New optical metrology technique offers 3-D surface measurement

Oct. 21, 2010—Focus-Variation, a new optical measurement technique pioneered by Alicona Imaging GmbH, Grambach, Austria, allows in-depth metrology of 3-D surfaces in the micro and nano range, and meets the ISO 25178 standard for topographical measurement.

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This image shows a 3-D view of a screw thread taken with the InfiniteFocus.

This screenshot shows a profile measurement taken of the screw (top photo) showing the types of measurements that can be made with the InfiniteFocus system.

The new metrology technique is at the heart of Alicona's 3-D surface measurement device InfiniteFocus, which can achieve measurement of most surfaces—including those with steep flanks and varying reflective properties—down to a vertical resolution of 10nm. The device provides dense measurements over large areas with a density of 2-16 Mio points of measurement.

Irrespective of whether it is used in the lab or during production, this technique works faster and more accurately than conventional methods with the potential to measure over a high vertical scan range (up to 20mm) and x-y range (up to 100mm x 100mm), enabling the robust measurement of relatively large components.

The InfiniteFocus system.

Robust optical surface measurement

The operating principle of Focus-Variation combines the small depth of field of an optical system with vertical scanning to provide topographical and color information from the variation of focus. Depending on the topography of a surface, the information from the variation of focus is analyzed in relation to the distance to the optics.

Using conventional optical measurement techniques, a high vertical resolution can only be reached with a small vertical scanning range, whereas the use of the Focus-Variation technique yields a high vertical resolution over the entire scanning range, allowing a dynamic of 1:200000.

Focus-Variation simultaneously captures the entire surface topographic information in combination with its true color information. Both the topographical and color information are registered to each other. This can be of major importance to understand if a particular surface feature is contamination or a real surface artifact. Additionally, a quality measure is determined for each measurement point.

The main component of InfiniteFocus is a precision optic system made consisting of various lens systems that can be equipped with different objectives allowing data collection with different resolutions. Using a beam-splitting mirror, light emerging from a white light source is inserted into the optical path of the system and focused onto the specimen.

Depending on the topography of the specimen, the light is reflected into several directions as soon as it hits the specimen. If the topography includes diffuse reflective properties, the light is reflected equally strong into each direction. In case of specular reflections, the light is reflected mainly in one direction. All rays emerging from the specimen and hitting the objective are bundled in the optics and gathered by a light sensitive sensor behind the beam-splitting mirror.

Due to the small depth of field of the optics, only small regions of the object are sharply imaged. To allow a complete detection of the surface with full depth of field, the precision optic is moved vertically along the optical axis. This means that each region of the object is sharply focused. A sensor captures a series of 2-D datasets during this scanning process. Thereby, all sensor parameters are optimized at each vertical position according to the reflective properties of the surface.

After the scanning process, the 2-D datasets are evaluated to generate 3-D information as well as an image with full depth of field. This is achieved by analyzing the variation of focus along the vertical axis. Due to the large amount of data mechanical restrictions can be eliminated allowing measurement results with high resolution. Once all height measurements are determined, an image with full depth of field is computed.

Not only is this innovative technology used in labs, it is inline capable as well. Depending on the desired measurement task and application, several sensors are offered in order to provide inline quality assurance. The full spectrum of measurement and analysis performance that can be achieved in the lab is also obtainable with the sensors used in the inline system. The robust and compact design of the sensors also allows operation in rough environments.

The Focus-Vvariation technique can be used in many different areas of materials evaluation, including:

• Surface Characterisation—Focus variation, unlike 2-D analysis, allows detailed characterization of materials surfaces. In addition to conventional Roughness (R_a) and profile (P_a) data, this technique can provide volume and area information. This in itself leads to the ability to characterize the surface and look at parameters such as material and void volume allowing studies of lubrication, porosity and wear that can be affected by the detailed understanding of these parameters.
• Evaluation of tolerances and wear analysis in 3-D—The technique allows verification of data based on CAD models and the display and measurement of worn materials. This can be used effectively in tool making, wear on bearings and failure analysis.
• Precision machining, milling and drilling, mold and tool making—Working in micro and nano means the technique can be used in tool and mold making. Particularly in the case with surfaces with a large lateral and vertical range.
• Accurate 3-D measurement of microgear wheels—This is a typical industrial application that can be easily achieved due to the distinctive measurement capabilities reached with Focus-Variation. Geometries with steep flanks of more than 80° can be measured accurately in seconds. Additionally, parameters such as the inner and outer diameter, flank angles and angles of angular geared components can be computed.

Visit Alicona's Web site for more information.