Physical Sputtering Process Research Articles

Space-resolved line shape model for sputtered atoms of finite-size targets

High-resolution emission spectroscopy provides valuable information on the physical sputtering process during plasma-wall interaction. Up to now, analyzing the observed spectral lines during sputtering did not account for the finite size of the targets. It becomes crucial if the size of the target becomes comparable with the distance the sputtered atoms travel before emitting the photons. So, for example, the generally used standard emission model based on an infinite target or the point source approximation breaks for observations using two lines of sight: parallel and perpendicular to the normal of the target. It is impossible to achieve consistent results for energy and angular distribution of sputtered atoms. The new space-resolved emission model for finite-size targets developed in this work removes this gap. It incorporates the space-velocity transformation for the distribution function and includes the finite lifetime of excited states. The model was validated using emission spectra of sputtered atoms from a polycrystalline tungsten sample bombarded by monoenergetic Ar+ with kinetic energies of 100 eV to 140 eV at normal incidence in the linear plasma device PSI-2. Using the new model enables the simultaneous fitting of the line shapes of sputtered tungsten for both observation angles. The optimization process is performed using the standard Thompson distribution by separating the energy-dependent parameter and the angular distribution.

Applying the plasma physical sputtering process to SRF cavity treatment: Simulation and Experiment Study

Plasma processing is a fast-emerging surface treatment technology for superconducting radio frequency (SRF) cavities: plasma interacts with the impurity contamination and tiny burrs, resulting in an etched clean-and-smooth RF surface. Among all the variations of plasma processing method, the physical sputtering process might be applied to SRF cavities but has no clear etching profile (including distribution uniformity, ion flux, energy, and etching rate). To investigate that, this paper utilizes a two-dimensional self-consistent model combining the fluid equations and the Monte Carlo collision method. With the model, we introduced the sputtering-yield probability distribution to evaluate the physical sputtering process quantitatively; optimized the plasma distribution characteristics with three electrode structures to attain uniformly distributed plasma; built the correlation between ion flux and ion-energy distribution function (IEDF) under varied power and frequency; and studied the physical sputtering process at different positions. In the end, we performed the plasma physical sputtering experiment on small niobium samples, seeing an etching and smoothing effect. This research successfully evaluated the physical sputtering process for SRF cavities, and indicated the feasibility of pure physical sputtering etching.

The effect of O2 impurity on surface morphology of polycrystalline W during low-energy and high-flux He+ irradiation

The interaction between the impurities (such as carbon, nitrogen, oxygen) and the plasma-facing materials (PFMs) can profoundly influence the performance and service of the PFMs. In this paper, we investigated the influence of oxygen (O2) impurity in the helium radio frequency (RF) plasma on the surface morphology of polycrystalline tungsten (W) irradiated at the surface temperature of 1450 ± 50 K and the ion energy of 100 eV. The pressure ratio of O2 to He (R) in RF source varied from 4.0 × 10−6 to 9.0 × 10-2. The total irradiation flux and fluence were ˜1.2 × 1022 ions·m-2·s-1 and ˜1.0 × 1026 ions·m-2, respectively. After He+ irradiation, the specimen surface morphology was observed by scanning electron microscopy. It was found that with increasing R from 4.0 × 10−6 to 9.0 × 10-2 the thickness of nano-fuzz layer at the W surface was thinner and thinner, accompanied by the formation of rod-like structures. The erosion yield increased from 5.2 × 10-4 to 2.3 × 10-2 W/ion when R varied from 4.0 × 10-6 to 9.0 × 10-2. The X-ray diffraction analysis shows that tungsten oxides were formed at the near surface of specimens when R exceeded 1.8 × 10-2. The erosion yield measurements revealed that in addition to surface physical sputtering process, the chemical erosion process could occur due to the interaction between oxygen-containing species and W at the surface. The results indicated that the presence of O2 impurity in He plasma can obviously affect the surface microstructure of W. The study suggested that O2 impurity can effectively reduce the growth of nano-fuzz structures.

