Noriaki Toyoda

Incident angle dependence of the sputtering effect of Ar-cluster-ion bombardment

Abstract Gas cluster ions impact onto a solid surface with low energy and with high density. At the moment of ion impact, multiple-collisions cause several unique effects, such as lateral sputtering, high-rate sputtering and surface cleaning and smoothing at normal incidence. We have irradiated Cu and Ag thin films with a 20 keV Ar cluster ion beam (mean cluster size is 3000) at several different incident angles. The sputtering yield of the cluster ion bombardment decreased with increase of the incident angle in proportion to cos θ, while that by monomer ion bombardment increased. The roughness of the Cu surface, bombarded with cluster ion, monotonically increased with increase of the incident angle. These characteristics of energetic cluster impact show quite a different dependence on the incident angle than those of monomer ion. The sputtering effect of the impact of energetic clusters is strongly dependent on the incident angle.

Surface treatment of diamond films with Ar and O2 cluster ion beams

Abstract Irradiation effects of Ar and O2 cluster ion beams were studied on Chemical Vapor Deposition (CVD) diamond films. When the acceleration energy of the O2 cluster ion was 20 keV, the sputtering yield was 400 atoms/cluster which is 13 times higher than that of Ar cluster ions because of the enhancement by chemical reactions. The average roughness of the diamond surface decreased with Ar cluster ion beams. This smoothing is attributed to the physical sputtering effect. However, a thin graphite layer was formed on the surface by contamination of monomer ion in the cluster beam, which decreases the optical transmittance of the diamond films. In contrast, the surface roughness was not improved but no graphite layer was formed with O2 cluster ions. By using both Ar and O2 cluster ion beams, a very flat diamond surface without a graphite layer on the surface can be fabricated.

Nano-Processing with Gas Cluster Ion Beams

Abstract This paper describes the fundamental principles and experimental status of gas cluster ion beam (GCIB) processing as a new technique with promise for practical industrial applications. A review is presented of the theoretical and experimental characteristics of new gas cluster ion bombardment processes and of related equipment development. The impacts of accelerated cluster ions upon substrate surfaces impart very high-energy densities in the impact regions of individual clusters and produce non-linear processes that are not present in the impacts of individual atomic ions. These unique bombardment characteristics are expected to facilitate new industrial applications that would not be possible by traditional ion beam processing. Among these are shallow ion implantation, high rate sputtering, surface cleaning and smoothing, and low-temperature thin film formation.

Reactive sputtering by SF6 cluster ion beams

Abstract Reactive gas cluster ion beams were formed by an adiabatic expansion of SF 6 with He mixture through a Laval nozzle and their reactive sputtering effects with solid surfaces have been studied. Si, W and Au samples were irradiated with SF 6 cluster ion beams at an energy of 20 keV. Due to the chemical reaction of SF 6 clusters with Si and W, sputtering yields of Si (1300 atoms/ion) and W (320 atoms/ion) were dramatically enhanced compared with those by Ar cluster ions (Si: 24, W: 35 atoms/ion). Sputtering yields of SF 6 cluster ions increased exponentially with the increase of acceleration energy, on the contrary, those of Ar cluster ions were proportional to the energy. Chemical reaction is the predominant sputtering process at an energy of around 5 keV. A Si(100) surface irradiated with SF 6 cluster ions was quite smooth, because of the smoothing effect of cluster ions.

Non-linear processes in the gas cluster ion beam modification of solid surfaces

The unique characteristics of gas cluster ion beam processing are reviewed. Cluster ion beams consisting of hundreds to thousands of atoms have been generated from various kinds of gas materials. Multiple collisions during the impact of accelerated cluster ions upon the substrate surfaces produce fundamentally non-linear bombarding processes. These bombarding characteristics can be applied to shallow ion implantation, high yield sputtering and smoothing, surface cleaning and low temperature thin film formation.

Surface processing by gas cluster ion beams at the atomic (molecular) level

Gas cluster ion beam techniques have been developed for atomic and molecular level surface modification processing. Shallow implantation, high yield sputtering, surface smoothing, and low damage surface cleaning have been demonstrated experimentally. This article reports recent results concerning surface treatments that are distinctly different from those produced by conventional monomer ion irradiation. Possible applications of gas cluster ion beam processing to new areas of surface modifications are suggested.

Atomic level smoothing of CVD diamond films by gas cluster ion beam etching

Abstract Chemical vapor deposited diamond films on silicon substrates were etched by a gas cluster ion beam. We found that a gas cluster ion beam of 10 17 ions/cm 2 would be effective to smooth the surface of the CVD diamond films. It was confirmed that atomic level smooth surfaces ( R a = 1.9 nm by AFM measurements) were formed by Ar gas cluster ion beam (Ar 3000 + ) etching. We believe that the gas cluster ion beam etching technique will be a key technology for diamond device fabrication.

Surface smoothing mechanism of gas cluster ion beams

Abstract Sputtering phenomena by gas cluster ions were modeled based on experimental results, and the surface smoothing effects with cluster ion beam were studied with Monte Carlo simulations. When a cluster ion impacts a slope, ejected atoms move down the slope, and consequently, the valley is filled by these dislocated atoms. An initially rough surface is made smooth by these effects. The dislocated atoms filling the valley are eventually removed with increasing dose, and finally, a very smooth surface with a thin, damaged layer can be obtained. Due to the small distance of the atomic motion induced by a cluster-ion impact, surfaces with narrow hill-and-valley are smoothed initially, and those with longer-scale roughness require more ion-dose to be smoothed.

Surface smoothing effects with reactive cluster ion beams

Abstract Surface smoothing effects of reactive cluster ion beams were studied and compared with those of Ar cluster ions. Si, SiC and W were irradiated with SF6 cluster ion beams. As these materials show reactive sputtering, sputtering yields become one or two orders of magnitude higher than that of Ar cluster ions, which have only the physical sputtering effect. The Au surface was smoothed with SF6 cluster ions at a normal incidence, however, the surface roughness of W was not improved with SF6 cluster ions. The surface smoothing effect with reactive sputtering is weaker than that of physical sputtering. The angular distribution of Au sputtered with SF6 cluster ion follows an under-cosine law, which is similar to that of Cu sputtered with Ar cluster ions. This under-cosine distribution of sputtered atoms is called the ‘lateral sputtering effect’. When W is irradiated with SF6 cluster ions, volatile materials such as WFx are produced by a chemical reaction with F, and they are thermally evaporated. As a result, the angular distribution follows the cosine law. Therefore, the ‘lateral sputtering effect’ is responsible for the surface smoothing effect.

Sputtering of elemental metals by Ar cluster ions

Abstract Energetic cluster bombardment effects have been examined for various materials. The sputtering yields of various materials with Ar cluster ions are two orders of magnitude higher than those with Ar monomer ions. The sputtering yield by cluster ion bombardment is proportional to the reciprocal of the sublimation energy of the target atoms. A dramatic reduction of Cu contamination on silicon surfaces has been obtained with Ar cluster ion bombardment at low ion dose. Low damage surface processing can be achieved, because the energy of each constituent atom is very low. This feature is quite suitable for low damage processing of electronic materials.