豊田 紀章

Surface smoothing by gas cluster ion beams (GCIB) was studied for compound semiconductor such as GaN and SiC. Average cluster size of Ar cluster ions was 2000atoms/cluster measured by time of flight (TOF). Since the total acceleration energy was 20keV, the energy per atom was 10eV/atom. This low-energy characteristic of gas cluster ion beams is desirable for compound semiconductors. GLIB irradiation was employed to remove the scratches of the mechanically polished SiC surface. After irradiation at acceleration energy of 15keV, the scratches was completry removed. The GaN film with initial average roughness of 4nm was also smoothed to that of 1.4nm by Ar cluster ion beams. Furthermore SiC substrates were irradiated with SF6 cluster ions. The sputtering yield of SiC with SF6 cluster ions was enhanced almost 3 times than that with Ar cluster ions.

O-2 cluster ion assisted deposition was demonstrated to form optical multilayer films for advanced optical communications. With O-2 cluster ion assisted deposition, high refractive index 2.20 at 550nm and very smooth surface (R-a = 0.37nm) of Ta2O5 films were realized. From a scanning electron microscope (SEM) image, dense film structure were observed without porous or columnar structures at the O-2 cluster ion assisted layers. The surface and interface of Ta2O5/SiO2 multilayer films were very flat due to surface smoothing effect of cluster ion beams, and it can be realized even though the beneath surface was rough. The shift of a center wavelength of multilayer film was quite small after an environmental test.

Ultra-smooth surfaces have been prepared using gas cluster ion beams (GCIB) processing. Ar cluster ions with average cluster size of several thousands of atoms were irradiated onto various targets at normal incidence. GCIB can easily produce subnanometer average roughness values and it can be applied for smoothing of small areas where mechanical polishing is not available. Scratches induced on surfaces by mechanical polishing can be removed with GCIB. From examination of the angular distributions of particles sputtered by Ar cluster ions, it has been shown that most of the particles are sputtered in lateral directions, an effect which enhances surface smoothing.

Secondary ion mass spectrometry (SIMS) with gas cluster ion beams was studied with experiments and molecular dynamics (MD) simulations to achieve a high-resolution depth profiling. For this purpose, it is important to prevent both ion mixing and the surface roughening due to energetic ions. As the Ar cluster ion beams shows high secondary ion yield and surface smoothing effects in the low-energy regime, it is suitable for the primary ion beam of SIMS. From MID simulations of Ar cluster ion impact on Si, ion mixing is heavier than than those for Ar monomer ions at the same energy per atom, because the energy density at the impact point is extremely high. However, the sputtering yields with Ar cluster ions are one or two orders of magnitude higher than that with Ar monomer ions at the same energy per atom. Comparing at the ion energy where the ion-mixing depths are the same by both Ar cluster and Ar monomer ions, cluster ions show almost 10 times higher sputtering yield than by Ar monomer ions. A preliminary experiment of SIMS with Ar cluster ion was performed and a mass resolution of several inn was achieved for a Ta film. (C) 2002 Published by Elsevier Science B.V.

Hard diamond-like carbon (DLC) films have outstanding characteristics with extremely thin film thickness, and will be required for various nano-scale devices. The requirements of this coating are high hardness, smooth surface, and low friction coefficient with thin film thickness below a few nm. The present films made with plasma enhanced CVD process will not be sufficient for hardness to achieve higher recording density in the future. To make films satisfying the above demands, a new method employing Ar cluster ion beam assisted deposition was proposed.

