# Patent

## Title
Memory device with negative thresholds

## Abstract

A method for data storage in a memory that includes a plurality of analog memory cells includes storing data in the memory by writing first storage values to the cells. One or more read reference levels are defined for reading the cells, such that at least one of the read reference levels is negative. After storing the data, second storage values are read from the cells using the read reference levels, so as to reconstruct the stored data. In another disclosed method, data is stored in the memory by mapping the data to first storage values selected from a set of the nominal storage values, and writing the first storage values to the cells. The set of nominal storage values is defined such that at least one of the nominal storage values is negative.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a divisional of U.S. patent application Ser. No. 12/019,011, filed Jan. 24, 2008, now U.S. Pat. No. 7,751,240 which claims the benefit of U.S. Provisional Patent Application 60/886,429, filed Jan. 24, 2007, whose disclosure is incorporated herein by reference.


FIELD OF THE INVENTION
The present invention relates generally to memory devices, and particularly to methods and systems for operating analog memory cells using negative thresholds.
BACKGROUND OF THE INVENTION
Several types of memory devices, such as Flash memories, use arrays of analog memory cells for storing data. Each analog memory cell stores a quantity of an analog value, also referred to as a storage value, such as an electrical charge. The storage value represents the information stored in the cell. In Flash memories, for example, each analog memory cell holds a certain amount of electrical charge. The range of possible analog values is typically divided into regions, each region corresponding to one or more data bit values. Data is written to an analog memory cell by writing a nominal analog value that corresponds to the desired bit or bits.
Some memory devices, commonly referred to as Single-Level Cell (SLC) devices, store a single bit of information in each memory cell, i.e., each memory cell can be programmed to assume two possible memory states. Higher-density devices, often referred to as Multi-Level Cell (MLC) devices, store two or more bits per memory cell, i.e., can be programmed to assume more than two possible memory states.
Flash memory devices are described, for example, by Bez et al., in “Introduction to Flash Memory,” Proceedings of the IEEE, volume 91, number 4, April, 2003, pages 489-502, which is incorporated herein by reference. Multi-level Flash cells and devices are described, for example, by Eitan et al., in “Multilevel Flash Cells and their Trade-Offs,” Proceedings of the 1996 IEEE International Electron Devices Meeting (IEDM), New York, N.Y., pages 169-172, which is incorporated herein by reference. The paper compares several kinds of multilevel Flash cells, such as common ground, DINOR, AND, NOR and NAND cells. An exemplary NAND Flash architecture is described by Takeuchi et al., in “A Double Level VTH Select Gate Array Architecture for Multi-Level NAND Flash Memories,” Digest of Technical Papers, 1995 Symposium on VLSI Circuits, Jun. 8-10, 1995, pages 69-70, which is incorporated herein by reference.
In many memory devices, data is stored in programming levels that are represented by positive threshold voltages (the minimal voltage that needs to be applied to the gate of the cell in order to cause the cell to conduct), and erased cells are set to a level that is represented by a negative threshold voltage. Several methods for sensing negative threshold voltages are known in the art, such as for verifying that cells have been properly erased.
For example, U.S. Pat. No. 7,031,210, whose disclosure is incorporated herein by reference, describes a method of measuring threshold voltages in a NAND Flash memory device. A test voltage is applied to a word line of selected memory cells to measure a distribution profile of threshold voltages of memory cells. A voltage summing up a pass voltage and an operation voltage is applied to word lines of deselected cells. The operation voltage is applied to a well and a common source line. A voltage summing up a pre-charge voltage and the operation voltage is applied to a bit line. Then, a voltage variation on the bit line can be detected to measure a threshold voltage of a memory cell. A negative threshold voltage can be measured by applying a positive voltage with reference to a voltage, as the threshold voltage of the memory cell, set by subtracting the operation voltage from the test voltage in accordance with the bit line voltage variation.
U.S. Pat. No. 6,288,944, whose disclosure is incorporated herein by reference, describes a NAND-type nonvolatile memory. The memory includes a sense circuit having a constant current supply source connected to a bit line to which memory cells are connected, and a sense transistor for sensing potential at the connection point thereof. The memory further includes a first reference potential on the opposite side from the bit line of the memory cells, and a second reference potential to which the source of the sense transistor is connected. During erase verification operations, the first reference potential and the second reference potential are controlled to a predetermined positive potential. By controlling the first reference potential to the positive potential, the control gate level of a memory cell can be equivalently brought to an erase-verify level (which is negative). By further controlling the second reference potential of the sense transistor to a positive potential, the equivalent threshold voltage of the sense transistor can be increased, or the equivalent trip level of the sense inverter increased.
U.S. Pat. No. 5,696,717, whose disclosure is incorporated herein by reference, describes a memory device that includes an array of NAND strings containing a plurality of memory cells and sense amplifiers for determining whether the memory cells have been properly erased and programmed during respective erase and program verification modes of operation. The memory includes an erase voltage adjusting circuit for setting a limit on a range of acceptable erase threshold voltages and a program voltage adjusting circuit for setting a limit on a range of acceptable program threshold voltages. The sense amplifier determines, during an erase verification operation, whether a memory cell in a NAND string has been erased to have a threshold voltage within a range of acceptable erase threshold voltages. The range of acceptable erase threshold voltages typically has an upper limit less than a predetermined reference potential (e.g., ground).
SUMMARY OF THE INVENTION
Embodiments of the present invention provide a method for data storage in a memory that includes a plurality of analog memory cells, the method including:
storing data in the memory by writing first storage values to the cells;
defining one or more read reference levels for reading the cells, such that at least one of the read reference levels is negative; and
after storing the data, reading second storage values from the cells using the read reference levels, so as to reconstruct the stored data.
In some embodiments, reading the second storage values includes applying respective gate voltages and source voltages to the cells and measuring source-drain currents flowing though the cells responsively to the applied source and gate voltages, and reading the second storage values using the at least one negative read reference level includes setting the gate voltages to be lower than the respective source voltages. In an embodiment, the cells are arranged in strings, and reading a second storage value from a given cell in a given string using the at least one negative read reference level includes:
applying a gate voltage to a gate of the given cell to be read, applying a source voltage that is greater than the gate voltage to a source of the string, and applying a pass voltage that is greater than the source voltage to gates of the cells other than the given cell in the given string;
pre-charging the given string to a pre-charge voltage that is greater than the source voltage; and
determining the second storage value responsively to a variation in the pre-charge voltage after a predetermined time interval.
In another embodiment, writing the first storage values includes selecting the first storage values from a set of nominal storage values, at least one of which is negative. In yet another embodiment, writing the first storage values includes applying respective gate voltages and source voltages to the cells and varying the gate voltages from first gate voltages to second gate voltages so as to add electrical charges to the cells, and writing the at least one negative nominal storage value to a given cell includes setting the first and second gate voltages applied to the given cell to be lower than the source voltage applied to the given cell. In a disclosed embodiment, storing the data includes verifying the first storage values written to the cells using one or more verification reference levels, at least one of which is negative.
In some embodiments, reading the second storage values includes raising at least one voltage applied to the cells by a known amount, and determining the second storage values responsively to the known amount and to the read reference levels. In an embodiment, writing the first storage values includes selecting the first storage values from a set of positive nominal storage values, and defining the read reference levels includes defining the at least one of the read reference levels to be negative when some of the first storage values become negative.
In some embodiments, the cells include NAND Flash cells. In a disclosed embodiment, defining the read reference levels includes evaluating a condition and including the at least one negative read reference level in the one or more read reference levels responsively to the evaluated condition.
There is also provided, in accordance with an embodiment of the present invention, a method for data storage in a memory that includes a plurality of analog memory cells, the method including:
defining a set of nominal storage values, at least one of which is negative;
storing data in the memory by mapping the data to first storage values selected from the set of the nominal storage values, and writing the first storage values to the cells; and
after storing the data, reading second storage values from the cells using one or more read reference levels, so as to reconstruct the stored data.
In some embodiments, writing the first storage values includes applying respective gate voltages and source voltages to the cells and varying the gate voltages from first gate voltages to second gate voltages so as to add electrical charges to the cells, and writing the at least one negative nominal storage value to a given cell includes setting the first and second gate voltages applied to the given cell to be lower than the source voltage applied to the given cell. In an embodiment, storing the data includes writing the negative at least one of the nominal storage values to some of the cells. In another embodiment, at least two of the nominal storage values are negative. In yet another embodiment, at least one of the read reference levels is negative.
In some embodiments, storing the data includes verifying the first storage values written to the cells using one or more verification reference levels, at least one of which is negative. In an embodiment, verifying the first storage values includes applying respective gate voltages and source voltages to the cells and measuring source-drain currents flowing though the cells responsively to the applied source and gate voltages, and verifying the first storage value using the at least one negative verification reference level includes setting the gate voltages to be lower than the respective source voltages.
Verifying the first storage values may include applying multiple Programming and Verification (P&V) iterations to the cells. Applying the multiple P&V iterations may include modifying parameters of a subsequent P&V iteration based on the source-drain currents measured in a previous P&V iteration. Modifying the parameters may include selecting a subset of the cells based on the source-drain currents measured in the previous P&V iteration, and applying the subsequent P&V iteration only to the selected subset.
in some embodiments, the cells include NAND Flash cells. In an embodiment, defining the nominal storage values includes evaluating a condition and including the at least one negative nominal storage value in the set of the nominal storage values responsively to the evaluated condition.
There is additionally provided, in accordance with an embodiment of the present invention, apparatus for data storage in a memory that includes a plurality of analog memory cells, the apparatus including:
programming circuitry, which is coupled to store data in the memory by writing first storage values to the cells; and
reading circuitry, which is coupled to define one or more read reference levels for reading the cells, such that at least one of the read reference levels is negative, and, after the data was stored, to read second storage values from the cells using the read reference levels, so as to reconstruct the stored data.
There is further provided, in accordance with an embodiment of the present invention, apparatus for data storage in a memory that includes a plurality of analog memory cells, the apparatus including:
programming circuitry, which is coupled to define a set of nominal storage values, at least one of which is negative, and to store data in the memory by mapping the data to first storage values selected from the set of the nominal storage values and writing the first storage values to the cells; and
reading circuitry, which is coupled, after the data was stored, to read second storage values from the cells using one or more read reference levels, so as to reconstruct the stored data.
There is also provided, in accordance with an embodiment of the present invention, apparatus for data storage, including:
a memory, which includes a plurality of analog memory cells;
programming circuitry, which is coupled to store data in the memory by writing first storage values to the cells; and
reading circuitry, which is coupled to define one or more read reference levels for reading the cells, such that at least one of the read reference levels is negative, and, after the data was stored, to read second storage values from the cells using the read reference levels, so as to reconstruct the stored data.
There is furthermore provided, in accordance with an embodiment of the present invention, apparatus for data storage, including:
a memory, which includes a plurality of analog memory cells;
programming circuitry, which is coupled to define a set of nominal storage values, at least one of which is negative, and to store data in the memory by mapping the data to first storage values selected from the set of the nominal storage values and writing the first storage values to the cells; and
reading circuitry, which is coupled, after the data was stored, to read second storage values from the cells using one or more read reference levels, so as to reconstruct the stored data.
The present invention will be more fully understood from the following detailed description of the embodiments thereof, taken together with the drawings in which:



