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DEFENSE & AEROSPACE
ELECTRONICS ASSEMBLY
SEMICONDUCTOR
WAFER MFG
DISK DRIVE INDUSTRY
ESD Control Measures in the Cleanroom
ESD standards or industry evaluation
practices for measuring the effectiveness of selected products used
in the control of ESD events or ESA include the following products and materials:
ESD Flooring Other Products and materials: Workstations, Conveyors, Racks, Plexiglas or Plastic Enclosures, Transfer Carts, Chairs, Garments,Ionizations,ESD Monitoring,ESD Materials
ANSI/ESD Association S7.1-2005 for Flooring Areas.
ANSI/ESD Association S7.1-2005
is an excellent evaluation method for measuring the point-to-point
and resistance-to-ground (RTG) resistance for evaluating a floor.
Conductive floors are designed to
measure at or below 1.0 x 10e6 ohms whereas topically treated floors’
point-to-point resistance is between 1.0 x 10e6 ohms to 1.0 x 1.0 x
10e9 ohms.
Typical office building air contains from 500,000 to 1,000,000 particles
(0.5 microns or larger) per cubic foot of air. A Class 100 cleanroom is
designed to never allow more than 100 particles (0.5 microns or larger)
per cubic foot of air. Class 1000 and Class 10,000 cleanrooms are
designed to limit particles to 1000 and 10,000 respectively.
A human hair is about 75-100 microns in diameter. A
particle 200 times smaller (0.5 micron) than the human hair can cause
major disaster in a cleanroom. Contamination can lead to expensive
downtime and increased production costs. In fact, the billion dollar
NASA Hubble Space Telescope was damaged and did not perform as designed
because of a particle smaller than 0.5 microns.
Background
Electrostatic charge generation control is necessary
in cleanrooms as it can reduce yield, disrupt automatic equipment
from electromagnetic interference (EMI), and reduce surface particle
collection by electrostatic attraction (ESA). ESA is becoming more of a
concern as static charges have been seen to actually bond contaminants
to surfaces of products or tooling. For example, for a 4” wafer,
charged to 1000 volts, a particle of 1 micron in diameter would have
the bonding force of over 830,000 pounds per square inch. Therefore,
electrostatic attraction between charged objects and particles can be
quite strong compared to gravitational, aerodynamic, or adhesion
forces.
With the heavy use of insulating materials such as glass,
Teflon, and polymers, items can become highly charged. Grounded
workstations are a potential concern, as is the use of stainless steel
work surfaces compared to static dissipative work surfaces. Conductive
work surfaces can be considered a current-carrying hazard to people
and ESD-sensitive devices.
Tools for the Control of ESD and ESA Issues
Clean room designers should gain a greater understanding of cleanroom
classifications and ESD to eliminate static electricity concerns. ESD control
measures have traditionally been utilized for defense, electronics
assembly, semiconductor, wafer manufacturing, and the disk drive
industry. In the ESA control arena, fiber optics, medical,
pharmaceutical, wafer, and disk drive companies have stepped up
programs to control ESA in their cleanrooms. Controlling both ESD
events and static electricity in the cleanroom is important for all
these industries. However, depending upon the sensitivity level of
electronic components or MR heads, ESD may be of greater concern
whereas ESA could weigh more heavily in wafer manufacturing and in areas
where contamination control is foremost.
Surface Resistance versus Relative Humidity (RH)
According to ANSI/ESD S541-2003 (Packaging and Materials ESD
Standard), surface resistance measurements at 1.0 x 1011 ohms for
materials are considered insulators. In the insulative range,
materials become nonconductive and hold static charges for several
seconds or more. Surface resistance rises and falls with relative
humidity and the RH at 1.0 x 1011 ohms is the cut-off for retention of
static dissipative properties. A low cut-off is desired for packaging
materials because dry air may be encountered in shipping and handling.
During unpacking, triboelectric charges (two surfaces rubbing
together) not drained to ground could cause damage by field induction.
ANSI/ESD S20.20-1999 recommends a
target RH between 30% - 70%. Below 30% RH, materials have a greater
tendency to charge.
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