Category Archives: Process Materials

The Semiconductor Industry Association (SIA) today announced worldwide sales of semiconductors reached $31.0 billion for the month of November 2016, an increase of 7.4 percent compared to the November 2015 total of $28.9 billion and 2.0 percent more than the October 2016 total of 30.4 billion. November marked the market’s largest year-to-year growth since January 2015. All monthly sales numbers are compiled by the World Semiconductor Trade Statistics (WSTS) organization and represent a three-month moving average.

“Global semiconductor sales continued to pick up steam in November, increasing at the highest rate in almost two years and nearly pulling even with the year-to-date total from the same point in 2015,” said John Neuffer, president and CEO, Semiconductor Industry Association. “The Chinese market continues to stand out, growing nearly 16 percent year-to-year to lead all regional markets. As 2016 draws to a close, the global semiconductor market appears likely to roughly match annual sales from 2015 and is well-positioned for a solid start to 2017.”

Month-to-month sales increased modestly across all regions: the Americas (3.3 percent), China (2.7 percent), Europe (2.5 percent), Asia Pacific/All Other (0.7 percent), and Japan (0.4 percent). Year-to-year sales increased in China (15.8 percent), Japan (8.2 percent), Asia Pacific/All Other (4.8 percent), and the Americas (3.2 percent), but fell slightly in Europe (-1.6 percent).

From the ground-breaking research breakthroughs to the shifting supplier landscape, these are the stories the Solid State Technology audience read the most during 2016.

#1: Moore’s Law did indeed stop at 28nm

In this follow up, Zvi Or-Bach, president and CEO, MonolithIC 3D, Inc., writes: “As we have predicted two and a half years back, the industry is bifurcating, and just a few products pursue scaling to 7nm while the majority of designs stay on 28nm or older nodes.”

#2: Yield and cost challenges at 16nm and beyond

In February, KLA-Tencor’s Robert Cappel and Cathy Perry-Sullivan wrote of a new 5D solution which utilizes multiple types of metrology systems to identify and control fab-wide sources of pattern variation, with an intelligent analysis system to handle the data being generated.

#3: EUVL: Taking it down to 5nm

The semiconductor industry is nothing if not persistent — it’s been working away at developing extreme ultraviolet lithography (EUVL) for many years, SEMI’s Deb Vogler reported in May.

#4: IBM scientists achieve storage memory breakthrough

For the first time, scientists at IBM Research have demonstrated reliably storing 3 bits of data per cell using a relatively new memory technology known as phase-change memory (PCM).

#5: ams breaks ground on NY wafer fab

In April, ams AG took a step forward in its long-term strategy of increasing manufacturing capacity for its high-performance sensors and sensor solution integrated circuits (ICs), holding a groundbreaking event at the site of its new wafer fabrication plant in Utica, New York.

#6: Foundries takeover 200mm fab capacity by 2018

In January, Christian Dieseldorff of SEMI wrote that a recent Global Fab Outlook report reveals a change in the landscape for 200mm fab capacity.

#7: Equipment spending up: 19 new fabs and lines to start construction

While semiconductor fab equipment spending was off to a slow start in 2016, it was expected to gain momentum through the end of the year. For 2016, 1.5 percent growth over 2015 is expected while 13 percent growth is forecast in 2017.

#8: How finFETs ended the service contract of silicide process

Arabinda Daa, TechInsights, provided a look into how the silicide process has evolved over the years, trying to cope with the progress in scaling technology and why it could no longer be of service to finFET devices.

#9: Five suppliers to hold 41% of global semiconductor marketshare in 2016

In December, IC Insights reported that two years of busy M&A activity had boosted marketshare among top suppliers.

#10: Countdown to Node 5: Moving beyond FinFETs

A forum of industry experts at SEMICON West 2016 discussed the challenges associated with getting from node 10 — which seems set for HVM — to nodes 7 and 5.

BONUS: Most Watched Webcast of 2016: View On Demand Now

IoT Device Trends and Challenges

Presenters: Rajeev Rajan, GLOBALFOUNDRIES, and Uday Tennety, GE Digital

The age of the Internet of Things is upon us, with the expectation that tens of billions of devices will be connected to the internet by 2020. This explosion of devices will make our lives simpler, yet create an array of new challenges and opportunities in the semiconductor industry. At the sensor level, very small, inexpensive, low power devices will be gathering data and communicating with one another and the “cloud.” On the other hand, this will mean huge amounts of small, often unstructured data (such as video) will rippling through the network and the infrastructure. The need to convert that data into “information” will require a massive investment in data centers and leading edge semiconductor technology.

