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Applied Surface Science 243 (2005) 113–124
Effects of adhesion layer (Ti or Zr) and Pt deposition
temperature on the properties of PZT thin films
deposited by RF magnetron sputtering
C.C. Mardarea,b,*, E. Joannia,c, A.I. Mardarea,b, J.R.A. Fernandesa,c,C.P.M. de Sad, P.B. Tavarese
aUOSE-INESC-Porto, Rua do Campo Alegre 687, 4169-007 Porto, PortugalbDepartamento de Fısica, Universidade do Porto, Rua do Campo Alegre 687, 4169-007 Porto, PortugalcDepartamento de Fısica, Universidade de Tras-os-Montes e Alto Douro, 5001-911 Vila Real, Portugal
dCEMUP, Rua do Campo Alegre 823, 4150-180 Porto, PortugaleDepartamento de Quımica e Centro de Quımica-Vila Real,
Universidade de Tras-os-Montes e Alto Douro, 5001-911 Vila Real, Portugal
Received in revised form 7 September 2004; accepted 8 September 2004
Available online 18 October 2004
Abstract
The effect of different bottom electrode structures (Pt/Ti/SiO2/Si and Pt/Zr/SiO2/Si) and Pt deposition temperatures on the
properties of ferroelectric lead zirconate titanate (PZT) thin films deposited by RF magnetron sputtering and crystallized either
in the furnace or by RTAwas investigated. The orientation of the films was strongly affected by all those parameters in the case of
Ti adhesion layer, whereas for Zr only a slight effect could be detected. The best ferroelectric properties were obtained for Pt/Ti
bottom electrodes with the Pt deposited at 500 8C and for Pt/Zr bottom electrodes with the Pt made at room temperature, in both
cases the PZT being crystallized in the furnace. The results are explained in terms of different stress levels and diffusion
processes taking place in the bottom electrode structures during their deposition and the crystallization of the PZT thin films.
# 2004 Elsevier B.V. All rights reserved.
PACS: 77.84.�s; 77.55.+f; 81.15.Cd
Keywords: PZT; Zirconium; Titanium; Ferroelectric properties; Sputtering
* Corresponding author. Tel.: +351 226082601;
fax: +351 226082799.
E-mail address: cezarina@hobbit.fc.up.pt (C.C. Mardare).
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved
doi:10.1016/j.apsusc.2004.09.050
1. Introduction
Ferroelectric lead zirconate titanate (PZT) thin film
capacitors have been studied with great interest in the last
decade because of their potential applicability in
.
C.C. Mardare et al. / Applied Surface Science 243 (2005) 113–124114
piezoelectric sensors, actuators and non-volatile memory
devices. For these capacitors, one of the most important
problems is the bottom electrode material and its
influence on the PZT properties. Platinum presents very
attractive properties as bottom electrode material for
ferroelectric capacitors due to its high electrical
conductivity, good stability to oxidation at high
temperatures and a high Schottky barrier height, which
makes the leakage currents to have low values. In spite of
those advantages, different materials like IrO2 [1], RuO2
[2], (La,Sr)CoO3 [3] or Ir [4] have been studied as Pt
replacements because of the large amount of experi-
mental data relating Pt and ferroelectric fatigue [5,6].
Another important issue is the poor adhesion of Pt to
silica; in order to overcome this problem, a Ti buffer
layer is commonly used. During crystallization of PZT
films deposited on Pt/Ti bottom electrodes, Ti diffuses
through Pt and then oxidizes, occasionally giving rise to
hillocks which lead to short-circuits between top and
bottom electrodes [7,8]. For improving Pt adhesion and
stability, different interlayers have been studied (TiN [8],
TiO2 [9], Ti/Ta [10], ZrO2 [11], Ru [12], Ta [13]). To the
best of our knowledge only one article has been
published regarding the use of Zr as adhesion/barrier
layer, but the study was made from the point of view of
diffusion mechanisms and oxidation taking place when
Ti, Zr and Ta react with PbO during deposition of
PbTiO3 at high temperature [14]. Like Ti, Zr is one of the
components of the PZT, but even though Zr is chemically
similar to Ti, the Zr atom is larger and its diffusion
behavior is expected to be different and this may
influence the ferroelectric properties of PZT thin films.
