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Warped wafers no longer a problem for acoustic scanner

Posted: 05 May 2016 ?? ?Print Version ?Bookmark and Share

Keywords:Sonoscan? silicon? wafer? transducer? surface tracker?

Sonoscan has introduced the C-SAM systems with its Quantitative Dynamic Z (Q-DZ) surface tracking feature for scanning warped wafers. The driving force behind this product is that silicon wafers are supposed to be 'flat' when they are being scanned by an acoustic micro imaging tool for defect detection or for measurement.

In some instances, the wafer is warped. Normally, the acoustic tool's transducer scans in a flat plane, and is focused and gated on a specific depth of interest. Warping of the wafer means that some die are partly or entirely out of focus and defects can be entirely missed.

Now Sonoscan has released a product to remedy this situation. The tool acquires data from the scans that not only reveal the features inside the wafer but also measures the warpage. The warpage information is used during the scan to instantly adjust the height of the transducer above the wafer surface and maintain the critical focus inside the wafer over its entire area.

Warped wafer

Figure 1: In the illustration, the warped wafer at left was scanned in a flat plane. White regions were too high, black regions were too low relative to a true flat surface. Only the grey zone between them was meaningfully imaged. The same wafer is shown at right. With Q-Dynamic Z surface tracking, all devices were imaged as though the wafer were flat.

The result is that the transducer scans a wafer whose topography resembles rolling hills, but the transducer sees it as a flat plain. All devices are imaged at the depth of interest, all internal features remain in focus and within the gate, and all rejects are identified without any false rejects due to warpage.

In addition, the Q-DZ outputs a series of contour coefficients/parameters that can be used to monitor processes in manufacturing, for example, encapsulant cure rates, internal stresses, etc.

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