Category Archives: Process Materials

For decades, scientists have tried to harness the unique properties of carbon nanotubes to create high-performance electronics that are faster or consume less power — resulting in longer battery life, faster wireless communication and faster processing speeds for devices like smartphones and laptops.

But a number of challenges have impeded the development of high-performance transistors made of carbon nanotubes, tiny cylinders made of carbon just one atom thick. Consequently, their performance has lagged far behind semiconductors such as silicon and gallium arsenide used in computer chips and personal electronics.

Now, for the first time, University of Wisconsin-Madison materials engineers have created carbon nanotube transistors that outperform state-of-the-art silicon transistors.

The UW-Madison engineers use a solution process to deposit aligned arrays of carbon nanotubes onto 1 inch by 1 inch substrates. The researchers used their scalable and rapid deposition process to coat the entire surface of this substrate with aligned carbon nanotubes in less than 5 minutes. The team's breakthrough could pave the way for carbon nanotube transistors to replace silicon transistors, and is particularly promising for wireless communications technologies. Credit: Stephanie Precourt

The UW-Madison engineers use a solution process to deposit aligned arrays of carbon nanotubes onto 1 inch by 1 inch substrates. The researchers used their scalable and rapid deposition process to coat the entire surface of this substrate with aligned carbon nanotubes in less than 5 minutes. The team’s breakthrough could pave the way for carbon nanotube transistors to replace silicon transistors, and is particularly promising for wireless communications technologies. Credit: Stephanie Precourt

Led by Michael Arnold and Padma Gopalan, UW-Madison professors of materials science and engineering, the team’s carbon nanotube transistors achieved current that’s 1.9 times higher than silicon transistors. The researchers reported their advance in a paper published Friday (Sept. 2) in the journal Science Advances.

“This achievement has been a dream of nanotechnology for the last 20 years,” says Arnold. “Making carbon nanotube transistors that are better than silicon transistors is a big milestone. This breakthrough in carbon nanotube transistor performance is a critical advance toward exploiting carbon nanotubes in logic, high-speed communications, and other semiconductor electronics technologies.”

This advance could pave the way for carbon nanotube transistors to replace silicon transistors and continue delivering the performance gains the computer industry relies on and that consumers demand. The new transistors are particularly promising for wireless communications technologies that require a lot of current flowing across a relatively small area.

As some of the best electrical conductors ever discovered, carbon nanotubes have long been recognized as a promising material for next-generation transistors.

Carbon nanotube transistors should be able to perform five times faster or use five times less energy than silicon transistors, according to extrapolations from single nanotube measurements. The nanotube’s ultra-small dimension makes it possible to rapidly change a current signal traveling across it, which could lead to substantial gains in the bandwidth of wireless communications devices.

But researchers have struggled to isolate purely carbon nanotubes, which are crucial, because metallic nanotube impurities act like copper wires and disrupt their semiconducting properties — like a short in an electronic device.

The UW-Madison team used polymers to selectively sort out the semiconducting nanotubes, achieving a solution of ultra-high-purity semiconducting carbon nanotubes.

“We’ve identified specific conditions in which you can get rid of nearly all metallic nanotubes, where we have less than 0.01 percent metallic nanotubes,” says Arnold.

Placement and alignment of the nanotubes is also difficult to control.

To make a good transistor, the nanotubes need to be aligned in just the right order, with just the right spacing, when assembled on a wafer. In 2014, the UW-Madison researchers overcame that challenge when they announced a technique, called “floating evaporative self-assembly,” that gives them this control.

The nanotubes must make good electrical contacts with the metal electrodes of the transistor. Because the polymer the UW-Madison researchers use to isolate the semiconducting nanotubes also acts like an insulating layer between the nanotubes and the electrodes, the team “baked” the nanotube arrays in a vacuum oven to remove the insulating layer. The result: excellent electrical contacts to the nanotubes.

The researchers also developed a treatment that removes residues from the nanotubes after they’re processed in solution.

“In our research, we’ve shown that we can simultaneously overcome all of these challenges of working with nanotubes, and that has allowed us to create these groundbreaking carbon nanotube transistors that surpass silicon and gallium arsenide transistors,” says Arnold.

The researchers benchmarked their carbon nanotube transistor against a silicon transistor of the same size, geometry and leakage current in order to make an apples-to-apples comparison.

They are continuing to work on adapting their device to match the geometry used in silicon transistors, which get smaller with each new generation. Work is also underway to develop high-performance radio frequency amplifiers that may be able to boost a cellphone signal. While the researchers have already scaled their alignment and deposition process to 1 inch by 1 inch wafers, they’re working on scaling the process up for commercial production.

