Downstream plasma transport and metal ionization in a high-powered pulsed-plasma magnetron

摘要

Downstream plasma transport and ionization processes in a high-powered pulsed-plasma magnetron were studied. The temporal evolution and spatial distribution of electron density (n_e) and temperature (T_e) were characterized with a 3D scanning triple Langmuir probe. Plasma expanded from the racetrack region into the downstream region, where a high n_e peak was formed some time into the pulse-off period. The expansion speed and directionality towards the substrate increased with a stronger magnetic field (B), largely as a consequence of a larger potential drop in the bulk plasma region during a relatively slower sheath formation. The fraction of Cu ions in the deposition flux was measured on the substrate using a gridded energy analyzer. It increased with higher pulse voltage. With increased B field from 200 to 800 Gauss above racetrack, n_e increased but the Cu ion fraction decreased from 42% to 16%. A comprehensive model was built, including the diffusion of as-sputtered Cu flux, the Cu ionization in the entire plasma region using the mapped n_e and T_e data, and ion extraction efficiency based on the measured plasma potential (V_p) distribution. The calculations matched the measurements and indicated the main causes of lower Cu ion fractions in stronger B fields to be the lower T_e and inefficient ion extraction in a larger pre-sheath potential.