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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?

The introduction by Sonoscan of a new automated bonded wafer inspection system sounds, on the surface, like the routine improvement of an existing system. However, when you look more closely at the capabilities, it becomes clear that the new system (Sonoscan calls it the AW300) marks a departure from ordinary non-destructive acoustic inspection and the emergence of a tool specifically designed to handle shrinking device sizes and expanding wafer sizes with ease and precision. It enables chip makers to achieve high yield of advanced devices at low cost.



Figure 1: New system acoustically inspects two 300mm wafers simultaneously.

What convinced Sonoscan to develop the new system (figure 1) was the unrelenting progress in several different corners of microelectronics. Devices are becoming smaller and consequently harder to manufacture. LEDs, for example, may have an area smaller than 1mm x 1mm, and are often made on 150mm wafers. Other types are made on 300mm wafers.

What this means for chip makers is that it will soon be feasible to fashion thousands or tens of thousands of chips on one wafer or bonded wafer pair. If someday 0.3mm die were made on a 300mm wafer, the yield would be hundreds of thousands of die per wafer. The manufacturer who can achieve this level of production, and who can maximize yield by the early and nondestructive identification and removal of defect chips, will have a good start on achieving a significant market advantage.

The most significant change from previous automated wafer inspection systems is that the new system simultaneously scans two wafers with a unique inertially balanced scanner mechanism. While having multiple transducers greatly increases throughput, it is increased even more by the maximum transducer speed of well beyond 1m per second. During each second, each transducer pulses ultrasound into, and receives return echoes from, thousands of x-y locations.

What the transducers are looking for is any internal gap-type feature. Gap-type features include cracks, voids, delaminations and non-bonded regions. The presence of any one of these anomalies turns a device into a reject. To some degree these terms overlap. An anomaly found at the bondline between two direct-bonded silicon wafers, for example, might be called a non-bond (an area that was simply never bonded) or a delamination (an area that was originally bonded but later became delaminated). The distinction might be important for process modification, but a defect of either type will be reject in order to achieve reliability.

Each pulse of ultrasound sent into the wafer pair or wafer will send back no reflection if it encounters only homogeneous material. The best example is two direct bonded silicon wafers used for SOI. If there are no defects, the only reflections are from the physical top and bottom of the wafer pair, and these reflections are excluded from the acoustic image by precise depth gating. If there are no reflections from the bondline depth, all of the devices can be considered free from bondline defects.

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