Thin-Film Deposition
to connect device structures to the “ outside world” requires the deposition and patterning of a metal layer. In fact , complex ICs have three and sometime four electrically isolated metallization layers. Electrical isolation of the metal layers in turn requires the deposition of intervening dielectric layers. Thin films are also deposit to prevent the interdiffusion of materials and to protect the device or circuit from contamination . In –use methods for depositing the required film are reviewed next.
Evoporation
Evoporation is one of the older and more straightforward methords of thin -film deposition. As envisiones in Fig. 4.12, the material to be evaporated is placed in or on a resistance heated
source holder inside a vacuum chamber. To evaporate AL, for example, a short piece of AL wire would be place on a tungsten or boat. The substrate on with the film is to be deposited is also positioned inside the chamber facing the source. The chamber is then evacuate, power supplied to the holder , and the source vaporized. Because of the reduced pressure, the source material travels unimpeded to the substrate and deposit as a thin film.
Generally speaking , hot-filament evaporation is subject to moderately high levels of contamination. Electron-beam evaporation, a variation of the process where the source is heated by an electron-beam, eliminates contamination but generates device-degrading x-rays.
Consequently, evaporation is seldom used in the production-line fabrication of modern ICs, although it is still extensively used in making simple devices where the cited problems are of minimal concern.
Sputtering
Sputtering, like evaporation, is performated in a vacuum chamber. The source material and the substrate (wafer) and placed on opposing parallel plates connected to a high-voltage power supply as pictured in Fig.4.13. During a deposition the chamber is fist evacuated
Figure4.13 schematic of a d.c. sputtering system. The source material covers the cathode while the wafer is mounted on the system anode. (From Jaeger 1988 by Addison-wesley Publishing Co., Inc. Reprinted by permission of the publisher.) of air and then a low-pressure amount of sputtering gas, typically Ar, is admitted in to the chamber. Applying an interelectrode voltage ionizes the Ar gas and creates a plasma between the plates. Since the plate covered with source material is maintained at the necative potential relative to the substrate, Ar* irons accelerated toward and into the source covered plate. The impacting Ar* irons are in turn cause source atoms or moreculas to be ejected from this plate. Being neutral, the ejected atoms or molecules readily traval to the substrate where they deposit to from the desired thin firm. A d.c. power supply can be used when depositing metals, and an RF supply is necessary to in traduce a gas of one of the components to assure the formation of a near- stoichiometric film. Providing low-temperature , low-contamination film with and acceptable throuthput , sputtering has become the chief commercial methord of depositing Al and other metals.
Chemical vapor deposition
In chemical vapor deposition the thin film is formed from one or more gas phase components. Either a compound decomposes to form the film or a reactions between gas components as takes place on form, Invariably the CVD reactions are surface catalyzed, preferentially taking place on the surface of wafers inserted into the gas stream. In-use CVD processes fall into one of three general categories. They are atmospheric pressure ( APCVD or simply CVD ). low-pressure (LPCVD). and plasma-enhanced (PECVD) pro-cesses. Atmospheric pressure depositions can be performed in relatively simple systems. Low-pressure often offers comparable kinetics with improved uniformity and less gas consumption. In plasma CVD the electrons in the plasma impart energy to the reaction gases,thereby enhancing the reactions and permitting very low deposition temperatures.
CVD reactors come in a variety of shapes and configurations; an example configuration employed in AP/LPCVD depositions is pictured in Fig. 4.14. CVD processing is routinely used to produce the masking and intermetallic films required in the formation of complex ICs. Polyerystalline Si. which functions as a pseudo-metal when heavily doped. is also deposited employing atmospheric and low-pressure CVD.APCVD. LPCVD. and PECVD are all typically used at some point in an IC process flow