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram that schematically illustrates a memory cell array, in accordance with an embodiment of the present invention;
FIGS. 2 and 3 are graphs showing threshold voltage distributions in a group of analog memory cells, in accordance with embodiments of the present invention;
FIG. 4 is a block diagram that schematically illustrates a memory system, in accordance with an embodiment of the present invention;
FIG. 5 is a flow chart that schematically illustrates a method for programming analog memory cells, in accordance with an embodiment of the present invention; and
FIG. 6 is a flow chart that schematically illustrates a method for reading analog memory cells, in accordance with an embodiment of the present invention.



DETAILED DESCRIPTION OF EMBODIMENTS
Overview
Some known data storage schemes use negative threshold voltages to mark erased cells. In such schemes, a block of memory cells is typically erased by setting the threshold voltages of its cells to a certain negative level. The cells may later be programmed by setting their threshold voltages to different positive levels that represent the data. Known schemes typically retrieve data from programmed cells by comparing their threshold voltages to read reference levels, whose values are non-negative.
Embodiments of the present invention that are described hereinbelow provide improved methods and systems for storing and retrieving data in analog memory cells. In some embodiments, a memory system comprises programming circuitry for storing data in the cells and reading circuitry for retrieving data from the cells. The programming circuitry programs the cells to assume multiple programming levels by writing respective nominal threshold voltages to the cells. At least one of the nominal threshold voltages is negative. In other words, one or more of the programming levels used for storing data in the cells are represented by negative threshold voltages. Such levels are referred to herein as “negative programming levels.”
Programming a cell typically comprises adding charge to the cell by changing its gate voltage from a first value to a second value, so as to alter its source-drain current. Programming a cell to a negative threshold voltage typically comprises adding charge to the cell when both the first and the second values of the gate voltage are lower than the source voltage of the cell.
The cells may be programmed using a Program and Verify (P&V) process, in which the threshold voltages written to the cells are verified against suitable verification reference levels (also referred to as verification thresholds, programming thresholds or P&V thresholds). When programming cells to the negative programming levels, the programmed threshold voltages can be verified by setting the verification reference levels to negative values.
In some embodiments, the reading circuitry reads data from the cells by comparing their threshold voltages to a set of read reference levels, at least one of which is negative. Reading a cell using a negative read reference level typically comprises measuring the source-drain current of the cell when its gate voltage is lower than its source voltage. Setting a negative verification reference level can be performed in a similar manner.
Negative read reference levels may be used for reading cells that store data in negative programming levels. Alternatively or additionally, negative read reference levels are also advantageous for reading cells having positive programming levels, for example when some low positive threshold voltages drift and become negative over time.
Unlike some known methods is which a negative storage value represents only an erased status of a given memory cell, the methods and systems described herein use negative storage values for storing useful data. The techniques described herein are able to program individual cells to assume negative threshold voltages that represent useful data, and to read useful data that is represented by negative threshold voltages. In some embodiments, multiple programming levels and/or multiple reference levels are negative.
In comparison with known memory devices, devices that store data in negative programming levels may have reduced operating voltages, and therefore improved reliability, increased lifetime, lower power consumption and lower interference between cells. For a given maximum threshold voltage, exploiting the negative side of the threshold voltage axis enables increasing the storage density and/or storage reliability of the memory device.
Memory Cell Array Configuration
FIG. 1 is a block diagram that schematically illustrates a memory cell array 10, in accordance with an embodiment of the present invention. In the present example, the array comprises a NAND Flash array. Array 10 comprises multiple analog memory cells 12. Each cell 12 comprises a a floating gate Metal-Oxide Semiconductor (MOS) transistor. A certain amount of electrical charge (electrons or holes) can be stored in a particular cell by applying appropriate voltage levels to the transistor gate, source and drain. The value stored in the cell can be read by measuring the threshold voltage of the cell, which is defined as the minimal voltage that needs to be applied to the gate of the transistor in order to cause the transistor to conduct. The read threshold voltage is indicative of the charge stored in the cell.
Memory cells 12 of array 10 are arranged in a grid having multiple rows and columns. The gates of the transistors in each row are connected by word lines 18, and the transistors in each column are connected to bit lines 16.
