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A research collaboration between Osaka University and the Nara Institute of Science and Technology for the first time used scanning tunneling microscopy (STM) to create images of atomically flat side-surfaces of 3D silicon crystals. This work helps semiconductor manufacturers continue to innovate while producing smaller, faster, and more energy-efficient computer chips for computers and smartphones.

Spatial-derivative STM images with 200x200 nm^2 at Vs = +1.5 V. Flat terraces become brighter and edges darker. The downstairs direction runs from left ((110) top-surface) to right ((-1-10) back-surface). Credit: Osaka University

Spatial-derivative STM images with 200×200 nm^2 at Vs = +1.5 V. Flat terraces become brighter and edges darker. The downstairs direction runs from left ((110) top-surface) to right ((-1-10) back-surface). Credit: Osaka University

Our computers and smartphones each are loaded with millions of tiny transistors. The processing speed of these devices has increased dramatically over time as the number of transistors that can fit on a single computer chip continues to increase. Based on Moore’s Law, the number of transistors per chip will double about every 2 years, and in this area it seems to be holding up. To keep up this pace of rapid innovation, computer manufacturers are continually on the lookout for new methods to make each transistor ever smaller.

Current microprocessors are made by adding patterns of circuits to flat silicon wafers. A novel way to cram more transistors in the same space is to fabricate 3D-structures. Fin-type field effect transistors (FETs) are named as such because they have fin-like silicon structures that extend into the air, off the surface of the chip. However, this new method requires a silicon crystal with a perfectly flat top and side-surfaces, instead of just the top surface, as with current devices. Designing the next generation of chips will require new knowledge of the atomic structures of the side-surfaces.

Now, researchers at Osaka University and the Nara Institute of Science and Technology report that they have used STM to image the side-surface of a silicon crystal for the first time. STM is a powerful technique that allows the locations of the individual silicon atoms to be seen. By passing a sharp tip very close to the sample, electrons can jump across the gap and create an electrical current. The microscope monitored this current, and determined the location of the atoms in the sample.

“Our study is a big first step toward the atomically resolved evaluation of transistors designed to have 3D-shapes,” study coauthor Azusa Hattori says.

To make the side-surfaces as smooth as possible, the researchers first treated the crystals with a process called reactive ion etching. Coauthor Hidekazu Tanaka says, “Our ability to directly look at the side-surfaces using STM proves that we can make artificial 3D structures with near-perfect atomic surface ordering.”

The same electrostatic charge that can make hair stand on end and attach balloons to clothing could be an efficient way to drive atomically thin electronic memory devices of the future, according to a new study led by researchers at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab).

In a study published today in the journal Nature, scientists have found a way to reversibly change the atomic structure of a 2-D material by injecting, or “doping,” it with electrons. The process uses far less energy than current methods for changing the configuration of a material’s structure.

Schematic shows the configuration for structural phase transition on a molybdenum ditelluride monolayer (MoTe2, shown as yellow and blue spheres), which is anchored by a metal electrodes (top gate and ground). The ionic liquid covering the monolayer and electrodes enables a high density of electrons to populate the monolayer, leading to changes in the structural lattice from a hexagonal (2H) to monoclinic (1T') pattern. Credit: Ying Wang/Berkeley Lab

Schematic shows the configuration for structural phase transition on a molybdenum ditelluride monolayer (MoTe2, shown as yellow and blue spheres), which is anchored by a metal electrodes (top gate and ground). The ionic liquid covering the monolayer and electrodes enables a high density of electrons to populate the monolayer, leading to changes in the structural lattice from a hexagonal (2H) to monoclinic (1T’) pattern. Credit: Ying Wang/Berkeley Lab

“We show, for the first time, that it is possible to inject electrons to drive structural phase changes in materials,” said study principal investigator Xiang Zhang, senior faculty scientist at Berkeley Lab’s Materials Sciences Division and a professor at UC Berkeley. “By adding electrons into a material, the overall energy goes up and will tip off the balance, resulting in the atomic structure re-arranging to a new pattern that is more stable. Such electron doping-driven structural phase transitions at the 2-D limit is not only important in fundamental physics; it also opens the door for new electronic memory and low-power switching in the next generation of ultra-thin devices.”

Switching a material’s structural configuration from one phase to another is the fundamental, binary characteristic that underlies today’s digital circuitry. Electronic components capable of this phase transition have shrunk down to paper-thin sizes, but they are still considered to be bulk, 3-D layers by scientists. By comparison, 2-D monolayer materials are composed of a single layer of atoms or molecules whose thickness is 100,000 times as small as a human hair.

“The idea of electron doping to alter a material’s atomic structure is unique to 2-D materials, which are much more electrically tunable compared with 3-D bulk materials,” said study co-lead author Jun Xiao, a graduate student in Zhang’s lab.

