| Semiconductors
In semiconductor industry, many of the process gases and their byproducts are hazardous to humans or the environment. These gases may be poisonous, like arsine (AsH3) and phosphine (PH3); they may be corrosive like hydrogen chloride (HCl). Some are pyrophoric, like silane (SiH4), they will spontaneously burn when exposed to air. Some gases are strong oxidizers like pure oxygen. Some gases will affect the environment, like chloroflurocarbons. The fundamental requirement for handling semiconductor process gas is containment. The gas must not unintentionally escape to the atmosphere. It must be consumed, neutralized, sequestered or scrubbed so that no significant hazardous or environmentally damaging material is ever released.
Compound Semiconductors
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TecHarmonic Product - Semiconductor Process Summary Table
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CVD, i.e. Chemical Vapor Deposition is chemical reactions, which transform gaseous molecules, called precursor, into a solid material, in the form of thin film or powder, on the surface of a substrate. The process is widely used to fabricate semiconductor devices. The by-products of the CVD process are volatile gases, which may be toxic, flammable or corrosive so they must be treated appropriately. Typical CVD precursor materials are Halides, Hydrides, Metalorganics compounds, Metal Alkyls, and others.
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MOCVD, i.e. Metal-Organic Chemical Vapor Deposition is a technique for depositing thin layers of atoms onto a semiconductor wafer. It involves passing metal oxides in an inert gas across a work piece to deposit a layer of metal oxide on the surface. MOCVD is used with III-V-semiconductors. Some of the source gases are very toxic, e.g. AsH3, PH3, etc.
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Ion Implant is a process that is widely used in the semiconductor industry. Ion implant is different from other semiconductor processes because it does not create a new layer on the wafer. Instead, it changes the electrical characteristics of precise areas within an existing layer on the wafer. In this method, impurities such as arsenic (As), phosphorous (P), boron (B), etc. are ionized then an accelerating column propels these ions to implant the wafers. The wafers show N-type (As, P) or P-type (B) electrical characteristics, depending on the impurities implanted.
Specialty gases such as arsine (AsH3), phosphine (PH3) and boron trifluoride (BF3), and solid substances such as metallic arsenic and red phosphorus are used as impurity sources.
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Etch is a process for removing material in a specified area through a chemical reaction. During the process, the solid is either dissolved in liquid chemicals (wet etching) or converted into gaseous compound (dry etching). One of the key processes in semiconductor manufacturing.
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Diffusion, the movement of one species through another in the direction of concentration gradient, is a natural chemical process. In semiconductor production, diffusion principles are used to introduce dopants into the wafer surface. The process is very dependent on temperature and time.
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VOC -Volatile Organic Compound. In the state of Pennsylvania a volatile organic compound is defined as an organic chemical species with a normal boiling point temperature below 100°C (i.e., the saturation temperature is less than 100°C when the saturation pressure is 101 kPa). Other states may have different (but similar) definitions.
The following website contains a list of VOC: http://www.ene.gov.on.ca/envision/monitoring/VOC_List.pdf
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Hydrides are inorganic compounds composed of hydrogen and other elements, often a metal. Examples include arsine (AsH3), phosphine (PH3), diborane (B2H6), germane (GeH4), stibine (SbH3), and silane (SiH4). The listed hydrides are highly toxic and flammable. They react violently with water and oxidizing agents and pose a dangerous fire risk. Phosphine, diborane, and silane are spontaneously flammable in air.
Certain hydride gases, notably arsine and phosphine, are commonly used as dopants in semiconductor research applications. Arsine is one of the most toxic gases known. It is a potent hemolytic agent (symptoms: red discoloration of the urine and sclera). Phosphine is extremely toxic to organs of high oxygen flow and demand. Thorough emergency planning for accidental releases shall be in place when such gases are to be used in the laboratory. Provision of air-supply respiratory protection may be called for as well as continuous system monitoring for releases.
- Metal hydrides, e.g. NaH, LiAlH4
- Nonmetal hydrides, e.g. B2H6 and other boranes, PH3, AsH3
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Flammables are chemicals in solid, liquid or gas state that have a flashpoint below 100°F. When they are near a spark, flame, or even static electricity, flammables burn at close to room temperature. Examples are: alcohol, propane, phosphorous, and many powder metals.
The following web site contains the list of Flammables substances: http://www.co.forsyth.nc.us/EnvAffairs/112(r)/112(r)_Flam_List.htm
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Pyrophoric chemicals can ignite spontaneously in air at or below a temperature of 130°F. Therefore, they should be stored in tightly closed containers under an inert atmosphere and any handling of them should be carried out under an inert atmosphere or liquid as well.
