IEEE TRANSACTIONS ON NANOTECHNOLOGY, VOL. 5, NO. 5, SEPTEMBER 2006
1
A Low-Power Nonvolatile Switching Element Based
on Copper-Tungsten Oxide Solid Electrolyte
Michael N. Kozicki, Member, IEEE, Chakravarthy Gopalan, Student Member, IEEE,
Muralikrishnan Balakrishnan, Student Member, IEEE, and Maria Mitkova
Index Terms—Copper electrodeposition, nonvolatile memory
devices, Raman spectroscopy, resistance change, solid electrolyte,
tungsten oxide, X-ray photoelectron spectroscopy.
I. INTRODUCTION
T
by way of electrodeposition at low voltage and current reduces
the resistance of the electrolyte by several orders of magnitude.
Our earlier work in this area concentrated on two-terminal devices, with active area as small as 40 nm, based on a silver doped
chalcogenide glass solid electrolyte film sandwiched between
a silver anode and an inert cathode [5]. However, we have recently added electrolytes consisting of transition metal oxides
combined with copper to our studies and it is this work that we
describe in this paper.
is a particularly desirable base
Tungsten trioxide
material for a solid electrolyte as it is compatible with
back-end-of-line (BEOL) processing in CMOS integrated
circuits where tungsten metal already plays a significant role.
Tungsten oxide formation techniques are compatible with
semiconductor processing and include wet chemical and
plasma oxidation, and physical vapor deposition. As we have
demonstrated, coupling this simple oxide with copper leads to
a solid electrolyte with desirable qualities for electrochemical
device operation. Copper is a particularly appropriate mobile
ion choice in the context of integration into CMOS processes
as many high performance ICs already use copper, along with
tungsten, in the upper levels of metal. Fig. 1 illustrates how
the integration of such a switching element could be achieved.
In this scheme, selected tungsten plugs which are normally
used to form the connections between one level of metal and
another are used as the lower electrode in the devices and the
copper interconnect is the upper electrode. The solid electrolyte
is placed between the two electrodes and is switched between
high and low resistance states by the application of appropriate
voltages on the electrodes. Note that during processing, only
one additional mask is required over the normal CMOS logic
to define which plugs have switching devices and which are
allowed to remain as simple connections between interconnect
layers and so the approach is inherently low in incremental
cost.
This paper focuses on the nature of copper-doped tungsten
oxide and on devices comprising this material. A comparison
is made between the quasi-static electrical characteristics of devices based on plasma-grown and deposited oxides, the copper
being added by photodissolution in both cases.
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Abstract—We describe the materials aspects and electrical
(Cu WO3 )
Cu switching elements.
characteristics of W
These materials are compatible with back-end-of-line processing
in CMOS integrated circuits where both tungsten and copper
already play a significant role. Devices based on Cu WO3 solid
electrolytes formed by photodiffusion of copper into tungsten
oxide switch via the electrochemical formation of a conducting
filament within the high resistance electrolyte film. They are
able to switch reversibly between widely spaced nonvolatile resistance states at low voltage ( 1 V) and current (
10 A).
Electrical characterization revealed that devices consisting of
plasma-grown oxides have a variable initial threshold voltage and
poor retention, whereas devices based on deposited oxide exhibit
a stable switching threshold and good retention, even at elevated
operating temperature ( 125 C). This difference in behavior
was attributed to the observation that the copper tends to oxidize
in the plasma-grown oxide whereas the copper in the deposited
oxide exists in an unbound state and is, therefore, more able to
participate in the switching process.
HERE is a considerable driving force within the semiconductor industry to create scalable elements that can
switch between widely spaced nonvolatile resistance states at
low power. Such elements could find widespread application
in next generation memory and reconfigurable logic [1]–[3].
Whereas there are several new technologies that show promise
in this respect, they typically lack complete scalability, i.e., they
have a physical size, programming voltage, or programming
current that is excessive for high density systems at the 65-nm
node and beyond. Many are also difficult to integrate due to
the complexities associated with the additional processing steps
and masking levels required and, therefore, will be expensive
to add to highly scaled CMOS. One potential approach to this
problem involves switching elements, known as Programmable
Metallization Cell (PMC) devices, which utilize the reduction of
nanoscale quantities of metal ions in solid electrolyte films [4].
The formation of a robust but reversible conducting pathway
II. DEVICES BASED ON SOLID ELECTROLYTES AND OXIDES
Manuscript received March 30, 2006; revised May 1, 2006. This work was
supported by Axon Technologies Corporation. The review of this paper was
arranged by Associate Editor B. G. Park.
The authors are with the Center for Solid State Electronics Research, Arizona
State University, Tempe, AZ 85287-6206 USA (michael.kozicki@asu.edu).
Color versions of Figs. 2–12 are available online at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TNANO.2006.880407
In our earlier work [4], [5], we presented the formation
and operation of PMC devices composed of a silver doped
chalcogenide glass solid electrolyte film (e.g., Ag–Ge–Se or
Ag–Ge–Se) sandwiched between a silver anode and an inert
cathode. For an applied bias in excess of a few hundred mV,
an electron current from the cathode reduces silver ions in the
1536-125X/$20.00 © 2006 IEEE
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IEEE TRANSACTIONS ON NANOTECHNOLOGY, VOL. 5, NO. 5, SEPTEMBER 2006
resistivity which were originally attributed to homogeneous
modifications or an unspecified breakdown mechanism. These
explanations were dismissed by Dearnaley and colleagues [8],
who developed a filamentary model for the changes in resistance. It was claimed that since the anode insulator interface is
not completely smooth, there will be places where the electric
field is locally high, resulting in the formation of a conductive
filament by field induced migration of material from the anode
into the oxide. The metallic filament propagates through the
insulator and thereby reduces the resistance of the oxide layer.
Subsequent switching in this “electroformed” structure was
thought to be via electrically stimulated rupture and healing
of the filamentary connection at locations along its length.
Thurstans and Oxley [9] proposed a model which took other
characteristics of the electroformed structure, in particular
the observed negative differential resistance, into account.
Their model features a conduction mechanism that depends on
trap-controlled thermally activated tunneling between isolated
segments of a ruptured metallic pathway. Recently, Baek et
al. [10] used such models to describe the unipolar switching
characteristics of their transition metal oxide-based devices. In
these structures, nickel oxide is switched to a low resistance
state using a relatively high voltage/low current “set” pulse
and returned to a high resistance state using a low voltage/high
current “reset” pulse of the same polarity. Although the effect
is attributed to charge trapping mechanisms in the electroformed oxide, it is perhaps more likely to be due to the rupture
and healing of a metallic pathway. We have observed similar
unipolar programming in PMC devices in which a metallic
filament is formed by the electrodeposition of the anode metal
in the electrolyte at a particularly high critical voltage but
can be disrupted using a current pulse of the same polarity
[11]. In this case, the erase (reset) current must be in excess
of the current used to write (set) the device so that the current
carrying capacity of the filament is exceeded and it is thereby
severed, however, the erase voltage must be below that required
for electrodepostion of the metal so that the break does not
spontaneously re-close. Note that for the devices described in
[4], the voltage threshold for electrodeposition is sufficiently
low that a reverse bias is required to erase the device; a forward
current in excess of the original programming current limit
leads to further electrodeposition and the device resistance falls
to a lower stable value rather than rising to the off state.
The consideration of tungsten oxide as a base glass for the
electrolyte in PMC devices led to a number of obvious questions in the context of the above discussion. The main issue was
whether the devices would behave like the chalcogenide-based
variants or more like the metal-insulator-metal structures described by Oxley. To address these issues, we engaged in a program of material analysis and device characterization, described
in the following sections.
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electrolyte while an equivalent number are injected from the
anode via oxidation of the silver. A metal-rich pathway forms
in the electrolyte film via this electrodeposition process. The
level and duration of the ion current above the electrodeposition
threshold determine the quantity of Ag electrodeposited and,
hence, the resistance of the pathway; the resistance of the
structure falls by many orders of magnitude even for applied
currents as low as a few microamperes. The low resistance
electrodeposit is electrically neutral and stable, leading to
retention in excess of 10 years [1], [4], however, applying a
bias with opposite polarity will break the link by a combination
of current-induced rupture and electrochemical oxidation of
the electrodeposit. Note that when a reverse bias is applied,
the break cannot “heal” by electrodeposition as there is a net
removal of metal from the electrolyte in a process which is opposite to that which formed the metal filament in the first place.
The reverse ion current flows until the previously deposited Ag
has been oxidized and returned to the silver electrode. Thus, the
resistance increases again until the high resistivity of the solid
electrolyte is achieved. The oxidation/reduction process is fast
(tens of nanoseconds or less for write and erase), consumes little
power (in the microwatt range), and can be cycled well beyond
conventional nonvolatile memory technology
[4], [5].
The electrical characteristics of PMC devices comprising
chalcogenide-based solid electrolytes result from the unique
nanostructure of the electrolyte material. We previously established that the dissolution of Ag into Se- or S-rich base
glasses produces a ternary that is a combination of a dispersed
superionic nano-crystalline Ag-rich phase within a glassy insulating Ge-rich phase [6], [7]. The Ag ion-containing regions
are typically less than 10 nm in diameter and are separated
by a few nanometers of the insulating phase [5], [6]. It is
this particular nanostructure that allows the films to exhibit a
resistivity of 100 cm or more (which yields a high device off
resistance) while retaining good ion availability and mobility.
The accessibility of mobile Ag ions for electrodeposition
throughout the electrolyte allows the devices to switch rapidly
as the growing electrodeposit will always have a local source of
ionic metal to feed its formation. The reduced ions are replaced
via the ion current from the anode and so the electrolyte does
not become depleted of ions following switching and contains
“excess” metal within the electrodeposited pathway. Indeed, it
is this addition of metal into the already metal ion-rich but nano
phase-separated electrolyte that leads to both the reduction in
resistance of the structure and the nonvolatility of the on state.
To complement our work on chalcogenide-based PMC
devices, we have recently examined elements that are more
closely related to the materials present in modern integrated circuits. Tungsten metal and its oxide and Cu are obvious choices
in this respect. There has been widespread interest in the use
of a variety of oxides in switching/memory applications for a
number of years. Several models exist that describe the electrical phenomenon seen in thin oxide films sandwiched between
two metal layers but as yet no complete or consistent model
describes all the observations. Films from tens to thousands of
nanometer thick when subjected to certain critical electric fields
have been observed to undergo permanent large reductions in
III. TUNGSTEN OXIDE CHARACTERISTICS
The electrical properties of
have been widely investigated because of the existence of electro- and photo-chromic
effects in materials based on this oxide [12]–[14]. It is a wide
is
bandgap dielectric—the electronic gap of amorphous
KOZICKI et al.: LOW-POWER NONVOLATILE SWITCHING ELEMENT BASED ON COPPER-TUNGSTEN OXIDE SOLID ELECTROLYTE
be characterized. Note that in the fabrication of device structures (discussed later), a patterned tungsten electrode was used
and the excess metal following photodiffusion was left in place
on the surface of the electrolyte to form part of the anode top
electrode.