The aim of the present work is to compare the
effectiveness of Zr and Ti as adhesion layers in
ferroelectic capacitors with Pt electrodes deposited at
different temperatures on SiO2/Si substrates. We also
studied the influence of the type of heat treatment
(furnace and Rapid Thermal Annealing (RTA)) on the
preferential orientation and ferroelectric properties of
Pb(Zr0.52Ti0.48)O3 thin films deposited at room
temperature by RF magnetron sputtering on Pt/Ti
and Pt/Zr bottom electrodes.
2. Experimental details
Substrates 1 cm � 1 cm were cut from silicon
wafers having (1 0 0) orientation. After cleaning and
heat-treating the substrates at 950 8C in a tubular
furnace in O2 flow for 72 h, a layer of approximately
1 mm thick SiO2 was obtained. The oxidized
substrates were coated with Ti/Pt/PZT or Zr/Pt/PZT
thin films, with all the layers being deposited by RF
magnetron sputtering in Ar at 1 � 10�2 mbar, using
2 in. diameter targets. The metallic film thickness
were measured in real time during deposition using a
quartz thickness monitor. Before each deposition
sequence, the chamber was evacuated to a base
pressure of at least 10�6 mbar.
The Ti and Zr films, 30 nm thick, were made in situ
at 200 8C, using an RF power of 150 W. The 300 nm
thick Pt films were made on top of Ti or Zr layers at
different temperatures (ambient, 200, 500, and
700 8C), using the same power of 150 W and without
breaking the vacuum between depositions. For
depositing the PZT thin films a sintered oxide target
with the morphotropic composition of Zr/Ti = 52/48
and with 10 mol% Pb excess was used. This
composition has both tetragonal-phase perovskite
(rich in Ti) and rhombohedric-phase perovskite (rich
in Zr), being optimal for the ferroelectric and
piezoelectric properties of the capacitors. Lead was
added in excess in order to compensate for losses
during the PZT deposition and crystallization pro-
cesses. The PZT thin films were deposited at room
temperature for 1 h, using an RF power of 100 W; in
this case a lower RF power was used because of the
risk of target damage. Before every deposition, the
targets were pre-sputtered for about 15 min in order to
avoid contamination and to have a reproducible
stoichiometry.
The PZT films were crystallized either in a tubular
furnace in O2 flow at 650 8C for 10 min using a heating
rate of 1 8C/s or by RTA at 650 8C for 1 min with a
heating rate of approximately 10 8C/s. The thickness
of the PZT films measured after crystallization using a
perfilometer (Dektak IIA) was 400 nm. For the
electrical characterization of the films, Al top
electrodes with an area of 0.1 mm2 were deposited
by thermal evaporation using a shadow mask.
The crystalline phases present and preferential orienta-
tion of the PZT thin films were investigated by X-Ray
Diffraction(SiemensD5000).Thesurfacemorphologyand
crosssectionofthefilmswerestudiedbyScanningElectron
Microscopy(Philips/FEIQuanta400).Asystemcomposed
of an amplifier, a function generator (HP 8116A) and an
C.C. Mardare et al. / Applied Surface Science 243 (2005) 113–124 115
Table 1
Preparation conditions for all the samples and sample identification
Adhesion
layer
Pt deposition
temperature (8C)
PZT heat
treatment
Sample
name
Ti Room Furnace TrF
RTA TrR
200 Furnace T2F
RTA T2R
500 Furnace T5F
RTA T5R
700 Furnace T7F
RTA T7R
Zr Room Furnace ZrF
RTA ZrR
200 Furnace Z2F
RTA Z2R
500 Furnace Z5F
RTA Z5R
700 Furnace Z7F
RTA Z7R
oscilloscope (LeCroy 9310M) was used for recording the
ferroelectric hysteresis loops. The evaluation of leakage
currentwascarriedoutwithapicoammeter (Keithley487).
The samples were poled using a 200 kV/cm field for 10 s,
10 min before starting the measurements, in order to allow
time for the capacitors to discharge. The current measure-
ments were performed 5 s after each 0.5 V increase in the
voltage.All theelectricalmeasurementswerecontrolledby
a computer running LabView1 programs. X-Ray Photo-
electron Spectroscopy (XPS) analysis was performed on
selected Pt/Ti and Pt/Zr films using an ESCALAB 200A,
(VG Scientific) with PISCES software. For analysis, an
achromatic Al(Ka) X-ray source operating at 15 keV
(300 W) was used, and the spectrometer, calibrated with
reference to Ag 3d5/2 (368.27 eV), was operated in CAE
mode with 20 eV pass energy. Data acquisition was
performed with a pressure lower than 10�8 mbar. Spectra
analysis was performed using peak fitting with Gaussian–
Lorentzian peak shape and Shirley type background
subtraction.
