Category Archives: Packaging Materials

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.

Scientists often discover interesting things without completely understanding how they work. That has been the case with an experimental memory technology in which temperature and voltage work together to create the conditions for data storage. But precisely how was unknown.

But when a Stanford team found a way to untangle the chip’s energy and heat requirements, their tentative findings revealed a pleasant surprise: The process may be more energy efficient than was previously supposed. That’s good news for next-generation mobile devices whose batteries would last longer if they were powering lower energy chips.

The group that made this discovery, led by Stanford electrical engineer H.-S. Philip Wong, is presenting the paper when the IEEE International Electron Devices Meeting (IEDM) brings leading researchers to San Francisco Dec. 5.

The new technology the team investigated is called resistive random-access memory, or RRAM for short. RRAM is based on a new type of semiconductor material that forms digital zeros and ones by resisting or permitting the flow of electrons. RRAM has the potential to do things that aren’t possible with silicon: for instance, being layered on top of computer transistors in new three-dimensional, high-rise chips that would be faster and more energy efficient than current electronics, which is ideal for smartphones and other mobile devices where energy efficiency is a vital feature.

But while engineers can observe that RRAM does store data, they don’t know exactly how these new materials work. “We need much more precise information about the fundamental behavior of RRAM before we can hope to produce reliable devices,” Wong said.

Jolting memory

So to help engineers understand some of the unknowns, Wong’s team built a tool to measure the basic forces that make RRAM chips work.

Graduate student Zizhen Jiang of the Stanford team explained the basics: RRAM materials are insulators, which normally do not allow electricity to flow, she said. But under certain circumstances, insulators can be induced to let electrons flow. Past research had shown how: Jolting RRAM materials with an electric field causes a pathway to form that permitted electron flows. This pathway is called a filament. To break the filament, researchers apply another jolt and the material becomes an insulator again. So each jolt switched the RRAM from zero to one or back, which is what makes the material useful for data storage.

But electricity is not the only force at play in RRAM switching. Pumping electrons into any material raises its temperature. That’s the principle behind electric stoves. In the case of RRAM, it was the elevated temperature caused by introducing voltage that induced filaments to form or break. The question was what voltage-induced temperature was needed to cause the switching. No one knew.

Before the new Stanford study researchers thought short bursts of voltage, sufficient to generate temperatures of about 1,160 degrees Fahrenheit – hot enough to melt aluminum – was the switching point. But those were estimates because there was no way to measure the heat generated by an electric jolt.

“In order to begin to answer our questions, we had to decouple the effects of voltage and temperature on filament formation,” said Ziwen Wang, another graduate student on the team.

Dissecting the heat needs

Essentially, the Stanford researchers had to heat the RRAM material without using an electric field. So they put an RRAM chip on a micro thermal stage (MTS) device – a sophisticated hot plate capable of generating a wide range of temperatures inside the material. Of course the objective was not merely to heat the material, but also to measure how filaments formed. Here they took advantage of the fact that RRAM materials are insulators in their natural state. That makes them digital zeros. As soon as a filament formed electrons would flow. The digital zero would become a digital one, which the researchers could detect.

Using this experimental model, the team put RRAM chips on the burner and cranked up the heat, starting at about 80 F – roughly the temperature of a warm room – all the way up to 1,520 F, hot enough to melt a silver coin. Heating the RRAM to various temperatures in between these extremes, the researchers measured precisely if and how RRAM switched from its native zero to a digital one.

To their pleasant surprise, the researchers observed that filaments could form more efficiently at ambient temperatures between 80 F and 260 F, which is hotter than boiling water – contrary to prior expectation that hotter was better.

If confirmed by subsequent research, this would be good news because in a working chip the switching temperature would be created by the voltage and duration of the electric jolt. Efficient switching at lower temperatures would require less electricity and make RRAM more energy efficient and extend battery life when used as the memory in mobile devices.

Much work remains to be done to make RRAM memory practical but this research provides the test bed to vary conditions systematically instead of relying on hit-and-miss hunches.

“Now we can use voltage and temperature as design inputs in a predictive manner and that is going to enable us to design a better memory device,” Wang said.