For each combustible gas or vapor, there is a particular mixture with air that contains just enough oxygen to sustain combustion. Some combustible gas mixtures ignite more easily than others. Further, mixtures of the same constituents in different proportion require different amounts of spark energy to cause combustion.
LOWER EXPLOSIVE LIMIT (LEL): The leanest mixture of gas or vapor in air where, once ignition occurs, the gas or vapor will continue to burn after the source of ignition has been removed.
UPPER EXPLOSIVE LIMIT (UEL): The richest mixture in which a flame will continue to burn after the source of ignition has been removed.
FLASH POINT: The lowest temperature at which a flammable liquid gives off enough vapors to form a flammable or ignitable mixture with air near the surface of the liquid or within the container used. Many hazardous liquids have flash points at or below room temperatures, and they are normally covered by a layer of flammable vapors that will ignite in the presence of a source of ignition.
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Corrosives are harsh substances that cause visible destruction of skin and eyes on contact and internal organs, if one swallows or inhales them. Corrosives are usually concentrated forms of irritants. Examples are: Hydrofluoric acid, Sulfuric acid, Caustic soda, etc.
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Toxic is defined by OSHA 29 CFR 1910.1200 App A as a chemical which falls in any of these three categories:
- A chemical that has a median lethal dose (LD50) of more than 50 milligrams per kilogram but not more than 500 milligrams per kilogram of body weight when administered orally to albino rats weighing between 200 and 300 grams each.
- A chemical that has a median lethal dose (LD50) of more than 200 milligrams per kilogram but not more than 1,000 milligrams per kilogram of body weight when administered by continuous contact for 24 hours (or less if death occurs within 24 hours) with the bare skin of albino rabbits weighing between two and three kilograms each.
- A chemical that has a median lethal concentration (LC50) in air of more than 200 parts per million but not more than 2,000 parts per million by volume of gas or vapor, or more than two milligrams per liter but not more than 20 milligrams per liter of mist, fume, or dust, when administered by continuous inhalation for one hour (or less if death occurs within one hour) to albino rats weighing between 200 and 300 grams each.
Examples used in semiconductor industry are Phosphine, Arsine, Boron Trichloride Ammonia, etc.
The following web site contains the list of Toxics substances: http://www.co.forsyth.nc.us/EnvAffairs/112(r)/112(r)_Toxics_List.htm
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Organometallics or Metalorganics are organic compounds comprised of a metal or nonmetal attached directly to carbon. Examples are Grignard compounds and metallic alkyls such as triethylaluminum and trimethylindium. Many organometallics are highly toxic or flammable. Many are also water-reactive and spontaneously combustible in air. Trialkyltins are the most toxic as a group. Most are highly reactive chemically. Special firefighting equipment (e.g., dry chemical powder fire extinguisher) may be needed where organometallics are handled.
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NOx, i.e. Nitrogen Oxides are gases containing nitrogen and oxygen that include NO, NO2, NO3, N2O, N2O3, N2O4, and N2O5. The first two, nitric oxide (NO), and nitrogen dioxide (NO2) are the primary NOx pollutants. Often cited as "NOX". Nitrogen oxides affect the respiratory apparatus. High concentrations of nitrogen oxides can be detected by the typical red-brownish color of the atmosphere of the cities with heavy traffic. The nitrogen oxides are highly reactive in the atmosphere and so are major sources of photochemical smog. Together with sulphur dioxide they are also a major cause of acid rain and acid deposition. Finally, they play a role in the complex chemistry that destroys the ozone layer. In the presence of CFC'and other chlorinated compounds they can actually reduce the attack on the ozone layer, while in the absence of these compounds they will destroy the ozone layer. The major sources of NOx are combustion processes (boilers, cars, etc.). About 60 percent of NO and NO2 emissions are from stationary sources (smokestacks) and most of the remaining 40 percent is from transportation exhaust, mainly automobiles.
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PFC - Perfluorinated Carbon is chemical compound that contains only fluorine and carbon. PFCs are not ozone-depleting substances. They do however have a high global warming potential. Perfluorinated carbons are highly volatile, linear, branched chain or cyclic perfluorinated carbons (C1 up to C6, fully saturated). This definition is covering the most common commercial PFC gases and liquids with boiling points up to 56°C even though there are other PFCs compounds, mainly used in closed system heat transfer applications. Examples are CF4, C2F6, C3F8, etc.
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