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Figure 1: Sample gas-distribution schematic for a 2-chamber system
© Glew Engineering Consulting, Inc.
In Part 1 and Part 2 of this blog series, I described some of the design challenges for the first parts of a gas distribution system: gas cabinets, liquid sources, valve manifold boxes (VMB) and the gas panel. The gas cabinets store the gas cylinders, the valve manifold box controls the distribution of gases to one or more process tools, and the gas panel controls the gas on one semiconductor equipment, e.g. plasma CVD, etch or CMP. In Part 3 I will cover the components necessary after the process tools, when the gas becomes exhaust: process pumps, abatement, and exhaust lines.
Vacuum Pumps for Semiconductor Processes
Any semiconductor processing that runs at a vacuum, or any pressure lower than atmospheric pressure, will require a pump to create that negative pressure and pull the gas through gas lines to the process tool. While the type of pump used is dependent on the pressure and flow rate necessary for the process, there are some general safety issues that a mechanical engineer should take into account when designing the system. NFPA 318 § 8.12, for instance, lays out the safety for pumps handling flammable or pyrophoric chemicals, requiring that the pumps “shall be of the dry type or use noncombustible oils.” (NFPA 318 § 8.12.1). Standard lubricants for machinery are flammable, so obviously mixing them with explosive gases would create an unnecessarily dangerous situation. Thus, it’s best to employ a dry pump like a scroll pump or a roots blower, or utilize an inert lubricant such as PTFE (Teflon). The chemical vapor deposition (CVD) chamber research lab we recently evaluated used silane and other flammable gases, so they selected a scroll pump as a backing pump and a roots blower as the process pump, allowing them to reach a very low vacuum pressure without risking a fire. Depending on the chemicals used, other NFPA requirements can include nitrogen purge, certain pump exhaust specifications, oil filters, and more.
Abatement of Waste Gases
The abatement system processes whatever dangerous gases are left remaining after passing through the process chamber and pump and convert them into a safe form. Most abatement systems operate with greater than 99% efficiency; that is, they remove 99% of the unwanted exhaust chemicals. Process exhaust might include not only unreacted precursor gases, but also the leftover other reactions. For instance, semiconductor fabs commonly use tetraethyl orthosilicate (TEOS) as a precursor for silicon dioxide formation for semiconductors. Acids and bases can both act as a catalyst, and the side product of the reaction is ethanol. So, a TEOS process might exhaust a mixture of gases that is volatile and flammable. So, a series of abatement chambers are necessary to convert gases into forms that are either safe enough to release to atmosphere or are capable of being stored in a replaceable waste container.
There are three main forms of waste gas abatement technology used by semiconductor fabs: dry, wet, and burn boxes. Dry abatement passes the waste gas through a bed of granular material, specifically selected to passively chemisorb those chemicals. Wet abatement performs similarly, but uses a liquid as the neutralizing medium; wet systems are very common, as they are most useful for neutralizing acids and bases. Finally, burn systems are the most energy intensive, using high temperatures, flames, or plasma to either ignite flammable substances or oxidize toxic chemicals. Depending on the chemicals used in and generated by a process, the mechanical engineer, materials scientist or chemical engineer designing the system might specify a combination of scrubber technologies. Sometimes one type of scrubber is effective for a deposition process, but another is necessary for the cleaning gas sent through the same chamber after the deposition. The system switches the exhaust from one exhaust path to another in a process called exhaust switching.
Abatement systems can be installed at the end of the exhaust line if space within the facility is limited. However, point-of-use (POU) abatement systems are generally the preferred option. Positioned directly after the pumps or process chambers, POU abatement minimizes the length of line through with exhaust gases travel. This lowers the chances of unwanted chemical reactions, corrosion, leaks, and precipitation, improving safety and maintenance. These two types of abatement are shown in the sample schematic at the beginning of the article.
The final step in the process is the exhaust of all gases to atmosphere. The apparent simplicity of this stage belies its importance in semiconductor fab safety. NFPA 318 §9 and SEMI S2 §22 are both entire sections devoted to the safety of exhaust ventilation systems. Often times the largest risk is due to the situation in many semiconductor fabs, in which it is necessary to connect exhaust ducts from multiple semiconductor process tools to a common large exhaust duct. While designing one workstation a mechanical engineer might fully account for the waste gases produced by that unit, but if those gases later combine and react with gases from other workstations then disaster can result. Dr. Glew has testified as a semiconductor gas systems expert witness on fab incidents and explosions originating with the exhaust system.
Semiconductor Equipment: A Complicated System
The gas distribution system for a semiconductor process is a complex set of components, where every part—from the gas cabinets through the valve manifold box and gas panels to the process chamber, and then out through the pump, abatement and exhaust—has its own set of safety requirements and regulations, as well as complex functional requirements. The rest of this blog series will focus on smaller details of the system.