Methods for Monitoring and Measuring Cleanliness of Surfaces

Chapter 3

Methods for Monitoring and Measuring Cleanliness of Surfaces Rajiv Kohli The Aerospace Corporation, NASA Johnson Space Center, 2525 Bay Area Blvd, Suite 600, Houston, TX 77058, USA

Chapter Outline 1. 2. 3. 4.

Introduction Types of Contaminants Product Cleanliness Levels Methods for Monitoring Surface Cleanliness 4.1. Surface Contaminant Sampling 4.1.1. Swab Sampling 4.1.2. Rinse Sampling 4.1.3. Placebo Sampling 4.1.4. Coupon Sampling 4.2. Direct Cleanliness Measurement Methods 4.2.1. White Glove Test 4.2.2. Visual Examination 4.2.3. Tape Test 4.2.4. Water-Break Test 4.2.5. Wettability and Contact Angle

108 109 110 110 114 114 115 115 116 116 116 116 118 119 120

4.2.6. Surface Tension (Dyne Solution) 4.2.7. Wipe Test 4.2.8. Direct Oxidation Carbon Coulometry 4.2.9. Evaporative Rate Analysis 4.2.10. Optically Stimulated Electron Emission 4.2.11. Grazing-Angle Reflectance Fourier Transform Infrared Spectroscopy 4.2.12. Surface Potential Difference 4.2.13. Indium Adhesion Test 4.2.14. Direct Microscopy Techniques 4.3. Indirect Cleanliness Measurement Methods

Developments in Surface Contamination and Cleaning, vol 4. DOI: 10.1016/B978-1-4377-7883-0.00003-1 Edited by Rajiv Kohli & K.L. Mittal. Copyright Ó 2012 Elsevier Inc. All rights reserved.

121 122

123 125

126

127 128 129 130 131

107

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4.3.1. Witness Surfaces 4.3.2. Particle Sizing and Counting 4.3.3. Indirect Microscopy Techniques 4.3.4. Molecular Contamination 4.4. Surface Analysis Methods 4.4.1. X-ray Photoelectron Spectroscopy (XPS) 4.4.2. Auger Electron Spectroscopy 4.4.3. Ellipsometry 4.5. Mass Spectrometry 4.6. Spectroscopic Methods 4.6.1. Ultraviolet (UV) Spectroscopy 4.6.2. Fourier Transform Infrared Spectroscopy

132 132

135 142 146

147 147 148 149 151 151

4.6.3. Raman Spectroscopy 4.6.4. Fluorescence Spectroscopy 4.6.5. Laser-Induced Breakdown Spectroscopy 4.7. Bulk Analytical Techniques 4.7.1. Ion Chromatography 4.7.2. Electrochemical Method 4.7.3. Radioactive Tracers 4.7.4. X-ray Diffraction 4.7.5. X-ray Fluorescence 4.7.6. Other Bulk Analysis Methods 4.8. Overview of Surface Cleanliness Measurement Methods 5. Summary Disclaimer Acknowledgment References

152 153

154 155 155 156 156 157 157 160

161 162 163 163 163

152

1. INTRODUCTION Workplace surfaces can become contaminated as a result of manufacturing processes that generate dusts, fumes, and vapors with significant consequences to the performance of the parts and to the health of workers exposed to the contaminants, such as in drug preparation, handling, and delivery. Airborne contaminants may settle on surfaces in the immediate vicinity of the process, or they can be transported to other remote areas to deposit on surfaces. Contamination can be spread by repeated contact between contaminated hands and surfaces, or by transfer of contaminated objects between different locations in the workplace. Contaminated material may become re-entrained in the air from a contaminated surface. The meaning of cleanliness can vary significantly according to the application for which the surface is intended. In the manufacturing of high-precision

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components, many operations require a precision cleaning step for the succeeding process step to be successful. For example, coating on a part will not adhere if the surface is not sufficiently clean. However, even the most precise cleaning method can leave some contamination on the surface. Thus, it is critical that the cleanliness of a surface be accurately monitored and verified to assure that it is within acceptable limits. Accurate and reliable cleanliness assessment also helps in protecting the environment against pollution from cleaning agents, by evaluating new cleaning methods, or by optimizing an existing cleaning process. The monitoring of surface contamination in the workplace is a necessary control measure to assess its effectiveness in preventing the spread of contamination in the work environment. Workplace exposure monitoring of contaminants is routinely performed by public health professionals to determine whether exposures are within established limits, such as the National Institute for Occupational Safety and Health (NIOSH) recommended exposure limits [1] or Occupational Safety and Health Administration (OSHA) personnel exposure limits [2].

2. TYPES OF CONTAMINANTS Surface contamination can be in many forms and may be present in a variety of states on the surface. It can generally be classified into the following categories. l

l

l

l

Particulate contamination generally refers to foreign matter present on the surface or in the environment, such as dust, hair, and fibers. Thin film or molecular contamination can cover some of, or the entire surface of, the exposed equipment and can be organic or inorganic. These types of contaminants include: acid gases (e.g., HF, HCl, HNO3, and other acids); bases (e.g., ammonia and amines); condensable compounds (e.g., silicone, xylene, and hydrocarbons); dopants (e.g., boron, phosphorous, and arsenic compounds); oxidants (e.g., ozone); and biotoxic substances [3,4]. These contaminants are of most concern for cleaning validation since their sources are likely to be from the manufacturing process itself, or from the air in the cleanroom. Examples include grease, oils, and surfactant/ chemical residues. Adsorption of contaminants, such as hydrocarbons or moisture, can be caused by exposure of the surface to the atmosphere. In addition, the presence of hydrocarbons is a potential fire hazard in highpressure liquid or gaseous oxygen service due to autoignition [5]. Ionic contamination refers to the presence of undesirable ions from process operations, human activity, environment, and materials that come in contact with the manufactured product. The ions can cause chemical, electrochemical, or galvanic corrosion of a part. Microbial contamination refers to any unwanted organism growing on the surface or on the residues left on the surface, and includes biological

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cultures, spores, or bacteria. This form of contamination is often found in the health industry and in food processing and delivery.

3. PRODUCT CLEANLINESS LEVELS Most precision technology applications require characterization of micro-sized and smaller particles, as well as nonvolatile residue (NVR). For example, civilian and defense space agencies in the United States (NASA, National Aeronautics and Space Administration; and DoD, Department of Defense) and Europe (ESA, European Space Agency) specify surface cleanliness levels for space hardware in the microparticle size range [6,7]. The cleanliness levels are based on contamination levels established in the industry standard IEST-STDCC1246D for particles from Level 1 to Level 1000 and for NVRs from Level AA5 (0.1 ng/cm2) to Level J (0.025 mg/cm2) [8]. Table 3.1. 1ists the allowable cleanliness levels. The cleanliness levels commonly used by NASA to specify particle and NVR contamination for hardware for gaseous and liquid oxygen service are 50A, 100A, and 300A [6], although for other applications stricter cleanliness levels may be specified, such as Level 10 for particles and Level A/5 or A/10 for NVR for the Genesis mission to collect solar wind particles [9]. Other contaminants, including metals, toxic and hazardous chemicals, radioactive materials, and biological substances, are specified for surfaces employed in specific industrial areas, such as semiconductor, metal processing, chemical production, nuclear industry, pharmaceutical manufacture, and food processing, handling, and delivery. For detailed characterization, it is necessary to resolve contaminant particles by their sizes. Discrete particles can be generally classified by size and resolution technique, as depicted in Table 3.2.

4. METHODS FOR MONITORING SURFACE CLEANLINESS Methods for measuring the degree of surface cleanliness can be categorized as direct or indirect. They are also either specific or nonspecific. For biological contamination, specific methods, which permit identification and quantification of a single species, are preferred by regulatory authorities [10]. Direct methods analyze the surface to determine whether the contamination of the surface exceeds a predetermined threshold. These methods either are dependent on human discretionary power, such as a magnified visual inspection, or require sophisticated and very expensive equipment, such as a scanning or transmission electron microscope with X-ray detectors, or a time-of-flight secondary ion mass spectrometer (TOFSIMS). Direct methods can provide information on the physical and chemical state and composition of the contaminants. However, these methods are generally limited to examination of a small surface area, which will not be

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TABLE 3.1 Product Surface Cleanliness Levels for Commercial and Noncommercial Applications Particulate level

NVR level

Cleanliness level

Particle size, mm

Maximum allowable count per 0.1 m2/or 0.1 L of gas or liquid

Level

Quantity mass/0.1 m2 or mass/0.1 L

1

1

1

AA5

10 ng

5

1

2.8

AA4.7

20 ng

2

2.3

AA4.3

50 ng

5

1

AA3.7

100 ng

1

8.4

AA3.3

200 ng

2

6.9

AA3

500 ng

5

2.9

AA2.7

1 mg

10

1

AA2.3

2 mg

2

53.1

A/100

5 mg

5

22.7

A/50

10 mg

15

13.3

A/20

50 mg

25

1

A/10

100 mg

5

166

A/5

200 mg

15

24.6

A/2

500 mg

25

7.2

A

1 mg

50

1

B

2 mg

5

1780

C

3 mg

15

264

D

4 mg

25

78.4

E

5 mg

50

10.7

F

7 mg

100

1

G

10 mg

15

4180

H

15 mg

25

1230

J

25 mg

50

169

100

15.8

200

1

10

25

50

100

200

(Continued )

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TABLE 3.1 Product Surface Cleanliness Levels for Commercial and Noncommercial Applicationsdcont’d Particulate level Cleanliness level

Particle size, mm

Maximum allowable count per 0.1 m2/or 0.1 L of gas or liquid

300

25

7450

50

1020

100

95

250

2.2

300

1

50

11,800

100

1090

250

26.3

500

1

50

95,800

100

8910

250

213

500

8.1

750

1

100

42,600

250

1020

500

38.7

750

4.7

1000

1

500

750

1000

NVR level

Level

Quantity mass/0.1 m2 or mass/0.1 L

representative of the whole sample, since contaminants are not distributed uniformly or homogeneously on the surface. This is a significant limitation in quantification of surface contamination on parts with complex surfaces, as well as porous surfaces and parts with deep holes. In the case of polymers, organic solvents used for cleaning and nonpolar contaminants can also diffuse into the sample. These contaminants are not amenable to analysis by direct methods.

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TABLE 3.2 Range of Particle Sizes and Selected Techniques for Resolution of Particles for Characterization Particle class

Particle size, nm

Resolution techniques

Macro

>50,000

Naked eye

Micro

>100e50,000

Conventional optical microscopy

Submicron

10e100

Near-field optical microscopy and super-resolution microscopy

Nano

>1e10

Electron and probe microscopy

Atomic

0.01e1

Electron and probe microscopy, holography, and resonance force microscopy

Subatomic

<0.01

Femtosecond to attosecond spectroscopy and atomic force microscopy

Indirect methods utilize certain properties of the surface as an indicator of the presence of a contaminant on the surface. These methods are based on extracting the contaminant from a specimen surface and analyzing the extract, except for volatile contaminants, such as hydrocarbons, which can be thermally evaporated from the surface and measured by a headspace sampling technique. Measurement of surface contaminants by indirect methods is not limited by porous or complex surfaces, since the contaminants can be extracted from the entire sample. Samples with absorbed or diffused contaminants can also be measured by indirect methods. One disadvantage of the indirect methods is the contaminants must be extractable and detectable by the selected analytical technique. This will require careful assessment of the extraction method and implementation of a precise procedure for extraction and recovery of the contaminants. The following factors should be considered in evaluating appropriate methods for surface cleanliness [10–12]. l l l l l l l l l l l

Objective, rapid, reliable, and reproducible; Sample processing time; Sufficiently low detection limit below the cleanliness requirement; Free from interferences; Ability to capture and detect a wide variety of contaminants; Ability to distinguish between contaminants quantitatively; Compatibility of test method and extraction media with the part surface; Independent of operator’s experience; Robustness and sample throughput; Infrastructure, equipment, and operating costs; Operator skills and training.

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There are a large number of techniques that are used for detection and analysis of surface contaminants [10–24]. These techniques range from simple wipe sampling and visual inspection to sophisticated electron and scanning probe microscopy with high spatial resolution and sensitivity for nanoscale analysis [25]. The technique chosen for a particular component is dependent on the sampling method, the physicochemical properties of the component, the residue limit, and the required detection range. All of these techniques have advantages and disadvantages related to the evaluation factors listed above. Recent developments in many of the techniques for characterization of particle contaminants have been discussed in a companion chapter in this volume [25]. In this chapter, we discuss methods for broadly characterizing surface cleanliness and some recent developments in these methods. No attempt has been made to describe each method. Detailed descriptions of the individual techniques, capabilities, and a variety of applications are given in the references cited, including individual publications, comprehensive reviews, government and industry standards, guidelines, practices, and manufacturer websites. All of these methods have been used for characterizing surfaces and surface cleanliness, ranging from gross contamination at the macroscale to particles and thin films at the nanoscale.

4.1. Surface Contaminant Sampling Most of the indirect methods require a contaminant sample that is extracted by sampling the surface of the part. Surface sampling can be carried out in a number of ways [10].

