(Invited) Opportunities and Challenges of Atom Switch for Low-Power Programmable Logic

Abstract

Opportunities and challenges of atom switch featuring Cu nanometer-scale conducting bridge in the solid-electrolyte are discussed with comparing the other emerging memories. Atom switch having the high on/off conductance ratio is suitable to directly replace the conventional CMOS switch element, realizing the low-power, nonvolatile programmable logic. Reliability challenges of the atom switch are also discussed in the view point of the stabilization of the on-and off-states. IntroductionWith the increasing requirement of tasks and functions in digital circuit, flexibility and energy-efficiency play important roles in addressing future low-power computing systems. A flexible hard wired-logic such as a field-programmable gate array (FPGA) becomes of great interest as an accelerator for the low-power systems since no redundant hardware, namely no instruction fetch mechanisms and cache, is advantageous to shift critical code from CPU to the accelerator and run those algorithms using multiple streaming processes. However, the FPGA with the large area suffers from large parasitic resistance and capacitance, limiting power/performance improvement.One solution for overcoming the issues of conventional FPGAs is a replacement of the CMOS switch element composed of SRAM and transmission gate (TMG) to a compact resistive-change switch [1]. Figure 1 shows the schematic image of the operation principle of atom switch. The solid-electrolyte is sandwiched between the active (Cu) anode and the inert cathode (Ru). When a positive voltage is applied to the Cu electrode, a nanometer-scale conducting Cu-bridge is formed and the switch is turned on. Cu + ions are supplied via electrolysis from the solid-electrolyte/Cu interface. A positive voltage applied to the inert electrode annihilates the Cu bridge and turns the switch off. Figure 2 (a) shows a tradeoff relationship between on/off ratio and endurance in emerging nonvolatile memories. Figure 2 (b) shows a concept of a crossbar switch integrated in BEOL [2, 3], where the atom switches are densely-packed and directly route signals flexibly. The replacement of the switch block reduces the chip area, lowering dynamic power consumption. The nonvolatility also decreases the static power since an efficient tuning of power-gating can be realized. For the switch application, atom switch having the high on/off conductance ratio is more suitable than the other memories. In the previous work, the 70 % dynamic power and 72 % area reductions were demonstrated in a 90 nm-node, test-vehicle [4]. In reality, once the FPGA is programmed, the configuration is not so much frequently re-programmed. Therefore, the >10 3 endurance of atom switch is acceptable for the programmable logic application so far.In this paper, an operation principle of the atom switch is reviewed and the application is discussed. For reliability challenge, a complementary atom switch (CAS) [5,6] and alloying electrode [7] are proposed. The pair of the switches works as one