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Comparison Between DC and HiPIMS Discharges. Application to Nickel Thin Films: Proceedings of the International Conference, ICERA 2018
Abstract
The study deals with a comparison between Direct Current (DC) and High Power Impulse Magetron Sputtering (HiPIMS) processes. We have first highlighted that the plasma of the DC discharge is composed mainly of gaseous species whereas the HiPIMS discharge leads to a plasma dominated by metal vapor and characterized by the presence of charged species of strong and low energy. For thin nickel (Ni) films, we have found the the use of HiPIMS produce denser and better crystallized layers improving the uniformity of the coating on substrates with complex geometries.
... This step also decreases the amount of surface defects by ionic peening. During the second step, a dense nickel layer [16] is deposited to minimize the oxidation of iron by preventing the diffusion of corrosive species. The last nickel oxide layer is an electronic insulating barrier [17]. ...
La détérioration des propriétés mécaniques de tôles de nickel ne présentant que quelques grains dans l'épaisseur (état multicristallin) est liée au retardement de l'activation des mécanismes de déformation dans les grains surfaciques tronqués. Des dépôts physiques en phase vapeur, par pulvérisation cathodique magnétron, d'un matériau sur un substrat de composition chimique identique (i.e. dépôt de nickel sur une tôle de nickel) ont été réalisés afin de générer une barrière adaptée à la fuite des dislocations au niveau des surfaces libres, responsable du fort adoucissement mécanique. La restructuration de la zone surfacique des tôles de nickel par une évolution graduelle des caractéristiques microstructurales du dépôt, agissant sur les mécanismes de déformation plastique et le mouvement des dislocations, a ainsi engendré une amélioration des performances mécaniques de ces multicristaux.
... This step also decreases the amount of surface defects by ionic peening. During the second step, a dense nickel layer [16] is deposited to minimize the oxidation of iron by preventing the diffusion of corrosive species. The last nickel oxide layer is an electronic insulating barrier [17]. ...
This paper develops a simple and versatile analytical method for characterizing residual stress associated with the deposition of thick, thin, single and multilayer coatings. This study is of great importance, since the generation of residual stress can lead to the formation of significant defects in the coating. To illustrate the effectiveness of our analytical method, two different processes were characterized. The coatings formed are intended for anti-corrosion applications. The first is a Physical Vapour Deposition (PVD) involving a High Power Impulsed Magnetron Sputtering (HiPIMS), the second is a sol-gel process. The approach is based on the optical recording of the curvature of a thin substrate, followed by its assessment using a fit of the digitalized image by a parametric model. The results provide experimental data on the development of stress during deposition. Both coatings studied have a low level of residual stress, which explains why they are flawless and which is interesting for the targeted anti-corrosion application. For the PVD coating, it was possible to link the development of the stress to the evolution of the film structure during growth. For the sol-gel process, the study of the stress allowed us to highlight the drying reaction mechanisms involved during the sol-gel transition and their influence on the gel structure.
Background: Oblique angle deposition is known for yielding the growth of columnar grains that are tilted in the direction of the deposition flux. Using this technique combined with high-power impulse magnetron sputtering (HiPIMS) can induce unique properties in ferromagnetic thin films. Earlier we have explored the properties of polycrystalline and epitaxially deposited permalloy thin films deposited under 35° tilt using HiPIMS and compared it with films deposited by dc magnetron sputtering (dcMS). The films prepared by HiPIMS present lower anisotropy and coercivity fields than films deposited with dcMS. For the epitaxial films dcMS deposition gives biaxial anisotropy while HiPIMS deposition gives a well-defined uniaxial anisotropy.
Results: We report on the deposition of 50 nm polycrystalline nickel thin films by dcMS and HiPIMS while the tilt angle with respect to the substrate normal is varied from 0° to 70°. The HiPIMS-deposited films are always denser, with a smoother surface and are magnetically softer than the dcMS-deposited films under the same deposition conditions. The obliquely deposited HiPIMS films are significantly more uniform in terms of thickness. Cross-sectional SEM images reveal that the dcMS-deposited film under 70° tilt angle consists of well-defined inclined nanocolumnar grains while grains of HiPIMS-deposited films are smaller and less tilted. Both deposition methods result in in-plane isotropic magnetic behavior at small tilt angles while larger tilt angles result in uniaxial magnetic anisotropy. The transition tilt angle varies with deposition method and is measured around 35° for dcMS and 60° for HiPIMS.
