Gas Cluster Ion Beam Processing Research Articles

Surface modification with gas cluster ion beams from fundamental characteristics to applications

Gas cluster ion beam (GCIB) processing has been applied for the ultra-smooth surfaces preparation recently. As GCIB exhibit significantly different physical phenomena from atomic or molecular ion beams, new fields of ion beams have been developed by using GCIB. In this paper, fundamental characteristics of GCIB will be summarized and surface smoothing applications are explained. Additionally, it has been shown that GCIB is suited for assisting ion beams during thin film depositions. Recent results of GCIB assisted depositions are also summarized in the latter part of this paper.

Gas-cluster ion-beam smoothing of chemo-mechanical-polish processed GaSb(100) substrates

Gas-cluster ion-beam (GCIB) processing of surfaces provides individual atoms within an accelerated gas cluster (∼1,500 atoms per cluster), an energy approximately equal to the individual bond energy of the target surface atoms. The gas-cluster beam is thus capable of providing smoothing and etching of the extreme surface of numerous semiconductors, metals, insulators, and magnetic materials. For semiconductor material systems, the gas-cluster processing effect on the surface and subsurface material is of critical interest for device and circuitry application integrity. In the case of III–V GaSb, chemo-mechanical or touch polishing is the final step in the semiconductor-wafer manufacturing process, often leaving scratches of various depths or damage on the polished surface. In this paper, we report the GCIB etching and smoothing of chemical-mechanical polished GaSb(100) wafers. Using a dual-energy, dual gas-cluster source process, ∼100 nm of material was removed from a GaSb(100) surface. Atomic-force microscopy (AFM) imaging and power spectral-density (PSD) analysis shows significant decrease in the post-GCIB root-mean-square (Rms) roughness and peak-to-valley measurements for the material systems. X-ray rocking-curve analysis has shown a 24-arcsec reduction in the full-width at half-maximum (FWHM) of the (111) x-ray diffraction peak of GaSb. High-resolution transmission-electron microscopy (HRTEM) shows the crystallinity of the subsurface of the pre- and post-GCIB surfaces to be consistent, following the 1 × 1016 ions/cm2 total-fluence processes, with dislocation density for both pre- and post-GCIB cases below the HRTEM resolution limit. X-ray photoelectron spectroscopy (XPS) indicates a strong Ga 3p electron binding-energy intensity for gallium-oxide formation on the GaSb surface with the use of an oxygen GCIB process. Analysis of the Ga 3p electron binding-energy peaks in the XPS data in conjunction with HRTEM indicates a higher Ga or GaSb content in the near-surface layer (less stoichiometric-oxide presence) with use of a CF4/O2 GCIB process. The same peak analysis indicates that the surface gallium-oxide state is nearly unchanged, except in thickness, with the use of an O2-GCIB second step. The material results suggest that GCIB provides a viable method of chemo-mechanical polish (CMP) damage removal on group III–V material for further device processing.

Gas cluster ion beam processing of gallium antimonide wafers for surface and sub-surface damage reduction

In order to bring low-power epitaxy-based gallium antimonide (GaSb) electronics and electro-optics to market, high-quality GaSb substrates with smooth surfaces and no surface damage are required. Here, a novel final polishing technique, gas cluster ion beam (GCIB) processing, is shown to improve the surface finish of chemical–mechanical polished (CMP) 50 mm (1 0 0) GaSb wafers by etching and smoothing CMP surface atoms through the sub-surface damage. For the first time, a fluorine-based gas cluster ion beam is reported for GCIB surface etching and smoothing of GaSb material. For the selected processing sequence, the surface roughness of a high-quality, 0.70 nm RMS GaSb wafer was reduced to 0.18 nm RMS without any observed changes in the full-widths at half-maximum (FWHM) of the (4 0 0) and (1 1 1) X-ray peaks of 14 and 20 arcsec, respectively. Results indicate that the GCIB process did not contribute to wafer surface or sub-surface polish damage. In a second case, a GCIB etch removed 200 nm of material from a non-optimal CMP (1 0 0) GaSb surface and reduced the full-width at half-maximum (1 1 1) X-ray peak from 76 to 52 arcsec in conjunction with a surface roughness decrease from 0.70 to 0.35 nm RMS. The data suggests that GCIB processing appears to be promising as a final GaSb wafer polish with an etch rate compatible for large scale manufacturing.