Characterization of these films was performed using Raman
spectroscopy and X-Ray Photoelectron Spectroscopy (XPS).
Raman spectra were performed in the micro Raman mode
using a 514.5 nm Ar+ laser, 100 objective, with an accumulation time of 100 s and a power of 6 mW. The experimental
curves were fitted using Labcalc software. X-ray Photoelectron
Spectroscopy (XPS) was carried out with high-resolution XPS
X-ray excitation and constant analyzer energy with
(Mg
8 eV pass energy, take-off angle of 90 , 240 W power). The
energy calibration of the spectrometer was performed using a
gold plate fixed to the sample. Raman data related to plasma
oxidized films are presented in Fig. 2. Since the films are predominantly amorphous, only breathing modes resulting from
the short-range order appear. The intensity of Raman signals
from the films is inherently weak because they are rather thin
but the deconvolution of the results permits collection of meaningful information relating to the vibration of the W-O bond.
This revealed a mode at 280 cm (bending mode), very weak
modes at 815 and 930 cm and one at 998 cm (stretching
film is illuminated with light, a mode at
mode). After the
, suggesting
450 cm appears that could be due to
some reduction has occurred [22]. Illumination with light
evidently causes some crystallization of the films, an effect
that is manifested in the appearance of a peak at 719 cm ,
bond. Such a structure has also
characteristic of the
been observed by Delichere et al. [23] who relate it to a higher
order of structural organization. When Cu is photodiffused into
the films, modes at intermediate frequency appear (440 and 460
and
cm ) that could be attributed to the vibrations of
modes [24]. This is significant as combinatorial electrochemical synthesis [24] has demonstrated that the formation of
Cu oxides is possible following introduction of Cu in the
films.
The XPS data of plasma oxidized films show that they are
indeed influenced by previous exposure to light Fig. 3(a) and
(b). Gaussian deconvolution of the W4f spectrum of the initial
and
films Fig. 3(a) shows well resolved doublet
caused by spin-orbit coupling with binding energies of 35.85
and 37.94 eV, which correspond to a typical 6 state of W.
However illumination with light causes a small shift of this douand
blet towards lower energies Fig. 3(b). The
show binding energies of 35.71 and 37.8 eV that could be an indication of some reduction of the oxide. We could not observe
a well expressed simultaneously occurring W4f double peak of
as was reported by Sun et al. [25] since the amount of the
5-valent W is small. We suggest that this is related to the fact
that there is no water included in our films. However the shift is
certainly an indication of the influence of light on our films. The
introduction of Cu causes significant changes in the XPS spectra
Fig. 4. The Gaussian deconvolution suggests the appearance of
4 binding energy values that could correspond to the 6 and
5 state of W, as expected. However, they are somewhat shifted
from the standard data for these particular doublets. It is diffi-
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3.25 eV [15]—but a large range of conductivity has been reported [16]. This is due to the fact that W can exist in different
valence states when oxidized, thus forming oxides with different stoichiometry and, hence, with different electrical conductivity. Different valence states can also be created by illumination with UV light with energy exceeding 3.3 eV [17]. In
can be altered by the introaddition, the conductivity of
duction of metal ions and this is of particular interest in the context of the current work [18]. As discussed earlier in this paper,
we have used silver as the mobile ionic species in the base glass
(and as the oxidizable electrode) in chalcogenide-based PMC
devices due to its excellent electrochemical qualities but copper
is also a good candidate due to its high ionic mobility in a variety of materials, including oxides [19]. To combine metals
and thereby form a stable solid electrolyte, we can
with
take advantage of the photosensitivity of the oxide. This effect
more commonly manifests itself as photochromism which can
be explained by a double charge-injection model [20], in which
and electrons into a
lattice forms a tunginjection of
sten bronze structure
,
. Note that
can
in tungsten oxide hydrate or
be partially reduced to
in 12-tungstates by electrons occupying the empty
orbital
is typically
in
due to the electron localization effect.
photochromic
but can actually be a variety of mobile ions,
. So, if the
film is prepared using a dry
including
reaction and a very thin UV transparent Cu film is evaporated
on top, illumination can cause the formation of a
bronze.
formation is concerned, although wet chemAs far as
ical or plasma oxidation of tungsten metal are simple room temperature processes, the thickness of the layer formed is limited to a few nanometers by the diffusion of the oxidant species
through the growing oxide. It is, therefore, necessary to use the
evaporation of tungsten oxide to form thicker films. To grow
thin tungsten oxide films on tungsten metal, we elected to use
plasma techniques as we discovered that the growth of layers by
wet chemical means was difficult to control and led to material
with highly variable characteristics. In addition, the wet grown
oxides also contain species that would possibly interfere with
the incorporation of Cu. The surface of 100-nm-thick tungsten
glow dismetal films was oxidized using a nitrous oxide
charge at an RF power of 250 W and a substrate heater temperature of 300 C for 10 min. in a PlasmaQuest plasma processing
system. The
flow rate was 20 sccm with 1000 sccm of helium carrier gas. The precise thickness of the grown oxide was
difficult to ascertain due to the roughening of the underlying
tungsten caused by the growth process but it was estimated using
ellipsometry and capacitance-voltage measurements to be in the
order of 3 to 4 nm thick. To achieve metal diffusion, 25-nm-thick
Cu layers were deposited by high vacuum evaporation and the
metal was photodiffused into the oxide using the 405-nm broadband UV light source of a Karl Suss MJB 3 mask aligner for 14
. This wavelength was
min at a power density of 4.5
[21], where
chosen as it is near the absorption edge of the
the light causes maximal change in the material. Blanket samples were fabricated for material characterization and the excess
surface metal was removed using a short treatment with iron nisolution so that only the Cu in the oxide would
trate
3
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Fig. 1. Schematic cross section of an integrated circuit, showing position of oxide switching elements on selected tungsten via plugs. Interconnect layers M1 and
M2 are separated by IMD. Through vias provide M1 to M2 connectivity.
WO
Fig. 2. Raman analysis of
formed by plasma oxididation of W (a) as
synthesized; (b) after illumination with UV light; (c) photo-doped with Cu.
cult to fully interpret these results but we believe that they could
be related to distortions in the amorphous structure of the films
and formation of clusters [26] in which some of the diffused Cu
is oxidized and the clusters become part of the overall structure.
This leads to the occurrence of bond overlapping and a shift in
the binding energies. Overall, even though the signal-to-noise
ratio is rather poor, the data related to Cu does indeed indicate
its presence in an oxidized state within the tungsten oxide matrix (see inset in Fig. 4).
We now turn to the deposited oxide samples. The evaporation
source was high purity tungsten oxide (99.99%) in 3–12 mm
sintered yellow-green pieces (Cerac, Inc.). Using high vacuum
thermal evaporation, 50- or 100-nm thick films were deposited
on the substrates described above. Following this, 25 nm of Cu
without breaking the deposition
was evaporated on the
system vacuum. The Cu was then photodiffused into the oxide
for 14 min and the excess metal was etched using the techniques
described earlier. The analysis of these films revealed some simwas clearly
ilarities with the plasma-grown oxides in that
present as before in the as-deposited films, indicating the forma, but the way the copper was incorporated in the detion of
posited oxide appeared different. Both Raman and XPS analysis
of the films gave little or no evidence of the formation of Cu-ox. We
ides after copper is photodiffused into the deposited
WO
Fig. 3. XPS spectra of
formed by plasma oxididation of W (a) as synthesized; (b) after illumination with UV light.
are currently investigating this in more detail but it would appear
that whereas at least some of the photodiffused Cu becomes oxidized in the plasma-grown oxide to form a two phase (copper
oxide/tungsten oxide) system, it exists in an unbound state in
the deposited material. This could be due to the fact that the
deposited oxides are in a highly stable state and do not readily
react with the diffusing copper, whereas the presumably substoichiometric plasma-grown oxides are much more reactive. In
any case, this dissimilarity is extremely significant in the context of the observed differences in the electrical characteristics
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KOZICKI et al.: LOW-POWER NONVOLATILE SWITCHING ELEMENT BASED ON COPPER-TUNGSTEN OXIDE SOLID ELECTROLYTE
WO
Fig. 4. XPS spectra of
formed by plasma oxididation of W after Cu
diffusion in the oxide film; inset—XPS spectrum of Cu-containing species in
the oxide film.
of devices based on plasma-grown and deposited oxides, as described in the following sections.
IV. DEVICE FABRICATION AND ELECTRICAL
CHARACTERIZATION
A. Devices Based on Plasma Grown Oxide
Tungsten oxide films were incorporated in PMC device structures using oxygen plasma treatment of a tungsten metal bottom
electrode as described in the previous section and with copper as
the mobile species and oxidizable top electrode. Schematics of
typical test device structures are shown in Fig. 5 (Fig. 5(a) represents devices based on grown oxide whereas Fig. 5(b) is the
deposited oxide variants described in the following section). A
100-nm-thick tungsten layer was deposited by chemical vapor
deposition on silicon dioxide grown on silicon substrates. The
tungsten was covered with 100 nm of silicon dioxide by chemical vapor deposition and via (through) holes were defined in
this dielectric using optical lithography and plasma
etching. The exposed tungsten in the holes was oxidized using
the plasma oxidation technique described in the previous section. A 25-nm-thick Cu metal layer was deposited using high
vacuum thermal evaporation and photodiffused into the
using a 405-nm broadband UV light source in a Karl Suss MJB
. The excess
3 mask aligner at an intensity of 4.5
Cu metal was not removed to ensure the presence of a Cu-rich
source on the electrolyte surface. A 50-nm-thick Cu metal layer
was then deposited using high vacuum thermal evaporation and
patterned using a lift off process to complete the oxidizable electrodes of the devices. Note that this device configuration was absolutely necessary to observe switching regardless of how the
base glass for the electrolyte was formed; a lack of Cu in the
top electrode results in no measurable switching activity. The
Fig. 5. Cross-sectional schematics of oxide-based switching devices. The active device area is the tungsten-electrolyte-Cu stack with area defined by the
dielectric. (a) Device based on grown
diameter of the through hole in the
oxide. (b) Device based on deposited oxide.
