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Substrate Manufacture
Polycrustalline Silicon Production |
To manufacture polycrystalline silicon, ultra-pure silicon is produced from raw quartzite (silica sand) that is melted and reduced to silicon in an electric arc furnace at over 1900ºC.
This metallurgical-grade silicon is drawn from the furnace and blown with oxygen or an oxygen-chloride mixture to
reduce the levels of impurities to achieve approximately 99% pure silicon. Next, the silicon is reacted with
hydrogen chloride gas in the presence of a copper-containing catalyst to form trichlorosilane (SiHCl3).
The trichlorosilane is reduced to very pure silicon by reacting it with hydrogen at high temperatures (about 1100ºC).
This "electronic grade" silicon has less than 1 ppb of impurities.
The following are the potential hazards of polycrustalline silicon production.
Silica (Crystalline)
Potential Hazard
- Possible employee exposure to crystalline silica used as a raw material.
Inhalation of silica can lead to chronic, accelerated or acute silicosis and is associated
with bronchitis and tuberculosis. Some studies also indicate an association with lung
cancer. Exposures to silicon dust may also occur; controls are
similar to those used for crystalline silica.
Possible Solutions
- Identify silica hazards and perform appropriate exposure evaluations.
- Identify and evaluate all potential exposure scenarios, for example: startup, operations, maintenance, cleaning, emergencies, and so forth.
- 29 CFR 1910.1000
Table
Z-3 provides permissible exposure limits for crystalline silica.
- OSHA
Technical Manual (OTM). OSHA Directive TED
01-00-015 [TED 1-0.15A], (1999, January 20).
- Provide appropriate ventilation to reduce silica concentration levels in the air.
- Maintain adequate housekeeping to remove unwanted silica dust and reduce
concentration levels.
- Use respiratory protection [29 CFR 1910.134] when necessary to further reduce exposure and protect employees.
Additional Information
OSHA Safety and Health Topics Pages:
OSHA eTools:
- Silica. The Silica eTool is an expert training and information aid. It includes current information
that will assist businesses and workers in identifying potential silica hazards in their workplaces by choosing appropriate
sampling and analytical techniques, comparing monitoring results with the silica exposure limits, and selecting appropriate
short and long-term control options.
Hydrogen Chloride Gas
Potential Hazard
- Possible employee exposure to hydrogen chloride gas. Hydrogen chloride is
irritating and corrosive to the eyes, skin, and mucous membranes. Exposure to high
concentrations can cause laryngitis, bronchitis, and pulmonary edema.
Possible Solutions
- Identify hydrogen chloride hazards and perform appropriate exposure
evaluations.
- Identify and evaluate all potential exposure scenarios, for example: startup, operations, maintenance, cleaning, emergencies, and so forth.
- 29 CFR 1910.1000
Table
Z-1 provides permissible exposure limits for hydrogen chloride.
- 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.
- Provide appropriate ventilation to reduce hydrogen chloride concentration
levels in the air.
- Provide PPE [29 CFR 1910
Subpart I] as appropriate to prevent eye and skin contact with hydrogen chloride.
- Use respiratory protection [29 CFR 1910.134]
when necessary to further reduce exposure and protect employees.
Additional Information
OSHA Safety and Health Topics Pages:
Single Crystal Ingot Growth |
 Almost all crystal growth is done by the Czochralski (Cz) method. This method begins by heating electronic-grade
polycrystalline silicon in a quartz crucible to 1200ºC in an argon atmosphere. Either radiofrequency (RF) or
resistance heating is used. A starter or "seed" crystal of silicon is placed onto the end of a rod and
dipped into the melt to form the crystal. The seed and crucible are rotated in opposite directions while the seed is
withdrawn. Silicon atoms attach to the rod and the crystal grows in size. Careful control of temperature, rotation
speed, and vertical withdrawal determines the size of the ingot. Different atmospheres (inert, oxidizing, reducing)
and pressures (vacuum, high pressure) also are maintained in the growth chamber depending on the type of crystal
desired.
Controlled amounts of impurities are added during crystal growth to establish the desired electrical properties
for the silicon. The melt is usually "doped" with elements like boron, phosphorous, arsenic, or antimony.
The following are the potential hazards of single crystal ingot growth.