Also, manufacturers seek increased visibility and better insights into the performance of their equipment and assets to minimize failures and reduce downtime. They wish to both cut their costs as well as grow their profits for the organization while ensuring safety for employees, the general public and the environment.

The Industrial Internet is transforming the way people and machines interact by using data and analytics in new ways to drive efficiency gains, accelerate productivity and achieve overall operational excellence. The advent of networked machines with embedded sensors and advanced analytics tools has greatly influenced the industrial ecosystem.

Today, the Industrial Internet allows you to combine data from the equipment sensors, operational data , and analytics to deliver valuable new insights that were never before possible. The results of these powerful analytic insights can be revolutionary for your business by transforming your technological infrastructure, helping reduce unplanned downtime, improve performance and maximize profitability and efficiency.

While solar cell technology is currently being used by many industrial and government entities, it remains prohibitively expensive to many individuals who would like to utilize it. There is a need for cheaper, more efficient solar cells than the traditional silicon solar cells so that more people may have access to this technology. One of the current popular topics in photovoltaic technology research centers around the use of organic-inorganic halide perovskites as solar cells because of the high power conversion efficiency and the low-cost fabrication.

Perovskites are a type of crystalline material that can be formed using a wide variety of different chemical combinations. Of the many different perovskites formulations that can be used in solar cells, the methylammonium lead iodide perovskite (MAPbI3) has been the most widely studied. Solar cells made of this material have been able to reach efficiencies exceeding 20% and are cheaper to manufacture than silicon. However, their short lifespans have prevented them from becoming a viable silicon solar cell alternative. In order to help create better solar cells in the future, members of the Energy Materials and Surface Sciences Unit at the Okinawa Institute of Science and Technology Graduate University (OIST) have been investigating the cause of rapid degradation of these perovskite solar cells (PSCs).

Dr. Shenghao Wang, first author of the publication in Nature Energy, suggests that the degradation of MAPbI3 perovskites may not be a fixable issue. His research reveals that iodide-based perovskites will universally produce a gaseous form of iodine, I2, during operation, which in turn causes further degradation of perovskite. While many researchers have pointed to other sources, such as moisture, atmospheric oxygen and heat as the cause of MAPbI3 degradation, the fact that these solar cells continue to degrade even in the absence of these factors led Wang to believe that a property intrinsic to these PSCs was causing the breakdown of material.

“We found that these PSCs are self-exposed to I2 vapor at the onset of degradation, which led to accelerated decomposition of the MAPbI3 perovskite material into PbI2.” Wang explained, “Because of the relatively high vapor pressure of I2, it can quickly permeate the rest of the perovskite material causing damage of the whole PSC.

This research does not rule out the probability of using perovskites in solar cells, however. Professor Yabing Qi, leader of the Energy Materials and Surface Sciences Unit and corresponding author of this work, expounds “our experimental results strongly suggest that it is necessary to develop new materials with a reduced concentration of iodine or a reinforced structure that can suppress iodine-induced degradation, in addition to desirable photovoltaic properties”.

These researchers at OIST are continuing to investigate different types of perovskite materials in order to find more efficient, cost-effective, and long lifespan perovskite material suitable for use. Their ultimate goal is to make solar cells that are affordable, efficient and stable so that they will be more accessible to the general population. Hopefully, better, cheaper solar cells will entice more people to utilize this technology.

Traditional computer memory, known as DRAM, uses electric fields to store information. In DRAM, the presence or absence of an electric charge is indicated either by number 1 or number 0. Unfortunately, this type of information storage is transient and information is lost when the computer is turned off. Newer types of memory, MRAM and FRAM, use long-lasting ferromagnetism and ferroelectricity to store information. However, no technology thus far combines the two.

To address this challenge, a group of scientists led by Prof. Masaki Azuma from the Laboratory for Materials and Structures at Tokyo Institute of Technology, along with associate Prof. Hajime Hojo at Kyushu University previously at Tokyo Tech, Prof. Ko Mibu at Nagoya Institute of Technology and five other researchers demonstrated the multiferroic nature of a thin film of BiFe1?xCoxO3 (BFCO). Multiferroic materials exhibit both ferromagnetism and ferroelectricity. These are expected to be used as multiple-state memory devices. Furthermore, if the two orders are strongly coupled and the magnetization can be reversed by applying an external electric field, the material should work as a form of low power consumption magnetic memory.

