Langmuir coined the term "plasma" to describe a region in a gas discharge that has an equal number of positive ions and electrons.
Definition: a system of particles(electrons, positively charged ions, and neutrals) whose properties and behaviour are dominated by collective long-range Coulomb interactions.
Generation of gas-phase plasma = ionization of a neutral gas by external sources of radiation, by energetic electron beams, by gas discharges, or simply by bulk heating.
Why is plasma regarded as the fourth state of the matter?
Solid: atoms are positioned in a rigid lattice structure. These atoms vibrate about their equilibrium position with certain amplitude.
Energy=>destroys the lattice structure
Liquid: atoms or constituents can move around but the motion of neighboring constituents are strongly correlated=>viscosity
Energy=>decrease in viscosity
Gas: constituents move independently
Energy=>ionization of neutral constituents
Plasma: ensemble of negatively charged electrons, positive ions, and residual neutrals.
Microscopic Processes in Plasma
non-thermal plasmas: collision between plasma electrons and other plasma constituents.
charge-carrier production
Plasma Characterization
External parameters:
for plasmas generated by electric discharge: characteristics of electrical power(amplitude, frequency,duty cycle)
Internal parameters:
Energy distribution of plasma electrons:
Thermal(hot) plasma : electron, ions, and neutrals have the same average energy or temperature
Non-thermal(cold) plasma: energy is preferentially channeled into the electron component of the plasma. so the plasma electrons are hotter than the ions and neutrals
Plasma Density
number of electrons or ions per cm3
in general, plasmas are electrically quasi-neutral, i.e. number of electrons=number of ions.
high electron density is a prerequisite for producing high densities of reactive neutral radicals
high plasma densities is important for plasma etching and PECVD.
10^8-10^10 /cm3 => low plasma density
10^10-10^12/cm3=> high plasma density
1 eV=1.6*10^(-19) J=11604 K
How plasmas are made?
Why is plasma regarded as the fourth state of the matter?
solid-->liquid-->gas-->plasma
Solid: atoms are positioned in a rigid lattice structure. These atoms vibrate about their equilibrium position with certain amplitude.
Energy=>destroys the lattice structure
Liquid: atoms or constituents can move around but the motion of neighboring constituents are strongly correlated=>viscosity
Energy=>decrease in viscosity
Gas: constituents move independently
Energy=>ionization of neutral constituents
Plasma: ensemble of negatively charged electrons, positive ions, and residual neutrals.
Microscopic Processes in Plasma
non-thermal plasmas: collision between plasma electrons and other plasma constituents.
charge-carrier production
- direct ionization of ground state atoms or molecules(electron energy>10eV)
- step-ionization of an excited atom or molecule, in particular a long-lived metastable state
Plasma Characterization
External parameters:
- plasma size(large volume, small volume),
- the material and geometry of electrode(metal, dielectric) ,
- gas composition and dynamics( the type and pressure of gas or gas mixture in which the plasma is ignited, gas flow)
for plasmas generated by electric discharge: characteristics of electrical power(amplitude, frequency,duty cycle)
Internal parameters:
- plasma density,i.e. densities of the charged particles(electrons and ions),
- the number densities of various excited species in the plasma, and
- the average energy and energy distribution of all these species.
Energy distribution of plasma electrons:
- assumed to be Maxwell-Boltzmann distribution for mathematical convenience
- Maxwell-Boltzmann distribution observed only when electron-electron interactions are important.
- Often, the observed distribution deviates significantly.
- energy distribution is furnished by the balance between the energy gain of electron due to the electric field and the energy loss in collision
Thermal(hot) plasma : electron, ions, and neutrals have the same average energy or temperature
Non-thermal(cold) plasma: energy is preferentially channeled into the electron component of the plasma. so the plasma electrons are hotter than the ions and neutrals
Plasma Density
number of electrons or ions per cm3
in general, plasmas are electrically quasi-neutral, i.e. number of electrons=number of ions.
high electron density is a prerequisite for producing high densities of reactive neutral radicals
high plasma densities is important for plasma etching and PECVD.
10^8-10^10 /cm3 => low plasma density
10^10-10^12/cm3=> high plasma density
1 eV=1.6*10^(-19) J=11604 K
How plasmas are made?