Metals and Salts
Potential Hazard
- Possible employee exposure to various metals and salts used for elemental
dopants, including phosphorous, boron, arsenic,
antimony, magnesium, etc.
Possible Solutions
- Identify metal hazards and perform appropriate exposure evaluations.
- Perform exposure measurements for the compounds used.
- Keep exposures below acceptable exposure levels.
- Address all dermal exposures.
- Provide appropriate ventilation to reduce concentration levels in air.
- Provide PPE [29 CFR 1910
Subpart I] as appropriate to prevent contact.
- Use respiratory protection [29 CFR 1910.134] when necessary to further reduce exposure and protect employees.
- Maintain adequate housekeeping to remove unwanted metals and reduce
concentration levels.
Additional Information
- 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.
OSHA Safety and Health Topics Pages:
Ingot Evaluation and Machining |
 Before the ingots are sliced into wafer substrates, the ends of the new single-crystal ingot are cropped using a water-lubricated single-bladed diamond saw. The
ingot is then placed on a lathe and ground to a uniform diameter. The ends of the cropped and ground ingots are
chamfered (beveled), using a dry belt sander, which reduces the possibility of shattering the ingot. The crystal
structure of the ingot is determined by x-ray diffraction, then a longitudinal section of the ingot cylinder is
removed by wet grinding to produce a "flat". This flat is used to mark the crystal orientation of the
ingot.
The following are the potential hazards of ingot evaluation and machining.
 To prepare the wafers, ingots are sliced into individual wafers with multiple-blade inner-diameter saws. This operation is done with wet
lubricants, and in some processes the wafers are stored in plastic reservoirs containing water or methanol. The
sliced wafers are mechanically lapped under pressure using a counter-rotating machine to achieve flatness and
parallelism on both sides of the wafer. Most lapping operations use slurries of either aluminum
oxide or silicon carbide. The edges of the individual
wafers are also rounded by the use of wet automatic grinders.
After lapping, wafers are etched with a solution containing nitric, acetic, and hydrofluoric acids. Etching may
be done in manual etch tanks or by automated etching machines. This etching process removes external surface damage
and reduces the thickness of the wafer.
 Next, the wafers are polished using an aqueous mixture of colloidal silica and sodium hydroxide. The wafers are
mounted onto a metal carrier plate that is attached by vacuum to the polishing machine. The polishing process
usually involves two or three polishing steps with progressively finer slurry, which decreases wafer thickness and
results in a mirror-like finish. Sometimes carrier pads must be stripped from the metal carrier plates. The pads are
usually stripped with solvents such as methylene chloride, methyl ethyl ketone, or a glycol ether mixture.
 Finally, the wafers are cleaned to remove any particles or residue remaining on the exterior surface of the
polished wafer. Various cleaning steps and solutions containing ammonia, hydrogen peroxide, hydrofluoric acid,
hydrochloric acid, and deionized water may be used.
The finished wafers are inspected and packaged for shipping. It should be noted that most semiconductor
manufacturers purchase wafers from firms that specialize in wafer production.
The following are the potential hazards of wafer preparation.
Methanol
Potential Hazard
- Possible employee exposure to methanol during wafer washing and storage.
Possible Solutions
- Identify methanol hazards and perform appropriate exposure evaluations.
- Perform exposure measurements.
- Keep exposures below acceptable exposure levels.
- Address all dermal exposures.
- Provide appropriate ventilation to reduce solvent concentration levels in the air.
- Provide PPE [29
CFR 1910
Subpart I] as appropriate to prevent eye and skin contact.
- Use respiratory protection [29 CFR 1910.134] when necessary to further reduce exposure and protect employees.
Additional Information
OSHA Safety and Health Topics Pages:
Acids
Potential Hazard
- Possible employee exposure to acids used during etching. Typical acids may
include HF, CH3COOH, and HNO3.
Possible Solutions
Additional Information
- 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.
Solvents
Potential Hazard
- Possible employee exposure to solvents used for stripping carrier pads.
Typical solvents may include methylene chloride, MEK, and glycol ethers.
Possible Solutions
Additional Information
Chemicals
Potential Hazard
- Possible employee exposure to additional chemicals used for final cleaning.
Typical chemicals may include NH3, H2O2, HF, and HCl.
Possible Solutions
Additional Information
-
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.
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