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Iñigo Bretos, Ricardo Jiménez, Aiying Wu, Angus I. Kingon, Paula M. Vilarinho,* and M. Lourdes Calzada* To the memory of Dr. Aiying Wu A novel solution method is developed that enables the processing of functional oxides under low-temperature conditions so that direct-large-area integration of active layers with flexible electronics is turned into reality. We demonstrate the concept on the most important multifunctional oxide, lead zirconate titanate. It reaches the lower limit temperature of crystallization at 300 °C, using a strategy that combines seeded diphasic precursors and photochemical solution deposition. Properties of these ferroelectric layers on flexible plastic fulfill the major requirements demanded for devices, showing a wider temperature range of applicability and functionality than those of the high-K dielectrics or organic ferroelectrics. This is a platform that can be used for many other functional oxide layers. In the case of the most widely used inorganic materials in electronics, i.e., semiconductors, significant efforts are being devoted to their low-temperature fabrication for a successful integration with flexible electronics.[1–6] Thus, thin-film transistors (TFTs) have been prepared directly onto polymers by solution methods, as amorphous,[7] or nanocrystalline[8] dielectric oxide layers at exceptionally low temperatures. But now the need for the direct integration of other active layers on polymers is mandatory to increase functionality of the flexible device. This is a major opportunity for ferroelectric oxide thin films, since their intrinsic multifunctionality (ferro-, pyro-, piezoelectricity) would allow diverse operations in electronic devices such as memories, sensors, actuators, transducers or detectors, making real applications not possible before (e.g., smart skin, flexible sensitive displays, photovoltaic cells or eye-type imagers).[8–14] But, inorganic ferroelectrics require high temperatures to obtain the needed crystalline phase[15,16] that exceed by far the thermal stability of plastics. This could be overcome with organic ferroelectrics that display much lower processing temperatures. However, they present fabrication and performance limitations (see Supporting Information, Table SI);[17–30] Dr. I. Bretos, Dr. R. Jiménez, Prof. M. L. Calzada Instituto de Ciencia de Materiales de Madrid (ICMM-CSIC) Cantoblanco, 28049, Madrid, Spain E-mail: [email protected] Dr. A. Wu, Prof. P. M. Vilarinho Departament of Ceramics and Glass Engineering CICECO, University of Aveiro, 3810-193, Aveiro, Portugal E-mail: [email protected] Prof. A. I. Kingon School of Engineering Brown University Providence, RI, 02912, USA
DOI: 10.1002/adma.201304308
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Activated Solutions Enabling Low-Temperature Processing of Functional Ferroelectric Oxides for Flexible Electronics
e.g., (i) narrow temperature range of applicability, (ii) low dielectric constant, (iii) high coercive fields or (iv) long switching times. Among the inorganic ferroelectrics, the lead zirconate titanate (Pb(Zr1–xTix)O3, PZT) is the one with the greatest commercial applications.[16] Therefore, the fabrication of PZT ferroelectric thin films at temperatures compatible with polymeric substrates would be of high technological interest. According to the literature (Figure 1), a low-temperature processing method for inorganic ferroelectric thin films,[30–44]
Figure 1. Key relevant challenges for the integration of ferroelectric oxide films with the Si-technology and flexible electronics. Shaded area (A) falls within the range of temperatures required for film integration into CMOS technology, with the switching charge densities demanded today for applications in NVFeRAMs (over 8.5 μC cm−2 until 2015).[45] Shaded area (B) denotes a region merging the thermal stability of available plastic substrates with polarization values usually found for organic ferroelectrics on rigid substrates.[20–29] Oxide dielectric semiconductors prepared at low temperatures[1–7] are positioned in the figure. Dielectric permittivities, K′, can be compared in this case; average values of K′ at room temperature of 15, 50 and 100 are reported in the literature for oxide semiconductors, organic ferroelectrics and high-temperature processed inorganic ferroelectrics, respectively. K′ values of ∼80 have been measured in the inorganic ferroelectric layers of this work, Table SII). A summary of the most relevant low-temperature methods reported for some ferroelectric oxide films are shown; acronyms correspond to the next ferroelectric compounds: PZT – Pb(Zr1–xTix)O3, PT – PbTiO3, PCT – (Pb1-xCax)TiO3 and BT - Bi4Ti3O12. Si, Gl and Pl indicate the PZT films of this work on silicon, glass and flexible polyimide, respectively.
