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Endurance/Retention Trade Off in HfOx and TaOx Based RRAM
Conference Paper · May 2016
DOI: 10.1109/IMW.2016.7495268
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Endurance/retention trade off in HfOx and TaOx based RRAM
M. Azzaz1,2, E. Vianello2, B. Sklenard2, P. Blaise2, A. Roule2, C.Sabbione2, S. Bernasconi2, C. Charpin2, C. Cagli2,
E. Jalaguier2, S. Jeannot1, S. Denorme1, P. Candelier1, M. Yu3, L.Nistor3, C. Fenouillet-Beranger2, L. Perniola2
1
2
STMicroelectronics, 850 Rue Jean Monnet, 38920 Crolles, France
CEA, LETI, Minatec campus, 17 Rue des martyrs, 38054 Grenoble, France
3
Applied Materials, 3050 Bowers Avenue, Santa Clara, CA United States.
E-mail: mourad.azzaz@cea.fr
Abstract— In this paper the memory performances of the
TiN/HfO2/Ti/TiN and TiN/Ta2O5/TaOx/TiN memory stacks are
compared. First, the bipolar switching parameters and the effect
of the compliance current on the memory window and endurance
are investigated. Then, the endurance and data retention
properties are compared at a given operating current (100µA).
Ta2O5 based memory stack exhibits a better memory window (2
decades) and data retention, while the HfO2 one shows good
endurance properties (108 cycles). Finally, thanks to ab initio
calculations using Density Functional Theory, the stability of the
conductive filament is investigated in both HfOx and TaOx
dielectrics.
Index Terms— RRAM, HfO2, Ta2O5, endurance, data
retention, Ab initio calculation.
I. INTRODUCTION
Oxide-based Resistive Random Access Memories
(RRAM) draw attention for high performance non-volatile
memory applications due to high speed, low power
consumption, promising endurance and thermal stability [1-3].
The HfOx and TaOx are considered as the most popular
materials for RRAMs and various companies and research
laboratories have adopted those materials as solution for the
resistive switching layer [4-6]. Thanks to the TaOx
performances, the mass production of the 180-nm RRAM
started in 2013 [7]. Recent results have demonstrated
promising endurance and retention performances on both HfOx
and TaOx switching layer [8-9]. However, a complete
comparison of these two memory stacks integrated on the
same test vehicle and an in-depth analysis of the programming
conditions is still missing.
In this paper for the first time, the comparison of the
memory performance for HfO2 and Ta2O5 based memory cells
is highlighted, the trades off between of operating current and
memory window, endurance and thermal stability are
investigated through extensive electrical characterization. In
the second part, Ab initio calculations using Density
Functional Theory are performed in order to investigate the
mechanisms responsible of the endurance and data retention
degradation. The calculations highlight the stability of the
conductive filament considering the interactions between the
dielectric and the active electrode on both HfO2 and Ta2O5
dielectrics.
II. EXPERIMENTAL
The devices studied in this work are composed by a RRAM
resistor in series with a NMOS transistor (Fig. 1.a) integrated in
65nm CMOS technology. Two different RRAM stacks are
studied. The first one is composed of a 5 nm thick HfO2
deposited by Atomic Layer Deposition (ALD) in-between Ti
Top Electrode (TE) (used as active electrode) and a TiN
Bottom Electrode (BE). In the second one, the resistive
switching layer is a 5nm-thick Ta2O5 stacked with a 5nm thick
TaOx (x<2) sandwiched in-between two TiN layers (TE & BE),
as shown in Fig. 1b. The stoichiometric (Ta2O5) and substoichiometric (TaOx) films were deposited by Physical Vapor
Deposition (PVD) using an Impulse chamber on Applied
Materials Endura platform. The resulting films were fine-tuned
so that x, or O:Ta ratio, can be precisely adjusted from 1.0 to
2.5. The degree of oxidation of the resulting films was tuned by
adjusting the O2 flow during the deposition step. The top and
bottom electrodes were deposited by PVD for both memory
stacks. The sub-stoichiometry of the TaOx is highlighted in Fig.