Previous scientists had speculated that ferroelectric BFO thin film, a close relative of BFCO, might be ferromagnetic as well, but they were thwarted by the presence of magnetic impurity. Prof. M. Azuma’s team successfully synthesized pure, thin films of BFCO by using pulsed laser deposition to perform epitaxial growth on a SrTiO3 (STO) substrate. They then conducted a series of tests to show that BFCO is both ferroelectric and ferromagnetic at room temperature. They manipulated the direction of ferroelectric polarization by applying an electric field, and showed that the low-temperature cychloidal spin structure, essentially the same as that of BiFeO3, changes to a collinear one with ferromagnetism at room temperature.

In the future, the scientists hope to realize electrical control of ferromagnetism, which could be applied in low power consumption, non-volatile memory devices.

BY DAN TRACY, Senior Director, Industry Research & Statistics, SEMI

Earlier this year, the Nikkei Asian Review (and other sources) reported the exit of Sumitomo Metal Mining (SMM) from the leadframe business. Leadframe makers headquartered in Japan have long had a prominent share of the global leadframe market, and SMM has been a top supplier for decades. And just several years ago, SMM and Hitachi Cable integrated their leadframe operations. According to the Nikkei Asian Review article, Chang Wah of Electronic Materials (Taiwan) is acquiring some of the SMM leadframe operations, while the article reports that SMM is negotiating the sale of its power semiconductor leadframe business to Jih Lin Technology (Taiwan).

SMM previously exited the bonding wire market in 2012, so this latest announcement reflects the company’s move away from commoditized material segments. According to SEMI’s own analysis, the leadframe market that has seen very little revenue growth over the past 12 years and it is an industry segment with a large supplier base, with over 30 suppliers globally. The basis for competition in the leadframe business has long been, generally  speaking, lowest price and shortest turn- around time. Leadframes are a commodity, though plating and etching capabilities can be a differentiator among the suppliers.

Over the past decade while production facilities in Japan and Southeast Asia closed, many leadframe suppliers shifted production to and increased capabilities in China. Also, China headquartered leadframe suppliers are numerous. These suppliers in China have typically focused on low-lead count and discrete leadframe products for domestic assembly plants, though some companies have expanded capabilities to produce higher value leadframe products. The market share of the China headquartered suppliers has gradually been growing.

Given the pricing pressures in the industry, the trend towards smaller, lower cost leadframes, and the transition to non-leadframe technologies, the long-term outlook for the leadframe market from a business perspective remains very challenging as overall revenue growth is unlikely. Expect further consolidation and the continued emergence of China suppliers in this longstanding packaging material segment.

The SEMI Strategic Material Conference 2017 will be held September 19-20 in San Jose, Calif.

Detection and measurement of fluorocarbons is key to both process control and safety.

BY STEPHEN D. ANDERSON, Sensor Electronics Corp., Savage, MN

Fluorocarbons (FCs) are widely used in the semicon- ductor industry in dry processing applications such as film etching, chemical vapor deposition (CVD), chamber cleaning, and as coolants for semiconductor manufacturing tools. Although toxicity levels are not well established, many FC compounds are considered somewhat toxic. Many FCs are also significant green- house gases, while others are flammable. Detection and measurement of FCs is key to both process control and safety. Examples of commonly used FCs in semiconductor manufacturing are given in Table 1.

Screen Shot 2017-04-21 at 7.52.41 AM

Table Notes:

1. Although the table may list an FC a snon-toxic, many are heavy gases that can cause asphyxiation. Others can cause severe frostbite. Some produce toxic byproducts, such as CO or HF if heated or burned.

2. The naming of organic compounds is often confusing, especially as to whether something is a fluorocarbon (FC) versus a perfluorocarbon (PFC). The latter usually refers to a compound in which all of the hydrogens have been replaced with fluorines. Then, trifluoromethane, for example, is an FC but not a PFC. Note also that the last two table entries don’t follow this rule with respect to their common names.

3. The last two table entries have the same formula, C5HF7 , but much different structures and properties. In fact the first listed is a chain (aliphatic) compound while the second is a ring (aromatic). Always buy using the CAS Number and not the formula.

Why infrared?

Among the available gas detection methods are:

  • Catalytic bead – Generates heat when exposed to combustible gas.
  • Electrochemical cell – Generates electrical current in response to specific gas.
  • Photoionization detector (PID) – Ionizes gas using UV light, measures ion current.
  • Pyrolyzer – Decomposes gas using heat, measures decomposition products.
  • Infrared absorption (IR) – Gas blocks infrared path from source to detector.
  • Metal oxide semiconductor (MOS) – Increases resistance in presence of gas.

