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SEMI E78-1102 © SEMI 1998, 2002 22 R2-3.6 Ionizers in equ ipment must deliver ionization over a wide range of hum idity and temperature conditions. Back-end assembly and test areas often do not have the level of temperat…

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SEMI E78-1102 © SEMI 1998, 2002 21
RELATED INFORMATION 2
STATIC CONTROL METHODS
NOTE: This related information is not an official part of SEMI E78 and is not intended to modify or supercede the official
standard. Determination of the suitability of the material is solely the responsibility of the user.
R2-1 Static Charge Control
R2-1.1 It is usually impossible to totally 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.
R2-2 Grounding Conductors and Static
Dissipative Materials
R2-2.1 An important consideration in selecting a
method is whether the charged material is a conductor
or an insulator. Static dissipative materials are created
by lowering the resistivity of insulating materials
through the addition of metal or carbon particles, or
other chemical additives. Static charge on a conductive
or static dissipative object can be easily controlled if the
object is provided with a path for the charge to flow to
earth ground. While charge is mobile in a conductor (or
in a static dissipative material), in insulators charge is
not mobile, and earth grounding is not an effective
means of eliminating the static charge.
R2-2.2 Equipment manufacturers can use both
conductive and static dissipative materials to reduce the
presence of static charge. If there is a path for the
charge to flow to earth ground, the static charge on
equipment, and materials handled by the equipment,
can be rapidly, and harmlessly, neutralized. Obviously,
the success of earth grounding depends on maintaining
the integrity of the ground path. This is sometimes a
problem when high-speed, moving parts of equipment
must be connected to earth ground.
R2-2.3 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 must also meet requirements for
avoiding micro-particle production and outgassing. As
long as the ground connection is maintained, these
“passive” procedures offer reasonable protection to the
equipment and product from sources of static charge.
R2-2.4 Unfortunately, these methods do not provide
complete protection from static-related problems. Even
when earth grounding is an option, it is subject to
human error. In applications where contamination is an
issue, additives and carbon particles used in static-
dissipative materials may become sources of
contamination themselves. When earth grounding or the
use of dissipative materials is either inappropriate or not
cost effective, ionization can be used.
R2-3 Ionization
R2-3.1 More often than not, the product itself uses
insulating materials, making earth grounding
unavailable as an option. While silicon is a
semiconductor, its oxide coating transforms it into an
insulator. Teflon is used in many chemical processes,
and quartz in high temperature processes. Epoxy and
ceramic packages are used for integrated circuits.
Insulators are easily charged, retain their charge for
long periods of time, and are often close to the product.
Dealing with static charge on insulators and isolated
conductors will often require the use of some type of
ionization. Ionizers are the most effective means of
dealing with static charges on insulators and isolated
conductors.
R2-3.2 For purposes of static charge control, ions are
molecules of the gases in air (nitrogen, oxygen, water
vapor, and carbon dioxide) that have lost or gained an
electron. Ions are present in normal outside air but are
removed when air is subjected to filtration and air
conditioning. Ionization systems work by increasing the
conductivity of the air with the ionized gas molecules.
When ionized air comes in contact with a charged
surface, the charged surface attracts ions of the opposite
polarity. As a result, the static electricity that has built
up on products, equipment and surfaces is neutralized.
R2-3.3 The most common methods of producing air
ions are radioisotopes and “corona discharge” resulting
from the electric field created when high voltage is
applied to a sharp point.
R2-3.4 The radioisotope most commonly used to
produce ionization is Polonium
210
, an alpha particle
emitter. The alpha particle collides with the surrounding
gas molecules, dislodging electrons, which results in
pairs of positive and negative ions.
R2-3.5 The corona discharge method produces a very
high electric field that interacts with the electrons in the
surrounding gas. The polarity of the ions depends on
the polarity of the high voltage on the emitter point.
Ions of opposite polarity to the charged surface are
required. Either polarity of static charge may be created
in the equipment or on the product.
SEMI E78-1102 © SEMI 1998, 2002 22
R2-3.6 Ionizers in equipment must deliver ionization
over a wide range of humidity and temperature
conditions. Back-end assembly and test areas often do
not have the level of temperature and humidity control
found in front-end wafer production. Ionizers installed
in the cramped spaces of production equipment will be
close to the product, in areas surrounded by grounded
metal parts. Ionizers should isolate the emitter points
from both the product and adjacent grounded surfaces.
Ionizers should produce sufficient ions to discharge
static on surfaces and products moving at high speeds
despite losses to ground. Most ionizers require
maintenance and periodic verification of their
performance.
