13th International Symposium on Process Systems Engineering (PSE 2018)

Kerul Suthar , ... Q. Peter He , in Computer Aided Chemical Engineering, 2018

4.3 Performance comparison of recursive VM methods

Here the recursive FVM is applied to the dataset. The performance is compared with other recursive VM methods. For recursive VM, the initial VM model is built based on the training data of 784 wafers, and is updated when new data becomes available. It can be seen that the recursive FVM outperforms other recursive VM methods in terms of both MAPE and R 2.

Table 2. Comparison of different recursive VM methods

Model MAPE %change R 2 %change
RPLS 5.67 13.4% 0.70 -9.1%
KF 5.07 1.4% 0.72 -6.5%
FVM 5.00 0% 0.77 0%

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Nanostructured Materials: Metrology

A. Jorio , M.S. Dresselhaus , in Reference Module in Materials Science and Materials Engineering, 2016

1 Definitions for Metrology and Nanometrology

Metrology is "the science of measurement, embracing both experimental and theoretical determinations at any level of uncertainty in any field of science and technology," as defined by the International Bureau of Weights and Measures ( BIPM, 2004). Metrology can be divided into three subfields: scientific metrology, applied metrology, and legal metrology. Legal metrology is the end of the line, concerning regulatory requirements of well established measurements and measuring instruments for the protection of consumers and fair trade. In applied metrology, the measurement science is developed toward manufacturing and other processes, ensuring the suitability of measurement instruments, their calibration, and quality control. Scientific metrology is the basis of all subfields, and concerns the development of new measurement methods, the realization of measurement standards, and the transfer of these standards to users. The metrology activity is coordinated by national laboratories, such as the National Institute of Standards and Technology (NIST, USA) and the National Institute of Metrology, Quality and Technology (Inmetro, Brazil), which are internationally coordinated by the BIPM. In parallel, standardization is coordinated by the International Organization for Standardization (ISO), together with other organizations like the Versailles Project on Advanced Materials and Standards (VAMAS), whose main objective is to support trade in high-technology products, through international collaborative projects aimed at providing the technical basis for drafting codes of practice and specifications for advanced materials.

The growing interest in applying nanomaterials to societal needs is now urging that increasing attention be given to the development of scientific and applied metrology to address nanomaterials as the newly developing field of nanometrology. This multidisciplinary field spans many disciplinary fields, such as chemistry, physics, materials science, biology, engineering, and nanoscience. Nanomaterials embrace the full range of traditional materials classes. The distinction between metrology in general and metrology on the nanoscale stems from the different properties of materials on the nanoscale as compared to their bulk counterparts. The Technical Committee for Nanotechnologies Standardization (TC-229) of ISO defines the field of nanotechnologies as the application of scientific knowledge to (1) understanding and control of matter and processes at the nanoscale, typically, but not exclusively, below 100 nanometers in one or more dimensions where the onset of size-dependent phenomena usually enables novel applications; (2) utilizing the properties of nanoscale materials that differ from the properties of individual atoms, molecules, and bulk matter, to create improved materials, devices, and systems that exploit these new properties (ISO, 2005). Specific tasks include developing standards for terminology and nomenclature; metrology and instrumentation, including specifications for standard reference materials; test methodologies; modeling and simulations; and science-based health, safety, and environmental practices. However, the fundamental aspects for the development of protocols and standards in nanomaterials, i.e., for building the basis for nanometrology, are still under construction.

The International System of Units (SI from the French Système International) is an evolving system, related to the physical understanding of nature, changing in accordance with advances in science and technology (Valdez, 2005). Today's SI is based on seven units: length (m), mass (kg), time (s), electric current (A), thermodynamic temperature (K), amount of substance (mol), and luminous intensity (cd), all the other units being derived from these. A fundamental goal of metrology is that different institutions should be able to calibrate these basic units, obtaining the same values within the same uncertainty. The historical way of doing that has been by using standard materials. The methodology today is trying to define the SI units based on fundamental constants. The meter convention was signed in 1875 as the distance between two lines made on a platinum–iridium prototype. The present definition dates from 1983: "The meter is the length of the path traveled by light in vacuum during a time interval of 1/299,792,458 of a second." This definition actually fixes the speed of light in vacuum. The ampere is defined as "the constant current, which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 meter apart in vacuum, would produce between these conductors a force equal to 2×10–7  N." This definition sets the permeability of vacuum at 4p×10–7  Hm-1. The definition of mass, however, still remains as the one adopted in 1901: "The kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram." The platinum–iridium international prototype is maintained at the BIPM (Paris). Considerable efforts are being made to define the kilogram in terms of fundamental constants, linking the kilogram to the Planck constant, the Avogadro constant, or the mass of an atom of 12C. The definition of the mole dates from 1971: "The mole is the amount of substance of a system that contains as many elementary entities as there are atoms in 0.012 kilogram of 12C." This definition refers to unbound atoms. As a consequence of the differences in binding energy, 0.012   kg of graphite has about 4×1014 more 12C atoms than the same mass in the gas phase. A given mass of diamond at room temperature contains about 1012 fewer atoms than the same mass of graphite (Valdez, 2005).