To detect FCs, electrochemical cells are eliminated, since none are designed to sense FCs. The PID can also be eliminated because UV light used is not energetic enough to ionize FCs. Catalytic beads are poisoned by halogen compounds and shouldn’t be used.

The Pyrolyzer can measure FCs. But, since it destroys the gas being measured, it cannot distinguish one FC from another. It is also difficult to make the Pyrolyzer explosion-proof or intrinsically safe.

MOS sensors require ambient air to operate, are easily contaminated, and are not specific.
IR alone has the ability to sense a specific FC gas.

The F-C bond

The common feature of FCs is the carbon-fluorine bond. The stretching vibrations of this bond result in infrared (IR) absorption at wavelengths ranging approximately from 7 to 10 micron [1]. The precise wavelength of absorption varies with the overall molecular structure, and is given in TABLE 2.

Screen Shot 2017-04-21 at 7.52.57 AM

For example, measuring the absorption at 10.4 micron can tell us how much C4F6 is present.
An entire branch of chemical study deals with deter- mining structure from absorption bands and vice-versa.

Infrared spectrometers, commonly used in these studies, have the ability to sweep through many wavelengths, looking at absorption versus wavelength.

The technique of gas detection by measuring absorption at one wavelength is termed NDIR (non-dispersive infrared). Non-dispersive means that a particular wavelength is selected using a fixed optical filter, in contrast to the variable mechanical filter used in an IR spectrometer. An NDIR is less flexible than an IR spectrometer, but has the advantage of no moving parts or complex optics, making it ideal for industrial environments.

NDIR

The principle of the NDIR is illustrated in FIGURE 1. The IR spectrum at specific points in the NDIR device is included (spectrum plots 1 – 4). The plots assume that target gas is present and absorbing at 7 micron.

Screen Shot 2017-04-21 at 7.53.06 AM

The IR source (spectrum 1) is a Graybody source (the term applied to a bsource with an emissivity less than 1), which provides IR light across a range of wavelengths from about 1 to 15 micron. A range of wavelengths is desired so that one source can be used to sense a variety of gases.
The light from the source passes through the target gas in a “waveguide” – a reflective chamber open to the atmosphere. The spectrum at the saveguide output (spectrum 2) shows a notch (attenuation) at 7 micron, due to absorption by the target gas.

The light is next applied to optical filters – the wavelength-selective parts of the NDIR. Each optical filter is a narrow bandpass filter, made from a window with various coatings that “create” optical interference except at wavelengths of interest. A typical filter bandwidth is 0.25 micron.
The target filter passes light in the range of about 6.9 micron to 7.1 micron, resulting in spectrum 3, which shows the effects of both the gas and filter. The NDIR usually includes a reference optical filter – a filter that passes light where the target gas is transparent – as a means of maintaining a fixed gain or sensitivity in the presence of varying Source light levels. In the example, the reference filter at 3 micron passes light in the range of about 2.9 micron to 3.1 micron, resulting in spectrum 4 (Note: The filter bandwidths in Fig. 1 are wider than actual for purposes of illustration).

The outputs of the two filters are applied to separate detectors (bolometer or thermopile) and converted to electrical signals.

The IR source is usually driven by a square wave to create a modulation in the IR output. The resulting AC signal allows for easier removal of offset and drift. For the highest possible modulation frequencies, newer MEMS- based sources with extremely small thermal mass, have recently become available.

Note that an NDIR can be created by other means. For example, the source might be an IR laser diode or IR light- emitting-diode. These alternate sources are generally at a disadvantage to the thermal source because of their limited bandwidth. Tunable laser diodes exist but are very expensive at present. Also, at the receiving end of the light path, an NDIR might use one or more photo- diodes rather than thermal detectors.

NDIR features

The light absorption by the target gas is exponentially related to gas concentration. This non-linearity is removed using a microcontroller algorithm that is generally different for each FC and each concentration range.

The NDIR also measures temperature and pressure, allowing for ideal gas law correction, which is used when the measurand is concentration rather than density. Temper- ature measurement also allows for temperature compensation of zero and span.

An NDIR device, as described here, typically measures concentration in the range of 100 ppm to 1000 ppm (by volume) or density in the range of 200 to 2000 milligram / liter. Typical accuracy is 5% of range. Special designs with long optical path length are now available for smaller concentrations.
NDIR maintenance is generally limited to periodic calibration of zero and span, and keeping the optical surfaces free of dust and obstruction. Because the NDIR
measures transmittance, it is inherently fail-safe:

No light = Lots of gas (or obstruction or Source fail) = Alarm

The simple and rugged optical system keeps unit cost and maintenance low, while increasing reliability.

