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NASA Ames Research Center

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1 NASA Ames Research Center
Phase-Change Nonvolatile Memory: Current Status and Possible Advances using Nanowires M. Meyyappan NASA Ames Research Center Moffett Field, CA Acknowledgement: Bin Yu, Xuhui Sun

2 Requirements for an Ideal Memory
Combine the best of these: - High speed of the Static Random Access Memory (SRAM) - Nonvolatile nature of flash memory - Density of DRAM Low Cost Scalability Some Candidates Magnetic RAM Ferroelectric RAM Ovonic Unified Memory (after Ovshinsky who proposed it in 1968) or Phase-change Random Access Memory (PRAM)

3 Phase Change Materials
• Phase change materials date back to 1960s - Mainstream optical storage media (CD-RW, DVD-RW) • Common phase-change material candidates - GeTe, GeSbTe, In2Se3, InSb, SbTe, GaSb, InSbTe, GaSeTe, … - Thermally induced phase change (orderly single crystalline or polycrystalline C-phase vs less orderly amorphous -phase)

4 Phase-Change Random Access Memory (PRAM)
Electrically operated phase-change Random Access Memory (PRAM) - Proposed nearly 3 decades ago - Binary or multiple resistive states of the programmable element to represent logic levels • PRAM advantages Simpler fabrication than FET-based NVMs Improved endurance (resistor-based) Faster read/write Binary or multiple resistive states Soft-error or radiation free operation

5 If that good, Why is it not on the market?
• PRAM Issues - Large programming current to generate the thermal energy needed for inducing the phase change - Joule heating induced power dissipation issues - Intercell thermal interference - Scaling difficulties due to the above In the beginning, reluctance to introduce an unknown complex alloy into main stream silicon processing At the same time, silicon flash memory started to advance rapidly All of this together put the brakes somewhat, slowing down developments

6 Why Renewed Interest in PRAM Now?
Difficulties scaling Flash Memory, leading to extensive search for alternatives Phase change material is no longer mysterious, much more known about these alloys in the last two decades Steady developments in addressing key issues - programming current reduction - endurance - intercell interferance - new architectures Major players involved - Intel, IBM, Samsung, Infineon, Philips…

7 PRAM Technology Advancement
IBM Infineon Macronix PCRAM Joint Project (2006 VLSI Symp) Pillar phase change memory, 180 nm CMOS N-doped GST pillar on top of W contact Current-confining pillar leads to self heating Reset current 900 µA (at 75 nm diameter) Endurance test 106 cycles

8 PRAM Technology Advancement
ST Microelectronics, VLSI Tech. Digest (2006) 90 nm PRAM, cell area 12 F2 Vertical PNP BJT Selector device base of BJT is the word line emitter connected to the bottom of PRAM cell Programming current ~ 400 µA Endurance 108 reset/set cycles

9 PRAM Technology Advancement
Cross bar array of PRAM devices Efforts to confine switching volume into nanometer scale Fabrication at 60 nm scale with UV nanoimprint lithography Lee et al Microelec. Eng. Vol. 84, (2007)

10 PRAM Technology Insight from 3-D Modeling
Current density and temperature at the GST/electrode interface are lower than in the center of the GST layer; due to larger heat dissipation on the TiN electrode. In the center, 616°C is reached in 3 ns. Temporal evolution of temperature and phase help to understand dynamics. Kim et al J. Appl. Phys. 101, (2007) (RWTH Aachen University)

11 PRAM Technology Advancement
Tip based memory, no separate selecting device Sharp scanning tip reduces the volume between the head that does R/W and the PRAM layer Slow scanning speed is to be compensated by parallel array of tips, just as in Millipede Modeling shows bits that are nm dia and tips ~ 20 nm dia are possible and can give Tbit/inch2 Wright et al, IEEE Trans. Nanotech. Vol. 5, p 50 (2006) U. of Exeter

