Category Archives: Energy Storage

(October 28, 2010) — Andrew Smith, Ventmark Technology Solutions, presents a 3D die stacking technology to address package miniaturization. Using bare die and vertical interconnect structures, this stacking technology permits the design of ultra-thin, near-chip-scale packaging (CSP) solutions without TSVs. Designers lacking custom integrated circuits (ICs) should look to new chip stacking technology to meet size and performance needs of integrating a range of devices into a small space.

Packaging technology’s evolution — from single-chip surface mount technology (SMT) packages to chip-on-board (COB) multi-chip modules and, most recently, package-on-package (POP) solutions — has greatly increased electronics density. Increasing demands for reduced size and increased functionality, however, often require higher levels of integration than current technologies support.

Figure 1. a) Wire bond fan out from a multi-level stack; b) staggered die stack with spacers and wire bond; c) die stack with vertical interconnects and top side passive devices.

With traditional stacked die structures, diminishing returns in real estate size are experienced as ever greater areas are consumed to accommodate the wire bond fan out from increased die count (Fig. 1a). Offset die and spacers (Fig. 1b) can also pose cost and complexity burdens.

The large upfront costs of developing a POP device make this technology most suitable for volume production applications, and leave other system designers with few options for downsizing.

3D integration with through-silicon via (TSV) technology offers the highest level of integration, but TSV is several years away from full commercial adoption. Currently, only CMOS image sensors are in volume TSV production, and integration of heterogeneous memory and microprocessors chips may not be available until 2014 [1]. Issues of cost reduction, thermal management, design and design for test must be overcome for complete TSV acceptance [2].

Bare die stacking using interposers and vertical interconnect structures (Fig. 1c) provides tight integration without TSV technology, or the upfront costs of a POP. Using traditional materials and processes in a non-traditional way, this approach leverages advances in wafer thinning, die bumping and flip-chip processes, in conjunction with high-density thick film ceramic to achieve a miniaturized system-in-package (SiP) solution at low cost.

Bare die stacking

As a flexible packaging technology, bare die stacking allows for the simple co-packaging of both identical and off-the-shelf heterogeneous die, as well as the incorporation of discrete and integrated passive devices. Applications include co-packing of microprocessors and memory (reduced size, improved performance), and custom memory stacks (greater memory/mm²). This die stacking concept also provides a highly scalable architecture, in X/Y and Z dimensions. As with other SiP approaches, functional design changes can be made at the SiP level without motherboard or other system-level redesigns, maintaining flexibility through the product life cycle. Creating a functional building block in a SiP device provides for device reuse across product lines, reducing front-end design effort in the product development cycle.

 

Figure 2. a) Vertical interconnect structure; b) Ayre hybrid with vertical interconnects and top side passive devices.

Vertical interconnect (VI) structures are a key element in the architecture of 3D die stacks, providing mechanical support within the stack and electrical connectivity between layers within the stack (Fig. 2). Using VI structures and eliminating wire bonds enables highly compact SiP devices in a variety of geometries created at low cost. These micro-miniature VI structures are interconnects fabricated from ceramic, and are fully customizable to the pin count and geometry required.

Because of increased power consumption/mm3, high thermal conductivity aluminum nitride materials provide additional benefits for the increased heat dissipation needed in SiP devices. VI structures can also be utilized to improve heat dissipation through the device with the addition of thermal vias, and offer an excellent temperature coefficient of expansion (TCE) match for improved reliability over a wide temperature range.

Vertical die stack case study

Vertical die stacking has application across a broad spectrum of system requirements, particularly where size and weight are at a premium, offering system designers a straightforward method to co-package critical elements of a design in a custom SiP to meet specific needs.

With materials inherently suited to high-temperature environments, vertical die stacking technology also provides a robust solution where size and ruggedness are critical, such as aero-engine instruments and down-well monitoring and logging. Typical applications include implantable medical devices, headsets, hand-held radios, wireless sensors, energy harvesting devices, body-worn devices, specialty memory product, harsh environment instruments, and hearing aids.

One such application is the Ayre Hybrid from On Semiconductor (Fig. 2). This micro hearing instrument packages a complete wireless audio system with DSP, near-field magnetic induction (NFMI) transceiver chip, memory and associated passive components into a device form factor of 1.85 × 36.8 × 6.48mm.