Arnold says it’s exciting to finally reach the point where researchers can exploit the nanotubes to attain performance gains in actual technologies.

“There has been a lot of hype about carbon nanotubes that hasn’t been realized, and that has kind of soured many people’s outlook,” he says. “But we think the hype is deserved. It has just taken decades of work for the materials science to catch up and allow us to effectively harness these materials.”

The researchers have patented their technology through the Wisconsin Alumni Research Foundation.

The researchers in Jonathan Claussen’s lab at Iowa State University (who like to call themselves nanoengineers) have been looking for ways to use graphene and its amazing properties in their sensors and other technologies.

Iowa State engineers are developing real-world, low-cost applications for graphene. CREDIT: Photos by Christopher Gannon/Iowa State University.

Iowa State engineers are developing real-world, low-cost applications for graphene. Credit: Photos by Christopher Gannon/Iowa State University.

Graphene is a wonder material: The carbon honeycomb is just an atom thick. It’s great at conducting electricity and heat; it’s strong and stable. But researchers have struggled to move beyond tiny lab samples for studying its material properties to larger pieces for real-world applications.

Recent projects that used inkjet printers to print multi-layer graphene circuits and electrodes had the engineers thinking about using it for flexible, wearable and low-cost electronics. For example, “Could we make graphene at scales large enough for glucose sensors?” asked Suprem Das, an Iowa State postdoctoral research associate in mechanical engineering and an associate of the U.S. Department of Energy’s Ames Laboratory.

But there were problems with the existing technology. Once printed, the graphene had to be treated to improve electrical conductivity and device performance. That usually meant high temperatures or chemicals – both could degrade flexible or disposable printing surfaces such as plastic films or even paper.

Das and Claussen came up with the idea of using lasers to treat the graphene. Claussen, an Iowa State assistant professor of mechanical engineering and an Ames Laboratory associate, worked with Gary Cheng, an associate professor at Purdue University’s School of Industrial Engineering, to develop and test the idea.

And it worked: They found treating inkjet-printed, multi-layer graphene electric circuits and electrodes with a pulsed-laser process improves electrical conductivity without damaging paper, polymers or other fragile printing surfaces.

“This creates a way to commercialize and scale-up the manufacturing of graphene,” Claussen said.

The findings are featured on the front cover of the journal Nanoscale‘s issue 35. Claussen and Cheng are lead authors and Das is first author. Additional Iowa State co-authors are Allison Cargill, John Hondred and Shaowei Ding, graduate students in mechanical engineering. Additional Purdue co-authors are Qiong Nian and Mojib Saei, graduate students in industrial engineering.

Two major grants are supporting the project and related research: a three-year grant from the National Institute of Food and Agriculture, U.S. Department of Agriculture, under award number 11901762 and a three-year grant from the Roy J. Carver Charitable Trust. Iowa State’s College of Engineering and department of mechanical engineering are also supporting the research.

The Iowa State Research Foundation Inc. has filed for a patent on the technology.

“The breakthrough of this project is transforming the inkjet-printed graphene into a conductive material capable of being used in new applications,” Claussen said.

Those applications could include sensors with biological applications, energy storage systems, electrical conducting components and even paper-based electronics.

To make all that possible, the engineers developed computer-controlled laser technology that selectively irradiates inkjet-printed graphene oxide. The treatment removes ink binders and reduces graphene oxide to graphene – physically stitching together millions of tiny graphene flakes. The process makes electrical conductivity more than a thousand times better.

“The laser works with a rapid pulse of high-energy photons that do not destroy the graphene or the substrate,” Das said. “They heat locally. They bombard locally. They process locally.”

That localized, laser processing also changes the shape and structure of the printed graphene from a flat surface to one with raised, 3-D nanostructures. The engineers say the 3-D structures are like tiny petals rising from the surface. The rough and ridged structure increases the electrochemical reactivity of the graphene, making it useful for chemical and biological sensors.

All of that, according to Claussen’s team of nanoengineers, could move graphene to commercial applications.

“This work paves the way for not only paper-based electronics with graphene circuits,” the researchers wrote in their paper, “it enables the creation of low-cost and disposable graphene-based electrochemical electrodes for myriad applications including sensors, biosensors, fuel cells and (medical) devices.”

Two scientists at the University of Central Florida have discovered how to get a solid material to act like a liquid without actually turning it into liquid, potentially opening a new world of possibilities for the electronic, optics and computing industries.

