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SEMI E78-1102 © SEMI 1998, 2002 4 Precise measurements will be difficult as the presence of the measuri ng instrum ent changes the field characteristics and may overstate the actual lev el of electrostatic fiel d. This i…
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.
SEMI E78-1102 © SEMI 1998, 2002 5
In
Ground
Electrometer
Faraday
Cup
Isolated
Inner Cup
Shielding
Outer Cup
Figure 1
Faraday Cup Charge Measurement
7.1.1 The relationship between ESD simulator testing
for product damage and charge measurements using the
Faraday cup is shown in Figure 2.
1000
VOLTS
DEVICE
UNDER
TEST
ESD
SIMULATOR
HBM, MM, CDM
100 NANOCOULOMB
DISCHARGE IMMUNITY
FARADAY
CUP
WAFER OR RETICLE CASSETTES
WAFERS, RETICLES, OR ICs
LESS THAN 100 NANOCOULOMB
ALLOWABLE CHARGE LEVEL
WAFER
RETICLE
IC
Figure 2
ESD Damage Testing
7.2 The instrument used for making electrostatic field
measurements is known as an electrostatic fieldmeter.
Instructions concerning its use should be obtained from
the instrument manufacturer and SEMI E43. The
measurement configuration shown in Figure 3
illustrates the effect of the instrument on the
measurement. In most cases the presence of the
fieldmeter will increase both the flux from the charged
surface and the divergence of the electric field lines.
The fieldmeter will generally indicate a higher value of
electric field than would be present without the
fieldmeter.
7.3 The instrumentation and test methods for
determining the ESD sensitivity of equipment are
described by IEC 6100-4-2 or other acceptable test
methods. The amount of static charge determined by
this test method is to be compared with the charge
measured on products and carriers with the Faraday
Cup test method. Figure 4 illustrates the two methods.
1999
+ + + + + + + + + + + + + + + +
2.54 cm
(1 inch)
Electrostatic
Fieldmeter
(volts/cm)
+ + + + + + + + + + + + + + + +
Charged
Surface
Electric Field Lines
Charged
Surface
Figure 3
Electrostatic Field Measurement
4000
V
OLTS
EQUIPMENT
UNDER
TEST
ESD
SIMULATOR
600 NANOCOULOMB
DISCHARGE IMMUNITY
FARADAY
CUP
WAFER CASSETTE
A
ND WAFERS
LESS THAN 600 NANOCOULOMB
A
LLOWABLE CHARGE LEVEL
Figure 4
ESD Immunity Testing
8 Safety Precautions
8.1 Personnel — Static charges can create safety
hazards during some semiconductor production
processes. ESA or ESD events that result in the
jamming or breakage of product in high speed
equipment may create a personnel hazard. ESD events
that produce sparks must be prevented in areas that use
flammable or explosive chemicals or gases. ESD events
to personnel are usually not harmful, but they may
result in an unwanted reflex, or “startle” reaction. This
reflex may create a personnel hazard, particularly in the
vicinity of moving equipment or where caustic
chemicals are in use. It may be necessary to use
additional static charge control methods, beyond those
used inside the equipment, to minimize these personnel
hazards.
8.2 Measurement Safety — Users should exercise
caution while making static charge measurements in the
vicinity of moving parts of production equipment, or in
areas where static potentials on ungrounded conductors
may exceed 30,000 volts. Refer to SEMI E43 for
additional measurement safety considerations.