Methods for forming semiconductor structures with buried isolation collars

A method for forming a semiconductor structure, comprising:

• patterning a hardmask to define an opening;
• implanting dopant ions through the opening to define a doped region in a substrate;
• narrowing the opening in the patterned hardmask;
• etching a surface of the substrate exposed through the narrowed opening in the patterned hardmask to a depth greater than a projected range of the ions to form a trench having sidewalls that extend from the surface to a bottom wall beneath the surface and to define a doped region extending about the sidewalls of the trench that is registered between the opening and the sidewalls of the trench;
• forming a porous region in the substrate about the sidewalls of the trench and at a depth between the surface of the substrate and the bottom wall of the trench;
• and
• converting the porous region to an insulator to define a buried isolation collar about the trench.

A method for forming a semiconductor structure, comprising:

• forming a trench having sidewalls that extend from a surface of a substrate to a bottom wall beneath the surface;
• filling the trench with an insulator;
• forming spacers on the sidewalls of the trench above the insulator;
• recessing the insulator to expose a portion of the sidewalls of the trench between the spacers and the insulator;
• introducing a dopant into the substrate through the exposed portion of the sidewalls to define a doped region;
• forming a porous region in the substrate about the sidewalls of the trench and at a depth between the surface of the substrate and the bottom wall of the trench;
• and
• converting the porous region to an insulator to define a buried isolation collar about the trench.

A method for forming a memory cell, comprising:

• forming a trench in a substrate;
• forming a first capacitor plate of a storage capacitor in the trench;
• forming a second capacitor plate buried in the substrate about the trench;
• forming a transistor having a first source/drain region coupled electrically with the first capacitor plate and a second source/drain region;
• forming a porous region in the substrate about sidewalls of the trench and at a depth between the second source/drain region and the second capacitor plate;
• and
• converting the porous region to an insulator to define a buried isolation collar electrically isolating the first source/drain region from the second capacitor plate.