Deposition is a broad term used in semiconductor processing that refers to the layering of additional material on the wafer surface. These layers may be applied at various stages during the manufacturing process in order to form a mask, to act as a new layer for further junction formation, or to form an insulating layer between two or more conductive layers. The general technique of deposition is known as chemical vapor deposition (CVD). CVD is commonly used to deposit layers of polycrystalline silicon, silicon dioxide, and silicon nitride on the substrate.
CVD is accomplished by placing the substrate wafers in a reactor chamber and heating them to a certain temperature. Controlled amounts of silicon or nitride source gases, usually carried by either nitrogen and/or hydrogen, are added to the reactor. Dopant gases may also be added if desired. A reaction between the source gases and the wafer occurs, thereby depositing the desired layer. Reaction temperatures between 500-1100ºC and pressures ranging from atmospheric to low pressure are used, depending on the specific deposition performed. Heating is usually accomplished with radiofrequency, infrared, or thermal resistance heating. Common source gases include silane, silicon tetrachloride, ammonia, and nitrous oxide. Some dopant gases that are used include arsine, phosphine, and diborane. The major categories of silicon CVD are shown in Table 6.
Epitaxy is a specific form of CVD that is used to form a thin elemental crystal layer on top of an identical substrate crystal. The main advantage of epitaxy is that a lightly doped layer of epitaxial silicon can be grown on top of a heavily doped silicon substrate, thus creating a layer of differing conductivity that can serve as an insulating layer. Silicon upon silicon is the most common epitaxial process. Usually, hydrogen chloride gas is first used to etch the wafers. Then gases such as silane, dichlorosilane, and trichlorosilane are used to deposit silicon. Light doping of the new crystal layer with additional gases may also be performed. The process is usually carried out at atmospheric pressure and temperatures between 900-1300ºC. Table 7 identifies the four major types of vapor phase epitaxy, parameters, and chemical reactions.
The following are the potential hazards of depostion.
- Flammable, Explosive and Pyrophoric Gases
- Toxic, Irritative and Corrosive Gases
- Radiofrequency (RF) and Infrared (IR) Radiation
- Thermal Burns
- Reaction-Product Residues
- Potential chemical exposures to maintenance personnel working on reaction chambers, pumps, and other associated equipment that may contain reaction-product residues. Substances such as HCl, arsine, phosphine, etc., may be found in deposition equipment.
- Occupational Health Guidelines for Chemical Hazards. US Department of Health and Human Services (DHHS), National Institute for Occupational Safety and Health (NIOSH) Publication No. 81-123, (1981, January). Provides a table of contents of guidelines for many hazardous chemicals. The files provide technical chemical information, including chemical and physical properties, health effects, exposure limits, and recommendations for medical monitoring, personal protective equipment (PPE), and control procedures.