Multi-state memory

• US 6,856,546 B2
• Filed: 11/13/2001
• Issued: 02/15/2005
• Est. Priority Date: 01/14/1992
• Status: Expired due to Term

• Current Assignee: SanDisk Technologies LLC (Western Digital Corporation)
• Original Assignee: SanDisk Corporation (Western Digital Corporation)
• Inventors: Fong, Yupin Kawing, Guterman, Daniel C.
• Primary Examiner(s): Nguyen, Viet Q.
• Application Number:
  US10/013,592
• Publication Number:
  US 20020163838A1
• Time in Patent Office: 1,190 Days
• Field of Search: 365/185.2, 365/185.18, 365/189.07, 365/189.09, 365/203, 365/205, 365/210, 365/185.01, 365/185.03, 365/185.22, 365/185.28, 365/185.19
• US Class Current: 365/185.2
• CPC Class Codes: G11C 11/5621 using charge storage in a floating gate
• CPC Class Codes: G11C 11/5628 Programming or writing circuits; Data input circuits
• CPC Class Codes: G11C 11/5635 Erasing circuits
• CPC Class Codes: G11C 11/5642 Sensing or reading circuits; Data output circuits
• CPC Class Codes: G11C 16/0425 comprising cells containing a merged floating gate and select transistor
• CPC Class Codes: G11C 16/0441 comprising cells containing multiple floating gate devices, e.g. separate read-and-write FAMOS transistors with connected floating gates
• CPC Class Codes: G11C 16/28 using differential sensing or reference cells, e.g. dummy cells
• CPC Class Codes: G11C 16/3418 Disturbance prevention or evaluation; Refreshing of disturbed memory data
• CPC Class Codes: G11C 16/3427 Circuits or methods to prevent or reduce disturbance of the state of a memory cell when neighbouring cells are read or written
• CPC Class Codes: G11C 16/349 Arrangements for evaluating degradation, retention or wearout, e.g. by counting erase cycles
• CPC Class Codes: G11C 16/3495 Circuits or methods to detect or delay wearout of nonvolatile EPROM or EEPROM memory devices, e.g. by counting numbers of erase or reprogram cycles, by using multiple memory areas serially or cyclically
• CPC Class Codes: G11C 2211/5634 Reference cells
• CPC Class Codes: G11C 27/005 with non-volatile charge storage, e.g. on floating gate or MNOS
• CPC Class Codes: G11C 29/00 Checking stores for correct operation ; Subsequent repair; Testing stores during standby or offline operation
• CPC Class Codes: G11C 29/76 using address translation or modifications
• CPC Class Codes: H01L 29/42324 Gate electrodes for transistors with a floating gate
• CPC Class Codes: H01L 29/42328 with at least one additional gate other than the floating gate and the control gate, e.g. program gate, erase gate or select gate
• CPC Class Codes: H01L 29/7885 Hot carrier injection from the channel
• 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

Maximized multi-state compaction and more tolerance in memory state behavior is achieved through a flexible, self-consistent and self-adapting mode of detection, covering a wide dynamic range. For high density multi-state encoding, this approach borders on full analog treatment, dictating analog techniques including A to D type conversion to reconstruct and process the data. In accordance with the teachings of this invention, the memory array is read with high fidelity, not to provide actual final digital data, but rather to provide raw data accurately reflecting the analog storage state, which information is sent to a memory controller for analysis and detection of the actual final digital data.

Claims

1. A multi-state memory, comprising:

a plurality of EEPROM memory cells, each of said memory cells coupled to a word line and to a bit line and able to store one of a plurality of multi-states, organized into a plurality of sectors;

at least one tracking cell for each of said multi-states, associated with each of said plurality of sectors;

read circuitry to read raw data associated with a programmed state of said tracking cell;

converter circuitry to convert said raw data to digital form;

circuitry to precharge said bit line coupled to a selected plurality of said memory cells and to then turn-on said word line coupled to selected said memory cells using a word line signal;

wherein discharge rate of potential on said bit line provides a measure of program drive for said memory cell; and

a memory controller to establish desired read points for each of a plurality of physical states, based upon digitally converted read said raw data from each said tracking cell.

2. The multi-state memory of claim 1, wherein said word line signal has at least one signal characteristic selected from a group consisting of (a) said signal is a ramp signal, and (b) said signal is a stair-step signal.

3. The multi-state memory of claim 1, wherein at least two said memory cells are coupled to be read and have sensed raw data digitized simultaneously.

4. The multi-state memory of claim 1, wherein at least 255 said memory cells are coupled to be read and to have sensed raw data digitized simultaneously.

5. The multi-state memory of claim 1, wherein said raw data is read at a clock rate selected according to at least one program drive characteristic of at least one said memory cell.

