Patent ID: 6846744

Claim:
A method for fabricating a bottle-shaped trench for DRAM devices, comprising: providing a substrate having thereon a pad layer; etching said pad layer and said substrate to form a deep trench having a vertical sidewall and bottom surface; depositing a conformal silicon nitride protection layer on said vertical sidewall and bottom surface of said deep trench; ion bombarding an upper portion of said conformal silicon nitride protection layer; selectively etching away said ion bombarded upper portion of said conformal silicon nitride protection layer to expose an upper portion of said vertical sidewall of said deep trench, wherein remaining said conformal silicon nitride protection layer constitutes a silicon nitride hard mask protecting a lower portion of said deep trench; oxidizing said exposed upper portion of said vertical sidewall of said deep trench thereby forming a collar silicon oxide layer above said silicon nitride hard mask; selectively etching away said silicon nitride hard mask; and using said collar silicon oxide layer as an etch mask to isotropic etching said vertical sidewall and said bottom surface of said deep trench that are not covered by said collar silicon oxide layer, thereby forming a bottle-shaped deep trench.