Time and energy resolved ion mass spectroscopy studies of the ion flux during high power pulsed magnetron sputtering of Cr in Ar and Ar/N2 atmospheres

Mass spectroscopy was used to analyze the energy and composition of the ion flux during high power pulsed magnetron sputtering (HIPIMS/HPPMS) of a Cr target in an industrial deposition system. The ion energy distribution functions were recorded in the time-averaged and time-resolved mode for Ar⁺, Ar²⁺, Cr⁺, Cr²⁺, N₂⁺ and N⁺ ions. In the metallic mode the dependence on pulse energy (equivalent of peak target current) was studied. In the case of reactive sputtering in an Ar/N₂ atmosphere, variations in ion flux composition were investigated for varying N₂-to-Ar flow ratio at constant pressure and HIPIMS power settings. The number of doubly charged Cr ions is found to increase linearly with increasing pulse energy. An intense flux of energetic N⁺ ions was observed during deposition in the reactive mode. The time evolution of ion flux composition is analyzed in detail and related to the film growth process. The ionization of working gas mixture is hampered during the most energetic phase of discharge by a high flux of sputter-ejected species entering the plasma, causing both gas rarefaction and quenching of the electron energy distribution function. It is suggested that the properties (composition and energy) of the ion flux incident on the substrate can be intentionally adjusted not only by varying the pulse energy (discharge peak current), but also by taking advantage of the observed time variations in the composition of ion flux.     

Studies of medium frequency high power density magnetron sputtering discharges

Optical emission spectroscopy and Langmuir probe have been used to study the power dependence of medium frequency, 100 kHz, pulsed magnetron sputtering discharge. Copper target was sputtered in the argon atmosphere. The examined power ranged from 0.5 to 4.5 kW which gave an average power density on target surface from 25 to 115 W/cm². Optical spectroscopy did not reveal any significant changes of copper ion contribution to the sputtering process. The electron temperature and plasma potential changed a little with applied power. The electron density depended linearly on the sputtering power.     

Mitigating the geometrical limitations of conventional sputtering by controlling the ion-to-neutral ratio during high power pulsed magnetron sputtering

High power pulsed magnetron sputtering has been used to grow thin chromium layers on substrates facing and orthogonal to the target. It is demonstrated that at low peak target current density, j_T < 0.6 A/cm² corresponding to a low ion-to-neutral flux ratio, films grown on substrates facing the target exhibit in-plane alignment. This is due to the rectangular shape of the target that yields an asymmetry in the off-normal flux of sputtered species. With increasing j_T the biaxial alignment degrades, as the major portion of the incoming flux (ions) can be effectively steered by the electric field of the substrate to remove asymmetry imposed by geometrical restrictions. Eventually, at j_T = 1.7 A/cm² a fiber texture is obtained. For films grown on substrates orthogonal to the target, the large column tilt characteristic for growth at low j_T, decreases with increasing ion content in the flux and almost disappears at the highest value of j_T. The latter indicates that material flux to the substrate is highly ionized so that deposition takes place along substrate normal despite the high nominal inclination angle. Thus, in the limit of high j_T the artifacts of conventional physical vapor deposition, resulting from the line-of-sight deposition, are effectively eliminated and the film growth proceeds more or less unaffected by the substrate orientation. Samples mounted orthogonally thus possess a similar texture, morphology, and topography as those facing the target.     

Deposition rate characteristics for steady state high power impulse magnetron sputtering (HIPIMS) discharges generated with a modulated pulsed power (MPP) generator

High power impulse magnetron sputtering (HIPIMS) pulses have been of great interest over the last decade. With such sputtering techniques a substantial amount of target material can be ionized and used for the engineering of surfaces and coatings. Depending on voltage, system configuration and target material, such discharges can be either transient or reach steady state currents during the pulse. The used HIPIMS power supply was a constant voltage supplies. Similarly, HIPIMS pulses with multiple steady state current phases can be generated using a modulated pulsed power (MPP) generator. A typical pulse consists of an ignition, low current and high current phase. The contribution of these phases to the deposition rate is presented. The ionization rate of single charge chromium ions has been found to increase linearly with increasing peak current density. An increase in deposition rate with lower magnetic field strength at the target surface can be attributed to a higher sputter yield due to a higher cathode voltage due to increasing system impedance in HIPIMS case, weaker trapping of deposition flux and to enhanced ion flux towards the substrate.     

