US 7366013 B2
A multiple level cell memory array has an area that can be programmed as single level cells. The cells to be programmed are initially programmed with the desire data into either the least significant or most significant bit of the cell. A second programming operation the programs reinforcing data that adjusts the threshold level of the cell to the appropriate level for the desired data.
1. A method for single level programming in a multiple level cell memory device, the method comprising:
writing desired data to one of either a least significant bit or a most significant bit of the cell; and
writing reinforcing data to the remaining bit of the cell such that a threshold voltage of the cell is adjusted to a voltage level required by the desired data.
2. The method of
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9. A method for single level programming in a multiple level cell NAND flash memory device, the method comprising:
determining a type of data to be programmed;
writing desired data to a least significant bit of a cell; and
writing reinforcing data that changes a threshold voltage of the cell to a voltage level required by the desired data.
10. The method of
11. The method of
12. A method for single level programming in a multiple level cell memory device comprising memory with a least significant page and a most significant page, the method comprising:
writing desired data to either the least significant page or the most significant page of at least one memory cell; and
writing reinforcing data to the remaining page of the at least one memory cell such that the remaining page is a non-adjacent page to the previously written page of each cell and is composed of reinforcing data to the desired data.
13. The method of
14. The method of
15. A memory device comprising:
a plurality of multiple level cells formed into an array of cells; and
at least one single level cell within the array of cells that is programmed and read as a single level cell in response to a memory driver that is adapted to write desired data to one of either a least significant bit or a most significant bit of the at least one single level cell and write reinforcing data to the remaining bit of the cell such that a threshold voltage of the cell is adjusted to a voltage level required by the desired data.
16. The device of
17. The device of
18. The device of
19. A flash memory device comprising:
a memory array comprising a plurality of cells that are programmable to multiple levels within a range of threshold voltages; and
control circuitry that is adapted to execute a single level programming method comprising writing desired data to one of either the least significant bit or the most significant bit of a dynamically selected cell, and writing reinforcing data to the remaining bit of the cell.
20. The device of
21. The device of
22. The device of
23. A memory system comprising:
a processor that generates memory signals and executes a memory driver; and
a memory device coupled to the processor and operating in response to the memory signals, the memory device comprising:
a plurality of multiple level cells formed as an array of cells; and
a control circuit that programs at least one multiple level cell in a single level format in response to the memory driver wherein the memory driver is adapted to write desired data to one of either a least significant bit or a most significant bit of the at least one multiple level cell and write reinforcing data to the remaining bit of the cell such that a threshold voltage of the cell is adjusted to a voltage level required by the desired data.
24. A memory module comprising:
a multiple level memory array arranged in rows and columns such that the rows comprise word lines coupled to control gates of a plurality of memory cells and the columns comprise bit lines coupled to series strings of memory cells, the memory array further arranged in memory blocks; and
control circuitry that is adapted to perform single level programming of at least one dynamically selected cell of the memory array by writing desired data to one of either a least significant bit or a most significant bit of the cell and writing reinforcing data to the remaining bit of the cell; and
a plurality of contacts configured to provide selective contact between the memory array and a host system.
25. The module of
26. A method for single level programming in a multiple level cell NAND flash memory device comprising a memory array, the method comprising:
determining a type of data to be programmed;
writing either multiple level cell data or single level cell data to at least one selected cell of the array in response to the type of data;
if the type of data is indicative of high reliability, writing desired data to a first bit of a cell and writing reinforcing data to a remaining bit of the cell that changes a threshold voltage of the cell to a voltage level required by the desired data; and
if the type of data is indicative of high memory density, writing the desired data in multiple level cell manner.
27. The method of
28. A memory system comprising:
a processor that generates memory signals and executes a high level memory driver and a low-level memory driver; and
a memory device coupled to the processor and operating in response to the memory signals, the memory device comprising:
a plurality of multiple level cells formed as an array of cells; and
a control circuit that programs a plurality of multiple level cells in a single level cell format in response to the high-level memory driver and the low-level memory driver wherein the low-level memory driver is adapted to write desired data to one of either a least significant bit or a most significant bit of each of the plurality of multiple level cells and write reinforcing data to the remaining bit of each of the plurality of multiple level cells such that a threshold voltage of each cell is adjusted to a voltage level required by the desired data.
