Category Archives: Process Materials

Scientists have developed a new method of characterizing graphene’s properties without applying disruptive electrical contacts, allowing them to investigate both the resistance and quantum capacitance of graphene and other two-dimensional materials. Researchers from the Swiss Nanoscience Institute and the University of Basel’s Department of Physics reported their findings in the journal Physical Review Applied.

Graphene consists of a single layer of carbon atoms. It is transparent, harder than diamond and stronger than steel, yet flexible, and a significantly better conductor of electricity than copper. Since graphene was first isolated in 2004, scientists across the world have been researching its properties and the possible applications for the ultrathin material. Other two-dimensional materials with similarly promising fields of application also exist; however, little research has been carried out into their electronic structures.

No need for electrical contacts

Electrical contacts are usually used to characterize the electronic properties of graphene and other two-dimensional materials. However, these can significantly alter the materials’ properties. Professor Christian Schönenberger’s team from the Swiss Nanoscience Institute and the University of Basel’s Department of Physics has now developed a new method of investigating these properties without applying contacts.

To do this, the scientists embedded graphene in the isolator boron nitride, placed it on a superconductor and coupled it with a microwave resonator. Both the electrical resistance and the quantum capacitance of the graphene affect the quality factor and resonance frequency of the resonator. Although these signals are very weak, they can be captured using superconducting resonators.

By comparing the microwave characteristics of resonators with and without encapsulated graphene, the scientists can determine both the electrical resistance and quantum capacitance. “These parameters are important in the determination of graphene’s exact properties and in the identification of limiting factors for its application,” explains Simon Zihlmann, a PhD student in Schönenberger’s group.

Also suitable for other two-dimensional materials

The boron nitride-encapsulated graphene served as a prototype material during the method’s development. Graphene integrated into other materials can be investigated in the same way. In addition, other two-dimensional materials can also be characterized without the use of electrical contacts; for example, the semiconductor molybdenum disulfide, which has applications in solar cells and optics.

By Walt Custer, Custer Consulting Group, and Dan Tracy, SEMI

SEMI’s year-to-date worldwide semiconductor equipment billings year-to-date through March show a 59.6 percent gain to the same period last year.

Understanding volatility in the electronic equipment supply chain can be valuable in forecasting future business activity.  A useful way to compare relevant electronic industry data series is by using 3/12 growth rates.  The 3/12 growth is the ratio of three months of data, compared to the same three months a year earlier.

Chart 1 compares the 3/12 growth rates of four data series:

  • World semiconductor equipment shipments (SEMI; www.semi.org)
  • Taiwan chip foundry sales (company composite maintained by Custer Consulting Group)
  • World semiconductor shipments (SIA, www.semiconductors.org & WSTS, www.wsts.org)
  • World electronic equipment sales (composite of 238 global OEMS maintained by Custer Consulting Group).

supply-chain-dynamics

Highlights

  • Semiconductor capital equipment sales are by far the most volatile of the four series in Chart 1, followed by foundry sales.
  • Foundry sales are a good leading indicator for semiconductor equipment shipments ─ leading SEMI equipment by 3-4 months on a 3/12 growth basis.
  • Foundry growth peaked in November 2016.
  • SEMI equipment growth appears to have peaked in February 2017.
  • Semiconductor shipments may have peaked in March 2017. March semiconductor revenues were up 18.5 percent in 1Q’17 vs 1Q’16 and, although still very strong, their rate of growth appears to have plateaued.

Note that 3/12 values greater than 1.0 indicate growth.  Declining 3/12 values (but greater than 1.0) indicate growth but at a slower rate.  Values below 1.0 indicate contraction.

Based upon Chart 1, semiconductor equipment 3/12 growth will likely reach zero in August or September of this year. Considering the unstable world geopolitical situation, uncertainty clearly exists.

SEMI members can access member-only market data and information at www.semi.org/en/free-market-data-semi-members.

Custer Consulting Group (www.custerconsulting.com) provides market research, business analyses and forecasts for the electronic equipment and solar/photovoltaic supply chains including semiconductors, printed circuit boards & other passive components, photovoltaic cells & modules, EMS, ODM & related assembly activities and materials & process equipment.