New ideas need new measurements and this is where the novel class of materials, the nanomaterials, is playing an important role. From the International Vocabulary of Basic and General Terms in Metrology (ISO 1993): "Measurement is the set of operations having the object of determining a value of a quantity." 'To measure' means 'to compare,' by experiment, the unknown value of a quantity with an appropriate standard unit, adopted by convention. The problem does not end with the definition of the meter. Length standards at different levels (from atomic to macroscopic distances) are needed to ensure traceability along all scales. The measurement accuracy is limited by instrumental uncertainty, such as counting electrons and missing one count by co-tunneling, and by the Heisenberg uncertainty principle. Nanotechnology opens new paths for achieving quantum limited sensitivity.

In this article we focus on scientific metrology to indicate some conceptual pathways for constructing the basis for applied and legal metrologies. Being impossible to give a broad coverage of all classes of nanomaterials, which include metals, ceramics, polymers, etc., here we use sp2 carbon nanostructures as an illustrative system for the development of nanometrology, with a focus on carbon nanotubes and graphene, as prototypes for one- and two-dimensional nanostructures. This is not the personal choice of the authors, but a generalized concept, and is the one adopted by the ISO-TC229. The sp2 carbon nanostructures are stable enough for manipulation, simple enough (just sp2 bonded carbon atoms) for modeling, and have been at the forefront of nanoscience and nanotechnology (Jorio et al., 2008; Novoselov et al., 2012). Since the properties of nanomaterials are strongly size dependent and are largely still in the discovery stage, the development of the science of nanometrology is especially challenging and rapidly evolving. Both the measurement process and the environment of a nanomaterial often perturb the properties of the nanosystem, and therefore establishing robust measurement protocols is also very challenging.

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Micro metal injection molding (MicroMIM)

V. Piotter , in Handbook of Metal Injection Molding, 2012

13.4.4 Metrology and handling

Metrology and quality assurance are crucial matters in micro-technology. When measuring the outer dimensions of green bodies and sintered parts one can rely on the measurement systems developed for application in micro-electronic or micro-electro-mechanical systems/micro-opto-electro-mechanical systems (MEMS/MOEMS) fabrication ( Lanza et al., 2008). For example, test stands based on coordinate measuring machine (CMM) units, or even white light interferometry and atomic force microscopy (AFM) are in use for geometrical inspection. These test systems are quite expensive. If an automated system is built up it can represent a useful quality inspection system. Reliable measurement and quality inspection is an important matter for micro-technology. As a result, a lot of research and development approaches are working in this field and no additional efforts are necessary for MicroPIM.

Much more complicated is the 'view into the body' to determine microcracks, cavities, and areas of powder/binder segregation (Hausnerova et al., 2010). This internal inspection has to be carried out rapidly, preferably online, to avoid excessive failure production. Therefore, the classical way of cutting and grinding, and the subsequent optical investigation of the crosssection, is not the most effective method of testing. Alternative methods, such as ultrasonic inspection and/or thermographical testing, show much more promise in terms of performance potential. As they are already under development, and are already in use for macroscopic PIM, they are not described in detail here.

Finally, the two-dimensional and three-dimensional inspection methods based on X-ray irradiation should be mentioned. MicroMIM parts profit by their small thicknesses, meaning that they can be irradiated without thorough energy dissipation and beam widening. A good description of the different approaches and their capabilities can be found in (Jenni et al., 2009). Using monochromatic synchrotron radiation a three-dimensional profile of the powder distribution over a whole MicroMIM sample can be generated and the determination of powder/binder segregation phenomena becomes possible (Heldele et al., 2006). Nevertheless, such investigations are costly and time consuming, meaning that faster and less complex variants have to be derived for industrial applications. More efficient methods have only recently begun to be developed (Albers et al., 2008). It is essential that this development continues as the down-scaling of functional test procedures has the potential to optimize the production of micro-parts and microsystems.

