Semiconductor device and fabrication method thereof

A method for forming a semiconductor device, comprising:

• providing a stacked substrate including a substrate, an insulating layer on a surface of the substrate, a semiconductor layer on a surface of the insulating layer;
• forming a plurality of first openings in the semiconductor layer to expose the insulating layer at a bottom surface of the plurality of first openings, wherein a first distance is defined between adjacent sidewalls of adjacent first openings;
• forming spacers on sidewall surfaces of each first opening in the semiconductor layer;
• etching the insulating layer and the substrate through the bottom surface of each first opening employing the semiconductor layer and the spacers as an etch mask to form a plurality of second openings through the insulating layer and into the substrate;
• etching sidewall surfaces of the substrate exposed in the second openings to define a second distance between adjacent substrate sidewalls of adjacent etched second openings, wherein the second distance is shorter than the first distance;
• forming an isolation layer in the plurality of second openings and the plurality of first openings;
• forming a gate structure on a surface portion of the semiconductor layer between adjacent isolation layers;
• and
• forming conductive structures on surface portions of the semiconductor layer on both sides of the gate structure, wherein the conductive structures penetrate through the isolation layer and into the substrate of the stacked substrate.