Dynamic random access memory with buried bit line and method for forming the same

A DRAM, comprising:

• a substrate comprising an active region, wherein the active region comprises two active pillars with neck channel regions, and a shallow recess is formed on surfaces of each of the neck channel regions;
• a buried bit line between the active pillars, and a top surface of the buried bit line is lower than a top surface of the substrate;
• an insulating structure over the buried bit line to separate the active pillars of the active regions an isolation structure to define the active region, and the isolation structure has a width less than a width of the insulating structure;
• and
• a plurality of buried word lines, wherein each of the buried word lines is contained in the shallow recess to surround the neck channel region of each of the active pillars, and the insulating structure is between the buried word lines,
• wherein the shallow recess is a concave recess defined in a sidewall of the neck channel region.