Semiconductor device and method of forming the same

A semiconductor device comprising:

• a substrate;
• a plurality of bit line structures formed on the substrate, each bit line structure extending in a first direction,
• wherein each bit line structure includes a first semiconductor layer, a conductive layer on the first semiconductor layer, and a capping layer on the conductive layer, wherein the conductive layer is disposed above the first semiconductor layer and below the capping layer;
• a plurality of word lines, each word line extending in a second direction different from the first direction;
• a plurality of contact structures, each provided between two adjacent bit line structures of the plurality of bit line structures,
• wherein each contact structure includes a lower contact portion and an upper contact portion, wherein a bottom surface of the upper contact portion contacts a top surface of the lower contact portion at a contact structure interface;
• and
• a plurality of spacer structures, each spacer structure located between a bit line structure and a contact structure and including at least an air gap, an insulating spacer between the air gap and the contact structure, and a capping spacer, wherein
• each capping spacer covers a top of a corresponding air gap, extends between a bit line structure and a contact structure from above the top of the corresponding air gap to a first height below the top of the corresponding air gap, and fills an area between the insulating spacer and the contact structure, wherein the first height is lower than a top of the conductive layer of a bit line structure adjacent to the capping spacer.