Semiconductor device having semiconductor layers disposed between upper portions of the isolation structures and the active regions

1. A semiconductor device, comprising:

a substrate;

a plurality of active regions disposed in the substrate, respectively extending along a first direction and arranged into an array;

a plurality of isolation structures disposed in the substrate between the active regions and respectively comprising an upper portion having a first thickness and a lower portion having a second thickness, wherein a sidewall of the upper portion comprises a first slope, a sidewall of the lower portion comprises a second slop, and the first slope and the second slope are different; and

a plurality of semiconductor layers disposed between the upper portions of the isolation structures and the active regions.

2. The semiconductor device according to claim 1 , wherein the substrate comprises silicon, the semiconductor layers comprises silicon, germanium, or silicon germanium.

3. The semiconductor device according to claim 1 , wherein bottom ends of the semiconductor layers are flush with boundaries between the upper portions and the lower portions of the isolation structures.

4. The semiconductor device according to claim 1 , wherein the second thickness is larger than the first thickness.

5. The semiconductor device according to claim 1 , wherein top ends of the semiconductor layers are not coplanar.

6. The semiconductor device according to claim 1 , further comprising voids formed in the lower portions of the isolation structures.

7. The semiconductor device according to claim 1 , further comprising:

a plurality of word lines disposed in the substrate, extending parallel along a second direction and cutting through the isolation structures and the active regions to divide each of the active regions into a center portion and two terminal portions;

a plurality of bit lines disposed on the substrate, extending parallel along a third direction and directly contacting the center portions of the active regions and the semiconductor layers at two sides of the center portions, wherein the first direction, the second direction, and the third direction are different directions, and the second direction and the third direction are perpendicular; and

a plurality of contact plugs disposed on the substrate and between the bit lines and directly contacting the terminal portions of the active regions.

8. The semiconductor device according to claim 7 , wherein the contact plugs are in direct contact with the semiconductor layers at one side of each of the terminal portions.

9. The semiconductor device according to claim 7 , wherein bottom ends of the isolation structures between the terminal portions are lower than bottom ends of the isolation structures at two sides of the center portions.

10. The semiconductor device according to claim 7 , wherein bottom ends of the semiconductor layers are lower than bottom surfaces of the contact plugs.

11. The semiconductor device according to claim 7 , wherein the word lines respectively comprise:

a conductive layer; and

an insulating capping layer disposed on the conductive layer, wherein top ends of the semiconductor layers are flush with top surfaces of the insulating capping layers, and bottom ends of the semiconductor layers are lower than bottom surfaces of the insulating capping layers.