CONTAMINATION AND ESD CONTROL IN HIGH-TECHNOLOGY MANUFACTURING

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CONTAMINATION AND ESD

CONTROL IN HIGH-TECHNOLOGY MANUFACTURING

ROGER W. WELKER R. NAGARAJAN CARL E. NEWBERG

A JOHN WILEY & SONS, INC., PUBLICATION

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CONTAMINATION AND ESD

CONTROL IN HIGH-TECHNOLOGY

MANUFACTURING

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CONTAMINATION AND ESD

CONTROL IN HIGH-TECHNOLOGY MANUFACTURING

ROGER W. WELKER R. NAGARAJAN CARL E. NEWBERG

A JOHN WILEY & SONS, INC., PUBLICATION

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Published by John Wiley & Sons, Inc., Hoboken, New Jersey.

Published simultaneously in Canada.

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Library of Congress Cataloging-in-Publication Data:

Welker, R. W.

Contamination and ESD control in high-technology manufacturing / by Roger W. Welker, R. Nagarajan, Carl E. Newberg.

p. cm.

ISBN-13: 978-0-471-41452-0 ISBN-10: 0-471-41452-2

1. Electronic apparatus and appliances—Protection. 2. Electric discharges. 3.

Electrostatics. 4. Contamination (Technology) 5. Cleanrooms. I. Nagarajan, R.

(Ramamurthy) II. Newberg, Carl E. III. Title.

TK7870.W36 2006 670.42—dc22

2005058119 Printed in the United States of America

10 9 8 7 6 5 4 3 2 1

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v

PREFACE

Contamination and electrostatic discharge (ESD) are now becoming recognized as factors affecting yield and reliability in an ever-increasing number of industries. Whereas contamination traditionally was recognized as affecting the semiconductor, disk drive, aerospace, pharmaceutical, and medical device industries, today such industries as automobile and food production are also discovering the benefits of contamination control. ESD control has experienced a similar growth in applications.

An engineer or scientist cannot obtain a degree in contamination or ESD control from a college or University. It is possible to obtain certification as an ESD control engineer or tech- nician through an independent certification agency. However, there is not a similar certification program for contamination control. Indeed, engineers and scientists with diverse degrees in mechanical engineering, chemistry, physics, microbiology, industrial engineering, electrical engineering, and many other fields are found in the ranks of those who consider them- selves contamination or ESD specialists. Despite the large number of degreed professionals working in contamination control and ESD control, these fields remain misunderstood and underappreciated. The misunderstandings often arise because of the interdisciplinary nature of contamination and ESD control. Because so many different academic disciplines are needed to provide a comprehensive understanding, the problems and solutions often appear confusingly complex. At the same time, the vast majority of contamination or ESD problems are solved using very simple analysis, making them appear to be child’s play.

In addition, there is a long-standing perception that what is good for contamination control is bad for ESD control, and vice versa. This book is an attempt to rectify these two problems.

● We begin with two general chapters on the fundamentals of contamination and ESD control.

● This is followed by a chapter on analysis methods useful for solving contamination and ESD problems.

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● We next begin to build the contamination and ESD control environment. We start with a description of cleanrooms, their components, and how processes are arranged within them. Construction of an ESD-protected environment is described similarly.

● The next subject covered is cleaning processes and the equipment used to support them. This subject is dealt with from the perspective of both the supplier and the user.

● Tooling is discussed in some detail. This involves material selection and evaluation problems common to all industries affected by contamination and ESD.

● We turn next to a discussion of consumable materials and supplies.

● We talk about contamination originating from people and how that is contained. This involves discussions of behavior and discipline.

● Finally, we discuss management of the cleanroom and ESD-protected workplace environments. Companies dealing with contamination and ESD range in size from those having a single sensitive facility to multinational corporations having cleanrooms and ESD-protected workplaces on virtually every continent.

The emphasis is to provide a working knowledge of contamination control and ESD control. This is done by introduction of control standards and examples of how they are employed. In this regard, the book is considered to be a “how-to” guide. It is filled with many case studies to illustrate and illuminate the lessons of contamination and ESD control engineering.

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FUNDAMENTALS OF CONTAMINATION CONTROL

1.1 INTRODUCTION

Contamination control is a process. It is the process of limiting contamination to below some tolerable amount. It does not mean absolute elimination of contamination. Indeed, due to the limitation of measurement capability, it is impossible to verify zero contamination. The best we can do is to reduce the amount of contamination to below the lower detec- tion limit of the measurement technique in use at the time.

