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Synthesis, Microstructure and Properties of Magnetron Sputtered Lead Zirconate Titanate (PZT) Thin Film Coatings

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Synthesis, Microstructure and Properties of Magnetron Sputtered Lead Zirconate Titanate (PZT) Thin Film Coatings coatings Review Synthesis, Microstructure and Properties of Magnetron Sputtered Lead Zirconate Titanate (PZT) Thin Film Coatings Youcao Ma 1 , Jian Song 2, Xubo Wang 1 , Yue Liu 2,* and Jia Zhou 1,* 1 State Key Laboratory of ASIC and System, School of Microelectronics, Fudan University, Shanghai 200433, China; [email protected] (Y.M.); [email protected] (X.W.) 2 State Key Laboratory of Metal Matrix Composites, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China; [email protected] * Correspondence: [email protected] (Y.L.); [email protected] (J.Z.) Abstract: Compared to aluminum nitride (AlN) with simple stoichiometry, lead zirconate titanate thin films (PZT) are the other promising candidate in advanced micro-electro-mechanical system (MEMS) devices due to their excellent piezoelectric and dielectric properties. The fabrication of PZT thin films with a large area is challenging but in urgent demand. Therefore, it is necessary to establish the relationships between synthesis parameters and specific properties. Compared to sol-gel and pulsed laser deposition techniques, this review highlights a magnetron sputtering technique owing to its high feasibility and controllability. In this review, we survey the microstructural characteristics of PZT thin films, as well as synthesis parameters (such as substrate, deposition temperature, gas atmosphere, and annealing temperature, etc.) and functional proper-ties (such as dielectric, piezoelectric, and ferroelectric, etc). The dependence of these influential factors is particularly emphasized in this review, which could provide experimental guidance for researchers to acquire PZT thin films with Citation: Ma, Y.; Song, J.; Wang, X.; expected properties by a magnetron sputtering technique. Liu, Y.; Zhou, J. Synthesis, Microstructure and Properties of Keywords: PZT thin films; magnetron sputtering; fabrication; microstructure; properties Magnetron Sputtered Lead Zirconate Titanate (PZT) Thin Film Coatings. Coatings 2021, 11, 944. https:// doi.org/10.3390/coatings11080944 1. Introduction Lead zirconate titanate Pb(ZrxTi1−x)O3 (PZT) ceramics has been widely employed in Academic Editors: Alessio Lamperti sensors, actuators, and transducers due to higher piezoelectric constant (compared with and Roman A. Surmenev PbTiO3, AlN), higher Curie temperature (compared with BaTiO3), and excellent tempera- ture stability (compared with NaxK − NbO )[1–5]. However, high-frequency ultrasound Received: 14 June 2021 1 x 3 transducer arrays for high resolution require thin piezoelectric ceramics which are difficult Accepted: 4 August 2021 Published: 7 August 2021 to manufacture and assemble with low cost and high efficiency [6,7]. PZT thin films could achieve these requirements and have the advantages of low volume and power consump- Publisher’s Note: MDPI stays neutral tion [8]. Therefore, PZT has been ranked as the most important piezoelectric thin film with regard to jurisdictional claims in coating (another one is aluminum nitride, AlN). Owing to its complex stoichiometry, the published maps and institutional affil- properties of PZT thin films (such as dielectricity, piezoelectricity, ferroelectricity) could be iations. tailored by controlling fabrication parameters. The characteristics of adjustable properties enable PZT thin films to be applied in a wider range of advanced micro-electric devices, including surface acoustic wave (SAW) devices, sensors, energy harvests, dynamic random- access memories (DRAMs), nonvolatile random-access memories (NVRAMs), transducers and so on [9–12]. Therefore, the focus of PZT research is directed towards synthesizing Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. thin films with desired properties by the optimization of fabrication parameters. This article is an open access article Among PZT thin films fabrication techniques, sol-gel, magnetron sputtering and distributed under the terms and pulsed laser deposition (PLD) are commonly used and categorized as a chemical method conditions of the Creative Commons (the first one) and a physical method (the latter two). The comparison of these three Attribution (CC BY) license (https:// common methods is summarized in Table1. The sol-gel (also called chemical solution creativecommons.org/licenses/by/ deposition, CSD) method is based on the reactions of precursor solutions and the schematic 4.0/). process is shown in Figure1a [ 13]. First, precursor solutions with proper stoichiometry of Coatings 2021, 11, 944. https://doi.org/10.3390/coatings11080944 https://www.mdpi.com/journal/coatings Coatings 2021, 11, x FOR PEER REVIEW 2 of 23 common