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

This article describes a large-diameter, surface-wave excited plasma (SWP) source designed for materials processing. The plasma reactor employs a laun cher of 2.45 GHz azimuthally symmetric surface waves in the field-free regi on of 24-pole line-cusp magnetic fields, generated by a set of permanent ma gnets surrounding the reactor chamber walls; the magnets also provide an el ectron cyclotron resonance (ECR) magnetic field of 875 G near the chamber w all surfaces. Langmuir probe and optical emission measurements were made fo r characterizing the plasma produced in Ar. After the microwave power was t urned on, the discharge was observed to start near the ECR region and then propagate toward the held-free region in the central area of the chamber. M oreover, the discharge was also observed to be excited by ECR at low microw ave-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 wher e the plasma electron density exceeds a 2.45 GHz microwave cutoff value of approximate to 7.4 x 10(10)cm(-3). As a result, overdense plasmas with larg e diameters could be successfully produced with moderate microwave input po wers less than or similar to 1 kW; the electron density was measured to be N-e similar to 1 -3.5 x 10(11)cm(-3) in Ar plasmas, with its spatial variat ion being less than or similar to 5% over a 26-cm-diam area. Furthermore, p reliminary experiments of SiO2 etching in pure CF4 plasmas showed that the SiO2 etch rate was typically similar to 300 nm/min with a good uniformity l ess than or similar to 3% over an 8-in.-diam wafer. (C) 2000 American Vacuu m Society. [S0754-2101(00)02403-9].