Reference scheme for a non-volatile semiconductor memory device

• US 20060274581A1
• Filed: 06/03/2005
• Published: 12/07/2006
• Est. Priority Date: 06/03/2005
• Status: Active Grant

• Current Assignee: Polaris Innovations Limited (Wi-LAN Inc.)
• Original Assignee: Qimonda AG (Infineon Technologies AG)
• Inventors: Redaelli, Marco, Ambroggi, Luca de
• Granted Patent: US 7,259,993 B2
• Time in Patent Office: Days
• US Class Current: 365/185.220
• CPC Class Codes: G11C 11/5642 Sensing or reading circuits; Data output circuits
• 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 16/3431 Circuits or methods to detect disturbed nonvolatile memory cells, e.g. which still read as programmed but with threshold less than the program verify threshold or read as erased but with threshold greater than the erase verify threshold, and to reverse the disturbance via a refreshing programming or erasing step
• CPC Class Codes: G11C 16/3436 Arrangements for verifying correct programming or erasure
• CPC Class Codes: G11C 16/349 Arrangements for evaluating degradation, retention or wearout, e.g. by counting erase cycles
• CPC Class Codes: G11C 2211/5634 Reference cells

Abstract

A non-volatile semiconductor memory device is provided comprising a memory area and a circuitry area. The memory area includes a plurality of memory cells and a set of array reference cells that are programmable to have a threshold voltage corresponding to an erased or a programmed state of a memory cell. In the circuitry area, additional main reference cells are provided, which are configured to also have a threshold voltage corresponding to an erased or programmed state of a memory cell. The main reference cells are used for setting of said array reference cells and said array reference cells are provided as a reference for reading or writing a state of said memory cells. A method is also provided for setting array reference cells in a non-volatile semiconductor memory device to a predefined threshold voltage.

Claims

1. A non-volatile semiconductor memory device comprising:

a memory area and a circuitry area, wherein said memory area comprises;

a plurality of memory cells arranged in a memory array; and

a set of programmable array reference cells that are provided as references for reading said memory cells, and wherein said circuitry area comprises;

a set of main reference cells that are provided as references for verifying a state of the array reference cells or the memory cells.

2. The non-volatile semiconductor memory device according to claim 1, wherein said set of main reference cells comprises:

a first main reference cell provided to have a threshold voltage corresponding to a programmed state of a memory cell; and

a second main reference cell provided to have a threshold voltage corresponding to an erased state of a memory cell, and wherein said set of programmable array reference cells comprises;

a first array reference cell provided to have a threshold voltage corresponding to a programmed state of a memory cell; and

a second array reference cell provided to have a threshold voltage corresponding to an erased state of a memory cell.

3. The non-volatile semiconductor memory device according to claim 1, wherein said set of main reference cells comprises:

a first main reference cell provided to have a threshold voltage corresponding to a programmed state of a memory cell; and

a second main reference cell provided to have a threshold voltage corresponding to an erased state of a memory cell, and wherein said set of programmable array reference cells comprises;

a read reference cell provided to have a threshold voltage defining a border between an erased state and a programmed state of a memory cell and that can be used for reading data from said memory cells.

4. The non-volatile semiconductor memory device according to claim 3, wherein said set of main reference cells further comprises a read reference cell provided to have a threshold voltage defining the border between an erased state and a programmed state of a memory cell and that is provided as a reference for verifying the state of said read reference cell comprised in said set of array reference cells.

5. The non-volatile semiconductor memory device according to claim 3, wherein said set of main reference cells further comprises a reference cell provided to have a threshold voltage corresponding to an over-erased state of a memory cell.

6. The non-volatile semiconductor memory device according to claim 3, wherein the memory cells are divided into a plurality of groups and wherein a set of array reference cells is provided for each group.

7. The non-volatile semiconductor memory device according to claim 6, wherein an additional status cell is provided in each of said set of programmable array reference cells to indicate by a predefined logical state if all cells of a predefined group of memory cells have an erased state.

8. The non-volatile semiconductor memory device according to claim 1, wherein said memory cells comprise multi-bit memory cells.

9. The non-volatile semiconductor memory device according to claim 8, wherein each memory cell comprises a transistor having an electric charge-trapping layer capable of storing two binary information bits.

10. The non-volatile semiconductor memory device according to claim 9, wherein said electric charge-trapping layer of said transistors is made of a nitride material.

11. The non-volatile semiconductor memory device according to claim 8, wherein each array reference cell comprises a transistor having an electric charge-trapping layer capable of storing two binary information bits, one of these bits defining the logical state of the reference cell.

