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Oxidation | Cleaning | Photoresist Application | Soft Bake | Mask Alignment and Photoexposure | Developing | Hard Bake
Etching | Photoresist Stripping | Doping (Junction Formation) | Deposition

Oxidation

The fabrication of an integrated circuit involves a sequence of processes that may be repeated many times before a circuit is complete. The device fabrication steps discussed in this and subsequent sections may be repeated anywhere from six to 15 times to achieve the desired product
Robotic arm transfers a wafer

Generally, the first step in semiconductor device fabrication involves the oxidation of the wafer surface in order to grow a thin layer of silicon dioxide (SiO2). This oxide is used to provide insulating and passivation layers.

  • The most common method of oxidation is thermal, and can be classified as either "dry" or "wet" oxidation. Wafers are loaded into quartz boats and slid into a furnace heated to approximately 1200C.
    • In dry oxidation, thin oxide layers are grown in an environment containing oxygen and hydrogen chloride near atmospheric pressure.
    • Thicker oxide layers require higher pressures and the use of steam (wet oxidation). Wet oxidation is performed by exposing the wafer to a mixture of oxygen and hydrogen in the furnace chamber. Water vapor is formed when the hydrogen and oxygen react.

The following are the potential hazards of oxidation.

Toxic Exhaust Gases

Potential Hazard

  • Possible employee exposure to corrosive exhaust gases, including hydrogen chloride. Gases such as hydrogen chloride can be irritating and corrosive to the eyes, skin, and mucous membranes. Exposure to high concentrations can cause laryngitis, bronchitis, and pulmonary edema.

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.

Cleaning

The need for a particulate and contamination-free wafer surface requires frequent cleaning. The major types of cleaning are:

  • Deionized water and detergent scrubbing.
  • Solvent: isopropyl alcohol (IPA), acetone, ethanol, terpenes.
  • Acid: HF, H2SO4 and H2O2, HCl, HNO3, and mixtures.
  • Caustic: NH4OH

The following are the potential hazards of cleaning.

Acids and Caustic Solutions

Potential Hazard

  • Possible employee exposure to acid and caustic solutions used during cleaning.

    • Typical acids may include mixtures of HF, H2SO4, H2O2, HCl, and HNO3.

    • Caustic solutions include mainly NH4OH.

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.

Photoresist Application

Following the creation of a silicon dioxide layer, the wafer is coated with a photosensitive material called a "photoresist." 

  • There are two types of photoresists: positive and negative. Positive photoresists undergo weakening when exposed to irradiation, whereas negative photoresists are strengthened. Most semiconductor processes use a positive resist.
  • The photoresist is applied by delivering a small amount of the liquid to the center of the wafer, then spinning the wafer at high speed to spread the material over the entire surface in a thin, uniform coating. Sometimes wafers are primed with an adhesive, hexamethyldisilazane (HMDS). Glycol ethers have been a popular solvent for carrying HMDS, although some manufacturers have switched to alternative solvents like xylene, n-butyl acetate, acetone, and 1,1,1-trichloroethane. Table 1 identifies the component makeup of various photoresist systems.

The following are the potential hazards of photoresist application.

Photoresist Chemicals

Potential Hazard

  • Possible employee exposure to photoresist chemicals (see Table 1).

Possible Solutions

  • Identify chemical hazards and perform appropriate exposure evaluations.
    • Perform exposure measurements for the chemicals used.

    • 29 CFR 1910.1000 Table Z-1 provides permissible exposure limits for various chemicals.
  • Address all dermal exposures.

  • Provide appropriate ventilation to reduce chemical 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.

  • Design and use specialized processing, material handling, and storage equipment to properly contain chemicals. Consider both normal use and emergency scenarios.

  • Install emergency facilities to provide immediate treatment in the event of an accidental exposure to corrosive materials. According to 29 CFR 1910.151, provide suitable facilities for quick drenching or flushing of the eyes and body for immediate emergency use whenever the eyes or body may be exposed to corrosive materials.

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:

Solvents

Potential Hazard

  • Possible employee exposure to solvents used for adhesive application.

