Reactive Sputtering Process Research Articles

On the role of chemisorption in the formation of the target surface compound layer during reactive magnetron sputtering

Modeling of the reactive sputter processes requires a description of the target processes which results in the formation of a compound layer on the target. The sputter removal of the formed compound is balanced by compound formation mechanisms. Chemisorption of reactive gas atoms and molecules was initially identified as one of these processes. Further improvement of these chemisorption based models was achieved by the inclusion of processes such direct reactive ion implantation, knock-on implantation of chemisorbed species, and the redeposition of sputtered material on the target. In a recent study, the importance of ion implantation was questioned and an alternative model proposed. The results of this model are confronted with existing literature to investigate its validity.

Effect of substrate and growth method on vanadium dioxide thin films by RF magnetron sputtering: Vanadium metal oxidation vs reactive sputtering

Vanadium dioxide (VO2) undergoes a metal–insulator phase transition at ∼70 °C and has attracted substantial interest for potential applications in electronics, including those in neuromorphic computing. The vanadium–oxygen system has a rather complicated phase diagram, and controlling the stoichiometry and the phase of thin films of vanadium oxides is a well-known challenge. We explore the novel combination of two methods of VO2 thin film deposition using off-axis RF magnetron sputtering on (100)- and (111)-oriented yttria-stabilized zirconia (YSZ) substrates: reactive sputtering of vanadium in an oxygen environment and sputtering of vanadium metal followed by oxidation to VO2. Interestingly, the reactive sputtering process on both substrate orientations yields the metastable semiconducting VO2 (B) phase, which is structurally stabilized by the YSZ surface. The metal sputtering and oxidation process on YSZ produces mainly the equilibrium monoclinic (or M1) phase of VO2 that exhibits a metal–insulator transition. Using this method, we obtained thin films of (010)-textured polycrystalline VO2 (M1) that show a metal–insulator transition with an on/off ratio larger than 1000.

A fluid model of pulsed direct current planar magnetron discharge

We simulated a pulsed direct current (DC) planar magnetron discharge using fluid model, solving for species continuity, momentum, and energy transfer equations, coupled with Poisson equation and Lorentz force for electromagnetism. Based on a validated DC magnetron model, an asymmetric bipolar potential waveform is applied at the cathode at 50–200 kHz frequency and 50–80% duty cycle. Our results show that pulsing leads to increased electron density and electron temperature, but decreased deposition rate over non-pulsed DC magnetron, trends consistent with those reported by experimental studies. Increasing pulse frequency increases electron temperature but reduces the electron density and deposition rate, whereas increasing duty cycle decreases both electron temperature and density but increases deposition rate. We found that the time-averaged electron density scales inversely with the frequency, and time-averaged discharge voltage magnitude scales with the duty cycle. Our results are readily applicable to modulated pulse power magnetron sputtering and can be extended to alternating current (AC) reactive sputtering processes.

Evidence of 1000 eV positive oxygen ion flux generated in reactive HiPIMS plasma

The arrival of highly energetic (near 1000 eV) positive atomic oxygen ions at the substrate region has been detected in a unipolar reactive high-power impulse magnetron sputtering process operated with an uncooled copper target in argon–oxygen mixtures. Examination of the ion fluxes from discharge plasma was performed with a magnetic sector mass-spectrometer and an electrostatic energy analyzer. The energy of fast positive O+ ions is close to the value of eV d (e—elementary charge, V d—discharge voltage), which indicates their connection to the well-studied fraction of negative O− ions, which undergo acceleration in the cathode sheath. After switching the oxygen gas supply off, the flux of energetic O+ species decreases gradually as the poisoned target surface layers become depleted of oxygen due to sputtering in pure argon. Presumably, the observed energetic O+ ions originate as a result of low-angle scattering of fast negative O− ions from other charged or neutral species in the plasma followed by electron detachment, ionization, or charge exchange.