Packaging options

Vertical die stacks can be provided in a variety of formats to suit user needs. Finished parts can be epoxy-encapsulated, JEDEC-compatible SMT devices suiting standard pick-and-place assembly, or “raw” die stacks to be direct mounted into a hybrid package or COB assembly.

Multi-level die stacks are assembled in panel arrays before dicing into individual stacks. Building multi-level stacks in parallel minimizes lead times and provides opportunity for in-process testing at the sub-assembly level, improving first pass yield.

Conclusion

Vertical die stacking is a 3D technology available today, offering high integration without TSVs. System designers can benefit from the technology’s flexibility, size and weight, and integration. Vertical die stacking provides a simple means to co-package off-the-shelf die and passives devices in a mature production environment.

Acknowledgment
Ayre is a trademark of On Semiconductor.

References
1. Jan Vardaman, “3D TSV Markets: Infrastructure Requirements for Growth,” p. 13 RTI 3D Integration Conference Dec. 2009
2. Phil Garrou, Ph.D., “The 4 Horsemen of 3D IC, Perspectives from the Leading Edge,” Oct. 16, 2009

Andrew Smith studied mechanical engineering at Abertay U., Dundee, Scotland and is an independent contractor working in microelectronics packaging. He is currently the principal at Ventmark Technology Solutions, 211 Giant Oak Avenue, Thousand Oaks, CA 91320 USA; ph.: 805-795-3968; [email protected].

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(October 5, 2010) — National Semiconductor Corp. (NYSE:NSM), designer and manufacturer of high-performance analog semiconductors, is collaborating with Suntech Power Holdings Co. Ltd. (NYSE:STP), producer of crystalline silicon (cSi) solar panels, to develop smart panel technology incorporating National Semiconductor’s SolarMagic power optimizer chipset into Suntech solar panels to improve the power output of solar systems.

"Smart panel technology is an important innovation that will help customers harvest more energy from their solar systems," said Shijun Cai, SVP of Suntech.

The companies announced the signing of a memorandum of understanding (MOU) in May 2009 for Suntech to evaluate National Semiconductor’s SolarMagic technology with the intention of jointly promoting the technology and developing future solutions. Suntech has completed its evaluation and the two companies have advanced to the development phase.

"Suntech was one of the first companies to recognize the value that integrated high-performance microelectronics bring to a photovoltaic (PV) panel. We are proud of the results of our collaboration with Suntech to enable a next-generation product that will embed SolarMagic technology inside the junction box and ultimately boost the performance of solar arrays," said Michael Polacek, SVP, key market segments and business development for National Semiconductor.

National Semiconductor will exhibit the SolarMagic power optimizer chipset in its booth #6219 at the Solar Power International conference in Los Angeles October 12-14, 2010.

National’s SolarMagic power optimizer chipset enables each solar module to produce the maximum energy regardless of whether other panels in the array are under-performing due to environmental mismatch. The technology enhances the energy harvest of each individual PV panel through a combination of advanced algorithms and leading-edge analog power management circuit techniques. In real-world tests involving shading and mismatch, Suntech modules with SolarMagic power optimizer technology were shown to recoup an average of 50% of lost energy, and in some cases captured as much as 75 percent of otherwise lost energy compared to standard panel performance.

Suntech recently utilized SolarMagic power optimizers with its panels on the eco-friendly Suntech Guosheng, a solar energy-powered water vessel ferrying passengers at the Shanghai World Expo.

National Semiconductor produces power management technology. Additional information is available at www.national.com.

Suntech Power Holdings Co., Ltd. produces solar products for residential, commercial, industrial, and utility applications. For more information, visit www.suntech-power.com.

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(September 30, 2010 – PRNewswire) — A revolutionary new spherical nanostructure, fully derived from very simple organic elements, yet strong as steel, has been developed and characterized at the laboratories of Ehud Gazit of Tel Aviv University and Itay Rousso of the Weizmann Institute of Science. Lightweight and exceptionally strong, easy and inexpensive to produce, friendly to the environment and biologically compatible, these promising bio-inspired nano-spheres have innumerable potential uses – from durable composite materials to medical implants.

