Silane Gas: The Dangers You Need To Know About
If you are in the semiconductor, solar, or battery fabrication industries, then silane gas might be essential to your process or production line. What you may not know, however, is how dangerous this gas can be...
Is silane a hazardous gas?
Silane gas is very harmful to breathe. Inhalation of high purity silane can cause silicic acid to form in the lungs, which is slightly toxic. Silane can also form silica fume irritants in air, which are separate compounds which are also hazardous.
Silane wants to burn
Silane is pyrophoric, which means it don't need an ignition source --- like a spark, hot surface, or open flame --- to start a fire. The presence of oxygen is enough to cause a massive fire.
Okay, silane gas is pyrophoric --- what are pyrophoric gasses?
Pyrophoric gas has the potential to ignite spontaneously when exposed to oxygen --- or even air at normal ambient temperature. Pyrophoric gas in its pure state ignites
How is pyrophoric gas different from flammable or extremely flammable gas?
Pyrophoric gas is not the same as flammable gas, even though they share several properties:
- Flammable gasses have flammable range in air at STP is any gas that can be ignitable in air at standard temperature and atmospheric pressure (20ºC and 101.3 kpa (14.7 psi)).
- Pyrophorics can have an explosive reaction when exposed to fresh air at any temperature or pressure.
- Both present occupational exposure risks and require:
- leak or safety devices,
- venting safety devices,
- pressure relief devices,
- precautionary measures,
- extreme caution,
- and/or personal protective equipment like eye protection and protective clothing
These measures can help to minimize the risk of everything from significant irritation to severe injury or death.
What's the difference between flammable and pyrophoric gas leaks?
If you release flammable gas into an environment, or if liquefied gas escapes from leaking containers: you could cause a leaking gas fire, or the gas could form explosive mixtures---or even poisonous gas.
If any exposed person doesn't withdraw immediately, or if that affected person breathes large amounts or has prolonged exposure to the flammable gas above acute exposure guideline levels, that individual could suffer all sorts of health problems.
And all of that can happen without a spark, hot surface, or open flame.
This is all also true if pyrophoric gas escapes. But if the pyrophoric concentration is above explosive limits, explosive decomposition can also happen without the presence of an ignition source.
What are the physical properties of silane?
Silane is a lot like methane, but methane is more stable during real-world use.
Which is not to say that silane is instable. Interestingly, it is a stable molecule---at least on paper.
The challenge in explaining silane's pyrophoricity is usually attributed to the fact that silane itself is stable, but that its sensitivity to impurities like moisture and surface imperfections and irregularities of its container surfaces causes it to become pyrophoric.
Silane: by the numbers
- Chemical formula: H 4 Si
- Vapor pressure: >1 atm
- Molecular weight: 32.117 g/mol -1
- Autoignition temperature: ~18ºC (about 64ºF)
- Lower combustion limit: ~1.37%
- Strong repulsive odor
- Colorless gas
Occupational Safety
- Extremely flammable
- Pyrophoric in air
NFPA Diamond for silane
Silane NFPA Diamond
Health Hazard: 2 — Can cause temporary incapacitation or residual injury
Flammability Hazard: 4 — Will vaporize and readily burn at normal temperatures.
Instability Hazard: 0 — Stable.
Special Hazards: None
Silane MSDS
And here is a link to an MSDS for silane.
Practical properties
The physical properties of silane at standard temperature and pressure are:
- At concentrations under 1.4% in air, silane gas is non-flammable
- Between 1.4%--4.1% in air, silane gas is flammable
- Anything over 4.1% in air, silane gas becomes a metastable mixture that is borderline pyrophoric to absolutely pyrophoric
- For a metastable mixture, ignition could be prompt (i.e., flame or deflagration) or delayed as a bulk auto-ignition (e.g., an explosion, a large fire)
Why would anyone want to use silane?
Silane was mainly of academic interest until the second half of the 20th century, when semiconductors started becoming commercially available.
Silane's primary use in modern society is as a precursor to elemental silicon. Mass-market semiconductor and solar panel manufacturing has led to enormous silane consumption.
