Category Archives: Process Materials

Taiwan is the world’s largest consumer of semiconductor materials for the seventh consecutive year, bringing new opportunities in this increasingly critical sector.  SEMICON Taiwan (13-15 September), held at Taipei’s Nangang Exhibition Center, will feature over 1,700 booths and 700 exhibitors, and more than 45,000 attendees from the global electronics manufacturing supply chain. This year, in addition to the much-anticipated Executive Summit, themed “Transformation: A Key to Solution,” 27 international forums will be held, exploring major issues. Speakers from TSMC, UMC, Powerchip, NVIDIA, Micron and Amkor will share their insights on trends and strategies of the next-generation electronics industry.

According to the SEMI Material Market Data Report, Taiwan’s semiconductor materials consumption was US$9.8 billion in 2016 − the world’s largest. Global semiconductor manufacturing equipment billings reached US$13.1 billion in Q1 2017, exceeding the record quarterly high set in Q3 2000. These figures signal that application drivers will continue to drive the development of a supply chain feeding their manufacturing processes, equipment and materials.

“As SEMICON Taiwan celebrates its 22nd year, the exhibition area will be expanded to closely align with the four major trends of applications in the market, which include Internet of Things (IoT), Smart Manufacturing, Smart Transportation, and Smart Medtech,” said Terry Tsao, president of SEMI Taiwan. “This year, SEMICON Taiwan aims to increasingly connect the entire manufacturing ecosystem vertically and horizontally. In addition, it will provide an overview of market trends and leading technologies in the industry, with forums and business matching activities which will enable collaboration and new opportunities.”

Theme Pavilions and Region Pavilions Focus on Opportunities

In addition to the eight customary theme pavilions, five new pavilions are featured this year, and to promote cross-border collaboration, eight regional pavilions are offered. The 21 pavilions include:

Theme Pavilions
  • Automated Optical Inspection (AOI)
  • Chemical Mechanical Planarization (CMP)
  • High-Tech Facility
  • Materials
  • Precision Machinery
  • Secondary Market
  • Smart Manufacturing & Automation
  • Taiwan Localization

 
New Theme Pavilions
  • Circular Economy
  • Compound Semiconductor
  • Flexible Hybrid Electronics/Micro-LED
  • Laser
  • Opto Semiconductor

 
Regional Pavilions
  • Cross-Strait
  • German
  • Holland High-Tech
  • Korean
  • Kyushu (Japan)
  • Okinawa (Japan)
  • Silicon Europe
  • Singapore

Co-located with SEMICON Taiwan 2017, the SiP Global Summit will discuss three key system-in-package topics:

  • Package Innovation in Automotive
  • 3D IC, 3D interconnection for AI and High-end Computing
  • Innovative Embedded Substrate and Fan-Out Technology to Enable 3D-SiP Devices

Participants will share trends on 2.5D/3D IC technologies, and the evolution and challenges of embedded technologies and wafer level packaging.

This is the first year that the International Test Conference (ITC) will be co-located with SEMICON Taiwan 2017, also marking the first time that ITC is held in Asia. The conference will focus on the rapid growth of emerging applications like IoT and automotive electronics, and how testing technologies are challenged by rapid advancements of manufacturing processes, 3D stacking and SiP.

For more information about SEMICON Taiwan 2017, please visit www.semicontaiwan.org or follow us on Facebook.

A little fluorine turns an insulating ceramic known as white graphene into a wide-bandgap semiconductor with magnetic properties. Rice University scientists said that could make the unique material suitable for electronics in extreme environments.

A proof-of-concept paper from Rice researchers demonstrates a way to turn two-dimensional hexagonal boron nitride (h-BN) – aka white graphene – from an insulator to a semiconductor. The magnetism, they said, is an unexpected bonus.

Because the atomically thin material is an exceptional conductor of heat, the researchers suggested it may be useful for electronics in high-temperature applications, perhaps even as magnetic memory devices.

The discovery appears this week in Science Advances.

“Boron nitride is a stable insulator and commercially very useful as a protective coating, even in cosmetics, because it absorbs ultraviolet light,” said Rice materials scientist Pulickel Ajayan, whose lab led the study. “There has been a lot of effort to try to modify its electronic structure, but we didn’t think it could become both a semiconductor and a magnetic material.

“So this is something quite different; nobody has seen this kind of behavior in boron nitride before,” he said.

