Tag Archives: letter-leds-tech

Reproducibility is a necessity for science but has often eluded researchers studying the lifetime of organic light-emitting diodes (OLEDs). Recent research from Japan sheds new light on why: impurities present in the vacuum chamber during fabrication but in amounts so small that they are easily overlooked.

Organic light-emitting diodes use a stack of organic layers to convert electricity into light, and these organic layers are most commonly fabricated by heating source materials in vacuum to evaporate and deposit them onto a lower temperature substrate.

While issues affecting the efficiency of OLEDs are already well understood, a complete picture of exactly how and why OLEDs degrade and lose brightness over time is still missing.

Complicating matters is that devices fabricated with seemingly the same procedures and conditions but by different research groups often degrade at vastly different rates even when the initial performance is the same.

Unable to attribute these reproducibility issues to known sources such as the amount of residual water in the chamber and the purity of the starting materials, a report published online in Scientific Reports on December 13, 2016, adds a new piece to the puzzle by focusing on the analysis of the environment in the vacuum chamber.

“Although we often idealize vacuums as being clean environments, we detected many impurities floating in the vacuum even when the deposition chamber is at room temperature,” says lead author Hiroshi Fujimoto, chief researcher at Fukuoka i3-Center for Organic Photonics and Electronics Research (i3-OPERA) and visiting associate professor of Kyushu University.

Because of these impurities in the deposition chamber, the researchers found that the time until an OLED under operation dims by a given amount because of degradation, known as the lifetime, sharply increased for OLEDs that spent a shorter time in the deposition chamber during fabrication.

This trend remained even after considering changes in residual water and source material purity, indicating the importance of controlling and minimizing the device fabrication time, a rarely discussed parameter.

Research partners at Sumika Chemical Analysis Service Ltd. (SCAS) confirmed an increase of accumulated impurities with time by analyzing the materials that deposited on extremely clean silicon wafers that were stored in the deposition chamber when OLED materials were not being evaporated.

Using a technique called liquid chromatography-mass spectrometry, the researchers found that many of the impurities could be traced to previously deposited materials and plasticizers from the vacuum chamber components.

“Really small amounts of these impurities get incorporated into the fabricated devices and are causing large changes in the lifetime,” says Professor Chihaya Adachi, director of Kyushu University’s Center for Organic Photonics and Electronics Research (OPERA), which also took part in the study.

In fact, the new results suggest that the impurities amount to less than even a single molecular layer.

To improve lifetime reproducibility, a practice often adopted in industry is the use of dedicated deposition chambers for specific materials, but this can be difficult in academic labs, where often only a limited number of deposition systems are available for testing a wide variety of new materials.

In these cases, deposition chamber design and cleaning in addition to control of the deposition time are especially important.

“This is an excellent reminder of just how careful we need to be to do good, reproducible science,” comments Professor Adachi.

Cree, Inc. (Nasdaq: CREE) introduces the XLamp XHP50.2 LED, which delivers up to seven percent more lumens and 10 percent higher lumens-per-watt (LPW) than the first generation XHP50 LED in the same 5.0 mm x 5.0 mm package. The new XHP50.2 LED enables lighting manufacturers to quickly improve the performance of existing XHP50 lighting designs. Capable of producing more than 2,500 lumens from its 6mm light emitting surface (LES), the XHP50.2 can reduce the size and cost of new designs and enable innovative solutions to address applications ranging from spot to street lighting.

“Arianna shares Cree’s vision that LEDs should not compromise quality or performance and should provide better lighting experiences in all aspects,” said Lorenzo Trevisanello, R&D manager of Arianna. “Our goals are to achieve the best cost-efficacy and versatility using the most efficient LEDs. Thanks to the XHP50.2 LED’s lumen density and proven reliability, even at high operating temperatures and drive currents, we are able to push the performance and size boundaries of our products even further.”

In addition to light output and efficacy enhancements, the XHP50.2 LED provides improvements to optical uniformity through secondary optics, enabling spot and portable lighting manufacturers to deliver better lighting experiences. The XHP50.2 LED has LM-80 data available immediately, reducing the time required to receive ENERGY STAR® and DesignLights Consortium® qualifications.

“Cree redefined High Power LED performance with the introduction of the industry’s first Extreme High Power LEDs,” said Dave Emerson, senior vice president and general manager for Cree LEDs. “Delivering the industry’s best lumen density and reliability, Cree’s XHP LED family allows our customers to achieve performance levels not possible with other LEDs at the lowest total system cost in a wide range of applications. With the launch of XHP50.2, Cree continues to redefine what is possible with high performance LEDs.”

