Category Archives: Process Materials

Band gaps, made to order


September 28, 2017

Control is a constant challenge for materials scientists, who are always seeking the perfect material — and the perfect way of treating it — to induce exactly the right electronic or optical activity required for a given application.

One key challenge to modulating activity in a semiconductor is controlling its band gap. When a material is excited with energy, say, a light pulse, the wider its band gap, the shorter the wavelength of the light it emits. The narrower the band gap, the longer the wavelength.

As electronics and the devices that incorporate them — smartphones, laptops and the like — have become smaller and smaller, the semiconductor transistors that power them have shrunk to the point of being not much larger than an atom. They can’t get much smaller. To overcome this limitation, researchers are seeking ways to harness the unique characteristics of nanoscale atomic cluster arrays — known as quantum dot superlattices — for building next generation electronics such as large-scale quantum information systems. In the quantum realm, precision is even more important.

New research conducted by UC Santa Barbara’s Department of Electrical and Computer Engineering reveals a major advance in precision superlattices materials. The findings by Professor Kaustav Banerjee, his Ph.D. students Xuejun Xie, Jiahao Kang and Wei Cao, postdoctoral fellow Jae Hwan Chu and collaborators at Rice University appear in the journal Nature Scientific Reports.

Their team’s research uses a focused electron beam to fabricate a large-scale quantum dot superlattice on which each quantum dot has a specific pre-determined size positioned at a precise location on an atomically thin sheet of two-dimensional (2-D) semiconductor molybdenum disulphide (MoS2). When the focused electron beam interacts with the MoS2 monolayer, it turns that area — which is on the order of a nanometer in diameter — from semiconducting to metallic. The quantum dots can be placed less than four nanometers apart, so that they become an artificial crystal — essentially a new 2-D material where the band gap can be specified to order, from 1.8 to 1.4 electron volts (eV).

This is the first time that scientists have created a large-area 2-D superlattice — nanoscale atomic clusters in an ordered grid — on an atomically thin material on which both the size and location of quantum dots are precisely controlled. The process not only creates several quantum dots, but can also be applied directly to large-scale fabrication of 2-D quantum dot superlattices. “We can, therefore, change the overall properties of the 2-D crystal,” Banerjee said.

Each quantum dot acts as a quantum well, where electron-hole activity occurs, and all of the dots in the grid are close enough to each other to ensure interactions. The researchers can vary the spacing and size of the dots to vary the band gap, which determines the wavelength of light it emits.

“Using this technique, we can engineer the band gap to match the application,” Banerjee said. Quantum dot superlattices have been widely investigated for creating materials with tunable band gaps but all were made using “bottom-up” methods in which atoms naturally and spontaneously combine to form a macro-object. But those methods make it inherently difficult to design the lattice structure as desired and, thus, to achieve optimal performance.

As an example, depending on conditions, combining carbon atoms yields only two results in the bulk (or 3-D) form: graphite or diamond. These cannot be ‘tuned’ and so cannot make anything in between. But when atoms can be precisely positioned, the material can be designed with desired characteristics.

“Our approach overcomes the problems of randomness and proximity, enabling control of the band gap and all the other characteristics you might want the material to have — with a high level of precision,” Xie said. “This is a new way to make materials, and it will have many uses, particularly in quantum computing and communication applications. The dots on the superlattice are so close to each other that the electrons are coupled, an important requirement for quantum computing.”

The quantum dot is theoretically an artificial “atom.” The developed technique makes such design and “tuning” possible by enabling top-down control of the size and the position of the artificial atoms at large scale.

To demonstrate the level of control achieved, the authors produced an image of “UCSB” spelled out in a grid of quantum dots. By using different doses from the electron beam, they were able to cause different areas of the university’s initials to light up at different wavelengths.

“When you change the dose of the electron beam, you can change the size of the quantum dot in the local region, and once you do that, you can control the band gap of the 2-D material,” Banerjee explained. “If you say you want a band gap of 1.6 eV, I can give it to you. If you want 1.5 eV, I can do that, too, starting with the same material.”

This demonstration of tunable direct band gap could usher a new generation of light-emitting devices for photonics applications.

Making a magnet from a piece of iron and a coil or wire, or another magnet, is a simple experiment. An external electric or magnetic field can align groups of atoms in the iron over time so that they take on their own permanent magnetic field. A similar accelerated process stores information on computer hard disks. A special case of magnetism, known as ferrimagnetism, could enable even faster switching of magnetism, leading to massive improvements in the way computers handle information.

Now, an international research group, led by Osaka University physicists, has provided new insight into how the composition of ferrimagnetic materials can affect their interactions with light. They recently reported their findings in Applied Physics Express.

“We know that laser pulses can reverse the magnetization in certain ferrimagnetic alloys, but light also affects other properties of the material,” coauthor Hidenori Fujiwara says. “To learn more about the interactions of the magnetism with light, we studied the spin dynamics of ferrimagnetic thin films containing different proportions of gadolinium.”

