豊田 紀章

We studied the effect of the duty ratio, i.e., the ratio of hill width to pitch, of patterned diamondlike carbon (DLC) surfaces on Ar gas cluster ion beam (GCIB) planarization effect. The patterns of 40 nm depth were fabricated on Si substrates by electron beam lithography and CHF(3) reactive-ion etching. The pitch of the line-and-space pattern was 300 nm and three duty ratios were adopted. Then, refilling materials were deposited to 50 nm thickness on the patterned substrates. The test samples were irradiated by Ar-GCIB and the resultant surface profiles were measured by atomic force microscopy. The acceleration energy for one cluster was 20 keV. The dose was set in the range from 5 x 10(14) to 5 x 10(16) ion/cm(2). Although there was a difference in the dose, the patterns clearly disappeared upon irradiating GCIB. The reduction rate of the peak-to-valley height decreased as the width of the hill increased. We indicated that GCIB irradiation is effective for the planarization of patterned surfaces with various duty ratios. (C) 2011 American Institute of Physics. [doi:10.1063/1.3556783]

Organic materials or polymers, which are widely used in electronic devices, are easily damaged by energetic ion or electron bombardment. Therefore, it is difficult to use conventional ion or electron beam processes for etching organic materials. In this study, a gas cluster beam possessing kinetic energy of the order of several hundred eV was used as a novel means to realize high-rate, low-damage etching. Polyethylene (PE) and polyvinyl chloride (PVC) were used as the target organic materials. Using a SF 6 cluster beam, the etching depth suddenly increased when the nozzle gas pressure exceeded 0.6 MPa, as in the case of cluster beam formation. When the SF 6 gas pressure in the nozzle was 1.2 MPa, the etching rates of PVC and PE were 2.88 μm/s and 1.62 μm/s, respectively. The dependence of the etching effect on cluster size was studied by varying the gas temperature. The etching depth of PVC increased with increasing average cluster size and intensity of the beam. The flow rate of the gas was constant hence, etching of the organic materials did not occur because of the individual impact of the molecular beam. In fact, it occurred because of the neutral cluster, which had a large total kinetic energy. The energy per molecule of the gas cluster beam is of the order of several tens of meV hence, high-rate, low-damage etching of organic materials can be potentially achieved. © 2011 Materials Research Society.

Organic materials have been widely used in various fields of electronic applications. However, they are difficult to process without damage by using a conventional ion beam which use energetic ions. In this study, gas cluster ion beam (GCIB), which shows low-damage process, was used for organic materials, and irradiation effect of size selected GCIB was studied with X-ray photoelectron spectroscopy (XPS). In the case of irradiation of 500 eV Ar ion (monomer ion) on polyimide, the intensities of both N-C=O and C-O bond decreased after irradiation. On the other hand, there was small change in the XPS spectra after 15 keV Ar-GCIB irradiation with the same ion dose. The etching rate of polyimide per one ion with 15 keV Ar-GCIB was almost 1.8×10 4 times higher than that with 500 eV Ar monomer ions. The damages in polyimide decreased with increasing the Ar cluster size owing to the reduction of energy per atom at acceleration voltage of 15 kV. After irradiation of size selected 5 kV Ar cluster ion, damage was almost negligible. Although, the surface became rough after irradiation of Ar-GClB, surface roughness and the change of chemical bond were very small with N 2-GCIB irradiation. Ar-GCIB irradiation on dye-sensitized solar cells (N719) showed that very low-damage process is possible with GCIB, and it indicated that GCIB is suitable for surface processing of organic materials used in electronic devices. © 2011 Materials Research Society.

Surface modification effects on patterned surface with gas cluster ion beam (GCIB) were studied by observation with a cross-sectional transmission electron microscope in order to use it for planarization of patterned media such as discrete track media (DTM) or bit-patterned media (BPM) for future hard disk drives. As a model structure of patterned media, line-and-space or bit patterns were fabricated on Si substrates, and subsequently amorphous carbon films were deposited on them. After Ar-GCIB irradiations on amorphous carbon, it was shown that GCIB preferentially removed bumps or crest on the surface of amorphous carbon at normal incidence. The required thickness for planarization was close to the initial peak-to-valley. At an incident angle of 57°, line-and-space patterns became sharp-pointed shape. On the contrary, line-and-space patterns were planarized without tiny asperity formation at 77°. These results indicate that quite effective planarization of patterned surface is possible using GCIB at normal or glancing angle irradiation. © 2011 Materials Research Society.

Gas cluster ions show dense energy deposition on a target surface, which result in the enhancement of chemical reactions. In reactive sputtering with gas cluster ions, the energy per atom or molecule plays an important role. In this study, the average cluster size (N, the number of atoms or molecules in a cluster ion) was controlled; thereby the dependences of the energy per molecule on the sputtering yields of carbon by CO(2) cluster ions and that of Si by SF(6)/Ar mixed gas cluster ions were investigated. Large CO(2) cluster ions with energy per molecule of 1 eV showed high reactive sputtering yield of an amorphous carbon film. However, these ions did not cause the formation of large craters on a graphite surface. It is possible to achieve very low damage etching by controlling the energy per molecule of reactive cluster ions. Further, in the case of SF(6)/Ar mixed cluster ions, it was found that reactive sputtering was enhanced when a small amount of SF(6) gas (similar to 10%) was mixed with Ar. The reactive sputtering yield of Si by one SF(6) molecule linearly increased with the energy per molecule. (C) 2010 Elsevier B.V. All rights reserved.

