Non-equal threshold voltage ranges in MLC NAND

• US 7,639,532 B2
• Filed: 10/10/2007
• Issued: 12/29/2009
• Est. Priority Date: 10/10/2007
• Status: Active Grant

• Current Assignee: Micron Technology, Inc.
• Original Assignee: Micron Technology, Inc.
• Inventors: Roohparvar, Frankie F., Sarin, Vishal, Hoei, Jung-Sheng
• Primary Examiner(s): NGUYEN, TAN
• Application Number:
  US11/869,868
• Publication Number:
  US 20090097311A1
• Time in Patent Office: 811 Days
• Field of Search: None
• US Class Current: 365/185.03
• CPC Class Codes: G11C 11/5628 Programming or writing circuits; Data input circuits
• CPC Class Codes: G11C 16/0483 comprising cells having several storage transistors connected in series
• CPC Class Codes: G11C 16/10 Programming or data input circuits
• CPC Class Codes: G11C 16/3418 Disturbance prevention or evaluation; Refreshing of disturbed memory data
• CPC Class Codes: G11C 27/005 with non-volatile charge storage, e.g. on floating gate or MNOS

Abstract

Memory devices adapted to process and generate analog data signals representative of data values of two or more bits of information facilitate increases in data transfer rates relative to devices processing and generating only binary data signals indicative of individual bits. Programming of such memory devices includes programming to a target threshold voltage range representative of the desired bit pattern. Reading such memory devices includes generating an analog data signal indicative of a threshold voltage of a target memory cell. Threshold voltage ranges of the memory cells have a larger range size for ranges that include lower threshold voltages and a smaller range size for ranges that include higher threshold voltages since program disturb is lower at higher threshold voltages.

Claims

1. A method of programming a multi-level cell memory, comprising:

assigning a plurality of threshold voltage ranges within each cell, a range for each level of the memory cell; and

sizing the plurality of threshold voltage ranges differently, each threshold voltage range representing a data bit pattern and where a largest of the threshold voltage ranges is at a lowest threshold voltage level, and a smallest of the threshold voltage ranges is at a highest threshold voltage level.

2. The method of claim 1, wherein sizing further comprises decreasing the threshold voltage ranges in size by level as the threshold voltage for the level increases.

3. A method of programming a multi-level cell memory, comprising:

adjusting threshold voltage ranges of a memory cell according to expected program

disturb within each range wherein a largest of the threshold voltage ranges is at a lowest threshold voltage level, and a smallest of the threshold voltage ranges is at a highest threshold voltage level.

4. The method of claim 3, wherein adjusting comprises assigning increasingly narrow threshold voltage ranges as threshold voltage values increase.

5. A method of programming a multi-level cell memory, comprising:

adjusting threshold voltage ranges of a memory cell according to expected charge loss or

gain within each range wherein a largest of the threshold voltage ranges is at a lowest threshold voltage level, and a smallest of the threshold voltage ranges is at a highest threshold voltage level.

6. A method of programming a memory device having an array of memory cells, comprising:

applying programming pulses to a target memory cell to increase a threshold voltage of the target memory cell, the threshold voltage being in one of a plurality of threshold voltage ranges of differing size depending upon their respective threshold voltages wherein a largest of the threshold voltage ranges is at a lowest threshold voltage level, and a smallest of the threshold voltage ranges is at a highest threshold voltage level; and

re-applying programming pulses to the target memory cell if its threshold voltage is less than a desired threshold voltage.

7. The method of claim 6, wherein applying further comprises applying pulses to place the threshold voltage of the target memory cell into ranges sized from a largest threshold voltage range for a lowest desired threshold voltage to a smallest threshold voltage range for a highest desired threshold voltage.

8. A memory device, comprising:

an array of memory cells each capable of storing multiple levels per cell; and

circuitry for control and/or access of the array of memory cells;

the control circuitry adapted to program a memory cell to a threshold voltage within one of a plurality of threshold voltage ranges, each of the plurality of threshold voltage ranges corresponding to a different data value and at least two of the threshold voltage ranges being of unequal size wherein a largest of the threshold voltage ranges is at a lowest threshold voltage level, and a smallest of the threshold voltage ranges is at a highest threshold voltage level.

9. The device of claim 8, wherein the plurality of ranges are sized from a largest threshold voltage range for a lowest threshold voltage to a smallest threshold voltage range for a highest threshold voltage.

10. A multi level cell memory device, comprising:

an array of memory cells to provide an analog data signal indicative of a threshold voltage of a target memory cell of the array;

circuitry for control and/or access of the array of memory cells; and

each memory cell capable of having a threshold voltage programmed within any one of a plurality of threshold voltage ranges, a threshold voltage range for each level of the cell, the threshold voltage ranges decreasing in size from a largest size for a lowest threshold voltage level to a smallest size for a highest threshold voltage range.

11. A storage device, comprising:

a solid state memory device adapted to receive and transmit analog data signals;

a controller for communicating with an external device; and

a read/write channel coupled to the controller and the memory device;

wherein the controller is further adapted to generate analog data signals for transmission having voltage levels indicative of threshold voltages of target memory cells, the voltage levels within ranges, each memory cell having a plurality of threshold voltage ranges of differing size depending upon the threshold voltage level and where a largest of the threshold voltage ranges is at a lowest threshold voltage level, and a smallest of the threshold voltage ranges is at a highest threshold voltage level.

12. The storage device of claim 11, wherein the voltage ranges decrease in width from a largest width at a lowest threshold voltage to a smallest width at a highest threshold voltage.

13. A storage device, comprising:

a solid state memory device adapted to receive and transmit analog data signals;

a controller for communicating with an external device; and

a read/write channel coupled to the controller and the memory device;

wherein the solid state memory device has a plurality of memory cells, each memory cell having a plurality of threshold voltage ranges, a range for each level of threshold voltage in the cell, the threshold voltage ranges of decreasing width from a widest range at a lowest threshold voltage range to a narrowest range at a highest threshold voltage range.

14. A solid state memory device, comprising:

an NAND memory array having a plurality of memory cells each storing a threshold voltage indicative of a level in one of a plurality of threshold voltage ranges; and

circuitry for control and/or access of non-volatile memory cells of the NAND memory array;

wherein the threshold voltage ranges are of decreasing size from a largest size for a lowest threshold voltage level to a smallest size for a highest threshold voltage range.

15. A solid state memory device, comprising:

a NAND memory array having a plurality of memory cells each storing a threshold voltage indicative of a level in one of a plurality of threshold voltage ranges; and

circuitry for control and/or access of memory cells of the NAND memory array;

wherein the circuitry for control and/or access of the memory cells is adapted to assign a plurality of threshold voltage ranges within each cell, a range for each level of the memory cell, and to size the plurality of threshold voltage ranges differently depending upon a bit pattern to be stored in the range and where a largest of the threshold voltage ranges is at a lowest threshold voltage level, and a smallest of the threshold voltage ranges is at a highest threshold voltage level.

16. A method of operating a memory, comprising:

programming a memory cell to a threshold voltage within one of a plurality of ranges, each of the plurality of ranges corresponding to a different data value and at least two of the ranges being of unequal size wherein a largest of the threshold voltage ranges is at a lowest threshold voltage level, and a smallest of the threshold voltage ranges is at a highest threshold voltage level; and

sensing an indication of the threshold voltage.

17. The method of claim 16, wherein the plurality of ranges decreases in width from a widest range at a lowest threshold voltage range to a narrowest range at a highest threshold voltage range.