Non-volatile semiconductor memory with large erase blocks storing cycle counts

• US 6,944,063 B2
• Filed: 01/28/2003
• Issued: 09/13/2005
• Est. Priority Date: 01/28/2003
• Status: Expired due to Term

• Current Assignee: Kioxia Corporation, SanDisk Technologies LLC (Western Digital Corporation)
• Original Assignee: Kabushiki Kaisha Toshiba (Toshiba Corporation), SanDisk Corporation (Western Digital Corporation)
• Inventors: Chen, Jian, Tanaka, Tomoharu
• Primary Examiner(s): Ho, Hoai
• Assistant Examiner(s): Pham, Ly Duy
• Application Number:
  US10/353,574
• Publication Number:
  US 20040145952A1
• Time in Patent Office: 959 Days
• Field of Search: 365/185.33, 365/185.11, 365/185.19, 365/185.23, 365/185.28, 365/200, 365/230.03, 365/230.06, 365/236, 365/238.5, 365/242, 365/185.01, 365/189.01, 365/185.09, 365/189.05, 365/218
• US Class Current: 365/185.33
• CPC Class Codes: G06F 11/1068 in sector programmable memories, e.g. flash disk G06F11/1072 takes precedence
• CPC Class Codes: G11C 11/5621 using charge storage in a floating gate
• CPC Class Codes: G11C 11/5635 Erasing circuits
• CPC Class Codes: G11C 16/0483 comprising cells having several storage transistors connected in series
• 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 16/3459 Circuits or methods to verify correct programming of nonvolatile memory cells
• CPC Class Codes: G11C 16/349 Arrangements for evaluating degradation, retention or wearout, e.g. by counting erase cycles
• CPC Class Codes: G11C 2211/5641 Multilevel memory having cells with different number of storage levels
• CPC Class Codes: G11C 2211/5646 Multilevel memory with flag bits, e.g. for showing that a "first page" of a word line is programmed but not a "second page"

Abstract

In a flash EEPROM system that is divided into separately erasable blocks of memory cells with multiple pages of user data being stored in each block, a count of the number of erase cycles that each block has endured is stored in one location within the block, such as in spare cells of only one page or distributed among header regions of multiple pages. The page or pages containing the block cycle count are initially read from each block that is being erased, the cycle count temporarily stored, the block erased and an updated cycle count is then written back into the block location. User data is then programmed into individual pages of the block as necessary. The user data is preferably stored in more than two states per memory cell storage element, in which case the cycle count can be stored in binary in a manner to speed up the erase process and reduce disturbing effects on the erased state that writing the updated cycle count can cause. An error correction code calculated from the cycle count may be stored with it, thereby allowing validation of the stored cycle count.

Claims

1. A method of operating a non-volatile memory having an array of memory cells organized into blocks of cells that are erasable together as a unit and which individually store a plurality of pages of user data, comprising:

(a) storing overhead data within the individual blocks of information relating to the operation of the block in which the overhead data are stored,

(b) an erase operation, including,

addressing one or more of the blocks for erase,

reading the overhead data from said one or more blocks and temporarily storing the read overhead data,

erasing the memory cells within said one or more blocks,

updating the read overhead data, and

programming the updated overhead data back into respective ones of said one or more blocks, thereby to leave the pages within the one or more blocks available for programming user data therein, wherein the updated overhead data are programmed into spare cells of a single page of the respective ones of said one or more blocks,

(c) thereafter programming user data into pages of said one or more blocks.

2. The method of claim 1, wherein the overhead data stored in the individual blocks includes a quantity indicting the number of erase operations to which the block containing the quantity has been subjected, and wherein updating the overhead data includes updating the erase operations number to include the current erase operation.

3. A method of operating a non-volatile memory having an array of memory cells organized into blocks of cells that are erasable together as a unit and which individually store a plurality of pages of user data, comprising:

(a) storing overhead data within the individual blocks of information relating to the operation of the block in which the overhead data are stored,

(b) an erase operation, including,

addressing one or more of the blocks for erase,

reading the overhead data from said one or more blocks and temporarily storing the read overhead data,

erasing the memory cells within said one or more blocks,

updating the read overhead data, and

programming the updated overhead data back into respective ones of said one or more blocks, thereby to leave the pages within the one or more blocks available for programming user data therein, and

(c) thereafter programming user data into pages of said one or more blocks, wherein programming user data includes programming header data in designated regions of the individual pages with information of the user data programmed in respective ones of the pages, and wherein the updated overhead data are programmed into spare cells of the header regions of more than one page within each of said one or more blocks.

4. The method of claim 3, wherein the header data stored in individual pages includes a redundancy code calculated from the user data of the respective pages in which the user data are programmed.

5. The method of claim 4, wherein the redundancy code includes an error correcting code.

6. The method of claim 3, wherein the overhead data stored in the individual blocks includes a quantity indicting the number of erase operations to which the block containing the quantity has been subjected, and wherein updating the overhead data includes updating the erase operations number to include the current erase operation.

