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A broken wafer is a loss and an opportunity. When a wafer breaks during fab processing, the first impulse is to clean up the mess and trash the broken pieces. But the fracture markings on the broken edges actually represent a detailed recording of the rapid sequence of events that took place during the breakage event. By examining the fracture markings with a microscope, it is often possible to discover how and why the wafer broke so that appropriate corrective and preventive actions can be taken.
The "Fracture Analysis Procedure" outlines the analysis procedure and gives two references where instructive descriptions and sample photos and drawings can be found. The photos on this web page show samples of the most commonly encountered fracture markings, rib lines and tear marks. The photo captions tell how to interpret rib lines and tear marks and describe four cases where fracture analysis was useful in solving a wafer fab breakage problem.
A wafer begins to break apart when the silicon at some surface location is subjected to a sufficiently large tensile stress. The tensile stress can arise due to wafer bending, uneven cooling, penetration, or during an impact when the silicon is compressed in one direction but pulled apart in other directions. The stress required to break a wafer is greatly reduced if the wafer surface has a stress concentrator such as a preexisting crack.
The goal of fracture analysis is to discover how and why a wafer broke so that future occurrences can be prevented. If a wafer broke because of a weakness, such as a scratch that occurred at a prior processing step, then that weakness can be eliminated. If a wafer broke because it was bent or penetrated by a piece of wafer handling equipment, then that equipment can be adjusted. A broken wafer is not only a loss but an opportunity for improvement.
Fracture Analysis Procedure
- Collect samples without destroying the evidence.
- Use a scale drawing to record and report the results.
- Examine the fracture markings on the broken edges to determine the direction of travel for each crack that split the wafer apart.
- Trace the crack(s) back to the approximate origin of the breakage.
- Examine the fracture markings to determine the exact origin.
- Examine the silicon at and around the origin for evidence of why the wafer broke.
Descriptions and photos of fracture markings are given in the following references:
- Lawrence Dyer, Jeffrey Seaman, and Dennis Olmstead, "Hertzian Contact and Thermal Fracture of Silicon Wafers During Processing", in Semiconductor Silicon 1990, Proceedings Volume 90-7, edited by H. R. Huff, K. G. Barraclough, and Jun-ichi Chikawa, (The Electrochemical Society, Pennington, NJ, 1990), pp. 156-167.
- Lawrence D. Dyer, "Fracture Tracing in Semiconductor Wafers", in Semiconductor Processing, ASTM Special Technical Publication 850, edited by D. C. Gupta, (American Society for Testing and Materials, Philadelphia, PA, 1984), pp. 297-308.
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