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is considered successful if the range of temperatures used are appropriate for integration into the CMOS technology, where the resulting films should ideally display values of the switching charge density close to those required for a practical application such as NVFeRAMs (shaded area A in Figure 1).[45] A further step beyond would involve the use of annealing temperatures close to those found in oxide semiconductors[1–7] or organic ferroelectrics[20–29] (shaded area B in Figure 1). Although these are still a long way off for inorganic ferroelectrics, the challenge at this point arises from the use of annealing temperatures within the thermal stability of polymer substrates, thus allowing the deposition of ferroelectric oxide layers directly onto flexible plastics with polarization values close (or even superior) to those of the organic counterparts (shaded area B in Figure 1). Among the different low-temperature deposition techniques reported in the literature for ferroelectric oxide thin films, special attention has been paid to those based on chemical solution deposition (CSD),[32–42] some of them referred to in Figure 1. During the last years, the use of UV-irradiation has opened new paths for the low temperature processing of inorganic materials within the CSD methodology;[46] the so-called photochemical solution deposition (PCSD).[34] Also, by using seeded diphasic sol-gel (SDSG) precursors,[35] an appreciable reduction on the crystallization temperature of ferroelectric oxides has been obtained. In the present work, the “photoactive sol”, hereinafter Ph, displays an increased absorption in the UV-range (see Supporting Information, Figure S1).[34] Very thin gel layers were dip-coated from this sol, irradiated under UV-light and annealed by rapid thermal processing (RTP), using an oxygen atmosphere. Elimination of organic compounds from these photosensitive systems is known to be accelerated by the irradiation due to (i) ozonolysis and (ii) the prompt dissociation of the alkyl group – O bonds with the subsequent formation of the metal-O-metal bonds (electronic excitation).[34,46] Incorporation of seeds into the photoactive sol (“photoactive sol + seeds”, hereinafter PhS) increases the number of nucleation sites in the resulting film, which produces a further reduction of the crystallization temperature.[35,47] The mechanism proposed for the low-temperature processing of the PZT thin films of this work is shown in Figure 2 and explained in deep in Figures S1 to S3. Silicon substrates (hereinafter Si) were first used to test the validity of the method. Ferroelectric PZT films are obtained by the two approaches (Ph and PhS solutions) at low temperatures; 400 °C and 300 °C for films derived from Ph and PhS solutions, respectively. A remarkable enhancement in the formation of the ferroelectric PZT perovskite is inferred for the films prepared from the PhS solution. The large UV-absorption of the gel layer together with the crystalline nanoseeds contained in it, decrease the Gibbs free energy barrier for the perovskite nucleation, leading to a significant reduction in the crystallization temperature of the films. This made the deposition of inorganic ferroelectrics on low-melting point substrates possible, such as those tested in this work, glass (Gl) or polyimide (Pl) (Figure S3). The proof of the applicability of this method to the emerging field of flexible electronics is presented here by the direct preparation of a ferroelectric PZT film onto a polyimide substrate. But, preparation of functional films on flexible substrates is much more stringent than on semiconductor rigid substrates with atomically-flat surfaces. A conditioning treatment of the
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Figure 2. Mechanisms for the low-temperature processing of inorganic ferroelectric thin films using the activated solutions of this work. Films are derived either from a photoactive sol (Ph), or from a photoactive sol with nanocrystalline seeds (PhS). The efficiency of the activated solutions for the low-temperature processing of functional oxide layers has been demonstrated in this work for different PZT compositions; e.g., (Zr1–xTix) O3 with x = 0.48 (morphotropic phase boundary, MPB, composition) and x = 0.70 (Ti-rich composition). Functionality of the low-temperature processed thin films is proved in this figure by the measurement of ferroelectric hysteresis loops in MPB Pb(Zr0.52Ti0.48)O3 layers onto Pt-coated silicon substrates. The mechanism proposed for the low-temperature processing of functional metal oxide films is explained in deep in Figures S1 to S3.