Irradiations of gas cluster ion beams (GCIB) on amorphous carbon films formed with chemical vapor deposition (CVD) were performed, and the affinity with lubricant was studied. The surface roughness of as-deposited amorphous CVD carbon film was improved with N(2)-GCIB irradiation. By reducing the formation of multiply charged cluster ions, large crater formations can be minimized, which helps to obtain smooth surface. It was also shown that nitrogen rich surface was formed on the amorphous CVD carbon with N(2)-GCIB. From the study of lubricant affinity and the surface free energy measurements, nitrogen containing functional groups were formed with N(2)-GCIB irradiation, as a result, adhesion between the lubricant and the amorphous CVD carbon film increased.

Fabrication of planarized discrete track media (DTM) by using gas cluster ion beams (GCIB) was demonstrated. Line-and-space patterns were fabricated using nanoimprint lithography and ion beam etching. These patterns were refilled by TiCr films, and the TiCr surface was planarized using Ar and N(2)-GCIBs. The GCIB process yielded excellent planarization of these patterns owing to the preferential modification of surface bumps and enhancement of the surface motion of atoms by GCIB irradiation. The flyability test of a slider flying at a 10-nm height on the DTM planarized with GCIBs indicated an almost 10% reduction in the acoustic emission (AE) output. Further, magnetic force microscope (MFM) and Kerr rotation measurements revealed that GCIB planarization did not produce any significant change in the magnetic characteristics of DTM. It was demonstrated that GCIB planarization is effective for fabricating DTM or similar structures such as bit-patterned media (BPM).

We studied Ar gas cluster ion beam (GCIB) planarization effect on patterned surfaces refilled with Cr, Ta and SiO(2). The patterns of 20 nm in depth were fabricated on Si substrate by using electron beam lithography and CHF(3)-reactive-ion-etching. The bit pattern pitches and the height of peak-to-valley were 150/200/300/400 nm and 20 nm, respectively. Then, refilling materials were deposited 30 nm in thickness on the patterned substrates. The test samples were irradiated by Ar-GCIB and the resultant surface profiles were measured by atomic force microscopy. Acceleration energy for a cluster was 20 keV. The dose was set from 2 x 10(15) to 5 x 10(15) ion/cm(2). Although there was a difference in the dose, the patterns disappeared clearly by irradiating GCIB. The reduction rate of peak-to valley height decreased with decreases of the pattern pitch. We indicated that GCIB irradiation is effective for the planarization of patterned surface refilled with Cr, Ta, and SiO(2).

A size-selected argon (Ar) gas-cluster ion beam (GCIB) was applied to the secondary ion mass spectrometry (SIMS) of a 1,4-didodecylbenzene (DDB) thin film. The samples were also analyzed by SIMS using an atomic Ar(+) ion projectile and X-ray photoelectron spectroscopy (XPS). Compared with those in the atomic-Ar(+) SIMS spectrum, the fragment species, including siloxane contaminants present on the sample surface, were enhanced several hundred times in the Ar gas-cluster SIMS spectrum. XPS spectra during beam irradiation indicate that the Ar GCIB sputters contaminants on the surface more effectively than the atomic Ar(+) ion beam. These results indicate that a large gas-cluster projectile can sputter a much shallower volume of organic material than small projectiles, resulting in an extremely surface-sensitive analysis of organic thin films. Copyright (C) 2010 John Wiley & Sons, Ltd.

A crystalline rutile titanium oxide (TiO2)-dispersed porous glass-ceramic was successfully prepared by the phase separation of aluminosilicate-based glass. A rutile TiO2 crystal was clearly observed after the heat treatment of 20TiO(2)center dot 29SiO(2)center dot 13Al(2)O(3)center dot 28CaO center dot 5MgO center dot 2P(2)O(5)center dot 2B(2)O(3)center dot 1ZrO(2) (mol%) glass at 900 degrees C for 15 h. The glass-ceramic obtained was then treated with an acid solution (3 mol/dm(3) nitric acid) to leach its soluble phases. The acid-treated sample became porous, and the glass-ceramic obtained had a large surface area of 74 m(2)/g. The porous glass-ceramic adsorbed methylene blue (MB) rapidly, and the adsorbed MB was photodecomposed by ultraviolet irradiation.

Surface planarization is important for fabrication of patterned media. One of the methods is smoothing of the patterned surface after deposition of refilling materials. However it requires two process steps. In this study, we studied planarization of patterned media by formation of refilling films with gas cluster ion beam (GCIB) assisted deposition to reduce the process step. Hard amorphous carbon films were deposited on line-and-space pattern (100 nm pitch, 20 nm in depth) by using Ar-GCIB assisted deposition. From the atomic force microscope and the cross-sectional transmission electron microscope observations, the line-and-space patterns were refilled with amorphous carbon films with Ar-GCIB assisted deposition and smooth surface was obtained. The thickness of the amorphous carbon film required for surface planarization was 32 nm, which was very small compared to the initial peak to valley (20 nm). By using this method, sputtering process for planarization can be omitted.

