Substrate Bias Research Articles

Effect of Substrate Bias Voltage on Microstructure and Mechanical Properties of Cr-Nb-Ti-Zr-N-O Ceramic Thin Films Produced by Reactive Sputtering

Hard coatings are applied in various applications to protect substrates from wear and corrosion. In the present study, multi-element ceramic films are deposited by reactive sputtering. The level of substrate bias voltage (−50, −125 and −200 V) is changed to investigate the structural and mechanical properties of Cr-Nb-Ti-Zr-N thin films. Chemical analysis (using EDS, XRD and Raman spectroscopy) reveals that these thin films (with a high amount of oxygen) are composed of a nanocomposite phase structure (amorphous and nano-crystalline phases). CrO2 and NbxN crystalline phases exist in an amorphous matrix in the coatings. By increasing the substrate voltage (from −50 to −200 V), the nitrogen content (from 30 to 40 at. %) increases, and CrxN crystalline phases are generated in S125 and S200. Morphological, topological and image analysis (employing FESEM and AFM) data show that the intermediate level of substrate bias voltage (sample S125) can produce a uniform surface with minimum defect density (15%). In addition, S125 has the minimum level of roughness (16.6 nm), skewness (0.2) and kurtosis (2.8). Therefore, the hardness, toughness and wear resistance (extracted from indentation and scratch tests) of this sample is maximum (H is 24.5 GPa and H/E is 0.107), while sample S50 shows complete fracture and delamination.

Improving the crystallinity and texture of oblique-angle-deposited AlN thin films using reactive synchronized HiPIMS

Many technologies, such as surface-acoustic-wave (SAW) resonators, sensors, and piezoelectric MEMS require highly-oriented and textured functional thin films. The best results are typically achieved for on-axis sputter geometries, but in some scenarios, this is not feasible, such as during co-deposition from multiple magnetrons or when coating substrates with high aspect ratios. Ionized physical vapor deposition (PVD) techniques such as HiPIMS can be used to accelerate the film-forming species onto the growing film using substrate-bias potentials, thus increasing adatom mobility and film texture. However, gas-ion incorporation can limit the feasibility of such synthesis approaches for defect-sensitive functional thin films. This work reports on the oblique-angle deposition of highly textured, c-axis oriented AlN (0002) films using reactive metal-ion synchronized HiPIMS. AlN thin films deposited using direct current magnetron sputtering (DCMS) and HiPIMS are discussed for comparison and the effect of ion irradiation through substrate biasing is investigated. We find that combining HiPIMS with a moderate substrate bias of only −30 V improves the crystalline quality and texture of the films significantly, while the process-gas incorporation and point defects formation can be further reduced by synchronizing the negative substrate bias potential to the Al-rich fraction of each HiPIMS pulse. In addition to a pronounced out-of-plane texture, the films show uniform polarization of the grains making this synthesis route suitable for piezoelectric applications. While the compressive stress in the films is still comparatively high, the results already demonstrate, that metal-ion synchronized HiPIMS can yield promising results for the synthesis of functional thin films under oblique-angle deposition conditions - even with low substrate-bias potentials.

Atomic layer annealing with radio-frequency substrate bias for control of grain morphology in gallium nitride thin films

Atomic layer annealing with radio-frequency substrate bias for control of grain morphology in gallium nitride thin films

Prediction of OES intensity ratios based on coating unit data in HPPMS processes by ANN

