Plasma Processing

Ions, Electrons Get Charged Up A plasma is a reactive gas consisting of ions, free electrons, free radicals, excited state species, and neutrals. A plasma can be generated at high pressure (1 atmosphere or higher) or low pressure (1 torr or less) by an electric discharge. A high-pressure plasma is created by a spark, arc, or corona discharge. The gas molecules in such a discharge are hot enough to melt or even burn metal, which is the principle behind arc welding and plasma cutting. A tom-pressure plasma is generated by applying an electric field (DC or AC) to a volume under partial vacuum. The few random electrons present in the gas are accelerated by the field. Collisions of these electrons with o t h e r gas molecules result in a cascade of ionization reactions. Under these conditions, the gas temperature is barely above ambient: however, the chemical reactivity of the various activated and disassociated gas molecules allows a low-temperature plasma to be used to perform a wide range of useful chemical processes without solvents or heat and with a minimum of reagent. Low-pressure plasma utilizes an electric field to create the same types of reactive intermediates that can be created thermally or chemically. However, there are critical differences that set these processes apart. These differences are responsible for the great flexibility of plasma processes for surface cleaning. The rate of reaction between a gas and a surface during processing (eg, c leaning, su r face mod i f i ca t i on , o r f i lm deposition) depends in part on the rate of adsorption of the activated gas molecules. This adsorption rate is inversely proportional to the substrate temperature. At the temperatures required for thermally activated chemistry, adsorption can be very slow. In a plasma, however, the ion and molecular temperature of the gas is very low. Only the electron temperature is high. This low ion and molecular temperature allows for high adsorption rates and therefore high reaction rates. The presence of low ion and high electron temperatures in plasma processes stems from the large mass difference between the two particles. The resting mass of the electron is about 10g, while the 2 7