Category Archives: Packaging Materials

At this week’s 2018 IEEE International Interconnect Technology Conference (IITC 2018), imec will present 11 papers on advanced interconnects, ranging from extending Cu and Co damascene metallization, all the way to evaluating new alternatives such as Ru and graphene. After careful evaluation of the resistance and reliability behavior, imec takes first steps towards extending conventional metallization into to the 3nm technology node.

For almost two decades, Cu-based dual damascene has been the workhorse industrial process flow for building reliable interconnects. But when downscaling logic device technology towards the 5nm and 3nm technology nodes, meeting resistance and reliability requirements for the tightly pitched Cu lines has become increasingly challenging. The industry is however in favor of extending the current damascene technology as long as possible, and therefore, different solutions have emerged.

To set the limits of scaling, imec has benchmarked the resistance of Cu with respect to Co and Ru in a damascene vehicle with scaled dimensions, demonstrating that Cu still outperforms Co for wire cross sections down to 300nm2 (or linewidths of 12nm), which corresponds to the 3nm technology node. To meet reliability requirements, one option is to use Cu in combination with thin diffusion barriers such as tantalum nitride (TaN)) and liners such as Co or Ru. It was found that the TaN diffusion barrier can be scaled to below 2nm while maintaining excellent Cu diffusion barrier properties.

For Cu linewidths down to 15–12nm, imec also modeled the impact of the interconnect line-edge roughness on the system-level performance. Line-edge roughness is caused by the lithographic and patterning steps of interconnect wires, resulting in small variations in wire width and spacing. At small pitches, these can affect the Cu interconnect resistance and variability. Although there is a significant impact of line-edge roughness on the resistance distribution for short Cu wires, the effect largely averages out at the system level.

An alternative solution to extend the traditional damascene flow is replacing Cu by Co. Today Co requires a diffusion barrier – an option that recently gained industrial acceptance. A next possible step is to enable barrierless Co or at least sub-nm barrier thickness with careful interface engineering. Co has the clear advantage of having a lower resistance for smaller wire cross-secions and smaller vias. Based on electromigration and thermal storage experiments, imec presents a detailed study of the mechanisms that impact Co via reliability, showing the abscence of voids in barrierless Co vias, demonstrating a better scalability of Co towards smaller nodes.

The research is performed in cooperation with imec’s key nano interconnect program partners including GlobalFoundries, Huawei, Intel, Micron, Qualcomm, Samsung, SK Hynix, SanDisk/Western Digital, Sony Semiconductor Solutions, TOSHIBA Memory and TSMC.

Researchers working at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) coupled graphene, a monolayer form of carbon, with thin layers of magnetic materials like cobalt and nickel to produce exotic behavior in electrons that could be useful for next-generation computing applications.

Andreas Schmid, left, and Gong Chen are pictured here with the spin-polarized low-energy electron microscopy (SPLEEM) instrument at Berkeley Lab. The instrument was integral to measurements of ultrathin samples that included graphene and magnetic materials. Credit: Roy Kaltschmidt/Berkeley Lab

The work was performed in collaboration with French scientists including Nobel Laureate Albert Fert, an emeritus professor at Paris-Sud University and scientific director for a research laboratory in France. The team performed key measurements at Berkeley Lab’s Molecular Foundry, a DOE Office of Science User Facility focused on nanoscience research.

Fert shared the Nobel Prize in Physics in 2007 for his work in understanding a magnetic effect in multilayer materials that led to new technology for reading data in hard drives, for example, and gave rise to a new field studying how to exploit and control a fundamental property known as “spin” in electrons to drive a new type of low-energy, high-speed computer memory and logic technology known as spintronics.

In this latest work, published online Monday in the journal Nature Materials, the research team showed how that spin property – analogous to a compass needle that can be tuned to face either north or south – is affected by the interaction of graphene with the magnetic layers.

The researchers found that the material’s electronic and magnetic properties create tiny swirling patterns where the layers meet, and this effect gives scientists hope for controlling the direction of these swirls and tapping this effect for a form of spintronics applications known as “spin-orbitronics” in ultrathin materials. The ultimate goal is to quickly and efficiently store and manipulate data at very small scales, and without the heat buildup that is a common hiccup for miniaturizing computing devices.

Typically, researchers working to produce this behavior for electrons in materials have coupled heavy and expensive metals like platinum and tantalum with magnetic materials to achieve such effects, but graphene offers a potentially revolutionary alternative since it is ultrathin, lightweight, has very high electrical conductivity, and can also serve as a protective layer for corrosion-prone magnetic materials.

