Magnetron sputtering - Milestones of 30 years

Since the introduction of the planar magnetron by J.S. Chapin in 1974 magnetron sputtering has become the most important technology for the deposition of thin films. Today it has conquered all industrial branches needing high-quality coatings for realization of new or improvement of existing products. The magnetron cathode combines the advantages of economic deposition even on large areas and the ability to coat very temperature sensitive plastic substrates. Main problems like poor target material utilization of the planar magnetron or process instabilities during deposition of highly insulating films have been solved by many innovations during the past 30 years. Novel films with even better quality seem to be possible with “High Power Impulse Magnetron Sputtering (HiPIMS)”. New attempts to increase sputter yield and thus film growth rate are “Sputter Yield Amplification (SYA)” or sputtering from hot targets. This paper gives a brief review on important milestones of the past three decades and outlines some ongoing developments.     

Magnetron sputtering – Milestones of 30 years

Since the introduction of the planar magnetron by J.S. Chapin in 1974 magnetron sputtering has become the most important technology for the deposition of thin films. Today it has conquered all industrial branches needing high-quality coatings for realization of new or improvement of existing products. The magnetron cathode combines the advantages of economic deposition even on large areas and the ability to coat very temperature sensitive plastic substrates. Main problems like poor target material utilization of the planar magnetron or process instabilities during deposition of highly insulating films have been solved by many innovations during the past 30 years. Novel films with even better quality seem to be possible with “High Power Impulse Magnetron Sputtering (HiPIMS)”. New attempts to increase sputter yield and thus film growth rate are “Sputter Yield Amplification (SYA)” or sputtering from hot targets. This paper gives a brief review on important milestones of the past three decades and outlines some ongoing developments.     

Reaktive Nanometer‐Multischichten als maßgeschneiderte Wärmequellen beim Fügen

Reactive nanometer multilayers as tailored heat sources for joining The use of traditional joining techniques like soldering or brazing for heat sensitive microstructures often results in damaging or stress induced deformation of the components. Therefore a technology would be desirable, where heat is produced locally and only for a short time. A very promising approach is the application of socalled reactive nanometer multilayers. Reactive nanometer multilayers consist of several hundreds or a few thousands of alternating layers wi^th thicknesses in the nanometer range that can exothermic react wi^th each other. After a local activation of the chemical reaction by an electrical spark or a laser pulse, a self‐sustaining reaction starts, which propagates parallel to the multilayer surface resulting in a stable intermetallic single layer. The peak temperature of the reaction can be above 1000 8C, but the maximum temperature is only reached for several ten milliseconds. Therefore, the heat remains localized in the vicinity of the solder layers surrounding the reactive multilayer. During the entire process the components to be joined exhibit temperatures close to room temperature. We will show new results concerning the fabrication of reactive nanometer multilayers by magnetron and ion beam sputter deposition and the fabrication of free standing nanometer multilayers.     

Novel coatings for unique applications

In the present article three different coatings systems are discussed based on reactive and non-reactive sputtering processes utilizing HiPIMS, pulsed DC and DC magnetron sputtering. The HiPIMS platform was used to develop a very hard TiB₂ coating with a low residual stress level. Pulsed DC magnetron sputtering was used to deposit 50 μm thick amorphous Al₂O₃ coating. Finally DC magnetron sputtering was used to deposit a Sr-Ti-O coating capable of releasing Sr facilitating accelerated bone formation and implant ingrowth.     

In‐Situ‐Charakterisierung optischer Mehrschichtsysteme in Großflächen‐ Produktionsanlagen

The paper discusses approaches to the insitu analysis of optical multi‐layer coating stacks such as mirror coatings, AR coatings or lowE layer stacks on large‐area substrates in production environments. The stable production of complex layer systems requires in‐situ analysis systems that are able to provide spectral information and optical performance data, but also yield thickness information for individual layers and thus aide the operating staff in detailed analyzing deviations from the production target stack. Solutions incorporating in‐situ optical reflectance, transmittance and ellipsometry measurements wi^th optical data being collected not only for the completed layer stack, but also at intermediate coating stages, are discussed and the accuracy, robustness and stability of different measurement systems and computation strategies are compared.     

