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Shallow trench isolation (STI) is extensively used as the isolation method beyond 0.18 μm generation. This study explored the formation of circular defects in high-density plasma (HDP) STI deposition. Circular defects were caused by the burst flow of silane reactive gas. The defect maps were coincident with the silane flow field. Fourier transform infrared and secondary-ion-mass spectroscopy data exhibited that the silane-burst flow formed a silicon rich oxide (SRO) film. This SRO film existed between the STI oxide and liner oxide. The circular defects were easily found using optical microscopy (OM) for STI with SRO film. Scanning electron microscopy and transmission electron microscopy photographs show that these defects include bubbles and concavities. When SRO fully covers the liner oxide, bubbles easily form by delamination between SRO film and liner oxide. This correlates with the high tensile stress produced by the SRO film. Besides this, higher STI deposition temperatures yield more bubbles. When SRO discontinuously forms on the liner oxide, the concavities were induced by the variation of STI deposition rate on SRO film and liner oxide. The surface charge difference between the SRO film and the liner oxide is the driving force for the generation of concavities. © 2003 American Vacuum Society.