Microwave Microcopy

Microwave microcopy is a technique where a probe, typically with a small tip or small aperture, is used to measure and determine the physical properties (e.g., impedance, dielectric properties, capacitance, conductivity, etc.) of a sample under test, or create an image of it, at sub-wavelength spatial scales. This means that if a microwave microscopy probe is scanned with a small step size, the properties of the sample under test can be mapped with extremely high resolution. In other words, the spatial resolution is dominated by the probe size, rather than the wavelength at the operating frequency.

There are a few ways in which microwave microcopy probes can be categorized. The first is whether they are used in contact or non-contact modes. Just like in near-field imaging, microwave microscopy probes are also very sensitive to changes in standoff. This is again due to the fields of the probe being highly variable as a function of distance in the near-field of such probes. While this issue can be avoided by using a contact probe, having the probe in contact with the sample can also cause wear and tear on the probe which can reduce performance, and essentially limits the application of the technique. Another way probes can be categorized is by whether they are designed to be aperture coupled probes or probes with tips. While aperture coupled probes can theoretically handle more power than tip-based probes and can therefore produce signals that better penetrate materials, they tend to have a lower spatial resolution than tip-based probes. A third way to categorize probes is by whether or not they have a resonating element or phase matching component integrated into them. Both of these probe additions can help enhance the sensitivity of the probe during measurement.

Another area of research has been the integration of microwave microscopy with atomic force microscopy and scanning tunneling microscopy, which allows for mapping of not only impedance, capacitance, and dielectric properties, but also dopant density of semiconductors.