Category Archives: OLEDs

Graphene, the two-dimensional powerhouse, packs extreme durability, electrical conductivity, and transparency into a one-atom-thick sheet of carbon. Despite being heralded as a breakthrough “wonder material,” graphene has been slow to leap into commercial and industrial products and processes.

Now, scientists have developed a simple and powerful method for creating resilient, customized, and high-performing graphene: layering it on top of common glass. This scalable and inexpensive process helps pave the way for a new class of microelectronic and optoelectronic devices–everything from efficient solar cells to touch screens.

The collaboration–led by scientists at the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory, Stony Brook University (SBU), and the Colleges of Nanoscale Science and Engineering at SUNY Polytechnic Institute–published their results February 12, 2016, in the journal Scientific Reports.

“We believe that this work could significantly advance the development of truly scalable graphene technologies,” said study coauthor Matthew Eisaman, a physicist at Brookhaven Lab and professor at SBU.

The scientists built the proof-of-concept graphene devices on substrates made of soda-lime glass–the most common glass found in windows, bottles, and many other products. In an unexpected twist, the sodium atoms in the glass had a powerful effect on the electronic properties of the graphene.

“The sodium inside the soda-lime glass creates high electron density in the graphene, which is essential to many processes and has been challenging to achieve,” said coauthor Nanditha Dissanayake of Voxtel, Inc., but formerly of Brookhaven Lab. “We actually discovered this efficient and robust solution during the pursuit of something a bit more complex. Such surprises are part of the beauty of science.”

Crucially, the effect remained strong even when the devices were exposed to air for several weeks–a clear improvement over competing techniques.

The experimental work was done primarily at Brookhaven’s Sustainable Energy Technologies Department and the Center for Functional Nanomaterials (CFN), which is a DOE Office of Science User Facility.

The graphene tweaks in question revolve around a process called doping, where the electronic properties are optimized for use in devices. This adjustment involves increasing either the number of electrons or the electron-free “holes” in a material to strike the perfect balance for different applications. For successful real-world devices, it is also very important that the local number of electrons transferred to the graphene does not degrade over time.

“The graphene doping process typically involves the introduction of external chemicals, which not only increases complexity, but it can also make the material more vulnerable to degradation,” Eisaman said. “Fortunately, we found a shortcut that overcame those obstacles.”

The team initially set out to optimize a solar cell containing graphene stacked on a high-performance copper indium gallium diselenide (CIGS) semiconductor, which in turn was stacked on an industrial soda-lime glass substrate.

The scientists then conducted preliminary tests of the novel system to provide a baseline for testing the effects of subsequent doping. But these tests exposed something strange: the graphene was already optimally doped without the introduction of any additional chemicals.

“To our surprise, the graphene and CIGS layers already formed a good solar cell junction!” Dissanayake said. “After much investigation, and the later isolation of graphene on the glass, we discovered that the sodium in the substrate automatically created high electron density within our multi-layered graphene.”

Pinpointing the mechanism by which sodium acts as a dopant involved a painstaking exploration of the system and its performance under different conditions, including making devices and measuring the doping strength on a wide range of substrates, both with and without sodium.

“Developing and characterizing the devices required complex nanofabrication, delicate transfer of the atomically thin graphene onto rough substrates, detailed structural and electro-optical characterization, and also the ability to grow the CIGS semiconductor,” Dissanayake said. “Fortunately, we had both the expertise and state-of-the-art instrumentation on hand to meet all those challenges, as well as generous funding.”

The bulk of the experimental work was conducted at Brookhaven Lab using techniques developed in-house, including advanced lithography. For the high-resolution electron microscopy measurements, CFN staff scientists and study coauthors Kim Kisslinger and Lihua Zhang lent their expertise. Coauthors Harry Efstathiadis and Daniel Dwyer–both at the College of Nanoscale Science and Engineering at SUNY Polytechnic Institute–led the effort to grow and characterize the high-quality CIGS films.

“Now that we have demonstrated the basic concept, we want to focus next on demonstrating fine control over the doping strength and spatial patterning,” Eisaman said.

