Tag Archives: letter-mems-tech

Columbia researchers have observed the fractional quantum Hall effect in bilayer graphene and shown that this exotic state of matter can be tuned by an electric field.

The fractional quantum Hall effect, which can occur when electrons confined to thin sheets are exposed to large magnetic fields, is a striking example of collective behavior where thousands of individual electrons behave as a single system. However, while the basic theory describing this effect is well established, many details of this collective behavior remain not well understood, in part because it is only observable in systems with extremely low disorder.

Graphene, an atomically thin sheet of carbon, is a promising material for study of the fractional quantum Hall effect both because it can it be a nearly defect-free crystal, and because researchers can ‘tune’ the charge density with an external metal ‘gate’ electrode and observe how the quantum states evolve in response. Over the past several years, a collaborative effort at Columbia University spanning researchers from Mechanical Engineering, Electrical Engineering and Physics, developed a series of breakthrough fabrication techniques in order to take advantage of this opportunity, allowing them to report the first observation of the fractional quantum Hall effect in graphene in 2009, and the first wide-range tuning of the effect in 2011.

An even more interesting system for study of the fractional quantum Hall effect is so-called bilayer graphene, which consists of two stacked graphene sheets. In this material, use of two metal gate electrodes (above and below) allows independent tuning of the charge density in each layer, which provides a completely new way to manipulate the fractional quantum Hall states. In particular, theory predicts that it should be possible to create exotic ‘non-abelian’ states that could be used for quantum computation.

While observation of the fractional quantum Hall effect in single layer graphene required simply making cleaner devices, observing this effect in bilayer graphene proved more difficult. “We knew that we could fabricate very clean bilayer graphene structures, but we suffered from our inability to make good electrical contact since bilayer graphene develops an electronic ‘band-gap’ under the high magnetic fields and low temperatures required for our experiments,” says Cory Dean, professor of Physics who recently moved to Columbia University, and lead author on the paper. A critical breakthrough was re-design of the devices so that the charge density in the contact regions could be tuned independently from the rest of the device, which allowed them to maintain good electrical contact even under large magnetic fields. “Once we had this new device structure the results were spectacular.”

Reporting in the July 4, 2014 edition of Science, the team demonstrates the existence of the fractional quantum Hall effect in bilayer graphene and shows evidence of a controllable phase transition by application of electric fields. One of the key questions towards understanding the fractional quantum Hall effect in any system is to identify the order associated with the ground state. For example, do all electrons associated within the collective state carry the same spin? In bilayer graphene this question is more complex since there are several degrees of symmetry at play all at once. In addition to spin, electrons can polarize by spontaneously residing entirely on one layer versus the other. This complexity provides an interesting new phase space to explore for new and unusual effects. In particular, several theories have predicted that application of electric fields to bilayer graphene could enable transitions between these ground state orders. “This is a new experimental knob that just is not available in other systems,” says James Hone, a professor of Mechanical Engineering and co-author on the paper. The team has confirmed for the first time that varying the applied electric field causes a phase transition, but the exact nature of these different phases remains an open question. “While theory expects that we can tune the ground state order, the complexity of the system makes it difficult to determine exactly which order is actually realized,” says Physics Professor and co-author Philip Kim.

“This is where the next phase of our research is headed,” says Dean. “The implications for this result could be far reaching,” he adds, “While we do not yet see any evidence of non-abelian states, the fact that we are able to modify the nature of the fractional quantum Hall effect by electric fields is a really exciting first step.”

While previous efforts have been able to demonstrate different aspects of the sample requirement, no other group has been able to bring this all together into a single device. Dean attributes this success to the unique collaborative environment fostered at Columbia University. “This is truly a remarkable environment,” he says, adding, “The open exchange of ideas across several disciplines makes the environment at Columbia a fertile ground for doing great science.” Device fabrication and initial testing was done at Columbia University. Measurement under large magnetic fields was then performed by the Columbia team with the aid of the National High Magnetic Field Laboratory user facility in Tallahassee, Florida. “We have established a fantastic relationship with the NHFML over many years,” says Dean. “The support provided by the NHMFL personnel at both the technical and scientific level has been invaluable to our efforts.”

Bending the rules


July 1, 2014

For his doctoral dissertation in the Goldman Superconductivity Research Group at the University of Minnesota, Yu Chen, now a postdoctoral researcher at UC Santa Barbara, developed a novel way to fabricate superconducting nanocircuitry. However, the extremely small zinc nanowires he designed did some unexpected — and sort of funky — things.

Chen, along with his thesis adviser, Allen M. Goldman, and theoretical physicist Alex Kamenev, both of the University of Minnesota, spent years seeking an explanation for these extremely puzzling effects. Their findings appear this week in Nature Physics.

“We were determined to figure out how we could reconcile the strange phenomena with the longstanding rules governing superconductivity,” said lead author Chen. “The coexistence of superconductivity with dissipation, which we observed, is counterintuitive and bends the rules as we know them.”

Typically superconductivity and dissipation are thought to be mutually exclusive because dissipation, a process in thermodynamic systems whereby electric energy is transformed into heat, is a feature of a normal — versus a superconductive — state.

“But we discovered that superconductivity and dissipation can coexist under rather generic conditions in what appears to be a universal manner,” Chen said.

After long and careful work, which involved both experimental and theoretical efforts, the researchers found an explanation that fits. Behind all of the observed phenomena is a peculiar nonequilibrium state of quasiparticles — electron-like excitations that formed in the nanowires Chen designed.

