Radio-frequency reactive sputter etching in magnetron fields

Abstract

The etch rates of materials in rf discharges can be enhanced at low target voltages and low discharge pressures by the application of magnetic fields. In a previous article magnetic enhancement in diode and hollow cathode systems was experimentally studied using an axial magnetic field, applied perpendicular to the target. The field in the present article is applied parallel to the target surface, in a magnetron configuration. Experimental results are given here for etch rate, uniformity, and power input. Etch rates of SiO2 were studied as a function of CF4 pressure, magnetron field, and mechanical discharge confinement. Various electrode shapes were used to confine the discharge and hence increase discharge ionization. Etch rate uniformity at low pressure was measured as a function of electrode confinement and magnetic field, and is discussed in terms of both the motion of glow electrons and the measured sheath (or dark space) heights. Etch rates increase with increasing gas pressure, electrode confinement, or magnetron field. At 2 Pa CF4 pressure and 975 Vpp at 13.56 MHz in a diode machine, etch rates increased by a factor of 10 using either electrode confinement or a 208 G magnetron field. Both methods improve electron trapping, thereby increasing ionization and etch rates. With applied voltage and gas pressure held constant, magnetron fields increase etch rate by a factor of 9 for a diode, 5 for an unconfined hollow cathode, and 1.7 for a target confined hollow cathode. Magnetron fields result in very poor etch rate uniformity, which can however be improved by adding electrode confinement to improve electron trapping. Etch power efficiency varied by only 33% over the magnetic field range studied here, indicating little magnetic field-induced depletion of chemically reactive species even in our most intense discharges. However, compared to the diode, power efficiency was up to two times greater in the electrode-confined hollow cathodes due to their more efficient utilization of these chemically reactive species.