Featuring Cree’s EasyWhite technology, which provides the industry’s best color consistency, the XLamp XHP50.2 LEDs are available in 2700K-6500K with high CRI options. Product samples are available now, and production quantities are available with standard lead times.

Outfitting the future


December 12, 2016

Wearable technology is about more than smartwatches or counting steps. Across North Carolina State University, researchers are using it to solve problems — monitoring heart rate and environmental dangers, powering electronic devices, delivering medications, building better prosthetics and improving safety.

wearable-tech-top-1500x650

They’re developing technologies that are functional, efficient, innovative and practical, and that could have an impact on countless lives.

Here are a few of the NC State projects at the forefront of this evolving field.

What’s NEXT in wearables

What if the clothes you already wear not only covered your body but also kept track of how it’s functioning — and all you had to do was put them on?

Finding innovative, useful and economical ways to integrate electronics into clothing is the mission of the College of Textiles’ Nano-Extended Textiles (NEXT) Research Group.

Headed by Jesse Jur, assistant professor in the Department of Textile Engineering, Chemistry and Science, the NEXT group seeks to create cost-effective, energy-efficient wearable technology that’s powered by the user’s own body.

Jur’s team has gained attention for projects like customizable, iron-on sensors that monitor the heart’s performance and transmit the readings to a smartphone, or that monitor environmental levels of potentially dangerous gases like carbon monoxide and ozone.

The NEXT group has also explored bioluminescence in fashion through a collaboration with recent College of Textiles graduate Jazsalyn McNeil, who joined the group as a “fusion designer” to meld her design sensibility with the group’s research. McNeil’s Pulse Dress incorporates screen-printed sensors that make LED lights blink with the wearer’s heartbeat. NEXT and McNeil hope that the eye-catching dress will both influence fashion and draw attention to the possibilities of wearable electronics.

Heating up wearable tech

In recent years, smartwatches have turned up on the arms of millions of people who want convenient ways to keep track of their fitness, but these still depend on conventional batteries. At NC State’s Center for Advanced Self-Powered Systems of Integrated Sensors and Technologies (ASSIST) — a National Science Foundation Nanosystems Engineering Research Center — researchers are developing innovative health-monitoring devices that are battery-free and body-powered.

“The goal of ASSIST is to make wearable technologies that can be used for long-term health monitoring, such as devices that track heart health or monitor physical and environmental variables to predict and prevent asthma attacks,” said Daryoosh Vashaee, an associate professor of electrical and computer engineering in the NC State College of Engineering.

Vashaee and a team of undergraduates and faculty members have developed a new approach for harvesting body heat and converting it into electricity to power wearable electronics. The prototype armbands and embedded sensors in T-shirts are lightweight, conform to the shape of the body and can generate far more electricity than previous lightweight heat-harvesting technologies.

“We want to make devices that don’t rely on batteries,” Vashaee said. “And we think this design and prototype moves us much closer to making that a reality.”

Taking the sting out of diabetes

For some people with serious health issues, wearable technology has the potential to offer more than bells and whistles — it could make their treatments easier and even save lives.

Zhen Gu, an associate professor in the UNC/NC State Joint Department of Biomedical Engineering, has developed a glucose-responsive insulin patch for people living with Type 1 Diabetes. At around the size of a penny, the thin, square patch contains more than a hundred tiny, painless needles that supply the wearer with insulin as needed. This potential treatment could help to ensure consistent blood-sugar levels — and spare patients regular injections.

Gu, who has been honored as one of MIT Technology Review’s “Innovators Under 35” for his work with innovative drug-delivery systems, received $4.6 million in funding from JDRF (formerly the Juvenile Diabetes Research Foundation) and multinational pharmaceutical company Sanofi for the project. The patch is currently in animal trials. Gu is also working on patches to deliver melanoma drugs directly to tumor sites and to deliver blood thinners as needed to prevent blood clots.

Walking wearables

Amputees have always been among the earliest adopters of wearable technology, as even minor advances in prosthetics can markedly improve their mobility. Helen Huang, associate professor of biomedical engineering and director of the Rehabilitation Engineering Core in the UNC/NC State Joint Department of Biomedical Engineering, has made it her mission to develop the next generation of powered prosthetic limbs.

Huang’s projects include software that allows powered prosthetics to tune themselves automatically, making the devices more responsive and lowering the costs associated with powered prosthetic use.