The quasiparticles are created by phase slips. In a superconductive state, when supercurrent flows through the nanowire, the quantum mechanical function describing the superconductivity of the wire evolves along the length of the wire as a spiral shaped like a child’s Slinky toy. From time to time, one of the revolutions of the spiral contracts and disappears altogether. This event is called a phase slip. This quirk generates quasiparticles, giving rise to a previously undiscovered voltage plateau state where dissipation and superconductivity coexist.

“The most significant achievement was making the nanowires smaller and cooler than anyone had done previously,” Kamenev said. “This allowed the quasiparticles to travel through the wire faster and avoid relaxation. This leads to a peculiar nonthermal state, which combines properties of a superconductor and a normal metal at the same time.”

In addition to discovering this unique phenomenon, the team also found another heretofore-unseen property in the voltage plateau. When a magnetic field is turned on in the voltage plateau state, rather than shrinking the superconducting region, which is what would usually occur, the superconducting area expands and is enhanced.

“This is an unexpected property of very small nanowires,” said Goldman.

This state appears to be universal for ultra-small superconducting circuitry like Chen’s, which features ideal contacts between the nano-elements and the leads. Such nanoscale superconductors may be key components in future superconducting computer systems.

“Our findings demonstrate that superconducting nanocircuits can be used as a simple, but rather generic platform to investigate nonequilibrium quantum phenomena,” Chen concluded.

“Now we need to explore the parameters of nanowires that give rise to the effect and those that don’t,” Goldman said. “We also need to examine the behavior of wires of different lengths and different materials in order to further define the parameters.”

Do not fold, spindle or mutilate. Those instructions were once printed on punch cards that fed data to mainframe computers. Today’s smart phones process more data, but they still weren’t built for being shoved into back pockets.

In the quest to build gadgets that can survive such abuse, engineers have been testing electronic systems based on a new materials that are both flexible and switchable – that is, capable of toggling between two electrical states, on-off, one-zero, the binary commands that can program all things digital.

Now three Stanford researchers believe that they’ve discovered just such a flexible, switchable material. It is a crystal that can form a paper-like sheet just three atoms thick. Computer simulations show that this crystalline lattice has the remarkable ability to behave like a switch: it can be mechanically pulled and pushed, back and forth, between two different atomic structures – one that conducts electricity well, the other that does not.

“Think of it like flicking a light switch on and off,” says Karel-Alexander Duerloo, a Stanford Engineering graduate student and first author of an article in Nature Communications.

So far this discovery only exists as a simulation. But co-author and team leader Evan Reed, Assistant Professor of Materials Science and Engineering, hopes this work will inspire experimental scientists to fabricate this super-thin crystal and use it to create electronic and other devices that would be as light and flexible as fibers.

Theoretically, such electronic materials have potential to reduce battery-draining power consumption in existing devices such as smart phones. This new, power-efficient material could also make it possible to create ‘smart’ clothing – imagine an ultralight cell phone or a GPS system integrated into your shirt.

Duerloo said this switchable material is formed when one atomic layer of molybdenum atoms gets sandwiched between two atomic layers of tellurium atoms.

Molybdenum and tellurium are elements that are currently used as additives for making alloys, such as steel. Tellurium is also an important component of many modern solar cells.

In his simulation, Duerloo relied on the fact that molybdenum and tellurium form a sheet-like crystal lattice that is just three-atoms thick. Notably, this atomic sandwich can form different crystalline structures that have useful properties: in one structure this lattice easily conducts electricity; in the other configuration it does not (see graphic: https://stanford.box.com/s/kf9r191eu56sy63119fq caption below).

Duerloo’s simulations show that it takes just a tiny effort to toggle the atomic structure of this three-layer amalgam from a non-conductive state into a conductive state. A gentle push switches the material back to the off state (see animation: https://stanford.box.com/s/k8d1p7frar3hl9pfa333 caption below).

These simulations, as yet unsupported by experimental confirmation, are at the leading edge of a new branch of materials science that delves into the behavior of monolayer substances.

The first and most famous monolayer is graphene, which was first observed in 2004. Graphene is a layer of carbon atoms that form a lattice that resembles chicken wire. Although it is just one atom thick, graphene is incredibly strong. A sheet of graphene could bear the weight of a cat without breaking this atomically thin lattice.

Graphene is also electrically conductive. That makes it potentially useful as a light, low power electronic component.

The discoverers of graphene shared a Nobel Prize in 2010, but even before this their work was so honored that other scientists had started looking for other monolayer materials with this interesting confluence of properties: strong, stable, crystalline structures that could conduct electricity.

To help find the most promising materials from a vast universe of molecular structures, a new discipline is rising: computational materials science.

“We’re like the advance scouts that survey the terrain and look for the best materials,” Reed said.

Now that they have simulated the potential of this molybdenum-tellurium crystal the Stanford researchers – the third team member is graduate student Yao Li — hope experimental scientists will explore possible uses of this three-atom thick switch.

“No would have known this was possible before because they didn’t know where to look,” Duerloo said.

By Pushkar P. Apte

Research forms the DNA of the semiconductor industry — few other industries invest as much as a percentage of revenue.  Semiconductor research has always driven the Moore’s Law Mantra of continuously making things “cheaper, better, faster, and smaller.”  This brought the industry into the realm of nanotechnology, where manipulation of just a few atoms yields complex quantum effects.   This requires multi-faceted innovation — in materials, processing and characterization techniques, and advanced patterning to fabricate these tiny devices.  To explore the various dimensions of this innovation, SEMICON West 2014 features a session on “Breakthrough Research Technologies.”