Hysteresis-free reactive high power impulse magnetron sputtering

High power impulse magnetron sputtering (HIPIMS) of an Al target in Ar/O₂ mixtures has been studied. The use of HIPIMS is shown to drastically influence the process characteristics compared to conventional sputtering. Under suitable conditions, oxide formation on the target as the reactive gas flow is increased is suppressed, and the hysteresis effect commonly observed as the gas flow is varied during conventional sputtering can be reduced, or even completely eliminated, using HIPIMS. Consequently, stoichiometric alumina can be deposited under stable process conditions at high rates. Possible explanations for this behavior as well as a model qualitatively describing the process are presented.     

Hysteresis behaviour of reactive high power impulse magnetron sputtering

Hysteresis behaviour during reactive High Power Impulse Magnetron Sputtering (HIPIMS) has been investigated in detail. Such analysis has been made possible by the recently developed plasma emission monitoring based reactive HIPIMS monitoring and control technology. Hysteresis curves were recorded at frequencies of 300, 450 and 600 Hz at an average power of 3.0 kW during reactive HIPIMS of Ti in Ar/O₂ atmosphere. It is shown that the target pulsing parameters, such as frequency, pulse voltage, and duty cycle do affect the overall shape of the hysteresis loop. Analysis of the hysteresis behaviour at different target pulsing parameters reveals how different regions of the hysteresis loop are affected by different pulsing parameters. The outcomes of this work demonstrate trends and explain relationships between the pulsing parameters and the hysteresis behaviour. Although the overall picture is rather complicated, it is quite clear that the hysteresis effect is induced by the same processes as in direct current magnetron sputtering, while the influence of the reactive ion implantation oxidation mechanism appears to be far more significant in reactive HIPIMS.     

Overview of optical properties of Al2O3 films prepared by various techniques

We study optical properties of Al₂O₃ films prepared by various techniques using spectroscopic ellipsometry. The film preparation techniques include conventional pulsed magnetron sputtering in various gas mixtures, high power impulse magnetron sputtering, annealing of as-deposited Al₂O₃ in an inert atmosphere and annealing of as-deposited Al in air. We focus on the effect of the preparation technique, deposition parameters and annealing temperature on the refractive index, n, and extinction coefficient, k, of stoichiometric Al₂O₃. At a wavelength of 550 nm we find n of 1.50-1.67 for amorphous deposited Al₂O₃, 1.65-1.67 for amorphous Al₂O₃ obtained by Al annealing, 1.46-1.69 for γ-Al₂O₃ and decreasing n for Al₂O₃ annealing temperature increasing up to 890 °C. The results facilitate correct interpretation of optical characterization of Al₂O₃, as well as selection of a preparation technique corresponding to a required Al₂O₃ structure and properties.     

Morphology of TiN thin films grown on SiO2 by reactive high power impulse magnetron sputtering

Thin TiN films were grown on SiO₂ by reactive high power impulse magnetron sputtering (HiPIMS) at a range of temperatures from 45 to 600 °C. The film properties were compared to films grown by conventional dc magnetron sputtering (dcMS) at similar conditions. Structural characterization was carried out using X-ray diffraction and reflection methods. The HiPIMS process produces denser films at lower growth temperature than does dcMS. Furthermore, the surface is much smoother for films grown by the HiPIMS process. The [200] grain size increases monotonically with increased growth temperature, whereas the size of the [111] oriented grains decreases to a minimum for a growth temperature of 400 °C after which it starts to increase with growth temperature. The [200] crystallites are smaller than the [111] crystallites for all growth temperatures. The grain sizes of both orientations are smaller in HiPIMS grown films than in dcMS grown films.     