Contamination can be defined in several different ways. One popular definition is that contamination is any form of matter or energy that has a detrimental effect on products or processes. This is a functional definition, since it assumes nothing about the nature of the contaminant, but rather, is concerned with what the contaminant does. Contamination can be matter, such as particles, films, ions, or gases. Contamination in the form of excess static electric charge can cause product damage due to the process of electrostatic discharge.

Contamination can also be electromagnetic radiation, such as the wrong wavelength of light. Some contamination engineers even include temperature or vibration outside specified limits as forms of contamination. Contamination can be in plasma, gaseous, liquid, or solid form and can appear within other solids, liquids, and gases.

1.1.1 Contamination Sources

It is often convenient to think of contamination in two broad categories: functional con- tamination and nuisance contamination. Functional contamination is contamination that has a detrimental effect on products or processes; nuisance contamination does not have a

Contamination and ESD Control in High-Technology Manufacturing, By Roger W. Welker, R. Nagarajan, and Carl E. Newberg

1

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directly detrimental effect on products or processes. Clearly, this distinction is useful, as it provides focus when investigating and eliminating problems caused by contamination. Our primary focus should be on functional contamination; however, nuisance contamination can become a problem if it is so extensive that it interferes with the orderly process of identifying and eliminating the source of a functional contaminant. An argument can be made that the indirect benefit of performing good nuisance contamination control is that it facil- itates our primary objective, control of functional contamination.

Contamination can originate from many different sources, take many forms, and appear in many places. Among the sources are the cleanroom, tooling, chemicals, processes, parts, consumable supplies, and people. There is a widely held belief within the contamination control community that people are the most significant source of contamination. This generalization must be approached with caution. For example, the relative contribution of people vs. tooling is process dependent. A factory where people do all material handling and manufacturing operations will have a different proportion of contamination from people than that of a fully automated process that has relatively few people present. A second example can be cited for the use of isolation enclosures and standard machine interfaces (SMIFs) around products and processes. Early in the promotion of SMIFs there was a widely communicated belief that uti- lizing SMIF technology would lessen the demands on and need for cleanrooms, would turn the cleanroom into a shirtsleeve environment, and would eliminate the need for continuous contamination monitoring. This has not been realized in the vast majority of cases because the isolation enclosure environment must be entered during maintenance and the isolation enclosure can actually amplify the contribution of tool-generated contamination, reempha- sizing the value of continuous monitoring.

Several different ways of visualizing the relative importance of various source categories to overall contamination are useful. Pareto diagrams (Figure 1.1) and pie charts (Figure 1.2) are particularly useful graphic representations, because they allow one to quickly visualize where problems originate and where corrective action must focus. Clearly, tooling and people are the most important sources for this hypothetical environment, accounting for more than half of all contaminants. How might a chart like this be generated?

One approach would be to examine all the failure analysis reports for field returns and select all failures where contamination was found and suspected to be a contributing factor

0 5 10 15 20 25 30 35 40 45

Tooling People

Piece Parts

Consumables

Facilities

Chemicals

Maintenance Source Category

Percent

FIGURE 1.1 Hypothetical contamination source distribution represented as a bar chart. This rep- resentation is particularly useful because it highlights problem areas.

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in the failure. Analysis of the materials of the contaminants could then be sorted into the appropriate source category. For example, clothing fibers or skin flakes are clearly associated with people contamination one failure producing this conclusion would be attributed to people. Analysis might also identify materials that could be associated with more than one category. For example, suppose that aluminum particles made of a particular alloy are iden- tified. If some of the piece parts and some parts of the tooling are made of this particular alloy of aluminum, it would not be possible to associate that failure unambiguously with either source category. In this case, because both categories could be the source, half of the failure would be attributed to each category.

Another approach might also be useful, especially where there are relatively few field failures. In this approach the yield loss is analyzed. Tests in which contamination can be a contributing factor to yield loss are then analyzed in the same manner as for field failures.

It can be reasonably argued that analyzing yield loss is a more productive approach to solving contamination problems than is analyzing field failures (being proactive rather than reactive). This is especially true where the tests are designed to be good predictors of field failures.

1.1.2 Contamination Adhesion Forces

The forces that affect adhesion of contamination also vary widely.

Van der Waals Forces which are created anytime that two bodies approach one another, are universal. As the bodies approach one another, the force of attraction increases.

This is theorized to be due to charge displacement: Like charges repel. Thus, electrons on one surface that are tightly held will repel electrons on a nearby surface that are less tightly held. This electron repulsion creates an oppositely charged surface, drawing the two surfaces together. At some point the materials approach each other so closely that they begin to repel each other, by either electron or proton repulsion. Figure 1.3 illustrates the attractive and repulsive forces of van der Waals attraction.