4.1.1. Swab Sampling Swab sampling involves wiping a predetermined area of the part with a swab that has been moistened with a solvent selected for its ability to extract the contaminant. Usually, the surface is carefully wiped in two directions with a certain number of strokes, using one side of the swab for each direction. Generally, swabbing proceeds from less contaminated to more highly contaminated areas in order to prevent recontamination by the material already collected on the swab. The swab is immersed in a known amount of a recovery solvent. Recovery levels are determined by the solubility of the compound in the swabbing solvent, the wiping procedure, and the physical nature of the surface. Ideally, they should be as close as possible to 100%, but greater than 70% is considered reasonable and as low as 50% is sometimes obtained. Lower values are generally considered unacceptable and require improved procedures. The concentration of contaminant in the recovery solvent is then determined by analytical techniques and the amount of contamination can be calculated. One advantage of swab sampling is that insoluble or slightly soluble residues on the surface are more readily removed by physically rubbing the surface

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as compared with rinsing. It also permits sampling from accessible locations that are difficult to clean. On the other hand, the small sampling area may not be representative of the entire surface if the contamination is not uniformly distributed over all contact surfaces. Other issues that need to be considered include the physical properties of the swabs and recovery levels. The swab material must not damage the surface or shed fibers, and it must not leach compounds, such as adhesive, that can interfere with the analytical procedures. The recovery level can be improved using swabs with two different solvents, such as swabbing with isopropyl alcohol (IPA) after swabbing with water, with the results from both swabs being combined.

4.1.2. Rinse Sampling In rinse sampling, a small sample of the solution collected from the last rinse cycle of the cleaning process is analyzed for the compound of interest and the recovery level calculated from the volume of the solution and the contact area. This method is commonly employed to prepare a sample for determination of the NVR level by the gravimetric method (Section 4.3.4.1). There are some concerns with rinse sampling. It is generally assumed that the target contaminant is efficiently extracted into the rinsing solution and that all parts of the hardware are cleaned equally. Since the surface contaminants are not measured directly, there is the possibility that unacceptably high levels of contaminants may be left in some areas, especially in parts with complex geometries. The choice of solvent can also affect the cleanliness verification outcome. A very poor solvent will result in low contamination of the final rinse even if large amounts of contaminants are left on the surface. An additional risk is incorrect application of an adequate procedure, which could mean the hardware is “verified” clean and used for some time before the contaminationinduced anomaly is detected, with disastrous consequences. Even with these concerns, rinse sampling does offer several advantages when implemented correctly. It is easy to collect a part of the final rinse solution from the equipment and it allows evaluation of contaminants from all parts of the surface that are difficult to reach with a swab. This makes rinse sampling ideal for clean-in-place (CIP) systems, sealed systems, or large-scale equipment that is difficult to disassemble or to be moved into a cleaning facility; instead, the equipment is cleaned and verified for cleanliness where it is assembled. 4.1.3. Placebo Sampling The placebo sampling method is used in the pharmaceutical industry and involves manufacturing a batch of product without the active ingredient or contaminant of interest (placebo) in the cleaned equipment. The final placebo is tested for the contaminant. If contamination occurs at one point in the process, the placebo products manufactured first are likely to be more contaminated than

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those that follow later. Hence, the results will be misleading. In addition, the batch of material used in manufacturing the placebo is wasted and the contamination levels in the placebo may be difficult to measure.

4.1.4. Coupon Sampling Coupon sampling methods involve the use of manufacturing equipment whose components can be removed and analyzed for contaminants. This approach is useful when incorporated with analytical techniques that can make measurements directly from the surface. The main problem with the coupon sampling is that the removable components are usually flat and come from locations that are not hard to clean; they are therefore not representative of a worst-case scenario. One of the major disadvantages of sampling, and of indirect measurement methods in general, is that the collected samples must be sent to a laboratory for analysis. The part cannot be declared clean until the analysis results are available. This may take some time, which can lead to process delays.

4.2. Direct Cleanliness Measurement Methods Direct methods are the preferred choice for surface cleanliness measurements since the surface is interrogated directly. The more common direct methods, both qualitative and quantitative, are discussed in the following sections.

4.2.1. White Glove Test The white glove test consists of wiping a clean, dry, white cloth or a whitegloved finger against the contaminated surface for some length, and then inspecting the wiped surface for visible contaminants. The contaminated surface must be flat. The part fails the test if excessive contaminants discolored the cloth. However, if the contaminants are clear, no discoloration will be observed on the wipe or the gloved finger. This is a qualitative and subjective test, and the assessment results can vary greatly from test to test and from operator to operator. Limiting results to a pass/fail basis prevents the operator from making a vague assessment of the cleanliness of the surface. This test can be used for qualitative cleanliness qualification of spaceflight hardware when the cleaning wiper is inspected at a distance and incident light level in accordance with the requirements specified for different visibly clean levels [6]. To meet a specified cleanliness level, the wiper must show no visible contamination on inspection under the conditions specified for that cleanliness level (Section 3). 4.2.2. Visual Examination For cleanliness qualification of spaceflight hardware for noncritical applications, NASA and other space agencies have specified direct visual examination

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of the surface for qualitative assessment of the level of cleanliness [6]. Visible cleanliness levels defined for hardware are: generally clean (GC), visibly clean (VC), and VC with ultraviolet (UV) light (VC þ UV). A Generally Clean (GC). Piece parts cleaned to GC level are examined to be free of manufacturing residue, dirt, oil, grease, processing debris, or other extraneous contamination. The GC level is specified for hardware that is not sensitive to contamination and is easily and quickly cleaned or recleaned. Examination of the surface for GC level does not require a specific intensity of light or a particular distance from the surface. B Visibly Clean (VC). Several levels of cleanliness have been defined within the VC cleanliness category based on incident light levels and inspection distances, as shown in Table 3.3. C VC with ultraviolet (UV) light (VC þ UV). For the VC þ UV cleanliness level, the hardware shall be free of all visible particulate and nonparticulate contamination augmented by inspection under UV light (320–380 nm wavelength). Many organic and some inorganic contaminants will fluoresce under UV light. Any evidence of fluorescence during UV inspection is cause for re-cleaning. If the surface to be inspected is inaccessible, a wipe test can be performed, and the wiping medium is inspected under UV light. For all VC levels, areas of suspected contamination may be inspected at closer distances than specified above. VC level inspections are limited to exposed and accessible surfaces. The use of inspection aids such as wipers, mirrors, borescopes, or tape lifts is permissible for those areas of suspect condition with limited or no direct line of sight. When inspection of piece parts at the minimum inspection distance specified for the required cleanliness level is impractical (for example, having to hold parts cleaned to level VC Standard 1.5–3 m away), closer inspection is permitted. When interior volumes do not provide sufficient access to physically conduct an inspection within the defined VC range, the inspection can be conducted at a distance that deviates from the defined range only to the extent required to physically perform the inspection.

TABLE 3.3 Inspection Distance and Light Intensity Parameters for the VC Cleanliness Levels for NASA Spaceflight Hardware VC cleanliness level

Inspection distance, m

Incident light intensity, lm/m2

VC Standard

1.5e3

500

VC Sensitive

0.6e1.2

500

VC Highly sensitive

0.15e0.45

1000

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Inspection is performed with the incident light at angles between 15 and to the surface. The contrast is enhanced by reducing the surrounding light levels to 50% for white light inspection and 10% or less for UV light inspection [26]. All surfaces are inspected and the light and distance are varied according to the cleanliness standard specified. 45

4.2.3. Tape Test The tape test is well suited for sampling smooth surfaces to determine the presence of particulate contamination 5 mm and larger, and sometimes even film residues [26,27]. In practice, a pressure-sensitive tape is applied to the surface and firmly pressed down. The tape is peeled off with the particles adhered to the tape. Evaluation of the tape for contamination can be performed in different ways. The tape can be compared with a standard chart of six dust classes from 0 to 5, as defined in Table 3.4. This method is commonly used for assessment of the cleanliness of steel surfaces prior to painting. The tape can be mounted on a counting slide, which allows counting, size measurement, and analysis of the particles. If an optical technique is used for evaluation, the adhering particles can be detected by transmission of the light through the tape and tape adhesive. Alternatively, the tape can be bonded to the slide and the tape backing is dissolved, leaving the particles embedded in the adhesive; and the air bubbles are eliminated with acrylic medium. The particles are detected by the light transmitted through the tape adhesive or by the light reflected off it.

l

l

The tape lift provides a rapid and simple technique for removing particles from a surface and determining their number and size distribution. By using TABLE 3.4 Dust Size Classes for the Dust Tape Test According to ISO 8502-3 [28] Class

Description of dust particles

0

Particles not visible under 10 magnification

1

Particles visible under 10 magnification but not with normal or corrected vision (usually particles less than 50 mm in diameter)

2

Particles just visible with normal or corrected vision (usually particles between 50 mm and 100 mm in diameter)

3

Particles clearly visible with normal or corrected vision (particles up to 0.5 mm in diameter)

4

Particles between 0.5 mm and 2.5 mm in diameter

5

Particles larger than 2.5 mm in diameter

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statistically determined sample size and locations, an estimate of the surface cleanliness level of large areas can be made. The selection of the tape and the verification of its effect on the cleanliness of the hardware are very important. The tape adhesive should have sufficient cohesion to avoid transfer of the adhesive to the surface under test. Cleaning may be required to remove any adhesive transferred to the surface. In addition, the tape should have low-outgassing characteristics, and as a minimum it should meet the requirements of less than 1.0% total mass loss (TML) and 0.1% collected volatile condensable materials (CVCMs). A commercial dust tape test kit (Fig. 3.1). contains a battery-powered portable microscope with a 10 magnifier with scaled lens and integrated light source; adhesive tape consisting of virtually colorless, transparent, self-adhesive, pressure-sensitive tape with adhesion strength of at least 190 N/m compliant with ASTM and ISO standards; standard chart with six dust classes ranging from 0 to 5, with descriptions for accurate class placement; and a comparator display board [29]. A tape with a low-adhesion gelatin layer has been developed to lift fingerprints, shoeprints, dust marks, and microcontaminants from almost every surface, including porous materials such as paper or cardboard [30]. This tape has wide application in forensics. Strippable coatings can also be used for removal of surface contamination in various applications, as discussed in a recent review [31].

4.2.4. Water-Break Test The water-break test uses running water, allowing it to form a sheet across the surface [32,33]. Breaks in the water indicate the presence of hydrophobic residues. The water break test is a fairly crude test, which is suitable for detecting films of process oils and heavy fingerprints. The test is often used for parts washing and is generally not suitable for precision-cleaning applications. It does not readily detect nonhydrophobic contaminants. A variation of the water-break test is the atomizer test that involves a gently sprayed water mist [34,35]. Any surfaces, where water repulsion occurs, indicate the presence of hydrophobic contamination. The atomizer test detects

FIGURE 3.1 Dust tape test kit. Courtesy of Elcometer, U.K.

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larger amounts of hydrophobic contaminants than the water-break test in which the kinetic energy of the flowing water may remove a hydrophobic residue. In contrast, the atomizer test enables the visualization of the atomized droplets of water being repelled by a hydrophobic contaminant. In practice, the atomizer test is applied immediately prior to painting or coating. A mist of distilled water is atomized onto the surface. If the water droplets tend to coalesce into large lenses lasting for 25 s without flashing out, the surface is considered as having satisfactorily passed the water-break test. If the water gathers into droplets within 25 s (if the surface shows a “water break” within that time), the surface is considered to have failed the test. If the water forms a continuous film by flashing out suddenly over a large area, this is considered evidence of the presence of a contaminant on the surface, and the surface is deemed to have failed the test. Failure to support an unbroken water film will require additional cleaning of the part. Multiple cleaning procedures may be required to achieve the required water break-free surface. An improved automated inspection system that includes an infrared (IR) camera has been developed for the wetted surface of the parts [36]. The IR camera offers greater contrast and is not subject to limitations of lighting angle and viewing orientation. Furthermore, by replacing manual inspection, the noncontact inspection system eliminates exposure of humans during manual inspection to hazardous chemicals used for the processing of parts. There are several variables that must be taken into consideration when the water-break test is used for validation of surface cleanliness [32]. These include the presence of hydrophobic and hydrophilic contaminants; presence of abrasive particles and smearing of the surface from abrasion; smearing from organic compounds used in processing; contaminants in the water used for testing; water temperature; and angle of the test surface. This makes the water-break test acceptable for qualitative testing in industrial and commercial applications, but it is not very useful for cleanliness testing in the electronics or other high-precision industries, where surface cleanliness is critical to product performance.

4.2.5. Wettability and Contact Angle Contact angles are a classical method of describing the adhesion of a contaminant liquid to a solid. They can be used to detect the presence of films, coatings, or similar contaminants, which have a surface energy that is different from the substrate. The detection capability of this method is directly related to the difference in their relative surface energies. Several standards have been issued for performing contact angle measurements for different applications [37–41]. When properly conducted, these tests can enable detection of hydrophobic organic contaminants at levels of fractions of monomolecular layer. In practice, a drop of liquid (water) is dispensed onto the surface of the part and the profile of the drop is analyzed to determine its shape. The contact angle is calculated from the tangent to the

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FIGURE 3.2 Contact angle as an indicator of surface cleanliness. The right figure has a low contact angle indicating a clean surface, while the left figure shows a high contact angle on a contaminated surface.

shape. Modern systems incorporate precision optics and charge-coupled device (CCD) cameras and integrated video imaging systems in conjunction with sophisticated software and precision mechanics to perform contact angle analysis quickly, easily, and accurately [42]. A perfectly clean metal surface would have a contact angle of 0 , while a contaminated surface exhibits a high contact angle, such as 90 or larger (Fig. 3.2). Wettability techniques to monitor surface cleanliness have been reviewed recently [43].