When chemistry graduate student Demetrius A. Vazquez-Molina took COF-5, a nano sponge-like, non-flammable manmade material and pressed it into pellets the size of a pinkie nail, he noticed something odd when he looked at its X-ray diffraction pattern. The material’s internal crystal structure arranged in a strange pattern. He took the lab results to his chemistry professor Fernando Uribe-Romo, who suggested he turn the pellets on their side and run the X-ray analysis again.

The result: The crystal structures within the material fell into precise patterns that allow for lithium ions to flow easily – like in a liquid.

The findings, published in the Journal of the American Chemical Society earlier this summer, are significant because a liquid is necessary for some electronics and other energy uses. But using current liquid materials sometimes is problematic.

For example, take lithium-ion batteries. They are among the best batteries on the market, charging everything from phones to hover boards. But they tend to be big and bulky because a liquid must be used within the battery to transfer lithium ions from one side of the battery to the other. This process stores and disperses energy. That reaction creates heat, which has resulted in cell phones exploding, hover boards bursting into flames, and even the grounding of some airplanes a few years ago that relied on lithium batteries for some of its functions.

But if a nontoxic solid could be used instead of a flammable liquid, industries could really change, Uribe-Romo said.

“We need to do a lot more testing, but this has a lot of promise,” he said. “If we could eliminate the need for liquid and use another material that was not flammable, would require less space and less packaging, that could really change things. That would mean less weight and potentially smaller batteries.”

Smaller, nontoxic and nonflammable materials could also mean smaller electronics and the ability to speed up the transfer of information via optics. And that could mean innovations to communication devices, computing power and even energy storage.

“This is really exciting for me,” said Vazquez-Molina who was a pre-med student before taking one of Uribe-Romo’s classes. “I liked chemistry, but until Professor Romo’s class I was getting bored. In his class I learned how to break all the (chemistry) rules. I really fell in love with chemistry then, because it is so intellectually stimulating.”

Uribe-Romo has his high school teacher in Mexico to thank for his passion for chemistry. After finishing his bachelor’s degree at Instituto Tecnológico y de Estudios Superiores de Monterrey in Mexico, Uribe-Romo earned a Ph.D. at the University of California at Los Angeles. He was a postdoctoral associate at Cornell University before joining UCF as an assistant professor in 2013.

The findings were pursued by a team lead by Uribe-Romo in collaboration with scientists at UCLA’s Department of Chemistry and Biochemistry. It’s a partnership the team is pursuing to see if COF-5 is indeed the material that could revolutionize battery and mobile device industries.

Applying an electric field to some materials causes their atoms to “switch” their electric polarization from one direction to another, making one side of the material positive and the other negative. This switching property of “ferroelectric” materials allows them to be used in a wide range of applications. For example, ferroelectric capacitors are used to store binary bits of data in memory devices.

The newly synthesized crystal is ferroelectric above room temperature (a-b, e-f) and turns into "plastic phase", meaning highly deformable, at higher temperature (a to c). The electric polarity of each molecule can be aligned in one direction by applying electric field as it cools (c to e). Credit: Harada J. et al., July 11, 2016, Nature Chemistry, DOI: 10.1038/NCHEM.2567

The newly synthesized crystal is ferroelectric above room temperature (a-b, e-f) and turns into “plastic phase”, meaning highly deformable, at higher temperature (a to c). The electric polarity of each molecule can be aligned in one direction by applying electric field as it cools (c to e). Credit: Harada J. et al., July 11, 2016, Nature Chemistry, DOI: 10.1038/NCHEM.2567

Researchers at Japan’s Hokkaido University have developed a novel ferroelectric plastic crystal that could accelerate the development of more flexible, cost-efficient and less toxic ferroelectrics than those currently in use.

The crystal is ferroelectric above room temperature, then turns into a plastic, more pliable phase at higher temperatures. At the higher temperatures, the molecules in the crystal have randomly different polarity axes, but they can be aligned in one direction by applying an electric field as the crystal cools, bringing it back to a ferroelectric state.

Being able to control the polarity in this manner addresses a major challenge previously faced by researchers working with organic compound-based ferroelectric crystals. These are less symmetric than inorganic crystals, and can thus be polarized only in one direction leading to a very weak overall polarization of randomly oriented crystals.

A distinct advantage of this particular crystal is its transition to a plastic state when heat is applied. This plasticity – as opposed to fracturing that occurs in regular organic and inorganic crystals when a mechanical stress is applied – makes it extremely advantageous for use as a thin ferroelectric film in devices, such as non-volatile ferroelectric random-access memory devices, which maintain memory when the power is turned off.

