Programmable memory array structure incorporating series-connected transistor strings and methods for fabrication and operation of same

• US 7,505,321 B2
• Filed: 12/31/2002
• Issued: 03/17/2009
• Est. Priority Date: 12/31/2002
• Status: Expired due to Term

• Current Assignee: SanDisk Technologies LLC (Western Digital Corporation)
• Original Assignee: SanDisk 3D LLC (Western Digital Corporation)
• Inventors: Koutnetsov, Igor, Scheuerlein, Roy E., Fasoli, Luca, Nallamothu, Sucheta, Walker, Andrew J., Chen, En-Hsing, Ilkbahar, Alper, Petti, Christopher
• Primary Examiner(s): Pham; Ly D
• Application Number:
  US10/335,078
• Publication Number:
  US 20040125629A1
• Time in Patent Office: 2,268 Days
• Field of Search: 365/185.17, 365/185.24, 365/185.11, 365/185.18, 365/185.05, 365/185.13, 365/130, 365/230.03
• US Class Current: 365/185.17
• CPC Class Codes: G11C 11/5621 using charge storage in a floating gate
• CPC Class Codes: G11C 16/0483 comprising cells having several storage transistors connected in series
• CPC Class Codes: H10B 43/30 characterised by the memory core region
• CPC Class Codes: H10B 51/30 characterised by the memory core region
• CPC Class Codes: H10B 69/00 Erasable-and-programmable ROM [EPROM] devices not provided for in groups H10B41/00 - H10B63/00, e.g. ultraviolet erasable-and-programmable ROM [UVEPROM] devices

Abstract

A three-dimensional flash memory array incorporates thin film transistors having a charge storage dielectric arranged in series-connected NAND strings to achieve a 4F² memory cell layout. The memory array may be programmed and erased using only tunneling currents, and no leakage paths are formed through non-selected memory cells. Each NAND string includes two block select devices for respectively coupling one end of the NAND string to a global bit line, and the other end to a shared bias node. Pairs of NAND strings within a block share the same global bit line. The memory cells are preferably depletion mode SONOS devices, as are the block select devices. The memory cells may be programmed to a near depletion threshold voltage, and the block select devices are maintained in a programmed state having a near depletion mode threshold voltage. NAND strings on more than one layer may be connected to global bit lines on a single layer. By interleaving the NAND strings on each memory level and using two shared bias nodes per block, very little additional overhead is required for the switch devices at each end of the NAND strings. The NAND strings on different memory levels are preferably connected together by way of vertical stacked vias, each preferably connecting to more than one memory level. Each memory level may be produced with less than three masks per level.

Claims

1. An integrated circuit comprising:

a three-dimensional memory array having a first plane of memory cells disposed above a first dielectric layer disposed above a substrate, and having a second plane of memory cells disposed above a second dielectric layer disposed above the first memory plane, said memory cells comprising modifiable conductance switch devices arranged in a plurality of series-connected NAND strings;

wherein each NAND string within a given memory plane includes a first switch device at a first end thereof for coupling the NAND string to an associated global bit line and further includes a second switch device at a second end thereof opposite the first end for coupling the NAND string to an associated bias node;

wherein a NAND string within the first memory plane and a NAND string within the second memory plane are vertically coupled to a global bit line disposed on a first interconnect layer, and are further vertically coupled to a bias node disposed on a second interconnect layer different than the first interconnect layer.

2. The integrated circuit as recited in claim 1 wherein the memory cells comprise devices having a depletion mode threshold voltage at least some of the time.

3. The integrated circuit as recited in claim 1 wherein the modifiable conductance switch devices comprise first transistors corresponding to a first data state, and second transistors corresponding to a second data state, each transistor having a respective threshold voltage which is determined during manufacture.

4. The integrated circuit as recited in claim 1 wherein the modifiable conductance switch devices comprise transistors having a respective threshold voltage which is modifiable post-manufacture.

5. The integrated circuit as recited in claim 1 wherein the modifiable conductance switch devices comprise transistors having a charge storage dielectric.

6. The integrated circuit as recited in claim 5 wherein the charge storage dielectric of the memory cell transistors comprises an oxide-nitride-oxide (ONO) stack.

7. The integrated circuit as recited in claim 6 wherein the memory cell transistors have a depletion mode threshold voltage when the charge storage dielectric has a minimum stored charge level.

8. The integrated circuit as recited in claim 6 wherein the memory cell transistors have a first depletion mode threshold voltage corresponding to an erased data state and have a second depletion mode threshold voltage corresponding to a programmed data state.

