4.16 Semiconductor Manufacturing
Categories 43 and 1891
4.16.1 Introduction
Categories 43 and 1891 account for point and area source pollutant emissions (PM, PM10, PM2.5, ROG, and TOG) from semiconductor manufacturing plants and other related integrated circuits manufacturing. Semiconductor manufacturing processes include blank wafer production, semiconductor fabrication, and assembly and packaging. (The following description of the semiconductor manufacturing processes was taken from the BAAQMD’s Permit Handbook, Section 7, Chapter 4.)
Blank Wafer Production
Silicon Crystal Growth: Molten silicon is introduced into a mold with a seed crystal of silicon. As the molten silicon cools, it crystallizes around the seed to “grow” a single crystalline ingot. Crystal growth is not in itself a source of air emissions and is not subject to permit requirements. Miscellaneous cleaning operations may result in organic or inorganic emissions. Organic cleaning operations may be exempt in accordance with District Regulation 2, Rule 1 - General Requirements, [Rule 2-1-118], 93. Abatement of organic or inorganic emissions may be required if emissions exceed the current best available control technology (BACT) trigger level.
Wafer Manufacturing: The cooled silicon ingot is shaped and sliced into round wafers, which are mechanically polished (“lapped”). These steps are subject to the particulate emission limits of [Regulation 6] if particulate emissions are produced and vented outside of the facility. Mechanical shaping, slicing and polishing may be exempt from permit requirements in accordance with District Regulation 2, Rule 1 - General Requirements, [Rule 2-1-121.1] or [2-1-125.1.2].
After polishing, wafers are etched in a chemical bath to remove surface imperfections. Depending on the type and concentration of etchants used, mists and aerosols may be produced. Finally, each wafer is polished to a smooth finish. The etching operations may be exempt from permitting requirements by District Regulation 2, Rule 1 - General Requirements [Rule 2-1-127.4], if the toxic risk provisions of [Rule 2-1-316] are satisfied.
If adhesives are used to fix wafers in position during polishing, the use of adhesives may be subject to VOC limits and other requirements in [Rule 8-51] 94. The application of adhesives may be exempt from permit requirements in accordance with [Rule 2-1-119.2], if the toxic risk provisions of District Regulation 2, Rule 1 - General Requirements [Rule 2-1-316] are satisfied. Miscellaneous cleaning operations may result in organic or inorganic emissions. Organic cleaning operations may be exempt in accordance with [Rule 2-1-118]. Abatement of organic or inorganic emissions may be required if emissions exceed the current best available control technology (BACT) trigger level.
Semiconductor Manufacturing
The processes used to form ICs on the wafer include:
Oxidation, where an inert layer of silicon dioxide is formed on the wafer by exposing the wafer to a heated oxygen environment.
Photoresist application, exposure and development, where solvent-based, light-sensitive resin solutions are uniformly applied to the wafer and then processed to leave a pattern of cured photoresist on the wafer which corresponds to the circuit image, while removing the non-image coating. Photoresist applicators may also apply non-light sensitive coatings (spin-on-glass, anti-reflective coatings) and solvents (edge bead remover, hexamethyldisilazane (HMDS)).
Etching, where reactive gases or liquids are used to remove the silicon dioxide layer from the wafer surface where it is not protected by cured photoresist, thereby exposing the underlying silicon for further processing; liquid etching is classified as a wet chemical station, as described below.
Photoresist stripping, where cured photoresist is removed from the wafer after it has allowed selective wafer surface processing; stripping solutions may be organic or inorganic depending on the composition of the underlying wafer surface, and therefore may be classified as either solvent stations or a wet chemical stations, as described below.
Doping (diffusion, ion implant), where the wafer is exposed to impurities that penetrate into the exposed silicon patterns to selectively modify the electrical conductivity of the silicon, thereby producing electronic components and circuits.
Layering (epitaxial growth, metallization films, chemical vapor deposition), where a doped wafer is covered with a uniform layer of silicon (to form a base for additional circuit layers) or metal (to form a conductive connection between the circuit layers and the external IC package).
Other processes include:
- Chemical mechanical polishing (CMP), where wafer surfaces are polished to maintain wafer flatness during processing.
- Solvent stations, where wafers or tools are cleaned by immersion in a solvent liquid or vapor or by being sprayed with a solvent liquid.
- Wet chemical stations, where wafers or tools are cleaned or etched by immersion in an inorganic solution or by being sprayed with such a solution.
