Vertical silicon etching by using an automatically and fast-controlled frequency tunable rf plasma source

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.