Category Archives: Materials

Nanoscientists at Northwestern University have developed a blueprint to fabricate new heterostructures from different types of 2-D materials. 2-D materials are single atom layers that can be stacked together like “nano-interlocking building blocks.” Materials scientists and physicists are excited about the properties of 2-D materials and their potential applications. The researchers describe their blueprint in the Journal of Applied Physics, from AIP Publishing.

Nanoscientists at Northwestern University have developed a blueprint to fabricate new heterostructures from different types of 2-D materials. The researchers describe their blueprint in the Journal of Applied Physics. In this image: Top: Vertical MoSe2-WSe2 heterostructure, radial MoS2-WS2 heterostructure, hybrid MoS2-WS2 heterostructure and Mose2-WSe2 alloy building block representations and crystal structure models Bottom: Vertical MoSe2-WSe2 heterostructure crystal structure model Credit: Cain, Hanson and Dravid

“We’ve outlined an easy, deterministic and readily deployable way to stack and stitch these individual layers into orders not seen in nature,” said Jeffrey Cain, an author on the paper who was formerly at Northwestern University but is now at Lawrence Berkeley National Laboratory and the University of California.

Cain explained that for nanoscientists, “the dream” is to combine 2-D materials in any order and collate a library of these heterostructures with their documented properties. Scientists can then select appropriate heterostructures from the library for their desired applications. For instance, the computer industry is trying to make transistors smaller and faster to increase computing power. A nanoscale semiconductor with favorable electronic properties could be used to make transistors in next-generation computers.

So far, nanoscientists have lacked clear methods for fabricating heterostructures, and have not yet been able to develop this library. In this work, the scientists looked to solve these fabrication issues. After identifying trends in the literature, they tested different conditions to map out the different parameters required to grow specific heterostructures from four types of 2-D materials: molybdenum disulfide and diselenide, and tungsten disulfide and diselenide. To fully characterize the atomically thin final products, the scientists used microscopy and spectrometry techniques.

The group was inspired by the science of time-temperature-transformation diagrams in classical materials, which maps out heating and cooling profiles to generate precise metallic microstructures. Based on this method, the researchers packaged their findings into one diagrammatic technique — the Time-Temperature-Architecture Diagram.

“People had previously written papers for specific morphologies, but we have unified it all and enabled the generation of these morphologies with one technique,” Cain said.

The unified Time-Temperature-Architecture Diagrams provide directions for the exact conditions required to generate numerous heterostructure morphologies and compositions. Using these diagrams, the researchers developed a unique library of nanostructures with physical properties of interest to physicists and materials scientists. The Northwestern University scientists are now examining the behaviors displayed by some materials in their library, like the electron flow across the stitched junctions between materials.

The researchers hope that their blueprint design will be useful for heterostructure fabrication beyond the first four materials. “Our specific diagrams would need revisions in the context of each new material, but we think that this idea is applicable and extendable to other material systems,” Cain said.

Scientists from the universities of Bristol and Cambridge have found a way to create polymeric semiconductor nanostructures that absorb light and transport its energy further than previously observed.

Image showing light emission from the polymeric nanostructures and schematic of a single nanostructure. Credit: University of Bristol

This could pave the way for more flexible and more efficient solar cells and photodetectors.

The researchers, whose work appears in the journal Science, say their findings could be a “game changer” by allowing the energy from sunlight absorbed in these materials to be captured and used more efficiently.

Lightweight semiconducting plastics are now widely used in mass market electronic displays such those found in phones, tablets and flat screen televisions. However, using these materials to convert sunlight into electricity, to make solar cells, is far more complex.

The photo-excited states – which is when photons of light are absorbed by the semiconducting material – need to move so that they can be “harvested” before they lose their energy in less useful ways. These excitations typically only travel ca. 10 nanometres in polymeric semiconductors, thus requiring the construction of structures patterned on this length-scale to maximise the “harvest”.

In the chemistry labs of the University of Bristol, Dr Xu-Hui Jin and colleagues developed a novel way to make highly ordered crystalline semiconducting structures using polymers.

While in the Cavendish Laboratory in Cambridge, Dr Michael Price measured the distance that the photo-exited states can travel, which reached distances of 200 nanometres – 20 times further than was previously possible.

200 nanometres is especially significant because it is greater than the thickness of material needed to completely absorb ambient light thus making these polymers more suitable as “light harvesters” for solar cells and photodetectors.

Dr George Whittell from Bristol’s School of Chemistry, explains: “The gain in efficiency would actually be for two reasons: first, because the energetic particles travel further, they are easier to “harvest”, and second, we could now incorporate layers ca. 100 nanometres thick, which is the minimum thickness needed to absorb all the energy from light – the so-called optical absorption depth. Previously, in layers this thick, the particles were unable to travel far enough to reach the surfaces.”

Co-researcher Professor Richard Friend, from Cambridge, added: “The distance that energy can be moved in these materials comes as a big surprise and points to the role of unexpected quantum coherent transport processes.”

The research team now plans to prepare structures thicker than those in the current study and greater than the optical absorption depth, with a view to building prototype solar cells based on this technology.

They are also preparing other structures capable of using light to perform chemical reactions, such as the splitting of water into hydrogen and oxygen.