High power pulse magnetron sputtering (HPPMS) processes are characterized by high peak powers and peak voltages. This results in a high fraction of ionized metal ions within the coating plasma. In order to analyze the correlations between parameters of the coating unit and the intensities of the excited and ionized plasma species, optical emission spectroscopy (OES) can be used. In those experiments, several process parameters are varied in a single coating process. Currently, the prediction of plasma parameters based on coating unit data follows deterministic models which cannot describe the complexity in total. Therefore, not all correlations can be fully understood. Artificial neural networks (ANN) can be used to identify correlations between process parameters and plasma species. This enables prediction of OES data based on data of the coating machine. In the present study different coating processes containing the elements Al, Cr, Ti, N and O were investigated. Current and voltages of the cathodes, substrate bias, chamber pressure, gas flows and the target compositions were used as input parameter of the ANN. Time resolved OES data of metal and gas species were used as output data. To determine the most appropriate training algorithm for the current predictions, multiple algorithms were employed. A good prediction accuracy of OES intensity ratios based on coating unit data for industrial scale coating processes for TiAlN was obtained. For CrAlON the prediction of the intensity ratios of the gas species showed good results. Nowadays a high amount of coating machine parameter variations in physical vapor deposition processes is needed in order to achieve tailored coating parameters. By using plasma diagnostics, such as OES, cost intensive coating deposition processes can be reduced significantly. A further shortening of process development time is possible by using ANN, since plasma compositions can be determined based on coating unit data.

Effect of substrate bias on structure and properties of (AlTiCrZrNb)N high-entropy alloy nitride coatings through arc ion plating

This study mainly focused on the effects of different substrate bias on the microstructure, elemental composition, mechanical and tribological properties of (AlTiCrZrNb)N coatings. (AlTiCrZrNb)N high-entropy nitride coatings with different substrate bias parameters were deposited on cemented carbide and silicon wafers using arc ion plating. The results showed that the bias voltage created a high-energy particle bombardment effect to reduce the number and volume of macroparticles brought by arc ion deposition technology; this optimized the surface quality of the coating. (AlTiCrZrNb)N coatings at different bias voltages exhibited a single-sided cubic solid solution structure. Substrate bias can significantly change the preferred orientation of coatings. As bias voltage increased from 0 V to −100 V, the preferred orientation of coatings changed from (111) to (200), and when negative voltage further increased to 200 V, the preferred orientation changed from (200) to (111). Appropriate substrate bias parameters can improve the residual stress, mechanical properties, and tribological properties of coatings. The highest hardness of the coating reached 34.85 ± 2.05 GPa. The lowest average friction coefficient was 0.26, and the lowest wear rate was 8.86 ± 1.05 × 10−6 mm3 ∙ N−1 ∙ m−1.

Corundum-structured AlCrNbTi oxide film grown using high-energy early-arriving ion irradiation in high-power impulse magnetron sputtering

Multicomponent or high-entropy oxide films are of interest due to their remarkable structure and properties. Here, energetic ion irradiation is utilized for controlling the phase formation and structure of AlCrNbTi oxide at growth temperature of 500°C. The ion acceleration is achieved by using a high-power impulse magnetron sputtering (HiPIMS) discharge, accompanied by a 10 μs-long synchronized substrate bias (Usync), to minimize the surface charging effect and accelerate early-arriving ions, mainly Al+, O+, Ar2+, and Al2+. By increasing the magnitude of Usync from −100 V to −500 V, the film structure changes from amorphous to single-phase corundum, followed by the formation of high-number-density stacking faults (or nanotwins) at Usync = −500 V. This approach paves the way to tailor the high-temperature-phase and defect formation of oxide films at low growth temperature, with prospects for use in protective-coating and dielectric applications.

Photocatalytic activity of TiO2 deposited by reactive HiPIMS with long target-to-substrate distance

