We usually think of three states of matter: solid, liquid, and gas. In typical critical and industrial cleaning applications, liquid is used to remove particulate and thin film contamination from solid objects. Solid media, alone or in combination with a liquid may also be used for cleaning. The gas phase, as in vapor degreasing, may be used to minimize cleaning agent contamination.

What is sometimes overlooked is that a fourth state of matter, plasma, has important applications in critical cleaning and contamination control. Many applications also involve surface modification. Plasmas are all around us; in fact we couldn’t exist without one object that consists of a plasma, the sun. The stars and most of the mass in interstellar space consist of plasma. The light emitted by fluorescent lights, neon signs, lightning, and visible flames is due to plasmas.

PLASMA FORMATION
Creating a plasma requires energy (Figure 1). If we add enough energy (heat) to a solid, it melts and forms a liquid. By adding more heat, a liquid evaporates or boils and forms a gas. With sufficient additional energy, via heat or electric field, the gas ionizes and forms a plasma.

Plasma is a mixture of electrons and ionized atoms or molecules. Plasma can contain free radicals. Plasma emits light; the wavelengths can range from visible to x-ray. The energy and momentum of plasma constituents as well as the associated UV light are useful for both cleaning and surface modification.

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