“People are dynamic — a patient’s physical condition may change as he or she becomes accustomed to a prosthetic leg, for example, or they may gain weight,” said Huang. “These changes mean the prosthetic needs to be re-tuned, and working with a prosthetist takes time and money.”

Huang’s team has also worked on technology that translates electrical signals in human muscles into signals that control powered prosthetic limbs — enabling sensors in the prosthetics to follow simple cues from the user’s brain such as “open hand” or “close hand.”

A bright idea for safety

For College of Textiles alumnus Jeremy Wall, a near miss with a car while he was riding his bike one night became an unexpected source of inspiration: He now heads a company, Lumenus, that’s developing clothing and accessories with embedded smart LED lighting.

Wall, a 2014 graduate in fashion and textile management, began working on his tech with the help of an undergraduate research scholarship while he was still a student. His goal was to help cyclists, motorcyclists and runners be more visible to motorists at night while staying stylish and functional during the day.

The company will soon hit the market with apparel and accessories including jackets, vests, leggings, backpacks and armbands. It’s also licensing its technology to companies such as backpack manufacturer Timbuk2 and working with the Department of Defense to develop sensors for military gear.

Lumenus has also created an app that adds extra features to the apparel. For example, the wearer can enter a destination on the app, and the LED lights on the garment will flash strategically at intersections or other potentially hazardous points along the route.

Wall recently returned to NC State for help getting his company off the ground, enlisting three College of Textiles undergraduates to work with Lumenus as part of their senior design project.

Leti, an institute of CEA Tech, has developed a new light-sensing device that integrates photodiodes below the buried oxide (BOX) of FDSOI transistors, making the transistors very sensitive to visible light.

Presented today during IEDM 2016 in the paper, “Extending the Functionality of FDSOI N- and P-FETs to Light Sensing,” the innovative device architecture uses capacitive coupling, which doesn’t necessarily require an electrical connection between the transistor and the diode. Leti said preliminary results show that sensitivity in the visible spectrum is already better than 0.1pW/µm2, with a wide dynamic range (seven orders of magnitude, i.e. similar to most advanced CMOS image sensors).

“FDSOI is a very versatile technology that already has been shown to be ‘faster, cooler, and simpler’ than FinFET, and which also may become smarter for More than Moore applications such as imaging,” said Lina Kadura, who presented the paper. “In fact, it may be smarter for sensing generally, because FDSOI transistors can be considered as very small footprint probes that are sensitive to the electric potential below the BOX.”

In addition to embedding more light-sensing functionality in circuits, potential future applications include leveraging pixel size in image sensors.

In other results of the study, Leti demonstrated for the first time that SRAM cell characteristics can be controlled by light illumination. Leti also said that with capacitive coupling, light absorption in the diode integrated below the BOX leads to light-induced voltage-threshold (VT) shift of the transistor above the BOX, which means that forward optical back-biasing and reverse optical back-biasing are possible, depending on the diode polarity. In addition, the response of the system is logarithmic with light illumination, similar to the response of human vision.

Detailing the molecular makeup of materials — from solar cells to organic light-emitting diodes (LEDs) and transistors, and medically important proteins — is not always a crystal-clear process.

To understand how materials work at these microscopic scales, and to better design materials to improve their function, it is necessary to not only know all about their composition but also their molecular arrangement and microscopic imperfections.

Now, a team of researchers working at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) has demonstrated infrared imaging of an organic semiconductor known for its electronics capabilities, revealing key nanoscale details about the nature of its crystal shapes and orientations, and defects that also affect its performance.

This image shows the crystal shape and height of a material known as PTCDA, with height represented by the shading (white is taller, darker orange is lowest). The white scale bar represents 500 nanometers. The illustration at bottom is a representation of the crystal shape. Credit: Berkeley Lab, CU-Boulder

This image shows the crystal shape and height of a material known as PTCDA, with height represented by the shading (white is taller, darker orange is lowest). The white scale bar represents 500 nanometers. The illustration at bottom is a representation of the crystal shape. Credit: Berkeley Lab, CU-Boulder

To achieve this imaging breakthrough, researchers from Berkeley Lab’s Advanced Light Source (ALS) and the University of Colorado-Boulder (CU-Boulder) combined the power of infrared light from the ALS and infrared light from a laser with a tool known as an atomic force microscope. The ALS, a synchrotron, produces light in a range of wavelengths or “colors” — from infrared to X-rays — by accelerating electron beams near the speed of light around bends.

The researchers focused both sources of infrared light onto the tip of the atomic force microscope, which works a bit like a record-player needle — it moves across the surface of a material and measures the subtlest of surface features as it lifts and dips.