Purdue researchers have discovered a new two-dimensional material, derived from the rare element tellurium, to make transistors that carry a current better throughout a computer chip.

Purdue researchers Wenzhuo Wu and Peide Ye recently discovered tellurene, a two-dimensional material they manufactured in a solution, that has what it takes to make high-speed electronics faster. Credit: Purdue University image/Vincent Walter

The discovery adds to a list of extremely thin, two-dimensional materials that engineers have tried to use for improving the operation speed of a chip’s transistors, which then allows information to be processed faster in electronic devices, such as phones and computers, and defense technologies like infrared sensors.

Other two-dimensional materials, such as graphene, black phosphorus and silicene, have lacked either stability at room temperature or the feasible production approaches required to nanomanufacture effective transistors for higher speed devices.

“All transistors need to send a large current, which translates to high-speed electronics,” said Peide Ye, Purdue’s Richard J. and Mary Jo Schwartz Professor of Electrical and Computer Engineering. “One-dimensional wires that currently make up transistors have very small cross sections. But a two-dimensional material, acting like a sheet, can send a current over a wider surface area.”

Tellurene, a two-dimensional film researchers found in the element tellurium, achieves a stable, sheet-like transistor structure with faster-moving “carriers” – meaning electrons and the holes they leave in their place. Despite tellurium’s rarity, the pros of tellurene would make transistors made from two-dimensional materials easier to produce on a larger scale. The researchers detail their findings in Nature Electronics.

“Even though tellurium is not abundant on the Earth’s crust, we only need a little bit to be synthesized through a solution method. And within the same batch, we have a very high production yield of two-dimensional tellurene materials,” said Wenzhuo Wu, assistant professor in Purdue’s School of Industrial Engineering. “You simply scale up the container that holds the solution, so productivity is high.”

Since electronics are typically in use at room temperature, naturally stable tellurene transistors at this temperature are more practical and cost-effective than other two-dimensional materials that have required a vacuum chamber or low operation temperature to achieve similar stability and performance.

The larger crystal flakes of tellurene also mean less barriers between flakes to electron movement – an issue with the more numerous, smaller flakes of other two-dimensional materials.

“High carrier mobility at room temperature means more practical applications,” Ye said. Faster-moving electrons and holes then lead to higher currents across a chip.

The researchers anticipate that because tellurene can grow on its own without the help of any other substance, the material could possibly find use in other applications beyond computer chip transistors, such as flexible printed devices that convert mechanical vibrations or heat to electricity.

“Tellurene is a multifunctional material, and Purdue is the birthplace for this new material,” Wu said. “In our opinion, this is much closer to the scalable production of two-dimensional materials with controlled properties for practical technologies.”

Researchers using powerful supercomputers have found a way to generate microwaves with inexpensive silicon, a breakthrough that could dramatically cut costs and improve devices such as sensors in self-driving vehicles.

“Until now, this was considered impossible,” said C.R. Selvakumar, an engineering professor at the University of Waterloo who proposed the concept several years ago.

High-frequency microwaves carry signals in a wide range of devices, including the radar units police use to catch speeders and collision-avoidance systems in cars.

The microwaves are typically generated by devices called Gunn diodes, which take advantage of the unique properties of expensive and toxic semiconductor materials such as gallium arsenide.

When voltage is applied to gallium arsenide and then increased, the electrical current running through it also increases – but only to a certain point. Beyond that point, the current decreases, an oddity known as the Gunn effect that results in the emission of microwaves.

Lead researcher Daryoush Shiri, a former Waterloo doctoral student who now works at Chalmers University of Technology in Sweden, used computational nanotechnology to show that the same effect could be achieved with silicon.

The second-most abundant substance on earth, silicon would be far easier to work with for manufacturing and costs about one-twentieth as much as gallium arsenide.

The new technology involves silicon nanowires so tiny it would take 100,000 of them bundled together to equal the thickness of a human hair.

Complex computer models showed that if silicon nanowires were stretched as voltage was applied to them, the Gunn effect, and therefore the emission of microwaves, could be induced.

“With the advent of new nano-fabrication methods, it is now easy to shape bulk silicon into nanowire forms and use it for this purpose,” said Shiri.

Selvakumar said the theoretical work is the first step in a development process that could lead to much cheaper, more flexible devices for the generation of microwaves.

The stretching mechanism could also act as a switch to turn the effect on and off, or vary the frequency of microwaves for a host of new applications that haven’t even been imagined yet.

“This is only the beginning,” said Selvakumar, a professor of electrical and computer engineering. “Now we will see where it goes, how it will ramify.”

An international research team led by physicists at the Technical University of Munich (TUM) has developed molecules that can be switched between two structurally different states using an applied voltage. Such nanoswitches can serve as the basis for a pioneering class of devices that could replace silicon-based components with organic molecules.

A research team at the Technical University of Munich has developed molecular nanoswitches that can be toggled between two structurally different states using an applied voltage. They can serve as the basis for a pioneering class of devices that could replace silicon-based components with organic molecules. Credit: Yuxiang Gong / TUM / Journal of the American Chemical Society

The development of new electronic technologies drives the incessant reduction of functional component sizes. In the context of an international collaborative effort, a team of physicists at the Technical University of Munich has successfully deployed a single molecule as a switching element for light signals.