SiO
probe pad window in the
to the tungsten layer was formed
using another lithography step in conjunction with wet-chemical etching. The quasi-static characteristics of the test devices
were obtained by connecting the device electrodes via tungsten
probes held in micromanipulators in a probe station to a semiconductor parameter analyzer (Agilent 4155C). Voltage doublesweeps were carried out starting at maximum reverse bias (tungsten electrode positive, copper electrode negative), sweeping
to an appropriate forward voltage (tungsten electrode negative,
copper electrode positive), and sweeping back again to the reverse bias starting point.
Fig. 6 shows a resistance-voltage plot from our earlier work
on large (1– m diameter) devices with a Cu/plasma-grown
electrolyte and Cu oxidizable electrode [4]. Three consecutive sweeps are shown from 0.75 V to 1.0 V to 0.75
V for a current limit of 0.5 A. The maximum off resistance
and the transition
for the first sweep is in excess of
to the low resistance state occurs at 0.9 V at which point the
current reaches its compliance limit. The current stays at the
compliance until the negative-going sweep reaches 0.25 V,
, more
at which point the measured on resistance is 500
than four orders of magnitude lower than the off state. Note
that the apparent rise in resistance between 0.25 and 1 V is an
artifact of the current compliance control in the measurement
instrument. The appearance of the lower threshold following
switching is typical of all PMC devices and represents the
minimum voltage required for electrodeposition once the
process has been initiated and a metal-rich region has formed
in the electrolyte. Electrodeposition within the oxide will
continue as long as the voltage drop is greater than 0.25 V
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Fig. 6. Resistance-voltage plot of a 1.0-m-diameter device with Cu/plasmaelectrolyte and Cu oxidizable electrode.
grown
Fig. 7. Resistance-voltage plot of a 240-nm-diameter device with Cu/plasmaelectrolyte and Cu oxidizable electrode.
grown
for this physical configuration. Note that the decreasing resistance due to the electrodeposition effect increases the current
flowing through the device until the compliance is reached.
At this point, the voltage drop falls to the threshold and the
electrodeposition stops. The on resistance is, therefore, defined
by the electrodeposition threshold divided by the current limit
.
It is clear from the plot of Fig. 6 that the first sweep is different from the following two. The off resistance is apparently
reduced by an order of magnitude following the first sweep and
the switching threshold moves from 0.9 V in the first sweep toward 0.7 V in the latter cycles. The same switching threshold
lowering effect was seen in all devices tested. In our experience,
elevated switching threshold is a result of “subsaturation” of the
electrolyte with metal ions. We believe that the reduced amount
of mobile copper in the doped plasma-grown oxides is due to
the two-phase nature of this electrolyte, in which some of the
copper is in the form of an oxide and, therefore, cannot readily
take part in the electrodepostion process. It is likely that the first
voltage sweep injects additional Cu from the anode to increase
the unbound copper concentration, thereby lowering the off resistance and write threshold for the following sweeps. However,
since this injection of Cu by the applied field will only increase
the concentration of metal along the volume of the filamentary
pathway (where the field is highest) and not in the rest of the
film, the metallic connection formed will be susceptible to dissolution by metal diffusion into the surrounding electrolyte due
to the high copper metal concentration gradient. This was confirmed by resistance-time measurements which showed an upward drift in the on-state resistance, which returns to the high
resistance off state after a few hours at room temperature and
after only a few minutes at 70 C.
The erase transition threshold occurs at 0.2 to 0.3 V in this
device, with full erase occurring beyond 0.5 V. The sudden increase in resistance at the erase transition threshold is due to the
current-induced breaking and voltage-induced electrochemical
oxidation of the conducting pathway, leading to a device resistance that is dominated by the high resistance of a portion of
now unbridged electrolyte. The subsequent resistance rise (and
any steps therein) is a result of the removal by electrochemical
oxidation of the remaining electrodeposited metal. Note that the
drop in off state resistance for increasing voltage magnitude in
either direction is due to the leakage current in the electrolyte.
Fig. 7 shows the <Au: Define R-V?> (R-V) characteristic
of a smaller Cu/plasma-grown
electrolyte device with a
240-nm-diameter for a 0.5- m current limit. This shows many
of the characteristics of the 1- m device but the write thresholds
are all considerably higher (around 2 V initially and decreasing
toward 1.7 V with increasing sweep number). The post-electrodeposition thresholds are also higher than before (close to 0.7
V), leading to higher on resistances as defined by the programming current relationship given earlier
. We believe that these higher thresholds are due to considerably lower Cu diffusion into the oxide in the small area devices during illumination. The optical attenuation of the reduced
intermetal dielectric (IMD) and the
dimension hole in the
nonconformal coverage of the copper in this reduced dimension
feature will lead to less light reaching the interface of the metal
and the oxide and this will reduce copper photodiffusion. The
lower copper diffusion, in addition to the oxidation of the Cu that
does enter the oxide, results in the higher switching threshold.
The lower Cu concentration does little to alter the erase initiation threshold, which occurs between 0.2 and 0.4 V and the
maximum off resistance of over
(the limit of the measurement system) is achieved beyond 0.5 V as before. This
is expected as the erase initiation occurs due to the current-induced breaking of the conducting filament and the nature of the
electrolyte has little effect on this.
Even though the devices based on plasma-grown oxide were
not ideal as switching elements, they showed the essential characteristics of PMC structures, with a large decrease in resistance
for a low write threshold and programming current and a return
to a high resistance state at a low negative bias. However, if they
are to be used in embedded memory or as switching elements
in reconfigurable logic, more work is required to improve their
stability.
WO
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KOZICKI et al.: LOW-POWER NONVOLATILE SWITCHING ELEMENT BASED ON COPPER-TUNGSTEN OXIDE SOLID ELECTROLYTE
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B. Devices Based on Deposited Oxide
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The substrates chosen for the deposited oxide-based device
experiments were similar to those described in the previous section. In this case, after the dielectric was etched to form the via
layers
holes and expose the underlying tungsten electrode,
50 or 100 nm thick were deposited under high vacuum using
an electron beam evaporator. Following this, 25 nm of Cu was
without breaking the deposition system
evaporated on the
vacuum and the Cu photodiffused into the oxide for 14 min
using the technique described earlier. Once again, the excess
Cu was not removed from the surface. To complete the device,
an additional 50 nm of Cu was deposited and the electrolyte/top
electrode stack was patterned using optical lithography and lift
off. A schematic of the device structure is shown in Fig. 5(b).
The electrical characterization using the set-up described earlier was performed at room temperature and at a range of elevated temperatures using a temperature controlled chuck on the
probe-station.
Fig. 8(a) gives a typical current-voltage plot of a 240-nm-didevice, comprising a 50-nm deameter
posited oxide. This was programmed at 1 A and exhibited a
write threshold of approximately 0.4 V and a lower electrodeposition threshold of 0.1 V, at which point the negative-going
sweep drops below the compliance current level. The device
returns to its high resistance state at 0.2 V. Fig. 8(b) shows
the resistance-voltage plot for the same device, showing an avand an on resistance
erage off resistance in the order of 2
. Note that the on resistance is dependent on proof 100
gramming current as before with the on resistance equal to the
lower threshold (0.1 V in this case) divided by the programming
current limit. Since this lower threshold for electrodeposition
is quite invariant, the on resistance is also highly reproducible,
typically to within 5% of the mean value for a given programming current. These results are similar in form to those obtained
with the plasma-grown oxide except that the write threshold is
not only lower for the much thicker deposited oxide but it is
also much more consistent, largely devoid of the higher initial
threshold effect; i.e., subsequent sweeps result in a switching
threshold that is very close to the first sweep value, with a median of 0.4 V and a standard deviation of approximately 50 mV.
In addition, the strong dependence of the write threshold on device diameter that was seen in the grown oxide devices is not
present in the deposited oxide variants. This suggests that the
deposited oxide in both the smaller and larger diameter devices
is equally saturated with “free” metal following the photodissolution step, which was evidently not the case in the grown material. Note that the lower off resistance, which has an average
for the 240-nm devices, is also indicative of
value around 2
higher mobile metal content than the grown oxide devices. This
is consistent with our material characterization work which suggested that the photodiffused copper formed a bronze with the
deposited tungsten oxide and, therefore, would be unbound and
mobile.
Fig. 9 is the resistance-voltage behavior of a 5- m-diameter
deposited oxide device (100-nm-thick
) at operating temperatures of 25 C, 70 C, and 135 C for a 10- A programming
current. The device switched from the high resistance range to
0
Fig. 8. Electrical behavior of a 240-nm-diameter Cu WO based device
formed from a 50-nm-thick oxide. (a) Current–voltage plot of a device programmed at 1 A. (b) Resistance voltage plot for the same device.
around 10
at all temperatures, with a lower electrodeposition threshold of 0.1 V as before. The 10 lower on resistance
is a direct result of the 10 higher programming current than
the case shown in Fig. 8 (i.e., in this case on resistance is given
). The apparent slight rise in write
by
threshold with temperature is probably due to the increase in the
probe contact resistance due to thermal expansion. The most
significant difference with temperature is the erase threshold,
which clearly drops from the room temperature initiation value
of 0.2 V as the temperature increases. This is consistent with
PMC device operation as the increased thermal energy reduces
the current required to break the conducting link. Fig. 10 gives
multiple sweep resistance-voltage plots of a 240-nm-diameter
) for a 100- A programming curdevice (100-nm-thick
rent at 135 C. The 100- A programming current gives an on
, as expected
.
resistance close to 1
With the exception of the first sweep, device operation is consistent with the larger device of Fig. 9 at 135 C, with a write
threshold close to 0.4 V, a lower threshold of 0.1 V, and an erase
IEEE TRANSACTIONS ON NANOTECHNOLOGY, VOL. 5, NO. 5, SEPTEMBER 2006
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8
Fig. 9. Resistance-voltage plots at different operating temperatures of 5-m
-diameter devices formed from 100 nm of
evaporated and photo-doped
with Cu.
WO
Fig. 10. Resistance-voltage plots at 135 C for a 240-nm-diameter device comphotodoped with Cu.
prising 100 nm of evaporated
WO
initiation threshold of 0.1 V. The apparent higher threshold
of 0.6 V for the first sweep is once again thought to be due to
probe resistance/thermal expansion effects as it was not repeatable (and, therefore, was not the same mechanism as that seen
in the grown oxide devices).
It is important to note that in the results shown in Figs. 8 to
10, the on resistance is independent of device diameter and electrolyte thickness. Insensitivity to device diameter is expected
when switching is due to the formation of a filamentary conducting pathway which is considerably smaller in width than
the electrolyte (device) diameter, The lack of sensitivity to electrolyte thickness is due to the fact that the on resistance depends
on electrodeposition threshold and programming current limit,
neither of which are influenced by device geometry. The on state
in the above devices is also quite ohmic, which suggests that the
filament is indeed metallic in nature, with good (low barrier)
contact to the tungsten and copper electrodes.