The classic approach to driving the structural transition of materials involves heating to above 500 degrees Celsius. Such methods are energy-intensive and not feasible for practical applications. In addition, the excess heat can significantly reduce the life span of components in integrated circuits.

A number of research groups have also investigated the use of chemicals to alter the configuration of atoms in semiconductor materials, but that process is still difficult to control and has not been widely adopted by industry.

“Here we use electrostatic doping to control the atomic configuration of a two-dimensional material,” said study co-lead author Ying Wang, another graduate student in Zhang’s lab. “Compared to the use of chemicals, our method is reversible and free of impurities. It has greater potential for integration into the manufacturing of cell phones, computers and other electronic devices.”

The researchers used molybdenum ditelluride (MoTe2), a typical 2-D semiconductor, and coated it with an ionic liquid (DEME-TFSI), which has an ultra-high capacitance, or ability to store electric charges. The layer of ionic liquid allowed the researchers to inject the semiconductor with electrons at a density of a hundred trillion to a quadrillion per square centimeter. It is an electron density that is one to two orders higher in magnitude than what could be achieved in 3-D bulk materials, the researchers said.

Through spectroscopic analysis, the researchers determined that the injection of electrons changed the atoms’ arrangement of the molybdenum ditelluride from a hexagonal shape to one that is monoclinic, which has more of a slanted cuboid shape. Once the electrons were retracted, the crystal structure returned to its original hexagonal pattern, showing that the phase transition is reversible. Moreover, these two types of atom arrangements have very different symmetries, providing a large contrast for applications in optical components.

“Such an atomically thin device could have dual functions, serving simultaneously as optical or electrical transistors, and hence broaden the functionalities of the electronics used in our daily lives,” said Wang.

A team led by Cory Dean, assistant professor of physics at Columbia University, and James Hone, Wang Fong-Jen Professor of Mechanical Engineering at Columbia Engineering, has definitively observed an intensely studied anomaly in condensed matter physics—the even-denominator fractional quantum Hall (FQH) state—via transport measurement in bilayer graphene. The study is published online in Science (October 6 issue).

“Observing the 5/2 state in any system is a remarkable scientific opportunity, since it encompasses some of the most perplexing concepts in modern condensed matter physics, such as emergence, quasi-particle formation, quantization, and even superconductivity,” Dean says. “Our observation that, in bilayer graphene, the 5/2 state survives to much higher temperatures than previously thought possible not only allows us to study this phenomenon in new ways, but also shifts our view of the FQH state from being largely a scientific curiosity to now having great potential for real-world applications, particularly in quantum computing.”

First discovered in the 1980s in gallium arsenide (GaAs) heterostructures, the 5/2 fractional quantum hall state remains the singular exception to the otherwise strict rule that says fractional quantum hall states can only exist with odd denominators. Soon after the discovery, theoretical work suggested that this state could represent an exotic type of superconductor, notable in part for the possibility that such a phase could enable a fundamentally new approach to quantum computation. However, confirmation of these theories has remained elusive, largely due to the fragile nature of the state; in GaAs it is observable only in the highest quality samples and even then appearing only at milikelvin temperaures (as much as 10,000 times colder than the freezing point of water).

The Columbia team has now observed this same state in bilayer graphene and appearing at much higher temperatures¾reaching several Kelvin. “While it’s still 100 times colder than the freezing point of water, seeing the even-denominator state at these temperatures opens the door to a whole new suite of experimental tools that previously were unthinkable,” says Dean. “After several decades of effort by researchers all over the world, we may finally be close to solving the mystery of the 5/2.”

One of the outstanding problems in the field of modern condensed matter physics is understanding the phenomenon of “emergence,” the result of a large collection of quantum particles behaving in concert due to interactions between the particles and giving rise to new characteristics that are not a feature of the individual parts. For instance, in superconductors, a large number of electrons all collapse to a single quantum state, which can then propagate through a metal without any energy loss. The fractional quantum Hall effect is another state in which electrons collude with one another, in the presence of a magnetic field, resulting in quasiparticles with potentially exotic quantum properties.

Very difficult to predict theoretically, emergence often challenges our foundational understanding of how particles behave. For example, since any two electrons have the same charge, we think of electrons as objects that want to repel each other. However, in a superconducting metal, electrons unexpectedly pair up, forming a new object known as a cooper pair. Individual electrons scatter when moving through a metal, giving rise to resistance, but spontaneously formed cooper pairs behave collectively in such a way that they move through the material with no resistance at all.

“Think of trying to make your way through a crowd at a rock concert where everyone is dancing with a lot of energy and constantly bumping into you, compared to a ballroom dance floor where pairs of dancers are all moving in the same, carefully choreographed way, and it is easy to avoid each other,” says Dean. “One of the reasons that makes the even-denominator fractional quantum Hall effect so fascinating is that its origin is believed to be very similar to that of a superconductor, but, instead of simply forming cooper pairs, an entirely new kind of quantum particle emerges.”

