Papers

ICMCTF2003 Session TS1: Principles of Pulsed Plasmas

Wednesday, April 30, 2003 8:30 AM in Room Sunrise

Wednesday Morning

Start	Invited?	Item
8:30 AM		TS1-1 Fundamentals of Pulsed Plasmas for Materials Processing A. Anders (Lawrence Berkeley National Laboratory) Pulsed plasmas offer a wider range of tunable parameters compared to continuously operated plasmas, and additionally new parameters appear such as pulse duty cycle. Therefore, new possibilities have arisen, helping to meet the demands of increasingly sophisticated materials processes, including thin film deposition, plasma etching, plasma cleaning of surfaces, and plasma immersion ion implantation. Pulsed plasmas are driven by external pulsed power sources, and one has to consider the power source and the plasma as a coupled system. The dynamic plasma impedance is a key quantity from an electrical engineering point of view. From a plasma physics point of view, one needs to consider the dynamics of plasma species, their density and energy distributions, ionization and recombination reactions, and, most importantly, the development of plasma sheaths, i.e. space charge layers that separate the quasi-neutral bulk plasma from biased electrodes or substrates. A very useful tool is the introduction of dimensionless scaling parameters which put the variety of plasma parameters in relation to characteristic quantities. This concept will be illustrated by a number of examples relevant to thin film deposition.
9:10 AM		TS1-3 Influence of Pulse Parameters on Plasma Properties and MgO Film Structure F. Richter, Yuhang Cheng, T. Welzel, T. Dunger, H. Kupfer (Chemnitz University of Technology, Germany) A pulsed mid frequency magnetron sputtering technique was used to deposit MgO thin films from a Mg target in an O₂/Ar atmosphere. The dependence of the plasma properties, film structure, and secondary electron emission properties on the pulse frequency (100-350 kHz) and duty cycle (50-90%) was systematically studied. The plasma properties were measured by using a Langmuir double probe. The crystalline structure and secondary electron emission (γ) coefficient of the films were characterised by using x-ray diffraction and a diode discharge devices, respectively. It was found that all films exhibit a strong (220) preferred orientation growth. As the pulse frequency is increased from 100 to 350 kHz, the time averaged electron temperature (T_e) and electron density (n_e) decreases and increases continuously, respectively. The intensity ratio of the (200) to (220) XRD peaks and γ coefficient increase gradually, reaching a maximum at a frequency of 250 kHz, then decrease rapidly. The increase of n_e and decrease of T_e with increasing pulse frequency causes an increase of the ion density and an decrease in the self substrate bias. The competition of these two mechanisms contributes to the change in the film structure with pulse frequency. The increase in the duty cycle results in a continuous increase in the n_e and defect density in the films, but a decrease in T_e, grain size, and γ coefficient. Suppression of the arcing events with decreasing duty cycle could correspond to the change of film structure. It should also be mentioned that the decrease of the duty cycle leads to an increase of the power in the "pulse on" period as the power supply was operated at a constant power mode, which may also contribute to an increase in the grain size and the decrease in the defect density in the films. The variation of the preferred orientation and the defect density in the films are the main reasons for the change of the γ coefficient with pulse frequency.
9:30 AM		TS1-4 Increasing the AlN Amount in Magnetron Sputtered Cr_1-xAl_xN PVD Coatings for High Temperature Applications by Means of Pulsed Power Supplies E. Lugscheider, K. Bobzin, M. Maes (Materials Science Institute, Aachen University, Germany) Chromium nitride coatings are frequently used in metal forming applications. Their hardness in comparison to other hard coatings like TiN and TiAlN coatings is considerably lower. The comparatively low hardness of this PVD coating, is favoured in many applications, where a high ductility is demanded. Another major benefit of chromium based coatings are the anti-adhesive characteristics. However the desire for more high temperature stable coatings is the basis for this research. In order to increase the high temperature stability of chromium nitride coatings, aluminum nitride is added to form Cr_1-xAl_xN coatings. Recent investigations show that an increased aluminium nitride content leads to an improved temperature stability of Cr_1-xAl_xN coatings. However, the deposition of aluminum nitride by metallic targets remains difficult, since an insulating layer is formed at the target during deposition. Former papers dealt with these insulating layers by sputtering the aluminium nitride in a R.F. mode. Although this is a very effective way to sputter non conductive PVD coatings. The sputter efficiency in terms of sputter rates are moderate. To increase sputter rates new technologies, based on bipolar power supplies, offer an alternative, in case of sputtering non conductive materials. Objective of this work is to look at the affect of the pulse sequence for reactively sputtered Cr_1-xAl_xN coatings. After varying the pulse sequence the coatings were then investigated with regards to their mechanical- and thermal properties as well as their sputter rates.
