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This blog series has been examining the design of gas distribution systems for semiconductor processes. The previous entry focused on the use of diaphragm valves and check valves to activate or deactivate the flow in a length of gas line; this entry will cover the design and application of pressure regulators. Mechanical engineers use these more complicated components to proportionally vary the gas pressure as needed; the regulators will maintain this constant set pressure in a line despite input or output pressure fluctuations.
Gas Pressure Regulators
Gas pressure regulators are specially-designed valves that autonomously ensure a constant pressure on either their inlet or outlet side. After the operator sets the desired pressure, the regulator measures the inlet or outlet pressure (depending on which needs to be controlled) and adjusts the pressure in the opposite side accordingly in order to maintain that pressure.
Downstream Pressure Regulators
Downstream pressure regulators are the most common type that engineers install in semiconductor equipment. “Downstream” refers to the regulator’s control over the output pressure, and the operator can use the regulator to control the amount of gas flowing through the system to the process chamber. Once the desired pressure is set, if the output pressure is too high then the proportional controller begins to close and lower the pressure, and vice versa.
Upstream pressure regulators
The counterpart to the downstream pressure regulator is the upstream version, which maintains a constant input pressure. These can be useful on tanks or chambers to ensure that they sustain a constant pressure. For instance, if pressure begins to build up in the tank, then the regulator will open and let more pressure out.
Pressure Regulator Design
Autonomous fluid pressure regulators (whether used on gas or liquid) have been in use for at least a century. The mechanisms that operate mechanical pressure regulators have not changed much since their initial design, and semiconductor processes still use these reliable regulators for many purposes, albeit with more precise hardware and higher-purity materials. Recently, engineers have also begun incorporating electronic controls into regulators as well, which provide different advantages.
Mechanical Pressure Regulators
The simplest iteration of a mechanical pressure regulator design uses a spring-loaded diaphragm connected to the outlet chamber; an increase in outlet pressure pushes the diaphragm upwards, which in turn pulls a flow-restricting poppet closed and reduces the pressure. The operator can set the desired outlet pressure by manually adjusting the spring’s tension. You can see an example of this type of regulator in the image above, from a 90-year-old regulator patent.
The patent shown above also incorporates a built-in pressure gauge, allowing the operator to check the outlet flow and adjust as need be. If a pressure regulator does not have a built-in gauge, it is generally a good idea for the mechanical engineer to place a separate gauge or transducer on the regulator, such that the operator can monitor the pressure and adjust the regulator as needed.
Electronic Pressure Regulators
Some modern pressure regulators forego the interlinked mechanical hardware solution in favor of an electronic system that links a pressure transducer to an electric proportional-control valve. While an electronic system can be more expensive and might not be as reliable as a totally-mechanical version, they offer much more precise control, especially at low flow rates. Additionally, electronic pressure regulators can provide steadier behavior; the math on mechanical regulators is complex, “with the intertia of the moving parts interacting with the acoustic resonation of hte gas in the pipeline”1, but electronic systems eliminate most of those moving parts.
Pressure Regulator Specification Options
Since the primary function of a pressure regulator is to adjust variable pressures up or down to a set constant pressure, the most important characteristics an engineer needs to select when specifying a regulator are the outlet gauge pressure and the regulator’s pressure range. The type of semiconductor process and the pressure requirements of the MFC will generally dictate the necessary outlet pressure, and the engineer should select a regulator with a range that encompasses this outlet pressure without significantly exceeding it. For instance, if the process requires gas at 30 psi gauge (psig), then the engineer should select a regulator with a range of 0-60 psig. A regulator with a 0-30 psig range would not offer enough adjustability around the target pressure, while a regulator with a 0-300 psig range would deny the user fine control over the pressure at operational levels. Additionally, the engineer must specify the inlet gauge pressure as well, to ensure that the regulator can handle whatever high pressure it will be receiving.
In addition to the above, pressure regulators also share the same standard specification options as flow restrictors and other semiconductor process components. Namely, the mechanical engineer needs to specify the fitting type and sex, as well the materials and finish needed to maintain gas purity.
Regulating Gas Flow in Semiconductor Processes
Accurately and predictably regulating the gas flow through semiconductor equipment is a matter of both performance and safety.
Some semiconductor processes can be very sensitive to small variances in gas flow rates, and any process with multiple gases usually needs precise control over their mixing ratios. As with any dynamic control system, regulators will oscillate as they attempt to match a pressure, and these oscillations can disrupt sensitive processes. Regulators require careful tuning, based on the viscosity and flow rate, in order to ensure the correct amount of damping. This will minimize the chance of overshooting the desired pressure and causing oscillatory behavior. Tight response time by the regulator gives the operator much more precise control over the system.
Safety Aspects of Pressure Regulation
Since compressed gas cylinders at high pressures are the most common source for these gases, it’s vital that the semiconductor equipment engineer reduce these pressures as soon as possible in the system. This will not only reduce the wear and tear on the valves and components in the system, it will also reduce the chance of leaks and make any leaks that do occur much less dangerous. Many engineers will design semiconductor equipment with multiple pressure regulators, so that the pressure will safely and predictably step down to the necessary process pressures.
1: Downie, N. A. Industrial gases. London New York: Blackie Academic & Professional, 1997. 211-212.