21.6 Conductive-AFM Tomography for 3D Filament Observation in

Conductive-AFM tomography for 3D filament observation in resistive switching devices
U.Celano1,2,*, L.Goux1, A.Belmonte1,2, A.Schulze1, K.Opsomer1, C.Detavernier3, O.Richard1, H.Bender1,
M.Jurczak1, W.Vandervorst1,2
1
imec, Kapeldreef 75, 3001 Leuven – Belgium
2
Katholieke Universiteit Leuven - Belgium
3
University of Gent - Belgium
*
[email protected]
Abstract
In this paper we demonstrate a novel characterization
technique for the observation of the conductive filament in
conductive bridging memory devices (CBRAM).
The
conductive filament is observed for a scaled memory element
programmed under 10µA operative current. After the
electrical programing, the C-AFM tomography enables the
3D analysis of the conductive filament within the switching
layer.
Introduction
Among different designs for non-volatile memories, the
conductive bridging device offers fast switching, high
endurance and good scalability (1-4). CBRAM operation
relies on the voltage-induced redox-based formation and
rupture of a Cu- or Ag-based conductive filament in an
insulating layer acting as a solid state electrolyte (2).
Although switching characteristics and reliability properties
are all related to the properties of the conductive filament
(CF), the observation of the CF in scaled devices is still
missing. This is due to the extremely challenging local
characterization and to the 3D-nature of the CF. So far,
attempts using in-situ TEM, C-AFM, EDX (5-8) have
obtained the observation of the CF only on dedicated test
structures. In this paper, for the first time, we unveil the 3Dstructure of a conductive filament (CF) for a scaled 1T1R
memory element using C-AFM tomography. We demonstrate
the application of this concept by imaging the CF for a
functional device as well as for a device stuck in its low
resistive state (LRS).
Methodology
The memory device in our work is a cross-bar structure
(Fig.1) stacked on top of a select transistor in a 1Transistor/1-Resistor (1T1R) scheme, as shown in Fig.2a.
The drain of the transistor is connected to the memory cell by
a TiN bottom electrode (BE) line which is planarized prior to
Al2O3\Cu\Au processing (Fig.2,3). In order to carefully
control the CF formation, we used the gate voltage to limit
the current at 10μA during forming and set switching
operations throughout cell cycling. A full resistive switching
cycle is reported in Fig.4. The CF is formed by applying a
positive voltage to the Cu electrode, the dissolution is done
with the opposite polarity. A memory device was left in its
LRS ~ 150kΩ before the C-AFM tomography. As
978-1-4799-2306-9/13/$31.00 ©2013 IEEE
traditionally C-AFM is used to resolve sub-nm electronic
defects in planar dielectrics (9), CF characterization by CAFM has been explored on surface observation. However
this approach presents several strong limitations: (i) the
analysis is confined to the surface, (ii) the CF is imaged only
in 2D (in-plane image), and (iii) the CF is not formed in a
real device configuration, and thus typically not
representative. Indeed when forming the CF in a real device,
the presence of the top electrode (Fig.5,6,7) shields the
switching layer and the CF, preventing any C-AFM analysis
(Fig.6). We overcome these limitations through the
development of C-AFM tomography which enables to
observe the conductive filament in full 3D. This is achieved
by collecting C-AFM images of the conductive filament at
different depths within the stack (Fig.8). Here we exploit the
hardness of in-house developed (10) conductive full-diamond
tips to induce a controlled material removal at each
consecutive AFM-scan (Fig.8,9) implementing in essence a
slice and view mode (11). To induce the controlled material
removal, a strong pressure is applied between diamond-tip
and sample (GPa) during the AFM scan which leads to a
removal rate down to ~ 0.1-0.3nm/scan providing sub-nm
depth resolution (12,13). The consecutive C-AFM images
collected during the slicing of the memory cell display the
evolution of the CF from the top (Fig.10) to the bottom
electrode (Fig.11). Collating all those images then leads to a
3D tomogram (Fig.12,13,14).