methods is summarized in Table 1. The sol-gel (also called chemical solution deposition, CSD) method is based on the reactions of precursor solutions and the sche- Coatings 2021, 11, 944 2 of 22 matic process is shown in Figure 1a [13]. First, precursor solutions with proper stoichi- ometry of metal ions were prepared while Pb was usually added in excess of 10%−20% to compensate the volatilization. Then, the solutions were dispersed on the surface of the substratemetal ions by werespin- preparedcoating method. while Pb Finally, was usually the sample added inwas excess annealed of 10% at− 20%a high to compensatetempera- turethe to volatilization. convert the solutions Then, the to solutionssolid oxide were films. dispersed This method on the is surfaceadvantage of theous substratedue to its by simplicity,spin-coating low method.cost, easy Finally, composition the sample changes was (Zr/Ti annealed ratio at and a high element temperature doping). to How- convert ever,the the solutions chemical to solidreactions oxide are films. hard Thisto control, method resulting is advantageous in the limited due to composition its simplicity, uniformitylow cost, and easy residual composition impurity changes that (Zr/Tilead to ratioproperties and element degradation. doping). Moreover However,, PZT the thinchemical films must reactions be deposited are hard layer to control, by layer resulting when inthe the total limited thickness composition exceeds uniformity100~200 nm, and leadingresidual to impurityrelatively that poor lead interface to properties bonding. degradation. Therefore, Moreover, physical PZTdeposition thin films methods must be havedeposited been introduced layer by layer to fabricate when the PZT total thin thickness films due exceeds to their 100~200 contro nm,llable leading stoichiometric to relatively compositionpoor interface and bonding.robust interface Therefore, bonding. physical deposition methods have been introduced to fabricate PZT thin films due to their controllable stoichiometric composition and robust Tableinterface 1. Comparisons bonding. of sol-gel, pulsed laser deposition (PLD) and magnetron sputtering tech- nique for the fabrication of PZT thin films. Table 1. Comparisons of sol-gel, pulsed laser deposition (PLD) and magnetron sputtering technique forType the fabrication Basic of PZT Process thin films. Advantages Disadvantages Cost: low Crystallinity: low Sol-gel Precursor solutions; Type Basic ProcessSubstrate: large Advantages Interface: Disadvantages weak (Chemical Spin-coating; Cost: low Crystallinity: low Precursor solutions;Thickness: thick Uniformity: low process)Sol-gel Annealing Substrate: large Interface: weak Spin-coating;Doping: convenient Impurity: yes (Chemical process) Thickness: thick Uniformity: low AnnealingCrystallinity: high Cost: high PLD Laser generation; Doping: convenient Impurity: yes Interface: strong Substrate: small (Physical Heat evaporation; Crystallinity: high Cost: high Laser generation; PLD Uniformity: Interface:high strong Thickness:Substrate: thin small process) Vapor depositionHeat evaporation; (Physical process) Impurity:Uniformity: no highDoping:Thickness: complex thin Vapor deposition Cost: low (industry)Impurity: no Doping: complex Magnetron Substrate:Cost: large low (industry) Ar Ionization; Doping: complex Substrate: large sputtering Ar Ionization;Crystallinity: high Doping: complex Magnetron sputteringBombardment; Crystallinity: highCost: high (research) (Physical Bombardment;Interface: strong Cost: high (research) (Physical process)Vapor deposition Interface: strongThickness: medium Vapor deposition Thickness: medium process) Uniformity:Uniformity: high high Impurity: noImpurity: no Figure 1. Schematic process of different fabrication methods for PZT thin films; (a) sol-gel including Figurespin-coating 1. Schematic and annealingprocess of baseddifferent on chemicalfabrication reactions. methods [13 for]; (PZTb) pulsed thin films; laser deposition(a) sol-gel includ- based on ingphysical spin-coating interactions and annealing by laser based heating. on Reprintedchemical reactions with permission. [13]; (b from) pulsed [14] laser Copyright deposition 2010 based Elsevier; on( cphysical) magnetron interactions sputtering by techniquelaser heating based. Reprinted on physical with interactions permission by ionfrom bombardment. [14] Copyright Reprinted 2010 Elsevier; (c) magnetron sputtering technique based on physical interactions by ion bombardment. with permission from [11] Copyright 2018 John Wiley and Sons. Reprinted with permission from [11] Copyright 2018 John Wiley and Sons . PLD and magnetron sputtering technique are two typical physical vapor deposition methodsPLD and for magnetron PZT thin films. sputtering The diagrammatic technique are sketch two oftypical PLD isphysical displayed vapor in Figuredeposition1b and methodsthe deposition for PZT is thin
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