“You could think about replacing computer hard disks with all solid state devices – no moving parts – using electrical signals alone,” said Andreas Schmid, a staff scientist at the Molecular Foundry who participated in the research. “Part of the goal is to get lower power-consumption and non-volatile data storage.”

The latest research represents an early step toward this goal, Schmid noted, and a next step is to control nanoscale magnetic features, called skyrmions, which can exhibit a property known as chirality that makes them swirl in either a clockwise or counterclockwise direction.

In more conventional layered materials, electrons traveling through the materials can act like an “electron wind” that changes magnetic structures like a pile of leaves blown by a strong wind, Schmid said.

But with the new graphene-layered material, its strong electron spin effects can drive magnetic textures of opposite chirality in different directions as a result of the “spin Hall effect,” which explains how electrical currents can affect spin and vice versa. If that chirality can be universally aligned across a material and flipped in a controlled way, researchers could use it to process data.

“Calculations by other team members show that if you take different magnetic materials and graphene and build a multilayer stack of many repeating structures, then this phenomenon and effect could possibly be very powerfully amplified,” Schmid said.

To measure the layered material, scientists applied spin-polarized low-energy electron microscopy (SPLEEM) using an instrument at the Molecular Foundry’s National Center for Electron Microscopy. It is one of just a handful of specialized devices around the world that allow scientists to combine different images to essentially map the orientations of a sample’s 3-D magnetization profile (or vector), revealing a its “spin textures.”

The research team also created the samples using the same SPLEEM instrument through a precise process known as molecular beam epitaxy, and separately studied the samples using other forms of electron-beam probing techniques.

Gong Chen, a co-lead author who participated in the study as a postdoctoral researcher at the Molecular Foundry and is now an assistant project scientist in the UC Davis Physics Department, said the collaboration sprang out of a discussion with French scientists at a conference in 2016 – both groups had independently been working on similar research and realized the synergy of working together.

While the effects that researchers have now observed in the latest experiments had been discussed decades ago in previous journal articles, Chen noted that the concept of using an atomically thin material like graphene in place of heavy elements to generate those effects was a new concept.

“It has only recently become a hot topic,” Chen said. “This effect in thin films had been ignored for a long time. This type of multilayer stacking is really stable and robust.”

Using skyrmions could be revolutionary for data processing, he said, because information can potentially be stored at much higher densities than is possible with conventional technologies, and with much lower power usage.

Molecular Foundry researchers are now working to form the graphene-magnetic multilayer material on an insulator or semiconductor to bring it closer to potential applications, Schmid said.

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.”

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.

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.

Technavio projects the global semiconductor glass wafer market to post a CAGR of more than 6% during the forecast period. The emergence of advanced and compact consumer electronic devices is a key driver, which is expected to impact market growth.

Consumer electronic devices have witnessed a massive transformation over the last five years. Feature phones have been replaced by smartphones, PCs by laptops, and now laptops are being replaced by tablets. Cathode ray tube (CRT) TVs are being replaced by light-emitting diode (LED) TVs and organic LED (OLED) TVs. Due to increase in unit shipments of tablets and smartphones over the last five years, the demand for ICs (including MEMS devices and CMOS image sensors) used in these devices is on the rise. As semiconductor glass wafers are integral to ICs, rising demand for ICs will generate strong demand for semiconductor glass wafers over the forecast period.

In this report, Technavio highlights the growing proliferation of IoT and connected devices as one of the key emerging trends to drive the global semiconductor glass wafer market:

Growing proliferation of IoT and connected devices

IoT is a network of interrelated computing devices comprising mechanical and digital machines or objects that possess the ability to transfer data over a network without human-to-computer interaction. More than 30 billion IoT devices, generating about 50 trillion GBs of data, are expected to be connected through IoT by 2022. IoT enables devices to collect data using sensors and actuators and transmits data to a centralized location on a real-time basis, which empowers the user to take an informed decision. Thus, the adoption of IoT is increasing in several market segments, such as consumer electronics, automotive, and medical.

According to a senior analyst at Technavio for semiconductor equipment research, “Sensors and MEMS are an integral part of IoT devices and are manufactured from semiconductor glass wafers. It is projected that a total of one trillion sensors will be produced in 2020 to support the demand for IoT devices. This will require a significant production of semiconductor glass wafers, which can be met by several fabs. Growing applications of IoT will drive the construction of fabs.”

A Columbia University-led international team of researchers has developed a technique to manipulate the electrical conductivity of graphene with compression, bringing the material one step closer to being a viable semiconductor for use in today’s electronic devices.