As in macroscopic PIM, handling, automation and the interfaces of the relevant production facilities play an important role in MicroMIM. Existing or soon to be developed tools can be used for MicroMIM on the condition that they are adapted to the desired small dimensions. This is a challenge for the whole micro fabrication world and considerable research and development efforts are underway from which MicroPIM will also benefit.

In the case of singular micro-parts, the precise positioning of gripper to part is essential as tolerances are 1   μm or less. This positioning of gripper to part has to be considered thoroughly during process planning (Freundt et al., 2008). This is also true for automated quality assurance. In the case of MicroPIM, the relatively low green strength, which might cause problems if mechanical grippers are used, has to be considered. Similarly, the higher weights due to the powder loading can be a disadvantage in the case of vacuum grippers. On the other hand, metal-filled components reveal some advantages for handling. For example, unlike plastic ones, they are not charged electrostatically. Thus, MicroMIM has an advantage over polymer micro-injection molding as regards the easier gripping or moving of parts.

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Diamond disc pad conditioning in chemical mechanical polishing

Z.C. Li , ... Q. Zhang , in Advances in Chemical Mechanical Planarization (CMP), 2016

13.3.2 Pad surface evaluation and measurement

Metrology plays a crucial role in enabling any type of CMP process control, and can be implemented in different ways based on the measurement techniques used, its location in the process flow, and the type and amount of data generated. During the CMP cycle, pad characteristics such as the thickness, the Young's modulus, and viscous properties of the pad tend to be dynamic. Therefore measurement of these properties is very important towards understanding polishing nonuniformity and the maintenance of acceptable WIWNU and wafer-to-wafer nonuniformity.

A destructive approach to determining the pad thickness is to measure it directly on a cut-off piece of the pad using a micrometer. Nondestructive tests were developed to monitor polishing pads since the late 1990s. Meikle disclosed methods and apparatus for measuring the change in the thickness of the polishing pad by using a laser beam detector [53–55] whereby pad thickness is measured in situ after a pad conditioning cycle. The measuring device as shown in Figure 13.9 is a laser position sensor or a laser interferometer with an emitter and a detector. A laser beam incident on the polishing pad reflects off the pad surface before and after the change in the pad thickness. The reflected beam is captured by the detector. A disadvantage of this invention is that the thickness data were obtained from the discontinuous points on the pad. As the polishing slurry interferes with the pad surface, it is difficult to determine which data point is valid. Furthermore, the thickness measurement is conducted after pad conditioning and cannot be achieved during the CMP cycle.

Figure 13.9. Laser sensor-based pad-monitoring method (after Ref.[55]).

 Zhang, X.H., Pei, Z.J., and Fisher, G.R., 2007, Measurement methods of pad properties for chemical mechanical polishing, Proceedings of the 2007 ASME International Mechanical Engineering Congress and Exposition (IMECE 2007), Seattle, WA, November 11–15, vol. 3, pp. 517–522.

Another invention using a laser sensor to monitor the pad thickness was reported by Chuang [56] as shown in Figure 13.10. The difference from the previous invention is that the measuring device is disposed on the polishing head (carrier) of the CMP machine monitoring the pad during a CMP cycle. The measuring device comprises a displacement sensor, a laser-emitting device, an interceptor, and a display device. The laser is emitted to the interceptor and reflected to the measuring device. The height of the pad surface (and hence the pad thickness) is detected. This invention achieves the in situ measurement during the CMP cycle.

Figure 13.10. Laser sensor-based pad-monitoring device installed in the polisher (after Ref.[56]).

 Zhang, X.H., Pei, Z.J., and Fisher, G.R., 2007, Measurement methods of pad properties for chemical mechanical polishing, Proceedings of the 2007 ASME International Mechanical Engineering Congress and Exposition (IMECE 2007), Seattle, WA, November 11–15, vol. 3, pp. 517–522.

Hong et al. [57] presented a linear multidimensional scanning device to monitor the polishing pad in a radial direction without overlapping the wafer as shown in Figure 13.11. The scanning device includes two sections. In the first section, it scans a first portion of the polishing pad that is in intermittent contact with the wafer. In the second section, it scans a second portion of the polishing pad that is never in contact with the wafer during the CMP cycle. After scanning the polishing pad surface, the profile is provided to the computer to determine if the pad needs to be replaced. Moreover, the thickness is monitored when CMP is running.

Figure 13.11. Linear multidimensional scanning device for monitoring pad surface (after Ref.[57]).

 Zhang, X.H., Pei, Z.J., and Fisher, G.R., 2007, Measurement methods of pad properties for chemical mechanical polishing, Proceedings of the 2007 ASME International Mechanical Engineering Congress and Exposition (IMECE 2007), Seattle, WA, November 11–15, vol. 3, pp. 517–522.