R2-4 Problem of Controlling Static Charge in
Manufacturing Equipment
R2-4.1 The interior of high speed production
equipment presents a challenge to most static control
methods. The cost of production space is high and
requires that equipment occupying the space be
compact and operate at as high a speed as practical.
Product is moved through small spaces at high speed by
a variety of robotic and other mechanisms. Triboelectric
charging (charge generation due to friction or contact
and separation of dissimilar materials) and contact with
ground are almost unavoidable. Grounding of
equipment parts that contact the product presents added
difficulties when the equipment parts are moving at
high speeds. Dissipating charge from insulating
surfaces and integrated circuit (IC) packages may be
difficult if the charged surfaces are not accessible.
Using ionizers in these confined spaces presents
challenges.
SEMI E78-1102 © SEMI 1998,2002 23
RELATED INFORMATION 3
EXAMPLE FOR ADDING ELECTROSTATIC COMPATABILITY
REQUIREMENTS TO PURCHASING DOCUMENTS FOR
SEMICONDUCTOR MANUFACTURING EQUIPMENT
NOTE: This related information is not an official part of SEMI E78 and is not intended to modify or
supercede the official standard. Determination of the suitability of the material is solely the responsibility of
the user.
R3-1 Purpose
Purchasing semiconductor manufacturing equipment
that meets SEMI E78 Electrostatic Compatibility
requirements can reduce the cost of ownership of the
equipment by reducing operating problems and product
defects, by eliminating costly equipment modifications
after delivery, and by making the equipment available
for use more quickly after delivery.
The purpose of this Related Information 3 is only to
describe an example of specifications for electrostatic
compatibility that are meant to form part of a general
purchasing document for production equipment or
minienvironments.
R3-1.1 Limitations — Related Information 3 is only
one possible example of defining E78 compliance
requirements for equipment. Users may include all, or
any part of it, modifying it as necessary in their
purchase documents. Users are responsible for
determining the appropriate level of static control
protection depending on their specific circumstances.
R3-2 Terminology (Reference: ESD
Association Glossary ADV1.0)
R3-2.1 conductive material — electrostatic conductive
materials have a surface resistance of < 1 × 10
4
Ω or a
surface resistivity of < 1 × 10
5
Ω/square when tested
according to ESD Association ESD STM11.11, or a
volume resistance of < 1 × 10
4
Ω or a volume resistivity
of < 1 × 10
4
Ω-cm when tested according to ESD
Association ANSI/ESD STM11.12. (Other national or
international (IEC) standards may be substituted).
R3-2.2 static dissipative material — electrostatic
dissipative materials have a surface resistance between
1 × 10
4
Ω and < 1 × 10
11
Ω or a surface resistivity of
between 1 × 10
5
Ω/square and < 1 × 10
12
Ω/square
when tested using ESD Association ESD STM11.11, or
a volume resistance of between 1 × 10
4
Ω and < 1 ×
10
11
Ω or volume resistivity between 1 × 10
4
Ω-cm and
< 1 × 10
11
Ω-cm when tested using ESD Association
ESD STM 11.12. (Other national or international (IEC)
standards may be substituted)
R3-3 General Static Control System
Description
R3-3.1 The static control system shall provide
electrostatic charge and ESD control to meet the
recommendations for Level 2 (or 1, 3, or 4 as agreed
upon) contained in SEMI standard E78 Electrostatic
Compatibility: Guide to Assess and Control
Electrostatic Discharge (ESD) and Electrostatic
Attraction (ESA) for Equipment. These
recommendations are defined in Section 12.5 Table 1.
R3-3.2 The static control system shall consist of the
following items, applied as needed to assure
compliance.
R3-3.2.1 Grounding of conductive equipment parts
when feasible.
R3-3.2.2 Static dissipative materials to replace
insulators when feasible.
R3-3.2.3 Ionization sources to control static charge on
process essential insulators or isolated conductors,
particularly when they are part of the product (e.g.,
oxide-coated silicon, epoxy-packaged devices, reticles).
R3-4 General Static Control System Design
Requirements
R3-4.1 Grounding
R3-4.1.1 Conductive equipment parts shall maintain a
resistance to chassis ground or to electrical ground of
less than 1 ohm. (Reference – ESD Association
standards ANSI EOS/ESD S6.1 Grounding --
Recommended Practice and ESD SP10.1 Automated
Handling Equipment. Other national or international
(IEC) standards may be substituted)
R3-4.1.2 In critical applications, as determined by the
user, constant monitoring of equipment grounding shall
be required. In some cases, local electrical safety
regulations may prohibit the use of this method. A
suitable test method to capture short interruptions of
ground connections may need to be used.
R3-4.1.3 Resistance measurements for moving
equipment parts shall be made with the equipment in
operation.