Van der Waals attractive forces are considered to be the weakest of the forces that bind contamination to surfaces. However, the force of van der Waals attraction should not be

Chemicals 3%

Consumables

13% Maintenance

2%

Facilities 3%

Other 8%

Piece Parts 16%

People 26%

Tooling 37%

FIGURE 1.2 Hypothetical contamination source distribution represented by a pie chart. This rep- resentation is useful in showing the relative importance of sources.

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underestimated. It is theoretically directly proportional to the surface area in contact between different surfaces. If the materials in contact are rigid and relatively nondeformable, as the surfaces approach one another, the attractive force between them reaches a maximum and sta- bilizes relatively quickly. Conversely, if one or more of the materials are deformable, they will change shape to accommodate the approach of the contacting surface. This will increase the area in contact and increase the adhesive force between them. (It is for this reason that elas- tomeric polymers can often be very difficult to clean.)

For a spherical particle contacting a flat surface, the adhesive force due to van der Waals interaction is given by

where A is the Hamaker constant, which depends on the materials of which the particle and surface are made, with a typical order of magnitude of 10¹⁹or 10¹⁸J; d is the diameter of the spherical particle; and x is the separation distance between the particle and the sur- face. The separation between the particle and the surface is never zero.

Electrostatic Attraction A second force binding particles to nonconducting surfaces is electrostatic attraction. This force is given by

where KEis a constant (9.0 10⁹N m²/C²in SI units), q is the charge on the particle in coulombs, and x is the particle diameter. For particles larger than 100 nm or so, the equi- librium charge, q, is roughly proportional to the square of the particle diameter, so the elec- trostatic force ends up being directly proportional to the particle diameter (like the van der Waals forces). Charged particles become attracted to an oppositely charged surface by simple coulombic attraction. However, both surfaces do not need to be charged. A common example occurs when particles are charged and are attracted to neutral surfaces.

Excess electrical charge plays a significant role in contamination. It can cause an ener- getic discharge [electrostatic discharge (ESD)] that causes damage. ESD is the usual concern

F K q

E x

E₂²

F Ad

adh 12x2 Repulsive

Force

Attractive 0

Interatomic distance

FIGURE 1.3 Van der Waals forces of attraction and repulsion.

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expressed regarding charge. However, charge also introduces a contamination concern. That is, charge on surfaces or particles can increase contamination of surfaces by a process called electrostatic attraction (ESA). The presence of excess charge on a surface creates an electrostatic field that will accelerate deposition of oppositely charged particles and thus accelerates contamination. The number of particles deposited on a surface is proportional to (1) particle charge and concentration, (2) the electrostatic charge per area on the surface, and (3) the duration of exposure.

Experiments have shown that charged particles are attracted to oppositely charged surfaces and to neutral surfaces but that charged surfaces have little effect on attraction of neutral airborne particles. Of course, since most particles become charged when they are generated, the latter case probably occurs seldom under normal circumstances in cleanrooms.

Figure 1.4 shows the situation schematically, where a particle of chargeq is attracted to a surface having a chargeQ (polarity opposite that of the particle) per area A. Figure 1.5 illustrates the situation where the surface is neutral but the particle is charged. The charge on the particle repels like charges on the surface. The surface thus acquires a net charge opposite in polarity to that on the particle locally around the vicinity of the particle.

Opposite charges attract. Thus, the charge on the particle causes the particle to suck itself to the surface, even when the surface started out neutral.

For the case shown in Figure 1.4, where the particle has charge q and the surface has charge Q of opposite polarity, the total number deposited on a surface is roughly

N cqEBAt

-q

+Q +Q

(a) (b)

FIGURE 1.4 Electrostatic attraction of a charged particle to an oppositely charged surface. (a) The particle is not yet affected by the charge on the surface; it has only a vertical component to its motion.

(b) The particle’s vertical motion has brought it into the electrostatic field surrounding the charged surface; the ESA of the particle imparts horizontal motion to the particle vis-à-vis the surface.

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where N number of particles deposited c airborne particle concentration q charge in coulombs per particle E electrical field strength on the surface B mechanical mobility of the particle

A area of the surface over which the charge Q is uniformly distributed t time

Figure 1.6 illustrates the effect of charge on contamination deposition on horizontal surfaces in vertical unidirectional-flow cleanrooms. Data are plotted from 0.01 to 10m (above approximately 5m, the effect of electrostatic charge on deposition rate is negligible).

The term nbdescribes the charge state of particles in terms of the Boltzmann equilibrium distribution. When nb 0, the particles are uncharged. This condition is expected to occur only rarely, as most mechanisms generating particles produce charge on the particles.