3. Results and discussion
Table 1 shows the names of the samples, the
deposition temperatures for the Pt films, the adhesion
layer used and the type of heat treatment for the PZT
thin films.
Fig. 1 shows the X-ray diffraction patterns for the
PZT films deposited on Pt/Ti bottom electrodes and
heat-treated at 650 8C either in furnace (a) or by RTA
(b). As one can see from the diffractograms, the PZT
films exhibited different predominant orientations as a
function of the deposition temperatures of the Pt and
the type of heat treatment (slow or rapid) used for their
crystallization.
Fig. 2 shows the X-ray diffractograms for the PZT
films deposited on Pt/Zr bottom electrodes and heat-
treated in the same way as the films deposited on Pt/Ti
(Fig. 2(a) furnace and 2(b) RTA).
The degree of orientation Dn for each crystalline
direction (n) for the PZT films was calculated using
the expression:
Dn ¼InðIC
ð101Þ=ICn ÞX
n
InðICð101Þ=IC
n Þ�100; (1)
where In is the intensity of each peak measured from
the X-ray pattern, ICn is the relative intensity of each
peak from the powder diffraction file card for
Pb(Zr0.52,Ti0.48) and ICð1 0 1Þ is the intensity of the main
(1 0 1) peak from the card.
In Fig. 3 are shown the orientations for the PZT thin
films deposited on platinum made at different
temperatures and with different adhesion layers,
calculated using expression (1).
In the case of Ti adhesion layer, for low Pt
deposition temperatures (ambient and 200 8C) and
slow heat treatment, the PZT films showed (1 0 0)
predominant orientation (D1 0 0 > 80%), whereas for
rapid heat treatment, the predominant orientation was
(1 1 1) (D1 1 1 > 70%). With the increase of Pt
deposition temperature (500 and 700 8C) the degree
of orientation for the PZT films decreased, indepen-
dently of the type of heat treatment, but D1 0 0 is still
bigger than 50%. From Fig. 3, one can also see that the
values of D1 0 1 for the PZT films made on Pt/Ti with
Pt deposited at high temperatures increased when
compared with the D1 0 1 values for the films made on
Pt deposited at low temperatures. In the case of Zr
adhesion layer, there is not any change in the
predominant orientation (1 0 0) of the PZT films,
D1 0 0 being bigger than 70% for all Pt deposition
C.C. Mardare et al. / Applied Surface Science 243 (2005) 113–124116
Fig. 1. X-ray diffraction graphs for the PZT thin films deposited on Pt/Ti bottom electrodes and heat-treated at 650 8C either in furnace (a) or by
RTA (b).
temperatures and for both types of heat treatment. One
can also notice a trend for the D1 0 1 values to grow
with the increase of the Pt deposition temperature and
heating rate.
The different PZT orientations are the final result
from a combination of different stress and diffusion
processes due to the type of heat treatment (furnace or
RTA), percentage of O2 during the crystallization, Pt
deposition temperature and adhesion layer (Ti or Zr).
As previously reported, Pt films made at low
temperatures are under compressive stress, whereas
the ones deposited at high temperature are under
tensile stress after cooling due to the larger thermal
expansion coefficient of Pt compared to the one for the
Si substrate (8.8 � 10�6 and 2.6 � 10�6 K�1 at room
temperature, respectively) [8,15–17]. For the deposi-
tion conditions used in this study, Pt films made at
room temperature and 200 8C must be under
compressive or low tensile stress whereas the ones
deposited at 500 and 700 8C must be under high tensile
C.C. Mardare et al. / Applied Surface Science 243 (2005) 113–124 117
Fig. 2. X-ray diffraction graphs for the PZT thin films deposited on Pt/Zr bottom electrodes and heat-treated at 650 8C either in furnace (a) or by
RTA (b).