Gas cluster ion beam (GCIB) impacts produce an intense energy density near the surface, which enhance chemical reactions at room temperature. Since the irradiated surface becomes highly reactive, Ar-GCIB irradiation at low vacuum (1.5x10(16) Torr) induces oxidation with residual water vapor very easily whereas Ar(+) irradiation does not cause oxidation. However, oxidation on Si surface was not observed after Ar-GCIB irradiation at high vacuum condition (9.3x10(-8) Torr). In the case of N(2)-GCIB irradiation, Si oxynitride layer was formed at low vacuum whereas Si nitride was formed with N(2)-GCIB irradiation under high vacuum at room temperature. Furthermore, the thickness of Si nitride layer increased with the acceleration energy.

Porous glasses (PGs) with 4 nm in diameter pores were prepared utilizing a spinodal-type phase separation of Na2O-B2O3-SiO2 glass, and their surface was modified using titanium tetraisopropoxide. Fourier transform infrared measurements revealed that the absorbance for strong hydrogen-bonding OH groups increased after the modification. Bond overlap population (BOP), which is directly related to the strength of the covalent bond, between O and H of OH groups were calculated using density functional theory and first-principles calculations. BOPs decreased by the introduction of Ti atoms on pores. The proton conductivity at 80 degrees C and 90% relative humidity of the titania-modified PG was about five times higher than that of unmodified PG.

The surface smoothing of polycrystalline materials is difficult owing to the etching rate dependence on crystalline orientation. In this study, surface smoothing with a gas cluster ion beam (GCIB) was studied in polycrystalline substrates such as poly-SiC (surface roughness, 0.3 nm) and yttrium aluminum garnet (YAG; surface roughness, 1.1 nm). Before irradiation, there were many scratches induced by mechanical polishing on poly-SiC. These scratches were removed after Ar-GCIB irradiation. Additional O-2-GCIB irradiation for asperity removal showed improvement in the average roughness of poly-SiC to 0.3 nm. Similarly, the scratches on the as-polished surface of YAG disappeared and the YAG surface roughness decreased to 0.7nm with Ar-GCIB irradiation without inducing an etching rate difference between grains. It was also shown that polycrystalline diamond or poly-Si films can be smoothed using GCIB. These results indicate that GCIB polishing is suitable for the surface planarization of polycrystalline materials. (C) 2010 The Japan Society of Applied Physics

Planarization of nano-structured surface has become important for various devices such as the bit patterned media or discrete track media for next generation storage devices. In this study, gas cluster ion beam (GCIB) assisted deposition was used to deposit amorphous carbon film on line-and-space pattern to realize simultaneous refilling and planarization. As GCIB irradiations induce dense energy depositions, the density of the amorphous carbon film deposited with GCIB assisted deposition became high compared to the other chemical vapor deposition (CVD) based deposition technique. The line-and-space pattern was refilled and planarized by deposition of amorphous carbon films by using Ar-GCIB assisted deposition. The thickness required for planarization with amorphous carbon deposition using Ar-GCIB assisted deposition was approximately the initial peak-to-valley height of the line-and-space pattern. Thus, very effective refilling and planarization is realized by using GCIB assisted deposition. (C) 2010 The Japan Society of Applied Physics

Cluster size effects of N(2)-GCIB on Si substrates were studied. After GCIB irradiations, two layers (amorphous Si and transition layer (partially modified crystalline Si)) were formed. The thickness of the amorphous Si layer and transition layer became thin in the case of N(2)-GCIB irradiations at the same energy/atom or energy/molecule. With increasing the N(2)-GCIB cluster size, the modified layer thickness became much thinner (similar to 2 nm). Thus, ultra-thin surface modification is achieved with N(2)-GCIB without inducing thick transition layer.

Irradiation effect of gas cluster ion beams (GCIB) on organic materials were studied with X-ray photoelectron spectroscopy by comparison to that with Ar-monomer ions. In the case of polyimide, the intensity of both N-C=O and -C-O- bond decreased with 500 eV Ar monomer ion irradiation. On the other hand, there was no significant change in the XPS spectra after Ar-GCIB irradiation. From the size-selected GCIB irradiation study, the damages in polyimide decreased with increasing the cluster size owing to the reduction of energy per atoms.

Surface modification and smoothing of patterned surfaces with gas cluster ion beams were studied. In this work, line and space patterns having various intervals and depths were created on amorphous carbon films by focused Ga(+) ion beams, and subsequently, Ar GCIB irradiations on the pattern were performed. When the acceleration voltage of Ar cluster ions was 20 kV, the grooves, whose interval was below 200 nm, were planarized. However, it required much higher ion dose for wider interval of patterns. It is estimated that the distance of lateral motions induced by one cluster ion impact defines the spatial wavelength dependence of smoothing. (C) 2009 Elsevier B.V. All rights reserved.

