Sputtering yields for low-energy Ar- and Ne-ion bombardment

The development of fluorescent lamps requires reliable experimental data of low-energy (30-300 eV) sputtering yields of the metallic components of lamps. An experimental study was carried out to measure the sputtering yields of molybdenum, tungsten and iron using the plasma chamber of a Secondary Neutral Mass Spectrometer (SNMS). The sputtering ion beam was extracted from radio frequency noble gas (Ar/Ne)-plasmas. The number of atoms sputtered was determined from the weight loss of the target by means of a microbalance. The sputtering yields data for Mo/Ar, W/Ar and Fe/Ar target/projectile combinations and their energy dependence were in good agreement with those calculated by computer simulations and measured by other groups. The sputter yield obtained for Fe/Ne target/projectile combination shows some deviation from the simulation results.     

Ion beam sputtering of ZnS thin films

Ion beam sputtering was used to deposit adherent high quality ZnS films non-reactively. The optical properties of films thicker than 80.0 nm were found to be equivalent to those of bulk ZnS with a packing density of unity. Unlike that of evaporated films, the refractive index was insensitive to the substrate temperature but for thinner films was dependent on the substrate condition prior to deposition. The deposition of fully dense films is qualitatively explained using Muller's model of ion-assisted deposition. In the case of ion beam sputtering, the high energy species comprise energetic sputtered atoms and/or molecules, reflected beam ions and charge exchange neutrals.     

The ion energy distributions and ion flux composition from a high power impulse magnetron sputtering discharge

The energy distribution of sputtered and ionized metal atoms as well as ions from the sputtering gas is reported for a high power impulse magnetron sputtering (HIPIMS) discharge. High power pulses were applied to a conventional planar circular magnetron Ti target. The peak power on the target surface was 1-2 kW/cm² with a duty factor of about 0.5%. Time resolved, and time averaged ion energy distributions were recorded with an energy resolving quadrupole mass spectrometer. The ion energy distributions recorded for the HIPIMS discharge are broader with maximum detected energy of 100 eV and contain a larger fraction of highly energetic ions (about 50% with E_i > 20 eV) as compared to a conventional direct current magnetron sputtering discharge. The composition of the ion flux was also determined, and reveals a high metal fraction. During the most intense moment of the discharge, the ionic flux consisted of approximately 50% Ti¹⁺, 24% Ti²⁺, 23% Ar¹⁺, and 3% Ar²⁺ ions.     

The synthesis of Nb3Ge by RF sputtering

The conditions necessary for the formation of Nb ₃ Ge by low-pressure rf sputtering with a superconducting transition temperatureT _c >21 K have been investigated. Samples have been deposited onto cooled substrates so that the film is first amorphous and then is crystallized by a subsequent annealing, and onto hot substrates, in which case the film is crystalline upon deposition. The highestT _c samples were obtained for a substrate temperature of 735±25° C. The optimum substrate temperature is the same as the optimum annealing temperature for crystallizing films which were first deposited onto cooled substrates. Special conditions are necessary for the formation of single-phase A15 samples of Nb ₃ Ge with an optimumT _c . We have utilized the collisions that the sputtered atoms undergo with the sputtering gas molecules to thermalize the sputtered atoms. We report here on sputtering in both krypton and in argon.     

Three-dimensional simulations of grid erosion in ion engines

Three-dimensional simulations have been conducted to analyze the wear of ion engine grids. Grid sputtering due to the impact of high-velocity ions and neutrals produced by charge-exchange and elastic collisions between mainstream ions and leaking neutrals was taken into account with redeposition of sputtered grid materials. Comparisons with experiments were performed, and it was found that the calculated grid mass losses agree well with those measured in experiments.     

Spatial and energy distributions of particles sputtered from NiPd single crystals

Molecular dynamic simulation has been used to study the sputtering of NiPd single-crystal disordered compounds. Spatial distributions, as well as the energy of Ni and Pd particles sputtered from the NiPd (001) face have been investigated, for Ar bombarding ions of energies E₀ ranging from 0.1 keV to 5 keV, incident at different angles. The results have shown a predominant exit both of Ni and Pd atoms near the same close-packed directions, in contrast to what is observed for ordered binary compounds. For unchanged composition of the surface, the emission of Ni in the <011> close-packed directions is significantly greater than the emission of Pd. When the composition of top layers was changed, however, the opposite is observed and the exit of Pd prevails. Energy dependence and energy spectra of sputtered components have been analysed as well as the origin and the number of the generation of ejected particles. The observed trends are explained by the particular behaviour of correlated collisions in single crystals.     

