- Email: [email protected]
Low temperature preparation of piezoelectric thin films by ultraviolet-assisted rapid thermal processing
Materials Science in Semiconductor Processing 5 (2003) 77–83
Low temperature preparation of piezoelectric thin films by ultraviolet-assisted rapid thermal processing L. Pardo*, R. Poyato, A. Gonza! lez, M.L. Calzada Depto. de Materiales Ferroelectricos, Insto. Ciencia de Materiales de Madrid, CSIC, Cantoblanco, 28049 Madrid, Spain
Abstract Ca- and La-modified lead titanate ferroelectric thin films were prepared by a sol–gel method from photosensitive solutions and with an ultraviolet (UV)-assisted rapid thermal processor, including an arrangement of excimer lamps. The diol-based route was used in the preparation of the precursor solutions, whose UV absorption and thermal decomposition were determined. Multiple deposition and crystallization steps of the deposited solution were used to promote preferential orientation. A comparative ferro and piezoelectric study of films prepared by conventional rapid thermal processing at 6501C and films prepared at 5501C with an intermediate step of UV irradiation at 2501C will be presented to assess the value of these films for their use in integrated piezoelectric sensors and microelectromechanical systems. Piezoelectric d33 and e31 coefficients were determined by double-beam laser interferometry and by the direct piezoelectric effect on cantilevers, respectively. The effect of the substrate and processing method on the preferential crystalline orientation of the films and the corresponding piezoelectric properties will be reported. The role of the composition will also be discussed. r 2002 Elsevier Science Ltd. All rights reserved. PACS: 77.80; 81.20; Lead titanate; Thin films; UV-assisted processing; Low-temperature processing; Texture; Piezoelectricity
1. Introduction Ferroelectric thin films are functional materials with a wide number of applications. Amongst those that have lately gained most attention in the scientific community are the microelectromechanical systems, making use of their piezoelectric properties [1]. The reduction of the processing temperature of the ferroelectric films below 6501C is still a challenge to make the ferroelectric films fully compatible with the technology of integrated circuits on silicon. Several methods have been tested to overcome the problem, following the usual physical and chemical deposition techniques. In particular, for chemical solution deposition routes, the common objective is to decrease the activation energy for
*Corresponding author. Tel.: +34-1-3349066; fax: +34-13720623. E-mail address: [email protected] (L. Pardo).
crystallization of the films, with methods such as the development of novel chemical routes [2,3]. Another method used to decrease the crystallization temperature of the films is the addition of a photoreactive compound, for example, ortho-nitrobenzaldehyde [4], to prepare photosensitive solutions. Starting from such solutions deposited by spin-on, thin PZT films have been obtained at moderate temperatures (6001C) under ultraviolet (UV) irradiation [4]. However, this method has not yet been fully exploited to attain a further crystallization temperature reduction, although it constitutes a promising alternative. Furthermore, a systematic study of the UV irradiation effects on the microstructure and properties of the films has not yet been carried out. This work reports obtaining sol–gel calcium and lanthanum-modified lead titanate (mPT) thin films by UV-assisted rapid thermal processing (RTP) at moderate temperatures. Their ferro–piezoelectric properties were measured and the results are shown here.
1369-8001/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 1 3 6 9 - 8 0 0 1 ( 0 2 ) 0 0 0 8 7 - 2
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L. Pardo et al. / Materials Science in Semiconductor Processing 5 (2003) 77–83
2. Experimental Sol–gel precursor solutions of (Pb0.76Ca0.24)TiO3 and (Pb0.88La0.08)TiO3 nominal compositions, hereinafter called Ca- and La-mPT, were prepared by a diol-based process previously developed by the authors [5,6] and slightly modified to enhance the solution rheology, thus allowing coating of full 400 silicon wafers. The advantages of this sol–gel method are the standard and lowcost chemical products used, the water compatibility, and thus lower toxicity of the products and of the resulting sol, the easy incorporation of modifiers into the system and the fabrication of crack-free films with thickness >1 mm. Lead acetate trihydrate, Pb(OOCCH3)2 3H2O, was dissolved in 1,3-propanediol, HO(CH2)3OH, and heated under reflux for 1 h in air. Then, titanium diisopropoxide bis-acetylacetonate, Ti(OC3H7)2(CH3COCHCOCH3)2, with a 1:5 molar ratio of Ti to 1,3propanediol, was added to the reaction flask and refluxed in air for 8 h. A lead and titanium sol was obtained after distilling off the byproducts of reaction. Modifier cations, Ca(II) and La (III), are added in the standard [5,6] procedure to the obtained sol as calcium acetate, Ca(CH3COO)2 xH2O, and lanthanum acetate, La(CH3COO)3 xH2O, dissolved in water. A high water to diol ratio was used in the lanthanum case due to the low solubility of the La compound. This standard solutions are hereinafter called Ca- and La-mPT. In this work, the calcium and lanthanum acetate solutions hereinafter called HAc solutions, were obtained by addition of the modifier cations dissolved