12 PRAM with Etched-down wires
M. Lankhorst et al., Nat. Mater., Vol. 4, 2005, P (Philips) Y. Chen et al , IEDM, 2006 (IBM) narrow line of GeSb Reset 90 µA for 60 nm2 Restraints to aggressive scaling Critical material dimension depends on “top-down” process Lithography resolution, etching-induced surface damage, line-edge roughness, difficulty to achieve high aspect ratio geometry, and more. Fabrication cost increases dramatically as critical size scale down

13 Limits of Phase Change Properties
Does phase change property exist when you reach extremely small particle size? Test cell: Aluminum top electrode 20 µm GST layer 100 nm p-doped silicon wafer In situ laser ablation to generate < 10 nm GST particles Phase change properties exist down to 10 nm particle size Projected reset current 1 µA

14 Why 1-D Phase-Change Nanowire?
Nanoscale Benefits Smaller cell volume, leads to direct reduction of energy needs Reduced melting point (30-50%) Reduced thermal conductivity (1-2 orders of mag.) Large aspect ratio (self-heating resistor) Perfect surface morphology (not etched) Growth Benefits Highly scalable critical size – diameter depends on catalyst size (down to ~ a few nm) Etching-free One-step LPCVD or MOCVD

15 Projected Nanowire-based Memory Features
Ultra-low reset current (< 10µA/cell) Possible very low-cost manufacturing Pitch depends on innovative cell design Break lithography limit with high-density template-guided large-scale self-assembly Vertical nanowire (NASA-Ames)

16 Nanowire Chemical Synthesis
Vapor-Liquid-Solid (VLS) Mechanism Schematic diagram of thermal evaporation CVD Carrier gas flow

17 Family of Phase-Change Materials
Binary Ternary Quaternary Ge Te Ge Sb Te Ag In Sb Te In Sb In Sb Te (Ge Sn)Sb Te In Se Ga Se Te Ge Sb (Se Te) Sb Te Sn Sb Te Te Ge Sb S Ga Sb In Sb Ge … … … Critical Parameters Melting Point Phase-change energy threshold Reliability/stability Multi-level storage Electrical Resistance Self-heating efficiency Programming energy Thermal Conductivity & Specific Heat (Red: nanowires synthesized at NASA-Ames)

18 Binary /Ternary Phase-Change Nanowires
X. Sun, B. Yu, M. Meyyappan, abstract submitted to MRS Spring Meeting, 2008

19 GeTe Nanowires: TEM, SAED, and EDS
<110> 40 nm (a) TEM image of an individual GeTe nanowire with a diameter of ~ 40 nm. The inset shows an SAED pattern of fcc cubic lattice structure. (b) EDS spectrum of the same GeTe nanowire. X. Sun et al., JPCC, 111, 2421 (2007)

20 In2Se3 Nanowires: TEM and EDS Spectra
d ~ 40nm In:Se2:3 TEM image and corresponding EDS spectra of an individual In2Se3 nanowire. Scare bar is 100 nm. X. Sun et al., APL, 89, (2006)

21 GeSb Nanowires: TEM, SAED and EDS
40 nm 100 nm Ge:Te≈1:1 Ge:Sb ≈ 7:93 ~ 9:91 Diameter range: nm

22 GeTe Nanowires: Melting Experiment and In-Situ Monitoring by TEM
Liquid GeTe In-situ Tm measurement of GeTe nanowire under TEM image monitoring (a) The GeTe nanowire is under room temperature. (b) The GeTe nanowire is heated up to 400C when the nanowire melts and its mass is gradually lost through evaporation. The remaining oxide shell can be seen from the image. X. Sun et al., J. Phys. Chem. C, Vol. 111 (2007)

23 PCM Nanowires: Melting Point
GeTe (d=70nm) In2Se3 (d=40nm) Bulk Tm 725°C 890°C Nanowire Tm 390°C 680°C Reduction 46% 24% Tm of bulk PCM Tm of PCM nanowires The melting temperature of the phase-change nanowire is identified as the point at which (1) the electron diffraction pattern disappears and (2) the nanowire starts to evaporate. This property is diameter-dependent: reduction even more significant for smaller diameters

24 Melting Point Reduction
 = ∆ = Deviation of melting point from the bulk value To = Bulk melting point  = Surface tension coefficient for a liquid-solid interface  = Material density r = Nanowire radius AR = Nanowire aspect ratio L = Latent heat of fusion