Exploring crystals composed of molecules similar to quinuclidine could lead to the discovery of more ferroelectric crystals, write the researchers in their paper published in the journal Nature Chemistry. Chemical modifications of the molecules’ constituent ions could also improve their performance, the researchers add.

A powerful new material developed by Northwestern University chemist William Dichtel and his research team could one day speed up the charging process of electric cars and help increase their driving range.

An electric car currently relies on a complex interplay of both batteries and supercapacitors to provide the energy it needs to go places, but that could change.

“Our material combines the best of both worlds — the ability to store large amounts of electrical energy or charge, like a battery, and the ability to charge and discharge rapidly, like a supercapacitor,” said Dichtel, a pioneer in the young research field of covalent organic frameworks (COFs).

Dichtel and his research team have combined a COF — a strong, stiff polymer with an abundance of tiny pores suitable for storing energy — with a very conductive material to create the first modified redox-active COF that closes the gap with other older porous carbon-based electrodes.

“COFs are beautiful structures with a lot of promise, but their conductivity is limited,” Dichtel said. “That’s the problem we are addressing here. By modifying them — by adding the attribute they lack — we can start to use COFs in a practical way.”

And modified COFs are commercially attractive: COFs are made of inexpensive, readily available materials, while carbon-based materials are expensive to process and mass-produce.

Dichtel, the Robert L. Letsinger Professor of Chemistry at the Weinberg College of Arts and Sciences, is presenting his team’s findings today (Aug. 24) at the American Chemical Society (ACS) National Meeting in Philadelphia. Also today, a paper by Dichtel and co-authors from Northwestern and Cornell University was published by the journal ACS Central Science.

To demonstrate the new material’s capabilities, the researchers built a coin-cell battery prototype device capable of powering a light-emitting diode for 30 seconds.

The material has outstanding stability, capable of 10,000 charge/discharge cycles, the researchers report. They also performed extensive additional experiments to understand how the COF and the conducting polymer, called poly(3,4-ethylenedioxythiophene) or PEDOT, work together to store electrical energy.

Dichtel and his team made the material on an electrode surface. Two organic molecules self-assembled and condensed into a honeycomb-like grid, one 2-D layer stacked on top of the other. Into the grid’s holes, or pores, the researchers deposited the conducting polymer.

Each pore is only 2.3 nanometers wide, but the COF is full of these useful pores, creating a lot of surface area in a very small space. A small amount of the fluffy COF powder, just enough to fill a shot glass and weighing the same as a dollar bill, has the surface area of an Olympic swimming pool.

The modified COF showed a dramatic improvement in its ability to both store energy and to rapidly charge and discharge the device. The material can store roughly 10 times more electrical energy than the unmodified COF, and it can get the electrical charge in and out of the device 10 to 15 times faster.

“It was pretty amazing to see this performance gain,” Dichtel said. “This research will guide us as we investigate other modified COFs and work to find the best materials for creating new electrical energy storage devices.”

If you’ve never had the plumber to your house, you’ve been lucky. Pipes can burst due to a catastrophic event, like subzero temperatures, or time and use can take a toll, wearing away at the materials with small dings and dents that aren’t evident until it’s too late.

But what if there were a way to identify those small, often microscopic failures before you had to call for help?

The Autonomous Materials Systems (AMS) Group at the Beckman Institute for Advanced Science and Technology has recently found a new way to identify microscopic damage in polymers and composite materials before total failure occurs.

Colorless, non-fluorescent microcapsules use a type of fluorescence called aggregation-induced emission (AIE), which becomes brighter as the indicator solidifies from solution and is visible under ultraviolet (UV) light. Credit: Autonomous Materials Systems Group, Beckman Institute for Advanced Science and Technology, University of Illinois

Colorless, non-fluorescent microcapsules use a type of fluorescence called aggregation-induced emission (AIE), which becomes brighter as the indicator solidifies from solution and is visible under ultraviolet (UV) light.
Credit: Autonomous Materials Systems Group, Beckman Institute for Advanced Science and Technology, University of Illinois

“Autonomous indication of small cracks has exciting potential to make structures safer and more reliable by giving time to intervene and repair or replace the damaged region prior to catastrophic failure,” said Nancy Sottos, professor of materials science and engineering, and one of the authors of “A Robust Damage-Reporting Strategy for Polymeric Materials Enabled by Aggression-Induced Emission,” recently published in ACS Central Science. The paper is part of a research project selected as a finalist for the Institution of Chemical Engineers (IChemE) Global Awards 2016.