stress after cooling. This stress state is independent of
the type of adhesion layer due to the big difference in
thickness between the Pt and Ti/Zr films (300 and
30 nm, respectively). Hence, the difference in the
orientation of the PZT films should be related with the
different diffusion behaviors of Ti and Zr through
the grain boundaries of the Pt films [7,14]. Other
factors are Pb diffusion from the PZT, and the related
stress changes that occur in Pt and PZT during the
crystallization process. Maeder et al. [14] found that
after annealing in O2 at 620 8C, the Zr layer under Pt
oxidizes in a dense, continuous film and it does not
diffuse significantly either in the Pt or to its surface,
due to the high reactivity of Zr with the oxygen. Their
results are confirmed by our observation that the main
orientation, (100), of the PZT films deposited over Pt/
Zr bottom electrodes did not present any substantial
modification either with the heating rate or with the
C.C. Mardare et al. / Applied Surface Science 243 (2005) 113–124118
Fig. 3. The orientations of the PZT thin films deposited on Pt made at different deposition temperatures with Ti or Zr adhesion layers and heat-
treated either in the furnace or by RTA.
Table 2
Percentage of Ti(Zr) on the surface of selected Pt films after heat
treatment
Adhesion
layer
Pt deposition
temperature (8C)
Type of heat
treatment
Percentage of
Ti(Zr) on
Pt surface
Ti Room RTA 3.8
Furnace 2.0
200 Furnace 3.1
500 Furnace 3.0
700 Furnace 1.6
RTA 2.3
Zr Room Furnace –
200 Furnace –
500 Furnace 0.3
700 Furnace 1.2
amount of O2 during the crystallization process. The
increase of the PZT (1 0 1) peak for high Pt deposition
temperatures can be related to the higher stability of
those Pt films when compared with the ones made at
lower temperatures. It has been reported that different
PZT orientations can be achieved as a result of
different types of external stresses applied to the
films during the crystallization process [18], and that
during the post-deposition annealing (e.g. for the PZT
crystallization), a volume shrinkage occurs around
300 8C for Pt films made at low temperatures, whereas
this phenomenon does not take place for the Pt films
made at high temperatures [8]. Combining this source
of stress in the Pt with the volume shrinkage that
occurs in PZT during the crystallization process, it is
obvious that the stress in the PZT film during the post-
deposition annealing process will be higher in the case
of Pt made at low temperatures than in the case of Pt
made at high temperatures. This is independent of the
adhesion layer and confirmed by the results presented
in Fig. 1 (Ti adhesion layer), where the decrease of
(1 0 0)/(1 1 1) peaks comes with the simultaneous
increase of (1 0 1) peak intensities.
Regarding Ti diffusion behavior in Pt grain
boundaries, many studies have been conducted
[7,14,19–22] but the amount of Ti/TiOx present
near/onto the Pt surface is dependent on the thickness
of the layers, deposition temperatures, annealing
atmospheres and time, and deposition methods,
making difficult to predict the outcome of a given
combination of new processing conditions. In the case
of this study, given the thickness of Pt, the percentage
of Ti on the Pt surface is expected to be small.
Table 2 shows the results from the XPS surveys
performed on the surface of Pt/Ti and Pt/Zr films
made at different Pt deposition temperatures after the
C.C. Mardare et al. / Applied Surface Science 243 (2005) 113–124 119
samples were heat-treated using the same conditions
as for the crystallization of the PZT films.
In the case of Pt/Zr samples, for low Pt deposition
temperatures (room temperature and 200 8C) no Zr
was present on the Pt surface. With the increase of Pt
deposition temperature (500 and 700 8C), some Zr
could be detected on the surface, but only in very small
amounts (0.31 and 1.32%, respectively). The conclu-
sion from these results is that the Zr diffusion in the
grain boundaries probably occurs mainly during the Pt
deposition and not during the post-deposition anneal-
ing, because for low bottom electrode deposition
temperatures the Pt grain size is much smaller (and
therefore the surface has more grain boundaries) and if
the diffusion would take place during the PZT heat
treatment, the amount should be bigger in this case and
not when the Pt was made at 500 or 700 8C (e.g. Figs. 6
and 7). After Pt deposition at high temperatures, the Zr
is probably already on the surface and the oxidation
occurs during the PZT annealing.
