Rail stack array of charge storage devices and method of making same

1. A monolithic three dimensional array comprising:

a) a first device level of charge storage transistors, the first device level comprising:

i) a plurality of spaced apart first rails extending in a first direction; and

ii) a plurality of spaced apart second rails formed over the first rails extending in a second direction different from the first direction,

wherein the first rails comprise first portions of the charge storage transistors and the second rails comprise second portions of the charge storage transistors; and

b) a second device level of charge storage transistors monolithically formed above the first device level.

2. The array of claim 1 , wherein the second device level comprises:

a plurality of spaced apart third rails extending in the first direction; and

a plurality of spaced apart fourth rails formed over the third rails extending in the second direction;

wherein the third rails comprise first portions of the charge storage transistors of the second device level and the fourth rails comprise second portions of the charge storage transistors of the second device level.

3. The array of claim 1 , wherein the first device level is disposed above a monocrystalline semiconductor substrate.

4. The array of claim 3 , wherein driver circuits are disposed in the monocrystalline semiconductor substrate.

5. The array of claim 1 , wherein:

the array comprises at least four device levels containing the charge storage transistors; and

adjacent device levels are separated by a respective interlevel insulating layer.

6. The array of claim 1 , further comprising:

an interlevel insulating layer located between the first and second device levels;

a via having a bottom end, the via extending between the first and second device levels through the interlevel insulating layer; and

a contact which extends between the first and second device levels in the via to contact an active region in one of the first rails at the bottom end of the via.

7. The array of claim 1 , wherein:

each first rail comprises a plurality of polysilicon active regions and each of the plurality of active regions is in contact with a charge storage region.

8. The array of claim 7 , wherein:

the first direction is substantially perpendicular to the second direction; and

each of the charge storage regions comprises an O—N—O dielectric stack.

9. The array of claim 8 , further comprising at least two conductive layers which contact each first rail.

10. The array of claim 9 , wherein:

the charge storage transistors are formed at intersections of each first rail and each second rail;

each transistor active region comprises source, channel and drain regions;

each transistor comprises a gate electrode disposed in one of the second rails.

11. The array of claim 10 , wherein the second rails comprise word lines.

12. The array of claim 11 , further comprising a third device level of charge storage transistors formed between the first and the second device levels.

13. A monolithic, three dimensional array of charge storage devices, comprising a plurality of device levels monolithically formed above a substrate, wherein:

each of the plurality of device levels comprises a plurality of spaced apart semiconductor active regions and a plurality of spaced apart rails;

the plurality of active regions are disposed at a first height;

the plurality of rails are disposed at a second height different than the first height; and

wherein the active regions comprise first portions of the charge storage devices and the rails comprise second portions of the charge storage devices.

14. The array of claim 13 , wherein:

each charge storage device comprises a first portion located in one active region and a second portion located in one portion of one rail;

each rail comprises a word line; and

a charge storage film is located in contact with the word line and in contact with the plurality of the active regions.

15. The array of claim 14 , wherein a space between said spaced apart rails is filled with an insulating material.

16. The array of claim 14 , wherein:

the active regions comprise polysilicon; and

the charge storage film comprises an O—N—O dielectric stack.

17. The array of claim 14 , wherein:

the charge storage devices are located in at least four device levels; and

adjacent device levels are separated by a respective interlevel insulating layer.

18. The array of claim 14 , further comprising at least two conductive layers which contact each active region.

19. The array of claim 18 , wherein:

the charge storage devices comprise charge storage transistors;

source, channel and drain regions of each transistor are located in each active region; and

each transistor gate electrode comprises a portion of the word line which intersects the transistor active region.

20. The array of claim 19 , wherein:

the semiconductor active regions are disposed in elongated semiconductor strips having two vertical side surfaces and a horizontal surface which contacts the charge storage film; and

the rails comprise elongated strips having two vertical side surfaces and a horizontal word line surface.

21. The array of claim 20 , wherein the charge storage devices are located at intersections of the elongated semiconductor strips and the rails.

22. The array of claim 13 , wherein the substrate is a monocrystalline semiconductor substrate.

23. The array of claim 13 , wherein driver circuits are disposed in the substrate.

24. The array of claim 14 , further comprising:

an interlevel insulating layer located between first and second device levels of the plurality of device levels;

a via having a bottom end, the via extending between the first and second device levels through the interlevel insulating layer; and

a contact which extends between the first and second device levels in the via to contact an active region at the bottom end of the via.

25. A method of making a monolithic three dimensional array of charge storage transistors disposed above a substrate, comprising:

a) forming a first device level of charge storage transistors, the first device level comprising:

i) a plurality of spaced apart first rails extending in a first direction; and

ii) a plurality of spaced apart second rails formed over the first rails extending in a second direction different from the first direction;

wherein the first rails comprise first portions of the charge storage transistors and the second rails comprise second portions of the charge storage transistors; and

b) monolithically forming a second device level of charge storage transistors above the first device level.

26. The method of claim 25 , wherein the second device level comprises:

a plurality of spaced apart third rails extending in the first direction; and

a plurality of spaced apart fourth rails formed over the third rails extending in the second direction;

wherein the third rails comprise first portions of the charge storage transistors of the second device level and the fourth rails comprise second portions of the charge storage transistors of the second device level.

27. The method of claim 25 , wherein:

the step of forming the first rails comprises:

forming a semiconductor layer above a substrate; and

patterning the semiconductor layer into a plurality of elongated strips having side walls and an upper surface; and

the step of forming the second rails comprises:

forming a word line film on a charge storage film; and

patterning the word line film to form the second rails.

