Pulsed DC Power Research Articles

New hybrid source of cold atmospheric plasma

New cold atmospheric plasma sources that combine microwave plasma with the hollow cathode plasma generated simultaneously by a common hollow electrode are described. A laboratory prototype of a hybrid hollow electrode activated discharge (H-HEAD) source with simple cylindrical electrode is presented. The hollow cathode plasma is generated by a pulsed DC power. The source is capable to generate over 15 cm long plasma plumes in open air at microwave power of 500 W at less than 500 sccm of argon flow. Parameters of the hybrid plasma plume are controlled by both the microwave generator and the hollow cathode power generator.

Mid-frequency reactive sputtering of dielectrics: Al 2O 3

Reactive sputtering of dielectrics is done today mainly by applying pulsed DC or AC power, all modulated in the range of 1 kHz–1 MHz frequencies (mid-frequency range). This frequency range is chosen to avoid arcing and, at the same time, to obtain a substantial deposition rate. Pulsed DC is used when a simple one-magnetron system is preferred. Unfortunately, such approach does not solve the ‘disappearing’ anode problem. The disappearing anode and arcing problems are solved simultaneously using mid-frequency AC power and either dual-anode (DAS) or dual-magnetron (DMS) systems. Applying biasing to conductive substrates in the above deposition techniques is a serious problem. In the case of the DASs and the DMSs, biasing could be applied by connecting the substrate to a center tap of an isolation transformer. Reactive sputtering of Al 2O 3 was investigated and analyzed in this work to develop this biasing approach.

Aluminum oxide films deposited in low pressure conditions by reactive pulsed dc magnetron sputtering

The reactive pulsed dc sputtering technique is widely used for the deposition of oxide films. The operating pressure for sputtering is commonly above 0.13 Pa. In this study, however, aluminum oxide (alumina) films were deposited at operating pressures from 0.06 to 0.4 Pa using a sputtering system equipped with a scanning magnetron cathode and a pulsed dc power supply. The pulsed dc power was found to be useful not only to reduce arcing, but also to sustain the discharge at low pressure. The electrical breakdown field, intrinsic stress, O/Al ratio, refractive index, and surface roughness were investigated. Both a low intrinsic stress and an O/Al ratio around the stoichiometry were required to get the film having a high breakdown field. A low operating pressure of 0.1 Pa was found to provide the necessary stress and O/Al ratio targets. A 50-nm-thick alumina film having a maximum breakdown field of 7.4 MV/cm was obtained.

Effect of power mode, target type and liquid nitrogen trap on the structure and tribological properties of MoS x coatings

MoS x coatings were prepared with an unbalanced DC magnetron-sputtering system by varying the DC power mode, target type and liquid nitrogen trap. The composition and surface morphology were examined by energy dispersive X-ray and scanning electron microscopy; structural characterization was determined by X-ray diffraction. The friction and wear properties were evaluated with a fretting tester in dry (<10% relative humidity) and humid (50% and 90% relative humidity) air. MoS x coatings deposited by simple DC power tended to form rougher surfaces, and had a higher edge orientation of crystallites with worse tribological performance than those by bipolar pulsed DC power. MoS x coatings made by a cold pressed target had a lower deposition rate, and were inclined to form (002) basal orientations parallel to the surface especially at higher argon pressures. The contents of oxygen and carbon in the MoS x coating with the use of liquid nitrogen trap were greatly reduced, but MoS x coatings had lower coefficients of friction and shorter wear life at higher relative humidity. The role of these deposition variables in the formation process is discussed.

Development of an industrialised DLC duplex treatment process

In the framework of the EC Brite project ‘IPEC’, a DLC duplex treatment process (plasma nitriding followed by plasma deposition of diamond-like carbon) was developed and scaled up to large reactor dimensions (2 m 3 volume). A basic feature of the process is the use of a low frequency pulsed dc power generator to control plasma conditions. The duplex process was optimised with respect to coating performance (adhesion, wear and friction behaviour, defects) and treatment time. The coated samples were characterised with the following techniques: nano-indentation (hardness), ball-on-disc (wear and friction), scratch test (adhesion), scanning electron microscopy and Raman spectroscopy (microstructure) and calo (thickness) tests. This paper presents the results of these investigations.

