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The gate oxide is the most fragile element of metal-oxide-semiconductor (MOS) transistor. Metallic contamination in the gate oxide leads to high leak current and even gate oxide breakdown. In this paper, we have investigated a failure case of NMOS gate oxide breakdown using secondary ion mass spectrometry (SIMS) because of its excellent sensitivity. The SIMS depth profiles at the test pad in the scribe line showed that the gate oxide breakdown was caused by tungsten (W) contamination. Further study indicated that W contaminated wafers during n-poly implantation by the re-deposition from the supporting disk of implanter. Based on the SIMS results, measures have been suggested to reduce the W contamination.