The researchers sequestered fluids containing turn-on fluorescence indicators in microcapsules, and then incorporated them into polymeric materials.

“We’ve developed microcapsules that are colorless and non-fluorescent when intact,” said Maxwell Robb, Beckman Institute Postdoctoral Fellow and a lead author on the paper. “We can embed them into materials, and when damage occurs, the microcapsules will release their payload and become fluorescent, indicating that repair is needed.”

Previous work led by Wenle Li, a postdoctoral research associate and co-first author of the study, had investigated another type of indicator within microcapsules, which underwent a chemical reaction upon release to produce a color change. However, the nature of the chemical reaction limited the system to a narrow range of materials.

The new method uses a type of fluorescence called aggregation-induced emission (AIE), which becomes brighter as the indicator solidifies from solution and is visible under ultraviolet (UV) light. The unique mechanism of indication, which relies on a physical change of state instead of a chemical reaction, enables excellent performance in a wide variety of materials and for visualizing different types of damage.

“The elegance of this system lies in its versatility as well as its sensitivity,” said Li. “We can easily visualize a fluorescence signal resulting from mechanical damage as small as two microns.”

The research is funded by BP, which is interested in coating oil and gas pipelines with a polymer coating that will be able to indicate damage. The goal is to target damage at its earliest stage to prevent further deterioration, improve safety and reliability, and reduce life cycle costs associated with regular maintenance and inspection.

Using instruments in Beckman’s Microscopy Suite, the group was able to study the microcapsules and coatings of various materials, image them, and correlate the fluorescence signals to 3D structures of the damaged coatings.

“This is incredibly interdisciplinary work,” said Robb. “Having knowledge about the aggregation-induced emission effect, and being able to design the chemistry of the microcapsule system was the starting point. Then there is the actual application of this technology into materials and coatings, which relies heavily on the expertise within materials science and engineering.”

The AMS Group includes Sottos, Jeffrey Moore, professor of chemistry, and Scott White, professor of aerospace engineering, who also co-authored the study. Their work has led to new discoveries in self-detecting and self-healing materials.

“To impact the coatings industry, materials with self-reporting capability must meet a few criteria: they must be simple, not change the way the materials are traditionally applied, and perform just as well,” said Moore. “Our approach hits this target – the new self-reporting function is realized by just one simple additive.”

The next steps for this research are to combine damage indication with self-healing materials.

“If you could couple this technology that lets you know that damage has occurred with a self-healing material that tells you when the damage has been healed, it could be really powerful,” said Robb.

“We have developed both turn-on fluorescence and color-changing indication systems. Our vision is to combine these multi-channel strategies to enable materials that monitor their mechanical integrity throughout the entire polymer lifecycle,” said Li.

Imagine an electronic newspaper that you could roll up and spill your coffee on, even as it updated itself before your eyes.

It’s an example of the technological revolution that has been waiting to happen, except for one major problem that, until now, scientists have not been able to resolve.

Researchers at McMaster University have cleared that obstacle by developing a new way to purify carbon nanotubes – the smaller, nimbler semiconductors that are expected to replace silicon within computer chips and a wide array of electronics.

Artistic rendition of a metallic carbon nanotube being pulled into solution, in analogy to the work described by the Adronov group. Credit: Alex Adronov, McMaster University

Artistic rendition of a metallic carbon nanotube being pulled into solution, in analogy to the work described by the Adronov group. Credit: Alex Adronov, McMaster University

“Once we have a reliable source of pure nanotubes that are not very expensive, a lot can happen very quickly,” says Alex Adronov, a professor of Chemistry at McMaster whose research team has developed a new and potentially cost-efficient way to purify carbon nanotubes.

Carbon nanotubes – hair-like structures that are one billionth of a metre in diameter but thousands of times longer – are tiny, flexible conductive nano-scale materials, expected to revolutionize computers and electronics by replacing much larger silicon-based chips.

A major problem standing in the way of the new technology, however, has been untangling metallic and semiconducting carbon nanotubes, since both are created simultaneously in the process of producing the microscopic structures, which typically involves heating carbon-based gases to a point where mixed clusters of nanotubes form spontaneously as black soot.

Only pure semiconducting or metallic carbon nanotubes are effective in device applications, but efficiently isolating them has proven to be a challenging problem to overcome. Even when the nanotube soot is ground down, semiconducting and metallic nanotubes are knotted together within each grain of powder. Both components are valuable, but only when separated.