Ferrimagnetic materials can be thought of as a mixture of electrons spinning at different sites in the material. Some of the spins might cancel each other out, but a certain residual magnetization will remain. Firing an ultra-fast laser pulse at the material may completely flip the spin direction, reversing the magnetism, or may disrupt the spins, causing a kind of wobbling known as spin precession. The type of behavior shown strongly depends on the material’s temperature and composition.

The researchers used an advanced synchrotron measurement setup developed in their previous studies to show that slightly varying the composition of an alloy dramatically changed its response to the laser pulse. Slightly more gadolinium in the films led to flipping of the magnetic spin; slightly less led to spin precession at room temperature.

The researchers’ setup could also visualize the wave-like nature of the spin precession over a few nanoseconds following the laser pulse. They showed that the angle of precision, or the angle of the spin wobble, was the largest reported to date.

“These are complex systems with many different interacting properties, but we have extracted some clear relationships between the composition of a ferrimagnetic alloy and its magnetic interactions with light,” coauthor Akira Sekiyama says. “Understanding these behaviors is important from a fundamental physics standpoint, and essential for applying these material systems in advanced electronic devices.”

Researchers from Finland and Taiwan have discovered how graphene, a single-atom-thin layer of carbon, can be forged into three-dimensional objects by using laser light. A striking illustration was provided when the researchers fabricated a pyramid with a height of 60nm, which is about 200 times larger than the thickness of a graphene sheet. The pyramid was so small that it would easily fit on a single strand of hair. The research was supported by the Academy of Finland and the Ministry of Science and Technology of the Republic of China.

A similar structure was made experimentally by using laser irradiation in a process called "optical forging." Credit: The University of Jyväskylä

A similar structure was made experimentally by using laser irradiation in a process called “optical forging.” Credit: The University of Jyväskylä

Graphene is a close relative to graphite, which consists of millions of layers of graphene and can be found in common pencil tips. After graphene was first isolated in 2004, researchers have learned to routinely produce and handle it. Graphene can be used to make electronic and optoelectronic devices, such as transistors, photodetectors and sensors. In future, we will probably see an increasing number of products containing graphene.

“We call this technique optical forging, since the process resembles forging metals into 3D shapes with a hammer. In our case, a laser beam is the hammer that forges graphene into 3D shapes,” explains Professor Mika Pettersson, who led the experimental team at the Nanoscience Center of the University of Jyväskylä, Finland. “The beauty of the technique is that it’s fast and easy to use; it doesn’t require any additional chemicals or processing. Despite the simplicity of the technique, we were very surprised initially when we observed that the laser beam induced such substantial changes on graphene. It took a while to understand what was happening.”

“At first, we were flabbergasted. The experimental data simply made no sense,” says Dr Pekka Koskinen, who was responsible for the theory. “But gradually, by close interplay between experiments and computer simulations, the actuality of 3D shapes and their formation mechanism started to become clear.”

“When we first examined the irradiated graphene, we were expecting to find traces of chemical species incorporated into the graphene, but we couldn’t find any. After some more careful inspections, we concluded that it must be purely structural defects, rather than chemical doping, that are responsible for such dramatic changes on graphene,” explains Associate Professor Wei Yen Woon from Taiwan, who led the experimental group that carried out X-ray photoelectron spectroscopy at the synchrotron facility.

The novel 3D graphene is stable and it has electronic and optical properties that differ from normal 2D graphene. Optically forged graphene can help in fabricating 3D architectures for graphene-based devices.

The basics of laser marking are reviewed, as well as current and emerging laser technologies.

BY DIETRICH TÖNNIES, Ph.D. and DIRK MÜLLER, Ph.D., Coherent Inc., Santa Clara, CA

Laser marking is established at multiple points in semiconductor production and applications continue to diversify. There are several laser technologies servicing the application space. This article reviews the basics of laser marking and the current and emerging laser technologies they utilize. It is intended to give a clear sense of what applications parameters drive the choice of laser (speed, cost, resolution, etc.), and provide those developing a new application some guidance on how to select the optimum technology.

Laser marking basics

Laser marking usually entails inducing a visible color or texture change on a surface. Alternatively, although less commonly, marking sometimes involves producing a macroscopic change in surface relief (e.g.engraving). To understand what laser type is best for a specific marking application, it is useful to examine the different laser/ material interactions that are generated by commonly used laser types.

Most frequently, lasers produce high contrast marks through a thermal interaction with the work piece. That is, material is heated until it undergoes a chemical reaction (e.g. oxidation) or change of crystalline structure that produces the desired color or texture change. However, the particulars of this process vary significantly between different materials and laser types.

CO2 lasers have been employed extensively for PCB marking because they provide a fast method of producing high contrast features. However, they are rarely selected when marking at the die or package level. This is because the focused spot size scales with wavelength due to diffraction. CO2 lasers emit the longest infrared (IR) output of any marking laser. Additionally, IR penetrates far into many materials, which can cause a substantial thermal impact on surrounding structures. Consequently, CO2 laser marking is limited to producing relatively large features where a significant heat affected zone (HAZ) can be tolerated.

Fiber lasers, which offer high power output in the near IR, have emerged over the past few years as one of the most cost effective tools for high-speed marking. Furthermore, the internal construction of fiber lasers renders a compact footprint, facilitating their integration into marking and test handlers. Cost and space savings are further enhanced when the output of a single, high power fiber laser is split, feeding two scanner systems.

