New Microscope for Materials

Researchers at Lawrence Berkeley National Laboratory (LBL) have developed a new thumb-sized microscope that operates on the same principles as a CD-player, using microwaves rather than visible light. Dubbed a Scanning Evanescent Microwave Probe (SEMP), this unique new instrument can be used to simultaneously characterize critical electronic properties along with topography in a wide assortment of materials. Xiao-Dong Xiang, a physicist in LBL's Materials Sciences Division, described the instrument in a paper he delivered at the APS Centennial meeting in Atlanta last March.

The SEMP uses near-field microwaves to measure the electrical impedance of materials with submicron resolution - a critical property for the electronics industry. By measuring the interaction between evanescent microwaves generated off an ultra sharp-tipped probe and the surface of the material, Xiang and his colleagues can not only map electrical impedance across the face of the material, they can simultaneously map the topography of its surface, another critical factor for manufacturing chips and other electronic devices.

The SEMP's probe is connected to a high quality-factor microwave resonator equipped with a thin metal shield designed to screen out all but the evanescent microwaves from being generated at its tip. "This feature is crucial for high resolution quantitative microscopy," says Xiang. "If both evanescent and propagating microwaves had to be considered and calculated, as is the case for all other types of microwave probes, the quantitative microscopy would be impossible." The interaction between evanescent microwaves and the sample surface gives rise to a resonance frequency and quality-factor changes in the resonator that are recorded as signals. These signals can be measured, and the measurements plugged into equations that translate them into a measurement of the sample's complex electrical impedance, with a spatial resolution of 100 nanometers.

The SEMP can be used on conductors and insulators as well as semiconductors. It has applications in any situation in which there is a need to characterize a material's electrical properties as a function of electric or magnetic fields, optical illumination, or temperature variations. The basic technology has been licensed to Ariel Technologies, but Xiang and his colleagues continue to refine the device, and are currently building a low-temperature version to enable them to study superconductors.