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2D Materials May Be Brittle

Tuesday, November 15th, 2016

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By Ed Korczynski, Sr. Technical Editor

International researchers using a novel in situ quantitative tensile testing platform have tested the uniform in-plane loading of freestanding membranes of 2D materials inside a scanning electron microscope (SEM). Led by materials researchers at Rice University, the in situ tensile testing reveals the brittle fracture of large-area molybdenum diselenide (MoSe2) crystals and measures their fracture strength for the first time. Borophene monolayers with a wavy topography are more flexible.

A communication to Advanced Materials online (DOI: 10.1002/adma.201604201) titled “Brittle Fracture of 2D MoSe2” by Yinchao Yang et al. disclosed work by researchers from the USA and China led by Department of Materials Science and NanoEngineering Professor Jun Lou at Rice University, Houston, Texas. His team found that MoSe2 is more brittle than expected, and that flaws as small as one missing atom can initiate catastrophic cracking under strain.

“It turns out not all 2D crystals are equal. Graphene is a lot more robust compared with some of the others we’re dealing with right now, like this molybdenum diselenide,” says Lou. “We think it has something to do with defects inherent to these materials. It’s very hard to detect them. Even if a cluster of vacancies makes a bigger hole, it’s difficult to find using any technique.” The team has posted a short animation online showing crack propagation.

2D Materials in a 3D World -222

While all real physical things in our world are inherently built as three-dimensional (3D) structures, a single layer of flat atoms approximates a two-dimensional (2D) structure. Except for special superconducting crystals frozen below the Curie temperature, when electrons flow through 3D materials there are always collisions which increase resistance and heat. However, certain single layers of crystals have atoms aligned such that electron transport is essentially confined within the 2D plane, and those electrons may move “ballistically” without being slowed by collisions.

MoSe2 is a dichalcogenide, a 2D semiconducting material that appears as a graphene-like hexagonal array from above but is actually a sandwich of Mo atoms between two layers of Se chalcogen atoms. MoSe2 is being considered for use as transistors and in next-generation solar cells, photodetectors, and catalysts as well as electronic and optical devices.

The Figure shows the micron-scale sample holder inside a SEM, where natural van der Waals forces held the sample in place on springy cantilever arms that measured the applied stress. Lead-author Yang is a postdoctoral researcher at Rice who developed a new dry-transfer process to exfoliate MoSe2 from the surface upon which it had been grown by chemical vapor deposition (CVD).

Custom built micron-scale mechanical jig used to test mechanical properties of nano-scale materials. (Source: Lou Group/Rice University)

The team measured the elastic modulus—the amount of stretching a material can handle and still return to its initial state—of MoSe2 at 177.2 (plus or minus 9.3) gigapascals (GPa). Graphene is more than five times as elastic. The fracture strength—amount of stretching a material can handle before breaking—was measured at 4.8 (plus or minus 2.9) GPa. Graphene is nearly 25 times stronger.

“The important message of this work is the brittle nature of these materials,” Lou says. “A lot of people are thinking about using 2D crystals because they’re inherently thin. They’re thinking about flexible electronics because they are semiconductors and their theoretical elastic strength should be very high. According to our calculations, they can be stretched up to 10 percent. The samples we have tested so far broke at 2 to 3 percent (of the theoretical maximum) at most.”

Borophene

“Wavy” borophene might be better, according to finding of other Rice University scientists. The Rice lab of theoretical physicist Boris Yakobson and experimental collaborators observed examples of naturally undulating metallic borophene—an atom-thick layer of boron—and suggested that transferring it onto an elastic surface would preserve the material’s stretchability along with its useful electronic properties.

Highly conductive graphene has promise for flexible electronics, but it is too stiff for devices that must repeatably bend, stretch, compress, or even twist. The Rice researchers found that borophene deposited on a silver substrate develops nanoscale corrugations, and due to weak binding to the silver can be exfoliated for transfer to a flexible surface. The research appeared recently in the American Chemical Society journal Nano Letters.

Rice University has been one of the world’s leading locations for the exploration of 1D and 2D materials research, ever since it was lucky enough to get a visionary genius like Richard Smalley to show up in 1976, so we should expect excellent work from people in their department of Materials Science and NanoEngineering (CSNE). Still, this ground-breaking work is being done in labs using tools capable of handling micron-scale substrates, so even after a metaphorical “path” has been found it will take a lot of work to build up a manufacturing roadway capable of fabricating meter-scale substrates.

—E.K.

Silicon as Disruptive Platform for IoT Applications

Monday, August 29th, 2016

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By Ed Korczynski, Sr. Technical Editor

Marie Semeria, chief executive officer of CEA-Leti (http://www.leti.fr/en), sat down with SemiMD during SEMICON West to discuss how the French R&D and pilot manufacturing campus—located at the foot of the beautiful French alps near Grenoble—is expanding the scope of it’s activities to develop systems solutions for the Internet-of-Things (IoT). Part-1 on hardware/software co-development was published last month.

Korczynski: Regarding ‘IoT’ applications, we expect that chips must be very low cost to be successful, and at the same time the ultimately winning solutions will be those that combine the best functionalities from different technology spaces each in a ‘sweet spot’ of cost to performance. It seems that being able to do it on SOI wafers could produce the right volumes.