Nagai et al. [58] used laser focus displacement meter (LFDM, LT-8110 laser sensor head, Keyence Corp.) to monitor the pad surface. The pad condition is observed without contacting the pad surface. The displacement and surface roughness of the pad are monitored in situ by the LFDM.

In addition, Fisher et al. [59] utilized ultrasound or electromagnetic radiation transmitters and receivers to cover any portion of the radial length of a polishing pad surface as shown in Figure 13.12. Signals from a single sensor or multiple sensors have a phase change or time delay compared to the reference signal that is obtained when the pad is new. The change in the pad thickness is measured by correlating it to the phase change (signal traveling distance difference). Every sensor combines a radiation transducer and a radiation receiver.

Figure 13.12. Ultrasound or electromagnetic sensors for monitoring pad surface (after Ref.[59]).

 Zhang, X.H., Pei, Z.J., and Fisher, G.R., 2007, Measurement methods of pad properties for chemical mechanical polishing, Proceedings of the 2007 ASME International Mechanical Engineering Congress and Exposition (IMECE 2007), Seattle, WA, November 11–15, vol. 3, pp. 517–522.

Adebanjo et al. [60] reported another nondestructive but contact method to measure in situ the thickness change of the polishing pad as shown in Figure 13.13. Two rigid planar members are placed on the conditioned and nonconditioned sections of the polishing pad, respectively. Measurements are made using a thickness gauge overhanging the depressed conditioned section by measuring the height difference between the planar members.

Figure 13.13. Contact method monitoring pad thickness change (after Ref.[60]).

 Zhang, X.H., Pei, Z.J., and Fisher, G.R., 2007, Measurement methods of pad properties for chemical mechanical polishing, Proceedings of the 2007 ASME International Mechanical Engineering Congress and Exposition (IMECE 2007), Seattle, WA, November 11–15, vol. 3, pp. 517–522.

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Characterization methods for high temperature polymer blends

L.A. Utracki , in High Temperature Polymer Blends, 2014

2.3 Methods of polymer characterization

Metrology is the science of measurement. Measurement is based on national or international measurement standards, usually set by national metrological institutes such as National Institute of Standards and Technology (NIST), National Physics Laboratory (NPL), BAM, NPL-I, etc. 40 , 41 Five independent units of measure are internationally recognized:

•

temperature interval (Kelvin, K)

•

linear distance (meter, m)

•

electrical current (ampere, A)

•

time (second, s)

•

mass (kilogram, kg).

These SI (Systèeme International) units are fundamental, the others being based on one or more of these. In addition, the SI recognizes candela (cd = luminous intensity) and mole (mol = number of substance elements corresponding to that of 12C atoms in 12   g). Standards are upgraded as scientific knowledge develops; for example, the current definition of meter is the length of the path travelled by light in a vacuum in 1/(299,792,458) s.

The measurement process should comprise:

(1)

sampling the test specimens

(2)

selection of the standard test method

(3)

calibration-traceability of measurements

(4)

statistical evaluation of data

(5)

validation by means of reference material, inter-laboratory comparisons, etc. 42

Since 1920 numerous national standardization organizations (e.g., Australia – AS/NZS, Britain – BSI, France – NF, Germany – DIN, Japan – JIS, Korea – KATS, Switzerland – SN, USA – ASTM) have been created, followed by the international standards bodies: European – EN, and International – ISO and IEC. The international organizations try to harmonize measurement procedures where possible.

Polymers are organic materials and their properties, starting with atomic composition up to performance of their multiphase systems, are well documented. However, specific standard test methods are often lacking. 43 Current test methods may be grouped into three categories, with diverse standard test methods available within each of them. These three categories are:

•

Determination of molecular composition and structure

•

Measurement of material properties

•

Materials performance.

In the category 'Determination of molecular composition and structure', standard test methods are:

•

Chemical composition by chemical analysis, spectrometry [Analytical spectroscopic methods ISO 6955; Fourier Transform Infrared Analysis (FT-IR) ASTM E1252], chromatography, microanalysis, microscopy, etc.

•

Determination of molecular structures on the nano- and micro-scale, using diffractometry, micrography, spectroscopy, scattering and other methods [ASTM 5017 NMR of LLDPE].

•

Surface and interphase characterization by spectroscopy (Auger, X-ray photoelectron, secondary ion mass [Surface chemical analysis ISO 17560; Surface Energy ASTM D5946; Surface Resistivity ASTM D257, IEC60093]), and topographic methods [Surface imperfections ISO 8785]. 44

•

Processability of polymers [Determination of properties of polymeric materials by means of a capillary rheometer ASTM D 3835–02].

