News in Brief Microscope // New Near-Field Scanning Microwave Microscope (NSMM) © Based on Material by NIST, USA
NIST’s ability to determine the composition and physics of nanoscale materials and devices is about to improve dramatically with the arrival of a new near-field scanning microwave microscope (NSMM).
"Basically, what we’re doing is using the very fine spatial resolution of scanning probe instruments such as scanning tunneling microscopes or atomic force microscopes (AFM) and combining it with the broadband compatibility of microwave measurements," says Mitch Wallis of the Radio-Frequency Electronics Group. “Our motivation is that we want to look at things like magnetic resonance or mechanical resonance on the nanoscale using microwaves. If you look at your cell phone or your computer, they’re all operating in the range of a few gigahertz. So we have to measure the nanoscale objects that make up those devices to get an understanding of how they perform at those frequencies. Otherwise, it’s going to be much harder to integrate them into useful commercial devices.”
In broad outline, a NSMM set-up consists of an atomic force microscope combined with a continuous or pulsed microwave signal applied to the AFM tip. The tip scans across the sample in a soft contact or at a distance of a few nanometers above the surface, emitting a microwave signal that is scattered by the material, altering its frequency, amplitude and other properties. The nature of the altered signal returning to the tip – which also serves as the receiving antenna – depends critically on variables such as permeability, permittivity, sheet resistance, dielectric constant, impedance, or the manner in which capacitance changes with voltage, which in turn are determined by the physical composition of the object under investigation.
"By measuring the frequency-dependent scattered signal, we have, in effect, another knob to turn,” says veteran researcher Pavel Kabos of the Advanced High-Frequency Devices Program. "And very recently we’ve been able to examine local properties of samples in extremely small dimensions, very close to the single-molecule level. This is of intense interest, for example, to microchip designers and fabricators who need to know the doping profile around a transistor gate or source or drain in a dynamic random-access memory chip."
Spatial information recorded by the scanning tip is merged with data from the returning microwave signal at each point in a designated area (typically a few micrometers square) to create a composite image. NSMMs can be tuned to produce images at depths ranging from sub-micrometer to 100 µm below the surface.