Effects of rf Frequency on Plasma Density in Capacitively Coupled Plasmas at Low Pressure Studied by Particle-in-Cell/Monte Carlo Collision Method

Capacitively Coupled Plasma (CCP) is widely used in deposition and etching processes of semiconductor manufacturing. The plasma density tends to increase with increasing the driving frequency [1]. This trends is attributed to the alpha mode of electrons. However, there is a paucity of literature on effects of reducing the drive frequency below the ion plasma frequency on the plasma density at pressures below 1 Torr [2]. In this study, effects of the driving frequency on the plasma density in capacitively coupled discharge plasmas was investigated using the Particle-In-Cell/Monte Carlo Collision (PIC/MCC) model. The PIC/MCC method is a model in which charged particle behavior is analyzed by the PIC method and particle collisions are analyzed by the MCC method.<br/>A one-dimensional axisymmetric PIC/MCC model was used to analyze an asymmetric capacitive discharge with a blocking capacitor. RF voltage was applied between the parallel plate electrodes. The electrode gap was 20 mm. Gas was Ar. The gas pressure was 200 mTorr. The rf amplitude Vrf is fixed at 400 V. The effects of the driving frequency frf on the electron density and discharge structures were examined at 3.39, 6.78, 10.17, 13.56 and 27.12 MHz.<br/>The electron density in the bulk plasma increases with increasing the driving frequency from 6.78 MHz to 27.12 MHz. The electron density in the bulk plasma were ~ 2×10¹⁶ m^-3 at frf = 6.78 MHz, ~ 6×10¹⁶ m^-3 at frf = 10.17 MHz, ~ 7×10¹⁶ m^-3 at frf = 13.56 MHz, and ~ 8×10¹⁶ m^-3 at frf = 27.12 MHz, respectively. The profiles of electron density between the electrodes spread as the rf frequency increased from 6.78 MHz to 27.12 MHz. These results shows that the sheath width becomes narrow and the bulk becomes wide as the rf frequency increases. The width of sheath are 8 mm at frf = 6.78 MHz and 3 mm at frf = 27.12 MHz, respectively. The absolute value of axial electric field just above the electrode surface was almost constant of 2×10⁵ V/m under all driving frequencies, whereas the profiles of axial electric field depends on the rf frequency. The slope of the axial electric field near the electrode increases with the rf frequency. The peak of ion energy distribution function (IEDF) also increases with the rf frequency . The peak of IEDF were ~ 40 eV at frf = 6.78 MHz, ~ 50 eV at frf = 10.17 MHz, ~ 60 eV at frf = 13.56 MHz, and ~ 70 eV at frf = 27.12 MHz, respectively.<br/>For the driving frequency of 3.39 MHz, the ion density started to oscillate at the driving frequency. Then the plasma becomes unstable due to the collapse of the plasma bulk after the generation. The details will be presented at the conference.<br/>This study was partly supported by JSPS KAKENHI Grant Number 20H00142.<br/>Reference<br/>[1] V. Vahedi, C. K. Birdsall, and M. A. Lieberman, Phys. Fluids B 5, 2719 (1993).<br/>[2] Kazuki Denpo, et al., Jpn. J. Appl. Phys., 60, 016002(2021).