In the case of Ti adhesion layer, because the Pt
grains are smaller (therefore having more grain
boundaries) and the Ti atom is smaller than the Zr
atom, the amount of Ti reaching the Pt surface is
bigger. The same trend for increasing the diffusion
with the Pt deposition temperature exists, but above
500 8C, due to the fact that the Pt is less porous and the
grain size is bigger, the amount of Ti on the surface
decreases. This happens with Ti and not with Zr
because the Ti diffusion in the Pt grain boundaries
probably continues during the heat treatment for the
PZT, whereas in the case of Zr, it diffuses significantly
only in the absence of oxygen (during Pt deposition).
For the films heat-treated by RTA the amount of Ti on
the Pt surface was bigger than for the films made in the
same conditions but annealed in the furnace, probably
due to the fact that during the rapid annealing the Ti
diffuses without having time to oxidize, hence not
blocking the flow of Ti through the grain boundaries as
in the case of furnace heat treatments.
As already discussed, the difference in the main
orientations of the PZT films made over Pt/Ti with Pt
deposited at low temperatures as a function of the type
of heat treatment should be connected with the
different stress in the films imposed by the heating
rates (1 8C/s in the furnace and 10 8C/s by RTA), but
the effect of the different degrees of Ti diffusion in the
Pt grain boundaries from the adhesion layer and also
from the PZT near the bottom electrode should not be
neglected. The PZT films heat-treated in the furnace
showed (1 0 0) preferential orientation, whereas the
ones heat-treated by RTA showed (1 1 1) preferential
orientation. With the increase of the Pt deposition
temperature (500 and 700 8C), even if the films are not
as strongly oriented as the ones made over the Pt
deposited at low temperature, there is still a difference
between the D1 1 1 values for the films crystallized in
the furnace or by RTA, the last ones being all the time
higher. For the PZT films made on Pt deposited at high
temperatures, the D1 0 1 values are increased when
compared with the values for the films deposited over
Pt made at low temperatures, but there is not a
significant change as a function of the type of heat
treatment. This increase should be connected with the
different stresses imposed by the Pt during the heat
treatments, since when deposited at high temperatures
the Pt is more stable and does not present a high
volume shrinkage during the PZT crystallization
process.
For the PZT films made over Pt/Zr, even if the main
orientation is (1 0 0) for all the conditions used in this
study, an increase of the D1 0 1 values can be seen for
the films heat-treated by RTAwhen compared with the
ones heat-treated in the furnace. In this case the
increase in orientation does not appear in the (1 1 1)
peak, as seen in the discussion about the Ti adhesion
layer. This happens probably because when Zr is used
as buffer, any Ti diffusing in the Pt originates only
from the PZT and not from the adhesion layer, as is the
case when the Pt/Ti bottom electrodes are used.
Considering the results obtained for all the films, the
different orientations should be related with the
different stress levels, as well as with the different
amounts of Ti in the Pt grain boundaries, whereas the
increase of each orientation for the PZT films heat-
treated by RTA should be related mainly to different
stress states and magnitudes.
The different orientations for the PZT films made
over Pt deposited at low temperatures (room and
200 8C) after the different types of heat treatments—
furnace (1 0 0) and RTA (1 1 1)—were probably
caused by the different stresses in the Pt films due
to Ti diffusion. In the case of furnace crystallization,
the Ti had more time to diffuse in the Pt due to the
longer exposure to high temperatures. At the same
time, the Ti oxidized in the Pt grain boundaries,
C.C. Mardare et al. / Applied Surface Science 243 (2005) 113–124120
Fig. 4. SEM images of (a) the surface of a Pt film deposited at
200 8C over Ti and (b) the cross section of a PZT film deposited over
Pt made under the same conditions.
exerting a compressive stress on the Pt film, which
would be already under compression during the heat
treatment for the PZT. In the case of RTA crystal-
lization, due to the very fast heat treatment, there
would be no time for Ti oxidation and the Pt would be
under a lower compression state than in the case of
furnace annealing, therefore the (1 1 1) orientation
would be predominant. This is in agreement with the
results reported by Qin et al. [18] who subjected the
substrate to external tensile and compressive stresses
during crystallization and obtained (1 0 0) orientation
for the films under compression and (1 1 1) orientation
for the ones under tension. In the case of Zr adhesion
layer, the films were (1 0 0) oriented independently of
the Pt deposition temperatures and the type of heat
treatment because for Pt deposited at high tempera-
tures over Zr, the Pt grain sizes were bigger and the
tensile stresses should be lower than for Ti, according
to the grain boundary relaxation model which predicts
an inverse dependence of stress on the grain size [11].
