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

Kengo Hanaoka,Kazunori Takahashi

doi:10.1063/5.0038596

Abstract

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.

Highlights

Low-pressure plasmas have been widely used in various applications, such as plasma processing devices for semiconductor manufacturing,1 space propulsion devices,2,3 and fusion plasmas.4 The source is often operated in the frequency range of 1 MHz–100 MHz and the rf power is coupled with the plasmas in capacitive, inductive, and wave modes
Similar behavior is seen for the rf powers supplied to the wafer stage, where the rf power for the wafer bias is applied in the etching step for 1 s and 0.2 s, for the cases of the rf pulse widths of 5 s and 1 s in the source, respectively, i.e., the initial 20% of the pulse width for the plasma production
These datasets shown here indicate that the fast- and automatically controlled rf system can be applied to the reactive plasmas for the etching and passivation cycle in the Bosch process, as well as the previously reported argon plasma

Summary

Introduction

Low-pressure plasmas have been widely used in various applications, such as plasma processing devices for semiconductor manufacturing, space propulsion devices, and fusion plasmas. The source is often operated in the frequency range of 1 MHz–100 MHz and the rf power is coupled with the plasmas in capacitive, inductive, and wave modes. Low-pressure plasmas have been widely used in various applications, such as plasma processing devices for semiconductor manufacturing, space propulsion devices, and fusion plasmas.. To transfer the rf power to the load, the load impedance has to be matched to the output impedance of the generator and the characteristic impedance of a power transmission line, which are typically 50 Ω. For the case of the plasma applications, the load impedance is changed by the existence of the plasma and its parameters. The impedance matching circuit (MC) including two variable capacitors and an inductor (if necessary) is generally used to adjust and match the impedance.. To tune the load impedance by using the MC, the capacitances are adjusted by mechanical motion, causing a poor temporal response of the impedance tuning and preventing the size of the MC from being compact The impedance matching circuit (MC) including two variable capacitors and an inductor (if necessary) is generally used to adjust and match the impedance. To tune the load impedance by using the MC, the capacitances are adjusted by mechanical motion, causing a poor temporal response of the impedance tuning and preventing the size of the MC from being compact

Methods

Results

Discussion

Conclusion