Particles with nb 1 correspond to the charge distribution that results when particles are exposed to a cloud of bipolar air ions, generally considered the minimum charge state of aerosol particles. The curve for particle deposition with nb 10 probably is a more realis- tic charge state for aerosols in the cleanroom.

The term E describes the charge state of surfaces in a cleanroom. E 100 V/cm (250 V/in.) lies between the charge states expected for rooms with air ionization, which typically will be controlled to less than100 and the actual charge state of surfaces in cleanrooms with no air ionization. Also shown in Figure 1.6 is the deposition velocity curve for nb 10 and E 1000 V/cm (2500 V/in.) which probably represents deposition rates that would be expected in many cleanrooms without air ionization.

(a) (b) (c)

FIGURE 1.5 Electrostatic attraction of a charged to a neutral insulating surface by induced surface charging. (a) A particle with chargeq approaches a neutral surface. (b) The charge on the particle locally induces a negatively charged region on the surface, producing an opposite polarity “image charge” on the surface. (c) The charged particle is attracted to the surface by its image charge.

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Much work has been done concerning the deposition of particles on charged silicon wafers in a cleanroom. The predictions of Liu and Ahn [1] and of Cooper et al. [2] were that deposition velocities (without electrostatic effects) would be near 0.001 to 0.01 cm/s. Pui et al. [3] confirmed this in the laboratory using monodisperse fluorescent particles having minimal charge (thus minimal electrostatic effects). Wu et al. [4] found that deposition velocities were about an order of magnitude higher for ungrounded wafers in a cleanroom than for grounded wafers. Cooper et al. used a minimal charge distribution (Boltzmann equilibrium distribution) and a Federal Standard 209 (FED-STD-209) class 100 (ISO 14644 class 5) cleanroom particle size distribution and predicted that an electric field as low as E 100 V/cm would produce an order-of-magnitude greater deposition with grav- ity plus diffusion than these two mechanisms combined produced without the electrostatic field. Further details are available in the book edited by Donovan [5].

Control The basic formula for the number of particles (of size d) attracted to the surface:

N cqEBAt suggests various alternatives for control [6]:

1. The concentration of airborne particles, c, should be kept to a minimum using stan- dard contamination control approaches.

0.01 0.1 1.0 10

Diameter (μm)

Deposition Velocity (cm/s)

100

10

1

0.1

0.01

0.001

0.0001

n_b = 0 E = 100 V/cm

n_b = 1 E = 100 V/cm

n_b = 10 E = 100 V/cm

n_b = 10 E = 1000 V/cm

FIGURE 1.6 Effect of particle charge and surface charge on surface contamination rates in clean- rooms. The lower curve, with n_b 0, E 100 V/cm, probably is not observed in practice since processes that produce particles usually charge them. The second curve, where n_b 1 and E 100 V/cm, probably represents the lower limit of deposition rates to be seen in cleanrooms. The upper curve, n_b 10, E 100 V/cm and nb 10, E 1000 V/cm, probably represents the range that includes most cleanrooms without air ionization.

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2. The particle charge q should be keep to a minimum by using air ionizers.

3. The electrostatic field E kQ/A should be minimized by preventing charging of sur- faces, by draining charge from the surface by grounding, by using a bipolar (positive and negative) air ionizer to neutralize charge on the surface, or by wet wiping with a grounded static-dissipative cloth. The beneficial effect of the use of bipolar air ionization in manufacturing cleanrooms has been demonstrated clearly [7].

4. The duration of exposure, t, should be minimized.

Charged surfaces increase the deposition of charged particles. Deposition rates per surface area are proportional to the electric field strength, particle charge, and particle concentration. Current standard contamination control procedures can minimize concentration. Air ionization and other static control techniques are able to reduce particle charge and surface charge, reducing particle contamination.

Capillary Attraction The third force binding contamination to surfaces is capillary attraction, in which a film forms between two bodies, concentrated as a meniscus between their contacting surfaces. This capillary film increases the surface area of contact between the two objects, increasing van der Waals forces attracting the objects. As the film dries, the adhesion force increases rapidly.

The force of capillary attraction is, to a first approximation, proportional to the surface tension of the liquid forming the capillary bridge between the particle and the surface. It should be kept in mind that adsorption by the liquid of materials on the particle, on the surface, or in the air can alter the surface tension of the liquid, so that the pure liquid surface tension might not be a good absolute predictor of the capillary force.

The effect of the formation of a capillary bridge between two objects and the resulting increase in attractive forces between them can have a profound influence on the ability to clean surfaces. The attractive forces are increased in direct proportion to the increase in surface area in contact, thus increasing the amount of force required to separate them.