Time-resolved optical emission spectroscopy of pulsed RF plasmas with copper magnetron sputtering

Time-resolved optical emission spectroscopy is used to investigate the discharge of the RF pulse sputtering of copper. The time integrated emission of a neutral copper atom in RF pulse sputtering was found to be superior to emission in continuous mode. The increase stemmed from the concentration of the power in the sharp pulse at the breakdown of the discharge. The integrated emission reached its maximum at 50% duty ratio. A sudden drop in the copper emission in continuous mode from that of a 98% duty ratio was correlated with an absence of the power peak of the pulse mode.     

The high power impulse magnetron sputtering discharge as an ionized physical vapor deposition tool

Various magnetron sputtering tools have been developed that provide a high degree of ionization of the sputtered vapor referred to as ionized physical vapor deposition (IPVD). The ions can be controlled with respect to energy and direction as they arrive to the growth surface which allows for increased control of film properties during growth. Here, the design parameters for IPVD systems are briefly reviewed. The first sputter based IPVD systems utilized a secondary plasma source between the target and the substrate in order to generate a highly ionized sputtered vapor. High power impulse magnetron sputtering (HiPIMS) is a recent sputtering technique that utilizes IPVD where a high density plasma is created by applying high power pulses at low frequency and low duty cycle to a magnetron sputtering device. A summary of the key experimental findings for the HiPIMS discharge is given. Measurements of the temporal and spatial behavior of the plasma parameters indicate electron density peak, that expands from the target with a fixed velocity. The discharge develops from an inert sputtering gas dominated to a sputtered vapor dominated during the pulse. The high electron density results in a high degree of ionization of the deposition material.     

Hysteresis and process stability in reactive high power impulse magnetron sputtering of metal oxides

In the further development of reactive sputter deposition, strategies which allow for stabilization of the transition zone between the metallic and compound modes, elimination of the process hysteresis, and increase of the deposition rate, are of particular interest. In this study, the hysteresis behavior and the characteristics of the transition zone during reactive high power impulse magnetron sputtering (HiPIMS) of Al and Ce targets in an Ar-O₂ atmosphere as a function of the pulsing frequency and the pumping speed are investigated. Comparison with reactive direct current magnetron sputtering (DCMS) reveals that HiPIMS allows for elimination/suppression of the hysteresis and a smoother transition from the metallic to the compound sputtering mode. For the experimental conditions employed in the present study, optimum behavior with respect to the hysteresis width is obtained at frequency values between 2 and 4 kHz, while HiPIMS processes with values below or above this range resemble the DCMS behavior. Al-O films are deposited using both HiPIMS and DCMS. Analysis of the film properties shows that elimination/suppression of the hysteresis in HiPIMS facilitates the growth of stoichiometric and transparent Al₂O₃ at relatively high deposition rates over a wider range of experimental conditions as compared to DCMS.     

高功率脉冲磁控溅射制备TiNx涂层研究

采用高功率复合脉冲磁控溅射技术(HPPMS)在316不锈钢、硬质合金基体上沉积了TiN薄膜,研究不同N2流量下TiNx膜层的沉积速率、硬度、晶体生长取向、摩擦磨损等性能,并在相同的平均靶电流下与直流磁控溅射制备的TiN薄膜对比.结果表明:HPPMS制备的膜层更加致密,在氩氮流量比为7.4∶1时膜层显微硬度达2470 HV,晶粒尺寸也明显小于直流磁控溅射制备的TiN,摩擦磨损性能也得到了改善.     

Fully dense, non-faceted 111-textured high power impulse magnetron sputtering TiN films grown in the absence of substrate heating and bias

We demonstrate the deposition of fully dense, stoichiometric TiN films on amorphous SiO₂ by reactive high power impulse magnetron sputtering (HiPIMS) in the absence of both substrate heating and applied bias. Contrary to the highly underdense layers obtained by reactive dc magnetron sputtering (dcMS) under similar conditions, the film nanostructure exhibits neither intra- nor intergrain porosity, exhibiting a strong 111 preferred orientation with flat surfaces. Competitive grain growth occurs only during the early stages of deposition (< 100 nm). The strong differences in the kinetically-limited nanostructural evolution for HiPIMS vs. dcMS are explained by high real-time deposition rates with long relaxation times, high ionization probabilities for Ti, and broad ion energy distributions.     