- Wipe cleaning, where tools and work surfaces are cleaned in place or at a dedicated station.
Assembly and Packaging
This is where wafers are cut into individual integrated circuits, which then are mounted into a package for assembly on a printed circuit board.
4.16.2 Methodology
Point Sources
Point Sources are operations that emit air pollution into the atmosphere at a fixed location within a facility, for which the Air District has issued a permit to operate, e.g. refinery cooling towers. These could also be a collection of similar equipment / sources located across multiple facilities, e.g. reciprocating engines.
During the permit to operate (PTO) issuance process, the BAAQMD collects information from the operating facility and/or determines from published literature, e.g. EPA’s AP-42, characteristics of a source including maximum throughput, emission factors for emitted pollutants, and control factors associated with downstream abatement devices. These characteristics are then stored for future use in the BAAQMD’s internal database. Facilities that hold a permit to operate are required to renew this permit periodically (this period varies based on facility and source type). Upon renewal, the facilities are requested to provide any updates to source characteristics as well as the source throughput for the last 12 months. This throughput, in combination with the emission factors and controls factors stored in the internal database, are used to estimate annual emissions at the source level. These source level emissions are then sorted and aggregated into categories.
Further speciation and quality assurance of emissions are performed as a part of the inventory process. The BAAQMD staff also perform a systematic crosswalk between CEPAM’s source category classification (Emission Inventory Code - EICs) and the District’s source category classification (category identification number - cat_ids), which ensures consistency in the annual emissions reporting process (CEIDARS) to California Air Resources Board. The last part of the inventory development process includes forecasting and back casting, and aggregation into sub-sectors and sectors for documentation purposes. For those years where no data is available, emissions data are backcasted to year-1990, as well as forecasted to year-2040 using either interpolation or another mathematical approach (see Trends section). Finally, emissions trends spanning from year 1990-2040 for each category and pollutant are evaluated for anomalies that are then investigated and addressed.
Category 43 is considered a point source category and follows the above methodology for emissions estimates.
Area Sources
Category 1891. This category estimates emissions from high global warming potential (GWP) 95 materials (perfluorocarbons, nitrogen trifluoride, and sulfur hexafluoride) in the semiconductor industry. These emissions are considered as area sources.
(a) Activity Data / Throughput
Category 43. This category contains point sources only where information on equipment, operating data, and throughput of each source are reported by individual semiconductor manufacturing plants and are stored in the District’s internal database.
Category 1891. These high GWP GHG emissions were estimated using data from the latest California Air Resources Boards (CARB) “Documentation of California’s 2000-2020 GHG Inventory”, California Greenhouse Gas Inventory by IPCC Category 96. The District does not have access the activity data and directly uses the statewide emissions for further processing.
(b) County Distribution / Fractions
Category 43. The county location of each company as reported in the District’s internal database was used to distribute emissions into each county.
Category 1891. The number of employees in each applicable Bay Area county’s, “Computer and Electronic Products” sector was used to determine the county fraction. This data was found in Table C-7 of the 2011 CDF Statistical Abstract report.
(c) Emission Factors
Perfluorocarbon emissions are considered total organic gas (TOG) emissions, with no reactive organic gas (ROG) component. They are also considered GHG’s with a range of global warming potential (GWP) values. The 2011 composite GWP value used for calculation purposes was 1,308.
(d) Control Factors
Category 43. District [Rule 8-30], 97 adopted in July 1983, set operating standards in solvent cleaning stations at semiconductor manufacturing facilities. An amendment to the regulation effective in November 1988 further set standards for the negative photoresist operations. October 1998 set requirements for solvent spray and solvent vapor units and fab area wipe cleaning operations. Additionally, there were further amendments to negative photoresist operations. Currently, it is estimated emissions are reduced by an overall control of approximately 85% due to this regulation.
(e) Speciation
The PM2.5/PM and the PM10/PM ratios applied to category 43 are based on an Air District internal speciation profile. Multiple data sources have been used for developing speciation profiles, such as Air District-approved source tests, PM speciation ratios used by other regional air quality agencies, and other relevant literature. These ratios are not necessarily consistent with the latest speciation profiles developed by CARB98 or the US Environmental Protection Agency99. For this category or group of categories, PM2.5 constitutes 42% of total PM and PM10 constitutes 70% of total PM. The Air District staff routinely review speciation profiles and may update ratios as needed for improving emissions estimates.
