Non-Volatile Memory Based on Solid Electrolytes

Transcription

1 Non-Volatile Memory Based on Solid Electrolytes Michael Kozicki Chakku Gopalan Murali Balakrishnan Mira Park Maria Mitkova Center for Solid State Electronics Research

2 Introduction The electrochemical redistribution of nanoscale quantities of metal in structures containing solid electrolytes is the basis of Programmable Metallization Cell (PMC) technology. Applications in electronics, MEMS, microfluidics, optics We can use this effect to form and dissolve conducting pathways on command so that we have an electronic switch that can be closed and opened rapidly using small voltages. Such switches have potential applications in non-volatile memory and programmable logic devices

3 Solid electrolytes Solid electrolytes behave like liquid electrolytes e - e - M M Mobile ions M M OR M + M + M + M + M + M + M+ M + M + M + M + M M M + M + M + Liquid Lateral/coplanar Vertical M + e - Mobile ions Ions move under the influence of an electric field Electrochemical reactions are possible with electron supply: M + + e - M reduction with oxidizable metal: M M + + e - oxidation Electrochemistry occurs at a few 100 mv

4 Room temperature electrolytes Silver or copper can be added to a variety of glasses to form good solid electrolytes base glasses can be a variety of chalcogenides (e.g., compounds of S or Se) or oxides base glasses can incorporate many tens of atomic % of metal ternaries are nanostructured materials with high ion mobility (up to 10-3 cm 2 /Vs) but relatively high resistivity (> 10 2 Ω.cm) metals and base glasses are relatively easy to process at low temperatures

5 Electrolyte preparation λ < 500 nm, ~1 J/cm 2 Ag or Cu Ge x S(e) 1-x or WO 3 W Si 25 nm 50 nm Ge x S(e) 1-x, x < 0.33 or WO 3 thin films deposited by PVD (other formation options exist) Ag or Cu deposited by PVD Metal driven into base glass using UV light and/or heat

6 Electrolyte example: Resistivity of Ge-S vs. Ag content T = 25 ºC Network modification Hi R >10 at.% Lo R Log ( ) (.cm ) (Bychkov) Concentration and distribution of Ag Percolation (as well as geometry) determines resistance of layer Reduction due to electrodeposition 0.5Ag 2 S-0.5GeS 2 (Belin) Ag saturated Ge-S (Elliot) Ag 2 S (Miyatani) Ag <10 at.% Ag (at %)

7 Inner Workings - Nanostructure Oxidizable electrode Inert electrode Glassy insulator <2 nm <10 nm 10 to 100 nm thick Superionic region

8 Inner Workings Switching Oxidizable electrode Inert electrode Glassy insulator Ions + I prog = na - ma Superionic region Electrodeposited metal V T = 0.25 V - Electrons

9 Information storage We store information as reduced metal in the solid electrolyte Write in forward bias to inject and reduce ions A few thousand electrodeposited atoms is fc charge range Ion mobility as high as 10-3 cm 2 /V.s and internal field around 10 5 V/cm leads to electrodeposit growth rates of 1 nm/nsec Erase in reverse bias to remove excess metal Decrease concentration by oxidation of electrodeposit Read options involve detection of amount of reduced metal in electrolyte Resistance change is large and easy to detect Other sensing options also exist

10 PMC test device layout Device array Cathode Via Anode Top electrode Anode layer Solid electrolyte Dielectric Inert electrode Device diameter, D

11 1.0E-06 Example of PMC device I-V 240 nm diameter, 50 nm Ag-Ge-Se on W cathode Current (A) 8.0E E E E E E-07 I prog = 1 µa R off = Ω R on = 10 5 Ω Full erase possible Break in pathway New/erased threshold Threshold with electrodeposit present -4.0E Voltage (V)

12 Example of PMC device I-V 240 nm diameter, 50 nm Ag-Ge-S on W cathode following 370 deg. C anneal 1.0E-06 Current(A) 8.0E E E E E+00 I prog = 1 µa R off = Ω R on = 10 5 Ω -2.0E E Voltage(V)