Researchers around the world have spent years trying to find effective and efficient ways to isolate carbon nanotubes and unleash their value.

While previous researchers had created polymers that could allow semiconducting carbon nanotubes to be dissolved and washed away, leaving metallic nanotubes behind, there was no such process for doing the opposite: dispersing the metallic nanotubes and leaving behind the semiconducting structures.

Now, Adronov’s research group has managed to reverse the electronic characteristics of a polymer known to disperse semiconducting nanotubes – while leaving the rest of the polymer’s structure intact. By so doing, they have reversed the process, leaving the semiconducting nanotubes behind while making it possible to disperse the metallic nanotubes.

The researchers worked closely with experts and equipment from McMaster’s Faculty of Engineering and the Canada Centre for Electron Microscopy, located on the university’s campus.

“There aren’t many places in the world where you can to this type of interdisciplinary work,” Adronov says.

The next step, he explains, is for his team or other researchers to exploit the discovery by finding a way to develop even more efficient polymers and scale up the process for commercial production.

One of the most critical issues the United States faces today is preventing terrorists from smuggling nuclear weapons into its ports. To this end, the U.S. Security and Accountability for Every Port Act mandates that all overseas cargo containers be scanned for possible nuclear materials or weapons.

Detecting neutron signals is an effective method to identify nuclear weapons and special nuclear materials. Helium-3 gas is used within detectors deployed in ports for this purpose.

The catch? While helium-3 gas works well for neutron detection, it’s extremely rare on Earth. Intense demand for helium-3 gas detectors has nearly depleted the supply, most of which was generated during the period of nuclear weapons production during the past 50 years. It isn’t easy to reproduce, and the scarcity of helium-3 gas has caused its cost to skyrocket recently — making it impossible to deploy enough neutron detectors to fulfill the requirement to scan all incoming overseas cargo containers.

Helium-4 is a more abundant form of helium gas, which is much less expensive, but can’t be used for neutron detection because it doesn’t interact with neutrons.

A group of Texas Tech University researchers led by Professors Hongxing Jiang and Jingyu Lin report this week in Applied Physics Letters, from AIP Publishing, that they have developed an alternative material — hexagonal boron nitride semiconductors — for neutron detection. This material fulfills many key requirements for helium gas detector replacements and can serve as a low-cost alternative in the future.

The group’s concept was first proposed to the Department of Homeland Security’s Domestic Nuclear Detection Office and received funding from its Academic Research Initiative program six years ago.

By using a 43-micron-thick hexagonal boron-10 enriched nitride layer, the group created a thermal neutron detector with 51.4 percent detection efficiency, which is a record high for semiconductor thermal neutron detectors.

“Higher detection efficiency is anticipated by further increasing the material thickness and improving materials quality,” explained Professor Jiang, Nanophotonics Center and Electrical & Computer Engineering, Whitacre College of Engineering, Texas Tech University.

“Our approach of using hexagonal boron nitride semiconductors for neutron detection centers on the fact that its boron-10 isotope has a very large interaction probability with thermal neutrons,” Jiang continued. “This makes it possible to create high-efficiency neutron detectors with relatively thin hexagonal boron nitride layers. And the very large energy bandgap of this semiconductor — 6.5 eV — gives these detectors inherently low leakage current densities.”

The key significance of the group’s work? This is a completely new material and technology that offers many advantages.

“Compared to helium gas detectors, boron nitride technology improves the performance of neutron detectors in terms of efficiency, sensitivity, ruggedness, versatile form factor, compactness, lightweight, no pressurization … and it’s inexpensive,” Jiang said.

This means that the material has the potential to revolutionize neutron detector technologies.

“Beyond special nuclear materials and weapons detection, solid-state neutron detectors also have medical, health, military, environment, and industrial applications,” he added. “The material also has applications in deep ultraviolet photonics and two-dimensional heterostructures. With the successful demonstration of high-efficiency neutron detectors, we expect it to perform well for other future applications.”

The main innovation behind this new type of neutron detector was developing hexagonal boron nitride with epitaxial layers of sufficient thickness — which previously didn’t exist.

“It took our group six years to find ways to produce this new material with a sufficient thickness and crystalline quality for neutron detection,” Jiang noted.

Based on their experience working with III-nitride wide bandgap semiconductors, the group knew at the outset that producing a material with high crystalline quality would be difficult.

“It’s surprising to us that the detector performs so well, despite the fact that there’s still a little room for improvement in terms of material quality,” he said.

One of the most important impacts of the group’s work is that “this new material and its potential should begin to be recognized by the semiconductor materials and radiation detection communities,” Jiang added.

