1.1 Introduction
A plasma is typically an ionized gas, and is usually considered to be a distinct state of matter or is called Řth state of matter" in contrast to solids, liquids and gases because of its unique properties. "Ionized" means that at least one electron has been dissociated from, or added to, a proportion of the atoms or molecules. The free electric charges make the plasma electrically conductive so that it responds strongly to electromagnetic fields. [1]
Plasma typically takes the form of neutral gas-like clouds, that is, in total including whole of the matters of electrons and ions, they are kept electrically neutral in macroscopic point of view, but may also include dust and grains (called dusty plasmas in this case). They are typically formed by heating and ionizing a gas, stripping electrons away from atoms, thereby enabling the positive and negative charges to move freely.
This state of matter was dubbed "plasma" by Irving Langmuir in 1928 [2] because it reminded him of a blood plasma. Langmuir wrote:
"Except near the electrodes, where there are sheaths containing very few electrons, the ionized gas contains ions and electrons in about equal numbers so that the resultant space charge is very small. We shall use the name plasma to describe this region containing balanced charges of ions and electrons."[1]Plasmas are the most common phase of matter. Some estimates suggest that up to 99% of matter in the entire visible universe is plasma, since the space between the stars is filled with plasma. However, more than 50% (some theories say 70-80% or more) of whole universe are said to be filled with "dark matter" which, currently, we cannot understand what they are, because we have no instrumental method to observe them at present. In the solar system, the planet Jupiter accounts for mostly nonplasma, only about 0.1% of the mass and 10-15% of the volume within the orbit of Pluto are plasma. Notable plasma physicist Hannes Alfvén noted that due to their electric charge, very small grains also behave as ions and form part of plasma [3].
Some examples of common plasma around us are shown in Table 1-1.
To completely describe the state of plasma, we would need to write down all the particle locations and velocities, and describe the electromagnetic field in the plasma region. However, it is generally not practical or necessary to keep track of all the particles in plasma. Therefore, plasma physicists commonly use less detailed descriptions mainly by two models; a fluid model and a kinetic model. Let us show the equations showing the plasma state. These are very complicated and the description of whole phenomena is outside the present scope. The readers should refer to more sophisticated books in order to lean the phenomena precisely.
1-1-1 Fluid modelFluid models describe plasmas in terms of smoothed quantities like density and averaged velocity around each position (see Plasma parameters in Table 1-2). One simple fluid model, magnetohydrodynamics, treats the plasma as a single fluid governed by a combination of Maxwell’s Equations and the Navier Stokes type Equations. A more general description is the two-fluid picture, where the ions and electrons are described separately. Fluid models are often useful when collisionality is sufficiently high to keep the plasma velocity distribution close to a Maxwell-Boltzmann distribution. Because fluid models usually describe the plasma in terms of a single flow at a certain temperature at each spatial location, they can neither capture velocity space structures like beams or double layers nor resolve wave-particle interactions:
1-2-2 Fluorescent lamps and neon signs
By using discharged plasma, there are many kinds of light sources such as fluorescent lamp, mercury lamp, lasers and so on.
Even in a fluorescent lamps, there are many kinds.
1. Low Pressure Discharge lamp:
Fluorescent lamps in a family use, low pressure sodium lamp, neon sign, low pressure xenon lamp, and so on.
2. High Pressure Discharge lamp or High Intensity Discharge lamp (HID):
High pressure mercury lamp, metal-halide lamp (include rare gases, Hg and metal-halogen), high pressure sodium lamp, high pressure sulfur lamp, and so on.1-2-3 Industrial application
There are tremendous amounts of applications of plasma techniques in the industrial fields. Precise descriptions will be given in the later sections. Here brief descriptions are shown. One of typical techniques is called the Chemical Vapor Deposition or thermal CVD method. If the plasma is employed instead of heating up the furnace, the process is realized even at the room temperature and the thin-film is deposited on the substrate. This method is called Plasma CVD. Low pressure, cold, weakly ionized plasma are used extensively in semiconductor device fabrication, especially for deposition and etching of thin films.
