High Sputtering Yield Research Articles

3D numerical simulation of gas heating effects in a magnetron sputter deposition system

3D numerical simulation of gas heating in a magnetron sputtering system is performed. Pressure, magnetron power density and location of the substrate plane in front of the target are shown to affect the gas temperature profile. For the pressure range under study, maximum gas temperature is shown to increase with pressure. By increasing the separation between the target and the substrate, the maximum gas temperature is shown to increase up to the point when most of the particles are assumed thermalized. Cu shows more gas heating than does Al due to its higher sputtering yield and energy transfer efficiency. Sputtered particles are shown to be the major source of gas heating.

Unexpectedly high sputtering yield of carbon at grazing angle of incidence ion bombardment

The relative sputtering yield of amorphous carbon with respect to polycrystalline nickel at Ar-ion bombardment was determined by means of Auger electron spectroscopy depth profiling as a function of the angle of incidence and projectile energy in the ranges of 49°–88° and 0.3–1keV, respectively. It was found that the relative sputtering yield YC∕YNi strongly increases with angle of incidence from 49° to 82°. At around 80° the sputtering yield of C is higher than that of Ni. Above 82° no dependence on the angle of incidence was found. The relative sputtering yield weakly depends on the energy of the projectile. The experimental results will be explained by the help of transport of ion in solid (TRIM) simulations.

Fundamental characteristics of liquid cluster ion source for surface modification

We have developed a cluster ion source using liquid organic materials, which has several advantages for surface modification based on the different properties of liquid cluster ions. When vapors of liquid materials such as ethanol were ejected through a nozzle into a high-vacuum region, clusters were produced by an adiabatic expansion phenomenon. The clusters produced were ionized by an electron bombardment method, and the cluster ion current increased with increase of an electron current for ionization. In order to achieve the effective transport of cluster ion beams, a unipotential or einzel lens was used. The current density was much larger than that without using the lens. Furthermore, the beam diameter could be controlled by adjusting the lens voltage, and it was changed between 5mm and 20mm. In addition, several kinds of substrates such as Si, SiO2, PC substrates were irradiated at different acceleration voltages with ethanol cluster ion beams, and chemical and physical sputtering with a high sputtering yield was achieved. The sputtered surfaces were very smooth at an atomic level. The liquid cluster ion beams have a high potential for surface etching with the atomic level flatness, which is not obtained by a conventional wet process.

A study on the reactive sputtering process with plasma chemistry

In this study, a completed mathematical model based upon earlier studies by Berg and Pekker for reactive sputtering is established to generically include the chemical reactions in plasma. For the purpose of parameter analysis, several representative chemical reactions were chosen and the governing equations were expressed in a non-dimensional form. Several non-dimensional parameters were also defined in line with their physical significances. Results show that the stability is characterized by the well-known hysteresis loop in the steady state solutions. Through parameter analysis, we found when the chemical reaction on substrate is moderate, a higher sputter yield of the compound leads to a more stable steady state at lower inflow rates. The presence of various gases in the plasma has different effects on the hysteresis. Results indicate the increase of particles such as the ion of atomic reactive gas and electrons could reduce the hysteresis and hence stabilize the process.

Further Roughing of Tungsten Surface due to Transient Heat Load by Laser Irradiation during Helium Plasma Exposure

