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Mobile ions may cause critical failures in integrated circuits. For silicon-on-insulator (SOI) wafers, the mobile ions affect not only the reliability of back end of the line but also the performance of the transistors. This paper presents a case study of potassium (K) contamination in the SOI wafer using dynamic secondary ion mass spectrometry. The results showed that the presence of K in chemical and mechanical polish slurry contaminated the porous interlayer dielectrics (ILD), penetrating below the surface due to their porous structure. The K contamination has been greatly reduced by capping the porous ILD with a high-density-oxide layer.