The scientists now need to probe more deeply into the fundamentals of the doping mechanism and more carefully study material’s resilience during exposure to real-world operating conditions. The initial results, however, suggest that the glass-graphene method is much more resistant to degradation than many other doping techniques.

“The potential applications for graphene touch many parts of everyone’s daily life, from consumer electronics to energy technologies,” Eisaman said. “It’s too early to tell exactly what impact our results will have, but this is an important step toward possibly making some of these applications truly affordable and scalable.”

For example, graphene’s high conductivity and transparency make it a very promising candidate as a transparent, conductive electrode to replace the relatively brittle and expensive indium tin oxide (ITO) in applications such as solar cells, organic light emitting diodes (OLEDs), flat panel displays, and touch screens. In order to replace ITO, scalable and low-cost methods must be developed to control graphene’s resistance to the flow of electrical current by controlling the doping strength. This new glass-graphene system could rise to that challenge, the researchers say.

A research team at Umeå University in Sweden has showed, for the first time, that a very efficient vertical charge transport in semiconducting polymers is possible by controlled chain and crystallite orientation. These pioneering results, which enhance charge transport in polymers by more than 1,000 times, have implications for organic opto-electronic devices and were recently published in the journal Advanced Materials.

Conjugated semiconducting polymers (plastic) possess exceptional optical and electronic properties, which make them highly attractive in the production of organic opto-electronic devices, such as for instance photovoltaic solar cells (OPV), light emitting diodes (OLED) and lasers.

Polythiophene polymers, such as poly(3-hexylthiophene), P3HT, have been among the most studied semiconducting polymers due to their strong optical absorbance and ease of processing into a thin film from solution. In both OPVs and OLEDs, charges must be transported in the out of plane (vertical) direction inside the polymer film.

However, until now the vertical charge carrier mobility of organic semiconductors, i.e. the ability of charges to move inside the material, has been too low to produce fast charge transport in electronic devices. Faster charge transport can occur along the polymer chain backbone. However, a method to produce controlled chain orientation and high mobility in the vertical direction has remained elusive until now.

In the present work, a team of chemists and materials scientists, led by Professor David R. Barbero at Umeå University, has found a new method to align chains vertically and to produce efficient transport of electric charges through the chain backbone. In this new study, moreover, high charge transport and high mobility were obtained without any chemical doping, which is often used to artificially enhance charge transport in polymers.

“The transport of electric charge is greatly enhanced solely by controlled chain and crystallite orientation inside the film. The mobility measured was approximately one thousand times higher than previously reported in the same organic semiconductor,” says David Barbero.

In what way will these results affect the field of organic electronics?

“We believe these results will impact the fields of polymer solar cells and organic photodiodes, where the charges are transported vertically in the device. Organic-based devices have traditionally been slower and less efficient than inorganic ones (e.g. made of silicon), in part due to the low mobility of organic (plastic) semiconductors. Typically, plastic semiconductors, which are only semi-crystalline, have hole mobilities about 10,000 times lower than doped silicon, which is used in many electronic devices. Now we show it is possible to obtain much higher mobility, and much closer to that of silicon, by controlled vertical chain alignment, and without doping,” says David Barbero.

The charge transport was measured using nanoscopic electrical measurements, and gave a mobility averaging 3.1 cm2/V.s, which is the highest mobility ever measured in P3HT, and which comes close to a theoretical estimation of the maximum mobility in P3HT. Crystallinity and molecular packing characterisation of the polymer was performed by synchrotron X-ray diffraction at Stanford University’s National Accelerator (SLAC) and confirmed that the high mobilities measured were due to the re-orientation of the polymer chains and crystallites, leading to fast charge transport along the polymer backbones.

These results, published in Advanced Materials, may open up the route to produce more efficient organic electronic devices with vertical charge transport (e.g. OPV, OLED, lasers etc.), by a simple and inexpensive method, and without requiring chemical modification of the polymer.

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.

