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Anil Saigal 05/01/2006 A novel technology utilizing energetic ionized
gas cluster ion beams (GCIB) has been successfully used to reduce the
surface roughness of SiC for electronic applications. Part I of the
article dealt with the basic concepts and GCIB equipment. Figure 3: 5mm x 5mm AFM image of untreated SiC wafer B surface. (Ra = 9.6 Ã…, Rmax = 248 Ã…) As described above and shown in Figure 3, many polishing scratches are visible on the as-received surfaces of the SiC wafers. Figure 4 shows processing with Ar clusters greatly improve surface topography by removing CMP scratches, but failed to improve the measured surface roughness. Ar cluster processing added high frequency roughness components to the surface, but a dual energy Ar process was able to slightly improve Ra.
Figure 4: 5mm x 5mm AFM images of SiC surfaces exposed to different GCIB Ar cluster energies: a) high energy low dose, Ra = 9.5 Ã…, Rmax = 178 Ã…, scratches still visible; b) high energy greater dose, Ra = 8.2 Ã…, Rmax =
105 Ã…, scratches gone but some high frequency roughness; c) dual dose,
high energy dose followed by low energy dose (condition A5) Ra = 7.0 Ã…, Rmax = 86 Ã…, smoothest argon result. A plot of Ra normalized to the untreated roughness value versus ion dose is shown in Figure 6. It shows the effect on surface roughness as a function of ion dose for 1) Ar single step processes and 2) Ar and O2 dual energy processes. In the case of Ar alone, there is an initial is an increase in roughness from 8.4 Ã… (as received) to 11 Ã… and then a gradual smoothing with increasing dose. Figure 5: 5mm x 5mm AFM images of SiC surfaces exposed to different GCIB O2 cluster energies: a) low energy, Ra = 9.1 Ã…, Rmax = 157 Ã…, scratches unaffected; b) high energy, Ra = 11.6 Ã…, Rmax = 355 Ã… Figure 6: Plot of relative Ra
(relative to as received roughness) vs. dose at high cluster energy (25
kV) with Ar clusters. Also shown is the effect of a dual energy
smoothing process, consisting of a dose at high energy followed by a
dose at lower energy for O2 and Ar. The greatest reduction in Ra and Rmax while utilizing only Ar was obtained using a dual energy smoothing process consisting of an initial dose at high energy followed by a dose at a moderate energy. As shown in Figure 7, this dual energy process produced a greater reduction in surface roughness than an equivalent dose using only high energy clusters. The result was a final Ra of 7.1 Ã… and Rmax of 86 Ã…. Figure 7: 5mm x 5mm AFM images of SiC surfaces exposed to GCIB polishing with O2 clusters at two energy levels. (Ra = 3.8 Ã…, Rmax = 69 Ã…), smoothest result. Processing with O2 clusters produced similar results. Low energy oxygen cluster doses alone did not significantly reduce surface roughness or remove polishing scratches. High energy oxygen doses were needed to alter the surface morphology and remove scratches. As with Ar clusters, these high-energy oxygen doses introduce a high frequency roughness component, which needs to be reduced by a subsequent lower energy oxygen dose. As with the argon dual energy process both low frequency polishing scratches and high frequency roughness were reduced. However, the oxygen process produced an overall smoother surface than the argon process with a final Ra of 3.8 Ã… and Rmax of 69 Ã… (Figure 7). Defect density (Nsp) and Chi Min values (Xmin) from RBS measurements of crystal lattice damage. Areas exposed to lower energy clusters had lower levels of damage than those exposed to higher energy clusters. The areas exposed to a dual energy process (high energy dose followed by lower energy dose) had still lower damage levels, similar to the unprocessed areas. Regions processed with only oxygen clusters had significantly lower damage levels than those processed with argon. CONCLUSIONS Gas cluster ion beam smoothing appears to be a viable process for improving the surface quality of electronic grade SiC. CMP polishing scratches were removed and maximum peak to valley height was decreased overall by over 60% by using a dual energy oxygen cluster ion beam process. (Anil Saigal is Chair and Professor of Mechanical Engineering at Tufts University. He can be reached at anil@lokvani.com. ) ![]() You may also access this article through our web-site http://www.lokvani.com/ |
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