The technique, detailed in a recent edition of the journal Science Advances, allows researchers to tune the infrared light in on specific chemical bonds and their arrangement in a sample, show detailed crystal features, and explore the nanoscale chemical environment in samples.

“Our technique is broadly applicable,” said Hans Bechtel an ALS scientist. “You could use this for many types of material — the only limitation is that it has to be relatively flat” so that the tip of the atomic force microscope can move across its peaks and valleys.

Markus Raschke, a CU-Boulder professor who developed the imaging technique with Eric Muller, a postdoctoral researcher in his group, said, “If you know the molecular composition and orientation in these organic materials then you can optimize their properties in a much more straightforward way.

“This work is informing materials design. The sensitivity of this technique is going from an average of millions of molecules to a few hundred, and the imaging resolution is going from the micron scale (millionths of an inch) to the nanoscale (billionths of an inch),” he said.

The infrared light of the synchrotron provided the essential wide band of the infrared spectrum, which makes it sensitive to many different chemicals’ bonds at the same time and also provides the sample’s molecular orientation. The conventional infrared laser, with its high power yet narrow range of infrared light, meanwhile, allowed researchers to zoom in on specific bonds to obtain very detailed imaging.

“Neither the ALS synchrotron nor the laser alone would have given us this level of microscopic insight,” Raschke said, while the combination of the two provided a powerful probe “greater than the sum of its parts.”

Raschke a decade ago first explored synchrotron-based infrared nano-spectroscopy using the BESSY synchrotron in Berlin. With his help and that of ALS scientists Michael Martin and Bechtel, the ALS in 2014 became the first synchrotron to offer nanoscale infrared imaging to visiting scientists.

The technique is particularly useful for the study and understanding of so-called “functional materials” that possess special photonic, electronic, or energy-conversion or energy-storage properties, he noted.

In principle, he added, the new advance in determining molecular orientation could be adapted to biological studies of proteins. “Molecular orientation is critical in determining biological function,” Raschke said. The orientation of molecules determines how energy and charge flows across from cell membranes to molecular solar energy conversion materials.

Bechtel said the infrared technique permits imaging resolution down to about 10-20 nanometers, which can resolve features up to 50,000 times smaller than a grain of sand.

The imaging technique used in these experiments, known as “scattering-type scanning near-field optical microscopy,” or s-SNOM, essentially uses the atomic force microscope tip as an ultrasensitive antenna, which transmits and receives focused infrared light in the region of the tip apex. Scattered light, captured from the tip as it moves over the sample, is recorded by a detector to produce high-resolution images.

“It’s non-invasive, and it provides information about molecular vibrations,” as the microscope’s tip moves over the sample, Bechtel said. Researchers used the technique to study the crystalline features of an organic semiconductor material known as PTCDA (perylenetetracarboxylic dianhydride).

Researchers reported that they observed defects in the orientation of the material’s crystal structure that provide a new understanding of the crystals’ growth mechanism and could aid in the design molecular devices using this material.

The new imaging capability sets the stage for a new National Science Foundation Center, announced in late September, that links CU-Boulder with Berkeley Lab, UC Berkeley, Florida International University, UC Irvine, and Fort Lewis College in Durango, Colo. The center will combine a range of microscopic imaging methods, including those that use electrons, X-rays, and light, across a broad range of disciplines.

This center, dubbed STROBE for Science and Technology Center on Real-Time Functional Imaging, will be led by Margaret Murnane, a distinguished professor at CU-Boulder, with Raschke serving as a co-lead.

At Berkeley Lab, STROBE will be served by a range of ALS capabilities, including the infrared beamlines managed by Bechtel and Martin and a new beamline dubbed COSMIC (for “coherent scattering and microscopy”). It will also benefit from Berkeley Lab-developed data analysis tools.

Less than a micrometre thin, bendable and giving all the colours that a regular LED display does, it still needs ten times less energy than a Kindle tablet. Researchers at Chalmers University of Technology have developed the basis for a new electronic “paper”. Their results were recently published in the high impact journal Advanced Materials.

Chalmers' e-paper contains gold, silver and PET plastic. The layer that produces the colours is less than a micrometre thin. Credit: Mats Tiborn

Chalmers’ e-paper contains gold, silver and PET plastic. The layer that produces the colours is less than a micrometre thin. Credit: Mats Tiborn

When Chalmers researcher Andreas Dahlin and his PhD student Kunli Xiong were working on placing conductive polymers on nanostructures, they discovered that the combination would be perfectly suited to creating electronic displays as thin as paper. A year later the results were ready for publication. A material that is less than a micrometre thin, flexible and giving all the colours that a standard LED display does.

