-
Pb(Zr,Ti)03 ceramic thick films for optical device
applications
J. Cardin, D Leduc, C Boisrobert, H.W. Gundel
To cite this version:
J. Cardin, D Leduc, C Boisrobert, H.W. Gundel. Pb(Zr,Ti)03
ceramic thick films for opticaldevice applications. SPIE
Proceedings Series, 2003, 5122, pp.371-376. .
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-
Pb(Zr,Ti)03 ceramic thick films for optical device
applications
J. Cardin, D. Leduc, C. Boisrobert, and H.W. Gundel*
Laboratoire de Physique des Isolants et d'Optronique EA 3254
(L.P.I.O.)Université de Nantes, Nantes, France
ABSTRACT
Ferroelectric Pb,ZrTi1O3 (PZT) has been prepared by chemical
solution deposition (CSD) and spin-coating technique,using acetate
and alkoxide precursors. Rapid thermal annealing has been employed
in order to obtain crystallization in theperovskite phase. Aiming
to study the optical properties of the films, PZT was deposited on
different glass substrates.Structural characterization ofthe films
was done by X-ray diffraction, morphology was investigated by SEM
micrography.Using standard photography analysis, the films were
qualified in terms of crack density, their appearance
stronglydepending on the type of substrate. Using a visible to the
near infrared spectrophotometer, the transmittance normal to
thesurface ofthe films was studied. Coupling oflaser light into the
films by the M-lines technique allowed the determination ofthe
refractive index and the thickness of the ferroelectric layer. A
waveguiding interferometer structure of Mach-Zehndertype was
realized by photolithography and wet chemical etching.
Keywords: PZT, thick films, glass substrate, transparency,
M-lines, wet chemical etching, optical waveguide
1. INTRODUCTION
The improvement of optical networks for high bit rate
telecommunication is mainly limited by the modulation
andamplification stages of the light signal and hence new materials
with improved properties are needed. An appropriatematerial would
have a high transparency in the visible and near infrared range, a
strong refractive index, a high electro-opticresponse and a
sufficient good long term stability.1'2 In order to realize a
typical multi-layer waveguide structure, integrationis necessary
which also implies elaboration of the material by thin film
techniques. Some ferroelectrics seem to meet thiscriteria, like
lithium niobate (LiNbO3) which is one of the most prominent member
of this class of materials. Otherperovskite type ferroelectrics,
like barium titanate (BaTiO3) or the large family ofPLZT solid
solutions [(Pb,La)(Zr,Ti)03]equally exhibit interesting properties
for electro-optic applications.37 In the present study, we
investigated the opticalproperties oflead zirconate titanate (PZT)
spin-coated on glass and the possibility to realize a Mach-Zehnder
interferometerstructure. The realization ofa PZT waveguide based on
the use ofglass substrates would allow a future development of
lowcost devices which is ofhigh interest for telecommunication
applications.
2. EXPERIMENTAL TECHNIQUE
The PZT thin films were elaborated by the Chemical Solution
Deposition (C.S.D.) technique using a modified MOD Sol-gelprocess.
The precursor solution was prepared at room atmosphere and
temperature. In order to obtain PZT (36/64), leadacetate
[Pb(CH3CO2)2,311201 was dissolved with an acetic acid solvent and
an appropriate ratio between zirconium alkoxide[(Zr(C3H70)4)] and
titanium n-propoxide [(Ti(C3H70)4)] adjusted. A 20 % lead excess
was used in order to prevent lossesduring the annealing process,
and ethylene glycol (HO-CH2-CH2-()H) was added to the solution in
order to reduce crackingof the films.8 The final solution was
filtered with a 200 nm seringe filter and was spin-coated onto the
substrates at 2500rpm duriig 20 seconds. Subsequently, the films
were dried at 120°C and submitted to a rapid thermal annealing
(RTA)
*Corresponding authors: Email:
hartmut.gunde1physique.univ-nantes.fr;
julien.cardinphysique.univ-nantes.fr;
httpi/www.sciences.univ-nantes.fr/physique/recherche/lpio;
Advanced Organic and Inorganic Optical Materials, Andris
Krumins, Donats Millers,Inta Muzikante, Andris Sternbergs, Vismants
Zauls, Editors, Proceedings of SPIEVol. 5122 (2003) © 2003 SPIE ·
0277-786X/03/$15.00
371
-
Classical photolithography and wet chemical etching wasemployed
for the realization of the waveguide structures,using a mixture of
50 ml HC1 (37%), 50 ml HNO3 (69%) et2 ml HF (48%) diluted to 50%. A
detailed study on theetching behavior of PZT ceramic thin films
shall bepublished elsewhere. 14
The structural properties of the PZT films havebeen determined
by X-ray diffraction andScanning Electron Microscopy.
