Capacitively coupled plasma
Encyclopedia
A capacitively coupled plasma (CCP) is one of the most common types of industrial plasma source
s. It essentially consists of two metal electrode
s separated by a small distance, placed in a reactor. The gas pressure in the reactor can be lower than atmosphere or it can be atmospheric
.
A typical CCP system is driven by a single radio-frequency
(RF) power supply, typically at 13.56 MHz. One of two electrodes is connected to the power supply, and the other one is grounded
. As this configuration is similar in principle to a capacitor
in an electric circuit, the plasma formed in this configuration is called a capacitively coupled plasma.
When an electric field
is generated between electrodes, atoms are ionized and release electrons. The electron
s in the gas are accelerated by the RF field and can ionize the gas directly or indirectly by collision
s, producing secondary electrons
. When the electric field is strong enough, it can lead to what is known as electron avalanche
. After avalanche breakdown
, the gas becomes electrically conductive due to abundant free electrons. Often it accompanies light emission from excited atoms or molecules in the gas. When visible light is produced, plasma
generation can be indirectly observed even with bare eyes.
A variation on capacitively coupled plasma involves isolating one of the electrodes, usually with a capacitor
. The capacitor appears like a short circuit
to the high frequency RF field, but like an open circuit
to DC
field. Electrons impinge on the electrode in the sheath
, and the electrode quickly acquires a negative charge (or self-bias) because the capacitor does not allow it to discharge to ground. This sets up a secondary, DC field across the plasma in addition to the AC
field. Massive ion
s are unable to react to the quickly changing AC field, but the strong, persistent DC field accelerates them toward the self-biased electrode. These energetic ions are exploited in many microfabrication
processes (see RIE) by placing a substrate on the isolated (self-biased) electrode.
CCPs have wide applications in the semiconductor processing industry for thin film deposition (see sputtering
, PECVD) and etching.
Plasma source
Plasma sources generate plasmas.Excitation of a plasma requires partial ionization of neutral atoms and/or molecules of a medium.There are several ways to cause ionization:collisions of energetic particles, strong electric fields acting on bond...
s. It essentially consists of two metal electrode
Electrode
An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit...
s separated by a small distance, placed in a reactor. The gas pressure in the reactor can be lower than atmosphere or it can be atmospheric
Atmosphere (unit)
The standard atmosphere is an international reference pressure defined as 101325 Pa and formerly used as unit of pressure. For practical purposes it has been replaced by the bar which is 105 Pa...
.
A typical CCP system is driven by a single radio-frequency
Radio frequency
Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
(RF) power supply, typically at 13.56 MHz. One of two electrodes is connected to the power supply, and the other one is grounded
Ground (electricity)
In electrical engineering, ground or earth may be the reference point in an electrical circuit from which other voltages are measured, or a common return path for electric current, or a direct physical connection to the Earth....
. As this configuration is similar in principle to a capacitor
Capacitor
A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...
in an electric circuit, the plasma formed in this configuration is called a capacitively coupled plasma.
When an electric field
Electric field
In physics, an electric field surrounds electrically charged particles and time-varying magnetic fields. The electric field depicts the force exerted on other electrically charged objects by the electrically charged particle the field is surrounding...
is generated between electrodes, atoms are ionized and release electrons. The electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
s in the gas are accelerated by the RF field and can ionize the gas directly or indirectly by collision
Collision
A collision is an isolated event which two or more moving bodies exert forces on each other for a relatively short time.Although the most common colloquial use of the word "collision" refers to accidents in which two or more objects collide, the scientific use of the word "collision" implies...
s, producing secondary electrons
Secondary electrons
Secondary electrons are electrons generated as ionization products. They are called 'secondary' because they are generated by other radiation . This radiation can be in the form of ions, electrons, or photons with sufficiently high energy, i.e. exceeding the ionization potential...
. When the electric field is strong enough, it can lead to what is known as electron avalanche
Electron avalanche
An electron avalanche is a process in which a number of free electrons in a medium are subjected to strong acceleration by an electric field, ionizing the medium's atoms by collision , thereby forming "new" electrons to undergo the same process in successive cycles...
. After avalanche breakdown
Avalanche breakdown
Avalanche breakdown is a phenomenon that can occur in both insulating and semiconducting materials. It is a form of electric current multiplication that can allow very large currents within materials which are otherwise good insulators. It is a type of electron avalanche...
, the gas becomes electrically conductive due to abundant free electrons. Often it accompanies light emission from excited atoms or molecules in the gas. When visible light is produced, plasma
Plasma (physics)
In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...
generation can be indirectly observed even with bare eyes.
A variation on capacitively coupled plasma involves isolating one of the electrodes, usually with a capacitor
Capacitor
A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...
. The capacitor appears like a short circuit
Short circuit
A short circuit in an electrical circuit that allows a current to travel along an unintended path, often where essentially no electrical impedance is encountered....
to the high frequency RF field, but like an open circuit
Open circuit
The term Open circuit may refer to:*Open-circuit scuba, a type of SCUBA-diving equipment where the user breathes from the set and then exhales to the surroundings without recycling the exhaled air...
to DC
Direct current
Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...
field. Electrons impinge on the electrode in the sheath
Debye sheath
The Debye sheath is a layer in a plasma which has a greater density of positive ions, and hence an overall excess positive charge, that balances an opposite negative charge on the surface of a material with which it is in contact...
, and the electrode quickly acquires a negative charge (or self-bias) because the capacitor does not allow it to discharge to ground. This sets up a secondary, DC field across the plasma in addition to the AC
Alternating current
In alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....
field. Massive ion
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
s are unable to react to the quickly changing AC field, but the strong, persistent DC field accelerates them toward the self-biased electrode. These energetic ions are exploited in many microfabrication
Microfabrication
Microfabrication is the term that describes processes of fabrication of miniature structures, of micrometre sizes and smaller. Historically the earliest microfabrication processes were used for integrated circuit fabrication, also known as "semiconductor manufacturing" or "semiconductor device...
processes (see RIE) by placing a substrate on the isolated (self-biased) electrode.
CCPs have wide applications in the semiconductor processing industry for thin film deposition (see sputtering
Sputtering
Sputtering is a process whereby atoms are ejected from a solid target material due to bombardment of the target by energetic particles. It is commonly used for thin-film deposition, etching and analytical techniques .-Physics of sputtering:...
, PECVD) and etching.
See also
- Inductively coupled plasmaInductively coupled plasmaAn inductively coupled plasma is a type of plasma source in which the energy is supplied by electric currents which are produced by electromagnetic induction, that is, by time-varying magnetic fields.-Operation:...
- Plasma etchingPlasma etchingPlasma etching is a form of plasma processing used to fabricate integrated circuits. It involves a high-speed stream of glow discharge of an appropriate gas mixture being shot at a sample. The plasma source, known as etch species, can be either charged or neutral...
- Plasma (physics)Plasma (physics)In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...
- Multipactor effectMultipactor effectThe multipactor effect is a phenomenon in radio frequency amplifier vacuum tubes and waveguides, where, under certain conditions, secondary electron emission in resonance with an alternating electric field leads to exponential electron multiplication, possibly damaging and even destroying the RF...
- List of plasma (physics) applications articles