If you were to point to one invention that triggered what has come to be known as the field of nanotechnology, then you would be on pretty safe ground to cite the work of IBM Zurich scientists in creating first the scanning tunneling microscope (STM) and later the atomic force microscope (AFM). Both of these were the first tools to give us the capabilities to investigate, characterize and manipulate matter on the nanoscale.
Now, once again, scientists at IBM Zurich have provided us with a new tool to see the nanoscale world: They’ve developed a technique that uses an AFM to measure the temperature of an object on the nanoscale. The resulting device could provide a greater understanding of the capabilities of nanoscale devices, such as a new generation of transistors based on nanomaterials.
While AFMs over the past three decades have been refined to conduct all sorts of measurements—serving, for example, as biosensors for measuring everything from glucose levels to water pollution—the ability for them to measure temperature has remained oddly elusive.
The problem has been, in part, was that scientist took a research direction toward nanoscale temperature measurement that left AFMs out of the loop.
“Previous research was focused on a nanoscale thermometer, but we should have been inventing a thermometer for the nanoscale—an important distinction,” said Fabian Menges, and IBM postdoc and co-inventor of the technique, in a press release. “This adjustment led us to develop a technique which combines local thermal sensing with the measuring capability of a microscope—we call it scanning probe thermometry.”
In this new scanning probe thermometer, described in the journal Nature Communications, the tip of an AFM is able to simultaneously measure two signals: a small heat flux and the resistance of the material to heat flow.
A material’s resistance to heat flow is pretty self-explanatory; heat flux is a measurement of the thermal energy being transferred through the surface of the material. It is the combination of these two measurements that makes it possible to determine how much heat is locally present on a nanoscale device. Previous attempts at taking these measurements only took into account the heat flux and neglected to factor in the heat resistance of the material. A description of the device and the technique can be seen in the video below.
“The technique is analogous to touching a hot plate and inferring its temperature from sensing the heat flux between our own body and the heat source,” explained Bernd Gotsmann, IBM scientist and co-inventor of the technique. “Essentially, the tip of the probe is our hand. Our perception of hot and cold can be very helpful to get an idea of an object’s temperature, but it can also be misleading if the resistance to heat flow is unknown.”
In an e-mail interview with IEEE Spectrum, Gotsmann and Menges said they have been applying the method to research transistor devices that are being evaluated as potential replacements or complements to traditional, silicon-based transistors.
Apart from confirming the thermal bottleneck issue—the problem that heat is more and more difficult to remove for smaller and smaller devices—the researchers found that thermoelectric effects have a surprisingly large effect on the temperature distribution on a chip.
From this initial application, the IBM researchers are confident they have struck upon a tool that will prove extremely helpful in characterizing transistors in a post-CMOS world.
"Not only is the scanning probe thermometer accurate, it meets the trifecta for tools: it's easy to operate, simple to build, and very versatile, in that it can be used to measure the temperature of nano- and micro-sized hot spots that can locally effect the physical properties of materials or govern chemical reactions in devices such as transistors, memory cells, thermoelectric energy converters or plasmonic structures. The applications are endless.”
Dexter Johnson is a contributing editor at IEEE Spectrum, with a focus on nanotechnology.