Densification and Evolution of Stress Development in Solution Derived PbZr 0.53 Ti 0.47 O 3 Thin Layers
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f stress from tensile, to progressively more compressive, on cooling. Although many studies have proved the stress effects and importance of film stress control in ferroelectric films, studies that have produced a fundamental understanding of stress development on heat treatment are very limited. Thus, in this paper the evolution of stress development for multideposited PZT thin layers (multilayers) on thermal cycling is presented with emphasis on consolidation behavior. Also, densification characteristics for the layers produced by different thermal processing methods are examined. EXPERIMENT Precursor solutions of PZT were prepared following the method described in the work of Lakeman [7], which was basically developed by Budd et al. [8]. Specimens for stress experiments were prepared by spin coating the sol onto 3 in. dia silicon (100) substrates. To improve increased sensitivity to bending stress, thin (7511m) wafers were selected. Uniformity was achieved by allowing the sol to coat the entire wafer at low rpm before slowly increasing the spin speed to 3000rpm for 50s. Stress was measured for an as-spun layer upon thermal cycling at 650"C, then after depositing two additional layers on the already densified coating, stress changes in a 3-layered coating were also examined. This process was repeated until the stress response in a total of nine layers was monitored. For the mutideposited coatings, pyrolysis was carried out at 300'C between each deposition. The stress in PZT multilayer was compared with the stress in lead titanate (PT) layers, whose preparation method is shown in the literature [6]. Stress was determined using a Tencor FLX-2980 equipped with a laser reflectance measurement system. Details on stress measurement are described in the work of Sengupta et al. [6]. The variation of film thickness on silicon substrates at a wide range of temperatures was monitored using in situ ellipsometry (Gaertner Ll16C, X•6328A) for single layers. For multilayers, a field emission scanning electron microscope (FEG-SEM, Hitachi S-4200) and a profilometer (Tensor Alpha Step 500) were employed. The influence of thermal processes on the densification behavior was investigated by measuring the thickness of the films prepared under different heating schedules; 450'C-lhr, then 650"C-30min, 650°C-3hr, and direct insertion at 650"C for 30min. In this study, 6-layered coatings were deposited on platinized silicon (Pt/Ti/SiO,/Si) and were subjected to heat treatment at 10°C/min except for the case of direct furnace insertion. The structure of the films was analyzed using an x-ray diffractometer (Seifert XRD 3000PTS, Cu K,,). RESULTS AND DISCUSSION Evaporation of liquid from within the polymeric network creates tension in the liquid due to the concave menisci developed at the liquid-vapor interface [9]. The liquid tension forces the compliant network into compression. The total stress in the layer is then approximately equal to the capillary pressure or tension exerted by the liquid. An as-spun PZT layer always developed a tensile
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