in water, H2O, and acetic acid, CH3COOH, with a 1:1 molar ratio of these solvents. These HAc solutions were also prepared with a 10% mol excess of PbO to compensate lead losses during the thermal treatment. The final solutions have a concentration of 0.3 mol/l and a density of B0.9 g/cm3. Three aliquots were separated from the former solutions and ortho-nitrobenzyl alcohol, O2NC6H4CH2OH (NBA), 1-hydroxy-cyclohexyl-phenyl-ketone, HOC6H10COC6H5 (HPK), and acetylacetone, CH3COCH2COCH3 (acac), were added to them in a 5% weight of the aliquot. These compounds were dissolved by stirring, obtaining transparent solutions after a few minutes. These solutions will be hereinafter called NBA, HPK and acac solutions, respectively. UV absorption of the solutions was measured after a dilution of 105 to 1 of water to solution. An aliquot of the solution was dried at 1001C for 12 h, and subjected to thermogravimetric and differential thermal analysis (TGA/DTA) in air, between room temperature and 10001C. Solutions were deposited by spin-on, at 2000 rpm for 45 s, onto 2 2 cm2 TiO2/Pt/TiO2/Ti/SiO2/(1 0 0)Si substrates, prepared by metallic evaporation and subse-
quent oxidation of the layers. The films were grown following a method of multiple deposition and subsequent crystallization. These steps were carried out four times to obtain films of B200 nm thickness for Ca-mPT samples and three times to obtain films of B150 nm thickness for La-mPT samples. Some of the films were prepared by pyrolysis of the deposited layers at 3501C for 1 min. and RTP crystallization at 6501C for 50 s using a heating rate of 301C/s in a JetStar 100 T processor (JIPELEC). For comparison, a La-mPT sample was also similarly processed at 5501C. Other films were prepared by pyrolysis at 1501C for 5 min, followed by UV irradiation at 2501C and RTP crystallization at 5501C, with the thermal schedule as in Fig. 1. This second process was carried out in a newly designed equipment (JetClip SG, JIPELEC), including an arrangement of high-intensity UV excimer lamps with emission centered at a wavelength of 222 nm and a power of 120 mW/cm2. These processing conditions were chosen after the study of the UV absorption and the thermal decomposition of the precursor solutions, reported in more detail elsewhere [7]. The thickness of the samples was measured with a Taylor–Hobson Talysurf 50 profilometer. Powder XRD analysis (Siemens D500) of the films was carried out at Bragg–Brentano geometry, with a decoupling between y and 2y angles of B31, to avoid the overlapping of the (1 1 1) diffraction peaks of the films and of the Pt bottom electrode. Separation of the peaks using pseudoVoigt functions allowed us a semiquantitative evaluation of the preferential crystalline orientation by calculation of peak intensity ratios with respect to the most intense peak of the perovskite in each case. Pt top electrodes with an area of 0.05 mm2 were deposited on the films by sputtering (BAL-TEC SCD 050) for the ferroelectric characterization. Ferroelectric hysteresis loops were measured with a modified Sawyer– Tower circuit using sinusoidal waves of 1 kHz. Measurement files were treated with our laboratory software and the loops were corrected by compensating the ohmic and capacitive currents, using a non-perturbative method [8]. The coercive fields, Ec ; were obtained as an average of the positive and the negative applied fields of the maxima of the current density vs. field (J2E) loops.
Fig. 1. Time–temperature profile of the UV-assisted RTP crystallization.
L. Pardo et al. / Materials Science in Semiconductor Processing 5 (2003) 77–83
The remanent polarization, Pr ; was calculated as the area below the current density peaks. Poling was carried out for the piezoelectric measurements: at 1501C with square pulses of 100 ms width and 10 ms period; at room temperature (RT) with a single triangular pulse. The poling voltage sign refers to the polarity of the voltage connected to the top electrode. Measurements of the piezoelectric d33 coefficients were carried out by double-beam laser interferometry [9] using Pt top electrodes of 0.3 mm2. Measurements of the e31 coefficient were made on cantilevers of 15 1.5 mm2 cut from the thin film samples, using top electrodes of 1– 2 mm2 area, by measurement of the electric charge created by the forced deflection of the end of the cantilever, being the other end tightly clamped, as detailed elsewhere [10].
3. Results and discussion Fig. 2 shows the UV absorption spectra of the five solutions of Ca-mPT prepared with (NBA, HPK and acac) and without (Ca-mPT and HAc) photoinitiators. The unmodified solution presents an intrinsic absorption. This can be explained by the characteristic p-p transition of the acetylacetonate ion, present in the solution from the Ti precursor, reported previously by other authors [11]. All the other solutions, except the HAc one, present enhanced absorption. This is due for the acac solution to the previously mentioned absorption mechanism, reinforced by the addition of the acetylacetone, and for the NBA and HPK solutions
5
furnace lamp: 222nm 0.3M Ca-mPT solutions
Absorption
4
HPK (248nm) NBA (266nm)
3
acac (268nm) Ca-mPT (264nm)
2
HAc (271nm) 1
0 200
250
300
350
400
wavelength (nm)
Fig. 2. UV absorption spectra of the five Ca-mPT sol–gel processed solutions with (HPK, NBA and acac) and without (Ca-mPT and HAc) photoinitiators. The wavelength of the local maxima is indicated for each curve.
