Session HW12: Inductively Coupled Plasmas

E-H Transitions in Ar/O2 and Ar/Cl2 Inductively Coupled Plasmas for Varying Antenna Aspect Ratio - Modeling

Electronegative plasmas sustained in halogen gases are used in the microelectronics industry for etching, and passivation.  Pulsed inductively coupled plasmas (ICPs) are prone to repetitive mode transitions from the capacitive (E-mode) to the inductive (H-mode) at the beginning of each power pulse due to the decrease in electron density in the attaching gas mixture during the interpulse afterglow.  The E-H transitions are sensitive to circuit and reactor configurations, and in particular the geometry of the antenna.  In this work, we discuss results from a computational investigation of E-H transitions in pulsed ICPs using flat-coils for different antenna aspect ratios (height vs width of windings of the coil), duty cycle and gas mixtures.  This investigation was performed using the Hybrid Plasma Equipment Model.  Typical conditions are Ar/Cl₂ and Ar/O₂ gas mixtures at tens of mTorr and a pulse repetition frequency of a few to 10 kHz.  We will discuss the plasma densities, energy distributions of ions to the window, and the power distribution between capacitive and inductive modes.  We found that the inductive coupling is a weak function of aspect ratio of the antenna.  Capacitive coupling is a sensitive function of aspect ratio, with low aspect ratio (i.e., long dimension parallel to window) producing more intense capacitive coupling.  Comparisons are made to experimental measurements of plasma properties.   

Volumetric Measurements of Electric and Magnetic Field, Current, Power deposition profiles in Ar/O2 Inductively Coupled Plasmas*

Electronegative inductively coupled plasmas (ICPs) sustained in halogen gases are used in the microelectronics industry for etching, and passivation. Pulsing is a primary control strategy to optimize conditions.  We report on time dependent measurements of electric and magnetic field, plasma current, and power deposition in pulsed ICPs sustained in tens of mTorr Ar/O₂ mixtures with the oxygen being a surrogate electronegative gas.  A 3D probe drive system measures changing magnetic fields throughout the plasma volume.  Alternatively an RF compensated Langmuir probe is used to measure electron and positive ion densities and electric potential. Probe inferred densities are calibrated against a microwave interferometer. Negative ion density is inferred from spectroscopic measurements, as well from combined laser photodetachment and Langmuir probe measurements. Results are reported for varying ratios of argon to oxygen concentrations. Comparisons are made to modeling results.   

Transient magneto-hydrodynamic modeling of inductively coupled plasma discharges

Inductively coupled plasma (ICP) generated by high power inductive discharges offers a large volume of contamination-free plasma for a considerable amount of time for various applications as it does not require electrodes to generate the plasma. However, many literatures involving experimental as well as computational work report complex 3D structures with undulating plasma-cold gas interfaces making the ICP unstable in the torch as well as the jet region which often leads to unsatisfactory application results.

The present work focuses on the computational study of magneto-hydrodynamic instabilities in ICP wind tunnels due to transient electro-magnetic fields and three-dimensional effects(created by non-axisymmetric coil configurations) in order to quantify its stability criteria. The MHD governing equations consists of a set of Navier-Stokes equations which are solved in a cell-centered finite volume flow solver and Maxwell equations which are solved in a mixed finite-element electromagnetic solver. Both sets of equations are coupled via Lorentz forces and Joule heating terms in the momentum and energy equations respectively.

The coupled multi-physics framework can perform simulations for 2D axi-symmetric as well as 3D ICP configurations. Preliminary results are obtained for a steady state 2D-axisymmetric plasma torch configuration. The results have been validated via code-to-code validation with another ICP solver CoolFluid. The future work will focus on performing three-dimensional transient ICP simulations to study the effects of transient electromagnetic field and actual coil configuration on the plasma flowfield.

    

Incorporating Electronegative Feedback Mechanisms in a Global Plasma Circuit Model for Pulsed Power Delivery

Pulsed inductively coupled plasmas (ICP) have been of interest in the semiconductor industry given their ability to reduce substrate damage and modulate key parameters such as electron density n_e, electron temperature T_e, and plasma potential V_p. For an electropositive plasma, in the power ON-Cycle of a pulse, these discharges are typically characterized by a sharp increase in T_e, and n_e, as well as a corresponding decrease or increase in the reflection coefficient (Γ), depending on the match settings_­. In previous work, a Matlab based Global Plasma Circuit Model (GPCM) has successfully been employed at characterizing these transients for argon. This work presents computational results from GPCM with an SF₆ Plasma and benchmarks them with hairpin and photodiode taken on the Inductively Coupled Argon Oxygen System (ICAROS). Modifications to the equivalent transformer model are also incorporated, such that electronegative effects such as electron attachment instabilities (EAI’s) can be accounted for. Finally, the role that match effects has on inducing these EAI’s are also explored, as it has been observed that modulating the topology of the match can change the behavior of instabilities.   

Negative hydrogen ion sources for the neutral beam systems at ITER

Large and powerful RF-driven ion sources are the front end component of the neutral beam injection systems of the international fusion experiment ITER. A low-pressure hydrogen plasma (0.3 Pa) is generated via inductively coupling (up to 800 kW RF power at 1 MHz) using a modular concept, i.e. eight drivers from which the plasma expands to a common expansion chamber of the size of 0.9 m×1.9 m. The generation of negative ions is based on the surface conversion process for which caesium is evaporated in the source. Negative ions are formed at the plasma grid, the first grid of the electrostatic accelerator system composed of 1280 apertures to extract a beam of 66 A H⁻ for 1000 s and 57 A D⁻ for 3600 s. In front of the plasma grid an ion-ion plasma evolves which is beneficial for reducing the current of co-extracted electrons often limiting the ion source performance. The presentation summarizes the status of the ion source development, the challenges still to overcome and gives insight into the complex plasma dynamics of these ion sources at the example of the size scaling experiments BATMAN Upgrade (1 driver) and ELISE (4 drivers) in operation at IPP. The route towards NBI systems at ITER, including the task of the neutral beam test facility (NBTF) will be briefly described as well.