Optical metrology is the science of making precise measurements using light-based techniques. It enables non-contact characterization and inspection across diverse fields, including semiconductor fabrication, nanotechnology research, biomedical imaging, precision manufacturing, and scientific studies. This article overviews key optical metrology techniques, their technical principles, and leading technology providers.
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Fundamentals of Optical Metrology
Optical metrology refers to the non-contact measurement of objects and physical properties. In production, automation, and testing, optical measuring techniques are employed to determine measurements and differences in speed, shape, or color automatically and precisely.
Optical metrology operates uniquely across various applications, leveraging the speed and stability of light. Instead of passively observing through lenses or studying sunlight interactions, engineers employing optical metrology actively direct ultra-precise lights and lasers toward objects. Specialized lenses then gather information about these objects based on the reflected light, enabling precise measurements and analyses.
Optical metrology techniques boast the following unparalleled advantages:
- Non-contact measurements prevent sample damage or distortion.
- High speed for dynamic processes or rapid acquisition.
- Broad working ranges from nanometer microscopy to large standoff measurements.
- Multi-parameter characterization of sample morphology, composition and inner structure.
- Extreme precision with sub-wavelength resolution.
High-Precision Optical Metrology Techniques
White Light Interferometry
White light interferometry (WLI) has become one of the most widely used optical metrology techniques over the past few decades. It uses broadband "white" light containing a range of wavelengths rather than a single-wavelength laser source.
In a typical WLI setup, light from a halogen bulb passes through a Michelson interferometer setup with one arm scanning vertically. Light-reflecting off the sample surface in this arm interferes with light from a fixed reference mirror. This creates an interference pattern dependent on the optical path difference. Each wavelength of white light produces a sinusoidal interference pattern, and the peak of high coherence occurs when the paths match. By scanning vertically and analyzing the interference intensity peak for each pixel, the surface height profile can be reconstructed with high precision.
Modern WLI systems use spectrometers and CCDs to capture interference data rapidly. In addition, they enable accurate 3D metrology of surface micro-roughness, step heights, and other structures. Applications span across quality control of precision machined and optical parts, defect detection in semiconductors, and biomedical research.
Deflectometry - Measuring Difficult Reflective Surfaces
Deflectometry offers a solution for optical techniques relying on scattering, absorption, or fluorescence effects, especially when dealing with smooth, mirror-like surfaces that may reflect light away from detectors.
In deflectometry, the sample surface essentially becomes part of the metrology system. A projected structured pattern of lines or grids is viewed through the reflective sample, with the resulting distortions revealing slope and topography information.
The angle-dependent scattering behavior maps the shape, curvature and defects of highly sensitive reflective components by tracking the deviations in the reflected patterns. This allows precise metrology of surfaces that may be difficult for other optical techniques.
Current applications of deflectometry include inspecting lithography mirrors, glass molding and more. The technique has also been adapted to work with display screens like smartphone displays, increasing accessibility.
Confocal Laser Scanning Microscopy
Confocal laser scanning microscopy (CLSM) facilitates the 3D localization of labeled target molecules in cells using an aperture to enhance the signal-to-noise ratio and improve sectioning. This technique is crucial in biological research, providing fine 3D images with improved contrast and sectioning capabilities.
The technology, developed in the 1950s by Minsky and later refined, has become a cornerstone in microbiology, enabling dynamic imaging of cellular and molecular processes. CLSM's compatibility with 3D live imaging and its efficiency in specimen preparation contribute to its status as an essential tool in biometrology research.
Holographic Imaging - Recording Optical Phase
Unlike traditional imaging, which captures light intensity, holography uses light interference to encode both intensity and optical phase information. This allows the reconstruction of the full complex optical wavefront.
In holography, laser illumination reflected by the sample interferes with a clean reference beam, creating an interference pattern that contains 3D information. This can be digitally captured via a camera sensor or photographic plate. Computational reconstruction then deciphers both intensity and optical phase data, enabling precise metrology of 3D structures, surface topographies, deformations, and more.
Quantifying the optical phase opens up quantitative imaging capabilities inaccessible by conventional microscopy. It provides metrology data with nanoscale precision, making holography invaluable for applications from biological imaging to micro-electromechanical systems.
Key Players in High-Precision Optical Metrology Devices
The advent of these remarkable optical metrology techniques has been enabled by continued innovation in systems, modules, and components. Some key providers known for their high-performance products include:
Bruker is a prominent 3D surface measurement and inspection solutions provider, delivering non-contact analyses with unparalleled accuracy. Their optical profiling metrology systems, rooted in ten generations of proprietary Wyko® white light interferometry (WLI) technology, are recognized for supporting cutting-edge research and quality control across various industries.