79
due to different process photoreactions that were also analysed previously by other authors [12]. Figs. 3(a) and (b) shows the TGA/DTA results for Hac and HPK Ca-mPT solutions. The major weight loss (mainly caused by the loss of photosensitive organics) occurs below 4001C. This is a common result for gels from all solutions [7]. The addition of photoinitiators increases the weight losses of the gels, which is an important drawback for the crystallization of the gel. The photosensitivity of all the solutions, with and without added photoinitiators, thus, has been proved. Since the use of HAc solutions, and due to their intrinsic photosensitivity, provides the simplest approach to the reduction of the processing temperature of the films by use of UV irradiation, this work will be focussed in the study of films processed from the HAc solutions. Table 1 shows the ratios of the perovskite XRD peak intensities of Ca- and La-mPT films from HAc solutions. The theoretical value of such intensities ratio for a polycrystalline sample with randomly distributed crystallites is 35/100/35 (JCPDS-ICDD file # 39–1336). Thus, all the samples show a /1 1 1S preferential orientation. For RTP thin films prepared by multiple deposition and crystallization on substrates with Ti on the top of the Pt, the heterogeneous nucleation at the interface substrate–ferroelectric film, and, to some extent, at the interface between layers, promotes a /1 1 1S preferential orientation [13]. The results in Table 1 show that this preferential orientation is partly inhibited in films processed by UV-assisted RTP. This is most probably due to the generation of new nucleation sites in the bulk of the film. Table 1 also shows the values of the remanent polarization (Pr ) obtained from the (J; E) ferroelectric hysteresis loops for these films. The La-mPT film prepared by RTP at 5501C shows only an incipient ferroelectric hysteresis loop, without clear maxima that would define coercive fields, and, thus, the calculation of Pr from this loop was not possible. This is shown in Fig. 4(a). This is due to the remaining amorphous, nonferroelectric, phase detected by XRD. Only the use of UV-assisted processing yielded ferroelectric films by RTP at 5501C; the (J; E) loop is shown for such a sample in Fig. 4(b). Fig. 5 shows the microstructure of similar Ca-mPT films processed at 5501C, by conventional RTP (Fig. 5(a)) and by RTP with UV irradiation (Fig. 5(b)), respectively. The microstructural features observed correlates well with the ferroelectric results. The film prepared without UV irradiation shows smaller grains (Fig. 5(a)), of a few tenths of nanometer size, and in some areas undefined grain boundaries, as correspond to a sample that is not well crystallized. In contrast, the film processed under UV irradiation (Fig. 5(b)) shows a more advanced formation stage and larger grain size, of the order of hundred nanometers.
L. Pardo et al. / Materials Science in Semiconductor Processing 5 (2003) 77–83
296
100
351
2.71
dry gel (100°C) from solution 1 (HAc/Ca-mPT) in air
EXO
80
energy(a.u.)
389 444 483
weight loss (%)
95
90
17.50
561 617 683
85
80
DTA
3.33
ATG
75 1.46 0
200
400
600
800
412
100
dry gel (100°C) from solution 4 (HPK/Ca-mPT) in air
5.77
EXO
95 90
energy(a.u.)
33.46
75
438
80
338
85
272
70
500
weight loss(%)
1000
T (°C)
(a)
656
65 60
DTA
7.69
55
ATG
50
0.77
45 0
200
400
(b)
600
800
1000
T(°C)
Fig. 3. TGA/DTA results for: (a) HAc and (b) HPK Ca-mPT solutions.
Table 2 shows the e31 piezoelectric coefficients for the poled films. Ca-mPT films have lower values than LamPT films. This result was expected, since it is well known that ceramics of (Pb0.76Ca0.24)TiO3 have the highest piezoelectric anisotropy and posses the lowest transversal piezoelectric effect among the mPT compositions. Remarkably, the La-mPT films processed at 5501C by UV-assisted RTP present comparable values to those prepared at 6501C by RTP, even poled at room temperature. Fig. 6 shows the d33 piezoelectric coefficient vs. voltage loops of the films, measured after poling at 1501C with +12 V. Changes of d33 are not remarkable when positive polarity field is applied indicating a
saturation of polarization. The most square loop, see Fig. 6(a), is found for the film presenting the most intense /1 1 1S preferential orientation (see Table 1), in which the application of a field of 190 kV/cm, opposite to the poling, still does not cause great decrease of the d33 coefficient (B30 pm/V). All the samples processed with UV-assisted RTP at 5501C show similar d33 remanent values to those RTP processed at 6501C.
4. Conclusions A set of precursor solutions of modified lead titanate films with and without the addition of photoinitiators
L. Pardo et al. / Materials Science in Semiconductor Processing 5 (2003) 77–83
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Table 1 XRD and ferroelectric characterization of the samples mPT
Crystallization process
Thickness (nm)
I100þ001 =I101þ110 =I111
Pr (mC/cm2)
Ca
RTP at 6501C UV-assisted RTP at 5501C
225 200
24/15/100 76/30/100
20.0 21.5
La
RTP at 6501C RTP at 5501C UV-assisted RTP at 5501C
160 200 160
82/0/100 98/20/100 100/48/–a
21.6 — 9.7
a
The perovskite 111 peak cannot be separated from the Pt one.
