metrology LEDs
depth and orientation; and there are four round, dark features, which are electrical connections below the die. The die themselves display a light-to-medium grey texture, indicating that they are well bonded to the substrate. There are no bright areas here that could indicate the presence of voids or disbonds (the latter are separations at an adhesive bond line in a bonded joint).
Figure 4. After the lens has been ground flat, topside acoustic imaging of this tiny LED can reveal essentially total delamination of the polymer from the substrate
solder between the heat sink and substrate, and between the substrate and the die. Such defects will be visible as bright features in the acoustic image. Meanwhile, scanning the top surface of the LED surface – so long as either no lens is in place, or it has been ground to create a flat scannable surface – can offer an insight into the internal features of the device.
Structural defects that are not related to heat removal can also be revealed in acoustic images. For example, images of one corner of an LED array can show features that are probably vertical cracks in the die (see Figure 3, which has a yellow arrow that marks out one of these features).
In addition, there are some lighter-coloured features of similar structure in same area, which are probably cracks that reflect ultrasound differently, due to variations in
Imaging from the topside can provide valuable insights into the workings of an LED. Our acoustic imaging tool has uncovered deficiencies in a single, 0.3 x 0.3 mm die that was inspected after removing part of the polymer lens covering the die.
Pulsing ultrasound through the remaining polished lens material revealed that the square shape of the die, at centre, is somewhat distorted, due to wires attached to its upper right and lower left corners (see Figure 4).
This image also shows the leads, which appear as large white features on the left and right. The red region completely surrounding the die reveals that the polymer is completely disbonded from the substrate below the die. It is possible that the disbond may already extend into the die attach material below the die, where it would reflect heat back to the die.
Expansion of the disbond under the die could also take place at a later time. This can cause individual LEDs to fail in service and to lower the efficacy of the luminaire.
Exposing these issues, which can ultimately lead to improvements in the quality of LED lighting, is one of the hallmarks of our acoustic imaging tool. As awareness of its capabilities grows, and engineers in the fabs increase their understanding of this instrument and its strengths, it will play an ever-increasing role in the solid-state lighting revolution.
© 2012 Angel Business Communications. Permission required.
Introduced in 2011,Sonoscan’s Gen6 high-end
laboratory acoustic microscope is best known for its highly intuitive Sonolytics software interface and
from being imaged in horizontal “slices.
its PolyGate module.Certain samples and materials benefit ”PolyGate permits automatic
and simultaneous imaging of a sample at up to 200 different gates (horizontal “slices”)
26
www.compoundsemiconductor.net October 2012
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