EHS Fab Hazards: A Review

Fabs are complex manufacturing facilities and, as such, have a number of safety hazards. For example, many of the substances used in semiconductor manufacturing are toxic, flammable or corrosive. The operation and maintenance of fabrication equipment can expose operators to other dangers, including electric shock, radiation and burns. To prevent accidents, workers must be aware of these risks, and follow safe working procedures. Some of the types of generic risks associated with some of the equipment, processes and materials used in IC fabrication will be discussed, but nothing can replace the actual specific training needed to operate specific equipment or meet hazards.

Although fabs can have many potential safety risks, the semiconductor industry ranks high in terms of protecting the health and safety of its workers. Among manufacturing industries, it is fourth highest in terms of its safety record. Only aerospace and nuclear industries provide similar worker exposure controls (Fig. 1 ). While many measures have been implemented with worker safety in mind, concerns for product purity have also resulted in the installation of many of the protection systems. Furthermore, there is an on-going, industry-sponsored system of worker injury surveillance, organized by the Semiconductor Safety Association, International SEMATECH, and the Environmental Safety and Health Committee of the Semiconductor Industry Association (SIA).

1. The semiconductor industry holds an enviable safety record. Among manufacturing industries, it is the fourth highest. Only aerospace and nuclear industries can boast of a similar accomplishment in this area.

Fab workers are trained in safe operating practices for specific processes. In addition, they must adhere to such general commonsense rules such as not working alone when performing hazardous tasks. Wearing effective safety apparel (including safety glasses and goggles, protective coats and aprons, and appropriate gloves for the job) also helps prevent accidents. For example, a full-face shield is always required when pouring chemicals. It is also good practice to wash hands with soap and water when leaving the workplace.

A well-run fab will monitor all its workstations to ensure safe practices are being followed and that unsafe conditions do not arise, and will have trained emergency-response teams in case of accidents. Biomonitoring (radiation badges, for example) is used for those who might be exposed to chemicals or physical hazards that could produce long-term ailments. Management has established good lines of communication between workers and supervisors to quickly identify and correct unsafe conditions, and thorough records are kept to help detect unusual circumstances that might point to potential safety problems.

Fabs are equipped with toxic gas sensors and alarms that are set off when the presence of certain gases is detected at ppm or ppb levels. Fail-safe mechanisms are employed on hazardous equipment such as gas valves and electrical interlocks, while sophisticated ventilation systems are ready to quickly clear the air. Workers know that they must leave the area if unusual odors are detected, notify their supervisors and, if appropriate, sound an alarm. However, safety concerns do not end there. Hazardous areas and containers of hazardous materials must have conspicuous warning labels drawing attention to the particular hazard they may pose. Figure 2 gives examples of some standard warning labels.

2. Various tools and materials in the fab can present hazards. To prevent accidents, these are tagged out (examples below) to ensure everyone is aware of the possible dangers.

These labels, which are posted on all equipment and containers holding gases and chemicals, provide information that workers are trained to understand and interpret (Fig. 3 ). For example, hazard levels are posted, ranked (from 4 to 0) by their severity in the following categories: fire, chemical reactivity, health, and specific types of hazardous conditions. The classification numbers used on these labels grade the hazard into one of five levels of danger:

3. Hazardous areas and containers of dangerous materials have conspicuous warning labels drawing attention to the problems that they pose. These labels, posted on all equipment and containers holding gases and chemicals, provide information that workers are trained to understand and interpret. Hazard levels are ranked from 4 to 0, by their severity.

0 — Materials that, on exposure, offer no hazard beyond that of an ordinary combustible material.

1 —Materials that cause irritation or only minor injury on exposure, even if no treatment is given. This includes those that require use of an approved canister-type gas mask.

2 — Materials that can cause temporary incapacitation or possible injury after intense or continued exposure, unless prompt medical treatment is given. This includes those that require use of respiratory protective equipment with an independent air supply.