0.9 0.6
2
J (A/cm )
0.3 0.0 -0.3 -0.6
1 kHz -0.9 -600
-400
-200
0
200
400
600
Field (kV/cm) 0.9
-6
Pr = 9.7 x10 C/cm
2
0.6
2
J (A/cm )
0.3 0.0 -0.3 -0.6
1 kHz -0.9 -600
-400
-200
0
200
400
600
Field (kV/cm)
Fig. 4. (J; E) ferroelectric hysteresis loops of HAc solutionderived La-mPt thin films processed at 5501C, by means of: (a) conventional RTP and (b) UV-assisted RTP.
(namely 1-hydroxy-cyclohexyl-phenyl-ketone, HOC6H10COC6H5, ortho-nitrobenzyl alcohol, O2NC6H4CH2OH and acetylacetone, CH3COCH2COCH3) was prepared by a diol-based sol–gel route. The UV-absorption measurements on these solutions indicate that all of them are photosensitive. The use of HAc solutions provides the simplest approach to the reduction of the processing temperature of the films by use of UV irradiation.
Fig. 5. SPM images, in tapping mode, of the topography of calcium modified lead titanate films processed from HAc solutions at 5501C, with: (a) UV-assisted RTP and (b) conventional RTP; observed area ¼ 500 500 nm2.
L. Pardo et al. / Materials Science in Semiconductor Processing 5 (2003) 77–83
82
Table 2 Poling conditions and piezoelectric e31 coefficient of the films mPT
Crystallization process
Poling conditions
e31 (C m2)
Ca
RTP at 6501C UV-assisted RTP at 5501C
1501C, +12 V, 5 min 1501C, 12 V, 5 min
0.40 0.24
La
RTP at 6501C UV-assisted RTP at 5501C
1501C, +12 V, 5 min RT, +6 V, 10 s
1.97 1.70
d33 (pm/V)
d33 (pm/V) 40
40
30 20
20
10
0
0
-20
-20
-10
-30
-40
-40
(a) -15
-10
-5
0
5
10
15
(b) -15
40
40
20
20
0
0
-20
-20
-40
-40 -8
(c)
-6
-4
-2
0
2
4
6
8
V (Volts)
-8
(d)
-10
-6
-5
-4
0
-2
0
5
2
10
4
6
15
8
V (Volts)
Fig. 6. d33 coefficient vs. voltage piezoelectric hysteresis loops of the poled HAc solution-derived thin films: Ca-mPT films (a) RTP at 6501C and (b) UV-assisted RTP at 5501C; La-mPT films (c) RTP at 6501C and (d) UV-assisted RTP at 5501C.
Ferroelectric thin films of calcium- and lanthanummodified lead titanate (mPT), were processed from HAc solutions by multiple spin-on deposition and crystallization at 5501C in a newly developed UV-assisted rapid thermal processor. The functional properties of these films are similar to those of RTP crystallized films at 6501C without UV irradiation. In particular, remanent piezoelectric coefficients of d33 B40 pC/N and, for LamPT, e31 B2 C/cm2 were measured for films processed at 5501C with UV irradiation. Effects of the UV irradiation on the film preferential orientation were observed, namely, the homogeneous nucleation throughout the film thickness disrupts the mechanism that generates the /1 1 1S preferential orientation, which is currently observed in films
grown on Pt substrates with Ti on the surface in absence of UV irradiation.
Acknowledgements Authors wish to thank R. Neuberger at EADS (Munich, Germany) for providing the platinized substrates, E. Lynch, S. O’Brien and P.V. Kelly (NMRC, Ireland) and H. Guillon (JIPELEC, France) for the processing of some of the samples, Igor Stolichnov (LC, EPFL, Switzerland) for the piezoelectric characterization of the samples, and C. Zaldo (ICMM-CSIC, Spain) for the UV-absorption measurements.
L. Pardo et al. / Materials Science in Semiconductor Processing 5 (2003) 77–83
This work was funded by Projects BRPR-CT98-0777 (MUVAST) and COST528 of the EC and MAT9991269CE of the Spanish CICYT. References [1] Muralt P. Integrated Ferroelectrics 1997;17:297–307. [2] Calzada ML, Jim!enez R, Gonzalez A, Mendiola J. Chem Mater 2001;13:3. [3] Celinska J, Joshi V, Narayan S, McMillan LD, Paz de Araujo CA. Integrated Ferroelectrics 2000;30:1. [4] Uozumi G, Kageyama K, Atsuki T, Soyama N, Uchida H, Ogi K. Jpn J Appl Phys 1999;38:5350–3. [5] Calzada ML, Sirera R. J Mater Sci Mater Elect 1996; 7:39–45.
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[6] Calzada ML, Alguero! M, Pardo L. J Sol–Gel Sci Technol 1998;13:837–41. [7] Gonzalez A, Poyato R, Pardo L, Calzada ML. Ferroelectrics 2002;271:45–50. ! [8] Ramos P. Ph.D. thesis. University of Autonoma de Madrid, 1996. [9] Kholkin AL, Wuthrich CH, Taylor DV, Setter N. Rev Sci Instrum 1996;67:1935. [10] Dubois MA, Muralt P. Sensors Actuators 1999;77: 106. [11] Barnum DW. J Inorg Nucl Chem 1961;21:221–37. [12] Soyama N, Sasaki G, Atsuki T, Yonozawa T, Ogi K. Proceedings of the ISAF’94, Pennsylvania State University, PA, USA. p. 408–11. [13] Gonzalez A, Poyato R, Jim!enez R, Mendiola J, Pardo L, Calzada ML. Surf Interf Anal 2000;29:325–9.
