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SEMI E129-1103 © SEMI 2003 20 R2-6 Neutralizing Charge on Insulators R2-6.1 Static charge on a conductive or sta tic- dissipative object can be co ntrolled if the obj ect is provided with a path for the charge to flow to…

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

SEMI E129-1103 © SEMI 2003 21

additional requirements to the ESD-protective

packaging properties. Some of these considerations

may actually preclude the ability to provide ESD

protection. Therefore, there may often be a tradeoff

between ESD protection, cleanliness, and other required

material properties.

R2-8.1.2 The dissipative property for ESD protection

requires some type of chemical, metallic or elemental

additive to the bulk of the material or the surface of an

ESD-protective item. Adding a highly conductive layer

within the structure of the material or item provides the

electrostatic-shielding property. Any additive or

treatment may contribute to contamination concerns for

application in cleanrooms.

R2-8.1.3 Making a material low charging (“antistatic”)

requires modification of the surface to decrease surface

resistance and reduce friction. Often, low charging

materials allow a transfer of materials to contacting

surfaces to make the actual surfaces similar in

chemistry. Materials with chemically similar surfaces

usually do not show high levels of charge generation

when placed into contact and then separated from each

other. A downside to this process is the potential

contamination issue that may be involved if the

materials are used in clean areas. Take care in selecting

low-charging materials if cleanliness is a major issue.

This concern includes ionic contamination, non-

volatile-residue and outgassing.

R2-8.2 Material Guidelines

R2-8.2.1 Physical Guidelines — The required material

physical properties depend on the application involved.

The selection process for a packaging material or

system should consider all of the physical attributes

generally associated with protecting an item during

transit or storage. The physical requirements normally

evaluated for any packaging system and associated test

methods are shown in Table R2-1.

Table R2-1 Packaging System Physical Properties

Physical Properties Test Method

Tensile Strength ASTM D882

Elongation ASTM D882

Tear Strength ASTM D1922

Puncture Resistance FED-STD-101/2065

Seam Seal Strength (if

applicable)

ASTM F88

Light Transmission ASTM F1003

Moisture Vapor Transmission

(if applicable)

ASTM F1249

Light Transmission Rate (if

applicable)

ASTM D1003

R2-8.2.2 ESD-Protective Packaging Guidelines — The

type of packaging required for ESD-susceptible items

depends on where the item is going. Packaging

materials used inside of an ESD-protected work area

are static dissipative or low charging. When sensitive

items are moved outside of the protected area, the level

of protection should increase to include electrostatic

discharge shielding. Some extremely sensitive items

may require additional EMI or RFI protection provided

by heavier layers of shielding. Materials that provide

increased shielding may also provide moisture barrier

protection for enclosed items. The necessary electrical

properties for packaging materials that are normally

evaluated are shown with their associated test methods

in Table R2-2.

Table R2-2 ESD-Protective Packaging Properties

Electrical Properties Test Method

Surface Resistance ESD STM11.11

Volume Resistance ANSI ESD STM11.12

Shielding ANSI ESD S11.31

Static Decay FED-STD-101/4046

R2-8.2.3 Contamination Control Guidelines —

Contamination issues are not limited to particles.

Plastic processing additives to plastic packaging and

handling materials, chemical and molecular

compatibility should also be of concern. Users can

evaluate plastic packaging and handling materials using

published standard test methods. There are multiple

test methods and procedures to identify contamination

properties, as shown in Table R2-3. The user is advised

to select the method best suited to their products that

require protection.

Table R2-3 Contamination Control Properties

Contamination Properties Test Method

Particles ISO Std 14644

IEST-STD-CC1246D

IDEMA M9

ARP 598

KSC-C-123

Non-Volatile Residue IDEMA M7

ASTM E1235

ASTM F331

IPA extraction method

Outgassing IDEMA M11 Dynamic

Headspace Analysis

ASTM E595

Ionics IDEMA M12

Extractable/leachable cation

levels by ion chromatography