MICROSCOPY & IMAGING
intensity envelope is narrow for high- magnification objects, it becomes wider for lower magnification as the objective’s numerical aperture (NA) and depth of field increase. Tis generally dictates the use of 50x magnification or higher, which severely limits the field of view. Data stitching is then required to map larger areas, significantly increasing measurement time as well as uncertainty. In WLI, the fringe envelope remains very narrow at all magnifications. Tis feature, combined with phase detection, enables subnanometre vertical resolution regardless of objective magnification. Users thus gain flexibility to use lower magnification objectives for greater fields of view, leading to much higher throughput while retaining highest Z accuracy measurements. Tere are two possible limits to the
lateral resolution of an optical system. Te first is pixel-limited resolution, where two adjacent features are imaged into a single camera pixel, making it impossible to distinguish between the features in the final digitised image. Another limitation to lateral resolution is diffraction, where there are at least two camera pixels for each feature, but features cannot be readily distinguished from each other. For visible-light 3D optical systems, this spatial resolution limit is usually about 350 to 400 nanometres. High-magnification objectives, such as 20x, 50x and 115x, typically produce diffraction-limited images. However, through multiple iterative scans and advanced analysis techniques, today’s top WLI profilers are now able to resolve features beyond the diffraction limit down to 75 nanometres.
Lead angle of screw threads can easily be measured at 60° with a WLI-based 3D optical profiler
Confocal microscopes produce a wider, weaker signal for lower magnification objectives
WLI profilers provide a constant, narrow signal for all objectives
MATCH YOUR APPLICATION REQUIREMENTS Ultimately, when choosing a technology or technique for your areal metrology, you should consider the type of data and repeatability needed for your metrology applications. Confocal systems are known for generating qualitatively beautiful intensity images of samples, but have difficulty producing repeatable quantitative topography data when compared to WLI-based optical profilers.
For samples with steep slopes, confocal methods have traditionally had an advantage linked to high NA from the objective. However, today’s top WLI profilometers are routinely able to measure slopes between 60 to 87°, such as those found in raw additive manufactured surfaces. In summary, today’s white light
interferometry profilers uniquely combine high-speed acquisition with large field of view while preserving sub-nanometre vertical resolution and 0.01 nanometre RMS data, regardless of the magnification. Tus, it is perfectly suitable for capturing surface texture and roughness at physical relevant scale. Confocal microscopes do not provide comparable resolution unless running at 50x magnification and higher, which limits your field of view, throughput and collection of quantitative statistics about what the surface topography looks like.
Robert Cid & Samuel Lesko are with Bruker.
www.bruker.com/contourx
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