1 - Helicon Plasma Sources

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This chapter emphasizes on the advantages of helicon discharges. It is mentioned that by 1993, helicon source had emerged as one of the major alternatives to the Reactive Ion Etching (RIE) discharge for high-throughput plasma-aided manufacturing. The purported advantages of helicon sources are mentioned as—high density, high efficiency, finite but low magnetic field, no internal electrodes, uniformity and quiescence, low pressure operation and several others. Regarding high density it is mentioned that typical helicon discharges in argon with 1–2 kW of RF (Radiofrequency) power have average plasma densities in excess of 1013 cm-3 and peak densities of order 1014 cm-3, about two orders of magnitude higher than in usual processing plasmas. However, densities in gases other than argon are considerably lower, the density drop when spread over a large area, and the quoted power is somewhat higher than normally used. When these factors are taken into account, this advantage of helicon sources is reduced to perhaps a factor of 3–10, which is still an appreciable gain. Ionization efficiency is a matter of conserving the primary electrons. Helicon sources employ the principle of the Penning discharge in using a magnetic field to confine electrons in the radial direction. Compared with the 875 G needed for the ECR source, however, the helicon source can be operated at 100–300 G, sufficient for trapping the electrons but not the ions. There is also a possibility of a low field mode of the helicon discharge, which requires only 10 to 40 G. Another important benefit of a magnetic field is the increased skin depth. It is stated that an ideal tool for plasma processing has to perform well according to the following four classical criteria—etch rate, anisotropy, uniformity and selectivity. This chapter has also emphasized the use of helicon discharges for production of ULSI circuits.