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Posts Tagged ‘TFET’

Technologies for Advanced Systems Shown at IMEC Tech Forum USA

Tuesday, July 14th, 2015

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By Ed Korczynski, Sr. Technical Editor

Luc Van den hove, president and CEO, imec opened the Imec Technology Forum – USA in San Francisco on July 13 by reminding us of the grand vision and motivation behind the work of our industry to empower individuals with micro- and nano-technologies in his talk, “From the happy few to the happy many.” While the imec consortium continues to lead the world in pure materials engineering and device exploration, they now work on systems-integration complexities with over 100 applications partners from agriculture, energy, healthcare, and transportation industries.

We are now living in an era where new chip technologies require trade-offs between power, performance, and bandwidth, and such trade-offs must be carefully explored for different applications spaces such as cloud clusters or sensor nodes. An Steegen, senior vice president process technology, imec, discussed the details of new CMOS chip extensions as well as post-CMOS device possibilities for different applications spaces in her presentation on “Technology innovation: an IoT era.” EUV lithography technology continues to be developed, targeting a single-exposure using 0.33 Numerical Aperture (NA) reflective lenses to pattern features as small as 18nm half-pitch, which would meet the Metal1 density specifications for the industry’s so-called “7nm node.” Patterning below 12nm half-pitch would seem to need higher-NA which is not an automatic extension of current EUV technology.

So while there is now some clarity regarding the pre-competitive process-technologies that will be needed to fabricate next-generation device, there is less clarity regarding which new device structures will best serve the needs of different electronics applications. CMOS finFETs using strained silicon-doped-with-Germanium Si(Ge) will eventually be replaced by gate-all-around (GAA) nano-wires (NW) using alternate-channel materials (ACM) with higher mobilities such as Ge and indium-gallium-arsenide (InGaAs). While many measures of CMOS performance improve with scaling to smaller dimensions, eventually leakage current and parasitic capacitances will impede further progress.

Figure 1 shows a summary of energy-vs.-delay analyses by imec for all manner of devices which could be used as switches in logic arrays. Spin-wave devices such as spin-transfer-torque RAM (STT-RAM) can run at low power consumption but are inherently slower than CMOS devices. Tunnel-FET (TFET) devices can be as fast or faster than CMOS while running at lower operating power due to reduced electrostatics, leading to promising R&D work.

Fig.1: Energy vs. delay for various logic switches. (Source: imec)

In an exclusive interview, Steegen explained how the consortium balances the needs of all partners in R&D, “When you try to predict future roadmaps you prefer to start from the mainstream. Trying to find the mainstream, so that customers can build derivatives from that, is what imec does. We’re getting closer to systems, and systems are reaching down to technology,” said Steegen. “We reach out to each other, while we continue to be experts in our own domains. If I’m inserting future memory into servers, the system architecture needs to change so we need to talk to the systems people. It’s a natural trend that has evolved.”

Network effects from “the cloud” and from future smart IoT nets require high-bandwidth and so improved electrical and optical connections at multiple levels are being explored at imec. Joris Van Campenhout, program director optical I/O, imec, discussed “Scaling the cloud using silicon photonics.” The challenge is how to build a 100Gb/s bandwidth in the near term, and then scale to 400G and then 1.6T though parallelism of wavelength division multiplexing; the best results to date for a transmitter and receiver reach 50Gb/s. By leveraging the existing CMOS manufacturing and 3-D assembly infrastructure, the hybrid CMOS silicon photonics platform enables high integration density and reduced power consumption, as well as high yield and low manufacturing cost. Supported by EDA tools including those from Mentor Graphics, there have been 7 tape-outs of devices in the last year using a Process Design Kit (PDK). When combined with laser sources and a 40nm node foundry CMOS chip, a complete integrated solution exists. Arrays of 50Gb/s structures can allow for 400Gb/s solutions by next year, and optical backplanes for server farms in another few years. However, to bring photonics closer to the chip in an optical interposer will require radical new new approaches to reduce costs, including integration of more efficient laser arrays.

