Cleanrooms: The Total Package
Thu, 11/16/2017 - 4:09pm by Greg Owen, PE, Jacobs.
When the topic of cleanroom design is mentioned, one automatically thinks of the cleanroom classification requirements of ISO 14644, or Federal Standard 209E. Both provide a historically reliable method of testing and classifying the airborne particulate contamination present in the "cleanroom." This evaluation is used to determine a metric called a Classification, or "Class" of the cleanroom. ISO 14644 loosely defines a cleanroom as:
"Cleanrooms and associated controlled environments provide for the control of airborne particulate contamination to levels appropriate for accomplishing contamination sensitive activities."
While this concept of cleanrooms only being particulate sensitive environments has been in general use since the mid-1950s, coupled with the advent of the "laminar flow" concept in the early 1960s, it does not fully address all of the contamination sources that can limit the user's ability to accomplish required sensitive activities. To achieve this broader, more holistic approach to the design of contamination control environments, a more encompassing definition and understanding of this type of a "cleanroom" is necessary:
"Contamination Control Environments, often referred to as a "cleanroom," is a purposely designed and constructed code compliant interior space that is intended to mitigate physical phenomena that are detrimental to the intended function and activities to be performed in the space."
The key differences in this definition is the inclusion of "mitigate physical phenomena that are detrimental to the intended function and activities to be performed in the space." A natural phenomenon is generally defined as an observable event which is not man-made. "Physical Phenomena" are any natural phenomenon involving the physical properties of matter and energy. These physical properties result in conditions that can adversely impact the activities performed in the cleanroom. Some examples of Physical Phenomena:
Particulate: Well-defined in ISO 14644 and FED STD 209E. The easiest form of contamination to control.
Temperature: Ambient and internal cleanroom temperatures need to be considered. Most, if not ALL, cleanrooms have internal temperature stability and tolerance ranges. Broad ranges of temperature stability will cause problems with alignment in photo processes. Wide tolerances in temperature will also result in temperature swings that impact processes such as photolithography and occupant comfort. Well-designed cleanroom air management systems can control temperatures to a tolerance of +/- 0.5oF with a stability rate of change of <0.5oF / Hr.
Humidity: Relative humidity is a relationship between the moisture content in the air and the maximum amount of moisture that the air can contain at a given temperature. An extreme example of a humidity sensitive space is a Lyophilization (Lyo) cleanroom. The freeze drying of drugs will be impacted if the humidity is not under tight control. In other clean environments, humidity that is too high may result in corrosion of exposed metal components, and too low in the generation of static electrical charges. Well-designed cleanroom outside air systems and controls can easily control humidity to +/- 1% RH.
Biological: Biological contaminants include microorganisms such as bacteria, viruses, yeasts, molds, and parasites that result in illness or injury; they can cause problems in cleanrooms/contamination controlled environments involved in the production of pharmaceuticals, medical devices and food. Vivariums also have concerns about the introduction and/or spread of biological contaminants. Control of this form of contamination is generally by UV sterilization of make-up air and thermal oxidization of exhaust.
Noise/sound:Sound and noise have the same impact on the activities carried out in a cleanroom. Regular sound is generally pleasing to our ears and mind, whereas unpleasant sound is referred to as "noise" and generally has an irregular pattern. Both sound and noise are propagated through sound waves that consists of a repeating pattern of high-pressure and low-pressure regions moving through a medium. Sound waves are sometimes referred to as a pressure wave, which can induce vibrations in equipment, resulting in harmonic sound amplification and generally making a space uncomfortable to the occupants. Generally, a quiet space provides a more enjoyable and collaborative environment. Sound mitigation is a well-defined science.