3 — Materials that can cause serious temporary or permanent injury with very short exposure, even though prompt medical treatment is given. This includes those that require protection from all body contact.

4 — Materials that can cause death or major injury with very short exposure, even though prompt medical treatment is given. This includes materials that are too dangerous to be approached without specialized protective equipment.

Safety information about chemicals is widely available through the Materials Safety Data Sheet (MSDS). By law, every hazardous chemical used in the facility must have an MSDS available for the workers. The suppliers of these materials provide the MSDS, and they also can be downloaded from websites. The information provided by the MSDS consists of:

• Chemical identity — the principal components of the material
• The threshold limit value (TLV) and permissible exposure limits (PEL)
• The health effects of overexposure
• Physical/chemical characteristics (whether it is combustible/explosive)
• Reactivity data
• Hazard data
• Common chemical or trade name and synonyms
• List of hazardous materials present over 1% and all carcinogens over 0.1% (in ppm and mg/m³
• Human organs impacted by overexposure and harmful effects
• Melting and boiling points, vapor pressure and specific gravity
• Flash point (lowest temperature it can be ignited with a flame)
• Auto-ignition point (lowest temperature it will ignite spontaneously in air)
• Chemical's stability

The process gases used in IC manufacturing have several hazardous characteristics, grouped into four categories: toxic, corrosive, flammable and pyrophoric.

Toxic gases are described as those dangerous to human life, such as arsine (AsH₃), phosphine (PH₃) and diborane (B₂ H₆). Corrosives are materials that destroy living tissue and equipment that comes in contact with them, such as HCl and HF. Flammables are substances that give off vapors that readily ignite if exposed to sparks, flames and other sources of ignition (such as H₂, PH₃ and SiHCl₃). And pyrophorics are materials that ignite spontaneously in air at or below 54°C. An example is silane (SiH₄), a gas widely used to deposit polysilicon, SiO₂ and Si₃N₄ by CVD.

A hazardous gas's toxicity level is quantified by specifying the maximum concentration that a human can be exposed to during a given time without suffering health damage. The lower the level of exposure that is allowed, the more toxic the material. These exposure levels are given as TLV-TWA and TLV-STEL.

The threshold limit value (TLV-TWA) is specified by the American Conference of Governmental Industrial Hygienists. It specifies the level under which someone can work for eight hours a day for an indefinite period without harmful effects (Table ). The threshold limit value-short term exposure limit (TLV-STEL) is a 15-minute time-weighted average exposure. STEL exposure should not be longer than 15 minutes, and not repeated more than four times per day. Health (IDHL) is a concentration representing a maximum level for which one could be exposed to for 30 minutes without impeding escape or causing any permanent health effects. The permissible exposure limit (PEL) is a standard for exposure set by the Occupational Safety and Health Act (OSHA). The PEL value is a time-weighted average exposure limit (typically for eight hours) or a ceiling exposure limit.

Many liquid chemicals used in a wafer fab are hazardous and corrosive. Corrosive chemicals can be either acids (pH<7) or bases (pH>7). When working with corrosive chemicals, they should be positively identified before use. Appropriate eye protection, as well as body-protecting apparel (acid-resistant aprons, gloves, sleeve guards, and boots that protect against chemical spills), must be worn when handling corrosive chemicals.

To avoid breathing in the vapors of corrosive chemicals, these are only used under a fume hood. Special attention is also given to chemical storage materials. For instance, HF is stored in plastic containers because it attacks glass. All workers are aware of the location of the nearest eye wash and chemical shower. Since most solvents have harmful vapors, and many are flammable, eye and body protection are obligatory to avoid exposure to, and the breathing of, solvent fumes. They are stored in a flammable materials storage cabinet, and kept away from open flames, sparks and heat. Solvents are disposed of into waste-solvent containers, not into acid drains, and care is exercised to not mix acid waste with solvent waste to avoid violent chemical reactions.