Alexander Mityashin, project manager thin film electronics, imec, explained why we need, “thin film electronics for smart applications.” There are billions of items in our world that could be made smarter with electronics, provided we can use additive thin-film processes to make ultra-low-cost thin-film transistors (TFT) that fit different market demands. Using amorphous indium-gallium-zinc-oxide (a-IGZO) deposited at low-temperature as the active layer on a plastic substrate, imec has been able to produce >10k TFTs/cm2 using just 4-5 lithography masks. Figure 2 shows these TFT integrated into a near-field communications (NFC) chip as first disclosed at ISSCC earlier this year in the paper, “IGZO thin-film transistor based flexible NFC tags powered by commercial USB reader device at 13.56MHz.” Working with Panasonic in 2013, imec showed a flexible organic light-emitting diode (OLED) display of just 0.15mm thickness that can be processed at 180°C. In collaboration with the Holst Center, they have worked on disposable flexible sensors that can adhere to human skin.

Fig.2: Thin-Film Transistors (TFT) fabricated on plastic using Flat Panel Display (FPD) manufacturing tools. (Source: imec/Holst Center)

Jim O’Neill, Chief Technology Officer of Entegris, expanded on the systems-level theme of the forum in his presentation on “Putting the pieces together – Materials innovation in a disruptive environment.” With so many additional materials being integrated into new device structures, there are inherently new yield-limiting defect mechanisms that will have to be controlled. With demand for chips now being driven primarily by high-volume consumer applications, the time between first commercial sample and HVM has compressed such that greater coordination is needed between device, equipment, and materials companies. For example, instead of developing a wet chemical formulation on a tool and then optimizing it with the right filter or dispense technology, the Process Engineer can start envisioning a “bottle-to-nozzle wetted surface solution.” By considering not just the intended reactions on the wafer but the unintended reactions that can occur up-steam and down-stream of the process chamber, full solutions to the semiconductor industry’s most challenging yield problems can be more quickly found.

—E.K.

5nm Node Needs EUV for Economics

Thursday, January 29th, 2015

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By Ed Korczynski, Sr. Technical Editor

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At IEDM 2014 last month in San Francisco, Applied Materials sponsored an evening panel discussion on the theme of “How do we continue past 7nm?” Given that leading fabs are now ramping 14nm node processes, and exploring manufacturing options for the 10nm node, “past 7nm” means 5nm node processing. There are many device options possible, but cost-effective manufacturing at this scale will require Extreme Ultra-Violet (EUV) lithography to avoid the costs of quadruple-patterning.

Fig. 1: Panelists discuss future IC manufacturing and design possibilities in San Francisco on December 16, 2014. (Source: Pete Singer)

Figure 1 shows the panel being moderated by Professor Mark Rodwell of the University of California Santa Barbara, composed of the following industry experts:

  • Karim Arabi, Ph.D. – vice president, engineering, Qualcomm,
  • Michael Guillorn, Ph.D. – research staff member, IBM,
  • Witek Maszara, Ph.D. – distinguished member of technical staff, GLOBALFOUNDRIES,
  • Aaron Thean, Ph.D. – vice president, logic process technologies, imec, and
  • Satheesh Kuppurao, Ph.D. – vice president, front end products group, Applied Materials.

Arabi said that from the design perspective the overarching concern is to keep “innovating at the edge” of instantaneous and mobile processing. At the transistor level, the 10nm node process will be similar to that at the 14nm node, though perhaps with alternate channels. The 7nm node will be an inflection point with more innovation needed such as gate-all-around (GAA) nanowires in a horizontal array. By the 5nm node there’s no way to avoid tunnel FETs and III-V channels and possibly vertical nanowires, though self-heating issues could become very challenging. There’s no shortage of good ideas in the front end and lots of optimism that we’ll be able to make the transistors somehow, but the situation in the backend of on-chip metal interconnect is looking like it could become a bottleneck.

