University of Idaho ECE Research Colloquium
March 8, 2007
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
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
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
Energy generation(solar cells)
Photodetectors
Environmentalpollutant detection
Energy storage(batteries)
Memory(CD’s, electronic)
Chalcogenide materials are key to
many new technology developments
Uses of Chalcogenide Materials
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)
How Does a Chalcogenide Material Act as a Memory?
Chalcogenide materials can be used as resistance variable memory cells: Logic ‘0’ state: Rcell> 200 kΩ Logic ‘1’ state: Rcell= 200 Ω to 100 kΩ
The resistance ranges vary quite a bit depending upon the material used.
‘0’ ‘1’
1 MΩ
Write, Vw
V
10 kΩ
V
Erase, Ve
OFF ON
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.
1.0
0.8
0.6
0.4
0.2
Dis
trib
utio
n
Resistance
ON OFF
1k to 200k 1M to 1G
Single Bit Test Structure
Top down viewDevice is here
Bottom electrode
Top electrode
Insulator
Memory cell
Metal-chalcogenide
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))
Ion-Conducting Memories
Resistance variable memory based on Ag+ mobility in a chalcogenide glass;
Ag is photodoped into a GexSe100-x based chalcogenide glass (x<33).
Ag
Ge30Se70
Visible light
(Ge40Se60)33 (Ag2Se)67
Developed by Axon Technologies (http://www.axontc.com)
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.
(Ge2Se3)33(Ag2Se)67
V
Ag electrode
+
-
-
+
Ion-Conducting Chalcogenide-Based Memories
Example material: Ge30Se70 photodoped with Ag
Ag
(Ge30Se70)67Ag33
W
V
From Kozicki, et al. NVMTS, Nov. 2004.
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.
Traditional Ion-Conducting Structure vs Stack Structure
Ag
Ge30Se70
Bottom electrode
Ag2+xSe
Top electrode
Ge40Se60
Bottom electrode
Traditional Ion-Conducting Memory Structure
Stacked Layer Ion-Conducting Memory Structure
Ag2Se-Based Ion-Conducting Memory(Instead of Photodoping with Ag)
Ge40Se60
V
W electrode
+
-
Ag2+Se
W electrode
20
15
10
5
0
-5
-10
Cur
rent
(m
icro
amps
)
-0.4 -0.2 0.0 0.2 0.4Voltage
‘1’Low R
‘0’High R VwVe
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.
Ge40Se60
V
W electrode
+
-
SnSe
W electrode
Ag
Ion-Conducting Memory Improvement Eliminate Ag photodoping
Use a metal-chalcogenide layer above a GexSe100-x glass with carefully selected stoichiometry
20
15
10
5
0
-5
-10
Cur
rent
(m
icro
amps
)
-0.4 -0.2 0.0 0.2 0.4Voltage
‘1’Low R
‘0’High R VwVe
Ge40Se60
V
W electrode
+
-
SnSe
W electrode
Ag
Metal Chalcogenide
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
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
Thermally Induced Phase Change
Creates Low R State
Creates High R State
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
Traditional Phase Change Structure Compared to a Stack Structure
Bottom electrode
Top electrode
Ge2Sb2Te5
Top electrode
SnTe
Bottom electrode
GeTe
Traditional Phase Change Memory Structure
Stacked Phase Change Memory Structure
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
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.
GeTe/SnTe TEM Image
WSnTe
Si3N4
GeTe
W
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.
10-11
10-10
10-9
10-8
10-7
10-6
10-5
10-4
Cur
rent
(A
)
1.41.21.00.80.60.4Voltage
Initial Electrical Characterization GeTe/SnTe Structure, +V+V is on the electrode nearest the SnTe Layer (top electrode)
Initial Electrical Characterization GeTe/SnTe Structure, -V
10-11
10-10
10-9
10-8
10-7
10-6
10-5
10-4
Cur
rent
(A
)
2.52.01.51.00.50.0Voltage (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.
Initial Electrical CharacterizationGe2Se3/SnTe Structure
10-11
10-10
10-9
10-8
10-7
10-6
10-5
10-4
Cur
rent
(A
)
1.41.21.00.80.60.40.2Voltage
+V -V
Initial Electrical CharacterizationGe2Se3/SnSe Structure
10-11
10-10
10-9
10-8
10-7
10-6
10-5
10-4
Cur
rent
(A
)
1086420Voltage
+V -VNo switching!