Cluster size effects of SiO2 thin film formation with size-selected O-2 gas cluster ion beams (GCIBs) irradiation on Si surface were studied. The cluster size varied between 500 and 20,000 molecules/cluster. With acceleration voltage of 5 kV, the SiO2 thickness was close to the native oxide thickness by irradiation of (O-2)(20,000) (0.25 eV/molecule), or (O-2)(10,000) (0.5 eV/molecule). However, it increased suddenly above 1 eV/molecule (5000 molecules/cluster), and increased monotonically up to 10 eV/molecule (500 molecules/cluster). The SiO2 thickness with 1 and 10 eV/molecule O-2-GCIB were 2.1 and 5.0 nm, respectively. When the acceleration voltage was 30 kV, the SiO2 thickness has a peak around 10 eV/molecule (3000 molecules/cluster), and it decreased gradually with increasing the energy/molecule. At high energy/molecule, physical sputtering effect became more dominant process than oxide formation. These results suggest that SiO2 thin film formation can be controlled by energy per molecule. (C) 2009 Elsevier B.V. All rights reserved.

The planarization of the bit-patterned surface using gas cluster ion beams (GCIBs) was studied. By applying the features of gas cluster ions, such as low-energy irradiation and surface smoothing effects, it is possible to carry out effective smoothing of a patterned surface. We fabricated a dot pattern on the Si substrate as a model structure for bit-patterned media by electron beam lithography and inductive coupled plasma etching. A carbon overcoat covered the Si dots. On Ar-GCIB irradiation at an acceleration voltage of 20 kV, the Si dots with a diameter of 150 nm were planarized. The required ion dose for planarization was 5 x 10(14) ions/cm(2). The cross-sectional transmission electron microscope images showed that only the carbon overcoat layer was planarized without a change in the structure of Si dots. Etching depth of the overcoat increased linearly with the ion dose, indicating good reproducibility of the method with precise control of residual thickness.

Photoelectron spectra of the valence band region in diamond, diamond-like carbon (DLC), glassy carbon, and graphite were measured in order to investigate the influence of the carbon atom coordination on photoelectron spectra. Differences in the photoelectron spectra were observed, depending on the coordination of the carbon atoms. The photoelectron spectra for DLC and glassy carbon were compared to those simulated using the photoelectron spectra for diamond and graphite. From this comparison, it was found that the photoelectron spectra in DLC could not be simply reproduced using those for diamond and graphite. In addition, the photoelectron spectra of the valence band region in DLC were compared to the density of states previously calculated using the molecular dynamics method. (C) 2009 The Japan Society of Applied Physics

Size effects of gas cluster ions on beam transport or collisions with residual gases, for amorphous layer formation and for sputtering effects were studied using a size-selected GCIB system. A GCIB releases its constituent atoms by collisions with residual gases, and as a result, it loses its acceleration energy. The number of released atoms is determined by the energy per atom and the cohesive energy of gas. By controlling the cluster size and electron ionization energy to carry out low-energy irradiation, formation of amorphous layer or crater-like damages on surfaces can be reduced. The dependence of the sputtering yield of Au on cluster sizes shows that a linear relationship exists between the number of Au atoms sputtered by one At atom and the energy per atom. It is shown that the cluster size, which defines the energy per atom, is the most important parameter affecting beam transport, low-damage processing and sputtering with GCIB. (C) 2009 Elsevier B.V. All rights reserved.

Surface planarization and modification of a patterned surface were demonstrated using gas cluster ion beam (GCIB). Grooves with 100-400 nm intervals were formed on amorphous carbon films using focused ion beams to study the special frequency dependence of the planarization. Also, line and space patterns were fabricated on Si substrates, and amorphous carbons were deposited as a model structure of discrete track media. Subsequently, surface planarization using Ar-GCIB was carried out. After GCIB irradiations, all of the grooves were completely removed, and a flat surface was realized. And it showed that GCIB irradiation planarized grooves without huge thickness loss. From the power spectrum density of an atomic force microscope, GCIB preferentially removed grooves with small intervals. It was found from energy dispersive x-ray spectroscopy that surface planarization without severe damage in the amorphous carbon and magnetic layers was carried out with GCIB. (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3073665]

In the secondary ion mass spectrometry (SIMS) of organic substances, the molecular weight of the intact ions currently detectable is at best only as high as 1000 Da, which for all practical purposes prevents the technique from being applied to biomaterials of higher mass. We have developed SIMS instrumentation in which the primary ions were argon cluster ions having a kinetic energy per atom, controlled down to 1 eV. On applying this instrumentation to several peptides and proteins, the signal intensity of fragment ions was decreased by a factor of 102 when the kinetic energy per atom was decreased below 5 eV; moreover, intact ions of insulin (molecular weight (MW): 5808) and cytochrome C (MW: 12 327) were detected without using any matrix. These results indicate that fragmentation can be substantially suppressed without sacrificing the sputter yield of intact ions when the kinetic energy per atom is decreased to the level of the target's dissociation energy. This principle is fully applicable to other biomolecules, and it can thus be expected to contribute to applications of SIMS to biomaterials in the future. Copyright (C) 2009 John Wiley & Sons, Ltd.

The paper reviews the development of cluster ion beam technology, including historical background, fundamental characteristics of cluster ion to solid surface interactions, emerging industrial applications, and identification of some of the significant events which occurred as the technology has evolved into what it is today. Processes employing ions of clusters comprised of a few hundred to many thousand atoms are now being developed into a new field of ion beam technology. Cluster-surface collisions produce important non-linear effects which are being applied to shallow junction formation, to etching and smoothing of semiconductors, metals, and dielectrics, to assisted formation of thin films with nano-scale accuracy, and to other surface modification applications.

