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SEMI E78-1102 © SEMI 1998, 2002 3 when discha rged directly t o a product or equipm ent part. 5.1.8 input and exit ports — The lo cations where product and/or prod uct carrie r s are placed to allow t he equipment to pro…
SEMI E78-1102 © SEMI 1998, 2002 2
discharged at a known voltage. This device is used to
establish the ESD damage threshold for semiconductor
products, or the effect of ESD on equipment. While the
amount of charge transferred is known (q = CV), the
maximum current that results is not. There is no
guarantee that the same amount of charge would
produce the same results if different values of
capacitance and voltage were used.
3.2 Location — The test methods and maximum
recommended levels of static charge on product,
reticles, and carriers are meant to be applied at the
input/exit ports of production equipment, and when
possible within the equipment. This document is not
meant to be applied in any way that affects the process
within the equipment.
3.3 Test Methods — The test methods referenced in
this document do not guarantee precise measurements
of static charge levels. The maximum static charge
levels recommended in this document have large
tolerances. (See Section 15.1.)
3.4 Static Charge Control — There are a variety of
static related issues in a semiconductor manufacturing
environment. The issues are complex due to the wide
range of electrostatic problems, and device or
equipment sensitivities to these problems. This guide
contains general recommendations. Users of this
document are cautioned that specific static related
problems may require or allow different levels of static
charge than are recommended in this document.
4 Referenced Standards
4.1 SEMI Standards
SEMI E10 — Standard for Definition and Measurement
of Equipment Reliability, Availability, and
Maintainability (RAM)
SEMI E33 — Specification for Semiconductor
Manufacturing Facility Electromagnetic Compatibility
SEMI E35 — Cost of Ownership for Semiconductor
Manufacturing Equipment Metrics
SEMI E43 — Guide for Measuring Static Charge on
Objects and Surfaces
4.2 ESD Association Standards and Advisories
1
ESD STM5.1 — Sensitivity Testing – Human Body
Model (HBM) - Component Level
ESD ADV1.0 — Glossary of Terms
1 ESD Association, 7900 Turin Road, Rome, NY 13440
ESD ADV 2.0 — Advisory for Protection and
Sensitivity Testing of Electrostatic Discharge
Susceptible Items - Handbook
ANSI/ESD STM5.3.1 — Sensitivity Testing - Charged
Device Model (CDM) - Component Level
ANSI/ESD STM5.2 — Sensitivity Testing -- Machine
Model (MM) - Component Level
4.3 Other Documents
IEC/TS 61000-4-2 — Transient Immunity Standard,
International Electrotechnical Commission (IEC)
2
89/336/EEC — European Union Directive on
Electromagnetic Compatibility
NOTE 1: As listed or revised, all documents cited shall be the
latest publications of adopted standards.
5 Terminology
5.1 Definitions
5.1.1 deposition velocity — Particle flux to a surface
(number of particles deposited per unit area per unit
time) divided by the particle concentration adjacent to
the surface boundary layer.
5.1.2 electromagnetic interference (EMI) — Any
electrical signal in the non-ionizing (sub-optical)
portion of the electromagnetic spectrum with the
potential to cause an undesired response in electronic
equipment.
5.1.3 electrostatic attraction (ESA) — The force
between two or more oppositely charged objects. The
result is increased deposition rate of particles onto
charged surfaces, or movement of charged materials.
5.1.4 electrostatic compatibility — Charge control
adequate for interequipment transfer of products,
reticles, and carriers without electrostatic problems.
5.1.5 electrostatic discharge (ESD) — The rapid
spontaneous transfer of electrostatic charge induced by
a high electrostatic field.
NOTE 2: Usually the charge flows in a spark between two
objects at different electrostatic potentials.
5.1.6 equipment interrupt — Any variance from the
specifications of equipment operation, whether or not
the equipment recovers automatically. Interrupts
include, but are not limited to, equipment stoppage,
equipment data errors, and physical mishandling of
products (reference SEMI E10).
5.1.7 ESD simulator — An instrument providing a
specified electrostatic discharge current waveform
2 IEC, 3, rue de Varembe, CH - 1211 Geneva 20 Switzerland
SEMI E78-1102 © SEMI 1998, 2002 3
when discharged directly to a product or equipment
part.
