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Performance improvement of HfO
2
-based
ferroelectric with 3D cylindrical capacitor
stress optimization
Cite as: J. Appl. Phys. 135, 235101 (2024); doi: 10.1063/5.0205852
View Online Export Citation CrossMar
k
Submitted: 28 February 2024 · Accepted: 21 May 2024 ·
Published Online: 17 June 2024
Wenqi Li,
1,2
Zhiliang Xia,
3
Dongyu Fan,
3
Yuxuan Fang,
1,2
and Zongliang Huo
3,a)
AFFILIATIONS
1
Institute of Microelectronics of the Chinese Academy of Sciences, Beijing 100029, China
2
University of Chinese Academy of Sciences, Beijing 100049, China
3
Yangtze Advanced Memory Industry Innovation Centre Limited Liability Company, Wuhan 430070, China
a)
Author to whom correspondence should be addressed: huozongliang@ime.ac.cn
ABSTRACT
To meet commercialization requirements, the distributions of materials in hafnium-based ferroelectric devices—including their phase and
orientation—need to be controlled. This article presents a method for improving the ferroelectric phase ratio and orientation by adjusting
the stress distribution of the annealing structure in a three-dimensional capacitor. In such a structure, stress can be applied in three direc-
tions: tangential, axial, and radial; there are, thus, more ways to regulate stress in three-dimensional structures than in two-dimensional
structures. This work sought to clarify the role of the stress direction on the proportions and orientations of ferroelectric phases. The results
of stress simulations show that a structure with an internal TiN electrode, but no filling provides greater axial and tangential stresses in the
hafnium-oxide layer. In comparison with the case of the hole being filled with tungsten, the proportion of the O phase is increased by
approximately 20%, and in experiments, the projection of the polarization direction onto the normal was found to be increased by 5%.
Axial and tangential stresses are regarded to be beneficial for the formation of the O phase and for improving the orientation of the polari-
zation direction. This work provides a theoretical basis and guidance for the three-dimensional integration of hafnium-based ferroelectric
materials.
© 2024 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license
(https://creativecommons.org/licenses/by/4.0/). https://doi.org/10.1063/5.0205852
I. INTRODUCTION
Since the discovery of ferroelectricity in hafnium-oxide-based
materials in 2011,
1
it has attracted considerable attention in the
semiconductor industry due to its excellent compatibility and scal-
ability.
2
The origin of ferroelectricity in hafnium-based materials
is a polar orthorhombic phase (O phase) called Pca2
1
, which is
a metastable phase in HfO
2
.
3,4
The formation of ferroelectric
hafnium oxide is a complex process that requires adjustments in
doping,
5–8
thickness,
9,10
stress,
11–14
and annealing.
15–17
Stress, in
particular, is a critical factor in the formation of hafnium-based fer-
roelectric materials.
In hafnium-based ferroelectric memory devices, there is a
coexistence of multiple phases in the ferroelectric layer,
18
which
raises the problem of the materials distribution in ferroelectric
capacitor arrays. In addition to the phase distribution, there is also
the distribution of the polarization direction of the ferroelectric
phase; this contributes to the total polarization only when the
polarization direction is perpendicular to the capacitive interface,
while the polarization contribution is zero when the polarization
direction is parallel to the capacitive interface. Therefore, hafnium-
based ferroelectric memory devices need to have an optimized
material distribution to be commercially viable. Stress, a key factor
in the formation of hafnium-based ferroelectric materials,
19
prom-
ises to be an effective means of controlling the distributions of fer-
roelectric materials.
Several studies have been conducted examining stresses in these
materials, primarily focusing on changing the substrate
20
or cover
21,22
materials in two-dimensional planar capacitors to examine the
Journal of
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J. Appl. Phys. 135, 235101 (2024); doi: 10.1063/5.0205852 135, 235101-1
©Author(s)2024
impact of the thermal-expansion coefficient on the ferroelectric
properties. Typically, ferroelectric hafnium oxide requires annealing
after metallization to achieve ferroelectricity. Hafnium-oxide films
have a different coefficient of thermal expansion from the cover
electrode, which is usually made of titanium nitride. This difference
in thermal strain generates stress inside the film when the tempera-
ture changes, and this plays a role in the annealing of the hafnium
oxide to crystallization.
