Investigation of reactive high power impulse magnetron sputtering processes using various target material-reactive gas combinations

The two most important issues limiting reactive high power impulse magnetron sputtering (HIPIMS) process applicability until recently were the absence of suitable reactive HIPIMS control means and the limited capability of HIPIMS power supplies in terms of arc handling. The significant advancement has been made recently by the development of the optical plasma monitoring (PM)-based process control technology for reactive HIPIMS [Surface & Coatings Technology 204 (2010) 2159-2164]. The initial studies of reactive HIPIMS processes however have only covered Ti-O₂ target material-reactive gas system.In this paper the recently developed PM-based active feedback control technology was applied to explore further reactive HIPIMS processes now using a variety of different target material and reactive gas combinations. Data for hysteresis behaviour and process control using either PM or constant gas flow methods for Ti-O₂, Ti-CO₂, Cr-O₂, Cr-C₂H₄, Al-O₂, and Zn:Al-O₂ material-gas systems is presented and compared. In all cases the processes were found to exhibit hysteresis behaviour. The magnitude and features of the hysteresis loop were found to depend strongly on a particular metal-reactive gas pair. Similar to AC and DC reactive sputtering processes the hysteresis behaviour in reactive HIPIMS was found to be more pronounced for the gases that have high chemical affinity for a metal sputter target. The PM-based process control technology monitoring either metal or gas plasma emissions was shown to provide accurate control and stable operation of reactive HIPIMS discharges.     

Control of reactive high power impulse magnetron sputtering processes

High power impulse magnetron sputtering (HIPIMS) is a technologically important physical vapour deposition (PVD) process that is able to provide a highly ionised flux of sputtered species. It is thought to be particularly important for applications where there is a need to coat 3D features (e.g. vias and trenches in semiconductor applications). HIPIMS may have other added benefits, as compared to DC or medium frequency AC/pulse-DC magnetron sputtering, related to better coating structure-property relationship control through self-species (sputtered metal) plasma/ion assistance.Many of the technologically important thin films (e.g. transparent conductive oxides, permeation barrier coatings, etc.) are sputtered from metal targets in a reactive gas atmosphere, usually Ar + O₂ or N₂, to ensure industrially relevant coating deposition rates. Enhanced structure-property relationship control of these thin film materials is highly desirable; hence, it also is desirable to use HIPIMS in a reactive deposition mode. Preliminary trials of reactive HIPIMS however have indicated that the control of this process using conventional means, such as conventional plasma emission monitoring (PEM) is difficult. Thus, the application of reactive HIPIMS is rather limited and the potential benefits are not realised, especially in the areas where precise process control and long term stability in a reactive environment are required.In this paper reactive HIPIMS process (Ti in Ar/O₂ atmosphere) is investigated and various control options are evaluated. The application of a recently developed PEM based reactive HIPIMS control method is reported. Performance of the developed technique is compared to that of the conventional PEM, Penning-PEM and λ-sensor based methods. It is shown that conventional PEM is impractical to control reactive HIPIMS, while the constant reactive gas flow method does not lead to a stable deposition process. The new PEM based process control technology was shown to provide precise control and stable operation of reactive HIPIMS discharges anywhere within the hysteresis loop. It was also found to be superior when compared to oxygen partial pressure control based techniques.     

Studies of hysteresis effect in reactive HiPIMS deposition of oxides

High power impulse magnetron sputtering (HiPIMS) has proven to be capable of substantial improvement of the quality of deposited coatings. Lately, there have been a number of reports indicating that the hysteresis effect may be reduced in HiPIMS mode resulting in an increase of the deposition rate of stoichiometric compound as compared to a direct current magnetron sputtering process in oxide mode. In this contribution, we have studied the hysteresis behaviour of Ti metal targets sputtered in Ar + O₂ mixtures. For fixed pulse on time and a constant average power, there is an optimum frequency minimizing the hysteresis. The effect of gas dynamics was analyzed by measurements of the gas refill time and rarefaction. Results indicate that the gas rarefaction may be responsible for the observed hysteresis behaviour. The results are in agreement with a previous study of Al oxide reactive process.     

