Cleanroom Air Usage in Automated Wet Benches
Minimizing exhaust levels in conditioned air processes can produce significant cost savings. Several approaches can be used to reduce exhaust levels of automated wet benches.
By LISE LAURIN
Wet benches are common equipment in cleanrooms worldwide and can account for over 20 percent of the air handling load. The way a wet bench is installed can contribute significantly to the air handling load and to the cost of running the cleanroom.
Many variables contribute to air handling efficiency. These include:
1. Protection of the operator from exposure to fumes.
2. Protecting the product from con tamination.
3. The source and required treatment of the incoming air.
4. The air path through the wet bench.
5. Design and control of exhaust.
6. Required special treatment of exhaust.
7. The economics of the various air flow designs and treatments.
The consequences involved with the first variable cause it to dominate the design of any air handling scheme. The easiest way to maintain a safe environment around an automated wet bench is to constantly draw copious amounts of cleanroom air through the wet bench into the exhaust system. As cleanroom air costs escalate and more stringent requirements are put on cleanroom effluent, the economic factor suggests that significant improvements can be made in the cost of ownership of wet benches.
The Cost of Incoming Air
In order to protect the product within the wet bench, the source of incoming air must be from an environment at least as clean as the cleanroom itself. Additional purification may occur within the wet bench (the minienvironment concept). Cleanroom air costs vary by location and cleanliness specifications. SEMI/Sematech quotes costs of $1.98 per CFM per year1 for a Class 10 cleanroom, while other sources indicate air conditioning costs alone can be as high as $50-60 per CFM per year.2 At $1.98 per CFM per year, reduction of cleanroom air usage in a wet bench is not a significant economic factor. At $50 per CFM per year, significant savings could result from a small decrease in average air flow rates.
Once the air leaves the wet bench any solvent or acid fumes must be treated before the air can be released into the environment. Effluent treatment costs are more significant than production costs, and can be between $0.30 per 10,000 ft3 air ($150 per CFM per year) and $30 per ft3 of air for VOC incineration.3 Any reduction in the amount of air moving through the wet bench can result in significant savings in reduced abatement costs.
By evaluating the different scenarios under which a wet bench must operate, it is possible to identify ways of reducing the amount of exhausted air. The highest risk to personnel is when the doors to the automated wet bench are open and cleanroom personnel are present. Air flow past the operator must be sufficient to draw fumes away, but must not create turbulence. Typical flow velocities required to maintain a unidirectional flow through a wet bench area are 50 to 150 linear feet per minute.
When the doors to the wet bench are closed, air flow requirements are less stringent. In this case, the only requirement is to ensure air flow is in one direction only: from the cleanroom through the wet bench and out to house exhaust. To maintain this direction of air flow, one needs to maintain the wet bench at a lower pressure than the cleanroom and the exhaust at a lower pressure than the wet bench. The actual air flow required to maintain this condition will depend upon the air path through the wet bench with its doors closed. This flow may be as little as 20 percent of the air flow (CFM) required with doors open.
Actual Conditions with Manual Damper Valve–Typical Case
Typical house exhaust variation is on the order of 䕂 percent. To compensate for this variation, the flow through a damper valve is usually set 30 percent higher than the flow required to maintain ideal conditions. For example, if 50 linear feet per minute are required for safety, the flow through the wet bench would have to be set at 65 linear feet per minute. When house exhaust levels fall by 30 percent, this will still ensure a minimum velocity of 50 linear feet per minute. This corresponds to an average static pressure setting which is 30 percent higher (more negative) than required for minimum air flow.
Figure 1 shows a 䔸 percent variation in house exhaust and the corresponding process pressure when using a manual damper valve. The red line shows the highest (negative) static pressure which can be guaranteed with this hardware. To maintain this level, the damper must be adjusted for 30 percent higher pressure.
