Influence of electrode materials on CeO x based resistive switching S. Kano a, C. Dou a, M. Hadi a, K. Kakushima b, P. Ahmet a, A. Nishiyama b, N. Sugii b, K. Tsutsui b, Y. Kataoka b, K. Natori a, E. Miranda c, T. Hattori a, and H. Iwai a a Frontier Research Center, Tokyo Institute of Technology b Interdisciplinary Graduate School of Science, Tokyo Institute of Technology c Department Enginyeria Electronica, Universitat Autonoma de Barcelona
Outline Background of Resistive Random Access Memory (ReRAM) Introduce of CeO x as a insulator Influence of bottom electrode for CeO x based ReRAM Resistance switching mechanism of CeO x /NiSi 2 structure ReRAM Conclusion 2
Background of Resistive Random Access Memory (ReRAM) Metal High Resistance Metal OFF State Reset Set Advantage Metal Low Resistance Metal ON State Non-volatility High speed operation Simple MIM structure Compatibility with CMOS process High integration Resistance switching (RS) mechanism has been considered oxygen ions migration Resistance ratio 10 8 10 6 10 4 10 2 10 0 Universal NVM Flash Memory Atomic switching ReRAM PRAM MRAM 10 1 10 2 10 3 10 4 10 5 Operation speed (ns) Ref. IEDM, VLSI 3
CeO x for ReRAM application Ce oxide Valence number fluctuation High oxygen ionic conductivity Ce 2 O 3 CeO 2 (Ce 3+ ) (Ce 4+ ) Ce 3+ CeO 2 Ce CeO 3+ 2 Fluorite structure Oxygen ions/vacancies are easily formed and eliminated Ce 3+ High speed operation large resistance ratio can been expected by using CeO x for ReRAM application 4
Effect of changing electrode Resistance ratio 10 4 10 3 10 2 HfO 2 based ReRAM Different electrode have important influence on RS characteristics 10 1 10 0 10 1 10 2 10 3 Operation speed (ns) Ref. IEDM, VLSI 5
Purpose of this work To investigate the influence of bottom electrode on CeO x based ReRAM Metal CeO x W, Ni, Ti, or NiSi 2 6
Fabrication process P + -Si substrate (Impurity concentration:10 18 cm -3 ) SPM&HF Cleaning Oxidation Metal (W, Ni, Ti, and NiSi 2 ) deposition Annealing at 500ºC for 1min (N 2 ) 20 m Insulator (CeO x ) evaporation at 300ºC Metal (W) deposition Metal etching Backside contact formation (Al) W CeO x W, Ni, Ti, and NiSi 2 Measurement 7
Resistance switching characteristics of using W, Ni, and Ti as a bottom electrode 10-2 Compliance Current@1mA 10-2 Compliance Current@0.75mA 10-2 10-12 -10 Initial Reset Set -5 0 5 Characteristics: W 10 Resistance ratio:2~150 (Read voltage at -0.1 V) Set voltage : 7~ 10 V Reset voltage: -7~-10 V 10-12 -10-5 0 5 Ni Initial Reset Set 10 10-12 -10 Ti Initial Reset Set -5 0 5 Difference of bottom electrode conduce different characteristics 10 8
Resistance switching characteristics of using NiSi 2 as a bottom electrode 10-2 Compliance Current@2mA Initial Reset Set -3-2 -1 0 1 2 3 4 5 Characteristics: Forming-free Significant changing of resistance Large resistance ratio ~10 4 (Read Voltage at -0.1 V) Set voltage : 7 V Reset voltage: -5 V Using NiSi 2 bottom electrode shows major different characteristics compared with other materials Resistance switching is caused by different mechanism 9
Resistance switching mechanism Buffer layer (CeO X ) Resistive Switching layer (SiO 2 ) Resistance switching of SiO 2 interfacial layer with the assistance of CeO X buffer layer SiO 2 (+) W CeO x O 2- NiSi 2 (-) O 2- SiO 2 interfacial layer is formed between CeO x and NiSi 2 W. Strydom, et al., Solid-State Electron. 9, pp. 947, (1987). Set process Reset process O 2- SiO 2 (-) W CeO x NiSi 2 (+) O 2-10
