onto cantilevers. Tips may range in size from 2μm to 10's of microns in height, with a radius of curvature as small as 2nm. Cantilevers are usually 100-200μm long and 500-2000nm thick. The spring constant of the cantilever varies with the cube of the thickness and the inverse cube of the length.
Spring constants are chosen according to factors such as the mode of scanning, the sample material, the image medium, and the general application. Currently the softest cantilevers are in the range of 0.01nN/nm and the stiffest may be hundreds of nN/nm. Additional factors in selection of a cantilever are the resonant frequency, the angular gain, and the level of viscous damping.
Silicon nitride probes are typically made using semiconductor photolithography techniques. Tips are integrated by depositing a layer of silicon nitride over an etched pit in a crystalline silicon surface. Because the pits are etched along the silicon crystalline planes, tips made in this fashion are pyramidal in shape. Silicon nitride probes are often used for contact mode applications since the cantilevers can be made thin for softer spring constants yet short enough to maintain reasonable sensitivity (angular gain). Short silicon nitride probes are also commonly used for AC mode imaging in fluids since their geometry makes them less sensitive to the effects of viscous damping.
Silicon probes are fabricated from single crystal silicon. Silicon tips are often conical in geometry with a high aspect ratio. Silicon probes are often much sharper than silicon nitride probes, but are easily broken. Since doped silicon is a semi-conductor, these probes can dissipate static charge much better than silicon nitride, which is a dielectric. While silicon probes can be used for contact application, they are most often used for AD modes in air due to their larger spring constants and higher resonant frequencies.
Tip Shape and Resolution
The X,Y step size of the scanner and the minimum tip radius of curvature determine the lateral resolution of the AFM image. Manufacturers report tip radii on the order of 2-60nm. These tips however may image at much higher resolutions than predicted by these geometries. This is possible because most tips have small asperities or defects on their surface. If the sample is very flat, these asperities with a much smaller radius of curvature may image the surface. A very low force setting is usually required to keep these asperities intact while scanning.
Selecting a Probe
The desirable properties for a probe depend on the imaging mode and the application. For both contact and non-contact imaging, the tip should be sharper than the smallest features on the sample. However, the sharpest tips are more expensive and less durable and should only be used when the best resolution is required.
Soft cantilevers are best for contact mode because they deflect without deforming the surface of the sample. Stiff cantilevers are best for non-contact mode because they have high resonant frequencies to give optimal results.