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March, 2014 How Acoustic Microscopes Became Smart By Tom Adams, Consultant, Sonoscan, Inc.
ing capabilities in several directions. By doing so they have made possible new kinds of analytical and screening techniques and become smarter. The changes in acoustic microscope technology involve both hardware and software. Typically acoustic microscope systems seek
T
out subsurface features, including anomalies, by pulsing ultrasound into the sample and turning the return echoes into pixels. Defects such as delaminations, voids and cracks send back the highest amplitude echoes and are bright white in the acoustic image; other material interfaces are some shade of gray. Laboratory microscopes image and analyze small numbers of components, while automated systems are used to scan large quanti- ties of components to sort out the bad ones.
Acoustic microscopes seek subsurface features, including anomalies, by pulsing ultrasound into the sample.
Lately, though, acoustic microscopes have
quietly been adding capabilities that find and image internal features better, faster, or in ways not possible before. There are four recent game- changers: customized transducers, scanning stacked die, adding transducers, and precise selec- tion of pixel size.
Customized transducers. Generic transducers have fixed frequencies, focal lengths and other fixed parameters. They are fine for many applica- tions, particularly at low frequencies up to about 50MHz. At higher frequencies, the relationship between focal length (the distance from the trans-
Customized ultrasonic transducers.
may need, for example, a customized 180MHz transducer that has a relatively short focal length but that can still reach the required depth in the sample.
Changed Focal Length Or the focal length may be changed in other
ways. Typically the focal length of a standard high frequency transducer is 0.5 inches (12.7mm), but a microscope user who needs very high resolution at a high frequency may require a different focal length.
This is why Sonoscan designs and manufac-
tures all of its C-SAM® system transducers from 50MHz up to the industry’s high of 400MHz. The company has also designed and manufactured over 2,500 customized transducers that have been opti- mized for specific applications. Engineers at Sonoscan have even turned out
he evolution of acoustic microscopes in recent years has enabled them to extend their non- destructive data-collecting and image-mak-
ducer at which resolution is highest) and the lens’s numerical aperture — in other words, the lens’s F- number is just as it is in optical systems. A trans- ducer with a longer focal length gives up some res- olution. At frequencies of 75MHz and above, the length of the water couplant path begins to affect performance because it absorbs more of the high- frequency ultrasound. A particular application
transducers with a frequency of 10MHz — a very low frequency at which ultrasound penetrates far- ther into materials — and with very long focal lengths. Such a transducer has been used to look through samples of silicon carbide that are 12 inches (304mm) thick. The image resolution is low, but when it is necessary to find internal features and defects without destroying the sample, they are the tools that work.
Scanning stacked die. This is a particularly dif- ficult task, one that defied solution for years. Recently Sonoscan, in cooperation with the Technical University of Dresden, has developed software that permits successful imaging. The problem with stacked die is that they are
very efficient at creating very large numbers of echoes that cannot easily be sorted out. Suppose a manufacturer has designed an 8-die stack, and wants to be sure there are no gap-type defects in the adhesive between any of the die. His target has 8 die, 8 layers of adhesive, and a substrate. A pulse of ultrasound travels through the
first die and strikes the top of the first adhesive layer. Part of the ultrasound is reflected, and part travels deeper, where it meets the interface between the bottom of the adhesive and the top of the second die. Here part of the ultrasound travels into the second die, but part is reflected upward. And of that part that is reflected upward, a portion is reflected downward by the bottom of die #1. This process is repeated as the pulse travels deeper, and soon the number of echoes arriving at the transducer is very large. How to sort out the right echo for imaging, let us say, the interface at the top of die #4?
Virtual Die Stack The new system uses information about the
materials and dimensions in the stack to create a Continued on next page
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