Definition of a plasma
A plasma is a gas of charged particles (both ions and electrons) and neutrals (atoms and molecules) but also of photons. More specific it can be characterized as a fully or partly ionized gas that is electrically neutral as a whole, i.e. the number of positive and negative charges is equal. It is often considered as the 4 th state of matter because it arises when supplying energy to a gas, though there is no abrupt phase transition like the transitions from solid to liquid and from liquid to gas. An alternative name for plasma is glow discharge owing to the characteristic glow from the plasma due to deexcitations of particles with the accompanied emission of photons. On Earth the plasma does not occur as a natural state, with exception of lightning flashes and flames, but in outer space plasma is the most common form of matter. Artificially generated gaseous plasmas have however numerous applications in the service of mankind. Plasma is found in so various applications as light sources, new kinds of television screens, in reactors for fusion experiments, etc. Probably the most common, and of the most economic importance, the plasma applications in material processing of solids, as well as of gases, are. Unlike the plasmas for fusion these plasmas are “cold”, i.e. not in thermodynamic equilibrium, where the gas is at low temperature while electrons have energies (temperatures) high enough to ionize, excite, dissociate, etc. part of gas particles.
Generation of plasmas
Plasmas for industrial applications in material processing are generated by different plasma sources.
A plasma can be generated by applying a voltage between two electrodes in a gas and at a certain voltage depending on the gas pressure and the distance between electrodes a breakdown will occur in the gas so that the gas becomes conducting due to the ionization. The ionization is caused by collisions between electrons, accelerated to the ionization energy by the electric field, and neutral particles, e.g. atoms. Every collision that generates one free electron can cause a new ionization but the first electron is also still free to collide again, so the ionization appears as an avalanche process. Eventually this process reaches a steady-state between the generation and the loss of the charged particles. The loss of the ions and electrons from the plasma volume can occur by recombination and diffusion to the plasma boundaries. The start of the ionization is enabled by the primary ions and electrons that always are present in any neutral gas for example due to ionization by cosmic radiation. Electrons with not enough energy to ionize an atom can change its electronic structure and excite it and when the atom deexcites a photon can be emitted. Recombination of charged particles and deexcitations contribute to a glow characteristic for the plasma systems.
In the simplest type of a glow discharge the applied voltage is a DC voltage and the two electrodes represent a cathode and anode respectively. The electric field is not distributed evenly between the electrodes which causes differences in the brightness of the glow. The most intense part of the discharge is the “negative glow” near, but separate from the cathode. The region between this glow and the cathode is “the cathode dark space” or “the space charge sheath” where the potential drops drastically. Due to no or very few collisions and hence no photon emission in this region it appears dark. Positive ions will be accelerated by the potential drop through the sheath and collide with the cathode surface. This can cause emission of secondary electrons that are repelled from the cathode into the negative glow and enhance ionization there. The ions can also knock out atoms from the cathode material and this effect is used in sputtering as a source of material to be deposited. If the distance between the cathode and the anode is long enough with respect to the width of the discharge another glow region, “the positive column”, can appear. At the anode there is also a dark space but very thin.
If the cathode is surrounded by a non-conducting material a plasma can not be sustained by a DC voltage because of charging of the electrode surface. In this case it is possible to power the electrode with radio frequency (RF) voltage to allow the discharge to be generated. The RF-discharges have usually more efficient ionization than the DC-discharges. The electrons have very low mass and they can easily follow the RF oscillations while the ions just follow the time average field. In the case of a conducting cathode a blocking capacitor between the cathode and the power supply can be used to build up a negative DC bias on the cathode (actually on both electrodes) and a space charge sheath can be formed between the electrodes and the plasma.In an RF-discharge the ions will be accelerated through this sheath like in the DC case.
Existence of sheaths in hollow electrode geometries can give rise to an “extra” discharge - the hollow cathode discharge (HCD) - that is utilized in the hollow cathode sources. In a two-electrode system with a hollow negative electrode (cathode) and a larger counter electrode (anode) the HCD can arise within the cavity in the cathode simultaneously with the “ordinary” discharge between the cathode and the anode if the distance of opposite walls in the cavity is roughly equal to the width of the negative glow. The origin of the HCD is an entrapment of electrons inside the hollow cathode when energetic electrons emitted from one cathode wall are accelerated across the sheath towards the opposite wall. When they reach the identical sheath on the opposite side with the same but opposite electric field they are reflected back. The electrons are trapped and forced to oscillate between the opposite sheaths. This mechanism is called the “hollow cathode effect”. During these oscillations electrons can undergo inelastic collisions with gas atoms and increase the probability for ionization giving a very dense plasma inside the cathode. This plasma is forced out of the cathode by flowing gas. The hollow cathode can also be powered by an RF power supply. The electrons can oscillate many times during one RF cycle giving a high plasma density. The hollow cathodes can have different geometries: tubes, arrays of tubes, or parallel plates (linear hollow cathodes).