Editorial Feature

An Introduction to Optical Metrology

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Metrology is the science of measurement. In today's world, there are various tools and technologies at our disposal for measuring an entity.

Importance of Optical Metrology

Being such a broad field consisting of numerous disciplines, few examples include clocks, sensors, microscopes, and thermometers. One very prominent part of metrology is the optical metrology. Optical measurement methods have gained prominence over the past couple of centuries. The development of optical microscopes in the early seventeenth century opened up new possibilities to comprehend nature at micron scales. A comparatively recent development in the field of optical metrology is the development of photography and spectroscopy in the 19th century which radically changed our way of how we characterize materials and structure of matter.

The importance of optical metrology can be assessed by the fact that a large number of Nobel Prizes have been awarded for the development of optical measurement techniques. A few examples include the development of interferometers by A. Michelson, Raman scattering by C. Raman, phase contrast microscopy by F. Zernike, nonlinear optics with lasers by A. Schawlow and ultraprecise laser spectroscopy by J. Hall and T. Hänsch. The realization of lasers in 1960 by Theodore Maiman lead to the use of optics for various applications including communication technology, characterization of matter and astrophysics. Optical measurement techniques gained enormous popularity due to its non-contacting and non-reinforcing nature. Some other features of optical metrology include the large scalability of the probing tool, high resolution, and flexible adaptability.

Drawbacks of Conventional Metrology Tools

The characteristics that give rise to the improvement in the performance of the systems are mostly dependent on the purpose of the measurement and the object or property under scrutiny. These characteristics are mostly associated with spatial and temporal resolution, precision, the degree of automation and robustness. For any manufacturing process, the yield depends on the DMAIC (Define, Measure, Analyze, Improve and Control) process. It’s an established methodology to achieve high process improvement and failure reduction. But for increasing need of “zero-defect” production especially in ultra-high precision processes such as employed in the semiconductor industry, the conventional metrology tools fail and cutting edge optical metrology techniques are required.

In the semiconductor industry, Moore’s law dictates the shrinkage of the device and feature size of the pattern with respect to the time. The feature size decreases with respect to the wavelength of light used to fabricate the structure. With this decrease in the feature size, the theoretical and practical constraints for their realization are increasing. To ensure the quality and consistency of the fabricated components, highly sophisticated optical metrology tools are employed. These optical metrology tools cover the full Range of yield applications by inspecting the quality of the prefabrication wafer, reticle qualification, and tool, process, and line monitoring. This results in fast corrective actions in case of a failure prediction and fast yield improvement which not only saves time but the cost since the materials and process involved in chip manufacturing is extremely costly. A few optical metrology tools are discussed in Table 1.

Table 1. Various optical metrology tools employed for measurements.

Method Quantity measured Quantity to be measured Light Source Measurement Scale
Classical Interferometry intensity, interference phase wavefront, distance, shape, refractive index coherent microscopic, nanoscopic
(digital) Holographic Interferometry intensity, interference phase, interference pattern distance, displacement, vibration, shape, refractive index, material fault coherent macroscopic, microscopic,
White-Light Interferometry contrast, intensity, interference phase distance, topography short-coherent microscopic, nanoscopic,
Speckle-Techniques intensity, interferencephase, interference pattern distance, displacement, vibration, shape, strain, material fault coherent macroscopic, microscopic,
Moiré-Techniques intensity, interferencephase, interference pattern displacement, strain, shape, material fault coherent, incoherent macroscopic, microscopic
Fringe Projection intensity, phase shape coherent, incoherent macroscopic, microscopic
Photo Elasticity intensity, interference phase, polarization birefringence, strain coherent macroscopic, microscopic
Microscopy, e.g. Confocal contrast, intensity, interference phase, polarization distances, refractive index, absorption, fluorescence incoherent, coherent microscopic, nanoscopic
Deflectometry contrast, intensity distance incoherent, coherent microscopic
Autofocus contrast, intensity distance incoherent macroscopisc, microscopic
Time of Flight intensity, interference phase distance coherent macroscopi

 

A Non-Destructive and Non-Invasive Approach

Most of these techniques are based on far-field optics, which suffer some serious drawbacks such as unwanted aberrations, noises, interference effects, environmental sensitivity, diffraction limited resolution and limited depth of field. There is a dire need of development of resolution enhanced metrology tools for inspection principles that can be used for industrial applications. In comparison to invasive ultra-high resolution tools such as scanning electron microscope (SEM) and atomic force microscope (AFM), optical metrology tools offer significantly lower resolution but still are of high importance by the virtue of their non-destructive, non-invasive and good in-line capability.

Conclusion

In conclusions, the next phase of optical metrology will be focused on the exploitation of near field phenomena which can provide many more details of the material and structures of interest. This will significantly reduce the resolution and this enables the manufacturer of real-time process correction for high much higher yield and faster production. It will also enable researchers who work with metamaterials and nanostructured surfaces to develop new materials with unconventional properties and functionalities.

Sources and Further Reading

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