In Fig. 4 are presented the SEM images of the
surface of a Pt film made at 200 8C over Ti (a) and a
cross section of a PZT film deposited over Pt made
under the same conditions and heat-treated by RTA
(b). The Pt films made at low temperatures over Ti
have a small grain size (around 100 nm) and the PZT
films deposited over these bottom electrodes present a
very smooth surface. On the other hand, when the Pt is
deposited at high temperature (700 8C) over Ti (Fig.
5(a)), the grain size is bigger (around 300 nm) and the
surface of the film is rough. This roughness is reflected
on the surface of the PZT film crystallized by RTA, as
seen in the cross section shown in Fig. 5(b).
Fig. 6 shows the surface of a Pt film made at low
temperature (200 8C) over Zr (a) and the cross section
of a PZT film deposited over a Pt(200 8C)/Zr bottom
electrode and crystallized by RTA (b). Again, the Pt
surface is very smooth, the grain size is small and the
PZT surface reflects the low roughness of the bottom
electrode. For Pt films deposited at high temperature
(700 8C) over Zr, the grain sizes are much bigger than
for films made over Ti (around 800 nm) as shown in
Fig. 7.
Fig. 8 shows the ferroelectric hysteresis loops for
the PZT films deposited over Pt made at different
temperatures and adhesion layers and crystallized at
650 8C either in the furnace or by RTA. All the
hysteresis loops were plotted at the maximum field
before dielectric breakdown and showed a slight
asymmetry due to the different top and bottom
electrodes (Al and Pt, respectively). Each graph
corresponds to a different Pt deposition temperature
and shows a comparison between the results obtained
for Ti and Zr adhesion layers for both types of heat
treatment.
The films crystallized over Pt made at room
temperature or 500 8C show the best ferroelectric
properties. The highest values for remnant polariza-
tion were obtained with Ti as adhesion layer and the
PZT films crystallized by RTA (around 30 mC/cm2),
C.C. Mardare et al. / Applied Surface Science 243 (2005) 113–124 121
Fig. 5. SEM images of (a) the surface of a Pt film deposited at
700 8C over Ti and (b) the cross section of a PZT film deposited over
Pt made under the same conditions.Fig. 6. SEM images of (a) the surface of a Pt film deposited at
200 8C over Zr and (b) the cross section of a PZT film deposited over
Pt made under the same conditions.
followed closely by the PZT/Pt/Ti samples crystal-
lized in the furnace. All the films made over Pt/Ti and
heat-treated by RTA show high values for the coercive
field, probably due to the significant fraction of (1 1 1)
orientation present in the films [23]. The results
obtained using Pt made at 700 8C are not as good as
the results for the other Pt deposition temperatures:
either the hysteresis loops are distorted or the
breakdown fields are low or the remnant polarization
values are small or the coercive field values are too
big. These results can be related to the surface
morphology shown in Fig. 5; the platinum surface is
very rough and the same degree of roughness is
preserved in the surface of the PZT.
In the case of Zr used as adhesion layer, the hysteresis
loops show smaller values for the coercive fields and the
remnant polarization values are also smaller when
compared with the equivalent films made over Pt/Ti, but
still in the normal range for sputtered PZT capacitors
(around 20 mC/cm2). Independently of the Pt deposition
temperature, the PZT films made over Pt/Zr and heat-
treated by RTA show weaker ferroelectric properties and
C.C. Mardare et al. / Applied Surface Science 243 (2005) 113–124122
Fig. 7. SEM image of the surface of a Pt film deposited at 700 8Cover Zr.
Fig. 8. Hysteresis loops for the PZT thin films made on Pt deposited at (a) r
adhesion layers and heat-treated at 650 8C in the furnace or by RTA.
the breakdown fields are smaller than those for the films
crystallized in the furnace.
