An essential research tool in nanoscience and nanotechnology, scanning tunneling microscopy (STM) is a powerful imaging technique that positions a sharp metal wire to within a nanometer above a material’s surface. As the wire probe scans the surface, a tunnel of electrons is sent between the probe tip and the surface. Changes in this current, which is sensitive to the presence or absence of atoms, are then measured. Collectively, these measurements generate an atomic-scale map of the surface, which can be used, for instance, to reveal critical defects in the integrated circuits that control personal computers and other electronic devices.
At the dawn of the nanotech era 25 years ago, the invention of STM enabled scientists to image atoms one-by-one on a surface, move them around at will, and assemble small nanostructures. While STM has propelled many advances in nanotechnology, its slow data collection rate has rendered it ineffective in imaging high-speed phenomena at the atomic scale. “The STM can’t go fast because its circuit limits you to a maximum measuring speed,” explains Kamil Ekinci, assistant professor of mechanical engineering. “If you want to scan the surface fast, you can’t see much because the STM technique has limited time resolution.”
Ekinci and his group—in collaboration with radio frequency expert Keith Schwab, a physicist at Cornell University—launched a National Science Foundation–funded project to scan the surface faster through the use of radio frequency waves. Their research on radio-frequency STM, published in Nature, demonstrates the use of radio frequency electronics techniques to increase STM data collection speed and improve imaging resolution.
“We sent radio waves—which have a high frequency—through the tip and sample of a standard STM,” Ekinci explains. “The radio waves reflected off of the tip and sample and we measured the much higher frequency response. As a result, we could see the surface clearly at a much faster speed—about 100 to 1,000 times faster than the conventional approach.”
This increase in the pace of data collection not only opens up the possibility of seeing much faster phenomena on surfaces at the atomic scale, but also paves the way for the efficient measurement of surface temperature variations at the nanoscale. Measurements of the temperature (based on the noise of the tunneling electrons) on the surface at atomic resolution could provide an important indicator of problematic hot spots within integrated circuits and devices. “As you scale circuits smaller and smaller, heat has trouble escaping efficiently,” says Ekinci. “We want to measure the surface temperature to within an atomic resolution to pinpoint the location of heat transfer problems.”
Because Ekinci’s first radio-frequency STM experiments lacked controls on surface cleanliness, temperature, and other environmental factors, his group is now building a new instrument enclosed in an ultra-high vacuum chamber. Ekinci expects new, more accurate results later in the year.