Chemically assisted physical sputtering of Tungsten: Identification via the [formula omitted] transition of WD in TEXTOR and ASDEX Upgrade plasmas

Tungsten (W) is a preferred plasma-facing material for areas of high particle and power impact in present day and future fusion devices. The lifetime of W plasma-facing components (PFCs) under steady-state conditions is limited by erosion processes induced by energetic impinging particles such as impurities (carbon C, oxygen O, nitrogen N, etc.) and hydrogen isotopes above the threshold energy for Physical Sputtering (PS). We discovered for the first time, in addition to the bare physical sputtering process of W, a second W erosion mechanism at PFCs in TEXTOR (limiter surfaces) and ASDEX Upgrade (divertor target plates) during deuterium plasma bombardment. The tungsten deuteride molecule WD has been identified spectroscopically via the 6Π→6Σ+ transition in parallel to ordinary neutral W line emission of the WI transition (5d5(6S)6s 7S3 → 5d5(6S)6p 7P4) at λ=400.9nm used as measure to quantify the bare PS process or, more precisely, the gross erosion of W. We identified the underlying process for the molecular release as Chemically Assisted Physical Sputtering (CAPS) as observed also in the case of beryllium or lanthanum. Measurements in TEXTOR and ASDEX Upgrade showed a dependence of the WD band emission on the surface temperature - connected to the deuterium content in the near W surface - as well as on the flux and energy of impinging energetic particles. A quantification of the released WD is not yet possible due to lack of appropriate molecular data for the conversion of photons into particles, but the spatially resolved ratio of atomic (WI) and molecular photon flux (WD) indicates that the release takes place at the same time, but differently distributed along the target plates and limiters reflecting the variation in the described critical parameters (impinging ion flux, ion impact energy, and material temperature) along the surface. The plasma conditions in front of the interaction zone are in both cases fully ionising with an electron temperature Te > 10eV, thus, recombination into WD can be excluded.

Sputtered nano-cobalt on H-USY zeolite for selectively converting syngas to gasoline

Selectively converting CO and H2 to gasoline product (isoparaffin and olefin) in one step still remains a great challenge. We demonstrate effective H-USY zeolite supported nano-cobalt bifunctional catalysts for this catalytic reaction, which are prepared by the novel physical sputtering process. Particles of the sputtered cobalt exist in nano-level and are well-dispersed on acid USY zeolite. Easy activation of the loaded nano-cobalt is also achieved in a low-temperature hydrogen reduction atmosphere. In the tandem catalytic reaction, the sputtered bifunctional Co/USY catalyst exhibits a much higher CO conversion and higher isoparaffin selectivity than the conventional impregnated one. Compared with H-Mor, H-Beta and other zeolites supported catalysts, H-USY zeolite supported cobalt catalyst shows the clearest promotional effect on the activity of Fischer-Tropsch synthesis. The described synthesis herein provides a new pathway to solve the problem caused by the strong metal-support interaction (MSI) in heterogeneous catalysis.

Etching properties of Na0.5K0.5NbO3 thin films by using inductively coupled CF4/Ar plasma

The etch process of the Na0.5K0.5NbO3 (NKN) thin film was performed in CF4/Ar plasma. We investigated the etch rate of the NKN thin films and the selectivity of NKN to SiO2 in an inductively coupled plasma. The maximum etch rate of the NKN thin films was 127.3nm/min in CF4/Ar (=4:16sccm) plasma and the selectivity of NKN to SiO2 was 0.31. We analyzed the XPS narrow scan spectra for the reaction on the surface of the NKN thin films. From the XPS data analysis, we were assumed that the byproducts were generated on the surface of the NKN thin films during the etching process, but the physical sputtering process effectively removed the byproducts. We analyzed the morphologies of the surface of the NKN thin films from AFM measurement. The Ar concentration affects the surface morphology greatly.

Simulation of focused ion beam etching by coupling a topography simulator and a Monte-Carlo sputtering yield simulator

For a realistic simulation of sputtering processes and the topography changes associated, we combined the 3-D topography simulator ANETCH with the Monte-Carlo ion implantation program MC_SIM. The coupling between the programs provides the possibility to study the results of physical sputtering processes for nearly arbitrary ion/target combinations without a priori knowledge about the respective yield from experiments. As a first application, simulations were carried out to optimize process parameters of sputtering experiments. In a second application, the topography of a trench after FIB preparation is compared to simulations. The side-wall evolution at an edge due to ion irradiation is studied as a third application.