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But fiber lasers have disadvantages, too. One reason for the low cost of many fiber lasers is that they are produced in high volumes with designs meant for general-purpose applications. For example, they usually produce a high quality beam with a Gaussian intensity profile. This is advantageous for many material processing applications, but not always for laser marking. In fact, a more uniform beam intensity distribution, called a flat-top profile, is sometimes more useful since it produces marks with a sharper, more abrupt edge (rather than a smooth transition from the marked to the unmarked region). Coherent recently introduced a new type of fiber (NuBEAM Flat-Top fiber technology) which enables efficient conversion of single-mode laser beams into flat-top beam profiles, specifically to address this issue.

Other quality criteria, such as high-purity linear polarization, and stability of pulse energy and pulse width, are difficult to achieve with low-cost fiber lasers. This limits their use in more stringent or sensitive marking applications. From a practical standpoint, most fiber lasers cannot be repaired in the field, but are replaced as a whole. This leads to longer equipment downtime and increased maintenance efforts as compared to traditional marking lasers based on diode-pumped, solid-state (DPSS) technology (specifically, DPSS is used here to refer to lasers with crystal resonators).

DPSS lasers also emit in the near infrared. Generally, these lasers are more expensive than a fiber laser of the same output power level. So, infrared DPSS lasers are most commonly used in applications having technical requirements that cannot be met by fiber lasers,such as high volume production of more advanced and expensive semiconductor devices.

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One advantage of DPSS laser technology is that it can be configured to directly produce a multi-mode beam profile which is essentially flat-top. The Coherent ❘ Rofin PowerLine E Air 30-1064 IC is an example which has found extensive use in semiconductor marking, since it provides an efficient way to rapidly produce very high contrast marks.

Another useful feature of DPSS lasers, which produce pulsewidths in the nanosecond regime, is that their output is much more stable than that of fiber lasers. This makes it much easier to reliably frequency double or triple their infrared light within the laser head, giving a choice of output in the green or ultraviolet (UV). Output at these wavelengths provides two significant benefits. First, they offer additional options in matching the absorption of the material to the laser wavelength. Stronger absorption generally yields higher marking efficiency and reduced HAZ, since the laser light doesn’t penetrate as far into the material. The second benefit of shorter wavelengths is the ability to focus to smaller spot sizes (because of their lower diffraction) and produce smaller, finer marks.

However, frequency multiplied DPSS lasers are generally more costly and voluminous than either fiber lasers or infrared DPSS lasers with comparable output power. Lower power translates into reduced marking speed.

Therefore, green and UV DPSS lasers are typically employed when they offer a significant advantage due to the particular absorption characteristics of the material(s) being marked.

Another emerging and important class of marking lasers has pulsewidths in the sub-nanosecond range. Due to the nature of the laser/material interaction at short pulsewidths, these lasers tend to produce the smallest possible HAZ with excellent depth control.

There are just a few products currently on the market that exploit this property. One example is the PowerLine Pico 10 from Coherent ❘ Rofin which generates 0.5 ns laser pulses in either the near IR (8 W total power) or green (3 W total power), at pulse repetition rates between 300 kHz and 800 kHz. This combination of output characteristics makes it capable of high speed marking of a wide range of materials where mark penetration depth must neces- sarily be shallow because of low material thickness, or to minimize HAZ.

Laser marking today

Typically, the first consideration in choosing a laser for a specific application is matching the absorption characteristics of the material with the laser wavelength. Similarly, desired feature size is also driven by laser wavelength, as well as by the precision of the beam scanning system. Next, HAZ constraints usually determine the maximum pulsewidth which can be used (although this choice is again highly material dependent). To see how these parameters interact in practice, it’s useful to review some real world applications.

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Epoxy-based molding compounds

The most commonly used molding compounds absorb very well in the near IR. Specifically, the near IR laser transforms the usually black molding compound into a gray/white powder, yielding high contrast marks. Plus, many IC packages have mold compound caps thick enough to easily tolerate a marking depth of 30 μm to 50 μm. As a result, many marking systems based on near IR lasers, both fiber and DPSS, are currently in use.

However, some semiconductor devices with small form factor have only thin mold compound caps to protect wire bonded silicon dies, and a marking depth of only 10 μm or less is required. Increasingly, green lasers are used for this type of shallow marking because of a stronger absorption at this wavelength by the epoxy matrix.

Ceramics

The process window when marking ceramics, such as used in packaging power semiconductors, high-brightness LEDs, RF devices, saw filters or MEMS sensors, is relatively narrow. Accurate focus and high pulse energy are critical to ensure reliable marking results, and ideally, the laser marker should have the capability to adjust the focus of the laser beam onto the ceramic surface in real time, in order to compensate for package height variations. Because of their more reliable interaction with ceramic materials, DPSS lasers based on Nd:YAG, which offer high pulse energies and relatively long pulses, are often still used for marking ceramic lids and substrates. Coherent ❘ Rofin has also developed a special fiber laser (the PowerLine F 20 Varia IC), which offers adjustable pulse widths up to 200 ns, specifically to improve process windows for marking applications of this type.