As consumers around the world have become increasingly dependent on electronics, the transistor, a semiconductor component central to the operation of these devices, has become a critical subject of scientific research. Over the last several decades, scientists and engineers have been able to both shrink the average transistor size and dramatically reduce its production costs. The current generation of smartphones, for example, relies on chips that each feature over 3.3 billion transistors.

Most transistors are silicon-based and silicon technology has driven the computer revolution. In some applications, however, silicon has significant limitations. These include use in high power electronic devices and in harsh environments like the engine of a car or under cosmic ray bombardment in space. Silicon devices are prone to faltering and failing in difficult environments.

Addressing these challenges, Jiangwei Liu, from Japan’s National Institute for Materials Sciences, and his colleagues describe new work developing diamond-based transistors this week in the journal Applied Physics Letters, from AIP Publishing.

“Silicon-based transistors often suffer from high switching loss during power transmission and fail when exposed to extremely high temperatures or levels of radiation,” Liu said. “Given the importance of developing devices that use less power and perform under harsh conditions, there has been a lot of interest within the broader scientific community in determining a way to build transistors that utilizes manufactured diamonds, which are a very durable material.”

And with this very interest in mind, the team developed a new fabrication process involving diamond, bringing “hardened electronics” closer to realization.

“Manufactured diamonds have a number of physical properties that make them very interesting to researchers working with transistors,” said Yasuo Koide, a professor and senior scientist at the National Institute for Materials Science leading the research group. “Not only are they physically hard materials, they also conduct heat well which means that they can cope with high levels of power and operate in hotter temperatures. In addition, they can endure larger voltages than existing semiconductor materials before breaking down.”

The research group focused their work on enhancement-mode metal-oxide-semiconductor field-effect transistors (MOSFETs), a type of transistor that is commonly used in electronics. One of the distinguishing features of transistors is inclusion of an insulated terminal called a “gate” whose input voltage determines whether the transistor will conduct electricity or not.

“One of the developments that makes our fabrication process innovative is that we deposited yttrium oxide (Y2O3) insulator directly onto the surface of the diamond [to form the gate],” said Liu. “We added the yttrium oxide to the diamond with a technique known as electron beam evaporation, which involves using a beam of electrons to transform molecules of yttrium oxide from the solid state to the gaseous state so that they can be made to cover a surface and solidify on it.”

According to Liu, yttrium oxide has many desirable qualities, including high thermal stability, strong affinity to oxygen and wide band gap energy, which contributes to its capabilities as an insulator.

“Another innovation was that the yttrium oxide was deposited as a single layer,” Liu said. “In our previous work, we have created oxide bi-layers, but a single layer is appealing because it’s less difficult and less expensive to manufacture.”

Liu and his colleagues hope to refine their understanding of electron movement through the diamond transistor with future research projects.

“We work with a type of manufactured diamond that has a hydrogen layer on its surface. One of the important challenges going forward will be to understand the mechanism of electron conduction through this carbon-hydrogen layer,” said Liu.

“Ultimately, our team’s goal is to build integrated circuits with diamonds,” Koide said. “With this in mind, we hope our work can support the development of energy-efficient devices that can function in conditions of extreme heat or radiation.”

Researchers have uncovered the exact mechanism that causes new solar cells to break down in air, paving the way for a solution.

Solar cells harness energy from the Sun and provide an alternative to non-renewable energy sources like fossil fuels. However, they face challenges from costly manufacturing processes and poor efficiency – the amount of sunlight converted to useable energy.

Light-absorbing materials called organic lead halide perovskites are used in a new type of solar cells that have shown great promise, as they are more flexible and cheaper to manufacture than traditional solar cells constructed of silicon.

However, perovskite cells degrade rapidly in natural conditions, greatly decreasing their performance in a matter of days. This is one reason they are not currently widely used.

Previously, a team led by scientists from the Department of Chemistry at Imperial discovered that this breakdown is due to the formation of ‘superoxides’ that attack the perovskite material. These superoxides are formed when light hitting the cells releases electrons, which react with the oxygen in the air.