By compressing layers of boron nitride and graphene, researchers were able to enhance the material's band gap, bringing it one step closer to being a viable semiconductor for use in today's electronic devices. Credit: Philip Krantz

By compressing layers of boron nitride and graphene, researchers were able to enhance the material’s band gap, bringing it one step closer to being a viable semiconductor for use in today’s electronic devices. Credit: Philip Krantz

“Graphene is the best electrical conductor that we know of on Earth,” said Matthew Yankowitz, a postdoctoral research scientist in Columbia’s physics department and first author on the study. “The problem is that it’s too good at conducting electricity, and we don’t know how to stop it effectively. Our work establishes for the first time a route to realizing a technologically relevant band gap in graphene without compromising its quality. Additionally, if applied to other interesting combinations of 2D materials, the technique we used may lead to new emergent phenomena, such as magnetism, superconductivity, and more.”

The study, funded by the National Science Foundation and the David and Lucille Packard Foundation, appears in the May 17 issue of Nature.

The unusual electronic properties of graphene, a two-dimensional (2D) material comprised of hexagonally-bonded carbon atoms, have excited the physics community since its discovery more than a decade ago. Graphene is the strongest, thinnest material known to exist. It also happens to be a superior conductor of electricity – the unique atomic arrangement of the carbon atoms in graphene allows its electrons to easily travel at extremely high velocity without the significant chance of scattering, saving precious energy typically lost in other conductors.

But turning off the transmission of electrons through the material without altering or sacrificing the favorable qualities of graphene has proven unsuccessful to-date.

“One of the grand goals in graphene research is to figure out a way to keep all the good things about graphene but also create a band gap – an electrical on-off switch,” said Cory Dean, assistant professor of physics at Columbia University and the study’s principal investigator. He explained that past efforts to modify graphene to create such a band gap have degraded the intrinsically good properties of graphene, rendering it much less useful. One superstructure does show promise, however. When graphene is sandwiched between layers of boron nitride (BN), an atomically-thin electrical insulator, and the two materials are rotationally aligned, the BN has been shown to modify the electronic structure of the graphene, creating a band gap that allows the material to behave as a semiconductor – that is, both as an electrical conductor and an insulator. The band gap created by this layering alone, however, is not large enough to be useful in the operation of electrical transistor devices at room temperature.

In an effort to enhance this band gap, Yankowitz, Dean, and their colleagues at the National High Magnetic Field Laboratory, the University of Seoul in Korea, and the National University of Singapore, compressed the layers of the BN-graphene structure and found that applying pressure substantially increased the size of the band gap, more effectively blocking the flow of electricity through the graphene.

“As we squeeze and apply pressure, the band gap grows,” Yankowitz said. “It’s still not a big enough gap – a strong enough switch – to be used in transistor devices at room temperature, but we have gained a fundamentally better understanding of why this band gap exists in the first place, how it can be tuned, and how we may target it in the future. Transistors are ubiquitous in our modern electronic devices, so if we can find a way to use graphene as a transistor it would have widespread applications.”

Yankowitz added that scientists have been conducting experiments at high pressures in conventional three-dimensional materials for years, but no one had yet figured out a way to do them with 2D materials. Now, researchers will be able to test how applying various degrees of pressure changes the properties of a vast range of combinations of stacked 2D materials.

“Any emergent property that results from the combination of 2D materials should grow stronger as the materials are compressed,” Yankowitz said. “We can take any of these arbitrary structures now and squeeze them and the strength of the resulting effect is tunable. We’ve added a new experimental tool to the toolbox we use to manipulate 2D materials and that tool opens boundless possibilities for creating devices with designer properties.”

Vladimir Mostepanenko, Chief Research Associate of KFU Cosmology Lab and Pulkovo Astronomical Observatory, explains, “Despite graphene layers’ extremely small width, it has proven to be a firm material which conducts electricity even under zero temperatures when density of charge carriers also equals zero. But something absolutely unexpected was that this residual conductivity can be expressed through fundamental physical constants – electron charge and Planck constant. Graphene has been used successfully in dozens of electronic devices and has been found in interstellar matter.”

Graphene’s unusual qualities led to speculation that the causality principle may not be observed for it. The authors, Vladimir Mostepanenko and Galina Klimchitskaya, proved that the principle is preserved for graphene. Through the direct analytic calculation it was shown that the real and imaginary parts of graphene conductivity, found recently on the basis of first principles of thermal quantum field theory using the polarization tensor in (2+1)-dimensional space-time, satisfy the Kramers-Kronig relations precisely.

The results are important for further inquiries into reflective and absorptive qualities of graphene.