Plasma Emission Monitoring Research Articles

高周波反応性スパッタリング法による窒化物薄膜作製プロセスにおよぼすターゲット金属の影響

In order to clarify the effect of target materials on the reactive sputtering process, we have characterized the hysteresis behavior of four kinds of target materials, Ti, Nb, Cr and Al. When highly reactive metals such as Ti and Nb were used as target, the hysteresis behavior appeared very clearly. On the other hand, the hysteresis became unclear when a low reactive metal such as Al was used. The hysteresis of Cr target was very small though the reactivity of Cr was not so low. This is attributed that the difference of sputtering yield between Cr metal and Cr-N nitride is smaller than that of the other metal.We have also investigated the optimum deposition conditions of sputtered Me-N films using the rf reactive sputtering apparatus equipped with a PEM (Plasma Emission Monitoring) control system for stabilizing the hysteresis behavior. The optimum condition for nitride films using highly reactive target metals such as Ti and Nb appeared to be near the unstable region of reactive sputtering process. The PEM system usedin the study was found to be very useful for depositing these nitride films. In the case of AIN films, the optimum condition was an excessive nitrogen partial pressure region. This difference in the optimum conditions is considered to be caused by the difference of reactivity of target metal and the stable metal-nitrogen alloy phase.

CrN films deposited by rf reactive sputtering using a plasma emission monitoring control

We have studied the influence of the nitrogen partial pressure ( P N 2 ) on composition, crystal structure, internal stress, resistivity and hardness of the rf reactive sputtered Cr–N films. Cr–N films have been deposited onto glass substrates by rf reactive magnetron sputtering using a plasma emission monitoring control system, where a signal in proportion to the light emitted by the sputtered Cr in the glow discharge plasma was used to control the admission of the reactive nitrogen into the system. Applied rf power and argon partial pressure ( P Ar) were kept constant at 140 W and 0.3 Pa, respectively. It is found that the composition of nitrogen in as-deposited films increases with increasing P N 2 . Stoichiometric CrN films seemed to grow at a P N 2 of about 1∼2×10 −2 Pa. The crystal structure of the film deposited at P N 2 lower than 1×10 −2 Pa is of hexagonal Cr 2N, while the film deposited at P N 2 higher than 1×10 −2 Pa is of single phase CrN with an NaCl-type structure. The CrN film with good mechanical property is found to grow within a very narrow region of P N 2 .

Strategies for high rate reactive sputtering

High rate reactive sputtering of heavily insulating dielectrics such as Si 3N 4 and TiO 2 is a key requirement for economic production of complex coating designs. Two approaches for achieving enhanced deposition rates for TiO 2 are contrasted. The use of closed loop process control through plasma emission monitoring has been demonstrated utilizing both DC and mid-frequency AC sputtering sources. Alternatively, high rate deposition of TiO 2 has been demonstrated via deposition from a sub-stoichiometric ceramic TiO 2 target, also utilizing DC and AC deposition. The use of mid-frequency AC is required to eliminate deposition non-uniformity due to insulating build up on anode surfaces. Data on deposition speeds, optical properties, long term deposition rate and uniformity stability are presented for both processes.

A novel reactive magnetron sputtering technique for producing insulating oxides of metal alloys and other compound thin films

Problems associated with reactive magnetron sputtering from elemental (i.e. non-compound) targets have been successfully solved in this work. The elements of this achievement are: (i) the use of mid-frequency (i.e. 40 kHz) AC power in the floating mode, between two magnetrons, allowed the reactive sputtering process to be arc-free and hence eliminating the undesired effects of arcing in reactive sputtering such as driving the process to become unstable, creating defects in the films and reducing the target lifetime. (ii) The combination of DC and mid-frequency AC power in a novel way using a filter to protect the DC power supply from the AC one (or the independently DC powered magnetrons method) permitted the composition of the produced films to be easily and independently manipulated by varying the magnitude of power applied to each magnetron. (iii) The use of very fast feedback methods to automatically control the admission rate of oxygen into the sputtering chamber (i.e. plasma emission monitoring or voltage control) allowed the stoichiometry of the deposited films to be independently controlled. This also allowed the deposition rate of the sputtered films to be high. (iv) Sputtering from two magnetrons made the production of alloys or multi-element compounds, which are either difficult or impossible to be formed from single targets, an easy task. (v) Substrate rotation enhanced atomic level mixing of the film constituents. The stoichiometry of the film was controlled by plasma emission monitoring or voltage control on one magnetron, and dopants were added by sputtering from the other magnetron. This means that the former magnetron served two purposes; the first was to sputter metal and oxidise it, and the second purpose was to oxidise the metal sputtered from the other magnetron. This novel technique opens the door wide for investigating virtually all potentially promising thin oxide films. Using this technique, a large range of alloy-oxide films was deposited at high rates. In fact, the independent control of both the metallic composition and stoichiometry was very valuable in identifying the optimum properties of these films. That is, giving transparent films of different refractive indices for optical applications. Furthermore, such a technique may also be capable of investigating other types of thin films (e.g. hard coatings, semiconducting films, superconducting films, etc.).