In the plasma CVD, the high energy electrons decompose the feed gas into radicals and ions. These radicals are adsorbed on the wafer surface, where they react to deposit a thin film through chemical reactions. A wide range of materials, such as SiO2, Si3N4, amorphous Si, carbon, and so on can be deposited with the use of plasma discharges.In order to make solar panel or liquid crystal display system, Si thin films of the large surface area are required. Plasma systems for realizing these purposes employ from DC discharge type though microwave discharge plasma. However, RF-discharge system with 13.56 MHz, capacitively coupled plasma (CCD) production system is popularly used especially in the industrial areas.
Dielectric thin film is used for LSI (Large Scale Integration) production. Diamond thin film is also easily produced by plasma CVD method under low pressure and low temperature conditions compared to natural diamond which requires 1500˚C or over and 60,000 atmospheric pressure. For this purpose, microwave plasma system without magnetic field is frequently employed.
Materials cut by chemical reaction is called "etching". In plasma etching, radicals excited by high energy electrons collide with material gas molecules to attach to the substrate and the vaporized molecule are produced by chemical reaction to be pumped out. The aspect ratio of plasma etching is as large as 20-50 (quite recently 80) is realized. Here, the aspect ratio is defined as the ratio of the hole depth vs. the hole width of the etched line or trough.
Sputtering is a process in which synthetic fabrics are coated with a fine layer of metal. Ions in the plasma are accelerated by electrostatic field to reach high enough energy, and accelerated ions collide with the solid target. This phenomenon is called "sputtering". The spattered particles are deposited onto the substrate to make thin film. This method is applied in many fields, such as to make tiny electronic device, plastic metals used for cars, window glass coating, lens coating and so on.1-2-4 Fusion energy researches
The process of nuclear fusion, evident in stars, including the sun, releases enormous amounts of energy. It occurs when the nuclei of light elements (such as hydrogen) are fused together at extremely high temperatures and pressures to form heavier elements (such as helium). For more than 50 years, with intensive work started in the late 1950’s, scientists have been working to develop practical methods for harnessing fusion reactions, with the hope of realizing the potential of this energy source (see Chap. 6). If this fusion energy is realized in the commercial base, CO2 problems which is now a source of environmental problems, will disappear forever.
1-3 Terrestrial Plasma
1-3-1 Lightning
Lightning is an example of plasma present near the earth’s surface. Typically, lightning discharges 30,000 amperes, at up to 100 million volts, and emits visible light, radio waves, x-rays and even gamma rays. Plasma temperatures in lightning stroke can reach about 30,000 Kelvin degrees and electron densities may exceed 1024 /m3.
An initial bipolar discharge (negative in the thundercloud and positive on the earth surface) starts from a negatively charged thundercloud. The discharge ionized channels are called "leaders". The negative charged leaders, called a "stepped leader", proceed generally downward in a number of quick jumps, each up to about 50 meters long [Fig. 1-1]. Along the way, the stepped leader may branch into a number of paths as it continues to descend. The progression of stepped leaders takes a comparatively long time (several ten milliseconds) to approach the ground. This initial phase involves a relatively small electric current (tens or hundreds of amperes), and the leader is almost invisible compared to the bright subsequent main lightning channel.When a stepped leader approaches the ground, the presence of opposite charges on the ground enhances the electric field. If the electric field is strong enough, a conductive discharge (called a positive streamer) can develop from these points. As the field increases, the positive streamer may evolve into a hotter, higher current leader which eventually connects to the descending stepped leader from the cloud. It has also possibility for many streamers to develop from many different objects simultaneously, but with only one connecting with the leader and forming the main discharge path results. Cartoon model shown in Fig.1-1 shows this phenomenon.
When the two leaders meet, the electric current greatly increases. The region of high current propagates back up the positive stepped leader into the cloud with a "return stroke" that is the most luminous part of the lightning discharge and is in a high density plasma state. The second leader called dart leader, comes down from the cloud through the same path made by the return stroke. Then second return stroke starts to come up to the cloud. The time duration between first stroke and second one is roughly 40~50 ms, depending on the weather condition.
