Semiconductor device and preparation method thereof

The invention discloses a semiconductor device and a preparation method thereof, and the preparation method comprises the steps: providing a substrate in which a word line structure is formed, a bit line supporting layer comprises first oxide layers and a first nitride layer, the bit line structure is formed in the first nitride layer, and the first oxide layers are formed at the two sides of the bit line structure and are located in the first nitride layer; patterning the support structure to form a first through hole corresponding to the bit line structure in the bit line support layer; etching the bit line supporting layer to a preset height along the first through hole, adjusting etching parameters and a selective etching ratio of etching gas to the oxide layer and the nitride layer, and continuously etching the bit line supporting layer until the bit line structure is exposed so as to form a polymer layer above the bit line structure, the polymer layer is formed on part of the side wall of the first thro