An important yield benefit is helping cleanrooms save energy and money.
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by Dan Hall, Progessive Technologies Inc.
Recent industry and government studies demonstrate the value of increasing worker safety and decreasing the tonnage of greenhouse gasses released during energy production.1 An exhaust balancing system promotes these goals while improving a facility's competitive advantage. Rapid methods of regulating house exhaust provide significant yield improvements in critical processes. They are inherently providing significant energy savings, as well. Extending the use of rapidly responding, point-of-use regulators throughout a semiconductor manufacturing facility can reduce the energy consumed by air handling and abatement equipment. As we move into the 300 mm era, where every savings must be realized to make the technology viable, automatic fab balancing systems offer a significant step forward in energy conservation, with the potential for as yet undiscovered yield improvement.
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Many processes now call for rapid point-of-use exhaust control to minimize process variation caused by house exhaust changes. By reducing one more variable, the addition of exhaust control often improves process robustness, resulting in an increased Cpk. For processes using rapidly responding regulators, the tight exhaust control minimizes the amount of airflow out of the tool, resulting in lower air handling costs. With escalating manufacturing costs and increasing emphasis on reducing energy consumption, facilities may want to investigate fab-wide exhaust control as a means to reduce total air handling.
One process that reaps tremendous improvements with the implementation of exhaust control is the resist coating process. Resist thickness uniformity relates to the speed of the airflow across the wafer. As the air speeds up, uniformity is degraded. The edge bead removal process, however, has a different relationship with airflow. The slower the airflow, the more likely it is for particles on the edge of the wafer to be swept up and redeposited on the wafer surface.
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One exhaust control method uses a two-stage control technique. This type of control responds within 50 milliseconds of a disturbance, establishing a constant condition (variation is typically less than ±2 percent) for the process tool. These regulators rely on a mechanical part powered by airflow and gravity. When air pressure within the building exhaust system fluctuates, a “floating piston” responds by using the energy of the fluctuation against itself, creating stable pressure or flow at the process. Two-stage regulators are ideal for resist coating because they respond so rapidly to fluctuations. Traditional controllers, such as a mechanized butterfly valve, respond so slowly that the process is complete by the time the controller brings the process under control after a disturbance. A two-stage regulator with variable set-point capability optimizes the airflow for each step, while minimizing overall airflow.
A second area that uses exhaust control for process improvement is the wet station area. When enclosed in a minienvironment, the airflow requirements for a wet station change instantaneously whenever the door to the minienvironment is opened or closed. With the door closed, unregulated exhaust will create a significant vacuum within the minienvironment. When the door is opened, air rushing in to fill the vacuum will deposit condensation from the door onto the wet deck and unprotected wafers.
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An exhaust regulator, however, can control the pressure within the minienvironment at just below room pressure. By responding immediately to a door opening, the regulator not only prevents particle redistribution, it maintains a safe environment for those using the wet bench-containing fumes at all times.
The two-stage regulator is ideal for this process as well.
By maintaining a constant pressure at the wet station, two-stage regulators allow a reduction in the airflow safety margin, resulting in up to 30 percent reduction in air volume. Because 21 percent of all cleanroom air is exhausted through a wet bench, these savings can be substantial.
Savings do not have to stop with wet benches, however. Similar safety and cost reduction benefits can be achieved in support cleaning equipment (consuming 10 percent of all cleanroom air), gas cabinets (14 percent), and process equipment cabinets (5 percent). Other areas that would see minimal process or safety improvements, such as enclosures and gas boxes (5 percent), would still benefit from optimized, controlled airflow.
How excess flow wastes energy
During the process of transforming silicon into valuable semiconductors, fabrication facilities push huge volumes of air—as much as 20 million pounds of air a day—through the building. This enormous quantity of air is known throughout the industry as “makeup air,” a name derived from the need to replace both clean and contaminated air removed by the exhaust system. Replacing this air uses a tremendous amount of energy. In fact, recent studies by both Sematech and the EPA have identified the HVAC system as the largest consumer of electricity in the energy-intensive semiconductor manufacturing process.
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Makeup air entering the facility must be meticulously prepared. First, the air-handling system refrigerates the air to remove moisture-containing contaminants; then it reheats, rehumidifies and filters the air to deliver it at precise specifications. To meet EPA, NFPA and general safety standards, the air leaving the facility must go through one of several abatement processes, such as burning or scrubbing, to remove toxins before they are released into the atmosphere. The amount of energy consumed by the makeup and abatement equipment correlates with the amount of air moving through the exhaust system. Experts agree that reducing the volume of air flowing through the HVAC system will reduce the facility's energy consumption.