Low temperature preparation of piezoelectric thin films by ultraviolet-assisted rapid thermal processing L. Pardo*, R. Poyato, A. Gonza! lez, M.L. Calzada Depto. de Materiales Ferroelectricos, Insto. Ciencia de Materiales de Madrid, CSIC, Cantoblanco, 28049 Madrid, Spain
Abstract Ca- and La-modified lead titanate ferroelectric thin films were prepared by a sol–gel method from photosensitive solutions and with an ultraviolet (UV)-assisted rapid thermal processor, including an arrangement of excimer lamps. The diol-based route was used in the preparation of the precursor solutions, whose UV absorption and thermal decomposition were determined. Multiple deposition and crystallization steps of the deposited solution were used to promote preferential orientation. A comparative ferro and piezoelectric study of films prepared by conventional rapid thermal processing at 6501C and films prepared at 5501C with an intermediate step of UV irradiation at 2501C will be presented to assess the value of these films for their use in integrated piezoelectric sensors and microelectromechanical systems. Piezoelectric d33 and e31 coefficients were determined by double-beam laser interferometry and by the direct piezoelectric effect on cantilevers, respectively. The effect of the substrate and processing method on the preferential crystalline orientation of the films and the corresponding piezoelectric properties will be reported. The role of the composition will also be discussed. r 2002 Elsevier Science Ltd. All rights reserved. PACS: 77.80; 81.20; Lead titanate; Thin films; UV-assisted processing; Low-temperature processing; Texture; Piezoelectricity
1. Introduction Ferroelectric thin films are functional materials with a wide number of applications. Amongst those that have lately gained most attention in the scientific community are the microelectromechanical systems, making use of their piezoelectric properties [1]. The reduction of the processing temperature of the ferroelectric films below 6501C is still a challenge to make the ferroelectric films fully compatible with the technology of integrated circuits on silicon. Several methods have been tested to overcome the problem, following the usual physical and chemical deposition techniques. In particular, for chemical solution deposition routes, the common objective is to decrease the activation energy for
*Corresponding author. Tel.: +34-1-3349066; fax: +34-13720623. E-mail address: [email protected] (L. Pardo).
crystallization of the films, with methods such as the development of novel chemical routes [2,3]. Another method used to decrease the crystallization temperature of the films is the addition of a photoreactive compound, for example, ortho-nitrobenzaldehyde [4], to prepare photosensitive solutions. Starting from such solutions deposited by spin-on, thin PZT films have been obtained at moderate temperatures (6001C) under ultraviolet (UV) irradiation [4]. However, this method has not yet been fully exploited to attain a further crystallization temperature reduction, although it constitutes a promising alternative. Furthermore, a systematic study of the UV irradiation effects on the microstructure and properties of the films has not yet been carried out. This work reports obtaining sol–gel calcium and lanthanum-modified lead titanate (mPT) thin films by UV-assisted rapid thermal processing (RTP) at moderate temperatures. Their ferro–piezoelectric properties were measured and the results are shown here.
1369-8001/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 1 3 6 9 - 8 0 0 1 ( 0 2 ) 0 0 0 8 7 - 2
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L. Pardo et al. / Materials Science in Semiconductor Processing 5 (2003) 77–83
2. Experimental Sol–gel precursor solutions of (Pb0.76Ca0.24)TiO3 and (Pb0.88La0.08)TiO3 nominal compositions, hereinafter called Ca- and La-mPT, were prepared by a diol-based process previously developed by the authors [5,6] and slightly modified to enhance the solution rheology, thus allowing coating of full 400 silicon wafers. The advantages of this sol–gel method are the standard and lowcost chemical products used, the water compatibility, and thus lower toxicity of the products and of the resulting sol, the easy incorporation of modifiers into the system and the fabrication of crack-free films with thickness >1 mm. Lead acetate trihydrate, Pb(OOCCH3)2 3H2O, was dissolved in 1,3-propanediol, HO(CH2)3OH, and heated under reflux for 1 h in air. Then, titanium diisopropoxide bis-acetylacetonate, Ti(OC3H7)2(CH3COCHCOCH3)2, with a 1:5 molar ratio of Ti to 1,3propanediol, was added to the reaction flask and refluxed in air for 8 h. A lead and titanium sol was obtained after distilling off the byproducts of reaction. Modifier cations, Ca(II) and La (III), are added in the standard [5,6] procedure to the obtained sol as calcium acetate, Ca(CH3COO)2 xH2O, and lanthanum acetate, La(CH3COO)3 xH2O, dissolved in water. A high water to diol ratio was used in the lanthanum case due to the low solubility of the La compound. This standard solutions are hereinafter called Ca- and La-mPT. In this work, the calcium and lanthanum acetate solutions hereinafter called HAc solutions, were obtained by addition of the modifier cations dissolved in water, H2O, and acetic acid, CH3COOH, with a 1:1 molar ratio of these solvents. These HAc