The ceramic substrates used with high-power LEDs often require tiny marks to identify individual devices. IR lasers are the preferred lasers for marking these ceramic substrates, providing their spot size is not too big for the layout to be marked. For very small marking features a green laser or UV laser is often required.

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Organic substrates

IC substrates or interposers are marked during production with traceable data matrix codes. The thin green solder resist layer on top of the substrate has to carry the mark, and care has to be taken that the copper underneath the solder resist is not exposed. Moreover, data matrix codes can be quite small, with cell sizes of only 125 μm or even less. Since the spot size of the focused laser beam must thus be much smaller than the cell size, the final spot diameter must be significantly less than 100 μm.

Defective IC substrates often are identified by marking large features (e.g., a cross) into the solder resist layer. Although the part is defective, the properties of the mark are still important. This is because it has to be reliably recognized by subsequent processing tools, and also, because any delamination of the solder resist layer might cause problems during succeeding processes.

IC strips have gold pads along their periphery which are used to identify parts found to be defective after die attach and wire bonding. For defective parts, the gold pad is marked by converting its color from gold to black or to dark grey.

Ideally, it is desirable to have one laser marker that can accomplish all three of these marking applications tasks. The green DPSS laser has become the standard laser marker for these applications, with UV lasers occasionally employed for high-end substrates.

Semiconductors

The growing demand for flip-chip devices, wafer-level packaging and defective die identification drives the need for direct marking of silicon, GaAs, GaN/sapphire or other semiconductors. Silicon is partially trans- parent in the near IR, and lasers at this wavelength are used whenever deep marks into silicon are required, such as placing wafer IDs near the wafer edge. Near IR laser markers are also selected for marking molded fan-out wafer level packaging wafers.

However, for marking either flip-chips or the backside of wafers, green lasers are preferred because of the strong absorption of this wavelength in silicon. Wafer backside marking requires only very shallow marks and the shallow laser penetration avoids potential damage to the circuitry on the reverse side of the flip-chip or wafer. The need for shallow marking also minimizes the laser power requirement. For example, Coherent ❘ Rofin provides a 6 W green laser (the PowerLine E 12 SHG IC) that is well suited for wafer backside marking, and can also mark the wafer through the tape whenever the wafer is mounted on a film frame.

Metals

Near IR lasers are widely used for marking the metal lids used with microprocessors and other high power consumption ICs.

Leadframes, which are plated with tin, silver or gold, are marked either before or after plating. Since leadframes are used for cost sensitive devices, capital investment is critical, and economical fiber lasers are often chosen for this reason.

Laser marking tomorrow

As packages get thinner and smaller, they will require shallower, higher resolution marks. Sub-nanosecond lasers are the most promising method for producing these types of marks, and are compatible with a wide range of materials. The diverse capabilities of this technology are shown in Figure 5, which depicts marking results on four different materials using a sub-nanosecond laser (Coherent ❘ Rofin PowerLine Pico 10-532 IC).

The first image is a flexible IC substrate; very thin solder resist layers and metal coatings make it important that the laser does not cause delamination. Here, the circular gold pad has been converted to black without delamination. In the next image, an IC substrate has been given a white mark, again without delaminating the solder resist.

The third image shows very small characters (< 150 μm) marked on the backside of a silicon wafer containing hundred thousands of tiny discrete semiconductor devices. Producing marks of this resolution through the film would be difficult to accomplish with a nanosecond pulsewidth laser.

The final image is a copper leadframe coated with thin silver film. Here, the goal is to produce a shallow mark with high contrast without engraving the under- lying material, which has been accomplished with the sub-nanosecond laser.

Conclusion

Semiconductor fabrication and packaging represent challenging marking applications, often requiring small, fine marks produced without a significant effect on surrounding material. An overall trend towards smaller and thinner device geometries will drive increased use of higher precision laser tools, such as those utilizing green and UV nanosecond lasers, and even sub-nanosecond lasers, while cost-sensitive applications will continue to utilize inexpensive fiber lasers.

A research group consisting of scientists from Tomsk Polytechnic University, Germany and Venezuela proved vulnerability of a two-dimensional semiconductor gallium selenide in air. This discovery will allow manufacturing superconducting nanoelectronics based on gallium selenide, which has never been previously achieved by any research team in the world.

The study was published in Semiconductor Science and Technology.

One of the promising areas of modern materials science is the study of two-dimensional (2D) materials, i.e. thin films consisting of one or several atomic layers. 2D materials due to their electrical superconductivity and strength could be a basis for modern nanoelectronics. Optic applications in nanoelectronics require advanced materials capable of ‘generating’ great electron fluxes upon light irradiation. Gallium selenide (GaSe) is one of the 2D semiconductors that can cope with this problem most efficiently.

‘Some research teams abroad tried to create electronic devices based on GaSe. However, despite extensive theoretical studies of this material, which were published in major scientific journals, the stability of the material in real devices remained unclear,’ says Prof. Raul Rodriguez, the Department of Lasers and Lighting Engineering.

