Figure 1: Analog Pressure Gauge
In the last few entries in this blog series, I’ve written about methods for controlling and regulating gas pressure and flow rates in a gas line with valves, pressure regulators and flow restrictors, but I haven’t yet discussed the gauges and sensors used to actually measure these values. I will cover mass flow controllers in a later blog, but in this and the next entry I will focus on pressure gauges and pressure transducers: first in the semiconductor fab, and next in semiconductor equipment.
The term “pressure gauge” is a catch-all for any device which measures and displays the pressure of a given volume of fluid. Pressure gauges can operate mechanically or electrically. Whereas some pressure gauges display the pressure value on an indicator at the point of use, others may transmit the data to a remote location.
The term “pressure transducer” refers to a specific type of pressure gauge which converts the pressure value to an electrical signal; these see widespread use in semiconductor processing, considering their increased accuracy over mechanical gauges and their ability to interface with electronic control systems.
A Brief Primer on Pressure Measurement
For those unfamiliar with pressure measurements, there are three specific types of pressure. As defined in the Dictionary of Scientific and Technical Terms (McGraw-Hill, 5th Ed.):
- Absolute Pressure: The pressure above the absolute zero value of pressure that theoretically obtains in empty space or at the absolute zero of temperature (6)
- Gage Pressure: The amount by which the total absolute pressure exceeds the ambient atmospheric pressure. (821)
- Differential Pressure: The difference in pressure between two points of a system, such as between the well bottom and wellhead or between the two sides of an orifice. (568)1
For example, atmospheric pressure in which we live is 0 gage pressure, and 14.7 PSI (101.3 kPa) absolute pressure.
If the pressure in a car tire is 30 psi gage, then it is 44.7 psi absolute pressure. The differential pressure between the tire and atmosphere is also 30 psi.
Specifying Pressure Gauges and Pressure Transducers
Pressure gauges and transducers require the same set of general specifications and component options as any other components in the gas distribution system of a semiconductor process. The mechanical engineer must specify the material of construction, material purity, surface finish, fitting type and sex, electrical connection, pressure range, and others. The component specifications for the pressure transducer must largely match the rest of the components in the system. As with pressure regulators, it’s important that the pressure range is kept as small as possible while still encompassing the operational values. This will allow the transducer to accurately detect small fluctuations in the pressure.
The mechanical engineer must also decide which type of device is most appropriate for the application. For each pressure gauge or transducer required on the system, the engineer must take into account:
- The required measurement precision and accuracy.
- The type of data needed from the device (e.g. a simple analog gauge or digital signal sent to a control workstation).
- The operating conditions: temperature, pressure, …
- The gases and materials that will be in contact with the pressure transducer.
- The type of pressure measurement: (1) gauge pressure, (2) absolute pressure, or (3) differential pressure.
Uses for Pressure Gauges and Transducers
Mechanical engineers typically use pressure gauges for a few purposes: monitoring, active control, and safety. Semiconductor equipment is no different, and it is common to see pressure gauges and transducers installed in multiple locations throughout a semiconductor fab.
Gas Cylinder Cabinet
Since gas cabinets store compressed gas cylinders at pressures as high as 3,000 PSI (200 atm), monitoring their pressure is important for the safety of the equipment and operators. Additionally, a significant drop in gas pressure can indicate that the supply is low and that cylinder needs replacing. Within the gas cabinets, each cylinder is attached to a pressure regulator and a pressure gauge, as shown in the opening image. Since precise pressure control or measurement isn’t usually necessary at this point, it is common to see simple analog gauges within gas cabinets. Lately, some fabs have also started installing transducers as well, to keep track of pressure over time and predict the next cylinder replacement date.
Valve Manifold Box
Valve manifold boxes (VMBs) bridge the space between gas cabinets and gas panels on the semiconductor tool, controlling the distribution of process gases to multiple semiconductor tools. The components within are substantially similar to those in a gas cabinet, but with lower pressures and lower flow rates. VMBs typically lower the gas pressures as they split gas lines to separate machines, so pressure must be monitored here as well. As a single spot where multiple inlet and outlet gas lines converge, VMBs let facility technicians or engineers monitor the gas pressures for multiple semiconductor tools; it also provides a single point where a technician can shut down the gas flow to multiple tools for maintenance or safety purposes.
Exhaust systems, as discussed in Part 3 of our blog series, require a negative gauge pressure, typically not more than a few inches of water. (Note than atmospheric pressure is equivalent to about 32 feet of water. Most scuba divers would recall this.) Keeping the exhaust at a slight vacuum is especially important between the process chamber and the abatement system, as waste gases can be volatile, flammable, corrosive, or a combination thereof. A negative gauge pressure, pulled by the blower system on the roof, ensures that any leak will cause air to flow into the exhaust duct from the room, rather than the reverse. Semiconductor fabs usually monitor exhaust pressure with a combination gauge and switch, like Dwyer’s Photohelic® series, which not only lets operators check the pressure at a glance but also provides a switch that can interface with a safety interlock system. With any loss of exhaust draw, the signal from the pressure switch will cause the safety supervisory system to shut the system down. This could mean a loss of vacuum pressure indicating that the exhaust system has lost power, or a pressure build-up that might indicate an obstruction in the ducts.
Figure 2: Analog pressure gauges
Non High-Purity Applications
Semiconductor fab operators must also measure and maintain pressure for the non-high-purity fluids that support the system, such as cooling water pressure, clean dry air (CDA) or pneumatic air or nitrogen. Since these systems are not high-purity, the types of pressure gauges used here must not be used in any high-purity applications in the fab. The image above shows simple dual and single pressure measurement gauges, which wouldn’t be compatible with a high-purity environment. (Bonus points for any reader that can identify the specific purpose of the gauge on the left).
Pressure Monitoring and Safety in a Semiconductor Fab
Mechanical engineers can also use pressure gauges and transducers within semiconductor process tools to control their operating pressures, which will be the subject of our next blog. However, at a factory level, pressure transducers and pressure gauges are most important for system monitoring and safety control. Even simple analog pressure gauges can provide vital feedback to operators and technicians, and upgrading to electronic transducers lets the mechanical engineer automate the fab’s safety measures.
1: Parker, Sybil P. McGraw-Hill Dictionary of Scientific and Technical Terms. 5th ed. New York: McGraw-Hill, 1994.