12. The non-volatile semiconductor memory device according to claim 11, wherein the second bit of the transistor of an erased array reference cell is set to a contrary logical state.

13. The non-volatile semiconductor memory device according to claim 1, wherein the memory device is configured to be operated in a test mode in which a setting of said main reference cells to a programmed or erased state is enabled;

and

wherein a setting of the main reference cells to a programmed or erased state is disabled after leaving the test mode.

14. A method of operating a non-volatile semiconductor memory device, said memory device comprising:

a memory area and a circuitry area, wherein said memory area comprises;

a plurality of memory cells arranged in a memory array; and

a set of programmable array reference cells that are provided as references for reading said memory cells, and wherein said circuitry area comprises;

a set of pre-programmed main reference cells that are provided as references for verifying the state of array reference cells or memory cells;

said method comprising;

setting at least one of said array reference cells to a predefined state using at least one of said pre-programmed main reference cells as a reference;

reading the state of at least one memory cell using the at least one of said array reference cells as a reference; and

verifying the state of at least one memory cell when erasing or programming the memory cell using at least one of said main reference cells as a reference.

15. The method according to claim 14, wherein setting a first of said array reference cells to an erased state comprises setting said first array reference cell to a programmed state and then setting said first array reference cell to an erased state.

16. The method according to claim 14, wherein:

a first reference cell of said set of main reference cells is pre-programmed to have a threshold voltage corresponding to a programmed state of a memory cell; and

a second reference cell of said set of main reference cells being pre-programmed to have a threshold voltage corresponding to an erased state of a memory cell;

said method comprising;

setting a first of said array reference cells to an erased state using said erased main reference cell as a reference;

setting a second of said array reference cells to a programmed state using said programmed main reference cell as a reference; and

reading at least one of said memory cells using said erased and programmed reference cells for generating a comparative value for the reading operation.

17. The method according to claim 16, wherein setting a first of said array reference cells and setting a second of said array reference cells are carried out when writing data to said memory cells.

18. The method according to claim 14, wherein:

a first reference cell of said set of main reference cells is pre-programmed to have a threshold voltage corresponding to a programmed state of a memory cell;

a second reference cell of said set of main reference cells is pre-programmed to have a threshold voltage corresponding to an erased state of a memory cell; and

said set of array reference cells comprising a read reference cell;

said method comprising;

generating a comparative value using said erased and programmed main reference cells;

setting said read reference cell to a threshold voltage defining a border between an erased state and a programmed state of a memory cell using said comparative value; and

reading at least one of said memory cells using said read reference cell as a reference.

19. The method according to claim 14, wherein:

a first reference cell of said set of main reference cells is pre-programmed to have a threshold voltage corresponding to a programmed state of a memory cell;

a second reference cell of said set of main reference cells is pre-programmed to have a threshold voltage corresponding to an erased state of a memory cell;

a third reference cell of said set of main reference cells is pre-programmed to have a threshold voltage defining the border between an erased state and a programmed state of a memory cell; and

said set of array reference cells comprising a read reference cell;

said method comprising;

setting said read reference cell to a read state using said third main reference cell as a reference; and

reading at least one of said memory cells using said read reference cell as a reference.

20. The method according to claim 14, further comprising:

assigning predefined memory cells to a group;

providing a status cell for said group of memory cells; and

setting said status cell to a predefined logical state when all memory cells assigned to said group have an erased state.

21. The method according to claim 20, wherein all cells of a group are coupled to a common wordline, each wordline being coupled to a status cell.

22. The method according to claim 21, further comprising:

reading said status cell prior to writing data to the cells assigned to said wordline; and

if the read state of the status cell is said predefined logical state, performing a re-programming of the programmed array reference using the programmed main reference cell.

23. The method according to claim 14, wherein the step of setting at least one of said array reference cells to a predefined state is repeated during the lifetime of said memory device.

24. The method according to claim 14, wherein the step of setting at least one of said array reference cells to a predefined state is carried out when a predefined user command provided for actions independent from the setting of array reference cells is launched.

25. A method for providing a non-volatile semiconductor memory device comprising:

providing a plurality of memory cells arranged in a memory array, a set of at least two programmable array reference cells, and a set of at least two programmable main reference cells;

setting at least one of said main reference cells to have a threshold voltage corresponding to an erased state; and

setting at least one other main reference cell to have a threshold voltage corresponding to a programmed state.

26. The method according to claim 25, wherein setting said at least one of said main reference cells and setting at least one other main reference cell are carried out during wafer sort.

27. The method according to claim 25, wherein the setting of said at least one of said main reference cells and the setting of at least one other main reference cell are carried out when testing the non-volatile semiconductor memory device.