4.2.6. Surface Tension (Dyne Solution) Cleaning of a surface by chemical, physical, or mechanical means results in an increase in surface energy. There are many commercial kits available for determining wettability by applying incrementally a sequence of graduated solutions to a surface and observing whether they “bead up” or not [44–46]. This is a common shopfloor test. A typical set has a range of solutions with surface tensions from 25 to 60 mN/m [or dynes/cm]. Test guidelines are documented in several standards [47,48]. Starting with a higher tension liquid, the solution is applied with a swab to the surface or to the substrate under test. If the solution does not maintain a continuous film over the surface, then the process is repeated with another liquid until a film remains continuous. The surface tension of this solution is called the critical wetting tension, or dyne value, for the surface. The use of liquid dyne solutions is not shopfloor friendly and has led to the development of dyne pens that are also based on a graduated dyne-level system. These pens are similar to a felt-tipped highlighter marker and contain the same solutions as in the bottles. The felt tip is swiped across the surface or substrate of interest and the results are interpreted in the same manner as the solutions and swab method. The difference is the felt-tipped pen offers convenience. 4.2.6.1. Dynamic Surface Tension Pure liquids and pure solvents have a unique surface tension value called static or equilibrium surface tension, which is the intermolecular force of attraction

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between adjacent molecules at the air/fluid interface. If anything contaminates the liquid or the formulation is changed by addition of surfactants or other chemicals, the intermolecular force at the air/liquid interface will change; the liquid has a “dynamic” surface tension. By measuring the changes in the surface tension, it is possible to determine in real time whether formulation changes are taking place and the extent of these changes. This is measured by the differential bubble pressure method [49–53]. The pressure required for bubble formation at a capillary tip immersed in the liquid is measured at different gas flow rates. The pressure and a calibration constant are used to calculate the dynamic surface tension at various surface ages. In practice, two probes of dissimilar orifice sizes (most commonly 0.5 mm and 4.0 mm) and various materials are inserted into a liquid, where the differential pressure value of the formed bubbles is measured. This value is directly proportional to the liquid surface tension. Since the method allows continuous bubbling, it also allows continuous in-process measurement. Dynamic surface tension has a wide range of applications, including online process control of contamination of cleaning liquids, assessment of active surfactants, additives in semiconductor processing liquid baths, and determination of critical micelle concentrations.

4.2.7. Wipe Test The wipe test is one of the most commonly employed methods worldwide for determining the cleanliness of a surface. It is used extensively for monitoring chemical and biological contamination, as well as particulate contamination, in a wide variety of applications [54–60]. A large number of standardized surface sampling methods have been developed specifying the wipe material, sampling protocol, and the analysis procedure. Examples of wipe methods include procedures specified by OSHA [61], NIOSH [62–64], and other methods specified by ASTM [65–68] and IEST [26]. Wipes are available commercially in a wide variety of synthetic and natural materials, such as rayon, poly (vinyl acetate) (PVAc), polyester, cotton, and paper. Some of the criteria for selection of the wipe medium include contaminant loading level; compatibility with the surface; durability; cost and availability; background levels; and the ability of the analytical laboratory to process the wipers, which requires the wipers be chemically digested and ashed. A new wipe colorimetric detection system for measuring isocyanate surface contamination (SWYPEÔ) has been developed and has been successfully tested in automotive repair and construction industry applications [69–71]. Wiping can be performed dry, or the wiper can be moistened with a wetting agent, such as water, alcohol, or other solvents and detergents. In general, the wet wipe method removes more particulate contaminants, but this often depends on the wipe media and the wetting liquid. For example, a filter paper wetted with alcohol collected up to 10 times more beryllium dust than a dry filter paper, compared with 6 times more recovery with

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a wetted PVAc wipe than a dry PVAc wipe [72]. Low surface-tension fluids, such as alcohol, give much higher recovery than fluids with high surface tension, such as water. A wide variety of surfaces and physical conditions are encountered in application of the wipe test. The surface may be hard or soft, smooth or rough, and porous or nonporous, and the substrate may be fragile. It may be heavily contaminated with oily residues, dust, and debris. Unfinished surfaces and sharp edges, pinch points, and other surface features can degrade or damage the wipe, with consequent loss of the contaminant recovered from the surface. If the surface is difficult to access, only part of the contaminant on the surface may be recovered. This may also make it difficult to follow the proper wiping technique prescribed by a standard, leading to reduced precision and misleading results [10]. As a direct method, the wipe test is qualitative in nature. For quantitative assessment, the contaminants on the wiper must be analyzed by chemical, or optical, or other ex-situ techniques, and the wipe test is considered an indirect assessment method. Conventional methods of determining elemental contamination on wipers use analytical techniques, such as inductively coupled plasma (ICP) or atomic absorption spectrometry (AAS). Other bulk analytical techniques including chromatography and mass spectrometry are used for organic contaminants. These methods analyze the solution from chemical digestion of the wiper. The analysis must be performed in a remote laboratory, which is costly and time consuming, and the analysis results are not available immediately. Recently, however, portable X-ray fluorescence (PXRF) and laser-induced breakdown spectroscopy (LIBS) have been developed for direct in-situ analysis of contaminants collected on wipe samples [73–80]. Although PXRF analysis of swipes is an established method, advantages of LIBS over this method include lower detection limits for many elements and the ability of LIBS to detect light elements, such as beryllium.

4.2.8. Direct Oxidation Carbon Coulometry Direct oxidation carbon coulometry (DOCC) is a direct quantitative cleanliness verification method that is cost-effective, rapid, easy to perform, surfacetexture independent, and adaptable to production environments [81–84]. The technique employs in-situ direct oxidation of surface carbon to carbon dioxide (CO2), followed by automatic CO2 coulometric detection. The amount of CO2 produced is proportional to the amount of surface carbon present on the sample. Differentiation between organic (hydrocarbons) and inorganic (carbonates, bicarbonates, and carbides) surface carbon, as well as allotropic forms of carbon (graphite and amorphous carbon), is controlled by the combustion temperature, since these compounds oxidize in specific temperature ranges (from ~473–573 K for hydrocarbons to ~973–1173 K for graphite, depending on thermodynamic and kinetic factors [85]).

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Figure 3.3 shows a schematic representation of the DOCC combustion system. Oxygen gas flows through pre-combustion scrubbers for the removal of interfering impurities prior to entering a quartz combustion tube. Organic surface carbon is oxidized to CO on samples placed into the first heated zone at 693–723 K of the quartz combustion tube, generally in about five minutes. The samples can be advanced into the second heated zone of the quartz combustion tube at 863 K for approximately an additional five minutes, where the inorganic surface carbon is converted to CO2. The oxidation times vary with sample type and temperature, but 10–15 min are usually sufficient. The second heated zone also contains a combustion catalyst, which converts any incompletely oxidized products (such as CO or volatile organic compounds) to CO2 before leaving the quartz combustion tube. The combustion gases are passed through a series of scrubbers that remove interfering gases, such as nitrogen and/or sulfur oxides. Excess oxygen gas sweeps the CO2 generated from the surface carbon oxidation reactions into the electrochemical cell of the automatic CO2 coulometer. The coulometry apparatus consists primarily of an electrochemical cell, where the titration takes place, and the associated electronics used to generate the titration current measure the total charge passing through the cell and digitally display the results. Figure 3.3 shows the titration cell and CO2 coulometer. The titration cell consists of a cathode compartment containing a platinum electrode, an anode compartment containing a silver electrode, and the titration solutions. The cathode chemistry involves a semi-aqueous solution containing an acid-based indicator and 2-aminoethanol. DOCC offers several advantages. The results are representative of the entire sample and detection levels down to 1 mg are achieved. The coulometric titration process is 100% efficient, giving highly accurate results. The system is adaptable to perform analysis of isolated internal surfaces, such as tubes, hoses, or fittings. Conversely, there are some disadvantages to DOCC. No

FIGURE 3.3 Schematic diagram of the DOCC system [82].

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elemental chemical information about the contaminants is obtained and determination of actual contaminant concentrations requires an estimate of the carbon content. It is a destructive technique that leaves an oxide scale on metal surfaces. The size of the parts that can be analyzed is dependent on the tube furnace capacity. Applications of DOCC include: cleanliness monitoring of surfaces of cold rolled steel, galvanized and aluminum parts, catalysts and glass; standardized monitoring of cleaning systems by determining the cleaning efficiency of a cleaning system; and predicting the effect of new materials, surface finishes, cutting liquids, or cleaning formulations on the component cleanliness.

4.2.9. Evaporative Rate Analysis Evaporative rate analysis has been developed as a nondestructive direct cleanliness verification method known as MESERAN (measurement and evaluation of surfaces by evaporative rate analysis) [86–89]. In this method, characterization of the contaminated surface of interest is carried out by depositing a chemical, consisting of a low boiling solvent or solvent combination, and a high-boiling but volatile 14C-labeled compound, on the test surface and observing the rate at which the low boiling solvent and the radiochemical evaporate from the surface from the detected beta emissions. Each test takes less than 3 min. Calibrations of various contaminants can be performed to develop calibration curves for the contaminants on substrates of interest to determine quantitative amounts of contamination detected down to 100 pg, or less than 1 monolayer. The calibration curves provide MESERAN numbers for evaluating cleaning effectiveness: MESERAN numbers less than 100 indicate good cleaning effectiveness (suitably clean surface); MESERAN numbers between 100 and 200 indicate moderate cleaning effectiveness (islands of contamination are present); and MESERAN numbers greater than 200 indicate poor cleaning effectiveness (gross contamination is present). Figure 3.4 shows an example of a calibration curve for Krytox fluorinated grease on a 304 L stainless steel substrate [88]. This analytical technique is used in a number of industrial and government facilities (within the United States and abroad) for research and development purposes, as well as for quality and production control [89,90]. MESERAN is routinely used for quantifying organic contamination on surfaces and the cross-link density (or degree of cure) in polymers. In addition, the MESERAN technique can be used for quantifying chemically active sites on surfaces [89]. The MESERAN method assumes the radiochemical is chemically compatible with the contaminants; the test solution solvent efficiently dissolves the contaminants within the time period of the solvent evaporation; and the test solution droplet covers all of the contaminants. Also, inadvertent contamination should be prevented and constant temperature, humidity, and pressure should be maintained for optimal reproducibility from test to test.

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FIGURE 3.4 MESERAN calibration curve for Krytox fluorinated grease on 304 L stainless steel [88].

4.2.10. Optically Stimulated Electron Emission Optically stimulated electron emission (OSEE), also known as photoelectron emission (PEE), is a direct method used for evaluation of cleanliness of surfaces contaminated with thin films [91–95]. It is an effective monitoring method for various applications, including: l

l

l

l

Evaluation of a cleaning process to assure the process is capable of achieving the required level of cleanliness on the part; Defining the quantitative level of cleanliness required to meet performance objectives; Online or in-process implementation for continuous monitoring of the part surface and the effectiveness of the cleaning process; Replacement of water break and other qualitative tests with a quantitative measurement of surface cleanliness.

The principle of this method is based on the fact that when a surface is illuminated with ultraviolet (UV) light of a particular wavelength, electrons are emitted from the surface. The emitted and subsequently scattered electrons can be collected across an air gap by a biased collector and measured as a current by the OSEE sensor instrument. Contamination reduces the electron emissions and, therefore, the current measured. The current is converted to voltage that can be correlated to contamination thickness. The equipment may be connected to a computer that color codes results for each particular part tested. This allows before and after comparisons of the cleaning effectiveness of a single cleaning solution, or side-by-side comparison of two parts cleaned in alternative cleaners. OSEE is simple to operate, fast, and relatively

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inexpensive [96,97]. In addition, it is quantitative, nondestructive, and noncontact. However, it requires a calibration for each part–contaminant combination to be tested. OSEE does not work well with contaminants that fluoresce. The range of thickness measurement depends on the emission level of the substrate and the attenuation level of the contaminants or coatings. OSEE is extremely sensitive to very low level changes in surface characteristics, which can also be a disadvantage. If the cleaned surface becomes oxidized before the OSEE measurement is made, it will give a false-positive reading. However, this can be overcome by altering the measurement procedure to include charge replacement [98]. The technique is capable of detecting the presence of a partial monolayer of thin film contamination and a spatial resolution of <1 mm. OSEE technology has been applied to detect contamination and/or thin film coatings in the aerospace industry, computer hard-disk manufacturing and metal finishing industry. The technique has also been applied to improve weldability of parts and for monitoring the presence and growth of oxides on bare copper on printed circuit boards. Other applications include: testing surface cleanliness prior to adhesive bonding; development of pre-weld cleaning/surface preparation methodology; inspection of surfaces for fingerprints; inspection of graphite/epoxy skins on aluminum-core honeycomb sandwich panels; detection of corrosive electrolyte on battery surface and development of battery cleaning procedure; cleanliness of neutron tubes; detection of nonvisible fingerprints on beryllium mirror surfaces; and verification of plasma-etch treatment of an elastomeric material [96,98,99,100]. The OSEE technique has already proven itself to be a very useful and reliable method for assuring product quality through proper quantitative monitoring of surface cleanliness levels. This test has been adapted at NASA to establish acceptance criteria for verification of NVR contamination for critical components of spaceflight hardware [6,101–103]. An example of a major shuttle element, where quality bonding is critical to performance reliability and safety, is the solid rocket motor. Inadequate bonding of the rubber installation to the case could result in exposure of the steel case to the hot gases from the burning propellant and result in burn-through, which could be disastrous. The use of OSEE has given NASA the capability of monitoring surface contamination down to 0.2 mg/0.1 m2, or NVR cleanliness level A/5 (Table 3.1).

