Session FR5: Modeling - Plasma Processing and Chemistry II

Two and Three Dimensional Inductive Coupled Plasma Remote Source Modeling with Single and Gas mixtures with Experimental Validation

 

The current industrial trend of plasma discharge is to use complex gas mixtures in certain ratios to enhance the reactor efficiency and optimize the process. This numerical study is focused to develop a comprehensive model of Inductively Coupled industrial plasma reactor using complex gas mixture to enhance the reactor efficiency. As a first step, the 2D axisymmetric and 3D models are developed to validate Argon and Oxygen discharge procedures. Mattson’s Suprema XP dual grid reactor has been used for the model development. Commercial modeling software, CFD-ACE+ was used for simulations of inductively coupled plasma reactor (without wafer bias) to address gas flow, heat transfer, plasma chemistry and electromagnetics in a coupled fashion. Experimental measurements are performed in Mattson’s Suprema Asher, a downstream ICP reactor with patterned grounded Faraday shield to reduce electron temperature and ion energy, as well as charge separation conductive grid between the plasma source and heated pedestal to reduce ion concentration on the wafer surface. Retractable Langmuir probe is inserted into plasma source through the special top cap with three openings, defining the vertical axes along which plasma density and electron temperature are repeatedly measured. Model calibration has been performed based on experimental data Both numerical and experimental data are presented in a comparative manner. Using the final model, a close match of result is observed leading to a proper validation. In the next step, we  developed 2D models to address the discharge of CF4/H2/O2, CF4/H2/He, H2/O2/HF. The hardware configuration of Mattson’s Suprema HMR with dual grid, and Suprema XP commercial reactors are used to develop the numerical model. Model predictions of plasma, ion and important active species densities, electron temperature, species flux uniformities over the pedestal are reported. This numerical models have been further used to simulate parametric process recipes.   

Kinetic Study of Effects of RF Pulsing in Dual Frequency Capacitively Coupled Plasma

Kinetic effects are important in low pressure capacitively coupled plasmas used for numerous plasma etch and deposition applications. In this work, dual frequency capacitively couple plasma sustained in Argon is characterized using electrostatic Particle-in-cell/Monte Carlo collision method for charged particles. Both the source (high frequency) and bias (low frequency) voltages are pulsed in the simulations. The objective of this work is to examine simultaneous pulsing of both the sources, and how pulsing design impacts targeted plasma applications where we would like to uniquely control the dynamics of plasma-surface interaction. Pulsing parameters (synchronous/staggered pulsing, pulse duty cycle and pulse frequency) are varied and temporal evolution of plasma characteristics such as plasma density, flux and energy of electrons and ions, density of excited neutrals, charge and displacement currents along with time-averaged particle distribution functions over pulsing cycle are studied. Especially, we looked at the staggered pulsing between source and bias voltages to study dynamic stabilization of plasma characteristics during offset period. Moreover, to understand complex nature of level-to-level pulsing, a comparative study is performed to study the impact of level-to-level pulsing over continuous excitation.   

Insights from Modeling Low-Pressure High-Voltage Dual-Frequency Capacitively Coupled Plasmas

Dual-frequency capacitively coupled plasmas are often utilized to improve control in etching processes in the semiconductor industry. Etching of high aspect ratio (HAR) features, with depths as much as 100 times their width, are particularly prone to distortions during the etch process such as tapering and twisting. To overcome this, higher ion energies are required, so HAR etching is generally performed with low pressure (mTorr) and high voltage (kV) dual-frequency capacitively coupled plasmas.

This low-pressure high-voltage regime is challenging for computational modeling. At these low pressures, kinetic effects become important and fluid or fluid-based hybrid models are no longer valid. The relatively high plasma densities used in industrial processes result in small Debye lengths, which must be spatially resolved. The high voltages produce energetic electrons, whose high velocities limit the timestep (through Courant-Friedrichs-Lewy constraints). In this work, Aleph, a particle-in-cell direct simulation Monte Carlo (PIC-DSMC) model, was used to investigate dual-frequency capacitively coupled Ar plasmas in conditions relevant to HAR etching. The upper electrode is powered with 1 kV at 40 MHz, and the lower electrode (i.e., the wafer) is powered at 3-6 kV at 2 MHz. The effect of pressure (4-8 mTorr) and gap size (3-7 cm) were explored. The resulting ion energy and angular distributions, electron energy and angular distributions, and the role of secondary electron emission in the plasma dynamics are discussed. The 40 MHz was shown to have some influence on the ion energies, and the fact that 40 MHz is a harmonic of 2 MHz was also shown to produce additional structures in the ion energy distribution.