The research team revealed the reasons behind this. They studied GaSe by means of Raman spectroscopy and x-ray photoelectron spectroscopy that allowed proving the existence of chemical bonds between gallium and oxygen. Photoluminescence in oxidized substance is absent that also proves the formation of oxides. It means that the scientists revealed that GaSe oxidizes quickly in air and loses its electrical conductivity necessary for creating nanoeletronic devices.

‘GaSe monolayers become oxidized almost immediately after being exposed to air. Further research of GASe stability in air will allow making proposals how to protect it and maintain its optoelectronic properties,’ emphasize the authors.

According to Prof. Rodriguez, for GaSe not to lose its unique properties it should be placed in a vacuum or inert environment. For example, it can be applied in encapsulated devices that are vacuum-manufactured and then covered with a protective layer eliminating air penetration.

This method can be used to produce next generation optoelectronics, detectors, light sources and solar batteries. Such devices of ultra-small sizes will have very high quantum efficiency, i.e. they will be able to generate large electron fluxes under small external exposure.

SEMICON Europa 2017 will take place in Munich for the first time, co-located with productronica (14-17 November in Munich, Germany). SEMICON Europa will showcase the critical issues shaping the entire electronics manufacturing supply chain. Fourexecutive keynotes will share their thought leadership on current opportunities for Europe: Maria Marced, president, TSMC Europe; Stefan Finkbeiner, CEO, Bosch Sensortec; and Frank M. Rinderknecht, founder and CEO of Rinspeed Inc.

“Innovations in semiconductor manufacturing are at the heart of the value chain driving innovations enabling key future growth drivers in Mobile, Automotive, Medical, passive and intelligent computing as well as AR and VR,” stated Laith Altimime, president, SEMI Europe. SEMICON Europa programs, sessions, and speakers will illuminate this year’s theme “Empowering Innovation and Shaping the Value Chain.”  Highlights of SEMICON Europa include:

  • Fab Management Forum: Quality Challenges – Solutions for Tomorrow ─ Topics include:Future of digital vehicles and requirements for quality and availability of semiconductors with Daimler AG, an analysis of Human failure and mindset change by European School of Management and Technology (ESMT) Berlin, and how innovative sensor and analytics solutions enable new applications in the fab of tomorrow by KINEXON GmbH.
  • Advanced Packaging Conference: Electronics Packaging and Test for Future Mobility ─With Yole Développement on the dynamics of the advanced packaging ecosystem, Robert Bosch GmbH on automotive, Infineon Technologies on packaging for automotive ─ challenges and solutions, RoodMicrotec GmbH on wafer and final test in the new era of electronics, and STMicroelectronics on packaging challenges for robust miniaturization.
  •  Power Electronics Conference: From Materials to Systems,The Latest Innovations ─Covering power electronics applications for Automotive by Fraunhofer Institute for Integrated Systems and Device Technology IISB, a forecast of the next five years to reveal how technology development will shape the power electronics market by Yole Développement, and  Cambridge University on Silicon and Wide bandgap devices in power electronics.
  • New! Materials Conference: Connected World ─ New Material Challenges and Solutions ─Includes a keynote by Christophe Maleville, SOITEC, on how to better optimize performance, power budget and cost to meet applications requirements; plus presentations from Volkswagen AG on the need for new industry alliances in automotive, FUJIFILM on maximum utilization of chemically amplified resist, and Dow Chemical on the information age and connectivity enabled by advanced electronic materials. The free Webinar “Connected World: New Material Challenges and Solutions – Market Update and Outlook“ is planned on 27 September.
  • New! European Connect2Car Forum ─ A new Forum in collaboration with SAE International. Insights for automotive OEM and supplier executives, consumer electronics leaders, mobile application developers, and aftermarket entrepreneurs focusing on enhancing the driver experience and accelerating the deployment of connected and autonomous vehicle technologies.
  • New! 2017FLEX Europe “Be Flexible” ─ New collaboration between FLEX and Fraunhofer EMFT. Insights on innovative solutions for flexible and stretchable systems by Würth Elektronik GmbH,  technology and applications of chip-film patch for hybrid systems in foil by IMS CHIPS, new capabilities and applications of flexible components by E Ink Corporation, and insight on how potentials of System-in-Package technologies will affect the future by Bosch.

SEMI and Messe München Joint Press Conference will take place on 14 November at 11:00-12:00, at Messe München Press Conference Center.

Scarce metals are found in a wide range of everyday objects around us. They are complicated to extract, difficult to recycle and so rare that several of them have become “conflict minerals” which can promote conflicts and oppression. A survey at Chalmers University of Technology now shows that there are potential technology-based solutions that can replace many of the metals with carbon nanomaterials, such as graphene.

They can be found in your computer, in your mobile phone, in almost all other electronic equipment and in many of the plastics around you. Society is highly dependent on scarce metals, and this dependence has many disadvantages.

Scarce metals such as tin, silver, tungsten and indium are both rare and difficult to extract since the workable concentrations are very small. This ensures the metals are highly sought after – and their extraction is a breeding ground for conflicts, such as in the Democratic Republic of the Congo where they fund armed conflicts.