745 J. Plasma Fusion Res. Vol.81, No.10 (2005) 745 746 Tungsten is one of the candidates for divertor materials because of its high thermal property and low sputtering yield. However, recent experimental observations have revealed that bubbles and holes are formed on the surface by helium plasma exposure even with an incident ion energy less than the threshold energy of sputtering [1]. The formation of holes and bubbles may lead to degradation of the superior properties of tungsten, especially, its thermal diffusivity. If the thermal diffusivity is degraded, transient heat loads owing to the ELMs (Edge Localized Modes) [2] and disruption may pose serious problems, such as the melting of high-Z materials. In this paper, the transient heat load to the tungsten target during exposure to helium plasmas was demonstrated using pulsed laser irradiation. Although the heat pulse duration was several ns, which is much shorter than the characteristic time of the ELMs (<~10 −4 s), the experimental observations indicate the effect of transient temperature increases on the surface modifi cation. Experiments were performed in the divertor simulator NAGDIS-II (NAGoya university DIvertor Simulator-II). A schematic view of the experimental setup is shown in Fig. 1. The helium plasma was irradiated to a W sample situated at about a 45-degree angle to the magnetic fi eld line. The specimen was powder metallurgy W 0.2 mm in thickness. Second-harmonic pulses of an Nd:YAG laser (Continuum: SLII-10), with a wavelength of 532 nm, were used for the photon source. The pulse duration was 5–7 ns, and the pulse interval was 0.1 s, suffi ciently longer than the characteristic thermal relaxation time. The laser beam was injected through a quartz viewing port at a 90-degree angle to the magnetic fi eld line. The laser-pulse energy was measured before the viewing port, and the energy per unit area at the target was deduced by taking into account the transmission rate of the viewing port and the angle of the laser beam to the tungsten sample. SEM images of the tungsten sample exposed to helium plasmas without laser irradiation are shown in Fig. 2 (a) (from top) and Fig. 3 (a) (cross section). The ion fl ux was Γi = 1.7 × 1023 m–2/s, and the incident ion energy was 27 eV. The exposure time to the helium plasma was 1800 s; the tungsten surface temperature measured with a radiation pyrometer was ~1700 K. Micron-bubbles and holes are observed on the surface, and the penetrating depth of the surface modifi cation was 1– 2 μm. Figure 2 (b) shows SEM images of the laser irradiated tungsten surface exposed to the helium plasma under the same

Molecular Depth Profiling of Histamine in Ice Using a Buckminsterfullerene Probe

We employ a buckminsterfullerene ion source to probe the distribution of histamine molecules at the water-ice/vacuum interface. The experiments utilize secondary ion mass spectrometry to detect molecular ions that are desorbed from a frozen aqueous histamine solution. The results show that this cluster ion probe induces an extraordinarily high sputter yield of 2400 ice molecules per impact event as determined by a quartz crystal microbalance. As a consequence of this high yield, we show that it is possible to produce molecular depth profiles of the top several hundred nanometers below the ice surface without destruction of the molecular ion signal by accumulation of beam-induced chemical damage. Similar profiles are reported for desorbed neutral molecular fragments by utilizing a high-power femtosecond-pulsed laser for photoionization. While this type of information could not be achieved using atomic projectiles, it is possible to remove the damage induced by such projectiles by subsequent cluster bombardment. These experiments are particularly important for organic surface analysis since they suggest that cluster ion probes may successfully be employed to remove overlayers that may mask the desired molecular information in static secondary ion mass spectral analysis.

Fundamentals of pulsed plasmas for materials processing

Pulsed plasmas use much higher power during each pulse compared to continuously operated plasmas. This feature and the appearance of additional new variables such as pulse duty cycle can lead to new, flexible, sometimes enabling processes for surface modification and thin film deposition, as discussed for plasma immersion ion processing, high power pulsed sputtering, thin film deposition with pulsed cathodic arcs and metal plasma immersion ion implantation and deposition. In all of these processes, transient sheaths are of great importance. The fundamentals of plasma sheaths are briefly reviewed. It is argued that the often-considered ion matrix sheath is only an educational case, whereas most realistic transient sheaths obey a time-dependent Child law. The recovery of pulsed sheaths is usually less considered than the formation of sheaths. If the pulse fall time normalized by the inverse ion plasma frequency is much greater than unity, sheath collapse can be simply described by a retreating Child law sheath. In the opposite case, electrons refill the ion space charge and complete recovery of the boundary layer is determined by ambipolar diffusion from the bulk plasma. The case of high power pulsed sputtering is discussed in greater detail, especially the appearance of a kink in the current–voltage characteristic, which changes its slope for current densities exceeding approximately 600 mA/cm 2. It is suggested that high pulse power opens the possibility that self-sputtering can occur during each pulse, especially for target materials of high sputter yield. The necessary condition for self-sputtering is αβγ>1, where α, β and γ are the ionization probability, ion return probability and sputter yield, respectively.

