WO2007143399A2 - Compensation de sensibilité de séquences de données en utilisant une tension différente - Google Patents

Compensation de sensibilité de séquences de données en utilisant une tension différente Download PDF

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Publication number
WO2007143399A2
WO2007143399A2 PCT/US2007/069590 US2007069590W WO2007143399A2 WO 2007143399 A2 WO2007143399 A2 WO 2007143399A2 US 2007069590 W US2007069590 W US 2007069590W WO 2007143399 A2 WO2007143399 A2 WO 2007143399A2
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Prior art keywords
voltage
volatile storage
data
storage elements
storage element
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PCT/US2007/069590
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English (en)
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WO2007143399A3 (fr
Inventor
Nima Mokhlesi
Yingda Dong
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Sandisk Corporation
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Priority claimed from US11/421,884 external-priority patent/US7310272B1/en
Priority claimed from US11/421,871 external-priority patent/US7450421B2/en
Application filed by Sandisk Corporation filed Critical Sandisk Corporation
Publication of WO2007143399A2 publication Critical patent/WO2007143399A2/fr
Publication of WO2007143399A3 publication Critical patent/WO2007143399A3/fr

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    • G—PHYSICS
    • G11—INFORMATION STORAGE
    • G11C—STATIC STORES
    • G11C11/00—Digital stores characterised by the use of particular electric or magnetic storage elements; Storage elements therefor
    • G11C11/56—Digital stores characterised by the use of particular electric or magnetic storage elements; Storage elements therefor using storage elements with more than two stable states represented by steps, e.g. of voltage, current, phase, frequency
    • G11C11/5621—Digital stores characterised by the use of particular electric or magnetic storage elements; Storage elements therefor using storage elements with more than two stable states represented by steps, e.g. of voltage, current, phase, frequency using charge storage in a floating gate
    • G11C11/5642—Sensing or reading circuits; Data output circuits
    • G—PHYSICS
    • G11—INFORMATION STORAGE
    • G11C—STATIC STORES
    • G11C16/00—Erasable programmable read-only memories
    • G11C16/02—Erasable programmable read-only memories electrically programmable
    • G11C16/04—Erasable programmable read-only memories electrically programmable using variable threshold transistors, e.g. FAMOS
    • G11C16/0483—Erasable programmable read-only memories electrically programmable using variable threshold transistors, e.g. FAMOS comprising cells having several storage transistors connected in series
    • G—PHYSICS
    • G11—INFORMATION STORAGE
    • G11C—STATIC STORES
    • G11C16/00—Erasable programmable read-only memories
    • G11C16/02—Erasable programmable read-only memories electrically programmable
    • G11C16/06—Auxiliary circuits, e.g. for writing into memory
    • G11C16/34—Determination of programming status, e.g. threshold voltage, overprogramming or underprogramming, retention
    • G11C16/3418—Disturbance prevention or evaluation; Refreshing of disturbed memory data
    • G—PHYSICS
    • G11—INFORMATION STORAGE
    • G11C—STATIC STORES
    • G11C16/00—Erasable programmable read-only memories
    • G11C16/02—Erasable programmable read-only memories electrically programmable
    • G11C16/06—Auxiliary circuits, e.g. for writing into memory
    • G11C16/34—Determination of programming status, e.g. threshold voltage, overprogramming or underprogramming, retention
    • G11C16/3418—Disturbance prevention or evaluation; Refreshing of disturbed memory data
    • G11C16/3427—Circuits or methods to prevent or reduce disturbance of the state of a memory cell when neighbouring cells are read or written
    • G—PHYSICS
    • G11—INFORMATION STORAGE
    • G11C—STATIC STORES
    • G11C16/00—Erasable programmable read-only memories
    • G11C16/02—Erasable programmable read-only memories electrically programmable
    • G11C16/06—Auxiliary circuits, e.g. for writing into memory
    • G11C16/34—Determination of programming status, e.g. threshold voltage, overprogramming or underprogramming, retention
    • G11C16/3436—Arrangements for verifying correct programming or erasure
    • G11C16/3454—Arrangements for verifying correct programming or for detecting overprogrammed cells
    • G—PHYSICS
    • G11—INFORMATION STORAGE
    • G11C—STATIC STORES
    • G11C16/00—Erasable programmable read-only memories
    • G11C16/02—Erasable programmable read-only memories electrically programmable
    • G11C16/06—Auxiliary circuits, e.g. for writing into memory
    • G11C16/34—Determination of programming status, e.g. threshold voltage, overprogramming or underprogramming, retention
    • G11C16/3436—Arrangements for verifying correct programming or erasure
    • G11C16/3454—Arrangements for verifying correct programming or for detecting overprogrammed cells
    • G11C16/3459—Circuits or methods to verify correct programming of nonvolatile memory cells
    • G—PHYSICS
    • G11—INFORMATION STORAGE
    • G11C—STATIC STORES
    • G11C2211/00—Indexing scheme relating to digital stores characterized by the use of particular electric or magnetic storage elements; Storage elements therefor
    • G11C2211/56—Indexing scheme relating to G11C11/56 and sub-groups for features not covered by these groups
    • G11C2211/562—Multilevel memory programming aspects
    • G11C2211/5621—Multilevel programming verification

Definitions

  • the present invention relates to technology for non- volatile memory.
  • Non-volatile semiconductor memory has become more popular for use in various electronic devices.
  • non-volatile semiconductor memory is used in cellular telephones, digital cameras, personal digital assistants, mobile computing devices, non-mobile computing devices and other devices.
  • Electrical Erasable Programmable Read Only Memory (EEPROM) and flash memory are among the most popular non-volatile semiconductor memories.
  • Both EEPROM and flash memory utilize a floating gate that is positioned above and insulated from a channel region in a semiconductor substrate.
  • the floating gate is positioned between the source and drain regions.
  • a control gate is provided over and insulated from the floating gate.
  • the threshold voltage of the transistor is controlled by the amount of charge that is retained on the floating gate. That is, the minimum amount of voltage that must be applied to the control gate before the transistor is turned on to permit conduction between its source and drain is controlled by the level of charge on the floating gate.
  • NAND structure which includes arranging multiple transistors in series between two select gates.
  • the transistors in series and the select gates are referred to as a NAND string.
  • Some EEPROM and flash memory devices have a floating gate that is used to store two ranges of charges and, therefore, the memory cell can be programmed/erased between two states (an erased state and a programmed state).
  • Such a flash memory device is sometimes referred to as a binary flash memory device.
  • a multi-state flash memory device is implemented by identifying multiple distinct allowed/valid programmed threshold voltage ranges separated by forbidden ranges. Each distinct threshold voltage range corresponds to a predetermined value for the set of data bits encoded in the memory device.
  • Errors can occur when reading non-volatile storage element due to at least two mechanisms: (1) capacitive coupling between neighboring floating gates and (2) changing conductivity of the channel area after programming (referred to as back pattern effect). Both of these issues are discussed below.
  • neighboring floating gates that are on the same bit line, neighboring floating gates on the same word line, or floating gates that are diagonal from the target floating gate because they are on both a neighboring bit line and neighboring word line.
  • the floating gate to floating gate coupling phenomena occurs most pronouncedly between sets of adjacent memory cells that have been programmed at different times. For example, a first memory cell is programmed to add a level of charge to its floating gate that corresponds to one set of data. Subsequently, one or more adjacent memory cells are programmed to add a level of charge to their floating gates that correspond to a second set of data. After the one or more of the adjacent memory cells are programmed, the charge level read from the first memory cell appears to be different than programmed because of the effect of the charge on the adjacent memory cells being coupled to the first memory cell. The coupling from adjacent memory cells can shift the apparent charge level being read a sufficient amount to lead to an erroneous reading of the data stored.
  • the effect of the floating gate to floating gate coupling is of greater concern for multi-state devices because in multi-state devices the allowed threshold voltage ranges and the forbidden ranges are narrower than in binary devices. Therefore, the floating gate to floating gate coupling can result in memory cells being shifted from an allowed threshold voltage range to a forbidden range.
  • Errors can also occur due to the back pattern effect.
  • memory cells are programmed in a certain order wherein the memory cells on the word line that is next to the source side select gate are programmed first. Subsequently, the memory cells on the adjacent word line are programmed, followed by the programming of memory cells on the next adjacent word line, and so on, until the memory cells on the last word line next to the drain side select gate are programmed.
  • the conductivity of the channel area under those word lines usually decreases as most of the cells will be programmed to one of the programmed states (while a smaller number, on average 25%, will stay in the erased state).
  • the IV characteristics change since less current will flow than compared to previous verify operation performed during programming.
  • the lowered current causes an artificial shift of the threshold voltages for the memory cells, which can lead to errors when reading data. This effect is referred to as the back pattern effect. -5-
  • the read process for a particular memory cell will provide compensation to an adjacent memory cell in order to reduce the coupling effect that the adjacent memory cell has on the particular memory cell.
  • a first voltage is used during a verify operation for unselected word lines that have been subjected to a programming operation and a second voltage is used for unselected word lines that have not been subjected to a programming operation.
  • One embodiment includes applying a particular voltage to a particular non-volatile storage element of a group of connected non-volatile storage elements, applying a first voltage to a first set of one or more nonvolatile storage elements of the group, applying a second voltage to a second set of one or more non-volatile storage elements of the group, applying a voltage that is different than the second voltage to a neighbor non-volatile storage element of the group in coordination with applying the particular voltage, and verifying programming of the particular non-volatile storage element based on the particular voltage.
  • the first set of one or more nonvolatile storage elements are on a first side of the particular non-volatile storage element and have already been subjected to one or more programming processes since a last erase of the group.
  • the first voltage is applied in coordination with the particular voltage.
  • the second set of one or more nonvolatile storage elements are on a second side of the particular non-volatile storage element and have not already been subjected to a programming processes since the last erase of the group.
  • the second voltage is applied in coordination with the particular voltage.
  • the neighbor non-volatile storage element has not been subjected to programming since erasing the group.
  • One embodiment includes applying a particular voltage to a particular non-volatile storage element of a group of connected non-volatile storage elements, applying a first voltage to a first set of one or more nonvolatile storage elements of the group, the first set of one or more non-volatile storage elements have already been subjected to one or more programming processes since a last erase of the group, applying a second voltage to a second set of one or more non-volatile storage elements of the group, applying a voltage that is different than the second voltage to a neighbor non-volatile storage element of the group in coordination with applying the particular voltage, verifying programming of the particular non-volatile storage element based on the particular voltage, and reading data from the particular nonvolatile storage element.