WO
Fig. 11. Data retention behavior of devices fabricated using Cu-doped
films programmed at 10 A and 1 V and probed using a 100 mV measurement
signal.
Another means for determining the suitability of the deposited oxide for nonvolatile switching applications was
measurement of retention of the devices over a range of temperatures. For this test, the devices were programmed using a
10- A current and the resistance measured using a 100 mV
dc probe signal at logarithmic intervals of time. The test was
conducted at room temperature and also at 70 C, and 135 C.
The results revealed that devices fabricated using photodiffused
films showed good state retention even at
Cu-saturated
135 C, as shown in Fig. 11. This particular test was terminated
at around 12 hours but in all the samples evaluated, there was
no indication of an upward drift in resistance. Indeed, in most
cases there was actually a small decrease in resistance for
the test conditions used, even though the high off state was
maintained.
Finally, the cycling endurance of the
device structure was assessed. A 500-nm-diameter device with
electrolyte was
a 100-nm-thick Cu-photodoped deposited
repeatedly written and erased at 135 C using a programmable
pulse generator. The pulse generator could be programmed to
provide variable numbers of 100 ms 1 V (write) followed by
100 ms 1.5 V (erase) pulse sequences. The voltages and pulse
lengths used were considerably in excess of what was required
to program the devices to ensure that they were fully written and
erased during each cycle, as confirmed by single pulse measurements and digital oscilloscope screen captures taken during cycling. A series resistor was used to limit the on current through
the device to approximately 5 A. Fig. 12 gives quasi-static resistance-voltage plots for voltage sweeps from 1 V to 1 V to
1 V with a current limit of 5 A. The plots were taken after
1, 10, 100, 1000, and 10 000 cycles, as indicated in the figure.
All curves below 10 000 cycles show good repeatability of the
off resistance, write threshold, and on resistance, all to within
(averaged
10% of the approximate median values of 400
, respectively. The erase porover 0–0.3 V), 0.4 V, and 20
tions of the individual curves (from 0 to 1 V) appear erratic
but the devices are actually in the off state following this part
KOZICKI et al.: LOW-POWER NONVOLATILE SWITCHING ELEMENT BASED ON COPPER-TUNGSTEN OXIDE SOLID ELECTROLYTE
WO
the Cu is not bound and is, therefore, more able to participate in
the switching process. We assume that this difference is a result
of the plasma-grown oxide being somewhat substoichiometric
compared to the stable deposited oxide but additional analysis
work is required to establish the exact mechanism of copper
incorporation. The devices based on deposited oxide had reproducible electrical characteristics, with critical parameters
such as off resistance, write threshold, and on resistance being
within 10% or less of their median values. The control of on
resistance by write current limit was evident in these devices
to 1
were demonstrated,
and on resistances from 100
suggesting that multi-level storage (two or more bits per cell)
might be possible with these devices. Retention and endurance
seemed promising for these early prototypes.
Of course, more material optimization and electrical characterization is required to improve write and erase speed and other
important factors such as endurance before these elements can
-range devices show promise as
be utilized. However, these
an easily integrated switching element as they can use existing
materials and compatible processing and involve as little as one
additional masking step over standard advanced CMOS.
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+
Fig. 12. Effects of write-erase cycling (each cycle was 100 ms at 1 V, 100
ms at 1.5 V) at 135 C of a 500-nm-diameter device comprising 100 nm of
photodoped with Cu. The device resistance-voltage plot was
evaporated
taken after various numbers of cycles as indicated in the plot.
0
9
of the sweep and can be subsequently fast cycled. The plot corresponding to 10 000 cycles shows a much lower off resistance
and even though the device switches at the expected threshold
on resistance, it cannot be cycled further.
and attains the 20
The failure mechanism that causes the “stuck bit” condition is
unknown but is currently under investigation.
V. CONCLUSION
To conclude, we have demonstrated the feasibility of fabricating solid-state switching elements using tungsten metal, a
solid electrolyte based on tungsten oxide with photodiffused
copper, and a copper electrode. These devices exhibit a high
off resistance and can be switched at low voltage to an on resistance state that is independent of device geometry (diameter
and electrolyte thickness) but is strongly governed by the write
current. The device can be returned to its high resistance state by
the application of a small reverse bias. The electrical characteristics, and in particular those of devices comprising deposited
tungsten oxide, are extremely similar to Programmable Metallization Cell devices based on chalcogenide electrolytes. We
can, therefore, conclude that the same low energy electrochemical processes that result in the formation and removal of a filamentary conducting pathway in the chalcogenide devices are
also responsible for the switching in the copper-doped oxide devices. The exact composition of this conducting pathway is currently unknown but the experimental evidence, which includes
the ohmic nature of the on state, suggests that it is the result of
the localized presence of Cu metal within the oxide.
Electrical characterization has revealed that devices consisting of plasma-grown oxides have a decreasing initial
threshold voltage and poor retention, whereas devices based on
deposited oxide posses a stable switching threshold and good
.
retention, even at elevated operating temperature
This difference in behavior was attributed to the observation
that the copper tends to oxidize in the plasma-grown oxide and
is, therefore, not in a mobile state whereas the copper-doped deposited oxide is “bronze-like,” comprising a structure in which
ACKNOWLEDGMENT
The authors would also like to thank C. Poweleit for preparation of the Raman spectra and T. Karcher for generation of
the XPS results. The authors gratefully acknowledge the help
of the staff and use of the facilities within the Center for Solid
State Electronics Research and the Center for Solid State Science at Arizona State University.
REFERENCES
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[6] M. N. Kozicki, M. Mitkova, J. Zhu, and M. Park, “Nanoscale phase
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[7] M. Mitkova, M. N. Kozicki, H. C. Kim, and T. L. Alford, “Thermal
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Films, vol. 449, pp. 248–253, 2004.
[8] G. Dearnaley, G. Stoneham, and D. V. Morgan, “Electrical phenomenon in amorphous oxide films,” Rep. Prog. Phys., vol. 33, pp.
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[10] I. G. Baek, M. S. Lee, S. Seo, M. J. Lee, D. H. Seo, D.-S. Suh, J. C.
Park, S. O. Park, H. S. Kim, I. K. Yoo, U.-I. Chung, and J. T. Moon,
“Highly scalable non-volatile resistive memory using simple binary
oxide driven by asymmetric unipolar voltage pulses,” in IEDM Tech.
Dig., 2004.
10
IEEE TRANSACTIONS ON NANOTECHNOLOGY, VOL. 5, NO. 5, SEPTEMBER 2006
Michael N. Kozicki (M’84) was born near Edinburgh, Scotland, and received the B.Sc. and Ph.D.
degrees from the University of Edinburgh in 1980
and 1985, respectively.
He was with Hughes Microelectronics Ltd. (now
Raytheon Systems Ltd.) before joining Arizona State
University (ASU) in 1985, where he is currently Professor of Electrical Engineering. He i holds several
key patents in integrated ionics, including those involving the storage of data by electrodeposition in
solid electrolytes. He is also a founder of Axon Technologies Corp., an ASU spin-out company formed to develop and license solidstate ionic technologies, and has served as Director of ASU’s Center for Solid
State Electronics Research.
Chakravarthy Gopalan (S’06 <Au: Please confirm
year?>) received the B.S. degree in electronics and
communication engineering from Madurai Kamaraj
University, Madurai, India, in 1999, and the Ph.D.
degree in electrical engineering from Arizona State
University, Tempe, in 2005.
He is currently working at Spansion, <Location?;
please update affiliations footnote on p.1 if necessary>. His current research interests include passive
array design and fabrication of programmable metallization cell memory devices as well as flash memory
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[11] W. C. West, “Electrically erasable non-volatile memory via electrochemical deposition of multifractal aggregates,” Ph.D. thesis, Arizona
State Univ., Tempe, May 1998.
[12] E. Ozkan, S.-H. Lee, C. E. Tracy, J. R. Pitts, and S. K. Deb, “Comparison of electrochromic amorphous and crystalline tungsten oxide
films,” Solar Energy Mater. Solar Cells, vol. 79, pp. 439–448, 2003.
[13] C. O. Avellaneda and L. O. S. Bulhoes, “Intercalation in WO(3) and
WO3: Li films,” Solid State Ion, vol. 165, pp. 59–64, 2003.
[14] A. Antonaia, M. C. Santoro, G. Fameli, and T. Polichetti, “Transport
mechanism and IR structural characterization of evaporated amorphous
WO films,” Thin Solid Films, vol. 426, pp. 281–287, 2003.
[15] K. Miyake, H. Kaneko, M. Sano, and N. Suedomi, “Physical and electrochromic properties of the amorphous and crystalline tungsten oxide
thick films prepared under reducing atmosphere,” J. Appl. Phys., vol.
55, pp. 2747–2753, 1984.
[16] M. Gillet, K. Aguir, C. Lemire, E. Gillet, and K. Schierbaum, “The
structure and electrical conductivity of vacuum-annealed WO thin
films,” Thin Solid Films, vol. 467, pp. 239–246, 2004.
[17] C. Bechinger, S. Herminghaus, and P. Leiderer, “Photoinduced doping
of thin amorphous WO films,” Thin Solid Films, vol. 239, pp.
156–160, 1994.
[18] R. C. Agrawal, M. L. Verma, and R. K. Gupta, “Electrical and electrochemical properties of a new silver tungstate glass system: x[0:75AgI :
o:25AgCl] : (1
x)[Ag O : WO ],” Solid State Ionics, vol. 171,
pp. 199–205, 2004.
[19] L. Chernyak, K. Gartsman, D. Cahen, and O. Stafsudd, “Electronic
effects of ion mobility in semiconductors: semionic behaviour of
CuInSe ,” J. Phys. Chem. Sol., vol. 56, pp. 1165–1195, 1995.
[20] B. W. Faughnan, R. S. Grandall, and P. M. Heyman, “Electrochromism
in WO amorphous films,” RCA Rev., vol. 36, pp. 177–197, 1975.
[21] K. Gesheva, A. Szekeres, and T. Ivanova, “Optical properties of chemical vapor deposited thin films of molybdenum and tungsten based
metal oxides,” Sol. Energy Mater. Sol. Cells, vol. 76, pp. 563–576,
2003.
[22] S.-H. Lee, H. M. Cheong, C. E. Tracy, A. Mascarenhas, D. K. Benson,
and S. K. Deb, “Raman spectroscopic studies of electrochromic aWO ,” Elelctrochim. Acta, vol. 44, pp. 3111–3115, 1999.
[23] P. Delichere, P. Falaras, M. Froment, A. Hugot-Le Goff, and B. Aguis,
“Electrochromism in anodic WO films 1: preparation and physicochemical properties of films in the virgin and colored states,” Thin Solid
Films, vol. 161, pp. 35–46, 1988.