9. The integrated circuit as recited in claim 6 wherein the memory cell devices have more than two nominal values of conductance, for storing more than one bit of data per memory cell.

10. The integrated circuit as recited in claim 1 wherein two NAND strings having word lines in common share a single global bit line.

11. The integrated circuit as recited in claim 10 wherein pairs of NAND strings are arranged so that a first control signal coupling one string of the pair to its associated global array line couples the other string of the pair to its associated bias node.

12. The integrated circuit as recited in claim 11 wherein an associated bias node for a given NAND string is also associated with another NAND string having different word lines.

13. The integrated circuit as recited in claim 1 wherein the first switch device of each respective NAND string comprises a transistor having a charge storage dielectric.

14. The integrated circuit as recited in claim 13 wherein the first switch device of each respective NAND string has a depletion mode threshold voltage.

15. The integrated circuit as recited in claim 1 wherein the substrate comprises a monocrystalline substrate including circuitry which is coupled to the memory array.

16. The integrated circuit as recited in claim 1 wherein the substrate comprises a polycrystalline substrate.

17. The integrated circuit as recited in claim 1 wherein the substrate comprises an insulating substrate.

18. The integrated circuit as recited in claim 1 wherein the switch devices and memory cell devices forming each NAND string are structurally substantially identical.

19. The integrated circuit as recited in claim 1 wherein a respective plurality of NAND strings on each of the first and second memory planes are coupled to a single global array line disposed on a single layer of the integrated circuit.

20. A computer readable medium encoding an integrated circuit, said encoded integrated circuit comprising:

a three-dimensional memory array having a first plane of memory cells disposed above a first dielectric layer disposed above a substrate, and having a second plane of memory cells disposed above a second dielectric layer disposed above the first memory plane, said memory cells comprising modifiable conductance switch devices arranged in a plurality of series-connected NAND strings;

wherein each NAND string within a given memory plane includes a first switch device at a first end thereof for coupling the NAND string to an associated global bit line and further includes a second switch device at a second end thereof opposite the first end for coupling the NAND string to an associated bias node;

wherein a NAND string within the first memory plane and a NAND string within the second memory plane are vertically coupled to a global bit line disposed on a first interconnect layer, and are further vertically coupled to a bias node disposed on a second interconnect layer different than the first interconnect layer.

21. The computer readable medium as recited in claim 20 wherein the modifiable conductance switch devices comprise transistors having a charge storage dielectric.

22. The computer readable medium as recited in claim 20 wherein two NAND strings having word lines in common share a single global bit line.

23. The computer readable medium as recited in claim 22 wherein pairs of NAND strings are arranged so that a first control signal coupling one string of the pair to its associated global array line couples the other string of the pair to its associated bias node.

24. The computer readable medium as recited in claim 23 wherein an associated bias node for a given NAND string is also associated with another NAND string having different word lines.

25. The computer readable medium as recited in claim 20 wherein the first switch device of each respective NAND string comprises a transistor having a charge storage dielectric.

26. The computer readable medium as recited in claim 25 wherein the first switch device of each respective NAND string has a depletion mode threshold voltage.

27. The computer readable medium as recited in claim 20 wherein the switch devices and memory cell devices forming each NAND string are structurally substantially identical.

28. An integrated circuit comprising:

a three-dimensional memory array having at least two planes of memory cells formed above a dielectric layer that is formed above a substrate, said memory cells comprising modifiable conductance switch devices arranged in a plurality of series-connected NAND strings; and

a plurality of global array lines disposed on a layer of the integrated circuit, each of which is shared by at least one NAND string on each of at least two memory planes;

a second plurality of array lines disposed on a second layer of the integrated circuit different than the first layer, each of said second plurality of array lines shared by a respective plurality of NAND strings on each of at least two memory planes;

wherein each NAND string includes a first switch device at a first end thereof for coupling the NAND string to an associated global array line; and

wherein the first switch device within a given NAND string is structurally substantially identical with the memory cell devices forming the given NAND string.

29. The integrated circuit as recited in claim 28 wherein at least two NAND strings on each of at least two memory planes make contact to an associated global array line by way of a shared zia.

30. The integrated circuit as recited in claim 28 wherein the modifiable conductance switch devices comprise transistors having a charge storage dielectric.

31. The integrated circuit as recited in claim 28 wherein at least one NAND string on each of at least two memory planes are coupled to an associated global array line by way of a single zia.

32. The integrated circuit as recited in claim 31 wherein at least four NAND strings on each of at least two memory planes are coupled to an associated global array line by way of a single zia.