Rotatable magnetron sputtering of aluminium in continuous and high power pulse modes using different strength magnetic arrays

This paper reports preliminary results of industrial size (152 mm target O.D.) rotatable magnetron sputtering of Al target in direct current (DC) and High Power Impulse Magnetron Sputtering (HIPIMS) modes using two standard commercially available magnetic arrays: standard strength array (as used for DC and AC processing) and a lower strength ‘RF’ array [i.e. as used for radio frequency (RF) magnetron sputtering]. A comparison of processes resulted in by combining the different magnetic arrays and power modes is made in terms of magnetic field distribution on the cathode surface, magnetron characteristics, process characteristics and deposition rates.Optical emission spectroscopy (OES) revealed enhanced sputtered Al flux ionisation in the HIPIMS discharge monitored 64 mm away from the target surface when using the ‘RF’ array. Importantly, the results of this work (at the processing conditions investigated) demonstrate that at the same average power the deposition rate of Al using HIPIMS in conjunction with the ‘RF’ array is substantially the same as that obtained for the ‘standard’ strength balanced array and DC power. This indicates that the magnetic field design of the ‘RF’ magnetic array affects favourably the sputtered flux transport perpendicular to the target surface by altering mass transport direction and minimising effects that reduce deposition rate (e.g. ion return effect). Arc rate is also reduced significantly (approximately ten times) if the low strength ‘RF’ array is used.     

Optical emission spectroscopy study for nitrogen-acetylene-argon and nitrogen-acetylene-helium 100 kHz and dc discharges

The optical emission spectroscopy was applied to investigate the middle frequency (100 kHz) and dc low pressure discharges, generated in the nitrogen-acetylene-argon and nitrogen-acetylene-helium mixtures, commonly used for deposits of carbon nitride thin layers. Changes in the emission intensities of the selected species: CN, CH, C, H, N₂, N₂⁺ as well as Ar, Ar⁺ and He, were studied as a function of the discharge current. Excitation processes occurring in the presence of argon and helium were compared and discussed. The N₂-C₂H₂-Ar and N₂-C₂H₂-He plasmas generated in the 100 kHz and dc glow discharges were characterized by the excitation (Ar, He, H), vibrational (CN, N₂) and rotational (CN, N₂⁺) temperatures. A significant deviation from the equilibrium state was observed for the plasma containing argon as well as helium.     

Structural and optical properties of yttrium trioxide thin films prepared by RF magnetron sputtering

Yttrium trioxide (Y₂O₃) thin films have been deposited on silicon (111) at different RF powers and the sputtering pressures by RF magnetron sputtering. The influences of the RF power and the sputtering pressures on the structural and optical properties of Y₂O₃ thin films were investigated by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), atomic force microscope (AFM) and spectroscopic ellipsometer (SE). The results show that chemical composition of as-deposited Y₂O₃ film is apparently close to the stoichiometric ratio and it is crystallized but crystallinity is poor. The monoclinic and cubic fluorite-like structure can coexist in as-deposited Y₂O₃ film. A four-layer-structured optical model consisting of silicon substrate, silicon dioxide (SiO₂) interlayer, Y₂O₃ layer and a surface roughness (SR) layer is built for interpreting preferably the results measured by spectroscopic ellipsometry. With the increase of RF power or decrease of sputtering pressure, the refractive index and optical bandgap of sputtered Y₂O₃ film is increased and the extinction coefficients is decreased.     

