This invention disclosed a novel manufacturing approach of collector and buried layer of a bipolar transistor. One aspect of the invention is that an oxide-nitride-oxide (ONO) sandwich structure is employed instead of oxide-nitride dual layer structure before trench etching. Another aspect is, through the formation of silicon oxide spacer in trench sidewall and silicon oxide remaining in trench bottom in the deposition and etch back process, the new structure hard mask can effectively protect active region from impurity implanted in ion implantation process.