“Switching with just a single molecule brings future electronics one step closer to the ultimate limit of miniaturization,” says nanoscientist Joachim Reichert from the Physics Department of the Technical University of Munich.

Different structure – different optical properties

The team initially developed a method that allowed them to create precise electrical contacts with molecules in strong optical fields and to control them using an applied voltage. At a potential difference of around one volt, the molecule changes its structure: It becomes flat, conductive and scatters light.

This optical behavior, which differs depending on the structure of the molecule, is quite exciting for the researchers because the scattering activity – Raman scattering, in this case – can be both observed and, at the same time, switched on and off via an applied voltage.

Challenging technology

The researchers used molecules synthesized by teams based in Basel and Karlsruhe. The molecules can change their structure in specific ways when they are charged. They are arranged on a metal surface and contacted using the corner of a glass fragment with a very thin metal coating as a tip..

This serves as an electrical contact, light source and light collector, all in one. The researchers used the fragment to direct laser light to the molecule and measure tiny spectroscopic signals that vary with the applied voltage.

Contacting individual molecules electrically is extremely challenging from a technical point of view. The scientists have now successfully combined this procedure with single-molecule spectroscopy, allowing them to observe even the smallest structural changes in molecules with great precision.

Competition for silicon

One goal of molecular electronics is to develop novel devices that can replace traditional silicon-based components using integrated and directly controllable molecules.

Thanks to its tiny dimensions, this nanosystem is suitable for applications in optoelectronics, in which light needs to be switched using variations in electrical potential.

A new class of adsorbent materials offer high capacity storage and safe delivery of dopant gases

BY J. ARNÓ, O.K. FARHA, W. MORRIS, P. SIU, G.M. TOM, M.H. WESTON, and P.E. FULLER, NuMat Technologies, Skokie, USA J. MCCABE, M. S. AMEEN, Axcelis Technologies, Beverly, MA

Metal-Organic Framework (MOF) materials are a new class of crystalline adsorbents with broad applicability in electronics materials storage, delivery, purification, and abatement. The adsorbents have unprecedented surface areas and uniform pore sizes that can be precisely customized to the specific properties of electronic gases. ION-X® is a sub-atmospheric dopant gas delivery system designed for ion implantation, and the first commercial product that uses MOFs (ION-X® is commercially available through an agreement between NuMat Technologies and Versum Materials). The performance of ION-X deliv- ering arsine (AsH3), phosphine (PH3), and boron triflu- oride (BF3) was evaluated in high current implanters at the Axcelis Advanced Technology Center and compared to the incumbent delivery systems. In-process and on-wafer results of the MOF-based dopant gases compared positively to conventional source gases. Flow, pressure, and beam stability were undistinguishable from conven- tional gas sources throughout the lifetime of the cylinder. Beam and wafer contamination levels (both surface and energetic) were below specification limits, matching the performance of the reference qualified products.

Dopant gas safety challenges

The storage and delivery of hazardous gases creates signif- icant environmental, health, and safety challenges. Their usage requires implementation of stringent safety control systems to minimize the risks of exposure to humans and the environment. The dangers associated with handling toxic gases are the result of both the inherent chemical hazard of the molecule and the kinetic energy stored in the vessel in the form of compression. In essence, the lethality of a toxic release is magnified exponentially by the energetic force of the high-pressure storage. Historically, one way to mitigate these risks was to dilute the hazardous material with inert gases in an effort to attenuate the toxicity effects. Depending on the concentration, this solution provides a safety factor improvement of 10 or 100 by virtue of reducing the molecular density of the hazardous gas to 10% or 1% mixtures, respectively. This approach is commonly used in the electronics manufacturing industry for gases that are known to have extreme toxicity. Hydride gases (i.e. arsine, phosphine, germane, or diborane) are examples of such highly toxic gases used as source materials in a number of electronic manufacturing processes. While this dilution method is effective at reducing the toxicity levels, these mixtures are typically produced at cylinder pressures significantly higher than the pressures of the pure toxic gases. In a release event, this solution reduces the lethality of the dose at the expense of a higher release rate.

In 1993, ATMI (now an Entegris company) introduced a different approach to reduce the toxic gas storage hazards [1]. The technology involves using nano-porous adsor- bents to condense the gas molecules onto their surfaces. This process effectively reduces the kinetic energy of the gas, thus reducing the pressure in the gas cylinder. The large available surface areas within these materials result in gas storage capacities comparable to the high-pressure cylinders. The intrinsic safety advantages of adsorbed gas cylinders are derived from the reduction in pressure within the cylinder. Typically, these vessels are filled to sub-atmospheric pressures (measured at room temperature) in order to inhibit an outward gas release in the event of a leak.

The first sub-atmospheric dopant gas delivery systems used zeolites (SDS® 1) while the second and third genera- tions (SDS® 2 and SDS® 3) evolved to activated carbon adsorbent materials. These gas cylinders store and deliver dopant precursor gases (primarily arsine, phosphine, and boron trifuoride) predominantly for ion implantation processes. In its third generation, and in order to further improve gas storage capacities, SDS 3 evolved by creating a highly dense monolithic adsorbent that nearly eliminated void volumes in the cylinder.

