The present disclosure provides a semiconductor device with an air gap and a boron nitride cap for reducing capacitive coupling in a pattern-dense region and a method for preparing the semiconductor device. The semiconductor device includes a first metal plug and a second metal plug disposed over a pattern-dense region of a semiconductor substrate. The semiconductor device also includes a third metal plug and a fourth metal plug disposed over a pattern-loose region of the semiconductor substrate. The semiconductor device further includes a boron nitride layer disposed over the pattern-dense region and the pattern-loose region of the semiconductor substrate. A first portion of the boron nitride layer between the first metal plug and the second metal plug is separated from the semiconductor substrate by an air gap, and a second portion of the boron nitride layer between the third metal plug and the fourth metal plug is in direct contact with the semiconductor substrate.