Conclusion: Due to the high discharge current and high ionized flux fraction, the HiPIMS process can suppress the inclined columnar growth induced by oblique angle deposition. Thus, the ferromagnetic thin films obliquely deposited by HiPIMS deposition exhibit different magnetic properties than dcMS-deposited films. The results demonstrate the potential of the HiPIMS process to tailor the material properties for some important technological applications in addition to the ability to fill high aspect ratio trenches and coating on cutting tools with complex geometries.
The high power impulse magnetron sputtering (HiPIMS) discharge is a recent addition to plasma based sputtering technology. In HiPIMS, high power is applied to the magnetron target in unipolar pulses at low duty cycle and low repetition frequency while keeping the average power about 2 orders of magnitude lower than the peak power. This results in a high plasma density, and high ionization fraction of the sputtered vapor, which allows better control of the film growth by controlling the energy and direction of the deposition species. This is a significant advantage over conventional dc magnetron sputtering where the sputtered vapor consists mainly of neutral species. The HiPIMS discharge is now an established ionized physical vapor deposition technique, which is easily scalable and has been successfully introduced into various industrial applications. The authors give an overview of the development of the HiPIMS discharge, and the underlying mechanisms that dictate the discharge properties. First, an introduction to the magnetron sputteringdischarge and its various configurations and modifications is given. Then the development and properties of the high power pulsed power supply are discussed, followed by an overview of the measured plasma parameters in the HiPIMS discharge, the electron energy and density, the ion energy, ion flux and plasma composition, and a discussion on the deposition rate. Finally, some of the models that have been developed to gain understanding of the discharge processes are reviewed, including the phenomenological material pathway model, and the ionization region model.
High power pulsed magnetron sputtering (HiPIMS) plasmas generate energetic
metal ions at the substrate as a major difference to conventional direct
current magnetron sputtering. The origin of these energetic ions in HiPIMS is
still an open issue, which is unraveled by using three fast diagnostics: time
resolved mass spectrometry with a temporal resolution of 2 $\mu$s, phase
resolved optical emission spectroscopy with 1 $\mu$s and the rotating shutter
experiment with a resolution of 50 $\mu$s. A power scan from dcMS-like to
HiPIMS plasmas was performed, with a 2-inch magnetron and a titanium target as
sputter source and argon as working gas. Clear differences in the transport as
well in the energetic properties of Ar$^+$, Ar$^{2+}$, Ti$^+$ and Ti$^{2+}$
were observed. For discharges with highest peak power densities a high
energetic group of Ti$^{+}$ and Ti$^{2+}$ could be identified. A cold group of
ions is always present. It is found that hot ions are observed only, when the
plasma enters the spokes regime, which can be monitored by oscillations in the
IV-characteristics in the MHz range. These oscillations are correlated with the
spokes phenomenon and are explained as an amplification of the Hall current. To
explain the presence of energetic ions, we propose a double layer (DL)
confining the hot plasma inside a spoke: if an atom becomes ionized inside the
spokes region it is accelerated because of the DL to higher energies whereas
its energy remains unchanged if it is ionized outside. In applying this DL
model to our measurements the observed phenomena as well as several
measurements from other groups can be explained. Only if spokes and a double
layer are present the confined particles can gain enough energy to leave the
magnetic trap. We conclude that the spoke phenomenon represents the essence of
HiPIMS plasmas, explaining their good performance for material synthesis
applications.
Ion energy distribution functions measured for high power impulse magnetron sputtering show features, such as a broad peak at several 10 eV with an extended tail, as well as asymmetry with respect to E×B, where E and B are the local electric and magnetic field vectors, respectively. Here it is proposed that those features are due to the formation of a potential hump of several 10 V in each of the traveling ionization zones. Potential hump formation is associated with a negative-positive-negative space charge that naturally forms in ionization zones driven by energetic drifting electrons.