Now, in a study published in Nature Communications, the team have determined how the superoxides form and how they attack the perovskite material, and have proposed possible solutions.

Working with Dr Christopher Eames and Professor Saiful Islam at the University of Bath, the team found that superoxide formation is helped by spaces in the structure of the perovskite normally taken up by molecules of iodide. Although iodide is a component of the perovskite material itself, there are defects where iodide is missing. These vacant spots are then used in the formation of superoxides.

The team found that dosing the material with extra iodide after manufacturing did improve the stability, but that a more permanent solution could be to engineer the iodide defects out.

Lead author of the new study, Nicholas Aristidou from the Department of Chemistry at Imperial, said: “After identifying the role of iodide defects in generating superoxide, we could successfully improve the material stability by filling the vacancies with additional iodide ions. This open up a new way of optimising the material for enhanced stability by controlling the type and density of defects present.”

Lead researcher Dr Saif Haque from the Department of Chemistry at Imperial added: “We have now provided a pathway to understand this process at the atomic scale and allow the design of devices with improved stability.”

Currently, the only way of protecting perovskite cells from degradation by air and light is to encase them in glass. However, perovskite solar cells are made from flexible material designed to be used in a range of settings, so the glass encasement severely limits their function.

Dr Haque said: “Glass encasement restricts movement and adds weight and cost to the cells. Improving the perovskite cell material itself is the best solution.”

The team hope to next test the stability of the cells in real-world settings. The cells would be exposed to a combination of both oxygen and moisture, testing the cells in more relevant scenarios.

Hafnia dons a new face


May 12, 2017

It’s a material world, and an extremely versatile one at that, considering its most basic building blocks — atoms — can be connected together to form different structures that retain the same composition.

Diamond and graphite, for example, are but two of the many polymorphs of carbon, meaning that both have the same chemical composition and differ only in the manner in which their atoms are connected. But what a world of difference that connectivity makes: The former goes into a ring and costs thousands of dollars, while the latter has to sit content within a humble pencil.

The inorganic compound hafnium dioxide commonly used in optical coatings likewise has several polymorphs, including a tetragonal form with highly attractive properties for computer chips and other optical elements. However, because this form is stable only at temperatures above 3100 degrees Fahrenheit — think blazing inferno — scientists have had to make do with its more limited monoclinic polymorph. Until now.

A team of researchers led by University of Kentucky chemist Beth Guiton and Texas A&M University chemist Sarbajit Banerjee in collaboration with Texas A&M materials science engineer Raymundo Arroyave has found a way to achieve this highly sought-after tetragonal phase at 1100 degrees Fahrenheit — think near-room-temperature and potential holy grail for the computing industry, along with countless other sectors and applications.

The team’s research, published today in Nature Communications, details their observation of this spectacular atom-by-atom transformation, witnessed with the help of incredibly powerful microscopes at Oak Ridge National Laboratory. After first shrinking monoclinic hafnium dioxide particles down to the size of tiny crystal nanorods, they gradually heated them, paying close attention to the barcode-like structure characterizing each nanorod and, in particular, its pair of nanoscale, fault-forming stripes that seem to function as ground zero for the transition.

“In this study we are watching a tiny metal oxide rod transform from one structure, which is the typical material found at room temperature, into a different, related structure not usually stable below 3100 degrees Fahrenheit,” said Guiton, who is an associate professor of chemistry in the UK College of Arts & Sciences. “This is significant because the high-temperature material has amazing properties that make it a candidate to replace silicon dioxide in the semiconductor industry, which is built on silicon.”

Watch through the microscope’s lens as hafnia atoms rearrange themselves at nanoscale levels in this video showing the same raw data seen by the team, courtesy of the UK Guiton Group.

The semiconductor industry has long relied on silicon dioxide as its thin, non-conductive layer of choice in the critical gap between the gate electrode — the valve that turns a transistor on and off — and the silicon transistor. Consistently thinning this non-conductive layer is what allows transistors to become smaller and faster, but Guiton points out there is such a thing as too thin — the point at which electrons start sloshing across the barrier, thereby heating their surroundings and draining power. She says most of us have seen and felt this scenario to some degree (pun intended), for instance, while watching videos on our phones and the battery simultaneously drain as the device in our palm noticeably begins to warm.