According to quantum mechanics, elementary particles fall into two categories, Fermions and Bosons, and behave in very different ways. Two Fermions, such as electrons, cannot occupy the same state, which is why, for example, the electrons in atoms fill successive orbitals. Bosons, such as photons, or particles of light, can occupy the same state, allowing them to act coherently as in the light emission from a laser. When two identical particles are interchanged, the quantum mechanical wave-function describing their combined state is multiplied by a phase factor of 1 for Bosons, and -1 for Fermions.

Soon after the discovery of the fractional quantum hall effect, it was suggested on theoretical grounds that the quasiparticles associated with this state behave neither as Bosons nor Fermions but instead what is called an anyon: when anyon quasiparticles are interchanged, the phase factor is neither 1 nor -1 but is fractional. Despite several decades of effort, there still is no conclusive experimental proof confirming that these quasiparticles are anyons. The 5/2 state¾-a non-abelian anyon¾-is thought to be even more exotic. In theory, non-abelian anyons obey anyonic statistics as in other fractional quantum Hall states, but with the special feature that this phase cannot simply be undone by reversing the process. This inability to simply unwind the phase would make any information stored in the system uniquely stable, and is why many people believe the 5/2 could be a great candidate for quantum computation.

“Demonstration of the predicted 5/2 statistics would represent a tremendous achievement,” says Dean. “In many regards, this would confirm that, by fabricating a material system with just the right thickness and just the right number of electrons, and then applying just the right magnetic fields, we could effectively engineer fundamentally new classes of particles, with properties that do not otherwise exist among known particles naturally found in the universe. We still have no conclusive evidence that the 5/2 state exhibits non-abelian properties, but our discovery of this state in bilayer graphene opens up exciting new opportunities to test these theories.”

Until now, all of those conditions have needed to be not only just right but also extreme. In conventional semi-conductors, the even-denominator states are very difficult to isolate, and exist only for ultra-pure materials, at extremely low temperatures and high magnetic fields. While certain features of the state have been observable devising experiments that could investigate the state without destroying it, has been challenging.

“We needed a new platform,” says Hone. “With the successful isolation of graphene, these atomically thin layers of carbon atoms emerged as a promising platform for the study of electrons in 2D in general. One of the keys is that electrons in graphene interact even more strongly than in conventional 2D electron systems, theoretically making effects such as the even-denominator state even more robust. But while there have been predictions that bilayer graphene could host the long-sought even-denominator states, at higher temperatures than seen before, these predictions have not been realized due mostly the difficulty of making graphene clean enough.”

The Columbia team built on many years of pioneering work to improve the quality of graphene devices, creating ultra-clean devices entirely from atomically flat 2D materials: bilayer graphene for the conducting channel, hexagonal boron nitride as a protective insulator, and graphite used for electrical connections and as a conductive gate to change the charge carrier density in the channel.
A crucial component of the research was having access to the high magnetic fields tools available at the National High Magnetic Field Laboratory in Tallahassee, Fla., a nationally funded user facility with which Hone and Dean have had extensive collaborations. They studied the electrical conduction through their devices under magnetic fields up to 34 Tesla, and achieved clear observation of the even-denominator states.

“By tilting the sample with respect to the magnetic field, we were able to provide new confirmation that this FQH state has many of the properties predicted by theory, such as being spin-polarized,” says Jia Li, the paper’s lead author and post-doctoral researcher working with Dean and Hone. “We also discovered that in bilayer graphene, this state can be manipulated in ways that are not possible in conventional materials.”

The Columbia team’s result, which demonstrates measurement in transport—how electrons flow in the system—is a crucial step forward towards confirming the possible exotic origin of the even denominator state. Their findings are reported contemporaneously with a similar report by a research group at University of California, Santa Barbara. The UCSB study observed the even denominator state by capacitance measurement, which probes the existence of an electrical gap associated with the onset of the state.

The team expects that the robust measurements they have now observed in bilayer graphene will enable new experiments that could definitively prove its non-abelian nature. Once this is established, the team hopes to begin demonstrating computation using the even denominator state.

“For many decades now it has been thought that if the 5/2 state does indeed represent a non-abelian anyon, it could theoretically revolutionize efforts to build a quantum computer,” Dean observes. “In the past, however, the extreme conditions necessary to see the state at all, let alone use it for computation, were always a major concern of practicality. Our results in bilayer graphene suggest that this dream may now not actually be so far from reality.”

Using a simple layer-by-layer coating technique, researchers from the U.S. and Korea have developed a paper-based flexible supercapacitor that could be used to help power wearable devices. The device uses metallic nanoparticles to coat cellulose fibers in the paper, creating supercapacitor electrodes with high energy and power densities – and the best performance so far in a textile-based supercapacitor.