Transport of sputtered atoms investigated by Monte Carlo method

Increasing attention has been paid to the sputtering process as a tool to deposit films and to the study of the interaction between the film properties and the deposition parameters. It is obvious that the energy and direction of these particles arriving at the substrate is in close relation with the transport process from the target to the substrate. This work deals with the computer simulation of the sputtered Ag atoms trajectories through the background gas in a diode-sputtering configuration. For that, we have developed a numerical model to simulate the transport process. We followed the three-dimensional trajectory of each sputtered atom separately and calculated the scattering angle and the energy loss if a collision took place. A statistical method, Monte Carlo simulations is used. The model predicts the flux of Ag atoms arriving at the substrate, their energies and angular distribution. The dependence of the deposition rates of Ag atoms on the gas pressure and the distance between target to substrate were investigated.     

Energies of particles at the cathode of a glow discharge

A Monte-Carlo calculation has been carried out to obtain the energy distribution of ions and neutrals at the cathode of a glow discharge, assuming charge exchange collisions in different fields. The comparison with the previously studied case of linear field is good. Depending on the parameter L/λ (ratio of length of cathode dark space to mean free path of atoms in the gas) the following information was obtained in addition: average number of collisions, relative contribution of number of collisions to the energy distribution, and average energy of the neutrals.     

In-situ analyses on the reactive sputtering process to deposit Al-doped ZnO films using an Al-Zn alloy target

The kinetic energies of generated ions were investigated during the reactive sputtering process to deposit Al-doped ZnO (AZO) films using an Al-Zn alloy target. The sputtering system was equipped with specially designed double feedback system to stabilise the reactive sputtering processes and analysis was performed with a quadrupole mass spectrometer combined with an energy analyser. Negative ions O⁻, O₂⁻, AlO⁻ and AlO₂⁻ with high kinetic energies corresponding to cathode voltage are generated at the partially oxidised target surface, after which some of the ions undergo subsequent charge exchange and/or dissociation. Positive ions O⁺, Ar⁺, Zn⁺ and Al⁺ with lower kinetic energies (around 10 eV) are generated by charge exchange of sputtered neutral O, Ar, Zn and Al atoms, respectively. As the target surface oxidises, cathode voltage decrease, the flux of high-energy negative ions increases and the electrical properties of the AZO degrade by ion bombardment as well as the AZO films that are deposited by conventional magnetron sputtering using an AZO target.     

Sputtering of ice films by the bombardment of Ar ions

The process of sputtering ice films covered on Au(1 1 1) surface at normal incidence by Ar ions with an initial energy of E₀ = 300 eV and 700 eV has been investigated by the molecular dynamic simulation. The mass spectrum and kinetic energy distributions have been calculated. Our simulations clearly show that the mass spectrum contains peaks of the water molecules, water clusters and Au atoms. These results are of interest for the study of mass spectrometry in films and surfaces.     

The synthesis of the Nb3(Al-Ge) system by sputtering

The variation of the superconducting transition temperatureT _chas been studied in the Nb₃Al _1–x Ge _x system for0x1 for samples produced by rf sputtering onto heated substrates. The effect that different sputtering gas pressures have upon the rate of energy loss of the sputtered atoms due to collisions with neutral sputtering gas atoms is considered. Also considered is how thermalization can be achieved in the fewest number of collisions by matching the mass of the sputtering gas atoms to that of the sputtered atoms. For the case of Nb ₃ (Al-Ge) we show that it is advantageous to use a mixture of sputtering gases so that the light Al atoms can be thermalized as well as the heavier Nb and Ge atoms. It is also thus shown that the same sputtering conditions that are optimal for forming high-T _cNb₃Ge onto heated substrates are not optimal for forming high-T _cNb ₃ Al.Supported by CUNY FRAP and National Science Foundation DMR 74-18138.     

