Blog - Sushma Charora

Introduction
Plasmas are basically conductive assemblies of charged particles, neutrals and fields which can carry current and even generate magnetic fields and due to this property they are efficient source of radiation and thus they find innumerable uses in areas where such control is needed or where the special source of energy or radiation are required.

Plasmas are usually generated when an electric field of sufficient energy accelerates the electrons to produce collision with neutral molecules resulting in dissociation, ionisation and excitation leading to impact ionisation. Plasma ignition besides depending on electric field of sufficient energy (breakdown voltage) required to begin impact ionisation also depends on
1. Ionisation potential of the gas.
2. AC power frequency
3. Temperature
4. Chamber and electrode geometries

RF Power and RF Plasma
The definition of RF power is the AC electrical power at Radio frequencies and electromagnetic radio waves are created when RF power travels through an unshielded conductor. As most industrial applications require the power with radio frequencies, which are in several MHZ, range the advantages of using a RF power sources over a conventional DC source and parts of a high frequency RF system are discussed below

Advantages of Using RF power source:
As most gases ionise more efficiently at Radio frequencies a RF power source is used for most industrial applications because of some inherent advantages over a DC source like
1. As the ions are much easier to ionise with RF power than a DC source so we get more number of ions for the same amount of power and thus they are much easier to produce and sustain a plasma while using a RF power source than a DC Source.
2. A RF power source creates electromagnetic waves inside the plasma chamber which couple energy more easily with the plasma inside the chamber.
3. In material processing industry particularly in deposition and etching systems the DC plasmas would cause charge build up on the electrodes and would extinguish the plasma but while using a RF power source there is no current flow through the electrodes and also the electrodes can be coated with an insulating material which allows cleaning of the chamber much more easier

A Typical RF power system consists of the following major subsystems
a) A RF generator system, which consists of an Oscillator, Power amplifier and a filter circuits. The basic purpose of RF generator is the energy source to create the plasma and it delivers an AC waveform at specified frequency and power level.
b) Transmission line system that consists of coaxial systems and RF feed thru arrangement. These transport RF energy from the generator to impedance matching unit are specially designed to carry RF energy with minimal loss. RF Very High Frequency RF Plasmas Page 2 of 5
feed thru couples from the RF matching network, which is at atmospheric pressure with the electrode in the Vacuum chamber.
c) An impedance matching network which includes phase and magnitude detector, motor control circuits and variable conductors or variable inductors. In order to maximise the power that the RF generator delivers to the plasma a matching network is used to match the impedance of the load with the output impedance of the load with the output impedance of the generator.
d) Electrodes. These electrodes are the final loads for the system and they create an oscillating electrical field in the vacuum space and the charged particles are set in motion by this electric field. In the AC system there is no anode or cathode but the anode is the ground electrode and the energised electrode is called as cathode.
e) Power measurement arrangement to measure the forward and reflected power, which is returned due to unmatched impedance.

Applications of RF Power systems
The below are the some of the applications of RF power systems:
• Most of the electrically conducting materials such as metals usually heated by a non-contact method using alternating magnetic field, the varying magnetic field induces an electric current inside the component just like a transformer and only the object which is required to be heated gets heated hence it is more efficient and consistent than radiative or resistive heating that is usually used. Industrial applications where RF has been used is for Induction heating, heat treatment of metals, heating for deformation operations like forging, brazing and soldering, shrink fitting of motor parts.
• It has also been used for drying of synthetic and natural fibres water in the product conducts.
• Lumber Treatment
• The radio frequency vacuum drying places wood between two large RF electrodes using the bipolar nature of water molecules means that they rotate quickly in response to the changing electric field Friction from moving water molecules creates steam that permeates out of the wood. Dries from the inside out has reduced drying time and it’s more efficient and gives higher quality.

High Frequency plasmas (> 50MHz)
Very High frequency radio waves usually in the range of 0.3 to 3 MHz usually come in the microwave ranges.
Microwave plasma have two appealing properties that are being used, they are:
1. Surface-wave sustainable mode: The plasmas that are excited by propagation of electromagnetic surface waves are called as surface wave sustained plasmas. These surface wave plasmas are further divided into two groups depending on the way the plasma grows its own guide wave ionisation or not if it can generate by its own it is called as Self guided plasma. The surface wave mode allows to generate the uniform plasmas in volumes whose lateral dimensions extends several wavelengths of the electromagnetic waves. Surface-wave-mode plasmas can be operated in a large variety of the geometries depending on the process gas and pressure range of the chamber.
2. Resonator mode: In the resonator mode the plasma density does not exceed the critical density. A standing electromagnetic wave is confined by a cavity that penetrates the plasma and sustains the plasma in the regions of highest field intensity. Plasmas that are generated in resonator mode are less resistant against detuning.
Microwave plasma in both the modes have the advantage that they have high degree of spatial localisation which allows to separate the location of plasma generation from the location of surface processing which helps in reducing the negative effects of the substrates released from the processed surface on the chemistry of the gas.

Advanced RF applications:
1. High Density Plasma
This technology enables highly directional processes like HDP CVD systems are used to fill deep trenches and HDP etch systems are used to create very high aspect ratio contacts.
The two high-density plasma sources are
a) Electron cyclotron resonance – It couples together an electric field operating at microwave frequencies of 2.45 GHZ and a steady state magnetic field. The microwave of 2.45 GHz chosen is because of the optimum electron orbit frequency. The microwave energy and magnetic field creates a resonance condition where the electrons get highly energised. This process has advantages that it operates at low pressures and low temperatures and produces high etch rates. It is used for an-isotropic etching of 0-0.25 micron particles.
b) Inductively Coupled plasmas – Electrons are mobilised by a magnetic field as they move at right angles to the direction of the magnetic field. The RF power is applied on the coil of the wire wrapped around outside of the plasma chamber, the coil creates a vertically oriented magnetic field inside the chamber and the electrons are trapped into a central helical path inside the chamber.
2. Use of High Frequency Plasmas in Weaponry - Very high-energy radio frequencies are also being used in the radio frequency weapons known as HERF (high Energy Radio Frequency weapons) also known, as HPRF or High Power Radio Frequency weapons are weapons that are designed to use high intensity radio waves to disrupt electronics of the enemy systems. They operate in a similar way to by inducing destructive voltage within the electronic wiring and they are usually directional and can be focused on a specific target.
The other key technology to weaponry is that of ENMOD, or Environmental Modification. This incorporates atmospheric plasmas artificially inserted in the atmosphere by the spraying of plumes of various particulate for macro conditioning of the Atmosphere, and the energizing or charging of these macro-systems takes place via tectonic energies, macro-solar power generation, or antennae broadcast energies.

References:
1. www.en.wikepedia.org
2. Use of Very High Frequency Plasma to Prepare a-Si:H Based Triple-Junction Solar Cells at High Deposition Rates Final TECHNICAL STATUS REPORT March 11 1998 - August 10 2001 S.J. Jones, T. Liu, D. Tsu and M. Izu Energy Conversion Devices, Inc.
3. A high-intensity, RF plasma-sputter negative ion source G. D. ALTON, R. LOHWASSER,B. CUI,Y. BAO,T. ZHANG,S. N. MURRAY, G. D. MILLS
4. RF - Spring 2001 - Introduction to Plasma lecture by Jim Jozwiak Selland College of Applied Technology