Non-Volatile Memory Based on Electrodeposition in Solid Electrolytes
M.N. Kozicki, M. Balakrishnan, C. Gopalan, M. Park, and M. Mitkova
Center for Solid State Electronics Research
Arizona State University
Tempe, AZ 85287-6206, USA
Phone - (480) 965-2572, Fax - (480) 965-8118, e-mail - michael.kozicki@asu.edu
Abstract
Programmable Metallization Cell (PMC) memory is based
on the electrochemical control of nanoscale quantities of
metal in thin films of solid electrolyte. A silver layer and an
inert electrode formed in contact with a Ag+-containing
electrolyte film creates a device in which information is
stored via the electrodeposition of silver. This results in large
non-volatile resistance changes.
This paper highlights
voltage, current, speed, endurance, and retention aspects of
PMC as well as the process temperature stability of Ag-Ge-S
electrolyte devices.
Keywords: Non-volatile memory, solid electrolytes,
chalcogenide glass, electrodeposition, resistance change.
Top
electrode
Silver
source
Electrolyte
Dielectric
Device
diameter (D)
Device
layers
50 nm
Bottom
electrode
Introduction
The semiconductor industry has acknowledged, via the
International Technology Roadmap for Semiconductors, that
there will be severe problems with the scaling of solid state
memory as we move toward the end of this decade. Physical
size reduction of memory based on charge storage (e.g.,
DRAM, Flash) will result in unacceptable retention or state
detection characteristics and the voltage, power, and cost
requirements of future systems rule out most other
approaches.
Programmable Metallization Cell (PMC)
memory, which is based on the electrochemical control of
nanoscale quantities of metal in thin films of solid electrolyte,
shows great promise as an ultra-scalable and manufacturable
solid state memory [1]. Key attributes are low internal
voltage, low power consumption, and the ability for the
storage cells to be physically scaled to minimum lithographic
dimensions. Electrolyte formation is a relatively simple
process, involving the dissolution of silver in a chalcogenide
(e.g., Ge-Se, Ge-S) glass. Standard processing equipment
may be utilized and no high temperature steps are necessary.
A silver-containing layer and an inert electrode formed in
contact with the electrolyte film (see Fig. 1) creates a device
in which information is stored via electrical changes caused
by the oxidation of the anode silver and reduction of silver
ions in the electrolyte. This occurs at an applied bias as low
as a few hundred mV and can result in a resistance change of
many orders of magnitude even for currents in the µA range.
A reverse bias of the same magnitude will reverse the process
until the excess silver has been removed, thereby erasing the
device. Since information is retained via metal atom
electrodeposition rather than charge storage, PMC memory is
non-volatile with excellent retention characteristics.
SiO2
Si
Fig. 1. Field emission scanning electron microscope photograph of a
cross section of a PMC memory device, showing the main layers.
This device was formed in a 40 nm via in a PMMA dielectric.
This paper shows the basic attributes of PMC memory
devices. Attention is given to current-voltage characteristics,
which illustrate low voltage and low current operation, as
well as speed, endurance, and elevated T retention
capabilities. Recent results that show the process temperature
stability of Ag-Ge-S electrolyte devices are also presented.
Device Fabrication and Results
Fig. 1 above illustrates the basic structure of a PMC
memory test device. The bottom electrode is typically
tungsten although it can be composed of various
electrochemically indifferent materials (e.g., nickel). The
dielectric can be a variety of materials although we have
concentrated on two: SiO2 for devices with D > 100 nm and
poly-methylmethacrylate (PMMA) for sub-100 nm devices
(the device of Fig. 1 is 40 nm in diameter and was fabricated
using electron-beam exposure of a PMMA dielectric film on
Ni). The solid electrolyte is typically formed after via etch by
depositing a 50 nm thick Ge-Se or Ge-S base glass layer
followed by a 25 nm thick layer of Ag, both by PVD. The Ag
1.0E-06
On state
Current (A)
8.0E-07
6.0E-07
4.0E-07
2.0E-07
Off state
0.0E+00
-2.0E-07
-0.3
-0.1
0.1
Voltage (V)
0.3
0.5
Fig. 2. Current-voltage plot of a W electrode device (D = 240 nm)
with a 1 µA current limit. Write threshold is 240 mV. Complete
erase occurs beyond -150 mV. Arrows indicate V sweep directions.
is then photodissolved into the base glass using uv light (405
nm, 1 J/cm2) at room temperature. The photodissolved Ag
will combine with Se- or S- rich base glasses to form a solid
electrolyte with high Ag ion mobility and availability [2,3].
