University of Idaho

1
Multi-Layer Phase-Change Electronic Memory Devices

• Kris Campbell
• Associate Professor
• Dept. of Electrical and Computer Engineering
  Dept. of Materials Science and Engineering
• Boise State University

2
Introduction

• Chalcogenide-based memories why do we need a
  new memory technology?
• Types of chalcogenide resistive memories ion
  conducting and phase-change
• Chalcogenide memory stack structures
• Tuning the phase-change memory operating
  parameters
• With materials
• Electrically
• Summary

3
What is a Chalcogenide Material?

• A Chalcogenide material contains one of the
  Group VI elements S, Se, or Te (O is usually
  omitted).
• Some examples of chalcogenides
• GeS germanium sulfide
• SnSe tin selenide
• ZnTe zinc telluride

4
Uses of Chalcogenide Materials
Memory (CDs, electronic)
Energy generation (solar cells)
Photodetectors
Chalcogenide materials are key to many new
technology developments
Environmental pollutant detection
Energy storage (batteries)
5
Why Are New Memory Technologies Under Development?

• Could replace both DRAM and Flash memory types
• DRAM has reached a size scaling limitation and is
  volatile
• Flash is prone to radiation damage, is high
  power, and has a short cycling lifetime
• Radiation resistant
• Scalable
• Low power operation
• Reconfigurable electronics applications
• Potential for multiple resistance states (means
  multiple data states in a single bit)

6
How Does a Chalcogenide Material Act as a Memory?

• Chalcogenide materials can be used as resistance
  variable memory cells
• Logic 0 state Rcellgt 200 kO
• Logic 1 state Rcell 200 O to 100 kO
• The resistance ranges vary quite a bit depending
  upon the material used.

7
ON and OFF State Distributions

• Resistance values in the ON and OFF states have a
  distribution of values
• Threshold voltages or programming currents for ON
  and OFF states also have a distribution of
  possible values.

8
Single Bit Test Structure
Device is here
Top down view
9
Types of Chalcogenide Resistive Memory

• Ion-Conducting
• Ions (e.g. Ag and Cu) are added to a
  chalcogenide glass
• Application of electric field causes formation of
  a conductive channel through glass (Kozicki, M.N.
  et al., Microelectronic Engineering 63, 485
  (2002))
• Thermally Induced Phase Change
• Crystalline to amorphous phase change low R to
  high R shift
• High current heats material to cause phase change
  (S.R. Ovshinsky, Phys. Rev. Lett. 21, 1450 (1968))

10
Ion-Conducting Memories

• Resistance variable memory based on Ag mobility
  in a chalcogenide glass
• Ag is photodoped into a GexSe100-x based
  chalcogenide glass (xlt33).

Visible light
Ag
Ge30Se70
Developed by Axon Technologies (http//www.axontc.
com)
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Ion-Conducting Memories - Operation

• A positive potential applied to the Ag electrode
  writes the bit to a low resistance state
• A negative potential applied to the Ag-containing
  electrode erases the bit to a high resistance
  state.

-

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Ion-Conducting Chalcogenide-Based Memories

• Example material Ge30Se70 photodoped with Ag

From Kozicki, et al. NVMTS, Nov. 2004.
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Why is Glass Stoichiometry Important For
Photodoping?

• Glasses in region I phase separate and form
  Ag2Se.
• Glasses in region II will not phase separate
  Ag2Se but will put Ag on the glass backbone.
• Photodoped Ge30Se70 will form 32 Ge40Se60 and
  68 Ag2Se.

Mitkova, M. et al., Phys. Rev. Lett. 83 (1999)
3848-3851.
14
Traditional Ion-Conducting Structure vs Stack
Structure
Top electrode
Ag
Ag2xSe
Ge30Se70
Ge40Se60
Bottom electrode
Bottom electrode
Traditional Ion-Conducting Memory Structure
Stacked Layer Ion-Conducting Memory Structure
15
Ag2Se-Based Ion-Conducting Memory(Instead of
Photodoping with Ag)
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Ion-Conducting Memory Improvement

• Ag2Se can be replaced with other
  metal-chalcogenides.
• Examples SnSe, PbSe, SnTe, Sb2Se3
• The Ge-chalcogenide must contain Ge-Ge bonds.
• GeSe-based materials are more stable than S or Te
  containing materials.

17
Ion-Conducting Memory Improvement

• Eliminate Ag photodoping
• Use a metal-chalcogenide layer above a GexSe100-x
  glass with carefully selected stoichiometry

Metal Chalcogenide
18
Ion-Conducting Memory Research Projects

• Investigate operational mechanism
• Influence of metal in the Metal-Se layer. Role of
  redox potential
• Glass rigid or floppy
• Type of mobile ion (e.g. Ag or Cu)
• Effects of these on memory properties
• switching speed
• power
• data retention
• resistance distribution
• thermal tolerance

19
What Are Phase-Change Materials?

• Materials that change their electrical resistance
  when they are switched between crystalline and
  glassy (disordered) structures.
• A well-studied example is Ge2Sb2Te5 (referred to
  as GST).