Fig. 9 presents the best and the worst leakage
current results obtained from all the films; they were
chosen independently of the Pt deposition temperature
and the crystallization type for PZT. As seen in the
figure, the samples T5F (PZT crystallized in the
furnace and made over Pt/Ti with Pt deposited at
500 8C) and ZrF (PZT crystallized in the furnace and
made over Pt/Zr with Pt deposited at room tempera-
ture) show the lowest leakage current values. The
highest values were obtained for the samples T7R
(PZT crystallized by RTA and made over Pt/Ti with Pt
deposited at 700 8C) and Z2R (PZT crystallized by
RTA and made over Pt/Zr with Pt deposited at 200 8C).
For the samples T5F and ZrF, the leakage current
density had low values, around 1.3 � 10�7 A/cm2 for
field values of 175 kV/cm, and increased significantly
for higher fields. In contrast, for the samples T7R and
oom temperature; (b) 200 8C; (c) 500 8C and (d) 700 8C with Ti or Zr
C.C. Mardare et al. / Applied Surface Science 243 (2005) 113–124 123
Fig. 9. Leakage current curves for the samples T5F, ZrF, T7R and
Z2R.
Z2R, with the increase of the electric field up to 137
and 100 kV/cm, respectively, the current density had a
big increase (from about 10�9 A/cm2 to 8.9 � 10�2
and 2.3 � 10�2 A/cm2, respectively) and above these
fields the current increased only moderately. As seen
in the figure, the films crystallized by RTA had higher
leakage current density values than the ones crystal-
lized in the furnace, this being a general trend for all
the films. This indicates a high Pb content in the as-
deposited films [24], because the short time of
exposure to high temperature during the RTA
decreased the amount of Pb lost when compared with
the films heat-treated in the furnace. There is a
tendency for the PZT films deposited over Pt/Zr
bottom electrodes to have higher values of leakage
current than the corresponding films made over Pt/Ti.
This is consistent with the results reported by Maeder
et al. [14], which observed that when Zr was used as
adhesion layer, the amount of Pb present in the bottom
electrode structure was smaller than in the case of Ti
adhesion layer so, in our case, the bigger amount of Pb
remaining in the films was probably responsible for
the bigger conductivity of the PZT films deposited
over Pt/Zr.
4. Summary
The effect of different bottom electrode structures
(Pt/Ti/SiO2/Si and Pt/Zr/SiO2/Si) and Pt deposition
temperatures on the properties of PZT thin films
deposited by RF magnetron sputtering and crystallized
either in the furnace or by RTA was investigated. The
orientation of the films deposited over Pt/Ti made at
low temperatures and crystallized in the furnace was
(1 0 0), changing to (1 1 1) when the PZT was
crystallized by RTA, probably because of the different
stresses in the Pt films due to Ti diffusion. When the
films were heat-treated slowly in the oxygen atmo-
sphere of the furnace, the Pt was under highly
compressive stress because of the migration and
oxidation of Ti in its grain boundaries, whereas when
the films were heat-treated by RTA in air, the stress in
Pt was less compressive due to the short exposure time
to the high temperatures. For higher Pt deposition
temperatures, the degree of PZT (1 0 0) orientation
decreased due to the higher Pt stability during the
subsequent heat treatment. For the films deposited
over Pt/Zr, Pt deposition temperatures and the
annealing method affected only slightly the (1 0 0)
PZT orientation due to the low diffusion rate, the Zr
tendency to oxidize in the early stages of the annealing
process and the effect of the bigger grain sizes.
The surface morphology of the films was strongly
influenced by the Pt deposition temperature: when Pt
was made at low temperatures, its grain size was small
and the surface of the PZT made over it was smooth,
whereas for high Pt deposition temperatures, the grain
size was bigger and the PZT surface was rough.
The best ferroelectric properties and the lowest
leakage currents were obtained for Pt/Ti bottom
electrodes with Pt deposited at 500 8C, and for Pt/Zr
with Pt deposited at room temperature, with the PZT
films being crystallized in the furnace in both cases.
The results indicate that Zr can be successfully used as
adhesion layer in PZT/Pt/Zr/SiO2/Si structures for
ferroelectric and piezoelectric devices, especially
when (1 0 0) orientation and lower coercive fields
are desirable.
Acknowledgements
C.C. Mardare and A.I. Mardare would like to
acknowledge Portuguese Foundation for Science and
Technology (FCT) for the financial support through
the PhD grants (SFRH/DB/12454/2003 and SFRH/
BD/11374/2002, respectively).
C.C. Mardare et al. / Applied Surface Science 243 (2005) 113–124124
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