TO MEET CLEANLINESS SPECIFICATIONS, the first thought is frequently “Build it in a Cleanroom.” However, not every critical component need be processed in a large, tightly specified cleanroom by gowned personnel. Sometimes mini-environments provide a superior and cost-effective option. We are expanding the concept of the mini-environment from that often used in microelectronics to encompass any small, atmosphere-controlled enclosure that allows processing without contaminatingthe product or the external environment.

Semiconductor wafer fabrication
While most processing is done in large air and humidity-controlled cleanrooms, critical processing of wafers is frequently performed in smaller self-contained enclosures that contain the wafers and robotically-operated processing tools. The super-critical cleanliness environments required for processing or transport are confined to small volumes, providing cleanliness levels unachievableeven with head-to-foot gowning of personnel.

Mini-environments for less critical applications or multi-use workspaces
A mini-environment approach to cleaning critical components can be employed even when required cleanliness levels are not so severe as for semiconductor wafer processing. Frequently, by using a mini-environment, a process can be performed in an area built to a lower cleanliness standard. This savescosts of building, facility maintenance, and gowning.

Another use for mini-environments is for multiple applications within the same workspace. Steve Silverman, of Bartlett Bay Consulting, says, “My emphasis is not multi-use cleanrooms. Instead, I advise my clients to use mini-environments and glove boxes. That way, two assemblers, or product lines in close proximity can be handling totally different processes with totally different requirements.That is a real strength of the glove box or the mini-environment.” [1]

Mini-environment examples
Functionally, a laminar flow workbench is a controlled mini-environment in an otherwise less controlled laboratory environment. Two commercially-availableexamples of glove box mini-environments for critical cleaning processes are aCO₂ cleaning system [2] and a solvent spray cleaning system. [3]

The CO₂ system consists of a snow-cleaning jet inside a benchtop chamber. It is not a true glove box since it has an open front. However, like a properly used glove box, it prevents outside atmosphere from entering or contaminants from leaving the chamber. A system controls the temperature and cleanliness of the internal atmosphere and collects airborne contaminants, removed from the component by the cleaning jet, before recirculating the air back through a heater/filter. In a cleanroom environment, it is expected to have little impact on the cleanliness or airflow of the surrounding area. In a non-clean environment, it avoids drawing outside atmosphere into the chamber. [4]

The solvent mini-environment consists of a solvent spray cleaning system in a stainless steel glove box. It provides a “cleanroom” environment to prevent recontamination after cleaning and also protects the worker from any vapors present during the cleaning process. When connected to a fume extracting unit with a carbon bed, the system becomes self contained providing a more favorable worker safety and environmental regulatory profile.

Therefore, when faced with cleaning specifications for a new product line, sometimes think small instead of big. A mini-environment may provide lower cost, higher performance, and better protection for both the product and the worker. Consider whether or not mini-environments have a place in your process flow, in and out of the cleanroom.

Note: Specific products are indicated as examples,not as endorsements.

References:

1. Steve Silverman, interview in “Clean Source, a BFK Solutions Newsletter”, 3, 2 (April 2006). Available at http://bfksolutions.com/Newsletter/Newsletter.html
2. PurCO2, CleanStation Precision CO2 Spray Cleaning System
3. Cobehn Systems, “Mini-Clean Room”
4. Product DataSheet, PurCO2 Div. of Cool Clean Technologies, Inc., http://www.purco2.com/PDF/PurCO2%20CS6000%20012206.pdf

 

Barbara Kanegsberg and Ed Kanegsberg are independent consultants in critical and precision cleaning,surface prepara-tion,and contamination control.They are the editors of “Handbook for Critical Cleaning,”CRC Press.Contact them at BFK Solutions LLC.,310-459-3614;info@bkfsolutions.com; www.bfksolutions.com.