In addition, they are difficult to recycle profitably since they are often present in small quantities in various components such as electronics.

Rickard Arvidsson and Björn Sandén, researchers in environmental systems analysis at Chalmers University of Technology, have now examined an alternative solution: substituting carbon nanomaterials for the scarce metals. These substances – the best known of which is graphene – are strong materials with good conductivity, like scarce metals.

“Now technology development has allowed us to make greater use of the common element carbon,” says Sandén. “Today there are many new carbon nanomaterials with similar properties to metals. It’s a welcome new track, and it’s important to invest in both the recycling and substitution of scarce metals from now on.”

The Chalmers researchers have studied the main applications of 14 different metals, and by reviewing patents and scientific literature have investigated the potential for replacing them by carbon nanomaterials. The results provide a unique overview of research and technology development in the field.

According to Arvidsson and Sandén the summary shows that a shift away from the use of scarce metals to carbon nanomaterials is already taking place.

“There are potential technology-based solutions for replacing 13 out of the 14 metals by carbon nanomaterials in their most common applications. The technology development is at different stages for different metals and applications, but in some cases such as indium and gallium, the results are very promising,” Arvidsson says.

“This offers hope,” says Sandén. “In the debate on resource constraints, circular economy and society’s handling of materials, the focus has long been on recycling and reuse. Substitution is a potential alternative that has not been explored to the same extent and as the resource issues become more pressing, we now have more tools to work with.”

The research findings were recently published in the Journal of Cleaner Production. Arvidsson and Sandén stress that there are significant potential benefits from reducing the use of scarce metals, and they hope to be able to strengthen the case for more research and development in the field.

“Imagine being able to replace scarce metals with carbon,” Sandén says. “Extracting the carbon from biomass would create a natural cycle.”

“Since carbon is such a common and readily available material, it would also be possible to reduce the conflicts and geopolitical problems associated with these metals,” Arvidsson says.

At the same time they point out that more research is needed in the field in order to deal with any new problems that may arise if the scarce metals are replaced.

“Carbon nanomaterials are only a relatively recent discovery, and so far knowledge is limited about their environmental impact from a life-cycle perspective. But generally there seems to be a potential for a low environmental impact,” Arvidsson says.

Facts:

Carbon nanomaterials consist solely or mainly of carbon, and are strong materials with good conductivity. Several scarce metals have similar properties. The metals are found, for example, in cables, thin screens, flame-retardants, corrosion protection and capacitors.

Rickard Arvidsson and Björn Sandén at Chalmers University of Technology have investigated whether the carbon nanomaterials graphene, fullerenes and carbon nanotubes have the potential to replace 14 scarce metals in their main areas of application (see table in attached image). They found potential technology-based solutions to replace the metals with carbon nanomaterials for all applications except for gold in jewellery. The metals which we are closest to being able to substitute are indium, gallium, beryllium and silver.

Modern life will be almost unthinkable without transistors. They are the ubiquitous building blocks of all electronic devices: each computer chip contains billions of them. However, as the chips become smaller and smaller, the current 3D field-electronic transistors (FETs) are reaching their efficiency limit. A research team at the Center for Artificial Low Dimensional Electronic Systems, within the Institute for Basic Science (IBS), has developed the first 2D electronic circuit (FET) made of a single material. Published on Nature Nanotechnology, this study shows a new method to make metal and semiconductor from the same material in order to manifacture 2D FETs.

In simple terms, FETs can be thought as high-speed switches, comprised of two metal electrodes and a semiconducting channel in between. Electrons (or holes) move from the source electrode to the drain electrode, flowing through the channel. While 3D FETs have been scaled down to nanoscale dimensions successfully, their physical limitations are starting to emerge. Short semiconductor channel lengths lead to a decrease in performance: some electrons (or holes) are able to flow between the electrodes even when they should not, causing heat and efficiency reduction. To overcome this performance degradation, transistor channels have to be made with nanometer-scale thin materials. However, even thin 3D materials are not good enough, as unpaired electrons, part of the so-called “dangling bonds” at the surface interfere with the flowing electrons, leading to scattering.

Passing from thin 3D FETs to 2D FETs can overcome these problems and bring in new attractive properties. “FETs made from 2D semiconductors are free from short-channel effects because all electrons are confined in naturally atomically thin channels, free of dangling bonds at the surface,” explains Ji Ho Sung, first author of the study. Moreover, single- and few-layer form of layered 2D materials have a wide range of electrical and tunable optical properties, atomic-scale thickness, mechanical flexibility and large bandgaps (1~2 eV).

The major issue for 2D FET transistors is the existence of a large contact resistance at the interface between the 2D semiconductor and any bulk metal. To address this, the team devised a new technique to produce 2D transistors with semiconductor and metal made of the same chemical compound, molybdenum telluride (MoTe2). It is a polymorphic material, meaning that it can be used both as metal and as semiconductor. Contact resistance at the interface between the semiconductor and metallic MoTe2 is shown to be very low. Barrier height was lowered by a factor of 7, from 150meV to 22meV.

