What Is Atomic Force Microscopy?

Atomic force microscopy (AFM) is a scanning probe technique for imaging, measuring, and manipulating surfaces at the nanoscale, capable of resolving features smaller than a single nanometer. The instrument works by raster-scanning a sharp tip mounted on a flexible cantilever across a sample surface, monitoring the forces between tip and sample, and using a feedback loop to reconstruct a three-dimensional topographic map. AFM was introduced by Gerd Binnig, Calvin Quate, and Christoph Gerber in 1986 and has since become one of the most widely used tools in nanoscience and materials engineering, in part because it operates on conducting and insulating samples alike, and in ambient air, liquid, or vacuum.

The forces sensed at tip-sample separations of a few angstroms to tens of nanometers include van der Waals attraction, Pauli repulsion, electrostatic, capillary, and magnetic forces. Detecting these forces requires a cantilever with a spring constant low enough that sub-nanoNewton forces produce measurable deflections. A laser beam reflected from the back of the cantilever onto a split photodetector transduces cantilever bending into an electrical signal with sub-angstrom sensitivity.

Operating Modes

AFM instruments operate in several distinct modes depending on the interaction regime exploited. In contact mode, the tip remains in continuous contact with the surface; a feedback loop holds the deflection constant by adjusting the vertical position of the piezo scanner, and the height adjustments form the image. Contact mode delivers high spatial resolution but can damage soft biological samples. In tapping mode (intermittent contact), the cantilever oscillates near its resonance frequency and briefly taps the surface at the bottom of each oscillation cycle; feedback on oscillation amplitude produces the height map while dramatically reducing lateral forces on the sample. Non-contact mode, in which the tip oscillates just above the surface in the attractive van der Waals regime, achieves true atomic resolution in ultra-high vacuum and has been used to image individual molecules and even covalent bonds. The Casimir effect, a quantum mechanical attractive force arising from vacuum fluctuations at sub-micron separations, becomes relevant in non-contact AFM measurements and must be accounted for in precision force calibration.

Magnetic Force Microscopy

Magnetic force microscopy (MFM) is an AFM variant that images nanoscale magnetic field distributions. In a standard two-pass protocol, the first scan records surface topography in tapping mode; a second scan at constant lift height above the surface detects only the long-range magnetostatic force gradient from the sample's stray field, using a tip coated with a magnetic thin film. As described in NIST's magnetic force microscopy resources, MFM provides sub-100 nm resolution of domain structures and is used extensively to characterize magnetic storage media, permanent magnets, and spintronic devices. Interpreting MFM contrast requires careful modeling of the tip's stray field to separate topographic artifacts from genuine magnetic signals.

Scanning Microwave Microscopy

Scanning microwave microscopy (SMM) combines an AFM with a microwave source, feeding a continuous or pulsed microwave signal through a coaxial probe to the AFM tip and measuring the reflected signal as the tip scans the surface. The technique is sensitive to local permittivity, conductivity, and carrier concentration at microwave frequencies, making it the ideal metrology tool for characterizing charge transport and interfacial properties at the nanoscale in a manner that neither contact-resistance nor optical methods can replicate. SMM has been applied to mapping dopant profiles in transistors, imaging two-dimensional materials, and probing ferroelectric domains. IEEE research on near-field microwave probes compatible with atomic force microscopy has advanced the probe designs needed to achieve nanometer spatial resolution at gigahertz frequencies.

Applications

Atomic force microscopy has applications in a range of fields, including:

• Semiconductor device inspection and dopant profiling
• Biological imaging of proteins, membranes, and DNA in liquid
• Materials science characterization of thin films and nanomaterials
• Nanoscale tribology and mechanical property mapping
• Data storage research including hard disk media and phase-change materials