The researchers found that adding fluorine to h-BN introduced defects into its atomic matrix that reduced the bandgap enough to make it a semiconductor. The bandgap determines the electrical conductivity of a material.

“We saw that the gap decreases at about 5 percent fluorination,” said Rice postdoctoral researcher and co-author Chandra Sekhar Tiwary. The gap gets smaller with additional fluorination, but only to a point. “Controlling the precise fluorination is something we need to work on. We can get ranges but we don’t have perfect control yet. Because the material is atomically thin, one atom less or more changes quite a bit.

“In the next set of experiments, we want to learn to tune it precisely, atom by atom,” he said.

They determined that tension applied by invading fluorine atoms altered the “spin” of electrons in the nitrogen atoms and affected their magnetic moments, the ghostly quality that determines how an atom will respond to a magnetic field like an invisible, nanoscale compass.

“We see angle-oriented spins, which are very unconventional for 2-D materials,” said Rice graduate student and lead author Sruthi Radhakrishnan. Rather than aligning to form ferromagnets or canceling each other out, the spins are randomly angled, giving the flat material random pockets of net magnetism. These ferromagnet or anti-ferromagnet pockets can exist in the same swatch of h-BN, which makes them “frustrated magnets” with competing domains.

The researchers said their simple, scalable method can potentially be applied to other 2-D materials. “Making new materials through nanoengineering is exactly what our group is about,” Ajayan said.

Co-authors of the paper are graduate students Carlos de los Reyes and Zehua Jin, chemistry lecturer Lawrence Alemany, postdoctoral researcher Vidya Kochat and Angel Martí, an associate professor of chemistry, of bioengineering and of materials science and nanoengineering, all of Rice; Valery Khabashesku of Rice and the Baker Hughes Center for Technology Innovation, Houston; Parambath Sudeep of Rice and the University of Toronto; Deya Das, Atanu Samanta and Rice alumnus Abhishek Singh of the Indian Institute of Science, Bangalore; Liangzi Deng and Ching-Wu Chu of the University of Houston; Thomas Weldeghiorghis of Louisiana State University and Ajit Roy of the Air Force Research Laboratories at Wright-Patterson Air Force Base.

Ajayan is chair of Rice’s Department of Materials Science and NanoEngineering, the Benjamin M. and Mary Greenwood Anderson Professor in Engineering and a professor of chemistry.

TECHCET CA—the advisory service firm providing electronic materials information—today announced that specialty chemical precursor market for the deposition of dielectrics and metals in integrated circuit (IC) fabrication is forecasted to increase at ~10% CAGR through the year 2021. TECHCET’s proprietary Wafer Forecast Model (WFM) shows that 3D-NAND devices are expected to grow at a rapid pace from 2016 and become one of the top three market segments by 2020. Logic ICs will continue to evolve, from 3D finFET devices to Gate-All-Around Nano-Wires (GAA-NW), enabled by new critical materials and manufacturing processes as detailed in new reports from TECHCET, “Advanced Insulating Dielectric Precursors,” and “ALD/CVD High-k & Metal Precursors.”

Precursors tracked by TECHCET for ALD/CVD of metal and high-k dielectric films on IC wafers include sources of aluminum, cobalt, hafnium, tantalum, titanium, tungsten, and zirconium. The total market for 2017 is now estimated to be US$435M, growing to US$638M in 2021. The top-2 suppliers are estimated to hold more than half of the total available market, with many players competing to supply the next enabling molecule. In particular, cobalt precursor demand is forecasted to reach >$80M in 2021 as foundries transition to below 14nm-node processing. As a potential conflict mineral, TECHCET tracks the sub-suppliers of cobalt.

“Metal precursors have had double-digit growth over an extended period of time, and we expect that to continue as the IC industry transitions to 10nm- and 7nm-node logic and 3D-NAND fabrication, with an average long term CAGR of 11% over 2013 to 2021,” says Dr. Jonas Sundqvist, lead author of the report, senior technology analyst with TECHCET and researcher with Fraunhofer IKTS. “Dielectric precursors growth today is clearly driven by dielectric PEALD deposition in multiple patterning, and by dielectric CVD in 3D-NAND.”

Precursors tracked by TECHCET for ALD/CVD/SOD of advanced dielectric films on IC wafers include multiple sources of silicon. The total market for 2017 is now estimated to be just over US$400M, growing to US$560M in 2021. Current growth over 10% is expected to slow slightly to be in the 8-10% range over 2019-2021. Anticipated near-term developments include transitions from CVD to ALD for several IC fab modules.