29. The device of
30. The device of
31. The device of
32. A flash memory system having a processor coupled to system memory and to a flash memory device, the system comprising:
a high-level driver that is executed by the processor and selects memory locations of the flash memory device to which data is written; and
a low-level driver, coupled to the high-level driver, that is executed by the processor to write single level cell data into multiple level cell memory locations as determined by the high-level driver in response to a type of data being written wherein the low-level driver is adapted to write the single level cell data to one of either a least significant bit or a most significant bit of each of the multiple level cell memory locations and write reinforcing data to the remaining bit of each of the multiple level cell memory locations such that a threshold voltage of each location is adjusted to a voltage level required by the desired data.
33. The system of
The present invention relates generally to memory devices and in particular the present invention relates to non-volatile memory devices.
Memory devices are typically provided as internal, semiconductor, integrated circuits in computers or other electronic devices. There are many different types of memory including random-access memory (RAM), read only memory (ROM), dynamic random access memory (DRAM), synchronous dynamic random access memory (SDRAM), and flash memory.
Flash memory devices have developed into a popular source of non-volatile memory for a wide range of electronic applications. Flash memory devices typically use a one-transistor memory cell that allows for high memory densities, high reliability, and low power consumption. Common uses for flash memory include personal computers, personal digital assistants (PDAs), digital cameras, and cellular telephones. Program code and system data such as a basic input/output system (BIOS) are typically stored in flash memory devices for use in personal computer systems.
As the performance and complexity of electronic systems increase, the requirement for additional memory in a system also increases. However, in order to continue to reduce the costs of the system, the parts count must be kept to a minimum. This can be accomplished by increasing the memory density of an integrated circuit.
Memory density can be increased by using multiple level cells (MLC). MLC memory can increase the amount of data stored in an integrated circuit without adding additional cells and/or increasing the size of the die. The MLC method stores two or more data bits in each memory cell.
MLC requires tight control of the threshold voltages in order to use multiple threshold levels per cell. One problem with non-volatile memory cells that are closely spaced, and MLC in particular, is the floating gate-to-floating gate capacitive coupling that causes interference between cells. The interference can shift the threshold voltage of neighboring cells as one cell is programmed. This is referred to as a program disturb condition that affects cells that are not desired to be programmed.
An MLC memory device also has a lower reliability than a single level cell (SLC) memory device due, in part, to the increased quantity of states requiring more closely spaced threshold voltages. A bad bit in a memory device used to store photographs can be tolerated more easily than a bad bit in a memory device that stores code. A bad bit in a photograph might only produce a bad pixel out of millions of pixels while a bad bit in code or other data could mean a corrupted instruction that affects the operation of an entire program.
Due to intense competition and consumer desire for longer battery life in electronic devices, manufacturers must constantly find ways to reduce the quantity of components in devices while maintaining reliability. For the reasons stated above, and for other reasons stated below that will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for a memory device having high density with high reliability.
The above-mentioned problems with flash memories and other problems are addressed by the present invention and will be understood by reading and studying the following specification.
The present invention encompasses a method for programming a multiple level, non-volatile memory device in a single level cell mode. The memory device has a plurality of memory cells that are organized into a plurality of memory blocks. Each memory block has a memory cell arrays comprised of bit line columns and word line rows.
The method includes writing the desired data to one of either the least significant bit or the most significant bit of a cell to be programmed. A second programming operation programs reinforcing data to the remaining bit of the cell. The reinforcing data adjusts the threshold level of the cell to the voltage required by the desired data.
Further embodiments of the invention include methods and apparatus of varying scope.
In the following detailed description of the invention, reference is made to the accompanying drawings that form a part hereof and in which is shown, by way of illustration, specific embodiments in which the invention may be practiced. In the drawings, like numerals describe substantially similar components throughout the several views. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. Other embodiments may be utilized and structural, logical, and electrical changes may be made without departing from the scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims and equivalents thereof.