MesOscopicoptical scanning ofthe films was performed with aMINOLTA
Dimage Dual Scan camera. Theoptical transmittance, normal to the
surface of thefilms, has been determined in a wavelength
rangebetween 200 nm and 2 .tm with a CARYspectrophotometer.
40 45 502 0 (°)
60 65 70
Figure 2: XRD pattern of PZT (36/64) films [, pyrochlorephase,
(xyz) perovskite phase)]
process during one minute. The crystallization behavior of the
films was studied for different annealing temperatures,ranging from
520°C to 620°C.
The PZT precursor solution was deposited on glass substrates of
25 x 25 mm2 area. Microscope slide glass was used inorder to adjust
the spin-coating parameters, and four types ofglass with different
thermal properties were used in order tostudy the filmability of
thematerial. Depending on thespin-coating velocity, thethickness of
one single layer ofPZT varied from 450 nm to550 nm. Thicker films
wererealized by multiple spin-coating, including drying andRTA heat
treatment of each individual coating. The thermal properties ofthe
substrates are shown in Table 1. As PZT has alinear thermal
expansion coefficient of 5,5. 10, glass substrates with bigger and
smaller coefficients were available.9
Table 1: Thermal properties ofthe glass substrates used for PZT
thin film deposition
Substrate Schott D263T Schott AF45 Corning 7059 Corning
1737FCoefficient oflinear 7,2.l0 4,5.10 4,6.l0 3,76.l0thermal
expansion (20°C-300°C) (20°C-300°C) (0°C-300°C) (0°C-300°C)Strain
point 529°C 627°C 565°C 666°C
Mirror
?J2 plate Beamsplitter Lense
Detector
He-Ne Laser
Motorized rotation stage
In order to determine the refractive index of thePZT thin films,
a M-lines measurement apparatuswas set up which applies the
principle ofdistributed coupling by evanescent fields to the
Figure 1: Schema ofthe rn-lines measurement set-up modes of the
guiding structure.'° A ZnSe prismhaving a refractive index higher
than that of the
ferroelectric, is pressed on the film (Figure 1) and is
separated from it by a small air gap. The incident light is
totallyreflected at the two sides ofthe prism, and leaves the prism
parallel to the incident direction. For some angles of
incidence,phase matching is obtained and coupling into the film due
tothe evanescent field excited in the gap becomes
possible.Determination ofthese synchronous angles allows to find
thecharacteristic propagation constants of the film.'3
Theattenuated totally reflected light as a function ofthe angle
ofincidence constitutes a dark line spectrum. Variation of the
Iangle of incidence was performed by rotating the ensemblePZT/prism
with a remote controlled motorized rotatingstage, synchronized to
the detector (jthoto diode). The light -source was a He-Ne laser of
632.8 urn wavelength, and thepolarization of the incident beam was
controlled by a half-wave plate associated to a polarizer.
20 25 30 35 55
372 Proc. of SPIE Vol. 5122
-
3. EXPERIMENTAL RESULTS AND DISCUSSION
The development of the X-ray diffraction pattern of a 2 tm thick
PZT (36/64) film deposited on a microscope slide glasssubstrate is
shown in Figure 2 as a function ofthe annealing temperature. At
520°C, only a pyrochiore phase ofthe PZT canbe detected. With
increasing annealing temperatures the respective peaks (at 20
29.5°, 34°, and 59°) decrease and arereplaced by the perovskite
pattern. At 580°C, the pyrochlore peaks become almost invisible.
Starting from 600°C,crystallization in the perovskite phase with a
preferential (110) orientation ofthe film (20 31°) was
obtained.
The microstructure ofthe PZT (36/64) films was studied by SEM
technique in the normal secondary electron mode and
thebackscattering mode. Figure 3 shows cross-sections of a 5-layer
PZT film annealed at 620°C, resulting in an overallthickness of
approximately 2.7 tm. In the normal mode (Figure 3a), the typical
PZT grain structure can be seen with thesubstrate on the right hand
side of the photo. As the SEM picture was focussed to the PZT, the
substrate does not appearclearly. Figure 3b was taken in the
backscattering mode, where the brightness is proportional to the
mass ofelements. At thesurface of the substrate, a brighter region
of approximately 500 nm thickness is visible which should
correspond to a
chemical element heavier thanglass. We suppose thatdiffusion of
lead from thePZT to the substrate duringthe thermal annealing
processcauses this zone (diffusionlayer). 15
The mesoscopic structure of aPZT mono-layer of approxi-mately
450 nm thickness wasanalyzed using opticalphotography. In Figure
4,deposition on four differentglass substrates is compared.Figures
4a and 4b show thePZT films spin-coated on aSchott D263T and a
Corning7059 glass substrate,presenting a high number ofcracks,
respectively. In thecase of Figures 4c and 4d,Schott AF45 and
Corning1737 glass substrates wereused, showing a morehomogeneous
surface withconsiderably less cracks. Asthe cracks appear
duringdrying and annealing of thefilms, we suppose that themismatch
between the thermalproperties of the glass and thePZT film is at
the origin ofthis phenomenon. This maycause desiccation
cracksduring the drying process andstrain cracks during
annealingand cooling of the films toroom temperature.
