Download Campbell - Microelectronics Research and Communications

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
University of Idaho
ECE Research Colloquium
March 8, 2007
Introduction
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
 Chalcogenide-based
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
Uses of Chalcogenide Materials
Energy generation
(solar cells)
Memory
(CD’s, electronic)
Chalcogenide
materials are key to
many new technology
developments
Environmental
pollutant detection
Photodetectors
Energy storage
(batteries)
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.
V
V
Write, Vw
10 kΩ
1 MΩ
Erase, Ve
‘0’
OFF
‘1’
ON
ON and OFF State Distributions
 Resistance values in the ON and OFF states have a
distribution of values;
1.0
Distribution
0.8
0.6
ON
OFF
1k to 200k
1M to 1G
0.4
0.2
Resistance
 Threshold voltages or programming currents for ON and
OFF states also have a distribution of possible values.
Single Bit Test Structure
Top electrode
Memory cell
Metalchalcogenide
Insulator
Bottom electrode
Device is here
Top down view
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).
Visible light
Ag
Ge30Se70
(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;
+Ag electrode
V
(Ge2Se3)33(Ag2Se)67
 A negative potential applied to the
Ag-containing electrode erases the
bit to a high resistance state.
-+
Ion-Conducting Chalcogenide-Based
Memories
Example material: Ge30Se70 photodoped with Ag
Ag
V
(Ge30Se70)67Ag33
W
From Kozicki, et al. NVMTS, Nov. 2004.
Why is Glass Stoichiometry Important
For Photodoping?
 Glasses in region I
Mitkova, M.; et al., Phys. Rev. Lett. 83 (1999)
3848-3851.
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.
Traditional Ion-Conducting Structure
vs Stack Structure
Ag
Ge30Se70
Bottom electrode
Traditional Ion-Conducting
Memory Structure
Top electrode
Ag2+xSe
Ge40Se60
Bottom electrode
Stacked Layer Ion-Conducting
Memory Structure
Ag2Se-Based Ion-Conducting Memory
(Instead of Photodoping with Ag)
20
‘1’
Low R
Current (microamps)
15
+
W electrode
10
V
Ag2+Se
5
Ge40Se60
0
-5
-10
-0.4
W electrode
‘0’
High R
-0.2
Ve
0.0
Voltage
-
Vw
0.2
0.4
Ion-Conducting Memory Improvement

Ag2Se can be replaced
with other metalchalcogenides.



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.
+
W electrode
Ag
V
SnSe
Ge40Se60
W electrode
-
Ion-Conducting Memory Improvement
 Eliminate Ag photodoping
 Use a metal-chalcogenide layer above a GexSe100-x
glass with carefully selected stoichiometry
20
+
‘1’
Low R
W electrode
Ag
V
SnSe
Metal Chalcogenide
Ge40Se60
W electrode
-
Current (microamps)
15
10
5
0
-5
-10
-0.4
‘0’
High R
-0.2
Ve
0.0
Voltage
Vw
0.2
0.4
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” JohnWiley and Sons, New York,
(1983) 12.
Low
Resistance
High
Resistance
Thermally Induced Phase Change
Creates High R State
Creates Low R State
Phase Change Memory IV Curve
 One
programming
voltage polarity.
 Current
requirement can
be high.
 Voltage
Polycrystalline
application must
go beyond VT
before switching
will occur.
Traditional Phase Change Structure
Compared to a Stack Structure
Top electrode
Top electrode
SnTe
Ge2Sb2Te5
Bottom electrode
Traditional Phase Change
Memory Structure
GeTe
Bottom electrode
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 (MCh).
 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
GeTe
W
SnTe
Si3N4
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.
Initial Electrical Characterization
GeTe/SnTe Structure, +V
+V is on the electrode nearest the SnTe Layer (top electrode)
-4
10
-5
10
-6
Current (A)
10
-7
10
-8
10
-9
10
-10
10
-11
10
0.4
0.6
1.0
0.8
Voltage
1.2
1.4
Initial Electrical Characterization
GeTe/SnTe Structure, -V
-V is on the electrode nearest the SnTe layer (top electrode)
-4
10
-5
10
-6
Current (A)
10
-7
10
-8
10
-9
10
-10
10
-11
10
0.0
0.5
1.0
1.5
Voltage (V)
2.0
2.5
Snap back at a higher V and higher I than the +V case.
Initial Electrical Characterization
Ge2Se3/SnTe Structure
-4
10
-5
10
-6
Current (A)
10
-7
10
-8
10
+V
-V
-9
10
-10
10
-11
10
0.2
0.4
0.6
0.8
Voltage
1.0
1.2
1.4
Initial Electrical Characterization
Ge2Se3/SnSe Structure
-4
10
-5
10
-6
Current (A)
10
-7
10
-8
10
+V
-V
-9
10
No switching!
-10
10
-11
10
0
2
4
6
Voltage
8
10
Initial Electrical Characterization
Ge2Se3/SnSe Structure
A 30nA pre-condition (+V),
Followed by -V
-7
-5
10
-8
Current (A)
Current (A)
10
10
-9
10
-10
10
-11
-7
10
-9
10
Switching!
-11
10
10
0.0
1.0
2.0
Voltage (V)
(a)
3.0
0.0
1.0
2.0
Voltage (V)
(b)
3.0
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
-4
+V
10
applied -5
10
-6
Current (A)
10
-7
10
-8
10
Ge2Se3/SnTe
Ge2Se3/SnSe
GeTe/SnTe
GST/SnTe
GST
-9
10
-10
10
-11
10
0.0
0.5
1.0
1.5
2.0
Voltage
2.5
3.0
3.5
Effects of M-Ch Layer on Switching
+V
applied
-4
10
-5
10
-6
Current (A)
10
-7
10
GeTe/ZnTe
GeTe/SnTe
-8
10
-9
10
-10
10
-11
10
0.5
1.0
1.5
2.0
Voltage
2.5
3.0
3.5
How are the Electrical Properties
Altered by Addition of Ag?
 Devices were tested with:
Ge2Se3/SnSe/Ag
 GeTe/SnSe/Ag