  • the reading of data includes applying a data dependent voltage to the neighbor non-volatile storage element based on a condition of the neighbor non-volatile storage element.
  • the first voltage is applied in coordination with the particular voltage.
  • the second set of one or more non-volatile storage elements have not already been subjected to a programming processes since the last erase of the group, the second voltage is applied in coordination with the particular voltage.
  • the neighbor non-volatile storage element is next to the particular non- volatile storage element.
  • One embodiment includes applying a particular voltage to a particular non-volatile storage element of a group of connected non-volatile storage elements, applying a first voltage to one or more non-volatile storage elements of the group that have already been subjected to one or more programming processes since a last erase of the group, applying a second voltage to a set of one or more non-volatile storage elements of the group that -7-
  • a dependent voltage based on a number of non-volatile storage elements in the set, applying (in coordination with the particular voltage) the dependent voltage to a non-volatile storage element that is a neighbor of the particular non-volatile storage element, and sensing a condition related to the particular non-volatile storage element and the particular voltage.
  • the first voltage is applied in coordination with the particular voltage.
  • the second voltage is applied in coordination with the particular voltage.
  • One example implementation comprises a plurality of non-volatile storage elements and a managing circuit in communication with the plurality of non-volatile storage elements for performing the processes discussed herein.
  • Figure 1 is a top view of a NAND string.
  • Figure 2 is an equivalent circuit diagram of the NAND string.
  • Figure 3 is a cross-sectional view of the NAND string.
  • Figure 4 is a block diagram of an array of NAND flash memory cells.
  • Figure 5 is a block diagram of a non-volatile memory system.
  • Figure 6 is a block diagram of a non-volatile memory system.
  • Figure 7 is a block diagram depicting one embodiment of the sense block.
  • Figure 7A is a block diagram of a memory array.
  • Figure 8 is a flow chart describing one embodiment of a process for programming non-volatile memory.
  • Figure 9 is an example wave form applied to the control gates of nonvolatile memory cells.
  • Figure 10 is a timing diagram that explains the behavior of certain signals during read/verify operations.
  • Figure 1OA depicts a NAND string.
  • Figure 1OB depicts a NAND string.
  • Figure 1OC is a flow chart describing one embodiment of a process for operating non-volatile storage.
  • Figure 1OD is flow chart describing one embodiment of a process of choosing a voltage for a word line next to the selected word line.
  • Figure 11 depicts an example set of threshold voltage distributions.
  • Figure 12 depicts an example set of threshold voltage distributions.
  • Figures 13A-C show various threshold voltage distributions and describe a process for programming non-volatile memory.
  • Figures 14A-G are tables depicting the order of programming nonvolatile memory in various embodiments.
  • Figure 15 is a flow chart describing one embodiment of a process for reading non-volatile memory.
  • Figure 16 is a flow chart describing one embodiment of a process for performing a read operation for non-volatile memory.
  • Figure 17 is a flow chart describing one embodiment of a process for recovering data.
  • Figure 18 is a flow chart describing one embodiment of a process for recovering data from multiple word lines.
  • Figure 19 is a flow chart describing one embodiment of a process for reading data from a lower page.
  • Figure 20 is a flow chart describing one embodiment of a process of reading data from an upper page.
  • Figure 21 is a flow chart describing one embodiment of a process for reading data.
  • Figure 22 is a flow chart describing one embodiment of a process for reading data from an upper page.
  • Figure 23 is a flow chart describing one embodiment of a process for reading data without using compensation.
  • Figure 24 is a flow chart describing one embodiment of a process for reading data while compensating for floating gate to floating gate (or dielectric region to dielectric region) coupling.
  • Figure 25 is a table depicting a process for determining data values.
  • Figure 26 is a flow chart describing one embodiment of a process for reading upper page data using a correction.
  • Figure 27 is a block diagram showing capacitive coupling between two neighboring memory cells. -10-
  • NAND flash memory structure which includes arranging multiple transistors in series between two select gates.
  • the transistors in series and the select gates are referred to as a NAND string.
  • Figure 1 is a top view showing one NAND string.
  • Figure 2 is an equivalent circuit thereof.
  • the NAND string depicted in Figures 1 and 2 includes four transistors, 100, 102, 104 and 106, in series and sandwiched between a first select gate 120 and a second select gate 122.
  • Select gate 120 gates the NAND string connection to bit line 126.
  • Select gate 122 gates the NAND string connection to source line 128.
  • Select gate 120 is controlled by applying the appropriate voltages to control gate 120CG.
  • Select gate 122 is controlled by applying the appropriate voltages to control gate 122CG.
  • Each of the transistors 100, 102, 104 and 106 has a control gate and a floating gate.
  • Transistor 100 has control gate IOOCG and floating gate 100FG.
  • Transistor 102 includes control gate 102CG and floating gate 102FG.
  • Transistor 104 includes control gate 104CG and floating gate 104FG.
  • Transistor 106 includes a control gate 106CG and floating gate 106FG.
  • Control gate IOOCG is connected to (or is) word line WL3
  • control gate 102CG is connected to word line WL2
  • control gate 104CG is connected to word line WLl
  • control gate 106CG is connected to word line WLO.
  • transistors 100, 102, 104 and 106 are each memory cells. In other embodiments, the memory cells may include multiple transistors or may be different than that depicted in Figures 1 and 2.
  • Select gate 120 is connected to select line SGD.
  • Select gate 122 is connected to select line SGS.
  • Figure 3 provides a cross-sectional view of the NAND string described above. As depicted in Figure 3, the transistors of the NAND string are formed in p-well region 140. Each transistor includes a stacked gate structure that consists of a control gate (IOOCG, 102CG, 104CG and 106CG) and a floating -11-
  • the control gates and the floating gates are typically formed by depositing poly-silicon layers.
  • the floating gates are formed on the surface of the p-well on top of an oxide or other dielectric film.
  • the control gate is above the floating gate, with an inter-polysilicon dielectric layer separating the control gate and floating gate.
  • the control gates of the memory cells (100, 102, 104 and 106) form the word lines.
  • N+ doped diffusion regions 130, 132, 134, 136 and 138 are shared between neighboring cells, through which the cells are connected to one another in series to form a NAND string. These N+ doped regions form the source and drain of each of the cells.
  • N+ doped region 130 serves as the drain of transistor 122 and the source for transistor 106
  • N+ doped region 132 serves as the drain for transistor 106 and the source for transistor 104
  • N+ doped region 134 serves as the drain for transistor 104 and the source for transistor 102
  • N+ doped region 136 serves as the drain for transistor 102 and the source for transistor 100
  • N+ doped region 138 serves as the drain for transistor 100 and the source for transistor 120.
  • N+ doped region 126 connects to the bit line for the NAND string
  • N+ doped region 128 connects to a common source line for multiple NAND strings.
  • Figures 1-3 show four memory cells in the NAND string, the use of four transistors is provided only as an example.
  • a NAND string used with the technology described herein can have less than four memory cells or more than four memory cells.
  • some NAND strings will include 8 memory cells, 16 memory cells, 32 memory cells, 64 memory cells, etc. The discussion herein is not limited to any particular number of memory cells in a NAND string.
  • Each memory cell can store data represented in analog or digital form.
  • the range of possible threshold voltages of the memory cell is divided into two ranges, which are assigned logical data "1" and "0.”
  • the threshold voltage is negative after the memory cell is erased, and defined as logic "1.”
  • the threshold voltage is positive after a program operation, and defined as logic "0.”
  • the memory cell will turn on to indicate logic one is being stored.
  • the threshold voltage is positive and a read operation is attempted by applying 0 volts to the control gate, the memory cell will not turn on, which indicates that logic zero is stored.
  • a memory cell can also store multiple states, thereby storing multiple bits of digital data.
  • the threshold voltage window is divided into the number of states. For example, if four states are used, there will be four threshold voltage ranges assigned to the data values "11,” “10,” “01,” and “00.”
  • the threshold voltage after an erase operation is negative and defined as "11.” Positive threshold voltages are used for the states of "10,” “01,” and "00.”
  • Another type of memory cell useful in flash EEPROM systems utilizes a non-conductive dielectric material in place of a conductive floating gate to store charge in a non-volatile manner.
  • a non-conductive dielectric material in place of a conductive floating gate to store charge in a non-volatile manner.
  • Such a cell is described in an article by Chan et al., "A True Single-Transistor Oxide-Nitride-Oxide EEPROM Device," IEEE Electron Device Letters, Vol. EDL-8, No. 3, March 1987, pp. 93-95.
  • a triple layer dielectric formed of silicon oxide, silicon nitride and silicon oxide (“ONO") is sandwiched between a conductive control gate and a -13-
  • the cell is programmed by injecting electrons from the cell channel into the nitride, where they are trapped and stored in a limited region. This stored charge then changes the threshold voltage of a portion of the channel of the cell in a manner that is detectable.
  • the cell is erased by injecting hot holes into the nitride. See also Nozaki et al., "A 1-Mb EEPROM with MONOS Memory Cell for Semiconductor Disk Application," IEEE Journal of Solid-State Circuits, Vol. 26, No. 4, April 1991, pp.
  • Figure 4 illustrates an example of an array of NAND cells, such as those shown in Figures 1-3.
  • a bit line 206 is coupled to the drain terminal 126 of the drain select gate for the NAND string 150.
  • a source line 204 may connect all the source terminals 128 of the source select gates of the NAND strings.
  • the array of memory cells is divided into a large number of blocks of memory cells.
  • the block is the unit of erase. That is, each block contains the minimum number of memory cells that are erased together.
  • Each block is typically divided into a number of pages.
  • a page is a unit of programming.
  • the individual pages may be divided into segments and the segments may contain the fewest number of cells that are written at one time as a basic programming operation.
  • One or more pages of data are typically stored in one row of memory cells.
  • a page can store one or more sectors.
  • a sector includes user data and overhead data.
  • Overhead data typically includes an Error Correction Code (ECC) that has been calculated from the user data of the sector.
  • ECC Error Correction Code
  • a portion of the controller calculates the ECC when data is being programmed into the array, and also checks it when data is being read from the array.
  • the ECCs and/or other overhead data are stored in different pages, or even different blocks, than the user data to which they pertain.