This paper discusses the principles and experimental status of gas cluster ion beam (GCIB) processing as a promising surface modification technique for practical industrial applications. Theoretical and experimental characteristics of GCIB processes and of related equipment development are described from the moment of neutral cluster formation, through ionization, acceleration and impact upon a surface. The impact of an accelerated cluster ion upon a target surface imparts very high energy densities into the impact area and produces non-linear effects that are not observed in the impacts of atomic ions. Unique characteristics of GCIB bombardment have been found to offer potential for various industrial applications that cannot be achieved by conventional ion beam processing. Among prospective applications are included shallow ion implantation, high rate sputtering, surface cleaning and smoothing, and low temperature thin film formation. Sputtering effects produced by cluster ion impact are particularly interesting. High sputtering yields and lateral distribution of sputtered atoms cause surface smoothing effects which cannot be achieved with monomer ion beams. Surface smoothing to atomic levels is expected to become the first production use of GCIB. (C) 2001 Elsevier Science B.V. All rights reserved.

OBJECTIVE: To identify a high-risk subgroup among patients with cytology-positive stage IIIA endometrial cancer. STUDY DESIGN: Fifty-four stage IIIA endometrial cancer patients who were positive only oil peritoneal cytology were divided into two groups based oil the cytologic pattern of their peritoneal smears. Iii group A, malignant cell clusters had well-defined edges, while the tumor cell clusters had scalloped edges in group B. The prognostic significance of these findings was investigated. RESULTS; The five-year disease-free survival rate was 97.5% in group A (n = 40) versus 50% in group B (n = 14). Multivariate analysis confirmed that the cytologic pattern had an independent influence oil survival. CONCLUSION: Positive peritoneal cytology composed of malignant cell clusters with well-defined edges has no impact oil survival. Only endometrial cancer patients who show tumor cell clusters with scalloped edges in peritoneal smears are worth considering for upstaging.

High-quality Ta2O5/SiO2 were deposited with oxygen gas cluster ion assisted deposition at low-temperature for advanced optical filters. With gas cluster ion assisted deposition, high refractive index and very smooth surface of Ta2O5 films were realized. The optimum cluster ion 2 energy and cluster ion current density for Ta2O5 films were found to be 7keV and 0.5muA/cm(2), respectively. The structure of film was very uniform and no porous or columnar structures were observed. The surface or interfaces of Ta2O5/SiO2 films were also very flat by surface smoothing effect of cluster ion beams. Very smooth surface can be realized even though the bottom surface was rough. There was no significant wavelength shift after an environmental test, which indicates that dense oxide films were formed at low-temperature with O-2 cluster ion assisted deposition.

Gas cluster ion beam (GCIB) processing has recently been introduced as a commercial tool for processing 'rough' surfaces, such as polished substrates or thin films. The physical interaction of a gas cluster ion beam with a surface is strikingly different from that of better-known 'monomer' ion beams. Clusters are formed by the adiabatic expansion of gas through a nozzle, ionization by electron impact, acceleration, and then impingement upon the surface to be processed. The physics of the surface interaction of the cluster beam strongly depends upon gas composition, cluster size, cluster size distribution, and beam energy. Typical argon GCIBs are composed of clusters ranging from several hundred to several thousand atoms in size. It has been previously shown that Ar clusters can be used to smooth surfaces at a sub-nanometer level. Argon cluster beam smoothing typically occurs in the energy range between 15 and similar to 30 keV. As such, the average energy per atom is of the order of 10 eV/atom upon cluster impact with the surface and subsequent dissociation. Ion cluster beams formed with reactive gases such as oxygen and nitrogen can also be formed, but at somewhat lower current densities than those obtainable with argon. Upon impact, reactive gas clusters undergo strong chemical reactions at the substrate surface. An extension of this chemical interaction is to utilize reactive clusters in an ion beam-assisted, thin-film physical vapor deposition process. This has been demonstrated with relatively low energy (E < <similar to> 10 keV) oxygen clusters in an electron-beam evaporator to form extremely low resistivity indium-tin oxide films on room-temperature substrates. This paper will describe the basics of GCIB formation and application to atomic scale smoothing of technologically interesting substrates and thin films, as well as reactive GCIB assisted deposition technology. The results presented demonstrate some of the unique physics and materials science that can be achieved with an emerging GCIB technology. (C) 2000 Published by Elsevier Science B.V. All rights reserved.