ε SiO2 Resistance change mechanism for set process Small E ε ( CeO x :28) Large E ( :3.9) (+) W CeO x O 2- SiO 2 NiSi 2 (-) O 2- Vo 2+ :Oxygen vacancy V Conductive filament d SiO E 2 SiO2 dceo x ECeO x 1 1.5 ESiO ESiO d 2 CeOx 7 1 1.5 dceo E x SiO 7 8 2 2 Dielectric constant of CeO x is much larger than that of SiO 2 Electric field induced at SiO 2 Hard breakdown at SiO 2 Set voltage (V) 6 4 2 T SiO 2 =1.5 nm E SiO 2 =16 MV/cm 0 5 10 15 Thickness of CeO x layer (nm) 11
Resistance change mechanism for reset process Reset process is caused by Re-Oxidation of SiO 2 (-) W CeO x O 2- SiO 2 NiSi 2 (+) O 2- Oxygen ions migration Fill in Breakdown spot Positive voltage applied to NiSi 2 bottom electrode O 2- migrate to the breakdown spot SiO 2 grows from NiSi 2 by local anodic oxidation 12
Stability and operation speed of W/CeO x /NiSi 2 structure ReRAM Resistance( ) 10 10 10 8 10 6 10 4 10 2 80 s ON/OFF~10 4 5 s V reset =-3.25V V set =3.9V Read Voltage@-0.1V 10 0 0 20 40 60 80 100 Cycle Good endurance Resistance ratio shows over 10 4 after 100 cycle Resistance( ) 10 10 10 8 10 6 10 4 10 2 ON/OFF~10 4 Read Voltage@-0.1V V reset =-3.5V V set =4.25V 10 10 0-1 10 0 10 1 10 2 10 3 working speed ( s) High speed operation Switching speed shows 200 ns 13
Conclusion CeO x with W top electrode and W, Ni, or Ti bottom electrode could not show large resistance ratio Resistance switching mechanism is attributed to the breakdown and re-oxidation of interfacial SiO 2 layer with the help of CeO x layer W/CeO x /NiSi 2 structure ReRAM shows forming-free, large resistance ratio, good endurance and high speed operation 14
Resistance switching mechanism Vo 2+ :Oxygen vacancy Metal Metal Applied voltage Oxygen migration and generate oxygen vacancy Conductive filament Formation of conductive filament Oxygen ions Migrate to CeO x Rupture Metal Oxygen ions migrate to Metal Recouping Changing voltage direction Rupture and recouping of conductive filament HRS Metal LRS Resistance changing Ref. [1] W.C. Chien, et al., IEDM pp. 440 (2010). [2] Y.Y.Chen, et al., IEEE International (2011). [3] Q. Lv, et al., J. Appl. Phys., 110, pp. 104511 (2011).
W/CeO x (13nm) /NiSi 2 structure 10-2 Compliance Current@2mA 10-12 NiSi 2 Initial Reset Set -6-4 -2 0 2 4 6 8 10
W/CeO x (3.25, 2 nm) /NiSi 2 structure 10-2 Compliance Current@5mA 10-2 Compliance Current@5mA 10-12 Initial Reset Set -3-2 -1 0 1 2 3 4 5 10-12 Initial Reset Set -2-1 0 1 2 3 4
TDDB W/CeO x (6.5nm) /NiSi 2 structure 6 5 4 3 2 1 0 10-1 10 0 10 1 10 2 10 3 10 4 Time (S)
Ce Resistance Ratio :1.5 10 3 @-0.1V Ce+SiO 2 (1.5nm) Resistance Ratio :1.6 10 4 @-0.1V 1 1 10-2 10-2 10-12 -5 0 5 10 10-12 -5 0 5 10
1 Si only 10-2 10-12 -5 0 5 10
W/CeOx(6.5nm)/NiSi 2 W/CeOx(13nm) /P + -Si 10-2 Compliance Current@2mA 1 Compliance Current@1mA 10-2 10-12 -4-3 -2-1 0 1 2 3 4 5 10-12 -5 0 5 10
W/HfO 2 /NiSi 2 W/HfO 2 /P + -Si Resistance Ratio:3 10 7 (Read Voltage at -0.1V) Resistance Ratio:2 10 5 (Read Voltage at -0.1V) 10-2 Compliance Current@1 A 10-2 Compliance Current@10 A 10-12 Initial Reset Set 10-14 -10-8 -6-4 -2 0 2 4 6 8 10 10-12 Initial Reset Set 10-14 -10-8 -6-4 -2 0 2 4 6 8 10
10-2 Compliance Current@2mA W W CeO x TiN Initial Reset Set -3 0 3 6 CeO x TiN SiO 2 TiO 2
Electron Beam Induced Current (EBIC) Electron beam irradiation Generate electron-hole pair Contribute to current observation large current J. Chen et al., Appl. Phys. Lett. 92 (2008) 262103. Contribute to breakdown spot Z. Wei, et. al., IEDM Tech. Dig., 721-723 (2011)
Local anodic oxidation Y. Ma, et al., Phys. Rev. B, 64, pp. 195324. (2001).