By Pete Singer

Semiconductor manufacturers use a variety of high global warming potential (GWP) gases to process wafers and to rapidly clean chemical vapor deposition (CVD) tool chambers. Processes use high GWP fluorinated compounds including perfluorocarbons (e.g., CF4, C2F6 and C3F8), hydrofluorocarbons (CHF3, CH3F and CH2F2), nitrogen trifluoride (NF3) and sulfur hexafluoride (SF6). Semiconductor manufacturing processes also use fluorinated heat transfer fluids and nitrous oxide (N2O).

Of these, the semiconductor industry naturally tends to focus its attention on CF4 since it is one of the worst offenders, with an atmospheric half-life of 50,000 years. “CF4 the hardest to get rid of and it’s one of the worst global warming gases,” said Kate Wilson, VP Marketing, Subfab Solutions – Semiconductor Division of Edwards. “We tend to use that as an indicator of how much of the other global warming gases, as well, are being emitted by the industry. If we’re dealing with that (CF4) well, we tend to be managing the rest of the gases pretty effectively.”

According to the Environmental Protection Agency (EPA), estimating fluorinated GHG emissions from semiconductor manufacture is complicated and has required a significant and coordinated effort by the industry and governments. It was historically assumed that the majority of these chemicals were consumed or transformed in the manufacturing process. It is now known that under normal operating conditions, anywhere between 10 to 80 percent of the fluorinated GHGs pass through the manufacturing tool chambers unreacted and are released into the air.

In addition, fluorinated GHG emissions vary depending on a number of factors, including gas used, type/brand of equipment used, company-specific process parameters, number of fluorinated GHG-using steps in a production process, generation of fluorinated GHG by-product chemicals, and whether appropriate abatement equipment has been installed. Companies’ product types, manufacturing processes and emissions also vary widely across semiconductor fabs.

The good news is that many companies in the semiconductor manufacturing industry have successfully identified, evaluated and implemented a variety of technologies that protect the climate and improved production efficiencies. Solutions have been investigated and successfully implemented in the following key technological areas:

  • Process improvements/source reduction
  • Alternative chemicals
  • Capture and beneficial reuse
  • Destruction technologies (known as abatement)

In 2011 the industry set new targets for 2020, which it summarizes as:

  • The implementation of best practices for new semiconductor fabs. The industry expects that the implementation of best practices will result in a normalized emission rate (NER) in 2020 of 0.22 kgCO2e/cm2, which is a 30 percent NER reduction from the 2010 aggregated baseline.
  • The addition of “Rest of World” fabs (fabs located outside the World Semiconductor Council (WSC) regions that are operated by a company from a WSC association) in reporting of emissions and the implementation of best practices for new fabs.
  • NER based measurement in kilograms of carbon equivalents per area of silicon wafers processed (kgCO2e/cm2), which will be the single WSC goal at the global level.

“We’re finding as we get down to the lower levels and different things come up as the highest priority in the fab where we’re moving into more and more lower usage processes, which are requiring abatement now in order to get those levels down to meet the targets of 2020 in the industry,” Wilson explained.

The main area for potential improvement now is etch, especially in older 200mm fabs where etch processes may not have been fitted with PFC abatement devices. This is particularly true for etch processes making extensive use of CF4. “The area where we still have the most gaps is clearly etch,” Wilson said. In CVD processes, most of the benefit was done by material shifts rather than actual abatement, although we clearly do need to abate the other gases in those processes. For the etch side, there are still quite a few customers that really only do the toxic emission abatement rather than the global warming gas emission abatement. But we do see, across almost all of our customer base, people have either fairly recently moved to fully abating all the PFC type gases or will be shortly.”

Wilson said some other gases have been coming up more recently in terms of things like N2O, which people are putting more focus on now as it’s becoming a larger part of the fab footprint of global warming materials.

For PFC abatement, Edwards offers the Atlas range of products, which destroys PFCs by burning them. This is followed by a wet scrub of the byproducts. This works quite well, but Wilson cautions that in can be tricky for some processes, such as chamber cleans with NF3. “If the burn is not correct and you get too hot, there’s actually the potential to create PFC’s. And so, it is quite critical to have well-controlled burn technology to make sure that you don’t actually cause issues where we didn’t have them before.”