As computer chips become smaller, faster and more powerful, their insulating layers must also be much more robust — currently a limiting factor for semiconductor technology. Guiton says this new phase of hafnia is an order of magnitude better at withstanding applied fields.

When it comes to watching hafnia’s structural transition between its traditional monoclinic state and this commercially desirable tetragonal phase at near-room temperature, Banerjee says it’s not unlike popular television — specifically, the “Hall of Faces” in the HBO show “Game of Thrones.”

“In essence, we have been able to watch in real time, on an atom by atom basis, as hafnia is transformed to a new phase, much like Arya Stark donning a new face,” Banerjee said. “The new phase of hafnia has a much higher ‘k’ value representing its ability to store charge, which would allow transistors to work really quickly while merely sipping on power instead of sapping it. The stripes turn out to be really important, since that is where the transition starts as the hafnia loses its stripes.”

Arroyave credits real-time atomic-scale information for enabling the group to figure out that the transformation occurs in a very different way at nanoscale levels than it does within the macroscopic particles that result in hafnia’s monoclinic form. The fact that it is nanoscale in the first place is why he says the transition occurs at, or much closer to, room temperature.

“Through synthesis at the nanoscale, the ‘height’ of the energy barrier separating the two forms has been shrunk, making it possible to observe tetragonal hafnia at much lower temperatures than usual,” Arroyave said. “This points toward strategies that could be used to stabilize a host of useful forms of materials that can enable a wide range of functionalities and associated technologies. This is just one example of the vast possibilities that exist when we start to explore the ‘metastable’ materials space.”

Banerjee says this study suggests one way to stabilize the tetragonal phase at actual room temperature — which he notes that his group previously accomplished via a different method last year — and big implications for fast, low-power-consumption transistors capable of controlling current without drawing power, reducing speed or producing heat.

“The possibilities are endless, including even more powerful laptops that don’t heat up and sip on power from their batteries and smart phones that ‘keep calm and carry on,'” Banerjee said. “We are trying to apply these same tricks to other polymorphs of hafnium dioxide and other materials — isolating other phases that are not readily stabilized at room temperature but may also have strange and desirable properties.”

Soitec, a designer and manufacturer of semiconductor materials for the electronics industry, has appointed Stephen Lin to the newly created position of vice president of strategic business development in China, a key region for the company’s future growth plans. Stephen Lin brings to Soitec nearly 30 years of experience leading semiconductor business operations in China and the U.S.

“Working with our executive team, Stephen is in charge of strengthening Soitec’s business interests within China as well as growing market demand for SOI wafer products,” said Thomas Piliszczuk, Soitec’s executive vice president, Business and Strategic Development. “Stephen will be instrumental in our efforts to continue growing China’s microelectronics ecosystem as he works closely with our customers as well as government agencies, institutions and the financial and investor communities.”

China is home to all key elements of the electronics value chain including semiconductor manufacturers, fabless device designers, and consumer end markets. Soitec is already highly engaged in China, working to expand the semiconductor ecosystem while also driving demand for silicon on insulator (SOI) wafer products with its direct and indirect customers. The company also collaborates closely with its Shanghai-based manufacturing partner Simgui and the National Silicon Industry Group (NSIG), which recently invested in Soitec.

Since beginning his semiconductor career at LSI Logic, Stephen Lin has held senior executive positions within several major electronics companies including NXP Semiconductors, Microsemi, Intel and Siemens. He also has launched start-up companies in China and the U.S. including Mobility Ventures. He earned his master in electrical engineering degree from McGill University in Quebec and his MBA from Santa Clara University in California. He is the author of multiple publications including “The Fabless Semiconductor China Handbook.”

Chemists have tried to synthesize carbon nanobelts for more than 60 years, but none have succeeded until now. A team at Nagoya University reported the first organic synthesis of a carbon nanobelt in Science. Carbon nanobelts are expected to serve as a useful template for building carbon nanotubes and open a new field of nanocarbon science.