[24] H. Rosen, E. M. Engler, T. C. Strand, V. Y. Lee, and D. Bethune,
“Raman study of lattice modes in the high-critical-temperature superconductor Y-Ba-Cu-O,” Phys. Rev B, vol. 36, pp. 726–728, 1987.
[25] S. H. Baeck, T. F. Jaramillo, C. Brändli, and E. W. McFarland, “Combinatorial electrochemical synthesis and characterization of tungstenbased mixed-metal oxides,” J. Comb. Chem., vol. 4, pp. 563–568, 2002.
[26] M. Sun, N. Xu, Y. W. Cao, J. N. Yao, and E. G. Wang, “A nanocrystalline tungsten oxide thin film: preparation, microstructure, and photochromic behavior,” J. Mater. Res., vol. 15, pp. 927–933, 2000.
[27] T. Nanba, M. Ishikawa, Y. Sakai, and Y. Miura, “Changes in atomic
and electronic structures of amorphous WO films due to electrochemical ion insertion,” Thin Solid Films, vol. 445, pp. 175–181, 2003.
0
solutions.
Murali Balakrishnan (S’) received the B.S. degree
in electronics and communication engineering from
the University of Madras, Chennai, India, in 1999,
and the M.S. degree in electrical engineering from
Arizona State University, Tempe, in 2003. He is currently working toward the Ph.D. degree at Arizona
State University.
His current research interests include fabrication
and characterization of programmable metallization
cell memory devices.
Maria Mitkova received the M.S. and Ph.D. degrees
from the Technological University, Sofia, Bulgaria,
in 1970 and 1976, respectively.
She was Professor at the Technological University
and Bulgarian Academy of Sciences and has worked
in the R&D of Ovonic memory devices in the Institute for Microelectronics. Since 1997, she has worked
in the USA at the University of Cincinnati, Cincinnati, OH, and Arizona State University, Tempe. Her
interests are in the field of amorphous semiconductors, their characterization and application as optical
and electronic memory media. She has specialized in a number of Ag-containing
chalcogenide systems.
IEEE TRANSACTIONS ON NANOTECHNOLOGY, VOL. 5, NO. 5, SEPTEMBER 2006
1
A Low-Power Nonvolatile Switching Element Based
on Copper-Tungsten Oxide Solid Electrolyte
Michael N. Kozicki, Member, IEEE, Chakravarthy Gopalan, Student Member, IEEE,
Muralikrishnan Balakrishnan, Student Member, IEEE, and Maria Mitkova
Index Terms—Copper electrodeposition, nonvolatile memory
devices, Raman spectroscopy, resistance change, solid electrolyte,
tungsten oxide, X-ray photoelectron spectroscopy.
I. INTRODUCTION
T
by way of electrodeposition at low voltage and current reduces
the resistance of the electrolyte by several orders of magnitude.
Our earlier work in this area concentrated on two-terminal devices, with active area as small as 40 nm, based on a silver doped
chalcogenide glass solid electrolyte film sandwiched between
a silver anode and an inert cathode [5]. However, we have recently added electrolytes consisting of transition metal oxides
combined with copper to our studies and it is this work that we
describe in this paper.
is a particularly desirable base
Tungsten trioxide
material for a solid electrolyte as it is compatible with
back-end-of-line (BEOL) processing in CMOS integrated
circuits where tungsten metal already plays a significant role.
Tungsten oxide formation techniques are compatible with
semiconductor processing and include wet chemical and
plasma oxidation, and physical vapor deposition. As we have
demonstrated, coupling this simple oxide with copper leads to
a solid electrolyte with desirable qualities for electrochemical
device operation. Copper is a particularly appropriate mobile
ion choice in the context of integration into CMOS processes
as many high performance ICs already use copper, along with
tungsten, in the upper levels of metal. Fig. 1 illustrates how
the integration of such a switching element could be achieved.
In this scheme, selected tungsten plugs which are normally
used to form the connections between one level of metal and
another are used as the lower electrode in the devices and the
copper interconnect is the upper electrode. The solid electrolyte
is placed between the two electrodes and is switched between
high and low resistance states by the application of appropriate
voltages on the electrodes. Note that during processing, only
one additional mask is required over the normal CMOS logic
to define which plugs have switching devices and which are
allowed to remain as simple connections between interconnect
layers and so the approach is inherently low in incremental
cost.
This paper focuses on the nature of copper-doped tungsten
oxide and on devices comprising this material. A comparison
is made between the quasi-static electrical characteristics of devices based on plasma-grown and deposited oxides, the copper
being added by photodissolution in both cases.
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Abstract—We describe the materials aspects and electrical
(Cu WO3 )
Cu switching elements.
characteristics of W
These materials are compatible with back-end-of-line processing
in CMOS integrated circuits where both tungsten and copper
already play a significant role. Devices based on Cu WO3 solid
electrolytes formed by photodiffusion of copper into tungsten
oxide switch via the electrochemical formation of a conducting
filament within the high resistance electrolyte film. They are
able to switch reversibly between widely spaced nonvolatile resistance states at low voltage ( 1 V) and current (
10 A).
Electrical characterization revealed that devices consisting of
plasma-grown oxides have a variable initial threshold voltage and
poor retention, whereas devices based on deposited oxide exhibit
a stable switching threshold and good retention, even at elevated
operating temperature ( 125 C). This difference in behavior
was attributed to the observation that the copper tends to oxidize
in the plasma-grown oxide whereas the copper in the deposited
oxide exists in an unbound state and is, therefore, more able to
participate in the switching process.
HERE is a considerable driving force within the semiconductor industry to create scalable elements that can
switch between widely spaced nonvolatile resistance states at
low power. Such elements could find widespread application
in next generation memory and reconfigurable logic [1]–[3].
Whereas there are several new technologies that show promise
in this respect, they typically lack complete scalability, i.e., they
have a physical size, programming voltage, or programming
current that is excessive for high density systems at the 65-nm
node and beyond. Many are also difficult to integrate due to
the complexities associated with the additional processing steps
and masking levels required and, therefore, will be expensive
to add to highly scaled CMOS. One potential approach to this
problem involves switching elements, known as Programmable
Metallization Cell (PMC) devices, which utilize the reduction of
nanoscale quantities of metal ions in solid electrolyte films [4].
The formation of a robust but reversible conducting pathway
II. DEVICES BASED ON SOLID ELECTROLYTES AND OXIDES
Manuscript received March 30, 2006; revised May 1, 2006. This work was
supported by Axon Technologies Corporation. The review of this paper was
arranged by Associate Editor B. G. Park.
The authors are with the Center for Solid State Electronics Research, Arizona
State University, Tempe, AZ 85287-6206 USA (michael.kozicki@asu.edu).
Color versions of Figs. 2–12 are available online at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TNANO.2006.880407
In our earlier work [4], [5], we presented the formation
and operation of PMC devices composed of a silver doped
chalcogenide glass solid electrolyte film (e.g., Ag–Ge–Se or
Ag–Ge–Se) sandwiched between a silver anode and an inert
cathode. For an applied bias in excess of a few hundred mV,
an electron current from the cathode reduces silver ions in the
1536-125X/$20.00 © 2006 IEEE
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IEEE TRANSACTIONS ON NANOTECHNOLOGY, VOL. 5, NO. 5, SEPTEMBER 2006
resistivity which were originally attributed to homogeneous
modifications or an unspecified breakdown mechanism. These
explanations were dismissed by Dearnaley and colleagues [8],
who developed a filamentary model for the changes in resistance. It was claimed that since the anode insulator interface is
not completely smooth, there will be places where the electric
field is locally high, resulting in the formation of a conductive
filament by field induced migration of material from the anode
into the oxide. The metallic filament propagates through the
insulator and thereby reduces the resistance of the oxide layer.
Subsequent switching in this “electroformed” structure was
thought to be via electrically stimulated rupture and healing
of the filamentary connection at locations along its length.
Thurstans and Oxley [9] proposed a model which took other
characteristics of the electroformed structure, in particular
the observed negative differential resistance, into account.
Their model features a conduction mechanism that depends on
trap-controlled thermally activated tunneling between isolated
segments of a ruptured metallic pathway. Recently, Baek et
al. [10] used such models to describe the unipolar switching
characteristics of their transition metal oxide-based devices. In
these structures, nickel oxide is switched to a low resistance
state using a relatively high voltage/low current “set” pulse
and returned to a high resistance state using a low voltage/high
current “reset” pulse of the same polarity. Although the effect
is attributed to charge trapping mechanisms in the electroformed oxide, it is perhaps more likely to be due to the rupture
and healing of a metallic pathway. We have observed similar
unipolar programming in PMC devices in which a metallic
filament is formed by the electrodeposition of the anode metal
in the electrolyte at a particularly high critical voltage but
can be disrupted using a current pulse of the same polarity
[11]. In this case, the erase (reset) current must be in excess
of the current used to write (set) the device so that the current
carrying capacity of the filament is exceeded and it is thereby
severed, however, the erase voltage must be below that required
for electrodepostion of the metal so that the break does not
spontaneously re-close. Note that for the devices described in
[4], the voltage threshold for electrodeposition is sufficiently
low that a reverse bias is required to erase the device; a forward
current in excess of the original programming current limit
leads to further electrodeposition and the device resistance falls
to a lower stable value rather than rising to the off state.
The consideration of tungsten oxide as a base glass for the
electrolyte in PMC devices led to a number of obvious questions in the context of the above discussion. The main issue was
whether the devices would behave like the chalcogenide-based
variants or more like the metal-insulator-metal structures described by Oxley. To address these issues, we engaged in a program of material analysis and device characterization, described
in the following sections.
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electrolyte while an equivalent number are injected from the
anode via oxidation of the silver. A metal-rich pathway forms
in the electrolyte film via this electrodeposition process. The
level and duration of the ion current above the electrodeposition
threshold determine the quantity of Ag electrodeposited and,
hence, the resistance of the pathway; the resistance of the
structure falls by many orders of magnitude even for applied
currents as low as a few microamperes. The low resistance
electrodeposit is electrically neutral and stable, leading to
retention in excess of 10 years [1], [4], however, applying a
bias with opposite polarity will break the link by a combination
of current-induced rupture and electrochemical oxidation of
the electrodeposit. Note that when a reverse bias is applied,
the break cannot “heal” by electrodeposition as there is a net
removal of metal from the electrolyte in a process which is opposite to that which formed the metal filament in the first place.