Wilson said another area where they have seen some issues with PFCs being created is with processing of carbon-doped materials, such as low-k dielectrics. “When they do the chamber clean, they’re cleaning off predominately silicon dioxide but there’s carbon in there so that can create PFCs and CF4 as well so there’s a requirement to look at abatement in those areas,” she said.

Another piece of good news is that no company in the supply chain is waiting for legislation to be enacted before they act themselves. “Right from consumers to the consumer manufacturers, the car manufacturers, consumer electric manufacturers, our direct customers, the equipment manufacturers plus the major players within semiconductor and flat panel display, it seems that at every level there’s a commitment that this is the right thing to do,” Wilson said. “At every level people are pushing to get the requirements more stringent and it’s almost not about legislation anymore, it’s about everybody actually thinks it’s a good idea and they want to do it.”

Across all process areas in the fab effective abatement technologies reduce the GHG emissions significantly. The reductions per process area are shown in the diagram.

Across all process areas in the fab effective abatement technologies reduce the GHG emissions significantly. The reductions per process area are shown in the diagram.

By Pete Singer

In order to increase device performance, the semiconductor industry has slowly been implementing many new materials. From the 1960s through the 1990s, only a handful of materials were used, most notably silicon, silicon oxide, silicon nitride and aluminum. Soon, by 2020, more than 40 different materials will be in high-volume production, including more “exotic” materials such as hafnium, ruthenium, zirconium, strontium, complex III-Vs (such as InGaAs), cobalt and SiC.

These new materials create a variety of challenges with regard to process integration (understanding material interface issues, adhesion, stress, cross-contamination, etc.). But they also create new challenges when it comes to material handling.

“As we go through technology node advancements, people are looking at the potential of different materials on the wafer,” notes Clint Haris, Senior Vice President and General Manager of the Microcontamination Control Division at Entegris (Billerica, MA). “They’re looking at different chemicals that are required to clean those materials to reduce defects and improve their operational yield, and what we’re increasingly seeing is that fabs are concerned with the fact that contamination can be introduced in the fluid stream anywhere in that long process flow.”

Haris said that part of their mission at Entegris is to make sure that the entire supply chain – from the development of a chemistry at the supplier to its use on a wafer in a fab – is working in harmony, particularly with regard to any materials that might “touch” the chemicals. “Not only do you want to filter and purify things throughout the whole fluid flow,” he said, “but you want to have that last filtration right before the fluid touches the surface of the wafer.”

The goal of filtration is, of course, to remove contaminants and particles before they reach the wafer, but the exact purity required can be a moving target. “Today we’re seeing a lot of these materials and liquids, which have a parts per trillion purity level, but there’s a desire to move to parts per quadrillion,” Haris said. That’s the equivalent of one drop in all the water that flows over Niagra Falls in one day.

In addition to the filtration challenge of achieving that level, there’s the question of do the analytical tools exist to actually measure contaminants at that level. The answer – not yet. “It’s actually a real issue where some of the metrology tools cannot meet our customers’ needs at those levels, and so one of the things that we’ve done is we’ve developed some techniques internally to enhance the capability of metrology,” Haris said. “We also work on how we prepare our samples so you can detect contamination at those levels.” Because that level of detection is so difficult — in some cases impossible – Haris said fabs are increasingly putting additional filters at the process tool and at the dispense nozzle to “protect against the unknown.”

Earlier this year, Entegris introduced Purasol™, a first-of-its-kind solvent purifier that removes a wide variety of metal microcontaminants found in organic solvents used in ultraclean chemical manufacturing processes. Using tailored membrane technology, the purifier can efficiently remove both dissolved and colloidal metal contaminants from a wide variety of ultra-pure, polar and non-polar solvents. “One of the main things that our customers are seeing is a concern with metal contamination in the photo process that can result in particular defects (see Figure), such as bridge defects,” Haris explained. Increasingly, fabs are moving from just filtration (removing particles) to purification (removing ions and metals), he added.

Illustration of metal contamination inducing defects on lithography process.

Illustration of metal contamination inducing defects on lithography process.

Entegris also recently acquired W. L. Gore & Associates’ water and chemical filtration product line for microelectronics applications. “This is a Teflon-based product line, which is used in ultrapure water filtration for semiconductor fabs, but it’s also a product that we’re selling into some of the fine chemical purification markets for some of the chemistries that are brought into the fabs,” Haris said. “We are focused on new product development and M&A to enhance our capability to support our customers as they overcome these contamination challenges..”

