What’s In a Semiconductor Gas Panel?
A gas panel is a term of art in the semiconductor equipment (tool) and semiconductor processing industry. Engineers refer to the box that controls the gases to a semiconductor processing chamber as a “gas panel.” One way to think about it is to consider a system’s gas panel to be its “control panel” for the gases. This the point where the supply lines from various gas sources converge, flow is regulated and directed to the appropriate destinations. Gas panels can range from the simple to the complex, with control for one gas or many. A typical piece of semiconductor processing equipment may have one line of inert gas, such as Argon, six lines of gas, including toxic and flammable gases, to control for each processing chamber. A semiconductor process tool typically has three to six chambers. Thus, the gas panels may become quite complex.
Gas panel complexity
For example, in one of our recent mechanical engineering consulting projects, we designed the gas panel for a CVD chamber. Each gas line had a “stick” with the following components: pressure regulator, pressure transducer, filter, mass flow controller, manual valve, three way pneumatic valves, a two way pneumatic valve, and lock out tag out “LOTO” capability. A gas panel may have dozens of gas lines, manifolded together to reach individual process chambers. The mechanical engineer who performs the design will undoubtedly use 3D CAD to create a realistic design model. The density of parts requires a detailed 3D CAD model to ensure that there are no interferences, and that one can access every components for assembly and service.
The differences in processes, chemicals, control requirements, safety regulations, and environment mean that gas panels typically have to be custom-made; a university-based photovoltaic laboratory will need a much different setup than an industrial semiconductor fab.
Even within semiconductor equipment, there are many different types of processes with different needs. Plasma etch, including capacitive plasma etch, inductively couple plasma etch, and high density plasma etch, are generally low flow at a few standard cubic centimeters per minute (SCCM) and operate in the mTorr vacuum range or lower. On the other hand, chemical vapor deposition (CVD) processes have more variation. Atmospheric CVD operates at atmospheric pressure, while sub-atmospheric pressure CVD operates at a slight vacuum, usually hundreds of Torr. Plasma CVD, sometimes referred to as plasma enhanced CVD (PECVD), requires pressures in the 10 Torr range, and the flows are significantly higher than for etch processes, some reaching liters per minute (SLM) range. High density plasma CVD (HDPCVD), is in the mTorr range, and the flows are lower than for PECVD. There are many other forms of semiconductor on this film processing, such as EPI silicon, atomic layer deposition (ALD), and sputtering, a common form of physical vapor deposition (PVD). We may review these and special gas panel requirements in later blogs in this series.
Gas panels exist on processing tools in other industries, usually associated with thin film processing. The solar panel (photovoltaic) industry, flat panel display industry, disc drive industry, and others all have gas panels in the capital equipment used to manufacture the products with thin films.
With such variety in design, high cost, and market size, there are a number of companies that manufacture gas panel components. The following are but a few examples among many. Parker-Hannifin, Swagelok, Fujikin and others manufacture a great variety of pneumatic and manual valves, pressure regulators, and fittings. Setra, Ametek and United Electric and others produce pressure transducers. Brooks, MKS and Fujikin make pressure transducers and mass flow controllers (MFCs). For gas filters, one may look to Entegris, Pall, Mott and others. A number of companies integrate components or sell complete systems and gas panels. All of the parts must be high purity, or ultra-high purity for use in semiconductor processing and clean rooms.
Function and Design of a Semiconductor Gas Panel
A gas panel acts as the control center for gas delivery in a process tool; it must ensure delivery of the correct chemicals to the tool in the right sequence, for a precise amount of time and at a specified mass flow rate. For this purpose, the designers must fine-tune the panel based on the requirements of the process. Pressure regulators and flow controllers must have a range large enough to meet the specifications of the process, but if the range is too large then the users will lose fine control over the flow rate and experience dynamic excursions. Valves and manifolds come in many configurations and their arrangement not only needs to deliver gases to the right spot, it also has to avoid mixing incompatible gases or risking backflow from one line to another.
Process variety causes gas panel variety
A process variation and requirements affect the selection of components in the gas panel. Simply, process variety causes gas panel variety and component variety. The process chamber pressures range from a few micro-Torr vacuum pressure to atmospheric pressure. Flow rates also vary widely, from as low as a few SCCM to 1,000 times higher in SLM. Similarly, some gas panels for some processes have minimal surface requirements, while others require fine surface finishes and special treatments, passivations or materials to avoid particle and ionic contamination. Outside of the process itself, the size of the equipment, clean room, or laboratory can impact the gas panel design; since space in clean rooms is expensive and thus limited, it might be necessary to upgrade from the ubiquitous variable compression ratio (VCR) fittings to the more compact but more expensive “surface mount” system.
Gas panel safety
In designing a gas panel, it’s also important to ensure the safety of the users. Some semiconductor processes use silane (SiH4) as a precursor for silicon or silicon dioxide. Over certain ranges, silane is a pyrophoric gas; silane will spontaneously combust in room-temperature air without any ignition source. For other metal hydride gases, such as germane, arsine and di-borane, the primary concern is the toxicity. With chemicals like these, leaks into areas with users are not permissible. But no matter how well-designed and assembled the components, fittings and tubes are, the designer must count on the possibility something going wrong at some point. The designer can plan for and document these potential failure modes in a Failure Mode Effects Criticality Analysis (FMECA) and in formal hazard reviews.
So, the gas panel must be contained in an enclosure that can safely exhaust a gas leak. Further, for certain hazard levels the system should detect a leak, and take appropriate actions. Documents like NFPA 318: Standard for the Protection of Semiconductor Fabrication Facilities and SEMI S2: Environmental, Health, and Safety Guideline for Semiconductor Manufacturing Equipment can serve as guidelines for safe design. However, while valuable, these guidelines are no substitute for input from experienced system designers.
There’s a great deal to know when it comes to designing a gas panel for semiconductor processing. Between the widely varying requirements for different chemicals and processes, the safety risks inherent with these types of systems, and the large list of component manufacturers and system assemblers, there are a lot of factors to consider. Gas panel components aren’t inexpensive, so there’s a balancing act in gaining the necessary functionality, modularity and serviceability while at the same time eliminating redundancies and unnecessarily complex parts. Equipment companies may have entire teams dedicated solely to designing their gas panels, so it can be a big task for a small research lab or a startup materials science firm to tackle.
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