“The ‘paper’ is similar to the Kindle tablet”, says Andreas Dahlin. “It isn’t lit up like a standard display, but rather reflects the external light which illuminates it. Therefore it works very well where there is bright light, such as out in the sun, in contrast to standard LED displays that work best in darkness. At the same time it needs only a tenth of the energy that a Kindle tablet uses, which itself uses much less energy than a tablet LED display”.

It all depends on the polymers’ ability to control how light is absorbed and reflected. The polymers that cover the whole surface lead the electric signals throughout the full display and create images in high resolution. The material is not yet ready for application, but the basis is there. The team has tested and built a few pixels. These use the same red, green and blue (RGB) colours that together can create all the colours in standard LED displays. The results so far have been positive, what remains now is to build pixels that cover an area as large as a display.

“We are working at a fundamental level but even so, the step to manufacturing a product out of it shouldn’t be too far away. What we need now are engineers”.

One obstacle today is that there is gold and silver in the display, which makes the manufacturing expensive.

“The gold surface is 20 nanometres thick so there is not that much gold in it”, says Andreas Dahlin. “But at present there is a lot of gold wasted in manufacturing it. Either we reduce the waste or we find another way to decrease the manufacturing cost”.

Andreas Dahlin thinks the best application for the displays will be well-lit places such as outside or in public places to display information. This could reduce the energy consumption and at the same time replace signs and information screens that aren’t currently electronic today with more flexible ones.

Samsung Electronics Co., Ltd. today announced a new line-up of chip scale package (CSP) LED modules for spotlights and downlights that features color tunability and increased design compatibility.

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“Our new CSP LED modules provide an optimal solution for lighting manufacturers who seek highly compatible and reliable LED components,” said Jacob Tarn, Executive Vice President, LED Business Team at Samsung Electronics. “Samsung will continue to strengthen its CSP technology leadership and spearhead new innovations in LED component technology to bring greater value to our customers.”

The new LED modules are Samsung’s first to incorporate CSP technology, which bring a wide range of lighting benefits such as significantly reducing the size of a conventional LED package. The combination of advanced flip chip and phosphor coating technology eliminates metal wires and plastic molds to enable more compact designs when manufacturing LED modules and fixtures.

In addition to their size advantage, Samsung’s new CSP LED modules deliver further characteristics that furnish seamless tunable color. A color-tunable LED module requires twice the number of LED packages in cool and warm temperature, which work in combination on the same board to create a range of tunable colors. In contrast to conventional plastic-molded LED packages that inevitably increase the size of the modules, Samsung’s ultra-compact chip scale LED packages allow the module size to remain unchanged.

Samsung’s new CSP LED modules are available in two form factors (19x19mm or 28x28mm) and are designed following Zhaga specifications, making them highly convenient in assembling. The modules also provide high-quality lighting in diverse beam angle options – spot, medium, wide – for improved compatibility with the optical solutions of Samsung’s partners. The new modules are based on CSP LED packages that have successfully completed 9,000 hours of LM-80 testing, a level of proven performance that reduces the time to market for lighting manufacturers.

Samsung is now sampling six models of the new CSP LED module in CRI 80 and 90 with varying lumen output, size and CCT specifications. The full line-up includes:

 Power

Form Factor 

Model

 Consumption

Lumen

(mm)

CCT
CO10 9.4 1050 lm 19×19 2700/3000/3500/4000K
CO20 18.3 2060 lm 19×19
CO30 27.4 3090 lm 28×28
CO40 36.5 4120 lm 28×28
TO10 9.2-9.8 1060 / 1150 lm 28×28

Color tunable between
2700K~5000K
TO20 17.7-18.4 1970 / 2190 lm 28×28

* Based on CRI 80

Scientists with the Energy Department’s National Renewable Energy Laboratory (NREL) for the first time discovered how to make perovskite solar cells out of quantum dots and used the new material to convert sunlight to electricity with 10.77 percent efficiency.

The research, Quantum dot-induced phase stabilization of a-CsPbI3 perovskite for high-efficiency photovoltaics, appears in the journal Science. The authors are Abhishek Swarnkar, Ashley Marshall, Erin Sanehira, Boris Chernomordik, David Moore, Jeffrey Christians, and Joseph Luther from NREL. Tamoghna Chakrabarti from the Colorado School of Mines also is a co-author.

