| When deprocessing, it is important to remove each layer totally before proceeding to the next layer. The SEM photo on the right (Figure 1) is an IC which has been etched for 15 minutes in wet nitride etch at 115-125°C. That etch time is about 2 minutes longer than necessary to remove 1 micron of nitride passivation. The over etch time assures that no nitride remains. Residual plasma nitride would act as a mask preventing uniform removal of underlaying oxide as shown below (Figure 2). |  Figure (1) This is a SEM photo of an IC after wet nitride etch. |
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| The SEM image on the right (Figure 2) is a piece of the same IC wafer
above. After the nitride passivation was removed, interlayer dielectric layers were
removed by etching in 49% HF at room temperature for about 7 seconds. The HF removed
the oxide portion of passivation and most of the interlayer oxide. The gate
polysilicon, metal 1, and metal 2 are all exposed. The most critical part of this process is the timing of the HF dip. There is little tolerance between just exposing the under layers and causing the second layer of metal to be undermined and float away. This problem is best handled by breaking up the etch time into 2-3 second segments. If the intent is to remove metal 2 leaving metal 1 intact, then metal etch should be used directly after wet nitride etch. |
 Figure (2) This is a SEM image after an HF dip to remove exposed interlayer dielectric. |
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| Wafer Failure Analysis - preliminary table of contents
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| For questions
or comments about the technical content of this site, Email: davidburgess@AcceleratedAnalysis.com |
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