The researchers, Prof. Gazit, Dr. Lihi Adler-Abramovich and Inbal Yanai from TAU’s Department of Molecular Biology and Biotechnology, working in collaboration with Dr. Itay Rousso and Nitzan Kol from the Weizmann Institute and David Barlam and Roni Shneck of Ben-Gurion University, used a simple dipeptide, consisting of only two amino acids, to form spherical nanostructures. Self-assembling under ambient conditions — without any heating or manipulation — this remarkable new material is the first bio-inspired nano-material known to date that is mechanically equal and even superior to many metallic substances. While demonstrating chemical properties similar to those of the ultra-rigid Kevlar(R) polymer, already used for bulletproof vests, the new substance is built from much simpler building blocks, enabling some important advantages: manipulation and deposition at the nano-scale, the fabrication of nano-materials of tubular, spherical and other geometries, and spontaneous formation by self-assembly. Here, indeed is a perfect building block for numerous applications.

Hard and strong as steel, this new nanostructure is an ideal element for the reinforcement of composite materials used in the space, aviation and transportation industries; biologically compatible yet extremely rigid and durable, it is an excellent candidate for replacing metallic implants; tough, light and impenetrable, it is an exceptional option for manufacturing bullet-proof vests.

The work was recently published in Angewandte Chemie.

The new nanotechnology development now emerging from Tel Aviv University is based on extensive research which began in Prof. Gazit’s laboratory in 2003. In an earlier achievement, the team was able to fabricate tubular nanostructures that assemble themselves into vast "forests" featuring exceptional mechanical and physical properties. This earlier work, based on the doctoral thesis of Dr. Lihi Adler-Abramovich, and published in 2009 in Nature Nanotechnology, may eventually generate self-cleaning windows and solar panels, as well as supreme energy storage devices with exceptionally high energy density.

The original paper can be found here: http://dx.doi.org/10.1002/anie.201002037 

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(August 19, 2010) — SouthWest NanoTechnologies Inc. (SWeNT), manufacturer of single-wall and Specialty Multi-Wall (SMW) carbon nanotubes (CNTs), is manufacturing specialty multi-wall carbon nanotubes for NanoRidge Materials Inc. These CNTs are being incorporated into enhanced body armor to improve protection of soldiers and law enforcement officers from small arms fire.

SWeNT’s SMW100 will be used in a highly advanced nanotechnology application to create stronger, lighter armor that fundamentally improves its resistance to impact and reduces the penetration depth of a bullet.

This new hybrid armor, which will be manufactured by NanoRidge customer Riley Solutions Inc. (RSI), has been selected by the Defense Advanced Research Program Agency (DARPA) to undergo rigorous testing and evaluation against the most destructive small arms fire.

"Once it has passed testing, the armor will provide U.S. military and law enforcement personnel better, lighter and less costly armor than has been available before," explains Kyle Kissell Ph.D, and RSI’s technical advisor. "NanoRidge selected SWeNT’s SMW100 after evaluating many different products and believes that its characteristics and commercial scalability will meet the needs of our nation’s protectors while saving lives."

“SouthWest NanoTechnologies is proud to be providing NanoRidge and Riley Solutions with SMW100 for use in these groundbreaking, nano-enhanced armor products," explains SWeNT CEO Dave Arthur. “Our patented CoMoCAT process enables us to produce the desired quality and at a cost and in quantities needed to meet the sizable demand that is expected.”

"SWeNT SMW100 is an excellent choice for this armor application because it is affordable, easy to disperse in polymers, and forms extremely robust networks that enhance the structural performance of the composites," says NanoRidge CEO Chris Lundberg. "Additionally, SWeNT’s domestic production and proven ability to deliver consistent quality are critical for the Department of Defense."

NanoRidge Materials, Inc. is a manufacturer of high-performance nanocomposite materials and composite components.

SouthWest NanoTechnologies Inc. (SWeNT) is a privately held specialty chemical company that manufactures high quality single-wall and specialty multi-wall carbon nanotubes, printable inks and CNT-coated fabrics for a range of products and applications including energy-efficient lighting, affordable photovoltaics, improved energy storage and printed electronics. For more information, please visit www.swentnano.com

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August 9, 2010 – Business execution, and not so much technological differentiation, separates the pack among firms developing nanotechnology-enabled batteries, according to a new report from Lux Research.