Now that the group has solved the problem of producing hexagonal boron nitride with sufficient thickness, as well as crystalline quality to enable the demonstration of neutron detectors with high efficiency, the next step is to demonstrate high-sensitivity of large-size detectors.

“These devices must be capable of detecting nuclear weapons from distances tens of meters away, which requires large-size detectors,” Jiang added. “There are technical challenges to overcome, but we’re working toward this goal.”

Recent breakthroughs in materials engineering of low-resistance W barriers/liners and bulk fill are making it possible to extend W use to next-generation devices.

BY JONATHAN BAKKE, Applied Materials, Santa Clara, CA

Tungsten (W), with its low resistivity and minimal electro-migration, has long been used for a variety of applications in fabricating semiconductor devices. For instance, it is used for logic contact, local interconnect (LIC), and metal gate (MG) fill as well as DRAM buried word line and contact and 3D NAND MG and contact. Sustained scaling, however, is posing challenges to its continued use with conventional process flows. Interconnect dimensions have shrunk to the point at which contact resistance is becoming an obstacle to realizing optimum transistor performance; fill integrity degrades as aspect ratios and the degree of re-entrance increase, making it difficult to ensure high-quality metallization.

At earlier nodes, larger dimensions made W fill possible using conformal CVD deposition. Now, overhang around the tops of ultra-small openings or bowing from the interconnect etch open preclude the conformal process from completely filling features without voids, while center seams are an inevitable result of conformal deposition, even in the absence of voids. These attributes render extremely small features vulnerable to breach during CMP, causing high resistance or complete failure of an inter- connect. High feature densities and lack of via redundancy in advanced chip designs mean that a single void can cause complete device failure and significant yield loss.

Fortunately, recent breakthroughs in materials engineering of low-resistance W barriers/liners and bulk fill are overcoming these limitations and making it possible to extend W use to next-generation devices. The former lower resistance by simplifying fill film requirements and enlarging the volume available for W fill; the latter eliminates undesirable seams to create more robust structures.

Low-resistance liners

To date, high-resistivity TiN has been predominantly used as an adhesion layer for CVD W and to block fluorine penetration during the bulk fill process. W does not grow directly on TiN; thus, it requires deposition of a nucleation layer before the fill step. As logic devices scale through the 10 nm node and beyond, the maximum critical dimension (CD) of the LIC willbe

Metal-organic deposition of thin W-based films offers an ideal solution, because it can eliminate high-resistance liners and nucle- ation layers while maintaining adhesion and fluorine-barrier properties equiv- alent to those of the current process flow. A new W liner has been developed that lowers line resistance for further device scaling: plasma-enhanced (PE) CVD W that nucleates on metal and oxides.

The PECVD W film is produced using a specialized chemical in the presence of reactive plasma that breaks down the ligands. The film composition is primarily W, and the atoms from the decomposed ligands are bonded to the W. The amorphous character of the film and the dopants in it from the ligand lead to good adhesion to dielec- trics and fluorine barrier properties in the 20-30Å range.

FIGURE 1 shows a simulation of a contact plug in the 4-30nm range. The model contains parallel and series resistors for the plug and through resistance. Features are assumed to be straight wall trenches. Resistance of 12 μΩ*cm is used for W at all thicknesses, which under-estimates the benefit of PECVD W. Scattering at film interfaces is not taken into account. The inflections in the curves (from right to left) occur when a film is removed due to volume constraints. It is clear that the benefit of PECVD W increases exponentially as CDs decrease, especially without the nucleation layer.

FIGURE 1. Plug resistance simulation demonstrates the significant benefit of PECVD W without a nucleation layer.

FIGURE 1. Plug resistance simulation demonstrates the significant benefit of PECVD W without a nucleation layer.

SiO2 trench structures with CDs ranging from 10nm to 150nm and a depth of 100nm were used to investigate W line resistance and evaluate gap-fill performance. As shown in FIGURE 2, line resistance in a ~10 nm CD dropped by nearly 90% compared with the conventional stack.

FIGURE 2. PECVD W plus gap fill reduces line resistance by nearly 90% over the conventional stack. The inset TEM shows conformal gap fill and CMP integration for PECVD W.

FIGURE 2. PECVD W plus gap fill reduces line resistance by nearly 90% over the conventional stack. The inset TEM shows conformal gap fill and CMP integration for PECVD W.