Energy losses of a gas cluster ion beam (GCIB) after collisions with residual gases were studied for Ar and CO(2) GCIBs. From the energy distribution of an Ar GCIB after collisions at various acceleration voltages and cluster sizes, it is found that GCIB lost its energy when its energy per atom was high. In addition, the energy loss of CO(2) GCIB was moderate compared to that of Ar GCIB at the same cluster size because of strong binging energy of CO(2) molecules. The energy loss mainly occurs as decrease of its cluster size. The number of the scattered atoms or molecules linearly increased with the energy per atom or molecule. The reduction of cluster size is determined by the energy / atom and the cohesive energy of the gas.

Precise surface processing of a silicon wafer was studied by using a gas cluster ion beam (GUB). The damage caused to the silicon surface was strongly dependent on irradiation parameters. The extent of damage varied with the species of source gas and the acceleration voltage (Va) of cluster ions. It also varied with the cluster size and residual gas pressure. The influence of electron acceleration voltage (Ve) used for ionization of a neutral cluster was also investigated. The irradiation damage, such as an amorphous silicon (a-Si) layer, a mixed layer of a-Si and c-Si (transition layer), and surface roughness, was increased with Ve. It is suggested that the increase in the amount of energy per atom was induced by high Ve, because of variation of the cluster size and/or cluster charge. An undamaged smooth surface can be produced by Ar-GCIB irradiation at low Ve and Va.

A new cluster time-of-flight secondary ion mass spectrometry (TOF-SIMS) was developed using a size-selected gas cluster ion as a projectile. Since a large gas cluster ion can generate many low-energy constituent atoms in a collision with the surface, it causes multiple and ultra low-energy sputtering. The mean kinetic energy of constituent atoms is provided by dividing the acceleration energy of the gas cluster ion by the number of constituent atoms. Therefore, the sputtering can be controlled to minimize the decomposition of sample molecules and substrate material by precisely adjusting the number of constituent atoms (the cluster size) and/or acceleration energy of the gas cluster ion. The cluster size was selected on the basis of the time-of-flight method using two ion deflectors attached along the ion-beam line. A high resolution of 11.7 was achieved for the cluster size/size width (M/Delta M) of Ar-cluster ions. (C) 2008 Elsevier B. V. All rights reserved.

We studied planarization of carbon overcoat by gas cluster ion beam (GCIB) for discrete track magnetic disks. We fabricated discrete tracks with variant pitches by focused ion beam on a 30-nm-thick carbon overcoat deposited on magnetic disks, and we planarized them with an Ar GCIB using lateral sputtering effect. It was found that planarization by GCIB was more effective for lower wavelengths than for those with a few hundreds nanometers. On the discrete tracks with 100-nm pitch and 20-nm depth, the initial peaks and valleys were removed by GCIB; however, on the ones with over 200-nm pitch, the initial peaks and valleys remained though the heights were drastically decreased. These results indicate the capability of GCIB planarization as a process for the discrete track media. Furthermore, cross sections were observed by transmission electron microscopy. The GCIB with relatively low dose could also planarize the 120-nm-pitch carbon tracks without etching the magnetic layer.

A gas cluster is an aggregate of a few to several thousands of gaseous atoms or molecules, and it can be accelerated to the desired energy after ionization. Since the kinetic energy of an atom in a cluster is equal to the total energy divided by the cluster size, a quite-low-energy ion beam can be realized. Although it is difficult to obtain low-energy monomer ion beams due to the space charge effect, equivalently low-energy ion beams can be realized by using cluster ion beams at relatively high acceleration voltages. The low-energy feature and the dense energy deposition at a local area are important characteristics of the irradiation by gas cluster ions. The diameter of a gas cluster with a cluster size of several thousands is only a few nanometers, so that thousands of atoms or molecules penetrate the target in an area that is only a few nanometers in diameter, which causes multiple collisions between target and cluster atoms. Therefore, all of the impinging energy of a gas cluster ion is deposited at the surface region, and this dense energy deposition is the origin of enhanced sputtering yields, crater formation, shock-wave generation, and other nonlinear effects. It is almost 20 years since the gas cluster ion beam (GCIB) equipment was first built in our laboratory. Since then, various kinds of GCIB equipment were constructed, and, currently, a GCIB system for 300-mm-diameter Si wafers is available. GCIB machines are being used for industrial applications, where a surface process is required. Surface smoothing, shallow doping, low-damage etching, trimming, and thin-film formations are promising applications of GCIBs. In this paper, the formation of the GCIB and the configuration of the GCIB equipment are summarized. Then, fundamental irradiation effects of the GCIB are discussed from the viewpoint of low-energy irradiation, sputtering, and dense energy deposition. Last, various applications of the GCIB are explained.

The process of sputtering by bombardment with gas cluster ions was investigated from the perspective of the kinetic energy per constituent atom (E-atom) of an incident cluster ion, which determines the threshold for the formation of craterlike defects by irradiation of an argon gas cluster ion beam (Ar-GCIB) onto a graphite surface. Furthermore, DNA molecules adsorbed on a graphite surface were preferentially sputtered by adjusting E-atom of the Ar-GCIB down to this threshold, while the substrate graphite surface retained its carbon lattice structure without the formation of craterlike defects These results indicate that a GCIB could be used as a primary ion beam for secondary ion mass spectrometry (SIMS), which would enable the preferential analysis of an adsorbed layer on a substrate without causing damage to the substrate.