9:50 AM		TS1-5 Discharge Characterisation during Plasma Electrolytic Oxidation of Aluminium A.L. Yerokhin (University of Hull, United Kingdom); L.O. Snizhko (Ukrainian State University for Chemical Technology, Ukraine); A. Leyland (The University of Sheffield, United Kingdom); N.L. Gurevina (Ukrainian State University for Chemical Technology, Ukraine); A. Pilkington (University of Hull, United Kingdom); A. Matthews (The University of Sheffield, United Kingdom) Plasma electrolytic oxidation (PEO) is a process of plasma-assisted electrochemical conversion of metal surfaces to produce layers of oxide ceramics. The plasma discharge during PEO occurs at the metal/electrolyte interface when the applied voltage exceeds a certain critical breakdown value (typically several hundreds of Volts) and appears as a number of discrete short-lived microdischarges moving across the metal surface. Thermal and chemical effects of the microdischarges onto the metal surface play an important role in the formation of phase, structural and stress states of the ceramic layers, determining their in-service performances. Major recent developments in the PEO technology have been aimed therefore at improving controllability of the plasma microdischarges by applying pulsed DC or AC biasing to the substrate. At the same time, a lack of knowledge about the microdischarge characteristics under such transient polarisation conditions allows neither simulation of the layer formation processes nor justification of the treatment regimes. In this work, the characteristics of microdischarges on the surface of Al during AC PEO treatment were studied by digital video imaging. The surface appearance was captured throughout the process with a ten-fold magnification at 24 Hz frequency. Obtained images of the surface microdischarges were then analysed using Scion Image software. As a result of the analysis, the following microdischarge characteristics have been evaluated for various stages of the surface layer formation: (i) the fraction of surface area being under the plasma discharge; (ii) microdischarge density and dimensional distribution; (iii) current density, fused volume and temperature fields developed in a single microdischarge. The results are compared with earlier estimations made for DC regimes of PEO, and the conditions of providing uniform microdischarge characteristics throughout the process are discussed.
10:10 AM		TS1-6 Spatial and Temporal Ion Distribution in High Power Pulsed Magnetron Sputtering J. Böhlmark, J. Alami, U. Helmersson (Linköping University, Sweden); A.P. Ehiasarian, W.-D. Münz (Sheffield Hallam University, United Kingdom) The spatial and temporal ion and neutral distribution in a high power pulsed magnetron discharge in an argon environment were investigated using a flat probe and optical emission and mass spectrometry. The high power pulses, with a peak value of 1-2 MW, were applied with a repetition frequency of 50 Hz resulting in a low duty factor of about 0,5 percent and an average power density at the target of about 10 Wcm^-2 as compared to a peak value of 2-3 kWcm^-2 in the pulse. The temporal particle distribution show a very interesting behavior with Ar ions and neutrals dominating the first part of the pulse, while later in the pulse it is a transition to metal ions and neutrals which in the end of the pulse totally dominate the plasma. The detail distribution is demonstrated as a function of pulse power, pulse shape, and sputtering gas pressure and composition.
10:30 AM		TS1-7 Influence of O₂ Flow Rate on the Structure and Properties of MgO Films Deposited by Dual Magnetron Sputtering Yuhang Cheng, H. Kupfer (Chemnitz University of Technology, Germany); U. Krause, T. Kopte, C. Peters (Fraunhofer-Institute für Elektronenstrahl- und Plasmatechnik (FEP), Germany); F. Richter (Chemnitz University of Technology, Germany) MgO films were deposited by a dual magnetron sputtering system with 400 mm target length using a sine wave power supply. A high-purity magnesium target was sputtered in a mixture of Ar and O₂ gas. Atomic force microscopy, x-ray diffraction, Ellipsometer, and a diode discharge device were used to characterize surface morphology, crystalline structure, optical properties, and secondary electron emission (g) coefficient of the films, respectively. The influence of O₂ flow rate with a constant Ar flow rate of 100 sccm on the structure and properties of the films were systematically studied. The increase of the O₂ flow rate from 5 to 14 sccm results in a significant decrease in the RMS surface roughness, the full width at half maximum of the (111) XRD peak and the g coefficient of the films, as well as a large increase in the intensity ratio of the (111) peak to (200) peak, which leads to the change in the growth texture of the film from a mixed (111) and (200) preferred orientation to a strong (111) preferred orientation. Further increase in the O₂ flow rate results in no significant change in the structure and properties of the MgO films. The changes in the preferred orientation and crystalline quality contribute to the evolution of the g coefficient with O₂ flow rate. In addition, similar deposition experiments have been done in another dual magnetron sputtering system with bigger magnetrons (610 mm target length) as well as with a unipolar pulsed magnetron of circular shape 150 mm in diameter. The results will be compared and conclusions be drawn about analogies and peculiarities of the processes under investigation.