Hydrofluoric acid (HF) is widely used by fabs to wet etch SiO₂ films and to clean diffusion tubes and other glassware. Yet HF has its own unique and especially dangerous safety hazards. HF is tricky stuff. No pain is felt when it first makes contact with the skin; however, severe and painful burns occur later as it penetrates the flesh, and then reacts with the calcium in bones. Workers are instructed to wear proper gloves that have been inspected for pinholes and tears when handling any unknown clear liquid, since HF and water resemble each other.

An electric shock can cause burns, muscle and nerve damage, heart failure, respiratory paralysis, and death. When a current in the body exceeds 100 mA, it interferes with the coordinated movement of the heart, causing it to enter a state of fibrillation. With cessation of circulation, death occurs within 3-5 minutes. If proper first aid is quickly provided, such as administering a large current pulse to the fibrillating heart (defibrillation), it is possible to restore it to it normal rhythm and recovery can be complete.

Electric shock is rarely a result of poor equipment design, mostly originating in electrical faults or human error. Additional dangerous conditions include inadequate safety signs, lack of ground fault indicators (where electricity and water can mix), and insufficient lockout/tagout measures to keep personnel from unguarded energy sources. At least two levels of safety interlocks (using keys, door switches and grounding bars) are usually designed into systems to prevent such mishaps. Since interlocks must be breached during maintenance procedures, there is always some risk. Overall, the rule is that no one works alone when dealing with hazardous equipment.

Because ion implanters are large tools, someone working inside can easily be overlooked. Thus, the requirement for working with a partner, as well for "tagging out" the system, ensuring that others will realize that someone is working inside. A common safety rule is for the worker to remove the implanter's door key, so the others cannot lock it and start up the system.

Several safety hazards are associated with light sources and radiation, with the most common being mercury arc lamps, lasers and X-rays.

Mercury arc lamps are designed to emit UV light for the lithography processes. Mercury (Hg) is a liquid at room temperature, and its vapor is highly toxic. The lamps contain mercury in a glass bulb, and must be carefully handled, requiring gloves during replacement because the oil in fingerprints that might be left on the lamp glass can cause uneven heating, resulting in a crack in the glass and an explosion. Workers are warned to never look directly at the light of mercury arc lamps without UV-protection goggles to avoid eye damage.

The excimer laser light sources of DUV steppers are dangerous, and can damage skin or eyes. When DUV lasers were located outside the cleanroom, workers could inadvertently walk into the beam path. Elaborate laser walls and curtains were needed during installation and maintenance to prevent this. Since lasers are now installed and maintained near the stepper, this hazard has been decreased.

Laser beams are not the only hazard. The gases used in excimer lasers contain fluorine and are toxic. Since the laser cavity must be periodically refilled, strict safety procedures are used during such refilling. Excimers also use high-voltage power supplies and, although modern supplies require minimal maintenance, they must be treated with respect when work is being done on the laser.

Ion implanters generate low levels of X-rays. Lead shielding (>6.5 mm thick) is used to absorb them, keeping the radiation level outside the implanter below the recommended limit of 0.25 mrem/hr. However, since implant maintenance technicians must work inside the tool, they wear X-ray monitor badges, and are routinely tested for blood hemoglobin efficiency (an early indicator of radiation exposure). Periodic radiation surveys are also incorporated into the implanter maintenance schedule.

Fire is every fab's nightmare, due to the many flammable chemicals and gases that can be ignited by sparks, flames or electric heaters. Electrical hazards can set fire to surrounding materials, as can hotplates and immersion heaters.

A variety of safety measures are employed to prevent fires, and to control or extinguish them if they arise. No installation lacks automatic sprinklers backed by an adequate water reservoir. Wherever possible, the fab itself is built with non-combustible or fire-resistant materials. The air handling system is designed to facilitate smoke removal. Human surveillance is continuous, and early intervention plans are regularly practiced to quickly douse any fire.

The most valuable safety feature of any installation, however, is its well-trained personnel. A staff that knows what to do in case of an emergency cannot be replaced by fail-safe devices or any other software or hardware system — these only supplement human efficiency.