EIS capacitor sensor, with TiO2 dielectric, applied in the evaluation of phosphate in wastewater

This work presents the development of an Electrolyte-Insulator-Semiconductor (EIS) device with a built-in reference electrode to detect phosphate in wastewater. The idea of developing this device comes from the need to improve the use of water processed by the effluent treatment plant. After treatment process, a fraction of the water can be used as wastewater, this water cannot be consumed, however it can be used for other purposes including cleaning public spaces, such as streets and squares. In the fabrication process of the EIS device, titanium oxide (TiO2) deposited on a silicon substrate was used as a sensor element, and titanium nitride (TiN) was used for the reference electrode. Both materials were deposited by the reactive sputtering process. Aluminum (Al) was used for the electrode on the back of the slide deposited by evaporation. Preliminary results of structural and electrical characterizations indicate that the manufactured device has good sensitivity to phosphate ions (66 mV/ppm). This sensitivity is due to the good quality of the film, which is shown using Raman spectroscopy, atomic force microscopy (AFM) and X-ray diffraction (XRD) techniques, in addition to electrical measurements.

Modeling of Reactive Sputtering-History and Development.

This work critically reviews the evolution of reactive sputtering modeling that has taken place over the last 50 years. The review summarizes the main features of the deposition of simple metal compound films (nitrides, oxides, oxynitrides, carbides, etc.) that were experimentally found by different researchers. The above features include significant non-linearity and hysteresis. At the beginning of the 1970s, specific chemisorption models were proposed. These models were based on the assumption that a compound film was formed on the target due to chemisorption. Their development led to the appearance of the general isothermal chemisorption model, which was supplemented by the processes on the surfaces of the vacuum chamber wall and the substrate. The model has undergone numerous transformations for application to various problems of reactive sputtering. At the next step in the development of modeling, the reactive sputtering deposition (RSD) model was proposed, which was based on the implantation of reactive gas molecules into the target, bulk chemical reaction, chemisorption, and the "knock-on effect". Another direction of the modeling development is represented by the nonisothermal physicochemical model, in which the Langmuir isotherm and the law of mass action are used. Various modifications of this model allowed describing reactive sputtering processes in more complex cases when the sputtering unit included a hot target or a sandwich one.

Formation of pure anatase TiO2 using the reactive pulsed dc magnetron sputtering method for controlling the target poisoning state

AbstractThe use of pulsed dc‐sputtering sources for reactive magnetron sputtering with oxygen offers a possibility to suppress the negative effects of target poisoning (such as arcing). This results in a wide process range for the selection of a desired operating point. The control of target poisoning plays a major role in maintaining constant coating properties and affects the stoichiometry of the reactive coating, as well as the coating rate and the economic impact of the coating process. In a hysteresis, the target poisoning during the reactive sputtering of titanium under oxygen addition proceeds nonlinearly. Without the use of a suitable target poisoning control technique, the sputtering process can abruptly change to an unstable state. As a result, variations of stoichiometry can occur during the deposition process. A proven method for maintaining a stable reactive sputtering process is the control of oxygen flow with the input variable target voltage. By determining the typical oxygen hysteresis at constant target power and constant argon flow, an operating point for the control loop is derived. The desired target voltage then serves as the input variable for the control loop of the target poisoning. The controlling technique for target poisoning is a basic requirement for the production of the photolytic active anatase phase of titanium dioxide (TiO2) using reactive magnetron sputtering. The photocatalytic equipment of surfaces with a titanium dioxide coating in the anatase phase can be realized with the reactive pulsed dc magnetron sputter ion plating process (DC‐MSIP). The pulsed DC‐MSIP process facilitates coating a variety of surfaces at temperatures below 200°C in an environmentally friendly manner.

Synthesis of CuO thin films by a direct current reactive sputtering process for CO gas sensing application

A direct current magnetron reactive sputtering, one of the well-known physical vapour deposition (PVD) techniques, was employed for the preparation of CuO thin films at room temperature for CO gas sensing application. The effect of the O2 gas flow ratio on the phase formation of copper oxide was studied by varying the O2 flow rate in the total flow of Ar/O2 gas mixture. Cu2O phase was found to form at a low O2 flow ratio of 10% and gradually converted into stable CuO phase with an increase in O2 flow ratio through the intermediate phase of Cu4O3. The films exhibited a granular morphology, and the average grain size increased with an increase in the O2 partial flow. Single-phase CuO thin film has been obtained with 40% of O2 gas flow ratio. The evolution of the copper oxide phases with increasing O2 partial flow was also confirmed using the Cu 2p and O 1s core-levels of X-ray photoelectron spectroscopy. The CO gas sensing characteristics of the CuO thin film were examined by varying the operating temperature in the range of 200 °C–400 °C. An optimized CO sensing response of 127% has been obtained at 375 °C towards 91 ppm concentration with a response/recovery time of 161 s/99 s.

