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SEMI E129-1103 © SEMI 2003 17 RELATED INFORMATION 2 STATIC CONTROL METHODS NOTICE: The m aterial contained i n this rela ted inform ation is not a n official part of SEMI E129 and is not intended to m odify or supersede …
SEMI E129-1103 © SEMI 2003 16
Euzent, B.L., Maloney, T.J., Donner II, J.C., “Reducing
Field Failure Rate with Improved EOS/ESD Design”,
Proceedings of the EOS/ESD Symposium, Las Vegas,
NV (1991), pp. 59-64.
Pierce, D.G., Shiley, W., Mulcahy, B., Wunder, M.,
“Electrical Overstress Testing of a 256K UVEPROM to
Rectangular and Double Exponential Pulses”,
Proceedings of the EOS/ESD Symposium, Anaheim,
CA (1988), pp. 137-146.
Renninger, R.G., Jon, M.C., Lin, D.L., Diep, T.,
Welsher, T.L., “ A Field-Induced Charged-Device
Model Simulator”, Proceedings of the EOS/ESD
Symposium, New Orleans, LA (1989), pp. 59-71.
NOTICE: Unless otherwise indicated, all documents
cited shall be the latest published versions.
R1-4 Acknowledgement
Contributed by Leo G. Henry, Ph.D., email:
leogesd@pacbell.
SEMI E129-1103 © SEMI 2003 17
RELATED INFORMATION 2
STATIC CONTROL METHODS
NOTICE: The material contained in this related information is not an official part of SEMI E129 and is not
intended to modify or supersede the guide in any way. These notes are provided as a source of information to aid in
the application of the guide, and are to be considered reference material. Determination of the suitability of the
material is solely the responsibility of the user. This related information was approved by full letter ballot
procedures on September 3, 2003.
R2-1 Static Charge Control
R2-1.1 It is usually impossible to eliminate static
electricity from work areas, but with proper use of
equipment and remedial procedures, most static
problems can be controlled. Many approaches to
controlling static charge have been tried over the years
and it is clear that there exists no single method for
controlling all static charge problems. However, it has
been shown that a consistently applied process for static
charge control greatly reduces problems associated with
static electricity.
R2-1.2 One overriding issue should not be forgotten in
the choice of static control materials and procedures.
This is the need for compatibility with the cleanliness
requirements of the area in which they are installed. A
variety of static control materials are available, but
some may not be cleanroom compatible due to issues of
particle shedding, outgassing, or their chemical
makeup. Each manufacturing area in the
semiconductor factory should contain only those static
control materials that are appropriate to the cleanliness
requirements of the area. Consult ESD TR11 for
additional information.
R2-2 Charge Generation
R2-2.1 Triboelectric Charging
R2-2.1.1 A fundamental law of nature states that an
electrostatic charge is generated whenever two
dissimilar materials contact and then separate. The
materials themselves, intimacy of contact, speed of
separation and a variety of other factors determine the
amount or level of electrostatic charge that is generated
during any given contact and separation event.
Predicting the level of charge ahead of time is nearly
impossible. Due to this unpredictability, the best that
can be done in a factory setting is to understand that
electrostatic charge generation will always occur.
However, electrostatic charge generation can be
measured and charge mitigation is normally a
possibility. Reducing charge generation requires
modification of surfaces to increase lubricity (i.e.,
reduce friction), increase conductivity (i.e., allows
charges to relax and flow back), and make the surfaces
chemically similar. Normally, more than one of the
above actions is required to provide a low charge-
generating system.
R2-2.1.2 While no intrinsic test method exists for the
determination of triboelectric charge-generation
properties, several practical evaluation techniques that
apply to many factory situations may be found in ESD
ADV 11.2.
R2-2.2 Effect of Humidity on Charge Generation
R2-2.2.1 It is generally understood that at constant
temperature and at increased relative humidity the
normally observed manifestations of static charge
generation, such as static cling and static shocks, are
greatly reduced. This does not mean that static charge
is not present in the environment. Even at 90% RH,
static charge may accumulate at a sufficient level to
cause problems with some processes or cause damage
to sensitive parts. Most factory processes, including
cleanrooms, are run at a humidity level of 30–70%.
Within this range, most of the humidity-dependant low-
charging materials will function within their intended
specifications. At lower humidity, surfaces dry out and
may become a source of charge generation.
R2-2.2.2 Specifying a humidity level for a factory is a
reasonably good idea if kept realistic. However,
maintaining a minimal level of humidity may be very
expensive in some areas of the world during normally
dry winter months. It could be more important and cost
effective to choose materials that provide the required
function for charge generation or dissipation at the
lowest expected humidity. Many parts of the world
experience humidity levels of 10% or less during the
winter months when outside air is brought inside and
then heated to warm interior spaces. In these locations,
which include the northern tier of US states and
Canada, much of northern Asia, as well as northern
Europe, careful attention to material selection is of
major importance to assure adequate dissipative and
low charge-generation performance.