Influence of additional magnetic field on plasma parameters in magnetron sputtering

With three additional magnetic rings being assembled outside the discharge room and connected with the magnetic field of the conventional unbalanced magnetron sputtering, a closed magnetic field configuration distribution had been formed in the whole discharge room and which can confine discharge plasma more effectively. The spatial distribution of the newly designed magnetic field configuration was simulated using the ANSYS software. Plasma potential, electron temperature, electron density and ion density in the discharge plasma were diagnosed by Langmuir probe and the optical emission line intensity ratios of Ar⁺/Ar and Cu⁺/Cu were studied by optical emission spectroscopy. The structure and morphology of the Cu films are measured by scanning electron microscopy. A comparative study of the new magnetic field configuration with the conventional unbalanced magnetic field configuration was conducted. The results showed that the application of the additional magnetic field can increase the plasma density, enhance the ionization degree of the sputtered Cu and decrease the plasma potential effectively. The characteristics of the deposited Cu film were also influenced by the new magnetic field configuration greatly.     

Control of the optical properties of silicon and chromium mixed oxides deposited by reactive magnetron sputtering

The development of mixed-oxide thin films allows obtaining materials with better properties than those of the different binary oxides, which makes them suitable for a great number of applications in different fields, such as tribology, optics or microelectronics. In this paper we investigate the deposition of mixed chromium and silicon oxides deposited by reactive magnetron sputtering with a view to use them as optical coatings with an adjustable refractive index. These films have been characterized by means of Rutherford backscattering spectrometry, Auger electron spectroscopy, X-ray diffraction, scanning electron microscopy, Fourier-transform infrared spectroscopy and spectroscopic ellipsometry so as to determine how the deposition conditions influence the characteristics of the material. We have found that the deposition parameter whose influence determines the properties of the films to a greater extent is the amount of oxygen in the reactive sputtering gas.     

Ultrafast pump-probe spectroscopy studies of CeO2 thin film deposited on Ni-W substrate by RF magnetron sputtering

This study presents the first investigation of rapid dynamical processes that occur in pure CeO₂ thin film, using ultra fast pump-probe spectroscopy at room temperature. For this purpose we have used a single (200) oriented CeO₂ film deposited on biaxially textured Ni-W substrate by RF magnetron sputtering technique. The ultrafast transient spectra show initial sharp rise transition followed by an exponential photon decay. This rise time is about 10 ps irrespective of the probe wavelengths range 500–800 nm. The initial decay constant (τ) at 500 nm probe wavelength is found to be 171 ps, while at 800 nm probe wavelength it is 107.5 ps. The ultrafast absorption spectra show two absorption peaks at 745 and 800 nm, and are attributed to the electronic transitions from ²F_7/2–²F_5/2 and ¹S₀–¹F₃ respectively. The relatively high intensity absorption peak at 745 nm indicates dominant f–f electronic transition. Further, the absorption peak at 745 nm splits into two distinct peaks with respect to delay time, and is attributed to the charge transfer in between Ce⁴⁺ and Ce³⁺ ions. These results indicate that CeO₂ itself is a potential candidate and can be used for optical applications.     

In situ optical spectroscopy during deposition of Ag:Si3N4 nanocomposite films by magnetron sputtering

In situ and real time surface differential reflectance (SDR) spectroscopy is employed to study the growth of metallic Ag and/or dielectric Si₃N₄ films during deposition by magnetron sputtering. The measurements during Si₃N₄ sputtering allow determining both the refractive index and the deposition rate. During Ag sputtering, the SDR presents a maximum in the visible range, typical of a surface plasmon resonance (SPR) indicating the 3D growth of silver nanoclusters. After a certain deposition thickness, the SDR change corresponds to a continuous layer growth and allows determining the Ag deposition rate. During Ag/Si₃N₄ alternate deposition, the SDR spectroscopy enables to follow the SPR modifications (position, amplitude and width) not only during the formation of the Ag nanoclusters but also during their capping by a Si₃N₄ matrix and during intermediate steps (holding time after the silver sputtering, Si₃N₄ target ignition and pre-sputtering before the Si₃N₄ deposition) where significant changes are detected. It suggests possible nanocluster reshaping or physicochemical processes occurring at the nanocluster interface during the different steps.