The transistors in a given column are arranged in groups, which are referred to as NAND strings 14. A given NAND string can be selected (i.e., connected to the bit line) by a string select switch 19. In a given string 14, the transistors are connected source-to-drain in series with one another. The first transistor in the string is connected to the bit line via switch 19. The last transistor is connected to a certain baseline voltage VB. The baseline voltage may comprise ground potential or any other suitable baseline voltage. Each NAND string typically comprises thirty-two transistors, although any other suitable NAND string size can be used.
The array is typically divided into multiple pages, i.e., groups of memory cells that are programmed and read simultaneously. Pages are sometimes sub-divided into sectors. In some embodiments, each page comprises an entire row of the array. In alternative embodiments, each row (word line) can be divided into two or more pages. For example, in some SLC devices each row is divided into two pages, one comprising the odd-order cells and the other comprising the even-order cells. In a typical implementation, a two-bit-per-cell memory device may have four pages per row, a three-bit-per-cell memory device may have six pages per row, and a four-bit-per-cell memory device may have eight pages per row.
As will be shown below, the methods and systems described herein are able to program cells 12 to assume negative threshold voltages. As a result, data can be stored and retrieved in programming levels that are located on the negative side of the threshold voltage axis. Programming a cell to assume a negative threshold voltage typically comprises adding charge to the cell by changing its gate voltage from a first value to a second value, so as to alter its source-drain current. Both the first and the second values of the gate voltage are lower than the source voltage of the cell. The methods and systems described herein are able to read data from a cell using negative read reference levels. Reading a cell using a negative read reference level typically comprises measuring the source-drain current of the cell when its gate voltage is lower than its source voltage.
Programming the cells is sometimes performed using an iterative Program and Verify (P&V) process. In a typical P&V process, an incrementally-increasing sequence of programming pulses is applied to the cells. The cell voltages are read between successive programming iterations and verified against the desired programming levels. The process continues until the cells reach their intended programming levels or until a maximum number of iterations have been performed. In some embodiments, cells 12 can be programmed to both positive and negative threshold voltages using an iterative P&V process.
Cells 12 are typically erased in blocks that contain multiple pages. The erasure operation typically involves applying a high negative voltage to an entire erasure block simultaneously.
Array 10 may comprise several thousand erasure blocks. In a typical two-bit-per-cell MLC device, each erasure block is on the order of 32 word lines, each comprising several thousand cells. Each word line of such a device is often partitioned into four pages (odd/even order cells, least/most significant bit of the cells). Three-bit-per cell devices having 32 word lines per erasure block would have 192 pages per erasure block, and four-bit-per-cell devices would have 256 pages per block. Alternatively, other block sizes and configurations can also be used.
Some memory devices comprise two or more separate memory cell arrays, often referred to as planes. Since each plane has a certain “busy” period between successive write operations, data can be written alternately to the different planes in order to increase programming speed.
Negative Programming Levels and Negative Read Thresholds
FIG. 2 is a graph showing threshold voltage distributions in a group of analog memory cells, in accordance with an embodiment of the present invention. The present example refers to a group of eight-level memory cells, such as a page or a block, in which each cell stores three bits of data. The figure shows eight threshold voltage distributions 20 denoted L1 . . . L8, each corresponding to a certain three-bit combination. The cells are programmed to the different levels by writing nominal threshold voltages 21 to the cells. Data is read from the memory cells by comparing their threshold voltages to a set of seven read thresholds 22.
In the example of FIG. 2, programming levels L1 and L2 are located on the negative side of the threshold voltage axis. In other words, the nominal threshold voltages corresponding to these levels are negative. In some embodiments, although not necessarily, one of the negative programming levels may be used to mark erased cells, either instead of or in addition to storing data. In the present example, two of the read thresholds that are used for reading the cells are negative.
The maximum threshold voltage used in the configuration of FIG. 2 is denoted VMAX in the figure. Since some of the programming levels are negative, VMAX can be reduced with respect to comparable eight-level schemes that store data only in positive levels. As a result of the lower VMAX, the memory cells have improved reliability, increased lifetime, lower power consumption and lower interference between cells.
Reducing VMAX enables a reduction of other voltages related to the operation of the memory cells. For example, some NAND Flash devices comprise memory cells that are arranged in strings. When a certain memory cell is read, a relatively high voltage (referred to as a pass voltage VPASS) is applied to the other cells in the string. Reducing VMAX enables a corresponding reduction of VPASS.
