Sputtering Theory Research Articles

Research on Equipment of Magnetron Sputtering and Deposition of ZnO Films

The parts of magnetron sputtering system were given in the paper. Meanwhile, we analyzed the working theory of magnetron sputtering. After pure zinc oxide (ZnO) film was grown, N-doped films was Prepared in NH3-O2-Ar atmosphere using zinc as a target, and Al-doped and N+Al codoped ZnO films were grown by co-sputtering technique using zinc and aluminum as targets. AFM showed the crystal quality of the Films. The results of experiment demonstrate that high-quality films have been achieved by this technique, and research on ZnO films can be done by the equipment.

Theory and molecular simulations of plasma sputtering, transport and deposition processes

The present review provides an overview of the basic theory of sputtering with recent models, focusing in particular on sputtered atom energy distribution functions. Molecular models such as Monte-Carlo, kinetic Monte-Carlo, and classical Molecular Dynamics simulations are presented due to their ability to describe the various processes involved in sputter deposition at the atomic and molecular scale as required. The sputter plasma, the sputtering mechanisms, the transport of sputtered material and its deposition leading to thin film growth can be addressed using these molecular simulations. In all cases, the underlying methodologies and some selected mechanisms are highlighted.

Quantum theory of electronic excitation and sputtering by transmission electron microscopy.

Many computational models have been developed to predict the rates of atomic displacements in two-dimensional (2D) materials under electron beam irradiation. However, these models often drastically underestimate the displacement rates in 2D insulators, in which beam-induced electronic excitations can reduce the binding energies of the irradiated atoms. This bond softening leads to a qualitative disagreement between theory and experiment, in that substantial sputtering is experimentally observed at beam energies deemed far too small to drive atomic dislocation by many current models. To address these theoretical shortcomings, this paper develops a first-principles method to calculate the probability of beam-induced electronic excitations by coupling quantum electrodynamics (QED) scattering amplitudes to density functional theory (DFT) single-particle orbitals. The presented theory then explicitly considers the effect of these electronic excitations on the sputtering cross section. Applying this method to 2D hexagonal BN and MoS2 significantly increases their calculated sputtering cross sections and correctly yields appreciable sputtering rates at beam energies previously predicted to leave the crystals intact. The proposed QED-DFT approach can be easily extended to describe a rich variety of beam-driven phenomena in any crystalline material.

Ion Irradiation Induce Surface Modifications of InSb(001)

Morphological development on InSb(001) surfaces, under ion irradiation, has been investigated by means of Atomic Force Microscopy in UHV conditions. The InSb(001) surface has been exposed to Ar+ beam with varying incident angles (from 0° to 80° off-normal) for energy 4.0 keV and fluence 4x1017 ion/cm2. Different kinds of nanostructures, depending on ion incident angle, have been observed on the irradiated surfaces such as nanocavities, or nanorings and nanodots, preferentially for close to normal incidence, and well-order ripples for the oblique incidence. The formation of ripple structures on InSb(001) surfaces as a function of the incidence angle has been observed. The orientation of the ripples has been found to be incident angle dependent and it is perpendicular, or parallel to the ion beam projection on the irradiated surface for angles of 45° and 80°, respectively.. The results have been discussed in terms of ballistic processes of sputtering and Bradley Harper theory for surface modifications.

Inelastic collision effects of high-energy neutrons in tungsten materials

Because the inelastic impact of high flux, high-energy neutrons on tungsten material in a commercial fusion reactor would drastically decrease the service life and steady-state functioning of the core plasma in the future, it is vital to investigate the damage process. In this paper, a particle sputtering theory approach is used to derive an equation for calculating the sputtering yield of high-energy neutrons by simulating the transport of primary knocked-out atoms (PKA) resulting from inelastic collisions of 14.1 MeV neutrons with tungsten materials at different depth levels, namely Y =∫E0Emaxf(E)*Y(E)dE, where f (E) is the energy probability distribution function of PKA, Y (E) is the sputtering yield at a given energy of PKA particles generated by inelastic scattering collisions, Emax is the maximum kinetic energy of PKA, and E0 is the minimum kinetic energy of PKA (generally taken as 0). According to the result, when the neutron wall load is 1 MW/m2, and the annual average equivalent material wastage is less than 1 nm, sputtering damage to tungsten material can be ignored. However, it must be kept in mind that the reflection of the PKA into the plasma will have an impact on the performance of the plasma confinement system and the damage caused by the PKA's internal energy.