Cluster size dependence of the surface morphology of gold was investigated using a size-selected gas cluster ion irradiation system equipped with a strong permanent magnet. In addition to the cluster size effects, the incident angle dependence of surface morphology was studied. At normal incidence, a gold surface was smoothed after irradiation with any size of cluster ions. At the incident angle of 45 degrees and 60 degrees, ripples with wavelength of several hundreds of nanometers were formed; however, there was no significant cluster size dependence of ripple structures. When the incident angle was increased to 70 degrees or 80 degrees, the direction of the ripples became parallel to the incident angles and the cluster size dependence became remarkable. (C) 2008 Elsevier B.V. All rights reserved.

As semiconductor devices become progressively smaller, it is important that wafers are fabricated with extremely uniform thicknesses. Although, GCIB technology can be used for corrective etching of Si wafers with high accuracy, an amorphous Si (a-Si) layer is formed in the process by the collision of cluster ions with the Si surface. We have shown that the a-Si layer formed on Si surfaces by Ar-GCIB and SF(6)-GCIB irradiation disappears completely and atomically smooth surfaces are formed after high-temperature annealing. In this study, the recovery of a surface damaged at various annealing temperatures was investigated in detail by TEM, AFM and SIMS. We found that the disappearance of the a-Si layer occurred by solid phase epitaxy (SPE) from low temperature and atomically smoothing at the surface occur by migration of Si atoms at high-temperature. Moreover, due to Ar-GCIB irradiation, At was found in high concentrations in the a-Si layer and crystal defects were formed on the surface after annealing. The Ar concentration on the Si surface could be decreased by irradiating SF(6)-GCIB after Ar-GCIB irradiation. (C) 2008 Elsevier B.V. All rights reserved.

The coordination of C atoms in diamond-like carbon (DLC) thin films formed by Ar gas cluster ion beam (GCIB) assisted deposition using fullerene as a carbon source was investigated using X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. From the curve fitting analysis of XPS spectra of the C 1s core level, the absolute sp(2) and sp(3) contents in the DLC films formed by Ar GCIB-assisted deposition were evaluated for the first time. The absolute sp(3) content of DLC films formed by the GCIB-assisted deposition at an acceleration voltage of 5 kV was the highest compared with that formed at 7 and 9 kV. In addition, the absolute sp(2) contents evaluated from XPS spectra were compared to the relative sp(2) contents evaluated from NEXAFS spectra. [DOI: 10.1143/JJAP.47.3380]

A cluster-ion irradiation system with cluster-size selection has been developed to study the effects of the cluster size for surface processes using cluster ions. A permanent magnet with a magnetic Field of 1.2 T is installed for size separation of large cluster ions. Trace formations at HOPG surface by the irradiation with size-selected Ar-cluster ions under an acceleration energy of 30 keV were investigated by scanning tunneling microscopy. Generation behavior of (he craterlike traces is strongly affected by the number of constituent atoms (cluster size) of the irradiating cluster ion. When the incident cluster ion is composed of 100 to 3000 atoms, craterlike traces are observed on the irradiated surfaces. In contrast, such traces are not observed at all with the irradiation of the cluster ions composed of over 5000 atoms. Such behavior is discussed on the basis of the kinetic energy per constituent atom of the cluster ion. To study GCIB irradiation effects on macromolecules, GCIB was irradiated on DNA molecules absorbed on graphite surface. Using GCIB irradiation, many more DNA Molecules were sputtered away compared with the monomer-ion irradiation. (C) 2008 Wiley Periodicals, Inc. Electron Comm Jpn, 91(2):40-45,2008; Published online in Wiley InterScience (www.interscience.wiley.coiii). DOI 10. 1002/ecj.10031

Mass-selected reactive gas cluster ion beams (GCIB) were formed using a permanent magnetic filter. Irradiations of CO(2) GCIB on amorphous carbon films and irradiations of SF(6) and SF(6)/Ar mixed GCIB on Si surfaces were performed to study the cluster size dependence on etching yields by reactive GCIB. The reactive sputtering yield of carbon by CO(2) GCIB was almost ten times higher than that by Ar GCIB. In the case of (CO(2))(20000) GCIB with energy of 20 keV (1 eV/atom), it showed the high sputtering yield of 200 atoms/ion, however, there was little crater formation on the carbon surface. It is thought that very soft etching without crater formation would take place in this condition. In the case of SF(6) GCIB on Si, the etching depth of Si showed maximum value when the fraction of SF6 to Ar was around 50%. As the etching yield was higher than pure SF(6) GCIB, there was a strong ion assisted etching effects in the case of Ar/SF(6) mixed cluster ion irradiations.

Processes employing clusters of ions comprised of a few hundred to many thousand atoms are now being developed into a new field of ion beam technology. Cluster-surface collisions produce important non-linear effects which are being applied to shallow junction formation, to etching and smoothing of semiconductors, metals, and dielectrics, to assisted formation of thin films with nano-scale accuracy, and to other surface modification applications. In 2000,a four year R&D project for development of industrial technology began in Japan under funding from the New Energy and Industrial Technology Development Organization (NEDO). Subjects of the projects are in areas of equipment development, semiconductor surface processing, high accuracy surface processing and high-quality film formation. In 2002, another major cluster ion beam project which emphasized nano-technology applications has started under a contract from the Ministry of Economy and Technology for Industry (METI). This METI project involved development related to size-selected cluster ion beam equipment and processes, and development of GCIB processes for very high rate etching and for zero damage etching of magnetic materials and compound semiconductor materials. This paper describes summery of the results.

