Comparing Focus-Variation Measurements to Classical Surface Metrology

Precise measurement solutions with high resolution are necessary when surface qualities and micro geometric features have to be checked. Compared to alternative optical technologies, Focus-Variation closes the gap between standard 3D coordinate measuring technology and classical surface metrology devices.

Profile Projectors

Profile projectors and other image processing systems are the forerunners of the current optical measurement systems and are still relevant for understanding optical measurement technology. Profile projectors expand the components‘ surface characteristics and project the image onto a screen. By the use of pattern matching, the image is compared to a proper reference.

Benefits of this approach are measurements that can be carried out within seconds, though the automatic measurement of geometric features is restricted to two-dimensional applications only. One key disadvantage is its sensitivity to object placement. Contradictory measurement results could be obtained, depending on alignment.

Structured Light

Structured light is based on a projector that lights up the measurement object with several bright and dark stripes and captures it with one or more cameras. The topography of the sample bends the stripe pattern of the projector. The distorted pattern is documented with a camera. Lastly, the topography is calculated via image processing.

One benefit of structured light is the high measurement speed when measuring big surfaces. Hence, the technology is mainly used for the measurement of oversized parts (e.g.: bodywork). The technology is of restricted appropriateness for high resolution sub-μm depth measurements as, for example, with roughness measurements. Additionally, the low depth of field and high sensitivity to changing surface characteristics significantly limit the application range.

Confocal Measurement

Confocal Measurement is characterized by high lateral resolution. Precisely at the focal point inside the detector, an added aperture is used to block light from above and below the focal plane. In that way, only light contained within the focal plane passes through the detector. Depth is measured by detecting the strongest signal. Confocal systems are mainly appropriate for measurements of very even surfaces that can be found on silicon structures or semiconductor geometries. The advantage of high resolution in z is accompanied with greater sensitivity towards vibrations.

White Light Interferometer (WLI)

White Light Interferometer (WLI) utilizes interference effects to define topographical features. One benefit is the high vertical resolution. While rough surfaces are hard to measure, the process is perfectly appropriate for the assessment of lenses and glass structures.

Focus-Variation

Focus-Variation collects not just data on depth, but also registered true color information of the surface. Roughness of micro and nanostructures are measured both profile and area based. Intricate geometries are measured via the Real3D technology from varied points of view which are then combined into a full 3D dataset. By measuring the dimension, form, roughness and position in one system, Focus-Variation lessens the gap between typical 3D coordinate measuring technology and classical surface metrology.

Unlike in profile projectors, an outline is not measured, but the components‘ 3D surface. Whereas confocal systems and interferometers measure intensity peaks or intensity modulation only in a very slim band around the central point of the system, Focus-Variation measures sharpness over a significantly bigger region. As a result, the technology is more tolerant against vibrations.

This information has been sourced, reviewed and adapted from materials provided by Alicona Imaging GmbH.

For more information on this source, please visit Alicona Imaging GmbH.

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