Modulated pulse power sputtered chromium coatings

Cr coatings were deposited using continuous dc magnetron sputtering (dcMS) and modulated pulse power sputtering (MPP) techniques in a closed field unbalanced magnetron sputtering system at equivalent average target powers. It was found that MPP sputtering exhibited higher deposition rates than in dcMS when the average target power density was above 14 W cm^− 2 for the Cr coating depositions. Plasma diagnostics confirmed a significant increase in the numbers of both target material (Cr) and gas (Ar) ions in the MPP plasma as compared to the dc plasma. The substrate peak current densities measured in the MPP depositions (104-324 mA cm^− 2) have been increased by over a factor of 50 to those in the dcMS conditions (2-5.5 mA cm^− 2). The enhanced ion flux bombardment from the highly ionized MPP plasma led to the formation of denser microstructure and finer grain size in the MPP Cr coatings than in the dcMS Cr coatings. In addition, MPP sputtered Cr coatings exhibited improved hardness and adhesion.     

A study on the microstructure and property development of d.c. magnetron co-sputtered ternary titanium aluminium nitride coatings Part II Effect of magnetron discharge power

Advanced ternary (Ti,Al)N coatings were produced by reactive magnetron co-sputtering technique with separate titanium and aluminium targets at a 30° magnetron configuration. The aluminium magnetron discharge power was adjusted from 0 to 6.0 W/cm² to investigate the effect of magnetron discharge power on the microstructure and property development of the coatings. It was found that increasing the aluminium magnetron discharge power caused the deposition rate and the aluminium content to increase, and the grain size and surface roughness of the coatings to decrease substantially. Tighter packing of the grain columns occurred and the microstructure changed from a porous zone 1 to a densified zone T structure, resulting in a continuous increase of the coating hardness. The major texture component of the coatings changed from the (111) to (200) orientations. The (101) orientations of the AlN structure also developed. It was found that the microstructure and hardness enhancement of the coatings was associated with an increased formation of the TiAlN and AlN phases and a densified, fine grain structure at higher magnetron discharge powers.     

Comparison of microstructure and mechanical properties of chromium nitride-based coatings deposited by high power impulse magnetron sputtering and by the combined steered cathodic arc/unbalanced magnetron technique

Sliding, abrasive, and impact wear tests were performed on chromium nitride (CrN)-based coatings deposited on mirror-polished M2 high speed steel substrates by the novel high power impulse magnetron sputtering (HIPIMS) utilising high peak cathode powers densities of 3000 W cm⁻². The coatings were compared to single layer CrN and multilayer superlattice CrN/NbN coatings deposited by the arc bond sputtering (ABS) technique designed to improve the coating substrate adhesion by a combined steered cathodic arc/unbalanced magnetron (UBM) sputtering process. The substrates were metal ion etched using non-reactive HIPIMS or steered cathodic arc at a substrate bias voltage of −1200 V. Subsequently a 2- to 3-μm thick CrN or CrN/NbN coating was deposited by reactive HIPIMS or UBM. No bias was used during the HIPIMS deposition, while the bias during UBM growth was in the range 75-100 V. The ion saturation current measured by a flat electrostatic probe reached values of 50 mA cm⁻² peak for HIPIMS and 1 mA cm⁻² continuous during UBM deposition. The microstructure of the HIPIMS coatings observed by transmission electron microscopy was fully dense in contrast to the voided columnar structure observed in conventional UBM sputtered CrN and CrN/NbN. The sliding wear coefficients of the HIPIMS CrN films of 2.3×10⁻¹⁶ m³ N⁻¹ m⁻¹ were lower by a factor of 4 and the roughness of the wear track was significantly reduced compared to the UBM-deposited CrN. The abrasive wear coefficient of the HIPIMS coating was 2.2×10⁻¹³ m³ N⁻¹ m⁻¹ representing an improvement by a factor of 3 over UBM deposited CrN and a wear resistance comparable to that of the superlattice CrN/NbN. The adhesion of the HIPIMS deposited CrN was comparable to state-of-the-art ABS technology.     

A TECHNICAL NOTE. INFLUENCE OF MEAN STRESS ON FATIGUE IN SEVERAL ALUMINIUM ALLOYS UTILIZING Kcmax THRESHOLD PROCEDURES

Recent interest in the constant K_max (K^c_max) threshold testing procedure has resulted in a more in-depth study of the influence of K_max level on fatigue response and ΔK_th in aluminium alloys. Under R^c= 0.1 conditions, which cause large amounts of closure, ΔK^th levels were typically 2 to 4 Mpam. However, under K^c_max test procedures, associated with no measurable closure at threshold, ΔK_th was typically 1 Mpam. A slight K^c_max level effect on ΔK_th was observed at high K_max values for some of the alloys, and was deemed to be a pure mean stress effect, separate from closure arguments.     