Semeria: Yes. It could be enough.

Korczynski: Do you have any feel in advance for how much area of silicon is needed? Some small ADC, an 8-bit micro-controller, and RF components may be done in different processes and then integrated. Is it possible that the total area of silicon needed could be less than a square millimeter?

Semeria: Yes.

Korczynski: Well, if they are that small then we have to remember how many units we’d get from just a single wafer, and there are 24 wafers in a batch…

Semeria: One batch can be enough for one market, depending upon the application.

Korczynski: If this is the case, then even though the concept of purely-additive roll-to-roll processes are attractive, oddly they may be too efficient and produce more units than the world can absorb. If we can do all that we need to do with established silicon wafer fab technology creating ICs smaller than a square millimeter then it will be very cost-effective.

Semeria: Leti’s strategy is to keep the performance of solid-state devices, so not to go to organic electronics. Use silicon as the differentiator to lower the cost, add more functions, and then miniaturize all that can be miniaturized. In this way we are achieving integration of MEMS with small electronics in arrays as small as one millimeter square. When you deal with such small die you can put them inside of flexible materials, inside of a t-shirt and it’s no problem. So that’s our strategy to keep small silicon and put it in clothes, in shoes, in windows, in glasses, and all sorts of flexible materials. When you are thinning substrates for bonding, then the thinned silicon is very flexible.

Korczynski: In 1999 I worked for one of the first companies selling through-silicon via technology, and it was all about backside thinning so I’ve played with flexible wafers.

Semeria: So you know what I mean.

Korczynski: Around 50 microns and below as long as you etch away any grinding defects from the backside it is very strong and very flexible (Fig. 1). At 50 microns the chip is still thick enough to be easily picked-and-placed, but it’s flexible. Below 10 microns the wafer is difficult to handle.

FIGURE 1: 50 micron thin silicon wafers can be strong and very flexible. (Source: Virginia Semiconductor)

Semeria: To maintain the advantage of cost for different applications spaces, we are developing the ‘chiplet’ approach which means a network of chips. It starts with a digital platform, then you add an active interposer to connect different dice. For example you could have 28nm-node on the bottom and a 14nm-node chip on top for some specific function. Then you can put embedded memory and RF connected through the interposer, and it’s the approach that we promote for the first generation of multi-functional integration on digital. Very flexible, cost-effective.

Korczynski: This is using some sort of bus to move information?

Semeria: Yes, this will be an electronic bus for the first generation, as we recently announced. Then a photonics interposer could be used for higher-speed data rate in a future generation. We have a full roadmap with different types of integration schemes. So it’s a way to combine all with silicon. Everything is intended to be integrated into existing 300mm silicon facilities. Some weeks ago we presented the first results showing silicon quantum bits built on 300mm substrates, and fully compatible with CMOS processing. So it’s the way we are going, taking a very disruptive approach using the foundation of proven 300mm silicon processing.

Korczynski: Interesting.

Semeria: For example, regarding driving assistance applications we have to consider fusion integration of different sensors, and complete coverage of the environment with low power-consumption. For computing capacity we developed a completely disruptive approach, very different from Intel and very different from nVidia which use consumer products as the basis for automotive application products. Specifically for automotive we developed a new probabilistic methodology to avoid all of the calculations based on floating-point. In this way we can divide the computing needs of the device by 100, so it’s another example of developing just the right device for the right application adapted for the right environment. So the approach is very different in development for IoT instead of mainstream CMOS.

Korczynski: For automotive there’s such a requirement for reliability, with billions of dollars at stake in product recalls and potential lawsuits, the auto industry is very risk-averse for very good reasons. So historically they’ve always used trailing-edge nodes, and if you want to supply to them you have to commit to 10 or maybe 20 years of manufacturing, and yet we still want to add in advance functionalities. The impression I’ve gotten is that the 28nm FD-SOI platform is fairly ideal here.

Semeria: FD-SOI is very reliable and very efficient. That’s why when we showed our demonstrator at the recent DAC it’s based on the STMicroelectronics micro-controller. It’s very reliable and adaptable for automotive applications.

Korczynski: Is it at 28nm?

Semeria: No, about 40nm now. The latest generation is not needed, because we changed the algorithms so we didn’t need so much capacity in computing. In IoT there is space to use 40nm or 32nm down to 28nm. It’s a great space to use ‘old technologies’ and optimize them with the right algorithms, the right signal-processing, and the right security. So it’s very exciting for Leti because we have all of the key competencies to be able to handle the IoT challenge, and there is a great ability to make various integration schemes depending upon the application. There is a very large space to demonstrate, and to develop new materials.

Korczynski: Does this relate to some recent work I’ve seen from Leti with micro-cantilevers?

Semeria: Yes, this is the work we are doing with CalTech on micro-resonators (Fig. 2).

FIGURE 2: MEMS/NEMS silicon cantilever resonator capable of detecting individual adhered molecules, for integration with digital CMOS in a complete IoT sensing system. (Source: Leti)

Korczynski: Thank you very much for taking the time to discuss these important trends.

Semeria: It is a pleasure.

—E.K.

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