ITO coating by reactive magnetron sputtering–comparison of properties from DC and MF processing

The aim of ITO process technology development is to obtain stable film properties with exceptionally low resistivity and high transmittance within the visible spectrum range. The challenge for this development is to upgrade such ITO technology for large area coating in production. Therefore, a comparison of processing with oxide or metallic targets for both DC and MF mode was performed. The comparisons with regard to ITO film properties are related to InSn alloy targets vs. ceramic ITO targets, single or dual magnetrons, process control mode, dynamic deposition rate and substrate temperature. Plasma Emission Monitor control works as an useful tool to deposit optimized ITO films having both low resistivity and high transmittance. Best results are obtained in dual DC mode with resistivities of less than 1.6×10−4 Ω cm at dynamic deposition rates of 110 nm×m/min.

Effect of partial pressure on the internal stress and the crystallographic structure of r.f. reactive sputtered Ti-N films

The influence of nitrogen partial pressure ( P N2) and argon partial pressure ( P Ar) on internal stress, crystallographic structure, and resistivity have been investigated for reactively sputtered Ti-N films in order to get some insight into the influence of deposition parameters. Ti-N films were deposited onto glass substrates by r.f. reactive magnetron sputtering using a plasma emission monitoring control system. The r.f. power applied was kept constant at 300 W during deposition, and the substrate temperature was room temperature. When P Ar is 0.4 Pa and P N2 is 1 × 10 −3 Pa or higher, films deposited become single phase of TiN, although preferential orientations of films change with P N2. Ti-N films deposited at a P N2 of 2–6 × 10 −3 Pa become a gold color, and the resistivity becomes minimum. The internal stress of deposited Ti-N films changes as a function of both argon and nitrogen partial pressure, and films deposited at a low P Ar tend to have high compressive stress. The internal stress is also related to preferred orientation of films. Films having the (001) preferential orientation show higher compressive stress than those having the (111) preferred orientation.

Chemical instability of the target surface during DC-magnetron sputtering of ITO-coatings

Indium–tin oxide (ITO) coatings on polymer film substrates should have the required properties already in their as-deposited state and should be chemically and thermally stable. The coatings can be produced either by reactive DC-magnetron sputtering of In/Sn (e.g., 80/20, wt.%) metallic alloy targets or by pseudo-reactive sputtering of ITO (In 2O 3SnO 2 90/10, wt.%) targets. In the former process (and to a lesser extent in the latter process), a surface instability of the target arises: black crystals of a dark phase grow on the target surface, changing the target voltage and causing pronounced arcing in the plasma. This harmful phenomenon grows worse during a sputter run and finally necessitates abortion of the run and mechanical cleaning of the target. This phenomenon has never been explained in literature. In this paper, it will be shown that the black crystals are In 2O and that they originate from an equilibrium reaction at the target surface in which In 2O 3 is decomposed into In 2O and O 2. This reaction is especially annoying with reactive processes in which optical plasma emission monitoring (PEM) is used as a control means to obtain the correct stoichiometry of the coatings at relatively high sputter deposition rates.

