Tag Archives: letter-leds-top

Scientists from Germany and Spain have discovered a way to create a BioLED by packaging luminescent proteins in the form of rubber. This innovative device gives off a white light which is created by equal parts of blue, green and red rubber layers covering one LED, thus rendering the same effect as with traditional inorganic LEDs but at a lower cost.

Increasingly popular LEDs, or light-emitting diodes, are the light of choice for the European Union and the United States when it comes to creating lighting devices of the future. This preference can be attributed to the fact that LEDs are more efficient than traditional incandescent bulbs and more stable than energy-efficient light bulbs.

Despite their advantages, however, LEDs are manufactured using inorganic materials that are in short supply -such as cerium and yttrium-, thus meaning that they are more expensive and difficult to sustain in the long run. Additionally, white LEDs produce a colour that is not optimal for eyesight since they lack a red component that can psychologically affect individuals exposed to them for long periods of time.

Now, however, a German-Spanish team of scientists has drawn inspiration from nature’s biomolecules in search of a solution. Their technique consists in introducing luminescent proteins into a polymer matrix to produce luminescent rubber. This technique involves a new way of packaging proteins which could end up substituting the technique used to create LEDs today.

“We have developed a technology and a hybrid device called BioLED that uses luminescent proteins to convert the blue light emitted by a ‘normal’ LED into pure white light”, explains Rubén D. Costa to Sinc, a researcher at the University of Erlangen-Nürnberg (Germany) and co-author of the study.

It is always necessary to have either a blue or an ultraviolet LED to excite the rubbers that are put over the LED in order to make it white. In other words, we can combine blue LED/green rubber/red rubber, or ultraviolet LED/blue rubber/green rubber/red rubber. The result is the first BioLED that gives off a pure white light created by similar parts of the colours blue, green and red, all while maintaining the efficiency offered by inorganic LEDs.

The authors clear up that the blue or ultraviolet LEDs are much cheaper than white ones, which are made of an expensive and scarce material known as YAG:Ce (Cerium-doped Yttrium Aluminium Garnet). The idea is replace it by proteins.

“The Bio-LEDs are simple to manufacture and their materials are low-cost and biodegradable, meaning that they can easily be recycled and replaced”, points out Costa, while also highlighting the high stability of these proteins that have “luminescent properties that remain intact during the months of storage under different environmental conditions of light, temperature and humidity”.

In fact, with this technique “we have been able to achieve a sustained use of proteins in optoelectronic devices with an excellent stability for the first time, something that had not happened in the last 50 years. This thus represents a major breakthrough in this field,” stresses Pedro B. Coto, another one of the authors who also conducts research at this German university.

Scientists are already working on optimising this new elastic material in order to achieve greater thermal stability and an even longer operating lifetime. They are addressing how to optimise the chemical composition of the polymer matrix in addition to using proteins that are increasingly more resistant to device operating conditions. The goal is to make this new BioLED more accessible on an industrial scale in the not too distant future.

Use of copper as a fluorescent material allows for the manufacture of inexpensive and environmentally compatible organic light-emitting diodes (OLEDs). Thermally activated delayed fuorescence (TADF) ensures high light yield. Scientists of Karlsruhe Institute of Technology (KIT), CYNORA, and the University of St Andrews have now measured the underlying quantum mechanics phenomenon of intersystem crossing in a copper complex. The results of this fundamental work are reported in the Science Advances journal and contribute to enhancing the energy efficiency of OLEDs.

Organic light-emitting diodes are deemed tomorrow’s source of light. They homogeneously emit light in all observation directions and produce brilliant colors and high contrasts. As it is also possible to manufacture transparent and flexible OLEDs, new application and design options result, such as flat light sources on window panes or displays that can be rolled up. OLEDs consist of ultra-thin layers of organic materials, which serve as emitter and are located between two electrodes. When voltage is applied, electrons from the cathode and holes (positive charges) from the anode are injected into the emitter, where they form electron-hole pairs. These so-called excitons are quasiparticles in the excited state. When they decay into their initial state again, they release energy.

Excitons may assume two different states: Singlet excitons decay immediately and emit light, whereas triplet excitons release their energy in the form of heat. Usually, 25 percent singlets and 75 percent triplets are encountered in OLEDs. To enhance energy efficiency of an OLED, also triplet excitons have to be used to generate light. In conventional light-emitting diodes heavy metals, such as iridium and platinum, are added for this purpose. But these materials are expensive, have a limited availability, and require complex OLED production methods.

It is cheaper and environmentally more compatible to use copper complexes as emitter materials. Thermally activated delayed fluorescence (TADF) ensures high light yields and, hence, high efficiency: Triplet excitons are transformed into singlet excitons which then emit photons. TADF is based on the quantum mechanics phenomenon of intersystem crossing (ISC), a transition from one electronic excitation state to another one of changed multiplicity, i.e. from singlet to triplet or vice versa. In organic molecules, this process is determined by spin-orbit coupling. This is the interaction of the orbital angular momentum of an electron in an atom with the spin of the electron. In this way, all excitons, triplets and singlets, can be used for the generation of light. With TADF, copper luminescent material reaches an efficiency of 100 percent.

Stefan Bräse and Larissa Bergmann of KIT’s Institute of Organic Chemistry (IOC), in cooperation with researchers of the OLED technology company CYNORA and the University of St Andrews, United Kingdom, for the first time measured the speed of intersystem crossing in a highly luminescent, thermally activated delayed fluorescence copper(I) complex in the solid state. The results are reported in the Science Advances journal. The scientists determined a time constant of intersystem crossing from singlet to triplet of 27 picoseconds (27 trillionths of a second). The reverse process – reverse intersystem crossing – from triplet to singlet is slower and leads to a TADF lasting for an average of 11.5 microseconds. These measurements improve the understanding of mechanisms leading to TADF and facilitate the specific development of TADF materials for energy-efficient OLEDs.

