Semiconductor structure and fabrication method thereof

A method for fabricating a semiconductor structure, comprising:

• providing a substrate having thereon an active area, a device isolation region surrounding the active area, a first bit line structure, a second bit line structure in proximity to the first bit line structure, and a conductive diffusion region in the active area between the first bit line structure and the second bit line structure;
• forming a contact hole between the first bit line structure and the second bit line structure, wherein the contact hole partially exposes the conductive diffusion region;
• depositing a plug layer within the contact hole, wherein the contact hole is completely filled with the plug layer with a sealed void at a bottom of the contact hole;
• and
• etching back the plug layer until the void is opened, thereby forming a buried plug layer at the bottom of the contact hole, wherein the buried plug layer has a U-shaped cross-sectional profile.

A semiconductor structure, comprising:

• a substrate;
• an active area in the substrate;
• a device isolation region surrounding the active area;
• a first bit line structure on the substrate;
• a second bit line structure in parallel with the first bit line structure on the substrate and in proximity to the first bit line structure;
• a conductive diffusion region in the active area between the first bit line structure and the second bit line structure;
• a contact hole between the first bit line structure and the second bit line structure, wherein the contact hole partially exposes the conductive diffusion region;
• a buried plug layer disposed in the contact hole and in direct contact with the conductive diffusion region, wherein the buried plug layer has a U-shaped cross-sectional profile;
• and
• a storage node contact layer disposed on the buried plug layer within the contact hole, wherein a gap is disposed between the storage node contact layer and the U-shaped cross-sectional profile of the buried plug layer.