Plasma etching in a multipolar discharge

Etching of silicon and SiO2 has been investigated in a dc plasma discharge confined by a multipolar surface magnetic field layer. The reactive plasma is produced by primary ionizing electrons drawn from heated tungsten filaments and confined by permanent magnets. Electrical probe measurements show that a uniform high-density plasma (1010–1011 cm−3) is sustained in SF6-O2 at very low pressure (0.2–2.0×10−3 Torr). Substrates are biased independently of plasma production by a low-frequency alternating voltage (0–400 V) applied to the substrate through a blocking capacitor. Anisotropic profiles are etched into Si in SF6-20% O2 with etch rates in excess of 1 μm/min at 2×10−3 Torr. The etch rate increases with increasing primary electron current (up to 3 A) and energy (up to 60 eV), gas pressure (up to 2.0×10−3 Torr), substrate bias voltage, and the addition of up to 20% O2. For higher ionizing electron energies (>60 eV) and higher gas pressure (>2.0×10−3 Torr), etching is partially blocked by residue formation. The etch anisotropy depends mainly on substrate bias, increasing for higher values of bias voltage. The Si:SiO2 etch selectivity is typically 10–20, becoming large with decreasing substrate bias and plasma ion density.