substrate previous to the deposition of the film is mandatory (Figure 3a, see also Supporting Information, Methods). Major aims of this are (i) improve the adherence between the plastic substrate and the metal electrode, (ii) increase the smoothness of the substrate surface, (iii) decrease the shrinkage of the substrate during annealing and (iv) avoid peeling/cracking of the film during crystallization. Crystalline PZT films are formed at a temperature of 300 °C (Gl@300 °C film) (Figure 3b). But even more impressive is the ferroelectric response of these layers directly deposited onto the flexible polyimide (Pl@350 °C film) (Figures 3c and 3d): remanent polarization values of Pr ∼15 μC cm−2 (Figure 3e, compensated loop). These are of the same order than those of PZT films conventionally prepared at temperatures over 600 °C on rigid silicon substrates[15,48–52] and surpass by far the switching charge density of 8.5 μC cm−2 demanded for applications in NVFeRAMs by the international technology roadmap of semiconductors (ITRS), which sets the target of the year 2015 for achieving this value[45] (see Figure 1). The variation of the dielectric constant, K′, with voltage of these low-temperature processed films on flexible polyimide corresponds to that of a typical ferroelectric, with values of K′ ∼80 at 0 V and 1 kHz, showing a low dispersion (Figure 4a). Tangent of dielectric losses, tanδ, decreases with frequency and have a value of ∼0.03 at 1 kHz (Figure 4b).
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COMMUNICATION Figure 3. Main features of the low-temperature solution processed inorganic PZT[Pb(Zr0.30Ti0.70)O3] ferroelectric layers on flexible plastic substrates. a) Conditioning treatment of the flexible polyimide substrate. b) Crystal structure by X-ray diffraction of the PZT films on rigid glass (Gl@300 °C) and flexible polyimide substrate (Pl@350 °C); the perovskite phase appears in the patterns of the films on both substrates at a lower temperature limit of 300 °C, but the large background coming from the amorphous substrates difficults the observation of the perovskite reflections. c) Photograph of a PZT film on flexible polyimide. The film has been directly deposited on the flexible polyimide substrate from the PhS solution, UV-irradiated and crystallized at low temperatures. d) Cross-section image obtained by scanning electron microscopy of the film onto the metalized polyimide substrate. e) Ferroelectric hysteresis loop of a PZT film on flexible polyimide (Pl@350 °C) with a thickness of ∼190 nm, showing values of Pr ∼ 15μC cm−2, from the compensated loop (see insets of e); the upper inset corresponds to the compensated ferroelectric hysteresis loop, showing the switching contribution to the polarization and the lower inset corresponds to the non-switching contribution to the polarization, which comes, mainly, from capacitance and conductivity). These values are close to those reported for PZT films processed at temperatures over 600 °C, Pr ∼ 20 μC cm−2, and are over those reported for organic ferroelectric films, Pr∼10 μC cm−2, both on rigid silicon substrates (Table SII).
However, just crystallinity and ferroelectric response is not enough for high performance devices. Reliability of ferroelectric thin films should fulfill four major requirements; (i) rapid switching of the polarization, (ii) long retention (stability of the remnant polarization with time), (iii) low fatigue (reduction of polarization with the number of switching cycles) and (iv) low leakage current density (see Table FEP9 of the ITRS 2012 edition[45]). The switching of these films is fast, ∼165 ns for a capacitor area of ∼2.6 10−4 cm2 (Figure 4c). The films retain the ∼70% value of Pr over 105 s (Figure 4d) and show a low fatigue up to 107 cycles (Figure 4e). Leakage current densities are low, ∼5 10−7 A cm−2 (Figure 4f). These ferroelectric characteristics are not only superior to those of organic ferroelectrics,[20–29] but also are close to those reported for high-temperature processed oxide films (Table SII).[15,16,48–52] Piezoelectric and pyroelectric responses of these films are shown in Figure S4. Finally, it should be noted that good mechanical flexibility is essential for applications in flexible electronics.[8,53] Actually,
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this is a major fact for ferroelectric/piezoelectric thin films, since the bending of the substrate can induce the poling or de-poling of the capacitor, and this can be used for tuning the sensitivity of the active ferroelectric film and the device application.[53] In addition, this is controlled by the displacement of the mechanical neutral plane in the device with the thickness and elastic modulus of the flexible substrate and encapsulation layer.[8] Experiments are in progress with these PZT films on polyimide for evaluating the variation of Pr under different front- and backward bending conditions. Preliminary results indicate that Pr values are quite stable for bending radii up to ∼20 mm. But still, the selection of appropriate encapsulation layers of the PZT active layer, which permits to attain a neutral mechanical plane into the bulk ferroelectric film with very small bending radii, thus preserving appropriate functional properties, and fabrication of laboratory-scale experimental setups for the measurement of the ferroelectric properties in these conditions are under development.
