Anomalous oxynitride loss after steam borophosphosilicate glass (BPSG) flow is observed for a borderless contact structure, and such loss engenders possible bit line shorts to word line. This phenomenon becomes more severe for BPSG film with higher phosphorous concentration. Phosphoric acid generation is deduced as the major mechanism. To enlarge process windows, a BPSG/undoped silicate glass stack has been proposed to withstand etch by acid. From practical application to 256 Mb dynamic random access memory process, this stack also shows a higher bit line contact yield and comparable contact sheet resistance with conventional stacks. More important, this technique could be fully integrated into incumbent ultralarge scale integrated technologies. (C) 2003 The Electrochemical Society.