In situ growth of lead zirconate titanate thin films by hybrid process: sol–gel method and pulsed-laser deposition
Introduction
Lead zirconate titanate (Pb(ZrxTi1 − x)O3, PZT) thin films have attracted great interest, because of their excellent ferroelectric, pyroelectric and piezoelectric properties. A large number of potential applications of PZT thin films have been reported, e.g. in nonvolatile ferroelectric random access memories (FeRAMs) [1], microelectro-mechanical systems (MEMS) [2], such as membrane-type micropumps [3], atomic force microscopy (AFM) cantilevers [4], [5] and microscanning mirror devices [6]. PZT films have been studied for over a decade, mostly in relation to FeRAMs for which the films are less than 0.5 μm in thickness [7]. However, for use in AFM cantilevers and microscanning mirror devices, a high-quality 3-μm-thick PZT film on an electrode/substrate is desirable to obtain a larger displacement [5], [6]. Many fabrication techniques, such as the sol–gel method, pulsed-laser deposition (PLD) and jet print deposition, have been used to fabricate PZT films for microactuators [8]. However, it is not easy to obtain thick PZT films with good electrical properties due to crack formation and the volatility of lead and lead oxide in films. In our previous work, we successfully fabricated crack-free PZT films of 3 μm thickness on Pt/Ti/SiO2/Si substrates using the sol–gel method [9]. However, a deposition of 3 μm requires more than 20 repetitions of the coating and firing procedures, which increases the risk of contamination and leads to Pb diffusion between the PZT layer and Pt/Ti bottom electrodes [10]. PLD is a promising technique for thick-PZT-film fabrication, because it offers the advantage of a high deposition rate (approximately 3 μm/h). In PLD, films can be crystallized by two different processing methods; namely, they can be crystallized in situ as they are deposited on hot substrates, or they can be deposited amorphously at lower substrate temperatures (or room temperature) and then crystallized by postdepositional annealing. To obtain well-crystallized PZT films on Pt/Ti/SiO2/Si substrates, substrate temperatures (in the case of in situ deposition) or postannealing temperatures (in the case of films deposited at room temperature) in the range 500–750 °C are required [11], [12], [13], [14], [26]. In our previous work, we successfully fabricated crack-free PZT films of 2 μm thickness on Pt/Ti/SiO2/Si substrates by PLD at room temperature, which were crystallized by subsequent annealing at 750 °C [15]. However, the temperature of postdeposition annealing in PLD is 750 °C, which is significantly higher than that used in the sol–gel method (600 °C). Thus the evaporation of lead and lead oxide from the surface of films at elevated temperatures is more acute than that in the case of the sol–gel process and leads to the formation of a thin pyrochlore layer on the surface of PZT films [16]. A hybrid process consisting of the sol–gel method and PLD is effective for obtaining thick PZT films with good properties, because it integrates the advantages of the sol–gel method and PLD [17]. In our previous work, we found that the temperature of postdeposition annealing in the hybrid process is lower than that in the case of direct film deposition by PLD on a Pt/Ti/SiO2/Si substrate, and the preferred orientation of the film obtained by the hybrid process can be controlled using the layer deposited by the sol–gel method. PZT films fabricated by the hybrid process exhibit better ferroelectric properties than those directly deposited by the PLD process [18]. However, in the hybrid process, PLD is performed at room temperature and the postprocess of amorphously deposited PZT films are still required, so that an in situ deposition technique is also desirable. Hence, in this study, PZT thin films were grown in situ on Pt/Ti/SiO2/Si substrates by the hybrid process consisting of the sol–gel method and PLD. The crystalline phases and preferred orientations, as well as the microstructures of PZT films were investigated. On the basis of the experimental results, the mechanism of the in situ growth of PZT films in the hybrid process is discussed.
Section snippets
Experimental details
Pt/Ti/SiO2/Si substrates were prepared by sputtering 0.05 μm of titanium and 0.15 μm of platinum on oxidized (1.8 μm of SiO2) silicon substrates.
First, a thin PZT layer was deposited on Pt/Ti/SiO2/Si(1 0 0) substrates using the sol–gel method. The precursor solution was prepared from lead acetate [Pb(CH3COO)2], zirconium–n-propoxide [Zr(C3H7O)4] and titanium tetraisopropoxide [Ti((CH3)2CHO)4]. 2-Propanol [(CH3)2CHOH] was used as the solvent. The solution composition was controlled at a ratio of
Crystalline phases and preferred orientations of PZT films
Fig. 1(a) shows the XRD pattern of the PZT layer deposited by the sol–gel method, which was dried at 400 °C for 20 min and then finally annealed at 600 °C for 30 min. The PZT layer mainly consists of the perovskite phase with a (1 0 0)-preferred orientation, and no peak of the pyrochlore phase was detected in the PZT layer. Fig. 2(a) shows the XRD pattern of the PZT layer deposited by the sol–gel method, which was dried at 250 °C for 20 min and then finally annealed at 600 °C for 30 min. The PZT
Conclusions
PZT thin films were grown in situ on Pt/Ti/SiO2/Si substrates at a substrate temperature of 460 °C by the hybrid process consisting of the sol–gel method and PLD. The crystalline structure and preferred orientation of the PZT films were examined by XRD analysis. The microstructures of the films were studied by TEM. XRD analysis indicated that the preferred orientation of PZT films could be controlled using the layer deposited by the sol–gel method. TEM images showed that the film had a
Acknowledgments
The authors thank Professor Tadao Watanabe for helpful advice, and Dr. L. J. Yan for assistance in the preparation of TEM samples. The support to this work by a Grant-in-Aid for Scientific Research (C) (No. 16560624) from the Japanese Ministry of Education, Culture, Sports, Science and Technology is gratefully acknowledged.
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