DFT calculation of the stability and mobility of noble gas atoms in silicon

We have investigated the stability and the migration of single noble gas atoms (He, Ne, Ar, Kr) in bulk silicon, by performing first-principles calculations. Our results indicate that all noble gas interstitials were found to be preferentially located in a tetrahedral site. Other possible sites have been studied too, like the hexagonal one which becomes unstable for large noble gas atoms such as Ar and Kr. Using nudged elastic band technique, we have determined the minimum energy path, and the associated migration energies. Our results are discussed and compared to other works.     

Spatial and temporal investigation of high power pulsed magnetron discharges by optical 2D-imaging

Optical 2D-imaging in combination with Abel inversion was used to study the spatial and temporal evolution of the plasma-induced emission of HIPIMS discharges. A titanium target, as well as an aluminium-doped zinc target was sputtered in an argon atmosphere of pressures 0.53 Pa and 1.33 Pa and an average power of 650 W and 400 W, respectively. The discharge was observed optically employing various wavelength filters to investigate the development of selected species, namely argon and titanium neutrals, as well as argon and zinc neutrals and ions.The argon neutral emission did not only differ substantially from the DC case but also underwent a significant development during the pulse ‘on’-time, showing a structure similar to an ion acoustic wave travelling away from the target, as well as rarefaction of the working gas while sputtering with high discharge peak currents. In addition, the intensity profile of argon and zinc ions revealed an increased width in front of the target which is then reflected by a wider sputter distribution of the target material. Indeed, it was found that the width of the metal neutral emission increased with increasing discharge current. Their emission revealed two distinct maxima and the loss of intensity in between these maxima could be explained by an increased ionisation of the sputtered metal flux.     

On eliminating deposition-induced amorphization of interfaces in refractory metal multilayer systems

Multilayer thin films with refractory metal components are susceptible to solid state amorphization. It is an objective to minimize this interfacial reaction in order to ensure smooth layering and compositionally abrupt interfaces. These features are desirable for maximum reflectivity in X-ray optic applications such as projection lithography as well as neutron supermirror applications. This goal is achievable when the deposition process is optimized to thermalize the sputtered neutrals. Low working gas pressures (below 5 m Torr) and large source-to-substrate distances (more than 12 cm) typically provide the proper conditions to form dense sputter deposits without adatom-induced, interfacial amorphization. The interfaces of Mo/Si and Ni/Ti multilayers are examined with high resolution electron microscopy for samples sputter deposited under thermalized conditions. It is shown to be possible to produce crystalline, refractory metal layers in the as-deposited structure without amorphous interfacial regions.     

A study of deposition of ITO films on organic layer using facing target sputtering in Ar and Kr gases

In this study, the effects of sputtering gases on the damage to an organic layer (aluminum (III) bis(2-methyl-8-quninolinato)-4-phenylphenolate (BAlq)) were investigated in the deposition of indium-tin oxide (ITO) films by using a facing target sputtering (FTS) system. Only a small improvement in the PL intensity of the BAlq layer was observed when the sputtering gas was changed from Ar to Kr gas. It was also found that suppression of the high-energy γ-electron bombardment to the substrate is very important to reduce the damages in the BAlq layers. In addition, a remarkable improvement in the PL intensity was observed by the increase in the sputtering gas pressure from 0.3 to 1.3 Pa. These results suggest that bombardment of reflected neutral gas atoms is not the main source of the damage that BAlq layer has, but the bombardment of high-energy sputtered atoms plays an important role for the remnant damages caused by the sputter-deposition of the ITO film at a low gas pressure, since the decrease in the PL intensity was almost totally suppressed by increasing the sputtering gas pressure above 1.1 Pa. Finally, low-damage sputter-deposition of ITO film with low resistivity on BAlq layer was achieved by completely suppressing the bombardment of high-energy particles, including γ-electrons, negative oxygen ions and high-energy sputter-emitted particles.     

Organic mass spectrometry with low-energy projectiles

Molecular dynamics computer simulations have been employed to elucidate mechanisms responsible for uplifting of a monolayer of benzene and polystyrene molecules adsorbed on Ag{1 1 1} by low-energy atomic and cluster Ar projectiles. The sputtering yield and mass distributions of ejected particles are analyzed depending on the type and the kinetic energy of a projectile. It is shown that the relative contribution of intact molecules can be greatly enhanced if the kinetic energy of atomic projectile is reduced below 60 eV. At these energies, however, the efficiency of desorption is low and the ejection process is limited only to loosely bound molecules. Much better results can be obtained for cluster projectiles containing hundreds of Ar atoms with the incident energy of a few eV per atom. The impact of such particles leads to a gentle and very efficient removal of intact organic molecules originally adsorbed at the surface.     

