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SEMI E129-1103 © SEMI 2003 19 • ESD STM7.1 can be used to measure the resistance to a groundable point and res istance point-t o -point for flooring, • ANSI ESD STM12 .1 can be used t o measure t he resistance to a groun…

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.

SEMI E129-1103 © SEMI 2003 19

• ESD STM7.1 can be used to measure the resistance

to a groundable point and resistance point-to-point

for flooring,

• ANSI ESD STM12.1 can be used to measure the

resistance to a groundable point for components of

chairs.

• Resistance characteristics of the wrist straps used

to ground personnel are covered in ANSI ESD

S1.1.

• Cleanroom garments may be tested with the

methods of ANSI ESD STM2.1.

R2-4 Static Charge Decay Time

R2-4.1 In some cases, it will be important to know the

actual time it takes to remove a known amount of

charge from an object.

• FED-STD-101 Method 4046 is often used for

homogeneous materials.

• ANSI ESD STM4.2 is used to measure decay time

of worksurface materials.

R2-4.2 Please be aware that static decay time may not

fully characterize an item or a material. However, the

information gathered in a decay time test may provide

valuable insight into the electrostatic performance of an

item or material when used in conjunction with other

data such as resistance or resistivity. Decay time is

only useful for materials with a high

resistance/resistivity. For materials with resistance less

than 10

Ω, decay time does not apply because the

equipment used is unable to measure decay times faster

than 0.01 s.

R2-5 Measuring Static Charge Levels on

Personnel

R2-5.1 Since personnel are mobile electrical

conductors, they pose a significant hazard in any ESD-

susceptible workplace. Thus, grounding of personnel is

among the most important of all static control technical

requirements. All of the industry program management

standards accepted around the world require grounding

of personnel as a foremost requirement. The most

important consideration for personnel in a properly

designed electrostatic protective area (EPA) is to

maintain the resistance to ground at < 35 MΩ for

operations handling parts sensitive to above 100-V

Human Body Model sensitivity. For areas handling

parts susceptible to less than 100-V HBM, the

resistance-to-ground specification should be reduced to

less than 10 MΩ to ground. Testing by the ESD

Association has shown that at 35 MΩ to ground, a

person can generate up to 100 V by vigorous movement

such as sliding feet on the floor. At 10 MΩ to ground,

the maximum voltage potential drops to about 6 V, a

level that is highly unlikely to contribute to damaging

even the most sensitive parts (consider the resistance

discussion above). Standard wrist strap test methods

are described and defined in ANSI ESD S1.1.

R2-5.2 For high-risk applications, advanced testing of

personnel grounding systems may be required. A

modern concept for continuous verification of

personnel grounding includes wrist strap constant

monitor systems. Choosing an appropriate system of

constant monitoring requires an understanding of the

actual performance measurement. To verify personnel

grounding, it is important to measure the condition of

the total system including the interaction of all the

contact resistances, together with wrist strap bands and

skin as well as ground cord, and shoes or shoe straps

with skin or stocking moisture. Make sure all

measurements include all the resistive interconnections

of the grounding systems. Do not do system

measurements without including all the components

and personnel.

R2-5.3 A common grounding appliance for personnel

is a wrist strap and connecting ground cord attached to

a common point grounding terminal at a workstation.

Since this methodology is most suitable for seated

personnel, it is a stated requirement in all of the

recognized static control programs (i.e., ANSI ESD

S20.20, JEDEC JESD625, IEC 61340-5-1, and EN

61340-5-1 (formerly CENNELEC 100015-1).

R2-5.4 Another common methodology for grounding

mobile personnel is a static control floor with static

control footwear. If this concept is used as the primary

grounding system, the resistance to ground has to be the

same as for a wrist strap. A useful test method for

resistance to ground of personnel through a floor-

footwear system is ANSI ESD STM97.1. An

alternative methodology in ANSI ESD S20.20 where

the resistance to ground of the floor-footwear system is

above 35 MΩ is to measure the voltage of a person

relative to ground using ESD STM97.2. This method

measures the charge generation characteristics of a

person moving about on a floor.

R2-5.5 Clothing coverings using approved static

control materials are often a requirement, especially in

cleanrooms. When ESD issues are involved, garments

need to be tested using approved test methods. A

useful method is ANSI ESD STM2.1.

R2-5.6 Additional static control information, useful

where personnel are concerned, may come from ESD

TR03 for gloves and finger cots, ESD TR05 for

garments, and ESD TR06 for triboelectrically charged

garments.

SEMI E129-1103 © SEMI 2003 20

R2-6 Neutralizing Charge on Insulators

R2-6.1 Static charge on a conductive or static-

dissipative object can be controlled if the object is

provided with a path for the charge to flow to earth

ground. Unfortunately, grounding 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.

R2-6.2 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. 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. Polytetrafluoroethylene (PTFE) 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.

R2-6.3 Dealing with static charge on insulators and

isolated conductors will often require the use of some

type of ionization or surface-resistance modification.

Ionizers are the most effective means of dealing with

static charges on insulators and isolated conductors if

the surface cannot be modified by coating or treatment

with some form of surface-dissipative or low-charging

“antistatic” agent.

R2-7 Ionization

R2-7.1 For purposes of static charge control, ions are

molecules of the gases in air (i.e., nitrogen, oxygen,

water vapor, 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 by supplying ionized gas

molecules. Another way of looking at the application

of ionization is to interpret the function as supplying

opposite polarity charge from the air to surfaces

charged to the opposite polarity. When ionized air

comes in contact with a charged surface, the charged

surface attracts ions of the opposite polarity. As a

result, the static charge that has built up on products,

equipment and surfaces is neutralized.

R2-7.2 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.

R2-7.3 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. Gas

molecules that lose an electron become positive ions.

The free electron is quickly captured by another neutral

gas molecule, creating a negative air ion. Note that the

process always creates pairs of positive and negative

ions.

R2-7.4 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.

Ionizers using the corona discharge method will often

include some type of monitoring and feedback circuitry

to assure that equal numbers of positive and negative

ions are created.

R2-7.5 Ions of opposite polarity to the charged surface

are required. Either polarity of static charge may be

created on facility and equipment surfaces, or on the

product. Ionizers may be installed on the ceiling of the

room or may be hung from the ceiling using appropriate

mounting methods. Ionizers are installed in

minienvironments, particularly in the product load and

unload areas. Ionizers may also be installed in the

interior of process equipment. No single ionizer type or

installation location is sufficient to solve all the static

problems in the semiconductor facility.

R2-7.6 Ionizers should 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 on products moving at high speeds despite

losses to ground. Most ionizers require maintenance

and periodic verification of their performance. The

appropriate ionizer installation will need to be

engineered for each specific application.

R2-8 Packaging Materials

R2-8.1 Types of Packaging

R2-8.1.1 Many forms of flexible and rigid packaging

are used to protect ESD-susceptible items. Bags,

boxes, wraps, tape and reel, pouches, cushioning foams,

totes, tubes and numerous other forms of containers are

available for any imaginable application. Each of these

container forms is available in an ESD-protective

version. Application in cleanroom environments adds