  • a sector of user data is typically 512 bytes, corresponding to the size of a sector in magnetic disk drives. Overhead data is typically an additional 16-20 bytes.
  • a large number of pages form a block, anywhere from 8 pages, for example, up to 32, 64, 128 or more pages.
  • a row of NAND strings comprises a block. -15-
  • Memory cells are erased in one embodiment by raising the p-well to an erase voltage (e.g., 20 volts) for a sufficient period of time and grounding the word lines of a selected block while the source and bit lines are floating. Due to capacitive coupling, the unselected word lines, bit lines, select lines, and source line are also raised to a significant fraction of the erase voltage. A strong electric field is thus applied to the tunnel oxide layers of selected memory cells and the data of the selected memory cells are erased as electrons of the floating gates are emitted to the substrate side, typically by Fowler- Nordheim tunneling mechanism. As electrons are transferred from the floating gate to the p-well region, the threshold voltage of a selected cell is lowered. Erasing can be performed on the entire memory array, separate blocks, or another unit of cells.
  • an erase voltage e.g. 20 volts
  • FIG. 5 illustrates a memory device 296 having read/write circuits for reading and programming a page of memory cells in parallel, according to one embodiment of the present invention.
  • Memory device 296 may include one or more memory die 298.
  • Memory die 298 includes a two-dimensional array of memory cells 300, control circuitry 310, and read/write circuits 365.
  • the array of memory cells can be three dimensional.
  • the memory array 300 is addressable by word lines via a row decoder 330 and by bit lines via a column decoder 360.
  • the read/write circuits 365 include multiple sense blocks 400 and allow a page of memory cells to be read or programmed in parallel.
  • a controller 350 is included in the same memory device 296 (e.g., a removable storage card) as the one or more memory die 298. Commands and Data are transferred between the host and controller 350 via lines 320 and between the controller and the one or more memory die 298 via lines 318.
  • the control circuitry 310 cooperates with the read/write circuits 365 to perform memory operations on the memory array 300.
  • the control circuitry 310 includes a state machine 312, an on-chip address decoder 314 and a power -16-
  • the state machine 312 provides chip-level control of memory operations.
  • the on-chip address decoder 314 provides an address interface between that used by the host or a memory controller to the hardware address used by the decoders 330 and 360.
  • the power control module 316 controls the power and voltages supplied to the word lines and bit lines during memory operations.
  • Fig. 5 some of the components of Fig. 5 can be combined.
  • one or more of the components of Fig. 5 (alone or in combination), other than memory cell array 300, can be thought of as a managing circuit.
  • one or more managing circuits may include any one of or a combination of control circuitry 310, state machine 312, decoders 314/360, power control 316, sense blocks 400, read/write circuits 365, controller 350, etc.
  • Figure 6 illustrates another arrangement of the memory device 296 shown in Figure 5.
  • Access to the memory array 300 by the various peripheral circuits is implemented in a symmetric fashion, on opposite sides of the array, so that the densities of access lines and circuitry on each side are reduced by half.
  • the row decoder is split into row decoders 33OA and 33OB and the column decoder into column decoders 360A and 360B.
  • the read/write circuits are split into read/write circuits 365 A connecting to bit lines from the bottom and read/write circuits 365B connecting to bit lines from the top of the array 300. In this way, the density of the read/write modules is essentially reduced by one half.
  • the device of Fig. 6 can also include a controller, as described above for the device of Fig. 5.
  • Figure 7 is a block diagram of an individual sense block 400 partitioned into a core portion, referred to as a sense module 380, and a common portion 390. In one embodiment, there will be a separate sense module 380 for each bit line and one common portion 390 for a set of multiple sense modules 380. In -17-
  • a sense block will include one common portion 390 and eight sense modules 380. Each of the sense modules in a group will communicate with the associated common portion via a data bus 372.
  • a data bus 372 For further details refer to U.S. Patent Application 11/026,536 "Non-Volatile Memory & Method with Shared Processing for an Aggregate of Sense Amplifiers" filed on 12/29/04, which is incorporated herein by reference in its entirety.
  • Sense module 380 comprises sense circuitry 370 that determines whether a conduction current in a connected bit line is above or below a predetermined threshold level.
  • Sense module 380 also includes a bit line latch 382 that is used to set a voltage condition on the connected bit line. For example, a predetermined state latched in bit line latch 382 will result in the connected bit line being pulled to a state designating program inhibit (e.g., Vdd).
  • Common portion 390 comprises a processor 392, a set of data latches 394 and an I/O Interface 396 coupled between the set of data latches 394 and data bus 320.
  • Processor 392 performs computations. For example, one of its functions is to determine the data stored in the sensed memory cell and store the determined data in the set of data latches.
  • the set of data latches 394 is used to store data bits determined by processor 392 during a read operation. It is also used to store data bits imported from the data bus 320 during a program operation. The imported data bits represent write data meant to be programmed into the memory.
  • I/O interface 396 provides an interface between data latches 394 and the data bus 320.
  • the operation of the system is under the control of state machine 312 that controls the supply of different control gate voltages to the addressed cell.
  • state machine 312 controls the supply of different control gate voltages to the addressed cell.
  • the sense module 380 may trip at one of these voltages and an output will be -18-
  • bit line latch 382 serves double duty, both as a latch for latching the output of the sense module 380 and also as a bit line latch as described above.
  • each processor 392 will include an output line (not depicted in Fig. 7) such that each of the output lines is wired-OR'd together.
  • the output lines are inverted prior to being connected to the wired-OR line. This configuration enables a quick determination during the program verification process of when the programming process has completed because the state machine receiving the wired-OR can determine when all bits being programmed have reached the desired level. For example, when each bit has reached its desired level, a logic zero for that bit will be sent to the wired-OR line (or a data one is inverted).
  • the state machine When all bits output a data 0 (or a data one inverted), then the state machine knows to terminate the programming process. Because each processor communicates with eight sense modules, the state machine needs to read the wired-OR line eight times, or logic is added to processor 392 to accumulate the results of the associated bit lines such that the state machine need only read the wired-OR line one time. Similarly, by choosing the logic levels correctly, the global state machine can detect when the first bit changes its state and change the algorithms accordingly.
  • the data to be programmed is stored in the set of data latches 394 from the data bus 320.
  • the program operation under the control of the state machine, comprises a series of programming voltage -19-
  • Each programming pulse is followed by a read back (verify) to determine if the cell has been programmed to the desired memory state.
  • Processor 392 monitors the read back memory state relative to the desired memory state. When the two are in agreement, the processor 222 sets the bit line latch 214 so as to cause the bit line to be pulled to a state designating program inhibit. This inhibits the cell coupled to the bit line from further programming even if programming pulses appear on its control gate. In other embodiments the processor initially loads the bit line latch 382 and the sense circuitry sets it to an inhibit value during the verify process.
  • Data latch stack 394 contains a stack of data latches corresponding to the sense module. In one embodiment, there are three data latches per sense module 380. In some implementations (but not required), the data latches are implemented as a shift register so that the parallel data stored therein is converted to serial data for data bus 320, and vice versa. In the preferred embodiment, all the data latches corresponding to the read/write block of m memory cells can be linked together to form a block shift register so that a block of data can be input or output by serial transfer. In particular, the bank of r read/write modules is adapted so that each of its set of data latches will shift data in to or out of the data bus in sequence as if they are part of a shift register for the entire read/write block.
  • a NAND flash memory is partitioned into 1,024 blocks. Other sized arrays can also be used.
  • the data stored in each block can be simultaneously erased.
  • the block is the minimum unit of memory cells that are simultaneously erased.
  • each block includes 8,512 columns corresponding to bit lines BLO, BLl , ... BL8511.
  • all the bit lines of a block can be simultaneously selected during read and program operations. Memory cells along a common word line and connected to any bit line can be programmed at the same time.
  • bit lines are divided into even bit lines and odd bit lines.
  • odd/even bit line architecture memory cells along a common word line and connected to the odd bit lines are programmed at one time, while memory cells along a common word line and connected to even bit lines are programmed at another time.
  • Figure 7A shows four memory cells connected in series to form a NAND string. Although four cells are shown to be included in each NAND string, more or less than four can be used (e.g., 16, 32, or another number).
  • One terminal of the NAND string is connected to a corresponding bit line via a drain select gate (connected to select gate drain line SGD), and another -21-
  • terminal is connected to c-source via a source select gate (connected to select gate source line SGS).
  • FIG. 8 is a flow chart describing one embodiment of a method for programming non-volatile memory.
  • memory cells are erased (in blocks or other units) prior to programming.
  • a "data load" command is issued by the controller and input received by control circuitry 310.
  • address data designating the page address is input to decoder 314 from the controller or host.
  • a page of program data for the addressed page is input to a data buffer for programming. That data is latched in the appropriate set of latches.
  • a "program" command is issued by the controller to state machine 312.
  • step 404 the data latched in step 404 will be programmed into the selected memory cells controlled by state machine 312 using the stepped pulses of Figure 9 applied to the appropriate word line.
  • the program voltage Vpgm is initialized to the starting pulse (e.g., 12V or other value) and a program counter PC maintained by state machine 312 is initialized at 0.
  • step 410 the first Vpgm pulse is applied to the selected word line. If logic "0" is stored in a particular data latch indicating that the corresponding memory cell should be programmed, then the corresponding bit line is grounded. On the other hand, if logic "1" is stored in the particular latch indicating that the corresponding memory cell should remain in its current data state, then the corresponding bit line is connected to Vdd to inhibit programming.
  • step 412 the states of the selected memory cells are verified using different voltages on the word lines, which means different voltage to the control gates of the memory cells of a NAND string. More details of the verify process are provided below. If it is detected that the target threshold voltage of a selected cell has reached the appropriate level, then the data stored in the -22-
  • corresponding data latch is changed to a logic "1.” If it is detected that the threshold voltage has not reached the appropriate level, the data stored in the corresponding data latch is not changed. In this manner, a bit line having a logic "1" stored in its corresponding data latch does not need to be programmed.
  • the state machine via the wired-OR type mechanism described above, knows that all selected cells have been programmed.
  • step 414 it is checked whether all of the data latches are storing logic "1.” If so, the programming process is complete and successful because all selected memory cells were programmed and verified. A status of "PASS" is reported in step 416.
  • the verification of step 412 includes providing a different one or more voltages to memory cells adjacent to the memory cells being programmed than that which is provided to the other unselected memory cells. For example, if memory cells on word line WLn are being programmed, then the voltage applied to memory cells on word lines WLn+ 1 will be different than the voltage applied to other unselected word lines. This compensation will be discussed in more detail below with respect to Figure 10.
