Semiconductor structure and method for forming the same

A method for forming a semiconductor structure, comprising:

• providing a substrate structure, the substrate structure comprising a substrate, a device region covering a portion of the substrate, and a trench region in direct contact with the device region and with the substrate, wherein the trench region comprises a trench structure, and the trench structure comprises a trench, a bottom-filling material filling up the trench, and a spacer filling the trench and surrounding the bottom-filling material;
• performing a first etching step to form a buffer space located in the trench and exposing an inclined wall configuration of the spacer;
• forming a dielectric layer filling the buffer space and covering the device region, the bottom-filling material and the inclined wall configuration;
• selectively removing the dielectric layer covering the device region;
• forming a top-filling material filling the buffer space and covering the device region, the bottom-filling material and the inclined wall configuration;
• and
• selectively removing the top-filling material so that the top-filling material fills up the buffer space, wherein there is a discontinuous grain interface disposed between the top-filling material and the bottom-filling material.