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Figure 4. Reliability of the PZT films on flexible plastic substrates. a,b) Variation of the dielectric constant, K’, (solid symbol) and loss tangent, tan δ, (open symbol) with a DC bias voltage at 10 kHz. c) Polarization switching as a function of time and measured at different pulse voltages. d) Retention of the polarization. e) Fatigue endurance of the polarization. f) Leakage current density as a function of the applied DC voltage. Measurements shown in the figure correspond to the Ti-rich PZT [Pb(Zr0.30Ti0.70)O3] films on flexible polyimide substrates (Pl@350 °C) with a thickness of ∼190 nm. A summary of the dielectric and ferroelectric properties of these films compared with those of organic ferroelectrics and high-temperature processed PZT films on rigid substrates is shown in Table SII. Piezoelectric and pyroelectric properties are shown in Figure S4.
In conclusion, a solution deposition approach based on the synergy between photochemical solution deposition (PCSD) and seeded diphasic sol-gel (SDSG) techniques makes it possible to overcome the problems traditionally associated with the low-temperature fabrication of ferroelectric oxide thin films, in particular the absence of a ferroelectric response, thereby allowing the use of these multifunctional oxide layers in emerging electronics, such as advanced devices supported on flexible polymeric substrates. The multifunctional properties and the reliability of the low-temperature processed PZT films directly deposited on polyimide reveal inorganic ferroelectrics as alternative materials of potential applicability in flexible electronics, not only competitive to the high-temperature processed oxide and organic ferroelectrics, but also to oxide and organic semiconductors.
Experimental Section Pb(Zr1-x,Tix)O3 (PZT) sols, with MPB (Pb(Zr0.52Ti0.48)O3) and Ti-rich (Pb(Zr0.30Ti0.70)O3) compositions, were prepared by sol-gel synthesis.[34–36] This sol is denoted “photoactive sol, Ph”. The seeded diphasic photoactive PZT precursor, “photoactive sol + seeds, PhS”, was prepared by adding a suspension of nanometric PZT powders to the
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former sol[35] (Figure S1). The precursors were dip-coated onto Pt-coated Si single crystals (Si), metalized glass (Gl) and flexible polyimide substrates (Pl) (see Supporting Information, Methods). The plastic substrates were subjected to a conditioning treatment summarized in Figure 3a and detailed in the Supporting Information. The deposited layers were dried on hot-plate (150 °C/600s). Dip-coating and drying were repeated several times (2-4) for each sample. UV-irradiation of these layers was carried out at 250 °C/3600s in a laboratory-scale equipment with a high-intensity UV excimer lamp (BlueLight Heraeus Excimer System, wavelength of 222 nm). The irradiated films were thermally treated by rapid thermal processing (RTP, JIPELEC JetStar 100T Processor) with a soaking time of 3600 s. Irradiation and RTP were carried out in 1 bar oxygen atmosphere. Longer RTP treatments (5 h) were tested for films crystallized at temperatures below 400 °C. The films obtained are denoted Si@T °C, Gl@T °C and Pl@T °C, where Si, Gl and Pl are the substrate and T is the processing temperature. The temperature during irradiation and RTP was controlled by the internal thermocouples of the equipments. External thermocouples were also used to test the working temperature. Coatings onto the substrate from OMEGALAQ Liquid Temperature Lacquers with melting points of 371 °C and 399 °C were also treated out in the irradiation and RTP systems. These studies indicated that the variation in temperature during processing was ±5 °C. Crystalline films with thickness between 70 and 200 nm were obtained, as measured from cross-section scanning electron microscopy (SEM) images and/or profilemetry. Planar capacitors were obtained by depositing top Pt electrodes of ∼3 × 10−8 m2 onto the film surfaces. Properties and reliability of the films were studied in these capacitors by experimental measurements detailed in the Supporting Information, Methods.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements This work was financed by Spanish Project MAT2010-15365 and Portuguese Project PTDC/CTM/108319/2008 together with FCT and FEDER support. The Spanish-Portuguese bilateral project PRIAIBPT-2011-0841 has also contributed to this work. I. B. acknowledges the support of the Juan de la Cierva Spanish program. We are indebted to UBE Europe GmbH for the kindly supply of UPILEX 75S/25S high temperature polyimide films. Received: August 27, 2013 Revised: October 21, 2013 Published online: December 12, 2013