1.c by the increasing of Ta5d and Ta6s components in the XPS
spectra for decreasing values of x.
(a)
(b)
(c)
Fig. 1. (a) Schematics of the studied RRAM cells integrated into a 1T1R
structure. (b) STEM of the TiN/Ta2O5/TaOx/TiN memory stack. (c)
XPS spectra of the TaOx layers obtained from various x values.
978-1-4673-8833-7/16/$31.00 ©2016 IEEE
III. RESULTS AND DISCUSSION
In this section an in-depth electrical characterization of the
memory performance is performed on Ta2O5/TaOx and HfO2/Ti
devices through quasi-static and pulsed measurements.
A. Switching parameters
Fig. 2.b shows the bipolar current-voltage characteristics of
the Ta2O5/TaOx RRAM devices using an operating current
compliance, Icc, of 100µA. The switching voltages Vforming, Vset
and Vreset are defined as the triggering voltage of the Forming,
SET and RESET transitions respectively. Fig. 2.b compares the
switching voltages of Ta2O5/TaOx and HfO2/Ti devices. The
Vset and Vreset are slightly higher for the Ta2O5/TaOx sample as
well as the Vforming voltage.
(a)
100µA
(b)
The LRS and HRS distributions reported in Fig. 3.b
confirm the better memory window obtained at Icc=100µA
(Fig. 3.a) for Ta2O5 devices (red symbols) as compared to HfO2
ones (blue symbols).
The impact of the compliance current on the endurance is
also investigated. Fig. 4 shows the endurance performances
varying the compliance current from 45µA to 1mA for both
HfO2 and Ta2O5 devices. The endurance is defined as the
number of cycles for which a memory window of about 5 is
achieved. The extraction is performed at 3σ of the HRS and
LRS distributions. The HfO2 devices show clear endurance
enhancement, up to 107 cycles for a compliance current of
330µA, whereas the Ta2O5 sample allows to reach only 104
cycles at a lower compliance current of 100µA. The current
compliance identified as the best trade-off between endurance
and memory window is 100µA for the Ta2O5 while for the HfO2
it is 330µA.
HRS/LRS ratio ≈ 5
@ 3σ
Ta2O5/TaOx
Fig. 2. (a) DC forming (green line) followed by 20 SET/RESET cycles at
100µA (gray lines) for the Ta2O5 based cell. Red and blue lines are the
median values of ET and RESET respectively. (b) Median value and
standard deviation of the switching voltages for both HfO2 and Ta2O5
samples.
B. Impact of the compliance current on the memory window
and endurance
Fig. 3.a shows the impact of varying the compliance
current, Icc from 45µA to 1mA, on the low and high resistance
levels for both the HfO2 (blue line) and the Ta2O5 (red line)
samples. Results are plotted after performing 1k cycles on 3
cells for each point. While for the HfO2 sample, the largest
memory window, of about two decades, is achieved for a high
compliance current of 330µA, for the Ta2O5 sample the highest
memory window (higher than two decades) is achieved for a
compliance current of 100µA.
(a)
(b)
LRS
HRS
Fig. 4. Endurance performances as a function of the compliance current for
both HfO2 and Ta2O5 devices. The endurance is defined as the number
of cycles for which a memory window higher than a factor 5, extracted
at 3σ of the HRS and LRS distributions, is achieved
C. Endurance at Icc=100µA
For the rest of the paper, a compliance current of 100µA is
used to compare the endurance and data retention proprieties of
the two simples. Fig. 5 shows the endurance tests performed on
both HfO2 (right) and Ta2O5 (left) memory stacks, on 10 cells,
without correction algorithm. The Ta2O5 stack maintains a
memory window of two decades up to 5×103 cycles, before to
close after 104 cycles due to the decreasing of HRS state.
However, regarding HfO2, a memory window of less than one
decade is maintained up to 108 cycles. Thus, the HfO2-based
memory stack has better endurance at Icc=100µA but for a
lower memory window as compared to the Ta2O5 one.
