By Paul Miller, contributing editor
While silicon wafer fabrication is largely a discrete manufacturing operation, there’s still a lot of process control going on in the utilities supporting a fab. The critical clean room heating, ventilation, and air conditioning (HVAC) systems present some especially interesting closed-loop control challenges.
Death by Dust
Semiconductor product yields depend 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 must also 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 a high velocity of 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 within ±1.5 °F. To maintain this accuracy, each recirculation air handler 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 recirculation air handlers (RAHs) are aligned in a grid high above the 125,000 square foot clean room (Figure 1). When the fab was originally built, nearly 100 air handlers were installed. Unfortunately, only a few were actually commissioned before the project was put on hold in the late 1990s when overcapacity hit the previously red-hot semiconductor manufacturing industry.
Figure 1: Recirculation air handlers (RAHs) are aligned in a grid to provide the fab’s cleanroom with tightly controlled and filtered air.
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, but 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 recirculation air handlers. For this accelerated project, it tapped Industrial Design Corp. (IDC), a global engineering firm for engineering procurement contractor, CH2M Hill.
“For our customers, time to market equals time to profit,” says Gary Mays, IDC’s instrumentation and controls manager. “Our first priority is to get our customer up and running as quickly as possible.”
To save time, IDC 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, IDC worked with Rockwell Automation to develop and implement an innovative intelligent motor control solution for the recirculation air handlers. The solution included Allen-Bradley PowerFlex intelligent, variable speed drives (Figure 2), a ControlLogix programmable automation controller, ControlNet networking, and FactoryTalk View human machine interface (HMI).
Figure 2: Intelligent drives control fan speeds to keep the cleanroom pressurized and save energy.
“The traditional problem with VFDs used in cleanroom environments was that they had bimetallic overloads that could trip and turn off,” says Steven Goh, of Rockwell. “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 Home-Run 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 instead 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 home run these instruments to a central control panel (Figure 3).
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 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, Rockwell’s variable frequency drives easily paid for themselves.”
Because space is at a premium inside a wafer fab, design firms look for innovative ways to reduce equipment footprints. IDC engineers designed the RAH control system to allow field I/O to be wired directly to the drive’s I/O board, essentially using the drive as a distributed I/O drop. Networking the drives I/O back to the controller eliminated the space required for new panels and conduit runs, saving valuable real estate.