Session FW1: Modeling - Plasma Processing and Chemistry I

Particle-in-Cell Modeling of Electron-Beam Generated Low Electron Temperature Plasma

Plasmas generated using electron beams are known to have low electron temperature (T_e) and plasma potential, which are particularly useful for atomic-precision plasma processing. Electron beam produced plasmas are typically confined using a magnetic field and operated at low gas pressures. Previous hybrid modeling of these plasmas indicated that plasma transport can be non-classical in this parameter regime. A self-consistent 2-dimensional particle-in-cell model of electron beam produced plasmas is described. The model examines the creation and evolution of plasma in low pressure (10 – 40 mTorr) Ar on injection of 2 keV electron beam. The plasma is well-confined by the magnetic field with the plasma more constricted around the beam axis at lower pressure and higher magnetic field. Physical and energy transport in the plasma (i.e., ambipolar diffusion and thermal conduction) are observed to scale differently with magnetic field and gas pressure. The charged species density is, for example, found to be more confined near the electron beam axis than T_e. The effect of gas pressure, magnetic field and beam current are examined in the paper. The impact of these parameters on electron density, plasma potential and T_e are found consistent with probe-based experimental measurements.   

Characterization of a transformer-coupled remote plasma source chamber using a fluids-based, multiphysics plasma model

The importance of remote plasma source (RPS) devices in semiconductor and thin-films manufacturing processes is growing rapidly. RPS's are found not only in subsidiary process like chamber clean or PFC abatement, but they are also being used more frequently in high-precision, primary manufacturing steps such as selective-etch or PECVD enhancement. As the application space for RPS's expands, so do the requirements placed on these devices. A new remote, transformer-coupled plasma (TCP) source with a toroidal chamber design is being developed with the intention of providing high flow rates of fluorine and oxygen based feedgases with optimized performance. An iterative design approach based solely on experimental data is crippled by long lead-times and laboratory resource availability. Thus, the goals of the present work are to use numerical modeling to characterize the plasmas within the RPS chamber and use the simulation results to optimize some of the fundamental design parameters. A "global model" of NF3 plasma is used to determine a simplified yet relevant set of reactions and collisions; then a full, 3D, steady-state model of the plasma is solved. Commercial modeling software, CFD-ACE+, was used for simulations of the plasma chamber to address gas flow, heat transfer, plasma chemistry and electromagnetics in a coupled fashion. Numerical results are correlated to experimental data, including Langmuir probe data. Then, the model is run parametrically with varying chamber topologies, to study the resulting plasma characteristics and the effect to feedgas dissociation efficiency. There is scarcity of plasma modeling in the literature for low-temperature, industrial plasmas in 3D, toroidal, transformer-coupled plasma chambers. It is the intention of the current work to elucidate both the challenges and efficacy of plasma modeling for these types of RPS devices.   

Numerical investigation of vacuum ultraviolet emission in Ar-O2 inductively coupled plasmas

The emission of vacuum ultraviolet (VUV) radiation is investigated in low-pressure Ar-O₂ inductively coupled plasmas via numerical simulations. Controlling VUV fluxes is important in a number of industrial and biomedical plasma applications because, depending on the process, VUV radiation may be desired, required to a certain degree, or unwanted. For this purpose, a self-consistent Ar-O₂ plasma-chemical reaction scheme has been implemented in a zero dimensional plasma chemistry model and is used to investigate VUV emission from excited O atoms (3s ⁵S⁰ and 3s ³S⁰) at 130 and 135 nm. The model has been successfully compared against experimental measurements of VUV emission, electron densities and excited state densities. In addition, VUV emission rates are investigated as a function of pressure, Ar-O₂ mixture, and continuous and pulsed power deposition. Moreover, the dominant reaction pathways leading to VUV emission have been identified and described. In general, when the dissociation degree of O₂ is large, VUV emission is high and is dominated by 130nm light from the decay of O(3s ³S⁰) to ground state. On the other hand, when the dissociation degree is low VUV emission rates are also low and are dominated by dissociative electron impact excitation processes.   

Effects of amplitude modulation discharge on behavior of oxygen ions in Ar/O2 capacitively coupled plasma studied by particle-in-cell/Monte Carlo collision model

Oxygen ions (O₂⁺, O^-) play key roles in fabrication of SiO₂ insulating films by plasma-enhanced chemical vapor deposition (PECVD) [1]. Here, we employed 1d3v PIC-MCC simulations to evaluate effects of amplitude modulation (AM) discharges [2] on behavior of oxygen ions in Ar/O₂ plasma. In AM discharges, the amplitude of the applied radio frequency (RF) voltage is varied sinusoidally. The results show that the O₂⁺ and O^- ion densities in the central plasma region and the ion energy distribution function (IEDF) of O₂⁺ ions incident on the grounded electrode vary with time for AM discharges. The variation widths increase with the modulation level, which indicates the modulation level is a good tuning knob to control ion density and IEDF. The results offer important information to establishing a deposition model for PECVD using AM discharges and achieving high quality SiO₂ films based on the model.

[1] Hu Li, et al., Jpn. J. Appl. Phys. 58 SEED06 (2019). [2] K. Abe, et al., Jpn. J. Appl. Phys. (2022) in press.

 

    

Plasma species and reaction dynamic-oriented global model studies for microscale argon discharges

Low temperature microplasma discharges have emerging applications in biomedicine, satellite propulsion, electric device and more[Garner et al, J. Appl. Phys 128, 210903 (2020) and Fu et al Plasma Res. Express 2, 013001 (2020)]. In this work, a global model is employed to explore the importance of plasma species (e, Ar⁺, Ar₂⁺, Ar^m, Ar^r, Ar^4p, Ar₂^*) and reaction dynamics for RF current source-driven argon discharges in a sub-millimeter gap. The model shows that Ar₂⁺ starts to be the main ion species above 100 Torr, and Ar₂* is the main metastable neutrals for pressure higher than 200 Torr. The wall loss process of Ar₂⁺ is less important compared to the recommendation reaction with electrons in the bulk. For fixed driving current and gap at atmosphere, the plasma density (mainly e and Ar₂⁺) increases first and then gets relatively flat with increasing driving frequency (0.1-2GHz), meanwhile, the main loss channel of Ar₂⁺ transits from wall loss to bulk recombination. We also intend to explore the similarity law of different plasma species densities, and their generation and loss process for various pressures, gap distances, and driving frequencies.     

Kinetics of non-equilibrium plasma in water vapor- and hydrocarbon-containing gaseous mixtures

The kinetics of non-equilibrium discharge plasma when the ion composition is dominated by water or hydrocarbon cluster ions are analyzed. Plasmas with water ions are formed in atmospheric discharges and discharges in the presence of liquid water or water vapor, including plasma applications for air purification, medicine and combustion. Reactions with hydrocarbon ions play an important role in interstellar chemistry, plasma enhanced chemical vapor deposition, synthetic diamond film deposition and plasma assisted combustion. The modeling of water ion chemistry and chemistry of hydrocarbon-containing plasmas requires large amount of data on the rates of ion formation and loss. This review provides the core database for the rate coefficients describing the kinetics of charged particles in plasmas with water and hydrocarbon ions as a function of temperature, pressure, and local electric field. Particular attention is given to cluster ions. The decay of non-equilibrium plasma with water and hydrocarbon ions is analyzed in combustible gaseous mixtures excited by a repetitively pulsed nanosecond discharge.