metrology LEDs
possible to perform manual and automated acoustic imaging with C-SAM systems. Today, the latter option is more common, with a technician placing one or more wafers on a tray, initiating the scan and examining the acoustic image to mark defective die for removal. Manual imaging makes sense when the wafer diameter is small, the value of each LED is high, and the application is critical. Typically up to sixteen 75 mm wafers can be scanned simultaneously. If the wafer is large, or if die are very small, automated inspection may be preferred.
Figure
2.The intended pathway for thermal dissipation from a single LED is downward (red arrows) to the heat sink at the
bottom.Gap-type defects,chiefly in the attachment materials,block heat and lead to early die
failure.Note that the gaps shown in this figure are greatly exaggerated in their vertical dimension; gaps that are effective heat blockers and that provide high-amplitude reflection for acoustic imaging can be as thin as a fraction of a micron
Imaging epiwafers...
Most LED wafers feature a stack of nitride-based epitaxial layers on a substrate, typically sapphire. When imaging these wafers, our C-SAM system tends to expose delaminations and other gap-type defects, either in the sapphire substrate or the epitaxial film. These defects and anomalies may be just 5 µm or so in size, so to operate with a sufficiently high spatial resolution the transducer pulses a high frequency, such as 230 MHz. It is
Today, the most common LED wafer is 75 mm (3 inches), but dimensions range from 50 mm (2- inch) to 150 mm (6 inches). Eventually, 200 mm (8- inch) and 300 mm (12-inch) wafers will be used in order to gain economies of scale. This means that wafers will accommodate more die than ever before – if they are 0.3 mm x 0.3 mm in size, a common dimension for today’s LEDs, then a 300 mm wafer could yield about 700,000 chips. Such high volumes will make manual inspection more difficult and favour automated inspection.
The defects that our C-SAM system identifies are often incredibly thin delaminations. When one of the thousands of ultrasonic pulses entering the wafer each second hits the interface between solid material and a delamination, more than 99.99 percent of the ultrasound is reflected to the transducer. This incredibly high level of reflection occurs when the gap is as thin as 0.01 µm, due to the massive difference in the material properties of the solid material and the air in the delamination. This scenario produces the highest amplitude signal and identifies an internal gap. Continued scanning reveals the total area of the delamination.
With manual imaging, a technician visually identifies defective LED die in the acoustic image, so that they can be discarded from production. In
Left: Based on Sonoscan’s well-known C-SAM acoustic microscope systems and launched in 2011,the AW300 300 mm bonded wafer inspection system automates the entire inspection system from carrier attachment and wafer selection through aligning and acoustic imaging to drying and sorting. Analysis software automatically measures the percentage of bonded and unbonded interface between the two wafers,and the sizes and number of voids. Accept/reject decisions are made automatically according to the user’s specific criteria
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www.compoundsemiconductor.net October 2012
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