Gas cluster ion beam applications and equipment

Interactions of gas cluster ions with solid surfaces, and prospective applications for those interactions, have been research topics of interest for several years. Substantial development efforts have been directed towards the problems associated with generating and transporting cluster ion beams having intensities sufficient to allow utilization in practical processing equipment. Unique smoothing, etching and chemical transformation effects which can be produced by cluster ion beams have been applied to a broad range of materials. Original gas cluster ion beam (GCIB) equipment which provided nanoampere level beam currents has evolved into automated milliampere level machines offering excellent process throughput, uniformity and reproducibility over large area targets. This paper will review cluster ion interactions, example applications and the present status of commercial GCIB processing equipment.

Gas Cluster Ion Beam Processing of GaSb and InSb Surfaces

ABSTRACTGas Cluster Ion Beam (GCIB) processing has recently emerged as a novel surface smoothing technique to improve the finish of chemical-mechanical polished (CMP) GaSb (100) and InSb (111) wafers. This technique is capable of improving the smoothness CMP surfaces and simultaneously producing a thin desorbable oxide layer for molecular beam epitaxial growth. By implementing recipes with specific gas mixtures, cluster energy sequences, and doses, an engineered oxide can be produced. Using GaSb wafers with a high quality CMP finish, we have demonstrated surface smoothing of GaSb by reducing the average roughness from 2.8Å to 1.7Å using a dual energy CF4/O2-GCIB process with a total charge fluence of 4×1015ions/cm2. For the first time, a GCIB grown oxide layer that is comprised of mostly gallium oxides which desorbed at 530°C in our molecular beam epitaxy system is reported, after which GaSb/AlGaSb epilayers have been successfully grown. Using InSb, we successfully demonstrated substrate smoothing by reducing the average roughness from 2.5Å to 1.6Å using a triple energy O2-GCIB process with a charge fluence 9×1015ions/cm2. In order to further demonstrate the ability of GCIB to smooth InSb surfaces, sharp ∼900nm high tips have been formed on a poorly mechanically polished InSb (111)A wafer and subsequently reduced to a height of ∼100nm, an improvement by a factor of eight, using a triple energy SF6/O2-GCIB process with a total charge fluence of 6×1016ions/cm3.

Computer modeling and electron microscopy of silicon surfaces irradiated by cluster ion impacts

A hybrid molecular dynamics model has been applied for modeling impacts of Ar and decaborane clusters, with energies ranging from 25 to 1500 eV/atom, impacting Si surfaces. Crater formation, sputtering, and the shapes of craters and rims were studied. Our simulation predicts that on a Si(1 0 0), craters are nearly triangular in cross-section, with the facets directed along the close-packed (1 1 1) planes. The Si(1 0 0) craters exhibit four fold symmetry. The craters on Si(1 1 1) surface are well rounded in cross-section and the top-view shows a complicated six fold or triangular image. The simulation results for individual gas cluster impacts were compared with experiments at low dose (10 10 ions/cm 2 charge fluence) for Ar impacts into Si(1 0 0) and Si(1 1 1) substrate surfaces. Atomic force microscopy and cross-sectional high-resolution transmission electron microscope imaging of individual gas cluster ion impacts into Si(1 0 0) and Si(1 1 1) substrate surfaces revealed faceting properties of the craters and are in agreement with the theoretical prediction. The sputtering yield from Si(1 0 0) surfaces bombarded with B 10 cluster ions, with total energy of 1–15 keV, was also calculated. The results of this study will be helpful for the research and development of a new low-damage gas cluster ion beam process technology.