The reverse ion current flows until the previously deposited Ag
has been oxidized and returned to the silver electrode. Thus, the
resistance increases again until the high resistivity of the solid
electrolyte is achieved. The oxidation/reduction process is fast
(tens of nanoseconds or less for write and erase), consumes little
power (in the microwatt range), and can be cycled well beyond
conventional nonvolatile memory technology
[4], [5].
The electrical characteristics of PMC devices comprising
chalcogenide-based solid electrolytes result from the unique
nanostructure of the electrolyte material. We previously established that the dissolution of Ag into Se- or S-rich base
glasses produces a ternary that is a combination of a dispersed
superionic nano-crystalline Ag-rich phase within a glassy insulating Ge-rich phase [6], [7]. The Ag ion-containing regions
are typically less than 10 nm in diameter and are separated
by a few nanometers of the insulating phase [5], [6]. It is
this particular nanostructure that allows the films to exhibit a
resistivity of 100 cm or more (which yields a high device off
resistance) while retaining good ion availability and mobility.
The accessibility of mobile Ag ions for electrodeposition
throughout the electrolyte allows the devices to switch rapidly
as the growing electrodeposit will always have a local source of
ionic metal to feed its formation. The reduced ions are replaced
via the ion current from the anode and so the electrolyte does
not become depleted of ions following switching and contains
“excess” metal within the electrodeposited pathway. Indeed, it
is this addition of metal into the already metal ion-rich but nano
phase-separated electrolyte that leads to both the reduction in
resistance of the structure and the nonvolatility of the on state.
To complement our work on chalcogenide-based PMC
devices, we have recently examined elements that are more
closely related to the materials present in modern integrated circuits. Tungsten metal and its oxide and Cu are obvious choices
in this respect. There has been widespread interest in the use
of a variety of oxides in switching/memory applications for a
number of years. Several models exist that describe the electrical phenomenon seen in thin oxide films sandwiched between
two metal layers but as yet no complete or consistent model
describes all the observations. Films from tens to thousands of
nanometer thick when subjected to certain critical electric fields
have been observed to undergo permanent large reductions in
III. TUNGSTEN OXIDE CHARACTERISTICS
The electrical properties of
have been widely investigated because of the existence of electro- and photo-chromic
effects in materials based on this oxide [12]–[14]. It is a wide
is
bandgap dielectric—the electronic gap of amorphous
KOZICKI et al.: LOW-POWER NONVOLATILE SWITCHING ELEMENT BASED ON COPPER-TUNGSTEN OXIDE SOLID ELECTROLYTE
be characterized. Note that in the fabrication of device structures (discussed later), a patterned tungsten electrode was used
and the excess metal following photodiffusion was left in place
on the surface of the electrolyte to form part of the anode top
electrode.
Characterization of these films was performed using Raman
spectroscopy and X-Ray Photoelectron Spectroscopy (XPS).
Raman spectra were performed in the micro Raman mode
using a 514.5 nm Ar+ laser, 100 objective, with an accumulation time of 100 s and a power of 6 mW. The experimental
curves were fitted using Labcalc software. X-ray Photoelectron
Spectroscopy (XPS) was carried out with high-resolution XPS
X-ray excitation and constant analyzer energy with
(Mg
8 eV pass energy, take-off angle of 90 , 240 W power). The
energy calibration of the spectrometer was performed using a
gold plate fixed to the sample. Raman data related to plasma
oxidized films are presented in Fig. 2. Since the films are predominantly amorphous, only breathing modes resulting from
the short-range order appear. The intensity of Raman signals
from the films is inherently weak because they are rather thin
but the deconvolution of the results permits collection of meaningful information relating to the vibration of the W-O bond.
This revealed a mode at 280 cm (bending mode), very weak
modes at 815 and 930 cm and one at 998 cm (stretching
film is illuminated with light, a mode at
mode). After the
, suggesting
450 cm appears that could be due to
some reduction has occurred [22]. Illumination with light
evidently causes some crystallization of the films, an effect
that is manifested in the appearance of a peak at 719 cm ,
bond. Such a structure has also
characteristic of the
been observed by Delichere et al. [23] who relate it to a higher
order of structural organization. When Cu is photodiffused into
the films, modes at intermediate frequency appear (440 and 460
and
cm ) that could be attributed to the vibrations of
modes [24]. This is significant as combinatorial electrochemical synthesis [24] has demonstrated that the formation of
Cu oxides is possible following introduction of Cu in the
films.
The XPS data of plasma oxidized films show that they are
indeed influenced by previous exposure to light Fig. 3(a) and
(b). Gaussian deconvolution of the W4f spectrum of the initial
and
films Fig. 3(a) shows well resolved doublet
caused by spin-orbit coupling with binding energies of 35.85
and 37.94 eV, which correspond to a typical 6 state of W.
However illumination with light causes a small shift of this douand
blet towards lower energies Fig. 3(b). The
show binding energies of 35.71 and 37.8 eV that could be an indication of some reduction of the oxide. We could not observe
a well expressed simultaneously occurring W4f double peak of
as was reported by Sun et al. [25] since the amount of the
5-valent W is small. We suggest that this is related to the fact
that there is no water included in our films. However the shift is
certainly an indication of the influence of light on our films. The
introduction of Cu causes significant changes in the XPS spectra
Fig. 4. The Gaussian deconvolution suggests the appearance of
4 binding energy values that could correspond to the 6 and
5 state of W, as expected. However, they are somewhat shifted
from the standard data for these particular doublets. It is diffi-
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3.25 eV [15]—but a large range of conductivity has been reported [16]. This is due to the fact that W can exist in different
valence states when oxidized, thus forming oxides with different stoichiometry and, hence, with different electrical conductivity. Different valence states can also be created by illumination with UV light with energy exceeding 3.3 eV [17]. In
can be altered by the introaddition, the conductivity of
duction of metal ions and this is of particular interest in the context of the current work [18]. As discussed earlier in this paper,
we have used silver as the mobile ionic species in the base glass
(and as the oxidizable electrode) in chalcogenide-based PMC
devices due to its excellent electrochemical qualities but copper
is also a good candidate due to its high ionic mobility in a variety of materials, including oxides [19]. To combine metals
and thereby form a stable solid electrolyte, we can
with
take advantage of the photosensitivity of the oxide. This effect
more commonly manifests itself as photochromism which can
be explained by a double charge-injection model [20], in which
and electrons into a
lattice forms a tunginjection of
sten bronze structure
,
. Note that
can
in tungsten oxide hydrate or
be partially reduced to
in 12-tungstates by electrons occupying the empty
orbital
is typically
in
due to the electron localization effect.
photochromic
but can actually be a variety of mobile ions,
. So, if the
film is prepared using a dry
including
reaction and a very thin UV transparent Cu film is evaporated
on top, illumination can cause the formation of a
bronze.
formation is concerned, although wet chemAs far as
ical or plasma oxidation of tungsten metal are simple room temperature processes, the thickness of the layer formed is limited to a few nanometers by the diffusion of the oxidant species
through the growing oxide. It is, therefore, necessary to use the
evaporation of tungsten oxide to form thicker films. To grow
thin tungsten oxide films on tungsten metal, we elected to use
plasma techniques as we discovered that the growth of layers by
wet chemical means was difficult to control and led to material
with highly variable characteristics. In addition, the wet grown
oxides also contain species that would possibly interfere with
the incorporation of Cu. The surface of 100-nm-thick tungsten
glow dismetal films was oxidized using a nitrous oxide
charge at an RF power of 250 W and a substrate heater temperature of 300 C for 10 min. in a PlasmaQuest plasma processing
system. The
flow rate was 20 sccm with 1000 sccm of helium carrier gas. The precise thickness of the grown oxide was
difficult to ascertain due to the roughening of the underlying
tungsten caused by the growth process but it was estimated using
ellipsometry and capacitance-voltage measurements to be in the
order of 3 to 4 nm thick. To achieve metal diffusion, 25-nm-thick
Cu layers were deposited by high vacuum evaporation and the
metal was photodiffused into the oxide using the 405-nm broadband UV light source of a Karl Suss MJB 3 mask aligner for 14
. This wavelength was
min at a power density of 4.5
[21], where
chosen as it is near the absorption edge of the
the light causes maximal change in the material. Blanket samples were fabricated for material characterization and the excess
surface metal was removed using a short treatment with iron nisolution so that only the Cu in the oxide would
trate
3
4
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Fig. 1. Schematic cross section of an integrated circuit, showing position of oxide switching elements on selected tungsten via plugs. Interconnect layers M1 and
M2 are separated by IMD. Through vias provide M1 to M2 connectivity.
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Fig. 2. Raman analysis of
formed by plasma oxididation of W (a) as
synthesized; (b) after illumination with UV light; (c) photo-doped with Cu.
cult to fully interpret these results but we believe that they could
be related to distortions in the amorphous structure of the films
and formation of clusters [26] in which some of the diffused Cu
is oxidized and the clusters become part of the overall structure.
This leads to the occurrence of bond overlapping and a shift in
the binding energies. Overall, even though the signal-to-noise
ratio is rather poor, the data related to Cu does indeed indicate
its presence in an oxidized state within the tungsten oxide matrix (see inset in Fig. 4).
We now turn to the deposited oxide samples. The evaporation
source was high purity tungsten oxide (99.99%) in 3–12 mm
sintered yellow-green pieces (Cerac, Inc.). Using high vacuum
thermal evaporation, 50- or 100-nm thick films were deposited
on the substrates described above. Following this, 25 nm of Cu
without breaking the deposition
was evaporated on the
system vacuum. The Cu was then photodiffused into the oxide
for 14 min and the excess metal was etched using the techniques
described earlier. The analysis of these films revealed some simwas clearly
ilarities with the plasma-grown oxides in that
present as before in the as-deposited films, indicating the forma, but the way the copper was incorporated in the detion of
posited oxide appeared different. Both Raman and XPS analysis
of the films gave little or no evidence of the formation of Cu-ox. We
ides after copper is photodiffused into the deposited
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Fig. 3. XPS spectra of
formed by plasma oxididation of W (a) as synthesized; (b) after illumination with UV light.
are currently investigating this in more detail but it would appear
that whereas at least some of the photodiffused Cu becomes oxidized in the plasma-grown oxide to form a two phase (copper
oxide/tungsten oxide) system, it exists in an unbound state in
the deposited material. This could be due to the fact that the
deposited oxides are in a highly stable state and do not readily
react with the diffusing copper, whereas the presumably substoichiometric plasma-grown oxides are much more reactive. In
any case, this dissimilarity is extremely significant in the context of the observed differences in the electrical characteristics
5
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KOZICKI et al.: LOW-POWER NONVOLATILE SWITCHING ELEMENT BASED ON COPPER-TUNGSTEN OXIDE SOLID ELECTROLYTE
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Fig. 4. XPS spectra of
formed by plasma oxididation of W after Cu
diffusion in the oxide film; inset—XPS spectrum of Cu-containing species in
the oxide film.
of devices based on plasma-grown and deposited oxides, as described in the following sections.
