Magnetic field effects on cylindrical magnetron reactive ion etching of Si/SiO2 in CF4 and CF4/H2 plasmas

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Abstract
Etching of Si and SiO2 has been investigated as a function of magnetic field strength in a radio-frequency cylindrical magnetron discharge using CF4 and CF4/H2 gas mixtures. Si and SiO2 etch rates increased with increasing magnetic field strength by a factor as large as 5 at a given power density. Futher increases in magnetic field strength, though, decreased etch rates in both CF4 and CF4/H2 plasmas. The density of CF, CF2, and F radicals, inferred from optical emission intensities, increased continuously as a function of magnetic field strength. Ion densities, measured by electrical probes, increased by a factor of ≊15 as the magnetic field strength was increased from 0 to 250 G in a CF4 discharge (0.45 W/cm2). However, the self-bias voltage on the powered electrode decreased rapidly with increasing magnetic field, from ∼1 kV at 0 G to almost 0 V at 250 G. The maximum etch rates occurred at an intermediate magnetic field strength where the self-bias voltage was between 25 and 50 V, which corresponds to conditions which maximize both the effectiveness of ion bombardment and ion density. With these conditions, the polymer thickness formed on the Si was a minimum and C/F ratio on the surface was a maximum. Highly anisotropic etching was also obtained near these conditions.

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