Chemical and physical sputtering effects on the surface morphology of carbon films grown by plasma chemical vapor deposition

We have studied the influence of chemical and physical sputtering on the surface morphology of hydrogenated carbon films deposited on silicon substrates by bias-enhanced electron cyclotron resonance chemical vapor deposition. Atomic force microscopy based power spectrum density (PSD) and roughness analysis have been used to investigate the film morphology. This study has been possible due to the appropriate choice of the experimental variables, in particular, gas mixture, resulting in either nitrogen-free (a-C:H) or nitrogenated carbon (a-CN:H) films, and substrate bias (Vb). Under these conditions, chemical sputtering is present for a-CN:H deposition but it is negligible for a-C:H film growth, while physical sputtering processes appear for both systems for Vb≤−85 V. When physical sputtering does not operate, the film growth with simultaneous chemical sputtering leads to a characteristic a-CN:H granular surface morphology. Furthermore, PSD analysis reveals that a spatial correlation of the a-CN:H film surface roughness, up to distances ∼300 nm, becomes a fingerprint of the coexistence of growth and chemical erosion processes on the film morphology. However, once physical sputtering takes place, the influence of chemical sputtering by reactive nitrogen species on the final surface morphology becomes negligible and both a-CN:H and a-C:H film morphologies are ultrasmooth.

Atomic scale simulation of physical sputtering of silicon oxide and silicon nitride thin films

Molecular dynamics simulations of energetic ion bombardments were carried out to investigate physical sputtering process. Bombardments of several ion species (He +, Ne +, Ar +, Kr +, Xe +) on a modeled amorphous silicon oxide substrate were simulated to find out the effect of plasma ion species on sputtering. Reflection characteristics of ions, fraction of energy deposited on substrate, and sputtering yield were obtained for each species, which collide on the surface at incident angle ( φ i) of 60° and with incident energy ( E i) of 100 eV. Simulations of Ar + bombardments on SiO 2 and Si 3N 4 substrates were carried out to compare the sputtering yield of two substrates at various incident angles ( φ i = 0 ° , 30°, 45°, 60°, 75°, 85°) and with incident energies ( E i=100 and 200 eV). The sputtering yield of silicon nitride was more than twice of that of silicon oxide in this range of incident energy. The preferential sputtering of O or N atoms rather than Si atoms was also observed in the physical sputtering of these multi-component substrates (SiO 2 and Si 3N 4).

Richard Edward Honig

Richard Edward Honig, a physicist who spent his career at RCA Laboratories in Princeton, New Jersey, died of cancer on 31 July 2001 in Haverford, Pennsylvania. He investigated the vaporization of solids and was a pioneer in the design of mass spectral techniques for the characterization of solid surfaces. Rick was born on 4 June 1917 in Göttingen, Germany. In 1933, his father, a professor of law at Göttingen University, moved the family to Istanbul, Turkey. There, Rick attended Robert College and graduated in 1938 with a BS in electrical engineering. He went on to MIT, receiving his MS in electrical engineering in 1939 and his PhD in molecular physics in 1944. His thesis at MIT, prepared under the guidance of Clark Goodman, involved mass spectrometric studies of light hydrocarbons.During Rick’s years as a graduate student, his interest in mass spectrometry emerged and developed into a passion that he would sustain for the rest of his life. After a brief four-year stint at Socony-Vacuum Oil Co (the predecessor of Mobil Oil Corp) in Paulsboro, New Jersey, Rick joined the research staff at RCA’s David Sarnoff Research Center (now the Sarnoff Corp) in Princeton in 1950. In 1951, he proposed studying the physical sputtering process using a mass spectrometer. With the help of RCA’s extensive machine shop capabilities, Rick constructed a 180° magnetic sector instrument. Thus began a long and productive period in Rick’s career during which he conducted fundamental experiments in secondary ion mass spectrometry (SIMS). In addition to SIMS, Rick used his early mass spectrometer to study the vaporization of many elements of interest to the electronics industry during the 1950s.During his sabbatical (1955–56) at the Free University in Brussels, Belgium, Rick continued to pursue his interests in the mass spectral characterization of solids vaporization. Rick’s concern with vapor pressure prompted him in 1957 to evaluate the information available at the time and publish the assembled data in chart form, which he updated in 1962 and 1969. His “vapor pressure charts” were instantly popular and continue to be displayed in many textbooks and laboratories throughout the world. His fundamental interest in solids analysis took his research into the areas of spark source mass spectrometry and laser desorption mass spectrometry. Rick and his RCA colleagues made many original and significant contributions to these fields as well. Among these are energy distributions of ions formed in the radio frequency spark source and the response of ion-sensitive plates for mass spectrography.Rick became head of RCA Laboratories’ newly reorganized materials characterization group in 1966 and held this position for the next 16 years. He built the group, directed its research, and promoted the careers of the group’s professional staff and technical associates. Under Rick’s leadership, RCA developed one of the most advanced and well-respected materials characterization laboratories in the world.Rick was an active participant in the activities of the American Society for Testing and Materials during the 1960s. He also was one of the founding fathers of the American Society for Mass Spectrometry and served as the society’s first vice president for programs (1968–70) and as its second president (1970–72).As a young man, Rick especially enjoyed gymnastics, ice skating, and tennis. In his later years, he enjoyed day outings and hiking with his family in the mountains of Switzerland and his native Germany. A longtime resident of Princeton, he was chairman of the language committee at the Princeton Adult School for many years.Rick’s friends and colleagues remember him with the greatest admiration and deepest appreciation for a lifetime devoted to the pursuit of scientific truth. His family and friends sorely miss him.Richard Edward HonigPPT|High resolution© 2002 American Institute of Physics.

