Large-diameter microwave plasma source excited by azimuthally symmetric surface waves

Abstract

This article describes a large-diameter, surface-wave excited plasma (SWP) source designed for materials processing. The plasma reactor employs a launcher of 2.45 GHz azimuthally symmetric surface waves in the field-free region of 24-pole line-cusp magnetic fields, generated by a set of permanent magnets surrounding the reactor chamber walls; the magnets also provide an electron cyclotron resonance (ECR) magnetic field of 875 G near the chamber wall surfaces. Langmuir probe and optical emission measurements were made for characterizing the plasma produced in Ar. After the microwave power was turned on, the discharge was observed to start near the ECR region and then propagate toward the field-free region in the central area of the chamber. Moreover, the discharge was also observed to be excited by ECR at low microwave-power levels, and by surface waves in the field-free region at above a critical power strongly depending on the gas pressure. Such a transition of plasma excitation from ECR to SWP was found to occur under conditions where the plasma electron density exceeds a 2.45 GHz microwave cutoff value of ≈7.4×1010 cm−3. As a result, overdense plasmas with large diameters could be successfully produced with moderate microwave input powers ≲1 kW; the electron density was measured to be Ne∼1−3.5×1011 cm−3 in Ar plasmas, with its spatial variation being ≲5% over a 26-cm-diam area. Furthermore, preliminary experiments of SiO2 etching in pure CF4 plasmas showed that the SiO2 etch rate was typically ∼300 nm/min with a good uniformity ≲3% over an 8-in.-diam wafer.