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SEMI E129-1103 © SEMI 2003 5 E43. There are curre ntly no industry standards f or determining the electric field sensitivity of reticles, but test met hods do exi st. 6.3.4 Finally, there is in creasing anecdotal evidenc…

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SEMI E129-1103 © SEMI 2003 4
5.1.6 ESD simulator — an instrument providing a
specified electrostatic discharge current waveform
when discharged directly to a product or equipment
part.
5.1.7 facility electrostatic levels — acceptable static
charge levels related to the major technology nodes of
product and reticle feature sizes.
5.1.8 minienvironment — a localized environment
created by an enclosure to isolate the product from
contamination and people.
5.1.9 product — any unit intended to become a
functional semiconductor device.
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 goals of this guide are to assist the user
in identifying static charge levels likely to cause
problems in the semiconductor manufacturing facility,
and to direct the user to static control methods
appropriate to mitigate these problems. This guide is
intended to provide the user with enough insight to
define test methodologies for measuring static charge
and for evaluating the methods to control it.
6.2 ESD Damage — Direct Discharge
6.2.1 When considering direct ESD damage to an
object (e.g., product, reticle, or equipment), the
important parameter is the current accompanying the
charge transfer to or from the object. The charge may
be transferred from facility and furniture surfaces,
personnel, equipment parts, carriers, packaging
materials, or anything else that contacts the object.
6.2.2 Established test methods exist for determining the
threshold of damage to a particular object. When testing
packaged devices, ESD simulators of various types are
used
. Refer to ESD Association standards ESD
STM5.1, ANSI ESD STM5.2, and ANSI ESD
STM5.3.1, JEDEC JESD22-A114 and JESD22-C101,
or MIL-STD 883 for further information concerning
device testing. There are no established standards for
ESD simulator testing of wafers, reticles, or
unpackaged semiconductor devices. ESD damage
thresholds for these items may be significantly different
than for packaged devices.
6.2.3 Once the damaging current level for a product is
determined using an appropriate ESD simulator, the
corresponding amount of charge is known from the
ESD simulator operating parameters.
6.2.4 In the manufacturing facility, it is important to
know the charge on any objects that might directly
contact the product. Charge measurement methods
using a coulombmeter and Faraday Cup are described
in SEMI E78 and SEMI E43 for isolated conductors
(including personnel), or small and moveable objects.
The measurement methods of SEMI E43 can be used to
establish that the charge levels on these objects will
pose a hazard to products or reticles from a direct ESD
event.
6.2.5 Electric field measurements on large and fixed
objects, or insulators are less useful in estimating
whether or not a damaging direct ESD event will occur.
On objects that cannot be conveniently measured with a
coulombmeter, Electrostatic Fieldmeter measurements
can be useful in estimating the ESD threat, even though
the measurement may be less quantitative than the
coulombmeter measurement.
6.3 ESD Damage — Induced Charge
6.3.1 Charge may be induced on an object that results
in ESD damage. Part of a product (e.g., epoxy
package) or reticle (e.g., quartz substrate) may become
charged and induce charge separation to occur on
another part of the product (e.g., lead pins) or reticle
(e.g., chrome traces). ESD will occur if the lead pins or
chrome traces contact ground. Using a coulombmeter
or Faraday Cup and the methods of SEMI E43, the end
user should test product or reticles to determine the
level of static charge at which ESD damage occurs.
6.3.2 Alternatively, either the product or reticles may
be handled in proximity to another charged object. The
field from this charged object induces charge on
product or reticles, and ESD can result if the product or
reticle contacts ground while in the presence of the
field. Using an electrostatic fieldmeter and the methods
of SEMI E43, the end user should test products and
reticles to determine the acceptable levels of electric
field from static charge.
6.3.3 It has been shown that a changing electric field
causes ESD damage to reticles without ground contact
occurring. A changing electric field can result at the
reticle when an object in proximity to the reticle
acquires a charge, the reticle or a charged object are in
motion with respect to each other, or grounding
conditions change the field between a charged object
and the reticle (for example, due to robot handling).
See references in Related Information 2. In areas of the
manufacturing facility that produce or handle reticles,
electric field from any charged object will need to be
limited to levels that do not cause reticle ESD damage.
Test methods for electric field are contained in SEMI
SEMI E129-1103 © SEMI 2003 5
E43. There are currently no industry standards for
determining the electric field sensitivity of reticles, but
test methods do exist.
6.3.4 Finally, there is increasing anecdotal evidence
that the presence of static charge on wafer surfaces is
becoming an ESD hazard as gate oxide thicknesses
become thinner. In the future, there may need to be
further limits on allowable static charge on wafer
surfaces to prevent ESD-related gate oxide damage
during front-end semiconductor manufacturing. Further
research is needed in this area.
6.4 Particle Attraction
6.4.1 Electrostatic attraction (ESA) of particles can
occur due to the electrostatic field created by the charge
on the surface of an object. Refer to SEMI E78 and
SEMI E43 for an analysis of this effect and its
measurement methods.
6.4.2 Particles may be attracted to charged facility
surfaces, or directly to charged products or reticles.
Subsequently, they may be dislodged from facility
surfaces and transfer to products or reticles. Once on
products or reticle surfaces they may cause either
random or repeating defects.
6.4.3 Electrostatic particle deposition velocity depends
only on electric field, particle size and particle charge.