Through a process called plasma-enhanced chemical vapor deposition, or PECVD, amorphous silicon can deposit metallurgical grade silicon particulates onto glass, plastic, or metal.
Right before the turn of the millennium, over 300 metric tons of silane were used annually. And its use has increased dramatically from there as semiconductors and microprocessors have become ubiquitous and the green energy industry has taken off.
How is silane formed?
There are a few ways to produce silane. Most commonly, it is a byproduct of the reaction of magnesium silicide and hydrogen chloride.
It can also be derived from metallurgical grade silicon by first treating metallurgical grade silicon with hydrogen chloride at high temperatures. As far as silicon compounds, forms, or derivatives go, creating silane with this method is comparatively complicated.
Are there only two ways to make silane?
Not at all. The two aforementioned are just the most commonly used in the laboratory and at commercial production facilities. There are further, even-more-complicated methods out there, too. Take a look at this patent application for example.
What are silane gas's non-pyrophoric hazards?
Aside from its pyrophoricity, silane gas has other chemical hazards that have to be taken seriously.
Inhaling silane creates toxic acid
Silane inhalation can cause the formation of silicic acid in the body, which is slightly toxic, and is classified as an irritant. Irritants can cause skin and tissue irritation, and can exacerbate underlying conditions.
Silica fume irritants formed by silane gas
Silane forms silica fume irritants when exposed to air. Silane gas has a sharp, acrid smell that's similar to acetic acid, but these fume irritants have little to no odor. Because silane reacts with air, it might seem simpler to just lump these two substances together and think that silane is odorless, but that's not the case. Silica fume irritants are a separate hazard for which odor isn't a viable detection method.
Silane gas could be carcinogenic
The IARC states:
“There is inadequate evidence in humans for the carcinogenicity of amorphous silica.”
In other words, silica fume irritants formed by silane gas in air could be carcinogenic, but evidence to that end is, at present, inadequate. Of course, there are lots of things out there that could be carcinogenic. It is very well possible that silane gas isn't one of those things. But evidence to the contrary isn't available.
Which leads directly into this article's free bit facility safety advice:
Free Safety Advice:
Assume that an unknown or exotic substance, tool, or process could kill someone—or you—if you make the mistake of assuming that it won’t.
As a general rule in facility safety, assume that an unknown or exotic substance, tool, or process could kill you if you make the mistake of assuming that it won't.
Leaked silane gas doesn't always explode. Sometimes it just burns everything nearby
Violent explosions and deflagrations are the biggest worry when dealing with pyrophoric gasses. But an explosion isn't the only way that a silane leak can ruin your day. Here are some others:
- Concentrated silane discharged slowly into well-mixed, air-diluted exhaust ducts will ignite if the silane concentration is >3%. The duct will get hot. Hot ducts that aren't supposed to be hot can cascade into a host of other serious problems.
- Small (< 40 lpm) leaks of concentrated silane in a gas cabinet purged with 500 cfm of air will burn smoothly without exploding. The cabinet's interior will become something like a kiln, heating everything inside to hundreds of degrees Celsius.
- However, a >40 lpm silane leak will, in fact, explode.
- Silane gas released through an open valve without an orifice can produce flames that extend eight feet from the valve. It will burn like a blowtorch.
- Imagine the width of a rail car, or a ladder on its side. Now imagine a blowtorch flame of similar size.
The importance of RFOs and EFS
If a pipe fails, devices called Restricted Flow Orifices (RFOs) and Excess Flow Switches (EFS) can help turn leaking containers and process lines safe. They work by stopping or limiting the flow of gas if there is too much pressure. RFOs and EFS need to be installed properly and maintained in order to work correctly and save lives.
How do Excess Flow Switches work?
Excess flow switches limit instantaneous gas release during a rupture event. They sense a sudden, uncharacteristic increase in line flow and close a valve ahead of the rupture. They limit the line's maximum flow in a failure situation.
How does a Restricted Flow Orifice work?