4.2.11. Grazing-Angle Reflectance Fourier Transform Infrared Spectroscopy Grazing-angle reflectance FTIR (GA-FTIR) spectroscopy is an established laboratory technique for the detection of low-level surface contamination at the monolayer level [96,10]. However, it had not previously been used for on-site, real-time analysis. Recently, portable FTIR instruments have been developed and successfully demonstrated for the detection and quantification of

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FIGURE 3.5 GA-FTIR device on a tripod oriented to analyze the underside of a wing of an A-10 aircraft [105].

hydrocarbon contamination on metal surfaces [104,105] and contamination by active pharmaceutical agents on glass and metal surfaces [106–108]. The method is based on absorption of a grazing-incidence infrared beam reflected from the surface of interest. The portable instruments are highly adaptable for in-situ field application. Figure 3.5 shows the instrument oriented to analyze the underside of a wing of an A-10 aircraft [105]. In this application, the GA-FTIR device could detect hydrocarbon contamination on a cleaned dichromate conversion-coated aluminum aircraft skin prior to coating application, indicating the inadequacy of the cleaning method. The FTIR device was also used to detect a trivalent chromium conversion coating and distinguish it chemically from a conventional dichromate conversion coating. In addition, the relative thickness of the coating could be determined by the intensity of the spectral peaks. GA-FTIR is a very sensitive method for detection of organic residues on metallic surfaces, capable of detecting contaminants to <1.0 mg/cm2. For pharmaceutical applications, surface loading of 0.05 mg/cm2 is a readily achievable limit of detection for this technique [109], well within the range of interest for pharmaceutical cleaning validation and other similar applications.

4.2.12. Surface Potential Difference Surface potential difference is a noninvasive cleanliness monitoring technique that is based on detecting changes in the work function and charge of materials [11,110–117]. In this method, a reference probe is held close to (but not contacting) the surface. Where the surface is completely uniform,

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a constant voltage is obtained that is proportional to the difference in work functions of the probe and the surface. When nonuniformity is encountered, the change in work function causes a change in voltage on the probe tip, which is proportional to the change in work function on the surface. Absolute values of the work function difference between the probe and a point on the sample can be measured and these values can be used to develop integrated images of the surface to compare samples to assess the impact of chemical contamination. Surface potential difference imaging is a fast, high-sensitivity technique for cleanliness monitoring. In semiconductor wafer manufacturing applications, complete wafer samples can be mapped in a few minutes. Inspection can help identify nonvisual defect locations in production. These locations can then be exported to analytical tools, such as XRF or time-of-flight secondary ion mass spectrometry (TOF-SIMS) for detailed analysis. Absolute measurements can be made for sample-to-sample comparison. Contaminants can be detected in the 109 at/cm2 range for Fe and Cu and 1011 at/cm2 for Al [116,117].

4.2.13. Indium Adhesion Test The principle of the indium adhesion test for surface cleanliness is based on adhesion of clean solid surfaces when placed in contact [118–123]. Strong adhesion bonds are formed, but these bonds are broken when the contact pressure is released. To separate the surfaces, a force must be applied that overcomes the adhesion force. If an organic contaminant is deposited on a clean metal surface, the adhesion force is reduced by nearly 2 orders of magnitude. By contacting a soft metal probe, such as indium, on the surface of the contaminated metal with a known force, an area of contact is established. The adhesion force is measured by withdrawing the indium probe from the surface and recording the force required for separation. On a contaminated surface, the separation force will be progressively lower with increasing amounts of contamination. Indium is an ideal metal as a probe for this test because it is very soft and it does not work harden; so it conforms to the surface and the area of contact is proportional to the adhesion force between the tip and the specimen surface [118,124]. It is also resistant to corrosion in air, making it possible to conduct the test in air. In practice, the tip of a clean indium probe is brought in contact with the test specimen and pressed against it at a constant speed until a predetermined force is reached [120,124,125]. The force is maintained for periods of 1–5 s. The probe is then withdrawn at the same speed from the surface and the force–time characteristics are recorded. The ratio of the tensile force to withdraw the probe and the applied force of the probe is an indication of the degree of cleanliness of the surface. The indium adhesion test is a sensitive direct test of surface contamination and is adaptable for production testing for rapid feedback of cleaning effectiveness. It can be applied to various substrates, including metals and

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alloys, ceramics, and polished and ground glasses, and to measurement of both hydrophobic and hydrophilic contaminants [120]. However, the force ratio must be established for different contaminants and substrates for use as a cleanliness verification method and the test surface must be accessible to the indium probe. In general, a high force ratio indicates a cleaner surface. For example, a rigorously cleaned glass surface gave values of the force ratio in the range 1.6–2.0 compared with values below 1.0 for a contaminated glass surface [120].

4.2.14. Direct Microscopy Techniques Optical and electron microscopic techniques are used as direct methods for detection and physical characterization (counting and dimensional size measurements) of particulate contamination on surfaces. 4.2.14.1. Optical Microscopy The far-field optical microscope is the standard tool for characterizing particle contaminants at the microscale, approximately 0.1–0.5 mm, limited by the wavelength of light. The microscope is routinely integrated in manufacturing processes as a direct method for contaminant identification and for defect characterization in many high-precision industries. At the same time, optical microscopy is also commonly employed as an indirect method for identification and characterization of contaminants extracted from the surface of a part, as well as for particle counting and sizing [24,126]. Modern optical microscopes are versatile instruments with a wide variety of illumination modes, including bright field, dark field, polarization, differential interference contrast, and fluorescence (Fig. 3.6), and they can be used in reflection or transmission modes. By using a motorized stage and available software, particle counting, imaging, and analysis can be accomplished in a highly efficient manner on large surface areas. These data can be used to verify the surface cleanliness of

FIGURE 3.6 Integrated circuit in (a) bright field, (b) dark field, and (c) differential interference contrast with reflected light. Courtesy of Nikon, Japan.

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a part. The operation and capabilities of optical microscopes, as well as new developments and applications, are described extensively on the websites of the major suppliers [127–130]. The use of optical microscopy for particle characterization is discussed in Chapter 4 of this volume [24]. 4.2.14.2. Electron Microscopy Techniques Scanning electron microscopy (SEM) rasters a focused electron beam across a sample surface, providing high-resolution and long depth-of-field images of the sample surface. This capability enables topographical rendition of the sample surface. SEM is one of the most widely used analytical tools in industry due to the extremely detailed images it can provide. SEM is routinely integrated in manufacturing processes as a direct method for contaminant identification and for defect characterization in many industries where higher resolution images are required than are possible with conventional optical microscopy. Transmission electron microscopy (TEM) and scanning TEM (STEM) are related techniques that use an electron beam to image a sample. A beam of high-energy electrons is transmitted through an ultrathin specimen, interacting with the specimen as it passes through, allowing for angstrom-level imaging and, with aberration correction, even sub-angstrom imaging [25]. Compared to SEM, TEM has better spatial resolution and is capable of additional analytical measurements, but it requires significantly more sample preparation. Although not employed as routinely as the SEM, TEM is starting to be integrated in semiconductor manufacturing processes as a direct method for contaminant identification and for defect characterization. This has been motivated by the progressively smaller contaminants and defect sizes that must be controlled. In addition, critical dimension metrology considerations for semiconductors dictate the need for in-line TEM and, possibly, even STEM. The principles, operation, and capabilities of electron microscopes are described extensively in Refs. [13,21–24]. The applications of electron microscopy for off-line contaminant analysis and some recent developments are discussed in Sections 4.3.2 and 4.3.3.2 and in Chapters 4 and 5 in this volume [24,25].

4.3. Indirect Cleanliness Measurement Methods Indirect methods rely on contaminants extracted from the surface of interest, which can then be measured by different techniques, although, as discussed above, field-portable instruments for some methods (such as XRF, LIBS, and GA-FTIR) have been developed and successfully used for direct in-situ analysis [73–80,104–106]. All these ex-situ methods assume that the sample being analyzed is representative of the entire surface, which may or may not be the case. The more common indirect methods, both qualitative and quantitative, are discussed in the following sections.

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4.3.1. Witness Surfaces In this method, particle deposition in controlled environments is determined by collecting particles and condensable molecular contaminants that deposit from the surrounding environment on a clean witness surface for a specified period of time or operational activity. The witness surface is subsequently analyzed by conventional methods to size and count the particles collected, measure film thickness, and determine the chemical nature of the contaminants [13,22–24,131,132]. This method provides a standard approach to measuring particle or molecular contaminant deposition in cleanrooms or other controlled environments. Witness surfaces typically lend themselves to traditional microscopic or image analysis techniques for sizing and counting particles on the surface, or other surface analysis techniques for physical and chemical characterization of the deposited contaminants. For cleanliness verification, the witness surface can also be a surface that best represents the part surface of interest, or it can be the part itself, which is subsequently evaluated by extracting a sample from the surface and analyzing the contaminants therein (Sections 4.1 and 4.3.4.1). Some considerations for selecting a witness surface include the available methods of analysis, precision and accuracy required, size of particles of concern, actual material of critical surfaces of concern, and cost. Additionally, certain surfaces may become charged, especially in dry environments, which will cause particle deposition. For monitoring vacuum environments, the witness surface must be made of low-outgassing, vacuum-compatible materials. Several types of witness surfaces are used for monitoring particle deposition. For microscopic counting and sizing, gridded membrane filters or counting slides are used. These can be evaluated directly after exposure; or they can be used for evaluation of the particles in a sample extracted from the surface and filtered to collect the particles on the gridded witness surface. Other witness surfaces include glass plates for particle fallout measurements by scattering; silicon wafers or disks for image analysis or other surface scanning methods; and optical surfaces (mirrors or lenses) for evaluation by reflectance or transmission measurements. 4.3.2. Particle Sizing and Counting Several techniques are commercially available for sizing, counting, and classification of particles as small as 1 nm in diameter [132–139]. Particle counting can be performed online to monitor the contamination level in air environments, such as in cleanrooms, or in cleaning solutions used for precision cleaning. Inline particle counting is also performed to determine the cleanliness of a solid surface. These measurement methods employ an automated optical particle counter (OPC), also referred to as a laser particle counter, if a laser light source is used for particle illumination. The OPC detects

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the particle by direct light scattering from the particle. The lower detection limit of the OPC is generally 100 nm in particle diameter [138]. Smaller particles down to 50 nm can be detected by the OPC by using a stronger laser light source, a more sensitive photo-detector, or both, but the cost of this instrument is considerably higher than instruments with 100 nm detection capability. Another particle counting technique is the condensation particle counter (CPC) or the condensation nucleus counter (CNC). This technique also involves optical detection based on light scattering, with the CPC or CNC detecting the particle by first condensing a vapor on the particle to form a droplet that grows to a larger size, which is then detected optically by light scattering. Very small particles can be detected by the CPC/CNC. As an example, ultrafine condensation nucleus counters (CNCs) can detect particles as small as 2 nm at a counting efficiency >70% [137]. Recently, the first CNC capable of detecting individual ions has been developed [140,141], and water-based CPCs have also been developed in which water vapor condenses onto the particles, growing them to a size where individual particles can be detected easily using an optical detector [142–146]. Water-based CPC instruments are available commercially that offer improved sensitivity, stability, and precision in comparison to conventional aerosol-based detectors [147]. Many of these methods do not provide information on size distribution of individual particles, while differential mobility size spectrometry can provide particle size distribution in the range <1–200 nm. To overcome this limitation, an online, semicontinuous double-size spectrometry method has been developed to provide information on size and effective density of single particles in the size range 1–50 nm [148]. This method involves size selection of the particles by an electrical mobility size classifier based on the Stokes diameter, followed by re-sizing in a hypersonic or supersonic impactor [136]. The impactor sizes the singly charged particles by their aerodynamic diameter, which depends on the particle mass and the Stokes diameter. The separation efficiency of the impactor is better than 80% for particles down to 3 nm diameter [148]. Most differential mobility particle-sizing instruments are designed for ambient pressure operation for aerosol applications. However, contamination from nanometer-sized particles formed at low pressures in semiconductor manufacturing is a significant concern, which can lead to irrecoverable yield loss. Several differential mobility analyzers have been developed for low-pressure operation for measurements of particle size distributions in the 3–20-nm-diameter range [149–151]. These instruments employ larger diameter and longer connecting tubes between the instrument and the vacuum pumps, together with mass flow controllers designed for low pressures. These features ensure a lowpressure drop and a large evacuation capacity. The instruments have successfully demonstrated accurate size classification of 3–20 nm particles at pressures as low as 200 Pa, approaching the pressures in semiconductor fabrication.

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More recently, a system for in-situ sizing of 3–200 nm particles has been designed and demonstrated for both low-pressure and high-temperature environments in synthesis reactors and semiconductor processing equipment [152]. This system utilizes an ejector–dilutor system to sample particles from the hot vacuum zone and to adjust particle concentrations to a suitable level for measurements with a scanning mobility particle sizer. The scanning mobility particle sizer consists of a differential mobility analyzer (DMA) to select particles of a given mobility and an ultrafine CPC to detect them. A hypersonic impactor samples the aerosol in parallel with the scanning mobility particle sizer. Particles are collected on TEM support grids for off-line measurements of morphology and chemical composition. Electron microscopy is regularly employed for counting and identification of asbestos fibers. Several standards and practices have been developed for SEM and TEM, using either a direct transfer or an indirect transfer method for specimen preparation [153–158]. In the indirect transfer method, the specimen preparation procedure involves ashing and dispersion of the collected particulate, so that all asbestos is measured, including the asbestos originally incorporated in particle aggregates or particles of composite materials. TEM specimens prepared using the indirect method do not represent the particles and fibers as they existed in the air (Fig. 3.7). However, the indirect method is applicable where detection and identification of asbestos fibers are likely to be prevented or hindered by other types of particulate in direct-transfer TEM specimen preparations.

FIGURE 3.7 Appearance of TEM asbestos specimens prepared by direct-transfer (left) and indirect-transfer (right) preparation methods from an air filter collected during abrasion of a gasket. Courtesy of E. Chatfield.