Oxygen is indispensable to animal and plant life, but its presence in the wrong places can feed a fire and cause iron to rust.

In the fabrication of solid state lighting devices, scientists are learning, oxygen also plays a two-edged role. While oxygen can impede the effectiveness of gallium nitride (GaN), an enabling material for LEDs, small amounts of oxygen in some cases are needed to enhance the devices’ optical properties. GaN doped with europium (Eu), which could provide the red color in LEDs and other displays, is one such case.

Last week, an international group of researchers shed light on this seeming contradiction and reported that the quantity and location of oxygen in GaN can be fine-tuned to improve the optical performance of Eu-doped GaN devices. The group includes researchers from Lehigh, Osaka University in Japan, the Instituto Superior Técnico in Portugal, the University of Mount Union in Ohio, and Oak Ridge National Laboratory in Tennessee.

Writing in Scientific Reports, a Nature publication, the group said that small quantities of oxygen promote the uniform incorporation of Eu into the crystal lattices of GaN. The group also demonstrated a method of incorporating Eu uniformly that utilizes only the oxygen levels that are inevitably present in the GaN anyway. Eu, a rare earth (RE) element, is added to GaN as a “dopant” to provide highly efficient red color emission, which is still a challenge for GaN-based optoelectronic devices.

The devices’ ability to emit light is dependent on the relative homogeneity of Eu incorporation, said Volkmar Dierolf, professor and chair of Lehigh’s physics department.

“Some details, such as why the oxygen is needed for Eu incorporation, are still unclear,” said Dierolf, “but we have determined that the amount required is roughly 2 percent of the amount of Eu ions. For every 100 Eu ions, you need two oxygen atoms to facilitate the incorporation of Eu to GaN.

“If the oxygen is not there, the Eu clusters up and does not incorporate. When the oxygen is present at about 2 percent, oxygen passivation takes place, allowing the Eu to incorporate into the GaN without clustering.”

The article is titled “Utilization of native oxygen in Eu(RE)-doped GaN for enabling device compatibility in optoelectronic applications.” The lead author, Brandon Mitchell, received his Ph.D. from Lehigh in 2014 and is now an assistant professor of physics and astronomy at the University of Mount Union and a visiting professor at Osaka University.

 

A comprehensive study

Gallium nitride, a hard and durable semiconductor, is valued in solid state lighting because it emits light in the visible spectrum and because its wide band gap makes GaN electronic devices more powerful and energy-efficient than devices made of silicon and other semiconductors.

The adverse effect of oxygen on GaN’s properties has been much discussed in the scientific literature, the researchers wrote in Scientific Reports, but oxygen’s influence on, and interaction with, RE dopants in GaN is less well understood.

“The presence of oxygen in GaN,” the group wrote in their article, which was published online Jan. 4, “…is normally discussed with a purely negative connotation, where possible positive aspects of its influence are not considered.

“For the continued optimization of this material, the positive and negative roles of critical defects, such as oxygen, need to be explored.”

The group used several imaging techniques, including Rutherford Backscattering, Atomic Probe Tomography and Combined Excitation Emission Spectroscopy, to obtain an atomic-level view of the diffusion and local concentrations of oxygen and Eu in the GaN crystal lattice.

Its investigation, the group wrote, represented the “first comprehensive study of the critical role that oxygen has on Eu in GaN.” The group chose to experiment with Eu-doped GaN (GaN:Eu), said Dierolf, because europium emits bright light in the red portion of the electromagnetic spectrum, a promising quality given the difficulty scientists have encountered in realizing red LED light.

The group said its results “strongly indicate that for single layers of GaN:Eu, significant concentrations of oxygen are required to ensure uniform Eu incorporation and favorable optical properties.

“However, for the high performance and reliability of GaN-based devices, the minimization of oxygen is essential. It is clear that these two requirements are not mutually compatible.”

Preliminary LED devices containing a single 300-nanometer active GaN:Eu layer have been demonstrated in recent years, the group reported, but have not yet achieved commercial viability, in part because of the incompatibility of oxygen with GaN.

To overcome that hurdle, said Dierolf, the researchers decided that instead of growing one thick, homogeneous layer of GaN:Eu they would grow several thinner layers of alternating doped and undoped regions. This approach, they found, utilizes the relatively small amount of oxygen that is naturally present in GaN grown with organo-metallic vapor phase epitaxy (OMVPE), the common method of preparing GaN.

“Instead of growing a thick layer of Eu-doped GaN,” said Dierolf, “we grew a layer that alternated doped and undoped regions. Through the diffusion of the europium ion, oxygen from the undoped regions was utilized to incorporate the Eu into the GaN. The europium then diffused into the undoped regions.”

To determine the optimal amount of oxygen needed to circumvent the oxygen-GaN incompatibility, the researchers also conducted experiments on GaN grown with an Eu “precursor” containing oxygen and on GaN intentionally doped with argon-diluted oxygen.

They found that the OMVPE- grown GaN contained significantly less oxygen than the other samples.

“The concentration of this oxygen [in the OMVPE- grown GaN] is over two orders of magnitude lower than those [concentrations] found in the samples grown with the oxygen-containing Eu…precursor,” the group wrote, “rendering the material compatible with current GaN-based devices.

“We have demonstrated that the oxygen concentration in GaN:Eu materials can be reduced to a device-compatible level. Periodic optimization of the concentration ratio between the normally occurring oxygen found in GaN and the Eu ions resulted in uniform Eu incorporation, without sacrificing emission intensity.

“These results appear to coincide with observations in other RE-doped GaN materials. Adoption of the methods discussed in this article could have a profound influence on the future optimization of these systems as well as GaN:Eu.”