What would a simple technique to remove thin layers from otherwise thick, rigid semiconductor crystals mean for the semiconductor industry? This concept has been actively explored for years, as integrated circuits made on thin layers hold promise for developments including improved thermal characteristics, lightweight stackability and a high degree of flexibility compared to conventionally thick substrates.

In a significant advance, a research group from IBM successfully applied their new “controlled spalling” layer transfer technique to gallium nitride (GaN) crystals, a prevalent semiconductor material, and created a pathway for producing many layers from a single substrate.

As they report in the Journal of Applied Physics, from AIP Publishing, controlled spalling can be used to produce thin layers from thick GaN crystals without causing crystalline damage. The technique also makes it possible to measure basic physical properties of the material system, like strain-induced optical effects and fracture toughness, which are otherwise difficult to measure.

The same 20-micron spalled GaN film, demonstrating the film's flexibility. Credit: Bedell/IBM Research

The same 20-micron spalled GaN film, demonstrating the film’s flexibility. Credit: Bedell/IBM Research

Single-crystal GaN wafers are extremely expensive, where just one 2-inch wafer can cost thousands of dollars, so having more layers means getting more value out of each wafer. Thinner layers also provide performance advantages for power electronics, since it offers lower electrical resistance and heat is easier to remove.

“Our approach to thin film removal is intriguing because it’s based on fracture,” said Stephen W. Bedell, research staff member at IBM Research and one of the paper’s authors. “First, we first deposit a nickel layer onto the surface of the material we want to remove. This nickel layer is under tensile strength — think drumhead. Then we simply roll a layer of tape onto the nickel, hold the substrate down so it can’t move, and then peel the tape off. When we do this, the stressed nickel layer creates a crack in the underlying material that goes down into the substrate and then travels parallel to the surface.”

Their method boils down to simply peeling off the tape, nickel layer and a thin layer of the substrate material stuck to the nickel.

“A good analogy of how remarkable this process is can be made with a pane of glass,” Bedell said. “We’re breaking the glass in the long direction, so instead of a bunch of broken glass shards, we’re left with two full sheets of glass. We can control how much of the surface is removed by adjusting the thickness of the nickel layer. Because the entire process is done at room temperature, we can even do this on finished circuits and devices, rendering them flexible.”

The group’s work is noteworthy for multiple reasons. For starters, it’s by far the simplest method of transferring thin layers from thick substrates. And it may well be the only layer transfer method that’s materially agnostic.

“We’ve already demonstrated the transfer of silicon, germanium, gallium arsenide, gallium nitride/sapphire, and even amorphous materials like glass, and it can be applied at nearly any time in the fabrication flow, from starting materials to partially or fully finished circuits,” Bedell said.

Turning a parlor trick into a reliable process, working to ensure that this approach would be a consistent technique for crack-free transfer, led to surprises along the way.

“The basic mechanism of substrate spalling fracture started out as a materials science problem,” he said. “It was known that metallic film deposition would often lead to cracking of the underlying substrate, which is considered a bad thing. But we found that this was a metastable phenomenon, meaning that we could deposit a thick enough layer to crack the substrate, but thin enough so that it didn’t crack on its own — it just needed a crack to get started.”

Their next discovery was how to make the crack initiation consistent and reliable. While there are many ways to generate a crack — laser, chemical etching, thermal, mechanical, etc. — it turns out that the simplest way, according to Bedell, is to terminate the thickness of the nickel layer very abruptly near the edge of the substrate.

“This creates a large stress discontinuity at the edge of the nickel film so that once the tape is applied, a small pull on the tape consistently initiates the crack in that region,” he said.

Though it may not be obvious, gallium nitride is a vital material to our everyday lives. It’s the underlying material used to fabricate blue, and now white, LEDs (for which the 2014 Nobel Prize in physics was awarded) as well as for high-power, high-voltage electronics. It may also prove useful for inherent biocompatibility, which when combined with control spalling may permit ultrathin bioelectronics or implantable sensors.

“Controlled spalling has already been used to create extremely lightweight, high-efficiency GaAs-based solar cells for aerospace applications and flexible state-of-the-art circuits,” Bedell said.

The group is now working with research partners to fabricate high-voltage GaN devices using this approach. “We’ve also had great interaction with many of the GaN technology leaders through the Department of Energy’s ARPA-E SWITCHES program and hope to use controlled spalling to enable novel devices through future partnerships,” Bedell said.