The ContourX-100 optical profilometer, a testament to their innovation, offers high-resolution measurements at an exceptional value, incorporating advancements such as a new 5 MP camera and an updated stage. Top of Form
Carl Zeiss AG
ZEISS, a renowned German company with over 160 years of expertise in optics and precision, holds a significant position in optical measuring technology. Their comprehensive range of optical and optoelectronic devices includes multi-sensor coordinate measuring machines, white-light sensors, and advanced 3D scanners, emphasizing high point density and speed for informative results.
ZEISS's recent acquisition of GOM strengthens its product portfolio in 3D digitization. With over 17,000 global installations, GOM provides software, machines, and systems for the automotive, aerospace, energy, and consumer goods industries. This acquisition enhances ZEISS's position in industrial metrology.
ZEISS ATOS LRX, an exceptional 3D scanner, features an ultra-bright laser light source and a large measuring area. With precise data capture (2 × 12 million coordinate points in a single scan) and seamless workflow in the ZEISS Quality Suite software, it meets high metrological demands in industrial applications, offering robustness and reliability in harsh environments.
KLA Instruments, a division of KLA Corporation, provides a comprehensive range of surface metrology and defect inspection solutions for chip and substrate manufacturing. Their metrology systems ensure precise measurements of pattern dimensions, film thicknesses, alignment, topography, and electro-optical properties.
Their Archer™ 750 overlay metrology system offers accurate on-product error feedback for advanced memory and logic devices. With wavelength tunability, high productivity, and advanced algorithms, it supports efficient inline monitoring and improved device overlay performance tracking.
FARO Technologies Inc
FARO® is a global leader in 3D measurement, imaging, and realization technology, offering high-precision solutions for engineering, manufacturing, and public safety industries. The company's goals include enabling faster and more accurate 3D documentation, minimizing measurement times, reducing costs, and mitigating product development risks.
One of FARO's notable products is the FARO® BuildIT Metrology Software. This software simplifies complex workflows in manufacturing environments, offering real-time guidance tools, easy-to-set-up automation, and advanced algorithms. It is a leading 3D metrology software platform for build, alignment, and inspection applications, drawing on over 20 years of expertise in delivering best-in-class measurement solutions to the manufacturing industry.
Driven by light's inherent speed, stability, and multi-functionality, optical metrology has become an indispensable contributor across diverse industries and scientific disciplines. As photonics, computation, and applications progress, high-precision optical measurement techniques will uphold critical quality, accuracy and progress standards.
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References and Further Reading
Osten, W. (2018, September). Optical metrology: the long and unstoppable way to become an outstanding measuring tool. In Speckle 2018: VII International Conference on Speckle Metrology (Vol. 10834, p. 1083402). SPIE. https://doi.org/10.1117/12.2322533
Engel, T. (2022). 3D-optical measurement techniques. Measurement Science and Technology. https://doi.org/10.1088/1361-6501/aca818
Canette, A., & Briandet, R. (2014). Microscopy| Confocal laser scanning microscopy. https://doi.org/10.1016/B978-0-12-384730-0.00214-7
White-light interferometry. [Online]. Available from: https://www.polytec.com/int/surface-metrology/technology/white-light-interferometry
Dr. Rüdiger Paschotta. (2023). Optical Metrology. https://doi.org/10.61835/0p9
Bruker. (2023). 3D optical profilers. [Online]. Available from: https://www.bruker.com/en/products-and-solutions/test-and-measurement/3d-optical-profilers.html
Bruker. (2023). ContourX-100. [Online]. Available from: https://www.bruker.com/en/products-and-solutions/test-and-measurement/3d-optical-profilers/contourx-100.html
GOM. (2023). ZEISS ATOS LRX. [Online]. Available from: https://www.gom.com/en/products/high-precision-3d-metrology/zeiss-atos-lrx
Carl Zeiss AG. (2023). Optical measurement. [Online]. Available from: https://www.zeiss.com.sg/metrology/campaigns/products/training/measurings-services/optical-measurement.html
FARO Technologies Inc. (2023). BuildIT metrology software. [Online]. Available from: https://www.faro.com/en/Products/Software/BuildIT-Metrology
KLA Corporation. (2023). Metrology. [Online]. Available from: https://www.kla.com/products/chip-manufacturing/metrology