Construction and Characterization of an Extended Helicon Plasma Source

A new helicon source has been developed for plasma processing applications. The source is a modification of the traditional cylindrical helicon source design to rectangular geometry. In order to accomplish this, the antenna used for launching helicon waves is stretched in the direction perpendicular to the static magnetic field lines. This source was coupled to a long rectangular slab chamber which is used for the actual material processing. A static magnetic field of -200 Gauss peak strength, pointing out from the source into the diffusion region, was applied to facilitate helicon wave propagation. 13.56 MHz rf power was used to excite the magnetized plasma along the slab, and a rectangular diffusion chamber was attached to the side of the new source. Langmuir probes were used extensively to characterize the plasma produced in the new chamber. Careful attention was given to rt and other perturbing effects on Langmuir probe traces. Probes were constructed to minimize perturbing effects, and measurements of electron energy distribution functions, plasma and floating potentials, and density are presented for a variety of conditions. The extended source is shown to produce large regions of 1012 cm-3 density plasma in argon under some weak magnetic field conditions. Magnetic induction probes were used to examine the structure of waves in the extended chamber. A 10 x 10 x 50 cm source, with an appropriate antenna is shown to excite waves of 12 cm wavelength for certain magnetic field configurations. The theory of wave propagation along magnetic field lines in rectangular geometry is presented here for the first time. Favorable comparisons between the theoretical model and experimental results indicate that the model may be of use for designing improved extended sources. This work shows that an extended helicon source can be used to generate large areas of uniform plasma in chambers of relatively small volume. Scaling of the slab source in either cylindrical ring- or rectangular-type chambers should have little effect on the physics of the source operation. Application of this technology may include areas outside of microelectronics processing, such as hardening layers for large objects, or plasma source ion implantation. Also included is a brief discussion of the work necessary to improve the applicability of this prototype tool for those plasma processing applications