What is a Scanning Probe Microscope?

A scanning probe microscope is any of several microscopes that produce three-dimensional surface images in very high detail, including atomic scale. Depending on the microscopy technique used, some of these microscopes can also measure physical properties of a material, including electrical current, conductivity, and magnetic fields. The first scanning probe microscope, called a scanning tunnel microscope (STM), was invented in the early 1980s. The inventors of the STM won the Nobel prize in physics a few years later. Since that time, several other techniques, founded on the same basic principles, have been invented.

All scanning probe microscopy techniques involve a small, sharp-tip scanning of the surface of the material, as data is digitally acquired from the scan. The tip of the scanning probe must be smaller than the features on the surface being scanned, in order to produce an accurate image. These tips must be replaced every few days. They are usually mounted on cantilevers, and in many SPM techniques, the movement of the cantilever is measured to determine the height of the surface.

In scanning tunnel microscopy, an electrical current is applied between the scanning tip and the surface being imaged. This current is kept constant by adjusting the height of the tip, thus generating a topographical image of the surface. Alternatively, the height of the tip may be kept constant while the changing current is measured to determine the height of the surface. Since this method uses electrical current, it is only applicable to materials which are conductors or semi-conductors.

Several types of scanning probe microscope fall under the category of atomic force microscopy (AFM). Unlike scanning tunnel microscopy, AFM can be used on all types of materials, regardless of their conductivity. All types of AFM use some indirect measurement of the force between the scanning tip and the surface to produce the image. This is usually achieved through a measurement of the cantilever deflection. The various types of atomic force microscope include contact AFM, non-contact AFM, and intermittent contact AFM. Several considerations determine which type of atomic force microscopy is best for a particular application, including the sensitivity of the material and the size of the sample to be scanned.

There are a few variations on the basic types of atomic force microscopy. Lateral force microscopy (LFM) measures the twisting force on the scanning tip, which is useful for mapping surface friction. Scanning capacitance microscopy is used to measure the capacitance of the sample while simultaneously producing an AFM topographical image. Conductive atomic force microscopes (C-AFM) use a conductive tip much as STM does, thus producing an AFM topographical image and a map of the electrical current. Force modulation microscopy (FMM) is used to measure a material's elastic properties.

Other scanning probe microscope techniques also exist to measure properties other than the three-dimensional surface. Electrostatic force microscopes (EFM) are used to measure the electrical charge on a surface. These are sometimes used to test microprocessor chips. Scanning thermal microscopy (SThM) collects data on thermal conductivity as well as mapping the topography of the surface. Magnetic force microscopes (MFM) measure the magnetic field on the surface along with the topography.