The new nanobelt, measuring 0.83 nanometer (nm) in diameter, was developed by researchers at Nagoya University’s JST-ERATO Itami Molecular Nanocarbon Project, and the Institute of Transformative Bio-Molecules (ITbM). Scientists around the world have tried to synthesize carbon nanobelts since the 1950s and Professor Kenichiro Itami’s group has worked on its synthesis for 12 years.

“Nobody knew whether its organic synthesis was even possible or not,” says Segawa, one of the leaders of this study who had been involved in its synthesis for 7 and a half years. “However, I had my mind set on the synthesis of this beautiful molecule.”

Carbon nanobelts are belt-shaped molecules composed of fused benzene rings, which are aromatic rings consisting of six carbon atoms. Carbon nanobelts are a segment of carbon nanotubes, which have various applications in electronics and photonics due to their unique physical characteristics.

Current synthetic methods produce carbon nanotubes with inconsistent diameters and sidewall structures, which changes their electrical and optical properties. This makes it extremely difficult to isolate and purify a single carbon nanotube that has a specific diameter, length and sidewall structure. Therefore, being able to precisely control the synthesis of structurally uniform carbon nanotubes will help develop novel and highly functional materials.

Carbon nanobelts have been identified as a way to build structurally uniform carbon nanotubes. However, synthesizing carbon nanobelts is challenging due to their extremely high strain energies. This is because benzene is stable when flat, but becomes unstable when they are distorted by fusion of the rings.

To overcome this problem, Guillaume Povie, a postdoctoral researcher of the JST-ERATO project, Yasutomo Segawa, a group leader of the JST-ERATO project, and Kenichiro Itami, the director of JST-ERATO project and the center director of ITbM, have succeeded in the first chemical synthesis of a carbon nanobelt from a readily available precursor, p-xylene (a benzene molecule with two methyl groups in the 1,4- (para-) position) in 11 steps.

The key to this success is their synthetic strategy based on the belt-shaped formation from a macrocycle precursor with relatively low ring strain. In their strategy, the team prepared a macrocycle precursor from p-xylene in 10 steps, and formed the belt-shaped aromatic compound by a coupling reaction. Nickel was essential to mediate the coupling process.

“The most difficult part of this research was this key coupling reaction of the macrocycle precursor,” says Povie. “The reaction did not proceed well day after day and it took me three to four months for testing various conditions. I have always believed where there’s a will, there’s a way.”

In 2015, Itami launched a new initiative in his ERATO project to focus particularly on the synthesis of the carbon nanobelt. At the so-called “belt festival,” various new synthetic routes for the carbon nanobelt were proposed and more than 10 researchers were involved in the project. On September 28, 2016, exactly a year after the start of the festival, the carbon nanobelt structure was finally revealed by X-ray crystallography in front of the Itami group members. Everyone held their breath while staring at the screen during X-ray analysis, and cheered when the cylindrical shape image of the carbon nanobelt appeared on the screen. Itami, Segawa and Povie expressed their joy with a high five (movie: https://www.youtube.com/watch?v=cABZla9w0uo).

“It was one of the most exciting moments in my life and I will never forget it,” says Itami. “Since this is the result of a decade-long study, I greatly appreciate all the past and current members of my group for their support and encouragement. Thanks to their skill, toughness, sense and strong will of all members, we achieved this successful result.”

The synthesized carbon nanobelt is a red-colored solid and exhibits deep red fluorescence. Analysis by X-ray crystallography revealed that the carbon nanobelt has a cylindrical shape in the same manner as carbon nanotubes. The researchers also measured its light absorption and emission, electric conductivity and structural rigidity by ultraviolet-visible absorption fluorescence, and Raman spectroscopic studies, as well as theoretical calculations.

“Actually, the synthesis part was finished last August but I could not rest until I was able to confirm the X-ray structure of the carbon nanobelt,” says Povie. “I was really happy when I saw the X-ray structure.”

The carbon nanobelt will be released to the market in the future. “We are looking forward to discovering new properties and functionalities of the carbon nanobelt with researchers from all over the world,” say Segawa and Itami.

