semi合集-English.pdf - 第563页

SEMI E78-1102 © SEMI 1998, 2002 21 RELATED INFORMATION 2 STATIC CONTROL METHODS NOTE: This related inform ation is not an official part of SEMI E78 and is not intend ed to modify or supercede the official standard. Deter…

SEMI E78-1102 © SEMI 1998, 2002 20

arms to open, dropping fully loaded wafer cassettes to

the floor. Costs were associated with loss of product.

R1-3.3.5 Automated material handling system “car”

became charged while coming in close proximity to

ionizer. Car came into contact with grounded object

during charging; creating data corruption which

resulted in system downtime, impact to factory

throughput, and cost associated with the replacement of

control electronics.

R1-3.3.6 Wafer taping/detaping tool generated charge

during normal operation. Chassis ground of the tool

was inadvertently removed, causing high charge to be

developed within the tool. Electrostatic Discharge

occurred at random time intervals within the tool.

Impact to equipment availability, and long solution

time.

R1-3.3.7 Wafers became charged during spin rinse

process. During transfer to wafer metrology tool

electrostatic discharge occurred, causing data

corruption. This resulted in unexpected tool lockups,

and reduced equipment availability.

R1-3.3.8 Ungrounded wall panels became charged and

generated ESD events. EMI produced from ESD events

coupled into photolithography equipment and created

data corruption. This resulted in impacts to equipment

utilization. Long solution time.

R1-3.3.9 Insulative ceiling panels became charged and

generated ESD events which produced high levels of

radiated and conducted EMI in a test area. EMI coupled

into tester/handler and produced data scramble. This

resulted in reduced equipment availability.

R1-3.3.10 Finished product became charged during

manual handling. Product came into direct contact with

test/handler equipment. This resulted in damaged

circuit cards which needed to be replaced, and

decreased equipment availability.

R1-3.3.11 Wafer transfer cart became charged while

rolling over temporary “insulative” floor. Cart came

into contact with plasma etcher control cabinet.

Resulting ESD event caused product loss and reduced

equipment availability.

R1-3.3.12 Wafer polisher robot arm became charged

during normal operation. Chassis ground wire for robot

left off. ESD event occurred causing data scramble

which resulted in process being aborted.

R1-3.4 Conclusion — Electrostatic Discharge (ESD)

affects semiconductor manufacturing equipment in

many ways. The issues are wide ranging from trivial

lock-ups and aborts of process equipment to factory

throughput and yields impacts. The scope of the ESD

problem is very broad and encompasses every aspect of

semiconductor manufacturing.

SEMI E78-1102 © SEMI 1998, 2002 21

RELATED INFORMATION 2

STATIC CONTROL METHODS

NOTE: This related information is not an official part of SEMI E78 and is not intended to modify or supercede the official

standard. Determination of the suitability of the material is solely the responsibility of the user.

R2-1 Static Charge Control

R2-1.1 It is usually impossible to totally 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.

R2-2 Grounding Conductors and Static

Dissipative Materials

R2-2.1 An important consideration in selecting a

method is whether the charged material is a conductor

or an insulator. Static dissipative materials are created

by lowering the resistivity of insulating materials

through the addition of metal or carbon particles, or

other chemical additives. Static charge on a conductive

or static dissipative object can be easily controlled if the

object is provided with a path for the charge to flow to

earth ground. 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.

R2-2.2 Equipment manufacturers can use both

conductive and static dissipative materials to reduce the

presence of static charge. If there is a path for the

charge to flow to earth ground, the static charge on

equipment, and materials handled by the equipment,

can be rapidly, and harmlessly, neutralized. Obviously,

the success of earth grounding depends on maintaining

the integrity of the ground path. This is sometimes a

problem when high-speed, moving parts of equipment

must be connected to earth ground.

R2-2.3 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 must also meet requirements for

avoiding micro-particle production and outgassing. As

long as the ground connection is maintained, these

“passive” procedures offer reasonable protection to the

equipment and product from sources of static charge.

R2-2.4 Unfortunately, these 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. When earth grounding or the

use of dissipative materials is either inappropriate or not

cost effective, ionization can be used.

R2-3 Ionization

R2-3.1 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. Teflon 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.

Dealing with static charge on insulators and isolated

conductors will often require the use of some type of

ionization. Ionizers are the most effective means of

dealing with static charges on insulators and isolated

conductors.

R2-3.2 For purposes of static charge control, ions are

molecules of the gases in air (nitrogen, oxygen, water

vapor, and 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 with the ionized gas molecules.

When ionized air comes in contact with a charged

surface, the charged surface attracts ions of the opposite

polarity. As a result, the static electricity that has built

up on products, equipment and surfaces is neutralized.

R2-3.3 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.

R2-3.4 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, which results in

pairs of positive and negative ions.

R2-3.5 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.

Ions of opposite polarity to the charged surface are

required. Either polarity of static charge may be created

in the equipment or on the product.

R2-3.6 Ionizers in equipment must 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 products moving at high speeds

despite losses to ground. Most ionizers require

maintenance and periodic verification of their

performance.

R2-4 Problem of Controlling Static Charge in

Manufacturing Equipment

R2-4.1 The interior of high speed production

equipment presents a challenge to most static control

methods. The cost of production space is high and

requires that equipment occupying the space be

compact and operate at as high a speed as practical.

Product is moved through small spaces at high speed by

a variety of robotic and other mechanisms. Triboelectric

charging (charge generation due to friction or contact

and separation of dissimilar materials) and contact with

ground are almost unavoidable. Grounding of

equipment parts that contact the product presents added

difficulties when the equipment parts are moving at

high speeds. Dissipating charge from insulating

surfaces and integrated circuit (IC) packages may be

difficult if the charged surfaces are not accessible.

Using ionizers in these confined spaces presents

challenges.