METROLOGY APPLICATION AREAS

Metrology

Metrology is the branch of science dedicated to measurement, which is defined as the process of comparing an unknown quantity, referred to as the measurand, with a standard of a known quantity.

From: Environmental Monitoring and Characterization, 2004

Related terms:ChemistryPhotonSemiconductorIndustryWavelengthX-RayElectron ParticleNanomaterialError

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Nanostructured Materials: Metrology

A. Jorio, M.S. Dresselhaus, in Encyclopedia of Materials: Science and Technology, 2010

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 is coordinated by national laboratories, such as the National Institute of Standards and Technology (NIST, USA) and the National Institute of Metrology, Standardization and Industrial Quality (Inmetro, Brazil), which are internationally coordinated by the BIPM. In parallel, normalization 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, and engineering. 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 control and utilize matter at the nanoscale, where size-related properties and phenomena can emerge. The nanoscale is the size range from approximately 1 nm to 100 nm (ISO 2005). Specific tasks include developing standards for: terminology and nomenclature; metrology and instrumentation, including specifications for reference materials; test methodologies; modelling and simulation; 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 largely undefined.

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 2×10−7N.” This definition sets the permeability of vacuum at 4π×10−7 H m−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 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 carbon nanotubes as an illustrative prototype of nanometrology efforts. This is not the personal choice of the authors, but a generalized concept, and is the one adopted by the ISO-TC229. Carbon nanotubes are stable enough for manipulation, simple enough (just sp carbon) for modeling, and have been at the forefront of nanoscience and nanotechnology (Jorio et al. 2008). 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.

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 sp2carbon nanostructures are stable enough for manipulation, simple enough (just sp2bonded 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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Masks for micro- and nanolithography

E. Gallagher, M. Hibbs, in Nanolithography, 2014

5.4 Mask metrology

Metrology is a general term for the science of measurement. Mask metrology is used to verify the quality of the mask pattern by measuring a selected group of features across the mask. This metrology sample is selected to be representative of the overall mask. The number of features included in the metrology sample must be large enough to ensure that the mask features meet specifications across the mask, but not so large that the metrology time is unreasonably long. The primary metrics that are measured on a photomask are: the feature dimension, the placement of the feature relative to an absolute grid, and the light modification properties. The corresponding metrology metrics are known as the CD, the IP, and phase and transmission.

5.4.1 Critical dimension (CD) metrology

The feature size, or CD, is measured with an optical microscope, an SEM, or an atomic force microscope (AFM). Historically, only a simple one-dimensional width of the feature was measured. As lithographic fidelity has improved, secondary features of the mask may transfer to the wafer so that line edge roughness, corner rounding, and general 2D shape fidelity have become important. Optical microscopes have insufficient resolution for many applications. AFMs have high resolution, but are very slow. SEMs offer high resolution and reasonable speed, and dominate mask CD metrologyapplications. SEMs optimized for top-down CD measurements of semiconductor patterns or masks are often called CDSEMs. There are several steps to the image acquisition and processing in a CDSEM. The electron beam is scanned across the mask, so that the mask surface emits secondary electrons. Three stages of CDSEM data extraction are shown in Fig. 5.3. First, the emitted electrons are recorded by detectors to create a two-dimensional image. Then, the contrast waveform is extracted from the bitmap and the edges of the features become visible as intensity peaks. Finally, the edges, are sharpened by applying algorithms so that the CD is given by the distance between the two edges. A known pitch standard is often used to calibrate the SEM output to a physical dimension. It is important to acknowledge that the underlying image is an approximation of reality; calibrating the results to standards is essential for accurate CD results.

5.3. CDSEM images showing (a) the normal field of view, (b) the extracted waveform, and (c) an algorithm is applied to the waveform to

As pattern sizes shrink through the continuing evolution of semiconductor technology, the requirements for accurate placement of mask features are also becoming tighter. For the most advanced semiconductor designs, pattern placement accuracy requirements are now in the 5–10 nm range. Because the mask is magnified 4 × to 5 × the size of the chip pattern, one might imagine that the mask IP tolerance would be correspondingly magnified; however, the mask contribution is only one small part of the total IP budget on the wafer. Consequently, the mask IP tolerances are also in the 5–10 nm range.