The group plans next to grow GaN quantum well structures and determine if they enable Eu to incorporate even more favorably and effectively into GaN. Toward that end, Dierolf and Nelson Tansu, professor of electrical and computer engineering and director of Lehigh’s Center for Photonics and Nanoelectronics, have been awarded a Collaborative Research Opportunity (CORE) grant from Lehigh.

Viewpoints: 2016 outlook


January 11, 2016

Each year, Solid State Technology turns to industry leaders to hear viewpoints on the technological and economic outlook for the upcoming year. Read through these expert opinions on what to expect in 2016.

New technologies will fuel pockets of growth in 2016

Plisinski_headshotBy Mike Plisinski, Chief Executive Officer, Rudolph Technologies, Inc.

While the 2016 outlook for the semiconductor industry as a whole appears increasingly uncertain, there are areas where significant growth remains likely. In particular, advanced packaging, driven by growing consumer demand in applications ranging from smartphones and tablets to the Internet of Things (IoT), shows great promise for continued innovation.

First, we see outsourced assembly and test (OSAT) manufacturers driving the development of new packaging technology. For example, we’ve seen major gains in the adoption of fan-out packaging and copper pillar technology, evidenced by ongoing capacity expansion, and the addition of new players—the most obvious perhaps being the large ongoing investment by a leading foundry in Asia where our inspection equipment has received a prominent role. We see more and more manufacturers choosing to add yield management and/or advanced process control (APC) software, to obtain a competitive advantage in not only cost, but also reliability. This is achieved by transforming ultra-large data sets into useful information used for predictive analytics (reducing costs) and analysis across the supply chain (improving reliability).

The growth in advanced packaging is also driving the adoption of sophisticated lithography techniques for these new technologies. Our JetStep advanced packaging stepper is now in high-volume manufacturing use at several top OSATs. The system allows our customers the flexibility to handle all of the current advanced processes within a single tool, which provides a compelling cost of ownership value. We also see emerging processes, such the adoption of rectangular panel substrates, in some packaging applications, certainly in fan-out, but also embedded and other packaging technologies. Rectangular panels promise significant gains in economy-of-scale and processing efficiency.

Lastly, expansion in radio frequency (RF) device capability continues to grow, with the increasing number of devices that communicate wirelessly and the increasing number of frequencies with which they communicate. Measuring the electrode and piezo layers of SAW/BAW filters will only grow as more and more filters are required in mobile devices. Beyond mobile, the expansion in RF is also driven by WiFi, Bluetooth and IoT requirements for connectivity, so we expect it to accelerate even as the smartphone growth curve flattens.

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Due to the further scaling and increasing complexity of transistors, the boundaries between back-end-of-line and front-end-of-line reliability research are gradually fading. Imec’s team leaders Kristof Croes and Dimitri Linten give their vision on the future of reliability research.

In April 2015, the 53rd edition of the IEEE International Reliability Physics Symposium (IRPS) took place, a top conference where experts in reliability of micro- and nanoelectronics meet. With 16 contributions as either an author or a co-author, imec was prominently present.

Dimitri Linten: “Our contributions to conferences such as IRPS highlight the unique role that imec plays in the field of reliability. And they show the importance of reliability research at imec for the development of new transistor and memory concepts. As scaling continues, a whole range of new technology options is being researched. New materials and architectures with often unknown failure mechanisms are being introduced. Reliability is one of the factors that determine which concept will finally have a chance. For example, one of the options is to replace silicon in the transistor’s channel by germanium or a III/V material since these materials provide a higher charge carrier mobility. But until now, these materials pose important challenges to the reliability of the transistors that are made of these materials. Or, researchers look at introducing either air gaps or ultralow-k materials as spacers between the transistor’s gate and drain in order to keep the capacity as low as possible. The integration of all these new materials is important, but their reliability is crucial as well: reliability before performance.”

Front of line Fig 1

Kristof Croes: “10 years ago, reliability was tested only in a final stage of a technology development. But due to the ever decreasing reliability margins, the reliability is now being tested from the very beginning. And this starts with an understanding of the physics behind the failure, for which we often collaborate with universities. Once we understand the failure of e.g. new materials, we can model our findings and predict the lifetime of the device.”

Front-end-of-line vs back-end-of-line

Traditionally, CMOS process engineers classify the semiconductor process in two main parts: the front-end-of-line (FEOL) and the back-end-of-line (BEOL). The FEOL comprises all the process steps that are related to the transistor itself, including the gate of the transistor. The BEOL comprises all subsequent process steps. In the BEOL, the various transistors are being interconnected through metal lines. The same classification is being used in reliability research. Consequently, FEOL and BEOL reliability is tested independently.

Kristof Croes: “This historical separation is being applied at imec as well, where reliability research within the process technology division is distributed among several groups. One group looks into the reliability of FEOL and memory chips. Another group investigates the BEOL reliability and chip packaging interaction. Today, in BEOL processes, electromigration (the movement of metal atoms as a result of an electric current), stress migration, time dependent dielectric breakdown (TDDB) and thermomechanical stress are the main failure mechanisms. We also look into 3D structures, where the impact and reliability of through-Si vias are important issues. In a 3D-stacked structure, for some applications, the Si wafer needs to be thinned down to about 5 micrometer. And this impacts the reliability. And there are thermal and thermo-mechanical influences related to the assembly of materials with completely different mechanical properties. All these failure mechanisms in the BEOL will become increasingly important for future technology nodes.”

Dimitri Linten: “We look into the time dependent breakdown (TDDB) of the gate stack, and into stress-induced leakage current (SILC) and hot carrier stress (HC). The bias temperature instability (BTI) is important as well, as it causes a shift of the threshold voltage (VT) of the transistor during the lifetime of the circuit. We also investigate memory elements, by testing and modelling the retention and endurance of the memories. ESD or electrostatic discharge is still one of the main important failure mechanisms at the level of the final ICs in a certain technology. In order to intercept the current that is released during an electrostatic discharge, protecting ESD structures are implemented in the FEOL.”