28. The method of claim 25 , wherein:

the step of forming the second rails comprises:

forming a word line film over a substrate; and

patterning the word line film to form the second rails; and

the step of forming the first rails comprises:

forming a semiconductor layer on a charge storage film above the second rails; and

patterning the semiconductor layer into a plurality of elongated strips having side walls and an upper surface.

29. The method of claim 25 , further comprising:

monolithically forming at least four device levels over each other; and

forming interlevel insulating layers between adjacent device levels.

30. The method of claim 25 , further comprising forming an insulating material between adjacent second rails.

31. The method of claim 25 , wherein:

each first rail comprises a plurality of polysilicon charge storage transistor active regions;

each second rail comprises a word line; and

the first direction is substantially perpendicular to the second direction.

32. The method of claim 31 , further comprising:

forming at least two conductive layers which contact each first rail; and

forming a charge storage film which contacts the active regions and the word line.

33. The method of claim 32 , wherein:

the charge storage film comprises an O—N—O dielectric stack;

each transistor active region comprises source, channel and drain regions; and

each transistor gate electrode comprises a portion of the word line which overlies the transistor active region.

34. The method of claim 33 , further comprising:

forming an interlevel insulating layer between the first and second device levels;

forming a via through the interlevel insulating layer which extends between the first and second device levels to an active region in a first rail; and

forming a contact in the via, such that the contact extends between the first and second device levels to contact the active region at a bottom of the via.

35. The method of claim 33 , wherein:

the step of forming the first rails comprises forming a first photoresist mask over a semiconductor layer and patterning the semiconductor layer using the first photoresist mask; and

the step of forming the second rails comprises forming a second photoresist mask over a word line layer and patterning the word line layer using the second photoresist mask.

36. The method of claim 25 , wherein the charge storage transistors are located at intersections of each first rail and each second rail.

37. The method of claim 25 , further comprising forming a third device level of charge storage transistors between the first and the second device levels.

38. An integrated circuit, comprising:

a three-dimensional memory array having more than one memory level of memory cells, each memory level of said integrated circuit comprising:

a plurality of rails on an insulating layer above the substrate, said rails running in a first direction, said rails comprising a lightly-doped semiconductor region disposed between heavily-doped semiconductor regions;

a charge storage film on the rails;

a plurality of word lines on the charge storage film, said word lines running in a second direction different than the first direction; and

an interlevel insulating layer above the word lines.

39. The integrated circuit of claim 38 , wherein the integrated circuit further comprises vias formed through at least one interlevel insulating layer making contact to portions of the rails.

40. The integrated circuit of claim 38 , wherein the charge storage film comprises a charge storage dielectric film.

41. The integrated circuit of claim 40 , wherein the charge storage dielectric film comprises silicon, oxygen and nitrogen.

42. The integrated circuit of claim 41 , wherein the charge storage dielectric film comprises a silicon oxide/silicon nitride/silicon oxide (ONO) stack.

43. The integrated circuit of claim 38 , further comprising a silicide layer formed on or within the word lines.

44. The integrated circuit of claim 38 , wherein the memory levels are monolithically formed over the substrate.

45. The integrated circuit of claim 38 , wherein the lightly-doped semiconductor region comprises a transistor channel region and the heavily-doped semiconductor regions comprise transistor source and drain regions.

46. The integrated circuit of claim 45 , wherein the rails comprise polysilicon rails.

47. A method for manufacturing an integrated circuit comprising a three-dimensional memory array having more than one memory level of memory cells, the method comprising:

forming a plurality of rails on an insulating layer above a substrate, said rails running in a first direction;

forming heavily-doped semiconductor regions in the rails such that a lightly-doped semiconductor region is disposed between the heavily-doped semiconductor regions;

forming a charge storage film on the rails;

forming a plurality of word lines on the charge storage film, said word lines running in a second direction different than the first direction;

forming an interlevel insulating layer above the word lines; and monolithically forming a second device level over the interlevel insulating layer.

48. The method of claim 47 , further comprising forming vias through the interlevel insulating layer.

49. The method of claim 48 , wherein the step of forming the vias comprises:

forming openings in the interlevel insulating layer to expose a portion of at least one rail or word line therebelow; and

filling the openings with a conductive material.

50. The method of claim 47 , wherein the charge storage film comprises a charge storage dielectric film.

51. The method of claim 50 , wherein the substrate is monocrystalline semiconductor.

52. The method of claim 51 , wherein the charge storage dielectric film comprises silicon, oxygen and nitrogen.

53. The method of claim 52 , wherein the charge storage dielectric film comprises a silicon oxide/silicon nitride/silicon oxide (ONO) stack.

54. The method of claim 47 , wherein the step of forming the plurality of rails comprises:

depositing a layer of silicon on the underlying insulating layer; and

masking and etching the silicon layer to form the plurality of rails.

55. The method of claim 54 , wherein the deposited layer of silicon comprises a polycrystalline silicon layer.

56. The method of claim 47 , further comprising the step of forming a suicide layer on or within the word lines.

57. The method of claim 56 , wherein the silicide layer is formed on a deposited polysilicon layer before etching the polysilicon layer to form the word lines.

58. The method of claim 47 , wherein:

the heavily-doped semiconductor regions comprise transistor source and drain regions; and

lightly-doped semiconductor channel regions are disposed between the source and drain regions.

59. The method of claim 47 , wherein the second device level comprises a plurality of charge storage transistors.