Dr. Thorsten Lill from Lam Research, who will present at this session, explains, “As device dimensions and their allowed tolerances approach the same order of magnitude as inter-atomic distances, atomic scale processing will soon be a necessity. We will discuss the framework of developing production-worthy atomic layer etch (ALE) to achieve layer-by-layer removal with atomic fidelity.”   This is nanotechnology at its best — where we are engineering materials by each atomic layer.  Precision engineering also requires precision simulation and modeling, as Dr. Peter Ventzek of Tokyo Electron will describe for plasma chemistries used in etching and other processes.

We also need novel techniques to characterize materials at the atomic level.  Dr. John Mardinly of ASU, also presenting at this session, describes one such technique: “Aberration-Corrected Transmission Electron Microscopy provides numerous advantages over conventional Field-Emission Transmission Electron Microscopes. Resolution is improved by 2-3 times, and imaging and analysis can be conducted with lower accelerating voltages so that specimen damage is reduced.” In conventional bright field imaging, delocalization is eliminated so that interfaces between materials are imaged precisely. In Scanning Transmission Electron Microscopes, the beam current density can be 8-10 times higher than in a convention Field-Emission Scanning Transmission Electron Microscope. This dramatically improves the visibility of features and enables chemical mapping through Electron Energy Loss Spectroscopy (EELS) or Energy-dispersive X-ray Spectroscopy (EDS) with atomic resolution.

Aside from technology drivers, new market forces are now re-shaping the industry: the rise of the individual consumer as the dominant end-user; the application of technology to diverse fields such as energy, transportation and healthcare; and the rapid proliferation of the Internet of Things.  These entail complex functionality, which often requires complementing nanoscale digital devices with “More-than-Moore” functions like biological sensors, better analog and power devices other silicon.  Prof. Oliver Brand will describe research advances in the “More-than-Moore” areas of chemical microsensors, inertial sensors and micromachined ultrasonic transducers at the Center for Microelectromechanical Systems (MEMS) and Microsystem Technologies on the Georgia Tech campus.

This multi-dimensional, fast-paced research requires academia, research institutes, and the industry to work closely together, not just to push the research, but also to implement it in the “real world” successfully.  Prof. Douglas A. Keszler from OSU will describe how the Center for Sustainable Materials Chemistry focuses on breakthrough research to enable next-generation capabilities in semiconductor and display manufacturing, highlighting the technologies of spin-outs Inpria and Amorphyx.  Dr. Michael Khbeis of the Washington Nanofabrication Facility (WNF) at the University of Washington will describe advances in packaging/3D integration, silicon photonics, magnetic materials, superconductivity, photovoltaics, and quantum information systems.

It is exciting to imagine what capabilities such breakthrough research will enable in the future. From cramming more than a billion electronic transistors on a thumbnail, decoding the human genome, and placing a powerful computer in everyone’s pocket, how will technology change the world?  Besides richer computing, communications, and entertainment, we can now also tackle large and meaningful topical challenges, such as climate change, energy and water conservation, implementation of renewables, and affordable, effective healthcare.  Information Technology and the Internet of Things are already making a dent in these challenges and enabling research as discussed will accelerate this and help us improve our lives and our planet.

University and research institutions are increasingly important to help accelerate technology advances. Learn more about the latest — from speakers working in research at Arizona State University, Georgia Institute of Technology, Lam Research/KU Leuven/imec Joint Project, Oregon State University, TEL, U. of Washington — at the Breakthrough Research Technologies session at SEMICON West 2014.

By Shannon Davis, Web Editor

Overheard @The ConFab: “I feel the best I’ve felt about semi since 2009.” –Mike Noonen, Silicon Catalyst

Monday’s research and development panel discussion at The ConFab 2014 started on that optimistic note as Moderator Scott Jones of AlixPartners led a discussion on Optimizing R&D Collaboration. Panelists Chris Danely of JP Morgan, Lode Lauwers of imec, Rory McInerney of Intel and Mike Noonen of Silicon Catalyst discussed where the next big growth drivers will come from and the ability of the industry to continue scaling and remain on Moore’s Law through the introduction of new technologies such as EUV, Advanced Packaging and 450mm. The panel also touched on the role startups will play and how increased collaboration can benefit the industry.

Here are highlights from Monday’s discussion.

How do you feel about the semiconductor cycle – is that at a positive point for innovation and small, start-up companies?

Mike Noonen: I feel the best about I’ve felt about semi since 2009. Without a doubt. When you combine that situation that we’re in with a couple driving forces, all of that has fundamental benefits to the semiconductor business at large. You take those mega trends that are not leading edge applications with the challenge of Moore’s Law – those are developing a whole host of innovation. We think this is a great time to think about how to reinvigorate startups – this is the best time to think about innovation.

From left to right: Panelists Chris Danely of JP Morgan, Mike Noonen of Silicon Catalyst, Lode Lauwers of imec, and Rory McInerney of Intel

From left to right: Panelists Chris Danely of JP Morgan, Mike Noonen of Silicon Catalyst, Lode Lauwers of imec, and Rory McInerney of Intel

Consolidation is a big theme right now. Is this something that’s holding us back the industry?

