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Technology: SiC Wafer Polishing With Gas Cluster Ion Beams - Part II

Anil Saigal

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.


Two, electronic grade, SiC wafers were used. One wafer (A) was processed with only Ar clusters while the second wafer (B) was polished with both Ar and O2 clusters.  A series of ion doses at different cluster energy levels was applied to separate process areas on each wafer.  On the second wafer in particular, a number of process areas were exposed to two consecutive ion doses in which the cluster energy and gas were varied. Cluster dose and cluster energy were the two GCIB process parameters that were varied.  

Surfaces were measured with a Digital Instruments atomic force microscope (AFM) equipped with analysis software, that was used to calculate surface roughness values such as arithmetic mean roughness (Ra) and maximum peak to valley height (Rmax).   Rutherford backscattering (RBS) channeling measurements were also performed on the wafers to measure the extent of near surface crystal lattice damage.  Ion channeling measurements were performed with He+ ions at 1.8 MeV after etching with hydrofluoric acid to remove any oxide layer.

Figure 3 shows a 5 um x 5 um AFM scan, which is typical of the surfaces of the SiC wafers.  This surface distinctly shows the effects of mechanical polishing.   The large number of surface scratches should be noted.  The more pronounced scratches are on the order of 50 Å deep.  The average virgin surface roughness Ra was 8.4 Å for wafer A and 9.6 Å for wafer B.

Figure 3: 5mm x 5mm AFM image of untreated SiC wafer B surface.  (Ra = 9.6 Å, Rmax = 248 Å)

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 Å, Rmax = 178 Å, scratches still visible; b) high energy greater dose, Ra = 8.2 Å, Rmax = 105 Å, scratches gone but some high frequency roughness; c) dual dose, high energy dose followed by low energy dose (condition A5) Ra = 7.0 Å, Rmax = 86 Å, smoothest argon result.

Figure 5 shows the effect of GCIB processing on the SiC wafer surface with O2 clusters.  At low cluster energy, the polishing scratches are less distinct.  At a higher energy the polishing scratches are completely gone, but a higher frequency roughness is now present.  Figure 5 shows that the low energy treatment slightly lowered the roughness while the high-energy process increased it.

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 Å (as received) to 11 Å 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 Å, Rmax = 157 Å, scratches unaffected; b) high energy, Ra = 11.6 Å, Rmax = 355 Å

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 lines are only a guide to the eye)

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 Å and Rmax of 86 Å.

Figure 7:  5mm x 5mm AFM images of SiC surfaces exposed to GCIB polishing with O2 clusters at two  energy levels. (Ra = 3.8 Å, Rmax = 69 Å), 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 Å and Rmax of 69 Å (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.

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. )

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