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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376 Proc. of SPIE Vol. 5122