Reactive High Power Impulse Magnetron Sputtering (HiPIMS) of TiO2 thin films was carried out to investigate the influence of ion parameters and the deposition temperature on the film crystallinity and photocatalytic performance. In order to limit unintentional substrate heating, a deposition setup with long target-to-substrate distance was used. Different HiPIMS pulse configurations, deposition temperatures and substrate bias were evaluated. TiO2 films prepared by pulsed dc magnetron sputtering were used as reference. Films deposited at room temperature were all found to be X-ray amorphous, and a minimum temperature of 200 °C was needed for film crystallization irrespective of the mode of operation. This is attributed to the relatively long target-to-substrate distance of 180 mm used in this work. The growth of a specific polymorph was shown to be dependent on the operation mode, where a high oxygen partial pressure was ideal for anatase formation. The photodegradation rates were, as expected, found to be highest for crystalline samples, where single- and mixed-phase films yielded similar rates. Furthermore, the photodegradation rates of HiPIMS films deposited without substrate heating could be enhanced up to 3 times as compared to the corresponding pulsed dc reference film. The ion assistance in HiPIMS is also beneficial at moderate temperatures, here 200 °C, where an improved crystallinity as compared to pdcMS, was observed.

Effect of tilted closed-field magnetron design on the microstructure and mechanical properties of TiZrNbTaN coatings

A common design of sputtering systems is to integrate many magnetron sources in a tilted closed-field configuration, which can drastically affect the magnetic field in the chamber and thus plasma characteristics. To study this effect explicitly, multicomponent TiZrNbTaN coatings were deposited at room temperature using direct current magnetron sputtering (DCMS) and high-power impulse magnetron sputtering (HiPIMS) with different substrate biases. The coatings were characterized by x-ray diffraction, scanning electron microscopy, nano-indentation, and energy dispersive x-ray spectroscopy. Magnetic field simulations revealed ten times higher magnetic field strengths at the substrate in single-magnetron configuration when compared to the closed-field. As a result, the substrate ion current increased ∼3 and 1.8 times for DCMS and HiPIMS, respectively. The film microstructure changed with the discharge type, in that DCMS coatings showed large sized columnar structures and HiPIMS coatings show globular nanosized structures with (111) orientation with a closed-field design. Coatings deposited from a single source showed dense columnar structures irrespective of the discharge type and developed (200) orientation only with HiPIMS. Coatings deposited with closed-field design by DCMS had low stress (0.8 to −1 GPa) and hardness in the range from 13 to 18 GPa. Use of HiPIMS resulted in higher stress (−3.6 to −4.3 GPa) and hardness (26–29 GPa). For coatings deposited with single source by DCMS, the stress (−0.15 to −3.7 GPa) and hardness were higher (18–26 GPa) than for coatings grown in the closed-field design. With HiPIMS and single source, the stress was in the range of −2.3 to −4.2 GPa with a ∼6% drop in the hardness (24–27 GPa).

Microstructure, mechanical properties and optical reflectance of TiNiN films deposited on silicon substrates using cathodic arc evaporation

Variations in applied substrate bias during cathodic arc deposition may impose intriguing consequences on the mechanical and optical properties of TiNiN coatings. Here, TiNiN films were fabricated onto Si substrates using a Ti95Ni5 (at.%) alloy target via cathodic arc evaporation employing a range of substrate bias voltages varying from 0 to -150 V. The aim of this study was to investigate the microstructures, mechanical performance, and optical properties of the TiNiN films in terms of the employed substrate potential. A significant enhancement (57%) in hardness for the TiNiN film was attained at -50 V as compared to zero substrate potential. This was principally attributed to the evolution of a fine, equiaxed nanocomposite structure, in association with the induction of a very high compressive residual stress at -50 V. In contrast, a maximum optical reflectance of 73% in the near-infrared range was determined for the coating synthesised at -150 V. It is speculated that incidence of fewer macroparticles on the film surface at elevated negative potentials, due to the induction of a higher repulsive force, may reduce the scattering of light incident on the coating's surface, promoting improved spectral reflectivity.

Back Gate Bias Influence on BESOI ISFET Sensitivity

This paper presents, for the first time, the BESOI MOSFET working as Ion Sensitive Field Effect Transistor (ISFET): the BESOI ISFET. Experimental measurements were performed with standard pH solution replacing the gate electrode. The threshold voltage becomes lower when the pH decreases (changes from basic to acid) as expected. Furthermore, the back (substrate) bias influence was studied, showing an increase in the device sensitivity (mV/pH) as a function of the back voltage. The best sensitivity value was obtained for 30 V applied in the back bias, reaching 33 mV/pH. These results demonstrates that the BESOI ISFET has electrical behavior compatible with other ISFETs.