  • step 418 the program counter PC is checked against a program limit value PCMAX.
  • a program limit value is 20; however, other numbers can also be used. If the program counter PC is not less than 20, then the program process has failed and a status of "FAIL" is reported in step 420. If the program counter PC is less than 20, then the Vpgm level is increased by the step size and the program counter PC is incremented in step 422. After step 422, the process loops back to step 410 to apply the next Vpgm pulse.
  • Figure 9 shows a series of program pulses that are applied to the word line selected for programming. In between program pulses are a set of verify pulses (not depicted). In some embodiments, there can be a verify pulse for -23-
  • each state that data is being programmed into there can be more or less verify pulses.
  • data is programmed to memory cells along a common word line.
  • one of the word lines is selected for programming. This word line will be referred to as the selected word line.
  • the remaining word lines of a block are referred to as the unselected word lines.
  • the selected word line may have one or two neighboring word lines. If the selected word line has two neighboring word lines, then the neighboring word line on the drain side is referred to as the drain side neighboring word line and the neighboring word line on the source side is referred to as the source side neighboring word line. For example, if WL2 of Fig. 2 is the selected word line, then WLl is the source side neighboring word line and WL3 is the drain side neighboring word line.
  • Each block of memory cells includes a set of bit lines forming columns and a set of word lines forming rows.
  • the bit lines are divided into odd bit lines and even bit lines.
  • Memory cells along a common word line and connected to the odd bit lines are programmed at one time, while memory cells along a common word line and connected to even bit lines are programmed at another time ("odd/even programming").
  • memory cells are programmed along a word line for all bit lines in the block (“all bit line programming").
  • the bit lines or block can be broken up into other groupings (e.g., left and right, more than two groupings, etc.).
  • Figure 10 is a timing diagram depicting the behavior of various signals during one iteration of a verify process. For example, if the memory cells are binary memory cells, the process of Fig. 10 may be performed during an iteration of step 412. If the memory cells are multi-state memory cells with -24-
  • Fig. 10 may be performed three times during an iteration of step 412.
  • the selected word line is connected to a voltage, a level of which is specified for each read and verify operation in order to determine whether a threshold voltage of the concerned memory cell has reached such level.
  • the conduction current of the memory cell is measured to determine whether the memory cell turned on in response to the voltage applied to the word line. If the conduction current is measured to be greater than a certain value, then it is assumed that the memory cell turned on and the voltage applied to the word line is greater than the threshold voltage of the memory cell. If the conduction current is not measured to be greater than the certain value, then it is assumed that the memory cell did not turn on and the voltage applied to the word line is not greater than the threshold voltage of the memory cell.
  • the conduction current of a memory cell is measured by the rate it discharges or charges a dedicated capacitor in the sense amplifier.
  • a memory array that uses all bit line programming can measure the conduction current of a memory cell by the rate it discharges or charges a dedicated capacitor in the sense amplifier.
  • the conduction current of the selected memory cell allows (or fails to allow) the NAND string that included the memory cell to discharge the bit line. The charge on the bit line is measured after a period of time to see whether it has been discharged or not.
  • a memory array that uses odd/even programming can measure the conduction current of a memory cell by determining whether the bit line has discharged. Figure 10 explains both examples. -25-
  • Figure 10 shows signals SGD, WL_unsel_S, WL_unsel_D, WLn+1, WLn, SGS, Selected BL, BLCLAMP, and Source starting at Vss (approximately O volts).
  • SGD represents the gate of the drain side select gate.
  • SGS is the gate of the source side select gate.
  • WLn is the word line selected for reading/verification.
  • WLn+1 is the unselected word line that is the drain side neighboring word line to WLn.
  • WL_unsel_S represents the unselected word lines on the source side of the selected word line.
  • WL_unsel_D represents the unselected word lines on the drain side of the selected word line, other than the drain side neighbor.
  • Selected BL is the bit line selected for reading/verification.
  • Source is the source line for the memory cells.
  • BLCLAMP is an analog signal that sets the value of the bit line when charged from the sense amplifier. Note that there are two versions of SGS, Selected BL and BLCLAMP depicted.
  • One set of these signals SGS (B), Selected BL (B) and BLCLAMP (B) depict a read/verify operation for an array of memory cells that measure the conduction current of a memory cell by determining whether the bit line has discharged.
  • Another set of these signals SGS (C), Selected BL (C) and BLCLAMP (C) depict a read/verify operation for an array of memory cells that measure the conduction current of a memory cell by the rate it discharges or charges a dedicated capacitor in the sense amplifier.
  • word lines WL_unsel_D are raised to Vrd2
  • the drain side neighboring word line WLn+1 is raised to VrdX
  • the selected word line WLn is raised to a verify level Vcgv (e.g., Vva, Vvb, or Vvc of Fig. 11) for a verify operation
  • BLCLAMP (B) is raised to a pre-charging voltage to pre-charge the selected bit line Selected BL(B) (e.g., to approximately 0.7 volts).
  • the voltages Vrdl, Vrd2 and VrdX act as pass voltages because they cause the unselected memory cells to turn on and act as pass gates.
  • Vrd2 is approximately 2.5- 4 volts lower than Vrdl; however, in other embodiments other values for Vrd2 can be used that are even lower than Vrdl or higher than Vrdl.
  • VrdX is equal to or slightly lower than Vrdl. In some embodiments, VrdX is greater than Vrd2 and lower (e.g., 2.4 - 3 volts lower) than Vrdl to accommodate the data dependent read processes described below. Applying Vrd2 lower than Vrdl compensates for the back pattern effect described above. At time t2, BLCLAMP (B) is lowered to Vss so the NAND string can control the bit line.
  • the source side select gate is turned on by raising SGS (B) to Vdd. This provides a path to dissipate the charge on the bit line. If the threshold voltage of the memory cell selected for reading is greater than Vcgr or the verify level applied to the selected word line WLn, then the selected memory cell will not turn on and the bit line will not discharge, as depicted by signal line 450. If the threshold voltage in the memory cell selected for reading is below Vcgr or below the verify level applied to the selected word line WLn, then the memory cell selected for reading will turn on (conduct) and the bit line voltage will dissipate, as depicted by curve 452.
  • the sense amplifier will determine whether the bit line has dissipated a sufficient amount. In between t2 and t3, BLCLAMP (B) is raised to let the sense amplifier measure the evaluated BL voltage and then lowered, as depicted in Fig. 10. At time t3, the depicted signals will be lowered to Vss (or another value for standby or recovery). Note that in other embodiments, the timing of some of the signals can be changed (e.g. shift the signal applied to the neighbor).
  • Vdd e.g., approximately 3.5 volts
  • unselected word lines WL_unsel_S are raised to Vrdl
  • unselected word lines WL_unsel_D are raised to Vrd2
  • the drain side neighboring word line WLn+ 1 is raised to VrdX
  • the selected word line WLn is raised to Vcgv (e.g., Vva, Vvb, or Vvc of Fig. 11) for a verify operation
  • BLCLAMP (C) is raised.
  • the sense amplifier holds the bit line voltage constant regardless of what the NAND sting is doing, so the sense amplifier measures the current flowing with the bit line "clamped" to that voltage.
  • BLCLAMP (C) rises at tl and does not change from tl to t3.
  • the sense amplifier will determine whether the capacitor in the sense amplifier has dissipated or charged a sufficient amount.
  • the depicted signals will be lowered to Vss (or another value for standby or recovery). Note that in other embodiments, the timing of some of the signals can be changed.
  • Vcgr e.g., Vra, Vrb, or Vrc of Fig. 11
  • Figure 1OA depicts a NAND string and a set of voltages applied to the NAND string during the verify operation depicted in Fig. 10.
  • the NAND string of Fig. 1OA includes eight memory cells 464, 466, 468, 470, 472, 474, -28-
  • Each of those eight memory cells includes a floating gate (FG) and a control gate (CG). Between each of the floating gates are source/drain regions 490.
  • FG floating gate
  • CG control gate
  • source/drain regions 490 there is a P-type substrate (e.g., Silicon), an N-well within the substrate and a P-well within the N-well (all of which are not depicted to make the drawings more readable).
  • the P- well may contain a channel implantation that is usually a P-type implantation that determines or helps to determine the threshold voltage and other characteristics of the memory cells.
  • the source/drain regions 490 are N+ diffusion regions that are formed in the P-well.
  • a drain side select gate 384 At one end of the NAND string is a drain side select gate 384.
  • the drain select gate 324 connects the NAND string to the corresponding bit line via bit line contact 494.
  • Source select gate 482 connects the NAND string to common source line 492.
  • the selected memory cell 470 receives the verify compare voltage Vcgv.
  • the unselected memory cells 464, 466 and 468 on the source side of selected memory cell 470 receive Vrdl at their control gates.
  • Memory cells 464, 466 and 468 have already been subjected to one or more programming processes that potentially caused programming of one or more pages of data stored in those memory cells since the last time that the NAND string of Fig. 1OA was erased.
  • the unselected memory cells 474, 476 and 478 on the drain side of selected memory cell 470 receive Vrd2 at their control gates.
  • unselected memory cells 474, 476 and 478 have not yet been subjected to one or more programming processes that potentially caused programming of one or more pages of data stored in those memory cells since the last time that the NAND string of Fig. 1OA was erased and, therefore, they are still in the erased state.
  • receiving Vrdl or Vrd2 is not the act of being subjected to a programming process.
  • Drain side neighbor memory cell 472 receives VrdX.
  • Memory cells 472, 474, 476 and 478 have not been subjected to a programming processes that potentially -29-
  • the unselected memory cells 464, 466 and 468 on the source side of selected memory cell 470 may be in states E, A, B, or C (see Figs. 11-13).
  • memory cells 472, 474, 476 and 478 on the drain side of selected memory cell 470 will be in the erased state E (see Figs. 11-13).
  • Memory cells 464, 466 and 468 are referred to be as being on the source side of selected memory cell 470 because they are on the same NAND string as selected memory cell 470 and on the same side of selected memory cell 470 as source side select gate 482.
  • Fig. 1OA shows three memory cells on the source side, one or more memory cells can be on the source side.
  • Memory cells 472, 474, 476 and 478 are referred to be as being on the drain side of selected memory cell 470 because they are on the same NAND string as selected memory cell 470 and on the same side of selected memory cell 470 as drain side select gate 484.
