We have written about pyrophoric materials in a number of Glew Engineering’s previous blogs on safety in semiconductor fabrication, but have yet to cover how to define it or its danger. At its simplest, a pyrophoric substance is any substance that spontaneously ignites in room temperature air. As one might imagine, spontaneous combustion on contact with the regular atmosphere we live in can be quite dangerous.
Last year, Glew Engineering assisted a research lab in designing a safe gas distribution system for their plasma-enhanced chemical vapor deposition chamber. During the initial work, the building administrators and safety managers were concerned about the lab’s proposed use of silane and germane, two pyrophoric and toxic materials. To them, “pyrophoic” was a mysterious and dangerous word that could spell disaster for their whole building. We reviewed standards and practices for handling semiconductor gasses, and helped to design and develop a safe strategy. The research lab uses much smaller quantities than in a a production semiconductor fab. Nonetheless, their concerns are justified.
Pyrophoric Chemical Reaction
Pyrophoricity is actually a special condition of a more general chemical characteristic called hypergolicity. A hypergolic chemical is a substance that spontaneously ignites when it comes into contact with an oxidizer. For example, liquid hypergolic fuels were once the preferred method of propulsion in rocket engines since they do not require a separate ignition source, and were stable on their own before they were combined with the oxidizer. One common example was hydrazine (H2NNH2) fuel combined with nitrogen tetroxide (N2O4) oxidizer.
Pyrophoric chemicals are a subset of hypergolic chemicals which can be oxidized (ignited) by air or water vapor at a reaction temperature at or below 55°C (130°F).
[See Fig. 1 Tall Fire from Chemical] This does not imply that pyrophoric materials possess some unique chemical property that differentiates them from less-reactive substances. Rather, every material has a heat of spontaneous ignition. Most people understand that paper can burn. For book paper, the spontaneous ignition temperature is 451°F. Some common materials like hay and coal have low ignition temperatures, and have been known to self-heat to the point of spontaneous combustion. Pyrophoric materials just have an ignition temperature so low that they instantaneously react with air and self-ignite. This characteristic makes them dangerous and also easy to underestimate, as it is such an unfamiliar quality to those without firsthand experience.
Pyrophoric Chemical Uses
There are numerous pyrophoric materials that exist as solids, liquids, or gases at room temperature. Many commonly-used metals and solid compounds are pyrophoric, especially when the surface area to volume ratio is very high, as with powders, dust, or shavings. This can be dangerous when the material is being machined for another purpose, and the shavings are expelled at high velocity. Some tools take advantage of the property, like fire strikers, which grind a pyrophoric metal into spontaneously-combusting shavings. Pyrophoric liquids are used less frequently. Their primary application is as igniters in liquid oxygen rocket engines, and occasionally as components in incendiary weapons. Even grain in grain elevators can self ignite, due to the large surface area.
Some staff at Glew Engineering have dealt with powdered aluminum, which can also be dangerous. When handling these materials, static electricity and sparks are to be avoided, and containers must be well grounded.
Pyrophoric gases are sometimes utilized in the semiconductor industry as precursors for depositing layers. Silane (SiH4) and germane (GeH4) are common. Silane is commonly used for forming silcon nitride Si3N4. Silane used to be common for forming silicon dioxide in the intermetal dielectric stack, until safer alternatives such as TEOS became common. Arsine (AsH3) and diborane (B2H6) are used as dopants. All four of these gases are pyrophoric, and some very toxic. While useful as sources for high-purity atomic deposition, pyrophoric gases can be incredibly dangerous. They are usually stored in pressurized gas cylinders, and in a larger fab might be piped from a central gas cabinet to multiple workstations. Every valve and fitting offers another chance for the gas to leak or air to enter, depending on whether the system is pressurized or under vacuum. Either way, mistakes can be catastrophic.
Pyrophoric Gas Hazards
Fig. 2 Chemical Storage Fire
Pyrophoric gases are very commonly used as a precursor gases in the semiconductor industry, and have been the cause of a number of deadly incidents. On reports shows 14 silane accidents over the last 40 years causing death or injury. These incidents were caused by factors as varied as incorrect equipment, missing (or even extraneous) equipment, improper installation, mishandling, contamination, and more.  The behavior of silane during rapid changes in temperature and pressure is still not completely understood. “Its behavior when released is unpredictable. As a result, silane has been involved in quite a number of significant incidents.” 
As a recognized hazard, the National Fire Protection Association regulates the use of pyrophoric materials in NFPA 45: Standard on Fire Protection for Laboratories Using Chemicals, NFPA 55: Compressed Gases and Cryogenic Fluids Code, and NFPA 318: Standard for the Protection of Semiconductor Fabrication Facilities. Furthermore, Semiconductor Equipment and Materials International provides best-practice guidelines in SEMI-S2Environmental, Health, and Safety Guideline for Semiconductor Manufacturing Equipment. However, knowledge of the codes alone is not enough to prevent all accidents. Using pyrophoric gases safely requires not just the correct storage, distribution, usage and exhaust equipment, it also imposes specific requirements on safety systems, facility equipment and building design. Lastly, it requires good engineering practice.
Pyrophoric Gas Safety
Glew Engineer Consulting’s work in the semiconductor industry makes us familiar with the dangers of utilizing pyrophoric gases like silane, germanium, and arsine. Safety standards and industry guidelines are a starting point, but nothing can replace years of experience working with these gases. If you have a project involving silane, germanium, arsine, or other pyrophoric gases, for your own safety and the safety of your co-workers, contact an expert. Your life and safety may depend on it.
 NFPA 318 Standard for the Protection of Semiconductor Fabrication Facilities (National Fire Protection Association 2015)