Figure 1: Gas distribution lines
For the first parts of this blog series, I wrote about the components and subsystems that makes up a high purity gas distribution system for a semiconductor process: Part 1 concerns gas panels, Part 2 gas storage, and Part 3 abatement and exhaust. But while I’ve covered the components individually, I haven’t looked at the gas lines that fluidically connect them as necessary for a functional system. Since the mechanical engineering of a single piece of tube is admittedly simple, the challenge in design comes in at a higher level. The focus at this stage is on the layout of all the semiconductor equipment and the varying gas line requirements for different gases. Figure 1 shows just how complicated this design can get.
Engineering Each Segment
The many different types of equipment and reactions used in semiconductor processing or other thin film applications require a broad variety of different gases and chemicals. The mechanical engineer must consider every gas individually, whether precursor or product, and account for their chemical properties in every line they use.
The gas lines used for semiconductor equipment have specific material properties due to their hazards, material compatibility, corrosion, and the high purity or ultra high purity requirements of the process. 316L stainless steel is a commonly-used material for process gas lines, but it can be processed with different properties for different applications. Standard 316L can be polished to 10 micro inches Ra, or 5Ra for high-purity systems. For greater purity requirements, a set of forging processes called vacuum induction melting and vacuum arc remelting (VIM VAR) can produce an alloy with extremely low impurity. For corrosive or high-temperature environments, it may be necessary for the mechanical engineer to upgrade to a super-alloy like C-22 Hastelloy™, an extremely corrosion- and wear-resistant nickel alloy.
Exhaust lines share many of the same considerations of hazardous gas transport, though they do not need to maintain the high-purity requirement. Exhaust gases might be flammable, corrosive, pyrophoric, inert or a combination thereof. For flammable exhaust, steel ducts are necessary and might even require internal sprinklers if the system doesn’t meet the requirements in NFPA 318 § 11.2.1. Acid exhaust, or corrosive exhaust, requires a non-reactive material such as polypropylene. In the event that the exhaust is both flammable and corrosive, then the exhaust line needs both materials: steel ducts lined with an polymer coating, such as PTFE.
Some gas lines may require thermal insulation or active heating. This is often required when using a liquid precursor; since the system had to raise the liquid to a certain temperature in order to vaporize it, it’s clear what could happen if the resulting gas drops back down below that temperature. Condensation within the gas line can be both costly and dangerous.
One of the prime tenets of engineering safety in semiconductor processing equipment is the elimination of situations wherein single point failure leads to a catastrophic or serious event. To protect the health and safety of the users, wherever possible there should be two levels of protection between them and any hazard. For certain gas lines that pass through an area occupied by people, this can be ensured by secondary containment of each line. For every ¼” gas line carrying a hazardous production material, the installers weld a coaxial ½” tube around it, creating an enclosed secondary volume into which a gas can leak without threatening users. This secondary containment should be continually exhausted during operation, and should be equipped with gas sensors that can detect any leak that might occur. Figure 3 is the sample schematic from the Part 3, updated to show coaxial secondary-containment lines in blue and normal lines in green. Since nitrogen gas (N2) is not a toxin or asphyxiant, it does not pose a risk if a leak occurs.
Figure 2: Sample 2-chamber gas distribution layout
Designing the equipment layout in a semiconductor fab or laboratory is a site-specific exercise, due to the great variety in processes, chemicals, environments, and whatever environmental or building regulations are in effect. Some underlying goals are common for every process though. Here are a few ideas that may be useful.
Reduce Gas Line Lengths
It’s generally in the best interest of system performance to keep gas lines as short as possible. By minimizing the distance gas has to travel between storage, gas panel, and process tool, the mechanical engineer can reduce the amount of contaminants picked up from the lines, lower the amount of gas that precipitates out as dust or liquid, maintain better accuracy and flow control, and minimize possible leak points. Further, by keeping more components in the same work space, it makes setting up controls and safety systems more efficient and cost-effective.
Eliminate Dead-legs in Gas Paths
“Dead leg” is a term for sections of process piping that have no through gas flow, resulting in static gas. It is important to design a system without dead-legs, or to minimize them if that is not possible. These dead-end sections fill with process gases that are then difficult to remove, even with multiple cycle purges. These static pockets of gas can precipitate into dusts or liquids, and over time build up from a maintenance headache to a serious safety hazard.
Modularity and Useful Equipment Life
Especially in a small processing company or laboratory, it can be hard to know whether the equipment under construction now will be performing the same function in 5 or 10 years. In some instances, the lab may plan on changes in the equipment requirements over the life of the equipment. For example, A CVD research lab that hired us as safety and design consultants planned on starting with only two gases, but wanted the capability to add more chemicals as their experimentation progressed. With a little thought and planning, and with minimal extra cost, one can increase the expansion capability of the gas panel by adding extra slots and control channels for future gas sticks. Reviewing potential needs and emphasizing modular design can yield future benefits and extend the equipment lifetime. SEMI recommends as a goal that semiconductor processing equipment be designed for a 10 year useful life. The same goal can be applied to the gas system within the semiconductor FAB.
How to Design the Physical Layout Semiconductor Equipment
Designing and laying out a semiconductor equipment system such that it does its job accurately and reliably while also meeting all safety codes is a challenging operation. A semiconductor fab giant may have all of the needed skills, but a smaller firm, startup, university or government agency can find itself without the knowledge necessary to put together the best system. Further, even if an organization has some knowledge of gas system design, they may not be current on the latest advances. Documents such as NFPA 318: Standard for the Protection of Semiconductor Fabrication Facilities (specifically, section 5.6 and 5.7) and ASME B31.3: Process Piping Guide can provide some generally guidance, but they can prove dense and difficult to interpret for someone with little prior exposure to the world of industry codes and standards. Further, they provide little guidance in high purity design, material and component selection, supplier selection, and working the contractors.