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. (C) 2000 Elsevier Science B.V. All rights reserved.

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. (C) 2000 Elsevier Science B.V. All rights reserved.

Polycrystalline Si (poly-Si) waveguides offer design flexibility and multilayered structures in Si-integrated photonic devices. However, as-deposited poly-Si surfaces are rough compared with single-crystalline Si, and a rough surface causes significant waveguide scattering loss at the surface. In this study, surface smoothing of poly-Si waveguides with a gas-cluster ion beam (GCIB) was demonstrated as a new smoothing technique. As the GCIB process is a directional ion-beam process, in principle it can be applied not only to plane surfaces but also to three-dimensional or non-flat structures, such as waveguide ridges. The initial average roughness of as-deposited poly-Si films (625 degreesC, 1 pm thick) ranged from 15 nm to 22 nm, and the grain sizes were distributed from 0.2 to 0.4 mum. This rough surface was dramatically smoothed to a roughness of 1.5 nm by Ar cluster ion irradiation. From the relation between the sputtered depth and the surface roughness, the sputtered depth must be greater than the height difference of the roughness (peak-to-valley) to obtain smooth surfaces. Optical transmission losses at lambda =1.54 mum were measured using cutback measurement from samples before and after the smoothing by GCIB. After surface smoothing with GCIB, the optical loss decreased from 85 dB/cm to 54 dB/cm.

Surface processing of microelectronic materials by bombardment with nanoparticles of condensed gases (i.e., clusters) in the form of an ion beam, makes possible etching and smoothing of those surfaces to very high figures of merit. As this is not possible with any conventional ion method, gas-cluster ion-beam systems have great potential in manufacturing. The formation of gas clusters and their collision with surfaces provides an interesting arena for novel physics and surface science. This paper outlines a physical model for the clusters and surface interactions, and provides examples of surface processing. In particular, the reduction of surface roughness while etching by cluster-ion bombardment is illustrated for various materials utilized in microelectronics.

Oxide film formation using high-intensity oxygen cluster ion beams has been developed. This deposition process uses large cluster ions, which can transport thousands of atoms per ion with very low energy per constituent atom. As a result, the interactions between the cluster and substrate atoms occur in the near-surface region, and cluster ions can deposit their energy with a high density in a very localized surface region. Enhancement of the oxidation reactions is clearly demonstrated. High-quality tin-doped indium-oxide (ITO) films, which are widely used in electrical and optical devices, are formed. Very smooth, highly transparent (> 80%) and low-resistivity (< 4 x 10(-4) Omega cm; which are the lowest values for films grown at room temperature) films, were obtained by the use of a 7 keV oxygen cluster ion beam. The energetic oxygen clusters collapsed at the surface and reacted with the metal atoms, and about 10% of them were incorporated when the kinetic energy of the cluster ion was above 5 keV. Oxidation reaction can be enhanced by energetic cluster ion bombardment, which offers a new technique for ion-assisted thin-film formation.

Ultra-shallow ion implantation by gas cluster ion beams has been demonstrated experimentally and confirmed by molecular dynamics simulations. Implantation of B10H14 in Si (100) at 2keV does not cause transient enhanced diffusion (TED) of boron atoms during annealing at 900 degrees C for 10sec. In order to reveal the diffusion mechanism of B atoms, the diffusivity of B atoms in ultra low-energy B10H14 ion implantation was measured by Secondary Ion Mass Spectroscopy (SIMS). B10H14 ions were implanted at 2, 3, 5 and 10keV. Subsequent annealing was performed at 900 degrees C and 1000 degrees C for 10 sec, respectively. In the case of the 900 degrees C annealing, TED was suppressed as the implant energy decreased and at energy less than 3keV, the TED of B atoms no longer occurred during annealing. High performance 40nm p-MOSFETs with ultra shallow junctions have been fabricated using B10H14 cluster ion implantation. The unique characteristics of gas cluster ion beam processes for sputtering has also been applied to very high-rate etching. Yields more than two orders of magnitude higher than those by monomer ions having the same energy and atomic scale smoothing of surfaces to average roughness less than 0.2nm have been demonstrated. This paper will discuss the status of cluster ion beam processes based upon our recent experimental and molecular dynamics simulation results.