In this paper, we describe a new sub-atmospheric gas delivery system (ION-X ®) that uses a novel ultra-high surface area class of materials called metal-organic frame- works (MOFs). In addition, the implant process perfor- mance using the new product delivering arsine, phosphine, and boron trifluoride was evaluated in a major ion implant OEM facility will be described.

MOF overview: The next generation in nano- porous adsorbents

MOF are three-dimensional crystalline structures assembled with metal-containing nodes connected by organic links (FIGURE 1). The resulting highly organized molecular structures generate nano-pores with record surface areas [2-4]. In addition, the large number of available metal nodes and organic linkers provide unpar- alleled molecular design flexibility to tailor the chemical and physical properties of the adsorbent material to fit the application. Since their discovery in the early 1990’s, MOFs have evolved from an academic curiosity to a widely recognized new class of materials with practical applications in energy, specialty chemicals, military, medical, pharmaceutical, and electronics industries. MOFs are one of the fastest growing classes of materials, with thousands of experimental structures now being reported.

For gas storage and delivery applications, MOFs’ design flexibility provides advantages over traditional adsorbents (FIGURE 2). Pore size, surface area, and chemical stability can be tailored to the specific properties of the adsorbed gases. Compared to zeolites and activated carbon adsorbents, MOFs have significantly larger surface areas (up to 7,000m2/g has been reported[5]. This property, combined with bulk density, is critical in gas storage applications where capacity is measured in terms of vessel volume rather than adsorbent mass. Pore size tunability is also an important parameter in efforts to match the dimensions of the MOF cavities to the molecular sizes of the target adsorbates. This parameter impacts adsorption capacities (how much gas can be loaded) and desorption characteristics (how much can be delivered as a function of pressure). Unlike the broad pore size distributions found in activated carbon adsorbents, MOFs’ crystallinity results in more “usable” pores. This pore size uniformity also results in higher gas quality, as impurities are selectively size excluded.

Preventing reactions between the adsorbent and the target gas is extremely important in electronics applications. Adsorbent/gas interactions will contribute to gas decomposition, leading to impurities and unwanted dopant gas composition changes that could affect the process. The molecular composition of zeolites and carbon adsor- bents are limited to a few elements (typically carbon, aluminum, and silicon) and their oxides. MOFs, on the other hand, can be synthetized from a large range of organic and inorganic constituents, offering more options for creating stable gas/ adsorbent interactions.

MOF-based gas delivery system for ion implant gases ION-X (FIGURE 3) is a sub-atmospheric dopant gas storage and delivery system designed for ion implantation [6]. ION-X uses individual MOF structures with tailored pore sizes to effectively and reversibly adsorb arsine, phosphine, and boron trifluoride gases. The pressure in filled ION-X cylinders is below one atmosphere, significantly reducing the health and environmental impact of an accidental gas release. Furthermore, MOFs’ ultra-high surface areas and uniform structures provide capacity and deliverable advantages compared to existing carbon adsorbent-based products (FIGURE 4). It is important to note that the first-generation ION-X cylinders utilize granulated MOFs with similar adsorbent bulk density to the first-generation carbon product: for the same mass of adsorbent, MOFs provide 40% to 55% higher gas delivery by virtue of their superior surface area and pose size uniformity. Analogous to the evolution of SDS®2, MOF densification inside the cylinder will further increase the gas capacity in next-generation ION-X products.

Implant performance characterization

The performances of ION-X dopant delivery systems were recently evaluated using a PurionH 300 mm high current ion implanter at Axcelis’ Advanced Technology Center (Beverly, MA, USA). The test plan included flow, mass spectral, and metal contamination analyses (both at the surface and at implanted depth). The experiments were repeated using commercially available and well-estab- lished sub-atmospheric dopant gas sources in order to provide a basis for comparison.

Cylinder installation and setup was seamless, requiring no modifications to the existing gas box hardware or software. Flow rate stability for all three gases (AsH3, PH3, and BF3) was demonstrated in the 3.5 to 8 sccm ranges down to cylinder pressures of 20 torr (spec limit). For arsine, the flow experiment continued through a full cylinder depletion, showing a stable flow rate down to cylinder pressure below 3 torr.

The beam energy, purity, and stability were evaluated by analyzing the mass spectra generated during the implantation processes. In all cases, the target dose was 5 x 1015 at/cm2 with beam energies of 40 keV, 20 keV and 15 keV for As+, P+, and BF¬2+ ion implants respectively. The stability and purity of the target doping ion beams were within specifications and very similar to the ones produced by the reference gas sources. Based on the mass spectra, ION-X did not generate any impurities derived from either gas or MOF decomposition.

Neutral and energetic metal contamination levels were thoroughly investigated in this study. All metal analyses were performed by sampling wafers produced using the recipes described in the previous paragraph. Vapor Phase Decomposition-inductively coupled Plasma-Mass Spectrometry (VPD-ICP-MS) was used to monitor the contamination from key trace metals at the wafer surface. Particular attention was placed on monitoring zinc and iron, metals used in the hydride and BF3 ION-X MOF adsorbents respectively. Results show that all metal levels were within specification limits and compared well to the levels detected in control wafers. In all cases, zinc and iron surface contamination levels were below their corresponding detection limits of 0.03 and 0.05 x 1010 atoms/cm2.