  • Fig. 1OA shows four memory cells on the drain side, one or more memory cells can be on the drain side; or two or more memory cells can be on the drain side.
  • Figure 1OB depicts the NAND string and a set of voltages applied to the NAND string during a read operation.
  • the selected memory cell 470 receives the read compare voltage Vcgr.
  • Vread Vrdl.
  • Drain side neighbor memory cell 472 receives VreadX on its control gate.
  • Figure 1OC is a flow chart describing one embodiment of a process for programming and reading.
  • all of the word lines for a block are programmed. Subsequent to that programming, all or a subset of the data may be read one or more times. In some embodiments, the word lines are -30-
  • step 500 memory cells connected to a first word line (e.g., WLO - See Fig. 7A) are programmed.
  • step 502 memory cells connected to a second word line (e.g., WLl) are programmed.
  • step 504 memory cells connected to a third word line are programmed. And so on, until memory cells connected to the last word line (e.g., the word line next to the drain side select gate) are programmed in step 506.
  • other orders of programming can also be used, including orders of programming that do not proceed from the source side select gate toward the drain side select gate.
  • any one or more memory cells of the block associated with any of the word lines can be read.
  • any one or more memory cells of the block associated with any of the word lines can be read.
  • a digital camera that stores a set of pictures. It is likely that the pictures will be stored across multiple blocks, thereby, programming all of the word lines prior to any read operations. Note that other order of operation different that as depicted in Fig. 1OC can be implemented.
  • Each word line may be subjected to one or more programming process.
  • a word line may be associated with multiple pages of data.
  • Each programming process may be for a separate page of data. That is, the process of Fig. 8 may be performed separately for each page of data.
  • each of steps 500-506 may include multiple programming processes.
  • all pages of data associated with a word line may be programmed together or a word line may only be associated with one page of data.
  • the voltage VrdX is dependent on the number of unselected word lines that have not yet been subjected to a program process. In one embodiment that programs from the source side to the drain side, VrdX is determined by the number of unselected word libes on the drain side of the selected word line. When considering a NAND string or other suitable grouping, the number of unselected memory cells on the drain side of the -31-
  • VrdX Vrdl-l(X)
  • X is equal to a small voltage in the range of .1 to .4volts that is chosen based on simulation and device characterization.
  • VrdX Vrdl-2(X)
  • VrdX Vrdl-3(X); and, so on.
  • Other formulas or mechanisms can also be used.
  • Figure 1OD depicts a flow chart describing one embodiment of a method for selecting the appropriate level for VrdX according to one embodiment where the voltage VrdX is dependent on the number of unselected word lines on the drain side of the selected word line.
  • the steps of Fig. 1OD are performed after step 402 and before step 404 of Figure 8.
  • step 402 A the block and word line associated with the page address is identified.
  • step 402B the word lines in the block identified in step 402A that have not yet been subjected to programming are determined.
  • step 402C the voltage VrdX for the drain side neighbor of the selected word line is selected based on the number of unselected word lines on the drain side of the selected word line.
  • the threshold voltages of the memory cells should be within one or more distributions of threshold voltages for programmed memory cells or within a distribution of threshold voltages for erased memory cells, as appropriate.
  • Figure 11 illustrates example threshold voltage distributions for the memory cell array when each memory cell stores two bits of data.
  • Figure 11 shows a first threshold voltage distribution E for erased memory cells.
  • Three threshold voltage distributions, A, B and C for programmed memory cells, are also depicted.
  • the threshold voltages in the E distribution are negative and the threshold voltages in the A, B and C distributions are positive.
  • Each distinct threshold voltage range of Figure 11 corresponds to predetermined values for the set of data bits.
  • the specific relationship between the data programmed into the memory cell and the threshold voltage levels of the cell depends upon the data encoding scheme adopted for the cells.
  • data values are assigned to the threshold voltage ranges using a Gray code assignment so that if the threshold voltage of a floating gate erroneously shifts to its neighboring physical state, only one bit will be affected.
  • One example assigns "11" to threshold voltage range E (state E), “10” to threshold voltage range A (state A), “00” to threshold voltage range B (state B) and “01” to threshold voltage range C (state C).
  • Gray code is not used.
  • Figure 11 shows four states, the present invention can also be used with other multi-state structures including those that include more or less than four states.
  • Figure 11 also shows three read reference voltages, Vra, Vrb and
  • Vrc for reading data from memory cells.
  • the system can determine what state the memory cell is in.
  • Figure 11 also shows three verify reference voltages, Vva, Vvb and Vvc.
  • Vva verify reference voltage
  • Vvb threshold voltage
  • Vvc verify reference voltage
  • memory cells can be programmed from the erase state E directly to any of the programmed states A, B or C.
  • a population of memory cells to be programmed may first be erased so that all memory cells in the population are in erased state E.
  • the process depicted in Figure 18, using the control gate voltage sequence depicted in Figure 9, will then be used to program memory cells directly into states A, B or C. While some memory cells are being programmed from state E to state A, other memory cells are being programmed from state E to state B and/or from state E to state C.
  • the amount of parasitic coupling to the adjacent floating gate under WLn-I is a maximized since the change in amount of charge on the floating gate under WLn is largest as compared to the change in voltage when programming from state E to state A or state E to state B.
  • the amount of coupling to the adjacent floating gate is reduced but still significant.
  • the amount of coupling is reduced even further. Consequently the amount of correction required to subsequently read each state of WLn-I will vary depending on the state of the adjacent cell on WLn.
  • Figure 12 illustrates an example of a two-pass technique of programming a multi-state memory cell that stores data for two different pages: a lower page and an upper page.
  • states are depicted: state E (11), state A (10), state B (00) and state C (01).
  • state E both pages store a "1.”
  • state A the lower page stores a "0" and the upper page stores a "1.”
  • state B both pages store "0.”
  • state C the lower page stores "1" and the upper page stores "0.” Note that although specific bit patterns have been assigned to each of the states, different bit patterns may also be assigned.
  • the cell's threshold voltage level is set according to the bit to be programmed into the lower logical page. If that bit is a logic "1," the threshold voltage is not changed since it is in the -34-
  • the cell's threshold voltage level is set according to the bit being programmed into the upper logical page. If the upper logical page bit is to store a logic "1,” then no programming occurs since the cell is in one of the states E or A, depending upon the programming of the lower page bit, both of which carry an upper page bit of "1.” If the upper page bit is to be a logic "0,” then the threshold voltage is shifted. If the first pass resulted in the cell remaining in the erased state E, then in the second phase the cell is programmed so that the threshold voltage is increased to be within state C, as depicted by arrow 534.
  • the memory cell is further programmed in the second pass so that the threshold voltage is increased to be within state B, as depicted by arrow 532.
  • the result of the second pass is to program the cell into the state designated to store a logic "0" for the upper page without changing the data for the lower page.
  • the amount of coupling to the floating gate on the adjacent word line depends on the final state.
  • a system can be set up to perform full sequence writing if enough data is written to fill up an entire page. If not enough data is written for a full page, then the programming process can program the lower page programming with the data received. When subsequent data is received, the system will then program the upper page. In yet another embodiment, the system can start writing in the mode that programs the lower page and convert to full sequence programming mode if enough data is subsequently received to fill up an entire (or most of a) word line's memory cells. More details of such an embodiment are disclosed in U.S. Patent Application titled "Pipelined Programming of Non-Volatile Memories -35-
  • Figures 13 A-C disclose another process for programming nonvolatile memory that reduces the effect of floating gate to floating gate coupling by, for any particular memory cell, writing to that particular memory cell with respect to a particular page subsequent to writing to adjacent memory cells for previous pages.
  • the non-volatile memory cells store two bits of data per memory cell, using four data states. For example, assume that state E is the erased state and states A, B and C are the programmed states. State E stores data 11. State A stores data 01. State B stores data 10. State C stores data 00. This is an example of non-Gray coding because both bits change between adjacent states A & B. Other encodings of data to physical data states can also be used.
  • Each memory cell stores two pages of data. For reference purposes these pages of data will be called upper page and lower page; however, they can be given other labels.
  • the upper page stores bit 0 and the lower page stores bit 1.
  • the upper page stores bit 1 and the lower page stores bit 0.
  • both pages store bit data 0.
  • the programming process of Figures 13A-C is a two-step process.
  • the first step the lower page is programmed. If the lower page is to remain data 1, then the memory cell state remains at state E. If the data is to be programmed to 0, then the threshold of voltage of the memory cell is raised such that the memory cell is programmed to state B'.
  • Figure 13A therefore shows the programming of memory cells from state E to state B'. State B' depicted in Figure 13A is an interim state B; therefore, the verify point is depicted as Vvb', which is lower than Vvb. -36-
  • a memory cell after a memory cell is programmed from state E to state B', its neighbor memory cell (WLn+ 1) in the NAND string will then be programmed with respect to its lower page. For example, looking back at Figure 2, after the lower page for memory cell 106 is programmed, the lower page for memory cell 104 would be programmed. After programming memory cell 104, the floating gate to floating gate coupling effect will raise the apparent threshold voltage of memory cell 106 if memory cell 104 had a threshold voltage raised from state E to state B'. This will have the effect of widening the threshold voltage distribution for state B' to that depicted as threshold voltage distribution 550 of Figure 13B. This apparent widening of the threshold voltage distribution will be remedied when programming the upper page.
  • Figure 13C depicts the process of programming the upper page.
  • the memory cell will remain in state E. If the memory cell is in state E and its upper page data is to be programmed to 0, then the threshold voltage of the memory cell will be raised so that the memory cell is in state A. If the memory cell was in intermediate threshold voltage distribution 550 and the upper page data is to remain at 1, then the memory cell will be programmed to final state B. If the memory cell is in intermediate threshold voltage distribution 550 and the upper page data is to become data 0, then the threshold voltage of the memory cell will be raised so that the memory cell is in state C.
  • FIG. 13A-C reduces the effect of floating gate to floating gate coupling because only the upper page programming of neighbor memory cells will have an effect on the apparent threshold voltage of a given memory cell.
  • An example of an alternate state coding is to move from distribution 550 to state C when the upper page data is a 1 , and to move to state B when the upper page data is a 0. -37-
  • Figures 13A-C provide an example with respect to four data states and two pages of data, the concepts taught by Figures 13A-C can be applied to other implementations with more or less than four states and different than two pages.
