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, ne, electron temperature, Te, plasma potential, Vp and floating potential, Vf have typically two-fold characteristic decay and rise times, which increase with distance from the target. In the magnetic trap, the initial ne 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 ne 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 Te 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 Te value of about 0.2–0.3 eV. In the on time, and outside the magnetic trap, bi-Maxwellian electrons are observed with Teh = 4–6 eV and Tec = 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 Te ∼ 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.