In the category 'Measurement of material properties', standard test methods are:

•

Mechanical: elasticity [DIN 16913–2; ISO 6721; ISO 1798 Plastics; Tensile Test Plastics – Microtensile ASTM D1708, Tensile Test Plastics – ASTM D638, ISO 527; Flexural Test ASTM D790 and ISO 178], plasticity [ISO 899 Plastics creep], hardness [ISO 2039 Plastics hardness], strength [ISO 75 Deflection of plastics], fracture mechanics [ISO 13586 Plastics LEFM], impact [Izod Impact (Notched) ASTM D256, ISO 180; Multiaxial Impact (Dynatup) ASTM D3763, ISO 6603, 7765].

•

Thermal: conductivity and specific heat [ASTM D2326 cellular plastics], enthalpy, expansion and compressibility [Thermal Expansion ASTM E831, ISO 11359 ASTM D696; D864 Plastics expansion; Compression Set ASTM D395; Compression Test ASTM D695, ISO 604], thermogravimetry [Compositional Analysis by TGA ASTM E1131, ISO 11358], dimensional stability [Dimensional Stability ASTM D1204]

•

Electrical: conductivity and resistivity [ISO 21318 for plastics; Volume Resistivity ASTM D257, IEC 60093], dielectric [Dielectric Constant/Dissipation Factor ASTM D150, IEC 60250; DIN 53483; EN 60811 Insulating materials; Dielectric Strength ASTM D149, IEC 60243], etc.

•

Magnetic [EN 62044 Measurement methods].

•

Optical: optical sensing, fiber optics, non-linear optics, optical measurements [ISO 3146 Plastics measurements in polarized microscope].

In the category 'Materials performance', standard test methods are:

•

Chemical aging, weathering and stabilization [Thermal Stability ASTM D3835, ISO 11443; Artificial weathering ISO 29664; QUV Accelerated Weathering ASTM D4329, D4587, ISO 4892, SAE J2020; Loss of plasticizers ISO 176, ISO 177], oxidation [Thermo-oxidative Stability ASTM D3012, GM9509P], hydrolysis [Water Absorption ASTM D570], accelerated aging [ISO 188 Accelerated Aging], flammability [ASTM D635, ISO 3795, 49CFR-571–302; Oxygen Index ASTM D2863].

•

Physical aging [ISO 291:2008 Plastics – Standard atmospheres for conditioning and testing; ISO 877].

•

Biogenic effects [Fungus resistance JIS Z 2911PP and PE degradation ASTM D3826; Antibacterial activity ISO 22196; Aerobic degradation of plastics ASTM D 5271, ASTM D 5511, ASTM D 5512, JIS K 6950, JIS K 6951, JIS K 6953].

•

Environmental impact [Plastics exposed to solar radiation, heat and light ISO 2578 and ISO 4892, ASTM D 5272].

•

Performance monitoring and control [Environmental stress cracking (ESC) ISO 22088; Non-destructive testing by ultrasound DIN EN 12668, ISO 10375].

Evidently, within each of the three test categories there are also standard procedures for testing specific polymers (e.g., PE [ASTM D3035], PP [ISO 1972; ISO 1873] or PVC [ASTM D3036]), or methods for carrying out specific types of test, e.g., Microbeam Analysis (MA):

•

ISO 22309:2006 MA – Quantitative Analysis using EDS

•

ISO 16700:2004 MA – Scanning Electron Microscopy (SEM) – Guidelines for Calibrating

•

ISO 15632:2002 MA – Specification for Energy Dispersive X-ray Spectrometers

•

ISO 14595:2003 MA – Electron Probe Microanalysis – Guidelines for the Specification of Certified Reference Materials (CRMs)

•

ISO 14594:2003 MA – Electron Probe Microanalysis – Guidelines for the Determination of Experimental Parameters for Wavelength Dispersive Spectroscopy

•

ISO/WD2 24173 MA – Guidelines for Electron Backscattered Diffraction Analysis.

Short descriptions of most of the aforementioned standards can be found on the internet. 45

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Volume 4

Rudolf W. Kessler , Waltraud Kessler , in Comprehensive Chemometrics (Second Edition), 2020

4.10.4.5.2 Critical assessment of primary, secondary and working standards in industry

In metrology (the science of measurement), a standard is an object, system, or experiment that bears a defined relationship to a unit of measurement of a physical quantity. There is a three-level hierarchy of physical measurement standards. At the top of the tree are the master standards—these are known as primary standards. Primary standards are made to the highest metrological quality and are the definitive definition or realization of their unit of measure. Secondary standards are calibrated with reference to a primary standard. The third level of standard, a standard which is periodically calibrated against a secondary standard, is known as a working standard. Working standards are used for the calibration of commercial and industrial measurement equipment.