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An automatic balancing exhaust system eliminates the previously discussed drawbacks. An automatic balancing system is a proactive system that isolates the individual processes from pressure fluctuations in the house exhaust system by placing active controllers at each branch or point of use of the exhaust system. Because of the need for rapid response, the design of this automatic system relies on two-stage regulators. Unlike the traditional system of mechanically locked valves, this point-of-use system corrects pressure fluctuations before they impair the process. The balancing system controls crosstalk as just another form of pressure fluctuation and proactively prevents the phenomenon from affecting the tools in the line. With this sophisticated control, a facility can fine tune a process that requires multiple set points and turn down exhaust when equipment is idle.
An important feature of the automatic balancing system is a microprocessor-based monitoring system that provides the operator with a centralized, real-time control base over an entire house exhaust network. A single monitoring instrument at each point of use serves as the operator's window into the process. A display reveals a 60-second continuous graphic representation of how the system is performing, allowing the operator to examine process pressure or exhaust flow. Then, if necessary, the operator can make real-time adjustments and see the system's response. When all the monitoring instruments link to a central computer, exhaust flow can be fine tuned. This flexible control is in contrast to today's “adjust and measure” policy.
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A controller at the exhaust output of each process tool regulates airflow at point of use throughout the facility. This system saves energy in three ways. First, precise regulation of exhaust pressure fluctuations (from ±30 percent down to ±2 percent) allows the operator to determine the minimum process or safety exhaust level and set the airflow at that level. This eliminates the need for a 30 percent margin in total flow to compensate for low exhaust swings. Second, the system reduces the excess volume of air flowing into the abatement equipment, decreasing the energy required to burn or scrub the exhaust. Third, by correcting crosstalk before it affects the process, the automatic fab balancing system eliminates the need for a traditional damper lockout procedure. Facilities can turn down exhaust levels when the equipment is idle (from 100 percent to approximately 10 percent of the flow rate required by the process) without affecting the other tools in the line.
Beyond the savings in energy consumption, an automatic balancing system provides yield benefits and reduces costs. Smooth uninterrupted scavenging of contaminated air out of the process eliminates particle and vapor contamination at the source, improving product quality and yield.
Dan Hall,director of R&D for Progressive Technologies, Inc. currently spearheads new product development. He has 16 years of design and development experience in the semiconductor, defense and industrial control industries. Dan holds a BSEE from the University of Delaware and an MSEE from the University of Michigan. Progressive Technologies designs and manufactures two-stage regulators and automatic fab balancing systems for the semiconductor and other clean room industries. Contact Dan at Progressive Technologies, Inc., 200 Ames Pond Drive, Tewksbury, MA 01876, 978-863-1000, fax 978-863-1099, or e-mail [email protected].
References:
- Proceedings of the EPA’S Semiconductor Energy Efficiency Opportunities workshop held in San Jose, CA, November 1997; SEMATECH’s Tool Exhaust Optimization PTAB held in Austin, TX, January 1998.
Why exhaust flow is required
Adequate exhaust flow is essential to the health and safety of employees conducting semiconductor processes in the cleanroom. Environmental health and safety standards require that sufficient air must flow through an open process into the exhaust system to scavenge away contaminated air and prevent backstreaming. Traditional HVAC systems cannot regulate exhaust pressure or maintain consistent flow at the point of use, so air pressure fluctuations in the main exhaust system affect the exhaust flow rate at all open processes and compromise the scavenging capability. The result is a high probability that house exhaust fluctuations (typically ± 30 percent of the average flow) will compromise the process exhaust flow. Prudent HVAC practice sets significantly higher cleanroom exhaust flow level standards to ensure a safe work environment despite these fluctuations. These exaggerated flow rates create excessive demands for makeup air to replace the air that has been exhausted. The equipment needed to process this large volume of air must be sized for the excess capacity, resulting in increased capital equipment costs and higher energy consumption.
Conventional HVAC systems control airflow with a combination of reactive electrical controls at the exhaust output and fixed mechanical valves at point of use. The technology for the mechanical valves, both dampers and butterfly valves, was first developed for fireplaces in medieval castles. Locked mechanical valves cannot respond to the inevitable fluctuation introduced into a building's exhaust system. Electronically controlled, variable-speed fans at the exhaust output respond but take seconds to regain control. In essence, a reactive system allows a problem to develop at the exhaust input and then attempts to correct it. Between the time the reactive system acknowledges a problem and then responds to it, exhaust backstreaming may already have contaminated the product and work environment.
Further complicating matters, all equipment of the same type typically ties into a common exhaust system using locked mechanical valves. In this design, increased airflow in one tool decreases airflow in adjacent tools—a phenomenon known as “crosstalk”. If a process requires variable flow rates to optimize yield and safety, the exhaust flow at the tool level becomes impossible to balance properly—the resulting flow rate is a tradeoff between the various process levels and the ability of the system to maintain sufficient flow.
Another drawback of this method is that the addition of any tool requires the local exhaust system to be rebalanced, resulting in a nonproductive operating expense and increased energy consumption. — DH