Pulsed dc magnetron-sputtering of microcrystalline silicon

Nominally undoped silicon films were prepared by pulsed dc magnetron sputtering in a pure Ar atmosphere (without hydrogen dilution) at substrate temperatures in the range of 25–450°C. An asymmetrical bipolar pulsed dc power generator was operated at pulse frequencies, f, in the range of 50–250 kHz. Crystalline films (μc-Si) were grown at 450°C. It is shown that with rising frequency the preferential orientation of the crystallites changes from (110) to (100). In addition, the Ar-content is strongly enhanced and this is connected with an enhancement of disorder in the films. Time-resolved Langmuir probe studies show that increasing the frequency causes an increase of the potential difference between the plasma and the grounded substrate. It is argued therefore that these effects arise from enhanced bombardment of the growing films with Ar + ions of high energy.

Studies of mid-frequency pulsed dc biasing

The application of mid-frequency (100–350 kHz) pulsed dc power at the substrate is a recent development in the magnetron sputtering field. It has been found that, unlike the dc case, if the bias is pulsed in this range, the current drawn at the substrate does not saturate, but continues to increase with increasing bias voltage. In addition, this effect becomes more marked as the pulse frequency is increased. For example, under a particular set of operating conditions, a threefold increase in ion current was observed at a bias voltage of −300 V when the bias was pulsed at 350 kHz, compared to the dc case. This phenomenon is believed to be due to the initiation of a second discharge at the substrate. Pulsing the substrate bias voltage, therefore, offers a novel means of controlling the ion current drawn at the substrate. Clearly, this has significant implications in relation to film growth, sputter cleaning, and substrate preheating processes. Consequently, the variation in ion current with pulse frequency and bias voltage has been studied for an unbalanced magnetron sputtering system. In addition, substrate heating rates, current–voltage wave forms and plasma characteristics have also been investigated. A series of TiO2 and TiN films were then grown under different bias conditions. Analysis of these films showed that the application of pulsed dc power at the substrate can significantly influence film structure and properties. In particular, shifts in crystalline structure and texture were observed.

Substrate effects during mid-frequency pulsed DC biasing

The use of pulsed DC power at the substrate is a recent development in the field of magnetron sputtering. Pulsing the substrate bias voltage in the mid-frequency range (100–350 kHz) has been found to significantly increase the ion current drawn at the substrate. For DC bias applications, it is normally found that the current drawn at the substrate saturates at bias voltages of the order of −100 V. Further increases in bias voltage do not lead to an associated increase in ion current. However, recent experiments have shown that if the bias voltage is pulsed, not only is the magnitude of the ion current greater than for the DC bias case, but this current also continues to increase as the bias voltage is increased. In addition, both of these effects become more marked as the pulse frequency is increased. For example, under a particular set of operating conditions, a three-fold increase was observed in the current drawn at the substrate as the bias voltage was increased from −100 to −300 V and the bias pulse frequency was increased from 0 to 350 kHz. Pulsing the substrate bias voltage, therefore, offers a novel means of controlling the ion current drawn at the substrate. Clearly, this has significant implications in relation to film growth, sputter cleaning and substrate pre-heating processes. Consequently, the variations in ion current with pulse frequency and bias voltage, and the associated substrate heating effects, have been studied for an unbalanced magnetron sputtering system. The influence of these parameters on the properties of reactively sputtered titania films is also reported.

Surface roughness control of the Al and Al2O3 thin films deposited by using pulsed DC magnetron sputtering

ABSTRACTThe thickness of the Al2O3 layer used in the magnetic tunneling junctions FM1/Al2O3/FM2 is less than 2 nm, here FM1 is for the ferromagnetic layer 1 and FM2 is for ferromagnetic layer 2. In order to obtain ultra-thin Al2O3 layer with higher breakdown voltage and pinhole free, extremely smooth surface roughness of this layer is required. The influence of the sputtering gas pressure, DC pulsed frequency, DC pulsed power, substrate bias and buffer layer on surface roughness and properties of Al thin films were studied. The single layer Al films are usually amorphous, texture (111) Al films can be obtained while using thin Ta 5 nm or Ta5/NiFe2 as underlayer. Very smooth Al thin film can be sputtered on Si/SiO2 (100) wafer with Ta/NiFe buffer layer at f=15 kHz (DC pulsed frequency) and with RF substrate biasing (Vpp is about 21 V). High quality MTJs with high MR ratio up to 44.6% and high field sensitivity up to 19.3%/Oe were finally demonstrated after optimization of thin film deposition process.