In industrial PAT applications, often working standards are adopted from primary and secondary standards which may describe a more complex functionality of the material rather than a defined standard measurement unit. In most cases it describes a bulk property of the material rather than a surface property, e.g. moisture, strength, etc. However, as often used for many years, these target values are key parameters which should be measured inline in the future using optical spectroscopy. Optical spectroscopy can measure the fundamental quality of a material describing its chemical composition as well as its morphological structure. But the penetration depth of the photon is often limited due to strong absorption and/or strong scatter, thus only surface properties are measured instead of bulk properties. This mismatch of information can lead to decisive errors in quantifying components in a dynamic, non-equilibrated environment.

In summary, the objective of PAT is to analyze and control the process and to manage the variability in the raw material, all intermediates and during processing, the objective of a quality control is to find data which are representative for a specified functionality of a final product. 9,17,52

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Radionuclide Standardization

Agustín Grau Malonda , Agustín Grau Carles , in Handbook of Radioactivity Analysis (Third Edition), 2012

5 Methods for Verifying and Improving the Quality of the Source

In radionuclide metrology, the maximum precision achieved is limited more by the quality of the sample than detector performance. A solid radioactive sample of high quality takes the form of a homogeneous and uniform thin layer of radioactive material. There are several methods to check this quality. First, we can view the structure and irregularities of the source under a microscope. The distribution of radioactivity and its homogeneity can be visualized by autoradiography, using, for example, a photographic film ( Nageldinger et al., 1998). Another way to determine the quality of a source is to measure their self-absorption from the beta efficiency variation of the sample (De Sanoit et al., 2001) or from the parameters that define a spectral alpha peak (Denecke et al., 2000).

Sample uniformity and homogeneity can be improved by depositing a wetting and seeding agent in the support of the source, before depositing the radioactive material. It is advisable to use commercial detergents as wetting agents, such as Tween 20, Catanac (Leprince and de Sanoit, 2002; Miguel et al., 1984), or tetraethylene glycol (TEG). To produce small crystals we use a seeding agent such as colloidal silica (Ludox), microspheres (K-007), or ion exchange resins (Merritt et al., 1960; Lachance and Roy, 1972; Lowenthal and Wyllie, 1973; Chen et al., 1989; Du et al., 1986; Blanchis et al., 1990).

The quality of a sample is also given by the drop drying procedure. A common procedure is to dry outdoors without airflows. Another method is to use a laminar flow, at room temperature or heating with an infrared lamp. Generally, we must apply a dry atmosphere, although in some cases it is useful to use the freeze-drying technique (Hutchinson, 1965; De Sanoit et al., 2004). Finally, we have also used dry warm nitrogen jets (Denecke et al., 2000).

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Radionuclide standardization

Agustín Grau Malonda , Agustín Grau Carles , in Handbook of Radioactivity Analysis: Volume 2 (Fourth Edition), 2020

5 Methods for verifying and improving the quality of the source

In radionuclide metrology, the maximum precision achieved is limited more by the quality of the sample than detector performance. A solid radioactive sample of high-quality takes the form of a homogeneous and uniform thin layer of radioactive material. There are several methods to check this quality. First we can view the structure and irregularities of the source under a microscope. The distribution of radioactivity and its homogeneity can be visualized by autoradiography, using for example a photographic film ( Nageldinger et al., 1998; De Senoit et al., 2004) as illustrated in Fig. 7.17. Another way to determine the quality of a source is to measure their self-absorption from the beta efficiency variation of the sample (De Sanoit et al., 2001) or from the parameters that define a spectral alpha peak (Denecke et al., 2000).

Figure 7.17. (A) Autoradiographs, radioactivity profiles and Nc/Nγ values for 65Zn sources dried by the vacuum freeze-drying method. (B) Autoradiographs, radioactivity profiles and Nc/Nγ values for 65Zn sources dried by evaporation at atmospheric pressure.

 From De Sanoit et al. (2004), reprinted with permission of Elsevier Science © 2004.