Modelling of sputtering yield amplification in serial reactive magnetron co-sputtering

Serial magnetron co-sputtering can be used to increase the deposition rate in reactive deposition of thin films. The increase in deposition rate is achieved by sputtering yield amplification through doping the sputtering target by a heavy element. The dopant is introduced by means of sputtering from an auxiliary target onto a rotating primary magnetron. During sputtering of the primary target, the dopant is implanted into the target surface. Here we present a model describing the serial co-sputtering technique. The model is based on the binary collision approximation and takes into account the dynamical sputtering and mixing at the target surface. As an example, W and Bi doping in reactive sputter deposition of Al₂O₃ is analyzed. W is shown to be very efficient dopant which can increase the deposition rate for oxide up to 100% with 1.6 at.% of W in the resulting coating. Doping by Bi is not very effective due to the low surface binding energy of Bi. The simulations show that sputtering yield amplification can be realized in the serial co-sputtering setup with rotating magnetrons.     

复合高功率脉冲磁控溅射技术的研究进展

高功率脉冲磁控溅射技术(HIPIMS)是一门新兴的高离化率磁控溅射技术.概述了HIPIMS的技术优势,包括高膜层致密度和平滑度、高膜基界面结合强度以及复杂形状工件表面膜层厚度均匀性好等.同时归纳了HIPIMS存在的问题,包括沉积速率及低溅射率金属靶材离化率低等.在此基础上,重点综述了近年来复合HIPIMS技术的研究进展,其中复合其他物理气相沉积技术的HIPIMS,包括复合直流磁控溅射增强HIPIMS、复合射频磁控溅射增强HIPIMS、复合中频磁控溅射增强HIPIMS、复合等离子体源离子注入与沉积增强HIPIMS等;增加辅助设备或装置的HIPIMS,包括增加感应耦合等离子体装置增强HIPIMS、增加电子回旋共振装置增强HIPIMS,以及增加外部磁场增强HIPIMS等.针对各种形式的复合HIPIMS技术,分别从复合HIPIMS技术的放电行为、离子输运特性,及制备膜层的结构与性能等方面进行了归纳.最后展望了复合HIPIMS技术的发展方向.     

CrAlYCN/CrCN nanoscale multilayer PVD coatings deposited by the combined High Power Impulse Magnetron Sputtering/Unbalanced Magnetron Sputtering (HIPIMS/UBM) technology

CrAlYCN/CrCN coating combining high hardness (H_p = 36 GPa) and low friction coefficient (µ = 0.42 against Al₂O₃) has been developed for machining of Si containing Al-alloys. The coating was deposited by the combined High Power Impulse Magnetron Sputtering/Unbalanced Magnetron sputtering, (HIPIMS/UBM) technology. Macroparticle free Cr⁺ ion flux was generated by HIPIMS discharge to sputter clean the substrates prior to the coating deposition. The use of HIPIMS for surface pre treatment resulted in excellent adhesion, scratch test adhesion critical load value of Lc = 55 N on HSS and Lc = 68 N due to the local epitaxial growth and extremely smooth coating surface, Ra = 0.012 μm due to the elimination of growth defects.The coating crystallised in fcc structure with a preferred {220} orientation. XTEM analysis revealed a nanoscale multilayer structure of the coating with carbon segregated at the column boundaries but also vertically to form a lateral phase at the interfaces between the individual nanolayers.Addition of C to CrAlYN/CrN increased the chemical inertness between cutting tool and workpiece material without deteriorating the oxidation resistance of the coating. Thermo gravimetric analysis showed that the temperature for the onset of rapid oxidation was as high as 940 °C.In dry milling of AlSi9Cu1 alloy, CrAlYCN/CrCN coated 8 mm diameter cemented carbide end mills outperformed non coated end mills by factor of 2.5 with effective hindered built up edge formation mechanism.     

Modelling of sputtering yield amplification effect in reactive deposition of oxides

Many reactive sputter deposition applications require high deposition rates. The primary limiting parameters in magnetron sputtering are the target power dissipation and sputtering yields of the target elements. In reactive deposition of oxides, the deposition rate is of particular interest due to the low sputtering yield of most commonly used oxides. Traditional high rate techniques rely on a feedback control of the oxygen partial pressure to prevent formation of oxide on the target and hence enable operation in the transition area. An alternative approach, based on target doping, is presented in this paper.By doping the sputtering target with heavy elements, it is possible to substantially enhance the sputtering yield and hence the deposition rate. Simulations of the partial sputtering yield values for aluminium from doped targets sputtered in reactive atmosphere have been carried out. The Monte Carlo based TRIDYN computer code has been used for simulations. The program has been used to find out optimum alloying conditions to obtain maximum partial sputtering yield for deposition of Al₂O₃. Our simulations indicate that the sputtering yield amplification in reactive sputtering may lead to much higher relative deposition rate increase than in a nonreactive case. The highest relative increase may be achieved in the transition region but substantial increase is predicted also in the oxide mode.     