Figure modified from Zallen, R. The Physics of
Amorphous Solids John-Wiley and Sons, New York,
(1983) 12.
Low Resistance
High Resistance
20
Thermally Induced Phase Change
21
Phase Change Memory IV Curve

• One programming voltage polarity.
• Current requirement can be high.
• Voltage application must go beyond VT before
  switching will occur.

Polycrystalline
22
Traditional Phase Change Structure Compared to a
Stack Structure
Top electrode
Top electrode
SnTe
Ge2Sb2Te5
GeTe
Bottom electrode
Bottom electrode
Traditional Phase Change Memory Structure
Stacked Phase Change Memory Structure
23
Phase-Change Memory Multi-Layer Stack Structures

• Tested Devices consist of a core Ge-chalcogenide
  (Ge-Ch) layer and a metal chalcogenide layer
  (M-Ch).

• Properties wanted
• Flexible operational properties tunable via
  materials selection or operating method
• Multiple resistance states
• Low power
• Large cycling lifetime

Device Dimensions 0.25 um via
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Initial Devices Tested

• Initial devices tested consisted of the stacks
• (1) GeTe/SnTe
• (2) Ge2Se3/SnTe
• (3) Ge2Se3/SnSe
• It was found that the material layers used had a
  significant effect on device operation.
• Campbell, K.A. Anderson, C.M. Microelectronics
  Journal, 38 (2007) 52-59.

25
GeTe/SnTe TEM Image
GeTe
W
W
SnTe
Si3N4
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Electrical Characterization Methodology

• Perform a current sweep with the top electrode
  potential either at a V or a -V.
• Perform limited cycling endurance measurements on
  single bit structures.

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Initial Electrical Characterization GeTe/SnTe
Structure, V

• V is on the electrode nearest the SnTe Layer
  (top electrode)

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Initial Electrical Characterization GeTe/SnTe
Structure, -V

• -V is on the electrode nearest the SnTe layer
  (top electrode)

Snap back at a higher V and higher I than the V
case.
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Initial Electrical CharacterizationGe2Se3/SnTe
Structure
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Initial Electrical CharacterizationGe2Se3/SnSe
Structure
No switching!
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Initial Electrical CharacterizationGe2Se3/SnSe
Structure

• A 30nA pre-condition (V),

Followed by -V
Switching!
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Movement of Sn Ions into Ge2Se3 Activates
Operation

• V drives Sn2 or Sn4 ions into the lower glass
  layer, thus allowing it to phase change.
• -V will not produce phase change since Sn ions do
  not move into lower glass.
• An activation (pre-conditioning) step of V at
  very low current (nA) will alter the Ge2Se3
  material, thus allowing phase change operation to
  occur with V.

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Initial Results Summary

• GeTe/SnTe phase change switching, /-V
• Ge2Se3/SnTe phase change switching, /-V
• Ge2Se3/SnSe phase change switching, V -V
  switching only possible after V, low current
  conditioning.
• Sn ions were moved into the Ge-Ch layer during V
  operation.
• Te ions were moved into Ge-Ch layer during -V
  operation.

34
Tuning the Switching Properties

• By selection of stack structure, we can create a
  device with selective operation (on only when
  activated).
• Operational mode depends on the voltage polarity
  used with the device.
• Can we tune the switching properties by altering
  the metal used in the metal chalcogenide layer or
  the electrode materials?

35
Tuning Operating Parameters with Materials

• Ge-Ch stoichiometry Ge-Ge bonds provide a
  thermodynamically favorable pathway for ion
  incorporation.
• Metal-Ch The redox potential, ionic radii,
  oxidation state, and coordination environment
  properties of the metal will impact the ability
  of the metal ion to migrate into and incorporate
  into the Ge-Ch material.
• Addition of other metal ions What happens upon
  the addition of small amounts of Cu or Ag?

36
Testing the Lower Glass and Metal Ion Influence

• We have subsequently tested the following stacks
  
• (1) GeTe/ZnTe metal ion influence
• (2) GeTe/SnSe lower glass influence
• (3) Ge2Se3/SnSe/Ag metal ion
• (4) GeTe/SnSe/Ag metal ion and lower glass
• (5) Ge2Sb2Te5 (GST)/SnTe lower glass
• Resistance switching is observed in all stacks
  but switching properties are different.

37
Current-Voltage Curves of Stack Structures
V applied
38
Effects of M-Ch Layer on Switching
V applied
39
How are the Electrical Properties Altered by
Addition of Ag?