Promise and potential for energy storage has attracted a number of firms, many of which are basing their work on nanomaterials such as lithium titanate and lithium iron phosphate nanoparticles with battery electrodes. But there is "little technological differentiation between firms targeting this segment," says Jurron Bradley, senior analyst with Lux Research, in a new report.

A123 Systems, for example, isn’t the only one who makes nanostructured lithium iron phosphate battery electrodes (target market: automotive), but it shines due to what he calls "solid business execution" — it was the only nanotech company to go public in 2009, and one of the year’s most successful IPOs in any tech category.

Others in the mix for the nanotech battery sector (see Bradley’s quadrated grid, below):

– Electrovaya. The company scored highest in "technical value" according to Bradley’s criteria, developing nanostructured polymer electrolytes for several types of battery cathodes. It also has a "relatively strong" revenue to employee calculation of >$41,000, as well as "a strong partnership list" that includes Tata Motors.

– K2 Energy Solutions. Bradley puts K2 in the "long shot" category. Despite recent development deals (e.g. a $30M Chinese JV and an undisclosed deal for scooters/bikes/etc.), this company has yet to land a significant partner in the "lucrative" automotive market, he points out.

– Altair Nanotechnologies. This company’s star has lost some luster — it’s 1Q cash burn rate was 5× its 2009 annual revenues, and its stock price hasn’t sniffed the Nasdaq-required $1 mark since late 2009 (it’s currently languishing around $0.40). At this point, Bradley says, one could also call ALTI another "long shot."

The full listings and summaries can be found in Lux Research’s new report: The governing green giants: Makers of cleantech nanointermediates on the Lux Innovation Grid.

Nano-enabled battery/electric vehicle applications. (Source: Lux Research Inc.)

 

(August 3, 2010) — A newly discovered nanomaterial could improve healthcare devices by increasing energy storage, help realize implantable microchips, or make better drugs. Scientists at the University of Texas have created silicon nanoneedles with modulated porosity. The nanoporous needles are flexible, semiconductors, biodegradable, and have a surface one hundred times larger that of solid nanowires.

Figure 1. A side view of a forest of bicolor nanoneedles. A central low porosity segment is green and two siding high porosity segments are red. An ultrathin porous wire crosses the picture sideways, in yellow.

These unique properties of the nanowires will provide a higher energy density when used as large surface anodes in lithium batteries; constitute the active elements of bioresorbable, flexible microchips for subcutaneous implants; or protect drugs while in the body and release them in a controlled manner to improve their therapeutic effect.

Figure 2. Bicolor nanoneedles seen from an angle. The high porosity segment is red and low porosity segment is green. The grass-like flexibility of the nanowires allows the tips to join.

“We have indicated that the novel combination of nanoscale dimensions of the needles with their flexibility, ability to conduct electricity, degrade in the body and have the ‘surface of a tennis court’ on the tip of your thumb is crucial to develop lithium batteries that can store more energy, produce integrated circuits (ICs) that can be implanted in the body, and deliver drugs more efficiently. All these components are necessary to design better healthcare devices” says Mauro Ferrari, chair of the NanoMedicine and Biomedical Engineering Department.

Figure 3. A forest of evenly spaced cylindrical nanoneedles. The diameter is 100nm and allows piercing of cell membrane without harming the cells.

Conducted by researchers at the department of NanoMedicine and Biomedical Engineering at the University of Texas, the study reports the use of a new, rapid and inexpensive etch mechanism that uses silver nanoparticles to form nanoneedles from silicon. The needles are synthesized in a solution of hydrogen peroxide. The porosity is controlled along the length of the needle by simply changing the concentration of peroxide over time. The porosity causes the needles to biodegrade in a predictable way over time, and gives them a surface 120 times larger than that of corresponding solid wires while maintaining the semiconductor and crystalline nature.

The study was supported by the National Institutes of Health (NIH), the Defense Advanced Research Projects Agency (DARPA) and the state of Texas.

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“We can control the size and shape of the porous structure using the same technology currently used to make computer microchips, and we are doing it to make needles with a tip smaller than 100nm. This will allow us to penetrate directly within many cells at once and delivery drugs very specifically inside them without killing the cell,” says Ciro Chiappini, leading author of the study and researcher at the University of Texas at Austin.