Au/SiO2 Nanosystems by XPS

Au/SiO2 nanocomposites were prepared by rf-sputtering of gold on amorphous silica substrates. Deposition experiments were carried out in Ar plasmas at temperatures as low as 60 °C. Particular attention was devoted to the combined influence of the applied rf power and total pressure on the chemico-physical properties of the final nanosystems. In particular, low pressures (&amp;lt;0.2 mbar) and high rf powers (&amp;gt;20 W) resulted in a higher sputtering yield, allowing the deposition of continuous gold films on silica. Conversely, decreased sputtering yields (at higher pressures and lower powers) enabled the preparation of discontinuous Au/SiO2 nanosystems. A thorough investigation of the structure-properties relationships was attained by means of a multi-technique characterization. In particular, laser reflection interferometry (LRI) was employed for an in situ monitoring of growth processes, while glancing-incidence x-ray diffraction (GIXRD) and transmission electron microscopy (TEM) provided valuable information on the system nanostructure. Moreover, x-ray photoelectron spectroscopy (XPS), UV-Vis spectroscopy, and atomic force microscopy (AFM) were used to investigate the chemical composition, optical properties, and surface morphology, respectively. This study is dedicated to an XPS investigation of the principal core levels (Au, Si, O) of Au/SiO2 nanosystems. In particular, detailed scans for the Au 4f, Si 2s, O 1s, and C 1s regions and related data for a gold film on silica and a discontinuous Au/SiO2 specimen are presented and discussed.

Stability analysis of reactive sputtering process

In reactive sputtering, the introduction of reactive gas would create a hysteresis transition from metal to compounds (oxides, nitrides and carbides, etc.) in both target and substrate. The hysteresis transition is characterized by a sudden change in partial pressure, cathode voltage, sputtering rate and fraction of compound formation. Therefore, the stability is an important issue of process control. In this article, a simple mathematical model based on Berg's original proposal is used to study the stability of steady state solutions. In order to facilitate the analysis, several non-dimensional parameters are identified and used for formulation. The stability is then investigated in two folds. For a small variation away from the steady state, the linear perturbation method is utilized to reduce the model to an eigenvalue problem. For large deviation from the steady state, a full numerical analysis is employed to obtain the steady state. Results show that an unsteady system converges to steady states relatively fast at low inflow rates and the positive range of eigenvalue are coherent with the presence of hysteresis loop. With the aid of the eigenvalue, it is also found that when the chemical reaction on the substrate is moderate, a higher sputter yield of the compound leads to a more stable steady state at lower inflow rates. Finally, a convergence parameter similar to the Lyapunov number is introduced for full numerical analysis to give a qualitative measure for the stability at steady state.

Cluster ion beam process technology

Since an initial study of gas cluster ion beam (GCIB) had started in Ion Beam Engineering Experimental Laboratory, Kyoto University, more than 15 years has passed. Some of the results are already been applied for an industrial use. Unique characteristics of GCIB bombardment have been found to offer potential for various industrial applications that cannot be achieved by conventional ion beam processing. 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 impacts of atomic ions. Among prospective applications are included shallow ion implantation, high-rate sputtering, surface cleaning and smoothing, and low-temperature thin film formation. Low-energy bombarding effects and sputtering effects produced by the cluster ion impact are particularly important. Cluster ion implantation has been applied to realize ultra-shallow junction formation. High-sputtering yields and lateral sputtering that cause surface smoothing cannot be achieved with monomer ion beams. The surface smoothing process to atomic levels becomes the first production use of GCIB. High-quality thin film formation using GCIB assisted deposition is also the characteristic that is explained mainly due to the very low energy and very high-density ion bombardment.