13 Final resistance vs. programming current Resistance (Ohms) 1.00E E E E E+01 Binary coding of full range split into 8 bands R on = V w /i p V w is around 100 mv Band width R = ±40% Range can be split into bands representing states/binary numbers 1.00E E E E E-02 Low power memory range Erasable antifuse range Current (Amps) 2 decades and R = ±25%would give 9 states (3+ bits) 4 decades and R = ±25%would give 18 states (4+ bits)

14 Example of R off /R on vs. operating T Resistance (Ohms) 1.0E E E E E E E+04 E a = 0.3 ev Device programmed at T E a < 0.1 ev I write = 1 µa Temperature (deg.c)

15 Input Cycling example around 10 billion cycles 75 nm diameter, 50 nm Ag-Ge-Se on Ni cathode Start of test 8.9 x 10 9 cycles Test circuit current ( µa) On Off E E E+11 Cycles 1.7 x cycles On transition Off transition 3.2 x cycles

16 Result of 25 nsec write pulse Current(A) 1.0E E E E E E+00 After 25 nsec pulse, on resistance = 1 MΩ Voltage(V) Before 25 nsec pulse, off resistance >13 MΩ Note: write current (and therefore on resistance) limited by test set-up parasitics

17 Low Programming Current (2 µa) Room Temperature Retention On resistance (ohms) 1.0E+08 R off /R on still >10x after 10 years 1.0E+07 <100 mv sense 1.0E E E E E E E+08 Time (seconds) 1 µm diameter, 50 nm Ag-Ge-Se on W cathode

18 Long Term Retention 1.00E ºC E a = 0.2 ev On resistance ( Ω ) 1.00E E RT Initial resistance Sweet spot? 1.00E Programming current (µa)

19 Retention example with elevated sense voltage 1.E+14 1.E+12 Out of range Resistance ( Ω ) 1.E+10 1.E+08 1.E+06 1.E+04 1.E mv sense 1.E+00 1.E-01 1.E+00 1.E+01 1.E+02 1.E+03 1.E+04 1.E+05 1.E+06 Time (s) 2.5 µm diameter, 50 nm Ag-Ge-S on W cathode

20 Scalability to <100 nm Ag Ag PMMA Ni 80 nm Ag 33 Ge 20 Se 47 Ag 33 Ge 20 Se 47 PMMA Ni 40 nm Si0 2 Si0 2 Devices fabricated using electron-beam lithography (EBL) using PMMA as dielectric

21 Nanoscale device example (40 nm) 40 nm diameter, 50 nm Ag-Ge-Se on Ni cathode fabricated using EBL 1.2E-03 Current (A) 1.0E E E E E E E E E-04 I prog = 1 ma R off = 10 7 Ω R on = 10 2 Ω 6 consecutive writre/erase operations -8.0E Voltage (V)

22 Alternative PMC device materials Electrolyte Although chalcogenide glasses such as Ge-S and Ge-Se have excellent ion mobilities, certain oxide glasses make good electrolytes and are very inexpensive to produce and integrate Considering their success in the field of electrochromics, transition metal oxides are a good choice Anode/mobile ions Silver is an ideal material for many reasons but copper has already found its way into the industry and hence is currently cheaper to implement

23 Oxide electrolyte/ag anode example: Tungsten oxide grown on W cathode µm diameter, WOx on W cathode Ag anode Current ( na) I prog = 500 na R off = Ω R on = 10 5 Ω 8 consecutive writre/erase operations Voltage (V)

24 Oxide electrolyte/cu anode example: Tungsten oxide grown on W cathode 1.E+11 1 µm diameter, WOx on W cathode Cu anode Resistance (Ohms) 1.E+10 1.E+09 1.E+08 1.E+07 1.E+06 1.E+05 I prog = 500 na, 3 consecutive writre/erase operations First sweep Second sweep Third sweep Voltage (V)

25 Oxide electrolyte/cu anode example: Tungsten oxide deposited on W cathode 5 µm diameter, 50 nm WOx on W cathode Cu anode I prog = 10 µa, 3 consecutive writre/erase operations

26 Summary of PMC characteristics Scalable Nanostructured materials allow small physical device size Scalable voltage, current, power, and energy! Flexible functionality Non-volatile, fast, good endurance Manufacturable? Simple structure and simple process BEOL additive with copper protocol Devices do not consume silicon real estate Potentially removes the barrier between memory and logic Opens up EPLD applications space