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2D Materials May Be Brittle

Tuesday, November 15th, 2016


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.”


“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.


3DIC Technology Drivers and Roadmaps

Monday, June 22nd, 2015


By Ed Korczynski, Sr. Technical Editor

After 15 years of targeted R&D, through-silicon via (TSV) formation technology has been established for various applications. Figure 1 shows that there are now detailed roadmaps for different types of 3-dimensional (3D) ICs well established in industry—first-order segmentation based on the wiring-level/partitioning—with all of the unit-processes and integration needed for reliable functionality shown. Using block-to-block integration with 5 micron lines at leading international IC foundries such as GlobalFoundries, systems stacking logic and memory such as the Hybrid Memory Cube (HMC) are now in production.

Fig. 1: Today’s 3D technology landscape segmented by wiring-level, showing cross-sections of typical 2-tier circuit stacks, and indicating planned reductions in contact pitches. (Source: imec)

“There are interposers for high-end complex SOC design with good yield,” informed Eric Beyne, Scientific Director Advanced Packaging & Interconnect for imec in an exclusive interview with Solid State Technology. ““For a systems company, once you’ve made the decision to go 3D there’s no way back,” said Beyne. “If you need high-bandwidth memory, for example, then you’re committed to some sort of 3D. The process is happening today.” Beyne is scheduled to talk about 3D technology driven by 3D application requirements in the imec Technology Forum to be held July 13 in San Francisco.

Adaptation of TSV for stacking of components into a complete functional system is key to high-volume demand. Phil Garrou, packaging technologist and SemiMD blogger, reported from the recent ConFab that Hynix is readying a second generation of high-bandwidth memory (HBM 2) for use in high performance computing (HPC) such as graphics, with products already announced like Pascal from Nvidia and Greenland from AMD.

For a normalized 1 cm2 of silicon area, wide-IO memory needs 1600 signal pins (not counting additional power and ground pins) so several thousand TSV are needed for high-performance stacked DRAM today, while in more advanced memory architectures it could go up by another factor of 10. For wide-IO HVM-2 (or Wide-IO2) the silicon consumed by IO circuitry is maybe 6 cm2 today, such that a 3D stack with shorter vertical connections would eliminate many of the drivers on the chip and would allow scaling of the micro-bumps to perhaps save a total of 4 cm2 in silicon area. 3D stacks provide such trade-offs between design and performance, so the best results are predicted for 3DICs where the partitioning can be re-done at the gate or transistor level. For example, a modern 8-core microprocessor could have over 50% of the silicon area consumed by L3-cache-memory and IO circuitry, and moving from 2D to 3D would reduce total wire-lengths and interconnect power consumptions by >50%.

There are inherent thresholds based on the High:Width ratio (H:W) that determine costs and challenges in process integration of TSV:

-    10:1 ratio is the limit for the use of relatively inexpensive physical vapor deposition (PVD) for the Cu barrier/seed (B/S),

-    20:1 ratio is the limit for the use of atomic-layer deposition (ALD) for B/S and electroless deposition (ELD) for Cu fill with 1.5 x 30 micron vias on the roadmap for the far future,

-    30:1 ratio and greater is unproven as manufacturable, though novel deposition technologies continue to be explored.

TSV Processing Results

The researchers at imec have evaluated different ways of connecting TSV to underlying silicon, and have determined that direct connections to micro-bumps are inherently superior to use of any re-distribution layer (RDL) metal. Consequently, there is renewed effort on scaling of micro-bump pitches to be able to match up with TSV. The standard minimum micro-bump pitch today of 40 micron has been shrunk to 20, and imec is now working on 10 micron with plans to go to 5 micron. While it may not help with TSV connections, an RDL layer may still be needed in the final stack and the Cu metal over-burden from TSV filling has been shown by imec to be sufficiently reproducible to be used as the RDL metal. The silicon surface area covered by TSV today is a few percents not 10s of percents, since the wiring level is global or semi-global.

Regarding the trade-offs between die-to-wafer (D2W) and wafer-to-wafer (W2W) stacking, D2W seems advantageous for most near-term solutions because of easier design and superior yield. D2W design is easier because the top die can be arbitrarily smaller silicon, instead of the identically sized chips needed in W2W stacks. Assuming the same defectivity levels in stacking, D2W yield will almost always be superior to W2W because of the ability to use strictly known-good-die. Still, there are high-density integration concepts out on the horizon that call for W2W stacking. Monolithic 3D (M3D) integration using re-grown active silicon instead of TSV may still be used in the future, but design and yield issues will be at least comparable to those of W2W stacking.

Beyne mentioned that during the recent ECTC 2015, EV Group showed impressive 250nm overlay accuracy on 450mm wafers, proving that W2W alignment at the next wafer size will be sufficient for 3D stacking. Beyne is also excited by the fact the at this year’s ECTC there was, “strong interest in thermo-compression bonding, with 18 papers from leading companies. It’s something that we’ve been working on for many years for die-to-wafer stacking, while people had mistakenly thought that it might be too slow or too expensive.”

Thermal issues for high-performance circuitry remain a potential issue for 3D stacking, particularly when working with finFETs. In 2D transistors the excellent thermal conductivity of the underlying silicon crystal acts like a built-in heat-sink to diffuse heat away from active regions. However, when 3D finFETs protrude from the silicon surface the main path for thermal dissipation is through the metal lines of the local interconnect stack, and so finFETs in general and stacks of finFETs in particular tend to induce more electro-migration (EM) failures in copper interconnects compared to 2D devices built on bulk silicon.

3D Designs and Cost Modeling

At a recent North California Chapter of the American Vacuum Society (NCCAVS) PAG-CMPUG-TFUG Joint Users Group Meeting discussing 3D chip technology held at Semi Global Headquarters in San Jose, Jun-Ho Choy of Mentor Graphics Corp. presented on “Electromigration Simulation Flow For Chip-Scale Parametric Failure Analysis.” Figure 2 shows the results from use of a physics-based model for temperature- and residual-stress-aware void nucleation and growth. Mentor has identified new failure mechanisms in TSV that are based on coefficient of thermal expansion (CTE) mismatch stresses. Large stresses can develop in lines near TSV during subsequent thermal processing, and the stress levels are layout dependent. In the worst cases the combined total stress can exceed the critical level required for void nucleation before any electrical stressing is applied. During electrical stress, EM voids were observed to initially nucleate under the TSV centers at the landing-pad interfaces even though these are the locations of minimal current-crowding, which requires proper modeling of CTE-mismatch induced stresses to explain.

Fig. 2: Calibration of an Electronic Design Automation (EDA) tool allows for accurate prediction of transistor performance depending on distance from a TSV. (Source: Mentor Graphics)

Planned for July 16, 2015 at SEMICON West in San Francisco, a presentation on “3DIC Technology Past, Present and Future” will be part of one of the side Semiconductor Technology Sessions (STS). Ramakanth Alapati, Director of Packaging Strategy and Marketing, GLOBALFOUNDRIES, will discuss the underlying economic, supply chain and technology factors that will drive productization of 3DIC technology as we know it today. Key to understanding the dynamic of technology adaptation is using performance/$ as a metric.