Materials processing by gas cluster ion beams

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.

Substrate smoothing using gas cluster ion beam processing

In order to smooth a wide variety of surface material types to within an angstrom of roughness without subsurface damage, a beam energy equivalent to the individual bond energy of the surface atoms would be required. This would ideally preserve the integrity of the underlying material matrix and eliminate both high and low frequency surface aberrations. A low energy ion beam of a few hundred eV would be difficult to produce with significant intensity, however, due to the space charge effect encountered from the increase in density of the ions produced (“beam blow-up”). The use of a gas cluster ion beam (GCIB) process, however, has proven to provide an effective atomic smoothing on numerous material surface compositions without causing subsurface damage.1–3 This paper provides a description of the recently developed GCIB surface smoothing and modification apparatus, a discussion of the mechanism for surface smoothing, and provides a focus on thin film Si, SiC, and semiconductor-on-insulator (SOI) material results.

Frequency response of common lead and shield type magnetic tunneling junction head

In this work, the frequency response in magnetic tunnel junction (MTJ) heads was studied. Both stray capacitance and junction resistance, forming a low-pass-filter, have to be reduced to improve the cutoff frequency in MTJ heads. By employing an Ar gas cluster ion beam (GCIB) process, junctions grown on the magnetic shield show a resistance area product as low as 3.6 /spl Omega//spl mu/m/sup 2/ and tunneling magneto-resistance over 14%. The dominant capacitance in common lead and shield MTJ heads was found mainly resulting from the shield-to-shield spacing, whose capacitance can be reduced by using an SiO/sub 2/ gap layer instead of Al/sub 2/O/sub 3/ layer and thus leading to an improved frequency response. Simple analysis indicates that a read amplifier design with low impedance could be helpful to realize a high data transfer rate, and a rate of around 800 Mbps for 100 Gbits/in/sup 2/ recording system can be thus expected.

Surface processing with gas-cluster ions to improve giant magnetoresistance films

The reduction of roughness, without introducing damage, of thin-film surfaces in giant magnetoresistance (GMR) applications will be essential in the development of advanced devices. Tools and methods to accomplish this are limited at present. Gas-cluster ion beam (GCIB) technology shows promise as a dry, low-temperature process that can provide substantial improvement, and can be integrated into GMR-film deposition-and-etch tools. In this work, we describe recent GCIB technique developments and processes for tantalum, alumina, permalloy, and other relevant materials. With argon GCIB it is possible to reduce the roughness of many films to well below a nanometer (root-mean-square), with the roughness falling exponentially with cluster dose. Prototype magnetic films for evaluation were fabricated on GCIB-smoothed alumina gap layers. Transmission electron microscopy revealed changes in roughness and grain morphology that may be correlated with magnetic properties.

Magnetic tunnel junctions on magnetic shield smoothed by gas cluster ion beam

In this work, a technique, gas cluster ion beam (GCIB), was introduced to smooth the bottom NiFe magnetic shield for magnetic tunnel junction (MTJ) read heads. The GCIB treatment can bring the surface roughness of the shield from 15 to 20 Å to around 5 Å, and the most of scratch marks can be removed. The efficiency of the GCIB process is dependent on the initial surface morphology. The MTJs grown on the magnetic shield smoothed by the GCIB show that the resistance area product RA is increased from 60 to ∼100 Ω μm2 with the GCIB dose up to 1×1016 ions/cm2, arising from a smooth insulating layer, meanwhile, the tunneling magnetoresistance (TMR) is almost constant or slightly decreases. This GCIB process can also improve breakdown voltage (approximately 0.019 V per 1015 ions/cm2) of the MTJs, and slightly increase the ferromagnetic coupling mainly due to the change of the surface morphology. Using this technology, an RA as low as 3.5–6.5 Ω μm2 together with a TMR of 14%–18% can be obtained for MTJs grown on the GCIB treated NiFe magnetic shield.

