Advances in Optical Metrology: Techniques and Applications

By Owais AliMay 1 2024Reviewed by Lexie Corner

Optical metrology, the science of making precise measurements using light, has become indispensable across the aerospace, automotive, and electronics industries, where precision and quality control are crucial. This article overviews the latest optical metrology techniques, applications, and future outlooks.

Image Credit: Simon Kadula/Shutterstock.com

The origins of optical metrology date back to pioneering work on optical interference by Thomas Young and the Michelson-Morley experiments in the 19^th century, demonstrating its applicability for dimensional measurements.

However, Theodore Maiman's invention of the laser in 1960 unleashed optical metrology's potential by providing an intense beam of highly coherent light ideal for interference measurements.

Over the following decades, the development of advanced laser sources, high-resolution cameras, fast computing hardware, and sophisticated algorithms refined optical techniques.

This steady progression has transformed optical metrology from a laboratory curiosity into an indispensable industrial technology critical for product development, quality assurance, and fundamental scientific research.¹

Latest Techniques in Optical Metrology

Phase-Shifting Interferometry

Phase-shifting interferometry (PSI) is an interferometric technique that achieves high measurement resolution and accuracy by analyzing the phase information from interfering wavefronts.

It captures multiple interferograms with a known 'phase shift' between them, typically using a laser source. The phase at each point is then calculated from the interferogram intensities using phase-shifting algorithms. This unwrapped phase map directly corresponds to the surface topography.

PSI's key advantage is its extremely high vertical resolution, down to sub-nanometer levels. However, it has a limited unambiguous depth range of a few hundred nanometers due to the 2π phase ambiguity, requiring additional techniques like multi-wavelength operation or null optics for larger surfaces.²

White-light Interferometry

Modern white-light interferometers use Michelson interferometry to precisely measure surface topographies. This method involves a light source with a short coherence length, where its collimated beam is split into a measurement and a reference beam. These beams are then reflected from the sample surface and a reference mirror.

After reflection, the beams recombine at the beam splitter and focus on a camera. The camera captures maximum intensity when the optical paths of both beams match, leading to constructive interference, thereby accurately mapping the surface height at each point.

White-light interferometry enables rapid and simultaneous measurement of large surface areas with high vertical resolution that remains constant regardless of the field of view.³

Computational Imaging with Structured Light

Computational imaging (CI) techniques combine structured illumination and computational reconstruction to enable novel imaging capabilities. Instead of directly imaging the scene, CI captures coded lower-dimensional measurements and uses prior information and algorithms to estimate the scene properties of interest.

A key example is the single-pixel camera, which can acquire high-resolution images from very few measurements by exploiting scene sparsity. When combined with structured light, CI enables enhanced capabilities, such as light field imaging, refocusing, robust depth estimation, and the ability to overcome the limitations of passive techniques.²

Encoded Search Focal Scan

Encoded Search Focal Scan (ESFS) is a new computational imaging technique that enables rapid topographic imaging at micro and nano scales.

Instead of capturing a full focal stack, it uses an encoded illumination scheme and binary search to find the focal plane corresponding to the surface height at each lateral position. This drastically reduces the number of images required for 3D reconstruction to log2(N), where N is the number of focal planes.

ESFS provides a simple way to achieve real-time, high-resolution 3D imaging over relatively large volumes.⁴

Holographic Imaging

Holographic imaging combines interferometry with microscopy to enable highly sensitive phase imaging of minute features and transparent objects.

It uses a common-path interferometer configuration, in which the input light field is split into object and reference wavefronts. These wavefronts are then overlapped to create an interference pattern recorded by a camera.

Digital holographic microscopes refine this process by using spatial filtering to produce a high-quality reference wave, ensuring the recorded hologram is sharp. This hologram is then digitally reconstructed to derive the phase profile, directly correlating to the object's optical thickness.

This method allows for multiple wavelengths, significantly extending the measurement range without ambiguity. The resultant holograph provides quantitative 3D phase maps with sub-wavelength accuracy, enabling the measurement of properties like refractive index variations inaccessible to conventional intensity imaging.⁵

Coherence Scanning Interferometry

Coherence Scanning Interferometry (CSI) uses a broadband or white light source instead of a laser. It exploits the limited coherence length of these sources to directly measure the optical path difference at each point by vertically scanning one arm and finding the position of maximum fringe contrast. This enables absolute surface measurement without 2π ambiguities over a large depth range.

CSI provides sub-nanometer resolution and is widely used for microscale 3D profilometry. However, it is sensitive to environmental disturbances, which can introduce measurement errors, and can be time-consuming for large surfaces requiring stitching multiple scans.²

Applications Across Industries

The exceptional precision, speed, and versatility of modern optical metrology techniques find diverse applications across various industries.

Aerospace Industry

In aerospace, advanced optical scanning and interferometry have become vital tools for verifying the integrity and complex geometries of components like turbine fan blades, compressor discs, and combustor chambers that must withstand extreme operating conditions.

