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Rapid acoustic inspection for 300MM wafer generation (Part 1)

Posted: 30 Dec 2011 ?? ?Print Version ?Bookmark and Share

Keywords:wafer inspection system? scanner? delaminations? bondline defects?

Ultrasound is reflected by interfaces between two materials. In wafer pairs bonded with an adhesive material or intermediate layer, there will be a small and predictable reflection from the interfaces on either side of the material. When an ultrasonic pulse strikes the interface between two solid materials, the amplitude of the reflection will be moderate, and in an acoustic image the interface will be some shade of gray. Where there is no interface, the acoustic image will be black.

However, where the pulse encounters the interface between a solid and the air in a gap-type defect, reflection is virtually total, and this area in the acoustic image will be bright white. In high-throughput automated inspection, of course, there might be no requirement for a visible acoustic image and no technician would be needed to observe the image; in that case defects are identified and defective die are recorded by software.

Figure 2: Two direct-bonded silicon wafers with a non-bonded region at the interface.

The types of defects that occur depend on the processes and materials in a particular application. In direct-bonded silicon wafers acoustic imaging may find non-bonded regions (figure 2) between the wafers, or cracks in the wafers. Non-bonds are sometimes caused by small particles that became lodged between the two wafers. Typically these non-bonds are roughly circular, the diameter of the circle being determined by the height of the particle.

Figure 3: Wall break in a MEMS device.

The same particle-caused non-bonds have been observed in bonded MEMS wafers. More common, though, are defects in the seal that surrounds the MEMS cavity (figure 3). The integrity of the seal is absolutely necessary for the MEMS device to function. The seals may be found acoustically to have one or more breaks, or to have voids that narrow the wall width and make it susceptible to failure in service.

Figure 4: Delamination among the built-up layers on an LED wafer.

LEDs wafers, and especially High-Brightness LED wafers, are in a different category. The base is typically a sapphire wafer, to which a very thin GaN wafer may be bonded. Numerous other layers are built up on these wafers. Defects may thus occur between layers and between the sapphire wafer and the very thin GaN wafer bonded to it. Acoustic imaging primarily looks for gaps between the die and between the built-up layers (figure 4), since these gaps can cause the layers to fall of when the wafer is diced. The risk of layer separation is even greater if the die are snapped apart.

About the author
Tom Adams is a consultant for Sonoscan Inc.

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