Accurate Thickness Measurements in Thin Films with Surface Analysis

Quantification in surface analysis using Auger electron spectroscopy and X-ray photoelectron spectroscopy has been the topic of significant work at NPL. A new approach to the quantification of materials that are homogeneous over the analysis volume has been developed using new average matrix relative sensitivity factors. These show agreement between theory and experiment at ~10% for all peaks and elements analysed. For samples that are not homogeneous, layer thicknesses are often required. For ultra-thin gate oxides, the International Technology Roadmap for Semiconductors requires 1.3% accuracy. For this purpose, angle-resolved XPS is a good candidate. In a wide study under the auspices of the Consultative Committee for Amount of Substance (CCQM), the accuracy of measurements of the thicknesses of SiO2 layers <8 nm thick on Si have been assessed. This study involved 45 sets of measurements in laboratories using MEIS, NRA, RBS, EBS, XPS, SIMS, ellipsometry, GIXRR, NR and TEM. The relative strengths and weaknesses become clear. These show that if XPS is used under reference conditions it can be reliable and fast with an accuracy, based on a calibration from the study, ~ 1%. Inter-method correlations as good as 0.05 nm are achieved over the 8 nm range. Furthermore, certain methods, thought to be accurate, suffer from incompleteness of the measurement method. For thicker layers, sputtering is generally used. Here a new method has been tested to generate a sputter yield database, for argon ions, of 26 critical elements. This database has been used to help evaluate a new semi-empirical theory of sputtering yields that includes terms missing from the current semi-empirical theories and removes errors that are up to, and may exceed, a factor of 5. The new theory agrees with published data at ~ 10% and shows why certain elements have anomalously high yields.

Effect of the Nanoscale Surface Relief on the Sputtering of Amorphous Carbon

In the binary collision approximation, the computer simulation of amorphous-carbon sputtering under bombardment with Ar ions with an energy of 1—10 keV is carried out with inclusion of the surface nanorelief of sinusoidal form. The angular characteristics of the target sputtering yield are obtained for different relief parameters. The simulation results are compared with analytical estimates and Monte Carlo calculations based on Sigmund sputtering theory. The simulation does not confirm a sharp increase in the sputtering yield of the undulating surface with increasing relief amplitude, which was predicted theoretically (Makeev, Barabasi). Significant discrepancies in the angular dependences of the sputtering yield are also observed. The calculation results are compared with existing experimental data.

Surface roughness evolution mechanism of the optical aluminum 6061 alloy during low energy Ar+ ion beam sputtering.

Ultra-smooth surfaces with low contamination and little damage are a great challenge for aluminum optical fabrication. Ion beam sputtering (IBS) has obvious advantages of low contamination and non-contact that make it a perfect method for processing aluminum optics. However, the evolution laws of aluminum surface morphology are quite different from conventional amorphous materials, which affects the roughness change and needs systematic research. Thus, in this paper, the roughness evolution of an aluminum optical surface (i.e., aluminum mirror) subjected to IBS has been studied with experimental and theoretical methods. The surface morphology evolution mechanisms of turning marks and second phase during IBS are revealed. The newly emerging relief morphology and its evolution mechanism are studied in depth. The experimental results find that IBS causes the coarsening of optical surfaces and the appearance of microstructures, leading to the surface quality deterioration. Turning marks have been through the process of deepening and vanish, while second phase generates microstructures on the original surface. The corresponding mechanism is discussed exhaustively. Preferential sputtering, curvature-dependent sputtering and material properties play important roles on surface quality deterioration. A modified roughness evolution mechanism and an improved binary sputtering theory are proposed to describe the polycrystalline sputtering phenomena. The current research can provide a guidance for the application of IBS in aluminum optics manufacture fields.

Ion irradiation induced surface composition modulation in equiatomic binary alloys

Composition modulation by Ar bombardment on the Co0.5Cu0.5 alloy and the CoAl B2 phase was investigated on atomic scale by molecular dynamics simulation. The Ar bombardment on alloys at 300 K revealed that this bombardment induces a surface composition modulation in the layer-by-layer mode. In both the Co0.5Cu0.5 alloy and the CoAl B2 phase, the element of higher-sputtering yield is accumulated on the top surface layer, whereas it is depleted in lower layers, which is puzzling in the framework the conventional sputtering theory. A phenomenological kinetic model derived from the MD simulation results considering both the rearrangement and the sputtering of the substrate atoms successfully demonstrated that the rearrangement of the substrate atoms plays a significant role in the observed composition modulation.