Electromagnetic interference:Electromagnetic interference (EMI) is a magnetic disturbance generated by an external source that affects an electrical circuit by conduction, electromagnetic induction, or electrostatic coupling. The disturbance may degrade the performance of the circuit or even stop it from functioning. In the case of electronic data, these effects can range from an increase in error rate to a complete loss of data. Both man-made and natural sources generate changing electrical currents and voltages that can cause EMI. Examples of sources of EMI are electrical power cables, transformers, large moving metallic objects (elevators, trains, trucks, subways), and solar activity. Construction techniques do exist to mitigate EMI impacts in a cleanroom, but the costs associated with this flexible design approach where any tool can be located anywhere (anything, anywhere) in the cleanroom is usually considered cost-prohibitive. EMI is best mitigated with passive shielding of sensitive equipment either at the room level or tool level. Active shielding systems are also available for extreme conditions.
Vibration:Vibration can be the result of seismic events, traffic (speed bumps, etc.) rotating equipment, the flow of fluids, walking of occupants, or other energy-imparting actions that can impact the supporting structure. Ambient vibrations are of the greatest concern and often the most difficult and costly to control. The good news is that many types of vibration sensitive equipment can have vibration economically mitigated through the use of active or passive isolation systems. The best approach is reasonable cost construction with the use of isolation systems as needed for sensitive equipment. Again, as discussed with EMI, the costs associated with the "anything, anywhere" approach to flexibility is usually considered cost prohibitive.
Light:Light is the visible form of electromagnetic radiation. In the photolithographic process used in the fabrication of semiconductors, certain light frequency will expose the resist (film) coating applied to the wafers and adversely impact the production of the devices. The mitigation of this contamination is through the use of light frequency blocking filters that generally mitigate to a very low level the light wave frequencies below 500nm. The result is the familiar "amber light' area found in many cleanrooms.
Radiation:Radiation, as a contaminate, can be harmful to humans and other living organisms and detrimental to many physical and material science activities performed in a cleanroom.
Static electricity:Static electricity is an imbalance in electrical charge that is developed through the repeated contact of materials with different electrical resistance. Static can cause substantial problems in the electronics industry and is mitigated through the use of static dissipative flooring, grounding straps, humidity control, and in some cases air ionization.
Molecular:Molecular contamination, or Airborne Molecular Contamination (AMC), consists of chemicals in the form of vapors or aerosols generally at a ppb level that have a detrimental effect on the activities performed in a cleanroom. These chemicals may be organic or inorganic in nature and include acids, bases, polymer additives, organometallic compounds, and dopants. The main sources for AMC are building and cleanroom construction materials, ambient environment, process chemicals, and operating personnel. An example of AMC is the obfuscation of a space telescope mirror due to sulfur compounds in the ambient environment. Molecular contamination is controlled through the use of selective chemical filtration in the outside supply air and the recirculation air systems. Additionally, careful selection of materials of construction is an effective means of actually eliminating AMC sources from the cleanroom.
The other key word in the definition is "mitigate." Mitigation is defined as the action of reduction of severity, seriousness, or painfulness. Since physical phenomena are a combination of naturally occurring and man-made conditions they cannot be totally eliminated; they can only be controlled to an acceptable level, i.e. mitigated.
While there is nothing inherently wrong in classifying a cleanroom by the airborne particulate count present in the space alone, a successful design must mitigate ALL forms of contamination that will adversely impact the functions carried out in the space. It is the designer's obligation to fully understand the activities to be carried out in the cleanroom and to aid their client in developing an understanding of the physical phenomena that will cause problems. A cleanroom that is functionally and promotes collaborative research needs to be a total cleanroom - one that addresses ALL forms of contamination that are detrimental to the intended use of the space.
Greg Owen, PE, is the Cleanroom Design Principal for Jacobs, a global provider of engineering and construction services. He has over 35 years' experience in the design and construction of contamination control facilities for semiconductor, solar, aerospace, pharmaceutical, and University. As a Mechanical Engineer with multi-discipline experience he has been involved in the design of over two million square feet of cleanrooms. firstname.lastname@example.org; www.jacobs.com