Two-dimensional graphene consists of single layers of carbon atoms and exhibits intriguing properties. The transparent material conducts electricity and heat extremely well. It is at the same time flexible and solid. Additionally, the electrical conductivity can be continuously varied between a metal and a semiconductor by, e.g., inserting chemically bound atoms and molecules into the graphene structure – the so-called functional groups. These unique properties offer a wide range of future applications as e.g. for new developments in optoelectronics or ultrafast components in the semiconductor industry. However, a successful use of graphene in the semiconductor industry can only be achieved if properties such as the conductivity, the size and the defects of the graphene structure induced by the functional groups can already be modulated during the synthesis of graphene.

In an international collaboration scientists led by Andreas Hirsch from the Friedrich-Alexander-Universität Erlangen-Nürnberg in close cooperation with Thomas Pichler from the University of Vienna accomplished a crucial breakthrough: using the latter’s newly developed experimental set-up they were able to identify, for the first time, vibrational spectra as the specific fingerprints of step-by-step chemically modified graphene by means of light scattering. This spectral signature, which was also theoretically attested, allows to determine the type and the number of functional groups in a fast and precise way. Among the reactions they examined, was the chemical binding of hydrogen to graphene. This was implemented by a controlled chemical reaction between water and particular compounds in which ions are inserted in graphite, a crystalline form of carbon.

This is a section of a graphene network with chemically bound hydrogen atom: the spectral vibrational signature of the single carbon-carbon bonds adjacent to the bound hydrogen atom is highlighted in different colors. Copyright: Frank Hauke, FAU

This is a section of a graphene network with chemically bound hydrogen atom: the spectral vibrational signature of the single carbon-carbon bonds adjacent to the bound hydrogen atom is highlighted in different colors. Copyright: Frank Hauke, FAU

Additional benefits

“This method of the in-situ Raman spectroscopy is a highly effective technique which allows controlling the function of graphene in a fast, contact-free and extensive way already during the production of the material,” says J. Chacon from Yachay Tech, one of the two lead authors of the study. This enables the production of tailored graphene-based materials with controlled electronic transport properties and their utilisation in semiconductor industry.

Researchers at North Carolina State University have developed a new approach for manipulating the behavior of cells on semiconductor materials, using light to alter the conductivity of the material itself.

“There’s a great deal of interest in being able to control cell behavior in relation to semiconductors – that’s the underlying idea behind bioelectronics,” says Albena Ivanisevic, a professor of materials science and engineering at NC State and corresponding author of a paper on the work. “Our work here effectively adds another tool to the toolbox for the development of new bioelectronic devices.”

The new approach makes use of a phenomenon called persistent photoconductivity. Materials that exhibit persistent photoconductivity become much more conductive when you shine a light on them. When the light is removed, it takes the material a long time to return to its original conductivity.

When conductivity is elevated, the charge at the surface of the material increases. And that increased surface charge can be used to direct cells to adhere to the surface.

“This is only one way to control the adhesion of cells to the surface of a material,” Ivanisevic says. “But it can be used in conjunction with others, such as engineering the roughness of the material’s surface or chemically modifying the material.”

For this study, the researchers demonstrated that all three characteristics can be used together, working with a gallium nitride substrate and PC12 cells – a line of model cells used widely in bioelectronics testing.

The researchers tested two groups of gallium nitride substrates that were identical, except that one group was exposed to UV light – triggering its persistent photoconductivity properties – while the second group was not.

“There was a clear, quantitative difference between the two groups – more cells adhered to the materials that had been exposed to light,” Ivanisevic says.

“This is a proof-of-concept paper,” Ivanisevic says. “We now need to explore how to engineer the topography and thickness of the semiconductor material in order to influence the persistent photoconductivity and roughness of the material. Ultimately, we want to provide better control of cell adhesion and behavior.”

A case study is presented based on the use of high throughput experimentation (HTE) for the discovery of new memory materials.