    • Glycol ethers have been a popular solvent. However, due to reproductive effects associated with exposures, they have been replaced with other chemicals.

    • Replacement solvents for glycol ethers have included chemicals such as xylene, n-butyl acetate, acetone, and 1,1,1-trichloroethane.

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.

Soft Bake

After photoresist application, the wafers are "soft baked" by placing them in an oven at moderate temperatures around 70-90C. This soft bake causes the photoresist to cure and the remaining solvents to evaporate.

The following are the potential hazards of soft baking.

Mask Alignment and Photoexposure


Stepper deep ultraviolet (DUV) tool

A photomask is aligned and placed on the coated wafer with precision instruments. The wafer and mask are then exposed to ultraviolet (UV) radiation from an intense mercury arc lamp. This causes exposure to the photo resist in places not protected by opaque regions of the mask. With a typical positive photoresist, the areas struck by light undergo a chemical reaction that will make the photoresist more soluble in an alkaline solution. UV exposure is the most common; however, x-ray and electron beam sources also may be used. Technician examining a reticle

The following are the potential hazards of mask alignment and photo exposure.

Ultraviolet (UV) Radiation

Potential Hazard

  • Possible employee exposure to ultraviolet (UV) radiation during photoexposure.

Possible Solutions

  • Identify UV hazards; perform exposure evaluations when applicable.

  • Enclose operations with UV emissions; provide shielding and interlocks as necessary.

  • Provide PPE [1910 Subpart I] as appropriate during operations when exposure is necessary.

  • Implement UV radiation safety programs to further identify and control UV hazards. Ozone gas may also be generated from the UV radiation.

  • Provide adequate ventilation to control ozone concentrations.

Additional Information

OSHA Safety and Health Topics Pages:

Mercury

Potential Hazard

  • Possible employee exposure to mercury from lamp rupture. Improper maintenance or infrequent bulb replacement can cause deteriorated or older lamps to rupture.

Possible Solutions

  • Implement preventive maintenance to inspect and replace lamps routinely.

  • Implement proper work practices to ensure that lamps are replaced carefully in order to avoid accidental breakage.

  • Store and dispose of lamps properly.

  • Respond to and clean up mercury spills properly.

  • Provide adequate PPE as necessary to minimize exposure.

Additional Information

OSHA Safety and Health Topics Pages:

Developing

Following exposure, the wafers are developed with aqueous solutions of either sodium hydroxide or potassium hydroxide. The developer is applied by either immersion, spraying, or atomization, causing the unpolymerized areas of the photoresist to be dissolved and removed. Various developer solutions are identified in Table 1. A solvent rinse (n-butyl acetate, IPA, acetone, etc.) is usually applied following the developer to remove any residual material.

The following are the potential hazards of developing.

Caustic Solutions and Aerosols

Potential Hazard

  • Possible employee exposure to caustic solutions and aerosols used during developing. Typical caustics include NaOH and KOH.

Possible Solutions

Hard Bake

After developing, an additional baking process or "hard bake" is performed to harden the remaining photoresist to a finish much like the enamel on an automobile. The photoresist is then ready to protect the underlying SiO2 during etching.

The following are the potential hazards of hard baking.

Etching

Etching removes layers of SiO2, metals, and polysilicon, according to the desired patterns delineated by the resist. The two major methods of etching are wet chemical etching or dry chemical etching.

Wet Chemical Etching: Wet etching is accomplished by submersion of the wafer in an acid bath. Common wet etchant chemical solutions are shown in Table 2. In general, etching solutions are housed in polypropylene, temperature-controlled baths. The baths are usually equipped with either a ring-type plenum exhaust ventilation or a slotted exhaust at the rear of the etch station. Vertical laminar-flow hoods are used to supply uniformly-filtered, particulate-free air to the top surface of the etch baths.

Dry Chemical Etching: Dry etching is commonly used due to its ability to better control the etching process and reduce contamination levels. Dry processing effectively etches desired layers through the use of gases, using either, a chemically reactive gas, or through physical bombardment of argon atoms.

