An Introduction to Optical Metrology

By Ibtisam AbbasiReviewed by Lexie CornerUpdated on Oct 29 2024

Optical metrology is a field of measurement science that utilizes light-based techniques to assess and characterize the dimensions, shape, surface quality, and other physical properties of objects and materials.¹ It has played a key role in the advancement of various industries such as electronics, biomedicine and healthcare, and manufacturing.

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Historical Significance and Evolution

Major Developments Before the 19^th Century

The origins of optical metrology can be traced back to early human innovations. The invention of spectacles in 1280 marked a turning point, as lens design and analysis became necessary for vision correction.

In 1608, the Netherlands saw the development of the first telescope and binoculars, though the term “telescope” was only later adopted following Galileo’s advancements with more powerful lenses. Contributions from Newton, Gregory, and others led to the invention of the first reflecting telescope in the 1660s.²

The compound microscope, also originating in the Netherlands in the 16^th century, saw notable performance improvements in the 19^th century, particularly through the contributions of Ernst Abbe.

Optical Metrology Developments After the 19^th Century

The early 20^th century brought about exciting advancements in optical microscopy, especially with the development of fluorescence microscopy. Initially, fluorescence images lacked clarity, but the addition of a small orifice—the pinhole—in 1950 led to the development of the confocal microscope, which gained popularity in the 1990s following the invention of lasers.³

Photography, introduced in the 1820s, enabled images to be captured on glass plates and, later, photographic film, adding a new method for assessing optical quality.² The concept of optical spectroscopy also emerged as a revolutionary tool for optical measurement, particularly in biomedical and materials sciences.

In the 20^th and 21^st centuries, the invention of lasers proved pivotal for light-based measurement systems, making lasers a cornerstone technology in optical metrology. The field continues to evolve, with advancements such as super-resolution 2D optical metrology now enabling researchers to measure sub-wavelength features with high precision.

Modern technologies like Artificial Intelligence (AI) and Machine Learning (ML) are also increasingly used to optimize performance, signaling promising developments in optical metrology.

Techniques and Applications

Various optical metrology techniques, particularly interferometry-based methods, are commonly used to measure surface roughness and other essential parameters due to their inherently high resolution and precision. Interferometry operates by measuring optical displacement, comparing the spatial distance between a reference light path and a measurement path against the known wavelength of light.

These methods are expected to be integrated into lunar rovers to capture high-resolution images, which would significantly contribute to advancements in space technology and deep-space exploration.⁵

Another optical technique used in metrology is spectrometry, specifically the transmission optical spectrometer. This instrument measures a sample's inherent properties, such as absorption or emission, in the near UV and visible spectrum. It evaluates the light's wavelength, which is a critical factor in optical coatings.⁶

Wavefront sensors (WFSs) are also essential for monitoring industrial processes through real-time wavefront sensing. These sensors measure distorted incoming wavefronts and relay the data to a deformable mirror to correct wavefront errors (WFEs).

The most widely used WFSs are the Shack-Hartmann (SH) sensor, which uses a micro-lens array placed at the pupil of the optical system, and the Pyramid sensor (PS), which captures four pupil images using a faceted prism positioned at the system's focus. These sensors are highly valued in metrology for optical platforms, real-time biomedical systems, and astronomy.⁷

Optical holography, introduced in 1948, enhanced traditional microscopy by capturing both the amplitude and phase of a wavefront, unlike conventional photography, which records only the amplitude. This technique finds applications in various optical information processing platforms, both traditional and modern.⁸

Recently, metasurfaces for holographic metrology have drawn attention for their potential to create ultra-compact optical surfaces, benefiting applications such as data storage, information encryption, microscopy, and 3D displays.⁹ Together, these advanced techniques have made optical metrology a key technology for precise measurements in industrial settings.

Major Challenges

Optical metrology is continually challenged to meet the demands of the semiconductor industry. The limited resolution of current optical imaging and measurement technologies necessitates new approaches to accommodate shrinking feature sizes and increasing integration density.

As precision requirements intensify at the nanoscale, high-precision alignment and positioning technologies become critical. Additionally, multiple patterning techniques in semiconductor manufacturing push optical sensors to their limits, requiring sub-nanometer precision for accurate measurements.