Tuning the microstructure and hydrogen permeability of VC/V composite membranes by radio frequency magnetron sputtering with applied substrate bias

Our recent work has shown that vanadium carbide (VC) films prepared by radio frequency magnetron sputtering (RFMS) can be used as H2 dissociation/recombination catalysts on vanadium (V) for high temperature hydrogen separation. During the deposition of VC on V by RFMS, substrate bias was further applied to tune the microstructure and hydrogen permeability of the VC/V composite membranes. It is found that moderate substrate bias during RFMS generates nanocrystalline VC films with hexagonal close packed V2C (hcp-V2C) phase, which exhibits finer grain size and higher carbon vacancy density compared to the films with face centered cubic VC (fcc-VC) phase prepared by RFMS without bias. The increase in carbon vacancy density results in an increase in the electron density around the Fermi level (EF) and a lowering of the d-band center for the VC, which contributes to its catalytic activity. The resulted VC/V composite membranes show significantly enhanced hydrogen permeability, particularly at all temperatures evaluated, above that of industrially used Pd. This work demonstrates that RFMS with applied substrate bias as microstructural engineering is an efficient method to prepare highly hydrogen-permeable VC/V composite membranes.

Magnetron sputtering of the high entropy alloy CoCrFeMnNi on 316L: Influence of substrate grain orientations

This study explores the influence of a 316 L stainless steel substrate on growth of CoCrFeMnNi coatings deposited using magnetron sputtering at different substrate bias. The study was carried out on a polycrystalline 316 L substrate where the growth behavior of the coating could be investigated on grains with different orientations. By combining electron backscatter diffraction (EBSD) before and after deposition and characterization of the same area, it was possible to determine growth behavior and surface morphologies on individual substrate grains. No strong influence of the substrate was observed at a floating bias. At a bias of -100 V, however, the coating was strongly influenced by the orientation of the individual substrate grains. Epitaxial growth with a smooth coating grain surface was observed on the <102>-oriented grains while a more columnar growth was observed on <111>-oriented substrate grains. Furthermore, a small difference in growth rate was observed on different substrate orientations. The growth behavior could be related to differences in energy transfer from incoming ions and diffusion rates during growth on different orientations.

High entropy alloy thin films on SS304 substrates: Evolution of microstructure and interface modulated by energetic condensation in nanoscale

High entropy alloys (HEAs), as a novel material in the 21st century, possess several advantages, such as excellent corrosion & oxidation resistance and high mechanical properties. HEA thin films show these favourable properties with lower material costs than their bulk counterparts. Studying the HEA film-substrate interface represents challenges but is of extreme importance for the understanding of growth mechanisms with important implications for film adhesion. However, most HEA films were deposited on monocrystalline silicon substrates with limited practical applicability. Further, where commercial stainless steel, aluminium or titanium alloy substrates were used, the microstructure and chemistry at the interface were neglected. Here, we deposited AlCrFeCoNiCu0.5 HEA thin films on stainless steel 304 (SS304) substrates using cathodic arc deposition with different substrate biases. The crystallography and microstructure were investigated using an X-ray and electron-microscopy based chatacterization. A transition of an incoherent to semi-coherent interface was observed from 0 V to -50 V of the substrate bias. Energy dispersive spectroscopy demonstrated a transition of Cr2O3 to aluminum oxide across the interface. The nanoindentation tests revealed the significant improvement of mechanical properties of SS304 with HEA coatings. High-strength HEA (8.0 ± 0.2 GPa) thin films with semi-coherent interfaces were manufactured on SS304.

