Ar Cluster Ion Research Articles

Low-Energy Irradiation Effects of Gas Cluster Ion Beams

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.

The effect of incident cluster ion energy and size on secondary ion yields emitted from Si

Secondary ions produced from a Si target bombarded with Ar cluster ions much larger than the molecular ions have been investigated. Secondary ions have been measured for a Si target bombarded with large Ar cluster ions at an incident energy between 7.5 and 27.5keV using a time-of-flight technique. The mean size of the Ar cluster ion was about 1000atoms/cluster, and then the collision energy per atom is between a few tens of eV and a few hundreds of eV. Secondary silicon cluster ions were detected under large Ar cluster ion bombardment. In this study, we succeeded to accurately measure the dependence of secondary ion spectra obtained by large Ar cluster ions on the incident cluster-ion size using a time-of-flight technique combined a primary-ion beam deflector and a secondary-ion deflector. The dependence of the secondary ion spectra under large Ar cluster ion bombardment on the incident cluster size is discussed.

Measurements of secondary ions emitted from organic compounds bombarded with large gas cluster ions

We propose to use cluster ions that are much larger than the molecular ions as primary ions for organic secondary ion mass spectrometry. Incident Ar cluster ion beams with energies from 10 to 20keV and a mean size of about 1000 atoms/cluster were used. Secondary ions were measured for a thin arginine film target (200nm) bombarded with large Ar cluster ions using a time-of-flight technique. Molecular ions of arginine and characteristic fragment ions were detected with high sensitivity. When large Ar cluster ions such as Ar1500 were incident on the arginine target, molecular ions of arginine were detected with little fragment ions. This indicates that large cluster ions can ionize arginine molecules without damaging them.

High-intensity Si cluster ion emission from a silicon target bombarded with large Ar cluster ions

Secondary ions emitted from Si targets were measured with a quadrupole mass spectrometer under large Ar cluster and monomer ion bombardment. Incident ion beams with energies from 7.5 to 25 keV were used and the mean size of the Ar cluster ion was about 1000 atoms/cluster. Si n + ions with n values up to n = 8 were detected under Ar cluster ion bombardment, whereas Si cluster ions were scarcely detected under Ar monomer ion bombardment. These cluster ion yields showed the power law dependence on the cluster size.

Secondary ion measurements for oxygen cluster ion SIMS

We have proposed to use oxygen cluster ions, which are much larger than molecular ions, as primary ions for secondary ion mass spectrometry (SIMS). A high intensity gas cluster ion source with a current density of a few μAs/cm 2 has been developed. Secondary ions emitted from Si have been investigated under O 2 and Ar cluster ion bombardment. A large enhancement of the yield of secondary ions produced by large O 2 and Ar cluster ions was found. The SIMS system utilizing large O 2 cluster ions must give both excellent depth resolution and high secondary ion yield.

Irradiation of silicon surface by Ar cluster ion beam: Cluster size effects

The influence of cluster size on damage layer thickness produced by Ar gas cluster ion beam (GCIB) irradiation is presented. The GCIB has a wide cluster size distribution which has been difficult to control. To obtain a narrow size distribution of the GCIB, the mass filter containing a strong permanent magnet is developed. In this study, the size-selected Ar-GCIB irradiates the Si substrates. The cluster size is varied between 500 and 20,000atoms/cluster. After irradiation, the damage layer thickness on Si substrates is measured by ellipsometry. The results of size-selected Ar-GCIB irradiation on Si substrates at acceleration energy of 5keV show that the damage decreases very quickly, from 7.9nm to 0.3nm with increasing the cluster size from 500 to 5000atoms/cluster. This tendency is in agreement with the results calculated by molecular dynamics simulation. These results suggest that the GCIB process is promising for low damage processing of semiconductor material.

Scanning Tunneling Microscopy Observation of Graphite Surfaces Irradiated with Size-Selected Ar Cluster Ion Beams

A cluster ion irradiation system with cluster size selection has been developed to study the effects of the cluster size in surface processing using cluster ions. A permanent magnet with a magnetic field of 1.2 T was installed for size separation of large cluster ions. Traces formed on a graphite surface by the impact with Ar cluster ions under an acceleration energy of 30 keV were investigated by scanning tunneling microscopy. The nature of the traces is strongly affected by the number of constituent atoms of the irradiating cluster ion. When the 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. This behavior is discussed on the basis of the kinetic energy per constituent atom of the cluster ion.