7. A method of operating a non-volatile memory having an array of memory cells organized into blocks of cells that are erasable together as a unit and which individually store a plurality of pages of user data, comprising:

(a) storing overhead data within the individual blocks of information relating to the operation of the block in which the overhead data are stored,

(b) an erase operation, including,

addressing one or more of the blocks for erase,

reading the overhead data from said one or more blocks and temporarily storing the read overhead data,

erasing the memory cells within said one or more blocks,

updating the read overhead data, and

programming the updated overhead data back into respective ones of said one or more blocks, thereby to leave the pages within the one or more blocks available for programming user data therein,

(c) thereafter programming user data into pages of said one or more blocks, and

(d) calculating a redundancy code from the overhead data that is stored within the individual blocks, and further storing the redundancy code within the individual blocks that contain the overhead data from which they were calculated, wherein reading the overhead data includes reading the redundancy code calculated from and stored with the overhead data and checking the read overhead data against the read redundancy code, and wherein programming the updated overhead data includes calculating a redundancy code from the updated overhead data and storing the calculated redundancy code in the same block as the overhead data.

8. The method of claim 7, wherein, if checking the read overhead data against the read redundancy code reveals that the overhead data is invalid, marking the block in which the read overhead data and redundancy code reside as unusable instead of erasing the block, updating the overhead data or programming the block with either updated overhead or user data.

9. The method of claim 8, additionally comprising marking the unusable block invalid by setting a flag as part of the overhead data within the unusable block.

10. A method of operating a non-volatile memory having an array of memory cells organized into blocks of cells that are erasable together as a unit and which individually store a plurality of pages of user data, comprising:

(a) storing overhead data within the individual blocks of information relating to the operation of the block in which the overhead data are stored,

(b) an erase operation, including,

addressing one or more of the blocks for erase,

reading the overhead data from said one or more blocks and temporarily storing the read overhead data,

erasing the memory cells within said one or more blocks,

updating the read overhead data, and

programming the updated overhead data back into respective ones of said one or more blocks, thereby to leave the pages within the one or more blocks available for programming user data therein, wherein programming the updated overhead data includes programming individual memory cell storage elements into one of exactly two storage states, thereby to store one bit of overhead data per storage element, and

(c) thereafter programming user data into pages of said one or more blocks, wherein programming user data includes programming individual memory cell storage elements into one of two or more states, thereby to store more than one bit of user data per storage element.

11. The method of claim 10, wherein erasing the memory cells includes driving the charge on memory cell storage elements to a first range, wherein programming the overhead data includes driving a portion of the memory cell storage elements to a second range according to the overhead data and wherein programming user data additionally includes programming individual memory cell storage elements into at least third and fourth charge ranges that are further removed from the first range than the second range.

12. The method of claim 10, wherein programming the user data includes applying successive programming pulses separated by a verification of the states of the cells until the desired programmed states are reached, the individual pulses increasing in magnitude by a first amount, and wherein programming the overhead data includes applying successive programming pulses separated by a verification of the state of the cells until the desired programmed states are reached, the individual pulses increasing in magnitude by a second amount that is higher than the first amount, thereby to program the overhead data faster than the user data.

13. The method of claim 11, wherein the array of memory cells is a NAND array.

14. A method of operating a non-volatile memory having an array of memory cells organized into blocks of cells that are erasable together as a unit and which individually store a plurality of pages that individually contain at least 512 bytes of user data, comprising:

(a) storing page overhead data within the individual pages of information relating to the user data stored therein,

(b) storing block overhead data within at least one page of the individual blocks of information relating to the block in which the overhead data are stored,

(c) an erase operation, including,

addressing one or more of the blocks for erase,

reading the block overhead data from said one or more blocks and temporarily storing the read block overhead data,

erasing the memory cells within said one or more blocks,

updating the read block overhead data, and

programming the updated block overhead data back into respective ones of said one or more blocks, thereby to leave the pages within the one or more blocks available for programming user data therein, wherein the updated block overhead data are programmed in more than one of the plurality of pages of the individual blocks, and

(d) thereafter programming user data into pages of said one or more blocks.

15. The method of claim 14, additionally comprising calculating a redundancy code from the block overhead data and storing the redundancy code within the individual blocks that contain the block overhead data from which they were calculated.

16. The method of claim 14, wherein, in a case where the block overhead data is lost prior to either updating the block overhead data or programming the updated block overhead data back into said one or more blocks, marking said one or more blocks as unusable instead of programming user data into pages of said one or more blocks.

17. The method of claim 14, wherein programming user data includes programming individual memory cell storage elements into one of two or more states, thereby to store more than one bit of user data per storage element.

18. The method of claim 14, wherein the array of memory cells is a NAND array.

19. The method of claim 14, wherein the block overhead data includes data of a redundancy code calculated from other of the block overhead data.

20. The method of claim 14, wherein the block overhead data includes data indicating a number of erase operations to which the block containing the indication has been subjected, and wherein updating the block overhead data includes updating the indication to include the current erase operation.