Moreover, for a given value of VMAX, the storage density of the memory cells can be increased by adding one or more programming levels on the negative side of the threshold voltage axis. Alternatively, moving programming levels to the negative side of the threshold voltage axis may be used to increase the separation between adjacent levels, thus reducing the likelihood of read errors and improving the reading performance.
FIG. 3 is a graph showing threshold voltage distributions 20 in a group of analog memory cells, in accordance with another embodiment of the present invention. In the scheme of FIG. 3, the memory cells comprise four-level cells, each storing two data bits. The memory cells are programmed using four distributions 20 denoted L1 . . . L4. The cells are read by comparing their threshold voltages to three read thresholds 22. As can be seen in the figure, level L1 comprises a negative level, and the read threshold separating between levels L1 and L2 is negative, as well.
The feature of reading memory cells with negative read thresholds is also advantageous in conventional storage schemes in which a single negative level is used for marking erased cells. For example, the threshold voltages of cells that were originally programmed to positive levels may drift and become negative over time. These drifts may be cause, for example, by cell aging, repetitive programming and erasure of the cells, interference from other cells, and other effects.
In FIG. 3, for example, distribution L2 may have been entirely positive when the cells were first programmed, and may have drifted and/or widened to become partly negative over time. Setting the read threshold separating between levels L1 and L2 to a non-negative value (e.g., 0V) will produce poor error probability, since the optimal position of this read threshold is negative. Setting the read threshold to the optimal negative value is important for maintaining adequate error performance. Thus, the ability to set negative read thresholds improves the reading performance of conventional memory cells, as well. The ability to use negative read thresholds enables tracking the optimal threshold positions along the entire life cycle of the memory cells.
The methods and systems described herein can be used for storing data in any suitable number of negative programming levels, and/or using any suitable number of negative read thresholds and verification thresholds.
System Description
FIG. 4 is a block diagram that schematically illustrates a memory system 24, in accordance with an embodiment of the present invention. System 24 stores data using negative programming levels and/or negative thresholds, using the techniques described herein. System 24 can be used in various host systems and devices, such as in computing devices, cellular phones or other communication terminals, removable memory modules (“disk-on-key” devices), digital cameras, music and other media players and/or any other system or device in which data is stored and retrieved.
System 24 comprises a memory device 28, which stores data in a memory cell array 32. The memory array comprises multiple analog memory cells 36. In the context of the present patent application and in the claims, the term “analog memory cell” is used to describe any memory cell that holds a continuous, analog value of a physical parameter, such as an electrical voltage or charge. Array 32 may comprise analog memory cells of any kind, such as, for example, NAND and NOR Flash cells, or any other suitable type of cells. In some embodiments, array 32 is an array of NAND Flash cells that are arranged in strings, such as in the configuration of FIG. 1 above.
The charge levels stored in the cells and/or the analog voltages or currents written into and read out of the cells are referred to herein collectively as analog values or storage values. Although the embodiments described herein mainly address threshold voltages, the methods and systems described herein may be used with any other suitable kind of storage values.
System 24 stores data in the analog memory cells by programming the cells to assume respective memory states, also referred to as programming levels. The memory states are selected from a finite set of possible states, and each state corresponds to a certain nominal storage value. For example, a 2 bit/cell MLC can be programmed to assume one of four possible memory states by writing one of four possible nominal storage values into the cell.
Memory device 28 comprises a reading/writing (R/W) unit 40, which converts data for storage in the memory device to analog storage values and writes them into memory cells 36. In alternative embodiments, the R/W unit does not perform the conversion, but is provided with voltage samples, i.e., with the storage values for storage in the cells. When reading data out of array 32, R/W unit 40 converts the storage values of memory cells into digital samples having a resolution of one or more bits.
The storage and retrieval of data in and out of memory device 28 is performed by a Memory Signal Processor (MSP) 44. MSP 44 comprises an interface 48 for communicating with memory device 28, and a signal processing unit 52, which processes the data that is written into and read from device 28. In some embodiments, unit 52 produces the storage values for storing in the memory cells and provides these values to R/W unit 40. Alternatively, unit 52 provides the data for storage, and the conversion to storage values is carried out by the R/W unit internally to the memory device.