By implanting conductive and charge storage materials in the paper, the technique creates large surface areas that function as current collectors and nanoparticle reservoirs for the electrodes. Testing shows that devices fabricated with the technique can be folded thousands of times without affecting conductivity.

“This type of flexible energy storage device could provide unique opportunities for connectivity among wearable and internet of things devices,” said Seung Woo Lee, an assistant professor in the Woodruff School of Mechanical Engineering at the Georgia Institute of Technology. “We could support an evolution of the most advanced portable electronics. We also have an opportunity to combine this supercapacitor with energy-harvesting devices that could power biomedical sensors, consumer and military electronics, and similar applications.”

The research, done with collaborators at Korea University, was supported by the National Research Foundation of Korea and reported September 14 in the journal Nature Communications.

Yongmin Ko, Minseong Kwon, Wan Ki Bae, Byeongyong Lee, Seung Woo Lee & Jinhan Cho, “Flexible supercapacitor electrodes based on real metal-like cellulose papers,” (Nature Communications, 2017) http://dx.doi.org/10.1038/s41467-017-00550-3

Yongmin Ko, Minseong Kwon, Wan Ki Bae, Byeongyong Lee, Seung Woo Lee & Jinhan Cho, “Flexible supercapacitor electrodes based on real metal-like cellulose papers,” (Nature Communications, 2017) http://dx.doi.org/10.1038/s41467-017-00550-3

Energy storage devices are generally judged on three properties: their energy density, power density and cycling stability. Supercapacitors often have high power density, but low energy density – the amount of energy that can be stored – compared to batteries, which often have the opposite attributes. In developing their new technique, Lee and collaborator Jinhan Cho from the Department of Chemical and Biological Engineering at Korea University set out to boost energy density of the supercapacitors while maintaining their high power output.

They began by dipping paper samples into a beaker of solution containing an amine surfactant material designed to bind the gold nanoparticles to the paper. Next they dipped the paper into a solution containing gold nanoparticles. Because the fibers are porous, the surfactants and nanoparticles enter the fibers and become strongly attached, creating a conformal coating on each fiber.

By repeating the dipping steps, the researchers created a conductive paper on which they added alternating layers of metal oxide energy storage materials such as manganese oxide. The ligand-mediated layer-by-layer approach helped minimize the contact resistance between neighboring metal and/or metal oxide nanoparticles. Using the simple process done at room temperatures, the layers can be built up to provide the desired electrical properties.

“It’s basically a very simple process,” Lee said. “The layer-by-layer process, which we did in alternating beakers, provides a good conformal coating on the cellulose fibers. We can fold the resulting metallized paper and otherwise flex it without damage to the conductivity.”

Though the research involved small samples of paper, the solution-based technique could likely be scaled up using larger tanks or even a spray-on technique. “There should be no limitation on the size of the samples that we could produce,” Lee said. “We just need to establish the optimal layer thickness that provides good conductivity while minimizing the use of the nanoparticles to optimize the tradeoff between cost and performance.”

The researchers demonstrated that their self-assembly technique improves several aspects of the paper supercapacitor, including its areal performance, an important factor for measuring flexible energy-storage electrodes. The maximum power and energy density of the metallic paper-based supercapacitors are estimated to be 15.1 mW/cm2 and 267.3 uW/cm2, respectively, substantially outperforming conventional paper or textile supercapacitors.

The next steps will include testing the technique on flexible fabrics, and developing flexible batteries that could work with the supercapacitors. The researchers used gold nanoparticles because they are easy to work with, but plan to test less expensive metals such as silver and copper to reduce the cost.

During his Ph.D. work, Lee developed the layer-by-layer self-assembly process for energy storage using different materials. With his Korean collaborators, he saw a new opportunity to apply that to flexible and wearable devices with nanoparticles.

“We have nanoscale control over the coating applied to the paper,” he added. “If we increase the number of layers, the performance continues to increase. And it’s all based on ordinary paper.”

In addition to those already mentioned, the research team included Yongmin Ko and Minseong Kwon from Korea University, Wan Ki Bae from the Photoelectronic Hybrids Research Center at the Korea Institute of Science and Technology, and Byeongyong Lee from Georgia Tech.

Physicists at the University of California, Riverside have developed a photodetector – a device that senses light – by combining two distinct inorganic materials and producing quantum mechanical processes that could revolutionize the way solar energy is collected.

Photodetectors are almost ubiquitous, found in cameras, cell phones, remote controls, solar cells, and even the panels of space shuttles. Measuring just microns across, these tiny devices convert light into electrons, whose subsequent movement generates an electronic signal. Increasing the efficiency of light-to-electricity conversion has been one of the primary aims in photodetector construction since their invention.

Lab researchers stacked two atomic layers of tungsten diselenide (WSe2) on a single atomic layer of molybdenum diselenide (MoSe2). Such stacking results in properties vastly different from those of the parent layers, allowing for customized electronic engineering at the tiniest possible scale.

