Optical Metrology – Understanding Focus Variation

Focus-Variation[1] integrates the small depth of focus of an optical system with vertical scanning to give color and topographical data from the variation of focus. The key component of the system is a precision optic that contains a variety of lens systems that can be fitted with different objectives, enabling measurements with different resolution.

When a beam splitting mirror is used, the light coming out from a white light source is inserted into the system’s optical path and then focused onto the specimen through the objective. Depending on the typography of the specimen, the same light is reflected into various directions once it hits the specimen through the objective.

If diffuse reflective properties are shown by the topography, then the light will be reflected equally strong into each direction. With regard to specular reflections, the light is scattered predominantly into one direction. All the rays arising from the specimen and hitting the objective lens are bundled in the optics and collected by a light sensitive sensor placed at the back of the beam splitting mirror. Only small areas of the object can be clearly imaged because of the small depth of field of the optics.

In order to completely detect the surface with full depth of field, the precision optic is vertically moved along the optical axis while continuously capturing the data from the surface. This way, each area of the object is clearly focused. Algorithms transform the acquired sensor data into 3D data and a true color image with full depth of field. This is obtained by examining the focus variation along the vertical axis.

InfiniteFocus is a 3D measurement device that is based on the operating principle of Focus-Variation. When using this measurement device, the following technical specifications emerge:

The vertical resolution can be as low as 10 nm and relies on the selected objective. The vertical scan range depends on the working distance of the objective and ranges between 4.5 and 23.5 mm. Compared to conventional methods, the vertical resolution is obtained irrespective of the scan height and results in a vertical resolution dynamic of 1: 500000.

The used objective determines the XY range, which usually ranges from 0.16 mm x 0.16 mm to 5.63 mm x 5.63 mm for one measurement. The XY range can be exceeded up to 100 x 100 mm by using a motorized XY stage and special algorithms.

Compared to other optical techniques that are limited to coaxial illumination, the maximum measurable slope angle does not only depend on the numerical aperture of the objective. Focus-Variation can be employed with many different illumination sources (such as a ring light), enabling the measurement of slope angles that go beyond 87°. Focus-Variation is applicable to surfaces that have a huge range of different optical reflectance values.

Since the optical technique is highly flexible in terms of using light, typical limitations such as measuring surfaces with strongly varying reflection properties even within the same field of view can be avoided. Specimen can differ from shiny to diffuse reflecting, from smooth to rough surface properties and from homogeneous to compound material. Focus-Variation overcomes the aspect of restricted measurement capabilities in terms of reflectance by a combination of illumination, controlling the integrated polarization and sensor parameters. Modulated illumination means that the intensity of the illumination is varying and not constant.

A signal generator can create the complex variation of the intensity. Through the continuously changing intensity, much more information is collected from the surface of the specimens.

Focus-Variation, apart from the scanned height data, also delivers a color image with full depth of field which is registered to the 3D points. This delivers an optical color image which eases measurements as far as the identification and localization of measurement fields or distinctive surface features are concerned. The visual correlation between the optical color image of the surface of the specimen and its depth information are usually connected to each other and are therefore an essential aspect of meaningful 3D measurement.

As the described method depends on analyzing the variation of focus, it is only relevant to surfaces where the focus varies sufficiently during the vertical scanning process. Surfaces that are meeting this requirement, such as transparent specimen or components with just a small local roughness are hardly measurable. Focus-Variation usually delivers repeatable measurement results for surfaces with a local Ra of 0.009 µm at a lc of 2 µm

The Focus-Variation technique is used to carry out high resolution 3D surface measurement for quality assurance in research and development activities in the laboratories as well as in production. Main applications are surface analysis and characterization in, for example, precision manufacturing, tool and mold making, automotive industry, tribology, corrosion, medical device development, electronics and all kinds of materials science. Due to its technical specifications, the Focus-Variation technique is used for measuring form and roughness.

Focus-Variation in Comparison

Whenever micro geometric features and surface qualities have to be examined, accurate measurement solutions with high resolution are essential. Focus-Variation, compared to alternative optical technologies, closes the gap between classical surface metrology devices and typical 3D coordinate measuring technology.

Profile Projectors

Image processing systems including profile projectors are the predecessors of current optical measurement systems and are still applicable for understanding optical measurement technology. Profile projectors enlarge the components' surface characteristics and project the image onto a screen.

The image is compared to a suitable reference through pattern matching. Benefits are measurements that can be made within seconds, albeit the automatic measurement of geometric features is restricted to 2D applications only. Sensitivity to object alignment is one major drawback. Based on its orientation, varying measurement results can be achieved.

Structured Light

Structured light is based on a projector that illuminates the measurement object with a number of dark and bright stripes and then captures it with at least a single camera. The topography of the sample distorts the projector’s stripe pattern; this distorted pattern is recorded with a camera and the topography is finally calculated through image processing.

A main benefit provided by structured light is the high measurement speed obtained when measuring large surfaces. Hence, the technology is mainly used for measuring extremely large parts (e.g. bodywork). However, the technology is limited to high-resolution sub-µm depth measurements as, for instance, with roughness measurements. Moreover, factors like high sensitivity to varying surface characteristics and the low depth of field considerably limit the application range.

Confocal Measurement

Confocal measurement is defined by its high lateral resolution. Right at the focal point within the detector, an extra aperture is employed to block light from both below and above the focal plane, allowing light within the focal plane to pass through the detector.

Depth is measured by detecting the strongest signal depth. In particular, confocal systems are suitable for measuring highly smooth surfaces that can be found on semiconductor geometries or silicon structures. The benefit of high resolution in z is accompanied with improved sensitivity towards vibrations.

White Light Interferometer (WLI)

WLI determines topographical features using interference effects. High vertical resolution is one major advantage. Although it can be difficult to measure rough surfaces, the method can be used for evaluating glass structure and lenses.

Focus-Variation

Focus-Variation collects depth information as well as the surface’s registered true color information. Roughnesses of nanostructures and microstructures are measured both profile and area based. The Real3D technology is used to measure complex geometries from different perspectives and these are subsequently integrated into a full 3D dataset.

By measuring position, roughness, form and dimension in a single system, Focus-Variation closes the gap between classical surface metrology and typical 3D coordinate measuring technology.

When compared to profile projectors, only the 3D surface of the components is measured, not the outline. While interferometers and confocal systems are capable of measuring intensity modulation or intensity peaks in a narrow band around the system’s focal point, Focus-Variation can measure sharpness over a significantly larger area. The technology is therefore more tolerant against vibrations.

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[1] ISO 25178-6: Geometrical product specifications (GPS) -- Surface texture: Areal -- Part 6: Classification of methods for measuring surface texture, Draft.

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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