The researchers have also shown that they can control the dissolution of the needles over several days and thus determine the life of the resulting implantable device. They can also control the color of the needles through porosity to design nanoneedles barcodes with codebars of different colors. Since porous silicon is not harmful to cells these barcodes can be used to tag cells and chemical reactions. “The barcodes are a very efficient system to identify cells in the natural environment without altering their functions, and we will use them to track movement of multiple cells at once. That is what I’m working on right now” says Jean Raymond Fakhoury, another author of the study.

The university research is the cover story in July 25th Advanced Functional Materials, http://www3.interscience.wiley.com/journal/123537708/abstract

(July 23, 2010) — A new paper from the lab of Rice University chemist James Tour demonstrates an environmentally friendly way to make bulk quantities of graphene oxide (GO), an insulating version of single-atom-thick graphene expected to find use in all kinds of material and electronic applications.

A second paper from Tour and Andreas Lüttge, a Rice professor of Earth science and chemistry, shows how GO is broken down by common bacteria that leave behind only harmless, natural graphite.

The paper appears online this week in the journal ACS Nano.

"These are the pillars that make graphene oxide production practical," said Tour, Rice’s T.T. and W.F. Chao Chair in Chemistry as well as a professor of mechanical engineering and materials science and of computer science. The GO manufacturing process was developed as part of a research project with M-I SWACO, a Houston-based producer of drilling fluids for the petrochemical industry that hopes to use graphene to improve the productivity of wells. 

View a webcast on Nanotechnology Safety from Small Times, on-demand at http://www.electroiq.com/index/webcasts/webcast-display/1960675815/webcasts/small-times/live-events/understanding-nanotechnology.html

Scientists have been making GO since the 19th century, but the new process eliminates a significant stumbling block to bulk production, Tour said. "People were using potassium chlorate or sodium nitrates that release toxic gases – one of which, chlorine dioxide, is explosive," he said. "Manufacturers are always reluctant to go to a large scale with any process that generates explosive intermediates."

Tour and his colleagues used a process similar to the one they employed to unzip multiwalled nanotubes into graphene nanoribbons, as described in a Nature paper last year. They process flakes of graphite (common pencil lead) with potassium permanganate, sulfuric acid and phosphoric acid, all common, inexpensive chemicals.

"Many companies have started to make graphene and graphene oxide, and I think they’re going to be very hard pressed to come up with a cheaper procedure that’s this efficient and as safe and environmentally friendly," Tour said.

The researchers suggested the water-soluble product could find use in polymers, ceramics and metals, as thin films for electronics, as drug-delivery devices and for hydrogen storage, as well as for oil and gas recovery. 

Though GO is a natural insulator, it could be chemically reduced to a conductor or semiconductor, though not without defects, Tour said.

With so many potential paths into the environment, the fate of GO nanomaterials concerned Tour, who sought the advice of Rice colleague Lüttge.

Lüttge and Everett Salas, a postdoctoral researcher in his lab and primary author of the second paper, had already been studying the effects of bacteria on carbon, so it was simple to shift their attention to GO. They found bacteria from the genus Shewanella easily convert GO to harmless graphene. The graphene then stacks itself into graphite.

"That’s a big plus for green nano, because these ubiquitous bacteria are quickly converting GO into an environmentally benign mineral," Tour said.

Essentially, Salas said, Shewanella have figured out how to "breathe" solid metal oxides. "These bacteria have turned themselves inside out. When we breathe oxygen, the reactions happen inside our cells. These microbes have taken those components and put them on the outside of their cells."

It is this capability that allows them to reduce GO to graphene. "It’s a mechanism we don’t understand completely because we didn’t know it was possible until a few months ago," he said of the process as it relates to GO.

The best news of all, Lüttge said, is that these metal-reducing bacteria "are found pretty much everywhere, so there will be no need to ‘inoculate’ the environment with them," he said. "These bacteria have been isolated from every imaginable environment – lakes, the sea floor, river mud, the open ocean, oil brines and even uranium mines."

He said the microbes also turn iron, chromium, uranium and arsenic compounds into "mostly benign" minerals. "Because of this, they’re playing a major role in efforts to develop bacteria-based bioremediation technologies."

Lüttge expects the discovery will lead to other practical technologies. His lab is investigating the interaction between bacteria and graphite electrodes to develop microbe-powered fuel cells, in collaboration with the Air Force Office of Scientific Research and its Multidisciplinary University Research Initiative (MURI).