5.1.8 input and exit ports — The locations where
product and/or product carriers are placed to allow the
equipment to process them, or where they are removed
from the equipment after processing.
5.1.9 minienvironments — A localized environment
created by an enclosure to isolate the product from
contamination and people.
5.1.10 product — Any unit intended to become a
functional semiconductor device.
5.1.11 sensitivity level 1 — Product, reticles, and
equipment are extremely vulnerable to damage and/or
problems from static charge.
5.1.12 sensitivity level 2 — Product, reticles, and
equipment are highly vulnerable to damage and/or
problems from static charge.
5.1.13 sensitivity level 3 — Product, reticles, and
equipment have nominal vulnerability to damage and/or
problems from static charge.
5.1.14 sensitivity level 4 — Product, reticles, and
equipment have negligible vulnerability to damage
and/or problems from static charge.
5.2 Description of Terms Specific to this Standard
5.2.1 carrier — A device for holding wafers, dies,
packaged integrated circuits, or reticles for various
processing steps in semiconductor manufacturing.
6 Requirements
6.1 Measurement Methods and Instrumentation — No
single method of testing for static charge can determine
a “safe” level. The amount of static charge, the
distribution of static charge on an object, and the nature
of the static discharge will all interact to determine if
the charge level is safe. It will be difficult to determine
levels that guarantee static related problems are totally
eliminated. The goal of this guide is to assist the user in
identifying static charge levels likely to cause problems
in process equipment. This guide should provide the
user with enough insight to define a test methodology
for each static problem and understand its limitations.
6.2 ESD Damage
6.2.1 When considering direct ESD damage to an
object (product, reticle, or equipment), the important
parameter is the current accompanying the charge
transfer to or from the object. Under a fixed set of test
parameters, the damaging amount of current due to the
charge transfer to or from the object can be determined.
Established test methods exist for determining the
threshold of damage to a particular object. ESD
simulators of various types are used for this purpose.
Refer to EOS/ESD Association Standards listed in
Section 4 for further information concerning device
testing.
6.2.2 The end user should determine what is damaging
current level due to charge transfer to product or
reticles that will be handled in a particular piece of
production equipment.
6.2.3 In the context of production equipment, it appears
important to know the charge on the product, its
carriers, and any other objects that might directly
contact the product. Charge is measured in coulombs,
or more conveniently in nanocoulombs (10
-9
coulombs)
for this purpose. The measurement is made with
instrumentation known as a Faraday Cup, as shown in
Section 7, Figure 1.
6.2.4 A charged object, like an integrated circuit, is
placed in the Faraday Cup and a reading is taken of the
charge on it. It will be necessary to obtain an instrument
with a large enough “cup” for wafers, cassettes, and
other equipment parts. It will also be necessary to get
the objects into the cup without altering their charge
levels. Further information on making these
measurements should be available from the
manufacturers of the measuring equipment.
6.2.5 The user should determine with an ESD
simulator what levels of ESD cause product, or reticle
damage. The equipment manufacturer will need to
determine with an ESD simulator what levels of static
charge cause equipment damage.
6.2.6 It will be the responsibility of the equipment
manufacturer to demonstrate that equipment operation
does not generate more than the allowable amount of
charge on product, carriers or equipment parts. This is
shown in Section 7, Figure 2.
6.3 Particle Attraction
6.3.1 Electrostatic attraction (ESA) of particles can
occur due to the electrostatic field created by the charge
on the surface of an object. Both the field strength and,
usually to a lesser degree, the divergence of the field
influence the electrostatic contribution to particle
deposition velocity. Electrostatic particle deposition
velocity also depends on particle size and particle
electrical charge. Unfortunately, even under controlled
laboratory conditions, accurate measurements of
electric field strength, particle size distribution, and,
especially, particle charge, are difficult. Of these three
parameters, electric field measurements are the most
likely to be available.
6.3.2 Measurements of electrostatic field can be made
with a commonly available electrostatic fieldmeter. The
units of electrostatic field are volts/cm (volts/inch).
