An automatically and fast-controlled frequency tunable radiofrequency (rf) system is employed to a plasma etching device, where the rf system contains two rf amplifiers operational in 37 MHz–43 MHz for a plasma source and a wafer stage. Both impedance matching circuits for the source and the wafer stage have no variable capacitors, leading a compact design of the rf system; the power reflection can be minimized by adjusting the frequencies. The rf frequency and the output power are automatically controlled so as to minimize a reflection coefficient and to maintain a constant net power corresponding to a forward power minus a reflected power for both the source and the stage. The source is operated with SF6 and C4F8 gases for silicon etching and passivation in the Bosch process, respectively. For both the cases, the impedance tuning can be accomplished within several ms and the net power is maintained at a constant level. By alternatively switching the SF6 and C4F8 plasmas with pulse widths of 5 s and 2 s, respectively, a vertical silicon etching is performed, where a scallop structure is clearly formed on the etching side wall. By shortening the pulse widths down to 1 s and 0.4 s for the SF6 and C4F8 plasmas, the size of the scallop structure is significantly reduced; the usability of the automatically and fast-controlled rf plasma source for the Bosch process is demonstrated.

A fast and automatically controlled frequency-tunable radiofrequency (rf) system is installed in an rf plasma thruster consisting of a stepped-diameter insulator source tube wound by a single-turn loop antenna and a solenoid providing a magnetic nozzle, and immersed in vacuum. The frequency and the output power are controlled so as to minimize the reflection coefficient and to maintain the net power corresponding to the forward minus reflected powers at a constant level. The reproducibility of the impedance matching and the stability of the net rf power are assessed, showing the fast impedance matching within about 10 msec and the long and stable delivery of the rf power to the thruster. When increasing the rf power up to 500 W, discontinuous changes in the source plasma density, the imparted thrust, and the signal intensity of the ion beam downstream of the thruster are observed, indicating effects of the discharge mode on the thruster performance and the ion energy distribution.

A frequency-tuned radiofrequency (rf) plasma source is automatically controlled to minimize the rf power reflection and to maintain the constant net rf power corresponding to the forward power minus the reflected power. The experiment is performed with a power amplifier operational for the frequency of 37-43 MHz, a compact helicon source consisting of a loop antenna, a solenoid, and an insulator tube. The rf power is supplied to the rf antenna via an impedance matching circuit consisting of only fixed small ceramic capacitors. It is demonstrated that the reflection coefficient of the rf power is minimized within ∼10 ms, and the net power is successfully maintained at a constant level.

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