solutions were also prepared with a 10% mol excess of PbO to compensate lead losses during the thermal treatment. The final solutions have a concentration of 0.3 mol/l and a density of B0.9 g/cm3. Three aliquots were separated from the former solutions and ortho-nitrobenzyl alcohol, O2NC6H4CH2OH (NBA), 1-hydroxy-cyclohexyl-phenyl-ketone, HOC6H10COC6H5 (HPK), and acetylacetone, CH3COCH2COCH3 (acac), were added to them in a 5% weight of the aliquot. These compounds were dissolved by stirring, obtaining transparent solutions after a few minutes. These solutions will be hereinafter called NBA, HPK and acac solutions, respectively. UV absorption of the solutions was measured after a dilution of 105 to 1 of water to solution. An aliquot of the solution was dried at 1001C for 12 h, and subjected to thermogravimetric and differential thermal analysis (TGA/DTA) in air, between room temperature and 10001C. Solutions were deposited by spin-on, at 2000 rpm for 45 s, onto 2 2 cm2 TiO2/Pt/TiO2/Ti/SiO2/(1 0 0)Si substrates, prepared by metallic evaporation and subse-
quent oxidation of the layers. The films were grown following a method of multiple deposition and subsequent crystallization. These steps were carried out four times to obtain films of B200 nm thickness for Ca-mPT samples and three times to obtain films of B150 nm thickness for La-mPT samples. Some of the films were prepared by pyrolysis of the deposited layers at 3501C for 1 min. and RTP crystallization at 6501C for 50 s using a heating rate of 301C/s in a JetStar 100 T processor (JIPELEC). For comparison, a La-mPT sample was also similarly processed at 5501C. Other films were prepared by pyrolysis at 1501C for 5 min, followed by UV irradiation at 2501C and RTP crystallization at 5501C, with the thermal schedule as in Fig. 1. This second process was carried out in a newly designed equipment (JetClip SG, JIPELEC), including an arrangement of high-intensity UV excimer lamps with emission centered at a wavelength of 222 nm and a power of 120 mW/cm2. These processing conditions were chosen after the study of the UV absorption and the thermal decomposition of the precursor solutions, reported in more detail elsewhere [7]. The thickness of the samples was measured with a Taylor–Hobson Talysurf 50 profilometer. Powder XRD analysis (Siemens D500) of the films was carried out at Bragg–Brentano geometry, with a decoupling between y and 2y angles of B31, to avoid the overlapping of the (1 1 1) diffraction peaks of the films and of the Pt bottom electrode. Separation of the peaks using pseudoVoigt functions allowed us a semiquantitative evaluation of the preferential crystalline orientation by calculation of peak intensity ratios with respect to the most intense peak of the perovskite in each case. Pt top electrodes with an area of 0.05 mm2 were deposited on the films by sputtering (BAL-TEC SCD 050) for the ferroelectric characterization. Ferroelectric hysteresis loops were measured with a modified Sawyer– Tower circuit using sinusoidal waves of 1 kHz. Measurement files were treated with our laboratory software and the loops were corrected by compensating the ohmic and capacitive currents, using a non-perturbative method [8]. The coercive fields, Ec ; were obtained as an average of the positive and the negative applied fields of the maxima of the current density vs. field (J2E) loops.
Fig. 1. Time–temperature profile of the UV-assisted RTP crystallization.
L. Pardo et al. / Materials Science in Semiconductor Processing 5 (2003) 77–83
The remanent polarization, Pr ; was calculated as the area below the current density peaks. Poling was carried out for the piezoelectric measurements: at 1501C with square pulses of 100 ms width and 10 ms period; at room temperature (RT) with a single triangular pulse. The poling voltage sign refers to the polarity of the voltage connected to the top electrode. Measurements of the piezoelectric d33 coefficients were carried out by double-beam laser interferometry [9] using Pt top electrodes of 0.3 mm2. Measurements of the e31 coefficient were made on cantilevers of 15 1.5 mm2 cut from the thin film samples, using top electrodes of 1– 2 mm2 area, by measurement of the electric charge created by the forced deflection of the end of the cantilever, being the other end tightly clamped, as detailed elsewhere [10].
3. Results and discussion Fig. 2 shows the UV absorption spectra of the five solutions of Ca-mPT prepared with (NBA, HPK and acac) and without (Ca-mPT and HAc) photoinitiators. The unmodified solution presents an intrinsic absorption. This can be explained by the characteristic p-p transition of the acetylacetonate ion, present in the solution from the Ti precursor, reported previously by other authors [11]. All the other solutions, except the HAc one, present enhanced absorption. This is due for the acac solution to the previously mentioned absorption mechanism, reinforced by the addition of the acetylacetone, and for the NBA and HPK solutions
5
furnace lamp: 222nm 0.3M Ca-mPT solutions
Absorption
4
HPK (248nm) NBA (266nm)
3
acac (268nm) Ca-mPT (264nm)
2
HAc (271nm) 1
0 200
250
300
350
400
wavelength (nm)
Fig. 2. UV absorption spectra of the five Ca-mPT sol–gel processed solutions with (HPK, NBA and acac) and without (Ca-mPT and HAc) photoinitiators. The wavelength of the local maxima is indicated for each curve.