    

Particle-in-Cell Techniques for Simulations of Magnetron Sputtering

Variable grid spacing, energy-conserving particle-in-cell (EPIC), and speed-limited particle-in-cell (SLPIC) techniques are investigated to speed up fully-kinetic simulations of magnetron sputtering. Fully-kinetic simulations of magnetron sputtering are needed to optimize the sputter-coating of thin films. Traditional PIC simulations of magnetron sputtering are computationally slow because the Debye length (~10^-5 m) is much smaller than the centimeter size device, and the plasma period (~10^-11 s^-1) is much shorter than the microsecond long dynamics. Variable grids speed up simulations by resolving the bulk of the plasma with larger cells and the high-density cathode sheath with small cells. EPIC speeds up simulations by relaxing the requirement to resolve the Debye length, which enables larger grid cells. SLPIC speeds up simulations by limiting the speeds of the fastest electrons, which enables larger timesteps. We have shown that SLPIC can quickly simulate electric discharge, collisions, and wall interactions, which are relevant to magnetron sputtering. We plan to compare these different PIC techniques and benchmark our results to both simulation and experimental data before exploring device optimization.   

Hybrid Plasma Modeling of Low-Pressure Oxygen Plasma in Capacitively Coupled Plasma Reactors

Dual-frequency low-pressure capacitively coupled plasmas (CCP) are widely used for reactive ion etching of dielectric thin films and chemical vapor deposition in the semiconductor industry. Computationally efficient fluid plasma simulations are generally used to understand plasma behavior in CCP reactors with respect to varying operating conditions which help us understand the effect of varying plasma operating conditions on the on-wafer process results. However, in low pressure conditions, the electrons mean free path can be long and kinetic effects are important. A full multi-dimensional multi-species particle plasma simulation is prohibitively expensive in terms of computational cost and time. In this paper, we discuss a hybrid plasma model where electrons are modeled as particles and ions and neutrals are modeled as fluid. We model oxygen only plasma that consists of multiple ionic species including those that are electronegative. The effect of gas temperature and assumed ion mobility and ion neutral collision cross-sections for the various ionic species on the new model results are analyzed and these results are compared with those of a fully particle simulation under the same operating conditions.   

Hybrid Plasma Simulation of RF Hollow Cathode Discharge at Moderate Pressure

  Plasma sources consisting of radio-frequency (RF) hollow cathode discharges (HCD) at moderate pressures have gained significance for advanced plasma processes. HCDs form in cylindrical cavities in the cathode. An array of such cavities can be used to create a large area HCD source. Under certain conditions the plasma in the hollow cavities becomes more intense. In this study, a single hollow cathode hole is simulated using a hybrid plasma model. The model includes continuity equations for charged and neutral species, drift-diffusion approximation for electron flux, the momentum conservation equation for ions, and the Poisson equation. A Monte Carlo model for secondary electrons is used to accurately compute production rates of species, which are coupled to the fluid plasma model. RF HCD behavior is simulated for different hollow cathode hole design and operating conditions. The plasma penetration inside the hollow cathode hole is enhanced depending on the pressure and hole size. The plasma enhancement due to RF sheath heating with increase in frequency as well as secondary electron emission has been explored synergistically. Additionally, the effect of the area ratio of the powered electrode to the grounded electrode on RF HCDs has been investigated.

    

Modelling of a Toroidal Wave Heated Plasma Source for the Remote Generation of Neutral Radicals

Remote plasma sources facilitate the production of neutral radical species for use in processing reactors which themselves cannot withstand exposure to charged species and radiative fluxes. Development of numerical models of power deposition in intermediate-to-high density plasmas, such as those used in remote plasma sources, is of importance to plasma-assisted material manufacturing and aerospace applications. In this work, results will be discussed from 2D fluid/Monte-Carlo simulations of a transformer-coupled radio-frequency (RF) toroidal remote plasma source sustained in argon and NF₃. A self-consistent treatment of the neutral gas temperature and associated inhomogeneous neutral depletion, is included in the model. Paschen breakdown and ignition criteria will be discussed through an analysis of the loop electric field topology for varying pre-ignition chamber pressures and gas admixtures. The inductive power deposition was investigated varying operating frequency, gas mixture, and chamber pressure. The degree to which these operating parameters affect ionization, and neutral species transport will also be discussed.