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By Paul Miller, Contributing Editor
While silicon wafer fabrication involves many discrete manufacturing operations, there also are process variables being controlled. The critical clean room heating, ventilation and air conditioning (HVAC) systems present some especially interesting closed-loop control challenges.
Semiconductor product yields depend heavily on environmental conditions inside the fab’s clean rooms. Not only must ambient temperature and humidity levels be maintained within very tight tolerances, airborne particulates also must be carefully controlled because they can render semiconductor components worthless.
For these reasons, clean rooms use extensive HVAC systems with high-performance filters to tightly control temperature and humidity, while filtering airborne particles. Typically, powerful recirculation air handlers (RAHs) provide high-velocity air at a uniform temperature. This creates a unidirectional or laminar flow of air through the clean room. The straight parallel streams of air at high velocity carry away airborne particles and prevent wafer contamination. The temperature in the clean room is controlled to ±1.5 ºF. To maintain this accuracy, each RAH relies on a cooling coil, a set of isolation dampers and a fan.
Teaching an Older Fab New Tricks
At one large wafer fabrication facility, more than 160 truck-sized RAHs are aligned in a grid high above the 125,000 ft2 clean room (Figure 1). When the fab was originally built, nearly 100 air handlers were installed. Unfortunately, only a few actually were commissioned before the project was put on hold in the late 1990s, when overcapacity hit the previously red-hot semiconductor manufacturing industry.
The minimal controls installed at the time were inefficient at best. Since the motors ran at a constant speed, workers had to climb several stories to adjust the air regulator that controlled the airflow to the damper solenoids. Not only was this a poor use of labor and a potential safety issue, it also resulted in wasted energy and unnecessary motor strain.
When the decision was made to reopen the fab, the wafer manufacturer also decided to update and coordinate the controls for its critical RAHs. For this accelerated project, it sought help from what was Industrial Design, a global engineering firm in Portland, Ore., for engineering procurement contractor, CH2M Hill.
gt;“For our customers, time to market equals time to profit,” says Gary Mays, the firm’s instrumentation and controls manager. “Our first priority is to get our customer up and running as quickly as possible.”
To save time, CH2M Hill initially considered using the existing motor starters. However, it quickly became apparent that this was neither the best control approach nor the most cost-effective solution for the air handlers.
Instead, it worked with Rockwell Automation to develop and implement an intelligent motor control solution for the RAHs. The solution included intelligent, variable-speed drives, a ControlLogix programmable automation controller, ControlNet networking, and FactoryTalk View human-machine interface (HMI).
“The traditional problem with VFDs used in clean room environments was they had bimetallic overloads that could trip and turn off,” says Steven Goh, marketing director, Asia Pacific, at Rockwell Automation. “In this case, intelligence means adding electronic overloads and presets to avoid those trips. Now, users know when something is beginning to happen to their motors, such as drawing higher current, and they can monitor 200-300 motors on one screen.”
No More Homerun Cabling
Using its existing motor starters, the fab previously would have needed to hardwire each starter back to the controller cabinet. By networking the smart drives, they could install one communications cable between the drive and controller cabinets. The temperature transmitter, control valve, damper position switches and damper solenoids in each RAH unit were all wired directly to the drive mounted on the RAH unit, eliminating the need to homerun these instruments to a central control panel.
Besides reducing wiring and installation costs, this approach created a centralized point for all drive programming and file storage, providing the flexibility and integrated capabilities that would carry the fab into the future.
Drives Pay Their Own Way
“When we evaluated our equipment options, Rockwell didn’t have the lowest initial purchase price,” says Mays. “But when we assessed the total cost to design, develop and deliver the solution, their variable-frequency drives easily paid for themselves.”
Because space is at a premium in a wafer fab, design firms look for innovative ways to reduce equipment footprints. CH2M Hill engineers designed the RAH control system to allow field I/O to be wired directly to a drive’s I/O board, essentially using the drive as a distributed I/O drop. Networking the drive’s I/O back to the controller eliminated the space required for new panels and conduit runs, saving valuable real estate.