Energetic metal contamination is of special interest in ion implantation as even low levels of impurities could affect the performance of the electronic devices. The depth profile of the metals used in ION-X’s MOFs composition were measured using Secondary Ion Mass Spectrometry (SIMS). Wafers used for SIMs analyses were doped using both ION-X and incumbent gas sources using the same ion implant tool and previously stated recipes. The zinc and iron metal concentration profiles for the hydride and boron implants were well within specifications and show no discernable differences between the incumbent and the MOF-based gas sources (FIGURE 5). These results, combined with the previous surface contamination tests, conclusively establish the gas and ion purity of the dopant species extracted from ION-X adsorbents. Moreover, the results are consistent with extensive gas analyses performed at NuMat after subjecting the MOF adsorbent materials to accelerated aging, vibration, and cycle testing.

Summary

This article provides process and on-wafer performance of ION-X, a new MOF-based dopant gas delivery system. The adsorbents used in these cylinders have surface areas, stability, purity, and pore sizes ideal for the storage and delivery of ion implant dopant gases. In-process and on-wafer performance of boron trifluoride, arsine, and phosphine dopant sources compared positively to conven- tional source gas cylinders. The issue of contamination was investigated in detail, demonstrating that the new adsorbents do not contribute to surface or energetic metal impurities. The results published in this article provide independent evaluation of the new product, supporting the safe use of this product in mainstream ion implant applications. To that end, ION-X is already qualified and being used at an electronics manufacturing site with confirmed high stability and purity performance.

References

  1. Olander, K. and Avila, A., “Subatmospheric Has Storage and Delivery: Past, Present, and Future”, Solid State Technology, Volume 57 (2014), pp 27-302.
  2. Y. Cui, B. Li, H. He, W. Zhou, B. Chen, and G. Qian, “Metal–Organic Frameworks as Platforms for Functional Materials,” Accounts of Chemical Research, vol. 49, pp. 483-493, 2016/03/15 2016.
  3. H. Furukawa, K. E. Cordova, M. O’Keeffe, and O. M. Yaghi, “The Chemistry and Applications of Metal-Organic Frameworks,” Science, vol. 341, 2013.
  4. P. Silva, S. M. F. Vilela, J. P. C. Tome, and F. A. Almeida Paz, “Multifunc- tional metal-organic frameworks: from academia to industrial applications,” Chemical Society Reviews, vol. 44, pp. 6774-6803, 2015.
  5. Omar K Farha et al., “Metal-Organic Framework Materials with Ultrahigh Surface Areas: Is the Sky the Limit?” J. Am. Chem. Soc. (2012), Vol. 134, pp 15016−15021
  6. G. M. Tom et al., “Utilization of Metal-Organic Frameworks for the Management of Gases Used in Ion Implantation”, 2016 21st International Conference on Ion ImplantationTechnology (IIT),Tainan, 2016, pp. 1-4.

Scientists at the Center for Functional Nanomaterials (CFN)–a U.S. Department of Energy (DOE) Office of Science User Facility at Brookhaven National Laboratory–have used an optoelectronic imaging technique to study the electronic behavior of atomically thin nanomaterials exposed to light. Combined with nanoscale optical imaging, this scanning photocurrent microscopy technique provides a powerful tool for understanding the processes affecting the generation of electrical current (photocurrent) in these materials. Such an understanding is key to improving the performance of solar cells, optical sensors, light-emitting diodes (LEDs), and other optoelectronics–electronic devices that rely on light-matter interactions to convert light into electrical signals or vice versa.

“Anyone who wants to know how light-induced electrical current is distributed across a semiconductor will benefit from this capability,” said CFN materials scientist Mircea Cotlet, co-corresponding author on the May 17 Advanced Functional Materials paper describing the work.

Generating an electrical current

When hit with light, semiconductors (materials that have an electrical resistance in between that of metals and insulators) generate an electric current. Semiconductors that consist of one layer or a few layers of atoms–for example, graphene, which has a single layer of carbon atoms–are of particular interest for next-generation optoelectronics because of their sensitivity to light, which can controllably alter their electrical conductivity and mechanical flexibility. However, the amount of light that atomically thin semiconductors can absorb is limited, thus limiting the materials’ response to light.

To enhance the light-harvesting properties of these two-dimensional (2D) materials, scientists add tiny (10-50 atoms in diameter) semiconducting particles called quantum dots in the layer(s). The resulting “hybrid” nanomaterials not only absorb more light but also have interactions occurring at the interface where the two components meet. Depending on their size and composition, the light-excited quantum dots will transfer either charge or energy to the 2D material. Knowing how these two processes influence the photocurrent response of the hybrid material under different optical and electrical conditions–such as the intensity of the incoming light and applied voltage–is important to designing optoelectronic devices with properties tailored for particular applications.

“Photodetectors sense an extremely low level of light and convert that light into an electrical signal,” explained Cotlet. “On the other hand, photovoltaic devices such as solar cells are made to absorb as much light as possible to produce an electrical current. In order to design a device that operates for photodetection or photovoltaic applications, we need to know which of the two processes–charge or energy transfer–is beneficial.”

