1. IBM Research scientist Nicolas Loubet holds a wafer of chips with 5nm silicon nanosheet transistors. (Image courtesy of IBM).

Semiconductor Lithography Plays Role in 5nm Transistors and Improved Li-Ion Battery

Large-scale adoption of wearable devices will depend on availability of two important technologies: improved batteries and higher density transistor circuits. These two technologies have been made possible by semiconductor lithography.

Announced at the 2017 Symposia on VLSI Technology and Circuits conference in Kyoto, Japan, IBM and its research alliance partners, GlobalFoundries and Samsung described a 5 nm transistor. This silicon nanosheet transistor was detailed in the Research Alliance paper Stacked Nanosheet Gate-All-Around Transistor to Enable Scaling Beyond FinFET.

Scientists working as part of the IBM-led Research Alliance at the SUNY Polytechnic Institute Colleges of Nanoscale Science and Engineering’s NanoTech Complex in Albany, N.Y., achieved the breakthrough by using stacks of silicon nanosheets as the device structure of the transistor, instead of the standard FinFET architecture, which is the blueprint for the semiconductor industry up through 7nm node technology. This paves the way for 30 billion switches on a fingernail-sized chip. Compared to the leading-edge 10nm technology available  today, a nanosheet-based 5nm technology can deliver 40% performance enhancement at fixed power, or 75% power savings at matched performance (Fig. 1).

IBM Research has explored nanosheet semiconductor technology for more than 10 years. Now, this current work will demonstrate the feasibility of designing and fabricating stacked nanosheet devices with electrical properties superior to current FinFET architecture.

To achieve a 5nm transistor, Extreme Ultraviolet (EUV) lithography was applied to the nanosheet transistor architecture. With EUV lithography, you can continuously adjust the width of the nanosheets, all within a single manufacturing process or chip design. This adjustability permits the fine-tuning of performance and power for specific circuits. This is not possible with today’s FinFET transistor architecture production that is limited by its current-carrying fin height. Therefore, while FinFET chips can scale to 5nm, simply reducing the amount of space between fins does not provide increased current flow for additional performance.

This new chip technology modifies Moore’s Law that breaks down into four parts: density, performance, power, and economy. Density, or the number of transistors per square inch of a chip, has gone from Gordon Moore’s original observation in 1965 of doubling every 12 months, to now taking three years. Performance improvements have experienced a similar slowing. Power, while less influential at its introduction, has grown in importance due to battery-hungry mobile devices. The economy, or cost per transistor, is the only element of the “law” that’s kept similar pace over the last 50 years. A future 5nm node chip with nanosheet transistors, and its scaled density, will deliver the expected value of performance, power, and economy.

2. Enovix 3D Silicon Lithium-ion Battery Platform. (Image courtesy of Enovix).

Semiconductor technology and conventional battery technology are rooted in two different fields of physical science; as a result, they are evolving at different rates. Computer processing capacity was said to double about every two years in accordance with Moore’s Law (not exactly). An exponential improvement is possible because processor performance is dictated by physics and determined by modern photolithographic techniques used in the fabrication of semiconductors.

Chemical ions transfer charge in batteries, and electrochemical reactions governed by electrode materials dictate the energy capacity.  With a conventional Li-ion battery platform, all critical performance characteristics—electrode materials, design, production, and packaging—are determined by chemistry. A change in one characteristic is inextricably linked with the others, and this narrows flexibility and limits performance improvement variables.

Enovix has built a new battery platform with an orthogonal approach that extends beyond chemistry to modern science and technology disciplines. Figure 2 illustrates how the Enovix 3D SiliconTM Lithium-ion Battery platform decouples the performance characteristics, which enables much greater innovation in materials, design, production, and packaging. With this approach, incremental improvements in each of these separate characteristics can collectively produce significant overall increases in battery energy density.

As a result, the Enovix 3D Silicon Lithium-ion Battery delivers an immediate improvement in performance for mobile devices, especially wearables. In addition, the new, modern platform enables continual innovation and significant battery performance improvements in a range of electric-powered mobile products for years to come.

3. Enovix 3D Silicon Lithium-ion Battery wafer processing. (Image courtesy of Enovix).

There are three active components in a rechargeable (secondary) Li-ion battery, the cathode (positive electrode), anode (negative electrode), and electrolyte. In a conventional Li-ion battery, the cathode and anode electrodes are fabricated by coating the active power materials in chemical slurries onto metal foil current collectors. Sheet like electrodes are then wound together, with a polymer separator between them that prevents shorting. Pores in the separator allow the electrolyte to convey ions between the anode and cathode. This naturally cylindrical “jelly roll” configuration is then packaged in a cylindrical metal can or flattened into a pseudo-prismatic configuration and packaged in either a polymer laminate pouch or metal case.

Within the resulting battery structure, the only materials that store energy are the active anode and cathode powders; all other materials are inactive. Collectively, the inactive materials typically comprise about 35% of the total battery weight and about 40% of the total battery volume. Although necessary for battery construction, from an energy density perspective, inactive materials are simply inert waste.

