Method for preparing semiconductor device with air spacer

A method for preparing a semiconductor device, comprising:

• forming a plurality of composite pillars over a substrate, wherein the composite pillars include conductive pillars and dielectric caps over the conductive pillars;
• transforming a sidewall portion of the conductive pillar into a first transformed portion;
• removing the first transformed portion such that a width of the dielectric cap is greater than a width of a remaining portion of the conductive pillar;
• forming a supporting pillar between adjacent two of the plurality of composite pillars;
• and
• forming a sealing layer at least contacts a top portion of the supporting pillar and a top of the dielectric cap, and air spacers are formed between the sealing layer, the supporting pillar and the remaining portions of the conductive pillars;
• wherein the sealing layer has an intervening portion contacting the top portion of the supporting pillar and the top portion of the dielectric cap, and the intervening portion has a bottom end lower than a bottom end of the dielectric cap.