Modelling of the hysteresis effect of target voltage in reactive magnetron sputtering process by using neural networks

Reactive sputtering process is very non-linear and usually exhibits hysteresis behaviour with respect to the reactive gas flow. Most of the problems encountered in the preparation of non-stoichiometric compound films by reactive sputtering are due to the hysteresis effect. Therefore, a considerable amount of effort has been devoted to find means for its elimination or ensuring a stable sputtering in the transition mode. This paper presents a new approach based on Artificial Neural Networks (ANNs) for modelling of the hysteresis effect of target voltage at different target power levels and reactive gas flow rates in reactive sputtering. Based on this model, it is possible to predict the target voltage in reactive magnetron sputtering processes, when the target power level, reactive gas flow rate and its direction are used as inputs to the artificial neural network (ANN). The proposed ANN is trained in different structures with the use of learning algorithms to obtain better performance and faster error convergence. Broyden–Fletcher–Goldfarb Shanno (BFGS) algorithm gives the best result among other learning algorithms used in the analysis. The training and test data required to develop the ANN model are obtained from the experimental studies. Both the training and the test results are in very good agreement with the experimental results obtained in this work.     

Use of a theoretical model to investigate RF and DC reactive sputtering of titanium and chromium oxide coatings

An analysis of RF and DC reactive sputtering techniques is presented. The transition between a metal sputtering mode and a compound sputtering mode is usually noticed with a metallic target and an argon+oxygen gas mixture. The so-called hysteresis effect often observed for small amounts of reactive gas is explained in recent models. By considering gas kinetics parameters, it is possible to evaluate quite simple relationships between the main processing parameters. These theoretical calculations enable the prediction and aid the understanding of instability phenomena observed in reactive sputtering. In this paper, the effects of some parameters on the position and size of instability regions are discussed, and the difference between DC and RF reactive deposition is investigated. Simulations and experimental results are compared for the case of titanium and chromium oxide thin films prepared by DC and RF reactive sputtering. The influence of sputtering power on the position of the hysteresis loop is analysed theoretically and experimentally, and the changes observed between the reactive sputtering of titanium and chromium oxide materials are also discussed.     

High power pulsed magnetron sputtering: A review on scientific and engineering state of the art

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.     

不同氩气气压下钒靶HIPIMS放电特性的演变

目的以V靶为例,研究高功率脉冲磁控溅射(HIPIMS)放电时不同工作气压下靶脉冲电流及等离子体发射光谱的表现形式和演变规律,为HIPIMS技术的进一步广泛应用提供理论依据。方法利用数字示波器采集HIPIMS脉冲放电电流波形,并利用发射光谱仪记录不同放电状态下的光谱谱线,分析不同气压下V靶HIPIMS放电特性的演变规律。同时,利用HIPIMS技术成功制备了V膜,并利用扫描电子显微镜观察了V膜的截面形貌。结果不同氩气气压下,随着靶脉冲电压的增加,靶电流峰值、靶电流平台值及靶电流平均值均单调增加,而且增加的速度越来越快,但靶电流峰值的增加速度明显高于平台值,这是由于脉冲峰值电流由气体放电决定所致。不同气压下,Ar0、Ar+、V0和V+四种谱线峰的光谱强度均随靶电压的增加而增加,相同靶电压时,其光谱强度随着气压的增加而增加。当气压为0.9 Pa、靶电压为610 V时,Ar和V的离化率分别为78%和35%。此外,利用HIPIMS技术制备的V膜光滑、致密,无柱状晶生长形貌特征。结论较高的工作气压和靶脉冲电压有利于获得较高的系统粒子离化率,但HIPIMS放电存在不稳定性。合适的工作气压是获得优质膜层的关键。     

Low temperature sputter deposition of SnOx:Sb films for transparent conducting oxide applications