Results
The C-AFM tomography starts with the removal of the
20nm-Au/20nm-Cu top electrode shielding the Al2O3. After
the controlled removal of the top electrode we slice in a
controlled manner through the Al2O3 with a rate of 0.7
nm/scan. The height measurements, in Fig.9, clearly indicate
that we dig into the dielectric with nm-steps from 5nm down
to ~1nm. After each slice the “probing C-AFM” cycle
identifies the properties of the CF at the exposed surface. For
instance the first C-AFM “view” on the Al2O3 (Fig.10)
clearly shows the planar extent of two CFs at the interface
closest to the Cu electrode. The subsequent views are collated
and interpolated into the final 3D tomogram (Fig.13,14).
Both figures show that the filament has a conical shape. We
observe a filament composed of two branches shrinking from
the Cu-top (spots ~ 20 and 12 nm) to bottom TiN-electrode
(spot ~2 and 8 nm size) e.g. the filament is wider on the side
of Cu. Similar shapes are found in all devices analyzed in
LRS.
A. Switching mechanism
The wider CF cross-section at the Cu-electrode interface
suggests CF growth from Cu anode down to TiN cathode
during forming, that is opposite to the direction expected
from the theory of electrochemical metallization memory cell
(2,3). We associate this result to the use of Al2O3 as
electrolyte material, and we propose the following scenario:
the forming voltage VF>2V required to generate a CF not
only allows injecting Cun+ ions in the Al2O3 layer but is also
sufficient to generate vacancy defects along the CF (Fig15a).
We previously performed ab-initio simulations (4) indicating
that Cun+ cations have better stability than Cu neutral species
in amorphous Al-O matrix, and that these cations drift by
forming bonds with oxygen (O) and hopping along chains of
O-sites (4). Such a drift process would be facilitated if Cun+
species would locally substitute Al-vacant sites. The doping
by Cun+ cations would require charge compensation through
hole defect or O-vacancy creation, increasing the electrical
conductivity of the CF. Therefore, the forming process may
correspond to a conical drift of Cun+ species embedded in a
local Al-Cu-O phase whereby conical shape is induced by
local field enhancement. This mechanism stops when Cufilament reaches the cathode. The final CF shape will depend
on further growth depending on current compliance. In this
scenario, the reset mechanism would correspond to a Jouleassisted drift back of Cun+ species. As reset voltage mainly
drops on the weakest part of the CF, closer to the TiN
interface (Fig.15e), reset would express as a CF recess
towards Cu electrode.
B. Correlation with device properties
C-AFM tomography shows that the total cross-section of the
CF is in the range of 10nm at the weakest part (Fig. 11,14),
which may be associated to the LRS resistance of ~105 Ω
measured before de-processing (note, the C-AFM artifact
fig.13b it leads to overestimation of the CF size). We have
reported excellent CBRAM switching functionality in this
range of LRS resistance previously (4), and more recently for
RLRS~1MΩ (14) which is expected to show narrower CFs.
Finally we used C-AFM tomography to understand the origin
of cycling degradation. Figs.17-19 illustrate the C-AFM
tomography analysis study of a device stuck in LRS i.e.
failing to reset. We observe the formation of a solid Cuplatelet running throughout the entire dielectric layer. This
platelet formation may be attributed to thermal-activated
diffusion of copper during switching, which takes place on a
preferential plane direction due to local minima for Cu
hopping (4). Thus creating a continuous Cu wall that does not
present any narrow part for reset.
Conclusion
We have demonstrated 3D C-AFM tomography as a novel
characterization technique for CF observation in CBRAM
devices. We can successfully show the CF in devices cycled
under real operative conditions and observe a conical shape
with sizes shrinking from ~30nm to ~10nm in the direction of
the cathode. The results are in agreement with a model taking
into account the slow migration and incorporation of Cun+
species into a Al-O based switching layer.