After a mask pattern has been created, IP is measured by an extremely accurate optical measuring device. The finished mask is placed on an interferometer-controlled stage, and an array of registration marks is viewed by a scanning laser beam. In order to support the required mask tolerances, subnanometer repeatability is required for the IP measurement. The two edges of the feature are identified and the location, or placement, of the image is calculated as shown in Fig. 5.4. The image size can also be extracted, but since the measurement is optical, this ancillary measurement may be insufficiently accurate for CD metrologyrequirements.

5.4. A schematic diagram showing the optical signal from a clear feature on a mask and the calculations for CD and IP.

A large number of registration marks, typically several dozen to a few hundred, will be measured on a photomask. This allows increased accuracy by averaging many measurements and provides a detailed view of localized mask distortions. The raw map of IP measurements is analyzed to remove errors with low spatial orders, such as whole-body displacement, rotation, magnification, and orthogonality. All of these low-order terms can be corrected by the wafer exposure equipment. The higher-order errors cannot be easily corrected, and limit the ultimate layer-to-layer overlay performance that can be achieved with the mask. Because of the extreme accuracy requirements of the IP measurement, the temperature and humidity environment inside the metrology equipment are tightly controlled. Even the small gravitational sag distortion of the mask induced when the mask is held horizontally during the measurement process is mathematically corrected to achieve the final sub-nanometer repeatability of the measurements.

5.4.3 Phase and transmission metrology

Photomasks that use a phase-shifting attenuator film require additional measurements to ensure that the optical phase shift and transmission of the film are within tolerances of their design values. Because the phase shift and transmission are both wavelength dependent, it is necessary to do the measurement with a light source having the same wavelength used to expose the pattern on the wafer. The same type of excimer laser used to expose the wafer could be used for phase and transmission metrology, but it is easier to use a broadband deuterium plasma lamp or mercury–xenon arc lamp with a monochromator to isolate the desired wavelength.

Once a beam of monochromatic light is produced, conventional dosi-metric methods can be used to measure the film transmission. Two different conventions are used to normalize the transmission measurement. Either the light transmission through the mask substrate and absorber film is measured relative to an equivalent air path, or the transmission through the film-covered region is normalized to the transmission through a clear region of the fused silica mask substrate. Although fused silica absorbs very little light at commonly used lithographic wavelengths, there are substantial reflective losses from the surface of the mask. For purposes of lithographic simulation programs, the transmission relative to a clear area of the mask is more useful than transmission relative to an air gap.

Optical phase metrology is more difficult than transmission metrology, and requires some sort of interferometric measurement. The most practical arrangement has been to use a shearing interferometer, which produces interference fringes by splitting the image of a small mask pattern and sending the two images down different light paths before recombining the two images with a small transverse offset between them. By analyzing the spatial offset between the interference fringes in different parts of the recombined image, the optical phase shift of the mask film can be derived. Commercial instruments are available that can automatically measure phase shift and transmission in patterned phase-shifting attenuator films.

Nanometrology

Jeremy J. Ramsden, in Nanotechnology, 2011

5.6 Summary

Metrology at the nanoscale imposes stringent requirements of accuracy and precision. A particular challenge is the fact that the size of the measuring instrument and the feature being measured become comparable. Should the nanoscale feature be absolutely small in the quantum sense, then we have the potential problem that its state is destroyed by the measurement. Given the identity of quantum objects, however, this does not usually pose a real practical difficulty—one merely needs to sample (and destroy) one of many. Regardless of the absolute size of the object being measured, if the measurement instrument has the same relative size as the object being measured, the measurement is subject to distortion. Previously this could always be resolved by shrinking the relative features of the measurement instrument. If, however, the object being measured is already at the lower size limit of fabrication (i.e., in the nanoscale) no further shrinkage of the measurement instrument is possible. Nevertheless, this does not pose an insuperable difficulty. Indeed, it has already been encountered and largely overcome whenever light has been used to measure features of an object smaller than the wavelength of the light.