In addition to developing quantum dot perovskite solar cells, the researchers discovered a method to stabilize a crystal structure in an all-inorganic perovskite material at room temperature that was previously only favorable at high temperatures. The crystal phase of the inorganic material is more stable in quantum dots.

Most research into perovskites has centered on a hybrid organic-inorganic structure. Since research into perovskites for photovoltaics began in 2009, their efficiency of converting sunlight into electricity has climbed steadily and now shows greater than 22 percent power conversion efficiency. However, the organic component hasn’t been durable enough for the long-term use of perovskites as a solar cell.

NREL scientists turned to quantum dots-which are essentially nanocrystals-of cesium lead iodide (CsPbI3) to remove the unstable organic component and open the door to high-efficiency quantum dot optoelectronics that can be used in LED lights and photovoltaics.

The nanocrystals of CsPbI3 were synthesized through the addition of a Cs-oleate solution to a flask containing PbI2 precursor. The NREL researchers purified the nanocrystals using methyl acetate as an anti-solvent that removed excess unreacted precursors. This step turned out to be critical to increasing their stability.

Contrary to the bulk version of CsPbI3, the nanocrystals were found to be stable not only at temperatures exceeding 600 degrees Fahrenheit but also at room temperatures and at hundreds of degrees below zero. The bulk version of this material is unstable at room temperature, where photovoltaics normally operate and convert very quickly to an undesired crystal structure.

NREL scientists were able to transform the nanocrystals into a thin film by repeatedly dipping them into a methyl acetate solution, yielding a thickness between 100 and 400 nanometers. Used in a solar cell, the CsPbI3 nanocrystal film proved efficient at converting 10.77 percent of sunlight into electricity at an extraordinary high open circuit voltage. The efficiency is similar to record quantum dot solar cells of other materials and surpasses other reported all-inorganic perovskite solar cells.

The research was funded in part by the Energy Department’s Office of Science and by the SunShot Initiative.

NREL is the U.S. Department of Energy’s primary national laboratory for renewable energy and energy efficiency research and development. NREL is operated for the Energy Department by The Alliance for Sustainable Energy, LLC.

The SunShot Initiative is a collaborative national effort that aggressively drives innovation to make solar energy fully cost-competitive with traditional energy sources before the end of the decade. Through SunShot, the Energy Department supports efforts by private companies, universities, and national laboratories to drive down the cost of solar electricity to $0.06 per kilowatt-hour. Learn more at energy.gov/sunshot.

Researchers have designed a device that uses light to manipulate its mechanical properties. The device, which was fabricated using a plasmomechanical metamaterial, operates through a unique mechanism that couples its optical and mechanical resonances, enabling it to oscillate indefinitely using energy absorbed from light.

This is an optically-driven mechanical oscillator fabricated using a plasmomechanical metamaterial. Credit: UC San Diego Jacobs School of Engineering

This is an optically-driven mechanical oscillator fabricated using a plasmomechanical metamaterial. Credit: UC San Diego Jacobs School of Engineering

This work demonstrates a metamaterial-based approach to develop an optically-driven mechanical oscillator. The device can potentially be used as a new frequency reference to accurately keep time in GPS, computers, wristwatches and other devices, researchers said. Other potential applications that could be derived from this metamaterial-based platform include high precision sensors and quantum transducers. The research was published Oct. 10 in the journal Nature Photonics.

Researchers engineered the metamaterial-based device by integrating tiny light absorbing nanoantennas onto nanomechanical oscillators. The study was led by Ertugrul Cubukcu, a professor of nanoengineering and electrical engineering at the University of California San Diego. The work, which Cubukcu started as a faculty member at the University of Pennsylvania and is continuing at the Jacobs School of Engineering at UC San Diego, demonstrates how efficient light-matter interactions can be utilized for applications in novel nanoscale devices.

Metamaterials are artificial materials that are engineered to exhibit exotic properties not found in nature. For example, metamaterials can be designed to manipulate light, sound and heat waves in ways that can’t typically be done with conventional materials.

Metamaterials are generally considered “lossy” because their metal components absorb light very efficiently. “The lossy trait of metamaterials is considered a nuisance in photonics applications and telecommunications systems, where you have to transmit a lot of power. We’re presenting a unique metamaterials approach by taking advantage of this lossy feature,” Cubukcu said.

The device in this study resembles a tiny capacitor–roughly the size of a quarter–consisting of two square plates measuring 500 microns by 500 microns. The top plate is a bilayer gold/silicon nitride membrane containing an array of cross-shaped slits–the nanoantennas–etched into the gold layer. The bottom plate is a metal reflector that is separated from the gold/silicon nitride bilayer by a three-micron-wide air gap.