An additional 50 nm of Ag is added to create a source for the
electrodeposition and this is capped with the top electrode
layer (this can also be a variety of metals but we tend to use
Au to prevent tarnishing of the silver during testing).
Fig. 2 above shows a 1 µA limit current-voltage curve for a
50 nm thick Ag-Ge-Se solid electrolyte on W (D = 240 nm,
SiO2 dielectric). The device switches from an Roff of >1010 Ω
(limit of the measurement apparatus) to an Ron near 200 kΩ at
240 mV. This result is typical for tungsten electrode devices
of 1 µm diameter and less [1] (Ni electrode devices, such as
that in Fig. 1, write at 180 mV). Ron is independent of area as
electrodeposition occurs in a region of only a few tens of nm
in diameter. For a current-limited write, Ron is inversely
proportional to the write current; if we use a 10 µA limit, the
on state resistance is closer to 20 kΩ. The electrodeposit is
essentially dissolved at -150 mV and the device returns to its
high resistance state. One consideration for Se-based devices
is that they do not tolerate processing temperatures above 230
°C without modification and so a low temperature back end of
1.0E-06
8.0E-07
Fig. 4. Switching/cycling characteristics after 1.25 x 1011 cycles
(time axis is 200 nsec/div). Switching is limited by the rise/fall time
of the input signal (35 nsec). The bold line is the smoothed signal.
line process is necessary. The Ag-Ge-S electrolyte devices
have much better process temperature stability with
acceptable changes following high temperature processing;
write voltage rises from 250 to 380 mV and erase from -100
to -170 mV for processing at 370 °C for 15 min (Fig. 3).
Fig. 4 shows the switching and cycling characteristics for a
µm scale Ag-Ge-Se device; the y-axis is the voltage across a
series resistor. The on state is around 1 MΩ (limited by test
system parasitics). Write and erase transitions occur within
the 35 nsec rise/fall time of the ±1 V input signal. Single
pulse measurements confirm that the actual write and erase
times are less than 25 nsec (system limit). The curve of Fig. 4
was obtained after 1.25 x 1011 cycles and the smoothed (noise
reduced) waveforms are essentially identical to those obtained
for this device at 108 and 2.5 x 1011 cycles, indicating no
discernible device degradation with cycling. Fig. 5 uses
extrapolated data (1 month actual test time) to illustrate Ron
dependence on write current for 10 years at room temperature
and at 70 °C. To maintain sub-100 kΩ Ron after 10 years at
70 °C, a write current in excess of 30 µA is required.
1.0E+06
On resistance (Ohms)
-4.0E-07
-0.5
Current(A)
6.0E-07
10 years @ 70 ºC
1.0E+05
10 years @ RT
1.0E+04
Initial resistance
1.0E+03
15
4.0E-07
20
25
30
35
40
Program m ing curre nt (uA)
2.0E-07
Fig. 5. On resistance after 10 years at room temperature (RT) and 70
°C for programming current limits from 15 to 40 µA.
0.0E+00
-2.0E-07
-4.0E-07
-0.5
References
-0.3
-0.1
0.1
Voltage(V)
0.3
0.5
Fig. 3. Current-voltage plot with a 1 µA current limit for a 50 nm
thick Ag-Ge-S electrolyte on W device (D = 240 nm) following a 15
minute inert ambient anneal at 370 °C. The write threshold is 380
mV and the device is completely erased beyond -170 mV.
[1]
[2]
[3]
R. Symanczyk et al., Proceedings of the Non-Volatile Memory
Technology Symposium, San Diego, CA, November 2003.
M.Mitkova and M.N. Kozicki, J. Non-Cryst. Solids, vol. 299302, 1023-1027 (2002).
M. Mitkova, M.N. Kozicki, H.C. Kim, and T.L. Alford, Thin
Solid Films, in press.