Optical Techniques Tackle Nanoscale Measurements

MARIE FREEBODY, CONTRIBUTING EDITOR, marie.freebody@photonics.com

Materials at the nanoscale can behave in surprising ways. While the properties of macro materials can often be predicted, the behaviors of a material invisible to the naked eye can differ greatly. Providing insight into this nanospace is essential for maximizing its commercial success.

The extremely high spatial resolution characterization capabilities of today’s nanomeasurement and test tools reveal vital information about correlations between structure and property. Only by studying, designing and modifying these nanostructures can we achieve the desired properties and performance from nanomaterials and devices.

“Any industries that design, manufacture or use nanomaterials and devices would be the beneficiaries,” said Anil Ghimire, applications scientist at Keysight Technologies Inc. in Santa Clara, Calif. “Examples include: semiconductors, MEMS, nano/microcomponents and devices, and energy (solar, battery, oil/gas). As the size, density, efficiency and performance requirements for such components and devices become more demanding, the benefits of nanomeasurement tools increase.”

Quick scan at 10 sec/frame scan rate of polydiethylsiloxane (PDES) phase images on silicon at different temperatures: 25 °C (a), 46 °C (b), 28 °C (c). Scan size: 4 μm. Courtesy of Keysight Technologies Inc.

In the semiconductor industry, device feature sizes are already at 14 nm and are rapidly approaching 10 nm. To analyze devices at this level requires 3D metrology, as well as electrical and thermal imaging.

Beyond semiconductors is the need in biology to know at the nanolevel the structure and chemical composition of a biological material. With research into using nanoparticles as drug-delivery vehicles and for medical diagnostics, nanotechnology test and measurement processes and equipment are increasingly important.

Tiny 2D materials make big impact

There’s quite a stir about some tiny 2D materials that are just one to a few atomic layers. Materials such as graphene are expected to make a huge impact on the world. This miniscule material is thought to have huge potential in industrial applications, including next-generation electronics, bendable displays and wearable technologies, thanks to its extraordinary electrical and optical properties. In addition to being the thinnest material possible, it is transparent, ultralightweight and can act as a perfect barrier and superb conductor.

Dried E. coli imaged in Quick Sense mode, which allows users to perform quantitative mapping of nanomechanical properties simultaneously. These 2-µm scans present adhesion (a) and stiffness (b) information of E. coli. Courtesy of Keysight Technologies Inc.

Since graphene was first isolated in 2004 by scientists at the U.K.’s University of Manchester, researchers around the world are utilizing its unique characteristics to improve existing materials and to transform technologies. Despite exciting prospects on the horizon, the industry may only have the confidence to invest if such materials can be made to produce reliable and repeatable results.

“Likewise, the market will only adopt these technologies if there is an inherent trust in these products. Such trust can only come from tried and tested quality assurance processes,” said David Yehudah Lewis, VP of Global Sales and Marketing at Nanonics Imaging Ltd. in Jerusalem. “At the heart of such processes is test and measurement equipment both in the laboratory and as well as on an industrial scale.”

Today’s nanomeasurement tools are stepping up to the challenge of transforming laboratory research instruments into devices for the real world. Traditionally, nanomeasurement tools were designed to carry out one task and this was how they were used. One tool would be used for imaging, while a separate tool would be used for various spectroscopies.

The future trend, however, is to integrate two or more tools to measure correlated information from a sample at the same measurement point in a way that could not have previously been possible by using the tools individually.

Nanonics’ MultiView 4000 Multiple Probe SPM Head. Courtesy of Nanonics Imaging Ltd.

“For example, one tool could acquire a sample’s local structure information, while the other simultaneously obtains chemical data from the same spot via spectroscopy,” Ghimire said.

Take, for example, the multiple optical approaches used in the pursuit of graphene-based technology. Here, several techniques have become vital to obtain an accurate understanding of a device’s characteristics. First, Raman and tip-enhanced Raman spectroscopy are important for establishing the thickness and state of the sample — whether it is conducting or semiconducting, for example.

Next, AFM (atomic force microscopy) and other SPM (scanning probe microscopy) techniques are important for establishing more detailed information about the device such as hardness, electrical and thermal conductivity, and optical properties.

Low-temperature atomic force microscopy/scanning probe microscopy/near-field scanning optical microscopy system. Courtesy of Nanonics Imaging Ltd.

“It is important to integrate multiple probes together to get a clear understanding of the transport properties of the device in certain areas,” Lewis said. “The ability to combine these test methodologies in a single platform is very important for getting a complete and accurate picture of the device and its properties, which is very important for quality assurance.”

The founder of Nanonics Imaging Ltd., professor Aaron Lewis, pioneered the field of near-field scanning optical microscopy (NSOM) and fountain pen nanochemistry (FPN). Today, the company offers its MultiProbe AFM along with a NanoToolKit of probes that can be brought into physical contact with one another to allow for electrical, thermal, optical, nanochemical writing and nano-optical measurements integrated with nano-Raman and nano-infrared.

Another major advance felt across countless industries was achieving resolution well below the optical diffraction limit. The optical feat earned three scientists the Nobel Prize in Chemistry in 2014 for their role in creating superresolution imaging techniques.

