Gas Cluster Ion Beam Irradiation Research Articles

Nanoscale smoothing of plasmonic films and structures using gas cluster ion beam irradiation

We present a novel method of using gas cluster ion beam irradiation (GCIB) to flatten and widen grains in silver films and structures, while simultaneously, reducing the film thickness with nanometer precision. Ultrathin Ag films produced by GCIB have lower absorbance and better adhesion compared to as-deposited films. By applying the technique post-fabrication to plasmonic color filters, waveguides and disks, we show that an enhanced surface plasmon resonance and propagation length can be achieved.

Ripple formations by gas cluster ion irradiation in reactive gas environments

Effects of background gas (N2 or SF6) on ripple formation by oblique incidence gas cluster ion beam (GCIB) irradiation were investigated. When N2 gas was introduced as background gas, both ripple structures and the sputtering yield of Si and SiO2 did not change by Ar-GCIB irradiation at 60° incidence. However, wavelength of ripple became larger when SF6 gas was introduced as background gas. Increase of the sputtering yield in SF6 environment correlates with the change of ripple structures. It is assumed that SF6 molecules adsorbed on ripple structures, and Si or SiO2 ripples were etched preferentially.

In situ X-ray photoelectron spectroscopy study of gas cluster ion beam etching of FeCo film

FeCo films of the type used in spin transfer torque magnetoresistive random access memory were etched by gas cluster ion beam (GCIB) irradiation with acetic acid vapor and characterized by in situ X-ray photoelectron spectroscopy. After 20 keV O2-GCIB irradiation with acetic acid vapor, etching depth enhancement (10.7×) was observed compared with the results without acetic acid vapor. The etching model of FeCo can be described as follows: (1) FeCo oxide formation with O2-GCIB irradiation, (2) acetic acid adsorption on FeCo oxide, (3) reactions between FeCo oxide and acetic acid, and (4) the desorption of volatile compounds by local and transient heating owing to O2-GCIB bombardment. Cross-sectional transmission electron microscopy, transmission electron diffraction analysis, and electron energy loss spectroscopy results showed no significant etching damage or oxidation of FeCo films after etching by O2-GCIB irradiation with acetic acid vapor. Therefore, the low-damage etching of FeCo can be performed by O2-GCIB irradiation with acetic acid vapor.

Sub-30 nm thick plasmonic films and structures with ultralow loss

We report an alternative method of producing sub-30 nm thick silver films and structures with ultralow loss using gas cluster ion beam irradiation (GCIB). We have direct evidence showing that scattering from grain boundaries and voids rather than surface roughness are the main mechanisms for the increase in loss with reducing thickness. Using GCIB irradiation, we demonstrate the ability to reduce these scattering effects simultaneously through nanoscale surface smoothing, increase in grain width and lower percolation threshold. Significant improvement in electrical and optical properties by up to 4 times is obtained, before deviation from bulk silver properties starts to occur at 12 nm. We show that this is an enabling technology that can be applied post fabrication to metallic films or lithographically patterned nanostructures for enhanced plasmonic performance, especially in the ultrathin regime.

Ultrafine Particle Removal Using Gas Cluster Ion Beam Technology

In this paper, the ultrafine particle removal using CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> gas cluster ion beam (GCIB) technology is investigated. The CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> GCIB is irradiated the sample at an angle of 0 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> with respect to the surface normal. The higher particle removal efficiency can be achieved at the higher kinetic energy of the gas cluster, and the inside space of line and space pattern particles can be removed. We also suggested that the CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> GCIB process has the high particle removal uniformity without redeposition of the removed particles. It is possible to remove the ultrafine particle as small as 12 nm in diameter (which is required for 2014 by ITRS 2011). The pattern damage is not observed for 45 nm poly-Si pattern. Moreover, the molecular dynamics simulation is performed to investigate the mechanisms of the particle removal by GCIB irradiation.

Irradiation Effects of Gas Cluster Ion Beams on Co–Fe Films

Irradiation effects of gas cluster ion beam (GCIB) were studied on Co3Fe7 films as a novel etching process. Etching depth of Co3Fe7 increased with increasing the acceleration voltage (Va) and the ionization electron voltage (Ve), however, there is a tradeoff between etching rate and the surface roughness. Because multiply charged GCIBs were formed at high Ve (200 V), the surface layer and interface became rough. When low Ve (50 V) was applied for ionization of Ar-GCIB, crystalline CoFe2O4 layer (10 nm thick) was formed by Ar-GCIB irradiations at room temperature. Oxidation was due to water vapors during GCIB irradiation, which will be eliminated in ultra high vacuum. It is expected that gentle etching of magnetic films is possible with Ar-GCIB ionized at low Ve.

Magnetic properties of Fe7Co3 films with gas cluster ion beam irradiations

Gas cluster ion beam (GCIB) irradiation was performed on Fe7Co3 films to examine the magnetic properties. After Ar ion beam etching, the coercive force (Hc) increased from the initial value; this may have been caused by irradiation damage from high-energy Ar ions. Hc decreased after Ar-GCIB irradiation (acceleration voltage (Va): 20 kV, ion fluence (F): 1 × 1015 ions/cm2). Since GCIB is an equivalent low-energy (several eV/atom) ion beam, it shows a damage-recovery effect. When the ionization electron voltage (Ve) was reduced from 200 to 60 V, Hc was observed to further decrease. Since the fraction of multiply charged Ar-GCIB decreased with decreasing Ve, severe damage of Fe7Co3 films can be minimized by employing low Ve.