25 Electron-Beam Based GST Nanowire Thermal Programming
After 5-sec e-beam localized thermal writing from crystal to amorphous Before After d ~ 55nm Thermally induced nano-encoding on an individual 1-D GST nanowire with scanning focused electron beam. A series of α-GST nanodots were created by highly localized thermal heating with an e-beam spot. The amorphous-to-crystalline boundary is marked by red dash line. X. Sun et al., APL, 90, (2007)

26 Nanowire Phase-Change Memory Prototype Fabrication/Measurement

27 In2Se3 Nanowire Memory Switching Behavior
In2Se3 nanowire phase change memory switching behavior as a function of reset/set pulse voltage Pulse width: (a) Reset at 20 nsec. (b) Set at 100 μsec. Could be further reduced via memory size scaling B. Yu et al., Appl. Phys. Lett. 91, (2007)

28 In2Se3 Nanowire Memory I-V R-V
Device characteristics in either state with successive measurement sweeps show stable resistive behavior Dynamic switching ratio (on/off resistance) is ~105

29 In2Se3 Nanowire Memory Repeated Reset-set Cycles
Reading Repeated resistance measurement of In2Se3 phase-change nanowire memory device. Device was switched between high- and low-resistive states using voltage pulses: LRS - HRS using 7V / 20 ns reset pulse; HRS - LRS using 5V / 100 S set pulse.

30 GeTe Nanowire Phase-Change Memory
@2.5V @1.1V GeTe nanowire phase change memory switching behavior as a function of reset/set voltage Pulse width: (a) Reset at 20 nsec. (b) Set at 20 μsec.

31 PRAM Performance Comparison: Nanowire vs. Thin Film
Storage Media In2Se3 nanowire In2Se3 Thin Film Resistive Switching Ratio 105 103 Reset Current 11 A 0.4 mA Reset Power/Energy 80 W / 1.6 pJ 16 mW / 1.12 nJ Set Power/Energy 0.25 nW / 25 fJ 14 W / 140 pJ Reference Our Work IEEE Trans. Mag. 41, 1034 (2005)

32 Why In2Se3 is Attractive? High resistance material, leading to inherently lower current Highly amenable for multilevel operation No need for doping and other strategies used with GST to increase resistance No phase segregation problems as are likely with ternary alloys

33 Nanowire PRAM Performance

34 Challenges ahead for Nanowire Approach
Controlled growth direction, diameter Overall process flow development; such things have never been done before Scalability modeling to demonstrate minimum pitch without interference If successful, Advantages would be Scalability at reduced cost (down to 5 nm) Thin film PRAM pitch is way larger than litho limit. NW-PRAM can be pushed to state-of-the-art litho limit (if catalyst is patterned) or lower (if catalyst self-assembled) Thin film PRAM footprint is large with large selecting device to deliver the needed current. NW-PRAM will also reduce the selecting device size and overall footprint.

35 Ultimate Phase-Change Memory ?
Technology Goal Technology Challenges Develop low-power high-density data storage using nanomaterial array, enabling 102~103X faster R/W, 10~15X lower write voltage, and 10~100X higher integration density Super-scalable R-switching nanowire memory Large-scale self-assembly / patterned assembly 3-D integration / selecting device Next-generation highly scalable, ultra-low power, resistive switching non-volatile memory chip technology based on phase-change nanomaterials Technical Approach Resistive Switching in PC Nanowire Anticipated Performance Metrics 3-D vertical nanowire array Non-charge-based (radiation-free) Significantly reduced thermal writing energy (102~103X) Super scalable memory cell Reduced thermal interference Multi-layer stacking for high integration density Binary or analog data storage Low temperature assembly compatible with Si-IC platform Target 0.5~1 V R/W operation 1 µA/cell reset current Erasing (reset) Programming (set) 1 TB/cm2 density <10-12 J/bit switch energy Nanowire before programming electrode Nanowire after programming Less than 10 ns write time 1010 cycle endurance High-resistance state Low-resistance state

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