The ultimate goal of nanometrology is to provide a list of the coordinates and identities of all the constituent atoms of a nanoscale structure, and all practical metrology is aimed at that goal. Approaches can be categorized as imaging or nonimaging, each of which category contains methods that can in turn be categorized as contact or noncontact. As one approaches the nanoscale, however, the distinction becomes harder to make. In another dimension, one can distinguish between topographical and chemical features; some techniques yield both. In particular, commercial scanning transmission electron microscopy is now able to yield the atomic coordinates of arbitrary three-dimensional structures.

Techniques able to make time-resolved measurements in situ are very useful for monitoring actual processes.

The representation of structure by a list of the atomic coordinates and identities will usually lead to vast, unmanageable data sets. Therefore, there is considerable interest in capturing the salient features of topography and chemistry by identifying regularities, possibly statistically.

The nano/bio interface presents perhaps the greatest challenges to nanometrology, not least because the introduction of living components into the system under scrutiny moves the problem into territory unfamiliar to most metrologists. However, important advances are being made in this domain, which has already allowed it to assume a far more quantitative nature than hitherto.

Perhaps the greatest current challenge is to devise instrumentation able to combine two or more techniques, enabling the simultaneous observation of multiple parameters in situ on the same sample.

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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.

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

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 isthat 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]).

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]).

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]).

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]).

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ABSOLUTE FLUX MEASUREMENTS

S.V. Bobashev, in Vacuum Ultraviolet Spectroscopy, 1999

8.2 Primary Radiator Radiometry

In metrology, the radiation detector standards are divided into primary standards and secondary, or transfer, standards. Usually the basis of primary standard operation is some fundamental law or phenomenon. A major requirement for a transfer standard is a property to be transferred from one place to another without noticeable changes in stability, sensitivity, and reliability. Transfer standards of chosen types are calibrated by comparison with primary radiometric standards and are used as working transfer standards for the calibration of detectors of the same type, or other types, for a variety of customers. Transfer detector standards can be calibrated with the help of both primary detector standards and primary radiator standards. Therefore, the accuracy of absolute measurements is connected with the development of more accurate primary radiator standards, primary detector standards, transfer detector standards, and the calibration technique.

Although the advantages of synchrotron radiation (SR) from synchrotrons and storage rings as primary radiation standard sources were recognized many years ago [9, 10], the application of electron accelerating installations for quantitative radiometry became a widespread practice only during the last decade. This is because of the many complications for development of the radiometric technique and instrumentation compatible with the specific parameters of SR. At present, an electron storage ring is considered to be an almost ideal source for quantitative radiometry of electromagnetic radiation in a wide spectral range from the infrared up to far VUV and the hard x-ray spectral range [6, 11, 12].

Electron storage rings as dedicated sources of SR have been built at many places around the world. Some of them are used as radiometric facilities in the VUV and the SXR. Calibrations have been made in the VUV and the SXR at the National Institute of Standards and Technology (SURF II, Gaithersburg), the Electrotechnical Laboratory (TERAS, Tsukuba), the Budker Institute of Nuclear Physics (VEPP-2M and VEPP-3, Novosibirsk), and the Physikalisch-Technische Bundesanstalt (BESSY I, Berlin). A small synchrotron is used at the Russian Research Institute for Optophysical Measurements (TROLL, Moscow) for calibration in the range 50–250 nm.

During the last few years, improvements in the measurement of storage ring parameters (current, energy of the orbiting electrons, as well as an accurate control of position of the orbital plane relative to the center of an aperture stop restricting the radiation flux used in calibration) provided high accuracy in determination of the absolute photon fluxes from storage rings. For all the previously mentioned electron storage rings, the spectral photon flux is greater than 10−6 W/cm2 ·

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