Co-authors of the first paper, "Improved Synthesis of Graphene Oxide," include postdoctoral research associates Dmitry Kosynkin, Jacob Berlin and Alexander Sinitskii; senior research scientist Lawrence Alemany; graduate students Daniela Marcano, Zhengzong Sun and Wei Lu and visiting research student Alexander Slesarev, all of Rice.

Salas, Tour, Lüttge and Sun are co-authors of the second paper, "Reduction of Graphene Oxide via Bacterial Respiration."

Funding for the projects came from the Alliance for NanoHealth, M-I SWACO, the Air Force Research Laboratory through the University Technology Corporation, the Department of Energy’s Office of Energy Efficiency and Renewable Energy within the Hydrogen Sorption Center of Excellence, the Office of Naval Research MURI program on graphene, the Air Force Office of Scientific Research and the Federal Aviation Administration.

Read the abstract for "Improved Synthesis of Graphene Oxide" at http://pubs.acs.org/doi/abs/10.1021/nn1006368.

Read the abstract for "Reduction of Graphene Oxide via Bacterial Respiration" at http://pubs.acs.org/doi/abs/10.1021/nn101081t.

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July 22, 2010 – Researchers at Oregon State U. have come up with yet another unique application for nanocoatings — help produce more electricity from sewage.

Their work, published a few weeks ago in Biosensors and Bioelectronics, focuses on the anodes of microbial electrochemical cells (MEC), the core of efforts to clean biowaste and produce useful levels of electricity, realizing twin goals of wastewater treatment and renewable energy. (OSU has been working on MECs for several years, hoping to develop systems for producing electricity from hydrogen fuel cells for automobiles.)

Bacteria from biowaste (i.e., sewage) is placed in an anode chamber, where they form a biofilm, consume nutrients, and grow, and that process releases electrons. Coating graphite anodes with a gold nanolayer increased the electricity production by 20×, they found; similar palladium coatings also produced an increase (50%-150%). They think iron nanoparticle coatings could produce similar electricity increases as gold — and cost a lot less. And a similar approach could be applied to producing hydrogen gas instead of electricity, toward use in hydrogen fuel cells e.g. in cars.

From the paper abstract:

Significant positive linear regression was obtained between the current density and the particle size (average Feret’s diameter and average area), while the circularity of the particles showed negative correlation with current densities. On the contrary, no significant correlation was evident between the current density and the particle density based on area fraction and particle counts. These results demonstrated that nano-decoration can greatly enhance the performance of microbial anodes, while the chemical composition, size and shape of the nanoparticles determined the extent of the enhancement.

More work is needed to get the process working beyond a lab environment, to lower its cost (e.g. identify the lowest-cost materials to use), and improve efficiency and electrical output even more. "We still need some improvements in design of the cathode chamber, and a better understanding of the interaction between different microbial species," added Frank Chaplen, an associate professor of biological and ecological engineering, in a statement. "But the new approach is clearly producing more electricity."

Ultimately, the researchers see the technology being used to reduce the cost of wastewater treatment, or in developing nations where wastewater treatment is impractical due to a lack of adequate power supply. Sewage treatment plants could be made to be completely self-sufficient in terms of energy usage, they say.

The research is supported by the Oregon Nanoscience and Microtechnologies Institute (ONAMI) and the National Science Foundation.

(July 20, 2010) — Nextreme Thermal Solutions will now include materials evaluation and characterization services in its consulting portfolio. With recent advances in materials sciences and nanotechnology, new materials are being developed that exhibit thermoelectric properties.

The assessment of thermoelectric materials is a challenge, particularly when investigating them over significant temperature ranges. Nextreme has capabilities to conduct measurements across a range of temperatures, below 200K to above 700K. Properties that can be measured include Seebeck coefficient, electrical resistivity and thermal conductivity. Equipped with thermal modeling, design, and engineering consulting services, Nextreme’s team of engineers can accelerate the development of new applications that are currently thermally constrained.

In addition to the evaluation of materials, Nextreme offers thermal modeling, design and engineering services to deliver fully optimized thermal management solutions. Nextreme routinely conducts analytical and numerical thermal modeling at all design levels from component to module to subsystem. Design services range from rudimentary analysis such as 1D and 3D modeling to more complex analyses that may involve the use of passive heat rejection systems, such as heat sinks and exchangers, or the use of active devices such as thermoelectric coolers to solve the overall thermal problem. Advanced analysis of complex systems, components or packages often require more detailed modeling to understand heat flow and thermal gradients.