The memory array of
The array is comprised of an array of floating gate cells 101 arranged in series strings 104, 105. Each of the floating gate cells 101 are coupled drain to source in each series chain 104, 105. A word line (WL0-WL31) that spans across multiple series strings 104, 105 is coupled to the control gates of every floating gate cell in a row in order to control their operation. In one embodiment, an array is comprised of 32 word lines. However, the present invention is not limited to any one word line quantity.
In operation, the word lines (WL0-WL31) select the individual floating gate memory cells in the series chain 104, 105 to be written to or read from and operate the remaining floating gate memory cells in each series string 104, 105 in a pass through mode. Each series string 104, 105 of floating gate memory cells is coupled to a source line 106 by a source select gate 116, 117 and to an individual bit line (BL1-BLN) by a drain select gate 112, 113. The source select gates 116, 117 are controlled by a source select gate control line SG(S) 118 coupled to their control gates. The drain select gates 112, 113 are controlled by a drain select gate control line SG(D) 114.
In the embodiment of
Each cell can be programmed as a single bit per cell (i.e., single level cell—SLC) or multiple bits per cell (i.e., multiple level cell—MLC). Each cell's threshold voltage (Vt) (determines the data that is stored in the cell. For example, in a single bit per cell, a Vt of 0.5V might indicate a programmed cell (i.e., logical 0 state) while a Vt of −0.5V might indicate an erased cell (i.e., logical 1 state).
A multilevel cell has multiple Vt windows that each indicate a different state. Multilevel cells take advantage of the analog nature of a traditional flash cell by assigning a bit pattern to a specific voltage range stored on the cell. This technology permits the storage of two or more bits per cell, depending on the quantity of voltage ranges assigned to the cell.
For example, a cell may be assigned four different voltage ranges of 200 mV for each range. Typically, a dead space or margin of 0.2V to 0.4V is between each range. If the voltage stored on the cell is within the first range, the cell is storing a 11. If the voltage is within the second range, the cell is storing a 01. This continues for as many ranges that are used for the cell. In one embodiment, 11 is the most negative threshold voltage range while 10 is the most positive threshold voltage range. Alternate embodiments assign the logical states to different threshold voltage ranges.
The embodiments of the present invention are not limited to two bits per cell. Some embodiments may store more than two bits per cell, depending on the quantity of different voltage ranges that can be differentiated on the cell. 100311 During a typical prior art programming operation, the selected word line for the flash memory cell to be programmed is biased with a series of programming pulses that start at a voltage that, in one embodiment, is greater than 16V with each subsequent pulse voltage increasing incrementally until the cell is programmed or a maximum programming voltage is reached.
A verification operation with a word line voltage of 0V is then performed to determine if the floating gate is at the proper voltage (e.g., 0.5V). The unselected word lines for the remaining cells are typically biased at approximately 10V during the program operation. In one embodiment, the unselected word line voltages can be any voltage equal to or greater than ground potential. Each of the memory cells is programmed in a substantially similar fashion.
A typical memory block may be comprised of 64 pages of single level cells. An MLC memory block is typically comprised of 128 pages. When one of these pages is accessed in a typical prior art read/program operation, the remaining pages in the block can experience a disturb condition. In both cases, the pages share common word lines and bitlines that can experience higher programming/read voltages whenever any one of the pages is programmed/read. These voltages can cause problems by disturbing the distributions for the cells that are not being accessed. The embodiments of the present invention, as discussed subsequently, use a scrambled logical addressing technique that substantially reduces or eliminates the disturb condition.
The numbers shown in each cell 301-308 represent the logical page numbers that are written to each particular cell 301-308. This particular pattern was chosen to minimize the floating gate-to-floating gate coupling that occurs when cells are programmed. The present invention, however, is not limited to any one programming pattern.
Each physical cell 301-308 is capable of holding two logical bits, as described previously regarding programming of MLC. Therefore, the addressing scheme of the present invention assigns two scrambled, logical addresses to each physical address. In other words, to increase the noise margin, no physical cell 301-308 in the SLC area of the array has two adjacent logical addresses.