Light and electricity dance a complicated tango in devices like LEDs, solar cells and sensors. A new anti-reflection coating developed by engineers at the University of Illinois at Urbana Champaign, in collaboration with researchers at the University of Massachusetts at Lowell, lets light through without hampering the flow of electricity, a step that could increase efficiency in such devices.

An array of nanopillars etched by thin layer of grate-patterned metal creates a nonreflective surface that could improve electronic device performance. Credit: Image courtesy of Daniel Wasserman

The coating is a specially engraved, nanostructured thin film that allows more light through than a flat surface, yet also provides electrical access to the underlying material – a crucial combination for optoelectronics, devices that convert electricity to light or vice versa. The researchers, led by U. of I. electrical and computer engineering professor Daniel Wasserman, published their findings in the journal Advanced Materials.

“The ability to improve both electrical and optical access to a material is an important step towards higher-efficiency optoelectronic devices,” said Wasserman, a member of the Micro and Nano Technology Laboratory at Illinois.

At the interface between two materials, such as a semiconductor and air, some light is always reflected, Wasserman said. This limits the efficiency of optoelectronic devices. If light is emitted in a semiconductor, some fraction of this light will never escape the semiconductor material. Alternatively, for a sensor or solar cell, some fraction of light will never make it to the detector to be collected and turned into an electrical signal. Researchers use a model called Fresnel’s equations to describe the reflection and transmission at the interface between two materials.

“It has been long known that structuring the surface of a material can increase light transmission,” said study co-author Viktor Podolskiy, a professor at the University of Massachusetts at Lowell. “Among such structures, one of the more interesting is similar to structures found in nature, and is referred to as a ‘moth-eye’ pattern: tiny nanopillars which can ‘beat’ the Fresnel equations at certain wavelengths and angles.”

Although such patterned surfaces aid in light transmission, they hinder electrical transmission, creating a barrier to the underlying electrical material.

“In most cases, the addition of a conducting material to the surface results in absorption and reflection, both of which will degrade device performance,” Wasserman said.

The Illinois and Massachusetts team used a patented method of metal-assisted chemical etching, MacEtch, developed at Illinois by Xiuling Li, U. of I. professor of electrical and computer engineering and co-author of the new paper. The researchers used MacEtch to engrave a patterned metal film into a semiconductor to create an array of tiny nanopillars rising above the metal film. The combination of these “moth-eye” nanopillars and the metal film created a partially coated material that outperformed the untreated semiconductor.

“The nanopillars enhance the optical transmission while the metal film offers electrical contact. Remarkably, we can improve our optical transmission and electrical access simultaneously,” said Runyu Liu, a graduate researcher at Illinois and a co-lead author of the work along with Illinois graduate researcher Xiang Zhao and Massachusetts graduate researcher Christopher Roberts.

The researchers demonstrated that their technique, which results in metal covering roughly half of the surface, can transmit about 90 percent of light to or from the surface. For comparison, the bare, unpatterned surface with no metal can only transmit 70 percent of the light and has no electrical contact.

The researchers also demonstrated their ability to tune the material’s optical properties by adjusting the metal film’s dimensions and how deeply it etches into the semiconductor.

“We are looking to integrate these nanostructured films with optoelectronic devices to demonstrate that we can simultaneously improve both the optical and electronic properties of devices operating at wavelengths from the visible all the way to the far infrared,” Wasserman said.

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.

Demand for LTPS TFT LCD shipments rose 30 percent in September 2015 to reach 51.6 million units, due to strong demand from Apple and Chinese brands. Total smartphone panel shipments grew 4 percent month over month to reach 160 million units in September 2015. While amorphous silicon (a-Si) thin-film transistor (TFT) liquid-crystal display (LCD) panels continue to lead the smartphone display market, low-temperature polysilicon (LTPS) TFT LCD panel shipment share is growing, according to IHS Inc., a of critical information and insight.