• Devices were tested with
• Ge2Se3/SnSe/Ag
• GeTe/SnSe/Ag

40
Ge2Se3/SnSe/Ag Device Multistate Resistance
Behavior
41
GeTe/SnSe/Ag Device Some Multistate Behavior
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Metal Ion Effects Summary

• The metal ion influences the possible multiple
  resistance states.
• Metal ion allows phase change switching in cases
  where the Ge-Ch normally does not switch.
• We can use the metal ion to alter the voltage
  needed to initiate snap back for phase change
  operation or alter the switching currents.
• Under investigation
• Switching speed and cycle lifetime
• Temperature dependence
• Resistance state retention
• Resistance stability of multistate behavior.

43
Electrical Characterization Lifetime Cycling

• Single bit testing is not ideal, however it does
  provide insight into how the material stack might
  perform over many cycles.

Agilent 33250A Arbitrary Waveform Generator
Agilent Oscilloscope
Micromanipulator
PCRAM Device
Micromanipulator
Rload
Rload is typically 10 kO to 1 kO depending on the
material under study.
44
Electrical Characterization Lifetime Cycling
GeTe/SnTe

• GeTe/SnTe initial tests show bits cycle gt 2
  million times.

Input (red) and V across load resistor (black)
45
Electrical Characterization Lifetime Cycling
Ge2Se3/SnTe

• Ge2Se3/SnTe initial tests show more consistent
  cycling than GeTe/SnTe structures.

Input (red) and V across load resistor (black)
Current through device (calculated by Vload/Rload)
46
Electrical Characterization Lifetime Cycling
Ge2Se3/SnSe

• gt 1e6 cycles
• Operation up to 135 C.

47
Ge2Se3/SnSe/Ag Device CyclingT 135C Rload
1kO
48
GeTe/SnSe/Ag Device Cycling T 30C Rload
1.5kO
49
Materials Questions We Need To Ask

• How are switching parameters altered by the
  materials and stack structure?
• Influence of Ge-Ch structure on switching?
• Properties of the M-Ch work function?
• Metal ion properties? How well does it fit
  into the glass structure? How mobile is the ion
  and what energy is required to cause it to move?
• Adhesion to electrodes?
• Knowing these answers will allow optimization for
  device electrical property tuning.

50
Tuning Operating Parameters Electrically

• Can we find electrical probing techniques that
  will
• Enable well separated resistance states?
• Improve data retention and temperature
  dependence?
• Create a wide dynamic range of allowed resistance
  values in a programmed state?
• What are the operating limitations in order to
  avoid losing the resistance state while in use in
  a circuit?

51
Multiple Resistance States Challenges

• Resistance range can vary as a function of
• Programming current
• Temperature
• Programming pulse parameters
• Retention time of the resistance value can also
  vary as a function of these parameters.
• How well does the resistance state get retained
  during operation as a resistor in a circuit?
• Quite often, due to the nature of the amorphous
  materials, the resistance values have a large
  spread. This overlap prevents reliable use of
  multistate programming with these materials. Can
  we use electrical techniques to help?

52
Example of Poor Programming Resistance
Distributions GeTe/SnSe
53
Electrical Control Reverse Potential
Programming Provides Multiple Resistance States
V
-V
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Electrical Control Summary

• Multistate resistance programming possible by
  programming with negative and positive potentials
  in the Ge-Ch/M-Ch stack structure.
• Electrically controlled activation of stack
  structure allows a device to be turned on when
  it is needed.

55
Summary

• Using Stacked Layers, we have more device
  operational flexibility
• We can control and tune operational parameters
• Threshold voltage, programming current, speed,
  retention, endurance
• Value of resistance states
• Number of possible resistance states
• We can electrically control device function
• Electrically activated devices
• Larger dynamic range between resistance states

56
Acknowledgements

• Collaborators
• Prof. Jeff Peloquin, Boise State University
  synthesis of materials.
• Mike Violette, Micron Technology equipment loan
  and use of analytical facilities for thin film
  characterization (SEM, ICP, TEM).
• Prof. Santosh Kurinec, Rochester Institute of
  Technology characterization of thin film stacks
  using XRD, RBS, Raman development of CMOS-based
  test array for materials stacks.
• Students
• Morgan Davis, Becky Munoz, Chris Anderson, Daren
  Wolverton.
• Funding This research was partially supported
  by a NASA Idaho EPSCoR grant, NASA grant
  NCC5-577.

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Phase-Change Memory Radiation Resistance
Phase-Change Memory
ON state Even if some regions in the crystalline
material are disturbed by SEE or TID, the
crystallinity in the rest of the cell will keep R
low.
OFF state Complete crystallization is not
induced by SEE or TID. Localized
crystallization can occur.
El-Sayed, S.M. Nuclear Instruments and Methods
in Physics Research B 225 (2004) 535-543.
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Ion-Conducting Memory Radiation Resistance
Ion-Conducting Memory
OFF State Material is disordered, SEE or TID
will not affect it.
ON State Ag filling the conductive channel
would have to be completely displaced from
contact with either electrode.