Materials Based on Amorphous Al2O3 and Composite W-Al2O3 for Solar Coatings Deposited by High-Rate Sputter Processes

In parabolic trough technology, the development of thermally and structurally stable solar coatings plays a key role in determining the efficiency, durability, and economic feasibility of tube receivers. A cermet-based solar coating is typically constituted by a thin film stratification, where a multilayer graded cermet is placed between an infrared metallic reflector and an antireflection filter. This work reports the realization of materials based on Al2O3 and W characterized by high structural and chemical stability in vacuum at high temperature, obtained through the optimization of high-deposition-rate processes. Al2O3 material, employed as the antireflection layer, was deposited through a reactive magnetron sputtering process at a high deposition rate. Cermet materials based on W-Al2O3 were deposited and employed as absorber layers by implementing reactive magnetron co-sputtering processes. An investigation into the stability of the realized samples was carried out by means of several material characterization methods before and after the annealing process in vacuum (1 × 10−3 Pa) at high temperature (620 °C). The structural properties of the samples were evaluated using Raman spectroscopy and XRD measurements, revealing a negligible presence of oxides that can compromise the structural stability. Spectrophotometric analysis showed little variations between the deposited and annealed samples, clearly indicating the high structural stability.

High-performance p-type transparent conducting CuI–Cu2O thin films with enhanced hole mobility, surface, and stability

We employed reactive magnetron sputtering and iodination process at room temperature to deposit CuI–Cu2O films with enhanced hole mobility, surface, and stability.

Multivariable Modeling and Feedback Control of Reactive Sputtering

A control design method to stabilize multivariable reactive sputter processes is developed. A new model is presented to describe the nonlinear deposition process with respect to the set argon gas flow, the set reactive gas flow and the generator power as the inputs. Outputs are the total pressure, the reactive gas pressure and the self-bias voltage. The control-oriented model is analyzed with regard to its static behavior and its stability properties. Based on this analysis, a novel design method for the process control is discussed. Experimental data illustrate the applicability of the developed control system to accomplish set-point following for the control loop despite the presence of uncertainties in the process parameters.

Моделирование реактивного распыления. Обзор

This paper takes a critical look at the evolution of reactive sputtering modeling over the past 50 years. The review summarizes the main regularities of the processes of film deposition of simple metal compounds (nitrides, oxides, oxynitrides, carbides, etc.) discovered experimentally by various researchers. The above features include significant non-linearity and hysteresis. In the 1970s, specific models of isothermal chemisorption were proposed. These models were based on the assumption that a compound film is formed on the target due to chemisorption. Their development led to the emergence of a general model of isothermal chemisorption, which was supplemented by processes on the surfaces of the vacuum chamber wall and the substrate. The model has undergone numerous transformations for application to a variety of reactive spray applications. At the next stage in the development of modeling a reactive sputtering (RSD) model was proposed based on the implantation of reactive gas molecules into the target, bulk chemical reaction, chemisorption, and the «knock-on» effect (implantation of adsorbed recoil atoms or molecules that received an impulse from an accelerated argon ion). Another direction in the development of modeling is represented by a non-isothermal physical and chemical model, in which the Langmuir isotherm and the law of mass action were used. Various modifications of this model made it possible to describe reactive sputtering processes in more complex cases, when a hot or sandwich target is included in the sputtered unit.

Enhancing ultraviolet response of NiO/Si heterojunction by high temperature anneal

In this work, NiO/Si heterojunction is fabricated by direct current reactive magnetron sputtering process and the effect of post anneal temperature on ultraviolet response of the heterojunction is investigated. The results indicate that the dark current is decreased and the light current is increased with annealing temperature increasing in the fabricated NiO/Si heterojucntion. When illuminated under 365 nm UV light with an intensity of 1.0 mW/cm2, the light to dark switch ratio of the fabricated NiO/Si heterojunction annealed at 800 °C is about 486.1. The corresponding ultraviolet response is enhanced by about 83.8 times compared to the heterojucntion without anneal. The switch-on and switch-off time of the 800 °C annealed NiO/Si heterojunction illuminated under 365 nm UV light are about 100 μs and 180 μs, respectively. It is expected that the NiO/Si heterojunction has enormous potential in multi-wavelength light detecting application.