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All TEM analytical methods incorporate a minimum fiber length of 0.5 mm, which provides consistency of analysis. This fiber length is a reasonable minimum length at which asbestos fibers can be reliably detected and identified in the TEM. The ability of the SEM method to detect and classify fibers with widths smaller than 0.2 mm is limited. If the fibers being sampled are predominantly less than 0.2 mm in width, the TEM method can be used to determine them. For off-line counting, optical and/or electron microscopy can be used on samples collected from a surface by the methods described in Section 3. Microscopic techniques enable chemical and physical characterization of the particles. Particle sampling, identification, and counting are discussed in detail in Chapter 1 in this volume [139].

4.3.3. Indirect Microscopy Techniques Optical and electron microscopy are well-established techniques used most commonly as indirect methods for off-line particle counting and sizing, as well as for chemical and structural characterization of surface contaminants. With a wide range of capabilities in spatial resolution and highly sensitive elemental analysis, these techniques can characterize particulate contaminants from micrometer to sub-angstrom sizes. 4.3.3.1. Optical Microscopy Optical microscopy is widely employed for characterization of micro-sized particles collected on witness surfaces described in Section 4.3.1. The techniques for particle identification and size analysis are discussed in more detail in Chapter 4 in this volume [23]. A major limitation of conventional far-field optical microscopy for biological applications is that the resolution is limited because of diffraction to a value of the order of a half-wavelength of the light used, approximately 200 and 500 nm for lateral and axial resolution, respectively [127–130]. Two major approaches have been developed to overcome the diffraction limit to achieve nanometer resolution in far-field optical fluorescence microscopy [25]. One approach makes use of nonlinear optics in which the fluorescent molecules are switched on or off from the ground state to the fluorescent state. Several super-resolution imaging techniques have been developed based on the specific nature of fluorescence to implement this approach. These techniques include two-photon microscopy; 4Pi confocal microscopy; standing-wave excitation, emission and excitation interference techniques; Fo¨rster (fluorescence) resonance energy transfer microscopy (FRET); stimulated emission depletion (STED) technique; combined STED and 4Pi confocal microscopy; ground state depletion microscopy (GSDM); saturated pattern extinction microscopy (SPEM); saturated structured illumination microscopy (SSIM); stochastic optical reconstruction microscopy (STORM); and photoactivation localization

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microscopy (PALM) [159–163]. With these techniques, spatial resolution of 20–30 nm has been demonstrated in far-field fluorescence microscopy, with an ultimate goal of 1–5 nm [129,164,165]. These techniques are employed extensively for imaging in biological applications and have been reviewed in detail [166–169]. The second approach to super-resolution is based on the concept of a lens made from metamaterials or materials with a negative refractive index [170]. In such a lens, a high-resolution image would be formed by surface plasmon polaritons at the interface between the positive and negative refractive index media in the compound lens. This approach is still evolving and practical implementation for surface analysis is likely to be sometime in the future. An alternative method to bypass the diffraction limit in optical systems is scanning near-field optical microscopy (SNOM), in which the light is confined to a probe with a nanometer-sized aperture and scanned very close to the sample. Under these conditions, the light is scattered from the near field and subwavelength imaging can be achieved [25]. The greatest advantage of SNOM probably rests in its ability to provide optical and spectroscopic data at high spatial resolution, in combination with simultaneous topographic information. Combining AFM and SNOM has proven to be an extremely powerful approach in certain areas of research, providing new information about a variety of specimen types that is simply not attainable with far-field microscopy [25]. SNOM can also be combined with laser ablation to obtain samples from the surface for in-situ chemical identification by mass spectrometry [25]. Some of the limitations of near-field optical microscopy include: l l

l

l l

Practically zero working distance and an extremely small depth of field. Extremely long scan times for high-resolution images or large specimen areas. Very low transmissivity of apertures smaller than the incident light wavelength. Only features at the surface of specimens can be studied. Fiber-optic probes are somewhat problematic for imaging soft materials due to their high spring constants, especially in shear-force mode.

SNOM is currently still in its infancy, and more research is needed toward developing improved probe fabrication techniques and more sensitive feedback mechanisms. The future of the technique most likely may be in refinement of apertureless near-field methods, some of which have already achieved resolutions of the order of 1 nm. However, typical resolutions for most SNOM instruments range around 50 nm, which is only 5 or 6 times better than that achieved by scanning confocal microscopy. This moderate increase in resolution comes at a considerable cost in time required to set up the SNOM instrument for proper imaging and in the complexity of operation. A major limitation of this method is that a SNOM image is

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obtained by point-by-point scanning, which is an indirect and rather slow process. Recently, a scanning system has been developed with scan speeds of 150 mm/s that can produce images of a 20 mm2 area in less than 10 ms [171]. Some of the recent developments are discussed in Chapter 5 in this volume [25]. 4.3.3.2. Electron Microscopy In addition to atomic level imaging, electron microscopy also enables elemental identification of nanosize samples. Table 3.5 lists the contrast mechanism and the chemical analysis capabilities of some common electron microscopy techniques. These techniques are routinely employed for contamination-related failure analysis in many different industries. The availability of commercial microscopes with a wide range of capabilities and TABLE 3.5 Electron Microscopy Techniques for Imaging Surface Structures Contrast mechanism

Resolution, nm

TEM

Diffraction and phase grating

0.1

Atomic resolution, thin films, and fine particles

AES

STEM

Diffraction and phase grating

0.1

Microdiffraction and microanalysis

AES; EELS

REM

Phase and diffraction

0.5

Bulk crystals

TRAXS; EELS AES; RHEED

SREM

Phase and diffraction

0.5

Bulk crystals and microdiffraction

TRAXS; EELS AES; RHEED

LEEM

LEED

5

No foreshortening effect

SEM

Secondary electron

1

Topography

EDS; WDS, Auger

SAM

Auger electron

2

Chemical mapping

Auger

Technique

Features

Chemical analysis

REM: reflection electron microscopy; SREM: scanning reflection electron microscopy; LEEM: lowenergy electron microscopy; SAM: scanning Auger microscopy; AES: Auger electron spectroscopy; EELS: electron energy-loss spectroscopy; TRAXS: total reflection angle X-ray spectroscopy; RHEED: reflection high-energy electron diffraction; EDS: energy dispersive spectroscopy; WDS: wavelength dispersive spectroscopy

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prices has made electron microscopy widely accessible and indispensable for surface analysis. One recent advance in electron microscopy is the development of in-situ or environmental capability that allows examination of samples in their native state under ambient pressure and elevated temperatures [25,172]. This capability is extremely valuable in surface contamination analysis because the contaminants can be analyzed directly, without any additional preparation that could alter surface of the sample. Electron microscopy techniques have been described extensively in Refs. [13,22,23,173]. The application of electron microscopy for characterizing nanosize particle contaminants is discussed in Chapter 4 in this volume [24]. Recent developments in high-resolution electron microscopy are discussed in Chapter 5 in this volume [25]. 4.3.3.3. High-Resolution X-Ray Spectrometry Semiconductor EDS and WDS are commonly employed as analytical tools for off-line chemical identification of contaminants, but these techniques cannot resolve closely spaced or overlapping X-ray peaks in complicated spectra, especially at low accelerating voltages [174,175]. For example, the severe peak overlaps between the Si Ka and the W Ma X-ray lines prevent their analysis by semiconductor EDS [176]. High-resolution X-ray detectors are required to meet the analysis requirements for small particles, particularly for those smaller than 0.1 mm. Silicon drift detector (SDD) technology is a recent advancement that can significantly improve light element sensitivity and provides high count rates for heavy elements as compared with Si(Li) detectors. For example, TEM/STEM analysis of nickel oxide films (~50 nm thickness) operated at 200 kV with an SDD showed count rates that never dropped below 80% of the maximum count rate over a tilt range 25 to þ25 compared with less than 30% for an Si(Li) detector [177,178]. SDD uses the same measurement principles of photoelectric absorption, inelastic scattering, and charge generation as the Si(Li) EDS, but it has a different design. The thickness of the SDD-EDS is nearly an order of magnitude less (~300 to 450 mm) than the thickness of the Si(Li) EDS detector (~3 mm); so the distance the charge travels is also correspondingly reduced by an order of magnitude. A complex pattern of discrete electrodes collects and directs the electrons to a central anode that is considerably smaller than the anode of an Si(Li) detector, thus reducing the noise. For most measures of spectrometric performance, SDD-EDS is equal to or better than Si(Li) EDS, including count rate and resolution. For the output count rate versus input count rate, the SDD exceeds the Si(Li) EDS performance by a factor of 5–10 for the same resolution, which can be enhanced by a factor of 200 with a cluster of SDDs [178]. This high throughput can benefit analytical measurements that are X-ray count limited, such as X-ray mapping. Similarly, the SDD-EDS with an active detector area of 10 mm2 achieves a resolution of 122.5 eV compared with 129 eV for Si(Li) EDS.

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Other leading techniques for developing high-resolution detectors are based on the use of semiconductor thermistors, superconducting edge transition detectors, superconducting tunneling junctions, and magnetic calorimeters [179–184]. With these methods, it is possible to obtain an energy resolution of 3–13 eV full width at half maximum (FWHM). This is an order of magnitude better than semiconductor EDS (~130 eV at FWHM) and is comparable to semiconductor WDS (~2 to 20 eV at FWHM). Of the techniques mentioned above, the superconducting edge transition detector is most often applied for X-ray microanalysis for SEM, as well as for TEM [174,175,185–189], and such detectors have been available commercially [190]. In a typical configuration of such a detector system, also known as hot-electron microcalorimeter EDS, the system consists of a metal film to absorb the X-rays, a thermometer to measure the temperature of the electrons in the absorber, and a coupling to a heat sink. When a particle or a photon interacts with the absorber, the incident energy is converted to heat. The corresponding temperature rise is measured by a superconductor tunnel junction that is cooled to well below the phase transition temperature. The amplitude of the current pulse through the junction is directly proportional to the incident energy (Fig. 3.8). The fundamental energy resolution limit of these detectors is of the order of 1–3 eV, depending on the absorber material. Detector technology can routinely achieve a resolution of 3–7 eV, which makes these devices very well suited to high-resolution X-ray detection. For example, on a TiN sample, it was possible

FIGURE 3.8 Schematic arrangement of a hot-electron microcalorimeter system.

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FIGURE 3.9 Microcalorimeter EDS spectrum of a tungsten particle on a silicon substrate [176]. Courtesy of NIST.

to resolve the La peak, both Lb peaks, and even the Lh peak for Ti using microcalorimeter EDS. Similar results were obtained with a 0.3 mm W particle on an Si substrate. As shown in Fig. 3.9, the Ma, Mb, Mg, and the Mz peaks for W were clearly resolved [176]. Microcalorimeter EDS offers one additional advantage. With its high-energy resolution, all peak shapes and integrated peak intensities are accessible, making it possible to measure chemical shifts in the X-ray spectra. This can provide chemical binding state information [176]. 4.3.3.4. Electron Diffraction Electron diffraction is similar to X-ray diffraction (XRD), except that it uses a beam of electrons to obtain a diffraction pattern from the sample of interest [173,191]. It is usually performed in a TEM, where the electrons pass through a thin film of the sample to be analyzed. The resulting diffraction pattern is then observed on a fluorescent screen, recorded on photographic film, or on imaging plates, or using a CCD camera. If the sample is tilted with respect to the incident electron beam, one can obtain diffraction patterns from several crystal orientations. Thus, the crystal structure can be mapped in three dimensions. Electron diffraction in a TEM requires that the sample be transparent to electrons at a thickness no greater than 100 nm. Specimen preparation must be done very carefully and it is time consuming. Also, the thin section of the sample can be damaged by the electron beam during examination in the TEM. Crystallographic structural information can be obtained with high accuracy by electron diffraction, but the right experimental conditions are essential and interpreting the data is nontrivial and nonroutine. Thus, XRD, rather than

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electron diffraction, is the preferred method to determine lattice parameters and atomic positions in surface analysis at the microscale. At the nanoscale, however, it is increasingly important to determine atomic ˚ –0.001 A ˚ [25]. Nanoarea positions with a precision of the order of 0.01 A electron diffraction in aberration-corrected STEM is the primary technique that has the potential to achieve this level of precision. Some of the recent developments in this area are discussed in Chapter 5 in this volume [25]. 4.3.3.5. Probe Microscopy Techniques Scanning probe microscopy (SPM) in its many manifestations and configurations, such as atomic force microscopy (AFM), scanning tunneling microscopy (STM), and scanning near-field optical microscopy (SNOM), provides atomic or near-atomic-resolution surface topography, which is ideal for determining angstrom-scale surface roughness on a sample. A sharp tip is scanned in proximity to the surface of the specimen and an image is obtained from the tip– surface interactions. Depending on the mode of operation, many different interactions can be imaged simultaneously, providing material properties along with topography and morphology information. Table 3.6 lists the contrast mechanism and resolution of some common SPM techniques. These techniques are not often employed for surface contamination analysis, primarily because of the limitation of achieving chemical contrast and thus the chemical identification of an individual atom. Recently, however, chemical contrast has been successfully demonstrated in STM and AFM.

TABLE 3.6 Common Scanning Probe Microscopy Techniques for Imaging Surface Structures Technique

Contrast mechanism

Resolution, nm

STM

Tunneling effect

02 (z) 1 (x, y)

High resolution

AFM

Atomic force

02 (z) 1 (x, y)

High resolution, nonconductive surface

FIM

Ionization

0.2

High resolution, depth profile

Atom probe mass spectrometry

SNOM

Photon

10

No surface damage

Raman; MS

FIM: field ion microscopy; MS: mass spectrometry

Features

Chemical analysis

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Chapter 5 discusses recent developments in probe microscopy, including two special techniques, scanning thermal microscopy and scanning acoustic microscopy, for characterization of particles and sub-surface features [25]. The application of AFM for monitoring contaminant particles is discussed in Chapter 6 in this volume [192].