This image shows an energy diagram of the WSe2-MoSe2 device. When a photon (1) strikes the WSe2 layer, it knocks loose an electron (2), freeing it to conduct through the WSe2 (3). At the junction between the two materials, the electron drops down into MoSe2 (4). The energy given off in the drop catapults a second electron from the WSe2 (5) into the MoSe2 (6), where both electrons are free to move and generate electricity. Credit: University Communications, UC Riverside.

This image shows an energy diagram of the WSe2-MoSe2 device. When a photon (1) strikes the WSe2 layer, it knocks loose an electron (2), freeing it to conduct through the WSe2 (3). At the junction between the two materials, the electron drops down into MoSe2 (4). The energy given off in the drop catapults a second electron from the WSe2 (5) into the MoSe2 (6), where both electrons are free to move and generate electricity. Credit: University Communications, UC Riverside.

Within atoms, electrons live in states that determine their energy level. When electrons move from one state to another, they either acquire or lose energy. Above a certain energy level, electrons can move freely. An electron moving into a lower energy state can transfer enough energy to knock loose another electron.

UC Riverside physicists observed that when a photon strikes the WSe2 layer, it knocks loose an electron, freeing it to conduct through the WSe2. At the junction between WSe2 and MoSe2, the electron drops down into MoSe2. The energy given off then catapults a second electron from the WSe2 into the MoSe2, where both electrons become free to move and generate electricity.

“We are seeing a new phenomenon occurring,” said Nathaniel M. Gabor, an assistant professor of physics, who led the research team. “Normally, when an electron jumps between energy states, it wastes energy. In our experiment, the waste energy instead creates another electron, doubling its efficiency. Understanding such processes, together with improved designs that push beyond the theoretical efficiency limits, will have a broad significance with regard to designing new ultra-efficient photovoltaic devices.”

Study results appear today in Nature Nanotechnology.

“The electron in WSe2 that is initially energized by the photon has an energy that is low with respect to WSe2,” said Fatemeh Barati, a graduate student in Gabor’s Quantum Materials Optoelectronics lab and the co-first author of the research paper. “With the application of a small electric field, it transfers to MoSe2, where its energy, with respect to this new material, is high. Meaning, it can now lose energy. This energy is dissipated as kinetic energy that dislodges the additional electron from WSe2.”

In existing solar panels models, one photon can at most generate one electron. In the prototype the researchers developed, one photon can generate two electrons or more through a process called electron multiplication.

The researchers explained that in ultrasmall materials, electrons behave like waves. Though it is unintuitive at large scales, the process of generating two electrons from one photon is perfectly allowable at extremely small length scales. When a material, such as WSe2 or MoSe2, gets thinned down to dimensions nearing the electron’s wavelength, the material’s properties begin to change in inexplicable, unpredictable, and mysterious ways.

“It’s like a wave stuck between walls closing in,” Gabor said. “Quantum mechanically, this changes all the scales. The combination of two different ultra small materials gives rise to an entirely new multiplication process. Two plus two equals five.”

“Ideally, in a solar cell we would want light coming in to turn into several electrons,” said Max Grossnickle, also a graduate student in Gabor’s lab and the research paper’s co-first author. “Our paper shows that this is possible.”

Barati noted that more electrons could be generated also by increasing the temperature of the device.

“We saw a doubling of electrons in our device at 340 degrees Kelvin (150 F), which is slightly above room temperature,” she said. “Few materials show this phenomenon around room temperature. As we increase this temperature, we should see more than a doubling of electrons.”

Electron multiplication in conventional photocell devices typically requires applied voltages of 10-100 volts. To observe the doubling of electrons, the researchers used only 1.2 volts, the typical voltage supplied by an AA battery.

“Such low voltage operation, and therefore low power consumption, may herald a revolutionary direction in photodetector and solar cell material design,” Grossnickle said.

He explained that the efficiency of a photovoltaic device is governed by a simple competition: light energy is either converted into waste heat or useful electronic power.

“Ultrathin materials may tip the balance in this competition by simultaneously limiting heat generation, while increasing electronic power,” he said.

Gabor explained that the quantum mechanical phenomenon his team observed in their device is similar to what occurs when cosmic rays, coming into contact with the Earth’s atmosphere with high kinetic energy, produce an array of new particles.

He speculated that the team’s findings could find applications in unforeseen ways.

“These materials, being only an atom thick, are nearly transparent,” he said. “It’s conceivable that one day we might see them included in paint or in solar cells incorporated into windows. Because these materials are flexible, we can envision their application in wearable photovoltaics, with the materials being integrated into the fabric. We could have, say, a suit that generates power – energy-harvesting technology that would be essentially invisible.”