Sputter Epitaxy of AlN and GaN on Si(111)

Sputter epitaxy is a low‐cost process suited for the deposition of group‐III‐nitride semiconductors and allowing the deposition on large substrate areas at lower growth temperatures than in metal–organic vapor phase epitaxy (MOVPE). High‐quality AlN, AlGaN, and GaN epitaxially grown on Si(111) substrates by reactive magnetron sputtering are demonstrated and details on process parameters are given. With an ammonia‐based reactive sputtering process in a high‐purity environment, AlN can be grown with high crystalline quality comparable to the best MOVPE‐grown samples regarding twist and tilt and with a very low surface roughness, free of the typical columnar structure of sputtered AlN and pits. Also AlGaN, typically required for strain engineering of GaN layers grown on Si, can be grown in the entire compositional range by co‐sputtering of Al and Ga. Thin undoped buffer layer samples show high breakdown field strengths well above 2.5 × 106 V cm−1 and demonstrate the possible use for field effect transistor (FET) buffer layers.

Study of ruthenium film grown in oxygen environment for x-ray optics application

Ru shows high damage threshold as compared to other standard materials Au, Rh and Pt for high heat load applications in high brilliance synchrotron radiation source and x-ray free electron laser. It is also a promising candidate to be used as a protective capping layer in multilayers for increasing their life time. In the present study, Ru thin films of 500Å thickness were deposited in oxygen environment at different substrate temperatures varying from 70°C to 500°C. In reactive ion beam sputtering process, the flow of Ar and reactive O2 gas was maintained in 4:1 ratio keeping total flow 3 SCCM constant. The deposited films were characterized using grazing incidence x-ray reflectivity (GIXRR), grazing incidence x-ray diffraction (GIXRD) and secondary ion mass spectroscopy (SIMS) techniques. A significant change in GIXRR profile was observed in the sample grown at 100°C and 500°C whereas the GIXRD measurements indicated no significant formation of ruthenium oxide at different temperatures. However, a strong peak of Ru2Si3 (222) at 500°C was observed indicating a strong interfacial reaction at ruthenium/ substrate interface.The experimental data suggested a weak signature of RuO2 phase in the films deposited at RT, 70°C and 100°C and therefore the effect of ruthenium oxide with 10% composition in pure ruthenium was calculated on optical performance in extreme ultra violet region of photon energy (85-115 eV).

직접 탄화수소 연료 적용을 위한 박막 고체산화물 연료전지 개발

Energy devices in modern society require high efficiency, carbon neutrality, and the capability of distributed power generation. A fuel cell is an energy conversion device, that satisfies all of these requirements. However, most fuel cells use hydrogen as a fuel, and more than half of hydrogen is currently produced through hydrocarbon reforming, resulting in significant energy loss. Additionally, the storage and supply of hydrogen require costly systems, and a large amount of energy is consumed during compression or liquidation processes. This paper develops a solid oxide fuel cell, that uses hydrocarbon directly as fuel to resolve this problem. A small amount of Ru is mixed with the Ni-based electrode, for the effective internal reforming of hydrocarbons. For rapid deposition of YSZ electrolytes, we developed a reactive sputtering process, using a DC power source. The developed thin-film solid oxide fuel cell, showed a performance of 76 mW/cm² at 500℃ using methane as fuel.

Analysis of TihxOy Films Produced by Physical Vapor Deposition Method

For decades, partially oxidized hydrides were commonly considered as undesirably contaminated phases and were avoided by scientists. Nevertheless, more recently, it was realized that in some hydrides and oxides, partial substitution of dissimilar H− and O2− anions allows one to obtain unique optical and electrical properties that might have appealing applications in commercial products. It was determined that specific properties of so called oxyhydride materials strongly depend on the used synthesis methods; therefore, there is a great interest in exploring various variants of oxyhydride formation. In the current study, TiHxOy films were deposited by a reactive magnetron sputtering process in Ar-O2-H2 gas mixtures. Color, transparency and crystal phase composition of the films coherently reacted to the Ar:O2:H2 gas ratio. Namely, the rise in partial hydrogen pressure promoted the formation of anatase phase TiO2 structure and darkening of the films. Interestingly, this had only minimal impact on the band gap values, but had a relatively strong negative effect on the photocatalytic activity of the films. The unaccustomed results stressed the difference between the partially reduced TiO2 with a significant amount of oxygen vacancies and synthesized TiHxOy films where some O2− ions are implicitly substituted by H− ions.

