Effects of In-Situ DC-Bias on the Composition, Microstructures and Dielectric Properties of RF Magnetron Reactive Sputte
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43 Mat. Res. Soc. Symp. Proc. Vol. 596 © 2000 Materials Research Society
EXPERIMENT Thin films of (Ba 65Sr,03 5)TiO 3 have been prepared by the rf magnetron reactive sputtering method using a single alloy target. The BST films were deposited on indium tin oxide (ITO)coated coming glasses. The film thickness is around 200nm. Pt films were deposited on BST thin films by dc magnetron reactive sputtering method as top electrodes. Post annealing was applied to the as-deposited films at 620'C for 2 hours in 1 atm oxygen atmosphere. BST films were also deposited on blank coming glass substrate for x-ray diffraction (XRD) analysis. Typical deposition conditions for this study are listed in Table I. The deposition rate was calculated from the film thickness measured with a TENCOR Alpha-step 250. The (Ba+Sr)/Ti composition ratio was analyzed with Electron Probe Microanalyzer (EPMA). Crystallinity of the films was investigated by an XRD analyzer, which provided information on microstructures and grain sizes. The dielectric constant was measured with an IP-4284A LCR meter at frequency 20Hz, bias voltage 0.1 Volt, and oscillation voltage 10mVolt.
A temperature range between 2 'C to 40 'C with a temperature step of 2 'C was
applied for the measurement. RESULTS AND DISCUSSION (Ba+Sr)/Ti composition ratio Many studies have reported that the thin film (Ba+Sr)/Ti atomic ratio was almost the same as with the target from which the film was deposited [1, 5-7]. These BST thin films were prepared using high substrate temperature, at least above 400'C, and the targets were around 3.5 S6 inches in diameter. These might be the two key reasons for easily getting similar thin film (Ba+Sr)/Ti atomic ratios as the target. However, lower substrate temperature and a smaller target size, i.e., 2 inches diameter, were applied in this study. Table II shows the thin film (Ba+Sr)/Ti atomic ratio dependence with in-situ negative DCbias and heat treatment. Thermal annealing at 620'C for 2 hours in 1 atm oxygen atmosphere was applied for the heat treatment. As-deposited BST thin films with no externally applied insitu DC-bias possess a Ba: Sr atomic ratio of around 0.67:0.33 and a (Ba+Sr)/Ti atomic ratio of 0.72. It was observed that the atomic ratio of Ba:Sr was very close to the target composition, 0.65:0.35. In contrast, the (Ba+Sr)/Ti atomic ratio was far from the target ratio. Accordingly, we modified the deposition technique to get better crystallinity and shift the (Ba+Sr)/Ti atomic ratio closer to the stoichiometric value. For the BST thin films, which were deposited with insitu negative DC-bias, the (Ba+Sr)/Ti atomic ratio was 0.74, which was slightly closer to the value of 1 compared to those of films deposited without externally applied DC-bias. After annealing at 620'C for 2 hours, the (Ba+Sr)/Ti atomic ratio was further increased to 0.83, which was much closer to the value 1. In this study, the (Ba+Sr)/Ti atomic ratios of BST thin films were all smaller than the value 1. This means that excess Ti atoms may exist in a form different from
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