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Thin wafer processing - yield enhancement through integrated metrology
Keywords: Thin wafer handling , yield enhancement, metrology
Thin wafer handling and processing is performed by temporary bonding to a rigid carrier wafer. The rigid carrier wafer gives mechanical support during wafer thinning and backside processing. Finally the thin wafer is debonded from the carrier wafer and attached to a dicing tape on film frame. While this technology has been demonstrated for a couple of years now in pilot line and small volume, it is an entirely different story to transfer such a technology to high volume manufacturing (HVM). Yield is the most important consideration for the transfer to HVM. At this point of the manufacturing flow the device wafers have seen complete front end processing and have a significant inherent value. Furthermore, wafer breakage does not only destroy the wafer, but it might be necessary to take the production tool down for chamber cleaning. However, wafer breakage itself is not the only concern. Defects at the wafer edge can nucleate micro cracks in the die, which can result in device failure at a later point of time. Thickness variations in the thin wafer create severe problems during TSV reveal and other downstream processes. The total thickness variation (TTV) of the thin wafer depends on the following: TTV of the carrier wafer, TTV of the adhesive film, the bond process and the thinning process (coarse grind, fine grind, stress relief etch). Current state-of-the-art processing enables 4-5µm cumulative TTV, with a roadmap target of 3µm. As the TTV of the thin wafer relies on so many contributing parameters, further process optimization has to be based on accurate metrology. A new metrology module integrated to the EVG850 platform allows measuring TTV of the carrier wafer, TTV of the adhesive film and TTV of the full stack. All these TTV parameters can be measured as a single measurement after bonding. In addition bow and warp of the temporarily bonded wafer stack are measured. A special metrology stage enables to scan the complete wafer at cycle times compatible with 100% inspection during production. Defects in the bond interface typically result in severe problems during downstream processing. Entrapped gas, delamination between adhesive film and wafer or coating defects can all potentially result in damage to the thin wafer. The integrated metrology module detects such defects simultaneously to measuring the TTV of the wafer. In this paper the potential defects during thin wafer processing are reviewed. A new integrated metrology module is presented, which allows 100% in-line inspection with full wafer scanning.
Daniel Burgstaller, Director of Technology
EV Group, Inc.
Tempe, AZ

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