R2-3 Grounding Conductors and Static
Dissipative Materials
R2-3.1 The primary method of controlling static charge
is grounding. If there is a path for the charge to flow to
earth ground, the static charge on facility surfaces,
SEMI E129-1103 © SEMI 2003 18
furniture, equipment, materials handled by the
equipment, and any ESD-protective materials can be
rapidly, and harmlessly, neutralized. Providing a path
to ground brings everything to the same “zero” voltage
electrical potential. All conductors in the environment,
including personnel, should be electrically connected
and attached to a known ground. It is important to note
that insulators cannot lose their electrostatic charge by
attachment to ground. Other charge-mitigating
techniques (discussed in later sections) are required to
neutralize excessive charge (i.e., electric field
reduction) on the necessary insulators used in the
electrostatic protective area (EPA), if their charge is
considered a risk to sensitive parts handled in that
location.
R2-3.2 Grounding methods and their measurement are
described in detail in ESD S6.1. Information on
electrical power grounding, which is a separate issue
not addressed in this document, may be found in
ANSI/NFPA 70, ANSI/IEEE STD 142, or appropriate
country guidelines.
R2-3.3 Grounded conductors cannot maintain a static
charge for very long. This is primarily dependent on
the overall electrical resistance of the system. System
resistance includes the resistance of the item being
grounded, the contact resistance at the interface
between items and the resistance in the actual
grounding path. Since resistance of the system ground
path determines the rate of discharge, it determines the
amount of protection provided to ESD-sensitive
devices. A permanent connection to ground will
certainly prevent any charge from building up on an
object and its subsequent transfer to an ESD-sensitive
device. However, if an ungrounded, charged object
contacts a material with too low a resistance to ground,
a damaging discharge may occur. Even if no direct
damage occurs, the ESD event creates EMI that may
affect the operation of nearby production equipment.
R2-3.4 It should be noted that attaching a large
metallic object (e.g., a stainless steel work surface) to
ground through a high resistance will not prevent a
discharge from occurring to the large metallic object.
The high capacitance of the metallic object, with
respect to the ESD-sensitive device, can still result in
ESD damage or EMI.
R2-3.5 To prevent these conditions from occurring, the
proper resistance for the ground path should be chosen.
A class of materials has been developed, called static
dissipative materials, which help to control discharge
rates when materials are placed in contact. Static
dissipative materials are created by lowering the
resistivity of insulating materials through the addition
of conductive particles, carbon particles, or chemical
additives. When connected to ground, these materials
slow the discharge rate from nanoseconds (typical for
metal-to-metal contact) to as long as hundreds of
milliseconds (depending on the size of the materials
involved). While a discharge occurs to allow charge
equalization with ground, a rapid, spark-producing
discharge may not occur.
R2-3.6 In modern production equipment, the frame and
other machine elements are normally metallic and thus
conductive. These machine elements are often covered
with static dissipative materials. In these design
situations, the conductive frame should be attached to
ground without any added resistance to provide a
ground plane. The dissipative surface over the
conductive frame provides charge-draining
characteristics suitable for reducing risk of damage
from rapid discharges.
R2-3.7 Static dissipative materials will need to retain
their dissipative properties over the range of
temperature and humidity conditions they will
encounter, and not change significantly over time. In
cleanrooms they should also meet requirements for
avoiding micro-particle production and outgassing.
R2-3.8 Measurements of the resistance and resistivity
of static dissipative materials are used to characterize
their effectiveness in a static control program.
Measurements should be made of the materials
themselves, as well as their performance in their
installation location. There are several measurement
methods available depending on the application.
Acceptable resistance ranges may depend on the ESD
sensitivity of the products or photomasks that are
handled in the area. These will need to be determined
by the end user and the material suppliers.
R2-3.9 See the following test methods for resistance
measurements of various items:
• ESD STM11.11: Surface Resistance Measurement
of Static Dissipative Planar Materials
• ANSI ESD STM11.12: Volume Resistance
Measurement of Static Dissipative Planar Materials
Both test methods characterize the resistivity or
resistance of either static dissipative or conductive
materials. These test methods can be used for
construction materials, work surfaces, furniture
components and most planar surfaces. They may also
be used for packaging and transport materials.
• ANSI ESD S4.1 can be used to measure the
resistance to a groundable point of these materials
once they are in place. This can be done for work
surfaces, construction materials (other than
flooring), and other planar surfaces on furniture or
equipment.