IV. DEVICE FABRICATION AND ELECTRICAL
CHARACTERIZATION
A. Devices Based on Plasma Grown Oxide
Tungsten oxide films were incorporated in PMC device structures using oxygen plasma treatment of a tungsten metal bottom
electrode as described in the previous section and with copper as
the mobile species and oxidizable top electrode. Schematics of
typical test device structures are shown in Fig. 5 (Fig. 5(a) represents devices based on grown oxide whereas Fig. 5(b) is the
deposited oxide variants described in the following section). A
100-nm-thick tungsten layer was deposited by chemical vapor
deposition on silicon dioxide grown on silicon substrates. The
tungsten was covered with 100 nm of silicon dioxide by chemical vapor deposition and via (through) holes were defined in
this dielectric using optical lithography and plasma
etching. The exposed tungsten in the holes was oxidized using
the plasma oxidation technique described in the previous section. A 25-nm-thick Cu metal layer was deposited using high
vacuum thermal evaporation and photodiffused into the
using a 405-nm broadband UV light source in a Karl Suss MJB
. The excess
3 mask aligner at an intensity of 4.5
Cu metal was not removed to ensure the presence of a Cu-rich
source on the electrolyte surface. A 50-nm-thick Cu metal layer
was then deposited using high vacuum thermal evaporation and
patterned using a lift off process to complete the oxidizable electrodes of the devices. Note that this device configuration was absolutely necessary to observe switching regardless of how the
base glass for the electrolyte was formed; a lack of Cu in the
top electrode results in no measurable switching activity. The
Fig. 5. Cross-sectional schematics of oxide-based switching devices. The active device area is the tungsten-electrolyte-Cu stack with area defined by the
dielectric. (a) Device based on grown
diameter of the through hole in the
oxide. (b) Device based on deposited oxide.
SiO
probe pad window in the
to the tungsten layer was formed
using another lithography step in conjunction with wet-chemical etching. The quasi-static characteristics of the test devices
were obtained by connecting the device electrodes via tungsten
probes held in micromanipulators in a probe station to a semiconductor parameter analyzer (Agilent 4155C). Voltage doublesweeps were carried out starting at maximum reverse bias (tungsten electrode positive, copper electrode negative), sweeping
to an appropriate forward voltage (tungsten electrode negative,
copper electrode positive), and sweeping back again to the reverse bias starting point.
Fig. 6 shows a resistance-voltage plot from our earlier work
on large (1– m diameter) devices with a Cu/plasma-grown
electrolyte and Cu oxidizable electrode [4]. Three consecutive sweeps are shown from 0.75 V to 1.0 V to 0.75
V for a current limit of 0.5 A. The maximum off resistance
and the transition
for the first sweep is in excess of
to the low resistance state occurs at 0.9 V at which point the
current reaches its compliance limit. The current stays at the
compliance until the negative-going sweep reaches 0.25 V,
, more
at which point the measured on resistance is 500
than four orders of magnitude lower than the off state. Note
that the apparent rise in resistance between 0.25 and 1 V is an
artifact of the current compliance control in the measurement
instrument. The appearance of the lower threshold following
switching is typical of all PMC devices and represents the
minimum voltage required for electrodeposition once the
process has been initiated and a metal-rich region has formed
in the electrolyte. Electrodeposition within the oxide will
continue as long as the voltage drop is greater than 0.25 V
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6
Fig. 6. Resistance-voltage plot of a 1.0-m-diameter device with Cu/plasmaelectrolyte and Cu oxidizable electrode.
grown
Fig. 7. Resistance-voltage plot of a 240-nm-diameter device with Cu/plasmaelectrolyte and Cu oxidizable electrode.
grown
for this physical configuration. Note that the decreasing resistance due to the electrodeposition effect increases the current
flowing through the device until the compliance is reached.
At this point, the voltage drop falls to the threshold and the
electrodeposition stops. The on resistance is, therefore, defined
by the electrodeposition threshold divided by the current limit
.
It is clear from the plot of Fig. 6 that the first sweep is different from the following two. The off resistance is apparently
reduced by an order of magnitude following the first sweep and
the switching threshold moves from 0.9 V in the first sweep toward 0.7 V in the latter cycles. The same switching threshold
lowering effect was seen in all devices tested. In our experience,
elevated switching threshold is a result of “subsaturation” of the
electrolyte with metal ions. We believe that the reduced amount
of mobile copper in the doped plasma-grown oxides is due to
the two-phase nature of this electrolyte, in which some of the
copper is in the form of an oxide and, therefore, cannot readily
take part in the electrodepostion process. It is likely that the first
voltage sweep injects additional Cu from the anode to increase
the unbound copper concentration, thereby lowering the off resistance and write threshold for the following sweeps. However,
since this injection of Cu by the applied field will only increase
the concentration of metal along the volume of the filamentary
pathway (where the field is highest) and not in the rest of the
film, the metallic connection formed will be susceptible to dissolution by metal diffusion into the surrounding electrolyte due
to the high copper metal concentration gradient. This was confirmed by resistance-time measurements which showed an upward drift in the on-state resistance, which returns to the high
resistance off state after a few hours at room temperature and
after only a few minutes at 70 C.
The erase transition threshold occurs at 0.2 to 0.3 V in this
device, with full erase occurring beyond 0.5 V. The sudden increase in resistance at the erase transition threshold is due to the
current-induced breaking and voltage-induced electrochemical
oxidation of the conducting pathway, leading to a device resistance that is dominated by the high resistance of a portion of
now unbridged electrolyte. The subsequent resistance rise (and
any steps therein) is a result of the removal by electrochemical
oxidation of the remaining electrodeposited metal. Note that the
drop in off state resistance for increasing voltage magnitude in
either direction is due to the leakage current in the electrolyte.
Fig. 7 shows the <Au: Define R-V?> (R-V) characteristic
of a smaller Cu/plasma-grown
electrolyte device with a
240-nm-diameter for a 0.5- m current limit. This shows many
of the characteristics of the 1- m device but the write thresholds
are all considerably higher (around 2 V initially and decreasing
toward 1.7 V with increasing sweep number). The post-electrodeposition thresholds are also higher than before (close to 0.7
V), leading to higher on resistances as defined by the programming current relationship given earlier
. We believe that these higher thresholds are due to considerably lower Cu diffusion into the oxide in the small area devices during illumination. The optical attenuation of the reduced
intermetal dielectric (IMD) and the
dimension hole in the
nonconformal coverage of the copper in this reduced dimension
feature will lead to less light reaching the interface of the metal
and the oxide and this will reduce copper photodiffusion. The
lower copper diffusion, in addition to the oxidation of the Cu that
does enter the oxide, results in the higher switching threshold.
The lower Cu concentration does little to alter the erase initiation threshold, which occurs between 0.2 and 0.4 V and the
maximum off resistance of over
(the limit of the measurement system) is achieved beyond 0.5 V as before. This
is expected as the erase initiation occurs due to the current-induced breaking of the conducting filament and the nature of the
electrolyte has little effect on this.
Even though the devices based on plasma-grown oxide were
not ideal as switching elements, they showed the essential characteristics of PMC structures, with a large decrease in resistance
for a low write threshold and programming current and a return
to a high resistance state at a low negative bias. However, if they
are to be used in embedded memory or as switching elements
in reconfigurable logic, more work is required to improve their
stability.
WO
WO
KOZICKI et al.: LOW-POWER NONVOLATILE SWITCHING ELEMENT BASED ON COPPER-TUNGSTEN OXIDE SOLID ELECTROLYTE
7
B. Devices Based on Deposited Oxide
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The substrates chosen for the deposited oxide-based device
experiments were similar to those described in the previous section. In this case, after the dielectric was etched to form the via
layers
holes and expose the underlying tungsten electrode,
50 or 100 nm thick were deposited under high vacuum using
an electron beam evaporator. Following this, 25 nm of Cu was
without breaking the deposition system
evaporated on the
vacuum and the Cu photodiffused into the oxide for 14 min
using the technique described earlier. Once again, the excess
Cu was not removed from the surface. To complete the device,
an additional 50 nm of Cu was deposited and the electrolyte/top
electrode stack was patterned using optical lithography and lift
off. A schematic of the device structure is shown in Fig. 5(b).
The electrical characterization using the set-up described earlier was performed at room temperature and at a range of elevated temperatures using a temperature controlled chuck on the
probe-station.
Fig. 8(a) gives a typical current-voltage plot of a 240-nm-didevice, comprising a 50-nm deameter
posited oxide. This was programmed at 1 A and exhibited a
write threshold of approximately 0.4 V and a lower electrodeposition threshold of 0.1 V, at which point the negative-going
sweep drops below the compliance current level. The device
returns to its high resistance state at 0.2 V. Fig. 8(b) shows
the resistance-voltage plot for the same device, showing an avand an on resistance
erage off resistance in the order of 2
. Note that the on resistance is dependent on proof 100
gramming current as before with the on resistance equal to the
lower threshold (0.1 V in this case) divided by the programming
current limit. Since this lower threshold for electrodeposition
is quite invariant, the on resistance is also highly reproducible,
typically to within 5% of the mean value for a given programming current. These results are similar in form to those obtained
with the plasma-grown oxide except that the write threshold is
not only lower for the much thicker deposited oxide but it is
also much more consistent, largely devoid of the higher initial
threshold effect; i.e., subsequent sweeps result in a switching
threshold that is very close to the first sweep value, with a median of 0.4 V and a standard deviation of approximately 50 mV.
In addition, the strong dependence of the write threshold on device diameter that was seen in the grown oxide devices is not
present in the deposited oxide variants. This suggests that the
deposited oxide in both the smaller and larger diameter devices
is equally saturated with “free” metal following the photodissolution step, which was evidently not the case in the grown material. Note that the lower off resistance, which has an average
for the 240-nm devices, is also indicative of
value around 2
higher mobile metal content than the grown oxide devices. This
is consistent with our material characterization work which suggested that the photodiffused copper formed a bronze with the
deposited tungsten oxide and, therefore, would be unbound and
mobile.