Capillary drying is the principal reason why “just-in-time” cleaning is very critical. Figure 1.7 illustrates the increase in surface area in contact due to formation of a capillary bridge between a particle and a surface.

The chemical history of the material forming the capillary bridge can have a profound effect on cleaning efficiency. For example, material forming the capillary bridge may be soluble in polar liquids such as water. Nonpolar solvents may not be able to dissolve the material forming the capillary bridge and thus may be found to be ineffective at cleaning a part whose prior history includes exposure to polar materials. Chemical history can also include exposure to elevated relative humidity. Contaminants may have solid chemicals adsorbed on their surface that do not wet the interface between the contaminant and the surface. If these solid chemicals are exposed to elevated relative humidity, they may absorb moisture and dissolve in the moisture, a process referred to as deliquescing. This concen- trated solution may then wet the interface between the contaminant and the surface, forming a capillary bridge. If the relative humidity later drops and the moisture evaporates, solid may precipitate out at the interface, forming a strong, solid bond.

Chemical Reactions A fourth force binding contamination to surfaces is the result of chemical reactions. Chemical reactions can result in adhesion between surfaces that are so strong that ordinary cleaning processes are rendered entirely ineffective. After chemical

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reactions have occurred, often the only way to clean is to find an alternative chemical reac- tion to reverse the chemical bond. This can be very difficult.

1.1.3 Contamination Control Methods

The effects of contamination are as varied as their sources and forms. Contamination can result directly in product failure or yield loss. Occasionally, the damage caused by contamination can be corrected by rework. Rework increases the cost of products. Contamination can also result in reliability loss. Many tests are performed to detect the presence and effects of contamination on products. If contamination was not a problem, these tests could be eliminated or reduced to a sampling frequency, further reducing production cost. Thus, the overall objective of contamination control is to optimize cost through maximization of yield and reliability.

Contamination is controlled using many different techniques:

● Development of contamination control plans. Here the activity focuses on identifying the known or suspect contamination tolerances of the product or processes. From these, contamination budgets can be developed and mitigation strategies planned. This plan- ning involves management early in the contamination control plan so that resources can be planned for and utilized effectively. This process is often referred to as the systems approach to contamination control.

Soluble material on surface

(a)

(b)

(c)

Soluble material dissolves – meniscus forms

Soluble material concentrates at interface, forms capillary

bridge

FIGURE 1.7 Effect of capillary bridge formation in the adhesion of contamination: (a) there is three-point contact; (b) the material is exposed to a solvent-rich atmosphere; (c) the solvent evaporates.

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● Construction and operation of clean facilities. The establishment of a contamination- controlled workplace is the first step in the implementation of an overall contamination control program. The plans may call for various architectural approaches, including the use of unidirectional-flow clean benches, minienvironments, glove boxes, and other nested architectures to achieve the desired level of cleanliness while minimizing capital and operating cost (see Chapter 4).

● Selection and approval of materials. These may be materials of construction for the workplace, materials for construction of tooling and fixtures, materials for consumable supplies and packaging, process chemicals, and so on. See Chapter 3 for guid- ance on material selection and ongoing control.

● Development of cleaning processes and the design of equipment to support the cleaning processes (see Chapter 5).

● Control of contamination from tools and fixtures. This aspect of control is especially, important in the modern highly automated workplace (see Chapter 6).

● Control of people, the most significant source of contamination in virtually all contamination-sensitive industries (see Chapters 9 and 10).

● Control of consumable supplies (e.g., gloves). See Chapter 8 for a general discussion.

● Continuous monitoring to verify compliance. This step can be indispensable in identifying problems and establishing proper control. (see Chapter 7).

1.2 GLOSSARY OF CONTAMINATION CONTROL TERMS

Many of the terms used in this book are common to descriptions of cleanrooms, tooling, piece parts, consumable supplies, and so on. A general knowledge of them is useful for any engineer or designer working in an industry where contamination is a potential issue. Where appropriate, common abbreviations and acronyms are included. In general, these acronyms are not used in the text but are provided for reference value. The terms listed below apply specifically to contamination control. Terms particular to electrostatic charge and discharge control are defined in Section 2.2. Additional terms are defined where appropriate, such as in Chapter 3 on analysis methods.

● Absorption: penetration of one substance into the interior of another: a sponge absorbs liquids. Chemical absorbents include activated charcoal and silica gel.

● Adsorption: a condition in which one substance is attracted to and held on the surface of another. Adsorption is responsible for chemical contamination of nonporous materials, such as machined metal parts.

● Aerosol: a quasistable gaseous suspension of liquid or solid particles about 100m in diameter or smaller.

● Airborne molecular contamination (AMC): vapor-phase contamination in air.

