Investigation of a pulsed magnetron sputtering discharge with a vacuum pentode modulator power supply

Abstract

A pulsed magnetron sputtering discharge at different modes of discharge ignition and a large range of duty cycles and pulse rates from single pulses to 50 kHz was investigated. A 2 kV vacuum pentode modulator was used for the generation of current pulses with amplitudes up to 2 A and target power densities up to 145 W/ cm 2 . Ignition of a pulsed discharge occurs with some delay, but still at the rise of the voltage pulse, if in the inter-electrode gap either a starting-up discharge with a current of some tens of milliamperes is sustained or if the residual concentration of charged particles after a previous pulse is still high enough. A spike of electron current to the substrate with energies up to 100 eV is generated at the current front of the high-current discharge when the voltage drops to its equilibrium value. These electrons are presumably secondary electrons. The effective cathode secondary emission and gas ionization take place at the current front when the voltage drop at the gap is higher than the equilibrium value. Therefore, the duration of the current front lasts only some microseconds and an excess concentration of charged particles is generated in the gap. During the high-current discharge stage the plasma expands with the velocity of ambipolar diffusion from the near-target region. The ion current to substrate reaches its maximum value after some tens of microseconds. For this reason, the pulse duration should not be lower than 20– 40 μs for ion-assisted deposition. In the afterglow period deionization of the post-discharge plasma takes place. Due to the surplus ion conductivity of the plasma the starting-up discharge is suppressed. With delay in self-restoring the starting-up discharge is some tens of microseconds. This time determines the duration of maximum pauses between the control pulses without the starting-up discharge. Thus, work in the middle-frequency range of pulse rates allows to exclude a starting-up discharge and to obtain the lowest delay for establishing the magnetron discharge at each of the following pulses.