When light is shined upon the device, the nanoantennas absorb all of the incoming radiation from light and convert that optical energy into heat. In response, the gold/silicon nitride bilayer bends because gold expands more than silicon nitride when heated. The bending of the bilayer alters the width of the air gap separating it from the metal reflector. This change in spacing causes the bilayer to absorb less light and as a result, the bilayer bends back to its original position. The bilayer can once again absorb all of the incoming light and the cycle repeats over and over again.

The device relies on a unique hybrid optical resonance known as the Fano resonance, which emerges as a result of the coupling between two distinct optical resonances of the metamaterial. The optical resonance can be tuned “at will” by applying a voltage.

The researchers also point out that because the plasmomechanical metamaterial can efficiently absorb light, it can function under a broad optical resonance. That means this metamaterial can potentially respond to a light source like an LED and won’t need a strong laser to provide the energy.

“Using plasmonic metamaterials, we were able to design and fabricate a device that can utilize light to amplify or dampen microscopic mechanical motion more powerfully than other devices that demonstrate these effects. Even a non-laser light source could still work on this device,” said Hai Zhu, a former graduate student in Cubukcu’s lab and first author of the study.

“Optical metamaterials enable the chip-level integration of functionalities such as light-focusing, spectral selectivity and polarization control that are usually performed by conventional optical components such as lenses, optical filters and polarizers. Our particular metamaterial-based approach can extend these effects across the electromagnetic spectrum,” said Fei Yi, a postdoctoral researcher who worked in Cubukcu’s lab.

By David W. Price and Douglas G. Sutherland

Author’s Note: The Process Watch series explores key concepts about process control—defect inspection and metrology—for the semiconductor industry. Following the previous installments, which examined the 10 fundamental truths of process control, this new series of articles highlights additional trends in process control, including successful implementation strategies and the benefits for IC manufacturing. 

Introduction

In a previous Process Watch article [1], we showed that big excursions are usually easy to detect but finding small excursions requires a combination of high capture rate and low noise. We also made the point that, in our experience, it’s usually the smaller excursions which end up costing the fab more in lost product. Catastrophic excursions have a large initial impact but are almost always detected quickly. By contrast, smaller “micro-excursions” sometimes last for weeks, exposing hundreds or thousands of lots to suppressed yield.

Figure 1 shows an example of a micro-excursion. For reference, the top chart depicts what is actually happening in the fab with an excursion occurring at lot number 300. The middle chart shows the same excursion through the eyes of an effective inspection strategy; while there is some noise due to sampling and imperfect capture rate, it is generally possible to identify the excursion within a few lots. The bottom chart shows how this excursion would look if the fab employed a compromised inspection strategy—low capture rate, high capture rate variability, or a large number of defects that are not of interest; in this case, dozens of lots are exposed before the fab engineer can identify the excursion with enough confidence to take corrective action.

Figure 1. Illustration of a micro-excursion. Top: what is actually happening in the fab. Middle: the excursion through the lens of an effective control strategy (average 2.5 exposed lots). Bottom: the excursion from the perspective of a compromised inspection strategy (~40 exposed lots).

Figure 1. Illustration of a micro-excursion. Top: what is actually happening in the fab. Middle: the excursion through the lens of an effective control strategy (average 2.5 exposed lots). Bottom: the excursion from the perspective of a compromised inspection strategy (~40 exposed lots).

Unfortunately, the scenario depicted in the bottom of Figure 1 is all too common. Seemingly innocuous cost-saving tactics such as reduced sampling or using a less sensitive inspector can quickly render a control strategy to be ineffective [2]. Moreover, the fab may gain a false sense of security that the layer is being effectively monitored by virtue of its ability to find the larger excursions. 

Micro-Excursions 

Table 1 illustrates the difference between catastrophic and micro-excursions. As the name implies, micro-excursions are subtle shifts away from the baseline. Of course, excursions may also take the form of anything in between these two.

Table 1: Catastrophic vs. Micro-Excursions

Table 1: Catastrophic vs. Micro-Excursions

Such baseline shifts happen to most, if not all, process tools—after all, that’s why fabs employ rigorous preventative maintenance (PM) schedules. But PM’s are expensive (parts, labor, lost production time), therefore fabs tend to put them off as long as possible.

Because the individual micro-excursions are so small, they are difficult observe from end-of-line (EOL) yield data. They are frequently only seen in EOL yield data through the cumulative impact of dozens of micro-excursions occurring simultaneously; even then it more often appears to be baseline yield loss. As a result, fab engineers sometimes use the terms “salami slicing” or “penny shaving” since these phrases describe how a series of many small actions can, as an accumulated whole, produce a large result [3].

