Figure 1: Gas storage cylinders and distribution
As discussed in Part 1 of this blog series, gas panels are the control center for a process that requires gas phase flow. As such, for any gas distribution system they are often the most complex and expensive component between the gas cylinder tank farm outside the semiconductor Fab (Figure 1) and the process chamber. The other parts of the system have their own requirements for safety and reliability, from the gas cabinet that holds the gas cylinders and on through the valve manifold box, process chamber, vacuum pump, abatement system, and exhaust ducts. As with the gas panel, the mechanical engineer must consider many requirements when designing the rest of the gas distribution system. For Part 2, I will look at the requirements for first leg of this complex system, from storage to gas panel.
Gas cabinets in Semiconductor Fabs
To safely contain gases in the event of a leak or fire, manufacturers and laboratories should store their dangerous chemicals, or Hazardous Production Material (HPM), in a “gas cabinet”. Besides the toxic, corrosive or explosive dangers each gas might present, the cylinders are also pressurized to as much as 3,000 psig, or 200 times atmospheric pressure. Commercial gas suppliers equip their gas cylinders with a built-in high pressure manual shutoff valve, but more components are necessary before the mechanical engineer can incorporate it into a gas distribution system. A gas cabinet’s plumbing will include a flow restrictor, a pressure regulator to reduce the 3,000 psig to working pressure, and cycle purge capability for cleaning the lines before cylinder exchange and maintenance.
Design and safety
A gas cabinet is not simply a box in which a lab can store their cylinders and forget about them; they are complex and complete safety systems in their own right. Per industry codes NFPA 30 (Flammable and Combustible Liquids), 55 (Compressed Gases and Cryogenic Fluid), 400 (Hazardous Materials Code), and 318 (Protection of Semiconductor Fabrication Facilities) and industry standard SEMI S2, there are some specific requirements to ensure the safety of the lab and the people using it. (See our previous blog on semiconductor safety). Fortunately, not all codes and standards are relevant to every situation. I’ve listed two examples below.
Gas cabinet wall thickness
Gas cabinet walls must be non-flammable and of a certain minimum thickness, to serve as a barrier for any fires that may ignite within, or to protect the gas cylinders from an external conflagration. NFPA 318 requires that all storage cabinets “be constructed of not less than 1.2 mm (18 gauge) steel.” (NFPA 318 § 184.108.40.206)
Gas Cabinet Exhaust
The gas cabinet must also have continuous exhaust with a sufficient flow rate, ensuring that any leaking gas out is quickly exhausted from the room. At the basic level, NFPA 318 § 5.5.3 sets two requirements:
“Hazardous chemical storage and dispensing rooms shall have mechanical exhaust ventilation as follows: 1) Mechanical exhaust ventilation shall be at a minimum rate of 0.31 m3/min•m2 (1 ft3/min•ft2) of floor area. 2) Exhaust and inlet openings shall be arranged to prevent accumulation of vapors.”
The engineer must design the exhaust fans and duct sizes to meet the flow rate specified in the first item. The second provides less guidance but is more complicated, requiring that the engineer minimizes dead zones in the gas cabinet where dangerous chemicals might accumulate. For a complex enough gas cabinet, this might require some computational fluid dynamics (CFD) modeling, to ensure sufficient flow rate throughout the cabinet. On-site inspection would follow, to confirm sufficient flow rates.
There are many other requirements set forth in the NFPA standards, including cycle purge requirements, gas leak detection, internal sprinklers, and others. The multitude of standards may be overwhelming, so coordination with a semiconductor equipment expert can be the best course of action.
Gas Cabinet Suppliers
As with gas panels, there are many manufacturers for the individual components that make up a gas cabinet, and some companies that build and supply complete turnkey systems. Companies like Air Products, Matheson Gas, PraxAir, and Semi-Gas are all well-known for their gas panel systems. Figure 2 below is an example of the plumbing needed in a gas cabinet, similar to what these companies sell as assembled units.
Figure 2: Standard design for gas panel “pigtail”
Liquid sources for Semiconductor Processes
Some process use chemicals that exist as a liquid at standard temperatures and pressures, and thus require evaporation before use. For example, many semiconductor processes make use of tin, and can use tin chloride (IV) as a liquid precursor. If the liquid has a low vapor pressure, changing it to a gaseous state is a simple matter of opening the storage container, called an “ampoule”, to the atmospheric or lower pressure of the system. The liquid will evaporate at a predictable rate and flow through the system. On the other hand, a gas with a higher vapor pressure will require additional energy. This can be as simple and self-explanatory as a heater or boiler, or it could involve bubbling a carrier gas through the liquid to facilitate evaporation, a system known as a bubbler. The most accurate, reliable and repeatable method is called injection, which takes a carefully controlled flow of liquid and injects a hot carrier gas into it, precisely and cleanly atomizing the precursor into the needed molecules.
Design and Safety
Since the gases that develop from these liquid precursors are usually dangerous, the liquid ampoules require the same type of enclosure system and safety precautions as a compressed gas cylinder: exhaust flow, purge line, gas sensors. There are some additional requirements of which the mechanical engineer needs to be conscious, including drip trays, liquid sensors and temperature regulation. For example, SEMI S2 § 15.1 recommends that each liquid source have the following:
“grounded or GFCI-protected heater; power interrupt; manual reset; automatic temperature controller; liquid level sensor; fail-safe over-temperature protection; proper construction materials; exhaust failure interlock; and overcurrent protection.”
Valve Manifold Boxes (VMB)
For a facility with multiple process tools, the gas distribution system will require a valve manifold box (VMB) between the gas cabinets and the gas panels on the process tools. VMBs centralize control and distribution for all of a facilities gas lines, making systems easier to maintain and upgrade. A VMB can also reduce the pressure and flow rate for every gas line, rather than requiring regulators in every gas panel. This centralization also lets facilities control processes at a high level, shutting down all gas flow at one point in the event of an emergency. With so many gas lines centralized in one spot however, and the necessary number of valves and fittings, VMBs also share the safety and design requirements of gas panels and gas cabinets.
It may seem like a lot of complication for what amounts to storage containers, but all of these requirements and regulations are based on the overarching needs for safety and reliability in a system. Even for a relatively simple system, like the single plasma enhanced CVD tool project we joined as mechanical safety consultants, it is important to keep safety on the forefront of engineering considerations. While the NFPA and SEMI standards may be dense and sometimes convoluted, the same experiences that led engineers to develop and codify these requirements also lets them design the most effective and efficient systems. If you’re working on semiconductor equipment, thin film processing, or photovoltaic fabrication, it’s in the best interests of your equipment, your health, and your wallet to work with an experienced professional.
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