Front of line Fig 2

FEOL and BEOL reliability: fading boundaries

As the dimensions of the transistor shrink, the impact of the FEOL on the BEOL reliability – and vice versa – increases. Kristof Croes: “A well-known example is self-heating in FinFETs. In planar CMOS processes, the heat that is released during the transistor’s operation is dissipated mainly through the Si substrate. But in a FinFET architecture, we have to take into account a higher thermal coupling towards the metal intercon- nects. The FinFETs warm up and heat the metal lines. And this impacts the reliability of the BEOL structure. In 3D technology, we thin down wafers with TSVs. After opening the TSVs, we can stack them on top of another wafer. The integration of the TSVs, the thinning and stacking of the wafers influence both the FEOL and the BEOL performance and reliability.”

Dimitri Linten: “Also the introduction of new architectures brings the reliability of FEOL and BEOL closer to each other. Think about vertical nanowires, potential successors of the FinFET because they promise a better electro- static channel control. One of the challenges in terms of reliability is to provide these structures with an ESD protection. While in more conventional structures, the FEOL is most sensitive to electrostatic discharge, the impact of electrostatic discharge on the BEOL becomes critical in vertical nanowires. In these 3D structures, we have to connect all the vertical nanowires through local interconnects and interconnects that will be located very close to each other. And these interconnects will put other requirements to the ESD protection circuit than we are used to. A possible solution is to consider a 3D stacking of ESD protection circuits on top of the transistor architecture.”

Another consequence of further scaling is an increase in the variability of the transistor parameters. In FEOL, variability is a well-known phenomenon.

Dimitri Linten: “Time dependent variability of BTI is a relatively new challenge for reliability research. For large transistors – the older generations – BTI translates into an average shift of the circuit’s threshold voltage of e.g. 50mV, the spec for BTI. But upon further scaling, there is no average shift anymore. Instead, there will be a static distribution of shifts. The variability becomes time dependent and the lifetime of the circuits will be spread. The imec FEOL reliability group is a world leader in this domain: we have developed a defect centric BTI model that has been adopted by market leaders in the semiconductor industry. On time dependent BTI, we closely collaborate with the design group in order to develop methodologies that take into account the time dependence.”

Kristof Croes: “Also in BEOL, variability becomes increasingly important. Think about via misalignment or line edge roughness of increasingly smaller metal lines. These issues degrade the reliability and the lifetime of the BEOL. To deal with the increasing variability, a powerful statistical toolbox is required. And this toolbox can be deployed for BEOL as well as for FEOL reliability research.”

Front of line Fig 3

When BEOL meets FEOL reliability

As dimensions are shrinking, the boundaries between FEOL and BEOL reliability are gradually fading.

Kristof Croes: “We are convinced that we should optimally attune the activities and tools used for reliability research. We have to bring the people from BEOL and FEOL reliability closer together. And we want to unite the researchers outside these groups that work on reliability. Reliability is a field of expertise and sharing problems often provides part of the solution. For future technology nodes and for develop- ments beyond scaling, this will increase the operational efficiency of reliability research. To strengthen this idea, we will organize an internal workshop at imec on September 4, with the help of our predecessors and colleagues Guido Groeseneken and Ingrid De Wolf. This will help our researchers to gain more insight into each other’s work and into the tools they use. Hopefully, this idea will be adopted outside of imec as well.”

Additional reading

Technical program of the 2015 IRPS conference with abstracts (http://www.irps.org/program/technical-program/15-program.pdf)

“New test allows to visualize in real-time crack formation of BEOL,” March issue of imec magazine (http://magazine.imec.be/data/57/ reader/reader.html#preferred/1/package/57/pub/63/page/2)

Hybrid optoelectronic devices based on blends of hard and soft semiconductors can combine the properties of the two material types, opening the possibility for devices with novel functionality and properties, such as cheap and scalable solution-based processing methods. However, the efficiency of such devices is limited by the relatively slow electronic communication between the material components that relies on charge transfer, which is susceptible to losses occurring at the hybrid interface.

A phenomenon called Förster resonant energy transfer (FRET) was recently theoretically predicted and experimentally observed in hybrid structures combining an inorganic quantum well with a soft semiconductor film. Förster resonant energy transfer is a radiationless transmission of energy that occurs on the nanometer scale from a donor molecule to an acceptor molecule. The process promotes energy rather than charge transfer, providing an alternative contactless pathway that avoids some of the losses caused by charge recombination at the interface.

Now researchers from the University of Cyprus and Cyprus University of Technology, along with colleagues from the University of Crete, Greece have conducted a comprehensive investigation on how various structural and electronic parameters affect FRET in structures of nitride quantum wells with light-emitting polymers. Based on their studies, the researchers discuss the process to optimize the energy transfer process and identify the limitations and implications of the Förster mechanism in practical devices. The work demonstrates the importance of understanding FRET in hybrid structures that could pave the way for developing novel devices such as high-efficiency LEDs and solar cells. The researchers present their work in a paper published this week in the Journal of Chemical Physics, from AIP Publishing.

In the top left is a schematic of Förster Resonant Energy Transfer from a near-surface nitride quantum well to a polymer overlayer. In the top right is fluorescence from solutions containing light emitting polymer materials. In the bottom left high resolution transmission electron microscope image from an InGaN/GaN quantum well. In the bottom right absorption and fluorescence spectra from various polymers used in our study. Credit: Grigorios Itskos/University of Cyprus, Cyprus

In the top left is a schematic of Förster Resonant Energy Transfer from a near-surface nitride quantum well to a polymer overlayer. In the top right is fluorescence from solutions containing light emitting polymer materials. In the bottom left high resolution transmission electron microscope image from an InGaN/GaN quantum well. In the bottom right absorption and fluorescence spectra from various polymers used in our study. Credit: Grigorios Itskos/University of Cyprus, Cyprus

“Pioneering theoretical and experimental work has demonstrated that energy can be efficiently transferred across hybrid semiconductors via the Förster mechanism. However, our understanding is not complete and many material and structural parameters affecting FRET in such hybrids remain unexplored. Our work employs for a first time a comprehensive approach that combines fabrication, theoretical modeling and optical spectroscopy to fully understand FRET in a nitride quantum well-polymer hybrid structure,” said Grigorios Itskos, the primary researcher and an assistant professor from the Department of Physics at the University of Cyprus.