2.2 Substrate surface damages by rf-sputtering

Electrical properties of (111) surfaces of n-type silicon exposed to sputtered metal atoms deposited in a rf-diode sputter system were studied. The metal-silicon junction formed by this process was investigated to evaluate surface properties of the junction. It was shown that the sputtering process introduced damage to the surface, causing charged centres to appear at the surface of the crystal. A theory is presented which explains most of the experimental results of the so damaged Schottky junction. Thermal activation energies are deduced from experimental data and compared to expected values predicted by the theory presented. The energies are found to vary between 0·3 and 0·7 eV depending on sputtering conditions.     

Generation processes of super-high-energy atoms and ions in magnetron sputtering plasma

Energy distribution function (EDF) of ion species (Ar⁺, Kr⁺, Xe⁺) in a rare gas magnetron plasma is measured at a substrate position, 0.1 m away from the target surface, by energy-resolved mass spectrometry. The measured ion EDF contains, besides a bulk low-energy part (<10 eV), a tail part of super-high energy on an order of 100 eV, depending on the mass ratio of ion species to target material (tungsten, permalloy (80% Ni, 20% Fe)). A weak electric field in a diffusion region of magnetron plasma cannot accelerate slow bulk ions of ∼0.2 eV to such high energies. Origin of large kinetic energies is attributed to the backscattering process on the target surface where, e.g., Ar⁺ ions impinging on the target are neutralized and reflected as fast Ar atoms of the kinetic energy approximately given by a two-body collision model. Subsequently, a part of fast atoms may be converted to fast ions in three possible collision processes in the diffusion region: (i) electron impact ionization (ii) resonant charge exchange, and (iii) ionization of slow atoms by fast atoms. Among them, the third process is found to be dominant from Monte Carlo simulations where the backscattering process is evaluated by the TRIM code. Furthermore, when the target mass is larger than the bombarding ion mass, the substrate is bombarded by the super-high-energy atoms having a flux 2-4 orders of magnitude larger than the fast-ion flux.     

Studies on sputtering process of multicomponent Zr-Ti-Cu-Ni-Be alloy thin films

Sputter deposition process of a multicomponent Zr-Ti-Cu-Ni-Be metallic alloy has been studied experimentally and by numerical simulations. Monte-Carlo simulations were performed using a model based on thermalization and diffusion of sputtered atoms. Incident energy and angle of sputtered atoms on substrate were obtained from simulations. The incident angular distribution was observed to be a normal distribution at all sputtering pressures. Average incident kinetic energy of the condensing atoms on the substrate was observed to be 0.2-0.3 eV indicating most of them are thermalized. Simulations were extended to predict compositional variations in films prepared at various process conditions. These results were compared with composition of films determined experimentally using Rutherford Backscattering Spectrometry (RBS). Contents of Zr, Ti, Cu and Ni quantified using RBS were in moderate agreement with the simulated composition. Be could not be quantified accurately by RBS largely due to very low energy peak of Be in the spectrum. These studies are shown to be useful in understanding the complexities in multicomponent sputtering.     

Substrate heating rates for planar and cylindrical-post magnetron sputtering sources

Substrate heating energies per atom deposited are reported for planar magnetron sputtering of aluminum, chromium, nickel, copper, molybdenum, indium, tantalum, tungsten and platinum in argon as well as for aluminum and chromium sputtered in O₂. Data are also reported for cylindrical magnetron sputtering of niobium, silver, tantalum, tungsten and Pb-Sn in argon as well as for molybdenum sputtered in neon, argon, krypton and xenon. The planar and cylindrical magnetron heating rates were comparable. The heating energies for argon sputtering ranged from 10–15 eV per deposited atom for the light metals to almost 100 eV atom^-1 for tantalum and tungsten. The implied reactive sputtering heating energies were about 500 eV molecule^-1 for Cr₂O₃ and 1150 eV molecule^-1 for Al₂O₃. Special experiments were conducted to examine the contributions to the substrate heating of plasma species and ion neutralization and reflection at the cathode. The data indicate that charged plasma species do not contribute significantly to the heating but that neutralized and reflected ions play a significant role in the planar as well as the cylindrical cases despite the differences in cathode geometry.