Plasmas are often called a fourth state of matter.  As we know, a solid substance in thermal equilibrium generally passes into a liquid state as the temperature is increased at a fixed pressure.  The liquid passed into a gas as the temperature is further increased.  At a sufficiently high temperature, the molecules in the gas decompose to form a gas of atoms that move freely in random directions, except for infrequent collisions between atoms.  If the temperature is further increased, then the atoms decompose into freely moving charged particles (electrons and positive ions), and the substance enters the plasma state.

Chemical etching also called wet solvent etching was the key technology to integrated circuit manufacturing up to the late 1960s. Although wet etching provides low cost and often infinite selectivity, it has a limitation for micron and sub-micron pattern sizes. The most serious limitation is that of isotropic etching, which results in undercutting of the mask material and hence limits the minimum size of the pattern.

    Chemically reactive plasma discharge are widely used to modify the surface properties of materials.  Plasma processing technology is vitally important to several of the largest manufacturing industries in the world.  Plasma-based surface processes are indispensable for manufacturing the very large-scale integrated circuits (ICs) used by the electronics industry.  Such processes are also critical for the aerospace, automotive, steel, biomedical, and toxic waste management industries.  

    Materials and surface structures can be fabricated that are not attainable by any other commercial method, and the surface properties of materials can be modified in unique ways.  For example, 0.2-mm-wide, 4-mm-deep trenches can be etched into silicon films or substrates.  A human hair is 50-100 mm in diameter, so hundreds of these trenches would fit endwise within a human hair.

Radio-frequency (RF) plasmas in halogen-containing gases are widely used for dry etching of materials in the fabrication of microelectronic devices. Because of the electronegative nature of the halogen atoms, large amounts of negative ions exist in such plasmas. Since negative ions are kept inside the bulk plasma by the sheath electric field, few of them can reach the surface of the substrate; thus the role of the negative ions in the surface reaction my not be very significant. However, the existence of the negative ions has a large effect on the transport of charged particles, the structure of the sheath, the electric field in the bulk plasma, etc, and hence the nature of plasmas containing negative ions is considerable different from that of plasmas not using electronegative gases. In plasma processing, when electronegative gases are used, negative ions vary the plasma potential and the behavior of electrons. Another problem is that negative ions cause dislocations on the surface during film formation.

CCP Discharge