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due to different process photoreactions that were also analysed previously by other authors [12]. Figs. 3(a) and (b) shows the TGA/DTA results for Hac and HPK Ca-mPT solutions. The major weight loss (mainly caused by the loss of photosensitive organics) occurs below 4001C. This is a common result for gels from all solutions [7]. The addition of photoinitiators increases the weight losses of the gels, which is an important drawback for the crystallization of the gel. The photosensitivity of all the solutions, with and without added photoinitiators, thus, has been proved. Since the use of HAc solutions, and due to their intrinsic photosensitivity, provides the simplest approach to the reduction of the processing temperature of the films by use of UV irradiation, this work will be focussed in the study of films processed from the HAc solutions. Table 1 shows the ratios of the perovskite XRD peak intensities of Ca- and La-mPT films from HAc solutions. The theoretical value of such intensities ratio for a polycrystalline sample with randomly distributed crystallites is 35/100/35 (JCPDS-ICDD file # 39–1336). Thus, all the samples show a /1 1 1S preferential orientation. For RTP thin films prepared by multiple deposition and crystallization on substrates with Ti on the top of the Pt, the heterogeneous nucleation at the interface substrate–ferroelectric film, and, to some extent, at the interface between layers, promotes a /1 1 1S preferential orientation [13]. The results in Table 1 show that this preferential orientation is partly inhibited in films processed by UV-assisted RTP. This is most probably due to the generation of new nucleation sites in the bulk of the film. Table 1 also shows the values of the remanent polarization (Pr ) obtained from the (J; E) ferroelectric hysteresis loops for these films. The La-mPT film prepared by RTP at 5501C shows only an incipient ferroelectric hysteresis loop, without clear maxima that would define coercive fields, and, thus, the calculation of Pr from this loop was not possible. This is shown in Fig. 4(a). This is due to the remaining amorphous, nonferroelectric, phase detected by XRD. Only the use of UV-assisted processing yielded ferroelectric films by RTP at 5501C; the (J; E) loop is shown for such a sample in Fig. 4(b). Fig. 5 shows the microstructure of similar Ca-mPT films processed at 5501C, by conventional RTP (Fig. 5(a)) and by RTP with UV irradiation (Fig. 5(b)), respectively. The microstructural features observed correlates well with the ferroelectric results. The film prepared without UV irradiation shows smaller grains (Fig. 5(a)), of a few tenths of nanometer size, and in some areas undefined grain boundaries, as correspond to a sample that is not well crystallized. In contrast, the film processed under UV irradiation (Fig. 5(b)) shows a more advanced formation stage and larger grain size, of the order of hundred nanometers.
L. Pardo et al. / Materials Science in Semiconductor Processing 5 (2003) 77–83
296
100
351
2.71
dry gel (100°C) from solution 1 (HAc/Ca-mPT) in air
EXO
80
energy(a.u.)
389 444 483
weight loss (%)
95
90
17.50
561 617 683
85
80
DTA
3.33
ATG
75 1.46 0
200
400
600
800
412
100
dry gel (100°C) from solution 4 (HPK/Ca-mPT) in air
5.77
EXO
95 90
energy(a.u.)
33.46
75
438
80
338
85
272
70
500
weight loss(%)
1000
T (°C)
(a)
656
65 60
DTA
7.69
55
ATG
50
0.77
45 0
200
400
(b)
600
800
1000
T(°C)
Fig. 3. TGA/DTA results for: (a) HAc and (b) HPK Ca-mPT solutions.
Table 2 shows the e31 piezoelectric coefficients for the poled films. Ca-mPT films have lower values than LamPT films. This result was expected, since it is well known that ceramics of (Pb0.76Ca0.24)TiO3 have the highest piezoelectric anisotropy and posses the lowest transversal piezoelectric effect among the mPT compositions. Remarkably, the La-mPT films processed at 5501C by UV-assisted RTP present comparable values to those prepared at 6501C by RTP, even poled at room temperature. Fig. 6 shows the d33 piezoelectric coefficient vs. voltage loops of the films, measured after poling at 1501C with +12 V. Changes of d33 are not remarkable when positive polarity field is applied indicating a
saturation of polarization. The most square loop, see Fig. 6(a), is found for the film presenting the most intense /1 1 1S preferential orientation (see Table 1), in which the application of a field of 190 kV/cm, opposite to the poling, still does not cause great decrease of the d33 coefficient (B30 pm/V). All the samples processed with UV-assisted RTP at 5501C show similar d33 remanent values to those RTP processed at 6501C.
4. Conclusions A set of precursor solutions of modified lead titanate films with and without the addition of photoinitiators
L. Pardo et al. / Materials Science in Semiconductor Processing 5 (2003) 77–83
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Table 1 XRD and ferroelectric characterization of the samples mPT
Crystallization process
Thickness (nm)
I100þ001 =I101þ110 =I111
Pr (mC/cm2)
Ca
RTP at 6501C UV-assisted RTP at 5501C
225 200
24/15/100 76/30/100
20.0 21.5
La
RTP at 6501C RTP at 5501C UV-assisted RTP at 5501C
160 200 160
82/0/100 98/20/100 100/48/–a
21.6 — 9.7
a
The perovskite 111 peak cannot be separated from the Pt one.
0.9 0.6
2
J (A/cm )
0.3 0.0 -0.3 -0.6
1 kHz -0.9 -600
-400
-200
0
200
400
600
Field (kV/cm) 0.9
-6
Pr = 9.7 x10 C/cm
2
0.6
2
J (A/cm )
0.3 0.0 -0.3 -0.6
1 kHz -0.9 -600
-400
-200
0
200
400
600
Field (kV/cm)
Fig. 4. (J; E) ferroelectric hysteresis loops of HAc solutionderived La-mPt thin films processed at 5501C, by means of: (a) conventional RTP and (b) UV-assisted RTP.