WIN Semiconductors Corp (TPEx:3105), the world’s largest pure-play compound semiconductor foundry, has released an optimized version of its 0.25µm gallium nitride technology, NP25, that provides superior DC and RF transistor performance. NP25 is a 0.25µm-gate GaN-on-SiC process, and offers users the flexibility to produce both fully integrated amplifier products as well as custom discrete transistors. In production since 2014, the optimized 0.25µm process offers enhanced RF performance with fast switching time, higher gain and increased power added efficiency for demanding power applications through Ku-band

Optimized NP25 transistors exhibit more ideal DC and RF IV characteristics and provide 2 dB higher maximum stable gain. Increased gain leads directly to higher power density and PAE under a range of tuning and bias conditions. This performance-optimized process is fully qualified and supported with a comprehensive design kit and transistor models.

The WIN NP25 technology is fabricated on 4-inch silicon carbide substrates and operates at a drain bias of 28 volts. At 10GHz, NP25 provides saturated output power of 5 watts/mm with 19 dB linear gain and over 65% power added efficiency. These performance metrics make the NP25 process well suited for a variety of high power, broad bandwidth and linear transmit functions in the radar, satellite communications, and wireless infrastructure markets.

EV Group (EVG), a supplier of wafer bonding and lithography equipment for the MEMS, nanotechnology and semiconductor markets, and SwissLitho AG, a manufacturer of novel nanolithography tools, today announced a joint solution to enable the production of 3D structures down to the single-nanometer scale. Initially demonstrated within the “Single Nanometer Manufacturing for Beyond CMOS Devices (SNM)” project funded by the Seventh Framework Program of the European Union, the joint solution involves SwissLitho’s novel NanoFrazor thermal scanning probe lithography system to produce master templates with 3D structures for nanoimprint lithography (NIL), and EVG’s HERCULES NIL system with SmartNIL® technology to replicate those structures at high throughput.

Target applications

EVG and SwissLitho will initially target the joint solution for developing diffractive optical elements and other related optical components that support photonics, data communications, augmented/virtual reality (AR/VR) and other applications, with the potential to expand into biotechnology, nanofluidics and other nanotechnology applications.

As part of the joint solution, SwissLitho’s NanoFrazor system will be used to create imprint masters. Compared to conventional approaches, including electron beam (e-beam) and grayscale lithography, the novel technology has the unique ability to print 3D structures with unsurpassed accuracy. EVG’s HERCULES NIL system will then be used to create working templates for production use, cost-effectively and at high throughput, using the company’s proprietary large-area nanoimprint SmartNIL technology.

Dr. Thomas Glinsner, corporate technology director at EV Group, noted, “SwissLitho’s NanoFrazor solution is highly complementary to EVG’s SmartNIL technology. Together we can offer a complete NIL solution for photonics and other applications involving 3D structure patterning, providing significant opportunity for both companies to expand our customer base and market reach. Our NILPhotonics® Competence Center will be the first point of contact for customers interested in this joint solution, where we will be able to offer feasibility studies, demonstrations and pilot-line production.”

A closer look at the technologies

Thermal scanning probe lithography, the technology behind the NanoFrazor, was invented at IBM Research in Zurich and acquired by SwissLitho AG. This maskless, direct-write lithography approach involves spin-coating a unique, thermally sensitive resist onto the sample surface before patterning. A heated ultra-sharp tip is then used to decompose and evaporate the resist locally while simultaneously inspecting the written nanostructures. The resulting arbitrary resist pattern can then be transferred into almost any other material using lift-off, etching, plating, molding or other methodologies.

“We developed our NanoFrazor line to provide a high-performance, affordable alternative and extension to costly e-beam lithography systems,” said Dr. Felix Holzner, SwissLitho CEO. “The technology allows manufacturing of the master with many ‘levels’ in a single step. In particular, 3D structures with single nanometer accuracy can be produced more easily and with greater fidelity compared to traditional e-beam or grayscale lithography methods. We look forward to working with customers to combine our technology with EVG’s successful SmartNIL process at their NILPhotonics Competence Center in Austria.”

The HERCULES NIL combines EVG’s extensive expertise in NIL, resist processing and high-volume manufacturing solutions into a single integrated system that offers throughput of up to 40 wph for 200-mm wafers. The system’s configurable, modular platform accommodates a variety of imprint materials and structure sizes–giving customers greater flexibility in addressing their manufacturing needs. In addition, its ability to fabricate multiple-use soft stamps helps extend the lifetime of master imprint templates.

Columbia Engineering researchers, led by Harish Krishnaswamy, associate professor of electrical engineering, in collaboration with Professor Andrea Alu’s group from UT-Austin, continue to break new ground in developing magnet-free non-reciprocal components in modern semiconductor processes. At the IEEE International Solid-State Circuits Conference in February, Krishnaswamy’s group unveiled a new device: the first magnet-free non-reciprocal circulator on a silicon chip that operates at millimeter-wave frequencies (frequencies near and above 30GHz). Following up on this work, in a paper (DOI 10.1038/s41467-017-00798-9) published today in Nature Communications, the team demonstrated the physical principles behind the new device.

