Sub-micron-scale surface roughness and contamination cause field emission that can lead to high-voltage breakdown of electrodes, and these are limiting factors in the development of high gradient RF technology. We are studying various Gas Cluster Ion Beam (GCIB) treatments to smooth, clean, etch and/or chemically alter electrode surfaces to allow higher fields and accelerating gradients, and to reduce the time and cost of conditioning high-voltage electrodes. For this paper, we have processed Nb, stainless steel and Ti electrode materials using beams of Ar, O2, or NF3+O2 clusters with accelerating potentials up to 35kV. Using a scanning field emission microscope (SFEM), we have repeatedly seen a dramatic reduction in the number of field emission sites on Nb coupons treated with GCIB. Smoothing effects on stainless steel and Ti substrates, evaluated using SEM and AFM imaging, show that 200-nm-wide polishing scratch marks are greatly attenuated. A 150-mm diameter GCIB-treated stainless steel electrode has shown virtually no DC field emission current at gradients over 20MV/m.

Surface processing with high-energy gas cluster ion beams (GCIB) is investigated for increasing the high voltage breakdown strength of RF cavities and electrodes in general. Various GCIB treatments were studied for Nb, Cu, Stainless Steel and Ti electrode materials using beams of Ar, Ar+H2, O2, N2, Ar+CH4, or O2+NF3 clusters with accelerating potentials up to 35kV. Etching using chemically active clusters such as NF3 reduces the grain structure of Nb used for SRF cavities. Smoothing effects on stainless steel and Ti substrates were evaluated using SEM and AFM imaging and show that 200nm wide polishing scratch marks are greatly attenuated. Using a combination of Ar and O2 processing for stainless steel electrode material, the oxide thickness and surface hardness are dramatically increased. The DC field emission of a 150-mm diameter sample of GCIB processed stainless steel electrode material was a factor of 106 less than a similar untreated sample.

Many studies have demonstrated that improving the surface smoothness and cleanliness of high voltage electrodes increases the voltage standoff capability, but none have specifically investigated the role of nano-scale and atomic level surface roughness. Using AFM imaging, we have studied the effect of gas cluster ion beams (GOB) on oxygen-free Cu electrode material that is used in high gradient RF cavities. Using Ar clusters accelerated by 30 kV, with a dose of 6 x 10(14) cm(-2), we have effectively removed an asperity that was 3500 (A) over circle wide and 350 (A) over circle high. Subsequent processing with 5 kV acceleration reduced the surface roughness from an Ra value of 13.2 (A) over circle to 4.8 (A) over circle. This demonstrates the effectiveness of GCIB for reducing sub-micron roughness to atom level smoothness. (c) 2005 Elsevier B.V. All rights reserved.

This paper investigates the use of gas cluster ion beam (GCIB) processing on SOI substrates for reducing the high frequency surface roughness of the starting material while improving the wafer uniformity. AFM images and power spectral density measurements of the pre- and post-GCIB surfaces are examined to determine the spatial-frequency range of the surface features most affected by the gas clusters.

In order to smooth a wide variety of surface material types to within an angstrom of roughness without subsurface damage, a beam energy equivalent to the individual bond energy of the surface atoms would be required. This would ideally preserve the integrity of the underlying material matrix and eliminate both high and low frequency surface aberrations. A low energy ion beam of a few hundred eV would be difficult to produce with significant intensity, however, due to the space charge effect encountered from the increase in density of the ions produced (“beam blow-up”). The use of a gas cluster ion beam (GCIB) process, however, has proven to provide an effective atomic smoothing on numerous material surface compositions without causing subsurface damage. 1–3 This paper provides a description of the recently developed GCIB surface smoothing and modification apparatus, a discussion of the mechanism for surface smoothing, and provides a focus on thin film Si, SiC, and semiconductor-on-insulator (SOI) material results.