4.3.4. Molecular Contamination Molecular contamination is very insidious and requires careful measurement to assess the presence of the contaminants and their extent on a surface. 4.3.4.1. Gravimetric Method for Nonvolatile Residue NVR refers to film-type contamination that is deposited on the surface. Table 3.1 specifies the product cleanliness levels in terms of the concentration of NVR per unit area of the surface or unit volume of a gas or liquid. NVR is measured by an indirect gravimetric method in which the contaminants are extracted from a given area (0.1 m2) of the surface using a specified solvent selected for its ability to extract the contaminants [193]. The extract is filtered and transferred to a pre-weighed dish and evaporated to a constant weight. The difference in the weight before and after extraction is the NVR expressed as ng/0.1 m2. This standard practice can also be applied to cleanroom gloves and wipers [194,195], as well as for gravimetric determination of NVR in permanent environmentally controlled facilities [196]. As noted in Section 3, NVR levels are specified for verification of the precision cleanliness of space hardware, as well as in other industries requiring high levels of cleanliness. 4.3.4.2. Determination of Volatile Condensable Material This method is a screening technique to determine the volatile content of materials when exposed to a vacuum environment [197–199]. Two parameters are measured: total mass loss (TML) and collected volatile condensable materials (CVCMs). An additional parameter, the amount of water vapor regained (WVR), can also be obtained after completion of exposures and measurements required for TML and CVCM. The test specimen is exposed under vacuum at <7  103 Pa to a temperature of 398 K for 24 h under carefully controlled conditions. The changes in the mass of the specimen and the mass of the outgassing products that condense on a collector at a temperature of 298 K are measured. Comparisons of material outgassing properties are valid at 398 K sample temperature and 298 K collector temperature only. Samples tested at other temperatures may be compared only with other materials, which were tested at that same temperature. This test is primarily a screening technique for materials. Many types of organic, polymeric, and inorganic materials can be tested. These include polymer potting compounds, foams, elastomers, films, tapes, insulations,

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shrink tubings, adhesives, coatings, fabrics, tie cords, and lubricants. The materials may be tested in the “as-received” condition or prepared for test by various curing specifications. The criteria used for the acceptance and rejection of materials are based on specific component and system requirements. Historically, a TML of 1.00% and a CVCM of 0.10% have been used as screening levels for rejection of spacecraft materials [200,201]. It is not necessarily valid for computing actual contamination on a system or a component because of differences in configuration, temperatures, and material processing. This method of screening materials is considered conservative because maximum operating temperatures in service are assumed not to exceed 325–335 K for most space applications. Some materials that have acceptable properties at the intended-use temperature may be eliminated because their properties are not satisfactory at the test temperature of 398 K. Also, materials that condense only below 298 K are not detected and additional tests are usually specified to qualify materials for a specific application. 4.3.4.3. Quartz Crystal Microbalance The quartz crystal microbalance (QCM) sensor is used to measure the molecular contamination of critical surfaces at one or more temperatures for an extended period of time [202–205]. This is a critical need for spacecraft where excess contamination from outgassing of materials and subsequent condensation on critical surfaces, such as solar arrays, moving mechanical assemblies, cryogenic insulation schemes, and electrical contacts, can cause catastrophic failures. Similarly, in the microelectronics industry, airborne molecular contaminants can cause unacceptable performance degradation of components, for example degraded absorptance, reflectance, or scattering characteristics of optical components. In practice, a piezoelectric crystal is exposed next to a “surface of interest” or in the plane where molecular flux is expected. It is then cooled to the temperature at which the crystal should condense, whatever molecular contaminant exists at that temperature. By measuring the frequency shift of the crystal and knowing the mass sensitivity, the mass accumulated can be determined. The technique is very sensitive and outgassing rates as low as 1014 g cm2 s1 have been measured on spacecraft in space [206]. QCM and thermoelectrically cooled QCM sensors are commercially available for a variety of space and ground applications [207]. 4.3.4.4. Surface Acoustic Wave The surface acoustic wave (SAW) device uses a sensor with a piezoelectric crystal vibrating at resonance frequencies as high as 600 MHz [203,208–211]. An alternating current applied to a transducer causes the surface of an exposed crystal to expand and contract and the generated acoustic waves propagate along the surface of the crystal to a sealed unexposed crystal. If a molecular

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contaminant deposits on the exposed crystal surface, the acoustic wave velocity is reduced and a frequency shift is observed. The difference in the frequency between the exposed and the unexposed crystal is an indicator of the mass of the contaminant deposited. The SAW device provides real-time detection of airborne molecular contaminant (AMC) deposition on surfaces, as well as chemical warfare agents, explosives, and other volatile organic compounds. AMC events can last from seconds to weeks, involving different sources and molecular species simultaneously. To effectively monitor and resolve these complex issues, high-sensitivity, real-time information provided by SAW sensors is particularly useful. The data are used to correlate AMC occurrences with cleanroom processes and events, such as process chemical migration, chemical outgassing, and introduction of outdoor air contamination into the facility. This can help to identify new contamination sources early. SAW instruments can measure mass changes of less than 0.2 ng/cm2 (<1 monolayer) at measurement intervals of one minute or less. Although SAW sensors can be small and portable, they are sensitive to moisture that can reduce their responsiveness in field applications. Combining the molecular identification capability of TOF-SIMS with the mass sensitivity and time resolution of the SAW creates a powerful tool for monitoring and diagnosing AMC problems. 4.3.4.5. Plasma Chromatography-Mass Spectrometry Plasma chromatography-mass spectrometry (PC-MS) has been developed as an ultra-sensitive analytical technique for characterization of trace levels of organic contaminants [212–214]. In this method, positive and negative ions are generated from the ion–molecule reactions initiated by radiation from a 63Ni source. A mobility spectrum is produced using a voltage gradient to move the charged particles generated in the reactor tube toward the drift spectrometer tube and by injecting a pulse of these ions through an injection grid into the spectrometer (Fig. 3.10). As this group of ions moves through the spectrometer in an inert nitrogen atmosphere toward the electrometer detector, separation of individual ionic types occurs because of their differing mobilities in N2. This action is similar to a TOF mass spectrometer, except that the ionic velocity at the atmospheric pressure PC conditions is determined by the collision interaction between an ion and neutral nitrogen molecules, rather than by the kinetic energy velocity of the ion as in mass spectroscopic conditions. Positive or negative ions can be independently observed by choosing the electrical field polarity. The primary advantage of the PC-MS is its extreme sensitivity to the detection of a large class of volatile chemicals. Typically, the instrument can detect the presence of picogram (1012 g) quantities of organic surface contaminants and it is not unusual to measure some compounds at the femtogram (1015 g) level [212]. A second advantage of this technique is its capability of compound characterization, as opposed to elemental analysis. In

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FIGURE 3.10 Schematic diagram of a plasma chromatography-mass spectrometry system for monitoring cleanliness of surfaces [212].

145

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addition, the PC/MS is a nonvacuum technique, operating at atmospheric pressure, and therefore the loss of volatile organic surface contaminants, which frequently occurs in the high-vacuum sample housings of other surface techniques, is not experienced with PC-MS. Also very little, if any, sample preparation is required. As an example, an ion mobility spectrum of a cleaned SiO2 wafer after storage in a polypropylene container for 5 days showed the presence of several compounds that had outgassed from the plastic container [213]. In the 1980s, a better understanding of PC-MS was achieved, suggesting PC-MS was more a spectrometric than a chromatographic process. The name was changed from plasma chromatography to ion mobility spectrometry (IMS) [214]. IMS is discussed in the next section. 4.3.4.6. Ion Mobility Spectrometry The basic principle of IMS involves the creation of gas phase ions by ionizing neutral molecules using different sources. The ions produced are separated by their drift velocities, which depend on the mass, charge, and shape of the ion [215–219]. This enables high-sensitivity analyses for a very wide range of compounds. The method offers many advantages including small size, instrument simplicity and ease of operation, portability, fast response, real-time monitoring capability, and short analysis time. These advantages outweigh the drawbacks and limitations of low selectivity and low resolving power, as well as interferences in highly contaminated environments. Fortunately, the conditions in semiconductor and other cleanrooms are relatively benign, so that IMS can be used successfully for detecting and monitoring AMCs in these facilities at sub-parts-per-billion levels [220]. IMS as an analytical technique is also used extensively in the pharmaceutical industry for cleaning validations, demonstrating that reaction vessels are sufficiently clean to proceed with the next batch of the pharmaceutical product. It is used to detect low or trace quantities of harmful substances and chemicals in the surrounding atmosphere in industrial and military applications [215–219]. Recent trends are to combine IMS with other detection techniques, such as conventional and time-of-flight mass spectrometry and gas or liquid chromatography. Ambient pressure ionization techniques that can be used for analysis of compounds with low volatility have also been developed [219].

4.4. Surface Analysis Methods There is a wide variety of surface analysis techniques that can detect, measure, and chemically identify contaminants. Each technique provides information about a different aspect of the surface, and by combining more than one technique, the surface can be completely characterized. In general, surface analysis techniques use expensive equipment in a remote laboratory

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and require highly skilled personnel for analysis and interpretation of the results.

4.4.1. X-ray Photoelectron Spectroscopy (XPS) This highly sophisticated and expensive measurement method uses special equipment to bombard the surface of interest with X-rays under vacuum conditions, causing electrons to be released from the surface. Since each element releases unique electrons under these conditions, the actual elemental composition of the surface can be quantified. In addition to elemental concentration, XPS can also provide information on the chemical state of the compound (e.g., CF vs. CF2) because the binding energy depends on the local chemical environment. The sampling depth is usually less than 10 nm. This method requires a very small, flat surface, and it is not only expensive but time consuming. Its application is limited to off-line surface analysis for organic and inorganic contaminants, but it can be used to calibrate and evaluate other, less sophisticated measurement methods. In addition, XPS has been used to validate the effectiveness of surface cleaning methods and to assess corrosion and adhesion failures. One limitation of XPS is that it requires a relatively large amount of sample (w5 mg) because of the comparatively large analysis area required (w10 mm diameter), even for state-of-the-art spectrometers. Recently, XPS has been successfully demonstrated as a nondestructive depth-profiling method that yields accurate depth information with nanometer resolution [221]. The method is based on the observation that controlled surface charging (CSC) causes line shifts that correlate directly with the vertical positions of the atoms. CSC is not affected by the surface roughness, and it is applicable to relatively thick structures. It can also differentiate spectrally identical atoms at different depth locations. The application of XPS for characterizing surface contaminants has been recently reviewed [222]. 4.4.2. Auger Electron Spectroscopy Auger Electron Spectroscopy (AES) is a surface-specific analytical technique that utilizes a high-energy, finely focused electron beam as an excitation source. Auger electrons are produced when the excited atoms release the extra energy to an electron that is then emitted as an Auger electron. AES collects and measures the kinetic energies of the emitted Auger electrons, which are characteristic of elements present at the surface and “near-surface” of a sample. This makes possible elemental composition analysis. The typical sampling depth of AES is 2 to 5 nm, making it a surface-sensitive analytical technique. The electron beam can be rastered or scanned over a large or small surface area, or it can be directly focused on a small surface feature. In the scanning mode, AES can be used for mapping chemical distribution. The ability to focus

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the electron beam onto diameters of 10 nm and less makes AES an extremely useful tool for elemental analysis of small surface features. It is a destructive analysis technique, but it is useful in identifying concentrated areas of contamination. It can be used quantitatively, when standards are available for quantification. Samples must be vacuum compatible and insulators are difficult to analyze due to sample charging. The nature of the Auger process results in broad peaks, so spectral overlapping can be a problem in data analysis. Moreover, the ability to raster the electron beam over an adjustable surface area provides control over the size of the analytical area. When used in combination with ion sputter sources, AES can perform large- and small-area compositional depth profiling, and when used with a focused ion beam (FIB), it is useful for analyzing cross-sections. The application of AES for characterizing surface contaminants has been recently reviewed [222].

4.4.3. Ellipsometry Ellipsometry is a technique for measuring film thickness [223–228]. It measures the change in polarization, as light reflects or transmits from a surface. The polarization change is represented as an amplitude ratio, J, and the phase difference, D. The measured response depends on optical properties and thickness of individual materials. Ellipsometry measurements are typically performed at oblique angles, where the largest changes in polarization occur. The typical range for spectroscopic ellipsometry measurements is 50–75 . With a motorized stage, the instrument can map film thickness as quickly as two points per second on samples as large as 300 mm in diameter. Ellipsometry is typically used for films whose thickness ranges from subnanometers to a few mm. If the material absorbs light, such as an organic film, thickness measurements will be limited to thin, semiopaque layers. This limitation can be overcome by making measurements at wavelengths with lower absorption. For metals that strongly absorb at all wavelengths, the maximum layer for thickness determination is typically about 100 nm. Although ellipsometry is primarily used to determine film thickness and optical constants, it is also applied to characterize composition, crystallinity, roughness, doping concentration, and other material properties associated with a change in optical response. Some of the advantages of ellipsometry are: l l l l l l l

Nondestructive technique Highly accurate and reproducible Can be used in any transparent and semi-transparent medium No reference material is necessary Very sensitive, especially for ultrathin films (<10 nm) Simultaneous multiple parameter determination Measures data at the wavelength of interest

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TABLE 3.7 Oil Contamination on 304 Stainless Steel Surface with and Without an Oxide Layer [229] No oxide layer

3 nm oxide layer

Contamination thickness, nm

D, deg

j, deg

D, deg

j, deg

0.00

103.07

30.12

94.64

31.16

1.00

101.25

30.45

93.03

31.49

2.00

99.49

30.78

91.49

31.80

3.00

97.79

31.10

90.00

31.11

4.00

96.13

31.42

88.56

32.41

5.00

94.53

31.74

87.15

32.70

10.00

87.20

33.22

80.75

34.10

15.00

80.92

34.58

75.26

35.36

20.00

75.56

35.80

70.58

36.49

Commercially produced surfaces, such as stainless steels, have a thin oxide layer remaining even after chemical or mechanical polishing. Ellipsometry has been effectively employed to determine oil contamination on commercial stainless steel surfaces [229]; evaluation of cleaning processes for copper interconnects [230]; surface cleanliness of single crystals such as Si, Ge, Pd, GaAs, and HgCdTe [231–235]; monomolecular thick layers of volatile contaminants adsorbed on gold contacts [236]; and investigation of metal corrosion layers [237–240]. As an example, Table 3.7 shows the changes in J and D due to oil contamination on a commercial 304 stainless steel surface with a nominal 3 nm oxide layer compared with a surface with no oxide layer [229]. The changes in J and D represent the sensitivity for detecting oil contamination. For a thickness of 5 nm, the values are 1.49 nm1 and 0.31 nm1, respectively, compared with 1.71 nm1 and 0.32 nm1 for the surface with no oxide layer. Changes in J and D can be detected to better than 0.01 in commercial instruments, making it possible to detect less than monolayer coverage of contamination on the surface.