Techniques for the sputtering of optimum indium-tin oxide films on to room-temperature substrates

New ion-assisted, successive-layer plasma anodisation processes are described for the preparation of indium-tin oxide films. They use unbalanced planar magnetron sputtering to provide controlled argon-ion bombardment of the growing film, and also use room-temperature substrates. In both processes the magnetron was used both to deposit the metal and to provide the activated reactive atmosphere, either by moving the substrate between different units or by controlled pulsing of the admission of the oxygen to change the function of the magnetron. These processes have been used to determine the optimum stoichiometry through control of the amount of oxygen incorporated into the metal oxide, and the optimum doping of the indium oxide with tin to produce films with the highest electrical conductivity and visible transparency. The experiments confirm results which were obtained with continuous deposition processes, which demonstrated that the addition of any tin will result in a loss of performance as compared to that achieved with control of the stoichiometry. The continuous processes used both oxide and metal targets with the supply of oxygen being carefully controlled, when necessary, from process parameters. We generally used plasma-emission monitoring (PEM) of the radiation emitted by the sputtered indium, when excited by the plasma, to indicate the partial pressure of oxygen and the state of the target. The continuous sputtering of a compacted oxide target was the least critical of the processes: only a small amount of flow-controlled oxygen had to be added to the sputtering atmosphere to give adequate electrically conducting and visibly transmitting films. Better films could be made more quickly reactively from metal targets, but were much more critical in the control of the conditions which they required. The use of successive ion-assisted plasma anodisation techniques made this a much less critical process, while in all cases the limiting resistivity was found to be around 4×10 6Ω·m. This was obtained via control of the stoichiometry through the oxygen content of an indium film. The role of tin was to make the achievement of optimum films more easily attainable.

Reactive sputter deposition of indium tin oxide films onto polymeric webs using optical emission spectroscopy and mass spectroscopy as process monitoring and control

Reactive sputter deposition of indium tin oxide (ITO) from an alloy target is an extremely sensitive process. The quality of the deposited ITO films is dependent upon the ability to maintain a certain partial pressure of oxygen in the sputter zone during the deposition process. Minor instabilities within the glow discharge plasma caused by contaminants outgassing from the substrate, low pump throughput, target poisoning due to the buildup of insulating layers on the target surface, or target arcing, can result in significant changes in the stoichiometry of the deposited layer, thus producing an inferior conducting film. These instabilities can lead to an uncontrollable transition between the metallic and fully reactive modes of the target that can force the process away from the desirable operating point. It has been shown previously that a plasma emission controlled circuit can be implemented to help with the control of the deposition of oxide films in this transition region. In this work we have configured a plasma emission monitor (PEM) with a residual gas analyzer (RGA) into a closed loop feedback circuit for the deposition of ITO onto moving flexible substrates. It was found that the stabilization of the operating point was achieved by controlling the admission of the reactive gas into the process zone using the chosen emission line of interest as the input signal. Furthermore, the operating point was determined without the continuous calibration of the PEM by choosing the RGA baseline as the normalizing signal. The result of these experiments are discussed and compared to the traditional method of producing an ITO coating using a constant pressure control process.

Plasma conditions for the deposition of wear-resistant carbon-coatings

The properties of sputter and microwave-ECR plasmas have been investigated with a plasma emission monitor (PEM) in the range between 280 and 820 run with various resolutions. Additional Langmuir probe measurements of the sputter plasma were performed to determine the spatial variation of electron density and temperature. Using helium as an additional measuring gas, the electron temperature has been determined by comparing the intensities of an argon and a helium emission line. The concentration of atomic hydrogen and CH radicals can be determined by actinometry. A comparison between the two types of plasmas shows similar electron temperature dependencies on the plasma power, the argon partial pressure and the acetylene partial pressure. The electron temperatures determined from PEM and Langmuir probe measurements correlate very well. When the microwave ECR plasma is used at the same time as the sputter plasma the electron temperature in the sputter plasma is reduced because of the charged particle current flowing out of the ECR plasma. The electron temperature in the ECR plasma varies only slightly when the sputter plasma is switched on but the concentration of atomic hydrogen increases because of the additional conversion of the acetylene on the target surface.