Etching, smoothing, and deposition with gas-cluster ion beam technology

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 ∼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<∼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.

Nano-processing with gas cluster ion beams

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.

Smoothing Thin Films with Gas-Cluster Ion Beams

ABSTRACTThin-film magnetic sensor and memory devices in future generations may benefit from a processing tool for final-step etching and smoothing of surfaces to nearly an atomic scale. Gas-cluster ion-beam (GCIB) systems make possible improved surface sputtering and processing for many types of materials. We propose application of GCIB processing as a key smoothing step in thin-film magnetic-materials technology, especially spin-valve GMR. Results of argon GCIB etching and smoothing of surfaces of alumina, silicon, permalloy and tantalum films are reported. No accumulating roughness or damage is observed. The distinct scratches and tracks seen in atomic-force microscopy of CMP-processed surfaces, are removed almost entirely by subsequent GCIB processing. The technique primarily reduces high spatial-frequency roughness and renders the topographic surface elevations more nearly gaussian (randomly distributed).

Low-energy cluster ion beam modification of surfaces

In cluster ion beam processes, multiple collisions during the impact of accelerated cluster ions upon the substrate surfaces produce fundamentally non-linear processes in a low-energy regime. The gas cluster ion beam process has been applied to produce ultra-shallow junctions (less than 7 nm in Si at 2 keV), very high-rate sputtering (more than two orders of magnitude higher than by monomer ions having the same energy), atomic scale smoothing of surfaces (to average roughness less than 0.2 nm) and reactive thin film formation at low substrate temperatures. This paper will review recent requirements for ion beam processing and the status of ion beam equipment development. Several characteristics of the gas-cluster ion beam processing are capable of meeting these recent requirements and will be demonstrated by our recent experimental and molecular dynamics simulation results.

Surface Smoothing of Polycrystalline Si Waveguides With Gas-Cluster Ion Beams

AbstractPolycrystalline 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°C, 1 μm thick) ranged from 15 nm to 22 nm, and the grain sizes were distributed from 0.2 to 0.4μm. 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 λ =1.54 μm 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.

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

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

Cluster ion beam processing

Recent requirements of materials processing by ion beams are now beyond their fundamental limits. Technologies which are much more controllable and versatile on an atomistic scale are needed. Gas cluster ion beam processing which has been developed at Kyoto University is one of the candidates to overcome the present limits. This paper discusses recent requirements in ion beam processing. Possible solutions by cluster ion beam processing are discussed in the following areas: (1) shallow junction formation; (2) high rate etching and atomically smooth surface formation; and (3) high quality thin film deposition. The gas cluster ion beam processing is discussed in comparison with traditional ion beam processing which is presently limited by available atomic and molecular ion beams.

Advances in Ion and Laser Beam Technology: Achievements of Japanese Government and University Projects

ABSTRACTThis paper reviews recent R&amp;D activity in advanced beam technologies funded by MITI and JST. A large-scale national project known as AMMTRA (Advanced Material-Processing and Machining Technology Research Association) funded by MITI has made a significant contribution to industrial applications of beam processing by developing high density ion and laser beam equipment. A subsequent program, the ACTA (Advanced Chemical Processing Technology Research Association) project involves development of multi-beam deposition systems for metal and dielectric materials formation including several systems which combine ion and laser beams, ion beams and CVD and plasma, and laser beams with sputtering. NEDO Proposal Based Project and JST (The Japan Science and Technology Corporation)-Exploitation and Application Study Project are fundamentally aimed to transfer technology from university to industry. Research in gas cluster ion beam technology is currently being conducted in the following areas: (i) shallow ion implantation for 0.1 μ m PMOS junction formation, (ii) high rate etching and smoothing for Si, diamond, SiC and metal oxide films and (iii) thin film depositions for optical filter and transparent conductive film. The gas cluster ion beam processes are discussed in comparison with traditional ion beam processing methods which are presently limited by available atomic and molecular ion beams.

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

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