A restricted flow orifice is a passive safety device. It constricts the flow of anything passing through it to some predetermined, maximum value. It's installed at a cylinder head, a discharge line, or anywhere else that a qualified process safety engineer decides that one should go.
So if, for example, you have a pyrophoric gas in a line supplying some quantity of gas: "N". And N quantity will cause an explosion during a rupture event.
But your tooling only needs ½N to function.
A process safety engineer might recommend installing a RFO at the cylinder head, which restricts the maximum output of the line to the amount your tooling needs: ½N. Now the line presents a reduced hazard while still serving its intended purpose.
Restricted Flow Orifices: the flat tire analogy
Think of a restricted flow orifice like a nail that gets stuck in your car tire. As long as the nail stays put in the tire, air can only escape as fast as it can get around the nail.
What size RFO should be used?
You should always use the smallest RFO that still meets your process requirements.
It's possible that your gas manufacturer can preinstall an RFO on your cylinder. If you have any questions about preinstalled RFOs, ask the gas's manufacturer.
As alwasy: Remember to document any questions you ask your supplier, and log any response you receive.
Silane cylinder storage
Typicallly, silane is stored with other pyrophoric gases and kept separate from flammable, oxidizing, and toxic gases. This is a precautionary measure. You don't want a secondary fire involving tanks, or any nearby cool containers being heated beyond their specific design intents.
Any gas cabinets should always be locked and have continuous ventilation that is alarmed and monitored.
Gas cabinets should have wet-fire sprinklers for cooling. Automatic dry chemical extinguishers are sometimes appropriate as an immediate precautionary measure, but you should consult with an expert in chemical hazards---like ReadyLimit!---to determine your specific process needs and help design for your facility's specific occupational safety concerns.
It goes without saying that no untrained persons should be allowed access to any gas cabinet---let alone a cabinet full of pyrophorics.
Using silane piping and equipment
Before starting your process, use a well-defined, approved process procedure. Process procedures---especially around pyrophorics---need to be followed rigorously. Mistakes can cost lives.
Here's a tip we always suggest to our clients:
Any purging, operations, or changing of lines that held silane---or any pyrophoric gas---must be done by > two> persons who have been properly trained. One person acts, the other observes.
Make sure that all valves are in the proper positions. Bypass or jumpers over interlocks should never be allowed.
Everyone who works around pyrophoric lines should learn the locations of the correct isolation valves. It might be a good idea to review isolation valve locations with your staff about as often as you practice fire evacuation and assembly procedures.
And don't forget to show any new hires where the isolation valves are, too---include it in your onboarding procedures.
Rules for pyrophoric piping systems
- Before any tool disassembly, everyone must use the correct cycle purging procedures.
- Do not disassemble any silane lines until the isolation valves are closed and purging has been established.
- Repeat: Purge before disassembly.
- When you're ready to take apart components that contained pure gases, look for silica near any fittings or nozzles.
- Never remove or disassemble check valves.
- Dual check valves should be required.
- If silane leaks, it generates heat and silica fume. If you see white fumes, it's a safe bet that silane is present.
- During the disconnection of silane lines, chambers, and collectors, make sure that there is continuous inert gas purging.
- Before any gas isolation valves are opened, check all reassembled pipes for leaks.
- Label all piping with name and direction of flow.
- Do not release silane into the fume hoods.
Silane safety practices
Here are some standard safety protocol that we always like to see.
Maintenance of silane tooling and supply
Establish and stick to an appropriate check and maintenance schedule. You'll want to monitor nozzles; unmanned hose holders; check for localized, slightly elevated temperatures; and scan for unrelated cylinders exposed nearby.
Rule of Two
Any task that involves interacting directly with a silane line should always be performed by two people---one to act, and one to observe.
Daily visual inspections of all piping systems.
Your EHS manager or an appropriate designee should perform a quick daily inspection of the facility's entire piping system. By quick, we mean superficial---not fast or careless.
There's no need to get up on a scissor lift and inspect every foot of pipe with a magnifying glass. But you'd be surprised at the sorts of problems that can be caught and addressed early just by doing brief facility walk-through.
