A plasma is a gas in which some electrons have been stripped away from some atoms to create positively charged ions and negatively charged electrons. Electric and magnetic fields are used to create plasmas and to control their behavior. Two early uses of plasma in semiconductor processing are sputtering and photoresist striping. In sputtering, a plasma is created with inert gas atoms, such as argon, and the argon ions are accelerated onto a target surface at high energy. The collisions knock target atoms off the surface, and at the low pressures involved, the freed target atoms travel by line of sight to the substrate. In photoresist stripping, the primary gas is oxygen. When oxygen is excited in the plasma, the resulting oxygen ions are extremely reactive with organic materials. Photoresist residue from photolithography processing is primarily organic chemicals, and the oxygen ions react with the residue to form volatile carbon dioxide.
Today, a major use of plasma is in plasma dry etching. Until the 1970s, patterns were generally created in thin films by wet etching processes. As feature sizes became too small to control with wet etching, plasma etching was introduced. Plasma etching provided the additional advantage that the nature of the etched sidewall could be controlled from generally rounded (isotropic etching) to sloped to essentially vertical (anisotropic etching).
Another application is plasma-enhanced chemical vapor deposition (PECVD or plasma CVD). In normal CVD, thermal energy at the substrate surface causes chemical reactions to occur there that result in deposition. When a plasma is used, some of the chemicals in the gas phase are stimulated or excited so that they are more reactive when they contact the heated substrate surface. As a result, higher growth rates can be achieved at lower temperatures than would be possible without the plasma.