MSP 44 communicates with a host 56, for accepting data for storage in the memory device and for outputting data retrieved from the memory device. MSP 44, and in particular unit 52, may be implemented in hardware. Alternatively, MSP 44 may comprise a microprocessor that runs suitable software, or a combination of hardware and software elements.
The configuration of FIG. 4 is an exemplary system configuration, which is shown purely for the sake of conceptual clarity. Any other suitable configuration can also be used. Elements that are not necessary for understanding the principles of the present invention, such as various interfaces, addressing circuits, timing and sequencing circuits and debugging circuits, have been omitted from the figure for clarity.
In the exemplary system configuration shown in FIG. 4, memory device 28 and MSP 44 are implemented as two separate Integrated Circuits (ICs). In alternative embodiments, however, the memory device and MSP may be integrated on separate semiconductor dies in a single Multi-Chip Package (MCP) or System on Chip (SoC). Further alternatively, some or all of the MSP circuitry may reside on the same die on which the memory array is disposed. Further alternatively, some or all of the functionality of MSP 44 can be implemented in software and carried out by a processor or other element of the host system. In some implementations, a single MSP 44 may be connected to multiple memory devices 28.
Data Storage and Retrieval Using Negative Programming Levels and/or Thresholds
As noted above, system 24 comprises programming circuitry that converts the data intended for storage into storage values, and writes the storage values to the memory cells. The programming circuitry may reside in the MSP, in the R/W unit of the memory device, or it may be partitioned between the two devices. In some embodiments, the programming circuitry writes the storage values into the cells in an iterative Program and Verify (P&V) process, as in known in the art.
In a typical P&V process, the programming circuitry applies an incrementally-increasing sequence of programming pulses to the cells, and verifies the programmed storage values between iterations by comparing them to one or more verification thresholds (also referred to herein as verification reference levels). Similarly, the reading circuitry reads the storage values of the cells by comparing them to one or more read thresholds (also referred to herein as read reference levels), and thus associates each cell with a certain programming level. The reading circuitry may reside in R/W unit 40 or jointly in the R/W unit and the MSP.
In some embodiments, the programming circuitry of system 24 applies a P&V process that programs the cells to both positive and negative threshold voltages, as derived from the data intended for storage. Programming a negative threshold voltage typically comprises adding charge to the cell by changing its gate voltage from a first value to a second value, so as to alter its source-drain current. Both the first and the second values of the gate voltage are lower than the source voltage of the cell. Programming a positive threshold voltage typically comprises adding charge to the cell when the gate voltage is higher than the source voltage of the cell. The programming circuitry applies the appropriate voltages to the cells in order to perform the desired operations.
As noted above, verifying the stored values comprises measuring the source-drain currents of the cells and comparing them with verification reference levels. In some embodiments, the programming circuitry determines parameters of a subsequent P&V iteration based on the source-drain currents measured in a previous iteration. For example, the programming circuitry may determine which of the cells have reached their intended target levels in the previous iteration, and continue to apply subsequent iterations only to the cells that have not yet reached their target levels.
In some embodiments, system 24 uses at least one negative read threshold for reading data from the memory cells. The reading circuitry may use any suitable method for setting negative read thresholds and for measuring negative threshold voltages. Typically, reading the storage value of a cell comprises measuring the current that flows between the source and drain of the cell. Reading a negative storage value from a cell thus comprises measuring the source-drain current of the cell when its gate voltage is lower than its source voltage. Typically, the R/W unit is able to read both positive and negative storage values, i.e., to measure the source-drain current both when the gate voltage is lower than the source voltage and when the gate voltage is higher than the source voltage. The reading circuitry applies the appropriate voltages to the cells in order to perform the desired operations.
Typically but not necessarily, setting a negative read threshold involves raising one or more of the voltages applied to the memory cell being read by a known amount, and determining the storage value stored in the cell based on the known amount and on the read thresholds. For example, the reading circuitry may raise the word line and bit line voltages of the array by a certain operation voltage in order to be able to measure negative threshold voltages.
In particular, when the memory comprises a NAND Flash memory whose cells are arranged in NAND string, the reading circuitry may read a cell whose threshold voltage is negative by:
(1) Applying a gate voltage Vg to the gate (word line) of the cell to be read, applying a pass voltage Vpass to the other gates (word lines) in the string, and applying a source voltage Vs to the source of the NAND string, such that Vs>Vg and Vpass>Vs.
(2) Pre-charging the bit line that is connected to the NAND string in question to a voltage Vb1, Vb1>Vs.
(3) Measuring the variation in the bit line voltage after a predetermined time interval.
Alternatively, the reading circuitry may set the word line voltages to zero, and raise the array potential to a certain positive potential. Further alternatively, the reading circuitry may apply any other suitable method for setting negative read thresholds and for comparing the storage values of the cells with the negative thresholds.
In some embodiments, systems 20 programs the cells so that at least one programming level that is used for storing data comprises a negative programming level. In other words, at least one programming level that is used for storing data corresponds to a negative nominal storage value. The programming circuitry may verify the negative storage values written to the cells by reading the storage values and comparing them with one or more verification thresholds, whose values are negative.
The verification operation is typically similar to the reading operation, except for using a different set of thresholds. Any of the above-mentioned techniques for reading negative threshold voltages can also be used for verification. In some embodiments, some hardware may be shared between the programming and reading circuitry.
FIG. 5 is a flow chart that schematically illustrates a method for programming analog memory cells, in accordance with an embodiment of the present invention. The data for storage is typically accepted from host 56. The method begins with the MSP defining a set of programming levels and an associated set of nominal storage values, at a level definition step 60. At least one of the nominal storage values in the set is negative.
The MSP defines a set of verification thresholds for verifying the storage values that correspond to the different programming levels, at a programming threshold definition step 64. Typically, the verification threshold that corresponds to a negative programming level is negative.
The programming circuitry stores data in the memory cells by programming the cells using the defined nominal storage values, at a programming step 68. As part of the programming process, the programming circuitry verifies the programmed storage values using the defined verification thresholds.
FIG. 6 is a flow chart that schematically illustrates a method for reading analog memory cells, in accordance with an embodiment of the present invention.
The method begins with the MSP defining a set of read thresholds, at a read threshold definition step 72. At least one of the read thresholds defined at step 72 is negative. The reading circuitry reads the memory cells using the defined read thresholds, at a reading step 76, so as to reconstruct the data. The MSP outputs the reconstructed data to the host.
In some cases, operations involving negative reference levels and/or negative programming levels may be less efficient than operations that involve only non-negative programming levels and reference levels, and therefore it may be advantageous to revert to negative voltage operation only when needed. In some embodiments, the MSP determines whether or not to use negative reference levels and/or negative programming levels during operations. The MSP may evaluate various types of conditions in order to determine whether or not it is preferable to use negative reference levels and/or negative programming levels.
For example, the MSP may decide to apply a negative reference level upon determining that a large number of cells in a given page have a negative threshold voltage, such as due to programming failures, temperature shift, leakage currents or for any other reason. The MSP may assess the number of cells in the page whose threshold voltage is negative by reading the cells with a read reference level set to zero, and counting the number of cells whose threshold voltage is below the reference level. In some embodiments, when the page is programmed, the MSP may store the number of programmed cells. The MSP may compare the number of cells having a negative threshold voltage with the number of programmed cells, in order to determine whether or not to read the page using a negative reference level.
As another example, the MSP may determine whether or not to employ a negative programming level when programming a page. The MSP may estimate the expected likelihood of errors, e.g., the distortion level or expected error probability in the page, if the page were to be programmed with only positive programming levels. The estimation may be based, for example, on the number of previous programming and erasure operations. If the likelihood of errors is relatively low, the MSP will use only positive programming levels. When the likelihood of errors is high, the MSP may program the page using programming levels, one or more of which is negative.
Although the embodiments described herein mainly address writing and reading data in solid-state memory devices, the principles of the present invention can also be used for programming and reading other types of storage devices, such as Hard Disk Drives (HDD).
It will thus be appreciated that the embodiments described above are cited by way of example, and that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and sub-combinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art.