“We used a systematic approach to optimize the FRET efficiency by tuning various parameters of the nitride quantum well component. The process allowed us to study unexplored aspects of the mechanism and identify competing mechanisms that limit the energy transfer efficiency in hybrid planar structures. The outcome of our investigation can guide future efforts towards a rational design of hybrid geometries that can optimize FRET and limit competing losses to render FRET-based devices feasible,” he said.

Itskos noted that the researchers chose to study structures based on nitrides because the material is well-researched and is used in niche applications such as blue light emitting LEDs. “However, the functionality [of nitride structures] can be further increased by combining them with other soft semiconductors such as light-emitting polymers. The spectral tunability and high light-absorption and emitting efficiency of the polymers can be exploited to demonstrate efficient down-conversion of the blue nitride emission, providing a scheme for efficient hybrid LEDs,” Itskos said.

In the study, the researchers initially sought to produce and study near-surface nitride quantum wells to allow a close proximity with the light-emitting polymer deposited on their top surface.

“The nanoscale proximity promotes efficient interactions between the excitations of the two materials, leading to fast Förster transfer that can compete with the intrinsic recombination of the excitations,” Itskos explained. Förster resonant energy transfer is a strongly distance-dependent process which occurs over a scale of typically 1 to 10 nanometers. The contactless pathway of energy transmission could avoid energy losses associated with charge recombination and transport in hybrid structures.

Using a sequence of growth runs, theoretical modeling and luminescence spectroscopy (a spectrally-resolved technique measuring the light emission of an object), the researchers identified the way to optimize the surface quantum well emission.

“We studied the influence of parameters such as growth temperature, material composition, and thickness of the quantum well and barrier on the optoelectronic properties of the nitride structures. Increase of the quantum confinement by reducing the width or increasing the barrier of the quantum well increases the well emission. However, for high quantum well confinement, excitations leak to the structure surface, quenching the luminescence. So there is an optimum set of quantum well parameters that produce emissive structures,” Itskos said. He also pointed out that the studies indicate a strong link between the luminescence efficiency of the nitride quantum well with the FRET efficiency of the hybrid structure, as predicted by the basic theory of Förster. The correlation could potentially provide an initial and simple FRET optimization method by optimizing the luminescent efficiency of the energy donor in the absence of the energy acceptor material.

“Our studies also indicated that electronic doping of the interlayer between the nitride quantum well and the polymer film reduces the efficiency of FRET. This constitutes a potential limitation for the implementation of such hybrid structures in real-world electronic devices, as electronic doping is required to produce efficient practical devices. Further studies are needed to establish the exact influence of doping on FRET,” Itskos noted.

He said the team’s next step is to perform a systematic study of hybrid structures based on doped nitride quantum wells to investigate the mechanisms via which electronic doping affects the characteristics of the Förster resonant energy transfer.

The use of sapphire in the manufacturing of Light Emitting Diodes (LEDs) is covered in the second part of a two part series.

BY WINTHROP A. BAYLIES and CHRISTOPHER JL MOORE, BayTech-Resor LLC, Maynard, MA

In Part 1 of this article, we discussed the optical and mechanical properties of sapphire and its use in the mobile device industry. In part 2, we will discuss the use of sapphire in the LED process including some of the newer technologies that produce these devices.

Solid state lighting (or “LED bulbs” as they are commonly known) have become a mainstream product in our culture. Their longer life time and lower power usage (along with the banning of incandescent bulbs) have ensured that more and more consumers are moving to this type of lighting. Like a fluorescent light (where the white light is produced by a phosphor coating excited by the excited gas molecules) solid state lights use a phosphor excited by the short wavelength light emitted by an LED. What you may not know is that about 8 out of every 10 LED bulbs sold uses sapphire as the starting material for their manufacturing process.

As we summarized in part 1, sapphire has some good points: hard, strong, optically transparent and chemically inert (there is a reason high end watches use sapphire crystals) and some bad points: hard, strong, and chemically inert (which is why sapphire crystals are more expensive than glass). What we did not discuss is that single crystal sapphire has turned out to be an ideal material on which to grow the layers of material needed to make an LED.

As FIGURE 1 shows an LED is made by growing epitaxial layers of Gallium Nitride (GaN), AlGan or InGaN on a substrate. Ideally one would use GaN as the substrate material (similar to growing epitaxial Si on Si for integrated circuits) as this would result in the highest quality material and thus the most efficient LED’s. Unfortunately GaN substrates are very difficult to make in any reasonable size and the costs have ruled out using this approach except in certain niche markets. The three main substrate alternatives have been silicon (Si), silicon carbide (SiC) and sapphire.

Sapphire 1

As a substrate material Si would be expected to be the best choice due to its high quality, low cost and ready availability. To date, the quality of GaN type layers grown on Si has not been sufficient for large scale manufacturing processes. Work continues on improving this process and although it may one day dominate the process it currently remains a small part of the business.

SiC substrates are higher cost than Si but have been successfully used for LED manufacturing processes. Much of the LEDs produced by Cree (who also manufacture SiC substrates) use this type of substrate. However, the higher cost and limited availability of 6 inch SiC material means that the majority of LED producers use sapphire.

