What Is Scanning Probe Microscopy?

Scanning probe microscopy is a family of surface characterization techniques in which a sharp physical probe is raster-scanned across a sample surface to map properties at the nanometer scale. Rather than using light or electrons to form an image, each method in the family measures a specific local interaction between the probe tip and the surface, such as quantum tunneling current, van der Waals force, magnetic force, or near-field optical coupling. The resulting data yield spatial maps of topography, electronic structure, mechanical stiffness, or chemical identity with resolutions that can reach the atomic scale under controlled conditions.

Scanning probe microscopy traces its origins to the invention of the scanning tunneling microscope (STM) by Gerd Binnig and Heinrich Rohrer at IBM Zurich in 1981, work recognized by the Nobel Prize in Physics in 1986. The atomic force microscope (AFM) followed in 1986, extending the technique to non-conducting samples and opening the field to biology, polymer science, and semiconductor metrology. The broader SPM family now encompasses dozens of variants that share the same fundamental architecture: a piezoelectric scanner, a sharp probe, and a feedback loop that keeps the tip-surface interaction at a set point while the scanner traces a pattern across the specimen.

Scanning Tunneling Microscopy

The STM applies a small bias voltage between a metallic tip and a conducting or semiconducting surface, separated by a gap of roughly one nanometer. Electrons tunnel quantum-mechanically across the gap, and the resulting current depends exponentially on tip-sample separation, giving the instrument extreme sensitivity to surface corrugation. At cryogenic temperatures in ultrahigh vacuum, the STM resolves individual atoms and has been used to image surface reconstructions, map local electronic density of states, and even position individual atoms with sub-angstrom precision. Because conduction is required, the STM is confined to electrically conducting samples, a limitation the AFM was designed to overcome.

Atomic Force Microscopy

The AFM replaces the tunneling current signal with a measurement of mechanical force. A microfabricated cantilever, typically silicon or silicon nitride, carries a sharp tip at its free end. As the tip approaches the surface, interatomic forces deflect the cantilever, and that deflection is detected by reflecting a laser beam off the cantilever's back surface onto a position-sensitive photodiode. The instrument can operate in contact mode, intermittent-contact (tapping) mode, or non-contact mode, each suited to different sample types and information goals. NIST's program in nanoscale property measurements by atomic force microscopy has advanced AFM from topography imaging alone to quantitative measurements of mechanical, electrical, magnetic, and chemical properties at nanometer resolution.

Specialized SPM Variants

Beyond STM and AFM, the SPM family includes magnetic force microscopy (MFM), which maps stray fields above magnetic domains; Kelvin probe force microscopy (KPFM), which measures surface potential; scanning near-field optical microscopy (SNOM), which breaks the diffraction limit using a sub-wavelength aperture on the probe tip; and scanning thermal microscopy, which maps thermal conductivity at the nanoscale. Each variant modifies the probe design, feedback signal, or operating environment to isolate a different surface property. NIST's scanning probe microscopy program for advanced materials and processes develops reference methods and calibration standards that underpin quantitative SPM measurements across these modes. A broader survey of the technique's evolution appears in Physical Review Letters coverage of SPM from its early demonstrations to widespread industrial use.

Applications

Scanning probe microscopy has applications in a range of fields, including:

• Semiconductor process control and defect inspection at the device level
• Biological imaging of proteins, cell membranes, and DNA strands in liquid environments
• Materials research characterizing thin films, nanoparticles, and 2D materials
• Nanofabrication and tip-induced lithography for prototype device fabrication
• Data storage research using probe tips to write and read nanoscale bit indentations