Figure 3: SEM micrographs of a PZT (36/64) film spin-coated on
microscope slide glass.Cross-section (a) in the SEM normal mode and
(b) in the backscattering mode
Figure 4: Optical photography of PZT films spin-coated on
different glass substrates andannealed at 620°C, (a) Schott D 263,
(b) Corning 7059, (c) Schott AF 45 and (d)Coming 1737F.
Proc. of SPIE Vol. 5122 373
-
A!45
.0,8
f
-
O,6
tO,4I
Optical transmission of a PZT mono-layer, deposited on the
different glass substrates, is shown in Figure 5 for heattreatment
temperatures ranging from 560°C to 620°C. Transmission has been
found to be between 30% and 80% in thevisible wavelength range and
up to 90% in the near infrared (2000 nm). In the case ofthe D263
glass substrate only, a lowertransparency was observed for the
films crystallized in the perovskite phase (600°C and 620°C). This
is supposed to be dueto light scattering, as in this case the PZT
films show an overall inhomogeneous aspect with many mesoscopic
cracks. Thehigher transparency of the same PZT film in the
pyrochlore phase (560°C and 580°C)) where no cracks have been
found,confirms this assumption. The transmission spectra of the PZT
films deposited on the 7059, AF45 and 1737F glass
substrates do not differ significantly. The UV cut offwavelength
ofthe PZT is around 350 nm, respectively, corresponding to a
band
750 gap energy of 3.5 eV, which is in accordance with the
intrinsic
--. -..-..limit oftransparency.'7 In the near infrared, and
particularly at the
700 ••'••.-..•--.. -._•••\. ./_•••_•".....-...
"telecommunication wavelength" (1 ,55 tim), transparency is
almost
.. : •- that ofthe substrate.
650 . . ..
600
TE
550
d.k0.n12 (n sin 0m)2 arctan g12sin93m2 2 _aretan g22 sinem 0: m
= 0 (1)
\ 1 (n3 sm 03m) ) \ 1 (n3 sm 3m) )where k0 is the wave vector
and n0, n2, and n3 represent the refractive index of the substrate,
the air gap, and the prism,respectively. The parameters gi and g2
are equal to one for TE polarization and equal to n1/n2 and n1/n0
for TM polarization.
D26 substrat
0,80
0,6
0
0,4
0.00,2
0,0
I..' —
——
Ii" ..- -- - -
I, ..,
I——
——-580CC560C
1,0
0,8
0
0,6
Co
04
00,2
Unlike the case of metalsubstrates,16 deposition on
glassresulted in more homogeneousfilms with less mesoscopiccracks
when the thermalexpansion coefficient of thesubstrate is smaller
than that ofthe PZT. This might be explainedby the different
thermal behaviorof the two types of substrateduring the annealing
process.Glass has a lower thermalconductivity than metal, and
theglass substrates used are thickerthan the metal substrates(0.7
mm and 0.2 mm). In thecase of the glasses investigated,homogeneous
and crack-freecrystallization of the PZT filmswas favored by using
thesubstrate with the smallestthermal expansion during theRTA heat
treatment process.
7059 substrat- _—;
I,I 600CCI ..580*C4 - ..-. 560*C400 600 800 1000 1200 1400 1600
1800 201
Wavelength (nm)
400 ' 600'
84;0'
id®•
ioo'
1400 1600 1800 20
Wavetength (nm)
DC
1,0
C 0,80
0,6
Co
0,4
0— — —620CC600CC
—. —- 580CC
—"—. 560CC
l737Fsubstrat . .r— ..
I i/'•.): ; I
I ':I'rI ———620CC
0,2 1 600CC. b —-—580C
I -"-. 560*C0,0 -.--'
400 600 800 1000 1200 1400 1800 1800 200(
Wavelength (nm)400 600 800 10110 1200 14ó0 1600 1800 200C
Wavelength (nm)
Figure 5: Optical transmission ofPZT films prepared on different
glass substrates and fordifferent annealing temperature.