IBS scientists used the chemical vapor deposition (CVD) technique to build high quality metallic or semiconducting MoTe2 crystals. The polymorphism is controlled by the temperature inside a hot-walled quartz-tube furnace filled with NaCl vapor: 710°C to obtain metal and 670°C for a semiconductor.

The scientists also manufactured larger scale structures using stripes of tungsten diselenide (WSe2) alternated with tungsten ditelluride (WTe2). They first created a thin layer of semiconducting WSe2 with chemical vapor deposition, then scraped out some stripes and grew metallic WTe2 on its place.

It is anticipated that in the future, it would be possible to realize an even smaller contact resistance, reaching the theoretical quantum limit, which is regarded as a major issue in the study of 2D materials, including graphene and other transition metal dichalcogenide materials.

The International Microelectronics And Packaging Society (IMAPS) will celebrate the 50th anniversary of its flagship technical conference – the IMAPS Symposium – from October 9 – 12, 2017, as microelectronics engineers and scientists gather at the Raleigh Convention Center near Research Triangle Park, North Carolina, USA to take part in the electronics industry’s largest technical conference dedicated to advanced microelectronics packaging technology. Researchers and exhibitors will showcase their work during a comprehensive conference program of technical papers, panels, special sessions, short courses/tutorials, and an exhibition that will spotlight premier work in the fields of microelectronics, semiconductor packaging and circuit design.

The 50th International Symposium on Microelectronics is an international technology forum for the presentation of applied research on microelectronics, consisting of more than 180 papers presented by researchers from corporations, universities and government labs worldwide, with five technical tracks: Chip Packaging Interactions; High Performance, Reliability, & Security; Advanced Packaging & Enabling Technologies; Advanced Packaging & System Integration; and Advanced Materials & Processes.

Keynote Presentations Lead Off the IMAPS Technical Program on Tuesday, October 10
Four keynote addresses from leading industry experts include:

“Packaging Challenges for the Next Generation of Mobile Devices,” by Ahmer Syed, Senior director of package engineering, Qualcomm Technologies

“Packaging without the Package – A More Holistic Moore’s Law,” by Subramanian (Subu) S. Iyer, distinguished chancellor’s professor in the Charles P. Reames Endowed Chair of the Electrical Engineering Department at the University of California at Los Angeles (UCLA) and Director of the Center for Heterogeneous Integration and Performance Scaling (CHIPS)

“Electronics Outside the Box: Building a Manufacturing Ecosystem for Flexible Hybrid Electronics,” by Benjamin Leever, senior materials engineer, Air Force Research Laboratory (AFRL) Soft Matter Materials Branch

“Transforming Electronic Interconnect,” by Tim Olson, founder & CTO, Deca Technologies

International Panel Session & Wine Reception on Wednesday, October 11
A panel session on “Global Perspectives on Packaging Requirements & Trends Towards 2025” will be moderated by Jan Vardaman, TechSearch International and Gabriel Pares, CEA-Leti. Panelist will include representatives from Asia (Yasumitsu Orii, NAGASE Group and Ton Schless, SIBCO), Europe (Steffen Kroehnert, Nanium and Eric Bridot, SAFRAN), and North America (David Jandzinski, Qorvo). The 90-minute panel session includes a wine reception.

Diversity Roundtable & Networking Discussions on Monday, October 9
Following the opening reception, IMAPS leaders will conduct a series of roundtable discussions designed to inspire conversations about overcoming diversity barriers, the strengths inherent in a diverse workforce, identifying and collaborating with a mentor, and more.

Posters & Pizza Session on Thursday, October 12
One of the fastest-growing segments of the IMAPS conference is the popular “Posters & Pizza” session held outside the exhibit hall, giving attendees the opportunity to interact one-on-one with presenters in a more informal setting.

Professional Development Courses (Short Courses & Tutorials) on Monday, October 9
Preceding the IMAPS Symposium technical program is a full day of professional development opportunities, presented as a series of 2-hour sessions in four tracks: Intro to Microelectronics Packaging; Next Generation Packaging Challenges; Baseline & Emerging Technologies; and Reliability. These short courses represent a unique opportunity, only available through IMAPS, for participants to personally interact with the instructors, and with each other in small groups from 10 – 30 people, led by industry experts in the field with ample time for questions and networking.

Student Opportunities at IMAPS
As part of its ongoing mission IMAPS invites students to participate in an informal networking event on Tuesday, October 10 with IMAPS industry leaders over lunch in the exhibit hall, giving them an chance to learn about career opportunities, navigating the hiring process, and other topics. In addition, the IMAPS Microelectronics Foundation sponsors a student paper competitionin conjunction with the Symposium that awards more than $3,500 in scholarships for outstanding student papers.

Social Events & an Introduction to the RTP/Raleigh Area’s Technology Community
In addition to the technical program, a variety of social events are planned around the IMAPS Symposia, including the Annual David C. Virissimo Memorial Fall Golf Classic, a charity golf outing scheduled for Monday, October 9 at NCSU’s Lonnie Poole Golf Course. Proceeds from the event benefit the IMAPS Microelectronics Foundation.