(namely 1-hydroxy-cyclohexyl-phenyl-ketone, HOC6H10COC6H5, ortho-nitrobenzyl alcohol, O2NC6H4CH2OH and acetylacetone, CH3COCH2COCH3) was prepared by a diol-based sol–gel route. The UV-absorption measurements on these solutions indicate that all of them are photosensitive. The use of HAc solutions provides the simplest approach to the reduction of the processing temperature of the films by use of UV irradiation.
Fig. 5. SPM images, in tapping mode, of the topography of calcium modified lead titanate films processed from HAc solutions at 5501C, with: (a) UV-assisted RTP and (b) conventional RTP; observed area ¼ 500 500 nm2.
L. Pardo et al. / Materials Science in Semiconductor Processing 5 (2003) 77–83
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Table 2 Poling conditions and piezoelectric e31 coefficient of the films mPT
Crystallization process
Poling conditions
e31 (C m2)
Ca
RTP at 6501C UV-assisted RTP at 5501C
1501C, +12 V, 5 min 1501C, 12 V, 5 min
0.40 0.24
La
RTP at 6501C UV-assisted RTP at 5501C
1501C, +12 V, 5 min RT, +6 V, 10 s
1.97 1.70
d33 (pm/V)
d33 (pm/V) 40
40
30 20
20
10
0
0
-20
-20
-10
-30
-40
-40
(a) -15
-10
-5
0
5
10
15
(b) -15
40
40
20
20
0
0
-20
-20
-40
-40 -8
(c)
-6
-4
-2
0
2
4
6
8
V (Volts)
-8
(d)
-10
-6
-5
-4
0
-2
0
5
2
10
4
6
15
8
V (Volts)
Fig. 6. d33 coefficient vs. voltage piezoelectric hysteresis loops of the poled HAc solution-derived thin films: Ca-mPT films (a) RTP at 6501C and (b) UV-assisted RTP at 5501C; La-mPT films (c) RTP at 6501C and (d) UV-assisted RTP at 5501C.
Ferroelectric thin films of calcium- and lanthanummodified lead titanate (mPT), were processed from HAc solutions by multiple spin-on deposition and crystallization at 5501C in a newly developed UV-assisted rapid thermal processor. The functional properties of these films are similar to those of RTP crystallized films at 6501C without UV irradiation. In particular, remanent piezoelectric coefficients of d33 B40 pC/N and, for LamPT, e31 B2 C/cm2 were measured for films processed at 5501C with UV irradiation. Effects of the UV irradiation on the film preferential orientation were observed, namely, the homogeneous nucleation throughout the film thickness disrupts the mechanism that generates the /1 1 1S preferential orientation, which is currently observed in films
grown on Pt substrates with Ti on the surface in absence of UV irradiation.
Acknowledgements Authors wish to thank R. Neuberger at EADS (Munich, Germany) for providing the platinized substrates, E. Lynch, S. O’Brien and P.V. Kelly (NMRC, Ireland) and H. Guillon (JIPELEC, France) for the processing of some of the samples, Igor Stolichnov (LC, EPFL, Switzerland) for the piezoelectric characterization of the samples, and C. Zaldo (ICMM-CSIC, Spain) for the UV-absorption measurements.
L. Pardo et al. / Materials Science in Semiconductor Processing 5 (2003) 77–83
This work was funded by Projects BRPR-CT98-0777 (MUVAST) and COST528 of the EC and MAT9991269CE of the Spanish CICYT. References [1] Muralt P. Integrated Ferroelectrics 1997;17:297–307. [2] Calzada ML, Jim!enez R, Gonzalez A, Mendiola J. Chem Mater 2001;13:3. [3] Celinska J, Joshi V, Narayan S, McMillan LD, Paz de Araujo CA. Integrated Ferroelectrics 2000;30:1. [4] Uozumi G, Kageyama K, Atsuki T, Soyama N, Uchida H, Ogi K. Jpn J Appl Phys 1999;38:5350–3. [5] Calzada ML, Sirera R. J Mater Sci Mater Elect 1996; 7:39–45.
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[6] Calzada ML, Alguero! M, Pardo L. J Sol–Gel Sci Technol 1998;13:837–41. [7] Gonzalez A, Poyato R, Pardo L, Calzada ML. Ferroelectrics 2002;271:45–50. ! [8] Ramos P. Ph.D. thesis. University of Autonoma de Madrid, 1996. [9] Kholkin AL, Wuthrich CH, Taylor DV, Setter N. Rev Sci Instrum 1996;67:1935. [10] Dubois MA, Muralt P. Sensors Actuators 1999;77: 106. [11] Barnum DW. J Inorg Nucl Chem 1961;21:221–37. [12] Soyama N, Sasaki G, Atsuki T, Yonozawa T, Ogi K. Proceedings of the ISAF’94, Pennsylvania State University, PA, USA. p. 408–11. [13] Gonzalez A, Poyato R, Jim!enez R, Mendiola J, Pardo L, Calzada ML. Surf Interf Anal 2000;29:325–9.