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Figure 1: Setra™ Pressure Transducer and Display
In the last blog, I introduced the design and use of pressure gauges and transducers and gave some examples of their use in a semiconductor fab in gas panels, valve manifold boxes, and exhaust systems. For those parts of the fab, pressure measurement is primarily useful for allowing operators to monitor gas pressures in real-time and for interfacing electronically with fab safety interlock systems and maintenance systems. Semiconductor equipment engineers, usually mechanical engineers or electrical engineers, have some of these same purposes in mind when they install pressure transducers directly on semiconductor tools, with the additional goal of granting the tool operator control over the pressures of incoming gas and in the process chamber.
Pressure Transducers in Gas Panels
As I wrote about in the first entry in this blog series, a semiconductor tool’s gas panel serves as the control center for cylinder gas delivery to the tool: it regulates mass flow and pressure and directs gases to the process chamber. Since the gas panel controls the sensitive mixture of process gases, sometimes at very low flow rates or pressures, its constitutive components must provide ultra-clean, precise, accurate and repeatable functionality. Therefore, the pressure transducers installed in a gas panel should offer more precise measurements, higher-resolution output, and faster response time than the transducers and gauges used elsewhere in the fab on gas cabinets or exhaust systems. However, they are still not as accurate as the pressure transducers used on the process chambers.
Inside the gas panel, the engineer will usually place a pressure transducer on each gas stick after the gas stick’s pressure regulator. The pressure transducer electronically monitors the pressure at the regulator’s output. For a more complicated system, the pressure transducer (pressure gauge) might be actively linked to the pressure regulator so they can work together to maintain the correct gas pressure. In either case, it’s important to keep the transducer and regulator close together, to reduce the lag between pressure adjustments at the regulator and their subsequent measurement by the transducer.
The designing mechanical engineer can direct the electronic output from the transducer to a digital display at the point of use inside the gas cabinet, or route it electronically to an external control station or to the semiconductor tool controller.
Figure 1 shows a Setra™ pressure transducer and its associated remote display unit (slightly worse for wear, after some time in operation). It is often convenient to display the pressure both at the transducer and remotely, allowing operators to instantly check the gas lines pressures whether they’re next to the gas panel or at their workstation. In most production semiconductor tools, the display and power supply functions incorporated into the display unit would instead be incorporated into process tool. On a recent consulting project with a semiconductor research lab, for instance, for each gas line we specified a Setra™ pressure transducer along with a small Setra™ digital display that piggybacks on the transducer’s data cable and displays the pressure. The accuracy of these units is on the order of 1% or better.
Figure 2: MKS™ Vacuum Pressure Transducer
Pressure Transducers on the Process Chamber
Aside from the gas panel, a mechanical engineer might also install pressure transducers directly on the semiconductor tool’s process chamber. These can be useful for both safety monitoring and control purposes.
For the research lab I mentioned above, we specified a combination pressure transducer and switch to act as a vacuum safety sensor for the chemical vapor deposition (CVD) chamber. Since their CVD chamber was designed for vacuum use and cannot safely contain over-pressurization, it’s crucial that the process shut down if the chamber ever loses vacuum. To accomplish this, we specified that the transducer’s output connect to the semiconductor tool’s safety interlock board. If the pressure transducer detects an absolute pressure higher than half an atmosphere, it will open its electronic circuit; this action will in turn cause the safety interlock board to cut power to the pneumatic valve control board, closing all valves in the system to halt gas flow.
Pressure transducers connected to the process chamber can also be essential tools for controlling more sensitive semiconductor processes. The MKS™ vacuum transducer shown in Figure 2, for instance, can detect minuscule pressure changes as small as a thousandth of a Torr. For reference, the pressure unit of measure known as the Torr, is already 1/760 of a standard atmosphere, or 1 mm of Hg. (The unit is named after the Italian scientist and fluid mechanics father Evangelista Torricelli, 1608 – 1647, a colleague of Galileo; Torricelli served as his assistant and then took over his teaching position after Galileo’s passing.)
A semiconductor engineer could use this transducer to precisely control the chamber’s vacuum throttling valve, in order to maintain a constant pressure. At the high vacuum pressure used in some semiconductor processes, the molecules in the chamber travel a distance greater than the chamber before they strike another molecule, in what is known as molecular flow. Known as the “mean free path”, this collision distance is highly dependent on pressure. Managing the mean free path is essential for controlling and predicting delicate processes.
That being said, most semiconductor equipment operates in the viscous pressure range, where gas molecules constantly collide with each other and the gas behaves as we observe in everyday life. The increased complexities and challenges of vacuum system design warrant their own blog series.
Figure 3: Analog Pressure Gauges
Similarly to the semiconductor fab, a semiconductor tool might also use non-high-purity gases. A semiconductor tool might use a pneumatic system to control its valves, for instance. The pneumatics would still require pressure gauges, but since its air is entirely separate from the process those gauges would not require high-purity materials or surface finishes. The NoShok™ gauge shown In Figure 3 would be the perfect low-cost device for this type of application. Obviously, exhaust systems are not considered high purity, but may be sensitive to corrosion. Those of you who moonlight as baristas may have noticed the “dual pressure gauge” shown above on the left as one from an espresso machine; it shows water pump pressure on the left and steam pressure on the right. Pressure gauges are common in everyday life, even if one does not notice them.
Pressure Measurement in Semiconductor Processes
Semiconductor processes (or any processes that involve transporting fluids from one place to another) rely on differential pressures to move gases from their supply cylinders and gas cabinets through the process tool and then out to the atmosphere as exhaust. The process gases will naturally try to move from the initial high pressure at the gas cylinder to the low pressure created by the tool’s vacuum pump. Knowing the gas pressures in both the distribution system in the semiconductor fab and in the semiconductor tool itself is vital in keeping the process functioning properly and safely. There are a wide variety of gauges, transducers and sensors for measuring gas pressure, and it’s important to choose the right type for each application in the system.