Cold discharge plasmas - Applications and diagnostics

At first, plasmas represented a very small branch of physics, and their field of application seemed modest (radio waves in the ionosphere, discharge tubes, lamps, arcs). But developments to control thermonuclear fusion in pinch, tokamak or stellarator reactors, to generate power through magnetohydrodynamics, or to understand plasmas in astrophysics and planetology (thermonuclear fusion in stars, interstellar plasma, terrestrial ionosphere and magnetosphere, lightning and aurora borealis) have stimulated a flourishing basic research.

Then other fields of technological application helped revive discharge physics. In addition to illuminated signs and discharge lamps, new optical, UV, incoherent or coherent X-ray sources (lasers) make extensive use of discharge plasmas. Plasma display panels are now appearing. Ion beam sources have been developed for satellite propulsion applications, as well as for materials processing and analysis. Arc plasmas are studied both for their importance in electrical engineering cut-off phenomena and as a thermal source for metallurgy. Plasma probes are widely used for spectroscopic analysis of the chemical composition of samples in aerosol form. Finally, reactive gas plasmas are now widely exploited for the richness of their physico-chemical processes, both in volume (plasma chemistry) and in interaction with materials (deposition, sputtering or etching, oxidation or reduction...).

In this respect, the spectacular development of microelectronics is exemplary. Initially, to produce transistors on silicon wafers, liquid-phase etching or thermal chemical vapor deposition processes were used. But this imposed a lower limit on the size of etched patterns (around 10 µm), and liquid processes could pose contamination problems, while high temperatures prevented the use of materials less thermally stable than silicon. This is where discharge plasmas have come into their own, combining :

• direct coupling of electrical power into the gas, thus avoiding excessive wall heating;

• the possibility of obtaining high degrees of dissociation of molecules at low pressure by creating active species (electrons, ions, atoms, radicals) out of thermodynamic equilibrium with respect to the gas or wall temperature;

• selective control of surface reactions by ionic bombardment or high-enthalpy species (atoms and free radicals).

Since then, the race to integrate more and more electronic devices on silicon wafers has pushed the art of developing new low- or high-frequency electrical discharge configurations (radio frequency at 13.56 MHz, microwave at 2.45 GHz) and plasma processes to achieve nanometre-level precision in...