A plasma is typically an ionized gas, and is usually considered to be a distinct state of matter or is called Řth state of matter" in contrast to solids, liquids and gases because of its unique properties. "Ionized" means that at least one electron has been dissociated from, or added to, a proportion of the atoms or molecules. The free electric charges make the plasma electrically conductive so that it responds strongly to electromagnetic fields. [1]
Plasma typically takes the form of neutral gas-like clouds, that is, in total including whole of the matters of electrons and ions, they are kept electrically neutral in macroscopic point of view, but may also include dust and grains (called dusty plasmas in this case). They are typically formed by heating and ionizing a gas, stripping electrons away from atoms, thereby enabling the positive and negative charges to move freely.
This state of matter was dubbed "plasma" by Irving Langmuir in 1928 [2] because it reminded him of a blood plasma. Langmuir wrote:
"Except near the electrodes, where there are sheaths containing very few electrons, the ionized gas contains ions and electrons in about equal numbers so that the resultant space charge is very small. We shall use the name plasma to describe this region containing balanced charges of ions and electrons."[1]Plasmas are the most common phase of matter. Some estimates suggest that up to 99% of matter in the entire visible universe is plasma, since the space between the stars is filled with plasma. However, more than 50% (some theories say 70-80% or more) of whole universe are said to be filled with "dark matter" which, currently, we cannot understand what they are, because we have no instrumental method to observe them at present. In the solar system, the planet Jupiter accounts for mostly nonplasma, only about 0.1% of the mass and 10-15% of the volume within the orbit of Pluto are plasma. Notable plasma physicist Hannes Alfvén noted that due to their electric charge, very small grains also behave as ions and form part of plasma [3].
Some examples of common plasma around us are shown in Table 1-1.
To completely describe the state of plasma, we would need to write down all the particle locations and velocities, and describe the electromagnetic field in the plasma region. However, it is generally not practical or necessary to keep track of all the particles in plasma. Therefore, plasma physicists commonly use less detailed descriptions mainly by two models; a fluid model and a kinetic model. Let us show the equations showing the plasma state. These are very complicated and the description of whole phenomena is outside the present scope. The readers should refer to more sophisticated books in order to lean the phenomena precisely.
1-1-1 Fluid modelFluid models describe plasmas in terms of smoothed quantities like density and averaged velocity around each position (see Plasma parameters in Table 1-2). One simple fluid model, magnetohydrodynamics, treats the plasma as a single fluid governed by a combination of Maxwell’s Equations and the Navier Stokes type Equations. A more general description is the two-fluid picture, where the ions and electrons are described separately. Fluid models are often useful when collisionality is sufficiently high to keep the plasma velocity distribution close to a Maxwell-Boltzmann distribution. Because fluid models usually describe the plasma in terms of a single flow at a certain temperature at each spatial location, they can neither capture velocity space structures like beams or double layers nor resolve wave-particle interactions:
1-2-2 Fluorescent lamps and neon signs
By using discharged plasma, there are many kinds of light sources such as fluorescent lamp, mercury lamp, lasers and so on.
Even in a fluorescent lamps, there are many kinds.
1. Low Pressure Discharge lamp:
Fluorescent lamps in a family use, low pressure sodium lamp, neon sign, low pressure xenon lamp, and so on.
2. High Pressure Discharge lamp or High Intensity Discharge lamp (HID):
High pressure mercury lamp, metal-halide lamp (include rare gases, Hg and metal-halogen), high pressure sodium lamp, high pressure sulfur lamp, and so on.1-2-3 Industrial application
There are tremendous amounts of applications of plasma techniques in the industrial fields. Precise descriptions will be given in the later sections. Here brief descriptions are shown. One of typical techniques is called the Chemical Vapor Deposition or thermal CVD method. If the plasma is employed instead of heating up the furnace, the process is realized even at the room temperature and the thin-film is deposited on the substrate. This method is called Plasma CVD. Low pressure, cold, weakly ionized plasma are used extensively in semiconductor device fabrication, especially for deposition and etching of thin films.