6. The multi-state memory of claim 1, wherein said raw data is read statically.

7. The multi-state memory of claim 6, further including:

a comparator to compare magnitude of said reference current with current read from said memory cell; and

a counter coupled to count clock pulses associated with said memory and to freeze counting when said reference current exceeds said currant read from said memory cell.

8. The multi-state memory of claim 7, wherein said reference current has a characteristic selected from a group consisting of (a) said reference current is a ramp function, and (b) said reference current is a step-function.

9. The multi-state memory of claim 8, wherein maximum amplitude of said reference current is scaled proportionally to a characteristic of at least one strongly erased said memory cell.

10. The multi-state memory of claim 1, wherein:

data from a sector to be read into said controller is taken first from said tracking cell for use in establishing discrimination points used to read remaining data from said sector into said controller.

11. The multi-state memory of claim 1, wherein:

said multi-state memory includes a plurality of tracking cells; and

further including subjecting at least chosen ones of said tracking cells to a erasure and writing regime to subject said tracking cells to an amount of wear commensurate with at least one of (a) data history and (b) wear experienced by at least some of said memory cells.

12. The multi-state memory of claim 11, wherein assignment of logical states to physical states within at least some chosen ones of said tracking cells is varied to improve wear management.

13. The multi-state memory of claim 12, wherein said assignment includes at least one assignment characteristic selected from a group consisting of (a) a continuous assignment, (b) a periodic assignment, and (c) an assignment based upon a random number.

14. The multi-state memory of claim 11, wherein:

tracking cells for a given logical state are reassigned to a same physical state for a given write cycle and are subjected to a same scrambling operation as memory cells in a remainder of a sector of said multi-state memory;

wherein said tracking cells reflect wear of an associated said sector and provide a mechanism to back convert from a physical state to a logical state.

15. The multi-state memory of claim 1, wherein:

data from a reference tracking cell is shifted into said controller and is processed, starting with at least a first group of tracking cells assigned to at least a first logical state, to identify a population center of tracking cells placed in said first logical state and thereby establish an acceptable compare point for said first state.

16. The multi-state memory of claim 15, wherein:

said acceptable compare point is an optimum compare point; and

processing shifted said data includes continuously summing each successive data from a number of cells in said population in said first logical state.

17. The multi-state memory of claim 15, wherein processing tracking cell data is carried out adaptively on a basis selected from a group consisting of (a) a continuous determination, and (b) an occasional determination.

18. The multi-state memory of claim 1, further including means for determining quality margin of data, concurrently with reading said data.

19. The multi-state memory of claim 1, further including using a counter value representing digitized analog data to define state ranges and state range error margins that define marginal data values.

20. The multi-state memory of claim 10, further including associating with quality of data information to indicate magnitude of a sensed cell parameter relative to a state population center to obtain thereby a direction of marginality.

21. The multi-state memory of claim 20, further including rewriting content of at least one said memory cell conditional upon obtained said direction of marginality.

22. The multi-state memory of claim 1, further including circuitry to carry out at least one procedure selected from a group consisting of (a) ranking a number of logical states according to logical state midpoints and establishing data rotation coding, (b) calculating midpoints between adjacent local states for use as comparison points, and (c) calculating side band quality points.

23. The multi-state memory of claim 1, further including:

a number of current sources equal to (N−

1) where N is a number of states for said multi-state memory; and

a data-in register for each cell in a chunk of cells, where size of said data-in register is 2^K=N;

wherein said controller writes information into said data register at start of a write operation for use in selecting one of (N−

1) current levels during verify, selection being dependant upon a particular state.

24. The multi-state memory of claim 1, further including:

a master current source tapped at (N−

1) locations to provide (N−

1) current magnitudes, where N is a number of states for said multi-state memory; and

a data-in register for each cell in a chunk of cells, where size of said data-in register is 2^K=N;

wherein said controller writes information into said data register at start of a write operation for use in selecting one of (N−

1) current levels during verify, selection being dependant upon a particular state.

25. The multi-state memory of claim 24, wherein said data-in register is a most significant bit portion of an existing readout register used with said multi-state memory.

26. The multi-state memory of claim 1, further including:

a sense amplifier comparator coupled to receive a reference current from a global reference circuit and to receive a bit-line coupled input signal from said cell; and

a multi-bit control gate register element (CGRE) coupled to receive an output from said sense amplifier comparator, a number of bits in said CGRE being selected according to desired sense resolution;

wherein a value stored in said CGRE is used to control a next control gate read signal generator to provide a next control gate read signal in a successive approximation fashion;

wherein if a current associated with said cell exceeds said reference current, said next control gate read signal is changed to reduce the associated said current.

27. The multi-state memory of claim 26, further including a plurality of sense amplifiers, of CGRE units and of next control gate read signal generators to sense 128 bits in parallel.

28. The multi-state memory of claim 27, further including means for dynamically steering selection of control gate read signal magnitudes.