The Linde Group is expanding production of the rare gases used by the semiconductor industry, including xenon, which is in increasing demand for etching 3D semiconductor structures.

This quarter, Linde expects to complete a xenon expansion project at its Alpha, N.J. plant, part of a xenon supply network that spans three continents. Of the three rare gases – neon, krypton, and xenon – xenon is the rarest. Besides extracting xenon from the atmosphere, Linde provides numerous services to conserve and optimize their process use of xenon, said Matt Adams, head of sales and marketing, electronic gases & specialty products. “We have been developing specialized solutions around xenon for 30 years. Recycling xenon can actually make some applications viable that may not have been previously, because there’s not enough product in the world,” he said.

Lithography gas expansions

With increasing demand for neon from DUV (deep UV) multi-patterning lithography and other excimer laser applications, Linde is expanding neon capacity at a newly installed neon production facility in La Porte, Texas.

Linde works with its customers to supply mixtures of neon, fluorine, and other gases for excimer laser patterning applications At SEMICON West, Linde (Bridgewater, N.J.) is discussing its expansion of lithography gas processing capacity at its Medford, Ore. facility.

“By investing in Medford for lithography gas production, it gives us another site and increases our business continuity plan. We work with the OEMs to make sure that we are changing as needed, to make sure that the tools and the fabs are working optimally,” Adams said.

Besides adding purification capacity, Adams said its continuity planning includes strengthening Linde’s supply chain in Europe and managing a portfolio of third-party sources.

With more than 60,000 employees worldwide and around $20 billion in annual revenues, Linde leads the industry in rare gases. “We continue to invest globally in our own sources, and at the same time develop additional supply capacity with our partners,” said Andreas Weisheit, head of Linde Electronics. For example, Linde has more than 35 captive air separation units (ASUs) for rare gas production, and manages a network of external suppliers.

The major lithography equipment and chip manufacturers work with Linde engineers to develop new technologies at Linde’s Centers of Excellence, including a center for laser gases in Alpha, N.J.

Linde spans the gamut of rare gas capabilities, including the design and manufacture of air separation units (ASUs) and rare gas extraction equipment, cryogenic engineering, purification capabilities for neon, krypton and xenon, and high-volume mixing and blending capabilities.

Rare gas production is a multi-stage process, Adams said. For example, a steel company that needs oxygen will have a Linde air separation unit onsite to extract the rare gases. This crude mixture, sometimes referred to as a soup of materials, is further refined — and in some cases undergoes cryogenic distillation — to extract the xenon, krypton and neon.

Because neon is the highest-volume rare gas, Linde has multiplesneon purifiers strategically located around the world. “That speaks to our business continuity planning, that we have these at separate locations. We’re able to process this crude neon into semiconductor grade neon. Of course, it’s similar with xenon and krypton,” he said.

Adams said the neon shortage has been addressed and supply and demand has come back into balance. “That can change with new and different applications. We are starting to see some tightening in the xenon market, due to some applications that are coming online that have a high xenon demand. Which is one of the reasons why we’re making the investment in Alpha, New Jersey,” he said.

Linde Electronics will be exhibiting at SEMICON West, booth number 5952 in the North hall in the Moscone Center. Its focus will be on the leadership that Linde Electronics brings to the semiconductor industry through such offerings as electronic specialty gases, on-site solutions, materials recycling and recovery and SPECTRA® nitrogen plants.

For more information, see The Linde Group online at www.linde.com/electronics.

Alpha Plant

Alpha Plant

A new low-temperature solution printing technique allows fabrication of high-efficiency perovskite solar cells with large crystals intended to minimize current-robbing grain boundaries. The meniscus-assisted solution printing (MASP) technique boosts power conversion efficiencies to nearly 20 percent by controlling crystal size and orientation.

The process, which uses parallel plates to create a meniscus of ink containing the metal halide perovskite precursors, could be scaled up to rapidly generate large areas of dense crystalline film on a variety of substrates, including flexible polymers. Operating parameters for the fabrication process were chosen by using a detailed kinetics study of perovskite crystals observed throughout their formation and growth cycle.

“We used a meniscus-assisted solution printing technique at low temperature to craft high quality perovskite films with much improved optoelectronic performance,” said Zhiqun Lin, a professor in the School of Materials Science and Engineering at the Georgia Institute of Technology. “We began by developing a detailed understanding of crystal growth kinetics that allowed us to know how the preparative parameters should be tuned to optimize fabrication of the films.”

