Semiconductor memory device and method of fabricating the same

1. A semiconductor memory device, comprising:

a semiconductor substrate that includes a trench;

a gate insulating layer that covers a bottom surface and an inner side surface of the trench;

a first gate electrode disposed in a lower region of the trench, the first gate electrode comprising a first metal;

a second gate electrode disposed in the trench and on the first gate electrode; and

a barrier layer disposed between the first gate electrode and the second gate electrode, the barrier layer comprising an oxynitride compound containing the first metal,

wherein the gate insulating layer comprises a first portion adjacent to the first gate electrode and a second portion adjacent to the second gate electrode, and

a thickness of the first portion is greater than a thickness of the second portion, when measured in a direction normal to a side surface of the trench.

2. The device of claim 1 , wherein the thickness of the first portion is 40% to 70% of the thickness of the second portion.

3. The device of claim 1 , wherein a width of the second gate electrode is greater than a width of the first gate electrode.

4. The device of claim 1 , wherein the second portion adjacent to the second gate electrode has a higher nitrogen concentration than the first portion adjacent to the first gate electrode.

5. The device of claim 1 , wherein the thickness of the second portion of the gate insulating layer decreases with increasing distance from the substrate.

6. The device of claim 1 , further comprising a first sub-insulating layer interposed between the second portion of the gate insulating layer and the second gate electrode.

7. The device of claim 6 , wherein an inner side surface of the first sub-insulating layer facing the trench is coplanar with an inner side surface of the first portion of the gate insulating layer.

8. The device of claim 1 , further comprising a capping layer disposed in the trench and on the second gate electrode,

wherein the gate insulating layer further comprises a third portion adjacent to the capping layer, and

the thickness of the second portion is greater than a thickness of the third portion, when measured in the direction normal to the side surface of the trench.

9. The device of claim 8 , further comprising a second sub-insulating layer interposed between the third portion of the gate insulating layer and the capping layer.

10. The device of claim 9 , wherein an inner side surface of the second sub-insulating layer that faces the trench is coplanar with an inner side surface of the first portion of the gate insulating layer.

11. The device of claim 8 , wherein a width of the capping layer is neater than a width of the second gate electrode.

12. The device of claim 1 , wherein a work function of the second gate electrode is lower than a work function of the first gate electrode.

13. The device of claim 1 , further comprising a liner layer between the barrier layer and the second gate electrode,

wherein the liner layer extends in a region between the gate insulating layer and the second gate electrode.

14. The device of claim 1 , further comprising an impurity injection region disposed in the active regions,

wherein the impurity injection region comprises a first impurity injection region between the gate lines and a second impurity injection region between the gate lines and the device isolation layer.

15. A semiconductor memory device, comprising:

a substrate that includes active regions arranged in a first direction and are defined by a device isolation layer;

a gate line buried in a trench in an upper portion of the substrate, the gate line crossing the active region in a second direction to divide the active region into first and second doped regions, the second direction crossing the first direction;

a gate insulating layer that separates the trench from the gate line;

a bit line disposed on the gate line, the bit line extending in a third direction crossing both of the first and second directions; and

a capacitor disposed on the substrate and connected to the second doped region,

wherein the gate line comprises:

a first gate electrode disposed in a lower region of the trench, a top surface of the first gate electrode containing oxygen and nitrogen atoms;

a second gate electrode on the first gate electrode; and

a capping layer on the second gate electrode, and

wherein a width of the second gate electrode is greater than a width of the first gate electrode.

16. The device of claim 15 , wherein the gate insulating layer comprises a first portion adjacent to the first gate electrode and a second portion adjacent to the second gate electrode, and

a thickness of the first portion is greater than a thickness of the second portion, when measured in a direction normal to a side surface of the trench.

17. The device of claim 16 , wherein the thickness of the second portion of the gate insulating layer decreases with increasing distance from the substrate.

18. The device of claim 16 , wherein the gate insulating layer further comprises a third portion adjacent to the capping layer, and

the thickness of the second portion is greater than a thickness of the third portion, when measured in the direction normal to the side surface of the trench.

19. The device of claim 15 , further comprising a first sub-insulating layer disposed between the second gate electrode and the gate insulating layer,

wherein a number of crystal defects of the first sub-insulating layer per unit area is less than a number of crystal defects of the gate insulating layer per unit area.

20. The device of claim 15 , further comprising a second sub-insulating layer disposed between the capping layer and the gate insulating layer,

wherein a number of crystal defects of the second sub-insulating layer per unit area is less than a number of crystal defects of the gate insulating layer per unit area.