Chemical - Plasma Etching: Plasma etching systems have been developed that can effectively etch silicon, silicon dioxide, silicon nitride, aluminum, tantalum, tantalum compounds, chromium, tungsten, gold, and glass. Two kinds of plasma etching reactor systems are in use — the barrel (cylindrical), and the parallel plate (planar). Both reactor types operate on the same principles and vary primarily in configuration only. The typical reactor consists of a vacuum reactor chamber made usually of aluminum, glass, or quartz. A radiofrequency (RF) energy source is used to activate fluorine-based or chlorine-based gases which act as etchants. Wafers are loaded into the chamber, a pump evacuates the chamber, and the reagent gas is introduced. The RF energy ionizes the gas and forms the etching plasma, which reacts with the wafers to form volatile products which are pumped away. Table 3 identifies the materials and plasma gases in use for etching various layers.

Physical Bombardment: Physical etching processes are similar to sandblasting; argon gas atoms are used to physically bombard the layer to be etched, and a vacuum pump system is used to remove dislocated material. Sputter etching is one physical technique involving ion impact and energy transfer. The wafer to be etched is attached to a negative electrode, or "target," in a glow-discharge circuit. Positive argon ions bombard the wafer surface, resulting in the dislocation of the surface atoms. Power is provided by an RF energy source. Ion beam etching and milling are similar physical etching processes which use a beam of low-energy ions to dislodge material. The ion beam is extracted from an ionized gas (argon or argon/oxygen) or plasma, created by an electrical discharge.

Reactive ion etching (RIE) is a combination of chemical and physical etching. During RIE, a wafer is placed in a chamber with an atmosphere of chemically reactive gas (CF4 or CCl4) at a low pressure. An electrical discharge creates an ion plasma with an energy of a few hundred electron volts. The ions strike the wafer surface vertically, where they react to form volatile species that are removed by the low pressure in-line vacuum system.

The following are the potential hazards of etching.

Acids

Potential Hazard

  • Possible employee exposure to acids used for wet chemical etching. Typical acids may include mixtures of HF, HCl, H2SO4, etc. (see Table 2).

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.

Reactive Gases

Potential Hazard

  • Possible employee exposure to fluorinated, chlorinated, and other reactive gases used for dry etching (see Table 3).

Possible Solutions

  • Identify gas hazards and perform appropriate exposure evaluations.

    • Identify and evaluate all potential exposure scenarios, for example: startup, operation, maintenance, cleaning, emergencies, and so forth.

    • See 29 CFR 1910.1000, Table Z-1, which contains permissible exposure limits for various substances.
  • Provide appropriate ventilation to reduce gas concentration levels in the air.

  • Provide PPE [29 CFR 1910 Subpart I] as appropriate to prevent contact with gases.

  • Use respiratory protection [29 CFR 1910.134] when necessary to further reduce exposure and protect employees.

  • Use gas monitoring systems with automatic shut-offs and alarm systems, as appropriate.

  • Design and use specialized processing, material handling, and storage equipment for gases. Consider both normal use and emergency scenarios. Process Safety Management (PSM) [29 CFR 1910.119] requirements may also apply.

Additional Information

OSHA Safety and Health Topics Pages:

Radiofrequency (RF) Radiation

Potential Hazard

  • Possible employee exposure to radiofrequency (RF) radiation used as an ionizing source for dry etching.

Possible Solutions

Photoresist Stripping

After etching, the resist has served its purpose and must be removed from the SiO2. "Plasma ashing" or "dry stripping" is usually the first step. The wafers are placed into a chamber under vacuum, and oxygen is introduced and subjected to radiofrequency power which creates oxygen radicals. The radicals react with the resist to oxidize it to water, carbon monoxide, and carbon dioxide. The ashing step is usually done to remove the top layer or "skin" of the resist, then additional wet or dry etching processes can be used to strip away the remaining resist (see Etching). Some wet and dry chemical constituents are shown in Table 4. After the stripping is complete, the wafers are rinsed with deionized water to remove any remaining chemicals or resist material.

The following are the potential hazards of photoresist stripping.

Acids

Potential Hazard

  • Possible employee exposure to acids used for wet chemical etching/stripping (see Table 4).

