Direct measurement and modeling of the redirected ion flux in a high-powered pulsed-plasma magnetron

Self-sputtering is a crucial feature in high-powered pulsed magnetron sputtering (HPPMS). A direct measurement of the recirculating ion fluxes to the target, however, has not been made until now using a specially designed magnetron system. A small orifice was drilled in the target, allowing plasma fluxes to penetrate and be diagnosed subsequently. Ion currents of the penetrating copper ions (Cu+) and argon ions (Ar+) were collected on biased grids, while Cu depositions were measured on witness Si wafers. Based on these measurements, fluxes of Cu+ ions and Ar+ ions were differentiated. For a tested condition, the ratio of Cu+ density to Ar+ density was determined to be 1.560.3, indicating a strong self-sputtering effect during HPPMS. Using a semiempirical plasma model, this ratio was predicted to be 1.4 within plasma, matching well with the measurement. The model calculates the evolution of various plasma species in the strong ionization region and thus allows a quick estimation of some key HPPMS parameters such as Cu+ ionization fraction and Cu+ to Ar+ density ratio in a time-resolved manner. The ion currents were observed to increase abruptly after a certain time delay, longer for a lower pulse voltage. This suggests a mechanism that the plasma is only ignited initially in a stripe along the sputtering "racetrack" where the magnetic field (B) is strong enough. At a higher pulse voltage, the ignition plasma stripe became longer and drifted faster parallel to the target toward the region of weak magnetic field. (C) 2015 American Vacuum Society.