Thus sapphire substrates account for the majority of LED devices produced [1]. Although not as cheap as Si they are cheaper than SiC, available from a number of manufacturers and are able to survive the high temperature processes needed to produce a short wavelength LED. FIGURE 2 schematically shows the production process for a typical non-patterned sapphire wafer.

Sapphire 2

The sapphire production process starts when a seed crystal and a mixture of aluminum oxide and crackle (un-crystallized sapphire material) is heated in a crucible. Included in this mix is a cookie-sized seed crystal which forms the pattern to be replicated as the crystal grows. Each furnace manufacturer has its own special recipe which heats the material using a specific temperature/ time profile based on the size of melt and the type of crystal to be grown. Once the correct growth temperature is reached the melt is cooled (this process can take two weeks depending on the amount of sapphire being produced) using another set of carefully controlled time/temperature profiles. When done correctly, the cookie-sized seed grows and produces a single-crystal sapphire boule. (FIGURE 3). In reality, two weeks is a long time and any number of can go arise during this process including gas bubbles, mechanical faults such as cracks and contamination. Each of these problems affects the sapphire and its crystal properties. Each crystal fault can become a nucleation site for defects in the epitaxy grown on wafers produced from the boule. There is a clear correlation between the time taken to grow a boule and the potential quality of the boule produced. Many of the problems encountered in the upscaling of the sapphire production process have come from trying to grow large boules at high speeds.

Sapphire 3

At this point in the process you have a boule which in fact has the wrong crystal orientation for growing GaN epitaxy. Unlike the Si crystal growth process where the cylindrical boules can be ground to size and then cut into wafers, sapphire boules are often cored at right angles to the boule axis. Some companies produce sapphire using a silicon like process [2] but the majority of sapphire produced has to be cored. Thus the next step in the process is to “core-drill” a boule to produce one or more smaller round cylinders (ingots) depending on the original boule size and the size of wafers to be produced.

The ability to grow large sized boules on a regular basis is not in question; most important is how much of that boule is bubble-, crack- and impurity-free. In some cases the boules are inspected with various metrology techniques to determine which sections of the boule can be used and which cannot. The section of the boules not used is recycled into the original growth process (unless contaminated). Obviously if one is producing 6 inch wafers larger volumes of the boule need to be defect free than if one is producing 2 inch or 4 inch. Currently most of the LEDs produced are produced on 4 inch wafers with a few newer 6 inch lines and a number of older 2 inch lines. 8 inch sapphire wafers do exists but are not in mass production at this time.

The process after this is very similar to that used in the silicon industry to produce the wafers which will be used as substrates. A diamond saw (remember, Sapphire is a very hard material) is used to cur the ingot into a number of thin disc shapes by cutting perpendicular to the ingot’s long sides. Each of these discs is then ground to its final size, surface-ground and mechanically and chemically polished to produce sapphire substrates. These substrates, after cleaning, can be used as starting material for the epitaxial process used to produce the LED structure. FIGURE 4 shows some pictures of typical 2, 4 and 5 inch sapphire substrates. As discussed earlier the more defect free the surface is the better the quality of epitaxial film that can be grown. The video listed in reference [3] produced by GTAT shows many of the steps discussed above.

Sapphire 4

Recently one further step has been taken to produce what are called patterned sapphire substrates (PSS). The multiple quantum well layer shown in Fig. 1 is the layer that generates light in an LED. As you can imagine this light is emitted in all directions. However, once packaged most LED’s emit light from only one surface of the device. In the case of Fig. 1, a typical package collects the light emitted from the top of the device. This of course means that all of the light emitted in any other direction is wasted. In particular, since sapphire is transparent, little of the light emitted toward the substrate can be used.

One obvious solution to this would be to coat the substrate with something that reflects the light (i.e. metal). Unfortunately this interferes with the epitaxial layer growth process, producing poor devices. One partial solution to the reflection problem is to pattern the sapphire surface such that it reflects light. This pattern can be a series of microscopic pyramidal structures or more rounded bump like structures on the surface. FIGURE 5 shows top and side view SEM pictures of some of the patterns produced by manufacturers. These patterns scatter the light and reflect some of it back towards the surface of the device increasing the light output from the LED. In addition to increasing the apparent light output a number of manufacturers have claimed that epitaxial layers grown on patterned substrates is of better quality than that grown on bare sapphire substrates.

Sapphire 5

Patterned substrates can be produced by the manufacturer of the sapphire substrates. However, factories now exist which begin with a non-patterned substrate and produce specific patterns (normally via chemical etch) for specific LED manufacturers.

Once valued only as a gemstone, sapphire is now an engineered material with a wide variety of industrial uses. These two article have concentrated on its use in mobile devices for everything from camera lens covers to touch sensors and touch screens to the starting material on which most of the solid state lights produced are made. Cost of the material continues to be a limiting factor in its widespread adoption for certain industries. However, as the technology for producing sapphire matures material costs are decreasing and in some ways sapphire substrates have become a commodity rather than a rarity.

Additional reading and viewing material

1. http://rubicontechnology.com/sites/default/files/Opportu- nities%20for%20Sapphire%20White%20Paper-Rubicon%20 Technology.pdf
2. http://www.arc-energy.com/products-services/CHES/Foundations/1
3. https://www.youtube.com/embed/mHrDXyQGSK0

Sapphire is hard, strong, optically transparent and chemically inert.

BY WINTHROP E. BAYLIES and CHRISTOPHER JL MOORE, BayTech-Resor LLC, Maynard, MA

Have you ever wondered what blue gemstone earrings, an LED lightbulb and an Apple Watch have in common? The answer (at least for this article) is that all depend on sapphire as part of their manufacturing process. In part 1 of the following two part article, we will discuss how sapphire is becoming an important part of the mobile device food chain. Part 2 will concentrate on how sapphire is used in LED production.