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Iñigo Bretos, Ricardo Jiménez, Aiying Wu, Angus I. Kingon, Paula M. Vilarinho,* and M. Lourdes Calzada* To the memory of Dr. Aiying Wu A novel solution method is developed that enables the processing of functional oxides under low-temperature conditions so that direct-large-area integration of active layers with flexible electronics is turned into reality. We demonstrate the concept on the most important multifunctional oxide, lead zirconate titanate. It reaches the lower limit temperature of crystallization at 300 °C, using a strategy that combines seeded diphasic precursors and photochemical solution deposition. Properties of these ferroelectric layers on flexible plastic fulfill the major requirements demanded for devices, showing a wider temperature range of applicability and functionality than those of the high-K dielectrics or organic ferroelectrics. This is a platform that can be used for many other functional oxide layers. In the case of the most widely used inorganic materials in electronics, i.e., semiconductors, significant efforts are being devoted to their low-temperature fabrication for a successful integration with flexible electronics.[1–6] Thus, thin-film transistors (TFTs) have been prepared directly onto polymers by solution methods, as amorphous,[7] or nanocrystalline[8] dielectric oxide layers at exceptionally low temperatures. But now the need for the direct integration of other active layers on polymers is mandatory to increase functionality of the flexible device. This is a major opportunity for ferroelectric oxide thin films, since their intrinsic multifunctionality (ferro-, pyro-, piezoelectricity) would allow diverse operations in electronic devices such as memories, sensors, actuators, transducers or detectors, making real applications not possible before (e.g., smart skin, flexible sensitive displays, photovoltaic cells or eye-type imagers).[8–14] But, inorganic ferroelectrics require high temperatures to obtain the needed crystalline phase[15,16] that exceed by far the thermal stability of plastics. This could be overcome with organic ferroelectrics that display much lower processing temperatures. However, they present fabrication and performance limitations (see Supporting Information, Table SI);[17–30] Dr. I. Bretos, Dr. R. Jiménez, Prof. M. L. Calzada Instituto de Ciencia de Materiales de Madrid (ICMM-CSIC) Cantoblanco, 28049, Madrid, Spain E-mail: [email protected] Dr. A. Wu, Prof. P. M. Vilarinho Departament of Ceramics and Glass Engineering CICECO, University of Aveiro, 3810-193, Aveiro, Portugal E-mail: [email protected] Prof. A. I. Kingon School of Engineering Brown University Providence, RI, 02912, USA
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Activated Solutions Enabling Low-Temperature Processing of Functional Ferroelectric Oxides for Flexible Electronics
e.g., (i) narrow temperature range of applicability, (ii) low dielectric constant, (iii) high coercive fields or (iv) long switching times. Among the inorganic ferroelectrics, the lead zirconate titanate (Pb(Zr1–xTix)O3, PZT) is the one with the greatest commercial applications.[16] Therefore, the fabrication of PZT ferroelectric thin films at temperatures compatible with polymeric substrates would be of high technological interest. According to the literature (Figure 1), a low-temperature processing method for inorganic ferroelectric thin films,[30–44]
Figure 1. Key relevant challenges for the integration of ferroelectric oxide films with the Si-technology and flexible electronics. Shaded area (A) falls within the range of temperatures required for film integration into CMOS technology, with the switching charge densities demanded today for applications in NVFeRAMs (over 8.5 μC cm−2 until 2015).[45] Shaded area (B) denotes a region merging the thermal stability of available plastic substrates with polarization values usually found for organic ferroelectrics on rigid substrates.[20–29] Oxide dielectric semiconductors prepared at low temperatures[1–7] are positioned in the figure. Dielectric permittivities, K′, can be compared in this case; average values of K′ at room temperature of 15, 50 and 100 are reported in the literature for oxide semiconductors, organic ferroelectrics and high-temperature processed inorganic ferroelectrics, respectively. K′ values of ∼80 have been measured in the inorganic ferroelectric layers of this work, Table SII). A summary of the most relevant low-temperature methods reported for some ferroelectric oxide films are shown; acronyms correspond to the next ferroelectric compounds: PZT – Pb(Zr1–xTix)O3, PT – PbTiO3, PCT – (Pb1-xCax)TiO3 and BT - Bi4Ti3O12. Si, Gl and Pl indicate the PZT films of this work on silicon, glass and flexible polyimide, respectively.