In the illustrated embodiment, logical pages 0 and 1 are programmed into cells 301, 302. Logical pages 2 and 3 are programmed into cells 303, 304 of row 1. Logical pages 4 and 5 are then programmed into cells 301, 302 of row 0. Logical pages 8 and 9 are programmed into cells 303, 304 of row 1. This pattern is repeated throughout the 128 pages of a memory block that is programmed in an SLC manner.
In order to program a physical cell in an MLC manner, each bit of data/code to be programmed is represented by two logical bits. In one embodiment, a logical 11 is assigned to represent a single level bit of 1 while a logical 10 is assigned to represent a single level bit of 0. This data pattern was chosen since the present embodiment of MLC memory, the threshold voltage of 11 is the most negative and the threshold voltage of 10 is the most positive thus substantially reducing the floating gate-to-floating gate interference and increasing the noise margin. Alternate embodiments may use other states besides logical 11 and/or 10.
The data to be stored 401 is received. In one embodiment, as described subsequently with reference to
In one embodiment, the LSB of the cell to be programmed is programmed first 403. However, an alternate embodiment may program the MSB first and the LSB last.
In the illustrated embodiment, the method first programs page 0 into the cell 301 (i.e., LSB). If the cell started from the erased state (i.e., a logical 11) and the data/code bit is a logical 0, then the cell 301 now has a programmed pattern of a logical 10.
When page 4 is to be programmed into the cell 301, it must be done so as to reinforce the desired data/code bit of 0. The reinforcing data is that data which changes the threshold voltage of each SLC programmed cell to the appropriate threshold voltage required for the desire single level data. Therefore, the reinforcing data must be determined 405 and then written into the cell 407. In the present example, a logical 10 is assigned to represent an SLC data bit of logical 0, so the cell is already at the proper threshold voltage. Therefore, when the cell is read, the threshold voltage to which logical 10 state is assigned will be detected and a logical 0 is read.
The above-described example, for purposes of clarity, does not discuss the programming of pages 1-3. By referring to
As another example, if a data/code bit of logical 0 is desired to be programmed into the cell 301 and the cell 301 has already been programmed from a previous page write operation with a logical 01, page 0 of the cell needs to program a logical 0 into the LSB of the cell 301. This is accomplished by raising the threshold voltage of the cell 301 from the logical 01 state to the level of the logical 10 state as seen in
This determination can be accomplished in various ways. In one embodiment, the physical address indicates whether the user is writing to an SLC area of memory or an MLC area. For example, an address that is in the first 32 MB of the memory array address space can be set aside for SLC data while the remainder of the array address space is MLC data. In such an embodiment, when the data is read, it is assumed that the multiple bits that are read map to a single level data bit.
In another embodiment, an indicator bit is included with the data to dynamically indicate whether to use the SLC driver or the normal MLC programming method. Alternate embodiments can use other forms to identify the type of programming.
The SLC driver 505 has been described with reference to
The processor 610 is coupled to the system memory 680. This memory block 680 includes all of the memory required by the system such as RAM, ROM, magnetic storage drives, or other forms of memory. A memory system may have any one of these types of system memory or all of them. In one embodiment, the low-level software driver of the present invention that enables SLC storage in MLC memory is stored in this system memory 680. The driver may be part of the system 620 operating system that is stored on a hard drive or ROM and eventually read into RAM for execution. The software modules illustrated in
The memory device includes an array of flash memory cells 630 as described above with reference to
An address buffer circuit 640 is provided to latch address signals provided on address input connections A0-Ax 642. Address signals are received and decoded by a row decoder 644 and a column decoder 646 to access the memory array 630. It will be appreciated by those skilled in the art, with the benefit of the present description, that the number of address input connections depends on the density and architecture of the memory array 630. That is, the number of addresses increases with both increased memory cell counts and increased bank and block counts.