Monday evening’s welcome reception will feature NC-themed entertainment from a local bluegrass band, and participants will also be able to view historical photos and other memorabilia spanning 50 years of IMAPS history.

There is also a scheduled tour of the nearby Micross Advanced Interconnect Technology (AIT) facility, one of the premier wafer bumping and wafer level packaging facilities in the U.S., with more than 20 years experience providing leading edge interconnect and 3D integration technologies (TSV, Si interposers, 3D IC) to worldwide customers.

New to the Symposium this year is a unique opportunity for IMAPS attendees to experience the vibrant technology community in the greater RTP/Raleigh area. IMAPS has invited local non-profit organizations that comprise the area’s rapidly-growing technology ecosystem to participate in a special area adjacent to the exhibit hall during the day of October 10, providing an opportunity for IMAPS Symposium attendees to network and interact.

To register for the IMAPS 50th International Symposium on Microelectronics, please visit the online registration site for more information, or contact Brianne Lamm, IMAPS Marketing & Events Manager, at [email protected] or 980-299-9873.

Many seashells, minerals, and semiconductor nanomaterials are made up of smaller crystals, which are assembled together like the pieces of a puzzle. Now, researchers have measured the forces that cause the crystals to assemble, revealing an orchestra of competing factors that researchers might be able to control.

The work has a variety of implications in both discovery and applied science. In addition to providing insights into the formation of minerals and semiconductor nanomaterials, it might also help scientists understand soil as it expands and contracts through wetting and drying cycles. In the applied realm, researchers might use the principles to develop new materials with unique properties for energy needs.

The results, published in the Proceedings of the National Academy of Sciences in July, describe how the arrangement of the atoms in the crystals creates forces that pull them together and align them for docking. The study reveals how the attraction becomes stronger or weaker as water is heated or salt is added, both of which are common processes in the natural world.

The multinational team, led by chemists Dongsheng Li and Jaehun Chun from the Department of Energy’s Pacific Northwest National Laboratory, explored the attractive forces between two crystal particles made from mica. A flaky mineral that is commonly used in electrical insulation, this silicon-based mineral is well-studied and easy to work with because it chips off in flat pieces with nearly-perfect crystal surfaces.

Forces and faces

Crystallization often occurs through assembly of multi-faceted building blocks: some faces on these smaller crystals line up better with others, like Lego blocks do. Li and Chun have been studying a specific crystallization process called oriented attachment. Among other distinguishing characteristics, oriented attachment occurs when smaller subunits of fledgling crystals align their best matching faces before clicking together.

The process creates various nonlinear forms: nanowires with branches, lattices that look like complicated honeycombs, and tetrapods — tiny structures that look like four-armed toy jacks. The molecular forces that contribute to this self-assembly are not well understood.

Molecular forces that come into play can attract or repel the tiny crystal building blocks to or from each other. These include a variety of textbook forces such as van der Waals, hydrogen bonding, and electrostatic, among others.

To explore the forces, Li, Chun and colleagues milled flat faces on tiny slabs of mica and put them on a device that measures the attraction between two pieces. Then they measured the attraction while twisting the faces relative to each other. The experiment allowed the mica to be bathed in a liquid that includes different salts, letting them test real-world scenarios.

The difference in this work was the liquid setup. Similar experiments by other researchers have been done dry under vacuum; in this work, the liquid created conditions that better simulate how real crystals form in nature and in large industrial methods. The team performed some of these experiments at EMSL, the Environmental Molecular Sciences Laboratory, a DOE Office of Science User Facility at PNNL.

Twist and salt

One of the first things the team found was that the attraction between two pieces of mica rose and fell as the faces twisted relative to each other, like when trying to make a sandwich out of two flat refrigerator magnets (go on, try it). In fact, the attraction rose and fell every 60 degrees, corresponding with the internal architecture of the mineral, which is almost hexagonal like a honeycomb cell.

Although other researchers more than a decade ago had predicted this cyclical attraction would happen, this is the first time scientists had measured the forces. Knowing the strength of the forces is key to manipulating crystallization in a research or industrial setting.

But other things were abuzz in the mica face-off as well. Between the two surfaces, the liquid environment housed electrically charged ions from salts, normal elements found during crystallization in nature. The water and the ions formed a somewhat stable layer between the surfaces that partly kept them separated. And as they moved toward each other, the two mica surfaces paused there, balanced between molecular attraction and repulsion by water and ions.

The team also found they could manipulate the strength of that attraction by changing the type of ions, their concentration, and the temperature. Different types of ions and their concentrations changed electrostatic repulsion between the mica surfaces. The size of the ions and how many charges they carried also created more or less space within the meddling layer.

Lastly, higher temperatures increased the strength of the attraction, contrary to how temperature behaves in simpler, less complex scenarios. The researchers built a model of the competing forces that included van der Waals, electrostatic, and hydration forces.

In the future, the researchers say, the principles gleaned from this study can be applied to other materials, which would be calculated for the material of interest. For example, manipulating the attraction might allow researchers to custom-build crystals of desired sizes and shapes and with unique properties. Overall, the work provides insights into crystal growth through nanoparticle assembly in synthetic, biological, and geochemical environments.