  • Figures 14A-F depict various tables that describe the order of programming according to various embodiments for the methods described by Figures 11, 12 and 13A-C.
  • Figure 14A is a table which describes the order for programming memory cells along a bit line for all bit line programming.
  • the block with four word lines includes four pages (page 0-3).
  • Page 0 is written first, followed by page 1, followed by page 2 and then followed by page 3.
  • the data in page 0 includes the data stored by all the memory cells connected to word line WLO.
  • the data in page 1 includes the data stored by the memory cells connected to word line WLl.
  • the data in page 2 includes the data stored by memory cells connected to WL2.
  • the data in page 3 includes the data stored by memory cells connected to word line WL3.
  • the embodiment of Figure 14A assumes full sequence programming, as described above with respect to Figure 11.
  • Figure 14B depicts the order of programming during odd/even programming when using the full sequence programming method described above with respect to Figure 11.
  • a block with four word lines includes eight pages of data.
  • the memory cells on even bit lines connected to word line WLO store data for page 0.
  • Memory cells on odd bit lines connected to word line WLO store data for page 1.
  • Memory cells on even bit lines connected to word line WLl store data for page 2.
  • Memory cells on odd bit lines connected to word line WLl store data for page 3.
  • Memory cells on even bit lines connected to word line WL2 store data for page 4.
  • Memory cells on odd bit lines connected to word line WL2 store data for page 5.
  • Memory cells on even bit lines connected to word line WL3 store data for page 6.
  • Memory cells on odd bit lines connected to word line WL3 store data for page 7.
  • Data is programmed in numerical order according to page number, from page 0 to page 7.
  • the table of Figure 14C describes the order for programming according to the two phase programming process of Figure 12 for a memory array that performs all bit line programming.
  • a block with four word lines is depicted to include eight pages.
  • the lower page of data forms page 0 and the upper page data forms page 1.
  • the lower page of data forms page 2 and the upper page data forms page 3.
  • the lower page of data forms page 4 and the upper page data forms page 5.
  • the lower page of data forms page 6 and the upper page data forms page 7.
  • Data is programmed in numerical order according to page number, from page 0 to page 7.
  • Figure 14D provides a table describing the order of programming the two-phase programming process of Figure 12 for a memory architecture that performs odd/even programming.
  • a block with four word lines includes 16 pages, where the pages are programmed in numerical order according to page number, from page 0 to page 15.
  • the lower page of data forms page 0 and the upper page data forms page 2.
  • the lower page of data forms page 1 and the upper page of data forms page 3.
  • the lower page forms page 4 and the upper page forms page 6.
  • the lower page forms page 5 and the upper page forms page 7.
  • the lower page forms page 8 and the upper page -39-
  • both lower and upper pages under each word line of the even bit lines are programmed before programming both pages of the odd bit lines for this same word line.
  • Figures 14F and 14G describe the order for programming memory cells utilizing the programming method of Figures 13A-C.
  • Figure 14F pertains to the architecture that performs all bit line programming.
  • the lower page forms page 0 and the upper page forms page 2.
  • the lower page forms page 1 and the upper page forms page 4.
  • the lower page forms page 3 and the upper page forms page 6.
  • the lower page forms page 5 and the upper page forms page 7.
  • Memory cells are programmed in numerical order according to page number, from page 0 to page 7.
  • the table of Figure 14G pertains to the architecture that performs odd/even programming.
  • the lower page forms page 0 and the upper page forms page 4.
  • the lower page forms page 1 and the upper page forms page 5.
  • the lower page forms page 2 and the upper page forms page 8.
  • the lower page forms page 3 and the upper page forms page 9.
  • the lower page forms page 6 and the upper page forms page 12.
  • the lower page forms page 7 and the -40-
  • each of the architectures having both even and odd bit lines can be implemented with all the even bit lines located physically together in, for example, the left side of the chip, and all of the odd bit lines located together in, for example, the right side of the chip.
  • memory cells are programmed along a NAND string from source side to the drain side.
  • the tables depict only an embodiment with four word lines. The various methods depicted within the tables can be applied to systems with more or less than four word lines. Examples of an architecture using odd/even programming can be found in U.S. Patent Nos. 6,522,580 and 6,643,188; both of which are incorporated herein by reference in their entirety. More information about an architecture that uses all bit line programming can be found in the following U.S.
  • architectures that program all bit lines together will read data from all bit lines together.
  • architectures that program odd and even bit lines separately will generally read odd and even bit lines separately.
  • the technology described herein for reading data can be used with all bit line programming or odd/even bit line programming.
  • Figure 15 is a flow chart describing one embodiment for reading data from non-volatile memory cells.
  • Figure 15 provides the read process at the system level.
  • a request to read data is received.
  • a read operation is performed for a particular page in response to the request to read data (step 598).
  • ECCs Error Correction Codes
  • the system will also create extra bits used for Error Correction Codes (ECCs) and write those ECC bits along with the page of data.
  • ECC Error Correction Codes
  • the ECC process used can include any suitable ECC process known in the art.
  • the ECC bits will be used to determine whether there are any errors in the data (step 602).
  • the ECC process can be performed by the controller, the state machine or elsewhere in the system. If there are no errors in the data, the data is reported to the user at step 604. For example, data will be communicated to a controller or host via data I/O lines 320. If an error is found at step 602, it is determined whether the error is correctable (step 606). The error may be due to the floating gate to floating gate coupling effect or other reasons. Various ECC methods have the ability to correct a predetermined number of errors in a set of data. If the ECC process can correct the data, then the ECC process is used to correct that data in step 608 and the data, as corrected, is reported to the user in step 610.
  • a data recovery process is performed in step 620.
  • an ECC process will be performed after step 620. More details about the data recovery process are described below. After the data is recovered, that data is reported at step 622. Note that the process of Figure 15 can be used with data programmed using all bit line programming or odd/even bit line programming.
  • Figure 16 is a flow chart describing one embodiment of a process for performing a read operation for a page (see step 600 of Fig. 15).
  • step 640 read reference voltage Vra is applied to the appropriate word line associated with the page.
  • step 642 the bit lines associated with the page are sensed to determine whether the addressed memory cells turn on or do not turn on based on the application of Vra to their control gates. Bit lines that conduct indicate that the memory cells were turned on; therefore, the threshold voltages of those memory cells are below Vra (e.g., in state E).
  • step 644 the result of the sensing for the bit lines is stored in the appropriate latches for those bit lines.
  • step 646 read reference voltage Vrb is applied to the word lines associated with the page being read.
  • step 648 the bit lines are sensed as described above.
  • step 650 the results are stored in the appropriate latches for the bit lines.
  • step 652 read reference voltage Vrc is applied to the word lines associated with the page.
  • step 654 the bit lines are sensed to determine which memory cells turn on, as described above.
  • step 656 the results from the sensing step are stored in the appropriate latches for the bit lines.
  • step 658 the data values for each bit line are determined. For example, if a memory cell conducts at Vra, then the memory cell is in state E.
  • processor 392 will store the determined data values in the appropriate latches for each bit line. In other embodiments, sensing the various levels (Vra, Vrb, and Vrc) may occur in different orders.
  • Steps 640-644 include performing the operation depicted in
  • Figure 17 includes a flow chart describing one embodiment of a process for recovering data (step 620).
  • Data may include an error due to the floating gate to floating gate coupling effect (or another cause).
  • the process of Figure 17 attempts to read the data while compensating for the floating gate to floating gate coupling effect (or another cause of error).
  • the compensation includes looking at the neighboring word line and determining how the programming of the neighboring word line has created a floating gate to floating gate coupling effect. For example, when reading data on word line WLn (e.g., WL2 of Fig. 7A), the process will also read the data of word line WLn+1 (e.g., WL3 of Fig. 7A). If the data on word line WLn+1 has caused an apparent change in the data on WLn, then the read process will compensate for that unintentional change.
  • word line WLn e.g., WL2 of Fig. 7A
  • the process will also read the data of word line WLn+1 (e.g.,
  • word line varies by array implementation and can be determined by characterizing the device.
  • Step 670 in Figure 17 includes performing a read operation for the neighboring word line WLn+1. This includes performing the process of Figure 16 for the neighboring word line. For example, if a page in word line WLl is being read, then step 670 includes performing the process of Figure 16 on word line WL2. The results of step 670 are stored in the appropriate latches in step 672. In some embodiments, the read operation performed for WLn+1 results in determining the actual data stored on WLn+1. In other embodiments, the read operation performed for WLn+1 results in a determination of charge levels on WLn+1, which may or may not accurately reflect the data stored on WLn+1.
  • An alternative embodiment includes margining of read voltages during the read of step 670 of Fig. 17. This margining of the read of step 670 would be done with the intent of making coupling corrections for the read of step 670. But such an embodiment may be inferior to not making the coupling correction during read of step 670, as explained above.
  • step 674 a read process is performed for the word line of interest WLn.
  • VreadX Vreadl.
  • Vreadl Vread (e.g., 5.5volts, or 6 volts , or another suitable value).
  • Vreadl Vread (e.g., 5.5volts, or 6 volts , or another suitable value).
  • step 674 are stored in the appropriate latches for bit lines with memory cells where neighbor cell WLn+1 was determined (in step 670) to be in state C. Therefore, the maximum compensation, CompC, is engaged for cells whose drain side neighbors had experienced the highest change in threshold voltage by being programmed from state E to state C. Note that these drain side neighbors were in State E during program/verify of WLn, but now are in State C. What has to be compensated for under all circumstances is the change in state of the drain side neighbor on WLn+1 experienced between the time of write of WLn and the present time of read of WLn. For other bit lines whose drain side neighbors are not being detected presently to be in state C, the data of this read of WLn which used Vread 1 on WLn+1 will be disregarded. -46-
  • step 678 a read process is performed for WLn.
  • the results of step 678 will be stored for bit lines with memory cells having neighboring memory cells (e.g., WLn+1) in state B. Data for other bit lines will be disregarded.
  • step 682 a read process is performed for WLn.
  • step 684 the results of step 682 will be stored for bit lines with memory cells having neighboring memory cells (e.g., WLn+1) in state A. Data for other bit lines will be disregarded.
  • step 686 a read process is performed for WLn.
  • Vread4 is identical in value to Vrdx used during programming or closer the Vrdx than Vread3 (e.g.,
  • Vreadl Vread2
  • Vread3 and Vread 4 can be determined based on device characterization, experimentation and/or simulation.