During normal operation with the doors closed, the flow is restricted to infiltration through unsealed areas. A manual damper valve is adjusted in the worst case for safety: 30 percent higher than the required flow rate with the doors open. When the doors are closed later, the wet bench internal pressure will experience a significant pressure drop, and the inside of the wet bench will begin to approach the negative pressure in the house exhaust system. The lower the pressure inside the wet bench, the more air will infiltrate through unsealed areas.
Over the life of the equipment, the infiltration flow will be the predominant challenger of effluent abatement equipment. With the process area near the pressure of the exhaust, the air consumption can be 500 percent greater than necessary.
This situation can cause contamination of the product within the wet bench. When all areas of the wet bench are near the pressure of the house exhaust, air flow will be by the easiest path, reducing isolation between process areas and allowing cross-contamination. Another source of contamination is from airborne droplets. In the lower pressure environment, evaporation of process chemicals will be accelerated. The evaporated vapor can then be recondensed on surfaces such as the access door. The air rushing in from the cleanroom to fill the vacuum in the wet bench when the access door is opened will then pull the droplets from the door and redeposit them within the wet bench, possibly on the product.
Burden of Excess Air on the House Exhaust System
Because the air moving through the house exhaust system is always fluctuating, the house exhaust system itself must be sized to handle the maximum amount of air which could flow through it. If the air flow through a wet bench could be capped at the minimum required for safety, the house exhaust system requirements could also be capped at this flow rate. However, because the average flow through a wet bench is 30 percent higher than the minimum safety requirement, the house exhaust system must be sized to handle more than 30 percent excess flow. This includes fans to move the air as well as scrubbers and effluent abatement equipment to remove effluent. When a cleanroom contains several wet benches, additional fans and scrubbers may be required to handle the extra exhaust.
One Alternative–Electronic Closed-loop Control of a Butterfly Valve
One method of limiting the amount of air flowing out of a wet bench is to carefully monitor the air flow into and out of the wet bench itself, and use an automated butterfly or damper valve to make adjustments as required. The system works to control the average pressure in the chamber. This method requires:
An anemometer built into the air flow path to measure velocity
A microprocessor-based feedback loop control system
A stepper motor-driven butterfly valve.
Widespread use of mechanized butterfly valves and electronic closed-loop control systems appear to make this alternative a comfortable solution. Integrated electronics allows integration into a fab-wide exhaust monitoring system, or into the wet bench controller, and the theory of operation and required maintenance are well understood.
Ideally, this approach could offer significant savings to the end-user. Specifically, the control system could be given a much lower set point to maintain ideal conditions, resulting in a lower average flow rate. Unfortunately, this potential savings is not achievable with this type of hardware because the safety requirements would be compromised.
Electronic closed-loop control is a four-step process: measure, analyze, command movement, move a mechanical part. This process takes time to complete (2 to 10 seconds for a typical mechanized butterfly valve). When a door is opened, the cleanroom air will rush into the wet bench to equalize the pressure differential created when the doors were closed. To reestablish the 50-150 feet per minute flow, the butterfly valve must make a correction of 500 percent. Furthermore, during that time, a rapid reduction in house exhaust can change the system dynamics, reducing the actual flow rate through the system by the same percentage as the house exhaust. For the period of time it takes to complete the feedback loop process the system is not maintaining a safe condition.
It can be seen that the process pressure can fall as low as 30 percent below set point with a 30 percent drop in house exhaust pressure. When a door is opened, the closed-loop control system has a far greater challenge to reestablish flow.
For this reason, a system with electronic closed-loop control will also need to be set 30 percent higher to maintain safe conditions. Figure 2 shows typical operating conditions with a mechanized butterfly valve. The red line again shows the maximum maintainable (negative) static pressure to maintain safety before a door is opened. This level is identical to the level with a manual damper valve.