Reactive pulsed magnetron sputtering process for alumina films

The pulsed magnetron sputtering (PMS) process is now among the leading techniques for the deposition of oxide films. In particular, the use of pulsed dc power has transformed the deposition of dielectric materials, such as alumina. The periodic target voltage reversals during the PMS process effectively discharge poisoned regions on the target. This significantly reduces the occurrence of arc events at the target and stabilizes the deposition process. Many researchers have now shown that pulsed dc reactive magnetron sputtering can be routinely used to produce fully dense, defect-free oxide films. Despite the success of the PMS process, few detailed studies have been carried out on the role played by parameters such as pulse frequency, duty cycle, and reverse voltage in the deposition process. In this study, therefore, alumina films were deposited by reactive pulsed dc magnetron sputtering. Operating conditions were systematically varied and the deposition process monitored throughout. The aim was to investigate the influence of the pulse parameters on the deposition process, and the interrelationships between the occurrence of arc events and the parameters chosen. As a result of this investigation, optimum conditions for the production of high-quality alumina films under hard arc-free conditions were also identified.

A study on the high rate deposition of CrN x films with controlled microstructure by magnetron sputtering

High rate deposition of CrN x films with control of microstructure was carried out by magnetron sputtering. For these purposes, the deposition processes parameters were varied: N 2 flow rate and especially substrate bias voltage, duty cycle and frequency using a pulsed DC power supply. The microstructure was analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM), and mechanical properties were evaluated by a microhardness test and adhesion test. The maximum deposition rate for CrN x compound films could be reached to nearly 90% compared with that for pure Cr coating due to the increase of ionization efficiency caused by a negative-pulsed DC bias. As N 2 flow rate is increased, the microstructure of CrN x films was changed from Cr+Cr 2N to CrN. Also, a phase transformation occurred between Cr 2N+CrN multi-phase and CrN mono-phase by control of a negative DC and/or pulsed DC bias voltage, duty cycle and frequency. Microhardness for CrN x films were measured to be up to 1600 kg/mm 2 and the maximum hardness value of 2250 kg/mm 2 was obtained for CrN x film deposited with a N 2 flow rate of 20 sccm at a negative DC bias of −100 V.

Frequency response in pulsed DC reactive sputtering processes

By simple arguments as well as results from a recently developed computer simulation model we have found out that for high frequency pulsed DC reactive sputtering the target poisoning does not reflect the periodicity of the pulsed DC power supply. The degree of target poisoning does not change markedly during a single duty cycle. The degree of poisoning essentially exhibits the same continuous time independent behavior as observed for the conventional continuous reactive sputtering process. Furthermore, it is shown that the distribution width of the transit times for sputtered atoms by far exceeds the period time for pulsed DC frequencies higher than 5–10 kHz. This causes a large overlap between sputter eroded material between consecutive pulses during processing resulting in an essentially continuous arrival rate of sputtered atoms to the substrate surface. This implies also that the deposition rate will be constant and will not follow the pulsed sputter erosion variation from the target. These findings show that, with respect to film stoichiometry and homogeneity, the high frequency pulsed DC reactive sputtering process behaves identically as the continuous reactive sputtering process. No chemical reaction effects or gas gettering variations will follow the periodicity of the pulsed DC power supply at high frequencies.