Acknowledgements
Research funded by a Ph.D. grant of the Agency for Innovation by
Science and Technology (IWT), we acknowledge the partial funding
by IMEC’s Industrial Affiliation program on RRAM.
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Fig. 1 Schematic illustration of the cross-point memory
device. The cell stack is presented in the inset.
Fig. 1 Schematic illustration of the cross-point
memory.
Fig. 3 Sectional view of the cross-point device,
perpendicular and parallel to the bottom electrode.
Fig. 2a Cross-sectional TEM of the integrated
1T1R device (schematic is shown in the inset).
Fig. 4 I-V curves of DC switching cycle for the
device under investigation, after forming the
devices is reset and finally is left in LRS ~150kΩ
(trace 3). The polarity to the top electrode is
positive for set and negative for reset.
Fig. 2b Cross-sectional HRTEM
image of the memory element.
Note, the area shown is the red
rectangle represented in Fig.2a.
Fig. 5 Traditional C-AFM on device
in LRS. In (a) the topography and in
(b) the current maps are presented.
Note, high current levels are
observed for both electrodes due to
the presence of the conductive
filament
bridging
these
two
terminals.
Fig. 6 C-AFM detail on the cross-point of a
memory device in LRS. Note, the presence of the
top electrode which is shielding the filament
observation.
Fig. 7 Tapping mode AFM image of the device
under investigation.
Fig. 8 Schematic illustration of the C-AFM
tomography concept. After top electrode
removal, C-AFM slices are scraped at
different heights of the conductive filament.
Fig. 9 AFM z-value, The height evolution
during the removal of the Al2O3 layer is reported
for different C-AFM slices.
Fig. 10 1st C-AFM slice after top electrode
removal. Note, the appearance of the two
branches of the conductive filament in the
middle of the active area.
Fig. 11 C-AFM current map close to the end of
the conductive filament. This is the last image of
the filament before the final removal of the
Al2O3.
Fig. 12 Stacking of the slices collected at
different heights of the conductive filament.
The consecutive images are aligned to correct
for possible drifts before the interpolation.
Fig. 13 (a) Sectional view of the 3D
reconstructed tomogram of the conductive
filament. In the bottom image the current level
corresponding to the Al2O3 material is removed,
in order to highlight the shape of the filament. (b)
A possible broadening effect of ~ 2nm on each
side of the CF can be due to the tunnel current
measured by the tip in proximity of the CF.
Fig. 14 Second branch of the CF showing a
clear conical shape. The wider part of the
filament is at the anode interface i.e. at the
interface of Cu injection.
Fig. 16 Superimposition of the
measured conductive filament with
our model illustration.
Fig. 15 Proposed model for the Cu migration and filament formation in low-mobility media. (a) Under the effect
of positive bias, Cu atoms are ionized and injected into the switching layer. (b) Due to the poor Cu mobility in
the switching layer the migration is highly hampered. (c) The Cu migration is easier in presence of vacancy
defects and along chains of O-sites of the Al-O bonding in the amorphous matrix. (d) Once the filament is
formed the voltage drop is completely across the filament and the electric field in the switching layer disappears.
Note, that the final CF can be constituted of different conductive species such as Cu ions, Cu , Al-Cu-O phases.
(e) Under the opposite polarity the filament rupture takes place due to electrochemical and joule heating assisted
processes that produce the rupture of the filament and the restoring of the high resistive state.
+
0
Fig.
19
Schematic
and
observation of Cu-platelet in
stuck LRS device. Three original
CFs join together inducing the
failure. This can be related to the
Cu sideways diffusion during
cycling.
Fig. 17 1st C-AFM slice at the top surface of a
device stuck in LRS. C-AFM image is taken after
electrode removal.
Fig. 18 By means of 3D C-AFM tomography it is
possible to observe the conductive filament
degeneration into a continuous conductive wall.
The latter shorts bottom and top electrodes and runs
through the entire switching layer as shown in the
sectional views.

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