Mechanical Closed-loop Pressure Regulator
A mechanical closed-loop, high-speed pressure regulator can also be used to maintain a constant pressure differential between the cleanroom and the wet bench. The mechanical closed-loop system controls in one step: the out-of-specification variable (i.e. the pressure change) changes the physical configuration of the regulator, and this physical change eliminates the pressure change and returns the flow to specification. A mechanical closed-loop pressure regulator in the exhaust line will prevent fluctuations in house exhaust from reaching the wet bench. In addition, when the wet bench doors are closed, the amount of air flow required to maintain a constant pressure differential will be reduced. In this approach, installation is straightforward: a single, mechanical closed-loop regulator is required in the exhaust line.
The stability and response rate of the mechanical closed-loop system maintains process pressure within 5 percent of setpoint, even with house exhaust fluctuations of 30 percent. This lets the user significantly lower the average air flow through the wet bench.
Mechanical closed-loop pressure regulators have been built which complete the response to a change within a tenth of a second. During this response time, the pressure is maintained within 5 percent of set point. Figure 3 shows actual process pressure variation using a mechanical closed-loop pressure regulator. In this case, the maximum maintainable (negative) static pressure corresponds to the product specification of -5 percent of set point.
The reaction time of the mechanical closed-loop pressure regulator allows a set point much closer to the ideal rate because house exhaust fluctuations will no longer affect the flow rate. With a set point producing a flow 5 percent higher than the ideal rate, savings from $6,000 to $45,000 could be realized. (See “Potential saving using 5 percent overdilution.”)
Additional reductions come from reduced air flow when the doors are closed. With the doors open five minutes every hour and assuming a conservative 20 percent reduction in air flow rate with doors closed, the following savings from $11,000 to $86,000 can be added to those above. (See “Potential savings based on a 20 percent reduction in air flow (doors closed).”)
This approach has the potential for significant savings to the end-user. The wetted path of the mechanical pressure regulator can be made of PVC or of stainless steel, providing compatibility with all common wet process fumes. Since there is only one mechanical component to the system, reliability is high.
In addition to cost savings, mechanical closed-loop pressure regulation offers benefits in reduced product contamination. By using separated pressure regulators for each station within a wet bench, each bath can be separately controlled to a preset pressure. In this configuration, fumes are contained within the station and will not be pulled across an incompatible bath. The problems associated with airborne droplets discussed earlier are also reduced. First, the pressure within the wet bench is closer to atmospheric pressure, reducing the rate of evaporation. Secondly, the velocity of the air rushing into the wet bench when a door is opened will be much lower due to the lower pressure differential between the wet bench and the cleanroom.
In 1990, Dr. Ohmi suggested: “Minimizing exhaust levels lowers the amount of external air taken in and reduces the power required for humidification and dehumidification. This is one of the major cleanroom energy conservation measures.”4 When you add the costs required to condition air which has moved through a wet bench, minimizing exhaust levels in this process can produce significant cost savings. n
References
1. Robert Sherman, “Technology Forum: Cost of Ownership–Clearing the Air,” SEMI/Sematech News, Nov. 1994, p. 12.
2. Phil Naughton, “Cleanroom HVAC Systems,” CleanRooms, Oct. 1992 p. 21.
3. Derrick Drohan, “VOC Abatement,” Semiconductor International, April 1993, p. 47.
4. Tadahiro Ohmi, et al, “Developing a Central Monitoring System for the ULSI Environment,” Microcontamination, Jan. 1990, p. 64.
The author would like to extend special thanks to Harold Fitch of Future Resource Development for his advice and encouragement with this article.
Lise Laurin is marketing product manager for Progressive Technologies Inc. (Andover, MA). She has a bachelor`s degree in Physics from Yale University and has worked in the semiconductor and semiconductor equipment industries for 14 years. Progressive Technologies Inc. specializes in point-of-use exhaust control for the semiconductor industry.
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Figure 2 shows typical operating conditions with a mechanized butterfly valve.
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Figure 3 shows actual process pressure variation using a mechanical closed-loop pressure regulator.
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Figure 4 highlights annual operating costs using various exhaust control methods.