Stress-dependent prediction of metastable phase formation for magnetron-sputtered V1−xAlxN and Ti1−xAlxN thin films

Metastable transition metal aluminum nitride (TMAlN, TM = Ti, V) thin films are today deposited utilizing ionized vapor phase condensation techniques where variations in ion flux and ion energy cause compressive film stress, in turn affecting Al solubility. While the metastable phase formation of TiAlN has been modeled, the influence of film stresses on phase formation has so far been overlooked. Using combinatorial deposition via magnetron sputtering, thermodynamic modeling and density functional theory calculations, we investigated the phase formation of V1−xAlxN and Ti1−xAlxN thin films at various substrate temperatures and deposition rates. Ab initio calculations indicate that the maximum solid solubility of Al in face-centered cubic (fcc) V1−xAlxN or fcc-Ti1−xAlxN shows a linear trend as a function of the magnitude of compressive stress. Here, we consider the influence of film stresses on the metastable phase formation of fcc-V1−xAlxN and fcc-Ti1−xAlxN for the first time. Specifically, experimental data from a single combinatorial deposition is utilized to predict the stress-dependent formation of metastable phases based on thermodynamic and ab initio data. Explicit consideration of stress extends the Al solubility limit to higher values for both Ti1-xAlxN and V1-xAlxN thin films, previously unobtainable by energetics, but accessible experimentally. These predictions are experimentally verified and thus provide guidance for experimental efforts with the goal of increasing the Al concentration in fcc-TMAlN thin films.

Micromorphology of sputtered aluminum thin films: A fractal analysis

Abstract In this article, the surface analyses of topography, microstructure and mechanical behavior of Al thin films sputtered on glass are presented to understand the influence of varying low substrate temperatures at high radio-frequency (RF) power. The films were prepared by RF magnetron sputtering process at different substrate temperatures of 55°C, 65°C, and 95°C and at a constant RF power of 350 W. The topography, microstructure and mechanical properties were characterized by atomic force microscope (AFM), field emission scanning electron microscope (FESEM) and nanoindentation method respectively. The results revealed that at a higher substrate temperature of 95°C the microstructure consists of dense and globular dominant aluminum structures and higher interface width (roughness) as compared to the other temperatures. Nanoindentation results showed that films deposited at 95°C exhibited the highest values of average hardness and elastic moduli and a lowest maximum depth of displacement. A fractal analysis on the topography and surface microstructure revealed a decrease of the fractal dimension with the increase in substrate temperature and that the films exhibited a mono-fractal behavior at all the substrate temperatures investigated in the present study. A description based on the sputtering theory for the monofractal nature of the aluminum films is provided.

Emission Theory of Amorphous Material Sputtering: the Dependence of the Sputtering Coefficient on the Angle of Primary Ion Beam Incidence

The dependences of the sputtering coefficient on the types of accelerated ions, their energy, and the angle of incidence on a target are calculated. For Ar–Si, Хе–С, and H–W systems, acceptable coincidence between the calculated and experimental data is obtained. Two mechanisms for secondary-particle ejection from solids are established; they determine the dependences of the base sputtering coefficient for the base on the energy and the angle of primary ion incidence on the target.

Smallest microhouse in the world, assembled on the facet of an optical fiber by origami and welded in the μRobotex nanofactory

In this study, the authors have demonstrated that it is possible to realize several three-dimensional (3D) micro- and nanostructures, by the fabrication of the smallest microhouse using a dual beam scanning electron microscope (SEM)/focused ion beam (FIB) Auriga 60 from Zeiss together with a six degree of freedom robot built with SmarAct components. In this new type of nanolab, cutting, etching, folding, assembling, and then welding thin membranes of silica on top of a cleaved optical fiber SMF28, or production of micro- and nanostructures, like the microhouse, are possible. The authors have experimentally shown that FIB can be used, in this new generation of micro/nanofactory, in combination with SEM, and gas injection system, in order to fabricate three-dimensional microstructures: a microhouse in this study, with ultrahigh accuracy assembly down to 10 nm. By using the theory of sputtering, the authors are able to propose a model of folding thin membranes of numerous materials such as metals, polymers, or crystals, i.e., silica, silicon, potassium tantalite, or lithium niobate. This method is usually described as origami in the literature [W. J. Aroa, H. I. Smith, and G. Barbastathis, Microelectron. Eng. 84, 1454 (2007); W. J. Aroa et al., J. Vac. Sci. Technol., B 25, 2184 (2007); and K. Chalapat et al., Adv. Mater. 25, 91 (2013)]. The experimental results indicate that the introduction of a microrobot inside the SEM vacuum chamber will provide the means to enlarge the scope of clean room facilities to build complex and smart 3D microsystems with heterogeneous materials, especially on the facet of an optical fiber in the lab on fiber new field. The authors propose a new way to easily manufacture many kinds of optical functions for light trapping based on nanoantennas, nanophotonic crystal, axicon or lattice, 3D biosensor with origami, and nanopatterning surfaces or carbon nanotubes, etc.