## Claims

1. A method for data storage in a memory that includes a plurality of analog memory cells, the method comprising:
defining a set of nominal storage values, at least one of which is negative;
storing data in the memory by mapping the data to first storage values selected from the set of the nominal storage values, and writing the first storage values to the cells; and
after storing the data, reading second storage values from the cells using one or more read reference levels, so as to reconstruct the stored data.

2. The method according to claim 1, wherein writing the first storage values comprises applying respective gate voltages and source voltages to the cells and varying the gate voltages from first gate voltages to second gate voltages so as to add electrical charges to the cells, and wherein writing the at least one negative nominal storage value to a given cell comprises setting the first and second gate voltages applied to the given cell to be lower than the source voltage applied to the given cell.

3. The method according to claim 1, wherein storing the data comprises writing the negative at least one of the nominal storage values to some of the cells.

4. The method according to claim 1, wherein at least two of the nominal storage values are negative.

5. The method according to claim 1, wherein at least one of the read reference levels is negative.

6. The method according to claim 1, wherein storing the data comprises verifying the first storage values written to the cells using one or more verification reference levels, at least one of which is negative.

7. The method according to claim 6, wherein verifying the first storage values comprises applying respective gate voltages and source voltages to the cells and measuring source-drain currents flowing though the cells responsively to the applied source and gate voltages, and wherein verifying the first storage value using the at least one negative verification reference level comprises setting the gate voltages to be lower than the respective source voltages.

8. The method according to claim 7, wherein verifying the first storage values comprises applying multiple Programming and Verification (P&V) iterations to the cells.

9. The method according to claim 8, wherein applying the multiple P&V iterations comprises modifying parameters of a subsequent P&V iteration based on the source-drain currents measured in a previous P&V iteration.

10. The method according to claim 9, wherein modifying the parameters comprises selecting a subset of the cells based on the source-drain currents measured in the previous P&V iteration, and applying the subsequent P&V iteration only to the selected subset.

11. The method according to claim 1, wherein the cells comprise NAND Flash cells.

12. The method according to claim 1, wherein defining the nominal storage values comprises evaluating a condition and including the at least one negative nominal storage value in the set of the nominal storage values responsively to the evaluated condition.

13. Apparatus for data storage in a memory that includes a plurality of analog memory cells, the apparatus comprising:
programming circuitry, which is coupled to define a set of nominal storage values, at least one of which is negative, and to store data in the memory by mapping the data to first storage values selected from the set of the nominal storage values and writing the first storage values to the cells; and
reading circuitry, which is coupled, after the data was stored, to read second storage values from the cells using one or more read reference levels, so as to reconstruct the stored data.

14. The apparatus according to claim 13, wherein the programming circuitry is coupled to write the first storage values by applying respective gate voltages and source voltages to the cells and varying the gate voltages from first gate voltages to second gate voltages so as to add electrical charges to the cells, and to set the first and second gate voltages applied to a given cell to be lower than the source voltage applied to the given cell so as to write the at least one negative nominal storage value to the given cell.

15. The apparatus according to claim 13, wherein the programming circuitry is coupled to write the negative at least one of the nominal storage values to some of the cells.

16. The apparatus according to claim 13, wherein at least two of the nominal storage values are negative.

17. The apparatus according to claim 13, wherein at least one of the read reference levels is negative.

18. The apparatus according to claim 13, wherein the programming circuitry is coupled to verify the first storage values written to the cells using one or more verification reference levels, at least one of which is negative.

19. The apparatus according to claim 18, wherein the programming circuitry is coupled to verify the first storage values by applying respective gate voltages and source voltages to the cells and measuring source-drain currents flowing though the cells responsively to the applied source and gate voltages, and to set the gate voltages to be lower than the respective source voltages so as to verify the first storage value using the at least one negative verification reference level.

20. The apparatus according to claim 19, wherein the programming circuitry is coupled to apply multiple Programming and Verification (P&V) iterations to the cells.

21. The apparatus according to claim 20, wherein the programming circuitry is coupled to modify parameters of a subsequent P&V iteration based on the source-drain currents measured in a previous P&V iteration.

22. The apparatus according to claim 20, wherein the programming circuitry is coupled to select a subset of the cells based on the source-drain currents measured in the previous P&V iteration, and to apply the subsequent P&V iteration only to the selected subset.

23. The apparatus according to claim 13, wherein the cells comprise NAND Flash cells.

24. The apparatus according to claim 13, wherein the programming circuitry is coupled to evaluate a condition and to include the at least one negative nominal storage value in the set of the nominal storage values responsively to the evaluated condition.

25. Apparatus for data storage, comprising:
a memory, which comprises a plurality of analog memory cells;
programming circuitry, which is coupled to define a set of nominal storage values, at least one of which is negative, and to store data in the memory by mapping the data to first storage values selected from the set of the nominal storage values and writing the first storage values to the cells; and
reading circuitry, which is coupled, after the data was stored, to read second storage values from the cells using one or more read reference levels, so as to reconstruct the stored data.