Influence of high power densities on the composition of pulsed magnetron plasmas

The application of high power pulses with peak voltage of −2 kV and peak power density of 3 kW cm−2 to magnetron plasma sources is a new development in sputtering technology. The high power is applied to ordinary magnetron cathodes in pulses with short duration of typically some tens of microseconds in order to avoid a glow-to-arc transition. High plasma densities are obtained which have been predicted to initiate self-sputtering. This study concerns Cr and Ti cathodes and presents evidence of multiply charged metal ions as well as of Ar ions in the dense plasma region of the high power pulsed magnetron discharge and a substantially increased metal ion production compared to continuous magnetron sputtering. The average degree of ionisation of the Cr metal deposition flux generated in the plasma source was 30% at a distance of 50 cm. Deposition rates were maintained comparable to conventional magnetron sputtering due to the low pressure of operation of the pulsed discharge—typically 0.4 Pa (3 mTorr) of Ar pressure was used.

Observations of the current–voltage characteristics of the discharge confirmed two modes of operation of the plasma source representing conventional pulsed sputtering at low powers (0.2 kW cm−2) and pulsed self-sputtering at higher powers (3 kW cm−2). The optical emission from the various species in the plasma showed an increase in metal ion-to-neutral ratio with increasing power. The time evolution within a pulse of the optical emission from Ar0, Cr0, Cr1+, and Cr2+ showed that at low powers Cr and Ar excitation develops simultaneously. However, at higher powers a distinct transition from Ar to Cr plasma within the duration of the pulse was observed. The time evolution of the discharge at higher powers is discussed.