Methods for forming semiconductor structures with buried isolation collars and semiconductor structures formed by these methods

A method for forming a semiconductor structure, comprising:

• implanting dopant ions through an opening in a hardmask to define a doped region in the substrate;
• narrowing the opening in the hardmask;
• etching the substrate through the narrowed opening in the hardmask to form a trench having sidewalls that extend to a base at a depth greater than a projected range of the dopant ions and to define a doped region extending about the sidewalls of the trench that is disposed between the narrowed opening and the base of the trench;
• converting the doped region into a porous region;
• oxidizing the porous region to form an insulator that defines a buried isolation collar;
• and
• after the porous region is oxidized, forming a first capacitor plate of a storage capacitor in the substrate below the buried isolation collar and extending about the sidewalls and the base of the trench.

A method for forming a semiconductor structure, comprising:

• etching a portion of a substrate exposed through an opening in a hardmask to form a trench having sidewalls that extend from a top surface of a substrate to a base beneath the top surface;
• forming a first capacitor plate of a storage capacitor in the substrate about the sidewalls and the base of the trench;
• depositing a first insulator in the trench;
• forming spacers on the sidewalls of the trench above the first insulator;
• recessing the insulator to expose a portion of the sidewalls of the trench between the spacers and the first insulator;
• introducing a dopant into the substrate through the exposed portion of the sidewalls to define a doped region;
• converting the doped region into a porous region;
• and
• oxidizing the porous region to form a second insulator that defines a buried isolation collar.