Seam-suppressed gap fill

Until now, feature dimensions have made W fill integration possible using nucleation followed by conformal CVD deposition – which always leaves a seam in features. At CDs

A new approach employs a unique, “selective” suppression mechanism that results in a bottom-up fill free of seams or voids. Pre-treating the nucleation layer creates preferred W growth from the bottom of the structure upwards and less on the field, minimizing the likelihood of void-creating pinch-off and seams (FIGURE 3). Experiments showed the process to be successful on structures with CDs ranging from 10nm to 150nm.

FIGURE 3. a.Cross-sectional TEM image of SSW partial fill of 30nm CD,100nm deep trench pattern with overhang created byAr sputter and PVD Ti. (b) TEM image of seamless SSW fill of the same structure. (c) TEM image of standard CVD W gap fill with seam.

FIGURE 3. a.Cross-sectional TEM image of SSW partial fill of 30nm CD,100nm deep trench pattern with overhang created byAr sputter and PVD Ti. (b) TEM image of seamless SSW fill of the same structure. (c) TEM image of standard CVD W gap fill with seam.

Electrical tests confirmed that SSW lowered line resistance compared to that of conventional CVD W (FIGURE 4). Post-CMP defect analysis by top-down view SEM revealed a narrow seam in conventional CVD W after W CMP (FIGURE 5a), while none is visible after SSW fill (FIGURE 5b).

FIGURE 4. Line resistance comparison of SSW and conventional CVD W on 10nm trench.

FIGURE 4. Line resistance comparison of SSW and conventional CVD W on 10nm trench.

Tungsten 5-1

FIGURE 5. Top-down SEM image of a) conventional CVD W process with visible seam in the center of the trench and b) SSW fill on the same structure.

FIGURE 5. Top-down SEM image of a) conventional CVD W process with visible seam in the center of the trench and b) SSW fill on the same structure.

Conclusion

For the next several nodes of logic and memory fabrication, W will remain an important material in interconnect and gate metallization. However, as scaling continues, transi- tions in process flows will be necessary to achieve low contact and line resistance while maintaining gap-fill integrity. A new W-based barrier/liner has been produced through precision materials engineering that improves device performance and integration while simplifying process flows. Similarly, a new SSW gap-fill process increases the volume of W (potentially lowering resistance), creates more robust features for post-fill integration, and relaxes requirements on CMP and dielectric etch steps, thus delivering performance, device design, and yield benefits.

For further detail on the processes presented in this article, see Bakke, J., et al., “Fluorine-Free Tungsten Films as Low Resistance liners for Tungsten Fill Applications” and Kai,W.,etal.,“ImprovingTungstenGap-FillforAdvance Contact Metallization,” presented at the 2016 IEEE Inter- national Interconnect Technology Conference.

Pixelligent Technologies, a developer of high-index advanced materials for solid state lighting and display applications and producer of PixClear products, announced today that it closed $10.4 million in new funding. The round was led by The Abell Foundation, The Bunting Family Office, and David Testa, the former Chief Investment Officer of T. Rowe Price. Funds will be used to complete the installation of additional manufacturing capacity, open new offices in Asia, and continue to drive innovation in lighting, display and optical applications.

To date Pixelligent has raised over $36.0M in equity funding and has been awarded more than $12M in U.S. government grant programs to support the development of its proprietary PixClear products and PixClearProcess. The Pixelligent nanotechnology platform includes proprietary nanocrystal synthesis, capping technology, high volume manufacturing and application engineering that supports ink jet, slot die, UV curing, spray coating, and numerous other manufacturing processes.

“We have clearly established Pixelligent as the leading high-index materials manufacturer for demanding solid state lighting and OLED display applications throughout the world. Pixelligent is partnering with leading advanced materials suppliers to deliver breakthrough performance that currently spans applications in 12 discrete markets including: lighting, displays, printed and flexible electronics, AR/VR, optically clear adhesives, MEMS, gradient index lenses, and others with a combined total over $9B in market opportunities. We have numerous commercial applications currently in the market and expect additional product introductions before the end of 2016,” said Craig Bandes, President & CEO of Pixelligent Technologies.

“We started our partnership with Pixelligent in 2011 when the company relocated to Baltimore City and have seen the company achieve all of their critical technology and manufacturing milestones, while establishing a global brand and presence. Our investment objective is to support leading edge companies that deliver breakthrough technology and products and create jobs in our local community. Pixelligent is at the forefront in delivering on the promise of the nanotechnology revolution. We are proud of what the team at Pixelligent has accomplished to date and we look forward to their continued growth and success,” said Eileen O’Rourke, CFO of The Abell Foundation.