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| Technology: SiC Wafer Polishing With Gas Cluster Ion Beams - Part I |
Anil Saigal
04/20/2006
Figure 1: Ion beam processes, a) Monomer ion beam, b) Gas cluster ion beam (GCIB) Figure 2: GCIB System
Background
SiC is being used in electrical switches and other electronic devices operating at high currents, power densities and temperatures. One requirement for these application is good surface finish. 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.
GCIB is a relatively new technique whereby energetic ionized clusters of condensed gas atoms, typically Ar, are directed towards the target. These clusters of tens to thousands of gas atoms or molecules are weakly held together by van der Waals forces. They are formed by homogeneous condensation occurring within the flow of a high-pressure gas expanding into a vacuum. Neutral clusters thus formed are then ionized and accelerated through a high voltage potential, producing an energetic beam of cluster ions. As shown in Figure 1, cluster ion beams have markedly different effects on the target material than do conventional monomer ion beams. Although the cluster ions are highly energetic (10 – 30 keV acceleration), this energy is shared between many atoms in the cluster giving each individual atom an average energy of 5-50 eV. This results in very little penetration and therefore a higher concentration of energy within a smaller volume. This produces much higher local impact temperatures and pressures than monomer ions as well as higher sputtering yields. Consequently, GCIB offers effective surface smoothing effects. GCIB has been used to produce exceedingly smooth surfaces with an average roughness of as low as two angstroms [1-10].
A significant smoothing and planarization effect is produced during gas cluster ion impacts on nonplanar surfaces. As atoms within a cluster are able to interact with each other as the cluster disintegrates upon impact, some of the total energy carried by the cluster is converted to individual atom energy directed along the plane of the target surface. The smoothing produced by gas cluster ion impacts is somewhat analogous to a balloon full of marbles thrown onto a plane surface under conditions whereby the balloon breaks and the marbles become scattered in all directions over the target due to interactions among themselves during the impact. This lateral sputtering effect interacts with surface features, preferentially etching peaks, to cause smoothing and planarization. Gas cluster ions have been experimentally demonstrated to produce atomic scale surface smoothing on virtually all solid inorganic materials. Examples include metals [Ta, Cu, Au, Ag], semiconductors and metal oxide superconductors such as YBa2Cu3O7-x. [4-10].
GCIB System
Figure 2 illustrates the gas cluster ion beam processing system ( Epion Corporation, Billerica, MA) used in this investigation. Gas is expanded in the nozzle and clusters are formed. The core of the beam enters the ionization section through a narrow skimmer where the clusters are ionized and then accelerated towards the target by an acceleration voltage. A permanent magnet deflects the lighter monomer species out of the beam, allowing only clusters to reach the target. Electrostatic scanning plates raster the beam to ensure a uniform dosage. The SiC wafer was covered with a tantalum aperture to further ensure uniform beam strength over the process area and to allow precise measurements of beam current and ion dosage. Typical processing chamber base pressure is 1x10-7 torr.
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