Effect of different annealing procedures on the microstructure and the electrical properties of CSD derived (Ba,Sr)TiO 3
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Effect of different annealing procedures on the microstructure and the electrical properties of CSD derived (Ba,Sr)TiO3 thin films. Sandip Halder1, Theodor Schneller1 and Rainer Waser1,2 1 Institute für Werkstoffe der Elektrotechnik- II, RWTH Aachen, Germany 2 Institut für Festkoerperforschung, Forchungzentrum Juelich, Juelich, Germany ABSTRACT Processing of BST thin films is becoming more and more important for microwave electronics and for its probable incorporation in future high density DRAM’s. A co-relation between processing and final device characteristics is of utmost importance. Differences in microstructure and electrical properties were observed when chemical solution deposited thin films were annealed using a conventional diffusion furnace, rapid thermal annealing furnace with different heating ramps, and a hot plate for pyrolysis prior crystallization. The solution was made with the propionate route and then deposited on Pt coated silicon wafers. Crosssectional SEM’s were performed on the different films. It was found that the microstructure depended on the annealing method of the film. The electrical properties of the films were also found to vary considerably. Frequency dependence of the dielectric constant was studied. The leakage study on different films was performed at different temperatures. INTRODUCTION Barium strontium titanate has been considered to be an important material for tunable microwave devices such as high-Q resonators, phase shifters and filters due to its large electric field tunability, relatively low dielectric loss and variable Curie temperature [1,2]. The Curie temperature can be varied by varying the amount for Sr substitution. Thin films of BST have been deposited by a number of methods like MOCVD[3], PLD[4], sputtering and CSD [5]. CSD is currently one of the preferred deposition methods for thin films because of its ease and high compositional control. To improve the material performance for the use in tunable devices it is important to study the effect of different processing parameters when depositing thin films by CSD. EXPERIMENTAL DETAILS The films of composition (Ba0.7Sr0.3)TiO3 were deposited on Pt coated Si substrates by a 0.1M solution. The details for the preparation of the precursor solution have been provided elsewhere [6]. The films were crystallized in three different ways. A flowchart depicting the different procedures for annealing the films is shown in figure 1. In the first method the films were dried and pyrolysed at 200oC and 400oC and then after the desired thickness was reached, it was crystallized at 700oC for 30 minutes (two-step process). In the second method the films were directly crystallized in the RTA for 10 minutes. The ramp rate was varied in the RTA and it was found that when the ramp rate was set to around 80 °C/sec the films attained a columnar structure. Faster ramp rates of about 160 °C/sec yielded non columnar grains and a lower dielectric constant. Finally in the third method the films were crystallized in the diffusion furnace
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