Selective Crystallization of A-SI:H Films on Glass
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mical vapor deposition on 7059 glass substrate at a substrate temperature of 250 °C. The thickness of the amorphous silicon was 1500 A. The films were first covered by 500 A Si0 2 deposited by magnetron sputtering and were then patterned by lithography to form various islands covered by the Si0 2 ; that is, some regions of amorphous Si samples were covered by SiO 2 (region B) and some regions were not (region A). These patterned films were exposed to an oxygen plasma in an electron cyclotron resonance (ECR) reactor. The samples were placed in the ECR plasma chamber in the same configuration as previously reported in our studies of ECR hydrogenation of thin film transistors. In this selective crystallization study, a 60 minute ECR plasma treatment was used with the microwave power at 700 watts, oxygen pressure at 4 x 10-4 Torr. and substrate temperature at 400 °C. The 500 A patterned SiO 2 was removed by etching in a buffered HF solution after oxygen plasma treatment. After this step, these plasma processed a-Si films were thermally annealed in a furnace at 600 °C for 6 hours. The annealed films were characterized using UV reflectance measurement, X-ray diffraction (XRD), and transmission electron microscopy (TEM). The electron transparent samples needed for TEM were prepared by using dilute HF to etch the film away from the substrate [6]. To explore the thermal budget space further, plasma processed a-Si films were also thermally annealed at 600 °C for other periods ranging from 2 hours to 20 hours. UV reflectance measurements were taken on all samples to determine the different crystallization times of the patterned regions. EXPERIMENTAL RESULTS In Figure 1, the UV reflectance spectra of a silicon film (region A of the sample) that was not covered by SiO 2 during the oxygen plasma exposure prior to 6 hours annealing at 600 °C is compared to that of a silicon film (re ion B of the sample) which received the same furnace annealing but was covered by the 500 A SiO 2 during oxygen plasma treatment. As a reference, the UV reflectance spectrum of a Si wafer is also shown (see Fig. l(a)). This single crystal Si spectrum shows two reflectance peaks at 276 nm and 365 nm which are due to optical transitions at the X point and along F-L axis of the Brillouin zone [8]. As shown in Fig. l(b), these characteristic reflectance peaks are also observed in region A of the sample. This indicates that region A film has been crystallized. In Fig. 1(c), no reflectance peak is discernable for the region B film. This demonstrates that region B of the sample remains amorphous. Both region A and region B of the annealed film were also characterized using x-ray diffraction. As shown in Figure 2, region A of the sample is crystallized while region B of the sample is still amorphous. To monitor the selective crystallization process and determine the crystallization thermal budget of different regions in an oxygen plasma treated sample, a reflectance peak height AR was defined as the difference between the minimum reflectance at around 240 nm an
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