Liquid FCs?

Several FCs are liquids at room temperature or have boiling points close to room temperature. NDIRs are ideal at sensing the liquid vapor, since the NDIR is easily made a part of the calibration setup. All that’s needed to calibrate is a controlled temperature and a table of vapor pressure versus temperature at equilibrium.

Conclusion

The NDIR is proving to be a the instrument of choice in detecting fluorocarbons. Its main features of selectivity, mechanical ruggedness, and operational simplicity are pushing aside other detection methods. Future NDIRs are expected to further this trend with improved ability to pinpoint a given FC gas in the presence of industrial cleaners and other interfering products.

References

1. http://www2.ups.edu/faculty/hanson/Spectroscopy/IR/IRfrequencies.html

STEPHEN ANDERSON, is an engineer at Sensor Electronics Corp., Savage, MN, phone 952-938-9486. He has a B. Chem and MSEE, both from the University of Minnesota and has been active in the process control industry for 35+ years.

2D materials may be brittle


December 21, 2016

This editorial originally appeared on SemiMD.com and was featured in the December 2016 issue of Solid State Technology. 

By Ed Korczynski, Sr. Technical Editor

International researchers using a novel in situ quantitative tensile testing platform have tested the uniform in-plane loading of freestanding membranes of 2D materials inside a scanning electron microscope (SEM). Led by materials researchers at Rice University, the in situ tensile testing reveals the brittle fracture of large-area molybdenum diselenide (MoSe2) crystals and measures their fracture strength for the first time. Borophene monolayers with a wavy topography are more flexible.

A communication to Advanced Materials online (DOI: 10.1002/adma.201604201) titled “Brittle Fracture of 2D MoSe2” by Yinchao Yang et al. disclosed work by researchers from the USA and China led by Department of Materials Science and NanoEngineering Professor Jun Lou at Rice University, Houston, Texas. His team found that MoSe2 is more brittle than expected, and that flaws as small as one missing atom can initiate catastrophic cracking under strain.

“It turns out not all 2D crystals are equal. Graphene is a lot more robust compared with some of the others we’re dealing with right now, like this molybdenum diselenide,” says Lou. “We think it has something to do with defects inherent to these materials. It’s very hard to detect them. Even if a cluster of vacancies makes a bigger hole, it’s difficult to find using any technique.” The team has posted a short animation onlineshowing crack propagation.

2D Materials in a 3D World -222

While all real physical things in our world are inherently built as three-dimensional (3D) structures, a single layer of flat atoms approximates a two-dimensional (2D) structure. Except for special superconducting crystals frozen below the Curie temperature, when electrons flow through 3D materials there are always collisions which increase resistance and heat. However, certain single layers of crystals have atoms aligned such that electron transport is essentially confined within the 2D plane, and those electrons may move “ballistically” without being slowed by collisions.

MoSe2 is a dichalcogenide, a 2D semiconducting material that appears as a graphene-like hexagonal array from above but is actually a sandwich of Mo atoms between two layers of Se chalcogen atoms. MoSe2 is being considered for use as transistors and in next-generation solar cells, photodetectors, and catalysts as well as electronic and optical devices.

The Figure shows the micron-scale sample holder inside a SEM, where natural van der Waals forces held the sample in place on springy cantilever arms that measured the applied stress. Lead-author Yang is a postdoctoral researcher at Rice who developed a new dry-transfer process to exfoliate MoSe2 from the surface upon which it had been grown by chemical vapor deposition (CVD).

The team measured the elastic modulus—the amount of stretching a material can handle and still return to its initial state—of MoSe2 at 177.2 (plus or minus 9.3) gigapascals (GPa). Graphene is more than five times as elastic. The fracture strength—amount of stretching a material can handle before breaking—was measured at 4.8 (plus or minus 2.9) GPa. Graphene is nearly 25 times stronger.

“The important message of this work is the brittle nature of these materials,” Lou says. “A lot of people are thinking about using 2D crystals because they’re inherently thin. They’re thinking about flexible electronics because they are semiconductors and their theoretical elastic strength should be very high. According to our calculations, they can be stretched up to 10 percent. The samples we have tested so far broke at 2 to 3 percent (of the theoretical maximum) at most.”