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.     

Surface modification of 316L stainless steel with magnetron sputtered TiN/VN nanoscale multilayers for bio implant applications

Nanoscale multilayered TiN/VN coatings were developed by reactive dc magnetron sputtering on 316L stainless steel substrates. The coatings showed a polycrystalline cubic structure with (111) preferential growth. XPS analysis indicated the presence of peaks corresponding to Ti2p, V2p, N1s, O1s, and C1s. Raman spectra exhibited the characteristic peaks in the acoustic range of 160–320 cm⁻¹ and in the optic range between 480 and 695 cm⁻¹. Columnar structure of the coatings was observed from TEM analysis. The number of adherent platelets on the surface of the TiN/VN multilayer, VN, TiN single layer coating exhibit fewer aggregation and pseudopodium than on substrates. The wear resistance of the multilayer coatings increases obviously as a result of their high hardness. Tafel plots in simulated bodily fluid showed lower corrosion rate for the TiN/VN nanoscale multilayer coatings compared to single layer and bare 316L SS substrate.     

Low-temperature deposited Titanium-doped zinc oxide thin films on the flexible PET substrate by DC magnetron sputtering

Transparent conducting Titanium-doped zinc oxide thin films (TZO) with high transparency and relatively low resistivity were firstly deposited on water-cooled polyethylene terephthalate (PET) substrates at room temperature by DC magnetron sputtering. The microstructure, optical and electrical properties of the deposited films were investigated and discussed. The XRD patterns show that all the deposited films are polycrystalline with a hexagonal structure and have a preferred orientation along the c-axis perpendicular to the substrate. The electrical resistivity decreases when the sputtering power increases from 45 W to 60 W. However, as the puttering power continue increases from 60 W to 90 W, the electrical resistivity increases rapidly. When the puttering power is 60 W, the films deposited on PET substrate have the lowest resistivity of 4.72 × 10⁻⁴ Ω cm and a relatively high transmittance of above 92% in the visible range.     

TiAlN-Beschichtungen mittels hoch-energetischer Kathodenzerstäubung

TiAlN-coatings by means of high-energy sputtering According to current knowledge, high-energy plasmas represent a promising tribological property profile for nitride hard material coatings and their applications. However, the increased ionization rate results in a high level of residual compressive stresses in TiAlN thin films (σ_TiAlN ∼ –6000 MPa), which can negatively influence the adhesion between substrate and coating and thus can lead to delamination and damage. In this context, the presented investigations prove that a hybrid process consisting of the synchronous use of direct current (DC) and high power pulse sputtering (HiPIMS) can be used to reduce the residual stress state in the coating. This allows to combine the advantages of both technologies and thus to qualify TiAlN thin films for an extended range of applications as tribological coatings for heat treated tool steels.     

Application of magnetron sputtering for producing bioactive ceramic coatings on implant materials

Radio frequency (RF) magnetron sputtering is a versatile deposition technique that can produce thin, uniform, dense calcium phosphate coatings. In this paper, principle and character of magnetron sputtering is introduced, and development of the hydroxyapatite and its composite coatings application is reviewed. In addition, influence of heat treatment on magnetron sputtered coatings is discussed. The heat treated coatings have been shown to exhibit bioactive behaviour both in vivo and in vitro. At last, the future application of the bioactive ceramic coating deposited by magnetron sputtering is mentioned.     

Electron field emission properties of carbon nanoflakes prepared by RF sputtering

Carbon nanoflakes (CNFs) with corrugated geometry were synthesized using RF sputtering process with Ar/CH₄ gas mixture. Transmission electron microscopic examination reveals that the introduction of H₂ in sputtering chamber leads to the preferential etching of amorphous carbons, while maintaining integrity for the nano-crystalline phases. The proportion of nano-sized crystalline clusters is thus increased, which improved the electron field emission (EFE) properties of the materials, viz. with turn-on field of E ₀ = 6.22 V/μm and FEE current density of J _e = 90.1 μA/cm² at 11.0 V/μm. The cathodes made of screen printing of CNFs-Ag paste exhibit even better EFE properties than the as-deposited CNFs. The EFE of the CNFs cathodes can be turned on at E ₀ = 5.71 V/μm, achieving J ₀ = 340.1 μA/cm² at 11.0 V/μm applied field. These results showed that the CNFs are inheritantly more robust in device fabrication process than the other carbon materials and thus possess better potential for electron field emitter applications.     

