Semiconductor device with thickening layer and method for fabricating the same

A semiconductor device, comprising:

• a substrate;
• a word line structure comprising:
• a word line dielectric layer positioned in the substrate and comprising a U-shaped cross-sectional profile;
• a word line conductive layer comprising a bottom conductive portion positioned on the word line dielectric layer and within the substrate, and a top conductive portion positioned on the bottom conductive portion and within the substrate;
• and
• a word line capping layer positioned on the word line conductive layer;
• a bottom thickening layer comprising a U-shaped cross-sectional profile, positioned between the bottom conductive portion and the top conductive portion, between the top conductive portion and the word line dielectric layer, and between the word line capping layer and the word line dielectric layer;
• a bottom capping layer positioned on the substrate and adjacent to the word line dielectric layer;
• and
• a top capping layer covering the bottom capping layer and the word line structure;
• wherein a top surface of the bottom thickening layer and a top surface of the word line dielectric layer are substantially coplanar and are at a vertical level higher than a top surface of the substrate.