Lighting up charge and energy transfer processes

In this study, the CFN scientists combined atomically thin molybdenum disulfide with quantum dots. Molybdenum disulfide is one of the transition-metal dichalcogenides, semiconducting compounds with a transition-metal (in this case, molybdenum) layer sandwiched between two thin layers of a chalcogen element (in this case, sulfur). To control the interfacial interactions, they designed two kinds of quantum dots: one with a composition that favors charge transfer and the other with a composition that favors energy transfer.

“Both kinds have cadmium selenide at their core, but one of the cores is surrounded by a shell of zinc sulfide,” explained CFN research associate and first author Mingxing Li. “The shell is a physical spacer that prevents charge transfer from happening. The core-shell quantum dots promote energy transfer, whereas the core-only quantum dots promote charge transfer.”

The scientists used the clean room in the CFN Nanofabrication Facility to make devices with the hybrid nanomaterials. To characterize the performance of these devices, they conducted scanning photocurrent microscopy studies with an optical microscope built in-house using existing equipment and the open-source GXSM instrument control software developed by CFN physicist and co-author Percy Zahl. In scanning photocurrent microscopy, a laser beam is scanned across the device while the photocurrent is measured at different points. All of these points are combined to produce an electrical current “map.” Because charge and energy transfer have distinct electrical signatures, scientists can use this technique to determine which process is behind the observed photocurrent response.

The maps in this study revealed that the photocurrent response was highest at low light exposure for the core-only hybrid device (charge transfer) and at high light exposure for the core-shell hybrid device (energy transfer). These results suggest that charge transfer is extremely beneficial to the device functioning as a photodetector, and energy transfer is preferred for photovoltaic applications.

“Distinguishing energy and charge transfers solely by optical techniques, such as photoluminescence lifetime imaging microscopy, is challenging because both processes reduce luminescence lifetime to similar degrees,” said CFN materials scientist and co-corresponding author Chang-Yong Nam. “Our investigation demonstrates that optoelectronic measurements combining localized optical excitation and photocurrent generation can not only clearly identify each process but also suggest potential optoelectronic device applications suitable to each case.”

“At the CFN, we conduct experiments to study how nanomaterials function under real operating conditions,” said Cotlet. “In this case, we combined the optical expertise of the Soft and Bio Nanomaterials Group, device fabrication and electrical characterization expertise of the Electronic Nanomaterials Group, and software expertise of the Interface Science and Catalysis Group to develop a capability at the CFN that will enable scientists to study optoelectronic processes in a variety of 2D materials. The new scanning photocurrent microscopy facility is now open to CFN users, and we hope this capability will draw more users to the CFN fabrication and characterization facilities to study and improve the performance of optoelectronic devices.”

The 64th annual IEEE International Electron Devices Meeting(IEDM), to be held at the Hilton San Francisco Union Square hotel December 1-5, 2018, has issued a Call for Papers seeking the world’s best original work in all areas of microelectronics research and development.

The paper submission deadline this year is Wednesday, August 1, 2018. Authors are asked to submit four-page camera-ready papers. Accepted papers will be published as-is in the proceedings. A limited number of late-news papers will be accepted. Authors are asked to submit late-news papers announcing only the most recent and noteworthy developments. The late-news submission deadline is September 10, 2018.

At IEDM each year, the world’s best scientists and engineers in the field of microelectronics gather to participate in a technical program consisting of more than 220 presentations, along with a variety of panels, special sessions, Short Courses, a supplier exhibit, IEEE/EDS award presentations and other events highlighting leading work in more areas of the field than any other conference.

This year, special emphasis is placed on the following topics:

  • Neuromorphic computing/AI
  • Quantum computing devices and links
  • Devices for RF, 5G, THz and mmWave
  • Advanced memory technologies
  • More-than-Moore devices and integrations
  • Technologies for advanced logic nodes
  • Non-charge-based devices and systems
  • Sensors and MEMS devices
  • Package-device level interactions
  • Electron device simulation and modeling
  • Advanced characterization, reliability and noise
  • Optoelectronics, displays and imaging systems

Overall, papers in the following areas of technology are encouraged:

  • Circuit and Device Interaction
  • Characterization, Reliability and Yield
  • Compound Semiconductor and High-Speed Devices
  • Memory Technology
  • Modeling and Simulation
  • Nano Device Technology
  • Optoelectronics, Displays and Imagers
  • Power Devices
  • Process and Manufacturing Technology
  • Sensors, MEMS and BioMEMS

Further information

For more information, interested persons should visit the IEDM 2018 home page at www.ieee-iedm.org.

SEMI, the global industry association representing the electronics manufacturing supply chain, today announced that the WT | Wearable Technologies Conference 2018 USA will co-locate July 11-12 with SEMICON West 2018 in San Francisco. The electronics industry’s premier U.S. event, SEMICON West — July 10-12 at Moscone North and South — will highlight engines of industry expansion including smart transportation, smart manufacturing, smart medtech, smart data, big data, artificial intelligence, blockchain and the Internet of Things (IoT). Click here to register.