Sample uniformity and homogeneity can be improved by depositing a wetting and seeding agent in the support of the source, before depositing the radioactive material. It is advisable to use commercial detergents as wetting agents, such as Tween 20, Catanac (Leprince and De Sanoit, 2002; Miguel et al., 1984) or tetra ethylene glycol (TEG). To produce small crystals we use a seeding agent such as colloidal silica (Ludox), microspheres (K-007) or ion exchange resins (Merritt et al., 1960; Lachance and Roy, 1972; Lowental and Wyllie, 1973; Chen et al., 1989; Du et al., 1986; Blanchis et al., 1990).

The quality of a sample is also given by the drop drying procedure. A common procedure is to dry outdoors without air flows. Another method is to use a laminar flow, both at room temperature or heating with an infrared lamp. Generally we must apply a dry atmosphere, although in some cases it is useful to use the freeze-drying technique (Hutchinson, 1965; De Sanoit et al., 2004). Finally, we have also used dry warm nitrogen jets (Denecke et al., 2000).

In the case of source preparation (De Sanoit et al., 2004), freeze-drying under vacuum consists of the dehydration of the radioactive drop by sublimation. The apparatus is composed of a 300   mm diameter stainless steel ice condenser chamber (ICC) with its associated cooling system and a vacuum pump (pump rate: 5.6   m3/h) in order to speed up the drying process. The experimental layout is shown in Fig. 7.18.

Figure 7.18. The ICC used for the freeze-drying method: (1) radioactive source shelf; (2) vacuum gauge; (3) microaeration valve; (4) ice condenser; (5) insulation; (6) ice condenser chamber; (7) acrylic cover; (8) pressure control valve; (9) vacuum pump; and (10) defrosting water valve.

 From De Sanoit et al. (2004), reprinted with permission of Elsevier Science.

In most cases, drying a liquid drop of a radioactive solution results in an agglomeration of crystals, the final size of which depends mainly on the time available for the crystals to grow. A usual practice was to dry the sources in a fume hood by air draft (Van der Eijk and Zehmer, 1977). The results were often unsatisfactory as large crystals were formed mainly at the boundary of the drop and at dust particle inclusions. In some cases the liquid withdrew to a much smaller area than the initial drop size, resulting in a very inhomogeneous distribution of the deposit. The uniformity of the deposit was improved by stirring the drop with a dry nitrogen jet. However, due to the long drying time of typically 10–20   minutes, crystals could still grow to large sizes. Employing a gas jet at an elevated temperature substantially accelerated the evaporation of the solvent. By using multiple gas jets and rotating the source at the same time, more turbulence was caused within the drop. This turbulence prevented the formation of a few large crystals at the three-phase boundary between the drop and the substrate. As a result of steady remixing, a large number of small crystals, uniformly distributed over the original drop size, were formed. Intense evaporation begins when temperature and gas flows are set high. In this phase, mainly water evaporates and only a film of the concentrated acid solution remains on the substrate. At this point, the heat input to the gas jets must be reduced to limit the temperature rise of the deposit and substrate and to reduce the buildup of material stresses in the deposit. These stresses may tear a thin foil substrate or reduce adherence between the deposit and the substrate.

The drying apparatus Figs. 7.19 and 7.20, consists of a turntable with variable speed, four gas jets with adjustable gas flow and temperature, a transparent bell jar and a vacuum pump. Gas jets at an elevated temperature impact directly onto the rotating liquid drop deposited on a substrate. Jet distances and positions are adjustable. A geared asynchronous motor using frequency control varies the rotation speed from 5 to 150   rpm. Placing the bell jar over the tabletop creates a closed and dust-free environment around the drop source. Pumping is needed to remove the water vapor from the closed container and enables control of pressure between 5 and 101.3   kPa, which also accelerates evaporation.

Figure 7.19. Construction of the hot gas jet. The heating element, a helix of resistive wire on a glass tube core, is centered inside the gas duct close to the outlet. Two thin-walled tubes thermally insulate the duct. A narrow nozzle forms the gas jet.

 From Denecke et al. (2000), reprinted with permission of Elsevier Science.

Figure 7.20. Principle of the accelerated drop drying: gas jets at an elevated temperature are impacting directly onto the rotating liquid drop deposited on a substrate. Jet distances and positions are adjustable.

 From Denecke et al. (2000), reprinted with permission of Elsevier Science © 2000.