10:50 AM		TS1-8 Concurrent Deposition and Time-resolved Plasma Diagnostics of Titanium-based Films Produced by Pulsed Magnetron Sputtering P.J. Kelly (University of Salford, United Kingdom); J.W. Bradley (UMIST, United Kingdom); P.S. Henderson (University of Salford, United Kingdom); H. Baecker (UMIST, United Kingdom); R. D. Arnell (University of Salford, United Kingdom) In the magnetron sputtering field, it is well-established that mid-frequency (20-350 kHz) pulsed processing offers many advantages over continuous DC processing for the reactive deposition of dielectric films. The correct selection of pulse parameters (frequency, duty, reverse voltage) can result in extended arc-free operating conditions, even during the deposition of highly insulating materials. Consequently, the pulsed sputtering technique is now being exploited commercially in multiple magnetron systems for many applications, including solar control and low-emissivity coatings, barrier layers on packaging, flat panel displays and solar cells. Despite this, to date, the interrelationships between the driving voltage waveform and pulsed plasma parameters, and their collective impact on film properties are not well understood. In this study, therefore, titanium and titanium dioxide films have been deposited by asymmetric bipolar pulsed sputtering. During the deposition process, time-resolved Langmuir probe measurements were taken and the driving voltage waveforms were recorded. The coatings were then characterised in terms of their structures and properties using a range of analytical and measurement techniques, including scanning electron microscopy, electron probe microanalysis, X-ray diffraction, micro-hardness testing, scratch adhesion testing, wear testing and surface profilometry. The optical properties of the TiO₂ films were also investigated. The variations in these properties with pulse frequency are reported and attempts are made to account for these variations in terms of the waveform dependant modulation of the plasma parameters during each pulse cycle.
11:10 AM		TS1-9 Interrelation of the ITO Film Properties with Pulsed Dual Magnetron Deposition Parameters A. Kolitsch, A. Rogozin, M. Vinnichenko, W. Moeller (Forschungszentrum Rossendorf, Germany) Middle frequency dual reactive magnetron sputter deposition was used in order to prepare the Indium tin oxide (ITO) films with an optimum of resistance and transmittance. The films were deposited on Si covered with 500 nm SiO₂ and soda lime glass substrates. The plasma ions and neutrals flow parameters have been studied during the deposition process by the Langmuir probe, HIDEN analyzer, and optical emission spectroscopy. Complex characterization of the ITO films have been carried out in order to relate their properties with pulsed magnetron operation parameters. The optical constants and the thickness of these films were determined by means of spectroscopic ellipsometry (SE) operating within the wavelength range l=300-1700 nm. The optical transmittance was measured for l=350-2000 nm. Four probe technique was used to determine the film specific resistance. The dependence of the ITO film properties on the O₂/Ar partial pressure relation was obtained similarly to known researches. The influence of the pulse duration and base pressure level on the film characteristics was detected. The decrease of the magnetron pulse on-time to the values of about 50 micro seconds, leads to strong increase of the films resistance at their high transmittance. The AFM data for these samples demonstrated five-times increase of the film grain sizes compared to those obtained at the on-time values 100 micro seconds. The observed decrease of optical transmittance in the near IR spectral range corresponds to the decrease of the film resistance. High optical transmittance at low resistance is found only at certain relation between the band gap values, and free electron plasma and relaxation frequency. The grading of optical constants was revealed by SE while the other techniques did not show depth grading of the film properties. Annealing of the ITO films in air atmosphere reduces the film resistance.