Synthesis of sputtered thin films in low energy ion beams

Magnetron sputtering is a process which gives a highly energetic depositing species. The growing film can be further bombarded with ions of the heavy gas used for sputtering by directing a plasma of it onto the surface. This can be done quite simply by using an unbalanced magnetron. The immersion of an insulating or isolated substrate-film combination in this plasma leads to a self-bias of around 30 V appearing on it's surface and a bombardment of low energy ions of the sputtering gas of several milli-amps per square centimetre. If the residual gas contains a reactive component, to form a compound film, then the gas is made much more reactive and less is needed to form the stoichiometric film. This can take place in a continuously operating system made stable using partial pressure control of the reactive gas with plasma emission monitoring or something similar. It can also be operated when the process of deposition is separated in time from the process of reaction and is repeated to build the film. We have called this process successive-plasma-anodisation (SPA) and it can be achieved by mechanically transferring the substrate between two magnetrons, one to deposit the metal film and one, which is unbalanced, to provide an oxygen plasma. It can also be operated by pulsing the reactive gas under carefully controlled conditions. Examples are given of the synthesis of compound films using low energy ion bombardment with these techniques and it is demonstrated that excellent films of a large range of oxides and nitrides can be made.

Composite DLC–metal/metal–carbide coatings produced by electron enhanced unbalanced magnetron sputtering

Composite coatings consisting of metal containing DLC were produced by unbalanced magnetron sputtering in a closed field magnetic configuration. Targets of Ti and W were poisoned with acetylene during the depositionprocess, enabling the formation of a carbon enriched surface layer, which was then sputtered to produce DLC–metal/metal–carbide composites. Target poisoning was controlled by plasma emission monitoring, and a hot filament was used to regulate the ion current to the substrates independently of the magnetron power and substrate bias. Coating morphology and composition were investigated using scanning electron microscopy techniques and glow discharge optical emission spectroscopy. Additionally the films were characterised by means of Raman spectroscopy. Microhardness, pin on disc, abrasive wheel, impact, and scratch tests were pelformed to evaluate the mechanical and tribological properties of the coatings. In pin on disc tests, dry sliding friction coefficients of less than 0·15 were recorded against an SAE 52100 steel ball, even after 1 km sliding distance. The implications of these extremely favourable tribological properties are discussed.

Effects of nitrogen pressure and ion flux on the properties of direct current reactive magnetron sputtered Zr–N films

Zr–N films have been deposited by direct current reactive magnetron sputtering using a plasma emission monitoring (PEM) control system, where a signal proportional to the light emitted by the sputtered Zr in the plasma created by the electrical discharge was used to control the admission of the reactive nitrogen into the system. The Zr line set point for PEM control, φ, was used as a parameter; as the partial pressure of nitrogen rises the Zr line emission falls due to target poisoning. The influence of the nitrogen partial pressure on the Zr–N film structure, reflectivity, resistivity, and internal stress was investigated. The effect of ion flux during film deposition on the film properties was also investigated. The Zr–N films deposited at the Zr line set point φ=60%–70% revealed the minimum resistivity and goldlike reflectivity spectra. These films showed a ZrN single phase. The internal stress of the films showed a maximum at φ=60%–70%, which corresponds to the condition for depositing minimum resistivity films. The external magnetic field scarcely affected the optical and the electrical properties of the deposited Zr–N films. The internal stress of the films deposited at the lower φ (higher nitrogen partial pressure) was only dependent on the ion flux.

Active process control of reactive sputter deposition

Control of the density, composition, ionisation rate and arrival energy of species is one of the main objectives of research in the development of reactive magnetron sputtering. The deposition of the latest generation of multilayer and multi-component coatings requires the independent control of deposited flux parameters using fast response and reliable control systems. A review of recent advances in process control showed the potential of techniques such as unbalanced magnetron sputtering in a closed magnetic field configuration, thermionically enhanced deposition and closed loop control with optical gas and metal plasma emission monitoring. These techniques were combined in an active control system. Special software was used to provide automatic computer aided process control in the deposition of multilayer and multi-component coatings. The system has been evaluated on a range of refractory and DLC coatings and recommendations on process control are given.