Co
C6)C
0)ta)
a)
-20 -19 -18 -17 -16 -15 -14 -13Angle of incidence ()
Figure 6: Typical TE dark line spectrum
A typical TE dark line spectrum from the PZT film obtained by
theM-lines technique is presented in Figure 6. Two absorption
linesappears at an angle of approximately -16.7° and -13.9°
andcorrespond to the excitation oftwo guided modes in the film.
-12 -11 -10 The refractive index n1 and the thickness d ofthe
ferroelectric filmare related to the synchronous angles O3 and the
mode order m viathe dispersion equation of a planar dielectric
waveguide18
374 Proc. of SPIE Vol. 5122
-
In order to determine n1 and d, equation (1) was solved
numerically using a modified Newton-Raphson method.'9 In thecase
ofthe PZT film deposited on the 1737F glass substrate, a refractive
index n1 =2.26 0.03 and a thickness d — 2 0.2Inn for a wavelength
of 632 nm were obtained. The results are in good agreement with
measurements at 1.3 jim and 1.55iim wavelength (n1 =2.26 and 2.20,
respectively), obtained from the same film with a commercial
measurement device.
Figure 7: Optical photography ofa Mach-Zehnder interferometer
structhre etched in a 2 jim thick PZT (36/64) film on glass
substrate.
In order to study the possibility of using PZT films on glass
substrates for waveguide applications, a Mach-Zehnderinterferometer
structure was realized by photolithography and wet chemicaletching,
which is shown in Figure 7. A linear waveguide (left hand part of
thestructure) is split into two parallel branches (central part
ofthe structure), one ofwhich can be supplied with electrodes, in
order to induce the electro-optic effects(linear Pockels effect or
quadratic Kerr effect). Due to the resulting phase shiftofthe light
with respect to the second branch, interference is obtained where
thebranches are reunified (right hand part of the structure). The
thickness of thePZT film is approximately 2 rim, the overall length
of the interferometerstructure is 14 mm. A SEM micrograph can be
seen in Figure 8, showing theright hand end ofthe two central
branches ofthe interferometer. The width of the
Figure 8: SEM micrograph of part of a etched structure is
approximately 30 j.tm. Light propagation within theMach-Zehnder
interferometer. waveguide was observed qualitatively, but could not
be recorded as yet.
4. CONCLUSIONS
PZT (36/64) films were realized by CSD, deposited by
spin-coating on glass substrates, and a typical
Mach-Zehnderwaveguide structure was obtained by wet chemical
etching. While the grain structure ofthe ceramic type PZT thin film
doesnot a priori hinder light propagation, up to now, macroscopic
cracks in the ferroelectric layers did not allow a
quantitativeanalysis ofthe attenuation in the guide. As could be
shown, those cracks result from the different thermal behavior of
thePZT and the glass substrate during the RTA heat treatment. For
certain substrates, however, the PZT elaboration routemight be
adapted in order to allow homogeneous and crack-free deposition of
the films. The results from the opticalcharacterization ofthe films
are encouraging for a future application in electro-optical
devices. The transparency normal tothe surface is high at the
telecommunication wavelength, and light propagation in the film
over a few centimeters wasobserved qualitatively. A M-lines
measurement technique was set up which shall allow a more
systematic study of therefractive index ofthe ferroelectric as a
ftmction ofthe material composition and the preparation route
parameters. A moreprofound knowledge on the related phenomena
combined with the possibility of tailoring the refractive index of
the filmswill be necessary to obtain monomode light propagation.
Preliminary results on a wet chemical etched Mach-Zehnder
typeinterferometer structure show a sufficient good resolution in
the micrometer region for a fimdamental investigation of
lightpropagation in the PZT films and a future study ofthe
electro-optic linear and non-linear effects.
ACKNOWLEDGMENT
The authors wish to thank Joel Charrier ofthe Laboratoire
d'Optronique CNRS-UMR 6082 in Lannion, France, for
helpfuldiscussion and for the complementary M-lines measurements
ofour PZT thin films.
REFERENCES
1. A. Carenco, "Composants actifs" in Systémes Optiques, G.
Roblin (ed.), EDP Sciences, pp. 93-143, 1991.2. J.P. Huignard,
"Presentation de l'effet electro-optique", in Optoelectronique 1,
P. Chavel, (ed.), EDP Sciences, pp. 289-294, 2002.3. R.H. Kim,
H.-H. Park, and G.-T. Joo, "The growth of LiNbO3 (006) on MgO
(00&) and LiTaO3 (012) substrates by sol-gel
procedure", Appi. Surf. Sd. 169, pp. 564-569, 2001.
Proc. of SPIE Vol. 5122 375
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