The ROG/TOG ratios applied to category 43 are based on an Air District internal speciation profile. Multiple data sources have been used for developing speciation profiles, such as Air District-approved source tests, TOG speciation ratios used by other regional air quality agencies, and relevant literature including latest speciation profiles developed by CARB100 and the US Environmental Protection Agency. For this category or group of categories, ROG constitutes 83% of TOG.
For category 1891, there are no PM emissions and there is no reactive organic gas (ROG) component in the Perfluorocarbon (TOG) emissions.
4.16.3 Changes in Methodology
Category 1891 follows the methodology for a special case category.
4.16.4 Emissions
A summary of emissions by category, county, and year are available via the associated data dashboard for this inventory publication.
4.16.5 Trends
(a) Historical Emissions / History
Category 43 Historical emissions for this category are derived from source-specific throughputs provided by the permitted facility, compiled/reported emission factors, and regulation-based control factors. This information is archived in the BAAQMD’s internal database which is queried to retrieve the data for historical and current years. Interpolation techniques to account for missing data are used when necessary, this is the case for years 1991-1992.
Category 1891. From 1990 – 2010, the growth profile was based on GHG emissions provided by CARB’s “Documentation of California’s Greenhouse Gas Inventory”, California Greenhouse Gas Inventory by IPCC Category. From 1984 – 1990, the growth profile was based on ABAG’s 2009 Information Employment growth profile. From 1975 - 1984, the growth profile assumed to grow linearly from zero, since prior to 1975, there were no emissions.
(b) Future Projections / Growth
Forecasting of point source emissions is done based on calculations as shown in the equation below using recently updated growth profiles and a base year of 2020. The growth profiles for the current base year inventory have been verified and updated to represent the most likely surrogate for growing emissions for a given category up to year 2040. Forecasting for point source emissions includes impact of in-place regulations, but does not include estimation of controls that will theoretically be implemented as part of future policy emission targets or proposed regulation and legislation.
\[ \text{PE} = \text{Gr} * \text{Ci} * \text{Ei} \] \(PE\) = projected emissions of pollutant i in a past or future year
\(Gr\) = growth rate by economic profile of industry or population
\(Ci\) = control factor of pollutant i based on adopted rules and regulations
\(Ei\) = base year emissions of pollutant i
Projections of emissions for both categories 43 and 1891 to year 2040 are assumed to be constant starting from 2020.
4.16.6 Uncertainties
The demand for semiconductor on global commerce may lead to uncertainties of emission projections.
4.16.7 Contact
Author: Michael Nguyen
Reviewer: Ariana Husain
Last Update: November 06, 2023
4.16.8 References & Footnotes
BAAQMD. 2017. District Regulation 2, Rule 1 - General Requirements, https://www.baaqmd.gov/~/media/dotgov/files/rules/regulation-2-rule-1/documents/20171206_fr_0201-pdf.pdf?la=en&rev=42c6de7702484ea58dfcbcecaeafa29f↩︎
BAAQMD. 2002. District Regulation 8, Rule 51 - Adhesive and Sealant Products, https://www.baaqmd.gov/~/media/dotgov/files/rules/reg-8-rule-51-adhesive-and-sealant-products/documents/rg0851.pdf?la=en&rev=7cf8dc673a4f41629984727defad9b55↩︎
BAAQMD. 2017. Greenhouse Gas Emission Estimates and Draft Forecasts, https://www.baaqmd.gov/~/media/files/planning-and-research/plans/2017-clean-air-plan/ghg_emissions_and_forecasts_draft.pdf?la=en↩︎
CARB. 2022. Documentation of California’s 2000-2020 GHG Inventory, https://ww2.arb.ca.gov/applications/california-ghg-inventory-documentation↩︎
BAAQMD. 1998. Regulation 8 Rule 30 - Semiconductor Wafer Fabrication Operations, https://www.baaqmd.gov/~/media/dotgov/files/rules/reg-8-rule-30-semiconductor-wafer-fabrication-operations/documents/rg0830.pdf?la=en&rev=6399035c055b4951b147495054fb5057↩︎
CARB. 2022.PMSIZE https://ww2.arb.ca.gov/speciation-profiles-used-carb-modeling↩︎
EPA. 2022. SPECIATE https://www.epa.gov/air-emissions-modeling/speciate↩︎
CARB. 2022.ORGPROF https://ww2.arb.ca.gov/speciation-profiles-used-carb-modeling↩︎