● Anemometer: an instrument for measuring air velocity.

● Anion: an atom or molecule with a net negative charge.

● Busy periods: During normal production operation.

● Cation: an atom or molecule with a net positive charge.

● Chimney effect: vertical movement of air due to buoyancy created by temperature differences.

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● Class: in the traditional U.S. federal standards, the airborne particle concentration limit, such as class 100 and class 10,000, as monitored using airborne optical particle counters. Class in FED-STD-209 is equal to the maximum allowable concentration of 0.5-m diameter and larger particles per cubic foot of air. Cleanroom classification were made metric in FED-STD-209 revE. In the metric version, the class is an M followed by a number, where the number is the approximate power-of-10 number of particles 0.5m and larger per cubic meter of air. In ISO 14644 class is n, where n is the power of 10 number of particles equal to or larger than 0.1m per cubic meter of air.

● Cleanroom: an enclosed area employing control over particulate matter and other forms of contamination in air, with airflow, relative humidity and temperature, and pressure control as needed. Clean benches, downflow units, minienvironments, and so on, are considered to be cleanrooms in this context.

● Colloid: a stable suspension of particles in a fluid.

● Contamination: an unwanted foreign substance or energy, including particles, organic and inorganic vapors, electromagnetic radiation, vibration, and electrostatic charge. (See also functional contamination and nuisance contamination.)

● Contamination control: the process of limiting contamination to within specified amounts.

● Critical and busy sampling: sampling that satisfies the criteria of critical location and busy periods.

● Critical location: as close to the product or process as possible without interfering physically with the movement of tools, people, or product.

● Critical operation: an operation in which contamination of the product or process results in yield loss or field failures in excess of desired amount. For example, in a disk drive manufacturing operation, critical operations are those in which customer heads or disks, or items that come in contact with heads or disks, are exposed.

● Critical surface: a surface that requires precision cleanliness.

● Deionized (DI): water or other liquids that have been purified to remove ionized material.

● Densitometer: a commercial instrument for measuring the opacity of photographic film.

● EDX (energy dispersive x-ray analysis): a technique for elemental analysis used dur- ing scanning electron microscopy.

● Factory environment: portions of a facility outside a contamination-controlled or static- safe workplace.

● FED-STD-209: the federal standard cleanroom Work Station Requirements, Controlled Environment, which describes and defines cleanrooms and is the basis for specifications for cleanrooms [8].

● Fiber: a particle with a length/width ratio in excess of 10 : 1. Some measurement stan- dards define a fiber as a particle with a length/width ratio greater than 3 : 1.

● FID (flame ionization detector): used in combination with gas chromatography to quantify volatile contaminants.

● FTIR (Fourier transform infrared spectroscopy): a technique for the identification of molecular contamination. It is most often used to analyze organic contaminants.

● Functional contamination: contamination that has a detrimental effect on product or processes.

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● GC/MS (gas chromatography/mass spectroscopy): a gas chromatograph allows the separation of complex mixtures of volatile molecules; mass spectroscopy is a detec- tion technique used to identify the separated molecules.

● HEPA (high-efficiency particulate air filters): filters that have at least 99.97% capture efficiency at the maximum penetrating particle size, usually rated at 0.3m.

● Hydrophilic: liking water. Hydrophilic substances tend to be wetted by water and occasionally also absorb water. The tendency of a material to absorb or repel water is an important consideration in the selection and efficacy of cleaning processes.

● Hydrophobic: hating water. Substances that are hydrophobic tend not to be wetted by water. The tendency of a material to absorb or repel water is an important consideration in the selection and efficacy of cleaning processes.

● ISO 14644: a standard being developed by the International Standards Organization to unify cleanroom standards on a worldwide basis.

● Level: the amount of contamination specified for a surface. Level is the size in micrometers above which less than 1 particle per square foot of surface is expected to be found. Defined in Military Standard 1246.

● Liquid-borne particle counter (LPC): an automated electronic device that separates, sizes, and counts individual particles suspended in liquids.

● Maximum penetrating particle size: in filtration, the particle size above and below which all particles of other sizes are collected with higher filtration efficiency.

● Micron: a unit of length equal to one millionth of a meter; more properly called a micrometer and abbreviated m.

● MIL-STD-1246: the U.S. military standard defining contamination per unit surface area.

● Molecular contamination: contamination in gaseous or molecular form.

● Nonparticulate matter: matter that does not have a definable length or width dimen- sion, such as a film or vapor.

● Nonvolatile residue (NVR): soluble or suspended matter remaining after controlled evaporation of a filtered volatile solvent. Filtration is normally performed through a 0.45- or 0.8-m filter prior to evaporation to distinguish between filterable particles and nonfilterable liquid or soluble contamination. Some laboratories do not filter prior to evaporation, so particle matter ends up being included in the NVR total.

