Amplitude Modulation Frequency Dependence of Ion Energy Distribution in Capacitively Coupled Discharge Plasma Studied by Particle-in-Cell/Monte Carlo Collision Method

SiO₂ films fabricated by plasma enhanced chemical vapor deposition (PECVD) are used as insulating films, passivation films and interlayer dielectric films in semiconductor devices. The conventional PECVD often shows low deposition rate, poor step coverage and poor film quality on side walls and bottom of trenches. High-precision control of the ion energy distribution (IED) and ion angular distribution (IAD) is important for such deposition of trenches. Moreover, it is difficult to measure IED and IAD simultaneously in experiments.<br/>Here, we focus on amplitude modulation (AM) discharge method [1. 2] to solve these problems. We investigated effects of amplitude modulation frequency (<i>f</i>_AM) on IED and IAD in capacitively coupled discharge plasma using Particle-In-Cell/Monte Carlo Collision (PIC/MCC) method.<br/>A two-dimensional axisymmetric PIC/MCC model was used to analyze an asymmetric capacitive discharge with a blocking capacitor. 13.56 MHz voltage was applied between the parallel plate electrodes. The amplitude of RF voltage was modulated by sin wave in frequency range of 20 kHz to 100kHz. The gas was Ar. The gas pressure was 1.33 Pa. IED and the IAD were analyzed at the surface of lower grounded electrode.<br/>The electron density in the bulk plasma is modulated in positive synchronization with the AM discharge voltage. The electron density at the center is constant of 2.20×10¹⁵ m^-3 in the CW discharge. In the AM discharges, the electron density at center region varies from 1.36×10¹⁵ to 2.45×10¹⁵ m^-3 for <i>f</i>_AM= 20 kHz and from 1.93×10¹⁵ to 2.14×10¹⁵m^-3 for <i>f</i>_AM = 100kHz. The variation for <i>f</i>_AM = 100kHz is 81% smaller than that for <i>f</i>_AM= 20 kHz. For all modulation frequencies, the absolute value of axial electric field Ez near the ground electrode is modulated in positive synchronization with the AM discharge voltage. Ez near the ground electrode for the CW discharge is -1.7×10⁴ V/m, whereas for the AM discharges, Ez varied from -1.1×10⁴ to -2.2×10⁴ V/m. The variations in axial electric field are almost same for <i>f</i>_AM= 20kHz to 100kHz.<br/>In AM discharge, the energy of IEDF peak is modulated in synchronization with the AM discharge voltage. When the discharge voltage is high, the ions have high kinetic energy. For the CW discharge, the energy of IEDF peak is constant at 95 eV, while for the AM discharges, it is in the range of 54 to 135 eV. IADF shows that most ions impinge onto the substrate at almost 90 degrees. There is a small angular difference in the full width at half maximum (FWHM) of IADF, depending on the modulation frequency. FWHM of IADF is in a range of 1.72° to 2.23° for CW discharge and 1.56° to 2.98° for AM discharges. AM discharges offers IADF control, which may improve step coverage and film quality on side walls of trences.<br/>This study was partly supported by JSPS KAKENHI Grant Number 20H00142.<br/>[1] Y. Watanabe, et al., Appl. Phys. Lett., 53 (1988) 1263.<br/>[2] K. Kamataki, et al. Thin Solid Films, 523 (2012) 76.