SnO_x:Sb films have been prepared by reactive dc magnetron sputtering from a metallic target, with the aim of evaluating the potential of SnO_x:Sb as an attractive low-cost alternative to In₂O₃:Sn (ITO) for TCO applications. The deposition was performed without any additional heating of the substrates. The films were subsequently analysed regarding their optical, electrical and structural properties. Our results show that there is only a narrow process window for the sputter deposition of transparent and conducting tin oxide films at low temperature. A sharp minimum in resistivity of 4.9 mΩ cm is observed at an oxygen content of approximately 17% in the sputtering gas. Under these deposition conditions, the SnO₂:Sb films turn out to be both highly transparent and crystalline. At lower oxygen content (10-15%) the SnO_x:Sb films are substoichiometric, as revealed by Rutherford backscattering, and show a low transmission and high resistivity due to numerous defects and the presence of the SnO phase. At higher oxygen content (> 17%) excess oxygen is incorporated into the films, which is attributed to an increase of oxygen ion bombardment. This leads to a degradation of the electrical properties and a decrease of the density of the films, whilst the optical transmittance slightly improves.     

Modeling reactive sputter deposition of titanium nitride in a triode magnetron sputtering system

In this paper, the so-called Berg's model was successfully employed in order to model the reactive sputter deposition of titanium nitride (TiN) by a triode magnetron sputtering (TMS) system. Such system consists of a grounded grid introduced between the target and the substrate. The grid acts as the anode, and the glow discharge is formed between the target and grid. The qualitative model was compared to experimental data. In addition, results from a conventional MS system were also compared to the ones from the modified TMS system. It was possible to observe that (a) the width of the hysteresis region is narrower for TMS for all modeled conditions; (b) the hysteresis width increases as a function of grid-to-target distance.     

Study of hybrid PVD–PECVD process of Ti sputtering in argon and acetylene

Hybrid PVD–PECVD process of target sputtering in hydrocarbon containing atmosphere combines aspects of both conventional reactive magnetron sputtering (PVD) and plasma enhanced chemical vapour deposition (PECVD). Such process is being typically used for deposition of metal carbides embedded in hydrogenated carbon matrix. Compared to the conventional co-sputtering of metal and carbon targets, in the hybrid deposition process the source of the carbon is dissociated hydrocarbon vapour in plasma. The aim of this paper is to study the extent of similarities or differences between this hybrid process and the conventional reactive magnetron sputtering. We have chosen the sputtering of titanium target in acetylene containing atmosphere as a representative of the hybrid processes. We focused on experimental measurements of the hybrid PVD–PECVD process behaviour, the time necessary for the process to achieve steady-state conditions and basic modelling of the process.     

High‐temperature oxidation behaviour of model Co?Re?Cr alloys at low oxygen partial pressure

The oxidation behaviour of four model Co? Re? Cr alloys and a commercial Co‐based alloy was investigated at 1000 °C and a low‐oxygen partial pressure of p(O₂) = 10^?16 bar, in order to prove the feasibility of a pre‐oxidation treatment. Under suitable conditions of the pre‐oxidation treatment, the oxidation of the highly reactive alloying element Cr is possible. All the studied alloys form a continuous and dense Cr₂O₃ scale on the metal surface. The transport of chromium to the surface occurs mainly from the Cr‐rich σ‐phase, which becomes completely dissolved in the surrounding matrix after long exposure times. As a result of the Cr₂O₃ scale, growth depletion of Cr occurs in the near surface region, leading to internal oxidation.     

Film characterization of titanium oxide films prepared by high-power impulse magnetron sputtering

Surface modification with a high power glow discharge is an emerging technology that can be used to improve the surface characteristics. Titanium oxide films are prepared using a high-power impulse magnetron sputtering (HPPS-M) glow discharge with a current density of 2 A/cm² and a power density of 1 kW/cm². Observing optical emission spectrum confirms that singly-ionized titanium ions are produced in the plasma. Ions are extracted from the HIPIMS glow plasma by a substrate placed near the plasma source. It is found that the substrate is immersed in the HPPS-M glow plasma. The film is deposited by a HPPS-M, and the results are compared to those of magnetron sputtering operated by a stationary dc power source. The deposition rate is lower by HPPS-M than that by DC-MS. The main structure of the films is rutile, however an anatase structure is also observed. The mixed structure is obtained at an oxygen rate as low as 5%. Anatase structure is not significantly observed in HPPS-M compared to that in DC-MS. The intensity of the XRD profiles becomes weaker with increasing the substrate position due to the collisions of metal species with the plasma species and the background gas particles. The deposition rate of the prepared titanium oxide film is significantly influenced by the production rate of titanium ions, distance of the substrate, and the gas mixture ratio. With regard to the effect of the gas ratio, the difference in the deposition rate is probably based on the argon ion density available to sputter titanium atoms that would eventually contribute to the titanium oxide film deposition.     

