The current-voltage characteristic of magnetron sputtering systems

Abstract

The I-V characteristic of dc and rf magnetron sputtering systems has been shown to fit an expression of the form I=β(V−V0)2, where V0 is the minimum voltage necessary to maintain a discharge. New experimental data is presented for a dc planar magnetron with Al, Cd2Sn, Cr, and CdSe targets in argon discharges and for a dc Research S-gun with an Al target in argon and argon/oxygen discharges. Literature data for planar, S-gun, and cylindrical magnetrons has also been shown to fit the above expression; for rf magnetrons, I is the rms current and V is the target self-bias voltage. V0 decreases from about 400 V at 0.1 Pa to 250 V at 1 Pa and then decreases at higher pressures. β increases from about 50 to ≤300 A/kV2 as the pressure increases from 0.1 to 10 Pa. The actual values of V0 and β depend on the system, target, and sputtering gas. It is shown that the I∝V2 dependence is due to a space-charge-limited electron current in the magnetron geometry which reduces the electron mobility by several orders of magnitude from the zero magnetic field value; the mobility ratio depends on the electron-atom collision frequency.