A semiconductor structure with an air gap, comprising: a substrate ; a dielectric stack (DL) comprising a first dielectric layer disposed on the substrate , a second dielectric layer disposed on the first dielectric layer , and a third dielectric layer disposed on the second dielectric layer ; a first conductive layer (MI1) disposed in the dielectric stack (DL); a second conductive layer (MI2) disposed in the dielectric stack (DL) and spaced apart from the first conductive layer (MI1), wherein the first conductive layer (MI1) and the second conductive layer (MI2) are coplanar; and a cross-like-shaped air gap (AG) disposed in the dielectric stack (DL) between the first conductive layer (MI1) and the second conductive layer (MI2), wherein the air gap (AG) has a widened middle portion (AG_M) in the second dielectric layer , a tapered upper portion (AG_U) in the third dielectric layer , and a tapered lower portion (AG_L) in the first dielectric layer , wherein the widened middle portion has a first width (W1), the tapered upper portion has a second width (W2), and the tapered lower portion has a third width (W3), and wherein the first width is greater than the second width and the third width, and wherein the third width is smaller than or equal to the second width, characterized in that the third dielectric layer comprises an extension portion that extends into the cross-like-shaped air gap (AG) and conformally covers a sidewall of the first dielectric layer adjacent to the tapered lower portion (AG_L) of the cross-like-shaped air gap (AG) and a sidewall of the second dielectric layer adjacent to the widened middle portion (AG_M) of the cross-like-shaped air gap (AG). a substrate ; a dielectric stack (DL) comprising a first dielectric layer disposed on the substrate , a second dielectric layer disposed on the first dielectric layer , and a third dielectric layer disposed on the second dielectric layer ; a first conductive layer (MI1) disposed in the dielectric stack (DL); a second conductive layer (MI2) disposed in the dielectric stack (DL) and spaced apart from the first conductive layer (MI1), wherein the first conductive layer (MI1) and the second conductive layer (MI2) are coplanar; and a cross-like-shaped air gap (AG) disposed in the dielectric stack (DL) between the first conductive layer (MI1) and the second conductive layer (MI2), wherein the air gap (AG) has a widened middle portion (AG_M) in the second dielectric layer , a tapered upper portion (AG_U) in the third dielectric layer , and a tapered lower portion (AG_L) in the first dielectric layer , wherein the widened middle portion has a first width (W1), the tapered upper portion has a second width (W2), and the tapered lower portion has a third width (W3), and wherein the first width is greater than the second width and the third width, and wherein the third width is smaller than or equal to the second width, characterized in that the third dielectric layer comprises an extension portion that extends into the cross-like-shaped air gap (AG) and conformally covers a sidewall of the first dielectric layer adjacent to the tapered lower portion (AG_L) of the cross-like-shaped air gap (AG) and a sidewall of the second dielectric layer adjacent to the widened middle portion (AG_M) of the cross-like-shaped air gap (AG). <EOS>