Robert Cid & Samuel Lesko on choosing the proper optical profiler for roughness and surface texture
Three-dimensional (3D) non-contact optical imaging has been a topography mainstay across a wide range of industries. Two widely used techniques are white light interferometry (WLI) and confocal microscopy, also known as laser scanning confocal microscopy (LSCM). Both provide a 3D surface representation from a scanned image and are ubiquitous for measuring nanometre-to-millimetre surface features. Though the principle of operation for each method delivers different advantages and disadvantages, WLI-based profilometers do provide some distinct metrology advantages over confocal microscopes for roughness and surface texture. Key to these advantages is the ability to maintain sub-nanometre vertical resolution and 0.01-nanometre RMS repeatability, regardless of magnification or field of view.
Principles of Measurement
Confocal microscopy was originally developed for imaging of biological cell and tissue samples with very little attention to metrology. In confocal microscopy, the sample is advanced vertically in steps such that each point on the surface passes through focus. A very small aperture is placed in front of the detector to admit light from a single point as it passes through focus. In LSCM, only one point is measured at a time, requiring raster scanning in the X and Y directions as well as in the Z axis to obtain data for each point on the surface. A limitation of this approach is that it becomes very time-consuming to capture data over a large field of view.
WLI-based 3D optical profilometry was, on the other hand, developed from the very beginning for industrial metrology applications. In WLI instruments, light approaching the sample is split and directed partly at the sample and partly at a high-quality reference surface. The light reflected from these two surfaces is then recombined. Where the sample is near focus, the light interacts to form a pattern of bright and dark Moiré that provides detail information on the surface shape. Using a specialised motor scanner synchronised with camera acquisition, the objective is scanned vertically with respect to the surface so that each point of the test surface passes through focus, capturing at a glance the Z position of all pixels within the field of view. This on-the-fly acquisition dramatically improves throughput versus confocal methods. A full 3D areal map is generated, enabling analysis of different parameters of interest, such as surface texture, roughness, or other critical geometric dimensions (diameter, width, spacing, etc.).
Vertical and Lateral Resolution
Vertical resolution is the most important performance characteristic in a surface profile measurement. By the nature of their operation, confocal systems must continuously move the stage around to raster scan the surface, so vertical resolution is limited by the axial point spread function. The height of each pixel location is found by detecting the peak intensity or by calculating the centre of mass of the intensity distribution around the focus position. While the 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. This 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. This 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.
There are two possible limits to the lateral resolution of an optical system. The 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.
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. Thus, 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