The Virtuous Cycle of Making and Measuring Nanostructures
Samuel Stavis, Leader, Nanostructure Fabrication and Measurement Group, National Institute of Standards and Technology (NIST)
In his 1959 lecture “There’s Plenty of Room at the Bottom,” Richard Feynman invited us to enter a new field of physics. He envisioned, with remarkable prescience, making, measuring and using new technology at the nanometer scale. The exact effect that his predictions had on future science is uncertain, but he certainly inspired science fiction writers, and their stories captured my imagination as an undergraduate student. Was it possible to create machines so small and put them to work?
I wanted to learn more and would soon have the opportunity, starting graduate school in the early years of the National Nanotechnology Initiative. After my coursework, I got to choose between the clean room or the machine shop to get going in the laboratory. I chose the clean room and put on a bunny suit, expecting to step into the machine shop of the future. Instead, I entered the lithography bay of a submicron facility from the late years of the disco era. I had to figure out how to make devices with a production tool called a wafer stepper, donated by the semiconductor industry after it was obsolete and stripped down for use by novices. And this stepper, which was just a few years younger than I, seemed like a modern marvel in comparison to the etcher that another graduate student had made for the facility. Despite my green misunderstanding of the when and how of nanotechnology, the stepper focused light to pattern a stencil, the etcher ignited a plasma to engrave the pattern, and I was forming structures that were smaller than I could see.
Over the next few years, I started to learn to make devices, microscopes and measurements. My friends who had read the same science fiction that I had asked what nanotechnology was really like in the laboratory. It seemed to me to be less like assembling molecular machines and more like patterning simple structures, such as films and slits, beams and channels, and pillars and holes, and then integrating those structures into devices that were just a bit more complex. But these research structures and devices could result in new functions, affecting how light shined, cantilevers vibrated and DNA squeezed into small spaces. Often, these new functions enabled new measurements. At the same time, it became clear to me that making good measurements, first on the devices and then with the devices, was essential to ensuring that nanotechnology was real science and not science fiction. At a conference, I learned that researchers at the National Institute of Standards and Technology (NIST) were also interested in new devices and good measurements.
I came to NIST as a postdoctoral researcher. In my first year at NIST, I attended a colloquium at the National Academy of Sciences on the promise and perils of nanotechnology, featuring a futurist named K. Eric Drexler who wrote influential work on the topic. He apologized for his overly dramatic depiction of molecular nanotechnology to the public. Now he tells me?! But, by then, I had developed a good sense of what nanotechnology was really like, at least in my own niche of research.
For a few years at NIST, while the CNST NanoFab was under development, I traveled back to graduate school to use that old stepper to make new devices. We published our last result from that lithography system just a few years ago, demonstrating a method to measure free energy (figure 1, left) that can power molecular devices and also answering the question of whether obsolete equipment could still yield significant advances. The answer is yes, when expertly and lovingly maintained, but nanofabrication facilities require investment and then reinvestment. In so doing, NIST has developed extraordinary instruments and staff over the years in the CNST NanoFab. The resulting capability is critical, enabling NIST researchers to push the limits of devices and measurements. I transitioned to making devices at NIST and learned the NIST way of making good, better and eventually the best measurements that are fit for their different purposes. I still love the challenge of estimating the uncertainty of a measurement, describing our incomplete knowledge of a quantity.
After several years at NIST, a virtuous cycle started to come into focus. As my colleagues and I worked on fabrication processes, device technologies and microscopy measurements at the nanometer scale, we benefited from useful interactions between the different types of work. In some experiments, microscopy revealed how a process or device worked. In other experiments, a nanostructure improved the capability of a microscope. In several experiments, we observed that we had completed multiple cycles of making and measuring things, getting better results and developing new capabilities with each cycle. This virtuous cycle is coming into sharper focus as NIST starts up the new Nanostructure Fabrication and Measurement Group, or Nanostructure Group for short. For many years, NIST has cultivated leading capabilities in fabrication and measurement of nanostructures, with some very productive overlap. The mission of this new group is to deliberately combine some of these capabilities and actively pursue their positive feedback, complementing the work of other groups in support of different aspects of the NIST mission at the nanometer scale.
The electron beam arrived early and stayed relevant in nanotechnology, presenting a good example of the virtuous cycle. Images of fine patterns made by electron-beam lithography and measured by electron microscopy appeared four decades ago (figure 2a, below), and even earlier. The smallest features are still impressive today but nowhere near the end of the story, leading to questions that NIST seeks to answer. How do we optimize the design of complex patterns? How well does a lithography system place the patterns? How can we measure and optimize the placement across a large area? Will an electron, ion or photon beam work better to make patterns in three dimensions? In a microscope image, what do the electrons, ions or photons that create signals reveal about nanostructures in three dimensions (figure 2b, below)? Can we compare these images to those from a scanning probe that traces the surface of the nanostructure? The answers to these questions are highly relevant to ongoing efforts to produce electronic and photonic devices. In this context, good measurements are often necessary in a timely manner, as work in a fabrication facility generally proceeds faster than work in a measurement laboratory. In some cases, a good measurement has to occur in real time, such as to focus an ion beam during a fabrication process (figure 2c, below).
The need for high speed across a large area is attracting attention to optical microscopy — the original microscopy. For four centuries, microscopists looked toward the bottom, but could not see all the way down. Four decades of research has demonstrated that the classical resolution limit of a few hundred nanometers is not so limiting, and that smaller structures can be super-resolved. Optical microscopes are also becoming increasingly capable and economical due to lights and cameras like those in smartphones. This leads to microscopes that are sensitive and precise, being able to detect many nanostructures simultaneously across a large area, but that are generally inaccurate.
Exploring the limits of optical microscopy is a perfect challenge and opportunity for NIST. Standard nanostructures with reference dimensions are essential to this work. NIST is making and measuring prototype standards (figure 3a, left) and establishing their traceability to the International System of Units (SI) by comparing data from optical microscopes and scanning-probe microscopes (figures 3b-c, left). This work has the potential to extend accuracy from NIST to external microscopists. Biomedical measurements are of interest to many, but the applications are diverse, including the fabrication and measurement of quantum devices (figure 4, below) and microelectromechnical systems, among others.
The last six decades have yielded amazing advances in making and measuring nanostructures and integrating them into useful devices. Whether patterning from the top down or assembling from the bottom up, nanotechnologists have increasing capabilities of control and corresponding needs for measurement. Microscopy is essential to validate and improve nanostructures while nanostructures can validate and improve microscopy, leading to positive feedback. NIST is focusing on this virtuous cycle to cultivate its capabilities and apply them to critical applications.
On the occasion of National Nanotechnology Day, I would like to conclude with a personal statement from the bottom. We have explored much of the nanometer scale, discovering important science and technology. And there is still plenty of room down here.
This post originally appeared on Taking Measure, the official blog of the National Institute of Standards and Technology (NIST) on October 9, 2020.
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About the Author
Samuel Stavis is a physical scientist and group leader at NIST. He received a B.S.E. in engineering physics from the University of Michigan and an M.S. and Ph.D. in applied physics from Cornell University.