Composition Control of Lead Or Bismuth Based Ferroelectric Thin Films Prepared by Sputtering Method
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using Pb, Ti, Zr and Bi metal targets. The same kind of substrates, sputtering gases and pressures were used. All depositions were carried out on unheated substrates. Power control and current control modes were compared to optimize the deposition conditions. In the case of experiments on ion beam sputtering, Pb, Bi, Ti metal targets and SrTiO 3 target were employed. The detailed mechanism of ion beam sputtering and the configuration of the system have been reviewed elsewhere 4' 5 . A typical gas pressure was 4-5* 10-2 Pa. The films were annealed using RTA and conventional furnace annealing. The composition of the films were determined by using plasma emission spectrometry (DCP). XRD and SEM were used to establish the crystal structures and examine the microstructures. The electrical properties of ferroelectric and antiferroelectric thin films such as hysteresis loops and capacitance-voltage characteristics were investigated using a Radient Technologies RT66A tester. RESULTS AND DISCUSSION
It is well known that excess Pb or Bi (typically 10 mol% excess) is necessary to prepare stoichiometric Pb or Bi based ferroelectric thin films. However, even though compensation is made, it is still difficult to precisely control the composition. First we have to optimize the deposition conditions such as sputtering power, gas pressure and composition, configuration of the targets and substrates, substrate temperature and so on. After the optimization there are still some factors that must be considered in order to obtain reproducibility and precise control of the composition. From empirical and theoretical points of view we focused on three items, including the target surfaces, the effects from negative ions and the compositions of as-deposited films. Target Surface The control of the target surface is important in order to achieve good reproducibility in film deposition because it has a great influence on the sputtering yield. Table I shows the compositions of PZT films deposited by rf sputtering with 10 mol% Pb rich sintered PZT targets. The depositions were carried out on unheated Si substrates at a power density of 1.6 W/cm 2 . The target compositions were found to have been transferred well to the films and good reproducibility was obtained. The same results were obtained for doped PZT targets. Elements such as Zn, Sr, Nb, Y, Mn, Co and W were sputtered from the oxide targets non-selectively and transported to the films without any effects due to atomic weight differences or sputtering yield except in the case of Pb and Bi. For the latter, a small change in composition was observed at the beginning of the depositions. It is considered that a nonstoichiometric layer containing either a deficiency or excess of Pb is formed during sintering of targets by evaporation of Pb from the surfaces. When the surface layer is removed, the reproducibility is easily achieved. In the case of powder targets with the same compositions, around 20 mol% excess Pb was observed in the films as shown in Table 1. This is probably caused by the la
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