Gas cluster ion beam (GCIB) was used for precise wafer fabrication process. GCIB realizes a quite low-energy ion beam and shows very precise mid good repeatability. To obtain thickness uniformity of Si over the whole wafer, small beam diameter (similar to 4mm) of GCIB was used. Thickness variations on the wafer can be reduced by location specific irradiation of collimated GCIB. By controlling the scan speed of GCIB irradiation based on the removal thickness at each irradiation position, thickness and height uniformity of Si can be unproved to several tens of run. In addition, etching. enhancement by using Ar/SF6 mixed cluster was studied.

Gas cluster ion beam (GCIB) shows characteristics of low-energy irradiation effect and dense energy deposition. GCIB-assisted deposition is expected to be used for high-quality fluoride film formation, although these films are sensitive to damage by ion irradiation. In this study, LaF3 and MgF2 films were deposited by SF6-GCIB-assisted deposition. When the acceleration voltage was 3-7 kV, the deposited LaF3 film had a high packing density, small columnar structure, and very flat surface. The average roughness of the LaF3 film surface was 0.25 nm by the surface smoothing effect of the GCIB. The LaF3 film density showed 5.90 g/cm(3), which was very close to the theoretical density of crystalline LaF3. The refractive index (n) of the LaF3 film increased from 1.62 to 1.72 with an increase in the ion current density. The extinction coefficient (k) was 1 X 10(-3) at a wavelength of 193 nm. By the deposition of a thin layer of Al2O3(extinction coefficient: 5 x 10(-3)) on the quartz substrate as an interfacial film, a strong adhesive fluoride film was obtained. (c) 2007 Published by Elsevier B.V.

Using an X-ray reflectivity method we estimated the density of several Diamond-Like carbon (DLC) films. Those films were about 100-nm-thick and formed on Si substrates by Gas-Cluster Ion Beam assisted (GCIB), RF-plasma-CVD (RF), ECR-plasma-CVD (ECR), ion plating (IP) and filtered-cathodic-vacuum-arc (FCVA) deposition methods. The X-ray reflectivity measurement using a conventional X-ray source revealed that the average density values of the GCIB and FCVA DLC films were larger than 2.7 g/cm(3), while those of other samples were less than 2.1 g/cm(3). We also performed the X-ray reflectivity measurement using a highly parallel X-ray microbeam, formed from synchrotron X-rays, of 2.0 mu m x 2.5 mu m in size which is about two orders of magnitude smaller than the conventional X-ray probe. The X-ray microbeam enables us to evaluate a local fluctuation of the film density and the thickness with a spatial resolution of the probe size. It is revealed that the density of some DLC films varies abruptly near the interface on a Si substrate. (c) 2007 Elsevier B.V. All rights reserved.

Gas cluster ion beam (GCIB) process has been used as a novel technique to realize very low-damage irradiations. The energy per atom of cluster ions can be reduced down to several eV/atom at several keV of total acceleration energy. In this study, size-controlled GCIB was formed using a strong permanent magnet, subsequently low-damage characteristics of large cluster ion irradiations were evaluated from the cross-sectional transmission electron microscope (TEM) observations of Si substrates. After size-controlled GCIB irradiation, the damaged or modified layer thickness was below several nm, when total acceleration energy was 5 keV and Ar cluster size was larger than 5000. Result obtained from spectroscopic ellipsometry also indicated the possibility of low-damage processing by size-controlled GCIB. (c) 2007 Elsevier B.V. All rights reserved.

Bombardment effects induced by mass-selected gas cluster ion beams (GCIB) were studied. In this study, a mass-selected GCIB system using a strong permanent magnet was employed and the cluster size effects of sputtering by GCIB irradiation were studied. The cluster size varied between 500 and 10,000 atoms/cluster. The sputtering yields of Au with mass-selected Ar-GCIB decreased with an increase in the cluster size because of a decrease in the energy per atom. The threshold energy for the sputtering of An by Ar-GCIB was approximately 0.6 eV/atom, which was significantly lower than that by Ar monomer ions (20 eV). Although the energy per atom of the cluster ions was low, dense energy deposition occurred near the surface and surface An atoms were effectively ejected. When the number of Ar atoms transferred by the clusters and the energy per atom were equivalent, the number of sputtered Au atoms was also same. (c) 2007 Published by Elsevier B.V.

Gas cluster ion beams (GCIB), containing huge aggregates with several thousands of atoms or molecules, were employed as assist ion beams for fluoride film formation. As gas cluster ions shows low-energy effects, chemical reaction enhancement and strong surface smoothing effects, GCIB-assisted deposition can be a desirable deposition technique for thin film multilayer structures for the ultra-violet region. In this work, SF6-GCIB was employed as the assist ion beam for deposition of fluoride films (MgF2 and LaF3). For fluoride films, it had been difficult to use energetic ions during deposition because of damage induced by ion bombardments. With SF6-GCIB irradiation, the surface roughness of LaF3 and MgF2 films decreased with increasing ion current density. From cross-sectional images, the film structure changed from columnar to bulk-like structure with high-dose SF6-GCIB irradiation. (C) 2007 Elsevier B.V. All rights reserved.