Borophene

“Wavy” borophene might be better, according to finding of other Rice University scientists. The Rice lab of theoretical physicist Boris Yakobson and experimental collaborators observed examples of naturally undulating metallic borophene—an atom-thick layer of boron—and suggested that transferring it onto an elastic surface would preserve the material’s stretchability along with its useful electronic properties.

Highly conductive graphene has promise for flexible electronics, but it is too stiff for devices that must repeatably bend, stretch, compress, or even twist. The Rice researchers found that borophene deposited on a silver substrate develops nanoscale corrugations, and due to weak binding to the silver can be exfoliated for transfer to a flexible surface. The research appeared recently in the American Chemical Society journal Nano Letters.

Rice University has been one of the world’s leading locations for the exploration of 1D and 2D materials research, ever since it was lucky enough to get a visionary genius like Richard Smalley to show up in 1976, so we should expect excellent work from people in their department of Materials Science and NanoEngineering (CSNE). Still, this ground-breaking work is being done in labs using tools capable of handling micron-scale substrates, so even after a metaphorical “path” has been found it will take a lot of work to build up a manufacturing roadway capable of fabricating meter-scale substrates.

—E.K.

When most living creatures get hurt, they can self-heal and recover from the injury. But, when damage occurs to inanimate objects, they don’t have that same ability and typically either lose functionality or have their useful lifecycle reduced. Researchers at the Beckman Institute for Advanced Science and Technology are working to change that.

For more than 15 years, Jeff Moore, a professor of chemistry, Nancy Sottos, a professor of materials science and engineering, and Scott White, a professor of aerospace engineering, have been collaborating in the Autonomous Materials Systems Group. Their work focuses on creating synthetic materials that can react to their environment, recover from damage, and even self-destruct once their usefulness has come to an end.

The trio of Beckman researchers are pioneers in what is now a dynamic and growing field. Their work on self-healing polymers was first presented in the journal Nature more than a decade-and-a-half ago. Prior to that, there had been just a few papers published on the subject of autonomous polymers. In the years since, research in the field has exploded, with hundreds of papers published.

Now, in a sweeping perspective article published this month in the journal Nature, the researchers, along with Beckman Postdoctoral Fellows Jason Patrick and Maxwell Robb, review the state-of-the-art autonomous polymers and lay out future directions for the field.

“What we’ve tried to capture for the first time is a vision of polymers as multifunctional entities that can manage their well-being,” Moore said.

The article is an overview of how their work has evolved from the development of self-healing polymers to a concentration on “life cycle control of polymers” — what he called “the healthy aging of materials.” He described the autonomous function of materials this way: “Live long, be fit, die fast, and leave no mess behind. … We want the materials to live as long as they can in a healthy state and, when the time comes, be able to trigger the inevitable from a functional state to recoverable materials resources.”

In the paper, the researchers identified five landscape-altering developments: self-protection, self-reporting, self-healing, regeneration, and controlled degradation.

Much of their work revolves around microcapsules, which are small, fluid-filled spheres that can be integrated into various material systems. The capsules contain a healing agent that is released automatically when exposed to a specific environmental change, such as physical damage or excessive temperature.

“You have capsules that remain stable in the material until the environment causes a stress that causes them to rupture,” explained Sottos. “A lot of different external stimuli can open up the capsules. You can have a thermal trigger, a mechanical trigger, and we’ve worked a lot on chemical triggers. They open up, release their contents, and the science is in what comes out and reacts.”

By developing new chemistries and ways to integrate microcapsules over the years, the researchers have created polymers that can do everything from re-filling minor damage in paints and coatings (self-protecting), changing color when undergoing stress (self-reporting), and re-bonding cracks or restoring electrical conductivity (self-healing).

The AMS Group also developed a way to efficiently fabricate vascular networks within polymers. These networks, which can include multiple channels that run throughout a material, are able to deliver healing agents multiple times, change thermal or magnetic properties, and facilitate other useful chemical interactions in a material.

A major development in their self-healing work focuses on repairing large-scale damage through the process of regeneration.

“Ballistic impacts, drilling holes in sheets of plastic, and these sorts of things, where a significant mass is lost … traditional self-healing has no way of dealing with that problem at all,” White said. “The materials that would be used to heal that hole would simply fall out, bleed out under gravity.”

So White and his collaborators came up with a two-channel healing system. When damage occurs on a large scale, a gel-like substance fills the space and builds upon itself, keeping the healing agents in place until they harden.