Fig. 9 is the resistance-voltage behavior of a 5- m-diameter
deposited oxide device (100-nm-thick
) at operating temperatures of 25 C, 70 C, and 135 C for a 10- A programming
current. The device switched from the high resistance range to
0
Fig. 8. Electrical behavior of a 240-nm-diameter Cu WO based device
formed from a 50-nm-thick oxide. (a) Current–voltage plot of a device programmed at 1 A. (b) Resistance voltage plot for the same device.
around 10
at all temperatures, with a lower electrodeposition threshold of 0.1 V as before. The 10 lower on resistance
is a direct result of the 10 higher programming current than
the case shown in Fig. 8 (i.e., in this case on resistance is given
). The apparent slight rise in write
by
threshold with temperature is probably due to the increase in the
probe contact resistance due to thermal expansion. The most
significant difference with temperature is the erase threshold,
which clearly drops from the room temperature initiation value
of 0.2 V as the temperature increases. This is consistent with
PMC device operation as the increased thermal energy reduces
the current required to break the conducting link. Fig. 10 gives
multiple sweep resistance-voltage plots of a 240-nm-diameter
) for a 100- A programming curdevice (100-nm-thick
rent at 135 C. The 100- A programming current gives an on
, as expected
.
resistance close to 1
With the exception of the first sweep, device operation is consistent with the larger device of Fig. 9 at 135 C, with a write
threshold close to 0.4 V, a lower threshold of 0.1 V, and an erase
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8
Fig. 9. Resistance-voltage plots at different operating temperatures of 5-m
-diameter devices formed from 100 nm of
evaporated and photo-doped
with Cu.
WO
Fig. 10. Resistance-voltage plots at 135 C for a 240-nm-diameter device comphotodoped with Cu.
prising 100 nm of evaporated
WO
initiation threshold of 0.1 V. The apparent higher threshold
of 0.6 V for the first sweep is once again thought to be due to
probe resistance/thermal expansion effects as it was not repeatable (and, therefore, was not the same mechanism as that seen
in the grown oxide devices).
It is important to note that in the results shown in Figs. 8 to
10, the on resistance is independent of device diameter and electrolyte thickness. Insensitivity to device diameter is expected
when switching is due to the formation of a filamentary conducting pathway which is considerably smaller in width than
the electrolyte (device) diameter, The lack of sensitivity to electrolyte thickness is due to the fact that the on resistance depends
on electrodeposition threshold and programming current limit,
neither of which are influenced by device geometry. The on state
in the above devices is also quite ohmic, which suggests that the
filament is indeed metallic in nature, with good (low barrier)
contact to the tungsten and copper electrodes.
WO
Fig. 11. Data retention behavior of devices fabricated using Cu-doped
films programmed at 10 A and 1 V and probed using a 100 mV measurement
signal.
Another means for determining the suitability of the deposited oxide for nonvolatile switching applications was
measurement of retention of the devices over a range of temperatures. For this test, the devices were programmed using a
10- A current and the resistance measured using a 100 mV
dc probe signal at logarithmic intervals of time. The test was
conducted at room temperature and also at 70 C, and 135 C.
The results revealed that devices fabricated using photodiffused
films showed good state retention even at
Cu-saturated
135 C, as shown in Fig. 11. This particular test was terminated
at around 12 hours but in all the samples evaluated, there was
no indication of an upward drift in resistance. Indeed, in most
cases there was actually a small decrease in resistance for
the test conditions used, even though the high off state was
maintained.
Finally, the cycling endurance of the
device structure was assessed. A 500-nm-diameter device with
electrolyte was
a 100-nm-thick Cu-photodoped deposited
repeatedly written and erased at 135 C using a programmable
pulse generator. The pulse generator could be programmed to
provide variable numbers of 100 ms 1 V (write) followed by
100 ms 1.5 V (erase) pulse sequences. The voltages and pulse
lengths used were considerably in excess of what was required
to program the devices to ensure that they were fully written and
erased during each cycle, as confirmed by single pulse measurements and digital oscilloscope screen captures taken during cycling. A series resistor was used to limit the on current through
the device to approximately 5 A. Fig. 12 gives quasi-static resistance-voltage plots for voltage sweeps from 1 V to 1 V to
1 V with a current limit of 5 A. The plots were taken after
1, 10, 100, 1000, and 10 000 cycles, as indicated in the figure.
All curves below 10 000 cycles show good repeatability of the
off resistance, write threshold, and on resistance, all to within
(averaged
10% of the approximate median values of 400
, respectively. The erase porover 0–0.3 V), 0.4 V, and 20
tions of the individual curves (from 0 to 1 V) appear erratic
but the devices are actually in the off state following this part
KOZICKI et al.: LOW-POWER NONVOLATILE SWITCHING ELEMENT BASED ON COPPER-TUNGSTEN OXIDE SOLID ELECTROLYTE
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the Cu is not bound and is, therefore, more able to participate in
the switching process. We assume that this difference is a result
of the plasma-grown oxide being somewhat substoichiometric
compared to the stable deposited oxide but additional analysis
work is required to establish the exact mechanism of copper
incorporation. The devices based on deposited oxide had reproducible electrical characteristics, with critical parameters
such as off resistance, write threshold, and on resistance being
within 10% or less of their median values. The control of on
resistance by write current limit was evident in these devices
to 1
were demonstrated,
and on resistances from 100
suggesting that multi-level storage (two or more bits per cell)
might be possible with these devices. Retention and endurance
seemed promising for these early prototypes.
Of course, more material optimization and electrical characterization is required to improve write and erase speed and other
important factors such as endurance before these elements can
-range devices show promise as
be utilized. However, these
an easily integrated switching element as they can use existing
materials and compatible processing and involve as little as one
additional masking step over standard advanced CMOS.
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+
Fig. 12. Effects of write-erase cycling (each cycle was 100 ms at 1 V, 100
ms at 1.5 V) at 135 C of a 500-nm-diameter device comprising 100 nm of
photodoped with Cu. The device resistance-voltage plot was
evaporated
taken after various numbers of cycles as indicated in the plot.
0
9
of the sweep and can be subsequently fast cycled. The plot corresponding to 10 000 cycles shows a much lower off resistance
and even though the device switches at the expected threshold
on resistance, it cannot be cycled further.
and attains the 20
The failure mechanism that causes the “stuck bit” condition is
unknown but is currently under investigation.
V. CONCLUSION
To conclude, we have demonstrated the feasibility of fabricating solid-state switching elements using tungsten metal, a
solid electrolyte based on tungsten oxide with photodiffused
copper, and a copper electrode. These devices exhibit a high
off resistance and can be switched at low voltage to an on resistance state that is independent of device geometry (diameter
and electrolyte thickness) but is strongly governed by the write
current. The device can be returned to its high resistance state by
the application of a small reverse bias. The electrical characteristics, and in particular those of devices comprising deposited
tungsten oxide, are extremely similar to Programmable Metallization Cell devices based on chalcogenide electrolytes. We
can, therefore, conclude that the same low energy electrochemical processes that result in the formation and removal of a filamentary conducting pathway in the chalcogenide devices are
also responsible for the switching in the copper-doped oxide devices. The exact composition of this conducting pathway is currently unknown but the experimental evidence, which includes
the ohmic nature of the on state, suggests that it is the result of
the localized presence of Cu metal within the oxide.
Electrical characterization has revealed that devices consisting of plasma-grown oxides have a decreasing initial
threshold voltage and poor retention, whereas devices based on
deposited oxide posses a stable switching threshold and good
.
retention, even at elevated operating temperature
This difference in behavior was attributed to the observation
that the copper tends to oxidize in the plasma-grown oxide and
is, therefore, not in a mobile state whereas the copper-doped deposited oxide is “bronze-like,” comprising a structure in which
ACKNOWLEDGMENT
The authors would also like to thank C. Poweleit for preparation of the Raman spectra and T. Karcher for generation of
the XPS results. The authors gratefully acknowledge the help
of the staff and use of the facilities within the Center for Solid
State Electronics Research and the Center for Solid State Science at Arizona State University.
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[2] T. Sakamoto, H. Sunamura, H. Kawaura, T. Hasegawa, T. Nakayama,
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[3] S. Kaeriyama, T. Sakamoto, H. Sunamura, M. Mizuno, H. Kawaura, T.
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10
IEEE TRANSACTIONS ON NANOTECHNOLOGY, VOL. 5, NO. 5, SEPTEMBER 2006
Michael N. Kozicki (M’84) was born near Edinburgh, Scotland, and received the B.Sc. and Ph.D.
degrees from the University of Edinburgh in 1980
and 1985, respectively.
He was with Hughes Microelectronics Ltd. (now
Raytheon Systems Ltd.) before joining Arizona State
University (ASU) in 1985, where he is currently Professor of Electrical Engineering. He i holds several
key patents in integrated ionics, including those involving the storage of data by electrodeposition in
solid electrolytes. He is also a founder of Axon Technologies Corp., an ASU spin-out company formed to develop and license solidstate ionic technologies, and has served as Director of ASU’s Center for Solid
State Electronics Research.
Chakravarthy Gopalan (S’06 <Au: Please confirm
year?>) received the B.S. degree in electronics and
communication engineering from Madurai Kamaraj
University, Madurai, India, in 1999, and the Ph.D.
degree in electrical engineering from Arizona State
University, Tempe, in 2005.
He is currently working at Spansion, <Location?;
please update affiliations footnote on p.1 if necessary>. His current research interests include passive
array design and fabrication of programmable metallization cell memory devices as well as flash memory
IE
Pr EE
int P
Ve ro
o
rs f
ion
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o:25AgCl] : (1
x)[Ag O : WO ],” Solid State Ionics, vol. 171,
pp. 199–205, 2004.
[19] L. Chernyak, K. Gartsman, D. Cahen, and O. Stafsudd, “Electronic
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2003.
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and S. K. Deb, “Raman spectroscopic studies of electrochromic aWO ,” Elelctrochim. Acta, vol. 44, pp. 3111–3115, 1999.
[23] P. Delichere, P. Falaras, M. Froment, A. Hugot-Le Goff, and B. Aguis,
“Electrochromism in anodic WO films 1: preparation and physicochemical properties of films in the virgin and colored states,” Thin Solid
Films, vol. 161, pp. 35–46, 1988.
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0
solutions.
Murali Balakrishnan (S’) received the B.S. degree
in electronics and communication engineering from
the University of Madras, Chennai, India, in 1999,
and the M.S. degree in electrical engineering from
Arizona State University, Tempe, in 2003. He is currently working toward the Ph.D. degree at Arizona
State University.
His current research interests include fabrication
and characterization of programmable metallization
cell memory devices.
Maria Mitkova received the M.S. and Ph.D. degrees
from the Technological University, Sofia, Bulgaria,
in 1970 and 1976, respectively.
She was Professor at the Technological University
and Bulgarian Academy of Sciences and has worked
in the R&D of Ovonic memory devices in the Institute for Microelectronics. Since 1997, she has worked
in the USA at the University of Cincinnati, Cincinnati, OH, and Arizona State University, Tempe. Her
interests are in the field of amorphous semiconductors, their characterization and application as optical
and electronic memory media. She has specialized in a number of Ag-containing
chalcogenide systems.