March, 2014
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How Acoustic Microscopes Became Smart Continued from previous page
virtual die stack that simulates as closely as possi- ble the physical die stack. The virtual die stack is “imaged” by a C-SAM system to nar- row down the parameters that will best image the physical stack. These parameters are used on the physical stack and further refined. A few more transitions between the virtual stack and the physical stack give the precise parameters that are needed for the interface at the top of die #4 or any other depth in the physical die stack. During development of the software, an
8-die stack was created in which an identify- ing number and three rectangular trenches were etched into each die. The shallow trench- es, being gaps, served as intended “defects,” as did the number that identified the die. This arrangement let developers know when the return echoes were indeed gated on the right interface. In the acoustic image from one
portion of a physical stack of eight die, the image was made by establishing initial imaging parameters with the virtual die stack, and refining them on the physical stack. The goal in this case was to image the features etched into die #7. Two numbers are visible: the “8” from die #8, and more brightly the “7” of die #7, circled in the image. The trenches etched into the silicon to the left and right of the numeral 7 are
The very short distances between the material interfaces in a die stack
also mandated customized transducers.
especially conspicuous. When used on a production die stack lacking inten- tional gap and etched numbers, this method will find real defects such as delaminations, and allow the needed process adjustments to be made. The very short distances be -
tween the material interfaces in a die stack also mandated the development of customized transducers that could handle the echoes that are very close- ly spaced in time.
Adding transducers. Newer auto- mated acoustic microscopes may have multiple scanning stages, with a transducer for each stage. If you are screening trays of components and have two stages, the automated sys- tem will image twice as many compo- nents as a single-transducer system in the same amount of time. But if the tray contains relatively large items whose area of interest is relatively small — lead frames, for example — the automated system can tell the transducer to scan only the areas of interest (the die and die attach) and skip all other areas. To date, Sono - scan has received reports of speed improvements of as much as 7X using this technique. A tray that has been scanned is
moved out immediately to the drying area, permitting the scanning of the next tray to begin at once. The batch and lot data are available immediate- ly, in the form of spreadsheets or visu- ally in the form of tray maps. Accept/reject criteria can be applied by software, or components can be sorted into other categories.
Precise Selection of Pixel Size. In the past, a standard field of view might have been composed of 1000 lines of 1000 pixels each, and yielding an acoustic image having 1 million
pixels. If higher resolution were needed, the size of the field of view remained the same, but the pixel count was increased to 2000 x 2000, producing an
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acoustic image having 4 million pixels. If even higher resolution was required, the count could be further increased to 4000 x 4000, but there were no intermediate steps in resolution. This restriction was abandoned in order
to allow the user to determine the pixel size and the number of pixels. A microscope user needing to find subsurface defects 25 microns wide can benefit by selecting 25 micron pix- els, and a field of view of any appropriate size. The acoustic images will show the defects, and time will be saved by not scan- ning at a lower pixel size. But a user who requires much higher resolution can select pixels sizes down to 1 micron. Contact: Sonoscan, Inc., 2149 E. Pratt
Acoustic image of die #7 of the eight dies in a test stack. A numeral and rectangular trenches have been etched into each die.
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