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is considered successful if the range of temperatures used are appropriate for integration into the CMOS technology, where the resulting films should ideally display values of the switching charge density close to those required for a practical application such as NVFeRAMs (shaded area A in Figure 1).[45] A further step beyond would involve the use of annealing temperatures close to those found in oxide semiconductors[1–7] or organic ferroelectrics[20–29] (shaded area B in Figure 1). Although these are still a long way off for inorganic ferroelectrics, the challenge at this point arises from the use of annealing temperatures within the thermal stability of polymer substrates, thus allowing the deposition of ferroelectric oxide layers directly onto flexible plastics with polarization values close (or even superior) to those of the organic counterparts (shaded area B in Figure 1). Among the different low-temperature deposition techniques reported in the literature for ferroelectric oxide thin films, special attention has been paid to those based on chemical solution deposition (CSD),[32–42] some of them referred to in Figure 1. During the last years, the use of UV-irradiation has opened new paths for the low temperature processing of inorganic materials within the CSD methodology;[46] the so-called photochemical solution deposition (PCSD).[34] Also, by using seeded diphasic sol-gel (SDSG) precursors,[35] an appreciable reduction on the crystallization temperature of ferroelectric oxides has been obtained. In the present work, the “photoactive sol”, hereinafter Ph, displays an increased absorption in the UV-range (see Supporting Information, Figure S1).[34] Very thin gel layers were dip-coated from this sol, irradiated under UV-light and annealed by rapid thermal processing (RTP), using an oxygen atmosphere. Elimination of organic compounds from these photosensitive systems is known to be accelerated by the irradiation due to (i) ozonolysis and (ii) the prompt dissociation of the alkyl group – O bonds with the subsequent formation of the metal-O-metal bonds (electronic excitation).[34,46] Incorporation of seeds into the photoactive sol (“photoactive sol + seeds”, hereinafter PhS) increases the number of nucleation sites in the resulting film, which produces a further reduction of the crystallization temperature.[35,47] The mechanism proposed for the low-temperature processing of the PZT thin films of this work is shown in Figure 2 and explained in deep in Figures S1 to S3. Silicon substrates (hereinafter Si) were first used to test the validity of the method. Ferroelectric PZT films are obtained by the two approaches (Ph and PhS solutions) at low temperatures; 400 °C and 300 °C for films derived from Ph and PhS solutions, respectively. A remarkable enhancement in the formation of the ferroelectric PZT perovskite is inferred for the films prepared from the PhS solution. The large UV-absorption of the gel layer together with the crystalline nanoseeds contained in it, decrease the Gibbs free energy barrier for the perovskite nucleation, leading to a significant reduction in the crystallization temperature of the films. This made the deposition of inorganic ferroelectrics on low-melting point substrates possible, such as those tested in this work, glass (Gl) or polyimide (Pl) (Figure S3). The proof of the applicability of this method to the emerging field of flexible electronics is presented here by the direct preparation of a ferroelectric PZT film onto a polyimide substrate. But, preparation of functional films on flexible substrates is much more stringent than on semiconductor rigid substrates with atomically-flat surfaces. A conditioning treatment of the
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Figure 2. Mechanisms for the low-temperature processing of inorganic ferroelectric thin films using the activated solutions of this work. Films are derived either from a photoactive sol (Ph), or from a photoactive sol with nanocrystalline seeds (PhS). The efficiency of the activated solutions for the low-temperature processing of functional oxide layers has been demonstrated in this work for different PZT compositions; e.g., (Zr1–xTix) O3 with x = 0.48 (morphotropic phase boundary, MPB, composition) and x = 0.70 (Ti-rich composition). Functionality of the low-temperature processed thin films is proved in this figure by the measurement of ferroelectric hysteresis loops in MPB Pb(Zr0.52Ti0.48)O3 layers onto Pt-coated silicon substrates. The mechanism proposed for the low-temperature processing of functional metal oxide films is explained in deep in Figures S1 to S3.
