Millivolt Switches past

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Transistor research based on graphene nanoribbons (Bokor and Fischer groups)

Millivolt Switches

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Past Achievements

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A. Graphene Nanoribbons

Research on graphene nanoribbon (GNR) based millivolt switches in the BETR Center is led by the Bokor group (and Prof. Felix Fischer), in collaboration with BETR industry affiliate TSMC. The BETR team has successfully synthesized several distinct GNRs with ultra-narrow width (0.7-3.0 nm), atomically smooth edges and uniform bandgap. Furthermore, as shown in Figure 1, fabrication processes were developed to integrate GNRs into functioning FETs with excellent switching behavior (ON/OFF ratios of ∼105 and ON-currents (Ion) of ∼60 nA).3, 5 However, since the bottom-up synthesis commonly takes place on catalytic metallic surfaces, the integration of GNRs into such devices requires their transfer onto insulating substrates, which remains one of the bottlenecks in the development of GNR-based electronics. In response, the Bokor group developed a method for the transfer-free placement of GNRs on insulators, involving growth of GNRs on a gold film deposited onto an insulating layer followed by gentle wet etching of the gold, which leaves the nanoribbons to settle in place on the underlying insulating substrate.5 Meanwhile, the team has also demonstrated transfer-free fabrication of ultrashort channel GNR FETs using this process. Importantly, this transfer-free process can scale up well to 12-inch wafers, which is extremely difficult for previous techniques.

Figure 1. Schematic representation (top) and ID−VG characteristics (bottom) of 7-AGNR FETs with Pd source−drain electrodes and local HfO2 back gate.

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B. Low-Temperature Semiconductor Processing

Processing of high-quality semiconductors at near-ambient temperatures has become increasingly important for both transparent/flexible electronics and monolithic 3D-CMOS architectures. While several solutions exist for n-type semiconductors, the Javey group recently reported a breakthrough for p-type semiconductors by successful thermal evaporation of tellurium thin films at cryogenic temperatures.6 Using this approach, the team fabricated high-performance wafer-scale p-type field-effect transistors on various transparent and flexible substrates (Figure 2). The transistors have excellent properties, including high effective hole mobilities and ON/OFF current ratios, even under substantial bending. Moreover, various functional logic gates and 3D circuits were demonstrated by integrating multi-layered transistors on a single chip using sequential lithography, deposition and lift-off processes.

Figure 2. Photographs of Te FETs fabricated by evaporation with a substrate temperature of −80 C on 4-inch quartz wafer (top, left), on PET (top, right), and on Kapton (bottom, left) while bent (the thickness of the Kapton substrate is 50 μm). Bottom, right. Effective mobility and on/off current ratio of Te FET (8 nm) on Kapton substrate under different bending radii.

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C. Non-Volatile Nano-Electromechanical (NEM) Switches

The Liu group has pioneered the use of non-volatile nano-electromechanical (NEM) switches (or relays) for energy efficient computing.1 NEM switches use electrostatic force to mechanically actuate a movable structure to make or break physical contact between current-conducting electrodes. Importantly, when the electrodes are separated physically by an air gap, no current flows across the gap, resulting in zero OFF-state current. Hence NEM switches have abrupt ON/OFF switching characteristics, in addition to robust operation across a wide temperature range, down to cryogenic temperatures.7 Moreover, NEM switches can be monolithically integrated with CMOS circuitry. This was demonstrated by the Liu group by implementation of non-volatile NEM switches using multiple metallic layers in the BEOL stack of a standard 65nm CMOS process, followed by a release etch after CMOS fabrication (Figure 3, left).8, 9 Low contact resistance and reconfigurable circuit functionality was verified by  operation of a vertical hybrid CMOS-NEM reconfigurable circuit. Program voltage is applied to either actuation electrode (Program 0 or Program 1) to electrostatically actuate the beam into contact with either D0 or D1 respectively. The contact adhesion force is designed to be higher than the spring restoring force to achieve non-volatile contacting states. A hybrid CMOS-NEM 2-input/1-output look-up table using 4×3 non-volatile NEM switch array is also demonstrated in Figure 3 (right).

Figure 3. Left. Cross-sectional SEM image of a vertical NEM switch fabricated in standard 65nm node, showing its five terminals in the as-fabricated state. Right. Input and output waveforms of a 2-input/1-output look-up table (LUT) based on non-volatile vertical NEM switches (top) and the corresponding truth table (bottom).

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References

  1. Z. A. Ye, S. Almeida, M. Rusch, A. Perlas, W. Zhang, U. Sikder, J. Jeon, V. Stojanović and T.-J. K. Liu, “Demonstration of Sub-50 mV Digital Integrated Circuits with Microelectromechnical Relays,” IEEE International Electron Devices Meeting, San Francisco, CA, Dec 2018.
  2. S.B. Desai, S.R. Madhvapathy, A.B. Sachid, J.P. Llinas, Q. Wang, G.H. Ahn, G. Pitner, M.J. Kim, J. Bokor, C. Hu, H.-S.P. Wong, and A. Javey, “MoS2 Transistors with 1-Nanometer Gate Lengths,” Science, vol. 354, pp. 99-102, Oct 2016.
  3. J.P. Llinas et al., “Short-Channel Field-Effect Transistors with 9-Atom and 13-Atom Wide Graphene Nanoribbons,” Nature Communications, vol. 8, pp. 633, Sep 2017.
  4. Y. Yang, R. Wilson, J. Gorchon, C.-H. Lambert, S. Salahuddin and J. Bokor, “Ultrafast Magnetization Reversal by Picosecond Electrical Pulses,Science Advances, vol. 3, pp. E1603117, Nov 2017.
  5. Z. Mutlu, J.P. Llinas, P.H. Jacobse, I. Piskun, R. Blackwell, M.F. Crommie, F.R. Fischer, and J. Bokor, “Transfer-Free Synthesis of Atomically Precise Graphene Nanoribbons on Insulating Substrates,” ACS Nano, vol. 15, pp. 2635-2642, Jan 2021.
  6. C. Zhao, C. Tan, D.H. Lien, X. Song, M. Amani, M. Hettick, H.Y.Y. Nyein, Z. Yuan, L. Li, M.C. Scott, and A. Javey, “Evaporated Tellurium Thin Films for p-type Field-Effect Transistors and Circuits,” Nature Nanotechnology, vol. 15, pp. 53-58, Jan 2020.
  7. X. Hu, S.F. Almeida, Z.A. Ye, and T.-J. King Liu, “Ultra-Low-Voltage Operation of MEM Relays for Cryogenic Logic Applications,” 2019 IEEE International Electron Devices Meeting (IEDM), San Francisco, CA, USA, pp. 34.2.1-34.2.4, Dec 2019.
  8. U. Sikder, G. Usai, T.-T. Yen, K. Horace-Herron, L. Hutin, and T.-K. Liu, “Back-End-of-Line Nano-Electro-Mechanical Switches for Reconfigurable Interconnects,IEEE Electron Device Letters, vol. 41, pp. 625-628, Apr 2020.
  9. U. Sikder, L. P. Tatum, T.-T. Yen, and T.-J. K. Liu, “Vertical NEM Switches in CMOS Back-End-of-Line: First Experimental Demonstration and Programming Scheme,” IEEE International Electron Devices Meeting (IEDM), San Francisco, CA, USA, pp. 21.2.1-21.2.4, Dec 2020.