Henry Chen, a Stanford alumnus who earned his PhD in Wang’s lab, gave this research a big assist and was a co-author on the paper. Chen, now with the Chinese memory chip-manufacturing firm GigaDevices Semiconductor Inc., helped develop the concepts and instruments that enabled the researchers to make the measurements being reported at IEDM.

In electronics, lower power consumption leads to operation cost savings, environmental benefits and the convenience advantages from longer running devices. While progress in energy efficiencies has been reported with alternative materials such as SiC and GaN, energy-savings in the standard inexpensive and widely used silicon devices are still keenly sought. K Tsutsui at Tokyo Institute of Technology and colleagues in Japan have now shown that by scaling down size parameters in all three dimensions their device they can achieve significant energy savings.

Tsutsui and colleagues studied silicon insulated gate bipolar transistors (IGBTs), a fast-operating switch that features in a number of every day appliances. While the efficiency of IGBTs is good, reducing the ON resistance, or the voltage from collector to emitter required for saturation (Vce(sat)), could help increase the energy efficiency of these devices further.

Previous investigations have highlighted that increases in the “injection enhancement (IE) effect”, which give rise to more charge carriers, leads to a reduction in Vce(sat). Although this has been achieved by reducing the mesa width in the device structure, the mesa resistance was thereby increased as well. Reducing the mesa height could help counter the increased resistance but is prone to impeding the (IE) effect. Instead the researchers reduced the mesa width, gate length, and the oxide thickness in the MOSFET to increase the IE effect and so reduce Vce(sat) from 1.70 to 1.26 V. With these alterations the researchers also used a reduced gate voltage, which has advantages for CMOS integration.

They conclude, “It was experimentally confirmed for the first time that significant Vce(sat) reduction can be achieved by scaling the IGBT both in the lateral and vertical dimensions with a decrease in the gate voltage.”

All our smart phones have shiny flat AMOLED displays. Behind each single pixel of these displays hide at least two silicon transistors which were mass-manufactured using laser annealing technologies. While the traditional methods to make them uses temperatures above 1,000°C, the laser technique reaches the same results at low temperatures even on plastic substrates (melting temperature below 300°C). Interestingly, a similar procedure can be used to generate crystals of graphene. Graphene is a strong and thin nano-material made of carbon, its electric and heat-conductive properties have attracted the attention of scientists worldwide.

High-resolution transmission electron microscopy shows that after just one laser pulse of 30 nanoseconds, the silicon carbide (SiC) substrate is melted and separates into a carbon and a silicon layer. More pulses cause the carbon layer to organize into graphene and the silicon to leave as gas. Credit: IBS

High-resolution transmission electron microscopy shows that after just one laser pulse of 30 nanoseconds, the silicon carbide (SiC) substrate is melted and separates into a carbon and a silicon layer. More pulses cause the carbon layer to organize into graphene and the silicon to leave as gas. Credit: IBS

Prof. KEON Jae Lee’s research group at the Center for Multidimensional Carbon Materials within the Institute for Basic Science (IBS) and Prof. CHOI Sung-Yool’s team at KAIST discovered graphene synthesis mechanism using laser-induced solid-state phase separation of single-crystal silicon carbide (SiC). This study, available on Nature Communications, clarifies how this laser technology can separate a complex compound (SiC) into its ultrathin elements of carbon and silicon.

Although several fundamental studies understood the effect of excimer lasers in transforming elemental materials like silicon, the laser interaction with more complex compounds like SiC has rarely been studied due to the complexity of compound phase transition and ultra-short processing time.

With high resolution microscope images and molecular dynamic simulations, scientists found that a single-pulse irradiation of xenon chloride excimer laser of 30 nanoseconds melts SiC, leading to the separation of a liquid SiC layer, a disordered carbon layer with graphitic domains (about 2.5 nm thick) on top surface and a polycrystalline silicon layer (about 5 nm) below carbon layer. Giving additional pulses causes the sublimation of the separated silicon, while the disordered carbon layer is transformed into a multilayer graphene.