Sapphire (chemical composition Al2O3) has a high melting point of 2040°C (3704°F) and is chemically resistant even at high temperatures. It is an anisotropic material meaning that its mechanical/thermal properties depend on the direction of the crystal plane that is cut and polished. An insulator with a 9.2 eV energy gap it is optically transparent. With a hardness of 9 on the Mhos scale, it is almost as hard and strong as diamond (10 Mhos).

To summarize, sapphire has some good points: hard, strong, optically transparent and chemically inert (there is a reason high end watches use sapphire crystals) and some bad points: hard, strong, and chemically inert (which is why sapphire crystals are more expensive than glass). That is, the very properties that make it ideal for applications needing mechanical strength and hardness mean that it is a difficult material to grow, machine and polish.

There are several places where sapphire can be (or is now) used in the manufacture of mobile devices. The most publicity in this area was generated in 2014 with significant speculation in both the trade magazines and newspapers (such as the Wall Street Journal) that the iPhone 6 would be released with a sapphire touch screen or at the very least a sapphire cover glass over the existing touchscreen. Part of this speculation was fueled by the large number (1700 to 2500 depending on source) of sapphire producing furnaces being installed at an Apple facility in Mesa Arizona. However, the sapphire iPhone 6 was not released due in part to the difficulties in growing and processing enough sapphire screens at a reasonable cost to supply the significant number of phones produced. There are now sapphire touch screen phones available from other suppliers and recently, the Apple Watch was released with a sapphire screen. In addition, many fingerprint sensors and camera cover glasses are now produced using sapphire as the cover material.

Requirements for sapphire material is clear (forgive the pun). For screens and cameras, it must be of good optical quality i.e. transmit light well and have low surface roughness. For fingerprint sensors, it needs consistent surface quality and electrical properties.

Production process

FIGURE 1 shows a schematic of the production process for sapphire used in a mobile device screen. The following paragraphs provide more detail on this process [1] as well as a few of problems encountered along the way.

Sapphire Fig 1

The sapphire production process starts when a seed crystal and a mixture of aluminum oxide and crackle (un-crystallized sapphire material) is heated using a specific temperature/time profile, then cooled (this process can take two weeks depending on the amount of sapphire being produced) using a carefully controlled set of time/temperature profiles. When done correctly, the cookie sized seed grows and produces a single-crystal sapphire boule. That at least is the theory. In reality, two weeks is a long time and any number of problems can go wrong during this process including gas bubbles, mechanical faults such as cracks and contamination. Each of these problems can affect the sapphire and its optical/electrical properties. There is a clear correlation between the time taken to grow a boule and the potential quality of the boule produced. Many of the problems encountered in the upscaling of the sapphire production process sprang from trying to grow large boules at high speeds.

It is at the next step in the process where boule size does matter. Typically, the boule will be drilled or cut to produce material near the size needed for the particular application. It makes a significant difference if the material is for a watch crystal (say 1.5 inch diameter ~ 1.7 square inches). Here you can “core-drill” a boule to produce a number of smaller cylinders. For a phone screen/cover plate (at 4 by 6 inch i.e. 24 square inches) a larger portion of the boule is needed for a box shape. The ability to grow large sized boules on a regular basis is not in question; most important is how much of that boule is bubble-, crack- and impurity-free. In some cases the boules are inspected with various metrology techniques to determine which sections of the boule can be used and which cannot. The section of the boules not used is recycled into the original growth process (unless contaminated).

Given the hardness of the sapphire, diamond wire saws or diamond core drills are used for cutting or coring the boules. The yield from any boule is a function of the original boule size, the size of the cores or slabs being produced and the volume of the boule free from imperfections. As was discussed earlier, and is typical of many processes, the larger the size of the piece the lower the yield.

The next step is to take the cylindrical cores (or rectangular slabs) and cut them into appropriate sized pieces. The thickness of the desired part and the amount the producer is willing to invest in high technology solutions determines what is done next. On one end of the technology scale, the parts are cut using a wire saw or an abrasive cutoff saw. On the other end of the scale, you can ion implant the surface to produce a damaged layer at a depth below the surface determined by the original ion energy. If the slab is heated after sufficient implantation is done, a thin sheet will separate from the surface. Both processes result in parts of the approximate size needed for the application; a discussion of the pros and cons of each approach is beyond the scope of this article.

The process after this point depends on the parts’ final application and their manufacturer. Given the difficulty of polishing a material this hard many of the bigger companies have developed proprietary process for grinding or mechanically polishing the sapphire parts to the desired shape and surface roughness/finish. From a mechanical strength standpoint, it is important that there be no significant scratching of the surface or chipping of the edges which could severely limit the mechanical strength of the final piece. From an optical standpoint, it is important to produce a uniform finish so as not to effect the overall appearance of the part. At this stage, the parts are then ground to their final size and any additional shaping of the part including holes/ profiles is done. FIGURE 2 shows a variety of sapphire parts at this stage of the process.

Sapphire Fig 2

In most sapphire part production these parts are next coated with a variety of optical and/or electrical and/ or chemical films again depending on their application. Because of its high index of refraction (1.76) a sapphire screen or watch crystal is highly reflective. For this application, the parts are typically coated with a series of films to produce an anti-reflection coating enhancing final screen readability. For parts that will be touched on a regular basis such as touchscreens or fingerprint sensors coatings, it is important that they be “self-cleaning.” In these cases, hydrophobic and oleophobic coatings are used to make sure your fingerprints are less likely to stay behind after the material has been touched. FIGURE 3 shows a series of parts after the coating and silk screening process. They are now ready for assembly into the mobile device.

Sapphire Fig 3

The use of sapphire in mobile devices is driven by two main concerns. One is that the final screen/sensor be mechanically stronger and harder than most glasses. There are a number of videos [2] available showing cement blocks being dragged over cell phones to show the sapphire screens’ scratchproof capabilities. The second (and not as well known) factor is the significant data showing that touch sensors made using sapphire have better performance characteristics due to its superior electrical properties and electrical uniformity. This allows the development of sensors which have improved performance in the field.