BY LARRY CHEN, MARK CLARK, CHARLENE CHEN, SUSAN CHENG and MILIND WELING, IMI Inc., San Jose, CA

The ever increasing demands for data translate into more sophisticated and specific thin film requirements for semiconductor materials. Each film layer has to not only demonstrate desired film properties, but also show good interfacial behavior with neighboring layers to contribute to the performance of the whole film stack or device. As a result, modern thin film material systems are including more elements from the periodic table with more complex compositions. The demand for short time to market has also increased, making the development of new materials even more difficult. In this paper, we present a case study of using high throughput experimentation (HTE) for the discovery of new memory materials. By using a combinatorial approach of sputtering technology, HTE can be applied to PVD chalcogenides and other materials targeted at memory semiconductors.
PVD background

Ever since the deposition of materials by magnetron sputtering was introduced by F. M. Penning, the technology has become a major method for industrial thin film deposition, which typically generates dense, hard, and robust thin film materials at relatively low production cost. The technology has been applied to major industries such as semiconductors, photovoltaics, optical coatings, displays, hard mechanical coatings, and so on. However, optimizing the magnetron sputtering processes has always been challenging to process and hardware design engineers, since material properties like density, crystalline structure, grain size, optical indices of a deposited film strongly depend on various process parameters, such as power, pressure, substrate temperature, sputter gas type, plasma type, sputter source to substrate distance, substrate bias, and pumping throughput. Additionally, the material properties heavily depend on the underlying layers, including the chosen substrate, below a film stack due to a texture effect in film structure and a formation of interfacial layers which comes from the intermixing of both materials. All the above parameters contribute to increasing the level of complexity of the development.

The semiconductor industry is constantly searching for new materials with unprecedented physical, optical, electrical, and mechanical properties, not only as a single film but also as a component of complex featured film stacks or functioning devices. This requires exploration of new materials not limited to pure or binary systems, but to ternary, quaternary systems and beyond. A very efficient solution to cope with the increasing complexity of development and the demand for short development time is a combinatorial approach.

The combinatorial approach can be defined as a process that couples the capability for parallel production of large arrays of diverse materials together with different high-throughput measurement techniques for various intrinsic and performance properties supported by data analytics for identifying lead materials [3]. For magnetron sputtering technology, the optimization of process param- eters has to be included as a major component of combinatorial approach. Considering all the multi-dimensional space of the development mentioned above, the combinatorial approach can be an excellent and efficient way of developing new materials in magnetron sputtering in terms of cost and time.

HTE methodology for PVD materials discovery

Platform Considerations As all process parameters in magnetron sputtering are somewhat correlated, it has been challenging for process engineers to come up with fully optimized process parameters for thin film production. In addition, semiconductor production facilities are typically optimized for consistent, efficient, high volume production of a single product at a time, and not for a wide range of simultaneous experiments. These factors make it challenging for memory manufacturers to test multiple materials, conditions and devices in an efficient manner, and without compromising either data quality or production throughput.

IMI’s high throughput experimentation (HTE) platform is set up for accelerated experimentation. Its combina- torial PVD tool typically has four sputter guns and one additional port at the center. All sputter guns can be equipped with various types of target materials including chalcogenides, puremetals, oxides, and nitrides, and each sputter source can be operated by different plasma modes independently, such as direct current (DC), pulsed direct current (PDC), and radio frequency (RF) with the ability to co-sputter with all four guns. The additional port at the center can be equipped with an ion beam source for ion beam assisted deposition, or ion beam cleaning, or an additional sputter gun which enables five gun co-sputtering operation. Process parameter windows can cover larger regimes than most production tool process parameters (Table 1).

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FIGURE 1 shows an example of a multi-target sputter chamber capable of controllably forming a variety of compounds in an array across a 300 mm substrate and an example substrate shown at right. The materials can also be deposited on a die-to-die basis (not shown) over a 300mm wafer test vehicle for direct device testing without the need for patterning. The effectiveness of the combinatorial screening can be increased by guiding the selection of material compositions using both semi-phenomenological and DFT-based modeling, as well as relating the experimental data to the results obtained from simulated annealing using ab-initio molecular dynamics and further DFT analysis of the simulated quasi-amorphous structures.

Deposition methodology

Two different methods can be used to deposit the combinatorial films of interest: site isolated spot and gradient approaches. For the site isolated spot approach, multiple numbers of spots were deposited on a substrate. Each individual spot represents a split condition from a design of experiment (DOE). Film composition can be controlled through the co-sputter of guns, which are equipped with targets consisting of different materials. Also, the process condition of each spot can be varied through the process parameter settings. All deposition conditions and procedures are fully automated.