Rory McInerney: I don’t think the industry is consolidating for us as much as we think. The big players are still HP, Lenovo, etc. The new players are Google, Facebook, Amazon, etc. – many didn’t exist 10 years ago. Within our world, there’s the traditional space, but there’s a ton of new stuff in the cloud and server segment.

Tell us some of the most exciting areas Intel is participating in.

Rory McInerney: On the data center side, we do want our 10 and 7nm, but one of the drivers of our business is the massive amount of data being generated around the world. There are tens of billions of devices that will be connected to the Internet in the few years. The only commonality in the [IoT] numbers is that they go up. All of them will have some element of connectivity and with that comes data. And that drives a virtual cycle. In our business, we love this – my point is, there’s a huge room for innovation. The innovation isn’t just the device but the software and application side.

How do investors view the emerging markets and trends? Do they see the opportunities or are they still focusing on traditional markets?

Chris Danely: From a broad perspective, the thing that an analyst looks at – are they playing to their strengths? You might have a company that starts out very successful, but they don’t play to their strengths and start to waste money. For example, Texas Instruments has taken their R&D down, but still outgrow the industry, because they play to their strengths. Another example is Intel – in the last 3 years, they were in the foundry business – we see a lot of potential to upset the apple cart in the foundry business. Nobody else could do this, but this is an area where we see them exploiting their strengths. Is the company playing to its strengths? We also look at ARM on servers – we don’t know if this is going to work or not, but I don’t think this changing the landscape of the industry. There’s still a bright future with semiconductor stocks.

How can executives communicate their R&D strategy better?

Chris Danely: I’ll use my personal experience – you want to keep that message very simple. Identify the growth trends. Make sure the message goes out continuously. Don’t be afraid to use a few buzz words/charts.

Lode Lauwers: If I may, Wall Street is looking in the short term. Time scale [for R&D] is close to 15 years. I don’t know if Wall Street has that visibility. I think a company should consider R&D as a long term investment. We go for long term engagements.

Rory McInerney: It’s a portfolio question in terms of R&D – you’re going to have your short term and your long term investments. I don’t think Wall Street is looking at all the details of investments. I think that our investments on the product side go out 10 years, but they’re small compared to our other investments.

Chris Danely: Wall Street has to consider about things on a six month basis.

Mike Noonen: Biotech, which has a very long time to market, is the second largest venture capital in the US. Biotech has remained lucrative and interesting in the US. In this area, companies go after a single application or problem, and it’s a vibrant and healthy investment. The take away is – it’s all about the economics. It might enable small start ups to innovate and then be acquired.

How should the industry leverage a company like imec?

Lode Lauwers: More than ever, you need to build partnerships. In this industry, we used to say, “Our company can work on its own.” Now, your ecosystem needs to become wider. Ten years ago, people were still sponsoring R&D. Now we are assessed in every individual area, deliverable by deliverable, on does it benefit, is there ROI. You need to be able to deliver relevant work. A company on its own doesn’t always have these abilities in house. Using imec, it’s like building on competences.

Do you see differences in how you approach partnerships?

Chris Danely: The CEOs and CFOs of semi companies are under pressure to not increase expenses, and that’s stifled risk-taking. Some are now approaching R&D through acquisition of startups with personnel – rather than partnerships.

Do you think these companies are larger – semi is a part of a much larger landscape – do you think this might drive the industry/change the landscape?

Rory McInerney: About 70-80 percent of cloud computing today is driven by the social media. That didn’t exist 5 years ago. There is a direct link between that and the changing semi landscape.

What is the biggest risk in the industry right now?

Chris Danely: Saturation. Semi companies are profitable, but we’re starting to see a lot of them, especially as fablite and fabless models are catching on.

Moderator Scott Jones of AlixPartners

Moderator Scott Jones of AlixPartners

The National University of Singapore, Singapore University of Technology and Design, and the A*STAR institutes: Institute of High Performance Computing and Institute of Materials Research and Engineering announced their recent discovery of quantum plasmonic tunneling.

Plasmon tunneling is a quantum-mechanical effect where electrons rapidly oscillate across very closely-spaced metal structures. Using a Titan scanning/transmission electron microscope (S/TEM), the scientists were able to not only observe this new phenomenon directly, but also control the frequency of the tunneling currents by placing single layers of different molecules between the closely-spaced metal particles. The speed of the switching will directly depend on the nature of the molecules used.

“In our research, we were able to demonstrate that the rapid current oscillations could take place over distances larger than a nanometer, which, although extremely small, opens up possibilities for new technological applications,” states one of the researchers and Science paper authors, Dr Michel Bosman, Institute of Materials Research and Engineering, A*STAR, Singapore.

Surface plasmons in metal particles can be introduced by simply shining light of the right color on them. By using the researchers’ approach, incoming light will then produce the small tunneling currents between the nearby metal particles. In effect, tiny electrical circuits are made that operate at enormously high speeds.

Today’s electrical circuits can operate up to GHz frequencies, but due to design issues, this is close to their inherent speed limit at room temperature. In order for devices to work faster, entirely new circuit designs are required. The research presented here shows a possible route for such optical circuits, by light-generated tunneling currents with operation speeds tens of thousands of times faster than today’s microprocessors.

Trisha Rice, vice president and general manager of Materials Science for FEI, comments, “This is incredible work being done by these researchers in Singapore, using the high-energy resolution of a monochromated Titan S/TEM to directly observe and control a quantum plasmonic tunneling event. Congratulations on this achievement and we look forward to learning of new and exciting results in this area.”