Interactions of argon cluster ion beams with silicon surfaces

The formation of irradiation damage for Si(0 0 1) surfaces by Ar cluster ion beams at different acceleration voltages was investigated by utilizing the Rutherford backscattering spectrometry (RBS) measurement. For the peak size of 10 000 atoms per cluster, the Si surfaces had no damage by the irradiation of the cluster ions, when the acceleration voltages were less than 5 kV. Furthermore, for the acceleration voltages larger than 5 kV, the amount of displacement atoms increased with an increase of the acceleration voltage. On the other hand, when the cluster size decreased, the threshold voltage for the damage formation decreased, and it was 2 kV for the peak size of 3000 atoms per cluster. To be compared with the experimental results, molecular dynamic (MD) simulation was used, and it showed that the irradiation of Ar cluster ions with large size and low incident energy did not produce any damage on the Si surfaces. In addition, the Si surfaces, which were previously damaged at a dose of 1 × 10 15 ions/cm 2 by the irradiation of high energy cluster ion beams such as 5 keV ion beams, were re-irradiated at a dose of 1 × 10 16 ions/cm 2 by low energy cluster ion beams such as 2 keV ion beams. The RBS channeling showed that the number of displacement atoms on the damaged surface decreased. The deposited energy on the Si surface was used to improve the crystalline state of the damaged surface, which suggests that the cluster ion irradiation has the ability to anneal the solid surfaces by adjusting the acceleration voltage and the cluster size.

Investigation of nanoprotrusion induced by isolated impact of Ar cluster ion beam on Si and GaAs crystal

Ar cluster was generated by an adiabatic expansion using a converging–diverging Laval nozzle at the pressure 5–7 bar and at room temperature. The size distribution of the cluster was estimated by a time-of-flight (TOF) spectroscopy and the mean size of the cluster at 5 bar was about 500 molecules. Ar cluster was ionized by electron impact and then irradiated on Si(100) and GaAs(100) semiconductor crystal surfaces at the very low fluence to investigate the interaction of cluster with the solid surface. From the isolated cluster impact at the acceleration 15–25 keV, nanoprotrusion with a few tens and hundreds of nanometer diameter and a few nanometer height was observed. This formation of out-grown nano-structure instead of inward crater is predicted due to the increase of the local pressure up to 10–100 GPa and local heating 104–105 K in ps by an impingement of cluster ion beam and re-bounce of liquid medium from the solid and subsequently quenching effect. In addition, the change of Si (100) surface roughness was investigated with the variations of Ar cluster ion dose, and from which phenomenological ESE model of cluster ion impact with solid surface, i.e., surface embossment(E), surface sputtering/smoothing(S), and surface etching(E), is proposed to happen in sequence.

Predissociation spectroscopy of the argon-solvated H5O2+ “zundel” cation in the 1000–1900 cm−1 region

Predissociation spectra of the H5O2+.Ar(1,2) cluster ions are reported in the 1000-1900 cm(-1) region. The weakly bound argon atoms enable investigation of the complex in a linear action mode, and the resulting spectra are much simpler than those reported previously in this region [Asmis et al., Science 299, 1375 (2003) and Fridgen et al., J. Phys. Chem. A 108, 9008 (2004)], which were obtained using infrared multiphoton dissociation of the bare complex. The observed spectrum consists of two relatively narrow bands at 1080 and 1770 cm(-1) that are likely due to excitation of the shared proton and intramolecular bending vibrations of the two water molecules, respectively. The narrow linewidths and relatively small (60 cm(-1)) perturbation introduced by the addition of a second argon atom indicate that the basic "zundel" character of the H5O2+ ion survives upon complexation.

Evaporatively cooled M+ (H2O)Ar cluster ions: infrared spectroscopy and internal energy simulations.

Rotationally resolved IR spectra of M+ (H2O)Ar cluster ions for M=Na, K, and Cs in the O-H stretch region were measured in a triple-quadrupole mass spectrometer. Analysis of the spectra yields O-H stretch vibrational band origins and relative IR intensities of the symmetric and asymmetric modes. The effect of the alkali-metal ions on these modes results in frequency shifts and intensity changes from the gas phase values of water. The A-rotational constants are also obtained from the rotational structure and are discussed. Experimentally, the temperatures of these species were deduced from the relative populations of the K-rotational states. The internal energies and temperatures of the cluster ions for Na and K were simulated using RRKM calculations and the evaporative ensemble formalism. With binding energies and vibrational frequencies obtained from ab initio calculations, the average predicted temperatures are qualitatively consistent with the experimental values and demonstrate the additional cooling resulting from argon evaporation.