SEMI E78-1102 © SEMI 1998, 2002 4
Precise measurements will be difficult as the presence
of the measuring instrument changes the field
characteristics and may overstate the actual level of
electrostatic field. This is shown in Section 7, Figure 3.
SEMI E43 describes measurement techniques using an
electrostatic fieldmeter.
6.3.3 Electrostatic deposition velocity depends only on
electric field, particle size and particle charge.
However, the concentration of particles deposited on a
surface also depends on the particle concentration in the
equipment area and the length of the exposure time
during which particle deposition occurs. Mechanisms
other than electrostatic deposition, such as gravitational
settling and diffusion, can also contribute to particle
deposition. The concentration of particles deposited by
these non-electrostatic mechanisms will also vary with
particle concentration in the equipment ambient and
exposure time.
6.3.4 Comparisons of the electrostatic deposition
velocity with the deposition velocities associated with
these other deposition mechanisms is the key for
determining threshold values of allowed electrostatic
field from the viewpoint of particle deposition. Such
comparisons are the basis for estimating the allowed
values of electrostatic field presented in Appendix A1-
2.2 and Related Information R1-2. Users and equipment
manufacturers should determine and agree on ambient
particle sizes and concentrations, and product exposure
times.
6.4 Equipment ESD
6.4.1 Equipment ESD immunity is being addressed in
general through a number of international standards
including IEC 61000-4-2 and BS EN 61000-6-2 for
European CE compliance. Measurements are made
using an ESD simulator which is described in these
standards.
6.4.2 The ESD simulator is used to create both a direct
discharge to the surface of the equipment and an air
discharge to a surface 10 cm (4 inches) away from the
equipment. The ESD simulator charges a 150 picofarad
capacitor (C) to a known voltage (V) and then
discharges it to produce a standardized discharge
waveform. Knowing the voltage and capacitance
involved in this test means the total charge can be
calculated by the equation q = CV. For example, a 4000
volt discharge (IEC 61000-4-2 test level) transfers a
charge, q = 600 nanocoulombs.
6.4.3 The value of the capacitor (C) in the ESD
simulator is specified in the international standards. It
should not be assumed that a different value of
capacitance and voltage that produce the same charge
transfer of 600 nanocoulombs would have the same
affect on a specific piece of equipment. Discharge
currents will vary with the impedance of the discharge
path and with the voltage. It is, however, impractical to
test all possible combinations. For the purposes of this
guide, the parameters of the ESD simulator specified in
the standards for equipment ESD immunity will be
used.
6.4.4 For true ESD immunity, an ESD event in
equipment must not disturb either the equipment it
occurs in, or another nearby piece of equipment. Charge
on product or carriers transferred from one piece of
equipment must not disturb the operation of subsequent
equipment. It will be a systems issue to make sure that
all equipment in a facility meets the required ESD
immunity standards.
6.4.5 The range of reactions in equipment to an ESD
event runs from transient errors that are automatically
corrected, to hard errors that cannot be corrected
without manual intervention or damage the equipment.
While small numbers of transient errors may be
acceptable from the point of view of equipment
operation, they may still cause unacceptable product
losses.
6.4.6 The user and manufacturer must determine with
an ESD simulator what levels of ESD cause equipment
interruptions. The user must determine if any of these
equipment interruptions caused by ESD are acceptable.
6.4.7 In setting a level to provide ESD immunity for
an individual piece of equipment from static charge on
products and carriers, the charge on these items should
be kept below the levels determined by ESD simulator
testing.
6.4.8 The Faraday Cup measurement can be used for
this purpose. If equipment has been tested for ESD
immunity and passes a 4000 volt test, then total charge
on product and carriers leaving this equipment should
be kept below 600 nanocoulombs. Any product
transferred at this level should not be handled by other
equipment with a lower ESD immunity. A possible
implementation of this test method is shown in Section
7, Figure 4.
7 Apparatus
7.1 ESD Damage — The apparatus for determining the
ESD damage thresholds for products will depend on the
test methods used. See Section 4 for additional
information. For measuring the charge generated on
product, reticles, or carriers, the Faraday Cup test
method is shown in Figure 1.