Initial Electrical CharacterizationGe2Se3/SnSe Structure
A 30nA pre-condition (+V), Followed by -V
10-11
10
-9
10-7
10-5
Cur
rent
(A
)
3.02.01.00.0Voltage (V)
10-11
10-10
10-9
10-8
10-7
Cur
rent
(A
)
3.02.01.00.0Voltage (V)
(a) (b)
Switching!
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.
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.
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?
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?
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.
Current-Voltage Curves of Stack Structures
10-11
10-10
10-9
10-8
10-7
10-6
10-5
10-4
Cur
rent
(A
)
3.53.02.52.01.51.00.50.0Voltage
Ge2Se3/SnTe Ge2Se3/SnSe GeTe/SnTe GST/SnTe GST
+Vapplied
Effects of M-Ch Layer on Switching
10-11
10-10
10-9
10-8
10-7
10-6
10-5
10-4
Cur
rent
(A
)
3.53.02.52.01.51.00.5
Voltage
GeTe/ZnTe GeTe/SnTe
+Vapplied
How are the Electrical Properties Altered by Addition of Ag?
Devices were tested with: Ge2Se3/SnSe/Ag
GeTe/SnSe/Ag
W
Sn-ch
W
Si3N4
Ge-ch
+
_
Ag
Ge2Se3/SnSe/Ag Device – Multistate Resistance Behavior
100
80
60
40
20
0
Cur
rent
(A
)
0.140.120.100.080.060.040.020.00Voltage (V)
5K5K
2K
700
1K
GeTe/SnSe/Ag Device – Some Multistate Behavior
100
80
60
40
20
0
Cur
rent
(A
)
0.350.300.250.200.150.100.050.00Voltage (V)
3k
1k
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.
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 33250AArbitrary Waveform
GeneratorAgilent Oscilloscope
Micromanipulator
Micromanipulator
Rload
PCRAM Device
Rload is typically 10 kΩ to 1 kΩdepending on the material under study.
Electrical Characterization – Lifetime Cycling – GeTe/SnTe
GeTe/SnTe – initial tests show bits cycle > 2 million times.
Input (red) and V across load resistor (black)
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 (calculatedby Vload/Rload)
Electrical Characterization – Lifetime Cycling –Ge2Se3/SnSe
> 1e6 cycles
Operation up to 135 °C.
8
6
4
2
0
Am
plitu
de (
V)
3210Time (ms)
Erase
Read
Write
Read
Vout Vin
Ge2Se3/SnSe/Ag Device CyclingT = 135°C; Rload = 1kΩ
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5V
olta
ge (
V)
4003002001000Time (s)
InputResponse after given number of cycles:
101
102
103
104
105
106
Write
Read
Erase
Read
GeTe/SnSe/Ag Device Cycling T = 30°C; Rload = 1.5kΩ
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5V
olta
ge (
V)
6005004003002001000Time (s)
InputResponse after given number of cycles:
101
102
103
104
105
106
Write
Erase
Read
Read
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.
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?
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?
Example of Poor Programming Resistance Distributions: GeTe/SnSe
104
2
4
6
105
2
4
6
106
2
Re
sist
an
ce (
Oh
ms)
86420Device Number
5678
104
2
3
4
5
678
105
Re
sist
an
ce (
Oh
ms)
86420Device Number
+ potential - potential Programming Current
100uA 1mA
Electrical Control: Reverse Potential Programming Provides Multiple Resistance States
104
105
106
107
108
109
Res
ista
nce
(Ohm
s)
86420Device Number
100A max Reverse potential 1mA max OFF
+V-V
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.
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
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.
Phase-Change Memory Radiation Resistance
OFF state:Complete crystallization is not induced by SEE or TID. Localized crystallization can occur.*
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.
Phase-Change Memory
* El-Sayed, S.M. Nuclear Instruments and Methods in Physics Research B 225 (2004) 535-543.
Metal 2
Metal 1
Rc1 Rc2 Chalcogenide
Crystalline
Amorphous
Ra1 Ra2
Metal 2
Metal 1
Rc1 Rc2
Ra1 Ra2
Chalcogenide
Crystalline
Amorphous
Ion-Conducting Memory Radiation Resistance
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.
Ion-Conducting Memory
(Ge2Se3)33(Ag2Se)67
V
Ag electrode
+
-
(Ge2Se3)33(Ag2Se)67
V
Ag electrode
+
-
