Reactive sputter etching in axial magnetic fields

Abstract

Magnetic fields are often applied to radio-frequency discharges in order to obtain high rate operation at low target voltages and low discharge pressures. It is known that discharge ionization is improved due to retention of electrons by magnetic fields. Here, a detailed comparison of axial magnetic enhancement and hollow cathode enhancement is made. Also included is the performance of a diode system. Our comparison criteria are (i) etch rate and (ii) surface power efficiency, for a given applied target voltage and gas pressure. High power efficiency is related to low substrate heating, which is important in high-rate semiconductor wafer processing. The axial configuration used here is one of three types of magnetic enhancement in common use (the others being magnetron and multipolar). It has the advantage of simple experimental design due to the uniform magnetic field across the target surface. Most of the results given here describe SiO2 etching in CF4 gas; however, some results for Si etching in SF6 are also given. Diode uniformity and reproducibility is poor in axial magnetic fields. This is not the case with the hollow cathodes, and we ascribe this difference to the difference in energy of typical electrons that are approaching target sheaths in the two cases. The rate enhancement available from axial magnetic fields in simple diode and target confined hollow cathode geometries is a factor of 2–3, for a given target voltage and at 1 Pa pressure of CF4. An even larger improvement of 16 times is obtained in the unconfined hollow cathode at low pressures. However, power efficiency is not improved by magnetic fields in any of the systems. These results can be understood in terms of the relative loss rates of electrons and neutrals from the discharge. Electron retention increases ionization, which raises both etch rates and input power. Neutral retention can increase rates, with little effect on input power. Thus, axial magnetic fields seem to be most appropriate where rate is not limited by the availability of active neutral species, and where electron energies approaching the target sheath are high.