For Modeling Low-Temperature, Non-Equilibrium Discharges
Tailor-Made to Simulate Low-Temperature Plasma Sources and Systems
The Plasma Module is tailor-made to model and simulate low-temperature plasma sources and systems. Engineers and scientists use it to gain insight into the physics of discharges and gauge the performance of existing or potential designs. The module can perform analysis in all space dimensions – 1D, 2D, and 3D. Plasma systems are, by their very nature, complicated systems with a high degree of nonlinearity. Small changes to the electrical input or plasma chemistry can result in significant changes in the discharge characteristics.
Plasmas – A Significant Multiphysics System
Low-temperature plasmas represent the amalgamation of fluid mechanics, reaction engineering, physical kinetics, heat transfer, mass transfer, and electromagnetics – a significant multiphysics system, in other words. The Plasma Module is a specialized tool for modeling non-equilibrium discharges, which occur in a wide range of engineering disciplines. The Plasma Module consists of a suite of physics interfaces that allow arbitrary systems to be modeled. These support the modeling of phenomena such as: direct current discharges, inductively-coupled plasmas, and microwave plasmas. A set of documented example models, with step-by-step descriptions of the modeling process, along with a user’s guide accompany the Plasma Module.
- ICP reactors typically operate at pressures in the millitorr range and produce much higher electron densities than capacitively coupled plasmas. Inductively coupled plasmas are popular because ion bombardment at low pressures results in a uniform etch rate on the surface of the wafer. The surface plot shows the electron number density inside a GEC ICP reactor.
- DIELECTRIC CURRENT DISCHARGES: A small gap is filled with a gas between two dielectric plates. Voltage is applied so that any free electrons will be accelerated and cause ionization. Shown is the mass fraction of electronically excited Argon atoms.
- MICROWAVE PLASMAS: In this cross-flow configuration, a TE mode wave enters from the top boundary and is absorbed when it interacts with the plasma. The white contour shows the location where the electron density is equal to the critical electron density. The wave is completely absorbed by the plasma.
Inductively Coupled Plasmas
Inductively coupled plasmas (ICP) were first used in the 1960s as thermal plasmas in coating equipment. These devices operated at pressures on the order of 0.1 atm and produced gas temperatures on the order of 10,000 K. In the 1990s, ICP became popular in the film processing industry as a way of fabricating large semiconductor wafers. These plasmas operated in the low-pressure regime, from 0.002–1 torr, and as a consequence, the gas temperature remains close to room temperature. Low-pressure ICPs are attractive because they provide a relatively uniform plasma density over a large volume. The plasma density is also high, around 1018 1/m3, which results in a significant ion flux to the surface of the wafer. Faraday shields are often added to reduce the effect of capacitive coupling between the plasma and the driving coil. The Inductively Coupled Plasma interface automatically sets up the complicated coupling between the electrons and the high frequency electromagnetic fields that are present in this type of plasma.
Direct Current Discharges
A specialized physics interface is available for modeling direct current (DC) discharges, which are sustained through secondary electron emission at the cathode due to ion bombardment. The interface allows for model inputs and contains the underlying equations and conditions for modeling this phenomenon. The electrons ejected from the cathode are accelerated through the cathode fall region into the bulk of the plasma. They may acquire enough energy to ionize the background gas, creating a new electron-ion pair. The electron makes its way to the anode, whereas the ion will migrate to the cathode where it may create a new secondary electron. It is not possible to sustain a DC discharge without including secondary electron emission.
You can use the Microwave Plasma interface to model wave heated discharges, which are sustained when electrons can gain enough energy from an electromagnetic wave as it penetrates the plasma. The physics of a microwave plasma are quite different depending on whether the TE mode (out-of-plane electric field) or the TM mode (in-plane electric field) is propagating. In neither case is it possible for the electromagnetic wave to penetrate into regions of the plasma where the electron density exceeds the critical electron density (around 7.6x1016 1/m3 for argon at 2.45 GHz). The pressure range for microwave plasmas is very broad. For electron cyclotron resonance (ECR) plasmas, the pressure can be on the order of 1 Pa or less. For non-ECR plasmas, the pressure typically ranges from 100 Pa up to atmospheric pressure. The power can range from a few watts all the way up to several kilowatts. Microwave plasmas are popular thanks to the cheap availability of microwave power.
Capacitively Coupled Plasma Analysis
Luke T. Gritter, Sergei Yushanov, Jeffrey S. Crompton, and Kyle C. Koppenhoefer AltaSim Technologies Columbus, OH
AltaSim Technologies provides engineering consulting services involving advanced multiphysics modeling such as capacitively coupled plasma (CCP). Plasma etching and deposition of thin films, critical processes in the manufacture of advanced microelectronic devices, commonly utilize CCP, in which the plasma is initiated and sustained by an ...
This model simulates a plasma at medium pressure (2 torr) where the plasma is still not in local thermodynamic equilibrium. At low pressures the two temperatures are decoupled but as the pressure increases the temperatures tend towards the same limit.
In-Plane Microwave Plasma
Wave heated discharges may be very simple, where a plane wave is guided into a reactor using a waveguide, or very complicated as in the case with ECR (electron cyclotron resonance) reactors. In this example, a wave is launched into reactor and an Argon plasma is created. The wave is partially absorbed and reflected by the plasma which sustains the ...
Dielectric Barrier Discharge
This model simulates electrical breakdown in an atmospheric pressure gas. Modeling dielectric barrier discharges in more than one dimension is possible, but the results can be difficult to interpret because of the amount of competing physics in the problem. In this simple model the problem is reduced to 1D by assuming the dielectric gap is ...
GEC ICP Reactor, Argon Chemistry
The GEC cell was introduced by NIST in order to provide a standardized platform for experimental and modeling studies of discharges in different laboratories. The plasma is sustained via inductive heating. The Reference Cell operates as an inductively-coupled plasma in this model. This model investigates the electrical characteristics of the ...
Computing the Ion Energy Distribution Function
One of the most useful quantites of interest after solving a self-consistent plasma model is the ion energy distribution function (IEDF). The magnitude and shape of the IEDF depends on many of the discharge parameters; pressure, plasma potential, sheath width etc. At very low pressures the plasma sheath is said to be collisionless, meaning that ...
Surface Chemistry Tutorial
Surface chemistry is often the most important and most overlooked aspect of reacting flow modeling. Surface rate expressions can be hard to find or not even exist at all. Often it is preferable to use sticking coefficients to describe surface reactions because they can be estimated intuitively. The tutorial model simulates outgassing from a ...
Benchmark Model of a Capacitively Coupled Plasma
The underlying physics of a capacitively coupled plasma is rather complicated, even for rather simple geometric configurations and plasma chemistries. This model benchmarks the Capacitively Coupled Plasma physics interface against many different codes.
Model of an Atmospheric Pressure Corona Discharge
This model simulates a negative corona discharge occurring in between two co-axially fashioned conductors. The negative electric potential is applied to the inner conductor and the exterior conductor is grounded. The modeled discharge is simulated in argon at atmospheric pressure.