substrate previous to the deposition of the film is mandatory (Figure 3a, see also Supporting Information, Methods). Major aims of this are (i) improve the adherence between the plastic substrate and the metal electrode, (ii) increase the smoothness of the substrate surface, (iii) decrease the shrinkage of the substrate during annealing and (iv) avoid peeling/cracking of the film during crystallization. Crystalline PZT films are formed at a temperature of 300 °C (Gl@300 °C film) (Figure 3b). But even more impressive is the ferroelectric response of these layers directly deposited onto the flexible polyimide (Pl@350 °C film) (Figures 3c and 3d): remanent polarization values of Pr ∼15 μC cm−2 (Figure 3e, compensated loop). These are of the same order than those of PZT films conventionally prepared at temperatures over 600 °C on rigid silicon substrates[15,48–52] and surpass by far the switching charge density of 8.5 μC cm−2 demanded for applications in NVFeRAMs by the international technology roadmap of semiconductors (ITRS), which sets the target of the year 2015 for achieving this value[45] (see Figure 1). The variation of the dielectric constant, K′, with voltage of these low-temperature processed films on flexible polyimide corresponds to that of a typical ferroelectric, with values of K′ ∼80 at 0 V and 1 kHz, showing a low dispersion (Figure 4a). Tangent of dielectric losses, tanδ, decreases with frequency and have a value of ∼0.03 at 1 kHz (Figure 4b).
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COMMUNICATION Figure 3. Main features of the low-temperature solution processed inorganic PZT[Pb(Zr0.30Ti0.70)O3] ferroelectric layers on flexible plastic substrates. a) Conditioning treatment of the flexible polyimide substrate. b) Crystal structure by X-ray diffraction of the PZT films on rigid glass (Gl@300 °C) and flexible polyimide substrate (Pl@350 °C); the perovskite phase appears in the patterns of the films on both substrates at a lower temperature limit of 300 °C, but the large background coming from the amorphous substrates difficults the observation of the perovskite reflections. c) Photograph of a PZT film on flexible polyimide. The film has been directly deposited on the flexible polyimide substrate from the PhS solution, UV-irradiated and crystallized at low temperatures. d) Cross-section image obtained by scanning electron microscopy of the film onto the metalized polyimide substrate. e) Ferroelectric hysteresis loop of a PZT film on flexible polyimide (Pl@350 °C) with a thickness of ∼190 nm, showing values of Pr ∼ 15μC cm−2, from the compensated loop (see insets of e); the upper inset corresponds to the compensated ferroelectric hysteresis loop, showing the switching contribution to the polarization and the lower inset corresponds to the non-switching contribution to the polarization, which comes, mainly, from capacitance and conductivity). These values are close to those reported for PZT films processed at temperatures over 600 °C, Pr ∼ 20 μC cm−2, and are over those reported for organic ferroelectric films, Pr∼10 μC cm−2, both on rigid silicon substrates (Table SII).
However, just crystallinity and ferroelectric response is not enough for high performance devices. Reliability of ferroelectric thin films should fulfill four major requirements; (i) rapid switching of the polarization, (ii) long retention (stability of the remnant polarization with time), (iii) low fatigue (reduction of polarization with the number of switching cycles) and (iv) low leakage current density (see Table FEP9 of the ITRS 2012 edition[45]). The switching of these films is fast, ∼165 ns for a capacitor area of ∼2.6 10−4 cm2 (Figure 4c). The films retain the ∼70% value of Pr over 105 s (Figure 4d) and show a low fatigue up to 107 cycles (Figure 4e). Leakage current densities are low, ∼5 10−7 A cm−2 (Figure 4f). These ferroelectric characteristics are not only superior to those of organic ferroelectrics,[20–29] but also are close to those reported for high-temperature processed oxide films (Table SII).[15,16,48–52] Piezoelectric and pyroelectric responses of these films are shown in Figure S4. Finally, it should be noted that good mechanical flexibility is essential for applications in flexible electronics.[8,53] Actually,
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this is a major fact for ferroelectric/piezoelectric thin films, since the bending of the substrate can induce the poling or de-poling of the capacitor, and this can be used for tuning the sensitivity of the active ferroelectric film and the device application.[53] In addition, this is controlled by the displacement of the mechanical neutral plane in the device with the thickness and elastic modulus of the flexible substrate and encapsulation layer.[8] Experiments are in progress with these PZT films on polyimide for evaluating the variation of Pr under different front- and backward bending conditions. Preliminary results indicate that Pr values are quite stable for bending radii up to ∼20 mm. But still, the selection of appropriate encapsulation layers of the PZT active layer, which permits to attain a neutral mechanical plane into the bulk ferroelectric film with very small bending radii, thus preserving appropriate functional properties, and fabrication of laboratory-scale experimental setups for the measurement of the ferroelectric properties in these conditions are under development.