The emergence of 2D materials introduces further challenges for optical metrology in quality control and industrial applications. The fabrication and integration of these materials into functional systems, such as metalenses, polarizing interfaces, optical insulators, holograms, data storage devices, and sensors, remain complex tasks.¹⁰

2D Materials Beyond Graphene: Properties and Potential

Recent Developments and Future Directions

Recent advancements in micro-optics and optical components—particularly in medical devices, electronic displays, and automotive parts—have created a demand for more advanced optical metrology tools.

Sensofar Metrology, a company based in Terrassa, Spain, recently introduced the Smart 2, an autonomous areal confocal profilometer. This device features three measurement systems within a single head: active illumination focus variation, confocal, and interferometry, setting new standards for automation in optical metrology.

LayTec, a German company, is developing various integrated optical metrology solutions and specialized algorithms. Their tools are incorporated into deposition systems, such as metal-organic chemical vapor deposition (MOCVD) systems, and are utilized in the front end of semiconductor device manufacturing.

In the future, optical measurement tools will become essential for downstream processes in semiconductor front-end production, including applications like wafer mapping to characterize wafers post-epitaxy. Optical techniques such as white light reflectance and photoluminescence are currently used to assess layer properties, enabling precise and reliable measurements.¹¹

Deep learning is also becoming integral to optical metrology, with experts leveraging it to refine techniques like interferometry. Machine learning and big data integration are expected to enhance optical metrology in fields such as autonomous vehicles, establishing it as a valuable tool in that industry.¹²

With these advancements, optical metrology is anticipated to become even more effective in areas like predictive maintenance and semiconductor manufacturing. Data-driven approaches, including DL and ML, are streamlining metrological processes.

New metrology techniques, sensor innovations, and advanced algorithms further validate the scientific community’s confidence in optical metrology, solidifying its role in modern industry.

Learn More: What Equipment is Used in Surface Metrology?

References and Further Reading

1. Bitri, R., et al. (2023). Advances in optical metrology: the impact of deep learning and quantum photonics. IEEE. Available at: https://doi.org/10.1109/iCCECE59400.2023.10238502
2. Mayo III, J. (2023). Before the laser: an optical metrology retrospective. In Applied Optical Metrology V. https://doi.org/10.1117/12.2676875
3. John Innes Center. (2023). A brief history of optical microscopy. [Online] John Innes Center. Available at: https://www.jic.ac.uk/blog/a-brief-history-of-optical-microscopy/ (Accessed on: October 06, 2024)
4. Wang, Y., et al. (2017). Review of surface profile measurement techniques based on optical interferometry. Optics and Lasers in Engineering. Available at: https://doi.org/10.1016/j.optlaseng.2017.02.004
5. Hall, L. (2024). A Lunar Long-Baseline Optical Imaging Interferometer: Artemis-enabled Stellar Imager (AeSI). [Online] NASA. Available at: https://www.nasa.gov/general/lunar_long_baseline_optical_imaging_interferometer/ (Accessed on: October 08, 2024)
6. Oxford Instruments. (2024). What is an Optical Spectrometer? [Online] Oxford Instruments. Available at: https://andor.oxinst.com/learning/view/article/what-is-an-optical-spectrometer (Accessed on: October 09, 2024)
7. Hénault, F., et al. (2024). Wavefront sensing with a Gradient Phase Filter. Optics Communications. https://doi.org/10.1016/j.optcom.2024.130910 (Accessed on: October 09, 2024)
8. Wen, X., et al. (2024). Quasicrystal metasurface for optical holography and diffraction. Light Sci Appl. https://doi.org/10.1038/s41377-024-01578-z
9. Deng, Z., et al. (2017). Metasurface optical holography. Materials Today Physics. Available at: https://doi.org/10.1016/j.mtphys.2017.11.001
10. Osten. W., et al. (2018).Optical metrology - the long and unstoppable way to become an  outstanding measuring tool. Proceedings of the SPIE:VII International Conference on Speckle Metrology. https://doi.org/10.1117/12.2322533
11. Castelo‐Porta, A. (2023). The future of optical measurement technology: A review of new developments in optical metrology and industry trends in the digitalization and integration of these solutions for in situ and inline measurements. PhotonicsViews. https://doi.org/10.1002/phvs.202300015
12. Zhang, Z., et al. (2023). Applications of data fusion in optical coordinate metrology: a review. Int J Adv Manuf Technol. https://doi.org/10.1007/s00170-022-10576-7

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