“We are excited that the WT | Wearables Technologies Conference has joined SEMICON West to co-locate in 2018,” said David Anderson, president of SEMI Americas. “Our strategic partnership brings new content and more value to our extended supply chain. Every day the semiconductor industry makes chips smaller and faster with ever-higher performance. These innovations enable new wearable applications for smart living, smart medtech and healthcare that are continuously improving our lives. The WT | Wearable Technologies Conference speakers at SEMICON West 2018 will demonstrate just how they use semiconductor technology to deliver leading-edge wearables.”

“It is a great pleasure to collaborate with the leading global electronics manufacturing association and its successful SEMICON West event,” said Christian Stammel, CEO of WT | Wearables Technologies. “Since the beginning of our platform in 2006, the semiconductor industry has been a major driver of wearables and IoT innovation. All major developments in the WT application markets like healthcare (smart patches), safety and security (tracking solutions), lifestyle and sport (smartwatches and wristbands) and in the industrial field (AR / VR) were driven by semiconductor and MEMS innovations. Our program of expert speakers at SEMICON West will share the latest insights in the wearables market as the SEMI and WT ecosystems explore collaboration and innovation opportunities.”

Technology trends in backplane technology are driving higher gas demand in display manufacturing. Specific gas requirements of process blocks are discussed, and various supply modes are reviewed.

BY EDDIE LEE, Linde Electronics, Hsinchu, Taiwan

Since its initial communalization in the 1990s, active matrix thin-film-transistor (TFT) displays have become an essential and indispensable part of modern living. They are much more than just televisions and smartphones; they are the primary communication and information portals for our day-to- day life: watches (wearables), appliances, advertising, signage, automobiles and more.

There are many similarities in the display TFT manufacturing and semiconductor device manufacturing such as the process steps (deposition, etch, cleaning, and doping), the type of gases used in these steps, and the fact that both display and semiconductor manufacturing both heavily use gases.

However, there are technology drivers and manufacturing challenges that differentiate the two. For semiconductor device manufacturing, there are technology limitations in making the device increasingly smaller. For display manufacturing, the challenge is primarily maintaining the uniformity of glass as consumers drive the demand for larger and thinner displays.

While semiconductor wafer size has maxed because of the challenges of making smaller features uniformly across the surface of the wafer, the size of the display mother glass has grown from 0.1m x 0.1m with 1.1mm thickness to 3m x 3m with 0.5mm thickness over the past 20 years due to consumer demands for larger, lighter, and more cost-effective devices.

As the display mother glass area gets bigger and bigger,so does the equipment used in the display manufacturing process and the volume of gases required. In addition, the consumer’s desire for a better viewing experience such as more vivid color, higher resolution, and lower power consumption has also driven display manufacturers to develop and commercialize active matrix organic light emitting displays (AMOLED).

Technology

Layers of display device

In general, there are two types of displays in the market today: active matrix liquid crystal display (AMLCD) and AMOLED. In its simplicity, the fundamental components required to make up the display are the same for AMLCD and AMOLED. There are four layers of a display device (FIGURE 1): a light source, switches that are the thin-film-transistor and where the gases are mainly used, a shutter to control the color selection, and the RGB (red, green, blue) color filter.

About backplane/TFT

The thin-film-transistors used for display are 2D transitional transistors, which are similar to bulk CMOS before FinFET. For the active matrix display, there is one transistor for each pixel to drive the individual RGB within the pixel. As the resolution of the display grows, the transistor size also reduces, but not to the sub-micron scale of semiconductor devices. For the 325 PPI density, the transistor size is approximately 0.0001 mm2 and for the 4K TV with 80 PPI density, the transistor size is approximately 0.001 mm2.

Technology trends TFT-LCD (thin-film-transistor liquid-crystal display) is the baseline technology. MO / White OLED (organic light emitting diode) is used for larger screens. LTPS / AMOLED is used for small / medium screens. The challenges for OLED are the effect of < 1 micron particles on yield, much higher cost compared to a-Si due to increased mask steps, and moisture impact to yield for the OLED step.

Mobility limitation (FIGURE 2) is one of the key reasons for the shift to MO and LTPS to enable better viewing experience from higher resolution, etc.

The challenge to MO is the oxidation after IGZO metalization / moisture prevention after OLED step, which decreases yield. A large volume of N2O (nitrous oxide) is required for manufacturing, which means a shift in the traditional supply mode might need to be considered.

Although AMLCD displays are still dominant in the market today, AMOLED displays are growing quickly. Currently about 25% of smartphones are made with AMOLED displays and this is expected to grow to ~40% by 2021. OLED televisions are also growing rapidly, enjoying double digit growth rate year over year. Based on IHS data, the revenue for display panels with AMOLED technol- ogies is expected to have a CAGR of 18.9% in the next five years while the AMLCD display revenue will have a -2.8% CAGR for the same period with the total display panel revenue CAGR of 2.5%. With the rapid growth of AMOLED display panels, the panel makers have accel- erated their investment in the equipment to produce AMOLED panels.

Types of backplanes

There are three types of thin-film-transistor devices for display: amorphous silicon (a-Si), low temperature polysilicon (LTPS), and metal oxide (MO), also known as transparent amorphous oxide semiconductor (TAOS). AMLCD panels typically use a-Si for lower-resolution displays and TVs while high-resolution displays use LTPS transistors, but this use is mainly limited to small and medium displays due to its higher costs and scalability limitations. AMOLED panels use LTPS and MO transistors where MO devices are typically used for TV and large displays (FIGURE 3).