Sputtering of SicGe1−c nanospheres

It is known that sputter yields of elemental materials depend on the target curvature. Here we explore to what extent this effect is modified for alloy targets using both Monte Carlo and molecular dynamics simulations. For the exemplary case of amorphous ${\mathrm{Si}}_{c}{\mathrm{Ge}}_{1\ensuremath{-}c}$ spheres, we find that, in the limit of small curvatures, the curvature dependence of the sputter preferentiality is negligible. This finding can be explained by a natural extension of the existing analytical theory of curvature-dependent sputtering for elemental materials. For large curvatures, however, the sputter preferentiality strongly depends on curvature. In this case, molecular-dynamics simulations also predict strong spike effects which increase the sputter yields. Sputter yield amplification---an increase in partial sputter yield with decreasing concentration of the element---occurs for Si-rich alloys.

Applying sputtering theory to directional atomic layer etching

Plasma assisted atomic layer etching (ALE) has recently been introduced into manufacturing of 10 nm logic devices. This implementation of ALE is called directional ALE because ions transfer momentum to the etching surface during the removal step. Plasma assisted directional ALE can be described as sputtering of a thin modified layer on the surface of the unmodified material. In this paper, the authors introduce a collision cascade based Monte Carlo model based on sputtering theory which has evolved for over 50 years [P. Sigmund, Thin Solid Films 520, 6031 (2012)]. To test the validity of this approach, calculated near threshold argon ion sputtering yields of silicon and chlorinated silicon are compared to published experimental data. The calculated ALE curve for Cl2/Ar ALE of tantalum is in good agreement with the experiment. The model was used to predict the presence of salient sputtering effects such as ion mass and impact angle dependence, as well as redeposition in directional ALE. Finally, the authors investigate time dependence of the synergy parameter for ion energies above the sputtering threshold of tantalum for Cl2/Ar ALE. The calculations show that close to 100% synergy can be obtained for short periods of time which opens a path to accelerate directional ALE. Very precise control of all process parameters as a function of time is prerequisite to realize this process space.

Emission theory of amorphous-material sputtering. Energy dependence of the sputtering coefficient

The applicability conditions for the Boltzmann kinetic equation in describing the sputtering process are analyzed. The application of the kinetic equation for calculation of the sputtering coefficient is questionable because of the small depth of origin of sputtered particles. A physical model of the emission sputtering theory for amorphous materials is proposed. The threshold sputtering energies and energy dependence of the sputtering coefficient for amorphous materials are calculated in the framework of the proposed model for the case of the normal incidence of primary ions in the energy range of 10 to 105 eV. The calculations are in good agreement with the experimental results.

Review Article: Tracing the recorded history of thin-film sputter deposition: From the 1800s to 2017