  • the process of Figure 17 is performed as part of the data recovery step 620 of Figure 15.
  • the process of Figure 17 can be used as the initial read process that is performed in response to a request to read data.
  • the system will perform a read operation in step 600.
  • step 600 is implemented by performing the process of Fig. 17.
  • An embodiment that uses the process of Fig. 17 to implement step 600 may not have the additional data recovery step 620, so if an error is not correctable the system would report the error.
  • Figure 18 is a flow chart indicating that the data recovery process
  • the method of Figure 17 can be performed for all the word lines of a block except for the last word line to be programmed. For example, if there are x+1 word lines, the recovery process can be used for word lines WLO through WLx-I. It would not be necessary to perform the recovery process for word line WLx (e.g., the word line closest to the drain) because that word line has no neighbor that was programmed after it that would cause the floating gate to floating gate coupling effect.
  • Figure 18 shows an embodiment with a recovery process performed for all the word lines sequentially, in one embodiment described above with respect to Figure 15, the recovery process can be performed for the word lines at separate times and only if there were ECC errors that were not correctable.
  • the process of Figure 16 would be amended to perform only steps 646, 648, 650, 658 and 660. Additionally, when recovering data (step 620), the process would perform the method of Figure 19 for recovering data for a lower page and the process of Figure 20 to recover data for an upper page.
  • a read operation is performed for the neighboring word line WLn+1 according to the method of Figure 16.
  • the read operation performed for WLn+1 results in determining the actual data stored on WLn+1.
  • the read operation performed for WLn+1 results in a determination of charge levels (or another condition) on WLn+1, which may or may not accurately reflect the data stored on WLn+1.
  • the results of that read operation are stored in the appropriate latches in step 732.
  • the data for the bit lines are sensed.
  • step 738 the results are stored in the appropriate latches.
  • the value of VreadX in step 734 should be the same as used during the verification process.
  • step 742 data is sensed as discussed above.
  • step 744 the results of the sense step 742 will be stored for bit lines associated with a neighboring cell storing data in state C.
  • step 948 the data will be sensed as discussed above.
  • step 950 the results of step 948 will be stored for bit lines associated with neighboring cells storing data in state B. Data for other bit lines will be discarded.
  • step 754 the data will be sensed as discussed above.
  • step 756 the results of step 754 will be stored for bit lines associated with neighboring cells storing data in state A. Data for other bit lines will be discarded.
  • step 760 the data will be sensed as discussed above.
  • step 762 the results of step 760 will be stored for bit lines associated with neighboring cells storing data in state E. Data for other bit lines will be discarded.
  • step 764 processor 392 will determine the data values based on the data stored from the sensing steps.
  • step 766 the determined data values from step 764 will be stored in latches for eventual communication to the user requesting the read of data.
  • steps 734-738 associated with state A could be performed between steps 762 and 764.
  • Other orders for performing the steps of Fig. 19, as well as the steps of other flow charts, can also be used.
  • step 764 When determining the data values in step 764, if a memory cell conducts in response to Vra, the lower page data is "1.” If the memory cell does not conduct in response to Vra and does not conduct in response to Vrc, then the lower page data is also "1.” If the memory cell does not conduct in response to Vra, but does conduct in response to Vrc , then the lower page data is "0.”
  • step 800 a read operation is performed for the neighboring word line WLn+1 using the method of Figure 16.
  • the read operation performed for WLn+1 results in determining the actual data stored on WLn+1.
  • the read operation performed for WLn+1 results in a determination of charge levels on WLn+1, which may or may not accurately reflect the data stored on WLn+1.
  • step 802 the results of step 800 are stored in the appropriate latches for each of the bit lines.
  • step 806 the data will be sensed as discussed above.
  • step 808 the results of step 806 will be stored for bit lines associated with neighboring cells storing data in state C. Data for other bit lines will be discarded.
  • step 812 the data will be sensed as discussed above.
  • step 814 the results of step 812 will be stored for bit lines associated with neighboring cells storing data in state B. Data for other bit lines will be discarded.
  • step 818 the data will be sensed as discussed above.
  • step 820 the results of step 818 will be stored for bit lines associated with neighboring cells storing data in state A. Data for other bit lines will be discarded.
  • step 824 the data will be sensed as discussed above.
  • step 826 the results of step 824 will be stored for bit lines associated with neighboring cells storing data in state E. Data for other bit lines will be discarded.
  • processor 392 determines the data values based on the stored sensed data. If a memory cell turned on in response to Vrb, then the upper page data is "1.” If a memory cell does not turn on in response to Vrb, then the upper page data is "0.” In step 830, the data values determined by processor 392 are stored in the data latches for communication to the user.
  • step 600 is implemented by performing the process of Figs. 19 and/or 20.
  • the system may not have the additional data recovery step 620, so if an error is not correctable the system would report the error.
  • Figures 19 and 20 are for reading data that are programmed using the upper page and lower page process of Figure 12. These two methods of Figures 19 and 20 can be used to read data programmed by all bit line programming or odd/even bit line programming. When used with all bit line programming, all bit lines are typically read simultaneously. When used with odd/even bit line programming, even bit lines are typically read simultaneously at a first time and odd bit lines are typically read simultaneously possibly at a different time.
  • Figures 21-26 describe processes used to read data that is programmed according to the method associated with Figures 13A-C.
  • the process of Figure 21 can be implemented as an overall process for reading data that is performed in response to a read request for a particular one or more pages (or other grouping) of data prior to, separate from and/or in conjunction with using ECCs.
  • the process of Figure 21 can be performed as part of data recovery step 620 of Figure 15.
  • any perturbation from floating gate to floating gate coupling due to programming the lower page of neighboring cells should be corrected when programming the upper page of the memory cell under question.
  • step 1060 of Figure 21 the process reads upper page data for the neighboring word line. If the upper page of the neighboring word line was not programmed (step 1062), then the page under consideration can be read without compensating for the floating gate to floating gate coupling effect (step 1064). If the upper page of the neighboring word line was programmed (step 1062), then the page under -53-
  • the read operation performed for neighboring word line results in a determination of charge levels on the neighboring word line, which may or may not accurately reflect the data stored thereon.
  • the selected word line to be read i.e. WLn
  • the selected word line to be read may itself have only lower page data. This can happen when the entire block has not yet been programmed. In such a situation it is always guaranteed that the cells on WLn+ 1 are still erased, and therefore, no coupling effect has yet plagued WLn cells. This means that no compensation is required. So the lower page read of a word line whose upper page has yet to be programmed can proceed as usual without the need for any compensation technique.
  • a memory array implementing the programming process of Figures 13A-C will reserve a set of memory cells to store one or more flags. For example, one column of memory cells can be used to store flags indicating whether the lower page of the respective rows of memory cells has been programmed and another column of memory cells can be used to store flags indicating whether the upper page for the respective rows of memory cells has been programmed. In some embodiments, redundant cells can be used to store copies of the flag. By checking the appropriate flag, it can be determined whether the upper page for the neighboring word line has been programmed. More details about such a flag and the process for programming can be found in United States Patent No. 6,657,891, Shibata et al, "Semiconductor Memory Device For Storing Multi-Valued Data," incorporated herein by reference in its entirety.
  • Figure 22 describes one embodiment of a process for reading the upper page data for a neighboring word line such as the drain side neighbor (step 1060 of Figure 21).
  • read reference voltage Vrc is applied to the word line associated with the page being read.
  • step 1104 the results of step 1102 are stored in the appropriate latches.
  • step 1106 the system checks the flag indicating upper page programming associated with the page being read.
  • the memory cell storing the flag will store data in state E if the flag is not set and in state C if the flag is set. Therefore, when that particular memory cell is sensed at step 1102, if the memory cell conducts (turns on), then the memory cell is not storing data in state C and the flag is not set. If the memory cell does not conduct, then it is assumed in step 1106 that the memory cell is indicating that the upper page has been programmed.
  • the flag can be stored in a byte. Rather than storing all bits in state C, the byte will include a unique 8-bit code representing the flag and known to the state machine 312, such that the 8-bit code has at least one bit in state E, at least one bit in state A, at least one bit in state B and at least one bit in state C. If the upper page has not been programmed, the byte of memory cells will all be in state E. If the upper page has been programmed, then the byte of memory cells will store the code. In one embodiment, step 1106 is performed by checking whether any of the memory cells of the byte storing the code do not turn on in response to Vrc.
  • step 1106 includes addressing and reading the byte of memory cells storing the flag and sending the data to the state machine, which will verify whether the code stored in the memory cells matches the code expected by the state machine. If so, the state machine concludes that the upper page has been programmed.
  • step 22 terminates with the conclusion that the upper page has not been programmed. If the flag has been set (step 1108), then it is assumed that the upper page has been programmed and at step 1120 voltage Vrb is applied to the word line associated with the page being read. In step 1122, the bit lines are sensed as discussed above. In step 1124, the results of step 1122 are stored in -55-
  • step 1126 voltage Vra is applied to the word line associated with the page being read.
  • step 1128 the bit lines are sensed.
  • step 1130 the results of step 1128 are stored in the appropriate latches.
  • processor 392 determines the data value stored by each of the memory cells being read based on the results of the three sensing steps 1102, 1122 and 1128.
  • step 1134 the data values determined in step 1132 are stored in the appropriate data latches for eventual communication to the user.
  • step 1132 processor 392 determines the values of the upper page and lower page data using well known simple logic techniques dependent on the specific state coding chosen. For example, for the coding described in Figure 13, the lower page data is Vrb* (the complement of the value stored when reading at Vrb), and the upper page data is Vra* OR (Vrb AND Vrc*).
  • Figure 23 is a flow chart describing one embodiment of a process for reading data of the word line under consideration when the system does not need to compensate for floating gate to floating gate coupling from a neighboring word line (see step 1064 of Figure 21).
  • step 1150 it is determined whether the read is for the upper page or lower page associated with the word line under consideration. If the read is for the lower page, then in step 1152 voltage Vrb is applied to the word line associated with the page being read.
  • step 1154 the bit lines are sensed.
  • the results of sensing step 1154 are stored in the appropriate latches.
  • the flag is checked to determine if the page contains upper page data.
  • step 1160 Vra is applied to the word line, the bit lines are re-sensed at step -56-
  • step 1162 determines a data value to be stored.