29. The multi-state memory of claim 26, further including:

a first CGRE unit and a first next control gate read signal generator used during programming, and

a second CGRE unit and a second next control gate read signal generator used during verification; and

a multiplexer unit to multiplex said first and said second CGRE and said first and said second next control gate read signal generator when changing between programming and verification operation.

57. The method of claim 56, wherein the method of claim 27 is earned out on 128 bits in parallel.

61. The method of claim 57, further including:

loading a voltage level corresponding to a to-be-programmed data level into an associated said CGRE unit; and

during program and verify operation, holding fixed a state specific magnitude of verification voltage control gate signal;

wherein all cells within a chunk of cells may be simultaneously verified such that further programming is locked-out on a cell-by-cell basis as each said cell passes verification operation.

62. The method of claim 57, further including providing on a steering bus a plurality of different magnitude voltage control gate voltage steps simultaneously;

and

enabling each said CGRE unit to decode to which magnitude of said steps an associated memory cell control gate is to be coupled, such decoding and coupling occurring in parallel.

63. The method of claim 57, further including multiplexing onto a steering bus a plurality of different magnitude voltage control gate voltage steps;

and

enabling each said CGRE unit to decode to which magnitude of said steps an associated memory cell control gate is to be coupled.

64. The method of claim 63, wherein said different magnitude voltage control gate voltage steps define a sub-set of voltages.

30. A method to implement multi-state memory, comprising the following steps:

providing a plurality of EEPROM memory cells, each of said memory cells coupled to a word line and to a bit line and able to store one of a plurality of multi-states, organized into a plurality of sectors;

providing at least one tracking cell for each of said multi-states, associated with each of said plurality of sectors;

reading read raw data associated with a programmed state of said tracking cell;

converting said raw data to digital form;

precharging said bit lines coupled to a selected plurality of said memory cells and turning-on said word line coupled to selected said memory cells using a word line signal;

wherein discharge rate of potential on said bit line provides a measure of program drive for said memory cell; and

using a memory controller to establish desired read points for each of a plurality of physical states, based upon digitally converted read said raw data from each said tracking cell.

31. The method of claim 30, further including providing said word line signal with at least one signal characteristic selected from a group consisting of (a) said signal is a ramp signal, and (b) said signal is a stair-step signal.

32. The method of claim 30, further including coupling at least two said memory cells to be read and have sensed raw data digitized simultaneously.

33. The method of claim 30, further including coupling at least 255 said memory cells to be read and to have sensed raw data digitized simultaneously.

34. The method of claim 30, further including reading said raw data at a clock rate selected according to at least one program drive characteristic of as least one said memory cell.

35. The method of claim 30, further including statically reading said raw data.

36. The method of claim 35, further including comparing magnitude of said reference current with current read from said memory cell;

and

coupling a counter to count clock pulses associated with said memory and to freeze counting when said reference current exceeds said current read from said memory cell.

37. The method of claim 36, further including providing said reference current with a characteristic selected from a group consisting of (a) said reference current is a ramp function, and (b) said reference current is a step-function.

38. The method of claim 37, further including scaling proportionally maximum amplitude of said reference current to a characteristic of at least one strongly erased said memory cell.

39. The method of claim 30, further including taking data from a sector to be read into said controller first from said tracking cell for use in establishing discrimination points used to read remaining data from said sector into said controller.

40. The method of claim 30, further including:

providing said multi-state memory with a plurality of tracking cells; and

subjecting at least chosen ones of said tracking cells a erasure and writing regime to subject said tracking cells to an amount of wear commensurate with at least one of (a) data history and (b) wear experienced by at least some of said memory cells.

41. The method of claim 40, further including varying assignment of logical states to physical states within at least some chosen ones of said tracking cells to improve wear management.

42. The method of claim 41, wherein said assignment includes at least one assignment characteristic selected from a group consisting of (a) a continuous assignment, (b) a periodic assignment, and (c) an assignment based upon a random number.

43. The method of claim 40, further including reassigning tracking cells for a given logical state to a same physical state for a given write cycle and subjecting said tracking sells to a same scrambling operation as memory cells in a remainder of a sector of said multi-state memory;

wherein said tracking cells reflect wear of an associated said sector and provide a mechanism to back convert from a physical state to a logical state.

44. The method of claim 30, further including shifting and processing data from a reference tracking cell into said controller, starting with at least a first group of tracking cells assigned to at least a first logical state, to identify a population center of tracking cells placed in said first logical state and thereby establish an acceptable compare point for said first state.

45. The method of claim 44, wherein:

said acceptable compare point is an optimum compare point; and

processing shifted said data includes continuously summing each successive data from a number of cells in said population in said first logical state.