Influence of inert gas pressure on deposition rate during pulsed laser deposition

The deposition rates of permalloy and Ag are monitored during pulsed laser deposition in different inert gas atmospheres. Under ultrahigh vacuum conditions, resputtering from the film surface occurs due to the presence of energetic particles in the plasma plume. With increasing gas pressure, a reduction of the particle energy is accompanied with a decrease of resputtering and a rise in the deposition rate for materials with high sputtering yield. In contrast, at higher gas pressures, scattering of ablated material out of the deposition path between target and substrate is observed, leading to a decrease in the deposition rate. While in the case of Xe and Ar these processes strongly overlap, they are best separated in He. A He pressure of about 0.4 mbar should be used to reduce the kinetic energy of the deposited particles, to reach the maximum deposition rate and to avoid implantation of the particles. This is helpful for the preparation of stoichiometric metallic alloy films and multilayers with sharp interfaces.

ビスマス鉄ガーネット薄膜の組成比と磁気光学特性

Nonthermodynamical Bi3Fe5O12 films were synthesized by the reactive ion beam sputtering technique, and the relation of the compositions of the starting materials 3(1+x)×Bi2O3 + 5 ×Fe2O3 powder, targets, and sputtered films was investigated. The compositions of the target and the film were different from that of the starting materials, which can be understood by taking the high volatility and high sputtering yield of Bi into consideration. By using a starting material with x = 0.15, BIG film with the composition Bi:Fe:O = 3.0:5.0:12.0 was successfully obtained. However, a film with x = 0.1 showed the largest θF value. From the point of view of the magneto-optical properties, the optimized compositions of the film may deviate form the stoichiometric compositions.

V2O5 thin films deposited by means of d.c. magnetron sputtering from ceramic V2O3 targets

The deposition of stoichiometric V2O5 films for abrasion-resistant V2O5/TiO2 anatase catalysts was investigated. Direct current magnetron sputtering from ceramic oxide targets in an argon/oxygen atmosphere was chosen as the deposition technique. Layers were prepared with different oxygen mole fractions in the plasma and the influence upon electronic and optical properties was investigated. Surface XPS measurements showed that stoichiometric layers are obtained for low oxygen flows, whereas for higher oxygen contributions reduced vanadium species are increasingly present. This departure from stoichiometry is ascribed to active re-sputtering of the deposited layer by O− ions, due to the higher sputter yield of oxygen compared with vanadium. Polycrystalline V2O5 layers were obtained at elevated substrate temperatures. Optical absorption coefficients in the visible region were found to correlate well with the layer stoichiometry, as deduced from the XPS lineshape analysis. Copyright © 2002 John Wiley & Sons, Ltd.

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.

Focused Ion Beam (FIB): More than Just a Fancy Ion Beam Thinner

The potential utility of FIB for routine (and novel) applications has come to forefront recently due to advances in ion optics which now allow formation of focused ion probe of better than ∼10-20 nm containing current density exceeding several A/cm2, with a liquid metal source (typically Gallium). The small ion probe size, coupled with shallow sputtering depth - yet high sputtering yield of ions, has opened several opportunities in machining, lithography and ion-assisted deposition.[1-3] These developments, including automation, multi-specimen stages, cross-compatible specimen holders for FIB/TEM/SEM, use of in-situ electron beam (so-called dual beam), coupled with innovations such as the “lift-off” process[4], have provided an invaluable set of tools for microelectronic defect characterization. However, re-deposition (contamination), ion implantation/damage especially for desirable thinner sections (&lt;∼50 nm) remain major concerns for further applications.While much of the excitement in TEM community for FIB is due to thin foil specimen preparation (especially in microelectronics), we have been exploiting the site-specific micromachining aspect of FIB beyond specimen preparation for TEM, which is the focus of this contribution. Two broad themes will be presented: One exploits the site-specificity of FIB in making thin sections (including lift-off) at and across localized deformation as in indentation response of micro/nanocomposites. The other involves FIB as a fabrication tool for sputtering/drilling arbitrary shapes and sizes down to 20-50 nm, to enhance functional aspects.