Guillorn extolled the virtues of embedded-memory to accelerate logic functions, as a great example of co-optimization at the chip level providing a real boost in performance at the system level. The infection at 7nm and beyond could lead to GAA Carbon Nano-Tube (CNT) as the minimum functional device. It’s limited to think about future devices only in terms of dimensional shrinks, since much of the performance improvement will come from new materials and new device and technology integration. In addition to concerns with interconnects, maintaining acceptable resistance in transistor contacts will be very difficult with reduced contact areas.

Maszara provided target numbers for a 5nm node technology to provide a 50% area shrink over 7nm:  gate pitch of 30nm, and interconnect level Metal 1 (M1) pitch of 20nm. To reach those targets, GLOBALFOUNDRIES’ cost models show that EUV with ~0.5 N.A. would be needed. Even if much of the lithography could use some manner of Directed Self-Assembly (DSA), EUV would still be needed for cut-masks and contacts. In terms of device performance, either finFET or nanowires could provide desired off current but the challenge then becomes how to get the on current for intended mobile applications? Alternative channels with high mobility materials could work but it remains to be seen how they will be integrated. A rough calculation of cost is the number of mask layers, and for 5nm node processing the cost/transistor could still go down if the industry has ideal EUV. Otherwise, the only affordable way to go may be stay at 7nm node specs but do transistor stacking.

Thein detailed why electrostatic scaling is a key factor. Parasitics will be extraordinary for any 5nm node devices due to the intrinsically higher number of surfaces and junctions within the same volume. Just the parasitic capacitances at 7nm are modeled as being 75% of the total capacitance of the chip. The device trend from planar to finFET to nanowires means proportionally increasing relative surface areas, which results in inherently greater sensitivity to surface-defects and interface-traps. Scaling to smaller structures may not help you if you loose most of the current and voltage in non-useful traps and defects, and that has already been seen in comparisons of III-V finFETs and nanowires. Also, 2D scaling of CMOS gates is not sustainable, and so one motivation for considering vertical transistors for logic at 5nm would be to allow for 20nm gates at 30nm pitch.

Kappurao reminded attendees that while there is still uncertainty regarding the device structures beyond 7nm, there is certainty in 4 trends for equipment processes the industry will need:

  1. everything is an interface requiring precision materials engineering,
  2. film depositions are either atomic-layer or selective films or even lattice-matched,
  3. pattern definition using dry selective-removal and directed self-assembly, and
  4. architecture in 3D means high aspect-ratio processing and non-equilibrium processing.

An example of non-equilibrium processing is single-wafer rapid-thermal-annealers (RTA) that today run for nanoseconds—providing the same or even better performance than equilibrium. Figure 2 shows that a cobalt-liner for copper lines along with a selective-cobalt cap provides a 10x improvement in electromigration compared to the previous process-of-record, which is an example of precision materials engineering solving scaling performance issues.

Fig. 2: ElectroMigration (EM) lifetimes for on-chip interconnects made with either conventional Cu or Cu lined and capped with Co, showing 10 times improvement with the latter. (Source: Applied Materials)

“We have to figure out how to control these materials,” reminded Kappurao. “At 5nm we’re talking about atomic precision, and we have to invent technologies that can control these things reliably in a manufacturable manner.” Whether it’s channel or contact or gate or interconnect, all the materials are going to change as we keep adding more functionality at smaller device sizes.

There is tremendous momentum in the industry behind density scaling, but when economic limits of 2D scaling are reached then designers will have to start working on 3D monolithic. It is likely that the industry will need even more integration of design and manufacturing, because it will be very challenging to keep the cost-per-function decreasing. After CMOS there are still many options for new devices to arrive in the form of spintronics or tunnel-FETs or quantum-dots.

However, Arabi reminded attendees as to why the industry has stayed with CMOS digital synchronous technology leading to design tools and a manufacturing roadmap in an ecosystem. “The industry hit a jackpot with CMOS digital. Let’s face it, we have not even been able to do asynchronous logic…even though people tried it for many years. My prediction is we’ll go as far as we can until we hit atomic limits.”


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