Memory devices having low permittivity layers and methods of fabricating the same

1. A memory device, comprising:

bit lines extending in a first direction on a substrate;

word lines configured to vertically cross the bit lines;

memory cells at intersections of the bit lines and the word lines;

a first low permittivity layer filling spaces between the bit lines and partially filling spaces between the memory cells on bottom surfaces of the word lines;

a first dielectric layer stacked on an upper surface of the first low permittivity layer between the memory cells;

a second dielectric layer filling spaces between the memory cells on upper surfaces of the bit lines; and

a second low permittivity layer stacked on an upper surface of the second dielectric layer and filling spaces between the word lines,

wherein the first and second low permittivity layers have lower permittivity than the first and second dielectric layers.

2. The device of claim 1 , wherein the upper surface of the first low permittivity layer includes a first surface located at the same level as the upper surface of the bit line, and a second surface located at a higher level than the first surface and between the memory cells.

3. The device of claim 2 , wherein the second surface of the first low permittivity layer defines a horizontal through hole, and the first surface of the first low permittivity layer includes a groove.

4. The device of claim 3 :

wherein the second dielectric layer contacts the first surface of the first low permittivity layer and fills the groove;

wherein a side of the second dielectric layer contacts a side of the first low permittivity layer; and

wherein the through hole is formed as an air gap that is an enclosed space.

5. The device of claim 4 :

wherein a first set of parallel sides of the memory cells each contact sides of the first low permittivity layer having the air gap and the first dielectric layer; and

wherein a second set of parallel sides, opposite the first set, of the memory cells each contact a side of the second dielectric layer.

6. The device of claim 1 , wherein the first dielectric layer is only between the first low permittivity layer and the word lines.

7. The device of claim 1 :

wherein a first set of parallel sides of the memory cells each contact the first low permittivity layer and the first dielectric layer, which are sequentially stacked; and

wherein a second set of parallel sides, opposite the first set, of the memory cells each contact the second dielectric layer.

8. The device of claim 1 :

wherein the first low permittivity layer and the second low permittivity layer have permittivity of no greater than 4; and

wherein the first low permittivity layer and the second low permittivity layer comprise one of include silicon oxide, fluorine-doped silicon dioxide, carbon-doped silicon dioxide, and porous silicon dioxide.

9. The device of claim 1 , wherein the first dielectric layer and the second dielectric layer comprise one of silicon nitride (SiN x ) and aluminum oxide (Al 2 O 3 ).

10. The device of claim 1 :

wherein the memory cells each include a diode and a variable resistance device, connected in series; and

wherein the variable resistance device comprises a first electrode, a variable resistor, and a second electrode, which are sequentially stacked.

11. The device of claim 10 , wherein the upper surface of the first low permittivity layer is located at a lower level than an upper surface of the first electrode.

12. The device of claim 10 , wherein a bottom surface of the second low permittivity layer is located at a higher level than a bottom surface of the second electrode.

13. A memory device, comprising:

bit lines extending in a first direction on a substrate;

word lines vertically crossing the bit lines;

memory cells at intersections of the bit lines and the word lines;

a first dielectric layer filling spaces between the bit lines and between the memory cells on bottom surfaces of the word lines, and defining an air gap;

a second dielectric layer configured to fill spaces between the memory cells on upper surfaces of the bit lines; and

a first low permittivity layer stacked on an upper surface of the first dielectric layer and filling spaces between the word lines,

wherein the first low permittivity layer has lower permittivity than the first dielectric layer and the second dielectric layer.

14. The device of claim 13 :

wherein the second dielectric layer is stacked on upper surfaces of the bit lines; and

wherein a first surface of the first dielectric layer located at the same level as the upper surfaces of the bit lines, contacts a side of the first dielectric layer, and becomes part of a boundary of the air gap.

15. The device of claim 13 :

wherein a first set of parallel sides of the memory cells contact a side of the first dielectric layer including the air gap; and

wherein a second set of parallel sides, opposite the first set, contact a side of the second dielectric layer.

16. The device of claim 13 , further comprising:

first bit lines and second bit lines extending in the first direction and vertically spaced apart on a substrate;

word lines between the first and second bit lines and configured to vertically cross the first and second bit lines;

first memory cells at intersections of the first bit lines and the word lines;

second memory cells at intersections of the word lines and the second bit lines;

a first low permittivity layer and a first dielectric layer stacked on a first separation space between the bit lines and between parallel sides of the first memory cells;

a second dielectric layer, a second low permittivity layer, and a third dielectric layer stacked on a second separation space between the other sides of the first memory cells, between the word lines, and between parallel sides of the second memory cells; and

a fourth dielectric layer and a third low permittivity layer stacked on a third separation space between the other sides of the second memory cells and between the second bit lines,

wherein the first, second and third low permittivity layers have lower permittivity than the first, second, third and fourth dielectric layers.

17. The device of claim 16 , wherein an upper surface of the second low permittivity layer includes a first surface located at the same level as upper surfaces of the word lines and a second surface located at a higher level than the first surface; and

wherein the second surface contacts a bottom surface of the third dielectric layer.

18. The device of claim 16 , wherein a bottom surface of the second dielectric layer contacts upper surfaces of the first bit lines and a side of the first low permittivity layer.

19. The device of claim 16 , further comprising an air gap inside the first low permittivity layer.

20. A memory device, comprising:

first bit lines and second bit lines extending in a first direction and vertically spaced apart on a substrate;

word lines between the first and second bit lines and configured to vertically cross the first and second bit lines;

first memory cells at intersections of the first bit lines and the word lines;

second memory cells at intersections of the word lines and the second bit lines;

a first dielectric layer configured filling a first separation space between the first bit lines and between parallel sides of the first memory cells, and defining an air gap therein;

a second dielectric layer, a first low permittivity layer, and a third dielectric layer stacked on a second separation space between remaining sides of the first memory cells, between the word lines, and between parallel sides of the second memory cells; and

a fourth dielectric layer and a second low permittivity layer stacked on a third separation space between remaining sides of the second memory cells and between the second bit lines,

wherein the first and second low permittivity layers have lower permittivity than the first, second, third and fourth dielectric layers.