Semiconductor device with thickening layer and method for fabricating the same

A method for fabricating a semiconductor device, comprising:

• providing a substrate with a source region and a drain region, forming a bottom capping layer on the substrate, and forming a word line trench through the bottom capping layer and extending to the substrate;
• conformally forming a word line dielectric layer on the word line trench;
• forming a bottom conductive portion on the word line dielectric layer and within the word line trench;
• conformally forming a layer of first thickening material on the bottom conductive portion, the word line dielectric layer, and the bottom capping layer;
• forming a top conductive portion on the layer of first thickening material and within the word line trench;
• conformally forming a layer of second thickening material on the top conductive portion and the layer of first thickening material;
• forming a layer of top insulating material on the layer of second thickening material, completely filling the word line trench removing portions of the second thickening material, the first thickening material, and the top insulating material to respectively form a top thickening layer, a bottom thickening layer, and a word line capping layer while concurrently recessing the word line dielectric layer;
• forming a top capping layer covering the bottom capping layer, the word line dielectric layer, the word line capping layer, the bottom thickening layer, and the top thickening layer;
• forming a bit line corresponding to the source region to penetrate through the top capping layer and the bottom capping layer and extend into the source region;
• and
• forming a cell contact corresponding to the drain region to penetrate through the top capping layer and the bottom capping layer and extend into the drain region.