Effects of Sputtering Bias on the Material Characteristics of Ag Nanotwinned Films

The mechanisms of the formation of nanotwinned structures have recently attracted attention due to the advantageous properties of the structures. This paper describes the microstructural evolution of (111)-oriented nanotwins into huge (200)-oriented grains with grain sizes exceeding the film thickness. The results reveal that the initiation temperature for abnormal grain growth can be improved to nearly 300 °C by altering the microstructure of the nt-Ag films with the application of substrate bias during deposition. Densification of the nt-Ag films by substrate bias reduces the number of fine equiaxed grains in the transition layer and hence reduces the overall grain boundary energy of the nt-Ag film. The lower number of discontinuities that form within the nanotwinned layer retards the lateral growth of the columnar grains. The combination of the effects is conducive to the thermal stability of Ag nanotwinned films, implying promising future applications in microelectronics.

A TCAD-Based Analysis of Substrate Bias Effect on Asymmetric Lateral SiGe HBT for THz Applications

In this work, a new asymmetric lateral silicon germanium heterojunction bipolar transistor (SiGe HBT) is proposed. This asymmetric structure allows one to modulate the carrier densities in the collector region by the application of substrate bias, which causes significant improvements in device performance. The open base breakdown voltage can be tuned from 2.2 to 3.6 V and the transit frequency ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <inline-formula> <tex-math notation="LaTeX">$ \textit{f}_{{\sf T}} $</tex-math> </inline-formula> </i> ) is improved up to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim$</tex-math> </inline-formula> 90% by varying the substrate bias. The bias-dependent variation in transit time is elaborately discussed using regional analysis. This work, for the first time, demonstrates an oscillation frequency ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$ \textit{f}_{\text{MAX}} $</tex-math> </inline-formula> ) of 2.7 THz achievable by tuning the substrate bias in an asymmetric silicon-on-insulator (SOI) lateral SiGe HBT.

Effect of substrate bias on the properties of plasma deposited organosilicone (pp-HMDSN) thin films

Effect of substrate bias on the properties of plasma deposited organosilicone (pp-HMDSN) thin films

Influence of Si segregates on the structural evolution, mechanical properties, and high-temperature fracture toughness of Cr-Si-B2±z coatings

The impact of Si-segregates and varying deposition conditions on the structural and mechanical properties of sputter deposited, high-temperature oxidation-resistant Cr-Si-B2±z coatings is studied from ambient, to elevated temperatures. Overstoichiometric, AlB2-structured Cr-Si-B2±z thin films with Si-content up to 15 at.% were synthesized on Ti-6Al-4V by magnetron-sputtering using a substrate bias of −120 V. The enhanced surface diffusion promotes mechanically superior, (001)-oriented coatings with hardness of H∼30 GPa up to a Si-content of 3 at.%. Higher Si-concentrations result in significant hardness loss to H∼20 GPa, related to a bias-independent solubility-limit in the CrB2-structure and the formation of mechanically-weak Si grain-boundary segregates. The as-deposited hardness of all Cr-Si-B2±z compositions is maintained after annealing to 800 °C, despite the initiation of material recovery. A B/Cr-ratio-independent oxidation resistance up to 1400 °C is demonstrated, underlining a minimum Si-content of 8 at.% to form a stable SiO2-based scale. In line with the room-temperature hardness, increasing Si-contents are accompanied by decreasing fracture toughness, reducing from KIC∼2.9 (Cr0.28B0.72) to ∼1.7 MPa√m (Cr0.24Si0.10B0.66). High-temperature cantilever bending up to 800 °C revealed a brittle-to-ductile-like transition for Cr0.28B0.72, resulting in an increased fracture toughness of KIC∼3.3 MPa√m. Si-alloyed coatings show decreasing fracture resistance up to 400 °C, whereas beyond, Si-segregates enable high-temperature plasticity and thus a significantly increased damage tolerance.