Deposition of titanium oxide films by reactive High Power Impulse Magnetron Sputtering (HiPIMS): Influence of the peak current value on the transition from metallic to poisoned regimes

In this study, reactive High Power Impulse Magnetron Sputtering (HiPIMS) experiments were carried out to synthesize titanium oxide films, using a 45 × 15 cm² titanium target in Ar/O₂ gas mixtures. The deposition process was studied as a function of the peak current (i_peak) at constant voltage during the pulse (1 kV) and constant average power (P_av). As the oxygen flow was increased, i_peak was kept constant (160, 300 or 400A) by adjusting the pulse duration and the average power (2 or 4 kW) by adjusting the pulse repetition frequency. For all experimental conditions, an abrupt transition from metallic towards poisoned regimes was observed. The transition curves exhibit hysteresis. As i_peak is increased from 160 A to 450 A, for P_av = 4 kW, the oxygen content (Ω) in the Ar/O₂ mixture needed to poison the target surface was reduced from Ω = 11.5% to Ω = 8.5%. These values are much smaller than those recorded for DC magnetron sputtering (DCMS) (Ω = 42%) and pulsed DCMS (Ω = 36%) experiments carried out at the same power. These results are explained by the enhancement of the ionization and dissociation rates of oxygen molecules with the increase of i_peak.     

不同靶基距下凹槽表面HIPIMS法制备钒膜的微观结构及膜厚均匀性

目的研究不同靶基距对高功率脉冲磁控溅射(HIPIMS)在凹槽表面制备钒膜微观结构和膜厚均匀性的影响,实现凹槽表面高膜层致密性和均匀性的钒膜制备。方法采用HIPIMS方法制备钒膜,在其他工艺参数不变的前提下,探讨不同靶基距对凹槽表面钒膜相结构、表面形貌及表面粗糙度、膜层厚度均匀性的影响。采用XRD、AFM及SEM等观测钒膜的表面形貌及生长特征。结果随着靶基距的增加,V(111)晶面衍射峰强度逐渐降低。当靶基距为12 cm时,钒膜膜层表面粗糙度最小,为0.434nm。相比直流磁控溅射(DCMS),采用HIPIMS制备的钒膜呈现出致密的膜层结构且柱状晶晶界不清晰。采用HIPIMS和DCMS方法制备钒膜时的沉积速率均随靶基距的增加而减少。当靶基距为8 cm时,采用HIPIMS方法在凹槽表面制备的钒膜均匀性最佳。结论采用HIPIMS方法凹槽表面钒膜生长的择优取向、表面形貌、沉积速率及膜厚均匀性均有影响。在相同的靶基距下,采用HIPIMS获得的钒膜膜厚均匀性优于DCMS方法。     

Thermal behaviour of hard nanocomposite coatings within the W-Si-N system in oxidant and protective atmospheres

This paper reviews the current knowledge on the thermal annealing of W-Si-N sputtered films in both protective and oxidant atmospheres. Firstly, sputter deposited W films are presented as a particular case of single nc-metallic films and their thermal behaviour is analyzed. Afterwards, the thermal stability and the oxidation resistance of W-Si-N system are considered. Special attention is paid to amorphous films. In a protective atmosphere, it is shown that after crystallization the hardness can be higher than those of as-deposited crystalline films with similar nanocomposite structure. The hardest films present a nc-W/a-Si₃N₄ nanocomposite structure type. The key factor for thermal oxidation resistance is the Si content. The higher the Si content the lower the oxidation rate is. The excellent thermal behaviour in oxidant atmospheres is attributed to either the formation of a protective surface layer of SiO₂ or the hindering of oxygen diffusion along the grain boundaries due to the Si-N phase.