23
When the temperature exceeds the film-
deposition temperature, compressive stress prevents volume expan-
sion, increasing the potential barrier for the T-phase-to-M-phase
transition and inhibiting the formation of the M phase.
24,25
When
the temperature drops below the deposition temperature, hafnium
oxide requires tensile stress to stretch its crystal lattice, reducing the
potential barrier from the T phase to the O phase, which helps the
crystalline phase transition of the T phase into the O phase, ulti-
mately resulting in ferroelectricity.
25,26
However, there have been few studies on the effect of stress on
ferroelectricity in 3D cylindrical capacitors.
19,27–30
It is important
to note that the stress distributions in 2D and 3D structures are dif-
ferent (Fig. 1). Planar capacitors experience mainly in-plane stress
in the xand ydirections, while their out-of-plane stress is negligi-
ble. However, cylindrical capacitors experience axial (l-axis),
tangential (θ-axis), and normal (r-axis) stresses. This study investi-
gated the impact of the stress distribution on the proportion of the
ferroelectric O phase and the orientation of the polarization direc-
tion in 3D cylindrical structures.
Herein, we present the results of simulations of the stress of
hafnium oxide under different annealing structures, followed by the
results of experiments. Wafers containing capacitor structures with
different filling conditions were annealed, and the phase distribu-
tions of the ferroelectric layers in the capacitor holes were charac-
terized in each case. The results of this study verify the role of
stress in a cylindrical capacitor.
II. SIMULATIONS
In a cylindrical capacitor, the stress can be adjusted by the
electrode thickness, filling structure, hole diameter, and other
dimensions. First, the stresses under different conditions were ana-
lyzed using stress simulations. As shown in Fig. 2, after the deposi-
tion of hafnium oxide in a hole, we carried out stress simulations
of two different structures: one with a single deposited layer of TiN
(case 1) and one filled with tungsten after the deposition of TiN
(case 2).
For these simulations, the interconnect and SiWB modules of
the Synopsys simulation software package were used to simulate
the stress state during annealing of hafnium-oxide layers in the two
structures. We added each layer of material at about 700 K on
wafer, including oxide, polysilicon, SiN, HfO
2
, TiN, and W; there-
fore, the structure is in a zero-stress state at this temperature.
Because each material has a different coefficient of thermal expan-
sion, each layer will produce different degrees of thermal strain
when the temperature changes, and thermal stresses will be gener-
ated due to the thermal-strain mismatch between the materials. It
has previously been shown that the formation of the hafnium-oxide
ferroelectric phase is affected by the tensile stresses generated
during the cooling phase of the annealing process;
23
we, thus,
focused on analyzing the state of stress on the hafnium-oxide layer
after returning to room temperature at 300 K.
As mentioned earlier, in a cylindrical capacitor structure, the
stress has three directions—axial, tangential, and radial—and differ-
ent structures will have different stress distributions. Figures 3(a)
and 3(b) show the stress distributions in the tangential, axial, and
radial directions for the two structures. In this examination, we are
mainly concerned with the stress condition in the ferroelectric film,
so a comparison of the stress distributions for the two cases was
made, and the results are shown in Fig. 3(c).
In Fig. 3(c), it can be seen that case 1 has relatively uniform
stresses in all three directions and has larger tangential and axial
stresses compared to case 2, which is in plane for the ferroelectric
layer. In contrast, case 2 has higher radial stresses but lower axial
and tangential stresses, which means that out-of-plane stresses are
more pronounced in case 2. Compared to the planar capacitor
structure, which has only in-plane stresses, it is hypothesized that
tangential and axial stresses play a greater role in increasing the
proportion of the O phase, meaning that the proportion of the O
phase is better in case 1 than in case 2.
FIG. 1. Schematic diagrams of stress directions in (a) a two-dimensional planar
capacitor and (b) a three-dimensional cylindrical capacitor.