The properties of giant magnetoresistance multilayers are a sensitive function of the vapor deposition process used for their synthesis. The highest magnetoresistance occurs when deposition results in interfaces that are flat and chemically separated. Molecular dynamics simulations have been used to explore the potential benefits of low energy xenon ion assistance during the physical vapor deposition of Ni/Cu/Ni multilayers grown in the [111] direction from thermalized metal fluxes characteristic of molecular beam epitaxy. The simulations indicated that the roughness of the interfaces was significantly reduced as the ion energy was increased from 0 to 5 eV. However, increasing the ion energy above 2 eV also resulted in significant copper–nickel intermixing at the nickel on copper interface. Interface flattening without intermixing could be achieved using a modulated low energy ion assistance strategy in which the first half of each new material layer was deposited without ion assistance, while the remainder of the layer was deposited with an optimum low ion energy assistance of 4 eV. Modulated low energy ion assistance during thermalized metal atom deposition was found to be a promising approach for creating metal multilayers with improved magnetoresistance. © 2000 American Institute of Physics.
High power impulse magnetron sputtering (HIPIMS) is a novel deposition technology successfully implemented on full scale industrial machines. HIPIMS utilizes short pulses of high power delivered to the target in order to generate high amount of metal ions. The life-span of ions between the pulses and their energy distribution could strongly influence the properties and characteristics of the deposited coating. In modern industrial coating machines the sample rotates on a substrate holder and changes its position and distance with regard to the magnetron. Time resolved measurements of the ion energy distribution function (IEDF) at different distances from the magnetron have been performed to investigate the temporal evolution of ions at various distances from target. The measurements were performed using two pressures, 1 and 3 Pa to investigate the influence of working gas pressure on IEDF. Plasma sampling energy-resolved mass spectroscopy was used to measure the IEDF of Ti1+, Ti2+, Ar1+, and Ar2+ ions in HIPIMS plasma discharge with titanium (Ti) target in Ar atmosphere. The measurements were done over a full pulse period and the distance between the magnetron and the orifice of the mass spectrometer was changed from 25 to 215 mm.
In this study, the effect on thin film growth due to an anomalous electron transport, found in high power impulse magnetron sputtering (HiPIMS), has been investigated for the case of a planar circular magnetron. An important consequence of this type of transport is that it affects the way ions are being transported in the plasma. It was found that a significant fraction of ions are transported radially outwards in the vicinity of the cathode, across the magnetic field lines, leading to increased deposition rates directly at the side of the cathode (perpendicular to the target surface). Furthermore, this mass transport parallel to the target surface leads to that the fraction of sputtered material reaching a substrate placed directly in front of the target is substantially lower in HiPIMS compared with conventional direct current magnetron sputtering (dcMS). This would help to explain the lower deposition rates generally observed for HiPIMS compared with dcMS. Moreover, time-averaged mass spectrometry measurements of the energy distribution of the cross-field transported ions were carried out. The measured distributions show a direction-dependent high-energy tail, in agreement with predictions of the anomalous transport mechanism.
Oscillating electric fields in the megahertz range have been studied in a high power impulse magnetron sputtering (HIPIMS) plasma with the use of electric field probe arrays. One possible reason for these oscillations to occur is charge perturbation—or so-called modified two-stream instabilities (MTSIs). It is known that MTSIs give rise to acceleration of the charged plasma species and can give a net transport of electrons across the magnetic field lines. Measurements of these oscillations confirm trends, specifically of the frequency dependence on ion mass and magnetic field strength as expected from the theory of MTSI waves. These results help to explain the previously reported anomalous fast electron transport in HIPIMS discharges, where classical theory of diffusion using collisions to transport electrons has failed.
An excellent adhesion of hard coatings to steel substrates is paramount in practically all application areas. Conventional methods utilize Ar glow etching or cathodic arc discharge pretreatments that have the disadvantage of producing weak interfaces or adding droplets, respectively. One tool for interface engineering is high power impulse magnetron sputtering (HIPIMS). HIPIMS is based on conventional sputtering with extremely high peak power densities reaching 3 kW cm − 2 at current densities of > 2 A cm − 2 . HIPIMS of Cr and Nb was used to prepare interfaces on 304 stainless steel and M2 high speed steel (HSS). During the pretreatment, the substrates were biased to U bias = − 600 V and U bias = − 1000 V in the environment of a HIPIMS of Cr and Nb plasma. The bombarding flux density reached peak values of 300 mA cm − 2 and consisted of highly ionized metal plasma containing a high proportion of Cr 1 + and Nb 1 + . Pretreatments were also carried out with Ar glow discharge and filtered cathodic arc as comparison. The adhesion was evaluated for coatings consisting of a 0.3 μ m thick CrN base layer and a 4 μ m thick nanolayer stack of Cr N ∕ Nb N with a period of 3.4 nm , hardness of HK 0.025 = 3100 , and residual stress of − 1.8 GPa . For HIPIMS of Cr pretreatment, the adhesion values on M2 HSS reached scratch test critical load values of L C = 70 N , thus comparing well to L C = 51 N for interfaces pretreated by arc discharge plasmas and to L C = 25 N for Ar etching. Cross sectional transmission electron microscopy studies revealed a clean interface and large areas of epitaxialgrowth in the case of HIPIMS pretreatment. The HIPIMS pretreatment promoted strong registry between the orientation of the coating and polycrystalline substrate grains due to the incorporation of metal ions and the preservation of crystallinity of the substrate. Evidence and conditions for the formation of cube-on-cube epitaxy and axiotaxy on steel and γ - Ti Al substrates are presented.