Antimicrobial Potential of Copper‐Containing Titanium Surfaces Generated by Ion Implantation and Dual High Power Impulse Magnetron Sputtering

The application of antimicrobial surfaces to titanium alloy (Ti) implants would be beneficial to prevent implant‐associated infections of joint endoprostheses and osteosyntheses. Copper (Cu) could be advantageously applied for this purpose, since it exhibits a well‐known antimicrobial activity and is a trace element in the human body, i.e., it is non‐toxic in small concentrations. This approach was evaluated with two plasma‐based surface modification procedures:

• 1) Implantation of Cu ions into Ti by means of plasma immersion ion implantation (PIII) and
• 2) Coating of Ti surfaces with Cu? Ti films by means of dual high power impulse magnetron sputtering (dual HiPIMS).

In this manner, the surfaces could be equipped with various amounts of Cu, as it was analyzed by X‐ray photoelectron spectroscopy (XPS). The surfaces released up to 8 mmol · L^?1 of Cu within 24 h, measured with atomic absorption spectroscopy (AAS). Hence, the surfaces possessed an antimicrobial potential against typical infect‐associated bacteria (Staphylococcus aureus). Surfaces with a higher Cu release prepared by HiPIMS technique revealed a higher antimicrobial effect, while surfaces implanted by PIII were less cytotoxic to osteoblasts (MG‐63 cells). These results show that Cu doped and coated implants could be useful for prevention and therapy of implant‐associated infections.     

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.     

Higher Tool Productivity due to New Generation of PVD Coatings

PVD coating technology has seen many new developments in the past few years. HIPIMS⁺ is an example of these developments with excellent results on tool productivity and lifespan. The technology combines the advantages of arc evaporation with those of magnetron sputtering and results in a dense and defect‐free coating with tuneable residual stress. The coatings have been tested successfully by international tool manufacturers. Additional developments can be seen in DLC coatings and the hydrogen‐free variety ta‐C. These coatings are suitable for low temperature deposition and machining of special materials.     

An All‐Solution‐Based Hybrid CMOS‐Like Quantum Dot/Carbon Nanotube Inverter

The development of low‐cost, flexible electronic devices is subordinated to the advancement in solution‐based and low‐temperature‐processable semiconducting materials, such as colloidal quantum dots (QDs) and single‐walled carbon nanotubes (SWCNTs). Here, excellent compatibility of QDs and SWCNTs as a complementary pair of semiconducting materials for fabrication of high‐performance complementary metal‐oxide‐semiconductor (CMOS)‐like inverters is demonstrated. The n‐type field effect transistors (FETs) based on I^? capped PbS QDs (V _th = 0.2 V, on/off = 10⁵, S _S‐th = 114 mV dec^?1, µ _e = 0.22 cm² V^?1 s^?1) and the p‐type FETs with tailored parameters based on low‐density random network of SWCNTs (V _th = ?0.2 V, on/off > 10⁵, S _S‐th = 63 mV dec^?1, µ _h = 0.04 cm² V^?1 s^?1) are integrated on the same substrate in order to obtain high‐performance hybrid inverters. The inverters operate in the sub‐1 V range (0.9 V) and have high gain (76 V/V), large maximum‐equal‐criteria noise margins (80%), and peak power consumption of 3 nW, in combination with low hysteresis (10 mV).     

Reactive deposition of Al-N coatings in Ar/N2 atmospheres using pulsed-DC or high power impulse magnetron sputtering discharges

In this paper, the metal to ceramic transition of the Al-N₂ system was investigated using classical reactive pulsed-DC magnetron sputtering and HIgh Power Impulse Magnetron Sputtering (HIPIMS) at a constant average current of 3 A. Optical emission spectroscopy measurements revealed more ionised aluminium species in the HIPIMS discharge compared to pulsed-DC sputtering. It also showed excited N⁰ and ionised N⁺ species in reactive Ar/N₂ HIPIMS discharges. The corresponding evolution of the consumed nitrogen flow as a function of the N₂ partial pressure revealed that a higher amount of reactive gas is needed to achieve stoichiometric AlN with HIPIMS. Electron probe micro-analysis and X-ray diffraction measurements confirmed that a partially poisoned aluminium target is enough to allow the deposition of stoichiometric hcp-AlN thin films via HIPIMS. To go further in the comparison of both processes, two stoichiometric hexagonal aluminium nitride thin films have been deposited. High power impulse magnetron sputtered hcp-AlN exhibits a higher nano-hardness (18 GPa) than that of the coating realised with conventional pulsed-DC sputtering (8 GPa).