Frequency-tuning radiofrequency plasma source operated in inductively-coupled mode under a low magnetic field

A radiofrequency (rf) inductively-coupled plasma source is operated with a frequency-tuning impedance matching system, where the rf frequency is variable in the range of 20–50 MHz and the maximum power is 100 W. The source consists of a 45 mm-diameter pyrex glass tube wound by an rf antenna and a solenoid providing a magnetic field strength in the range of 0–200 Gauss. A reflected rf power for no plasma case is minimized at the frequency of ∼25 MHz, whereas the frequency giving the minimum reflection with the high density plasma is about 28 MHz, where the density jump is observed when minimizing the reflection. A high density argon plasma above $1\times {10}^{12}$ cm⁻³ is successfully obtained in the source for the rf power of 50–100 W, where it is observed that an external magnetic field of a few tens of Gauss yields the highest plasma density in the present configuration. The frequency-tuning plasma source is applied to a compact and high-speed silicon etcher in an Ar–SF₆ plasma; then the etching rate of $8\mu$m min⁻¹ is obtained for no bias voltage to the silicon wafer, i.e. for the case that a physical ion etching process is eliminated. (paper)