Most devices are reciprocal: signals travel in the same manner in forward and reverse directions. Nonreciprocal devices, such as circulators, on the other hand, allow forward and reverse signals to traverse different paths and therefore be separated. Traditionally, nonreciprocal devices have been built from special magnetic materials that make them bulky, expensive, and not suitable for consumer wireless electronics.

The team has developed a new way to enable nonreciprocal transmission of waves: using carefully synchronized high-speed transistor switches that route forward and reverse waves differently. In effect, it is similar to two trains approaching each other at super-high speeds that are detoured at the last moment so that they do not collide.

The key advance of this new approach is that it enables circulators to be built in conventional semiconductor chips and operate at millimeter-wave frequencies, enabling full-duplex or two-way wireless. Virtually all electronic devices currently operate in half-duplex mode at lower radio-frequencies (below 6GHz), and consequently, we are rapidly running out of bandwidth. Full-duplex communications, in which a transmitter and a receiver of a transceiver operate simultaneously on the same frequency channel, enables doubling of data capacity within existing bandwidth. Going to the higher mm-wave frequencies, 30GHz and above, opens up new bandwidth that is not currently in use.

“This gives us a lot more real estate,” notes Krishnaswamy, whose Columbia High-Speed and Mm-wave IC (CoSMIC) Lab has been working on silicon radio chips for full duplex communications for several years. His method enables loss-free, compact, and extremely broadband non-reciprocal behavior, theoretically from DC to daylight, that can be used to build a wide range of non-reciprocal components such as isolators, gyrators, and circulators.

“This mm-wave circulator enables mm-wave wireless full-duplex communications, Krishnaswamy adds, “and this could revolutionize emerging 5G cellular networks, wireless links for virtual reality, and automotive radar.”

The implications are enormous. Self-driving cars, for instance, require low-cost fully-integrated millimeter-wave radars. These radars inherently need to be full-duplex, and would work alongside ultra-sound and camera-based sensors in self-driving cars because they can work in all weather conditions and during both night and day. The Columbia Engineering circulator could also be used to build millimeter-wave full-duplex wireless links for VR headsets, which currently rely on a wired connection or tether to the computing device.

“For a smooth wireless VR experience, a huge amount of data has to be sent back and forth between the computer and the headset requiring low-latency bi-directional communication,” says Krishnaswamy. “A mm-wave full-duplex transceiver enabled by our CMOS circulator could be a promising solution as it has the potential to deliver high speed data with low latency, in a small size with low cost.”

The team, funded by sources including the National Science Foundation EFRI program, the DARPA SPAR program, and Texas Instruments, is currently working to improve the linearity and isolation performance of their circulator. Their long-term goal is to build a large-scale mm-wave full-duplex phased array system that uses their circulator.

As microchips become ever smaller and therefore faster, the shrinking size of their copper interconnects leads to increased electrical resistivity at the nanoscale. Finding a solution to this impending technical bottleneck is a major problem for the semiconductor industry.

One promising possibility involves reducing the resistivity size effect by altering the crystalline orientation of interconnect materials. A pair of researchers from Rensselaer Polytechnic Institute conducted electron transport measurements in epitaxial single-crystal layers of tungsten (W) as one such potential interconnect solution. They performed first-principles simulations, finding a definite orientation-dependent effect. The anisotropic resistivity effect they found was most marked between layers with two particular orientations of the lattice structure, namely W(001) and W(110). The work is published this week in the Journal of Applied Physics, from AIP Publishing.

The measured resistivity of epitaxial tungsten layers with (001) and (011) crystal orientation vs thickness d. The tungsten Fermi surface is color coded according to the wave vector dependent Fermi velocity vf. At small thickness, where surface scattering dominates, W(011) is nearly twice as conductive as W(001). Transport simulations indicate that this is due to the anisotropy in the Fermi surface. These results indicate how narrow wires in future computer chips can be made two times more conductive, effectively reducing the required electric power by 50 percent. Credit: Daniel Gall, Rensselaer Polytechnic Institute

The measured resistivity of epitaxial tungsten layers with (001) and (011) crystal orientation vs thickness d. The tungsten Fermi surface is color coded according to the wave vector dependent Fermi velocity vf. At small thickness, where surface scattering dominates, W(011) is nearly twice as conductive as W(001). Transport simulations indicate that this is due to the anisotropy in the Fermi surface. These results indicate how narrow wires in future computer chips can be made two times more conductive, effectively reducing the required electric power by 50 percent. Credit: Daniel Gall, Rensselaer Polytechnic Institute

Author Pengyuan Zheng noted that both the 2013 and 2015 International Technology Roadmap for Semiconductors (ITRS) called for new materials to replace copper as interconnect material to limit resistance increase at reduced scale and minimize both power consumption and signal delay.

