Desktop SEMs produce fast images

Desktop SEM systems are designed primarily for rapid imaging.

-- Test & Measurement World, 7/1/2010 12:00:00 AM

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The push to reduce the size of inspection systems for increased portability and flexibility has led to the development of desktop machines based on technologies such as AOI (automated optical inspection) and AXI (automated x-ray inspection) as well as on SEMs (scanning electron microscopes). Many of these smaller systems are used for offline, rather than inline, inspection tasks. In SEMs, electrons image a sample to distinguish characteristics on a much smaller scale than is possible with optical microscope techniques. Since this process must occur in a vacuum, most desktop SEM applications are by necessity restricted to offline inspection, said Phenom-World's Jack McFarland, GM for North America.

Desktop SEM systems are designed primarily for rapid imaging, said McFarland. For example, the moving parts in MEMS need to be inspected quickly at around 5000X magnification. Desktop SEM systems produce high-quality images that are comparable to those from large SEM systems, but they do so much faster and for one-tenth to one-twentieth of the cost.

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For example, the Phenom desktop SEM can load and image a sample in under 30 s. "It can image at magnifications up to 24,000X, but usually you're looking for magnification between [the point] where optical runs out of steam at about 1000X, up to about 5000X," McFarland said. "You do lose some flexibility for dealing with difficult samples, but 80% of the time the desktop meets your needs." He explained that in the other 20% of cases, a desktop SEM is ineffective "usually due to the need for much higher magnifications, larger samples, wet samples, or where elemental analysis is required."

In crystalline silicon PV (photovoltaic) solar cells, desktop SEMs can reveal variances in the mono- or multicrystalline structure, which affect cell quality, as well as over-etching, which can cause capacity loss. Courtesy of Phenom-World.

But by focusing on the most prevalent uses and giving up a little flexibility, customers can gain a machine that fills the majority of their needs and is much simpler to operate than a typical large SEM. Although a few desktop SEMs are placed on the factory floor, they are used mostly in research and failure analysis labs where there's not a lot of space, and where they can be moved around easily.

I In wafer inspection, desktop SEMs can only look at pieces of wafers due to the sample chamber's small size, usually 1 to 2 in. in diameter, said McFarland. "For PCBs, you'd probably do a cross-section and look for a noncontinuous layer, for example, where there could be a short," he said.

Only some technologies in desktop SEMs will likely contribute to even smaller footprints in the future, but their capabilities will continue to improve, said McFarland. "For example, the navigation camera at the front end of the Phenom will get a lot better and smaller, so we may end up with a combination optical/SEM inspection device," he said. Vacuum technologies are improving but the mechanics and electronics involved will only shrink slightly in the near future. "Most likely, we will keep the Phenom at the same footprint while we increase its software capabilities and make it even faster, rather than trying to add capabilities from the big SEMs."