Micro-excursions are typically brought to an end because: (a) a fab detects them and puts the tool responsible for the excursion down; or, (b) the fab gets lucky and a regular PM resolves the problem and restores the tool to its baseline. In the latter case, the fab may never know there was a problem.

The Superposition of Multiple Simultaneous Micro-Excursions

To understand the combined impact of these multiple micro-excursions, it is important to recognize:

  1. Micro-excursions on different layers (different process tools) will come and go at different times
  2. Micro-excursions have different magnitudes in defectivity or baseline shift
  3. Micro-excursions have different durations

In other words, each micro-excursion has a characteristic phase, amplitude and wavelength. Indeed, it is helpful to imagine individual micro-excursions as wave forms which combine to create a cumulative wave form. Mathematically, we can apply the Principle of Superposition [4] to model the resulting impact on yield from the contributing micro-excursions.

Figure 2 illustrates the cumulative effect of one, five, and 10 micro-excursions happening simultaneously in a 1,000 step semiconductor process. In this case, we are assuming a baseline yield of 90 percent, that each micro-excursion has a magnitude of 2 percent baseline yield loss, and that they are detected on the 10th lot after it starts. As expected, the impact of a single micro-excursion is negligible but the combined impact is large.

Figure 2. The cumulative impact of one, five, and 10 simultaneous micro-excursions happening in a 1,000 step process: increased yield loss and yield variation.

Figure 2. The cumulative impact of one, five, and 10 simultaneous micro-excursions happening in a 1,000 step process: increased yield loss and yield variation.

It is interesting to note that the bottom curve in Figure 2 would seem to suggest that the fab is suffering from a baseline yield problem. However, what appears to be 80 percent baseline yield is actually 90 percent baseline yield with multiple simultaneous micro-excursions, which brings the average yield down to 80 percent. This distinction is important since it points to different approaches in how the fab might go about improving the average yield. A true baseline yield problem would suggest that the fab devote resources to run experiments to evaluate potential process improvements (design of experiments (DOEs), split lot experiments, failure analysis, etc.). These activities would ultimately prove frustrating as the engineers would be trying to pinpoint a dozen constantly-changing sources of yield loss.

The fab engineer who correctly surmises that this yield loss is, in fact, driven by micro-excursions would instead focus on implementing tighter process tool monitoring strategies. Specifically, they would examine the sensitivity and frequency of process tool monitor inspections; depending on the process tool, these monitors could be bare wafer inspectors on blanket wafers and/or laser scanning inspectors on product wafers. The goal is to ensure these inspections provide timely detection of small micro-excursions, not just the big excursions.

The impact of an improved process tool monitoring strategy can be seen in Figure 3. By improving the capture rate (sensitivity), reducing the number of non-critical defects (by doing pre/post inspections or using an effective binning routine), and reducing other sources of noise, the fab can bring the exposed product down from 40 lots to 2.5 lots. This, in turn, significantly reduces the yield loss and yield variation.

Figure 3. The impact of 10 simultaneous micro-excursions for the fab with a compromised inspection strategy (brown curve, ~40 lots at risk), and a fab with an effective process tool monitoring strategy (blue curve, ~2.5 lots at risk).

Figure 3. The impact of 10 simultaneous micro-excursions for the fab with a compromised inspection strategy (brown curve, ~40 lots at risk), and a fab with an effective process tool monitoring strategy (blue curve, ~2.5 lots at risk).

Summary

Most fabs do a good job of finding the catastrophic defect excursions. Micro-excursions are much more common and much harder to detect. There are usually very small excursions happening simultaneously at many different layers that go completely undetected. The superposition of these micro-excursions leads to unexplained yield loss and unexplained yield variation.

As a yield engineer, you must be wary of this. An inspection strategy that guards only against catastrophic excursions can create the false sense of security that the layer is being effectively monitored—when in reality you are missing many of these smaller events that chip away or “salami slice” your yield.

References:

About the Author: 

Dr. David W. Price is a Senior Director at KLA-Tencor Corp. Dr. Douglas Sutherland is a Principal Scientist at KLA-Tencor Corp. Over the last 10 years, Dr. Price and Dr. Sutherland have worked directly with more than 50 semiconductor IC manufacturers to help them optimize their overall inspection strategy to achieve the lowest total cost. This series of articles attempts to summarize some of the universal lessons they have observed through these engagements.