4.5. Mass Spectrometry Mass spectrometry (MS) is an off-line analytical technique used for chemical identification and determination of elemental concentrations of surface contaminants. It is particularly well suited for trace and ultratrace analysis due to

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its very high sensitivity and low detection limits and the ability to analyze very small quantities of a sample. The basic principle of MS involves ion generation from the contaminants in the sample, which are separated by their mass-tocharge ratio in a mass separator and detection of separated ions in the detector. There are a wide variety of MS techniques for analysis that differ primarily in their ion sources, the method of ionization, and the instrumentation used for application of the technique [241–243]. These techniques include inductivelycoupled MS (ICP-MS), laser-assisted ICP-MS, spark source MS (SSMS), laser ionization MS (LIMS), secondary ion MS (SIMS), glow discharge MS (GDMS), matrix-assisted laser desorption/ionization MS (MALDI-MS), electrospray ionization MS (ESI-MS), laser microprobe MS (LMMS), and sputtered neutral MS (SNMS). Of these techniques, SIMS is most commonly employed for surface analysis, as well as for depth profiling to track ion-implanted species and trace contaminants that may have diffused below the surface. SIMS modes of operation relevant to surface contamination analysis are static SIMS for identification of organic and inorganic species; dynamic SIMS for depth profiling; and time of flight SIMS (TOF-SIMS) to obtain molecule-specific chemical information from surfaces with molecular layer surface sensitivity. Increased lateral resolution has also been possible with a new SIMS microprobe. With its high sensitivity at high mass resolution (no mass interference), the NanoSIMS instrument allows trace element imaging and quantification with 50 nm SIMS lateral resolution, even in electrically insulating materials [244]. Figure 3.11 shows high-sensitivity depth profiling of

FIGURE 3.11 High-sensitivity depth profiling of phosphorus on a silicon substrate. A sputter rate higher than 0.5 mm/min and a detection limit of 5  1013 at/ cm3 (1 ppb) were achieved [246].

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phosphorous implants in silicon. High mass resolution is used to separate 30SiH from 31P. NanoSIMS imaging has been used for measurements of elemental distributions in aerosol particles [245,246]. Laser microprobe mass spectrometry (LMMS) has wide applications in the microanalyses of various complex objects. Among the obvious virtues of LMMS are its versatility, its high spatial resolution, and the possibility for obtaining not only elemental, but also structural information from trace metals and organic and inorganic compounds. However, like all other MS techniques, it requires the contaminants to be exposed to a vacuum, thereby affecting the composition of the sample; the molecular structure of the stable compounds is destroyed under normal conditions by the action of the high power laser irradiance. This can be avoided by the use of condensed matter, such as xerogels [247,248]. The application of xerogel matrices makes it possible to perform complex examinations of samples. One drawback of MS is that there is a significant time delay in returning analysis data because the samples must be analyzed in a remote laboratory. A new innovative cluster beam SIMS technique has been developed that uses liquid metal ion sources that significantly increase the secondary ion yield of specific molecules [25]. This has resulted in increased resolution of the SIMS technique for sub-micrometer imaging and even three-dimensional molecular depth profiling of surface, as discussed in Chapter 5 in this volume [25]. There has been a growing trend in miniaturization of mass spectrometers with obvious advantages of portability and ease of use in space, environmental, clinical, and other demanding applications [249–251]. Mass spectrometry and its wide range of applications for surface analysis are described in recent publications [22,241–243]. The application of SIMS for characterizing surface contaminants has been recently reviewed [222].

4.6. Spectroscopic Methods Spectroscopic methods are a versatile means for determining the structure of surface contaminants. Some of the more common techniques are discussed here.

4.6.1. Ultraviolet (UV) Spectroscopy This method has been used to measure flux residue left on printed circuit boards in the electronics industry and has also been adapted to detect oils and greases on metal parts [13,18,20]. This method requires the use of extraction equipment and a UV spectrometer, which are moderately expensive. In addition, the method requires that the contaminant to be analyzed has a unique absorption wavelength that can be identified in the ultraviolet spectrum. A calibration curve is created by measuring samples of the solvent containing known concentrations of the contaminant at the unique wavelength. The method is only usable in the concentration ranges where the calibration curve is straight.

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Parts that are to be analyzed are extracted in a known amount of solvent to remove any of the contaminant. Typically, agitation or sonication is required during the extraction, which must be done in the same manner for each sample for the results to be meaningful. The solvent extract is analyzed in the UV spectrometer at the unique wavelength. The absorbance is compared to the calibration curve to find the concentration of the contaminant in the extraction solvent. Based on the total volume of solvent and this concentration, the actual amount of contamination is derived. This method must be conducted in a laboratory and requires a skilled operator.

4.6.2. Fourier Transform Infrared Spectroscopy Fourier transform infrared (FTIR) spectroscopy provides specific information about chemical bonding and molecular structures of organic compounds and some inorganic materials. It is especially useful for identification of unknown compounds when reference IR spectra are available. When exposed to infrared radiation, chemical bonds vibrate at characteristic frequencies in the IR regime, and they absorb the radiation at frequencies that match their vibration modes. By measuring the radiation absorbed as a function of frequency, a spectrum is obtained that can be used to identify functional groups and compounds. FTIR is a nondestructive technique capable of identifying organic functional groups and often specific organic compounds and extensive spectral libraries are available for compound identification. The technique is suitable for volatile contaminants since it does not require vacuum conditions. FTIR is not very suitable for inorganic contaminants, primarily due to the lack of available spectral libraries for inorganic compounds. It is not very surface sensitive. The detection limits are 0.1 to 1 weight percent. The minimum analysis area is ~15 mm  15 mm. IR does not provide a quantitative measure of functional group concentrations, and peaks are often hard to distinguish from background features. IR spectroscopy also lacks the sensitivity to detect some functional groups whose presence has been identified by other analytical techniques. FTIR has been discussed extensively [12–20,23]. 4.6.3. Raman Spectroscopy Raman spectroscopy is similar to FTIR in that it also measures molecular vibrations to determine the chemical structure of a sample and identify the compounds present. However, the method yields better spatial resolution and enables the analysis of smaller samples. In the laser Raman technique, a beam of monoenergetic visible light is focused onto the sample through the objective lens of an optical microscope. The spectrum of the inelastic (energy shifted) scattered light arising from the normal linear Stokes–Raman frequency shifts is used for analysis. The Raman frequency shifts generated

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FIGURE 3.12 Raman image of polystyrene (PS) beads demonstrates the spatial resolution of the technique at sub-micrometer scale. Courtesy of Horiba, Japan.

are characteristic of the molecular vibrational and rotational modes of the sample and are independent of the exciting wavelength. Thus, the Raman spectra provide a molecular fingerprint for identification and characterization of the sample. In fact, Raman spectroscopy can be used for surface imaging to map the distribution of contaminants with sub-mm spatial resolution (Fig. 3.12). Raman spectroscopy is a useful technique for qualitative analysis of organic molecules, polymers, biomolecules, and/or inorganic mixed materials in the bulk and in individual particles, and it can also be used for semi-quantitative and quantitative analysis. Recent developments in Raman spectroscopy are discussed in Chapter 5 in this volume [25].

4.6.4. Fluorescence Spectroscopy Fluorescence spectroscopy is based on a light-emitting process that is triggered by the absorption of the excited radiation of an appropriate wavelength that is one of the most efficient interactions between light and substance. In practice, a radiation source, such as UV or laser radiation, is directed on the surface of interest. The radiation from the surface is detected and a signal is generated. The signal is compared to a standard that has been cleaned to an acceptable cleanliness level. The spectral distribution of intensity of fluorescence emitted by a molecule or an atom immediately after optical excitation is specific to the substance even in very low concentrations. In order to achieve high spatial resolution, the measuring head of the instrument can be manually or automatically focused on any number of locations on the part to record the spectra, thus creating a contamination map. Fluorescence spectroscopy is most commonly employed in the biomedical field for assessing dermal exposure to contaminants, as well as contamination in healthcare institutions and surface cleanliness measurements in metalworking and other industrial applications [252–258]. It is a fast, noninvasive technique that can be used for online or offline contamination identification and analysis.

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4.6.5. Laser-Induced Breakdown Spectroscopy LIBS is a minimally destructive, atomic emission spectroscopic technique in which successive nanosecond laser pulses ablate a small amount of material from the surface of interest [259–263]. The resulting microplasma is used to interrogate a material (solid, liquid, or gas). Excited gas phase atoms, ions, and molecular fragments formed within the plasma emit fluorescent radiation that is characteristic of the material being sampled. If the same point on the surface is ablated successively, the LIBS spectra can be collected to provide in-situ cross-sectional analysis of the material. Raman microscopy can achieve very high spatial resolution (~1 mm) and gives spectra largely free of contamination from surrounding material. By combining these techniques, insitu elemental analytical data and chemical composition of the contaminant of interest can be obtained. The combined LIBS–Raman technique has been used for identification and analysis of contaminants in pigment in artworks [264,265]. For example, the original white paint on a Byzantine icon was analyzed to be a hydrated lead carbonate (2PbCO3.Pb(OH)2), but that the subsequent restorative work used zinc oxide paint [265]. LIBS can be combined with laser cleaning applications or other conventional cleaning methods for monitoring and controlling the inspection and cleaning process for culturally relevant structures and artwork [266–270]. LIBS analyses have been performed at longer distances (50–100 m) for various applications, such as hazardous material detection [76] and industrial monitoring [271]. The LIBS technique can be used to obtain information on the size, number density, mass and composition of a wide range of contaminant particles, including metal hydrides, coal particles, halons, pigments, and single elements, such as As, Be, Cd, F, Fe, Mn, Ni, Pb, and Hg [272]. Particles as small as 150 nm have been analyzed, corresponding to an absolute detectable mass of 1015 g. This shows the very high sensitivity of the LIBS technique for particle characterization. LIBS features several key advantages for the analysis of surface contaminants in industrial applications, as well as on cultural heritage objects and artworks. The analysis can be performed in-situ and only requires an optical contact with the object. Its ability to detect light elements, such as Be, and having very low detection limits for many elements is an advantage over other in-situ techniques, such as X-ray fluorescence. The technique does not require sampling, and little or no sample preparation. Furthermore, LIBS is a very rapid technique (limited by readout speed of the detector), as the information is recorded with a single laser pulse measurement. The technique is very slightly destructive, as the material ablated from the surface is minimal. In addition, LIBS allows depth profiling of a surface by applying successive laser pulses on the same spot. The equipment required to perform LIBS is technologically mature, and a variety of commercial systems are available. The equipment can be easily ruggedized. A disadvantage of LIBS for in-situ measurements is that

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the surface concentration may be below the instrument detection limit. This may be a serious issue because of the small area sampled by each laser pulse (<1 mm2). More recently, LIBS has been used to analyze airborne particles collected on filters and biological substances and elements collected on wipers as contamination wiped from various surfaces [73,75,76]. An advantage of wipe samples rather than direct analysis of the surface contaminant in-situ lies in a reduction in spectral interferences from the underlying substrate materials, such as painted and bare metal surfaces, wood, and plastic. Wipers are typically composed mainly of the elements C, O, N, and H, which have few atomic emission lines in an LIBS spectrum. LIBS detection limits, determined for the elements Ag, As, Ba, Be, Cd, Cr, Cu, Hg, Mn, Ni, Pb, Sr, and Zn on wipes (2 cm2 area), ranged from 0.002 mg for Be to 1.46 mg for Pb [80]. Another recent innovation has been to combine LIBS with SNOM to map and correlate elemental chemical compositions of surfaces with the surface topography [273–276]. In this method, surface topography is mapped by scanning the surface with a SNOM probe. The probe is then positioned over the feature of interest, such as a contaminant particle, that is ablated with a normal or ultrafast laser pulse. The LIBS spectrum of the feature is obtained from the optical emissions resulting from the microplasma plume.

4.7. Bulk Analytical Techniques Several analytical techniques are used for bulk analysis of contaminants. The more common ones are discussed here.