“Typically, when we think of nanoscale test and measurement, we might think of microscopes that allow one to achieve the nanoscale resolution needed to image ultrasmall features,” said Kartik Srinivasan, a nanophotonics expert and project leader at the Center for Nanoscale Science and Technology (CNST) at the National Institute of Standards and Technology (NIST) in Maryland. “Along with continuing developments in various forms of electron and ion microscopy that push the forefront of spatial resolution, there have been many important developments in optical microscopy.

Near-field scanning optical microscopy nanophotonics setup. Courtesy of Nanonics Imaging Ltd.

“From the perspective of nanophotonic technologies, these various microscopy techniques help us understand whether the devices look the way they are supposed to — for example, have the intended dimensions, shape and so on,” he continued.

With nanotechnology and measurement methods advancing quickly, there is great faith in one day mastering the mysteries of the nanodomain.

“As we understand the nanoworld better by leveraging ever-more advanced technologies that are capable of achieving higher measurement accuracy and sensitivity,” Ghimire said, “the uncertainties and challenges will become increasingly manageable and easier to overcome.”

Taking tiny measurements is a tall order

Working at the nanoscale poses particular challenges, which always increase when measurement scales shrink significantly and new factors come into play. Achievable resolution, measurement accuracy and sensitivity are very much affected by thermal, mechanical and electrical disturbances, not to mention the inherent limitations of the various techniques themselves.

(left) Alex Liddle, (middle) Kartik Srinivasan, (right) Samuel Stavis. Courtesy of K.Dill/NIST.

Indeed, a system’s measured properties should be independent of the specific tools used to make those measurements. However, due to their size, this is often not the case. Photonics Spectra talked with several leading experts in the field of nanoscale test and measurement from the Center for Nanoscale Science and Technology (CNST) at the National Institute of Standards and Technology (NIST) in Maryland. Alex Liddle leads the Nanofabrication Research Group; Kartik Srinivasan is a nanophotonics expert and project leader; and Samuel Stavis is an expert in nanoparticle manufacturing metrology.

Q: Why is measuring at the nanoscale so tricky, and what strategies can be used to overcome the challenges? Liddle: Making an accurate measurement at the nanoscale — one that provides a correct absolute value — is very difficult, for the rather obvious reason that measuring very small things is just intrinsically harder. There’s just a lot less signal from a nanoscale object. However, in manufacturing, sometimes it is okay to have measurements that are not accurate, but that are precise — meaning repeatable. Such measurements can provide all the information necessary to make sure a process is staying on track.

Srinivasan: There are many challenges in measuring nanoscale systems. Ideally, the system’s properties should be independent of the specific tools used to measure it and the measurement should be a probe that at most weakly perturbs the system. Due to their size, nanoscale systems can often be more strongly influenced by the probe. For example, trying to measure the evanescent field near a waveguide will tend to perturb that field.

There are many different strategies for addressing this challenge. Of course, developing measurement methods that are suitably minimally perturbative is one focus. This goes together with the development of simulation tools that can help researchers better understand how the measurement probe couples to the system. Moreover, one can design test structures to be fabricated at the same time as the target system that can be measured to reveal the desired information about a given fabrication process; for example, the optical loss and whose coupling to the measurement probe is well understood.

Q: How can nanotechnology be taken out of the laboratory and into commercial markets?

Liddle: There are two areas of nanotechnology that require test and measurement. First, measurements are needed to understand what is going on at the nanoscale in the first place. These are the kinds of measurements that are typically time-consuming, detailed and difficult to make — the kind that are typically done in a research environment. However, once the basic material, device or process knowledge has been developed, a different kind of measurement is needed for manufacturing.

These measurements need to be fast, cost-appropriate, simple and reliable, and enable, for example, closed-loop process control. Often, we find we have the right tools for the first class of measurements, but the second can be particularly challenging. If these measurements can be done well, they lead to much higher material or device yields and, as more and more process data becomes available, to more rapid improvements in any given product.

Stavis: Nanoparticle technologies are now emerging in commercial products. However, current methods for nanoparticle test and measurement, among other manufacturing challenges, are still limiting their commercial impact. Innovations in test and measurement, and eventually standardization of characterization methods, are necessary to improve quality control of nanoparticle products.

We can use nanofabrication processes to build accuracy into devices for nanoparticle test and measurement. We often develop these measurement devices for use with optical microscopes. We can then use nanofabrication processes to build calibration devices for optical microscopes to ensure their accuracy in nanoparticle imaging and tracking measurements.

Q: What exciting new technologies are you are working on and what applications stand to benefit?

Stavis: We are working on a variety of device technologies, typically fluidic, for nanoparticle characterization. We are also working on optical devices for microscope calibration. Our focus is on developing measurement devices and methods that are scalable for widespread application in research and development, as well as mass production of nanoparticles.

Almost anything is possible for an expert to accomplish with enough money, but making measurement systems accessible and economical for profitable manufacturing of nanoparticles is a unique challenge that the CNST can address through our mission and expertise in nanofabrication and microscopy.

Srinivasan: We are working on a variety of nanophotonic device technologies. One type of device uses nonlinear optics to shift the frequency of light to and from the telecommunications band with high efficiency and very low added noise, and can even work at the single-photon level. Another type of device generates a broad comb of optical frequencies for a single frequency input. Both types of devices are generally applicable to applications in communications and time and frequency metrology.

Other researchers in the CNST are working on fast and flexible waveguide probes that can address some of the inline control and test needs I mentioned above. Ultimately, the goal is to develop test and measurement systems for photonic circuits that are analogous to those already in place for electronic ones.