Optimum Incident Angle of Ar Cluster Ion Beam for Superhard Carbon Film Deposition

Diamond-like carbon (DLC) films with super hardness (>50 GPa) were deposited using Ar cluster ion beams at various oblique incident angles irradiated towards substrates on which C60 was simultaneously deposited. When the incident angles of Ar cluster ions were from 0° to 20°, the DLC films had a super hardness of approximately 50 GPa, the hardness three times higher than that of conventional DLC films. Furthermore, the films had a flat surface and a low sp2 content, which was estimated with a near edge X-ray absorption fine structure (NEXAFS). However, in the range of angles above 40°, the carbon films had a lower hardness, a rougher surface, and a higher sp2 content. Therefore, the incident angles of the Ar cluster ions for obtaining super hardness ranged from 0° to 20°.

A new secondary ion mass spectrometry (SIMS) system with high-intensity cluster ion source

Secondary ion mass spectrometry (SIMS) with gas cluster ion beams (GCIB) was investigated with experiments and molecular dynamics (MD) simulations. A new cluster SIMS system with high-intensity cluster ion source has been developed. Ar cluster ion beam with the average size of 2000 was utilized as a primary ion beam. The beam diameter is less than 1 mm at the target position, and a current density of 10 μA/cm 2 is obtained. The etch rate of Si with this current density is more than 20 nm/min, which is far beyond the etch rate in recent low energy SIMS system. The secondary ion intensity linearly increases with the acceleration voltage. A threshold voltage of a few keV for secondary ion emission was found. These results are consistent with previous sputtering experiments.

Characterization of hydrated Na+(phenol) and K+(phenol) complexes using infrared spectroscopy

Hydrated alkali metal ion-phenol complexes were studied to model these species in aqueous solution for M=Na and K. IR predissociation spectroscopy in the O-H stretch region was used to analyze the structures of M+(Phenol)(H2O)n cluster ions, for n = 1-4. The onset of hydrogen bonding was observed to occur at n=4. Ab initio calculations were used to qualitatively explore the types of hydrogen-bonded structures of the M+(Phenol)(H2O)4 isomers. By combining the ab initio calculations and IR spectra, several different structures were identified for each metal ion. In contrast to benzene, detailed in a previous study of Na+(Benzene)n(H2O)m [J. Chem. Phys. 110, 8429 (1999)], phenol is able to bind directly to Na+ even in the presence of four waters. This is likely the result of the sigma-type interaction between the phenol oxygen and the ion. With K+, the dominant isomers are those in which the phenol O-H group is involved in a hydrogen bond with the water molecules, while with Na+, the dominant isomers are those in which the phenol O-H group is free and the water molecules are hydrogen-bonded to each other. Spectra and ab initio calculations for the M+(Phenol)Ar cluster ions for M=Na and K are reported to characterize the free phenol O-H stretch in the M+(Phenol) complex. While pi-type configurations were observed for binary M+(Phenol) complexes, sigma-type configurations appear to dominate the hydrated cluster ions.

Surface smoothing with large current cluster ion beam

A cluster is an aggregate of a few to several thousands atoms. When many atoms constituting a cluster ion bombard a local area, high-density energy deposition and multiple-collision processes are realized. Because of the interactions, cluster ion beam processes can produce new surface modification effects, such as surface smoothing, high rate sputtering and very shallow implantation. Especially, the cluster ion beam can smooth surfaces of diamond films that are very hard materials and difficult to machining with usual ion processes. High ion dose is needed to realize the nano-level smoothing and etching of hard materials using cluster ion beam and large current is needed to increase the productivity of processing. In order to achieve large current cluster ion beam, the cluster generation was studied and the beam current reached 600 μA. The size distributions of large current Ar cluster ion beams were measured using compact time-of-flight (TOF) system. A surface of diamond film could be smoothed with the large current cluster ion beam and the surface roughness ( R a) became 5.1 nm from 32 nm.