W
+
Ag
Sn-ch
Ge-ch
Si3N4
W
_
Ge2Se3/SnSe/Ag Device – Multistate
Resistance Behavior
100
700
Current (A)
80
60
1K
40
20
2K
5K
5K
0
0.00
0.02
0.04
0.06 0.08 0.10
Voltage (V)
0.12
0.14
GeTe/SnSe/Ag Device – Some Multistate
Behavior
100
Current (A)
80
60
1k
40
3k
20
0
0.00
0.05
0.10
0.15
0.20
Voltage (V)
0.25
0.30
0.35
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 33250A
Arbitrary Waveform
Generator
Agilent Oscilloscope
Micromanipulator
PCRAM Device
Micromanipulator
Rload
Rload is typically 10 kΩ to 1 kΩ
depending 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
(calculated
by Vload/Rload)
Electrical Characterization – Lifetime
Cycling –Ge2Se3/SnSe
 > 1e6
Erase
6
Amplitude (V)
cycles
 Operation
up to 135
°C.
8
Vout
Vin
4
Write
2
Read
Read
0
0
1
2
Time (ms)
3
Ge2Se3/SnSe/Ag Device Cycling
T = 135°C; Rload = 1kΩ
1.5
Write
1.0
0.5
Voltage (V)
Input
Response after given
number of cycles:
1
10
2
10
3
10
4
10
5
10
6
10
Read
0.0
Read
-0.5
-1.0
Erase
-1.5
0
100
200
Time (s)
300
400
GeTe/SnSe/Ag Device Cycling
T = 30°C; Rload = 1.5kΩ
1.5
Write
1.0
Voltage (V)
Input
Response after given
number of cycles:
1
10
2
10
3
10
4
10
5
10
6
10
0.5
Read
0.0
Read
-0.5
-1.0
Erase
-1.5
0
100
200
300
400
Time (s)
500
600
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
+ potential
Programming Current
100uA
1mA
- potential
5
10
8
7
6
5
2
Resistance (Ohms)
Resistance (Ohms)
6
10
6
4
2
5
10
6
4
4
3
2
4
10
8
7
6
5
2
4
10
0
2
4
6
8
Device Number
0
2
4
6
8
Device Number
Electrical Control: Reverse Potential
Programming Provides Multiple Resistance States
9
10
8
Resistance (Ohms)
10
100A max +V
Reverse potential 1mA max -V
OFF
7
10
6
10
5
10
4
10
0
2
4
6
Device Number
8
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
Phase-Change Memory
ON state:
OFF state:
Complete crystallization is not induced
by SEE or TID.
Localized crystallization can occur.*
Metal 2
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.
Metal 2
Rc1
Rc2
Rc1
Chalcogenide
Rc2
Crystalline
Ra1
Crystalline
Ra2
Ra1
Amorphous
Metal 1
* El-Sayed, S.M. Nuclear Instruments and Methods
in Physics Research B 225 (2004) 535-543.
Chalcogenide
Ra2
Amorphous
Metal 1
Ion-Conducting Memory Radiation
Resistance
Ion-Conducting Memory
OFF State: Material is
ON State: Ag filling the
disordered, SEE or TID will not
affect it.
conductive channel would have
to be completely displaced from
contact with either electrode.
+
+
Ag electrode
Ag electrode
V
(Ge2Se3)33(Ag2Se)67
V
(Ge2Se3)33(Ag2Se)67
-
-