ABSTRACT
Chemical reactions in plasmas were studied for decades before the first practical use of oxygen plasma for removal of photoresists in microelectronics fabrication. Fluorine-containing plasmas were used in the early seventies for etching in silicon device fabrication. Heavy use of plasmas for deposition and etching of dielectrics and metals followed well before the science of the process was understood. In modern IC fabrication about a third of the processing steps use plasmas. For a review of the historical aspects, the reader is referred to books on the subject [1-4]. 7.1 Plasma Processing
A broad definition of plasma, as it applies to physical sciences, is a partially ionized gas composed of negatively charged electrons, positively charged ions, and neutrals whose collective properties and behavior are dominated by collective long-range Coulomb interactions [2]. Plasmas in the gas phase are generated by a variety of mechanisms such as ionization, radiation, electron beams, and more commonly application of an electric field. These plasma types
differ by electron concentration and average electron energy, kTe, where k is the Boltzman constant and Te is the electron temperature. The average energy of plasma species is expressed in terms of this electron temperature, Te. Plasmas may be classified as thermal or nonthermal. In thermal, or hot, plasmas, all three constituents, electrons, ions, and neutrals, have the same energy or temperature. Plasma filling free space has a density of ne ≈ 10/cm3 and a kTe of about 25 eV. In nonthermal plasmas, the energy is mainly in the electrons, the plasma electrons being hotter than the other species. Nonthermal plasmas are used for technological applications in which the temperature may be close to room temperature, electron densities are of the order of 108 to 1013/cm3, and electron energies are of the order of few to 10 eV. Glow discharge plasmas are nonequilibrium plasmas because the electron temperature is greater than the gas temperature. Plasma is weakly ionized and as a whole is almost neutral. Atoms, molecules, and ions are responsible for the sputtering or etching reactions. Only in high-density plasmas can the ionization reach 10%. Plasmas with densities of 1010/cm3 and higher are referred to as high-density plasmas and are used for plasma deposition and etching in the semiconductor industry. Such high densities of electrons are needed to produce high densities of reactive radicals that are needed for these processes.