“TFT-LCD, based on a-Si substrate, has been the leading panel technology for mobile phones because it is easy to manufacture and costs less to produce than other display technologies. However, since Apple adopted LTPS for its popular iPhones, demand for the new technology has continued to increase,” said Brian Huh, senior analyst for IHS Technology. “While LTPS panels cost greater, they boast lower power consumption and higher resolution compared to a-Si LCD panels. Greater demand for higher definition screens, especially in China, has also increased the adoption of LTPS LCD mobile phone displays.”

Based on the latest information in the IHS Smartphone Display Shipment Tracker, the market share for the a-Si TFT LCD panel fell 10 percent month over month, but the panel still comprised the majority of smartphone display shipments, reaching 79.6 million in September 2015. Active-matrix organic light-emitting diode (AMOLED) panel shipments grew 7 percent to reach just 25 million units.

As a point of differentiation in the smartphone display market, Samsung Electronics adopted AMOLED-based LTPS displays in 2009. At that time Samsung Display was not looking to expand its customer base because Samsung Electronics digested almost all of the company’s AMOLED capacity. However as Samsung Electronics’ AMOLED smartphone business began to decline last year, Samsung Display has been expanding its customer lineup. “Since the end of last year, Samsung Display has been actively and aggressively promoting AMOLED displays to other electronics companies, especially in China, and AMOLED panel shipments for Chinese brands have increased remarkably since September,” Huh said.

By Sue Davis, Director of Business Development & Senior Analyst, Techcet

IDTechEx Printed Electronics USA 2015, held in Santa Clara, CA Nov 18-19, is one mega conference with 8 co-located tracks ranging from sensor technology & wearables to IoT, energy harvesting & storage to electric vehicles, 3D printing and graphene. IDTechEx completely occupied the Santa Clara Convention Center; throughout the day attendees and exhibitors commented the attendance was indeed up over prior years. To the dismay of some late arrivals, parking spaces were at a premium.

A venue with >200 exhibitors showcasing new technologies and applications connected conference attendees with equipment and materials suppliers, OEMs, end users, research institutes and academia.

Raghu Das, CEO of IDTechEx, kicked off the conference by sharing a key trends including:

  • Structural electronics are here now!
  • The Fashion industry is converging with technology (and evidenced by a number of exhibitors from this sector)
  • Stretchable electronics R&D has ramped significantly in the last 12 months
  • Printed and flexible electronics manufacturing is becoming center stage

Dr. Mounir Zok, a keynote speaker and biomedical engineering specialist for the US Olympic committee started his talk with a quote: “The blink of an eye dictates gold vs no medal.” He emphasized that technology is a key enabler to continually improve sports performance.

I had the opportunity to meet with several exhibitors:

  • Keith McMillen, founder and CEO of BeBop Sensors and avid musician, shared his journey of developing cylindrical sensors to analyze a violinist’s bow movement led to utilizing this technology for the Internet of Things and the founding of BeBop Sensors. Smart fabric is the core for Bebop’s sensor platform.
  • Dream car in every facet; aesthetics, functionality and environment understates the design of the Blade Keith Czinger, CEO and Founder of Divergent, discussed the foundation for Blade’s development was deeply rooted in reducing environmental impact while ensuring high performance.
  • Printed Circuit Boards (PCBs) – manufactured via additive 3D printing technology vs. conventional processing labor, material and time intensive processes was demonstrated at NanoDimesion’s booth. Simon Fried, CMO and Co-Founder of NanoDimension discussed the benefit of 3D printed circuit boards (prototyping in hours vs weeks, design flexibility, process repeatability, …). In addition to development the 3D printers, NanoDimension has developed a line of specialty inks.

Another show highlight was Demonstration Street, a dedicated area on the show floor for product demonstrations in various stages of development – prototype to commercialization- featured printed flexible displays including posters, e-readers, audio paper, interactive games, OLED displays, electronics in fabrics, interactive printed controls and menus, printed RFID and more.

Stay tuned: Day 2 promises to be equally exciting! The main challenge is navigating IDTechEx to see all the great technology.

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

Due to the growth of the semiconductor business, the wider adoption of Cu pillar solutions and the introduction of Flip Chip technology for LED and CMOS Image Sensors (CIS) applications, the Flip Chip market is expending. Under this context, more and more industrial companies including OSATs, IDMs IC foundries and bumping house undertake in this market.