● Nuisance contamination: contamination that does not have a functional effect on a product or process but which interferes with the discovery of functional contamination or interferes with the orderly management of a cleanroom.

● Optical particle counter (OPC): automatic electronic devices that size and count individual particles. The abbreviation OPC is limited to airborne particle counters.

● Outgassing: the process of production of matter in gaseous form.

● Particulate matter: matter with definable width and thickness.

● Scanning electron microscope (SEM): an instrument used to physically characterize the shape and size of an object. It produces an image using a beam of electrons and is capable of resolving structures smaller than 1m.

● Semiclean zone: an area with restrictions on contamination from behavior and/or materials and/or attire but without an airborne particle classification.

● Surface contamination rate (SCR): the rate at which surfaces accumulate contamina- tion. Contamination on surfaces is described by contamination levels.

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● Tooling: any mechanism or device used in processing, handling, and assembly, including robots, hard automation, materials-handling systems, and processing equipment.

● Turbidimeter: an instrument for measuring the quantity of contamination suspended in a liquid by extinction and scattering.

● Turbulence: in cleanrooms, any flow that is not unidirectional. Turbulent flow is gen- erally rotational and includes flow directions and magnitudes not in compliance with specified limits. Not to be confused with the fluid dynamic definition of turbulent.

Today, cleanrooms that are not described as having unidirectional airflow are referred to as non-unidirectional- or mixed-flow cleanrooms.

● ULPA (ultralow penetration filter): usually, a filter with better than 99.997% particle removal efficiency at the most penetrating particle size, usually rated at 0.12m.

● Unidirectional: in cleanrooms, airflow characterized by straight streamlines that flow parallel to one another. Previously, the word laminar was used to describe this type of flow. Because the cleanroom use of the term laminar did not fit the fluid mechanical definition of laminar flow, the term unidirectional airflow is now preferred.

● Viable contamination: contamination by bacteria, spores, or viruses.

● Wipe-down: any procedure for manual cleaning of objects within or to be placed within a cleanroom. A complete wipe-down procedure must include descriptions of cleaning supplies, chemicals, procedures, and acceptance criteria.

● Witness plate: a bare, clean, unpatterned silicon wafer, disk, microscope slide, or part used as a surrogate for production parts, assemblies, and other critical surfaces in a cleanroom, for the purpose of measuring surface contamination rates.

1.3 SPECIFYING CONTAMINATION IN AIR AND ON SURFACES

Contamination in air is specified in units of contamination per unit volume. For example, airborne particle contamination is usually expressed in particles per cubic foot or cubic meter, airborne molecular contamination in parts per billion or micrograms per cubic meter, and airborne bacteria in colony-forming units per cubic foot or cubic meter of air. In general, the methodologies are intended to demonstrate compliance with requirements of ISO 14644 or FED-STD-209.

Contamination on surfaces is described in units of contamination per unit area.

Contamination on surfaces is expressed in two different ways. Aerial density, in particles or mass per unit surface area, is analogous to particles per unit volume used to describe airborne particle concentrations. An alternative way of describing surface contamination is defined in MIL-STD-1246, which defines a model particle size distribution and a cleanli- ness term, level, the particle size at which one particle is expected per square foot of sur- face area. MIL-STD-1246 also defines nonvolatile residue in mass per unit area, outgassing in percent weight loss, volatile condensable matter, and condensed volatile condensable matter. The standard also defines percent obscuration, a term used almost exclusively in the aerospace industry [9].

It is important to differentiate terminology to clarify the difference between volumetric contamination in the air in the cleanroom vs. aerial contamination on surfaces. The two terms used to classify airborne and surface-borne contamination are class and level, respec- tively. Class refers to the concentration of airborne particulate matter (or other airborne

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contaminants) per unit volume, whereas level refers to the concentration of contaminants on surfaces per unit area. The terms are not interchangeable and there is no simple analyt- ical relationship between them, although several attempts have been made to establish such a correlation [10].

MIL-STD-1246 is a method of specifying surface contamination levels. The particulate portion of the specification is based on a particle size distribution where the log of the concentration of the particles per unit surface area is plotted vs. the log of the square of the particle size (Figure 1.8). The model particle size distribution assumes that the maximum particle concentration occurs at 1m.