NTU (Singapore) ITRI (Taiwan) Samsung (Korea) Macronix (Taiwan) NCTU (Taiwan) Stanford (USA) Stanford (USA) NTU (Singapore) Hynix (Korea) Renesas (Japn) Samsung (Korea) Insulator (nm) HfO x (4) HfO x (3) Ta/Ta 2 O 3 WO x HfO 2 (80) AlO X N Y (15) AlO x HfO x /AlO y (4.2) TiO 2 /Al 2 O 3 Ta 2 O 5 /TiO 2 (10) Ta 2 O 3 /TaO x TE/BE Ni/p + -Si Ti/- TiN/- TiN/W Ni/Pt Al/- CNT/CNT Ni/p + -Si TiN/TiN Ru/Ru Pt/Pt Set Voltage speed 2.5 V - 1.4 V 40 ns 2.5 V 10 ns 1.8 V 50 ns 4.0 V 100 ns 2.2 V - 15 V 50 ms 1.4 V 10 ns 3.0 V 10 ns 3.6 V 10 ns -4.5 V 10 ns Reset Voltage speed 1.5 V - -1.5 V 40 ns -2.5 V 10 ns -1.2 V 50 ns -4.0 V 100 ns -0.8 V - -10 V 50 ms 0.5 V 30 ns -4.0 v 10 ns 2.5 V 10 ns 7.0 V 10 ns Endurance - 10 10 10 6 >10 6 10 6 10 5 10 4 10 6-10 5 >10 12 Resistance Ratio >10 3 50 10 10 10 3 10 2 >10 10 5 - >10 2 >10 2 Type unipolar and bipolar bipolar bipolar bipolar bipolar bipolar bipolar unipolar bipolar unipolar bipolar Forming free free - - - free - free - - - Mechanism Schottky current transport - Oxigen migration Space charge limited current Metal firament Frenkel- Poole emission Oxygen vacancies filaments Joule heat induced redox Oxygen vacancies filaments Oxigen migration Oxigen migration Structure 1R 1T-1R 1T-1R 1R 1S(Ni/TiO 2 /Ni)-1R 1T-1R 1R 1D-1R 1T-1R 1T-1R 1T-1R distinction Flexible Vertical
Endurance: >10 7 Cycles (Flash 10 3 ~10 7 ) Resistance Ratio: R off / R on > 10 Scalability: F<22 nm and / or 3-D stacking Write voltage: 1~5 V (Flash >5V) Read voltage: 0.1~0.5 V Write speed: <100 ns (Flash >10 s, DRAM >100 ns, SRAM, 10 ns) Retention: >10 yrs
10-2 10-2 Compliance Current@2mA 10-12 -10-5 0 5 Ni Initial Reset Set 10 Initial Reset Set -3-2 -1 0 1 2 3 4 5
10-2 Compliance Current@1mA Compliance Current@0.75mA 10-2 10-12 -10 W Initial Reset Set -5 0 5 10-12 Ti Initial Reset Set -5 0 5 10