46. The method of claim 44, further including carrying out adaptively processing of tracking cell data on a basis selected from a group consisting of (a) a continuous determination, and (b) an occasional determination.

47. The method memory of claim 30, further including determining quality margin of data, concurrently with reading said data.

48. The method of claim 30, further including using a counter value representing digitized analog data to define state ranges and state range error margins that define marginal data values.

49. The method of claim 48, further including associating with quality of data information to indicate magnitude of a sensed cell parameter relative to a state population center to obtain thereby a direction of marginality.

50. The method of claim 49, further including rewriting content of at least one said memory cell conditional upon obtained said direction of marginality.

51. The method of claim 30, further including carrying out at least one step selected from a group consisting of (a) ranking a number of logical states according to logical stale midpoints and establishing data rotation coding, (b) calculating midpoints between adjacent local states for use as comparison points, and (c) calculating side band quality points.

52. The method of claim 30, further including:

providing (N−

1) values of reference current where N is a number of states for said multi-state memory; and

providing a data-in register for each cell in a chunk of cells, where size of said data-in register is 2^K=N;

wherein said controller writes information into said data register at start of a write operation for use in selecting one of (N−

1) current levels during verify, selection being dependent upon a particular state.

53. The method of claim 52, wherein verification includes simultaneously comparing all cells within a chunk of cells to a corresponding reference target value, and if verification is successful locking-out further programming.

54. The method of claim 53, wherein said locking-out occurs on a cell-by-cell basis.

55. The method of claim 30, further including:

providing a sense amplifier comparator coupled to receive a reference current from a global reference circuit and to receive a bit-line coupled input signal from said cell; and

coupling a multi-bit control gate register clement (CGRE) to receive an output from said sense amplifier comparator, a number of bits in said CGRE being selected according to desired sexist resolution; and

using a value stored in said CGRE is used to provide a next control gate read signal in a successive approximation fashion;

wherein if a current associated with said cell exceeds said reference current, said next control gate read signal is changed to reduce the associated said current.

56. The method of claim 55, wherein:

said method of claim 55 is repeated for a total of six passes; and

a last stored value in said CGRE is used as a digital equivalent of a memory state for a floating gate of said memory cell.

58. The method of claim 56, including dynamically steering selection of control gate read signal magnitudes.

59. The method of claim 55, further including using a regime of control gate read signals that include a constant voltage component.

60. The method of claim 55, further including providing a first CGRE unit and a first next control gate read signal generator for programming, and providing a second CGRE unit and a second next control gate read signal generator for verifying;

and

multiplexing said first and said second CGRE and said first and said second next control gate read signal generator when changing between programming and verification operation.

65. A method of reading programmed states of a selected plurality of an array of non-volatile erasable and re-programmable memory cells that are operably connected with respective bit lines and which have word lines extending thereacross, comprising:

pre-charging at least some of the bit lines that are operably connected with the selected memory cells to a pre-charged level,

thereafter changing voltages of those of the word lines that are operably connected with the selected memory cells from non-selected levels to selected levels, while maintaining others of the word lines at non-selected levels,

thereafter allowing the pre-charged bit lines to discharge through the selected memory cells to a predetermined level, and

individually measuring durations of the discharge of the pre-charged bit lines to the predetermined level, thereby to provide individual measures of the programmed states of the selected memory cells.

66. The method of claim 65, wherein changing voltages of those of the word lines that are operably connected with the selected memory cells includes ramping up the voltages of those ward lines.

67. The method of claim 65, wherein changing voltages of those of the word lines that are operably connected with the selected memory cells includes stepping up the voltages of those word lines.

68. The method of claim 65, additionally comprising storing the individual measures of the programmed stares of the selected memory cells as digital values in registers.

69. The method of claim 68, wherein changing voltages of those of the word lines that are operably connected with the selected memory cells includes changing the voltages over time.

70. The method of claim 65, wherein the durations of the discharge of the pre-charged bit lines are measured by a digital counter, thereby to provide the individual measures of the programmed state of the selected memory cells as binary representations.

71. The method of claim 70, which additionally comprises storing an output of the digital counter in a plurality of registers, and thereafter disconnecting individual ones of the registers from the counter in response to the discharge of the pre-charged bit lines of corresponding ones of the selected memory cells to the predetermined level, thereby leaving the binary representations of the programmed states of the selected memory cells in individual registers.

72. A method of reading programmed states of a selected plurality of an array of erasable and re-programmable non-volatile memory cell, comprising:

applying a voltage condition to an element of the individual selected memory cells,

removing the voltage condition from the selected memory cells,

after removal of the voltage condition, monitoring a parameter that changes for the selected memory cells in accordance with their individual programmed states, and

measuring relative durations of the changing parameter of the selected memory cells, thereby to provide indications of their programmed states.

73. The method of claim 72, wherein the changing parameter that is measured includes a capacitive discharge time.