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In part 6 of our blog series on the design of high purity semiconductor equipment, we’ll be discussing gas valves. While mass flow controllers fine-tune the amount of gas delivered to a process, and pressure regulators fine tune the gas pressure, it is the manual and pneumatic valves that control on or off state of gas flow. Valves operate as a binary (on/off) means of controlling the flow for every segment of gas line from storage to gas panel to exhaust. Since they have such a vital role in the system, there are many different options to consider when selecting which valves to use.
Figure 1 above shows the variety of valves used in a semiconductor tool’s dedicated gas panel; which might have 3-5 of these “gas sticks”. Further types can be found in other sections of the gas distribution system in the semiconductor fab (factory).
The most commonly-used valve in the semiconductor equipment is the diaphragm valve, so-called because it uses an elastic diaphragm to seal off the flow. In many cases there are multiple diaphrapms on top of each other for safety and strength. This type of valve not only provides much faster and more precise actuation than a gate valve or ball valve, but the diaphragm also separates the fluid from the actuation components of the valve. Considering the corrosive and reactive hazardous production materials used in many semiconductor processes, this separation is vital to the longevity of the valve hardware. Furthermore, this also maintains the purity of the gas, which is vital in semiconductor manufacturing.
Semiconductor equipment engineers also commonly use check valves in both semiconductor equipment and the semiconductor fab infrastructure. There are a few variations on the hardware that makes up a check valve, but the function is always the same: to allow fluid flow in one direction but prevent it in the other. Check valves are not controlled by the user, but rely on the fluid flow itself to open and close as appropriate. These are usually considered a safety feature. In Figure 2, for instance, the check valve is placed at the downstream end of the gas stick, to prevent any blowback or other gases from damaging the sensitive mass flow controller (MFC).
Although it’s possible to use electromechanical or hydraulic power to actuate valves, in the semiconductor industry the predominant types of control valves are actuated by pneumatics. Manual valves can’t be operated by the control system, and hence are not generally considered part of it. However, these are still used for some safety applications.
Manual valves use a lever or wheel to open and close their valve members. Since these require a person to actuate them, they aren’t feasible for use everywhere in a large system nor appropriate for automatic safety measures, or even process control. However, they can be useful when strategically placed. Mechanical engineers will typically install a manual valve on gas lines between the gas cabinet and the gas panel, so that a single valve can control the gas flow to the entire process in the event that gas needs to be stopped for maintenance or service. Furthermore, manual valves allow lockout-tagout (LOTO) functionality, allowing users to padlock the valve in the closed position for maintenance or safety reasons. “Lock out tag out” is a safety requirement of SEMI and OSHA, so that someone servicing the equipment is not exposed to hazards. Simply, one writes a note, known as a “tag,” and then locks out the hazard. Others know who locked out the system, and when. LOTO is used for electrical, mechanical, or any hazard, not just gas or chemical hazards. You can see a LOTO manual valve on the image above in the first position on the gas stick; with the rest of the gas panel components downstream, a technician can use this one valve to shut down the gas flow for any component that needs service.
Pneumatic actuation is the most commonly-used method for valves in semiconductor processing. Pneumatic valves are controlled by air pressure delivered from a central pilot valve manifold, usually through polyethylene or polyurethane tubing. Pneumatic control systems offer reliable, fast, and repeatable actuation. Safety is also built-in, as any pressure loss in the lines or power loss to the pilot manifold will drop the valves to their safe, normally-open or normally-closed state. Pneumatic valves can be made to be LOTO capable, but it is less common.
Other Valve Options
Beyond the type of valve member and actuation, there are a few other valve specifications that the mechanical engineer must consider when choosing a valve for semiconductor applications. For ordering purposes, the customer specifies these options by stringing together a series of manufacturer-defined number and letter codes into a single part number. These include some of the following choices: fitting type such as VCR; fitting sex, such as male or female; valve diameter, such as 1/4 inch; material of construction, such as 316L VIM VAR stainless steel; and other options.
For a pneumatically actuated valve, the engineer must choose which state the valve defaults to when it is receiving no pressure. For safety factors, most valves in semiconductor processes are “normally closed”, meaning that with no air pressure they stay shut; as mentioned in our blog on semiconductor safety interlocks (part 5), this stops the flow of hazardous gases in emergency situations. Some valves are better suited to a normally-open configuration, if they involve exhaust or purge capabilities. The mechanical engineer or semiconductor equipment engineer must consider the safety situations for each valve individually as they design the system.
Most valve manufacturers offer their standard diaphragm valves in low-pressure and high pressure versions. Gas cylinders require high pressure valves. On the other hand, gas panels are usually rated for a low pressure of operations. The destinction is most important for penumatically or electrically operated valves. The higher the fluid pressure, the more force that it takes to actuate the valve. A high pressure gas can often force a low pressure pneumatic valve open, On the other hand, many manual valves are rated for high pressure, and can be used for low pressure, without a distinction.
Material and Finish
As discussed in the Material section of our semiconductor gas lines blog (part 4), gas distribution lines come in multiple types of materials and surface finishes. Any valve installed on those lines must ensure the same level of purity, so it’s important to pay attention to valve specification sheets and make sure the materials meet your requirements.
A number of different fitting types are used in semiconductor processes, the most common being Swagelok’s proprietary swaging system and Variable Compression Ratio (VCR) fittings. Some manufacturers only offer VCR face seal or tube stub (for swaging or welding) ports on their valves, but others may offer National Pipe Thread (NPT) ports or other types. Most companies also offer valves designed for use with surface mount systems, if a more compact distribution system is necessary. Also, for anyone designing a system, it’s vital to pay attention to the gender of the ports on the different components; one doesn’t want to reach the assembly stage and find themselves trying to connect two male ports together.
Valves in the Semiconductor Fab
When designing the gas distribution system of a semiconductor fab or research lab, the mechanical engineer or semiconductor equipment engineer must give careful consideration to selection of components. The high purity valves are no exception, and their selection is even more important when safety is concerned; valve failures can be catastrophic. High purity valves have an important role in the distribution system, and selecting the wrong option on a single valve might foul up the process or bring about unwanted safety issues.