3-D microscopes based on WLI use specialized objective lenses to create a unique signal that represents an object’s shape with nanometer-scale vertical resolution, which has the advantage of being independent of system magnification. A continuum of signals are collected during the vertical scan and then analyzed to yield height information. Despite that, these specialized objective lenses deliver excellent 3-D topography—imaging of a material’s surface isn’t always straight forward.
A sample viewed through a specialized objective in a WLI-based 3-D optical microscope is obscured by the light beam from the reference mirror and the interference signal. The ratio of sample to mirror reflectivity determines the clarity of a sample’s image. For samples of very low reflectivity, in comparison with the mirror’s reflectivity, the image clarity may be very poor.
Traditionally, the specialized objectives used in a WLI system don’t provide the same image of a sample as brightfield objectives do. Thus, one method to obtain a 3-D color image is to do a two-scan measurement: one scan with an interferometric objective delivering 3-D measurement data based on finding a best focus position for each pixel, and a second scan with a brightfield objective during which imaging information is collected only at the previously predetermined best-focus positions and then overlaid on the 3-D measurement data.
However, two scans extend the measurement time. It’s therefore desirable to obtain both the color image and the 3-D topography data of an object with a single scan using only interferometric objectives. There are a few issues that must be overcome in an image obtained using an interferometric objective: the presence of fringes and the sometimes poor visibility of features on the sample surface. These two issues can be dealt with by a combination of image processing and the utilization of special side illumination.
A powerful addition to WLI 3-D microscopes is this external side illumination source, which enables excellent direct imaging, especially of low reflective and diffuse surfaces. Without such side illumination, the only illumination of the sample on a WLI microscope is a narrow light beam coming through the imaging objective. This kind of illumination isn’t a good substitute for natural light and, hence, the resulting image from such illumination is often said to be “unrealistic” or “not natural.” This is only further complicated by the aforementioned interference signal and reference mirror light reflection. With the addition of external illumination, samples are illuminated by more natural, diffuse light, and the resulting sample appearance is more true to life.
The combination of engineered side illumination and special image processing algorithms significantly improve observation of samples, particularly those with low reflectivity and diffuse surfaces. Improved imaging helps with finding focus and areas of interest on the sample, and also helps with retrieving the sample’s high-fidelity color or greyscale image—all of which make the WLI microscope much more intuitive to operate, even for novice users. These improvements in imaging significantly redefine the functionality of WLI-based 3-D microscopy, a technology perceived as one that produced excellent metrology, but had limited imaging capabilities.
Color-coded printed circuit board 3-D surface topography map. Typical display for 3-D microscopes based on WLI. Image: Bruker Corp.
Color-coded printed circuit board 3-D surface topography map. Typical display for 3-D microscopes based on WLI. Image: Bruker Corp.
Now high-fidelity imaging and precise surface-topography measurement capabilities are making 3-D microscopes based on WLI a more attractive metrology tool. The value of WLI-based 3-D measurements in assessing engineered surfaces has been proven. This metrology technique has been widely adopted by many industries, ranging from semiconductor and microelectronics fabrication, to precision manufacturing for automotive and aerospace parts and medical devices, just to name a few.
Imaging also serves key inspection needs not met by metrology alone. To produce images, some optical microscopy systems utilize multiple cameras or illuminations and require several scans. But the best technology implementations require only one scan, and these are the fastest and the least prone to alignment error. The newest 3-D optical microscopy technology, such as that built into Contour Elite 3-D optical microscopes from Bruker (Tucson, Ariz.), adds imaging and display capabilities with the ease-of-use advantages usually only available with confocal microscopy, giving users the ability to easily and quickly obtain not only high-resolution and repeatable surface topography measurements, but also high-quality images and 3-D visualization of sample topography.
High-fidelity images provide valuable information from the sample measurement that compliments the optical metrology data. In addition to revealing specific surface details that otherwise would be difficult or impossible to see, this new technology also enables users to segment data based on color information to rapidly select areas of interest and collect critical metrology data from specific regions. The additional and complementary information leads to better problem solving in engineered surface applications.
Combining high-end metrology with the ability to see, recognize and display what was measured is important, not only for understanding the data captured, but also for communicating what the data means. In addition to great precision topography measurement of surfaces, 3-D microscopes based on WLI now can provide great color or grayscale imaging of the sample. Beyond topography, image representation of the sample provides additional avenues for feature or defect characterization of the sample. Accurate metrology data, along with crisp surface images, give users the complete surface characterization story.
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