Use solid lockout/tagout procedures.
A thorough and up-to-date safety plan is a critical component of any high technology fabrication process.
Safety starts with the basics: Lockout/tagout procedures to protect workers from hazards like pyrophoric gas (silane) in the event of an emergency or accident.
Lockout refers to isolating hazardous energy sources by locking them out of operation, and tagout refers to manually disconnecting power sources when they are not required for work activities.
Tags should be attached to equipment that has been locked out, indicating that it cannot be used until properly released via a written procedure or verbal confirmation between worker and supervisor.
Locking out hazardous energy sources helps prevent accidents during maintenance activities, as well as dangerous incidents caused by human error.
Train and retrain on process controls.
The best amorphous silicon process plan in the world isn't worth much if staff doesn't know how to control the tools to which they're assigned.
A proper process hazard analysis performed by a qualified, experienced consultant---like ReadyLimit!---can refine your production and minimize human error.
Learn to recognize silane leaks.
Pure silane smells horrible. And if it hasn't already exploded, it produces thick white fumes.
Learn locations of emergency equipment.
Everyone onsite needs to be trained in the location and use of emergency equipment, be familiar with initial evacuation procedures, and know who to pay attention to during an evacuation. Evacuation procedures don't end as soon as you're out of the building. You may need to know the procedure for an initial downwind evacuation, too, depending on the spilled material, vapor cloud drift, and entrance or egress for any tank truck that might need access.
Quantity and use of silane onsite
To prevent ignition events, limit the local silane stock in pipes and chambers to the minimum amount that standard production requires.
Wherever possible, limit the maximum concentration of silane in enclosures to less than 0.4% by volume.
Never collect or store silane mixtures in any device; always purging first is the best approach.
Avoiding silane explosions and fires
Silane supply from bulk storage to your tooling should have dual check valves and RFOs at cylinders and at each branch connection.
Exhaust treatment systems
Silane exhaust abatement systems using adsorption or combustion are common.
It's important to limit the normal and maximum composition and flow-rate of exhaust gases. In other words, always purge slowly and carefully.
Suggestions for silane exhaust adsorption systems
Gas detection is needed to determine breakthrough of gases.
Suggestions for silane exhaust combustion systems.
We like to see these things in place on any combustion system for silane exhaust:
- A flame detector
- An air flow-rate sensor
- An exhaust temperature monitor
- Adequate gas detection
Silane leak and detection alarms
There are two types of detection tool that we recommend:
- Direct gas detectors
- Indirect flame detectors.
Toxic gas detectors will not work.
Any detector should be interlocked with all others to trigger a gas supply system shutdown.
All systems shall be fail-safe.
Silane leak detection
Systems should be regularly maintained, calibrated and tested.
Monitoring systems must have a backup power source---either UPS or battery.
Any safety system must be reliable. In other words, no home-made detection systems should ever be allowed.
Fire detection and suppression
The only safe way to put out a silane fire is to isolate the source and wait for it to burn-out.
Fire detection and suppression systems must have an alternate, remote, manual activation to allow operation in event of heat from an external fire.
Never disable any fire protection systems.
Emergency response
Make sure your local emergency response service is familiar with handling pyrophoric incidents. Beyond that, make sure your staff is familiar with our top five emergency response suggestions for any pyrophoric fire, deflagration, or explosion:
- Let it burn.
- Do not attempt to extinguish the fire.
- It is not your job to fight fire.
- Assume that any tanks engulfed by flame will explode.
- Let the fire burn.
First things first: perform a Process Hazards Analysis
Before you make any changes at your facility, you should complete a Process Hazards Analysis on your entire system.
Any process hazard analysis must be performed by qualified staff. Or an expert consultant. Like ReadyLimit!
TL;DR
- Wherever feasible, use the minimum possible amount of silane.
- Until you've purged the silane lines, don't open any of them..
- Minimize occupational safety risks by physical and/or operational means.
- Update occupational safety risk assessments regularly.
- Drop us a line. We're good at this. We've solved your silane issue before.