All over the world researchers are investigating solar cells which imitate plant photosynthesis, using sunlight and water to create synthetic fuels such as hydrogen. Empa researchers have developed such a photoelectrochemical cell, recreating a moth’s eye to drastically increase its light collecting efficiency. The cell is made of cheap raw materials – iron and tungsten oxide.

Rust – iron oxide – could revolutionize solar cell technology. This usually unwanted substance can be used to make photoelectrodes which split water and generate hydrogen.  Sunlight is thereby directly converted into valuable fuel rather than first being used to generate electricity. Unfortunately, as a raw material iron oxide has its limitations. Although it is unbelievably cheap and absorbs light in exactly the wavelength region where the sun emits the most energy, it conducts electricity very poorly and must therefore be used in the form of an extremely thin film in order for the water splitting technique to work. The disadvantage of this is that these thin-films absorb too little of the sunlight shining on the cell.

Microspheres to collect the sunlight

Empa researchers Florent Boudoire and Artur Braun have now succeeded in solving this problem. A special microstructure on the photoelectrode surface literally gathers in sunlight and does not let it out again. The basis for this innovative structure are tiny particles of tungsten oxide which, because of their saturated yellow colour, can also be used for photoelectrodes. The yellow microspheres are applied to an electrode and then covered with an extremely thin nanoscale layer of iron oxide. When external light falls on the particle it is internally reflected back and forth, till finally all the light is absorbed. All the entire energy in the beam is now available to use for splitting the water molecules.

In principle the newly conceived microstructure functions like the eye of a moth, explains Florent Boudoire. The eyes of these night active creatures need to collect as much light as possible to see in the dark, and also must reflect as little as possible to avoid detection and being eaten by their enemies. The microstructure of their eyes especially adapted to the appropriate wavelength of light. Empa’s photocells take advantage of the same effect.

In order to recreate artificial moth eyes from metal oxide microspheres, Florent Boudoire sprays a sheet of glass with a suspension of plastic particles, each of which contains at its center a drop of tungsten salt solution. The particles lie on the glass like a layer of marbles packed close to each other. The sheet is placed in an oven and heated, the plastic material burns away and each drop of salt solution is transformed into the required tungsten oxide microsphere. The next step is to spray the new structure with an iron salt solution and once again heat it in an oven.

“Capturing light” simulated on the computer

Now, one could interpret these mixing, spraying and burning processes as pure alchemy – a series of steps that is eventually successful by pure chance. However in parallel to their practical experiments, the researchers have been running calculations modelling the process on their computers and have thus been able to simulate the «capturing of light» in the tiny spheres. The results of the simulation agree with the experimental observations, as project leader Artur Braun confirms. It is clear to see how much the tungsten oxide contributes to the photo current and how much is due to the iron oxide. Also, the smaller the microspheres the more light which lands on the iron oxide underneath the tiny balls. As a next step the researchers plan to investigate what the effect of several layers of microspheres lying on top of each other might be. The work on moth eye solar cells is still in progress.

Storing gas on a sorbent provides an innovative, yet simple and lasting solution.

BY KARL OLANDER, Ph.D. and ANTHONY AVILA, ATMI, Inc., an Entegris company, Billerica, MA

The period following the introduction of subatmospheric pressure gas storage and delivery was punctuated by continuous technical innovation.

Even as the methodology became the standard for supplying ion implant dopants, it continued to rapidly evolve and improve. This article reflects on the milestones of the last 20 years and considers where this technology goes from here.

From the beginning, the semiconductor industry’s concern over using highly toxic process gases was evident by the large investment being made in dedicated gas rooms, robust ventilation systems, scrubbers, gas containment protocols and toxic gas monitoring. While major advances have been made in the form of automated gas cabinets and valve manifold boxes, gas line components, improved cylinder valves and safety training, the underlying threat of a catastrophic gas release remained.

Risk factors targeted

The underlying risk with compressed gases is twofold: high pressure, which provides the motive force to discharge the contents of a cylinder, and secondly, a relatively large hazardous production material inventory, which can be released during a containment breach. Pressure also is a factor in component failure and gas reactivity, e.g., corrosion. Mitigating these issues would considerably increase safety.

FIGURE 1. The stages of developing a new chemical precursor for use in commercial IC production.

FIGURE 1. The stages of developing a new chemical precursor for use in commercial IC production.

Analysis of the risks suggested an on-demand, point-of-use gas generator would improve safety by both reducing operating pressure and gas inventory[1]. The challenges associated with this approach include complexity of operation and gas purity, especially in a fab or process tool setting. Chemical generation of arsine, while possible, per equation [A], also substituted a highly reactive toxic solid for arsine[2]. Considerable safety and environmental issues accompanied the operation of such a generator. An on-demand, point-of-use electrochemical approach for supplying arsine, per equation [B], would also eliminate the need for high pressure storage if the associated operational issues could be overcome. Numerous attempts at developing a commercial electrochemical generator just never proved successful[3].

[A] KAsH2 + H2O —> AsH3/H2O + KOH
[B] As(s) + 3H2O + 3e(-) —> AsH3(g) + 3OH(-)

Innovation from a simple(r) solution

Pressure swing adsorption processes utilize the selective affinity between gases and solid adsorbents, and are widely used to recover and purify a range of gases. Under optimal conditions, the gas adsorption process releases energy and produces a material that behaves mores like a solid than a gas.

