Industrial MicroscopyGets Down to Work
Paula M. Powell
It's a three-dimensional world for industrial microscopy these days, whether it is used to help monitor thin-film deposition processes or to inspect bumps on a circuit board. Regardless of the technique, building the 3-D image usually involves capturing an image in one X-Y plane, translating the sample along the Z-axis to take other images and stitching these together into a composite image. How successful that process is depends on several factors, from the accuracy of the translation stage to the focusing capabilities of the system.
Jason Wickersham, product manager with Olympus Industrial America Inc. in Orangeburg, N.Y., uses laser scanning confocal microscopy to illustrate how critical focus capability is. As to be expected, the position of best focus also will be the brightest. Record the Z-position that produces the brightest intensity for each point and use this data to build the 3-D representation of the sample surface. The resulting image will have an extended focal depth such that the entire specimen is in focus. The concept is fairly simple, but Wickersham noted that the key to success, at least in the industrial world, is not just to stitch well, but also to do it relatively fast.
In semiconductor manufacturing, microscopy can help pinpoint a variety of features and defects including film defects, PCB circuit traces and broken bond wires (shown). Courtesy of Olympus Industrial America Inc.
At present, several techniques are vying for prominence in emerging microsurface mapping applications. Just as one method appears to reach a level of sophistication critical for a specific application, another comes along designed to play off the other’s weaknesses to capture market share. Take, for instance, recent work by Maitreyee Roy and colleagues at the University of Sydney in Australia. They have developed a technique based on low-coherence scanning interference microscopes, the most promising property of which is the ability to overcome ambiguity problems inherent to monochromatic interferometric systems. Roy said that, although the technique uses an optical sectioning property similar to that used with confocal microscopy techniques, it relies on coherence effects rather than the physical apertures to enhance the lateral and longitudinal resolution.
One trend to note is that it may be possible to incorporate a new industrial microscopy method without a complete retooling of the inspection station. Consider, for example, the need to characterize the complex topography of microelectromechanical systems. Optimization of the manufacturing process requires that engineers understand and monitor height and thickness characteristics of various layers, as well as their homogeneity, as they are deposited on a substrate.
According to engineers at Carl Zeiss MicroImaging in Thornwood, N.Y., a basic reflected light microscope can be adapted to measure surface tomography on the nanometer scale with the addition of a device that allows one to make total-interference-contrast measurements. The emphasis is on developing an industrial system that has the flexibility to do both optical analysis and height measurement.
The following series of articles examines fundamental issues common to all industrial microscopy applications — how to inspect and map complex microstructures with the required resolution and speed. These sometimes-conflicting demands will only continue to grow in importance as end users in diverse industries tackle increasingly difficult inspection tasks with industrial microscopes.
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