To achieve rapid, uniform deposition of an amorphous hydrogenated silicon (a-Si:H) film, a capacitively coupled plasma (CCP) is often used at an intermediate pressure (>100 Pa), with a silane (SiH ₄ )-based mixture. At these pressures, heavy particle interactions (such as ion–ion, ion–neutral, and neutral–neutral reactions) contribute significantly to the formation of precursor radicals. By adding a consideration of the thermal diffusion effects to the neutral transport equation, the chemical processes have been numerically analyzed with variation in the number fraction of SiH ₄ and electrode spacing using a two-dimensional fluid model of radio frequency discharges in a cylindrically symmetric CCP reactor. The non-uniformity of the deposition rate profiles increases consistently as electrode spacing increases, although the non-uniformity of the plasma parameters decreases with the increase of electrode spacing. The simulated deposition rate profiles match well with the experimental data for the change of electrode spacing. Based on the validation of our model, we propose predictive designs to potentially improve the reactor and process by modifying the thermal and electrical surface conditions.

The change of the ion transport is investigated with the variation of the focus ring property at the wafer edge of a capacitively coupled plasma under an intermediate pressure of a few Torr. The particle fluxes and the ion trajectories at different locations are investigated with the variations of the gap size between the wafer edge and the focus ring, the focus ring height, and the permittivity of the focus ring. The incident angle and the particle fluxes to the wafer edge increase with the gap size. Conversely, the particle fluxes to the wafer edge decrease with the increase in the focus ring height. The incident angle of ions still keeps normal to the surface at the wafer edge, but on the left side of the focus ring, it increases dramatically with the increase in the focus ring height. With the change of the permittivity of the focus ring, it is possible to control the ratio of the ion flux to the neutral flux on the focus ring surface by enhancing only the ion flux independently.

The wide applicability of capacitively coupled plasma (CCP) deposition has increased the interest in developing comprehensive numerical models, but CCP imposes a tremendous computational cost when conducting a transient analysis in a three-dimensional (3D) model which reflects the real geometry of reactors. In particular, the detailed flow features of reactive gases induced by 3D geometric effects need to be considered for the precise calculation of radical distribution of reactive species. Thus, an alternative inclusive method for the numerical simulation of CCP deposition is proposed to simulate a two-dimensional (2D) CCP model based on the 3D gas flow results by simulating flow, temperature, and species fields in a 3D space at first without calculating the plasma chemistry. A numerical study of a cylindrical showerhead-electrode CCP reactor was conducted for particular cases of SiH ₄ /NH ₃ /N ₂ /He gas mixture to deposit a hydrogenated silicon nitride (SiN _x H _y ) film. The proposed methodology produces numerical results for a 300 mm wafer deposition reactor which agree very well with the deposition rate profile measured experimentally along the wafer radius.

In this study, we focus on the difference in the spatial distribution of the plasma parameters between SiH₄/He capacitively coupled plasma (CCP) and SiH₄/Ar CCP. The SiH₄/He mixture is modeled using the chemical reactions that were successfully derived in our previous studies. The chemical reaction model of the SiH₄/Ar mixture built in this study is based on the detailed set of chemical reactions in Ar. The spatial distribution of the plasma parameters is examined with the aid of a 2D fluid model. The electron and radical densities of SiH₄/Ar CCP are higher than those of SiH₄/He CCP. In addition, dilution with Ar results in more uniform reaction rates, which leads to a more uniform deposition profile. Because helium requires higher threshold energies for excitation and ionization, dilution with He had little effect on the precursor production. As a result, the concentration of Si₂H₆ observed in the inter-electrode region when using Ar for dilution was observed to be about ten times higher than the concentration of Si₂H₆ observed for He. This high concentration played a large role in influencing the formation of important radicals that determine the deposition rate as well as the difference in the deposition rate profile between Ar and He as diluents. The higher concentration of Si₂H₆ when using Ar means that the production rate of Si₂H₅ is higher in Ar. An examination of the effect of the dilution gas on the deposition rate profile indicated that the deposition rate profile with Ar is 100% more uniform and the deposition rate nearly 87% higher than for dilution with He.

The effect of reactor dimension on deposition rate profiles is analyzed with a two-dimensional (2D) fluid simulation of a capacitively coupled plasma (CCP) reactor to deposit a hydrogenated silicon nitride (SiN _x H _y ) film with a SiH ₄ /NH ₃ /N ₂ /He gas mixture. We focus on the complex function of electrode spacing to reveal the physical relation between reactor geometry and deposition rate profiles. The simulation demonstrates that the localization of electron density is concentrated close to the powered electrode periphery for electrode spacing of 9 mm. However, the plasma distribution becomes bulk dominated with electrode spacing of 15 mm by relaxing the localization. As a result, the increase in the electrode spacing creates a more uniform electron power density profile, and the deposition rate profile of SiN _x H _y film changes from convex to concave in a radial direction. The change in the deposition rate profile is validated through comparison with the experimental observation, which agrees well with the simulation results with errors of less than 5%. The deposition rate profile with electrode spacing of 9 mm is very sensitive to the non-uniform gas density condition applied to the showerhead inlet. However, the deposition rate profile with electrode spacing of 15 mm is not sensitive to the inlet gas profile because of the increasing residence time. The increase of the electrode spacing promotes molecule–molecule gas phase reactions and consequently weakens the effect of the inlet boundary condition.

The adjustment of the gas pressure has been shown to improve the deposition rate and uniformity of a plasma process. This led us to investigate the effect of the gas pressure in a 300 mm wafer reactor. We numerically simulated SiH₄/He capacitively coupled plasma discharges for the deposition of a hydrogenated amorphous silicon film. The results indicated that an increase in the gas pressure leads to uniform dissipation of the power coupled to the plasma and deposition profiles. By toggling the sidewall condition from grounded to dielectric while varying the gas pressure, we observed a modification of the plasma distributions and deposition profiles. Based thereupon, we concluded that the combination of high pressure with narrow electrode spacing can guarantee more efficient and uniform deposition. Additionally, this result was experimentally validated using the plasma deposition of hydrogenated amorphous carbon from the mixture C₃H₆/Ar/He. Even though the mixture differed from that we adopted in the simulation, the combination of high pressure with narrow electrode spacing still induced uniform deposition.