Early work at reversibly adsorbing toxic materials on a highly porous substrate showed promise. In 1988, the Olin Corporation described an arsine storage and delivery system where the gas was [reversibly] adsorbed onto a zeolite, or microporous alumino- silicate, material[4]. A portion of the stored gas could be recovered by heating the storage vessel to develop sufficient arsine pressure to supply a process tool. In 1992, ATMI supplied a prototype system based on the Olin technology to the Naval Research Lab in Washington, D.C.

The breakthrough that lead to the first commercial subatmospheric pressure gas storage and delivery system occurred when ATMI reported the majority of the adsorbed gas could be supplied to the process by subjecting the storage vessel to a strong vacuum. Using vacuum rather than thermal energy simplified the process, providing the means for an on-demand system[5]. Using a sorbent had the effect of turning the gas into something more akin to a “solid.” That characteristic, coupled with the absence of a pressure driver, delivered an inherently safe condition. The vacuum delivery condition also helped define where the technology would find its first application: ion implantation[6].

Safe and efficient gas storage and delivery

In 1993, prototype arsine storage and delivery cylinders based on vacuum delivery were beta tested at AT&T in Allentown, PA[g] [f]. The system was trademarked Safe Delivery Source®, or SDS®. Papers were presented on safe storage and delivery of ion implant dopant gases the following year in Catania, Sicily at the International Ion Implant Technology Conference[7].

The goal to find a safer method to offset the use of compressed gases was realized: (1) gas is stored at low pressure (ca. 650 Torr at 21°C) and (2) the potential for large and rapid gas loss is averted. Leaks, if they occur, whether by accidental valve opening or a containment breach, would be first inward into the cylinder. Once the pressure equalizes, gas loss to the environment would be governed mainly by diffusion as the gas molecules remain associated with the sorbent. The SDS package, while not a gas generator per se, effectively functions like one.

FIGURE 2. Cutaway view of SDS3 carbon pucks within a finished cylinder.

FIGURE 2. Cutaway view of SDS3 carbon pucks within a finished cylinder.

While subatmospheric pressure operation is an artifact of having to “pull the gas” away from the sorbent, it has become synonymous with safe gas delivery. The optimization work which followed focused on reducing pressure drop in the gas delivery system by improving conductance in valves, mass flow controllers and delivery lines. A restrictive flow orifice was no longer required. The new gas sources proved to work best when in close proximity to the tool.

The years after this technology introduction also saw considerable efforts to improve the sorbent; ultra-pure carbon replaced the zeolite-based material used in the first generation SDS (SDS1), roughly doubling the deliverable quantities of gas per cylinder. These granular carbon sorbents in the SDS2 were later replaced by solid, round monolithic carbon “pucks” in SDS3 (FIGURE 2), which necessitated the cylinder be built around the sorbent[8]. This improvement again roughly doubled gas cylinder capacity.

Recognized in international standards

In 2012, the United Nations (U.N.) recognized the uniqueness of adsorbed gases and amended the Model Regulation for the Transport of Dangerous Goods by creating a new “condition of transport” for gases adsorbed on a solid and assigning a total of 17 new identification numbers and shipping names to the Dangerous Good List. Adoption is expected to occur by 2015. A few of the additions are noted here.

Arsine   – UN 2188 – compressed
Arsine, adsorbed – UN 3522 – SDS
Phosphine – UN 2199 – compressed
Phosphine, adsorbed – UN 3525 – SDS

FIGURE 3. The evolution of a SAGS Type 1 gas package.

FIGURE 3. The evolution of a SAGS Type 1 gas package.

In recent years, fire codes have been updated through the definition and classification of subatmospheric Gas Systems, or SAGS, based on the internal [storage] pressure of the gas.9 Systems based on both sub-atmospheric pressure storage and delivery are designated as Type 1 SAGS. It is important to note that the UN definition for adsorbed gases, and the resulting new classifications mentioned above, only applies to Type 1 SAGS, defined as follows:

3.3.28.5.1 Subatmospheric Gas Storage and Delivery System (Type 1 SAGS). A gas source package that stores and delivers gas at sub-atmospheric pressure and includes a container (e.g., gas cylinder and outlet valve) that stores and delivers gas at a pressure of less than 14.7 psia at NTP.

It is also worth mentioning that sub-atmospheric pressure gas delivery can also be achieved using high pressure cylinders by embedding a pressure reduction and control system. The Type 2 SAGS typically employs a normally closed, internal regulator[s] that a vacuum condition to open. This is not a definition of sub-atmospheric storage and delivery, but of sub-atmospheric delivery only.

3.3.28.5.2 Subatmospheric Gas Delivery System (Type 2 SAGS). A gas source package that stores compressed gas and delivers gas subatmospherically and includes a container (e.g., gas cylinder and outlet valve) that stores gas at a pressure greater than 14.7 psia at NTP and delivers gas at a pressure of less than 14.7 psia at NTP.

In general, Environmental Safety and Health managers, risk underwriters and authorities having jurisdiction recognize the importance of SAGS and requires recommend their use whenever process conditions allow[10].

Expanding SAGS into new applications

Taking the lessons learned from SDS2/SDS3 in ion implant operations, along with key findings from
other applications like HDP-CVD (the SAGE package) and combined with sorbent purification and carbon nanopore size tuning, SAGS Type 1 packages are poised to offer their safety advantages in new and emerging areas, as well as add even more safety and efficiency benefits. Currently, a new package called Plasma Delivery SourceTM (PDSTM) is available for high flow rate applications, while maintaining all the safety attributes of the SAGS Type 1 package.

