Solution Deposition of Ferroelectric Thin Films
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MRS BULLETIN/JUNE 1996
ing reagents such as alkoxide (M(OR)n where M is a metal and R is an alkyl group), carboxylate (M(OOCR),,) and J3diketonate (MO,(CH3COCHCOCH3),,) compounds of lead, zirconium, and titanium are employed. The most frequently used solution-preparation approaches may be grouped into three categories: (1) sol-gel processes that use 2-methoxyethanol as a reactant and solvent, 910 (2) hybrid processes that use chelating agents such as acetic acid 1112 or diethanolamine to reduce alkoxide reactivity, and (3) metalorganic decomposition (MOD) approaches that use large, waterinsensitive carboxylate compounds.1314 During solution preparation, the chemical interactions that occur between the starting reagents will depend on the characteristics of the compound, primarily its reactivity and solution-preparation conditions, such as the reflux temperature. For the long-chain carboxylate compounds used in MOD processing, such as zirconium ethylhexanoate and titanium di-methoxy di-neodecanoate, precursor reactivity is low, and chemical interactions between the species are minimal. In contrast, in sol-gel processes that use alkoxide compounds, the reactivity of the starting reagents is high, and the structure of the species formed in solution can bear little resemblance to those of the starting compounds. In these instances, the species that are formed are frequently oligomeric in nature, and may contain more than one type of cation. The nature of the precursor species formed in these reactions can impact film shrinkage, crystallization temperature, and microstructure. A typical flow diagram for solution synthesis and film deposition is illustrated in Figure 1. Here processing variables are shown on the left, material states and characteristics at different pro-
cess points are indicated in the boxes, and processing behavior is indicated on the right. The three classes of solutionpreparation approaches all have in common the synthesis of a precursor solution that is used to deposit an amorphous thin film, which is subsequently crystallized into the desired perovskite phase by heat treatment. However, the modifications in starting compound characteristics that occur during solution synthesis can have substantial effects on film processing since they affect the "structural evolution" of the film from the asdeposited to the crystalline state by impacting processes such as condensation, gelation, and densification. Even though it has proved difficult to elucidate the mechanisms by which precursor properties affect film microstructure, solution-deposition routes have been developed to the extent that PZT thin films with uniform microstructures and good ferroelectric properties have been reproducibly prepared by numerous research groups (see Figure 2). 2-Methoxyethanol Solution Synthesis The 2-methoxyethanol process11'10 is considered a sol-gel process since alkoxide starting reagents are used and the key reactions leading to the formation of the precursor species are hydrolysis and condensation, whereby metal-oxyge
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