This paper reviews the contribution made to the observation and understanding of the basic physical processes occurring in an important type of magnetized low-pressure plasma discharge, the pulsed magnetron.In industry, these plasma sources are operated typically in reactive mode where a cathode is sputtered in the presence of both chemically reactive and noble gases typically with the power modulated in the mid-frequency (5–350 kHz) range. In this review, we concentrate mostly, however, on physics-based studies carried out on magnetron systems operated in argon. This simplifies the physical–chemical processes occurring and makes interpretation of the observations somewhat easier.Since their first recorded use in 1993 there have been more than 300 peer-reviewed paper publications concerned with pulsed magnetrons, dealing wholly or in part with fundamental observations and basic studies. The fundamentals of these plasmas and the relationship between the plasma parameters and thin film quality regularly have whole sessions at international conferences devoted to them; however, since many different types of magnetron geometries have been used worldwide with different operating parameters the important results are often difficult to tease out. For example, we find the detailed observations of the plasma parameter (particle density and temperature) evolution from experiment to experiment are at best difficult to compare and at worst contradictory.We review in turn five major areas of studies which are addressed in the literature and try to draw out the major results. These areas are: fast electron generation, bulk plasma heating, short and long-term plasma parameter rise and decay rates, plasma potential modulation and transient phenomena. The influence of these phenomena on the ion energy and ion energy flux at the substrate is discussed. This review, although not exhaustive, will serve as a useful guide for more in-depth investigations using the referenced literature and also hopefully as an inspiration for future studies.
Mass and energy spectra of negative ions in magnetron sputtering discharges have been investigated with an energy-dispersive mass spectrometer. The dc magnetrons have been operated in the same reactive Ar/O2 atmosphere but with three different target materials: Cu, In, and W. Besides negative ions of the working gas, a variety of target metal containing negative molecular ions were found in the discharge. Their occurrence is strongly dependent on the target material. It has been correlated to the electron affinity and the bond strength of the molecules which has been calculated by density functional theory. Energy spectra of the negative ions exhibit three contributions that are clearly distinguishable. Their different origin is discussed as electron attachment in the gas phase and at the target surface, and molecule fragmentation during transport from target to substrate. The latter two contributions again significantly deviate for different target material. The high-energy part of the spectra has been analyzed with respect to the energy the particles gain upon release from the surface. It suggests that bigger molecules formed on the surface are released by ion-assisted desorption.
T h e o p e n -a c c e s s j o u r n a l f o r p h y s i c s Abstract. Time-resolved emissive probe measurements have been performed to study the spatio-temporal development of the plasma potential in an asymmetric bipolar pulsed magnetron discharge. The influence of the substrate potential as well as of the substrate position has been investigated while the further conditions were the same. To access the entire potential range which was between −100 V and + 400 V and to obtain sufficient time-resolution of the emissive probe, different heating currents had to be used. The plasma potential has been found to be typically close to zero in the 'on' phase, about + 40 V in the stable 'off' phase and up to + 400 V at the beginning of the 'off' phase, which is in agreement with the results of other authors. However, the positive values in the 'off' phase are generally lower than those reported and stay mostly below the target potential. This is explained by macroscopic considerations of the quasineutrality of the plasma taking into account a magnetic and geometrical shielding of the target, acting as an anode in the 'off' phase, and the potential and position of the substrate holder and environment. New Journal of Physics
Langmuir probes are important means for the characterization of plasma discharges. For measurements in plasmas used for the deposition of thin films, the Langmuir double probe is especially suited. With the increasing popularity of pulsed deposition discharges, there is also an increasing need for time-resolved characterization methods. For Langmuir probes, several single-probe approaches to time-resolved measurements are reported but very few for the double probe. We present a time-resolved Langmuir double-probe technique, which is applied to a pulsed magnetron discharge at several 100 kHz used for MgO deposition. The investigations show that a proper treatment of the current measurement is necessary to obtain reliable results. In doing so, a characteristic time dependence of the charge-carrier density during the “pulse on” time containing maximum values of almost 2∙1011cm−3 was found. This characteristic time dependence varies with the pulse frequency and the duty cycle. A similar time dependence of the electron temperature is only observed when the probe is placed near the magnesium target.
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