The effect of the high pulse current and the duty cycle on the deposition rate in high power pulsed magnetron sputtering (HPPMS) is investigated. Using a Cr target and the same average target current, deposition rates are compared to dc magnetron sputtering (dcMS) rates. It is found that for a peak target current density I<sub>T<sub> pd </sub></sub> of up to 570 mA cm <sup>-2</sup> , HPPMS and dcMS deposition rates are equal. For I<sub>T<sub>pd</sub></sub>≫570 mA cm <sup>-2</sup> , optical emission spectroscopy shows a pronounced increase of the Cr <sup>+</sup>/ Cr <sup>0</sup> signal ratio. In addition, a loss of deposition rate, which is attributed to self-sputtering, is observed.
In plasma-based deposition processing, the importance of low-energy ion bombardment during thin film growth can hardly be exaggerated. Ion bombardment is an important physical tool available to materials scientists in the design of new materials and new structures. Glow discharges and in particular, the magnetron sputtering discharge have the advantage that the ions of the discharge are abundantly available to the deposition process. However, the ion chemistry is usually dominated by the ions of the inert sputtering gas while ions of the sputtered material are rare. Over the last few years, various ionized sputtering techniques have appeared that can achieve a high degree of ionization of the sputtered atoms, often up to 50% but in some cases as much as approximately 90%. This opens a complete new perspective in the engineering and design of new thin film materials. The development and application of magnetron sputtering systems for ionized physical vapor deposition (IPVD) is reviewed. The application of a secondary discharge, inductively coupled plasma magnetron sputtering (ICP-MS) and microwave amplified magnetron sputtering, is discussed as well as the high power impulse magnetron sputtering (HIPIMS), the self-sustained sputtering (SSS) magnetron, and the hollow cathode magnetron (HCM) sputtering discharges. Furthermore, filtered arc-deposition is discussed due to its importance as an IPVD technique. Examples of the importance of the IPVD-techniques for growth of thin films with improved adhesion, improved microstructures, improved coverage of complex shaped substrates, and increased reactivity with higher deposition rate in reactive processes are reviewed.
This study investigates the understanding of the impact on the pulse width duration (30 and 100 μs) regarding the plasma dynamic of an HiPIMS discharge by investigating time-resolved and time-integrated ion energy distribution functions. In order to establish an objective comparison, the cathode voltage and the mean power were kept constant at −600 V and 7 W.cm⁻², respectively. Time average measurements revealed that for a pulse as long as 100 μs, the ion flux at the substrate is mostly composed of argon ions (18% higher). Instead, for a lower pulse width of 30 μs, the ions flux at the substrate seems to be dominated by chromium ions (up to 7%). Furthermore, the huge difference between these discharges resides in the lowest energy part of the IEDFs of singly charged chromium ions where the intensity is actually more important in short HiPIMS pulse. Our results reveal that long HiPiMS pulses are characterized by a longer rarefaction effect close to the chromium target vicinity as compared to shorter pulses. In order to explain these phenomena, time-resolved measurements were carried out and highlighted the fact that low energy ions cannot escape the cathode environment due to the applied high voltage acting as a low-energy ions trap. Current characteristics can be correlated to time-resolved ions fluxes measurements and support this explanation. These findings may help the community to find out that working with shorter pulses (tON ) can have an important benefit to improve the material properties of most common thin films, but also to enhance the adhesion of coatings on substrates by a metallic pre-treatment.