Thermal Stability of Sputtered Tungsten Nitrides for Solar Thermal Applications

In this work, tungsten nitrides sputtered at different powers supplied to a W target (300 W, 500 W, 700 W) and proposed for solar thermal applications as part of solar absorbers, as active and robust materials for capacity energy storage and as plasma-facing materials were annealed in vacuum at medium-high temperatures (470 °C, 580 °C) and characterized by means of X-ray diffraction (XRD), AFM, micro-Raman, FTIR, UV–VIS–-NIR, sheet, surficial energy and wetting angle measurements. From the overall set of analyses, some important modifications and differences between samples after annealing emerged (which will be useful for selecting them for specific applications) and have been correlated to sputtered W metallic clusters’ ability to adsorb, form complexes with and react with the strong N2 triple bond under the various plasma conditions of a reactive sputtering process. In particular, the 300 W film of poor crystalline quality as deposited, after annealing released entrapped nitrogen and retained its W2N structure up to a temperature of 580 °C. Despite there being no phase transition, there was an increase in sheet resistance, which is detrimental because the preservation of metallic character is an important requisite for the proposed applications. The 500 W film had a stable crystalline structure and a metallic character unmodified by increasing temperature. The 700 W film, whose structure as deposited was almost amorphous, underwent the most severe modification after annealing: crystallizing, disproportioning and giving rise to a composite and porous nature (W + WNx) not ideal for spectrally selective coating applications, but useful for tailoring capacitive energy storage devices, or for catalysts for hydrogen evolution reactions (as an alternative to platinum) in alkaline water electrolysis.

High Rate Deposition of Piezoelectric AlScN Films by Reactive Magnetron Sputtering from AlSc Alloy Targets on Large Area.

This paper reports on the deposition and characterization of piezoelectric AlXSc1-XN (further: AlScN) films on Si substrates using AlSc alloy targets with 30 at.% Sc. Films were deposited on a Ø200 mm area with deposition rates of 200 nm/min using a reactive magnetron sputtering process with a unipolar–bipolar hybrid pulse mode of FEP. The homogeneity of film composition, structural properties and piezoelectric properties were investigated depending on process parameters, especially the pulse mode of powering in unipolar–bipolar hybrid pulse mode operation. Characterization methods include energy-dispersive spectrometry of X-ray (EDS), X-ray diffraction (XRD), piezoresponse force microscopy (PFM) and double-beam laser interferometry (DBLI). The film composition was Al0.695Sc0.295N. The films showed good homogeneity of film structure with full width at half maximum (FWHM) of AlScN(002) rocking curves at 2.2 ± 0.1° over the whole coating area when deposited with higher share of unipolar pulse mode during film growth. For a higher share of bipolar pulse mode, the films showed a much larger c-lattice parameter in the center of the coating area, indicating high in-plane compressive stress in the films. Rocking curve FWHM also showed similar values of 1.5° at the center to 3° at outer edge. The piezoelectric characterization method revealed homogenous d33,f of 11–12 pm/V for films deposited at a high share of unipolar pulse mode and distribution of 7–10 pm/V for a lower share of unipolar pulse mode. The films exhibited ferroelectric switching behavior with coercive fields of around 3–3.5 MV/cm and polarization of 80–120 µC/cm².

Tailoring superhydrophilicity and photocatalysis of sputter deposited TiO2/WO3 thin films on super white glass by process and chemical composition optimization

Super white cover glass with durable self-cleaning ability is critical for solar panels to function properly. In this regard, an ultrahigh vacuum coater was used to deposit TiO2/WO3 nanocomposite thin films with WO3 molar fractions of 0.14, 0.32 and 0.88 onto super white glass using a pulsed-DC reactive sputter process. The purpose of this study is through processing controls (poisoned degree and film annealing) and adjustments of the film's chemical composition to tailor photocatalytic performance of the deposit films. Annealing could cause sodium to diffuse out from super white glass (Na = 2.9 wt%) into the films, reacting itself with WO3 to form water-soluble sodium tungstate (Na2WO4). Thus, through composition optimization and the films deposited at fully-poisoned modes and then adequately annealed, an anatase-dominated, mesoporous coating that yields the most active and durable superhydrophilicity and photocatalysis is fabricated, potentially for self-cleaning cover glass applications. The evidence for the mesoporous film to yield highly effective light-induced superhydrophilicity and anti-coking abilities is given, and the mechanism for it to yield the best photoactivity above the other films is clarified by atomic-scaled analyses of transmission electron microscopy, X-ray absorption spectroscopy and Rutherford backscattering spectroscopy.