(a)
(b)
HfO2/Ti
@100µA
Ta2O5/TaOx
3
Fig. 3. (a) LRS and HRS median values (10 cycles) and standard deviation
for both HfO2 and Ta2O5 samples for different compliance currents.
(b) LRS and HRS distributions in standard deviation scale for the 103
first cycles of both HfO2 (blue symbols) and Ta2O5 devices (red
symbols). Measurements are performed with fixed pulse conditions:
SET: 2V, 10µs; RESET: 1.3V, 10µs: Icc=100µA
Fig. 5. Endurance test for (a) Ta2O5 devices measured with fixed pulse
conditions: SET: 2V, 10µs; RESET: 1.3V, 10µs for (b) for HfO2
devices measured with fixed pulse conditions: SET: 2V, 10µs;
RESET: 1.4V, 10µs. Red and blue symbols correspond to the average
of LRS and HRS on 25 cells, respectively.
D. Data retention at Icc=100µA
Fig. 6.a shows LRS and HRS evolution versus bake time at
250°C for the Ta2O5 devices. The HRS state is stable in
temperature while the LRS state increases slightly during the
bake. The memory window is maintained higher than 5 up to
104s (~3 hours) at 250°C.
Fig. 6.b and Fig. 6.c show the average of the LRS state
during bake at different temperatures for Ta2O5 and HfO2
respectively. Ta2O5 samples have a better LRS stability at all
the investigated temperatures compared to the HfO2 where the
LRS fails faster towards high resistance level. The extracted
failure times based on the failure criteria defined as 5×Rinit
(Rinit: RLRS before bake) are plotted in Arrhenius plots (Fig. 7).
Higher activation energy (Ea) is extracted for the Ta2O5 from
the Fig. 6 and more than 10 years retention at ~117°C is
extrapolated for the Ta2O5.
(a)
(b)
Ta2O5/TaOx
The switching mechanism is believed to be due to the
formation of a conductive filament of O vacancies as shown
schematically in Fig. 8.a and Fig. 8.b for LRS and HRS
respectively. O atoms that have been removed from their site to
form vacancies are stored in the first atomic layers of the
electrode [10]. The stability of the conductive filament will be
investigated using first principle calculations by considering
the interactions between the dielectric and the oxidized
electrode. The oxidized TiOx electrode is generated by the
reaction with the active Ti top electrode for the HfO2 sample,
while the oxidized TaOx electrode is deposited by PVD for the
Ta2O5 sample.
(a)
(b)
250°C @ Icc=100µA
Fig. 8. Schematic of the switching mechanism. (a) LRS is achieved by a
conductive filament of O vacancies and (b) HRS is due to the
dissolution of a region of the filament.
HRS
LRS
IV. PHYSICAL INTERPRETATIONS
Ta2O5/TaOx
HfO2/Ti
(c)
LRS
LRS
Fig. 6. (a) Retention characteristics at 250°C for Ta2O5/TaOx devices for
different cells programed at Icc=100µA. The red and blue curves show
the average trend of 20 cells for LRS and HRS respectively. Evolution
of LRS average resistance of 20 cells for Ta2O5 (b), HfO2 (c) at 3
different temperatures with Icc=100µA.
LRS state
Fig. 7. Extracted failure time based on the criteria shown in Fig. 6. A higher
activation energy (Ea) is extracted for Ta2O5 (from an Arrhenius law).