How gases are used

This shift in technology also requires a change in the gases used in production of AMOLED panels as compared with the AMLCD panels. As shown in FIGURE 4, display manufacturing today uses a wide variety of gases.

These gases can be categorized into two types: Electronic Specialty gases (ESGs) and Electronic Bulk gases (EBGs) (FIGURE 5). Electronic Specialty gases such as silane, nitrogen trifluoride, fluorine (on-site generation), sulfur hexafluoride, ammonia, and phosphine mixtures make up 52% of the gases used in the manufacture of the displays while the Electronic Bulk gases–nitrogen, hydrogen, helium, oxygen, carbon dioxide, and argon – make up the remaining 48% of the gases used in the display manufacturing.

Key usage drivers

The key ga susage driver in the manufacturing of displays is PECVD (plasma-enhanced chemical vapor deposition), which accounts for 75% of the ESG spending, while dry etch is driving helium usage. LTPS and MO transistor production is driving nitrous oxide usage. The ESG usage for MO transistor production differs from what is shown in FIGURE 4: nitrous oxide makes up 63% of gas spend, nitrogen trifluoride 26%, silane 7%, and sulfur hexafluoride and ammonia together around 4%. Laser gases are used not only for lithography, but also for excimer laser annealing application in LTPS.

Silane: SiH4 is one of the most critical molecules in display manufacturing. It is used in conjunction with ammonia (NH3) to create the silicon nitride layer for a-Si transistor, with nitrogen (N2) to form the pre excimer laser anneal a-Si for the LTPS transistor, or with nitrous oxide (N2O) to form the silicon oxide layer of MO transistor.

Nitrogen trifluoride: NF3 is the single largest electronic material from spend and volume standpoint for a-Si and LTPS display production while being surpassed by N2O for MO production. NF3 is used for cleaning the PECVD chambers. This gas requires scalability to get the cost advantage necessary for the highly competitive market.

Nitrous oxide: Used in both LTPS and MO display production, N2O has surpassed NF3 to become the largest electronic material from spend and volume standpoint for MO production. N2O is a regional and localized product due to its low cost, making long supply chains with high logistic costs unfeasible. Averaging approximately 2 kg per 5.5 m2 of mother glass area, it requires around 240 tons per month for a typical 120K per month capacity generation 8.5 MO display production. The largest N2O compressed gas trailer can only deliver six tons of N2O each time and thus it becomes both costly and risky
for MO production.

Nitrogen: For a typical large display fab, N2 demand can be as high as 50,000 Nm3/hour, so an on-site generator, such as the Linde SPECTRA-N® 50,000, is a cost-effective solution that has the added benefit of an 8% reduction in CO2 (carbon dioxide) footprint over conventional nitrogen plants.

Helium: H2 is used for cooling the glass during and after processing. Manufacturers are looking at ways to decrease the usage of helium because of cost and availability issues due it being a non-renewable gas.

Gas distribution at the fab

N2 On-site generators: Nitrogen is the largest consumed gas at the fab, and is required to be available before the first tools are brought to the fab. Like major semiconductor fabs, large display fabs require very large amounts of nitrogen, which can only be economically supplied by on-site plants.

Cryogenic liquid truck trailers: Oxygen, argon, and carbon dioxide are produced at off-site plants and trucked short distances as cryogenic liquids in specialty vacuum-insulated tankers.
Compressed gas truck trailers: Other large volume gases like hydrogen and helium are supplied over longer distances in truck or ISO-sized tanks as compressed gases.

Individual packages: Specialty gases are supplied in individual packages. For higher volume materials like silane and nitrogen trifluoride, these can be supplied in large ISO packages holding up to 10 tons. Materials with smaller requirements are packaged in standard gas cylinders.

Blended gases: Laser gases and dopants are supplied as blends of several different gases. Both the accuracy and precision of the blended products are important to maintain the display device fabrication operating within acceptable parameters.

In-fab distribution: Gas supply does not end with the delivery or production of the material of the fab. Rather, the materials are further regulated with additional filtration, purification, and on-line analysis before delivery to individual production tools.

Conclusion

The consumer demand for displays that offer increas- ingly vivid color, higher resolution, and lower power consumption will challenge display makers to step up the technologies they employ and to develop newer displays such as flexible and transparent displays. The transistors to support these new displays will either be LTPS and / or MO, which means the gases currently being used in these processes will continue to grow. Considering the current a-Si display production, the gas consumption per area of the glass will increase by 25% for LTPS and ~ 50% for MO productions.

To facilitate these increasing demands, display manufacturers must partner with gas suppliers to identify which can meet their technology needs, globally source electronic materials to provide customers with stable and cost- effective gas solutions, develop local sources of electronic materials, improve productivity, reduce carbon footprint, and increase energy efficiency through on-site gas plants. This is particularly true for the burgeoning China display manufacturing market, which will benefit from investing in on-site bulk gas plants and collaboration with global materials suppliers with local production facilities for high-purity gas and chemical manufacturing.