MICROSCOPY & IMAGING
PE FIND THERFECT PROFILER
Robert Cid & Samuel Lesko on choosing the proper optical profiler for roughness and surface texture
T
hree-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. Tough 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
Patterned sapphire substrate (PSS) image taken showing capability of WLI 3D profilers to provide high-speed precision metrology measurements of steep angles
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. Te 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. Tis 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. Te 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
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