  • Vrb or Vra
  • step 1170 If it is determined that the page address corresponds to the upper page (step 1150), an upper page read process is performed at step 1170.
  • the upper page read process of step 1170 includes the same method described in Figure 22, which includes reading the flag and all three states since an unwritten upper page may be addressed for reading, or another reason.
  • Figure 24 depicts a flow chart describing one embodiment of a process for reading data while compensating for floating gate to floating gate coupling effect (see step 1066 of Figure 21).
  • the system determines whether to use compensation for the floating gate to floating gate coupling. This is performed separately for each bit line.
  • the appropriate processor 392 will determine which bit lines need to use the compensation based on the data from the neighboring word lines. If a neighboring word line is in state E or B (or has charge apparently indicating state E or B), then the particular word line being read need not compensate for the floating gate to floating gate coupling effect. The assumption is that if it is in state E it hasn't contributed to any coupling because the threshold hasn't moved since the current word line was written. If it is in state B, it got there from B', and the movement from B' to B is small and can be neglected. In -57-
  • this small movement can be compensated for by the application of a proportionately small ⁇ VREAD.
  • the process of step 1200 can be performed concurrently with step 1060.
  • Figure 25 provides a chart explaining steps to perform a determination whether to use an offset for a particular bit line.
  • the first step is to perform a read process using Vra on the word line.
  • the second step is to perform a read using Vrb.
  • a latch stores a 1 if the memory cell is in state E and a 0 if the memory cell is in states A, B, C or.
  • the latch will store a 1 for states E and A, and store a 0 for states B and C.
  • the third step of Figure 25 includes performing an XOR operation on the inverted results from the second step with the results from step 1.
  • a read is performed using Vrc at the word line.
  • a latch stores a 1 for states E, A and B, and stores a 0 for state C.
  • the results of step 4 and step 3 are operated by a logical AND operation. Note that steps 1 , 2 and 4 may be performed as part of Figure 22. Steps 3 and 5 of Figure 25 can be performed by dedicated hardware or by processor 392.
  • the results of step 5 are stored in a latch with 1 being stored if no compensation is needed and 0 being stored if compensation is needed. Thus, a compensation will be required for those cells that are read on WLn that have neighboring memory cells on WLn+ 1 that are in the A or C state.
  • This approach requires only one latch to determine whether to correct WLn or not, in contrast to some previous methods that store the full data from WLn+ 1, requiring two or more latches.
  • step 1202 of Figure 24 it is determined whether the page being read is the upper page or lower page. If the page being read is the lower page, then Vrb is applied to the word line WLn associated with the page being read and Vread4 is applied to the drain side neighbor word line WLn+ 1 during a read process in step 1204. Note that for the state coding described in Figure 13, reading at Vrb is sufficient to determine the lower page -58-
  • step 1208 the results of step 1206 are stored in the appropriate latches associated with the bit lines.
  • Vrb will be applied to the word line WLn for the page being read and Vread3 is applied to the drain side neighbor word line WLn+1 during a read process (e.g., see Fig 10).
  • step 1212 the bit lines are sensed.
  • step 1214 the results of the sensing of step 1212 are used to overwrite the results stored in step 1208 for the bit lines for which it was determined at step 1200 to use compensation. If the particular bit line is determined not to have to use compensation, then the data from step 1212 is not stored.
  • processor 392 will determine whether the data is 1 or 0 for the lower page. If the memory cell turned on in response to Vrb, then the lower page data is 1; otherwise, the lower page data is 0. At step 1218, the lower page data is stored in the appropriate latches for communication to the user.
  • step 1202 If it is determined at step 1202 that the page being read is the upper page, then the upper page correction process is performed at step 1220.
  • Figure 26 provides a flow chart describing the upper page correction process.
  • Vrc is applied to the word line associated with the page being read and Vread4 is applied to the drain side neighbor word line WLn+1 as part of a read process.
  • step 1252 the bit lines are sensed.
  • step 1254 the results of the sensing step are stored in the appropriate latches.
  • step 1256 Vrc is applied to the word line associated with the page being read and Vread3 is applied to the drain side neighbor word line WLn+1 as part of a read process.
  • step 1258 the bit lines are sensed.
  • step 1260 the results of the sensing step 1258 are used to overwrite the results stored in step 1254 for any bit line for which the compensation is required (see step 1200).
  • Vrb is applied to the word line and Vread4 is applied to the drain side neighbor word line WLn+1 during a read process.
  • bit lines are sensed.
  • step 1274 the results of sensing step -59-
  • step 1272 are stored.
  • step 1276 Vrb is applied to the word line associated with the page being read and Vread3 is applied to the drain side neighbor word line WLn+1 during a read process.
  • step 1278 the bit lines are sensed.
  • step 1280 the results of step 1278 are used to overwrite the results stored at step 1274 for those bit lines for which the compensation is required (see step 1200).
  • step 1282 Vra is applied to the word line associated with the page being read and Vread4 is applied to the drain side neighbor word line WLn+1 as part of a read process.
  • step 1284 the bit lines are sensed.
  • step 1286 the results of the sensing step 1284 are stored in the appropriate latches.
  • step 1288 Vra is applied to the word line associated with the page being read and Vread3 is applied to the drain side neighbor word line WLn+1 as part of a read process.
  • step 1290 the bit lines are sensed.
  • step 1292 the results of step 1290 are used to overwrite the results stored in step 1286 for those bit lines for which the compensation is required (see step 1200).
  • step 1294 the processor 392 determines the data values in the same manner as previously described another method known in the art.
  • step 1296 the data values determined by the processor 392 are stored in the appropriate data latches for communication to the user. In other embodiments the order of reading (Vrc, Vrb, Vra) may be changed.
  • the read process will be pipelined such that the state machine will execute a next page sensing while the user is transferring out the previous page of data.
  • the flag fetch process may interrupt the pipelined read process. To avoid such an interruption, one embodiment contemplates reading the flag for a given page when that page is read and using the wired-OR detection process to check the flag (rather than reading the flag and sending it -60-
  • step 1060 of Figure 21 reading the neighboring word line
  • the process first reads data using Vrc as the reference voltage.
  • Vrc the reference voltage
  • the wired-OR line indicates that each state stores data 1
  • the upper page has not been programmed; therefore, no compensation is needed and the system will read without compensating for the floating gate to floating gate coupling (step 1064).
  • the flag is a one-byte code that includes data in each data state
  • at least the flag memory cells would have data in state C if the flag is set.
  • the state machine concludes that the flag has not been set; therefore, the upper page for the neighboring word line has not been programmed and compensation for floating gate coupling is not needed.
  • FIG. 27 graphically explains the concept of floating gate to floating gate coupling.
  • Fig. 27 depicts neighboring floating gates 1302 and 1304, which are on the same NAND string.
  • Floating gates 1302 and 1304 are situated above NAND channel/substrate 1306, which has source/drain regions 1308, 1310 and 1312.
  • Above floating gate 1302 is control gate 1314 that is connected to and part of word line WLn.
  • Above floating gate 1304 is control gate 1316 that is connected to and part of word line WLn+ 1.
  • floating gate 1302 will likely be subject to coupling from multiple other floating gates, for simplicity
  • Fig. 27 only shows the effects from one neighboring memory cell. Specifically, Fig.
  • FIG. 27 shows three components of coupling provided to floating gate 1302 from its neighbor: rl, r2 and Cr.
  • the component rl is the coupling ratio between the neighboring floating gates (1302 and 1304), and is calculated as the capacitance of the -61-
  • the component r2 is the coupling ratio between the floating gate 1302 and the drain side neighbor control gate 1316, and is calculated as the capacitance of floating gate 1302 and control gate 1316 divided by the sum of all capacitive couplings of floating gate 1302 to all the other electrodes surrounding it.
  • the component Cr is the control gate coupling ratio and is calculated as the capacitance between floating gate 1304 and its corresponding control gate 1316 divided by the sum of all capacitive couplings of floating gate 1302 to all the other electrodes surrounding it.
  • the amount of required compensation is the amount of required compensation
  • ⁇ Vread can be calculated as follows:
  • ⁇ VTn+1 is the change in threshold voltage of the drain side neighbor memory cell between the time of program/verify of WLn and the present time.
  • ⁇ VTn+1, and rl are the root causes of the word line to word line parasitic coupling effect that is mitigated by the present method.
  • ⁇ Vread is the compensation that is brought to bear in order to combat this effect.
  • Compensation for coupling described herein can be achieved by utilizing the same parasitic capacitance between neighboring floating gates as well as capacitance between the floating gate and the neighboring control gate. Since the control gate/floating gate stack is typically etched in one step, the compensation tracks the variations in spacing from memory cell to memory cell. Thus, when two neighbors are farther apart, the coupling is smaller and so will the required compensation for this effect be naturally smaller. When -62-
  • etch back depth In some devices, the control gate partially wraps around the floating gate. The amount of overlap is called "etch back." Variations in etch back depth can effect the amount of coupling. With the above-described compensation scheme, the effect of the compensation will similarly vary with etch back depth.

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Abstract

Selon l'invention, des erreurs peuvent se produire lors de la lecture de la tension de seuil d'un élément programmé de mémorisation non volatile en raison d'au moins deux mécanismes : (1) un couplage capacitif entre des grilles flottantes voisines et (2) un changement de conductivité de la zone de canal après programmation (appelé effet de séquence retour). Pour prendre en compte le couplage entre des grilles flottantes voisines, le procédé de lecture d'une cellule mémoire particulière procurera une compensation à une cellule mémoire adjacente afin de réduire l'effet de couplage que présente la cellule mémoire adjacente sur la cellule mémoire particulière. Afin de prendre en compte l'effet de séquence retour, une première tension est utilisée pendant une opération de vérification pour des lignes de mots non sélectionnées qui ont été soumises à une opération de programmation, et une seconde tension est utilisée pour des lignes de mots non sélectionnées qui n'ont pas été soumises à une opération de programmation. La combinaison de ces deux techniques permet une mémorisation et une récupération plus précises des données.
PCT/US2007/069590 2006-06-02 2007-05-23 Compensation de sensibilité de séquences de données en utilisant une tension différente WO2007143399A2 (fr)

Applications Claiming Priority (4)

Application Number Priority Date Filing Date Title
US11/421,884 2006-06-02
US11/421,884 US7310272B1 (en) 2006-06-02 2006-06-02 System for performing data pattern sensitivity compensation using different voltage
US11/421,871 2006-06-02
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