However, the number of particles deposited on a
surface also depends on the particle concentration in the
area and the length of the exposure time during which
particle deposition occurs. SEMI E78 contains
information to relate allowable electric field to ambient
particle concentration and exposure time.
6.4.4 The measurement methods of SEMI E43 can be
used to establish that the electric field from any facility
surface meets the requirements of this document.
6.4.5 Charge is difficult to evaluate on large objects,
especially insulators. Electric field measurements on
these objects may be useful in estimating the risk that a
damaging direct ESD event might occur. However,
electric field measurements on insulators are highly
qualitative and only provide a figure of merit as to the
threat that these charges may represent to the ESD-
sensitive device.
6.5 Equipment ESD
6.5.1 Equipment ESD immunity has been established
at levels considerably higher than those that result in
damage to product and reticles. If facility static charge
limits shown in Table 1, of Section 12.5 are used to
protect product and reticles, they will provide sufficient
protection for the equipment.
6.5.2 Equipment ESD immunity is addressed, in
general, through a number of international standards
including SEMI E78, IEC 61000-4-2, and BS
EN50082-2 for European CE compliance.
Measurements are made using an ESD simulator, which
is described in these standards.
7 Apparatus
7.1 ESD Damage — For measuring the charge
generated on product, reticles, or carriers, the Faraday
Cup test method is shown in Figure 1. Additional
information on this test method is contained in SEMI
E43.
In
Ground
Electrometer
Faraday
Cup
Isolated
Inner Cup
Shielding
Outer Cup
Figure 1
Faraday Cup Charge Measurement
7.2 When the object whose charge is to be measured is
conductive, a nanocoulombmeter may be used.
Additional information on this test method is contained
in SEMI E43.
7.3 The instrument used for making electrostatic field
measurements on large objects or surfaces is known as
an electrostatic fieldmeter. Instructions concerning its
use should be obtained from the instrument
manufacturer and SEMI E43. The measurement
configuration is shown in Figure 2.
1
9
9
9
+ + + + + + + + + + + + + + + +
2.54 cm
(1 inch)
Electrostatic
Fieldmeter
(volts/cm)
+ + + + + + + + + + + + + + + +
Charged
Surface
Electric Field Lines
Charged
Surface
Figure 2
Electrostatic Field Measurement
7.4 For small objects or surface areas, an electrostatic
voltmeter is appropriate.
SEMI E129-1103 © SEMI 2003 6
8 Safety Precautions
8.1 Personnel — Static charges can create safety
hazards during some semiconductor production
processes.
8.1.1 ESA, ESD, and EMI events that result in the
jamming or breakage of product in high-speed
equipment may create a personnel hazard.
8.1.2 ESD events that produce sparks must be
prevented in areas that use flammable or explosive
chemicals or gases.
8.1.3 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.
8.1.4 EMI resulting from ESD events may cause
unpredictable behavior of robotics or other moving
equipment that put personnel at risk.
8.1.5 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 V. Refer to SEMI E43 for
additional measurement safety considerations.
9 Test Specimen
9.1 The user, material supplier, facility
designer/builder, and equipment manufacturer should
agree upon and document:
• Type(s) of testing to be performed
• Location of the testing (e.g., in a test chamber or in
the actual use location)
• Who will do the testing
• Number and type of test samples
• Number of measurements
• Acceptable test results
9.2 The user, material supplier, facility
designer/builder, and equipment manufacturer should
agree upon and document all appropriate environmental
conditions (e.g., temperature, humidity, dew point,
airflow).
9.3 The user, material supplier, facility
designer/builder, and equipment manufacturer should
agree upon and document the operating history of
equipment prior to, or during testing (e.g., warm-up
time, type of carrier, number of products processed,
operating speed).
10 Preparation of Apparatus and Sample
10.1 Depending on the type of testing to be done,
consult the appropriate testing document for apparatus
and sample preparation. See Section 4.
11 Calibration and Standardization
11.1 Depending on the type of testing to be done,
consult the appropriate testing document for apparatus
calibration and verification. See Section 4.
12 Procedures
12.1 See Sections 6 and 7 and the appropriate test
methods of Section 4.
12.2 ESD Damage
12.2.1 Users should establish product damage
thresholds for their products. Measurement methods
for integrated circuits are described in SEMI E78
Related Information 1 and the documents contained in
Section 4. Appropriate measurement methods for ESD
damage to wafers, reticles, and other items may be
adapted from the instrumentation used in these test
methods.
12.2.2 In place of using the test methods referenced in
Section 12.2.1, users may decide to follow the
recommendations for acceptable electrostatic levels
contained in Section 12.5 Table 1, which are based on
product and reticle geometry. See Appendix 1 for more
information.
12.2.3 Measurements of ESD damage levels are made
in units of coulombs, or more conveniently in
nanocoulombs (nC = 10
-9
coulombs).
12.2.4 The Faraday Cup method is used to determine
the static charge levels on products, carriers and
equipment parts. See Section 7.1. Each item should be
transported to the Faraday Cup in a way that does not
alter its charge level. Consult the measurement
equipment manufacturer’s instructions for
recommendations on how to achieve this.
12.3 Electrostatic Field
12.3.1 Users should work with cleanroom designers,
material suppliers, equipment manufacturers and reticle
suppliers to determine ambient particle levels, product
exposure times during processing, and reticle damage
levels due to electric field.
12.3.2 Electrostatic field measurements should be
made at a minimum on all surfaces within the facility