Near Edge X-Ray Absorption Fine Structure Study for Optimization of Hard Diamond-Like Carbon Film Formation with Ar Cluster Ion Beam

Diamond-like carbon (DLC) film deposited using C60 vapor with simultaneous irradiation of an Ar cluster ion beam was characterized by a near edge X-ray absorption fine structure (NEXAFS), in order to optimize the hard DLC film deposition conditions. Contents of sp2 orbitals in the films, which were estimated from NEXAFS spectra, are 30% lower than that of a conventional DLC film deposited by a RF plasma method. Those contents were obtained under the flux ratio of the C60 molecules to the Ar cluster ions to range from 1 to 20, at 5 keV of Ar cluster ion acceleration energy. Average hardness of the films was 50 GPa under these flux ratios. This hardness was three times higher than that of a conventional DLC film. Furthermore, the lowest sp2 content and above-mentioned high hardness were obtained at room temperature of the substrate when the depositions were performed in the range of the substrate temperature from room temperature to 250°C.

Modeling of surface smoothing process by cluster ion beam irradiation

Smoothing effect is one of the advantages of cluster ion beam. It is important to estimate an ion dose to achieve required surface smoothness. The smoothing rate with cluster ion beam depends on surface profiles. In this work, modeling of surface smoothing process with cluster ion beams was examined. Ar cluster ions with an energy of 20 keV was irradiated on rough Si Surface. Fast Fourier transform was applied to atomic force microscope data of 100 × 100 μm 2, and power spectra were calculated. Smoothing rate depends on the wave number as well as the ion dose. Relationship between smoothing rate and wave number was derived. The surface smoothing process was modeled with the use of the wave number dependence of the smoothing rate. The calculated and the experimental surface profiles are in good agreements.

NEXAFS study on substrate temperature dependence of DLC films formed by Ar cluster ion beam assisted deposition

For the optimization of the synthesis condition of the production of diamond-like carbon (DLC) thin films by Ar gas cluster ion beam (GCIB) assisted deposition of fullerene, the local structure of DLC thin films was investigated by measuring near-edge X-ray absorption fine structure spectra of the carbon K-edge over the excitation energy range 275–320 eV, using synchrotron radiation. The DLC thin films were formed by GCIB assisted deposition at substrate temperatures ranging from room temperature to 250 °C. The sp 2 content estimated from the analysis of the peak corresponding to the transition of the excitation electron from a carbon 1s orbital to a π * orbital was found to increase with the substrate temperature during GCIB assisted deposition.

Influence of residual Ar + in Ar cluster ion beam for DLC film formation

In order to study the influences of residual Ar monomer ion (Ar +) on sp 2 content and hardness of diamond like carbon (DLC) films formed by Ar cluster ion beam assisted deposition, Ar cluster ion, Ar + and their mixed ions (Ar cluster ion and Ar +) bombardments were performed during evaporation of C 60. From near edge X-ray absorption fine structure (NEXAFS) and Raman spectroscopy measurements, lower sp 2 content in the carbon films was obtained with Ar cluster ion bombardment than that with Ar + and mixed ion. Furthermore higher hardness and smooth surface were shown with Ar cluster ion bombardments. Therefore it was important to reduce Ar + in Ar cluster ion beams to obtain hard DLC films with flat surface.

Optimization of flux ratio of C 60 molecule to Ar cluster ion for diamond-like carbon film deposition by NEXAFS measurement

To optimize the flux ratio of C 60 to Ar cluster ion during diamond-like carbon (DLC) film deposition, DLC films deposited under various flux ratios were characterized with Raman spectrometry and near edge X-ray absorption fine structure (NEXAFS). From Raman spectrometry, DLC films were deposited when the flux ratio of C 60 to Ar cluster ion was between 0.7 and 4. When the ratio was over 10, C 60 films were mainly deposited. As it was difficult to do precise characterization of DLC with Raman spectrometry, NEXAFS measurements were performed for the films deposited in the DLC deposition range of flux ratio. From NEXAFS spectra, there was no significant difference in sp 2 contents when the flux ratios were between 1 and 4. Hardness of these DLC films was between 40 and 45 GPa. Therefore very hard DLC film could be deposited with Ar cluster ion beam assist deposition over wide range of flux ratios, and this deposition process was very stable against changes of the flux ratio.