FIG. 2. Structural diagrams of (a) case 1: internal TiN electrode without filling
and (b) case 2: internal TiN electrode with W filling.
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©Author(s)2024
III. EXPERIMENTS
In this work, oxide, polysilicon, and silicon nitride were
sequentially deposited on the substrate, and capacitor holes were
then etched. About 10-nm-thick film of silicon-doped hafnium
oxide was then deposited in the capacitor holes by atomic-layer
deposition (ALD); the doping ratio of silicon to hafnium was
between 3% and 4%.
1
Then, an about 10-nm-thick film of titanium
nitride was deposited by ALD. Some of the capacitor holes were
then filled with tungsten by further deposition. The capacitors were
then rapid thermal annealed at 950 °C. The process flow is shown
in Fig. 4(a), and a transmission electron microscope (TEM) image
of the resulting structures is shown in Fig. 4(b).
IV. RESULTS AND DISCUSSION
A. Physical phase ratio
Precession electron diffraction (PED) is a technique used to
collect electron diffraction patterns in a TEM. This method involves
rotating the incoming electron beam at a specific tilt angle with
respect to the optical axis to obtain electron diffraction patterns.
These patterns can provide information on phases and their
orientations.
Figures 5(a) and 5(c) display TEM images of the two capacitor
hole structures. PED was used to analyze the phase distributions in
the ferroelectric layers of these structures. The results are shown in
Figs. 5(b) and 5(d), in which different colors indicate the phases,
including polysilicon, TiN, W, and the hafnium-oxide layer. The
legend shown in Fig. 5(e) indicates that the hafnium-oxide layer
contains M, O, and T phases; the figure displays the distributions
of these phases in single holes. It can be observed that the O phase
is dominant, with some M phase and a small amount of T phase.
FIG. 3. Stress-distribution diagrams in
the tangential, axial, and radial direc-
tions for (a) case 1 and (b) case 2. (c)
Bar chart of stresses in the three direc-
tions in the hafnium-oxide layer for the
two cases.
FIG. 4. (a) Process flow for the creation of the structures and (b) the TEM
image of the structures.
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By conducting PED analysis on several capacitive holes and
determining the physical phase ratio in each hole, the average
phase ratios of hafnium oxide in the capacitive arrays could be
calculated. Figure 5(f ) displays the proportions of each phase in
the capacitive array for the two structures. The bar-graph values
represent the average proportion of each phase, indicating that
case 1 has a higher level of O phase and a lower level of M phase.
In Sec. II, we analyzed the stress distributions under the two
schemes. It was found that case 1 has higher tangential and axial
stresses, while case 2 has higher radial stresses. In this experi-
ment, other conditions were kept constant to confirm that the
change in the phase ratio in the ferroelectric film is caused by
the stress. In the 3D holes, axial and tangential stresses are more
conducive to the formation of the O phase and suppress the M
phase.
B. Polarization orientation
The lattice orientation of the O phase was analyzed using
PED. This method allows the extraction of lattice information,
including the phase structure and placement, for each pixel point.
The Euler angle provides crystal-placement information by decom-
posing the angular displacement into three sets of angles rotating
around three mutually perpendicular axes. For the orthogonal
phase Pca2
1
, these axes are its a,b, and caxes. To obtain the effec-
tive polarization strength of the O-phase contribution, we calculate
the angle between the Euler angle and the capacitance normal.
Figures 6(a) and 6(b) show the angular distributions between
the caxis of the polarization direction of the O phase and the
normal direction of the capacitor in the hole. The polarization
intensity contributed by each pixel point can be obtained by
FIG. 5. (a) TEM image of a single
hole for case 1 and (b) its PED phase-
distribution diagram, (c) TEM image of
a single hole for case 2 and (d) its
PED phase-distribution diagram, and
(e) a legend for PED phase-distribution
diagrams. (f ) Statistical chart showing
the average proportions of phases in
cases 1 and 2.