The use of Nextreme’s engineering services can enable rapid and successful prototyping and accelerate a program as well as reduce program delays. Services are tailored to needs and budget. "Initial evaluation and integration of micro-scale thermoelectrics is often a challenge for organizations without thermal engineering expertise or when existing resources are limited," said Dave Koester, VP of engineering at Nextreme.

Nextreme uses its thin-film thermoelectric technology to produce discrete and integrated cooling and power generation devices. Nextreme currently offers several thermoelectric coolers, such as the OptoCooler HV14 and UPF40, that are capable of cooling and heating in ranges from 0.4 to 4W, with plans to provide higher heat pumping in the near future. Modules for electronics cooling and power generation are available for order now and pricing is available upon request. Nextreme Labs is currently working in laser diode cooling, thermal margining, and clean energy harvesting solutions for wireless sensor networks and remote power management. For more information, go to www.nextreme.com.

by Michael A. Fury, Techcet GroupClick to Enlarge

April 20, 2010 – The fifth and final day (Friday 4/9) of the MRS Spring 2010 meeting in San Francisco called for a stretch, as flexible and stretchable electronics provided a focal point for the day. Highlights included: Ge-Si integration, carbon nanotubes in organic photovoltaics, energy storage using paper, printable GaN semiconductors, and stretchable circuit boards and conductors.

(Underscored codes at the beginning of papers reviewed refer to the symposium, session and paper number; additional presentation details can be found in the MRS Spring 2010 program.)

I6.5. Krishna Saraswat at Stanford presented a scheme for germanium integration on silicon for high-performance MOSFETs and optical interconnects. The 4% lattice mismatch between Si and Ge allows defect free Ge layers on ~2nm thick to be grown on Si. By growing thin Ge layers at 400°C and annealing in H2 at 850°C, the defects can migrate to the surface and release. Growing the Ge between islands of SiO2 allows a defect-free Ge layer to be fabricated by repeating the growth/anneal cycle until the SiO2 has been over-filled and the individual Ge wells grow together on the surface. Using this substrate, candidate device structures were shown for light emitters, detectors and modulators, the design elements needed for integrated optical interconnects.

HH16.1. Teresa Barnes at NREL is developing flexible and solution-processible transparent carbon nanotube (CNT) contacts for organic photovoltaics (OPV), with the objective of replacing the glass/ITO/hole transport layers with polymer/SWNT. The CNT layers are deposited by ultrasonic spray coating and often out-perform ITO expectations for contact sheet resistance and transparency. The OPV device design is optimized around the CNT material, rather than attempting a drop-in substitution for the ITO film.

HH16.2. Ajay Virkar at Stanford described a hybrid organic-CNT composite material for transparent electrodes in OPV. Theoretically, a CNT monolayer can have a sheet resistance of 10 Ω/sq at 95% transmission, comparable to ITO with 10-40 Ω/sq at 95% T. In practice, the contact resistance between contiguous CNT is as high as 1GΩ, so the best observed CNT TCO films have been ~80 Ω/sq at 85% T. This group has developed an overcoat that acts as a nano-solder at the CNT contact points, enabling a lower resistance with thinner layers and ≥95% T.

Other MRS blogs:
Day 4: TSVs, wafer bonding, CNTs, ALD for rare-earth HK, graphene
Day 3: Nanoimprint litho, 32nm memories, FET/Si/CNT sensors
Day 2: CVD for Cu, low-k etch stop, future FETs, graphene "atom hopping"
Day 1: Charge-trapping NVM, organics, graphene, PV

HH16.3. Kamil Mielczarek at UT Dallas demonstrated a polymeric parallel tandem OPV with transparent MWCNT as the interlayer electrode. The MWCNT are drawn as free-standing ribbons, but morphologies with high optical transmission tend to have lower conductivity, and vice versa. The spectral efficiency of the MWCNT is superior to Ag and more cost-effective. Additional work is planned to optimize the MWCNT processes to better match the electrical performance of Ag, which is commonly used as the common interlayer electrode.