Also, in addition to the inherent safety, PDS employs a pneumatic operator (valve) to the cylinder which further minimizes the opportunity for human error. In an emergency, such as a toxic gas alarm, pressure excursion, loss of exhaust, etc., gas flow at the source can be quickly stopped and the cylinder isolated. Cycle/purge operations are made safer as human involvement is minimized. Human-initiated events, like over-torqueing the valve, failing to close the valve or even back-filling a cylinder with purge gas, are prevented.

SDS1 SDS2 SDS3
Arsine 200 559 835
Phosphine 85 198 385

Expanding the use of SAGS beyond the domain of ion implant involves successfully navigating key process factors such as operating pressure, flow rates, proximity to the tool and purity. One approach includes coupling the PDS cylinder and gas cabinet together to yield a plug and play “smart” delivery system. Unlike high pressure systems, which are more concerned with excess flow situations, knowing and controlling pressure allows a SAGS cabinet to operate at a reduced risk. This enables linking cabinet ventilation rates with the system operating pressure. During normal operating conditions, the exhaust rate could be reduced by up to 80 percent because the system is operating sub-atmospherically. Should the operating pressure exceed a preset threshold, the exhaust flow would automatically revert to a higher range or the cylinder valve would close.

The future, therefore, could see these PDS packages extended to another level by incorporating them into smart delivery systems, which will further reduce risk, maximize efficiency, improve cost of ownership and expand the footprint for SAGS into new applications like plasma doping, solar, epitaxy and etch.

Summary

During the last 20 years, the semiconductor industry undertook a large effort to develop safer gas delivery technologies to reduce risks associated with dopants used in ion implant. Many technologies were considered, including chemical and electrochemical gas generators, complexing gases with ionic liquids or mechanically controlling cylinder discharge pressure using embedded regulator devices.

In the end, storing gas on a sorbent provided an innovative, yet simple and lasting solution. Gas-sorbent interactions are well understood, reproducible and can be achieved with a minimum of moving parts. Gas release risks, driven by pressure, are all but removed from consideration. And any potential for human error continues to be a target for improvement wherever toxic gases are used.

References

1. Proc. Natl. Acad. Sci. USA 89 pp 821-826, 1992.
2. Appl. Phys. Lett., 60 1483
3. Electron Transfer Technology, US Patent 59225232
4. Olin Corporation, US Patent US4744221A
5. Advanced Technology Materials, US Patent US5518528 6. Many thanks to Dan McKee and Lee Van Horn for being the first of many early adopters.
7. Proceedings of the Tenth International Conference on Ion Implantation Technology, 1994, pp 523-526.
8. DOT-SP 13220.
9. NFPA 318, Standard for the Protection of Semiconductor Fabrication Facilities 2012 Edition. 10. SAGS in the FAB, SST reference

ATMI is a wholly owned subsidiary of Entegris, Inc. ATMI, Safe Delivery Source, SDS, Plasma Delivery Source and PDS are trademarks of Entegris, Inc. in the U.S., other countries, or both. All other names are trademarks of their respective companies.

A route for constructing protein nanomachines engineered for specific applications may be closer to reality.

Biological systems produce an incredible array of self-assembling, functional protein tools. Some examples of these nanoscale protein materials are scaffolds to anchor cellular activities, molecular motors to drive physiological events, and capsules for delivering viruses into host cells.

Scientists inspired by these sophisticated molecular machines want to build their own, with forms and functions customized to tackle modern-day challenges.

This is a computational model of a successfully designed two-component protein nanocage with tetrahedral symmetry. Credit: Dr. Vikram Mulligan

This is a computational model of a successfully designed two-component protein nanocage with tetrahedral symmetry.
Credit: Dr. Vikram Mulligan

The ability to design new protein nanostructures could have useful implications in targeted delivery of drugs, in vaccine development and in plasmonics — manipulating electromagnetic signals to guide light diffraction for information technologies, energy production or other uses.

A recently developed computational method may be an important step toward that goal. The project was led by the University of Washington’s Neil King, translational investigator; Jacob Bale, graduate student in Molecular and Cellular Biology; and William Sheffler in David Baker’s laboratory at the University of Washington Institute for Protein Design, in collaboration with colleagues at UCLA and Janelia Farm.

The work is based in the Rosetta macromolecular modeling package developed by Baker and his colleagues. The program was originally created to predict natural protein structures from amino acid sequences. Researchers in the Baker lab and around the world are increasingly using Rosetta to design new protein structures and sequences aimed at solving real-world problems.

“Proteins are amazing structures that can do remarkable things,” King said, “they can respond to changes in their environment. Exposure to a particular metabolite or a rise in temperature, for example, can trigger an alteration in a particular protein’s shape and function.” People often call proteins the building blocks of life.

“But unlike, say, a PVC pipe,” King said, “they are not simply construction material.” They are also construction (and demolition) workers — speeding up chemical reactions, breaking down food, carrying messages, interacting with each other, and performing countless other duties vital to life.

Reporting in the June 5 issue of Nature, the researchers describe the development and application of new Rosetta software enabling the design of novel protein nanomaterials composed of multiple copies of distinct protein subunits, which arrange themselves into higher order, symmetrical architectures.