Novel precision machining using gas cluster ion beams (GCIB) is proposed to fabricate a small lens or free-surface mold. Properties of GCIB such as surface smoothing, high-rate etching and ease of forming a collimated beam are promising for novel precision machining. In this study, GCIB machining of a small mold (0.56 mm in diameter) and a free-surface mold was demonstrated. Before irradiation, a hillock (height 195 nm and width 240 gm) was observed on the center of the mold. By collimated GCIB irradiation with precise ion dose and position controls, the difference between designed and measured shape was decreased from 195 to 26 nm. Also, free-surface mold fabrication was demonstrated using position controlled GCIB. The error of etching depth from the designed shape was 10 nm for free-surface mold. (C) 2007 Elsevier B.V. All rights reserved.

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 associated with the impacts of atomic ions. These unique characteristics of gas cluster ion (GCIB) bombardment have been found to offer potential for various industrial applications. Prospective applications for these effects include low energy ion implantation, high rate sputtering, surface cleaning and smoothing, and low temperature thin film formation. This paper reviews recent R & D trends of GCIB processes. (C) 2007 Elsevier B.V. All rights reserved.

The surface structures of Si (100) wafers subjected to gas cluster ion beam (GCIB) irradiation have been analyzed by cross-sectional transmission electron microscopy (XTEM) and atomic force microscopy (AFM). GCIB irradiation is a promising technique for both precise surface etching and planarization of Si wafers. However, it is very important to understand the crystalline structure of Si wafers after GCIB irradiation. An Ar-GCIB used for the physically sputtering of Si atoms and a SF6-GCIB used for the chemical etching of the Si surface are also analyzed. The GCIB irradiation increases the surface roughness of the wafers, and amorphous Si layers are formed on the wafer surface. However, when the Si wafers are annealed in hydrogen at a high temperature after the GCIB irradiation, the surface roughness decreases to the same level as that before the irradiation. Moreover, the amorphous Si layers disappear completely. (c) 2007 Elsevier B.V. All rights reserved.

Cluster size dependence on energy distribution and velocity of gas cluster ion beam (GCIB) after collision with residual gas was studied using size-controlled GCIB system to make it clear the irradiation energy and cluster size under poor vacuum conditions. From the energy distribution measurements, an Ar cluster ion which was accelerated at high voltage (30 kV) was easier to lose its kinetic energy than that accelerated at low voltage (10 kV). Also, at the same acceleration voltage, the smaller Ar. cluster ion lost its energy rapidly than larger one. These results indicated that the energy loss during beam transport is determined by the velocity of cluster ions. If cluster was originated from the gas which has strong binding force such as CO2, large cluster ions kept their kinetic energy at constant even after frequent collisions with residual gas. (c) 2007 Elsevier B.V. All rights reserved.

A cluster-ion irradiation system with cluster-size selection has been developed to study the effects of the cluster size for surface processes using cluster ions. A permanent magnet with a magnetic field of 1.2 T is installed for size separation of large cluster ions. Trace formations at HOPG surface by the irradiation with size-selected Ar-cluster ions under acceleration energy of 30 keV were investigated by a scanning tunneling microscopy. Generation behavior of the crater-like traces is strongly affected by the number of constituent atoms (cluster size) of the irradiating cluster ion. When the incident cluster ion is composed of 100 - 3000 atoms, crater-like traces are observed on the irradiated surfaces. In contrast, such traces are not observed at all with the irradiation of the cluster-ions composed of over 5000 atoms. Such the behavior is discussed on the basis of the kinetic energy per constituent atom of the cluster ion. To study GCIB irradiation effects against macromolecule, GCIB was irradiated on DNA molecules absorbed on graphite surface. By the GCIB irradiation, much more DNA molecules was sputtered away as compared with the monomer-ion irradiation.

Gas Cluster Ion Beam (GCIB) process is applied for surface smoothing of one dimensional photonic band gap structure. After argon GCIB irradiations, hillocks on the Si3N4 surface were preferentially removed and smooth surface was realized. Reflectance spectra of PBG structure after Ar-GCIB irradiation was much closer to that of the theoretical line than that of the as-deposited sample. GCIB process will be a candidate for precise pattern formation which is required for Si micro photonics.

This paper reviews gas cluster ion beam (GCIB) technology, including the generation of cluster beams, fundamental characteristics of cluster ion to solid surface interactions, emerging industrial applications, and identification of some of the significant events which occurred as the technology has evolved into what it is today. More than 20 years have passed since the author (I.Y) first began to explore feasibility of processing by gas cluster ion beams at the Ion Beam Engineering Experimental Laboratory of Kyoto University. Processes employing ions of gaseous material clusters comprised of a few hundred to many thousand atoms are now being developed into a new field of ion beam technology. Cluster-surface collisions produce important non-linear effects which are being applied to shallow junction formation, to etching and smoothing of semiconductors, metals, and dielectrics, to assisted formation of thin films with nano-scale accuracy, and to other surface modification applications.