Sputtering of hydrocarbons by ion-induced breaking of chemical bonds

While the physical sputtering of atoms caused by keV and MeV ions has been studied extensively both by molecular dynamics (MD) simulation and experiments, the mechanisms leading to atom and molecule erosion at energies ∼1–100 eV are not very well understood. We now describe how low-energy hydrogen ions can cause erosion of carbon atoms and hydrocarbon molecules by entering the region of a carbon–carbon chemical bond and hence breaking it, a process we call `swift chemical sputtering'. In the particular case of hydrogen bombardment of carbon-based materials, we further show that this can lead to erosion yields far exceeding those expected for a physical sputtering process.

Energy distributions of CD4 and CD3 chemically released from graphite by D+ and D0/Ne+ impact

Abstract Time-of-flight (TOF) experiments were performed to determine the energy distributions of CD 4 and CD 3 chemically released from graphite by energetic D + ion impact or by a synergistic reaction of Ne + ions and D atoms at temperatures near 800 K and room temperature (RT). The energy distribution of CD 4 chemically released from graphite at 800 K by 400 eV D + impact can be fitted by two Maxwell–Boltzmann (MB) distributions, one corresponding to the target temperature (800 K) and a faster one (40%) corresponding to 2000 K. In the synergistic reaction of D 0 atoms and 3 keV Ne + ions impacting on graphite at 830 K nearly all formed CD 3 are released by an MB distribution corresponding to 830 K. At room temperature, the time-dependent behavior of the CD 4 signal, after stopping the D + or the Ne + irradiation in the synergistic reaction case, indicates that no physical sputtering process is involved in this reaction. During synergistic reaction of graphite at room temperature with D 0 /Ne + , the released CD 4 can be fitted by two MB distributions corresponding to 1200 and 285 K.

Chemical and physical sputtering of aluminium and gold samples using Ar–H 2 DC-glow discharges

We present a series of sputtering experiments on aluminium samples performed with an Ar–H 2 DC-glow discharge at varying Ar–H 2 gas-composition, driven at a discharge voltage of –300 V and a pressure of 0.2 mbar, in conjunction with measurements of the corresponding ion-energy distributions of the ions bombarding the discharge cathode (Ar +, Ar 2+, ArH +, H 2 + and H 3 +). Similar measurements on gold samples, which have been published, have shown that the Au-sputtering efficiency of an Ar–H 2 glow discharge as a function of gas-composition could be adequately described by the corresponding change in the measured ion-energy distributions, under the assumption of a purely physical sputtering process. The experiments presented here show that this is not the case for aluminium (effectively Al 2O 3). In this case, a measured optimal gas-composition of 80% H 2 was found for Al-sputtering, while the energy-distributions suggest an optimum at 20% (as for gold). This clearly suggests that hydrogen-enhanced chemical sputtering is taking place.

Be+ and BeF + Implants in GaAs Using Ions Produced from Solid‐Gas Reaction

Be implantation is widely used for p‐type doping in III–V semiconductors. Physical sputtering of solid Be may result in some beam instability and low beam current, while solid sources such as require heating to achieve high vapor pressure. This work discusses implantation with Be+ and ions produced via plasma‐etching of solid Be with fluorine‐containing gases. Etching of Be with produced Be+ and ions with currents 10 times and 55 times higher than that of Be+ from the physical sputtering process, respectively. Activation of Be implanted as Be+ from the new process is comparable to those of the other processes. Activation of Be implanted as exhibited higher activation energy and lower activation efficiency. The implant still provides reasonable activation for source/drain and buried layer formation under acceptable annealing conditions. The lower effective Be+ implant energy of the implant allows shallow implant depths to be realized.