Their most recent work is concerned with how to deal with material systems when they have reached the end of their useful life. This work involves making materials that can self-destruct when a specific environmental signal is given (triggered transience). The researchers believe that triggers such as high temperature, water, ultraviolet light, and many others may one day be used to make obsolete devices degrade quickly so that they can be reused or recycled, thus reducing electronic waste and boosting sustainability.

Autonomous polymers are beginning to make their way into the commercial sector. Commercialization efforts have produced materials such as wear-resistant mobile device cases and automotive paints that can self-repair minor scratches. And more self-healing products are slowly coming to market including a microcapsule-based powder coating produced by the Champaign-based start-up company Autonomic Materials Inc.

While the practical application of many of these techniques still face challenges, Moore, Sottos, White, and their colleagues continue to work toward the creation of smart materials that can function independently, self-heal, and disintegrate once they are no longer useful, offering the eventual promise of safer, more efficient, and longer-lasting products that require fewer resources and produce less waste.

Graphene, a material that could usher in the next generation of electronic and energy devices, could be closer than ever to mass production, thanks to microwaves.

A new study by an international team of researchers from UNIST and Rutgers University has proved that it is now possible to produce high quality graphene, using a microwave oven. The team reports that this new technique may have solved some of graphene’s difficult manufacturing problems. The findings of the research have been published in the September issue of the prestigious journal Science.

Reducing graphene oxide sheets (prGON) into pristine graphene, using 1-to-2 second pulses of microwaves. Credit: UNIST

Reducing graphene oxide sheets (prGON) into pristine graphene, using 1-to-2 second pulses of microwaves. Credit: UNIST

This study was jointly conducted by Dr. Jieun Yang, an alumna of UNIST, Prof. Hyeon Suk Shin (School of Natural Science) of UNIST, Prof. Hu Young Jeon (School of Natural Science) of UNIST, Prof. Manish Chhowalla of Rutgers University, and five other researchers from Rutgers University, New Brunswick, NJ, United States.

Graphene comes from a base material of graphite, the cheap material in the ‘lead’ of pencils. The structure of graphite consists of many flat layers of graphene sheets. One of the most promising ways to achieve large quantities of graphene is to exfoliate graphite into individual graphene sheets by using chemicals. However, the oxygen exposure during the process may cause some inevitable side reactions, as it can ultimately be very damaging to the individual graphene layers.

Indeed, oxygen distorts the pristine atomic structure of graphene and degrades its properties. Therefore, removing oxygen from graphene oxide to obtain high-quality graphene has been a significant challenge over the past two decades for the scientific community working on graphene.

Dr. Yang and her research team have discovered that baking the exfoliated graphene oxide for just 1-to-2 second pulses of microwaves, can eliminate virtually all of the oxygen from graphene oxides.

“The partially reduced graphene oxides absorb microwave energy, produced inside a microwave oven ,” says Dr. Yang, the lead author of the study. She adds, “This not only efficiently eliminates oxygen functional groups from graphene oxides, but is also capable of rearranging defective graphene films.”

The results indicate that the new graphene exibits substantially reduced oxygen concentration of 4% much lower than the currently existing graphene with an oxygen content in the range of 15% to 25%.

Prof. Shin states, “Countries around the world, such as South Korea, U.S., England, and China have been investing heavily in research for the affordable, mass commercialization of graphene.”

He adds, “The current method for mass-producing high-quality graphene lacks reproducibility, but holds huge untapped market potential. Therefore, securing the fundamental technology for mass production of graphene is an extremely important matter in terms of commercializing future promising industries.”

The study’s co-author, Prof. Manish Chhowalla is an associate chair in the Department of Materials Science and Engineering in Rutgers’ School of Engineering and Director of the Rutgers Institute for Advanced Materials, Devices and Nanotechnology. Prof. Chhowalla has been working on a joint research project with Prof. Shin and Prof. Jeon of UNIST. Dr. Jieun Yang, a former student of Prof. Shin is now working as a post-doctoral associate in Chhowalla’s group at Rutgers University.

The next time you place your coffee order, imagine slapping onto your to-go cup a sticker that acts as an electronic decal, letting you know the precise temperature of your triple-venti no-foam latte. Someday, the high-tech stamping that produces such a sticker might also bring us food packaging that displays a digital countdown to warn of spoiling produce, or even a window pane that shows the day’s forecast, based on measurements of the weather conditions outside.

Engineers at MIT have invented a fast, precise printing process that may make such electronic surfaces an inexpensive reality. In a paper published today in Science Advances, the researchers report that they have fabricated a stamp made from forests of carbon nanotubes that is able to print electronic inks onto rigid and flexible surfaces.

