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 imbal-
ance 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 dis-
sipative 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 det-
rimental 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 sys-
tems. 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.