11:30 AM		TS1-10 The Industrial Application of Pulsed DC Bias Power Supplies in Closed Field Unbalanced Magnetron Sputter Ion Plating K.E. Cooke, J. Hamsphire, W. Southall, D.G. Teer (Teer Coatings Ltd, United Kingdom) The advantages of using pulsed DC power supplies in the reactive magnetron deposition of dielectric coatings from conductive or partially conductive sputtering targets are well recognised, particularly in terms of the elimination of arcing and enhanced process stability. Less attention has been paid, however, to the application of pulsed DC power to the biasing of substrates in ion plating processes. This paper draws on our seven years of practical experience of using a range of pulsed DC bias supplies in a wide variety of industrial coating equipment exploiting closed field unbalanced magnetron sputtering technology. The effect of pulsed plasma characteristics on surface cleaning, interface formation and the coating of components is investigated, including the appropriate selection of parameters such as voltage, frequency, pulse width and pressure. The effectiveness of such pre-cleaning is evaluated, for example, by the subsequent scratch adhesion testing of the coatings. The synergistic interaction of the bias with DC and pulsed DC closed field magnetron sources is explored. Practical considerations, such as the processing of thermally sensitive materials and the preservation of critical surface characteristics, are also discussed.
11:50 AM		TS1-11 Alumina Deposition with Pulsed Plasma in CFUBMS S.L. Carrera, J.J. Moore, G.G.W. Mustoe (Colorado School of Mines) Asymmetric bipolar pulsed DC-magnetron sputtering is a thin film deposition method commonly employed for industrial coating of large areas with dielectric materials. DC-magnetron sputtering can be used to deposit oxides, however pulsing the plasma stabilizes the deposition process by reducing the amount of arcing in the process. Frequencies and duty cycles are pulsing parameters that can be adjusted to explore the effect on the microstructure and mechanical properties of the film. Also, manipulation of the substrate bias can potentially significantly alter the microstructure of the alumina film. This study aims to adjust these parameters in a closed-field unbalanced magnetron sputtering (CFUBMS) environment and ultimately optimize the film. Ion bombardment of growing thin films is a useful processing parameter often used in reactive sputtering and other plasma processes. A Hiden electrostatic quadrupole mass spectrometer and energy analyzer was used to measure the effect of frequency, duty cycle and bias on the ion energy at the substrate, which correlates well to the target voltage waveforms. Thin film materials were characterized using standard techniques.
12:10 PM		TS1-12 High-power Pulsed dc Magnetron Discharges for Ionized High-rate Sputtering J. Vlcek, J. Lestina, A.D. Pajdarova, P. Belsky, M. Kormunda, J. Musil (University of West Bohemia, Czech Republic) High-power pulsed dc magnetron discharges for ionized high-rate sputtering of thin films were systematically investigated. The depositions were performed using an unbalanced circular magnetron with a copper target of 100 mm in diameter at a fixed argon pressure p = 1 Pa. The magnetron was driven by a pulsed dc power supply working in the frequency range from 0.5 to 50 kHz with a maximum pulse voltage and a maximum pulse current of 1 kV and 120 A, respectively. We report on time-resolved optical emission spectroscopy, and time- and energy-resolved mass spectroscopy of the discharge plasma generated by a magnetron with a standard planar Cu target at a repetition frequency of the pulses f_r = 1 kHz, a length of the voltage pulse of 200 µs (a 20 % duty cycle) and at preset values of the average pulse current I_da in the range from 5 to 50 A. The deposition rate of Cu films formed on a grounded substrate was higher than 2µm/min at a maximum average target power loading in a pulse of 680 W/cm². It was found that a quick rise in the target power loading during a pulse leads to intensive sputtering of the Cu target and to very effective ionization of sputtered Cu atoms in the discharge. In addition, the density of Ar atoms decreases at high values of the target power loading due to rarefaction of argon gas. As a result, Cu⁺ ions become strongly dominant in total ion fluxes on the substrate after an initial phase of the high-power pulses investigated. It is also shown that the same magnetron equipped with a grooved Cu target makes it possible to significantly decrease the magnetron voltage. Then the ionized high-rate sputtering, requiring a high average target power loading in a pulse, can be realized using the grooved Cu target at the repetition frequencies in the range from 10 to 50 kHz. Let us note that the needed high I_da values cannot be achieved with the use of a standard planar Cu target at these frequencies.