The downside of using sapphire remains its cost. Estimates [3] have reported sapphire costs 2 to 10 times the price of an equivalent glass part. Although these costs are coming down, in price sensitive applications glass continues to dominate at this time and it is expected that only higher end phones will use sapphire screens.

In the second part of this article, we will discuss the importance of sapphire in the LED industry and the difference in process needed for this material.

Additional reading/viewing material

1. http://www.businessinsider.com/how-sapphire- glass-screens-are-made-2014-9
2. Video Aero Gear’s Flight Glass SX Sapphire Crystal vs a Concrete
3. http://seekingalpha.com/article/2230553-ignore- the-sapphire-threat-corning-is-on-a-roll

Growing Conference Business at Extension Media Brings Experienced Events Producer Onboard

SAN FRANCISCO, October 28, 2015 – Extension Media announced today the addition of Sally L. Bixby as Senior Events Director for Extension Media’s fast-growing conference division. She will be based in the downtown Portland, Oregon office where Extension Media has editorial staff.

Ms. Bixby is an accomplished corporate events producer with nearly 16 years of in-depth experience in operations and marketing, holding senior staff positions in multiple events projects. To date, she has managed more than 450 business conferences in North America alone and produced several internationally as well. She brings to the role a significant track record of increasing event attendance, managing large- and small-scale budgets and driving lead generation for companies such as: AMD, Avnet, Curtiss-Wright, Intel, Kontron, MathWorks and more. Throughout her career, Ms. Bixby has cultivated relationships in the embedded systems, semiconductor and medical electronics industries, as well as academia and several professional organizations, building mutually beneficial and long-term business relationships.

“We are thrilled that Sally is leading the conference operations management team and will also be focusing her energy on growing the conference and exhibition side of our business, adding several events aimed at the embedded and growing IoT market segments as well as the semiconductor manufacturing and design market,” said Vince Ridley, president and founder of Extension Media. “Her professionalism and passion for delivering successful events will benefit both Extension Media and our clients. Sally’s attention to exceeding expected goals make her an ideal fit.”

“I look forward to expanding the conference business at Extension Media, connecting knowledgeable, responsive leaders and influencers,” said Ms. Bixby. “Recent experience creating a successful China-U.S. IoT Summit for a Fortune 100 company – that resulted in 120% of the attendee goal and a 10.5% budget savings – has me looking forward to helping our clients achieve impressive results.”

Prior to joining Extension Media, Ms. Bixby was an independent senior events producer running her own company, EventBelle Productions. In 2014 and 2015, she managed all operations, budgets and the VIP program for The ConFab, the preeminent semiconductor manufacturing conference and networking event for leaders and decision-makers addressing the economics of semiconductor manufacturing.

About Extension Media
Extension Media is a privately held company operating more than 50 B2B magazines, engineers’ guides, email newsletters, web sites and conferences that focus on high-tech industry platforms and emerging technologies such as: chip design, semiconductor and electronics manufacturing, embedded systems, software, architectures and industry standards.

Extension Media produces industry leading events including The ConFab, the Internet of Things Developers Conference (IoT DevCon) and the Multicore Developers Conference (Multicore DevCon), and publishes Embedded Systems Engineering, EECatalog.com, Embedded Intel® Solutions, EmbeddedIntel.com, Chip Design, ChipDesignMag.com, Solid State Technology, Solid-State.com and SemiMD.com.

Extension Media Contacts
Vince Ridley
[email protected]
415-255-0390
Sally L. Bixby
[email protected]
503-705-8651

Orlando, FLorida – At the Meeting of the International Microelectronics Assembly and Packaging Society (IMAPS 2015), imec and CMST (imec’s associated lab at Ghent University) present a novel technology for thermoplastically deformable electronics enabling low-cost 2.5D free-form rigid electronic objects. The technology is under evaluation in Philips LED lamp carriers, a downlight luminaire and a omnidirectional lightsource, to demonstrate the potential of this technology in innovative lighting applications.

Miniature LED dome test vehicle with integrated low power LEDs. (a) Device before forming. (b) Device after vacuum forming using a 40 mm half sphere.

Miniature LED dome test vehicle with integrated low power LEDs. (a) Device before forming. (b) Device after vacuum forming using a 40 mm half sphere.

Thanks to its energy-efficiency, excellent light quality, and high output power, light emitting diode (LED) technology is becoming the sustainable light source for the 21st century. But in addition, it also allows to design unprecedented, innovative lighting solutions. Imec and CMST’s new thermoplastically deformable electronic circuits now add a new dimension to the possibilities to fabricate novel lamp designs as well as smart applications in ambient intelligence and wearables.

The innovative technology is based on meander-shaped interconnects, a robust technique to realize dynamically stretchable elastic electronic circuits including LEDs. These are then embedded in thermoplastic polymers (e.g. polycarbonate). Following production on a flat substrate, using standard printed circuit board production equipment, the circuit is given its final form using thermoforming techniques such as vacuum forming, high pressure forming or even injection molding. Upon cooling, the thermoplastic retains its shape without inducing large internal stresses in the circuits. The method, based on standard available production processes, does not require large investments, reducing the cost of fabrication. The resulting designs have a low weight and low complexity, a high resilience, a low tooling and material cost, and a higher degree of manufacturer independence due to the standard industrial practices that are used.

The production process was developed in collaboration between the industrial and academic partners involved in the FP7 project TERASEL: imec, CMST (Ghent University), ACB, Holst Centre, Niebling Formtechnologie; Sintex NP and Philips Lighting BV. TERASEL is a European effort focusing on the development, industrial implementation and application of large-area, cost-effective, randomly shaped electronics and sensor circuit technologies.