Thin films, ubiquitous in today's world, have a documented history of more than 5000 years. However, thin-film growth by sputter deposition, which required the development of vacuum pumps and electrical power in the 1600s and the 1700s, is a much more recent phenomenon. First reported in the early 1800s, sputter deposition already dominated the optical-coating market by 1880. Preferential sputtering of alloys, sputtering of liquids, multitarget sputtering, and optical spectroscopy for process characterization were all described in the 1800s. Measurements of threshold energies and yields were carried out in the late 1800s, and yields in reasonable agreement with modern data were reported in the 1930s. Roll-to-roll sputter coating on flexible substrates was introduced in the mid-1930s, and the initial demonstration of sustained self-sputtering (i.e., sputtering without gas) was performed in 1970. The term magnetron dates to 1921, and the results of the first magnetron sputtering experiments were published in the late 1930s. The earliest descriptions of a parallel-plate magnetron were provided in a patent filed in 1962, rotatable magnetrons appeared in the early 1980s, and tunable “unbalanced” magnetron sputtering was developed in 1992. Two additional forms of magnetron sputtering evolved during the 1990s, both with the goal of efficiently ionizing sputter-ejected metal atoms: ionized-magnetron sputtering and high-power impulse magnetron sputtering, with the latter now being available in several variants. Radio frequency (rf) glow discharges were reported in 1891, with the initial results from rf deposition and etching experiments published in the 1930s. Modern capacitively-coupled rf sputtering systems were developed and modeled in the early 1960s, and a patent was filed in 1975 that led to pulsed-dc and mid-frequency-ac sputtering. The purposeful synthesis of metal-oxide films goes back to at least 1907, leading to early metal-oxide and nitride sputtering experiments in 1933, although the term “reactive sputtering” was not used in the literature until 1953. The effect of target oxidation on secondary-electron yields and sputtering rates was reported in 1940. The first kinetic models of reactive sputtering appeared in the 1960s; high-rate reactive sputtering, based on partial-pressure control, was developed in the early 1980s. While abundant experimental and theoretical evidence already existed in the late 1800s to the early 1900s demonstrating that sputtering is due to momentum transfer via ion-bombardment-induced near-surface collision cascades, the concept of sputtering resulting from local “impact evaporation” continued in the literature into the 1960s. Modern sputtering theory is based upon a linear-transport model published in 1969. No less than eight Nobel Laureates in Physics and Chemistry played major roles in the evolution of modern sputter deposition.

Formation of nanoripples on amorphous alumina thin films during low-energy ion-beam sputtering: Experiments and simulations

The formation of nanopatterns induced by low-energy (0.5--1.5 keV) ${\mathrm{Xe}}^{+}$ ion-beam sputtering of amorphous alumina thin films is investigated by atomic force microscopy and grazing incidence small-angle x-ray scattering. The observed dependence of the surface morphology on ion incidence angle, temperature, ion energy, and fluence is compared with the predictions of linear and nonlinear continuum theoretical models. The results show that ion-induced mass redistribution stabilizes the surface at near-normal and very grazing incidence angles, while curvature-dependent erosion governs the formation of periodic nanoripples in the range of incidence angles between ${50}^{\ensuremath{\circ}}$ and ${65}^{\ensuremath{\circ}}$. Surface-confined ion-induced viscous flow is shown to be the dominant relaxation mechanism during erosion. Moreover, pattern evolution with ion fluence (pattern ordering and asymmetry of the ripple profile, in particular) suggests that nonlinear effects that are ignored by the Sigmund's collision cascade theory of sputtering contribute strongly to the observed dynamics of ripple formation.

Size and Charge Distributions of Stable Clusters Formed in Ion Sputtering of Metals

A theory of ion sputtering of metals in the form of neutral and charged clusters with their subsequent fragmentation into the stable state is developed. The theory is based on simple physical assumptions and is in good agreement with experiment. Results are presented in the form of formulas convenient for practical application. As an example, calculations of the total yield of stable neutral and charged clusters of silver, indium, and niobium are carried out.

Ion beam sputtering of Ti: Influence of process parameters on angular and energy distribution of sputtered and backscattered particles

In the present study, the influence of ion energy and geometrical parameters onto the angular and energy distribution of secondary particles for sputtering a Ti target with Ar ions is investigated. The angular distribution of the particle flux of the sputtered Ti atoms was determined by the collection method, i.e. by growing Ti films and measuring their thickness. The formal description of the particle flux can be realized by dividing it into an isotropic and an anisotropic part. The experimental data show that increasing the ion energy or decreasing the ion incidence angle lead to an increase of the isotropic part, which is in good agreement with basic sputtering theory. The energy distribution of the secondary ions was measured using an energy-selective mass spectrometer. The energy distribution of the sputtered target ions shows a maximum at an energy between 10eV and 20eV followed by a decay proportional to E−n, which is in principle in accordance with Thompson’s theory, followed by a high energetic tail. When the sum of incidence angle and emission angle is increased, the high-energetic tail expands to higher energies and an additional peak due to direct sputtering events may occur. In the case of backscattered primary Ar ions, a maximum at an energy between 5eV and 10eV appears and, depending on the scattering geometry, an additional broad peak at a higher energy due to direct scattering events is observed. The center energy of the additional structure shifts systematically to higher energies with decreasing scattering angle or increasing ion energy. The experimental results are compared to calculations based on simple elastic two-particle-interaction theory and to simulations done with the Monte Carlo code SDTrimSP. Both confirm in principle the experimental findings.