The new technique is reported July 7 in the journal Nature Communications. The research has been supported by the Air Force Office of Scientific Research (AFOSR) and the National Science Foundation (NSF).

Georgia Tech Research Scientist Ming He adjusts the equipment for the meniscus-assisted solution printing (MASP) technique used to fabricate perovskite films for solar cells. (Credit: Rob Felt, Georgia Tech)

Georgia Tech Research Scientist Ming He adjusts the equipment for the meniscus-assisted solution printing (MASP) technique used to fabricate perovskite films for solar cells. (Credit: Rob Felt, Georgia Tech)

Perovskites offer an attractive alternative to traditional materials for capturing electricity from light, but existing fabrication techniques typically produce small crystalline grains whose boundaries can trap the electrons produced when photons strike the materials. Existing production techniques for preparing large-grained perovskite films typically require higher temperatures, which is not favorable for polymer materials used as substrates – which could help lower the fabrication costs and enable flexible perovskite solar cells.

So Lin, Research Scientist Ming He and colleagues decided to try a new approach that relies on capillary action to draw perovskite ink into a meniscus formed between two nearly parallel plates approximately 300 microns apart. The bottom plate moves continuously, allowing solvent to evaporate at the meniscus edge to form crystalline perovskite. As the crystals form, fresh ink is drawn into the meniscus using the same physical process that forms a coffee ring on an absorbent surface such as paper.

“Because solvent evaporation triggers the transport of precursors from the inside to the outside, perovskite precursors accumulate at the edge of the meniscus and form a saturated phase,” Lin explained. “This saturated phase leads to the nucleation and growth of crystals. Over a large area, we see a flat and uniform film having high crystallinity and dense growth of large crystals.”

To establish the optimal rate for moving the plates, the distance between plates and the temperature applied to the lower plate, the researchers studied the growth of perovskite crystals during MASP. Using movies taken through an optical microscope to monitor the grains, they discovered that the crystals first grow at a quadratic rate, but slow to a linear rate when they began to impinge on their neighbors.

“When the crystals run into their neighbors, that affects their growth,” noted He. “We found that all of the grains we studied followed similar growth dynamics and grew into a continuous film on the substrate.”

The MASP process generates relatively large crystals – 20 to 80 microns in diameter – that cover the substrate surface. Having a dense structure with fewer crystals minimizes the gaps that can interrupt the current flow, and reduces the number of boundaries that can trap electrons and holes and allow them to recombine.

Using films produced with the MASP process, the researchers have built solar cells that have power conversion efficiencies averaging 18 percent – with some as high as 20 percent. The cells have been tested with more than 100 hours of operation without encapsulation. “The stability of our MASP film is improved because of the high quality of the crystals,” Lin said.

Doctor-blading is one of the conventional perovskite fabrication techniques in which higher temperatures are used to evaporate the solvent. Lin and his colleagues heated their substrate to only about 60 degrees Celsius, which would be potentially compatible with polymer substrate materials.

So far, the researchers have produced centimeter-scale samples, but they believe the process could be scaled up and applied to flexible substrates, potentially facilitating roll-to-roll continuous processing of the perovskite materials. That could help lower the cost of producing solar cells and other optoelectronic devices.

“The meniscus-assisted solution printing technique would have advantages for flexible solar cells and other applications requiring a low-temperature continuous fabrication process,” Lin added. “We expect the process could be scaled up to produce high throughput, large-scale perovskite films.”

Among the next steps are fabricating the films on polymer substrates, and evaluating other unique properties (e.g., thermal and piezotronic) of the material.

Brewer Science Inc. today announced from SEMICON West the extension of its partnership with Arkema to develop second-generation directed self-assembly (DSA) materials using high-x (chi) block copolymers. These new materials target advanced-node wafer patterning processes, because they enable even smaller feature sizes than first-generation DSA materials. As such, they provide a cost-effective solution to achieving device nodes down to 5nm and beyond, thereby enabling the continuation of Moore’s law.

“There have been very high expectations that DSA would solve all patterning issues,” said Darron Jurajda, Business Unit Manager, Brewer Science Inc. “Like all worthwhile technologies, there are many challenges to be solved before going into production. Leveraging our earlier DSA collaboration with Arkema offers the best path for implementing the next generation of materials. Together, we look forward to unlocking DSA’s full potential in accordance with industry timelines for manufacturing.”