The “More than Moore” market research and strategy consulting company Yole Développement (Yole) explored this industry and proposes today a detailed technology and market report, entitled “Flip Chip: Technologies & Market Trends”. Yole’s team is daily discussing with the leaders of the Advanced Packaging industry. Based on these interactions, the consulting company highlights the evolution of the technical needs and market trends. These major results make Yole’s analysts to think that full capacity should be reached in 2017.

What are the required investments to support this growth? Are there competitive technologies such as TSMC’s new solution, high-performance integrated fan-out wafer level packaging (InFO-WLP), that could answer the market needs and compete Flip Chip technology?

Under “Flip Chip: Technologies & Market Trends” report, Yole’s advanced packaging team provides an overview of Flip Chip technology and market trends. The company reviews the competitive landscape including player dynamics and key market trends; they also detail the Flip Chip market capacity and wafer forecast. Yole’s report also includes a detailed technology roadmap.

“Based on the discussions we had with the major advanced packaging companies, at Yole, we think that demand for Flip Chip is expected to reach the current maximum capacity in 2017,” said Santosh Kumar, Senior, Technology & Market Analyst, Advanced Packaging & Semiconductor Manufacturing at Yole. And he adds: “Therefore, new investment will be needed starting in 2018.”

Since Cu pillar processing can be performed by standard foundries and IDMs, the supply chain may see some slight modification. Yole’s analysts expect higher investment in Cu pillar 12” line wafer bumping lines from wafer foundries such as TSMC and SMIC. This change will affect OSATs’ wafer bumping revenue since foundries will gain market share.

OSATs will maintain their strong position in wafer bumping and assembly thanks to of their huge experience and low cost solutions. Their business model enables them to better control the supply chain, as they provide for the complete set of flip-chip services: package design and qualification, wafer bumping, substrate in-sourcing, assembly and final test.

However, big IDM companies like Intel and Samsung maintain their dominance in terms of wafer bumping capacity.

flip chip bump

“At Yole, we expect that even in 2020 Intel will remain the highest-capacity player in Cu pillar wafer bumping,” commented Thibault Buisson, Technology & Analyst, Advanced Packaging at Yole. Foundries and OSATs are also establishing joint ventures for wafer bumping to provide turnkey solutions to customers from chip fabrication to assembly at competitive cost.

And what about the Chinese companies? Do they have a role to play in the Flip Chip market? Chinese players are significantly increasing their presence in wafer bumping and Flip Chip assembly by mergers and acquisitions. JCET acquired STATS ChipPAC and FCI was acquired by Tianshui Huatian Technology Company.

In that context, Yole’s report, Flip Chip: Technologies & Market Trends report gives insights on the future strategies that players may adopt. A detailed description of this report is available on www.i-micronews.com, advanced packaging reports section.

By Dr. Harry Zervos, Principal Analyst, IDTechEx

Flexible electronic devices are starting to experience significant proliferation, with more and more devices with innovative form factors being brought to market, from small components such as disposable sensors that have been in the market for quite some time now, all the way to new flexible smart phones currently being demonstrated by consumer electronics giants like Samsung and LG.

While printing technologies enable lower manufacturing costs and superior performance in many applications, vacuum deposition still claims significant market share in flexible electronics, although sometimes a combination of both can be the ideal combination.

From test strips to OLEDs 

Glucose test strips are a great example of the prevalence of both printed and vacuum deposited devices. Over ten billion test strips are being manufactured worldwide, in order to cater for the needs of the ever-increasing number of people living with diabetes. Although each manufacturer/brand has its own technology and design, the following cross-section shows the key parts of a test strip.  Manufacturers follow both thick film (screen printing) and thin film (sputtering) techniques for depositing the circuit in test strips, each of the techniques with its own merits.