The physical and chemical aspects of plasma–surface interaction in high-power impulse magnetron sputtering (HiPIMS) discharges are overviewed. The data obtained by various plasma diagnostic methods representing the important sputtering discharge regions, namely the cathode vicinity, plasma bulk, and substrate vicinity, are reported. After a detailed introduction to the problem and description of the plasma characterization methods suitable for pulsed magnetron discharge analysis, an overview of the recent plasma diagnostics achievements in both non-reactive and reactive HiPIMS discharges is presented. Finally, the conclusions and perspectives suggesting possible directions and research strategies for increasing our knowledge in this domain are given.
In this study, a systematic investigation on the deposition of Cr–CrOx bi-layer film was performed by magnetron DC sputtering. The X-ray photoelectron spectrometer (XPS) examining the bare Cr film showed that the peaks of Cr 2P3/2 and Cr 2P1/2 appeared in the Cr thin film associated with the presence of a 12 nm oxide layer. The transmission was reduced to zero as the Cr film exceeded 100 nm in thickness. The reflection saturated at a value of ≈55% when the thickness of the Cr film reached 30 nm. The optical density exceeded 3.50 with a Cr film thickness over 150 nm. In order to reduce the reflection of the film to a level of ≤4%, a Cr–CrOx bi-layer thin film was prepared. Overall, a Cr–CrOx bi-layer film with the Cr layer 130 nm and the CrOx layer 40 nm in thickness reported a transmission of zero, a reflection of 3.82% and an optical density of 4.04, all meeting the requirements of anti-reflection black matrix (BM) for display applications.
Time-resolved mass spectrometry (MS) study of a high-power impulse magnetron sputtering discharge (HiPIMS) operating in the 'short-pulse' regime (5 µs) at 1 kHz of the repetition frequency is undertaken. Several time-resolved effects related to both Ti+ and Ar+ ion energy distribution functions (IEDF) are found. In particular, the dynamics of both the low- (0–5 eV) and high- (5–30 eV) energy regions presented in Ti+ IEDFs is clarified. According to our results the sputtered and ionized Ti arrive at the virtual substrate position in the form of two waves, with the first one representing the high-energy Ti+, and the second one responsible for the low-energy Ti+. An essential decrease in the population of the energetic Ti+ group is observed at the moment of the arrival of the low-energy group, which is explained by the charge exchange processes, as well as by the refilling process afterwards. The role of Ar metastables presumably generated at the end of the plasma pulse for further Ti ionization is stressed as well. The time-averaged IEDFs for Ti+ and Ar+ are additionally analysed. The effective ion temperatures are calculated for these species for the above-mentioned energy ranges. A considerable increase in the effective ion temperature for the high-energy Ti+ is found, which is directly related to the elevation of the high-energy tail in the time-averaged IEDFs with increasing discharge voltage. Possible mechanisms for such an elevation are discussed.
The energy distribution of ions in the plasma during unbalanced magnetron sputtering was measured with an energy-resolved mass spectrometer. Typical energy distributions of the main ion species were measured for sputtering of a Cu target in an Ar atmosphere in the pressure range from about 0.1 to 10 Pa. The effects of the discharge pressure and of the distance between target and mass spectrometer on the ion energy distribution were studied. A low energy (thermalised) peak in the ion energy distribution function was observed at an energy corresponding to the plasma potential. In addition, a high energy tail was observed at energies of about 3–30 eV above the low energy peak, not only for sputtered ionised atoms but also for the Ar ions. The ion energy distribution of this high energy tail can be fitted with an exponential function; the mean energy was about 1–5 eV. The integrated intensity of the high energy tail was a function of the pressure-distance product p × d, where d is the distance of the mass spectrometer from the magnetron. The function is close to exponential decrease with increasing p × d product for the high energy tail of the Cu+ energy distribution. A similar exponential decrease was observed also for the high energy tail of the Ar+ energy distribution with the exception of lowest pressures (
High-power pulsed magnetron discharges have drawn an increasing interest as an approach to produce highly ionized metallic vapor. In this paper we propose to study how the plasma composition and the deposition rate are influenced by the pulse duration. The plasma is studied by time-resolved optical emission and absorption spectroscopies and the deposition rate is controlled thanks to a quartz microbalance. The pulse length is varied between 2.5 and 20 μs at 2 and 10 mTorr in pure argon. The sputtered material is titanium. For a constant discharge power, the deposition rate increases as the pulse length decreases. With 5 μs pulse, for an average power of 300 W, the deposition rate is ∼ 70% of the deposition rate obtained in direct current magnetron sputtering at the same power. The increase of deposition rate can be related to the sputtering regime. For long pulses, self-sputtering seems to occur as demonstrated by time-resolved optical emission diagnostic of the discharge. In contrary, the metallic vapor ionization rate, as determined by absorption measurements, diminishes as the pulses are shortened. Nevertheless, the ionization rate is in the range of 50% for 5 μs pulses while it lies below 10% in the case of a classical continuous magnetron discharge.