A. John Hart, the Mitsui Career Development Associate Professor in Contemporary Technology and Mechanical Engineering at MIT, says the team’s stamping process should be able to print transistors small enough to control individual pixels in high-resolution displays and touchscreens. The new printing technique may also offer a relatively cheap, fast way to manufacture electronic surfaces for as-yet-unknown applications.

“There is a huge need for printing of electronic devices that are extremely inexpensive but provide simple computations and interactive functions,” Hart says. “Our new printing process is an enabling technology for high-performance, fully printed electronics, including transistors, optically functional surfaces, and ubiquitous sensors.”

Sanha Kim, a postdoc in MIT’s departments of Mechanical Engineering and Chemical Engineering, is the lead author, and Hart is the senior author. Their co-authors are mechanical engineering graduate students Hossein Sojoudi, Hangbo Zhao, and Dhanushkodi Mariappan; Gareth McKinley, the School of Engineering Professor of Teaching Innovation; and Karen Gleason, professor of chemical engineering and MIT’s associate provost.

A stamp from tiny pen quills

There have been other attempts in recent years to print electronic surfaces using inkjet printing and rubber stamping techniques, but with fuzzy results. Because such techniques are difficult to control at very small scales, they tend to produce “coffee ring” patterns where ink spills over the borders, or uneven prints that can lead to incomplete circuits.

“There are critical limitations to existing printing processes in the control they have over the feature size and thickness of the layer that’s printed,” Hart says. “For something like a transistor or thin film with particular electrical or optical properties, those characteristics are very important.”

Hart and his team sought to print electronics much more precisely, by designing “nanoporous” stamps. (Imagine a stamp that’s more spongy than rubber and shrunk to the size of a pinky fingernail, with patterned features that are much smaller than the width of a human hair.) They reasoned that the stamp should be porous, to allow a solution of nanoparticles, or “ink,” to flow uniformly through the stamp and onto whatever surface is to be printed. Designed in this way, the stamp should achieve much higher resolution than conventional rubber stamp printing, referred to as flexography.

Kim and Hart hit upon the perfect material to create their highly detailed stamp: carbon nanotubes — strong, microscopic sheets of carbon atoms, arranged in cylinders. Hart’s group has specialized in growing forests of vertically aligned nanotubes in carefully controlled patterns that can be engineered into highly detailed stamps.

“It’s somewhat serendipitous that the solution to high-resolution printing of electronics leverages our background in making carbon nanotubes for many years,” Hart says. “The forests of carbon nanotubes can transfer ink onto a surface like massive numbers of tiny pen quills.”

Printing circuits, roll by roll

To make their stamps, the researchers used the group’s previously developed techniques to grow the carbon nanotubes on a surface of silicon in various patterns, including honeycomb-like hexagons and flower-shaped designs. They coated the nanotubes with a thin polymer layer (developed by Gleason’s group) to ensure the ink would penetrate throughout the nanotube forest and the nanotubes would not shrink after the ink was stamped. Then they infused the stamp with a small volume of electronic ink containing nanoparticles such as silver, zinc oxide, or semiconductor quantum dots.

The key to printing tiny, precise, high-resolution patterns is in the amount of pressure applied to stamp the ink. The team developed a model to predict the amount of force necessary to stamp an even layer of ink onto a substrate, given the roughness of both the stamp and the substrate, and the concentration of nanoparticles in the ink.

To scale up the process, Mariappan built a printing machine, including a motorized roller, and attached to it various flexible substrates. The researchers fixed each stamp onto a platform attached to a spring, which they used to control the force used to press the stamp against the substrate.

“This would be a continuous industrial process, where you would have a stamp, and a roller on which you’d have a substrate you want to print on, like a spool of plastic film or specialized paper for electronics,” Hart says. “We found, limited by the motor we used in the printing system, we could print at 200 millimeters per second, continuously, which is already competitive with the rates of industrial printing technologies. This, combined with a tenfold improvement in the printing resolution that we demonstrated, is encouraging.”

After stamping ink patterns of various designs, the team tested the printed patterns’ electrical conductivity. After annealing, or heating, the designs after stamping — a common step in activating electronic features — the printed patterns were indeed highly conductive, and could serve, for example, as high-performance transparent electrodes.

Going forward, Hart and his team plan to pursue the possibility of fully printed electronics.

“Another exciting next step is the integration of our printing technologies with 2-D materials, such as graphene, which together could enable new, ultrathin electronic and energy conversion devices,” Hart says.