Structural modification in reactive-ion-etched i-InP and n+-InP studied by Raman scattering

Structural and electrical property modifications in i-InP and n+-InP by reactive ion etching have been characterized by using Raman scattering to observe changes in the positions and intensities of intrinsic phonons and coupled phonon-plasmon bands. Different etch gas compositions (Ar, He, CH4/H2, CH4/Ar, CH4/He, and CH4/H2/Ar), bias potentials (0–500 V), and etch times were examined. Electrical property changes were followed by correlating results with a one-sided abrupt junction model. Ar and He, which interact with the material by physical sputtering processes, were found to induce the greatest structural and electrical modifications. Ar-etched samples exhibited the greatest structural modification, with evidence of structural damage coming from far beyond the calculated penetration depth of low-energy Ar+, while samples etched in methane-based plasmas showed very little structural perturbation. Etching in all of the gas mixtures used produced some degree of electrical modification in n+-InP, with He plasmas causing the most significant changes. In general, samples etched with lower ion energies and sputter components exhibited the least structural and electrical modification.

Effects of Ar+ angle of incidence on the etching of Si with Cl2 and low-energy Ar+ ions

Experiments have been performed in a beam apparatus where an Ar+ beam and a Cl2 beam are directed simultaneously at a Si target. The apparatus allows independent variation of the ion angle-of-incidence and the detection angle. Time-of-flight (TOF) distributions of the reaction product SiCl are measured for various Cl2 to Ar+ flux ratios and various angles-of-incidence. These distributions can for the larger part well be fitted with collision cascade distributions, showing that at the parameter values chosen, the etch process in Si(Cl2,Ar+) is a chemically enhanced physical sputtering process. The observed variations of the binding energy U0 obtained from the TOF distributions support this mechanism. A discussion in relation to similar studies on the etch rate and the composition of the surface and the altered layer leads to a consistent picture in support of the above model.

Ultrastructural comparison of ion beam and radiofrequency plasma etching effects on biological tissue sections.

Three dry etching techniques (Ar+ ion beam, O2+ ion beam, O2 radiofrequency electrodeless discharge) were compared with respect to preferential etching and damage to the ultrastructure of glutaraldehyde-fixed Epon-embedded frog skeletal muscle sections. SEM and TEM studies were performed on both unstained and stained (osmium tetroxide, uranyl acetate) sections. Etching effects were observed to differ for the various ion beam or plasma etching techniques. Whereas selective retention of electron dense structures (e.g. Z lines, nuclear heterochromatin) was observed for oxygen plasma etching, preferential etching of these components was observed using O2+ ion beam bombardment. Selectively etched Z lines and etch-resistant nucleoli were observed for both reactive (O2+) and inert (Ar+) ion beam sputtering after sufficiently high ion doses. The above suggest that selective etching under keV ion beam irradiation is related more to physical sputtering processes (momentum transfer) than to the chemical reactivity of the incident ion. Heavy metal post-fixation and staining had no qualitative effect on the nature of the selective etching phenomena. The above findings are significant in that they potentially influence both electron and ion microprobe measurements of etched biological specimens.

Analysis of secondary ion species sputtered from SiC surface during He + and D + ion irradiation

Physical and chemical sputtering processes on the surface of silicon carbide irradiated by energetic deuterium and helium ions were investigated with the use of a modified ion microprobe mass analyzer. Measurements of the energy distribution curves of sputtered ions were performed using a changing sector electric field and the absolute energy was calibrated by adapting a retarding grid method. Secondary ions created through a chemical sputtering process such as SiD + and CD + in the case of D + irradiation on the silicon carbide surface showed two peaks existing in the energy distribution curve, while a single peak was observed for Si + and C + which were generated through a physical sputtering process by He + or D + irradiation. The energy distribution curves of secondary ion species, especially chemically sputtered ion species, were found to be strongly influenced by the target temperature. The amount of each secondary ion obtained from the area of the energy distribution curve for D + or He + irradiation decreased as the target temperature increased, except Si + ions which increased gradually up to 800°C and then decreased sharply above this temperature.