effects of adhesion layer (ti or zr) and pt deposition temperature on the properties of pzt thin...

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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: [email protected] (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


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


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


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


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,


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),


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


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


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).


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effects of adhesion layer (ti or zr) and pt deposition temperature on the properties of pzt thin...

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