Plasma, the fourth state of matter, is a partially ionized gas mixture consisting of ions, free radicals, neutral byproducts, and photons. Plasma is an electrically neutral mixture of physically and chemically active gas phase species capable of surface modification. The surface modification is the result of the interaction between the gas phase active species and a solid surface. The active species within the plasma are cable of both physical work via ion assisted sputtering and chemical work through radical or byproduct chemical reaction. As a result, plasma can perform numerous surface modification processes including surface activation, contamination removal, cross linking, etch by chemical reaction, and physical bombardment.
There are two key mechanisms in plasma processing: a physical mechanism performed by ions and a chemical mechanism performed by free radicals and byproducts. Physical plasma is a result of the charged ionic species gaining sufficient kinetic energy from the applied electric field to perform sputtering. Through energy transfer, partial molecules and atoms are sputtered from the surface, resulting in contaminant removal from the substrate surface. Physical bombardment also changes the topography of the surface, increases the surface roughness at the molecular level, and can aid in interface adhesion. Chemical plasma relies on the chemically active free radicals and byproducts generated in the plasma to diffuse to the sample surface. These free radicals and byproducts decrease the activation energy in a chemical reaction, resulting in material removal. The volatile chemical reaction byproducts are removed from the sample surface and the process chamber by the vacuum system. Isolation of a particular plasma mechanism is determined by the plasma mode, where the mechanism selection is based upon the packaging application.