Evaporation Processes
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MRS BULLETIN/DECEMBER 1988
technologies had to await the development of vacuum techniques and therefore dates to the post World War II era — 1946 and later. This development proceeded at an exponential pace in thin films and is covered in an excellent review by Glang 6 on evaporated films, in the Handbook of Thin Film Technology,7 and in the classic text by Holland. 8 More recent references in the Science and Technology of Surface Coatings9''9* include material on PVD techniques as well as the other techniques for surface coatings. The work on mechanical properties of thin films has been reported in several review articles.10"15 Evaporation Processes All deposition processes consist of three major steps. For PVD processes these steps are: 1. Generation of the depositing species. This involves a transition from the condensed phase to the vapor phase. For deposition of compounds, it involves a reaction between the components of the compound, some of which can be introduced into the chamber as a gas or vapor. In evaporation processes, the vapor species are generated by heating the material to be evaporated using resistance, induction, electron beam or laser beam heating sources. 2. Transport of the species from the source to the substrate, where molecular or viscous flow regimes can apply. Molecular flow, where the mean free path is larger than the source-to-substrate distance, occurs at low partial pressures of the depositing species and residual gas in the system. It is responsible for the line-of-sight feature typical of evaporation-deposition processes and low pressure magnetron-type sputtered d e p o s i t i o n p r o c e s s e s . Viscous flow occurs at higher partial pressures, 20120 mtorr, typical of diode sputter deposition. It also occurs when a substantial partial pressure of an inert gas is inten-
tionally added in the evaporation deposition process to cause gas-scattering of the depositing species and increase the throwing power of the process. An additional feature in step 2 is the absence or p r e s e n c e of a plasma in the source-to-substrate region and the plasma's excitation mode, e.g., dc, rf, or m i c r o w a v e . The excitation m o d e is important because it controls the electron energy and distribution function and consequently the plasma volume chemistry. The ionization probability peaks at low electron energies (50100 eV). Therefore, processes which involve low electron energies (such as plasma-assisted evaporation), where the electron energies generating the plasma can be independently controlled, offer a more versatile, richer plasma volume chemistry than processes such as sputtering, where the electron energies are dictated by other considerations such as target voltage (500-100 V) which control the sputtering rate. In sputter deposition, the electron energies cannot be controlled independently of other process p a r a m e t e r s . The p r e s e n c e of a plasma is optional in the evaporation process but is an integral part of the sputtering process. 3. Film growth on the substrate.
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