Using a cylindrical Langmuir probe, time-resolved measurements of plasma parameters near the substrate were carried out in a high power impulse magnetron sputtering (HIPIMS) discharge. Two different target materials (Ti and Cr) were used and a magnetron was operated at a pressure of 0.28 and 2.66 Pa, frequency of 100 Hz, pulse duration of 70 µs and a duty cycle of 0.7%.
The results show that a high density plasma (n ~ (0.1–0.8) × 10¹⁸ m⁻³) is generated near the substrate in the studied pressure range. A strong dependence of the plasma density on the target material is observed at the same value of the discharge current. This phenomenon is thought to be due to the effect of the sputtering yield of the target material on the ionization and transport processes in the discharge. The plasma dynamic is studied through the temporal evolution of the electron energy distribution function.
The energy distribution and composition of the ion flux on a substrate during reactive magnetron sputtering of TiN and TiWN films were studied by the energy resolved mass spectroscopy. The entrance flange of the probe Hiden EQP500 was positioned in a distance of 50 mm from the Ti or WTi (70:30 at.%) target 100 mm in diameter. The sputtering was carried out in a mixture of argon and nitrogen of various compositions at pressures from 0.05 to 10 Pa and discharge currents from 0.5 to 7 A. The energy spectra of ions at low pressures were characterized by extended high-energy tails. The high energy of sputtered (metal) atoms follows from their distribution at the cathode after being sputtered. The high-energy gas ions (Ar+, N2+, N+) stem from two sources. One is the transfer of energy in the collisions with the sputtered metal atoms. The other is the reflection of the energetic ions from heavy elements in the target. A strong reduction of the ion energy at the substrate was found when the pressure was increased from 0.5 to 10 Pa. As a consequence of a loss of energy in many collisions the high-energy portion of the ion energy spectra diminished and the energy spectra of various kinds of ions became similar. Nevertheless, the reflected ions were still apparent, albeit at lower intensity. The TRIM Monte-Carlo simulation showed that the flux of the fast reflected ions and flux of sputtered atoms are of the same order of magnitude, indicating thus the important role of the former species in forming the film properties at low pressures. The analysis of the composition of the ion flux during sputtering in a mixture of nitrogen and argon revealed that the ratio of ion fluxes TiN+/Ti+ reached maximum of approximately 0.13, while WN+/W+ was up to 0.3.
The influence on thin film density using high power impulse magnetron sputtering (HiPIMS) has been investigated for eight different target materials (Al, Ti, Cr, Cu, Zr, Ag, Ta, and Pt). The density values as well as deposition rates have been compared to results obtained from thin films grown by direct current magnetron sputtering (DCMS) under the same experimental conditions. Overall, it was found that the HiPIMS deposited coatings were approximately 5–15% denser compared to the DCMS deposited coatings. This could be attributed to the increased metal ion bombardment commonly seen in HiPIMS discharges, which also was verified using a global plasma model to assess the degree of ionization of sputtered metal. One key feature is that the momentum transfer between the growing film and the incoming metal ions is very efficient due to the equal mass of film and bombarding species, leading to a less pronounced columnar microstructure. As expected the deposition rates were found to be lower for HiPIMS compared to DCMS. For several materials this decrease is not as pronounced as previously reported in the literature, which is shown in the case of Ta, Pt, and Ag with rateHiPIMS/rateDCMS ~ 70–85%, while still achieving denser coatings.