High-chi block copolymers will further extend DSA’s advantages, achieving feature sizes that meet the requirements for 5nm and beyond. Extending their partnership allows these companies to build on their knowledge base, giving them a head start on developing high-chi materials.

As feature sizes shrink more aggressively with each node, it has become cost prohibitive to create them using existing patterning processes, such as EUV, self-aligned double patterning and self-aligned quad patterning. This presents a challenge for foundries and integrated device manufacturers preparing to ramp to 7nm and 5nm processes. DSA provides an alternative solution to achieving fine feature patterning; can be explored for minimal investment; and is cost efficient in final production. Development of high-chi materials also expands the opportunity for implementing DSA in other applications, including photonics, membrane applications and other areas of microelectronics.

The original collaboration between the two companies combined Brewer Science’s know-how in patterning and process integration with Arkema’s leading-edge expertise in block copolymer development to develop polystyrene-polymethyl methacrylate DSA materials, which are now production-ready to manufacture sub-22nm features.

A new type of semiconductor may be coming to a high-definition display near you. Scientists at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) have shown that a class of semiconductor called halide perovskites is capable of emitting multiple, bright colors from a single nanowire at resolutions as small as 500 nanometers.

A 2-D plate showing alternating cesium lead chloride (blue) and cesium lead bromide (green) segments. Credit: Letian Dou/Berkeley Lab and Connor G. Bischak/UC Berkeley

A 2-D plate showing alternating cesium lead chloride (blue) and cesium lead bromide (green) segments. Credit: Letian Dou/Berkeley Lab and Connor G. Bischak/UC Berkeley

The findings, published online this week in the early edition of the Proceedings of the National Academy of Sciences, represent a clear challenge to quantum dot displays that rely upon traditional semiconductor nanocrystals to emit light. It could also influence the development of new applications in optoelectronics, photovoltaics, nanoscopic lasers, and ultrasensitive photodetectors, among others.

The researchers used electron beam lithography to fabricate halide perovskite nanowire heterojunctions, the junction of two different semiconductors. In device applications, heterojunctions determine the energy level and bandgap characteristics, and are therefore considered a key building block of modern electronics and photovoltaics.

The researchers pointed out that the lattice in halide perovskites is held together by ionic instead of covalent bonds. In ionic bonds, atoms of opposite charges are attracted to each other and transfer electrons to each other. Covalent bonds, in contrast, occur when atoms share their electrons with each other.

“With inorganic halide perovskite, we can easily swap the anions in the ionic bonds while maintaining the single crystalline nature of the materials,” said study principal investigator Peidong Yang, senior faculty scientist at Berkeley Lab’s Materials Sciences Division. “This allows us to easily reconfigure the structure and composition of the material. That’s why halide perovskites are considered soft lattice semiconductors. Covalent bonds, in contrast, are relatively robust and require more energy to change. Our study basically showed that we can pretty much change the composition of any segment of this soft semiconductor.”

In this case, the researchers tested cesium lead halide perovskite, and then they used a common nanofabrication technique combined with anion exchange chemistry to swap out the halide ions to create cesium lead iodide, bromide, and chloride perovskites.

Each variation resulted in a different color emitted. Moreover, the researchers showed that multiple heterojunctions could be engineered on a single nanowire. They were able to achieve a pixel size down to 500 nanometers, and they determined that the color of the material was tunable throughout the entire range of visible light.

The researchers said that the chemical solution-processing technique used to treat this class of soft, ionic-bonded semiconductors is far simpler than methods used to manufacture traditional colloidal semiconductors.

“For conventional semiconductors, fabricating the junction is quite complicated and expensive,” said study co-lead author Letian Dou, who conducted the work as a postdoctoral fellow in Yang’s lab. “High temperatures and vacuum conditions are usually involved to control the materials’ growth and doping. Precisely controlling the materials composition and property is also challenging because conventional semiconductors are ‘hard’ due to strong covalent bonding.”

To swap the anions in a soft semiconductor, the material is soaked in a special chemical solution at room temperature.

“It’s a simple process, and it is very easy to scale up,” said Yang, who is also a professor of chemistry at UC Berkeley. “You don’t need to spend long hours in a clean room, and you don’t need high temperatures.”

The researchers are continuing to improve the resolution of these soft semiconductors, and are working to integrate them into an electric circuit.