Observations of the long-term plasma evolution in a pulsed dc magnetron discharge

Abstract

Using a time-resolved Langmuir probe the temporal evolution of the plasma parameters in a pulsed dc magnetron discharge was determined for a number of positions both inside and outside the magnetic trap. The discharge was operated at a pulse frequency of 2 kHz with a 50% duty cycle, a titanium target and at a fixed argon pressure of 0.53 Pa.The electron density, n e , electron temperature, T e , plasma potential, V p and floating potential, V f have typically two-fold characteristic decay and rise times, which increase with distance from the target. In the magnetic trap, the initial n e decay rate is ∼18 µs, lengthening to 120 µs. However, in the plasma bulk, this figure is about 250 µs at all times during the plasma decay. The n e decay rate on the discharge centreline is somewhat slower at about 600 µs. During the transition from duty on to off phases a 'density-wave', initiated at the cathode, is observed to propagate downstream at about 1000 m s −1 (a velocity close to the calculated ion acoustic speed).The results reveal a complex picture in terms of the electron energies. In the off time, the electrons are characterized by a single Maxwellian distribution function and the initial T e decay times are between 6 and 10 µs. However, these lengthen dramatically to values between 600 and 800 µs depending on position to give a final T e value of about 0.2-0.3 eV. In the on time, and outside the magnetic trap, bi-Maxwellian electrons are observed with T eh = 4-6 eV and T ec = 1-2 eV. A simple model, taking into account the effect of the magnetic field on electron collection at the probe, has been used to show that the relative density of hot electrons is as high as 40%. On magnetic field lines that connect with the walls, bi-Maxwellian electrons exist during the whole on period; however, on the centreline their temperatures converge after about 100 µs to form a single distribution with T e ∼ 3 eV.Plasma potential measurements show that the strong axial electric fields in the plasma in the on time totally collapse in the off time. The general structure of the plasma and possible generation mechanisms for the two electron temperature groups is briefly discussed.

Plasma parameters of pulsed-dc discharges in methane used to deposit diamondlike carbon films

“…Sheridan et al 51 reported them for dc discharges in helium and Godyak et al 52 analyzed different non-Maxwellian plasmas with probe diagnostics. In the field of pulsed plasmas, Bäcker and Bradley 37 thoroughly investigated the effects of incorporating magnetrons into the plasma parameters, such as, for instance, the generation of bi-Maxwellian discharges induced by the presence of a magnetic field.…”

“…Bäcker and Bradley 37 supposed that secondary electrons from the cathode were firmly pushed into the bulk of the plasma by the contracting sheath, leading to an increase in T e . Using Langmuir probe diagnostics, Glocker showed that ac discharges resulted in electrons that were more energetic than in dc-powered plasmas.…”

“…Much progress has been reported concerning time-resolved tracking of electron temperature, plasma density, and potentials in pulsed plasmas. [36][37][38] Such analysis increases our understanding of the plasma kinetics in discharges with time-varying patterns and involving a huge variety of gases. Hydrocarbons are the main precursors for the synthesis of DLC films: propane ͑C 3 H 8 ͒, 39 benzene ͑C 6 H 6 ͒, 40 acetylene ͑C 2 H 2 ͒, and CH 4 are of particular importance.…”

Electromagnetic Boundary Layer Flow Control Facility Development Using Conductive Particle Seeding

“…Recent research involving ionization with high voltage fields 24,25 or magnetrons 26 has shown that pulsed electrical discharges may be superior to a DC discharge. Unlike DC, the pulsed discharges do not permit charge to build up on the surfaces of the electrodes which is a precursor to arcing.…”

Investigation of the time evolution of plasma parameters in a pulsed magnetron discharge

“…It is caused by small ratio of working time and time of periode t w /T . The repetition frequencies in the order of kHz (with duty cycles about 50 %), so-called mid-frequency discharge, were used in many cases with different types of target materials (Ti targets e. g. in [11]). The measured electron density reached n e ≈ 10 16 m −3 in almost all mentioned works.…”