In their study, Zheng and co-author Daniel Gall chose tungsten because of its asymmetric Fermi surface — its electron energy structure. This made it a good candidate to demonstrate the anisotropic resistivity effect at the small scales of interest. “The bulk material is completely isotropic, so the resistivity is the same in all directions,” Gall said. “But if we have thin films, then the resistivity varies considerably.”

To test the most promising orientations, the researchers grew epitaxial W(001) and W(110) films on substrates and conducted resistivity measurements of both while immersed in liquid nitrogen at 77 Kelvin (about -196 degrees Celsius) and at room temperature, or 295 Kelvin. “We had roughly a factor of 2 difference in the resistivity between the 001 oriented tungsten and 110 oriented tungsten,” Gall said, but they found considerably smaller resistivity in the W(011) layers.

Although the measured anisotropic resistance effect was in good agreement with what they expected from calculations, the effective mean free path — the average distance electrons can move before scattering against a boundary — in the thin film experiments was much larger than the theoretical value for bulk tungsten.

“An electron travels through a wire on a diagonal, it hits a surface, gets scattered, and then continues traveling until it hits something else, maybe the other side of the wire or a lattice vibration,” Gall said. “But this model looks wrong for small wires.”

The experimenters believe this may be explained by quantum mechanical processes of the electrons that arise at these limited scales. Electrons may be simultaneously touching both sides of the wire or experiencing increased electron-phonon (lattice vibrations) coupling as the layer thickness decreases, phenomena that could affect the search for another metal to replace copper interconnects.

“The envisioned conductivity advantages of rhodium, iridium, and nickel may be smaller than predicted,” said Zheng. Findings like these will prove increasingly important as quantum mechanical scales become more commonplace for the demands of interconnects.

The research team is continuing to explore the anisotropic size effect in other metals with nonspherical Fermi surfaces, such as molybdenum. They found that the orientation of the surface relative to the layer orientation and transport direction is vital, as it determines the actual increase in resistivity at these reduced dimensions.

“The results presented in this paper clearly demonstrate that the correct choice of crystalline orientation has the potential to reduce nanowire resistance,” said Zheng. The importance of the work extends beyond current nanoelectronics to new and developing technologies, including transparent flexible conductors, thermoelectrics and memristors that can potentially store information. “It’s the problem that defines what you can do in the next technology,” Gall said.

Advanced Linear Devices Inc. (ALD), a designer of analog semiconductors, today announced a zero-gate threshold voltage EPAD P-Channel MOSFET Array launching the industry’s first precision sub-threshold circuit. The MOSFET currently has the industry’s lowest operating voltage of 0.2 Volt (V) and current of less than one nano amp (nA). These chips enable the operating regions required for the next generations of development in energy harvesting, Internet of Things (IoT) sensors applications.

The ALD310700A/ALD310700 quad zero threshold MOSFET is intended for the development of small signal precision applications utilizing 0.00V Zero Threshold Voltage. The circuit is ideal for designs requiring very low operating voltages of < +0.5V power supplies. Allowing circuits to operate in the subthreshold region for the first time ever, expands the MOSFET’s operating range into never-before achieved signal levels.

The new MOSFET simplifies circuit biasing schemes and reduces component counts while providing greater precision and sensitivity of sensor applications for IoT engineers. The P-Channel MOSFET can work in conjunction with ALD N-Channel Zero Threshold MOSFET devices in matched sensor applications. The ALD310700A/ALD310700 is the P-channel version of the popular ALD110800A/ALD110800 Precision Zero Threshold N-channel device. Together, these two MOSFET series deliver complementary precision performance. These complementary paired devices enable the design of 0.5% precision current mirrors, current sources, and circuits referenced to power or ground sources including differential amplifier input stages, transmission gates and multiplexers.

Notable device features

  •     Precision offset voltages (VOS): 2mV max.; 10mV max.;
  •     Low minimum operating voltage of less than 0.2V;
  •     Ultra-low minimum operating current of less than 1nA:
  •     Matched and tracked temperature characteristics.

“These devices operate at a point with 100 times lower power than comparable MOSFET arrays. More importantly they enable the next generation of applications at power levels and precision that were impossible until now,” said Robert Chao, President, and Founder, Advanced Linear Devices Inc. “These arrays offer circuit designers working on IoT nodes that require matched sensor activity a method to collect power from supercapacitors or deep cycle batteries.”

As an example, some potential energy harvesting sources, such as thermal electric generators that yield just 0.2V, produce such low levels of energy that they are barely useful for driving power in electronic circuitry. The ALD P-Channel Zero-Threshold (VGS(th)=0.00) EPAD MOSFET arrays can be coupled with a low voltage step-up converter to give low-level power sources a greater range as an energy harvesting source.

This device is available in a quad version and is a member of the EPAD® Matched Pair MOSFET Family. The parts can be ordered directly from ALD or DigiKey and Mouser. Prices start at $2.00 at 100 pieces.