Screen printing technology involves printing patterns of conductors and insulators onto the surface of planar solid (plastic or ceramic) substrates based on pressing the corresponding inks through a patterned mask. Each strip contains printed working and reference electrodes with the working one coated with the necessary reagents and membranes, with the reagents commonly dispensed by ink jet printing technology and deposited in the dry form. With thin film deposited electrodes, sputtering or laser ablation is commonly utilized. Lifescan for instance, a Johnson & Johnson company, mostly prints electrodes whereas Roche utilizes laser ablation in its Indianapolis plant. Along with the very specialized organic materials utilized in assays in the actively sensing part of the test strips, advanced devices integrating thin film technology utilize gold nanoparticles and mesoporous Pt electrodes, and even the use of carbon nanotubes and graphene has been demonstrated in certain designs.

OLED displays are a good example where the advent of printing techniques is meant to bring about much larger displays, manufactured at lower costs but for the time being, the OLED industry makes displays that are almost exclusively vacuum evaporated. Optimized solution processed materials are also becoming available but for now, vacuum deposited options perform better. Sunic, Aixtron, Canon Tokki and ULVAC are some of the companies that actively design and market equipment and materials for industrial vacuum technology in OLED applications.

Most of these companies, along with others such as Applied Materials are active in making more than just the active OLED layers, providing equipment for TFT deposition, encapsulation, etc.

The opportunity here is significant: The OLED market is meant to reach over $50bn in the next decade, with flexible and rigid plastic OLED displays surpassing 16 billion by 2020.

Flexible encapsulation & thin film PV

Encapsulation of flexible versions of OLED displays is set to become an exciting market: flexible barrier films – whether utilizing CVD or PVD processes or even in cases when ALD is utilized to make high quality, defect free layers- are hugely benefiting from vacuum deposition techniques and have created encapsulation materials that can reach the water vapor transmission rates required to allow flexible OLED displays the necessary lifetimes required to become commercially viable. Encapsulation for flexible OLED devices is a market that is expected to reach almost $340m by 2022 according to IDTechEx Research in the report “Barrier Layers for Flexible Electronics 2016-2026: Technologies, Markets, Forecasts”.

Flexible versions of thin film photovoltaics also require stringent encapsulation, but thin films have had harsh competition from low cost crystalline silicon cells from China, that have significantly reduced their market share in recent years. Just over 7% of the overall market for PV this year is expected to be thin-film based, according to research from SPV Research.

It is interesting to point out that manufacturing of all thin films for solar cell applications is fully vacuum based: PECVD for amorphous silicon platforms, sputtering or co-evaporation tends to be the preferred deposition techniques for CIGS technologies while CdTe leader First Solar has developed and optimized its own unique vacuum deposition technique, High Rate Vapor Transport Deposition (HRVTD). In this process, co-developed with NREL in an effort that started back in the early 1990’s, the material to be deposited is carried on a gas stream in powder form, then heated and vaporized as it passes through a membrane before depositing on a glass substrate. The technology can deposit a thin uniform layer of CdTe (or CdS, a common material system used as a buffer layer in CdTe cells) on 8 square feet of glass in less than 40 seconds, a deposition rate much higher than other rival thin film solar technologies that proved to be key in First Solar’s success in improving yield and output and consequently lower production costs for its thin film solar cells.

Conclusions 

The conclusion is simple: commercializing flexible or printed electronics will invariably require a deeper understanding of vacuum deposition technologies. Printing techniques are not the only manufacturing option that can allow for the freedom in design that the advent of flexibility in form factor is ushering in. In fact, vacuum deposition technologies are currently enabling the proliferation of a wide range of components and devices, from encapsulation films to thin flexible batteries to transparent conductive films and backplane elements. In many cases, having reached economies of scale, vacuum deposited devices have reached attractive cost structures that make it harder for printed versions to compete, having to “dig deep” in order to bring forward additional selling points than just reductions in cost.

Printed Electronics USA 2015 taking place in Santa Clara, CA on the 18th and 19th of November this year is going to focus on the importance of vacuum deposition, with both the conference as well as the trade show featuring contributions from end users, device manufacturers and manufacturing equipment suppliers of vacuum deposition technologies.