Application of Cluster ION Beam Smoothing to SiC and YBCO Surfaces
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313 Mat. Res. Soc. Symp. Proc. Vol. 585 © 2000 Materials Research Society
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Bim Fig. 1. Schematic sketch of Houston 30 kV GCIB. through a Laval nozzle into a high-vacuum chamber after being collimated by a skimmer. The cluster size distribution can be changed by the gas pressure and temperature at the nozzle The clusters are subsequently ionized by electron bombardment and then accelerated and focused by a series of plates at high potential. A small ionization vacuum gauge tube measures the neutral beam intensity. The ion beam is electro statically scanned during irradiation to ensure uniform coverage of the sample surface. The current of the scanned beam was about 0.5 microAmps, with a beam spot size of 1.5 cm 2 corresponding to 1El6 Ar atoms/cm 2/s. The YBCO thin films were epitaxially deposited upon a MgO substrate. The bulk SiC samples were provided commercially. Electrical resistance of these films at microwave input power was determined by frequency-domain and pulse time-domain measurements using a 14GHz TE011 dielectric cavity. Irradiated films were furnace annealed at 4800C to remove any residual ion-induced damage in the near surface. The bulk 6H-SiC was [1000] oriented with an n-type resistivity of 0.3-0.2 ohm-cm. RESULTS AND DISCUSSION The sputtering yield per ion is about a factor of 100 greater for clusters than monomer ions, independent of the conductivity of the target 5. The removal rate for SiC is measured to be approximately 5 nm per 1El15 ions/cm2 . Fig. 2a shows the surface topography of a SiC wafer. Surface masking was accomplished during irradiation such that A) marks the original surface, and B) after receiving 5E15(22 nm removed), and C) after a dose of 1E16/cm 2 (42 nm removed). The step height between the sections B C is -20 nm and is marked on the image. A TEM image showing the surface defects in the bulk SiC crystal before irradiation is presented in figure 2b. Fig. 2a shows AFM images of surfaces of a SiC wafer before and after irradiation with Ar clusters at energy of 20keV. The area marked (A) is the original surface of the wafer and the area marked (C) a region after receiving a dose of 1 E16 cm 2 . The comparison of these regions clearly demonstrates the sputtering and smoothing effect of cluster ion irradiation. For this example, the average surface roughness of the SiC was reduced from 8.3 to 2.9 ± 1 nm following irradiation at normal incidence with the scanned cluster beam. An expanded view of the respective regions are also shown in the figure. Inspection of the surface at intermediary doses indicated that the effect was fairly proportional to dose. However, an interesting artifact was
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Figure 2.
(a) AFM images of a SiC wafer (regions A and C are shown in more detail in the smaller figures); (b) TEM image.
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b) After irradiation
a) Before irradiation
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