1 Nanoelectronic Memory Devices: Space-Time-Energy Trade-offs Ralph Cavin and Victor Zhirnov Semiconductor Research Corporation

2 Main Points  Many candidates for beyond-CMOS nano-electronics have been proposed for memory, but no clear successor has been identified.  Methodology for system-level analysis  How is maximum performance related to device physics? SRC/NSF A*STAR Forum on 2020 Semiconductor Memory Strategies: Processes, Devices, and Architectures, Singapore, October 20-21,

Space-Time-Energy Metrics  Essential parameters of the memory element are:  cell size/density,  retention time, access time/speed  operating voltage/energy.  None of known memory technologies, perform well across all of these parameters  At the most basic level, for all memory elements, there is interdependence between operational voltage, the speed of operation and the retention time.  Cell dimensions are also part of the trade-off, hence the Space-Time-Energy compromise 3

Space-Action Principle for Memory 4 The Least Action principle is a fundamental principle in Physics Plank’s constant h=6.62x Js (  h)

Three essential components of a Memory Device:  1) ‘Storage node’  physics of memory operation  2) ‘Sensor’ which reads the state  e.g. transistor  3) ‘Selector’ which allows a memory cell in an array to be addressed  transistor  diode  All three components impact scaling limits for all memory devices 5

Three Major Memory State Variables  Electron Charge (‘moving electrons’)  e.g. DRAM, Flash  Electron Spin (‘moving spins’)  (STT-) MRAM  Massive particle(s) (‘moving atoms’)  e.g. ReRAM, PCM, Nanomechanical, etc. 6 Note: Electrical I/O always wanted

DRAM schematic 7 Storage: N carriers Barrier+Selector E b, eVMax. retention 0.61 ms ms ms 1.1~1 h 1.6>10 years Problem 1: In Si devices E bmax <E g =1.1 eV Only volatile memory possible with FET barrier

Volatile electron-based memory: DRAM 8 E b,tr E b,C dCdC a Selector Sensor Storage Node 25fF Problem 2a: External sensing requires large N el Problem 2b: Large N el requires large size of storage node (capacitor) Problem 2c: Series resistance of the storage capacitor increases with scaling V cap = cm 3 N el ~10 5 (a=10 nm, K=100)

9 Charge-based injection “easy” in DRAM: Barrierless transport… Write (>25fF) a~10 nm t w ~0.1-1 ns K=100 Dominates at a <10 nm

DRAM summary 10 -Low barrier height-Volatility a =15 nm Vol cap ~10 6 nm 3 E w ~3  J DRAM inherent issues: Selector Sensor - Remote sensing – Large size of Storage node E  t  V~10 -9 J-ns-nm 3 Vol FET =10 5 nm 3 E w ~ J Vol cap ~5  10 5 nm 3 Vol FET =10 4 nm 3 a =30 nm E  t  V~10 -8 J-ns-nm 3 T a =1 sT a =10y

Flash in the limits of scaling 11 Selector Storage Node Sensor ~10 nm ~6nm ~20nm N el ~10 Vol storage =2000 nm 3 Vol FET ~3 a 3 ~3000nm 3 N el ~10

Voltage-Time Dilemma u For an arbitrary electron-charge based memory element, there is interdependence between operational voltage, the speed of operation and the retention time. u Specifically, the nonvolatile electron-based memory, suffers from the “barrier” issue: v High barriers needed for long retention do not allow fast charge injection v It is difficult (impossible?) to match their speed and voltages to logic 12

Flash Summary 13 E~ J t ~1  s=1000 ns E  t  V~10 -9 J-ns-nm 3 The minimum space-action metric is approximately the same as for the DRAM Vol storage =2000 nm 3 Vol FET ~3 a 3 ~3000nm 3

14 Conclusion on ultimate charge- based memories u All charge-based memories suffer from the “barrier” issue: v High barriers needed for long retention do not allow fast charge injection v It is difficult (impossible?) to match their speed and voltages to logic n Voltage-Time Dilemma Non-charge-based NVMs? The Choice of Information Carrier

Spin torque transfer MRAM (Moving spins): Energy Limit 15 D. Weller and A. Moser, “Thermal Effect Limits in Ultrahigh-Density Magnetic Recording”, IEEE Trans. Magn. 36 (1999) 4423 t store >10 y E b = KV > ~1.4 eV (f 0 ~ c -1 ) the anisotropy constant of a material volume

FET selector is biggest component of STT- MRAM in the limits of scaling nm V=1500 nm 3 N spin ~10 5 J-G. Zhu, Proc. IEEE 96 (2008) 1786 Selecting FET Storage Node E b = KV >1.4eV (the anisotropy constant of a material) K~0.1-1 J/cm 3 volume Vol FET ~3 a 3 ~3000nm 3

STT-RAM summary 17 (Alternative estimate based on optimistic write current/write time projections: I w ~10 7 A/cm 2 and t w ~1 ns E w ~10 7 A/cm 2  11nm 2  1V  1 ns~ J V=1500 nm 3 (e.g. 11nm 2  2 nm) E  t  V~ J-ns-nm 3 This looks a little better than the electron-based we looked at earlier! E w ~N spin  E b ~10 5  ~ J Vol FET ~3 a 3 ~3000nm 3

Scaled ReRAM (courtesy Dr. In YOO/Samsung) 18

Ultimate ReRAM: 1-atom gap A B E b =0.38 eV d t =0.075 nm d t <<a V=0.5 V ON/OFF~1.61

Ultimate ReRAM: 2-atom gap E b =2.63 eV d t =0.37 nm d t <a V=0.5 V ON/OFF~476 B A

Ultimate Atomic Relay: 4-atom gap Energy E b (energy barrier for diffusion) eV

Ultimate Atomic Relay: 4-atom gap Energy E b (energy barrier for diffusion) eV (E b =0.5 eV, n>5)

Ultimate ReRAM: A summary V=1nm 3 N at ~100 (64) E~N at *1eV~ J t w ~1 ns (can be shown) E  t  V~ J-ns-nm 3 Vol FET ~3 a 3 ~3000nm 3 E  t  V~ J-ns-nm 3 without FET with FET

Summary 24 DRAM Flash STT-RAM ReRAM V stor, nm N carriers 100 E w, J t w, ns Biggest component Storage Node Selector 1 ns 10 3 ns 1 ns Sensor FET Space- Action, J-ns-nm ~ ~ ~10 -9 Constraints by sensor not considered Main constraints due to sensor ~ without FET Vol FET ~3000nm 3 With FET

Summary u Memory cell design is a tradeoff between physical variables needed to achieve long retention times, and short write/read times. u A global metric, space-action, for all memory categories provides insights into most promising extremely-scaled memory devices based on fundamental physics u Scaling Limits of semiconductor component often dominate overall scaling for the memory cell u Our preliminary study suggests a good potential for ReRAM (some constraints are not considered) u Today’s memory technology meets Feynman’s challenge of placing the 24 volumes of Encyclopedia Britannica (~200 MB) on the head of a pin (~.025 cm^2). v Library of Congress (10 Terabytes) on 1 cm^2 by 2020? 25