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Figure 4. Reliability of the PZT films on flexible plastic substrates. a,b) Variation of the dielectric constant, K’, (solid symbol) and loss tangent, tan δ, (open symbol) with a DC bias voltage at 10 kHz. c) Polarization switching as a function of time and measured at different pulse voltages. d) Retention of the polarization. e) Fatigue endurance of the polarization. f) Leakage current density as a function of the applied DC voltage. Measurements shown in the figure correspond to the Ti-rich PZT [Pb(Zr0.30Ti0.70)O3] films on flexible polyimide substrates (Pl@350 °C) with a thickness of ∼190 nm. A summary of the dielectric and ferroelectric properties of these films compared with those of organic ferroelectrics and high-temperature processed PZT films on rigid substrates is shown in Table SII. Piezoelectric and pyroelectric properties are shown in Figure S4.
In conclusion, a solution deposition approach based on the synergy between photochemical solution deposition (PCSD) and seeded diphasic sol-gel (SDSG) techniques makes it possible to overcome the problems traditionally associated with the low-temperature fabrication of ferroelectric oxide thin films, in particular the absence of a ferroelectric response, thereby allowing the use of these multifunctional oxide layers in emerging electronics, such as advanced devices supported on flexible polymeric substrates. The multifunctional properties and the reliability of the low-temperature processed PZT films directly deposited on polyimide reveal inorganic ferroelectrics as alternative materials of potential applicability in flexible electronics, not only competitive to the high-temperature processed oxide and organic ferroelectrics, but also to oxide and organic semiconductors.
Experimental Section Pb(Zr1-x,Tix)O3 (PZT) sols, with MPB (Pb(Zr0.52Ti0.48)O3) and Ti-rich (Pb(Zr0.30Ti0.70)O3) compositions, were prepared by sol-gel synthesis.[34–36] This sol is denoted “photoactive sol, Ph”. The seeded diphasic photoactive PZT precursor, “photoactive sol + seeds, PhS”, was prepared by adding a suspension of nanometric PZT powders to the
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former sol[35] (Figure S1). The precursors were dip-coated onto Pt-coated Si single crystals (Si), metalized glass (Gl) and flexible polyimide substrates (Pl) (see Supporting Information, Methods). The plastic substrates were subjected to a conditioning treatment summarized in Figure 3a and detailed in the Supporting Information. The deposited layers were dried on hot-plate (150 °C/600s). Dip-coating and drying were repeated several times (2-4) for each sample. UV-irradiation of these layers was carried out at 250 °C/3600s in a laboratory-scale equipment with a high-intensity UV excimer lamp (BlueLight Heraeus Excimer System, wavelength of 222 nm). The irradiated films were thermally treated by rapid thermal processing (RTP, JIPELEC JetStar 100T Processor) with a soaking time of 3600 s. Irradiation and RTP were carried out in 1 bar oxygen atmosphere. Longer RTP treatments (5 h) were tested for films crystallized at temperatures below 400 °C. The films obtained are denoted Si@T °C, Gl@T °C and Pl@T °C, where Si, Gl and Pl are the substrate and T is the processing temperature. The temperature during irradiation and RTP was controlled by the internal thermocouples of the equipments. External thermocouples were also used to test the working temperature. Coatings onto the substrate from OMEGALAQ Liquid Temperature Lacquers with melting points of 371 °C and 399 °C were also treated out in the irradiation and RTP systems. These studies indicated that the variation in temperature during processing was ±5 °C. Crystalline films with thickness between 70 and 200 nm were obtained, as measured from cross-section scanning electron microscopy (SEM) images and/or profilemetry. Planar capacitors were obtained by depositing top Pt electrodes of ∼3 × 10−8 m2 onto the film surfaces. Properties and reliability of the films were studied in these capacitors by experimental measurements detailed in the Supporting Information, Methods.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements This work was financed by Spanish Project MAT2010-15365 and Portuguese Project PTDC/CTM/108319/2008 together with FCT and FEDER support. The Spanish-Portuguese bilateral project PRIAIBPT-2011-0841 has also contributed to this work. I. B. acknowledges the support of the Juan de la Cierva Spanish program. We are indebted to UBE Europe GmbH for the kindly supply of UPILEX 75S/25S high temperature polyimide films. Received: August 27, 2013 Revised: October 21, 2013 Published online: December 12, 2013
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