MIL-STD-1246 also specifies cleanliness for nonvolatile residues. A typical MIL-STD- 1246 specification could be MIL-STD-1246 level X, Y, Z, where

X numerical particle cleanliness level, corresponding to the particle size occur- ring at 1 particle per unit area, as shown in Table 1.1

Y nonvolatile cleanliness level in g/cm², as shown in Table 1.2

Z alternative or additional cleanliness levels, consisting of one or more abbreviations from the following list and the maximum limits described:

PAC percent area covered

PC particle count specified independent of Table 1.1

00 1 2

1 5 10

25 50

100 200

300 500

7501000 3

Number of Particles, log(N)

4 5 6 7 8 9

1 2 3 4

Particle Size, (log x)² (μm)

5 6 7 8 9

FIGURE 1.8 MIL-STD-1246 size distribution model.

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TABLE 1.1 Particle Cleanliness Levels

Level Particle Size (m) Count per 1 ft² Count per 0.1 m² Count per Liter

1 1 1.0 1.08 10

5 1 2.8 3.02 28

2 2.3 2.48 23

5 1.0 1.08 10

10 1 8.4 9.07 84

2 7.0 7.56 70

5 3.0 3.24 30

10 1.0 1.08 10

25 2 53 57 530

5 23 24.8 230

15 3.4 3.67 34

25 1.0 1.08 10

50 5 166 179 1,660

15 25 27.0 250

25 7.3 7.88 73

50 1.0 1.08 10

100 5 1,785 1,930 17,850

15 265 286 2,650

25 78 84.2 780

50 11 11.9 110

100 1 1.08 10

200 15 4,189 4,520 4,190

25 1,240 1,340 12,400

50 170 184 1,700

100 16 17.3 160

200 1.0 1.08 10

300 25 7,455 8,050 74,550

50 1,021 1,100 10,210

100 95 103 950

250 2.3 2.48 23

300 1.0 1.08 10

500 50 11,817 12,800 118,170

100 1,100 1,190 11,000

250 26 28.1 260

500 1.0 1.08 10

750 50 95,807 105,000 958,070

100 8,919 9,630 819,190

250 214 231 2,140

500 8.1 8.75 81

750 1.0 1.08 10

1000 100 42,658 46,100 426,580

250 1,022 1,100 10,220

500 39 42.1 390

750 4.8 5.18 51

1,000 1.0 1.08 10

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CVCM volatile condensable material collected in accordance with ASTM E595 VCM volatile condensable material determined by methods other than ASTM E595 NTU nephelometric turbidity units

TML total mass loss in accordance with ASTM E595

1.4 SOURCES OF CONTAMINATION

One classification scheme often found useful for addressing contamination problems is to classify them generically according to source categories. Here we present a useful scheme for classifying sources of contamination. Each source can have a chemical composition or fingerprint that points to it uniquely or to one or two of the many other source categories.

Facility The facility of a cleanroom, sometimes referred to as the cleanroom ambient environment, is often an important source category. The facility is often detected as a source during at-rest certification and during surveys using airborne optical particle counters (OPCs). When this occurs, an OPC is used to track the source of contamination to its origin by identifying the highest concentration point along the cleanroom ceiling, walls, and floors.

Causes of failures in a cleanroom facility include underpressurization of the room with respect to the factory, failures of seals in the ceiling filter grid, holes in filters, and fan failures.

Failures also can be associated with walls, floors, doors and other architectural features if these degrade or become damaged in use. When these types of failures occur, the composition of the contamination usually matches the composition of materials in the factory or outside air and often includes terrestrial dust, factory emissions, automobile exhaust particles, and so on.

The cleanroom ambient environment can also be a contributing factor in the failure to adequately control contaminants generated within a cleanroom, due to inadequate or misdirected airflows. In this case, the problem usually must be clarified using flow visuali- zation to locate the improper airflows. The composition of the contamination can be anything

TABLE 1.2 Nonvolatile Residue Cleanliness Levels Limit, NVR

Level g/cm² mg/L

A/100 0.01 0.1

A/50 0.02 0.2

A/20 0.05 0.5

A/10 0.1 1.0

A/5 0.2 2.0

A/2 0.5 5.0

A 1.0 10

B 2.0 20

C 3.0 30

D 4.0 40

E 5.0 50

F 7.0 70

G 10.0 100

H 15.0 150

J 25.0 250

CONTAMINATION AND ESD CONTROL IN HIGH-TECHNOLOGY MANUFACTURING

CONTAMINATION AND ESD

CONTROL IN HIGH-TECHNOLOGY MANUFACTURING

CONTAMINATION AND ESD

CONTROL IN HIGH-TECHNOLOGY

MANUFACTURING

CONTAMINATION AND ESD

CONTROL IN HIGH-TECHNOLOGY MANUFACTURING

CONTENTS

PREFACE

FUNDAMENTALS OF CONTAMINATION CONTROL