In the plasma CVD, the high energy electrons decompose the feed gas into radicals and ions. These radicals are adsorbed on the wafer surface, where they react to deposit a thin film through chemical reactions. A wide range of materials, such as SiO2, Si3N4, amorphous Si, carbon, and so on can be deposited with the use of plasma discharges.In order to make solar panel or liquid crystal display system, Si thin films of the large surface area are required. Plasma systems for realizing these purposes employ from DC discharge type though microwave discharge plasma. However, RF-discharge system with 13.56 MHz, capacitively coupled plasma (CCD) production system is popularly used especially in the industrial areas.
Dielectric thin film is used for LSI (Large Scale Integration) production. Diamond thin film is also easily produced by plasma CVD method under low pressure and low temperature conditions compared to natural diamond which requires 1500˚C or over and 60,000 atmospheric pressure. For this purpose, microwave plasma system without magnetic field is frequently employed.
Materials cut by chemical reaction is called "etching". In plasma etching, radicals excited by high energy electrons collide with material gas molecules to attach to the substrate and the vaporized molecule are produced by chemical reaction to be pumped out. The aspect ratio of plasma etching is as large as 20-50 (quite recently 80) is realized. Here, the aspect ratio is defined as the ratio of the hole depth vs. the hole width of the etched line or trough.
Sputtering is a process in which synthetic fabrics are coated with a fine layer of metal. Ions in the plasma are accelerated by electrostatic field to reach high enough energy, and accelerated ions collide with the solid target. This phenomenon is called "sputtering". The spattered particles are deposited onto the substrate to make thin film. This method is applied in many fields, such as to make tiny electronic device, plastic metals used for cars, window glass coating, lens coating and so on.1-2-4 Fusion energy researches
The process of nuclear fusion, evident in stars, including the sun, releases enormous amounts of energy. It occurs when the nuclei of light elements (such as hydrogen) are fused together at extremely high temperatures and pressures to form heavier elements (such as helium). For more than 50 years, with intensive work started in the late 1950’s, scientists have been working to develop practical methods for harnessing fusion reactions, with the hope of realizing the potential of this energy source (see Chap. 6). If this fusion energy is realized in the commercial base, CO2 problems which is now a source of environmental problems, will disappear forever.
1-3 Terrestrial Plasma
1-3-1 Lightning
Lightning is an example of plasma present near the earth’s surface. Typically, lightning discharges 30,000 amperes, at up to 100 million volts, and emits visible light, radio waves, x-rays and even gamma rays. Plasma temperatures in lightning stroke can reach about 30,000 Kelvin degrees and electron densities may exceed 1024 /m3.
An initial bipolar discharge (negative in the thundercloud and positive on the earth surface) starts from a negatively charged thundercloud. The discharge ionized channels are called "leaders". The negative charged leaders, called a "stepped leader", proceed generally downward in a number of quick jumps, each up to about 50 meters long [Fig. 1-1]. Along the way, the stepped leader may branch into a number of paths as it continues to descend. The progression of stepped leaders takes a comparatively long time (several ten milliseconds) to approach the ground. This initial phase involves a relatively small electric current (tens or hundreds of amperes), and the leader is almost invisible compared to the bright subsequent main lightning channel.When a stepped leader approaches the ground, the presence of opposite charges on the ground enhances the electric field. If the electric field is strong enough, a conductive discharge (called a positive streamer) can develop from these points. As the field increases, the positive streamer may evolve into a hotter, higher current leader which eventually connects to the descending stepped leader from the cloud. It has also possibility for many streamers to develop from many different objects simultaneously, but with only one connecting with the leader and forming the main discharge path results. Cartoon model shown in Fig.1-1 shows this phenomenon.
When the two leaders meet, the electric current greatly increases. The region of high current propagates back up the positive stepped leader into the cloud with a "return stroke" that is the most luminous part of the lightning discharge and is in a high density plasma state. The second leader called dart leader, comes down from the cloud through the same path made by the return stroke. Then second return stroke starts to come up to the cloud. The time duration between first stroke and second one is roughly 40~50 ms, depending on the weather condition.