Ion beams are used intensively for analytical applications. In order to meet the demands of the recent remarkable progress in material science, new ion beam analysis techniques based on large gaseous cluster ion beams are proposed. Cluster ion beams, which provide an equivalent low energy beam, offer many advantages for analytical technique. For instance, sputtering yields of the cluster ions are one or two orders of magnitude higher than monomer ions and smooth, flat surfaces can be maintained during the sputter depth profiling with cluster ions. These advantages are the result of multiple-collision and high local density energy deposition of cluster ions. Gaseous clusters such as argon or oxygen, as utilized in the analysis are generated by supersonic expansion. The beam current of the cluster ions is a few mu A which is sufficient for most analyses.

Results of the surface smoothing of a CVD-diamond membrane by gas cluster ion beams are presented. An as-deposited diamond membrane with a surface roughness of 400 Angstrom Ra was irradiated by Ar cluster ions with a energy of 20keV. A very smooth surface of 30 Angstrom Ra was obtained at a dose of 3X10(17) ions/cm(2). This result can be clarified by computer simulation which shows that the surface smoothing of the diamond membrane was improved by a lateral sputtering of the cluster ions. However, a thin graphite layer was formed on the surface by contamination of monomer ions in the cluster beam, which decreased the transparency of the diamond membrane. A subsequent irradiation with O-2 cluster ions removed these graphite layers.

Irradiation effects of Ar and O-2 cluster ion beams were studied on Chemical Vapor Deposition (CVD) diamond films. When the acceleration energy of the O-2 cluster ion was 20 keV, the sputtering yield was 400 atoms/cluster which is 13 rimes 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 O-2 cluster ions. By using both Ar and O-2 cluster ion beams, a very flat diamond surface without a graphite layer on the surface can be fabricated. (C) 1999 Elsevier Science B.V. All rights reserved.

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 him formation. (C) 1998 Elsevier Science S.A. All rights reserved.

This paper discusses angular distributions of sputtered particles from the target in cluster ion beam irradiation. For a copper target, the angular distribution of Cu particles with Ar cluster ion beams bombarding at a normal incidence was found to follow under-cosine law, whereas for Ar monomer ions the distribution follows the cosine law, as predicted from the linear cascade collision theory. From our molecular dynamics simulations, the sputtering mechanism with Ar cluster ions is as follows. At the impact of a cluster on a target a crater is formed. Numerous atoms acquire high lateral momentum and are ejected in the lateral direction. Subsequently, these particles cause an under-cosine distribution, and thus the behavior is drastically different from that with monomer ion bombardment. Experimental results show that the angular particle distribution is not strongly dependent on the incident angle above 10 degrees, but is very sensitive to the incidence angle below that. Microscopic images of a trace observed with scanning tunneling microscopy (STM) reinforce these results. (C) 1998 Elsevier Science S.A. All rights reserved.

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. (C) 1998 Elsevier Science S.A. All rights reserved.

Smoothing high-temperature superconductor (HTS) surfaces, especially HTS thin-film surfaces, is crucial for HTS thin-film device processing. In this letter, we describe a method to planarize the surface of a YBa2Cu3O7-delta HTS film down to a smoothness with a standard deviation of 1 nm or better. The method includes first smoothing the HTS surface by ion cluster beam bombardment, followed by annealing in oxygen ambient to regrow the damaged surface layer. Additional YBCO layers can be grown epitaxially on the treated surface, even without removing the top surface layer, which contained some residual damage after annealing. This method can be integrated into HTS circuit fabrication as a key step of planarization. (C) 1998 American Institute of Physics.