Possible Solutions

Solvents

Potential Hazard

  • Possible employee exposure to solvents used for stripping and rinsing (see Table 4).

Possible Solutions

Radiofrequency (RF) Radiation

Potential Hazard

  • Possible employee exposure to radiofrequency (RF) radiation used as a power source for dry stripping.

Possible Solutions

Doping (Junction Formation)

Dopants are impurity elements added to the semiconductor crystal to form electrical junctions or boundaries between "n" and "p" regions in the crystal. An n-type region is an area containing an excess of electrons for conduction of electricity. A p-type region contains an excess of electron holes or acceptors. The difference in electric potentials between the two regions facilitates the flow of electrons through the circuit. The junctions form the essential element for all semiconductor functions. The most common doping methods include diffusion and ion implantation. Materials used for dopants mainly include compounds of antimony, arsenic, phosphorous, and boron, in gaseous, liquid, and solid physical states. Table 5 identifies various dopants used for both diffusion and ion implantation.

Diffusion

Diffusion occurs when impurity atoms or molecules migrate from an area of high concentration to an area of low concentration. Diffusion usually occurs in two steps: predeposition and drive-in.

During predeposition, the impurity dopant is added to the wafer substrate. Predeposition is done in a furnace at temperatures around 1000-1250C. The dopant is introduced into the furnace, and may be in the form of a gas, solid, or liquid. Gaseous dopants are mixed with an inert carrier gas, such as nitrogen or argon, and introduced into the furnace. Solid dopants are often applied in a powder form. The solid is heated and a stream of carrier gas moves the dopant into the furnace. Liquid sources are used by bubbling an inert carrier gas through the liquid dopant, and the gas saturated with the liquid is added to the furnace.

The wafers are then put into a second furnace at higher temperatures (about 1300C) to "drive-in" the dopant. The drive-in process usually occurs in an oxidizing atmosphere so that a protective layer of SiO2 is grown over the diffused layer.

Ion Implantation

ImplantDuring ion implantation, the dopants are ionized (stripped of electrons), accelerated using an electric field, and deposited in the silicon wafer. Upon striking the wafer, the dopant is embedded at various depths, depending on its mass and energy.

Typically, a gaseous dopant is ionized by electric discharge or by heat from a hot filament. The ions are separated using an electromagnetic field that bends the positively-charged particles to a selected band. This ion band is then passed through a high-current accelerator. The high-velocity beam of ions is focused on the wafer, causing the dopant ions to strike the wafer surface and penetrate. Sometimes a mask is used to implant a designated pattern on the wafer. As with diffusion, ion implantation allows the formation of junctions by changing the conductivity characteristics of precise regions in the wafer.

Implantation can damage the surface of the wafer. A high-temperature annealing step (800-1000C) is performed to return the wafer to its original condition and to further incorporate the dopant atoms into the silicon crystal lattice. Stack furnaces, high-energy lasers, electron beams, or flash lamps can be used for annealing.

The following are the potential hazards of doping.

Toxic, Irritative, and Corrosive Gases and Liquids

Potential Hazard

  • Possible employee exposure to toxic, irritative, and corrosive gases and liquids (see Table 5).

Possible Solutions

Reaction-Product Residues

Potential Hazard

  • Potential chemical exposures to maintenance personnel working on reaction chambers, pumps, and other associated equipment that may contain reaction-product residues. Substances such as arsenic, arsine, phosphine, etc., may be found in ion implantation equipment.

Possible Solutions

Additional Information

  • Arsenic. OSHA Safety and Health Topics Page.

Deposition

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.

Technician loading a wafer cassette into a CVD tool

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 500-1100C 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 900-1300C. Table 7 identifies the four major types of vapor phase epitaxy, parameters, and chemical reactions.

The following are the potential hazards of depostion.

Reaction-Product Residues

Potential Hazard

  • 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.

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.
Oxidation | Cleaning | Photoresist Application | Soft Bake | Mask Alignment and Photoexposure | Developing | Hard Bake
Etching | Photoresist Stripping | Doping (Junction Formation) | Deposition