There are several types of scanning microscopes including the scanning electron microscope, scanning tunneling microscope, and atomic force microscope. Typically, scanning microscopes consist of a probe or a beam of electrons that scans the surface of a sample. The interaction between the scanning microscope and the sample produces measurable data, such as the change in current, probe deflection, or the production of secondary electrons. This data is used to create an image of the surface of the sample at the atomic level.
The scanning electron microscope is one of several types of scanning microscopes used to image a sample. The microscope detects signals resulting from the interaction of its electron beam with the atoms on the surface of the sample. Several types of signals are usually produced including light, x-rays, and electrons.
There are several types of electrons that can be measured by this microscope including transmitted electrons, back-scattered electrons, and secondary electrons. Typically, scanning electron microscopes have a detector for secondary electrons, which are dislodged electrons produced from a primary source of radiation, namely the electron beam. The secondary electrons give information about the physical structure of the surface at the atomic level. Generally, the microscope images an area of 1-5 nanometers.
Scanning microscopes that utilize a probe, such as the scanning tunneling microscope, produce higher resolution images than the scanning electron microscope. The scanning tunneling microscope features a conducting tip that is placed very close to the sample. A voltage difference between the conducting tip and the sample causes electrons to tunnel from the sample to the tip.
As the electrons cross, a tunneling current is formed and measured. As the conducting tip is moved, the current changes, reflecting differences in height or density on the surface of the sample. With this data, an image of the surface at the atomic level is constructed.
The atomic force microscope is another scanning microscope that features a probe. It consists of a cantilever and a sharp tip that is placed near the surface of the sample. As the tip approaches the sample, forces between the tip and the sample cause the cantilever to deflect. Typically forces include mechanical contact force, van der Waals force, and electrostatic force.
Typically, the cantilever deflection is measured using a laser that is focused on the top surface of the cantilever. The deflection reveals the physical shape of the surface at a particular point. Both the sample and the probe are moved to scan the entire surface. An image is constructed from the data obtained by the laser.