Dynamic simulations of pulsed reactive sputtering processes

In the reactive sputtering process numerous parameters interact in a complex and nonlinear way. Thus, if any parameter is changed during operation, it is not an easy matter to predict the response of the other involved parameters. The static behavior of the reactive sputtering process has previously been carefully investigated. However, studies of the time dependent behavior of this process are rare. It is important to study the dynamic behavior to gain a more complete understanding of the process. Furthermore, the increased use of pulsed power sources has set focus on the process dynamics. A model describing the time-dependent behavior of the pulsed reactive sputtering process is proposed in this work. The model suggests that the time to reach process equilibrium may vary substantially depending on the process history. This memory effect has been experimentally verified. Finally, the simulations clearly point out that for high repetition frequencies (&amp;gt;1 kHz) of the pulsed direct current (dc) reactive sputtering process there will be no time-dependent variations of the chemical compositions either at the target surface or at the growing film at the substrates. At these high frequencies the chemistry of the process will act identical as to the continuous dc reactive sputtering process having the average pulsed dc power supplied to the target.

Using pulsed direct current power for reactive sputtering of Al2O3

Application of steady direct current (DC) power to the reactive sputtering of dielectrics such as Al2O3 is seriously hampered by arcing. This arcing can be alleviated to a great extent when pulsed dc power is applied. By pulsed dc power we mean that the power is applied for a short “on” period, and then removed for a short “off” period. During the “off” period the plasma can discharge the surfaces, provided certain conditions are met. The dependence of adequate discharging, and thus arc prevention, on the duration of the “on” and “off” periods is examined. In addition, the dynamics of plasma density loss in the “off” period and its reestablishment in the initial part of the “on” period are discussed. Reactive sputtering takes place only during the “on” period, and part of this period is lost for effective sputtering due to the time required for full plasma reestablishment. This produces a dependence of the deposition rate on both the duty cycle and the frequency of pulsing, but not on the power consumed. This dependence is examined and the power efficiencies of alternating current and pulsed power dc reactive sputtering are compared and the differences between single and dual cathode systems in published results is discussed.

High-rate reactive DC magnetron sputtering of oxide and nitride superlattice coatings

Over the past 10 years, there have been three major advancements in reactive sputtering technology that now make it possible to deposit both conductive and non-conductive fully-dense films at high rates. These three advances are unbalanced magnetron sputtering, partial pressure control of the reactive gas, and pulsed dc power. Multicathode unbalanced magnetron sputtering systems provide a dense secondary plasma that is used for producing a well-adhered, fully dense film that is difficult to achieve with conventional magnetron sputtering. Online automatic partial pressure control of the reactive gas prevents the poisoning of the target surface during deposition, which leads to compound film deposition rates that approach or are equal to those for the pure metal rate. Pulsed dc power, where the polarity of the voltage on the sputtering target is alternately switched briefly between negative and positive, prevents arcing on the target surface during the deposition of nonconducting films. With both pulsed dc power and partial pressure control of the reactive gas, films such as aluminum oxide can now be deposited reactively at rates up to 78% of the pure metal rate. The reactive unbalanced magnetron sputtering process is used to deposit polycrystalline nitride superlattice films such as TiN/NbN or TiN/VN with hardnesses exceeding 50 GPa, which is more than double the hardness of either component in the multilayered film. The nitride superlattice work is being extended to oxide films, and initial results are encouraging. Nanometer scale, multilayer Al 2O 3/ZrO 2 and Y 2O 3/ZrO 2 films have been deposited at high rates. The Al 2O 3/ZrO 2 films are amorphous and optically clear, whereas the Y 2O 3/ZrO 2 films are crystalline as well as being optically clear.

Morphology of industrial diamond powders : F.Marian et al. (DACIA Synthetic Diamond Factory, Romania.)

A286 (AISI 660), an austenitic precipitation hardening stainless steel has good strength and corrosion resistance, but its low hardness restricts their use in applications involving abrasive wear. In order to improve the hardness without loss of corrosion resistance properties of A286 stainless steel, plasma nitrocarburizing process was carried out at temperatures ranging from 350 °C to 500 °C for 4 and 8 h. The gas mixture composition employed during the process was 78% nitrogen, 20% hydrogen and 2% acetylene gas. Plasma was generated using a pulse DC power supply having a frequency of 30 kHz.After plasma nitrocarburizing, a dual layer was formed consisting of nitrogen and carbon in the top layer followed by carbon in the bottom layer. Compared with the untreated A286 stainless steel specimens, the microhardness values measured from the plasma nitrocarburized specimens treated at 400 and 450 °C increased by a factor of two and three respectively. The electrochemical corrosion behavior of the plasma nitrocarburized A286 specimens has been studied in 3.5 wt.% NaCl solution. The results show that specimens treated at 400 °C for 4 h has better pitting corrosion resistance and higher surface hardness compared with that of the untreated A286 stainless steel. This is mainly attributed to the presence of carbon and nitrogen in the modified layer existing as a solid solution.