Calculations based on density functional theory (DFT) in
the generalized gradient approximation (GGA) have been
carried out with PWSCF, part of the QUANTUM ESPRESSO
package [11]. The plane-wave energy cutoff was chosen as 50
Ry for Ti-O and Hf-O systems and 60 Ry for Ta-O systems in
order to converge the total energy within 10 meV/atom. Γcentered uniform k meshes with resolution 2
0.05 Å were
used to sample the Brillouin zone. Monoclinic HfO2
(spacegroup P21/c) and orthorhombic λ-Ta2O5 (spacegroup
Pbam, [12]) were considered to study the energetics of oxygen
vacancies. Defect calculations were performed in a 2x2x2
supercell with 96 atoms for HfO2 and a 2x2x3 with 168 atoms
for Ta2O5. HfO2 contains two different O sites: 3-fold (O3) and
4-fold (O4) coordinated [13]. λ-Ta2O5 is composed of 2D
Ta2O3 layers connected by 2-fold O atoms (O inter. layer). The
Ta2O3 layers contain two different O sites: 2-fold (O2) and 3fold coordinated (O3) [12]. Fig. 9 shows the 3-fold and 4-fold
O vacancies in Ta2O5 and HfO2 respectively. The formation
energy of an oxygen vacancy ( ) is given by:
μ ,
where
is the total energy of the supercell containing
the oxygen vacancy,
is the total energy of the defectfree supercell and μ is the O chemical potential. The choice of
μ allows to investigate the reaction energies between a
defective oxide (i.e. containing oxygen vacancies) and an
oxidized electrode. Fig. 10 shows the evolution of the
formation energy of O vacancies in (a) HfO2 and (b) Ta2O5 as a
function of the O chemical potential referred to that of
0). The shaded area shows the
molecular oxygen (Δμ
relevant range of μ corresponding to TiOx in the case of HfO2
and TaOx in the case of Ta2O5. In such conditions, it turns out
that the VO formation energy varies between 0.52 and 0.6 eV in
Ta2O5 while it varies between 0.95 and 1.4 eV in HfO2,
depending on the O content of the electrode.
(a)
(b)
Fig. 9. Supercells of (a) Ta2O5 and (b) HfO2 containing an O vacancy (VO3
and VO4 respectively). The vacancy position is shown in orange.
This ~0.5 eV difference in the formation energy suggests
that vacancies are less stable in HfO2 than in Ta2O5, and could
be responsible of the degraded retention in HfO2/Ti RRAM. On
the other hand, the low formation energy in Ta2O5 will favor O
vacancies formation (in particular during cycling) that would
lower the endurance, as observed in our memories. Fig. 11
shows a schematic representation of the energy barriers in
HfO2 and Ta2O5. The higher formation energy of VO in HfO2
thermodynamically promotes HRS. In contrast, in Ta2O5 the
conductive filament is more stable but the low formation
energy of VO limits the endurance.
(a)
(b)
Fig. 10. Formation energies of O vacancies as a function of O chemical
potential in (a) HfO2 and (b) Ta2O5. The shaded areas correspond to
the O chemical potential ranges of (a) O in TiOx, x varying between 1
and 1/4 and (b) O in TaOx, x varying between 1 and 2. In these
conditions, the formation energy of O vacancy ranges between 0.95
and 1.4 eV in HfO2 and between 0.52 and 0.6 eV in Ta2O5 (see inset).
(a)
(b)
Fig. 11. Schematic diagram highlighting the differences between (a)
HfO2/Ti and (b) Ta2O5/TaOx RRAM.
V. CONCLUSION
In this work, we have compared the memory performances
of HfO2 and Ta2O5 based RRAM memory stacks. Good
memory window (2 decades) and retention (10 years at 117 °C)
are demonstrated at 100µA operating current for Ta2O5
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devices, endurance is limited to 104 cycles. Better endurance
performances (up to 108 cycles without degradation of the
memory window) are demonstrated for the HfO2 memory
stack, at the cost of degraded memory window (less than one
decade) and thermal stability (10 years at 78°C). The
endurance/retention trade off may be explained by the oxygen
vacancy formation energies in the two materials calculated by
ab initio calculations. The higher VO formation energy in HfO2
thermodynamically promotes the HRS, thus explaining the
lower LRS thermal stability in HfO2/Ti RRAMs. On the other
hand, the lower endurance of the Ta2O5 cells can be attributed
to the lower VO formation energy in Ta2O5 that will favor O
vacancies formation during cycling.
ACKNOWLEDGMENT
This work has been partially supported by the European
621217 PANACHE project.
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