With the new software the scientists were able to create five novel, 24-subunit cage-like protein nanomaterials. Importantly, the actual structures, the researchers observed, were in very close agreement with their computer modeling.

Their method depends on encoding pairs of protein amino acid sequences with the information needed to direct molecular assembly through protein-protein interfaces. The interfaces not only provide the energetic forces that drive the assembly process, they also precisely orient the pairs of protein building blocks with the geometry required to yield the desired cage-like symmetric architectures.

Creating this cage-shaped protein, the scientists said, may be a first step towards building nano-scale containers. King said he looks forward to a time when cancer-drug molecules will be packaged inside of designed nanocages and delivered directly to tumor cells, sparing healthy cells.

“The problem today with cancer chemotherapy is that it hits every cell and makes the patient feel sick,” King said. Packaging the drugs inside customized nanovehicles with parking options restricted to cancer sites might circumvent the side effects.

The scientists note that combining just two types of symmetry elements, as in this study, can in theory give rise to a range of symmetrical shapes, such as cubic point groups, helices, layers, and crystals.

King explained that the immune system responds to repetitive, symmetric patterns, such as those on the surface of a virus or disease bacteria. Building nano-decoys may be a way train the immune system to attack certain types of pathogens.

“This concept may become the foundation for vaccines based on engineered nanomaterials,” King said. Further down the road, he and Bale anticipate that these design methods might also be useful for developing new clean energy technologies.

The scientists added in their report, “The precise control over interface geometry offered by our method enables the design of two-component protein nanomaterials with diverse nanoscale features, such as surfaces, pores, and internal volumes, with high accuracy.”

They went on to say that the combinations possible with two-component materials greatly expand the number and variety of potential nanomaterials that could be designed.

It may be possible to produce nanomaterials in a variety of sizes, shapes and arrangements, and also move on to construct increasingly more complex materials from more than two components.

The researchers emphasized that the long-term goal of such structures is not to be static. The hope is that they will mimic or go beyond the dynamic performance of naturally occurring protein assemblies, and that eventually novel molecular protein machines could be manufactured with programmable functions.

The researchers pointed out that although designing proteins and protein-based nanomaterials is very challenging due to the relative complexity of protein structures and interactions, there are now more than a handful of laboratories around the world making major strides in this field. Each of the leading contributors have key strengths, they said. The strengths of the UW team is in the accuracy of the match of the designed proteins to the computational models and the predictability of the results.

Faster electronic device architectures are in the offing with the unveiling of the world’s first fully two-dimensional field-effect transistor (FET) by researchers with the Lawrence Berkeley National Laboratory (Berkeley Lab). Unlike conventional FETs made from silicon, these 2D FETs suffer no performance drop-off under high voltages and provide high electron mobility, even when scaled to a monolayer in thickness.

Faster electronic device architectures are in the offing with the unveiling of the world’s first fully two-dimensional field-effect transistor (FET) by researchers with the Lawrence Berkeley National Laboratory (Berkeley Lab). Unlike conventional FETs made from silicon, these 2D FETs suffer no performance drop-off under high voltages

Ali Javey, a faculty scientist in Berkeley Lab’s Materials Sciences Division and a UC Berkeley professor of electrical engineering and computer science, led this research in which 2D heterostructures were fabricated from layers of a transition metal dichalcogenide, hexagonal boron nitride and graphene stacked via van der Waals interactions.

“Our work represents an important stepping stone towards the realization of a new class of electronic devices in which interfaces based on van der Waals interactions rather than covalent bonding provide an unprecedented degree of control in material engineering and device exploration,” Javey says. “The results demonstrate the promise of using an all-layered material system for future electronic applications.”

Javey is the corresponding author of a paper describing this research in ACS Nano titled “Field-Effect Transistors Built from All Two-Dimensional Material Components“. Co-authors are Tania Roy, Mahmut Tosun, Jeong Seuk Kang, Angada Sachid, Sujay Desai, Mark Hettick and Chenming Hu.

FETs, so-called because an electrical signal sent through one electrode creates an electrical current throughout the device, are one of the pillars of the electronics industry, ubiquitous to computers, cell phones, tablets, pads and virtually every other widely used electronic device. All FETs are comprised of gate, source and drain electrodes connected by a channel through which a charge-carrier – either electrons or holes – flow. Mismatches between the crystal structure and atomic lattices of these individual components result in rough surfaces – often with dangling chemical bonds – that degrade charge-carrier mobility, especially at high electrical fields.

“In constructing our 2D FETs so that each component is made from layered materials with van der Waals interfaces, we provide a unique device structure in which the thickness of each component is well-defined without any surface roughness, not even at the atomic level,” Javey says. “The van der Waals bonding of the interfaces and the use of a multi-step transfer process present a platform for making complex devices based on crystalline layers without the constraints of lattice parameters that often limit the growth and performance of conventional heterojunction materials.”

Javey and his team fabricated their 2D FETs using the transition metal dichalcogenide molybdenum disulfide as the electron-carrying channel, hexagonal boron nitride as the gate insulator, and graphene as the source, drain and gate electrodes. All of these constituent materials are single crystals held together by van der Waals bonding.

For the 2D FETs produced in this study, mechanical exfoliation was used to create the layered components. In the future, Javey and his team will look into growing these heterogeneous layers directly on a substrate. They will also look to scale down the thickness of individual components to a monolayer and the lengths of the channels to molecular-scale dimensions.

This research was funded by the U.S. Department of Energy’s Office of Science.