Morphology control in Ni/Ti multilayer neutron mirrors by ion-assisted interface engineering and B4C incorporation

The optical contrast and minimum layer thickness of Ni/Ti broadband neutron multilayer supermirrors is usually hampered by an interface width, typically 0.7 nm, caused by nanocrystallites, interdiffusion, and/or intermixing. We explore the elimination of nanocrystallites in combination with interface smoothening by modulation of ion assistance during magnetron sputter deposition of 0.8 to 6.4 nm thick Ni and Ti layers. The amorphization is achieved through incorporation of natural B4C where B and C preferably bond to Ti. A two-stage substrate bias was applied to each layer; -30 V for the initial 1 nm followed by -100 V for the remaining part, generating multilayer mirrors with interface widths of 0.40-0.45 nm. The results predict that high performance supermirrors with m-values as high as 10 are feasible by using 11B isotope-enriched B4C combined with temporal control of the ion assistance.

Influence of the etching process on the coating performance in dry tribological contacts

Physical vapor deposition (PVD) coatings are utilized for wear protection of gears. The use of PVD coatings in unlubricated contacts is also the subject of several research efforts. A promising approach to reduce wear in dry tribological contacts is the combination of MoS2 and WS2 embedded in CrAlN. One possible way to deposit these coatings is the application of High Power Pulse Magnetron Sputtering (HPPMS) process combined with direct current Magnetron Sputtering (dcMS). A challenge by depositing these coatings with an HPPMS/dcMS process is a sufficient adhesion between the coating and the substrate. The present study focuses on the influence of the etching process on the performance of CrAlN + Mo:W:Sg coatings in dry tribological contacts. Before coating deposition, the substrates were pretreated inside the vacuum chamber by different etching processes. During booster etching, which increases the Ar ionization, the substrate bias UB was varied between UB = −150 and UB = −250 V with steps of ΔUB = −50 V. Furthermore, an HPPMS metal ion etching process was used after booster etching at UB = −150 V for these processes. For HPPMS metal ion etching, the number of HPPMS cathodes was varied between one and two. With respect to the etching process, the morphology, surface roughness, compound adhesion by means of the scratch test as well as the coefficient of friction, and the wear volume of the subsequently deposited coatings were investigated. When increasing the substrate bias UB during booster etching, a reduced compound adhesion was found. Moreover, an increased wear volume was observed after pin-on-disk (PoD) tests. For metal ion etching with one activated HPPMS cathode, the wear volume after the PoD test was decreased compared to the pure booster etching processes. When increasing the number of HPPMS cathodes during metal ion etching, the compound adhesion decreased slightly. The wear volume after PoD tests, in contrast, increased. When only using one HPPMS cathode, the lowest wear volume of all investigated coatings was achieved. The present investigations show the potential of tailoring the etching process in order to increase the performance of PVD coatings. A customized HPPMS metal ion etching process can be used to improve the properties of well-established hard PVD coatings.

HiPIMS deposition of superconducting Nb thin films onto Cu substrates

The use of energetic condensation techniques, such as HiPIMS, has the potential to significantly improve the superconducting performance of coated Cu cavities by producing bulk-like Nb thin films. However, in order to benefit from these techniques, a deeper understanding of the effects of, and the relationship between, the different deposition parameters, the physical properties of the deposited films and the superconducting performance of the films, is still required. This contribution details the effects of different HiPIMS deposition parameters on the morphological, crystallographic and superconducting properties of Nb thin films deposited onto electropolished Cu substrates. Higher duty cycles (8 – 20 %) than previous HiPIMS investigations were used in the majority of the coatings. The films displayed more bulk-like crystallographic properties, indicated here by the lattice parameter, with a “transition zone” apparent in the early stages of film growth. A reduction in the surface roughness as well as interfacial voids, compared to typical DC MS Nb films, was also observed. The superconducting performance of the films, specifically the first magnetic flux entry field, showed a marked reliance on the substrate bias and the film thickness. Conversely, the HiPIMS duty cycle appears to have a comparatively insignificant effect.