Cubic boron nitride thin film deposition by unbalanced magnetron sputtering and dc pulsed substrate biasing

Boron nitride (BN) thin films were deposited by unbalanced magnetron sputtering. An asymmetric bipolar pulsed dc power supply was used to bias the substrate with a 218 kHz signal. BN was deposited at different negative bias voltages and films with over 80% of the zinc-blende cubic phase, as measured by Fourier transform infrared (FTIR) spectroscopy, were obtained. For a given set of deposition conditions a window of maximum negative bias voltage was defined inside which the cubic phase is stabilized. This negative bias voltage was used to derive the momentum imparted to the growing film per arriving boron atom, and it is shown that a threshold value for the formation of the cubic phase (cBN) is consistent with previous results using ion beam assisted deposition. Several films were deposited on aluminum nitride and diamond thin films previously deposited on crystalline silicon in order to study the effect of the substrate on the stabilization of the cBN phase. A twofold reduction in the full width at half maximum (FWHM) of the 1060 cm−1 FTIR peak is obtained for films deposited on diamond, which is indicative of better ordering of the B–N cubic bonding. At high negative bias voltages the FWHM increases toward the value for BN films deposited onto crystalline silicon which is expected since the greater energetic bombardment produces excessive damage to the film.

Asymmetric bipolar pulsed power: a new power technology

There is increasing commercial interest in processes involving reactive PVD deposition of insulating films, including deposition of wear resistant coatings; insulating films for microcircuits (including devices such as thin films heads) or electronic devices such as capacitors; sophisticated architectural glass coatings; coatings on polyester film for architectural glass laminates or oxygen barriers for food packaging; heat reflecting coatings for high efficiency lamps or induction furnace heat shields; deposition of barrier and functional layers for flat panel displays, including the ITO glass used in LCD displays; and myriad other similar functional and decorative applications. Reactive sputtering using single cathodes and pulsed dc power has been used, and satisfactory results obtained. An intrinsic problem with single targets, however, is related to the coating of the anode with the deposited insulator, a problem solved by using dual sources. Dual target systems can use sinusoidal power, but increased flexibility is obtained using pulsed power and multiple cathodes. This article describes the power supply technology required for pulsed multiple cathode sputtering, and describes a new pulsed power supply which has been developed for dual cathode sputtering.

TiAlN and TiAlCN deposition in an industrial PaCVD-plant

An industrial PaCVD-plant was equipped with an AlCl 3-generator. By using Ar, H 2, N 2, CH 4, TiCl 4 and AlCl 3, TiAlN- and TiAlCN-films could be deposited on hard metal and steel substrates. The plasma was generated by a DC-pulse power supply with frequencies up to 50 kHz. The reactor size was 350 mm in diameter and 900 mm in height. During one batch 1200 indexable inserts could be coated. The growth rates were about 1–3 μm h −1. The deposited films show a fine structure and Cl-concentrations below 3%. The measured critical loads were between 30 and 40 N. Wear test results show an increase in tool life up to several 100% compared with uncoated or TiN-coated tools.

Very-high-rate reactive sputtering of alumina hard coatings

A very-high-rate reactive magnetron sputtering deposition process for alumina hard coatings at substrate temperatures ≤250°C has been developed. Utilizing pulsed DC power to sputter Al+AlO x off the target surface and partial pressure control of the reactive gas to maintain a certain partial pressure value (accuracy of better than 0.005 mtorr), fully dense, transparent alumina coatings could be produced at 76% of the metal deposition rate. The coatings have an elastic modulus of 140 GPa, a hardness of 12 GPa, a chemical composition close to stoichiometric, and a refractive index of 1.65. The influence of the O 2 partial pressure on the deposition rate, optical and mechanical properties is discussed.