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Rapid acoustic inspection for 300MM wafer generation (Part 2)

Posted: 09 Jan 2012 ?? ?Print Version ?Bookmark and Share

Keywords:wafer inspection? transducer? acoustic imaging?

The AW300 wafer inspection system from Sonoscan was designed to handle and inspect 300mm wafers, as the name suggests, although it can also inspect 200mm wafers. Its multiple transducers and two stages handle two wafers at a time, and are completely automated. Wafer handling, wafer inspection, and data analysis are all automated. Carriers of wafers enter the system, and when the carriers emerge the complete analysis of accept and reject, according to the user's definitions, is available.

Two of the key elements in achieving this high performance are precision in handling and high resolution in acoustic imaging. When a FOUP or other carrier type arrives at the in-line system, the robot that handles the wafers first checks the carrier to see how many wafer pairs it contains and how they are arranged. If it is a 25-wafer carrier that holds only 10 wafers, the robot takes note of this. If there are gaps in the carrierwafers in slots 16 and 18, for example, but none in slot 17this is also noted. In some cases the FOUP itself may have been left out of alignment by a previous handling operation. The robot also rotates and positions the wafer into the correct orientation for scanning.

The first move is to the scanning stage, where the robot places the wafer pair very precisely. Next a gentle vacuum is applied evenly to the bottom surface of the wafer pair in order to help hold it in position during ultrasonic scanning. At this point the transducer and its waterfall mechanism begin raster-scanning back and forth across the wafer at speeds up to 1 meter or beyond per second. The waterfall mechanism provides a constant tiny stream of water to provide coupling between the transducer and the wafer. The coupling is required because ultra high frequency ultrasound does not travel through air. It is the alternative to an immersion bath that is more cumbersome, slows the scan speed, and complicates handling of the wafers.

Figure: Defects (white) near the edge of a bonded wafer pair. (Acoustic image courtesy SonoLab, www.sonoscan.com)

As the transducer races back and forth across the wafer pair, data from thousands of x-y coordinates per second is collected. Software identifies defects that the user is interested in: particles, non-bonds and delaminations between direct-bonded silicon wafers (figure 1), gaps in cavity seals in MEMS wafers, delaminations between built-up layers on sapphire LED wafers, etc. What is being built up is a large data set that can be used to identify all die that fail to meet the user's criteria. On small wafers with relatively large die, a technician might observe the acoustic image onto which the wafer map has been overlaid in order to spot and remove the defect die, but at 300mm, especially with very small die, both the identification and removal are automated.

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