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SEMI E129-1103 © SEMI 2003 18 furniture, e quipment , materials handl ed by the equipment , and any ESD-p r otective materials can be rapidly, and harmlessly, neutralized. Providing a path to ground brin gs everything to…

SEMI E129-1103 © SEMI 2003 17

RELATED INFORMATION 2

STATIC CONTROL METHODS

NOTICE: The material contained in this related information is not an official part of SEMI E129 and is not

intended to modify or supersede the guide in any way. These notes are provided as a source of information to aid in

the application of the guide, and are to be considered reference material. Determination of the suitability of the

material is solely the responsibility of the user. This related information was approved by full letter ballot

procedures on September 3, 2003.

R2-1 Static Charge Control

R2-1.1 It is usually impossible to 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. However, it has

been shown that a consistently applied process for static

charge control greatly reduces problems associated with

static electricity.

R2-1.2 One overriding issue should not be forgotten in

the choice of static control materials and procedures.

This is the need for compatibility with the cleanliness

requirements of the area in which they are installed. A

variety of static control materials are available, but

some may not be cleanroom compatible due to issues of

particle shedding, outgassing, or their chemical

makeup. Each manufacturing area in the

semiconductor factory should contain only those static

control materials that are appropriate to the cleanliness

requirements of the area. Consult ESD TR11 for

additional information.

R2-2 Charge Generation

R2-2.1 Triboelectric Charging

R2-2.1.1 A fundamental law of nature states that an

electrostatic charge is generated whenever two

dissimilar materials contact and then separate. The

materials themselves, intimacy of contact, speed of

separation and a variety of other factors determine the

amount or level of electrostatic charge that is generated

during any given contact and separation event.

Predicting the level of charge ahead of time is nearly

impossible. Due to this unpredictability, the best that

can be done in a factory setting is to understand that

electrostatic charge generation will always occur.

However, electrostatic charge generation can be

measured and charge mitigation is normally a

possibility. Reducing charge generation requires

modification of surfaces to increase lubricity (i.e.,

reduce friction), increase conductivity (i.e., allows

charges to relax and flow back), and make the surfaces

chemically similar. Normally, more than one of the

above actions is required to provide a low charge-

generating system.

R2-2.1.2 While no intrinsic test method exists for the

determination of triboelectric charge-generation

properties, several practical evaluation techniques that

apply to many factory situations may be found in ESD

ADV 11.2.

R2-2.2 Effect of Humidity on Charge Generation

R2-2.2.1 It is generally understood that at constant

temperature and at increased relative humidity the

normally observed manifestations of static charge

generation, such as static cling and static shocks, are

greatly reduced. This does not mean that static charge

is not present in the environment. Even at 90% RH,

static charge may accumulate at a sufficient level to

cause problems with some processes or cause damage

to sensitive parts. Most factory processes, including

cleanrooms, are run at a humidity level of 30–70%.

Within this range, most of the humidity-dependant low-

charging materials will function within their intended

specifications. At lower humidity, surfaces dry out and

may become a source of charge generation.

R2-2.2.2 Specifying a humidity level for a factory is a

reasonably good idea if kept realistic. However,

maintaining a minimal level of humidity may be very

expensive in some areas of the world during normally

dry winter months. It could be more important and cost

effective to choose materials that provide the required

function for charge generation or dissipation at the

lowest expected humidity. Many parts of the world

experience humidity levels of 10% or less during the

winter months when outside air is brought inside and

then heated to warm interior spaces. In these locations,

which include the northern tier of US states and

Canada, much of northern Asia, as well as northern

Europe, careful attention to material selection is of

major importance to assure adequate dissipative and

low charge-generation performance.

R2-3 Grounding Conductors and Static

Dissipative Materials

R2-3.1 The primary method of controlling static charge

is grounding. If there is a path for the charge to flow to

earth ground, the static charge on facility surfaces,

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.