Techniques like white light interferometry enable comprehensive inspections of micro-scale cooling holes, thin leading and trailing edge radius measurements, and surface finish characterization—all critical for ensuring operational safety and attaining optimal performance.⁶

Semiconductor Manufacturing

The perpetual miniaturization of integrated circuit features has placed enormous demands on semiconductor metrology, with device nodes approaching just a few nanometers.

Optical techniques like X-Ray fluorescence, atomic force microscopy, and critical dimension scanning electron microscopy, which offer resolutions between 1 nm and 20 nm, are indispensable across all stages of semiconductor production.

They are crucial for measuring wafer geometry, film thickness, and composition, as well as for inspecting defects and conducting critical dimension metrology on final chips. They also ensure the structural integrity and precise dimensions of silicon wafers and nanostructures, critical as the industry transitions to 300 mm wafers and features down to 3 nm.^7,8

Biomedical Engineering

Optical metrology is also making significant inroads into biomedical engineering for precise characterization and quality control of medical devices, implants, tissue engineering substrates, and more.

For instance, non-destructive 3D optical metrology techniques like digital holography enable quantitative measurement of biomaterials and micro-components under physiological conditions with sub-nanometer resolution, providing invaluable input for design optimization and virtual simulations.⁹

Integration with Other Technologies

Modern industrial optical metrology systems enhance workflows by integrating augmented reality (AR) visualization, which allows the real-time overlay of detailed 3D measurement data directly onto the inspected components.

AR streamlines the interpretation and communication of results by instantly highlighting geometric deviations in their proper spatial context, eliminating the need for cross-referencing between separate datasets and physical parts.¹⁰

Robotic automation is being applied to execute automated inspection routines, feeding components into optical metrology systems and handling them during measurement. This increases throughput and enables precise traceable positioning for repeatable results.

Advanced machine learning techniques, such as deep learning, are applied to intelligently analyze and interpret the enormous datasets generated by modern high-resolution optical metrology systems.

In addition to pattern recognition and defect identification, AI/ML techniques enable adaptive optimization of system parameters to handle variabilities that hinder deterministic modeling robustly. This approach unlocks more consistent and reliable operations across parts and environments outside ideal calibration conditions.²

The Future of Optical Metrology

Optical metrology will continue to advance rapidly, driven by demands for ever-higher precision and new application domains.

The adoption of advanced illumination sources like supercontinuum lasers, which provide intense ultrashort pulsed beams spanning a broad spectrum, is enabling long-distance metrology, enhanced resolution, speed, and surface discrimination capabilities over traditional narrow-band lasers and LEDs. Their coherence properties simplify optical designs while improving performance.¹¹

Additionally, the trend toward miniaturization and commoditization is transforming optical metrology from specialized laboratory equipment into widespread, everyday tools.

However, several key challenges remain to be addressed, including the need for personnel training on increasingly complex systems, standardization to ensure interoperability, and efficient data management pipelines for handling increasingly massive 3D datasets.² Overcoming these hurdles will be critical for optical metrology to achieve its full transformative potential across industries.

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References and Further Reading

1. Ellis, J. D., Haitjema, H., Jiang, X., Joo, KN., Leach, R. (2020). Advances in optical metrology and instrumentation: Introduction. JOSA A. doi.org/10.1364/JOSAA.405559
2. Marrugo, AG., Gao, F., Zhang, S. (2020). State-of-the-art active optical techniques for three-dimensional surface metrology: a review. JOSA A. doi.org/10.1364/JOSAA.398644
3. Polytec. (2024). White-light interferometry. [Online] Polytec. Available at: https://www.polytec.com/int/surface-metrology/technology/white-light-interferometry
4. Vilar, N., Artigas, R., Duocastella, M., Carles, G. (2024). Fast topographic optical imaging using encoded search focal scan. Nature Communications. doi.org/10.1038/s41467-024-46267-y
5. Engel, T. (2022). 3D optical measurement techniques. Measurement Science and Technology. doi.org/10.1088/1361-6501/aca818
6. Alicona Imaging GmbH. (2024). Optical 3D metrology solutions for quality assurance in the aerospace industry. [Online] Alicona Imaging GmbH. Available at: https://www.alicona.com/en/solutions/industries/aerospace
7. University Wafer. (2024). Semiconductor Metrology. [Online] University Wafer. Available at: https://www.universitywafer.com/semiconductor-metrology.html
8. Queengate. (2024). Semiconductor metrology: positioning is key. [Online] Physicsworld. Available at: https://physicsworld.com/a/semiconductor-metrology-positioning-is-key/
9. Sezdi, M. (2019). Biomedical metrology. Bioelectronics and Medical Devices. doi.org/10.1016/B978-0-08-102420-1.00019-4
10. Radiant Vision Systems. (2019). Enabling Display Measurement within Augmented & Virtual Reality Headsets. [Online] Association for Advancing Automation. Available at: https://www.automate.org/vision/tech-papers/enabling-display-measurement-within-augmented-and-virtual-reality-headsets
11. Ray, P. (2024). Hyperspectral long-distance metrology using a femtosecond laser supercontinuum. ETH Zurich. doi.org/10.3929/ethz-b-000667524

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