High power pulsed magnetron sputtering (HPPMS) is an emerging technology that has gained substantial interest among academics and industrials alike. HPPMS, also known as HIPIMS (high power impulse magnetron sputtering), is a physical vapor deposition technique in which the power is applied to the target in pulses of low duty cycle (< 10%) and frequency (< 10 kHz) leading to pulse target power densities of several kW cm− 2. This mode of operation results in generation of ultra-dense plasmas with unique properties, such as a high degree of ionization of the sputtered atoms and an off-normal transport of ionized species, with respect to the target. These features make possible the deposition of dense and smooth coatings on complex-shaped substrates, and provide new and added parameters to control the deposition process, tailor the properties and optimize the performance of elemental and compound films.
Residual stresses in films produced by physical vapour deposition (PVD) techniques result from the contribution of thermal, intrinsic and extrinsic stresses. Tensile intrinsic stresses are usually observed in not fully dense films deposited by thermal evaporation from non-energetic particles. Compressive intrinsic stresses develop in relatively dense films deposited at low temperatures under energetic particle bombardment. Various models proposed in the literature to explain the origin of intrinsic stresses in PVD films are presented. In addition to thermal and intrinsic stresses, extrinsic stresses may be found in porous films exposed to room air or polar molecule vapour. Stress data for films deposited by magnetron sputtering and thermal evaporation are presented and discussed.
Low-energy ion-assisted magnetron sputter deposition has been used for the synthesis of highly reflective Ni/V multilayer soft X-ray mirrors. A low ion energy and a high ion-to-metal flux ratio were employed in order to stimulate the adatom mobility while minimizing ion-induced intermixing at the interfaces. An analytic model, based on the binary collision approximation, was used in order to gain insight into low-energy ion–surface interactions as a function of ion energy and ion-to-metal flux ratio. The model predicted a favorable region in the ion energy-flux parameter space where only surface atomic displacements are stimulated during growth of Ni and V for multilayers. For a series of Ni/V multilayer mirrors with multilayer periods about Λ = 1.2 nm, grown with a continuous ion assistance using energies in the range 7–36 eV and with ion-to-metal flux ratios ΦNi = 4.7 and ΦV=20.9, specular and diffuse X-ray scattering analyses revealed that ion energies of ∼27–31 eV produced the best trade-off between reduced interfacial roughness and intermixing. However, it was also concluded that an interface mixing of about ± 1 atomic distance is unavoidable when a continuous flux of assisting ions is used.To overcome this limitation, a sophisticated interface engineering technique was employed, where the first 0.3 nm of each layer was grown with a high-flux low-energy ion assistance and the remaining part was grown with a slightly higher ion energy. This method was demonstrated to largely eliminate the intermixing while maintaining the smoothening effect of ion assistance. Two Ni/V multilayer soft X-ray mirror structures, one with 500 periods designed for near-normal incidence and one 150 periods reflecting polarizer at the Brewster angle, were grown utilizing the interface engineering concept. Both the near-normal incidence reflectivity as well as polarizability were improved by a factor of 2 as compared to previously reported data for an X-ray energy of E = 511 eV.
A model describing film growth from hyperthermal (~1-103 eV) species impinging on substrates is presented. The model involves a shallow subsurface implantation process called ``subplantation,'' energy loss, preferential displacement of atoms with low displacement energy Ed, leaving the high-Ed atoms intact, sputtering of substrate material, and inclusion of a new phase due to incorporation of a high density of interstitials in a host matrix. Epitaxial or preferred orientation may result from the angular dependence of the Ed and the boundary conditions imposed by the host matrix, i.e., the ``mold'' effect. The discussion focuses on deposition of carbon diamondlike films, but examples of other systems, such as Si, Ge, and Ag, are provided as well. The model is supported by classical-ion-trajectory calculations and experimental data. The calculations probe the role of ion range, local concentration, backscattering coefficient, sputtering yield, and ion-induced damage in film evolution. The experimental data emphasize in situ surface-analysis studies of film evolution. The physical parameters of the deposition process that are treated are as follows: (i) nature of bombarding species (C+ versus C-, C- versus C-2, CnH+m, Ar+, and H+), (ii) ion energy, (iii) type of substrate, and (iv) substrate temperature during deposition.
A model for film growth from hyperthermal (≊1–103 eV) species impinging on substrates is proposed. The process includes subsurface implantation, energy loss, preferential displacement of atoms with low displacement energies (Ed) leaving high Ed atoms undisplaced, and sputtering of substrate material. Epitaxial growth and preferred orientation result from the angular dependence of the Ed and the host mold effect. The model, supported by ion trajectory calculations and experimental data, is applied to diamond film formation from C+ ions.