The memory device 600 reads data in the memory array 630 by sensing voltage or current changes in the memory array columns using sense/buffer circuitry 650. The sense/buffer circuitry, in one embodiment, is coupled to read and latch a row of data from the memory array 630. Data input and output buffer circuitry 660 is included for bi-directional data communication over a plurality of data connections 662 with the controller 610. Write circuitry 655 is provided to write data to the memory array.
Control circuitry 670 decodes signals provided on control connections 672 from the processor 610. These signals are used to control the operations on the memory array 630, including data read, data write (program), and erase operations. The control circuitry 670 may be a state machine, a sequencer, or some other type of controller.
The flash memory device illustrated in
In some embodiments, memory module 700 will include a housing 705 (as depicted) to enclose one or more memory devices 710, though such a housing is not essential to all devices or device applications. At least one memory device 710 is a non-volatile memory [including or adapted to perform elements of the invention]. Where present, the housing 705 includes one or more contacts 715 for communication with a host device. Examples of host devices include digital cameras, digital recording and playback devices, PDAs, personal computers, memory card readers, interface hubs and the like. For some embodiments, the contacts 715 are in the form of a standardized interface. For example, with a USB flash drive, the contacts 715 might be in the form of a USB Type-A male connector. For some embodiments, the contacts 715 are in the form of a semi-proprietary interface, such as might be found on COMPACTFLASH memory cards licensed by SANDISK Corporation, MEMORYSTICK memory cards licensed by SONY Corporation, SD SECURE DIGITAL memory cards licensed by TOSHIBA Corporation and the like. In general, however, contacts 715 provide an interface for passing control, address and/or data signals between the memory module 700 and a host having compatible receptors for the contacts 715.
The memory module 700 may optionally include additional circuitry 720 which may be one or more integrated circuits and/or discrete components. For some embodiments, the additional circuitry 720 may include a memory controller for controlling access across multiple memory devices 710 and/or for providing a translation layer between an external host and a memory device 710. For example, there may not be a one-to-one correspondence between the number of contacts 715 and a number of I/O connections to the one or more memory devices 710. Thus, a memory controller could selectively couple an I/O connection (not shown in
The additional circuitry 720 may further include functionality unrelated to control of a memory device 710 such as logic functions as might be performed by an ASIC (application specific integrated circuit). Also, the additional circuitry 720 may include circuitry to restrict read or write access to the memory module 700, such as password protection, biometrics or the like. The additional circuitry 720 may include circuitry to indicate a status of the memory module 700. For example, the additional circuitry 720 may include functionality to determine whether power is being supplied to the memory module 700 and whether the memory module 700 is currently being accessed, and to display an indication of its status, such as a solid light while powered and a flashing light while being accessed. The additional circuitry 720 may further include passive devices, such as decoupling capacitors to help regulate power requirements within the memory module 700.
The high-level driver 801 works in conjunction with an operating system or an application to manage a flash memory integrated circuit. The FTL 801 manages data in a flash device so that it appears that the data is written to a particular location when in fact the data is being stored in different locations of the flash. The FTL 801 moves the data to different physical locations in the flash memory array in order to achieve more even wear characteristics. This allows the flash device to appear as a disk drive or other block storage device to the processor.
The low-level drivers 802 are responsible for taking the remapped addresses and data from the FTL 801 and performing the actual programming/reading of the data. In one embodiment, the embodiments of the SLC programming in an MLC flash memory device are performed by a low-level memory driver.
In summary, the embodiments of the present invention create an MLC non-volatile memory device with at least one SLC memory cell in the same memory array as a plurality of MLC data. The programming/reading of SLC data in the MLC device is determined dynamically by control circuit/processor executing a low-level software driver in response to data reliability or address. This gives the end user of the MLC non-volatile memory device the option of storing sensitive code data in the SLC area for higher reliability and other, less sensitive data in the denser MLC area of the array.
Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement that is calculated to achieve the same purpose may be substituted for the specific embodiments shown. Many adaptations of the invention will be apparent to those of ordinary skill in the art. Accordingly, this application is intended to cover any adaptations or variations of the invention. It is manifestly intended that this invention be limited only by the following claims and equivalents thereof.