Development of gas cluster ion beam irradiation system with an orthogonal acceleration TOF instrument

Surface damage induced on biomolecules with gas cluster ion beam (GCIB) irradiation is significantly lower than with atomic or small cluster ion beams, and for this reason, surface analysis techniques such as secondary ion mass spectrometry (SIMS) have become one of the most important applications of GCIB, particularly for microscale chemical imaging of biomolecular species. Because of the low duty‐cycle in time‐of‐flight (TOF)‐SIMS, only less than 0.1 % of the incident ion beam is used for analysis, meaning that analysis with high spatial resolution can practically be extremely lengthy. The duty cycle can be significantly improved with the orthogonal acceleration (oa) TOF method because with this method secondary ion mass spectra can be measured at high mass resolution without requiring a pulsed primary ion beam. In this study, we developed a gas cluster ion irradiation system mounted on an oa‐TOF instrument and investigated the sputtering yield and secondary ion yield of arginine. Copyright © 2012 John Wiley &amp; Sons, Ltd.

Dependence of Pin Surface Roughness for Friction Forces of Ultrathin Perfluoropolyether Lubricant Film on Magnetic Disks by Pin-on-Disk Test

We fabricated supersmooth probes for use in pin-on-disk sliding tests by applying gas cluster ion beam irradiation to glass convex lenses. In the fabrication process, various changes were made to the irradiation conditions; these included one-step irradiation of Ar clusters or two-step irradiation of Ar and N2clusters, with or without Ar cluster-assisted tough carbon deposition prior to N2irradiation, and the application of various ion doses onto the surface. We successfully obtained probes with a centerline averaged surface roughness that ranged widely from 1.08 to 4.30 nm. Using these probes, we measured the friction forces exerted on magnetic disks coated with a molecularly thin lubricant film. Perfluoropolyether lubricant films with different numbers of hydroxyl end groups were compared, and our results indicated that the friction force increases as the surface roughness of the pin decreases and that increases as the number of hydroxyl end groups increases.

Effect of duty ratio of patterned surface on planarization by gas cluster ion beams

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 CHF3 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 5×1014 to 5×1016 ion/cm2. 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.

Surface Characterization and Lubricant Performance on Amorphous Carbon Films Irradiated With Gas Cluster Ion Beams

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 N2 -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 N2 -GCIB. From the study of lubricant affinity and the surface free energy measurements, nitrogen containing functional groups were formed with N2-GCIB irradiation, as a result, adhesion between the lubricant and the amorphous CVD carbon film increased.

Fabrication of Planarized Discrete Track Media Using Gas Cluster Ion Beams

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 N2-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).

Planarization of Nonmagnetic Films on Bit Patterned Substrates by Gas Cluster Ion Beams

We studied Ar gas cluster ion beam (GCIB) planarization effect on patterned surfaces refilled with Cr, Ta and SiO2. The patterns of 20 nm in depth were fabricated on Si substrate by using electron beam lithography and CHF3 -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 × 1015 to 5 × 1015 ion/cm2. 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 SiO2.

Refilling and Planarization of Patterned Surface with Amorphous Carbon Films by Using Gas Cluster Ion Beam Assisted Deposition

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.

Modification and smoothing of patterned surface by gas cluster ion beam irradiation

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.

Cluster size dependence of SiO 2 thin film formation by O 2 gas cluster ion beams

Cluster size effects of SiO 2 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 SiO 2 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 SiO 2 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 SiO 2 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 SiO 2 thin film formation can be controlled by energy per molecule.

Planarization of amorphous carbon films on patterned substrates using gas cluster ion beams

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.

Cluster size dependences of bombardment effects using mass-selected gas cluster ion beams

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 Au by Ar-GCIB was approximately 0.6eV/atom, which was significantly lower than that by Ar monomer ions (20eV). Although the energy per atom of the cluster ions was low, dense energy deposition occurred near the surface and surface Au 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.

Molecular dynamics study of glancing angle gas cluster irradiation on irregular-structured surfaces

Surface modification processes resulting from glancing angle gas cluster ion irradiation were studied by molecular dynamics (MD) simulations. Large Ar2000 gas clusters accelerated with 20keV were irradiated onto Si(100) substrates to which were attached small Si block-shaped (size of 4096, 16384 or 65536 atoms) structures. The MD simulations revealed that when an Ar cluster is irradiated at 80° of incident angle, the cluster slides on the substrate surface without damage and impacts on the wall of the block structures causing multiple collisions, and dynamic deformation of the blocks. Especially, in the case of the impact on the small surface block (Si4096), a large part of the block was sputtered as a large chunk, and the target substrate was smoothed with only one cluster impact. The evaluation of the damage distribution showed that silicon interstitials, caused by the multiple collision mechanism, reside within 15Å from the surface, which suggests that low-damage and high-performance surface modification processing would be realized by utilizing a glancing angle gas cluster ion beam irradiation technique.

Low-damage processing using size-controlled gas cluster ion beams

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 5keV 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.