Vincent Difilippo

Ion beam nanosmoothing of sapphire and silicon carbide surfaces

The surfaces of single-crystal wafers of sapphire and silicon carbide with microelectronic-grade high polish were exposed to a gas-cluster ion beam (GCIB) and significant reductions in roughness were observed. Atomic-force microscopy revealed that the typical initial surfaces consisted of a fine but small random roughness together with relatively large and sharp asperities. The latter were removed efficiently and GCIB smoothing process improvements are reported. The SiC wafers also have a high density of shallow scratch marks and these too were removed, with the average roughness Ra falling below 4 angstrom after the best process. Analysis of the SiC by Rutherford backscattering spectroscopy in channeling mode revealed that when the GCIB process was adjusted so that asperities and scratch marks were removed, there was no increase in near- and at-surface damage. In particular, no lattice damage was observed of the sort typically caused by ion implantation prior to annealing. Significantly, it was found that oxygen gas cluster ion beams provided superior results with SiC as compared with argon GCIB. Surface smoothing mechanisms are proposed to explain these results.

Gas Cluster Ion Beam Polishing of Orthopedic Implants

One of the major causes of failure of orthopedic implants used in hip replacement surgery is the generation of excessive wear particles. These wear particles accumulate in the joint and initiate an autoimmune reaction which can weaken the bone-implant joint, leading to failure and revision surgery. The relatively poor implant surface finish created by current mechanical polishing techniques has been identified as a major cause of wear particle generation. Gas cluster ion beams (GCIB) have been successfully used to reduce the surface roughness of orthopedic implants. In GCIB, a high-pressure gas, such as argon, is supersonically expanded through a nozzle into vacuum. This adiabatic expansion results in the condensation of clusters consisting of tens to thousands of gas atoms weakly held together by Van der Waals forces. These clusters are then ionized and accelerated towards the target substrate. Upon impact, they create a strong lateral sputtering effect resulting in a net smoothing of the surface. Lapped flat coupons and production femoral balls made of ASTM 1537 cobalt-28 chromium-6 molybdenum surgical implant alloy were processed by GCIB utilizing gas clusters of argon and oxygen. Surface morphology was characterized before and after processing by atomic force microscopy and white light optical profilometry. As shown in Figure 1 GCIB processing was able to successfully remove asperities and create a tribologically improved surface on both flat test coupons and on production components.Copyright