Embedded Memory past

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Electrical pulse-induced ultrafast magnetic switching (Bokor and Salahuddin groups)

Embedded Memory & Spin-Based Logic

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Past Achievements

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A. Ultrafast Magnetic Switching

The Bokor and Salahuddin groups are  developing current-driven, ultra-high speed magnetic elements for logic and memory with switching energies at the sub-femtojoule level.4, 5 Magnetic systems are attractive logic switches since their non-volatility can be used to reduce static power losses. However, the low speed of magnetic switching has severely limited device applications. The BETR research team demonstrated that ultrafast switching of magnetic materials is possible by hot electrons that are excited via electrical pulses.4 Moreover, a further breakthrough was recently achieved by the observation of spin-orbit torque (SOT) switching of a ferromagnet with picosecond electrical pulses.5 In detail, it was discovered that photoconductive switches can be used to apply 6-ps-wide electrical pulses and deterministically switch the out-of-plane magnetization of a common thin cobalt film via spin–orbit torque (Figure 1). First results indicate that the magnetization switching consumes less than 50 pJ in micron-sized devices. Scaling down the device dimensions to 20 nm dimensions gives estimated switching energies of a few fJ, which is the current size of state-of-the-art magnetic memory devices.

Figure 1. Ultrafast SOT switching set-up. Photogenerated ~6-ps-duration electrical pulses are guided and focused by a coplanar waveguide into the magnetic stack, resulting in ultrafast SOTs. The sampled picosecond current pulse is shown at the back of the figure. The solid green line is a guide to the eye.

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B. NEM-Based Relays for Embedded Memory

The Liu and Stojanović groups have teamed up with support by BETR industry affiliate SK Hynix to explore system-based integrated circuit implementations in the so-called “edge computing” scenarios, where the sensory and computation functions are severely energy limited. For example, the team has investigated the use of NEM-based relays for applications as look-up tables (LUTs) and for embedded memory, including arrays of reconfigurable NEM-based interconnects for novel memory applications.3 For this, vertically oriented relays were designed and fabricated by a standard 65nm CMOS process, using the multiple interconnect layers in the back-end-of-line (BEOL) process (Figure 2).6, 7 NEM switches are thus monolithically integrated with CMOS circuitry by performing a release etch after CMOS fabrication process with relatively low thermal budget. An analysis of the performance of NEMS switch-based embedded memories to increase the capacity and lower the energy-consumption per node is shown in Figure 2. The results demonstrate that with proper process node scaling, the NEMory can achieve very competitive metrics compared to CMOS-based memories and ReRAM.

Figure 2. Top. NEM switches fabricated using multiple layers in a standard BEOL process in 65nm CMOS technology showing simulated structure of a vertically oriented body-biased relay (left) and schematic cross-section after sacrificial oxide etch (right). Bottom. NEMory performance comparison with ReRAM and CMOS.

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C. Magnetoelectric Spin-Orbit (MESO) Logic

The Ramesh group is exploring pathways to drastically reduce the voltage requirement for electric field switching of multiferroics. Together with colleagues from Intel, the team developed a new logic computing concept based on a magnetoelectric spin–orbit (MESO) device with magnetoelectric switching nodes and spin–orbit-effect readout (Figure 3).8 The ultimate goal is to switch magnets purely with a voltage of just 100 mV, or below. When successful, this will represent a 35-fold reduction in voltage amplitude compared to state of the art and a corresponding 1000-fold reduction in switching energy. This will also represent a paradigm shift in that no current will be necessary for switching magnetization.

Figure 3. a) MESO logic transduction for a cascadable charge-input and charge-output logic device. b) MESO device comprising a spin-injection layer for spin injection from the ferromagnet to the topological material, an interconnect made of a conductive material, and contacts to the power supply and ground.

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References

  1. E. A. Tremsina, N. Roschewsky, and S. Salahuddin, “Micromagnetic Analysis and Optimization of Spin-Orbit Torque Switching Processes in Synthetic Antiferromagnets,” Journal of Applied Physics, vol. 126, pp. 163905, Oct 2019.
  2. N. Roschewsky, E. Walker, P. Gowtham, S. Muschinske, F. Hellman, S. Bank, and S. Salahuddin, “Spin-Orbit Torque and Nernst Effect in Bi-Sb/Co Heterostructures,” Physical Review B, vol. 99, pp. 195103-195108, May 2019.
  3. K. Kato, V. Stojanović, and T.-J. K. Liu, “Embedded Nano-Electro-Mechanical Memory for Energy-Efficient Reconfigurable Logic,” IEEE Electron Device Letters, vol. 37, pp. 1563–1565, Dec. 2016.
  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. K. Jhuria, J. Hohlfeld, A. Pattabi, E. Martin, A. Y. Arriola Córdova, X. Shi, R. Lo Conte, S. Petit-Watelot, J. C. Rojas-Sanchez, G. Malinowski, S. Mangin, A. Lemaître, M. Hehn, J. Bokor, R. B. Wilson, and J. Gorchon, “Spin–Orbit Torque Switching of a Ferromagnet with Picosecond Electrical Pulses,” Nature Electronics, vol. 3, pp. 680-686, Oct 2020.
  6. U. Sikder, G. Usai, T.-T. Yen, K. Horace-Herron, L. Hutin, and T.-J.K. Liu, “Back-End-of-Line Nano-Electro-Mechanical Switches for Reconfigurable Interconnects,” IEEE Electron Device Letters, vol. 41, pp. 625-628, April 2020.
  7. 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, pp. 21.2.1-4, Mar 2021.
  8. S. Manipatruni, D.E. Nikonov, C.-C. Lin, T.A. Gosavi, H. Liu, B. Prasad, Y.-L. Huang, E. Bonturim, R. Ramesh, and I.A. Young, “Scalable Energy-Efficient Magnetoelectric Spin-Orbit Logic,” Nature, vol. 565, pp. 35-43, Jan 2019.