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Chemical Imaging Analysis

Freddy Adams , Carlo Barbante , in Comprehensive Analytical Chemistry, 2015

The New Role of Metrology

The role of metrology as the science of measurement needs to be identified in connection with imaging analysis. In chemical analysis, the goal is to produce data as close to the truth as possible (trueness), which can also be (re)produced by other analysts around the world (traceability). The two main goals of metrology are the 'accuracy' and 'traceability' of chemical results. Metrology itself cannot ensure the truth of a chemical analysis, but rather a consensus value based on the measurement of the same analyte(s) in a given sample by a large number of laboratories (or by using a primary method). A true value is not intrinsically achievable, hence, it is common practice to report the uncertainty of an average value (of the entire analytical procedure, not of an individual analytical measurement!). In imaging analysis, which involves complex data sets (images or sets of images), the measurements should be carried out in such a manner that they are confident (that a consensus can be obtained) and ideally, traceable.

Chemical imaging will need to comply with the metrological requirements, because, in simple terms, metrology is defined as the science of the measurements (chemical ones in this case). Presently, methodological concepts are largely lacking to do this. Instead of concentrating on the individual measurements in the data set more comprehensive tools will need to be developed, using chemometric information or information derived from various imaging tools. For elemental imaging analysis, attempts are made to develop XRF into a reference-free, primary method. Isotope dilution is a possible primary method of analysis in MS. Some of the methods now available reach the fundamental limits of atom or molecule detection, where only statistical considerations govern the degree of confidence.

Analytical chemistry, particularly when we consider it in the context of imaging analysis, cannot be identified as a particular subject within metrology, as a part of chemical metrology, On the other hand we need to keep in mind that chemical imaging needs to conform itself eventually to metrological principles. If, for the present time, for the multitude of individual measurements in a complex data set this is not intrinsically achievable, it might be possible to link the uncertainty of an average value (not of every individual analytical signal measured) to metrological concepts. This can be achieved by summing up the spectroscopic data of every data point of a data set or, at least to sum up the elements of specific parts of an image data set (Regions Of Interest, ROIs). These ROIs are defined on the basis of information that is available in the entire data set. Statistical (chemometric) tools, or supporting information from various other imaging tools, should be able to recognise these significant subsets in the overall data collection. The resulting sum spectra and their representation of the average composition in ROIs are more easily to deal with for quantitative evaluation than the individual spectra because they have a better signal-to-noise ratio.

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Sputter Processing

Andrew H. Simon , in Handbook of Thin Film Deposition (Third Edition), 2012

Resistance/Four-Point Probe Measurement

The most established metrology technique for metallic thin films is the four-point probe resistance measurement, in which four in-line probe tips are used to measure sheet resistance. For a rectangular block of conducting material of resistivity ρ, length L, thickness t, and width w, the resistance R will be given in the formula R=ρL/(tw).

For the special case where the sample's width is equal to the length, w=L, the resistance expression simplifies to R=ρ/t. The resistance in this case is referred to as the sheet resistance per unit square and is quoted in terms of Ω/square. We thus see that if the bulk resistivity of the material is known with certainty, the thickness can be determined using the four-point probe sheet resistance measurement. Alternatively, if the thickness is measured using some separate technique, the four-point probe measurement can be used to determine the bulk resistivity of the film.

Typically the two outer probes are operated in current source mode, with the two inner probes measuring the voltage drop across the current path in the sample (Figure 4.16). This arrangement eliminates any confounding effects due to contact resistance. Assuming that the dimensions of the sample are much greater than the probe-tip spacing, the geometrical correction factor to convert the current and voltage measurements is R=4.532V/I, where V is the voltage between the inner-probe tips, and I is the current forced through the outer probe tips. Probe-tip spacings of ~0.5–2.0   mm are typical for semiconductor applications.

Figure 4.16. Probe configuration for a four-point probe resistance measurement. For thin metal films, the outer probes are typically operated in current source mode, with the inner two probes used for voltage measurement. For a thin metal film, the sheet resistance is measured from the voltage and current values (see text for details).

Commercially available four-point probe tools for the semiconductor industry will typically have a user-selectable probing pattern that samples the wafer center, equally azimuthally spaced points at the wafer edge (at a user-specified maximum radius r), and concentric rings of points at some fractional radii in-between (most often r/3 and 2r/3). This concentric-ring sampling enables standardized measurements of sputtered-film uniformity which are widely accepted in the industry for process benchmarking.

Modern commercial instruments can be programmed to adjust the probe current automatically so as to give a suitably large inner-probe voltage reading that results in minimal error, enabling measurements ranging from ~1   mΩ/square to ~1   MΩ/square. One limitation for four-point probe measurements is that the probe tips can punch through films ~50   Å or thinner, thus leading to spurious readings indicative of the substrate or prior layer rather than the film itself. Similarly, it should be noted that if one is measuring a multilayer film stack, the measured film of interest should be the lowest resistance film in the stack.

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