4.7.1. Ion Chromatography Ion chromatography separates, identifies, and quantifies ions. The analysis begins with a sample, typically a water medium containing ions of interest, but it can also be a chemical medium, such as isopropyl alcohol. A portion of the matrix is injected into the system and combined with an eluent stream that carries the sample to the analytical column. The analytical column separates the ions of interest in the sample into narrow bands within the stream of the eluent. The eluent then sweeps these groups of ions into a suppressor device, which electrolytically transforms the eluent into pure water, leaving just the ions of interest in pure water to be swept downstream to the conductivity detector. The detector indentifies the ions based on their conductivity relative to the water eluent. At this point, all interfering ions have been removed and the sensitivity of the detector has been maximized, allowing for detection of very low partsper-billion and parts-per-trillion levels of ions. Ion chromatography for cleanliness verification has been reviewed previously [277].

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4.7.2. Electrochemical Method Recently, a field method for wipe testing has been developed that uses an electrochemical technique for quantification of polychloride biphenyls (PCBs) in transformer oils, soils, water, and surface wipers [278]. The wipe sampling procedure is the same as for other analysis methods, but the analytical test uses a sodium reaction to remove chlorine from the PCBs, which is then quantified using a chloride-specific electrode instead of a gas chromatograph. The instrument to perform this test, called the L2000 PCB/Chloride Analyzer, uses an ion-specific electrode to quantify the contaminants in the sample. The usable measurement range is 2–2000 ppm for oils and soils, 20 ppb–2000 ppm for water, and 2–2000 mg/100 cm2 for wiper samples. An oil sample requires about five minutes for analysis, while water, soil, and surface tests take about ten minutes each. The rate of throughput can be as high as 10 wiper samples per hour, when sample preparation and analyses are done indoors [279]. The advantage to this method of surface testing over traditional techniques is that the analysis may be performed on-site without sending a sample to the laboratory. The method has some disadvantages: l

l

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l l l

Limited usefulness when PCB concentrations in the soil are less than 10 ppm; Inability to distinguish PCB contamination from other chlorinated organic compounds; Sensitivity to changes in temperature require frequent recalibration of the instrument; Outdoor use in cold weather may reduce the reliability of the results; Wet soil samples, if not properly handled, will yield unreliable results; Known or potential interferences include: other chlorinated organic compounds that are preferentially soluble in a nonpolar solvent; iodine; and bromine.

4.7.3. Radioactive Tracers Several methods are available that utilize radioactive tracers for monitoring surface cleanliness [280–289]. In fact, the original RCA cleaning method [290] was developed from the tracer technique by deliberately contaminating etchant and cleaning solutions with various isotopes, including 198Au, 64Cu, 59Fe, 51Cr, 65 Zn, 122Sb, 124Sb, 54Mn, and 99Mo [281]. Most of these methods involve the use of a specific radioactive isotope as a tracer in the surface-extracting solution and either the surface of the part or the cleaning solution is monitored for residual tracer activity. Gamma and beta radiation can be readily measured by conventional techniques, including gamma-ray spectrometry and scintillation counting, while gas proportional particle counters and solid-state detectors can be used for alpha radiation [291,292]. The tracer technique offers high reliability, high-throughput sampling and analysis, easy operation, and no interference from stable isotopes. The technique has been used for monitoring trace

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metal contamination in semiconductor and optical device processing [284–288]; platinum surface contamination in hospital pharmacies [289]; testing of medical personnel’s exposure to cytotoxic drugs from spill and leakage in drug preparation systems (technetium test method) [293,294]; alpha particle emission from Sn-based alloys’ solder bumps and hafnium in high-k gate materials [292]; and other industrial applications, such as automobile engine surface wear and leak detection. The radioactive tracer technique can also be used as a direct test for the surface cleanliness measurement method. As discussed in Section 4.2.9, the MESERAN technique employs 14C as a volatile tracer that is added to the surface and the rate of evaporation measured. The resulting MESERAN number is an indicator of the degree of contamination present on the surface.

4.7.4. X-ray Diffraction X-ray diffraction (XRD) is a powerful nondestructive technique for characterizing crystalline materials. It provides information on structures, phases, preferred crystal orientations (texture), and other structural parameters, such as average grain size, crystallinity, strain, and crystal defects. X-ray diffraction peaks are produced by constructive interference of a monochromatic beam of X-rays scattered at specific angles from each set of lattice planes in a sample. The peak intensities are determined by the atomic positions within the lattice planes. Consequently, the X-ray diffraction pattern is the fingerprint of periodic atomic arrangements in a given material. An online search of a standard database for X-ray powder diffraction patterns enables quick phase identification for a large variety of crystalline samples [191]. 4.7.5. X-ray Fluorescence X-ray fluorescence (XRF) is a nondestructive analytical technique that allows qualitative and quantitative characterization of solids, liquids, and powders [295,296]. It uses X-ray excitation to provide inorganic elemental analysis from Na up to Bk. Micro-XRF combines these properties with microscopic analysis, so that individual particles down to 10 mm and surface features can be analyzed, and elemental distribution images can be generated. Instruments equipped with a CCD camera and a high-brightness micro-X-ray beam with diameters ranging from 1.2 mm down to 10 mm can accurately direct the beam and irradiate a very small area of a sample for microanalysis. Automated sample scanning provides detailed images of elemental distribution over areas as large as 10 cm  10 cm, which is not achievable with other techniques. Optical and elemental views can be seamlessly merged, with the shortest possible acquisition times. As an example, Fig. 3.13a shows a white particle (~5 mm) in a factory producing resin that was identified as a titanium species using micro-XRF (Fig. 13b) and confirmed as TiO2 by complementary analysis with a Raman microscope (Fig. 13c) [296].

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FIGURE 3.13 White contaminant particle found in a factory producing resin identified as a titanium species by micro-XRF, and TiO2 by Raman microscopy. (a) Optical image. (b) Micro-XRF image. (c) Raman spectrum of the particle and reference TiO2 spectrum. Courtesy of Horiba, Japan.

The use of an electron beam for SEM-EDS analysis means the technique is suitable for surface analysis only. By contrast, XRF benefits from the penetrating nature of the primary X-ray beam, allowing visualization and characterization of nonvisible features, and higher detection limits compared with SEM-EDS since more atoms are analyzed. One commercial instrument offers the capability for analysis of even large samples in full vacuum or localized vacuum modes [296]. The full vacuum mode offers the highest sensitivity to light elements. It provides optimized analysis conditions for pharmaceutical tablets, mineral sections, and light element alloys. With the localized vacuum

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mode, it is possible to analyze biological materials and other samples that cannot be subjected to a full vacuum. XRF is generally employed where simple sample preparation and nondestructive measurements are requirements for analysis. Micro-XRF is used to provide qualitative, quantitative, and elemental imaging analysis in a wide variety of applications including forensics, industrial quality control, failure analysis, in-situ (with a portable system) and off-line contaminant analysis, historical artifact authentication, restriction of use of hazardous substances (RoHS) compliance, electronics, and coating thickness measurements. Recently, micro-XRF has been combined with micro-computed tomography (micro-CT) to extend the capabilities of nondestructive threedimensional (3D) imaging by adding true 3D chemical analysis capabilities to high-resolution micro-tomography [297]. The instrument combines a micro-CT scanner, which provides high-resolution morphological information and absorption-corrected maps for chemical analysis and a full-field 3D microXRF scanner for reconstruction of 3D chemical composition inside the sample with a 3D spatial resolution down to 13 mm. All chemical elements from chlorine to uranium can be detected. Figure 3.14 shows a micro-CT scan of a filter with dust particles, which are clearly visible.

FIGURE 3.14 Micro-CT scan of a used filter from a vacuum cleaner with dust particles. (a) SEM secondary electron image. (b) X-ray image through the sample; (c) 3D rendered front view with the particles shown in red; (d) semi-transparent 3D rendered side view. Courtesy of SKYSCAN, Kontich, Belgium.

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As noted earlier, portable XRF instruments are commercially available for elemental field analysis for contaminants [74,77–79,298]. One disadvantage of portable XRF is that it cannot be employed effectively when analyte concentrations are below parts-per-million levels. It also suffers from the lack of availability of standards to produce calibration curves. Another portable technique that has been recently developed for field elemental analysis applications is tungsten coil atomic absorption spectrometry [299]. Detection limits of this technique are in the 0.04–1500 mg/L range. 4.7.5.1. Total Reflection X-ray Fluorescence A special XRF technique is total reflection XRF (TXRF) that is used for trace metal analysis on Si wafers in the semiconductor industry [300,301]. It utilizes extremely low-angle X-ray excitation of a polished sample surface. The incident angle of the X-ray beam is below the critical angle for the substrate and limits excitation to the outermost surface layers of the sample, making TXRF a highly surface-sensitive technique. Due to its unique configuration, the main advantage of TXRF over conventional XRF is reduced measurement background contributions by elimination of sample scattering, resulting in increased elemental measurement sensitivity. The sampling depth for conventional TXRF is 3–8 nm, with a detection limit of 109–1012 at/cm2. However, this technique cannot detect low-Z elements, such as Li, Na, and Al. Recently, sweeping TXRF has been developed in which hundreds of point measurements are made on a whole wafer surface within 30 min, as compared to a few point measurements with standard TXRF [300,301]. The advantage is that spatial information on the distribution and concentration of the contaminants on the surface can be mapped. The mapping capability facilitates contamination control when there is a nonuniform distribution of metallic contamination across the surface of the wafer. Also, using an iridium anode instead of a tungsten anode TXRF has been successfully applied to analyze new high-k semiconductor materials, such as hafnium-based films. Figure 3.15 shows a wide view of Ir–La spectrum for hafnium silicate contaminated with 5  1011 atoms of Ti, Cr, Fe, Ni, and Cu. The Hf–La and the Ka peaks of Ti, Cr, and Fe are observed clearly without interference. The iridium source can give low detection limits around 1  1010 at/cm2 for the surface metal analysis.

4.7.6. Other Bulk Analysis Methods Other well-established bulk analysis methods include chromatographic techniques (gas chromatography and liquid chromatography) that are extensively employed for contaminant identification and quantification in biomedical and environmental applications [255]. For analysis of inorganic contaminants, atomic absorption spectroscopy and inductively coupled plasma spectrometry are most commonly used. The principles, operations, and applications of these techniques have been described extensively [13].

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FIGURE 3.15 Hafnium silicate spectrum with Ir–La source irradiation. The sample was a 2 nm hafnium silicate film intentionally contaminated with 5  1011 atoms of Ti, Cr, Fe, Ni, and Cu [300].

4.8. Overview of Surface Cleanliness Measurement Methods Surface contamination is often a combination of different types of contaminants, depending on the origin of the surface. For example, sterile surgical instruments may be contaminated by biological substances, such as protein residues, due to ineffective cleaning processes, as well as dust particles from inadequate storage of the cleaned instruments. Cytotoxic platinum compounds can leave trace metal contamination on various surfaces in hospitals during drug preparation and delivery. Contaminant particles, AMCs, and ionic contamination are of concern in microelectronics processing. Workplace dermal exposure is a health concern in many applications where contaminant particles, volatile compounds, and biological substances may be present. These contaminants are present in different amounts from gross to trace amounts and can range in size from microscale to nanoscale. No single method is sufficient to characterize all contaminants, and a combination of methods is required for this purpose and for continued monitoring of the cleanliness of the surface. The applications of the characterization methods discussed in this chapter are broadly summarized for the key types of contaminants. Sampling of the contaminants for analysis is performed by the methods discussed in Section 3 and in Chapter 1. l

Particles l Counting and sizing: visual examination, microscopy, optical particle counters, and size classification techniques; l Elemental analysis: electron microscopy with analytical detectors; l Bulk analysis by spectroscopic techniques, mass spectrometry, X-ray diffraction, and fluorescence;

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Imaging: optical and electron microscopy and surface analytical techniques. Films l Thickness: ellipsometry, X-ray techniques, and surface analytical techniques; l Composition: carbon coulometry, surface analytical techniques, and mass spectrometry; l Surface deposition: surface acoustic wave, quartz crystal microbalance, gravimetric method, optically stimulated electron emission, and MESERAN. Ionic l Composition: ion chromatography. Metals l Distribution (mapping): surface potential difference, X-ray fluorescence, and surface analytical techniques; l Elemental analysis: electron microscopy with analytical detectors and surface analytical techniques; l Bulk analysis: mass spectrometry, spectroscopy, X-ray diffraction, and fluorescence. Organic l Distribution (mapping): spectroscopic techniques; l Composition: spectroscopic techniques, mass spectrometry, and chromatography. Biological contaminants l Counting and sizing: microscopy; l Identification [302–309]: visual examination with dyes (e.g., CoomassieÒ, Amido black 10B), ATP (adenosine triphosphate) bioluminescence, microscopy, spectroscopy with fluorescent dyes (e.g., ruthenium-based stain SYPRO Ruby, silver stains, DAPI (40 ,6-diamidino-2-phenylindole dihydrochloride), and dansyl polymyxin), Raman spectroscopy, MALDI-TOF-MS, and radioactive tracers. l

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5. SUMMARY There are a large number of direct and indirect methods for measuring and monitoring the cleanliness of surfaces for various applications. These methods range from direct visual examination of the surface of interest to extracting the contaminants from the surface for off-line analysis by one or more analytical techniques. Recent developments in the more common cleanliness assessment methods are discussed in this chapter. With the current capabilities of these cleanliness assessment methods, it is possible to characterize surface cleanliness from the macroscale to the nanoscale for the major types of contaminants, namely, particles, thin films or molecular contamination, ionic contamination, and biological contaminants.

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DISCLAIMER Mention of commercial products in this chapter is for information only and does not imply recommendation or endorsement by The Aerospace Corporation. All trademarks, service marks, and trade names are the property of their respective owners.

ACKNOWLEDGMENT The author is very grateful to Jody Mantell of the University of Houston at Clear Lake for help with locating obscure reference articles.

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