Practical aspects for the use of plasma emission monitoring in reactive magnetron sputtering processes

The sensitivity of Plasma Emission Monitoring (PEM) depends on both target material and oxygen mass flow at least for indium and tin target. Optical emission intensity and cathode voltage were recorded for several argon and oxygen flows in various tin and indium oxide thin film fabrication experiments, with the magnetron working in constant current mode. The results obtained indicate marked differences between indium and tin targets, as far as their reactive sputtering behaviour is concerned. A theoretical explanation is proposed for the observed target voltage variations with increasing oxygen mass flow in relation to the corresponding optical emission intensities.

Preparation of W-C:H coatings by reactive magnetron sputtering

The reactive d.c.-magnetron sputter technique in conventional and unbalanced modes was used to prepare tungsten-containing hydrodcarbon (W-C:H) coatings which have low friction coefficients and high wear resistances. Reproducible deposition processes stable over a long time without poisoning could be realized using a plasma emission monitor (PEM) control unit. From investgigation of light emission spectra of the magnetron discharge the intensive tungsten line at 401 nm was selected as most suitable for process control. The dependences of characteristic process parameters such as target voltage, optical emission intensity or deposition rate on the reactive gas flow were quite different from those known for the reactive sputter deposition of TiN or other nitrides and oxides. For both tungsten and WC targets the PEM intensity decreased monotonically with increasing acetylene flow. However, the deposition rates on substrates with floating potential increased. An additional r.f. excitation of the substrate electrode caused only slight changes in rate and metal content in the coatings. It can be concluded that the sputter process is dominant for W-C:H growth. The contribution of direct plasma polymetrization at the substrate is small. From-X-ray diffraction investigations it was concluded that the W-C:H coatings contain microcrystallites of the metastable cubic tungsten carbide WC 1 − x . The Vickers hardness and Young's modulus depend on the atomic ratio of tungsten to carbon.

Control of reactive d.c. magnetron sputtering of SnO2 by means of optical emission

Abstract One procedure to stabilize reactive d.c. magnetron discharges is the so-called plasma emission monitoring (PEM). Here the intensity of the plasma emission of a characteristic target line is used as measuring signal for a closed loop to control the reactive gas flow. Up to now, the PEM system has been used for the reactive deposition of a series of materials such as TiN, TiO 2 and indium tin oxide. Advantages of PEM control not only resulted from the stability of the discharge but equally from the extension of the reactive working point in the region of the transition mode and in enhanced deposition rates. For the production of a heat-reflecting film system on glass it is SnO 2 that is mainly used for dielectric sublayers. In an attempt to use PEM control for the reactive deposition of SnO 2 a non-monotonic relation between film composition and line intensity was measured in contradiction to the results for the above-mentioned materials. A similar behaviour is obtained for the relation between line intensity and film deposition rate. In either case the line intensity approached a maximum. This behaviour can be explained by a strong increase in the excitation of the sputtered metal particles in the oxygen-containing plasma. In this way, the decrease in the metal rate, and consequently the intensity of the metal line, are overcompensated. A concept and an instrumental solution are described which ensured the control of the reactive sputter process of SnO 2 under the conditions mentioned. The method can be transferred to other materials that exhibit a similar behaviour.

Reactive d.c. high-rate sputtering as production technology

We discuss process parameters that influence the deposition of layers with dedicated stoichiometry in terms of the fundamentals of reactive d.c. sputter processes with magnetron sources. We introduce the term “basic stability” of reactive sputter arrangements and describe their variants. To make the industrial production of films from chemical compounds profitable, however, it is necessary to work in the unstable transition mode. We discuss methods for process stabilization and present new approaches by using a plasma-emission monitor (PEM) and a self-stabilizing arrangement for the long-term deposition of insulating films. When working with the PEM control loop, the reactive d.c. sputter process can be stabilized even at close coupling between target and substrate. To exemplify various production techniques with different requirements, we compare indium-tin oxide (ITO), Cr-Si-O and TiN layers. The PEM control allows the deposition of ITO layers at a defined degree of oxidation to obtain minimum resistivities of ϱ < 4.5 × 10-4 Ω cm at a high transparency. The self-stabilizing coating technique yields homogeneous, very thick, hard Cr-Si-O antiwear coatings at rates much higher than those of metal sputtering. For optimized TiN layers on tools, the plasma density is of decisive importance in attaining a fine-grained structure. A large increase in the power density on the target and the use of a bias for film deposition under ion action yield film properties of outstanding quality.