Comparative ion yields by secondary ion mass spectrometry from microelectronic films

Secondary ion mass spectrometry (SIMS) is reported from multiple-element multiple-matrix ion implants. The implants include a thirteen element metal set and a six element gas set implanted into films of interest for microelectronics (silicon, silicides, wiring layers, liner metals, inorganic dielectrics, and polymer dielectrics.) Using these standards, this study performs a broad comparison of ion yields using a metric defined as the normalized useful yield. We find that the yield of K+ with O2+ primaries is constant for almost all matrices, in keeping with a concept of ion yield saturation. The yield of Cl− with Cs+ primaries approaches a yield saturation limit for titanium but the ion yield falls for materials with higher sputter yields, becoming 3.5× lower from copper. The variations in negative ion yields from matrix to matrix are much larger, with anomalies more pronounced, for weaker-yielding ions, becoming 50× for C− from Ti to Cu. In this article we document sets of ion implants, show some of the SIMS profiles, and note trends in ion yields and implications for SIMS analysis.

Desorption of large organic molecules induced by keV projectiles

In order to understand the emission of organic molecules in sputtering, classical molecular dynamics (MD) is used to model the 5 keV Ar atom bombardment of polystyrene oligomers adsorbed on Ag{1 1 1}. The analysis of the results shows that a significant fraction of the trajectories generates high action events in the sample. These events are characterized by the simultaneous motion of several hundreds of substrate atoms and, oftentimes, by high emission yields of substrate atoms, clusters and polystyrene molecules. Collision trees representing the energetic part of the cascades confirm that high sputtering yields of molecules occur when a large portion of the primary particle energy is quickly dissipated in the upper layers of the silver substrate. This class of events where high action occurs in the surface region might explain the ejection of organic species with a mass of several kilodaltons such as biomolecules and synthetic polymers. In the simulation, these events are capable of desorbing polystyrene molecules of ∼2 kDa.

Cluster size effect on reactive sputtering by fluorine cluster impact using molecular dynamics simulation

The mechanism of high-yield sputtering induced by reactive cluster impact was investigated using molecular dynamics (MD) simulations. Various sizes of fluorine clusters were radiated on clean silicon surface. At an incident energy of 1 eV/atom, F atom and F 2 molecule are only adsorbed on the surface and sputtering of Si atom does not occur. However, fluorine cluster, which consists of more than several tens molecules causes sputtering. In this case, most of Si atoms are sputtered as fluorinated material such as SiF x . This effect is due to the fact that cluster impact induces high-density particle and energy deposition, which enhances both formation of precursors and desorption of etching products. The deposition of atoms and energy becomes denser as the incident cluster size increases, so that larger clusters have shown higher sputtering yield.

Theoretical sputtering yields of Al and Mg targets in physical vapor deposition processes

Calculations of sputtering yields are performed for aluminum and magnesium target materials with the use of a homemade software based on the analytical expression of Garcia-Rosalès. The theoretical yield of magnesium is about twice the value related to aluminum in accordance with the experimental ratio of the Mg/Al deposition rates. We show that the deposits of the same materials must be subjected to densification followed by resputtering phenomena when a bias voltage is set to the substrates. These predictions are confirmed by atomic force microscopy observations, especially for magnesium deposits owing to their texture and the high sputtering yield of this material.

Spectroscopic investigation on the impurity influxes of carbon and silicon in the ASDEX upgrade experiment

Emission profiles of carbon and silicon along the heat shield (HS) were measured before and after siliconization of the vessel. Siliconization does not affect the divertor and therefore large changes in impurity concentrations and influxes cannot be attributed to it. A strong transient increase of the silicon concentration was observed immediately after the siliconization lasting for a few discharges. However, no large change was observed in the silicon influx from the HS, excluding it as possible source of the concentration increase. The fast decrease in the silicon concentration can be attributed to strong erosion at the outer limiter. The impurity influxes for C 2+ and Si 2+ from the HS are estimated and compared with the corresponding core concentrations. The relative contribution to the plasma impurity content from divertor, HS and outer protection limiter is discussed. Zeeman splitting analysis permits one to identify the emitting region of carbon radiation. Anomalously high silicon sputtering yields are measured, which do not however cause high core silicon content.