JJ6.1. Liangbing Hu at Stanford showed the latest work in paper energy storage devices. The Al and Cu metal current collectors account for 20%-30% of the weight in Li-ion batteries, designed for high energy density, and supercapacitors, designed for discharge power delivery. These metals can be replaced with CNT or Ag nanowires inks screen printed on paper. Ag is the better conductor, but can be matched if you use all metallic CNT. CNT supercapacitors show excellent performance with a specific capacitance of 200 F/g, a specific energy of 47 Wh/kg (comparable to that of rechargeable batteries), a specific power of 200,000 W/kg, and a stable cycling life over 40,000 cycles.

JJ6.2. Martin Kaltenbrunner at Johannes Kepler University showed a novel power supply for stretchable electronics based on Xanthan electrolyte gel power cells, in which the anode and cathode, each ~1cm2, are laid out with a lateral separation of 0.3cm rather than on top of each other in order to eliminate the risk of shorting over time. Open circuit voltages of 1.47V and short circuit currents of up to 40mA have been achieved, with a capacity of 3mAh/cm2 active cell area.

JJ6.5. Keyan Zang at IMRE in Singapore presented a simple release method for high-quality printable GaN semiconductors. The GaN device is fabricated on a substrate with an embedded SiO2 layer designed for ease of undercut and release. The finished device is overcoated with PDMS, the SiO2 is undercut with HF, and the PDMS/GaN device is transferred to a Si or flex substrate. This scheme is an extension of what is becoming a common strategy for fabricating high performance devices for flexible substrate applications.

JJ6.6. Wayne Chen at UCSD described a process for layer transfer of high quality, single crystalline (110) InP for flexible applications. The method uses an implanted H+ layer in a geometry that avoids critical device regions, as implant damage is difficult to anneal out. The resulting devices can be lifted off the donor substrate using a ‘smart cut’ process and dual flip transfer methods. Areas up to 2mm2 and 6μm thick have been transferred without introducing defects or degrading device performance.

JJ7.2. Darryl Cotton at Cambridge showed some multilayered gold-elastomer structures in PDMS for a stretchable circuit board. The 50nm Au layer is deposited on a 5nm Cr adhesion layer on the PDMS. At 20% strain the resistance rises 2×-5×, and increases ~10% after 1000 stretch cycles. Using a photo-patternable PDMS, one can fabricate sloped vias as small as 300μm square for multilayer interconnects. A four-level touch pad with contacts to a flex circuit element was demonstrated.

JJ7.3. Frederick Bossuyt, at U Gent & IMEC outlined a journey from single conductive layer to double conductive layer stretchable electronics. Single layer designs require a zero-resistance crossover resistor at every wiring intersection. Polyimide is used as the mechanical support for copper on one side and Ag flex paste on the other. This material set can accommodate vias <100μm in diameter, with a 200μm line/space minimum pitch. In meander areas, the PI base needs to be wider than the metal traces, in order to prevent Ag paste bleed over the edges and shorts after repeated flexing.

JJ7.4. Jaewook Jeong at Seoul National University showed a novel implementation of silver electrodes on elastomeric substrates for stretchable electronics applications. Ag 700nm thick was deposited on PDMS substrates with micro-roughness (1-1.5μm) and macro-waviness (200μm deep, 400μm p-p). The micro-roughness reduces mechanical stress as the substrate stretches, and the waviness allows the metal to flex rather than pull apart. With the waves, a 50% strain results in a 3× increase in resistance. Without the waves, the maximum strain possible without failure is 35%, with an 8x resistance increase.

JJ7.5. Adam Robinson, at Cambridge showed a method for printed stretchable conductors using silver-based ink compatible with PDMS substrates. An organometallic Ag ink was chosen for its cure temperature of 130°C. However, the ink dewets during the cure process. The PDMS surface wetting was modified by molding 2μm diameter pillars 2μm high into the PDMS surface as surface tension breaks. Pillar pitches of 20μm, 10μm and 6μm showed an increasing efficacy of line-width preservation for the SonoPlot-printed lines. This technique produced printed silver conducting features of >100nm thickness, which can be stretched up to strains of 20% over 1,000 cycles without loss of conductivity.

Michael A. Fury, Ph.D, is senior technology analyst at Techcet Group, LLC, P.O. Box 29, Del Mar, CA 92014; e-mail [email protected].