In the gradient approach, non-uniform film in terms of composition and thickness is intentionally generated on top of a substrate by co-sputtering through an open large area aperture. A semi-empirical model is used for the control of non-uniformity. The modeling also helps in controlling the film composition throughout a target’s lifetime. In this approach, composition gradients and the thickness gradients can be generated by a single film deposition on a substrate. Theoretically, an infinite number of variations can be analyzed within a film, which is only limited by the spatial resolution of metrologies.

Characterization and device performance

Once films have been deposited via PVD, characterization can be carried out, including testing of physical, optical and electrical parameters. These can range from general film characteristics including composition, thickness and crystallinity, to device-specific electrical parameters such as leakage, threshold voltage, and On/ Off ratio.

Measuring and analyzing large numbers of data generated from HTE methodology can be time- consuming. By using the automated metrology tools and a unified database system, measurements and analysis steps can be expedited to limit bottlenecks and deliver data most efficiently. A multi-stage approach can also help to prioritize and focus experimental resources on the most promising candidates.

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HTE vs traditional methods

Key benefits of the HTE approach include the expedited learning cycle, cost reduction, and improved data quality. For semiconductor applications, a single 200mm or 300mm wafer can hold more than 30 splits, which can lead to a reduction in cycle of learning time (one device wafer instead of more than 30). Additionally, as all spots on a single wafer go through the same follow-up device fabrication steps together, data can be free from unexpected fluctuations of subsequent steps. Overall, the HTE approach can expedite the learning cycle by 5 ~ 10 times compared to single substrate based approach. A comparison of both HTE with traditional methods is summarized in Table 2.

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A case study in NVM

New materials for memory elements such as non-volatile memory (NVM) selectors must meet a wide range of performance parameters (FIGURE 3 shows a typical memory cell with the selector element called out), in order to reduce sneak currents and manage variability in memory arrays.

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Table 3 lists some of the key parameters desired in a memory selector material.

Of course, optimizing all of these parameters simultaneously in a single element or compound (and one that is practical for high volume memory manufacturing) is challenging. IMI’s HTE methodology enables rapid and simultaneous optimization of key trade-offs between performance, reliability and integration, in the quest for an ideal selector.

HTE for NVM selector materials

Use of a HTE methodology allows rapid screening of NVM selector candidate material compounds, compo- sitions and stacks. IMI has conducted multiple customer engagements in memory selector materials screening, and a typical experimental workflow is outlined in FIGURE 4, showing progression from PVD deposition, through physical and electrical characterizations of films and devices.

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This experimental process can be carried out multiple times, through subsequently more advanced stages on a fewer number of samples, as promising candidates are narrowed down and further optimized. FIGURE 5 shows a possible strategy for testing a series of candi- dates through three different stages. In the earlier stages, a wide range of options could be screened quickly, but the more extensive (and time consuming) characterization and analysis can be saved for later stages, when only the best performing candidates are already selected. This enables the best use of deposition and testing resources, leading to optimal results in an efficient timeframe.

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Fast and high-quality experimental results

IMI has extensive experience in working both on dynamic random access memory (DRAM) as well as NVM materials. In DRAM, the company has worked on development of dielectric, electrode and interface layer materials. IMI’s process engineers, materials scientists and electrical engineers work upfront with a customer on the design of experiments to ensure the delivery of rapid cycles of learning with the most efficient use of resources.

A typical customer project might range between a few months up to a year or more, encompassing hundreds or even thousands of different experiments. In NVM selectors alone, IMI has conducted:
• 2500+experiments on Metal Chalcogenides
• 2000+ experiments on MIEC
• 1000+experiments on Transition Metal Oxides

Conclusion

High throughput experimentation can offer rapid, high quality materials data when effectively applied to PVD memory selector development. However it does require an advanced platform, and a facility and team experienced in efficient deposition and testing of the materials and devices. Materials and device expertise is also helpful in managing and optimizing the experimental workflow for maximum efficiency and high quality data.