Biomedical Engineering Reference
In-Depth Information
19.1 PLASMA SCIENCE AND TECHNOLOGY
Plasma is often called as the fourth state of matter. Generally, a solid substance melts into a liquid
when the temperature is increased at a fi xed pressure. The liquid then transforms into a gas as the
temperature is further increased. Under high temperature, the molecules in the gas decompose to
form atoms that move freely in random directions in space. If the temperature is further increased,
the atoms decompose into freely moving charged particles, and the substance enters the plasma
state. Thus, under certain conditions, a plasma can be thought of as a collection of electrons, single-
and multiple-charged positive and negative ions along with neutral atoms, excited particles, elec-
tromagnetic radiation, molecules, and molecular fragments. In plasma, the densities of the excited
particles, ions, and electrons as well as the intensity of the electromagnetic radiation far exceed
those that are found in more mundane situations encountered elsewhere. An important feature of
plasma is that the positive and negative particles are in a state of charge equilibrium, and the sum
of the positive and negative charges in a suffi ciently large volume is equal to zero. In fact, plasma
technology is a dry, environmental-friendly, and cost-effi cient process in a myriad of applications.
19.1.1 P
LASMA
S
OURCES
To produce a plasma, electron separation from atoms or molecules in the gas state, or ionization, is
required. Ionization occurs when an atom or a molecule gains enough energy from an outside excita-
tion source or via interactions (collisions) with each other. In fact, to have a stable plasma discharge,
the choice of heating mechanisms and geometric confi gurations for sustaining discharge is impor-
tant. In most practical situations, the plasma is produced by an electrical discharge. Various forms of
discharges such as radio frequency (RF), microwave, glow, and arc discharges have been developed.
Generally, these discharges can be separated into two categories: gaseous and metallic plasma dis-
charges. The RF, glow, and cathodic arc discharges are discussed in the following sections in detail.
19.1.1.1
Radio Frequency Discharge
RF discharges can be classifi ed into two types according to the method of coupling the RF power
with the load: capacitive coupling and inductive coupling. The setup typically consists of an RF
generator, a matching network, and an antenna as shown in Figure 19.1. Commonly, the generator
RF supply
Matching network
C1
C2
Plasma
Ground anode
RF cathode
FIGURE 19.1
Schematic diagram of a capacitively-coupled plasma (CCP) plasma source with an equivalent
electrical circuit.