FIG. 6. O-phase polarization-direction distribution diagrams for (a) case 1 and (b)
case 2 and (c) a legend for PED polarization-direction distribution diagrams. (d)
Statistical chart showing the polarization-angle contributions in cases 1 and 2.
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J. Appl. Phys. 135, 235101 (2024); doi: 10.1063/5.0205852 135, 235101-4
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projecting the caxis of the lattice of each pixel point in the normal
direction. The polarization intensity contributed by each pixel
point in a single capacitor hole can be summed and averaged to
indicate the contribution to the polarization intensity from the fer-
roelectric O phase in a capacitor. Similar to counting the percent-
age of each phase in the capacitor array, the polarization-angle
contribution in each capacitor was counted to obtain Fig. 6(d).Itis
evident that the average value of the polarization-angle contribu-
tion is larger and more concentrated for case 1. This aligns with
the trends of tangential and axial stresses. The improvement in
grain orientation is speculated to result from the contributions
from tangential and axial stresses, resulting in the O-phase caxis
being aligned along the normal direction.
C. Calculation of polarization intensity
In Secs. IV A and IV B, the proportion of the O phase in each
capacitor hole and the projection value of the corresponding caxis
of the O phase to the radial direction of the capacitor hole were
obtained. In this section, we report the polarization intensity as
estimated using the proportion of the O phase and the projection
value of the caxis. If there exists a capacitor hole whose O-phase
proportion is 100% and whose caxis points in the outward direc-
tion with a projection value of 100%, it will be capable of contrib-
uting all the polarization intensities of the O phase itself to the
capacitor; the values obtained from first-principles calculations in
this situation are generally 50–66 μC/cm
2
.
31–34
Therefore, the polar-
ization intensity can be estimated using the method presented in
Fig. 7(a).
Figure 7(b) displays the remnant polarizations for the two
types of ferroelectric capacitor, as estimated from the PED charac-
terization and the polarization strength of a single-crystal cell. It is
evident that case 1 exhibits superior polarization strength. This is
primarily because case 1 experiences higher axial and tangential
stresses, resulting in a greater proportion of the ferroelectric O
phase. Additionally, the polarization direction of the O phase in
case 1 is closer to vertical than in case 2.
This estimation disregards the issues of dead layers, domain-
wall pinning, and interactions between ferroelectric domains.
35
Further investigation is required to bridge the gap between the esti-
mated and actual measured values. As the primary objective of this
work was to examine the impact of stress on the physical phase, the
electrical aspect was not considered.
V. CONCLUSION
This paper presents the results of stress simulations and exper-
iments to elucidate the relationship between stress and the genera-
tion of the ferroelectric phase in three-dimensional structures. It
was found that both axial and tangential stresses contribute to the
generation of the ferroelectric phase. Specifically, the tangential
stress is the largest when only one layer of TiN is deposited, and
the radial stress increases significantly when the hole is filled with
W. Additionally, it was found that the proportion of the O phase
exceeds 70% when only one layer of TiN is deposited for annealing.
These results suggest that the situation with only one layer of TiN
is the best for obtaining ferroelectricity with anneal in a cylindrical
capacitor. The stress simulations showed that the magnitude of tan-
gential stress is related to the filling structure and its thickness.
Therefore, if the TiN layer is further thinned, it is presumed that
the tangential tensile stress of HfO
2
will continue to increase.
ACKNOWLEDGMENTS
This work was supported by the National Key Research and
Development Program of China under Grant No. 2023YFB4402500.
AUTHOR DECLARATIONS
Conflict of Interest
The authors have no conflicts to disclose.
Author Contributions
All the authors contributed to this work.
Wenqi Li: Conceptualization (equal); Data curation (equal);
Formal analysis (equal); Writing –original draft (equal). Zhiliang
Xia: Methodology (equal); Project administration (equal);
Resources (equal). Dongyu Fan: Software (equal). Yuxuan Fang:
Visualization (equal). Zongliang Huo: Resources (equal);
Supervision (equal).
DATA AVAILABILITY
The data that support the findings of this study are available
from the corresponding author upon reasonable request.
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