Enovix now uses a patented 3D cell architecture instead of the 2D microfabrication that was initially used to miniaturize electronic devices. The Enovix 3D cell architecture is inherently rectangular and utilizes thinner deposited metal (rather than thicker foil) current collectors, which reduces inactive material and eliminates the dead inner-cell winding core. This significantly increases the proportion of active- to-inactive materials, so that only about 25% of the total battery weight and volume is inactive.

The Enovix 3D cell architecture also provides several safety features not obtainable with a conventional Li-ion battery structure:

·     Conformal ceramic separators that tolerate higher temperatures than conventional polymeric sheets.

·     Excess capacity in the patented silicon anode reduces the risk for lithium metal plating during overcharge conditions.

·     Distributed collectors limit current, and if an electrical short occurs, physically segregated cathodes limit propagation of material breakdown.

·     High thermal dissipation mitigates local hot spots.

·     Collectively, the features of the Enovix 3D cell architecture reduce the danger of thermal runaway.

The Enovix 3D Silicon Lithium-ion Battery is fabricated using a photolithographic mask and etch process applied to a solar-grade silicon (SGSi) substrate. A low-cost wafer production process—initially pioneered for photovoltaic solar cells—enables reliable, affordable high-volume battery production (Fig. 3).

Graphite, a crystalline structure of pure carbon, is used as an anode compound for the conventional Li-ion batteries used in most mobile devices. Lithium is absorbed by graphite when the battery is charging and emitted as the battery is discharged (in use).

4. Enovix 3D Silicon Lithium-ion Battery energy density comparison. (Image courtesy of Enovix).

In contrast, a silicon anode has the potential to store ten times more lithium than a graphite anode, which, in theory, can increase the energy capacity of a conventional Li-ion cell by up to 40%. However, silicon’s absorption of lithium during charging can lead to its expansion, fracture, and eventual disconnection from the electrode, causing a rapid decrease in the ability of the cell to accept a charge. Silicon-graphite composite anode materials have shown promise for practical application in Li-ion batteries, but any benefit of the silicon is proportionally reduced by the ratio of graphite to silicon.

As part of its 3D architectural approach, Enovix developed a patented anode comprised of micro-structured porous silicon. The 100% silicon anode delivers a significant energy density increase over a graphite anode at millimeter cell thickness, and it overcomes the problems of first-cycle loss, battery swelling, and poor cycle life. Enovix has been able to achieve this improvement in large part because of its 3D cell structure, which also enables high-volume, low-cost automated production. Furthermore, this structure enables its battery platform to fully utilize future advances in a broad range of cathode materials.

Rather than working on cell chemistry, Enovix is focused on improving the cell structure. Its 3D cell structure is inherently flat, and the wafer production and packaging process is very similar to that used to produce integrated circuits. Therefore, the Enovix 3D Silicon Lithium-ion Battery can be configured in a similar manner to the other microelectronic components and optimized for mobile, portable product performance. Figure 4 shows the initial energy density improvement from the Enovix 3D Silicon Lithium-ion Battery (blue), relative to a small conventional Li-ion battery (red). Collectively, the improvements from the Enovix 3D Silicon Lithium-ion Battery orthogonal platform deliver 1.5X to 3X greater energy density at cell thicknesses of a few millimeters. The overall increase in energy density with this 3D Silicon Lithium-ion Battery materially reduces present energy storage limitations with ultrathin millimeter thickness cells for wearable device form factors.

5. Enovix 3D Silicon Lithium-ion Battery product roadmap. (Image courtesy of Enovix).

In addition to an immediate increase in energy density, a long-term advantage of the Enovix 3D Silicon Lithium-ion Battery platform is its ability to enable continual innovation and produce significant battery performance improvements for years to come. The conventional Li-ion battery, introduced in 1991, has historically delivered only a 5% average annual increase in energy density. This is a major reason it has lagged the increased energy requirements of innovative mobile products.

Enovix can continue to incorporate improvements from conventional Li-ion cells, such as cathode materials. But it also liberates a 100% silicon anode from the constraints of the conventional Li-ion battery structure. Additionally, in a continuous improvement cycle similar to that experienced with semiconductors and solar cells, advances in photolithography and etch processes enable continued improvement in spatial efficiency—the ratio of active-to-inactive materials. Collectively, the advantages of the Enovix orthogonal platform are projected to deliver two to three times the average annual increase in energy density of a conventional Li-ion battery for the foreseeable future.

Figure 5 shows projected energy density trends for the Enovix 3D Silicon Lithium-ion Battery, compared to the historical rate of increase from a conventional Li-ion battery. As Enovix extends beyond wearables into larger mobile markets—smartphones, tablets, and notebook computers—production volume will increase, and unit cost will decline. The combination of higher energy density, improved safety, and lower cost will eventually make Enovix an attractive choice for electric vehicles.

Key specifications for the Enovix 3D Silicon Lithium-ion Rechargeable Battery, which is presently in the pilot-production (pre-commercialization) phase.

Volumetric Energy Density: 800 WHr/L

Gravimetric Energy Density: 321 WHr/kg

Nominal Voltage: 3.45 V

Voltage Range: 2.5 – 4.2 V

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