Unique characteristics of gas cluster ion beam processing are reviewed. Cluster ion beams consisting of a few hundreds to thousands of atoms have been generated from various kinds of gas materials. Multi-collisions during the impact of accelerated cluster ions upon the substrate surfaces produce fundamentally low energy bombarding effects in a range of a few eV to hundreds of eV per atom at very high density. These bombarding characteristics can be applied to shallow ion implantation high yield sputtering and smoothing, surface cleaning and low temperature thin film formation.

It has been observed that a cluster ion which contains several thousands of atoms produces unique sputtering effects. It has a high yield and a strong smoothing effect for various materials such as CVD diamond films, In this work, the sputtering yield, surface smoothing and angular distribution of the sputtered atoms have been measured, and the different sputtering mechanisms of cluster ions have been revealed. Cu films Were irradiated with Ar cluster ion beams accelerated up to 20keV, at several incident angles. From the energy and incident angle dependence of the sputtering yield, it was seen that the energy density is responsible for the sputtering. The Cu surfaces irradiated with Ar cluster ions were observed by Atomic Force Microscope. The surface roughness was smallest at normal incidence, and the surface roughness increased with the incident angle. A ripple structure was observed at incident angle of 60 degrees. The angular distribution of the sputtered atoms displayed an under-cosine shape at normal incidence, and many sputtered atoms were distributed in a lateral direction. a single trace of an Ar cluster ion impact measured by STM confirmed this result.

Reactive gas cluster ion beams were formed by an adiabatic expansion of SF6 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 SF6 cluster ion beams at an energy of 20 keV. Due to the chemical reaction of SF, 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 SF6 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 SF6 cluster ions was quite smooth, because of the smoothing effect of cluster ions.

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 theta, 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 processing with gas cluster ion beams which contain hundreds or even many thousands of atoms or molecules has been studied. The sputtering yields of various materials with Ar cluster ions are about one order of magnitude higher than those produced by Ar monomer ions with the same energy. Also, the sputtering yields of Si and W are chemically enhanced with SF6 cluster ion beams. The surface roughness of Cu dramatically decreased when bombarded with Ar cluster ion beams compared with Ar monomer ion beams, and there is no roughening mechanism. This smoothing effect of gas cluster ion beams is applicable for very hard materials, such as CVD diamond films.

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.

Reactive gas cluster ion beam etching, which has many advantages for plasma etching, is proposed. The anisotropic etching of Si is realized with a reactive cluster ion beam. Various kinds of cluster ion beams, from gaseous materials such as Ar, O-2, N-2, SF6, N2O, and CO2, can be generated by expanding them through a Laval nozzle into a high vacuum. The etching rate of W and Si bombarded by SF6 cluster ions was quite high as the result of reactive sputtering. Surprisingly, the sputtering yield reaches 2200 Si atoms/ion, when Si is sputtered by SF6 cluster ions, with an average size of 2000. This yield is two orders of magnitude larger than the value reported for reactive sputtering using monomer ions. 2.2 mu m of Si was etched by SF6 cluster ions, with an energy of 25 keV, at a dose of 5 x 10(15) ions/cm(2). Due to the low ion dose, the charge induced damage is reduced. The selectivity of Si to SiO2 increases with decreasing incident energy of cluster ions and finally infinite selectivity can be achieved at 5 keV. This energy corresponds to 2.5 eV per SF6 molecule, when the cluster size is 2000. A 0.5 mu m hole was etched by a SF6 ion cluster ion beam and anisotropic etching was demonstrated. (C) 1996 American Vacuum Society.

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. (C) 1996 American Vacuum Society.

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-300(+)) etching. We believe that the gas cluster ion beam etching technique will be a key technology for diamond device fabrication.