“This research shows that the laser material interaction technology can be a powerful tool for next generation of two dimensional nanomaterials,” said Prof. Keon. Prof. Choi added: “Using laser-induced phase separation of complex compounds, new types of two dimensional materials can be synthesized in the future.” IBS Prof. Keon is affiliated with the School of Materials Science and Engineering, KAIST and Prof. Choi with the School of Electrical Engineering and Graphene Research Center, KAIST.

University of Texas at Dallas physicists have published new findings examining the electrical properties of materials that could be harnessed for next-generation transistors and electronics.

Dr. Fan Zhang, assistant professor of physics, and senior physics student Armin Khamoshi recently published their research on transition metal dichalcogenides, or TMDs, in the journal Nature Communications. Zhang is a co-corresponding author, and Khamoshi is a co-lead author of the paper, which also includes collaborating scientists at Hong Kong University of Science and Technology.

In recent years, scientists and engineers have become interested in TMDs in part because they are superior in many ways to graphene, a one-atom thick, two-dimensional sheet of carbon atoms arranged in a lattice. Since it was first isolated in 2004, graphene has been investigated for its potential to replace conventional semiconductors in transistors, shrinking them even further in size. Graphene is an exceptional conductor, a material in which electrons move easily, with high mobility.

“It was thought that graphene could be used in transistors, but in transistors, you need to be able to switch the electric current on and off,” Zhang said. “With graphene, however, the current cannot be easily switched off.”

Beyond Graphene

In their search for alternatives, scientists and engineers have turned to TMDs, which also can be made into thin, two-dimensional sheets, or monolayers, just a few molecules thick.

“TMDs have something graphene does not have — an energy gap that allows the flow of electrons to be controlled, for the current to be switched on and off,” Khamoshi said. “This gap makes TMDs ideal for use in transistors. TMDs are also very good absorbers of circularly polarized light, so they could be used in detectors. For these reasons, these materials have become a very popular topic of research.”

One of the challenges is to optimize and increase electron mobility in TMD materials, a key factor if they are to be developed for use in transistors, Khamoshi said.

In their most recent project, Zhang and Khamoshi provided the theoretical work to guide the Hong Kong group on the layer-by-layer construction of a TMD device and on the use of magnetic fields to study how electrons travel through the device. Each monolayer of TMD is three molecules thick, and the layers were sandwiched between two sheets of boron nitride molecules.

The behavior of electrons controls the behavior of these materials,” Zhang said. “We want to make use of highly mobile electrons, but it is very challenging. Our collaborators in Hong Kong made significant progress in that direction by devising a way to significantly increase electron mobility.”

The team discovered that how electrons behave in the TMDs depends on whether an even or odd number of TMD layers were used.

“This layer-dependent behavior is a very surprising finding,” Zhang said. “It doesn’t matter how many layers you have, but rather, whether there are an odd or even number of layers.”

Electron Physics

Because the TMD materials operate on the scale of individual atoms and electrons, the researchers incorporated quantum physics into their theories and observations. Unlike classical physics, which describes the behavior of large-scale objects that we can see and touch, quantum physics governs the realm of very small particles, including electrons.

On the size scale of everyday electrical devices, electrons flowing through wires behave like a stream of particles. In the quantum world, however, electrons behave like waves, and the electrical transverse conductance of the two-dimensional material in the presence of a magnetic field is no longer like a stream — it changes in discrete steps, Zhang said. The phenomenon is called quantum Hall conductance.

“Quantum Hall conductance might change one step by one step, or two steps by two steps, and so on,” he said. “We found that if we used an even number of TMD layers in our device, there was a 12-step quantum conductance. If we applied a strong enough magnetic field to it, it would change by six steps at a time.”

Using an odd number of layers combined with a low magnetic field also resulted in a 6-step quantum Hall conductance in the TMDs, but under stronger magnetic fields, it became a 3-step by 3-step phenomenon.

“The type of quantum Hall conductance we predicted and observed in our TMD devices has never been found in any other material,” Zhang said. “These results not only decipher the intrinsic properties of TMD materials, but also demonstrate that we achieved high electron mobility in the devices. This gives us hope that we can one day use TMDs for transistors.”