Abstract:
Methods of erasing a sector of multi-level flash memory cells (MLB) having three or more data states to a single data state are provided. The present invention employs an interactive sector erase algorithm that repeatedly erases, verifies, soft programs, and programs the sector in two or more erase phases to achieve highly compact data state distributions. In one example, the algorithm essentially erases all the MLB cells of the sector to an intermediate state and corresponding threshold voltage value using interactive erasing, soft programming and programming pulses in a first phase. Then in a second phase, the algorithm further erases all the MLB cells of the sector using additional interactive erasing and soft programming pulses until a final data state is achieved corresponding to a desired final threshold voltage value of the cells. Optionally, the algorithm may include one or more additional phases of similar operations that successively bring the memory cells of the sector to a compacted common erased state in preparation for subsequent programming operations. In one aspect of the method, the actual threshold values and/or data states chosen for these phases may be predetermined and input to the memory device by the user.

Description:
FIELD OF INVENTION  
       [0001]     The present invention relates generally to memory devices and the like, and in particular to a method of erasing sectors of cells with multiple-level data states in flash memory devices.  
       BACKGROUND OF THE INVENTION  
       [0002]     Many different types and styles of memory exist to store data for computers and similar type systems. For example, random access memory (RAM), dynamic random access memory (DRAM), static random access memory (SRAM), read only memory (ROM), programmable read only memory (PROM), electrically programmable read only memory (EPROM), electrically erasable programmable read only memory (EEPROM) and flash memory are all presently available to accommodate data storage.  
         [0003]     Each type of memory has its own particular advantages and disadvantages. For example, DRAM and SRAM allow individual bits of data to be erased one at a time, but such memory loses its data when power is removed. EEPROM can alternatively be easily erased without extra exterior equipment, but has reduced data storage density, lower speed, and higher cost. EPROM, in contrast, is less expensive and has greater density but lacks ease of erasability.  
         [0004]     Flash memory, has become a popular type of memory because it combines the advantages of the high density and low cost of EPROM with the electrical erasability of EEPROM. Flash memory can be rewritten and can hold its contents without power, and thus is nonvolatile. It is used in many portable electronic products, such as cell phones, portable computers, voice recorders, etc. as well as in many larger electronic systems, such as cars, planes, industrial control systems, etc.  
         [0005]     Flash memory is generally constructed of many memory cells where, generally, single bits of data are stored in and read from respective memory cells. The cells are generally programmed by hot electron injection and erased by Fowler-Nordheim tunneling or other mechanisms. As with many aspects of the semiconductor industry, there is a continuing desire and effort to achieve higher device packing densities and increase the number of memory cells on a semiconductor wafer. Similarly, increased device speed and performance are also desired to allow more data to be stored on smaller memory devices.  
         [0006]     Individual flash memory cells are organized into individually addressable units or groups, which are accessed for read, program, or erase operations through address decoding circuitry. The individual memory cells are typically comprised of a semiconductor structure adapted for storing a bit of data and includes appropriate decoding and group selection circuitry, as well as circuitry to provide voltages to the cells being operated upon.  
         [0007]     The erase, program, and read operations are commonly performed by application of appropriate voltages to certain terminals of the memory cell. In an erase or write operation the voltages are applied so as to cause a charge to be removed or stored in the memory cell. In a read operation, appropriate voltages are applied so as to cause a current to flow in the cell, wherein the amount of such current is indicative of the value of the data stored in the cell. The memory device includes appropriate circuitry to sense the resulting cell current in order to determine the data stored therein, which is then provided to data bus terminals of the device for access by other devices in a system in which the memory device is employed.  
         [0008]     Programming circuitry controls a bit of a cell by applying a signal to a wordline, which acts as a control gate, and changing bitline connections such that the bit is stored by the source and drain connections. Programming a cell using a suitable mechanism such as hot electron injection, generally increases the threshold voltage of a cell. Erasing is performed as a blanket operation wherein an array or sector of cells can be simultaneously erased and typically produces a lower threshold voltage in the cell.  
         [0009]     In the blanket erasing of flash memory, cells within an array or sector are typically erased concurrently and can be accomplished by one or more applications of short erase pulses. After each erase pulse, an erase verification or read can be performed to determine if each cell in the array is now “erased” (blank), or yet remains “un-erased” or “under-erased”, (e.g., whether the cell has a threshold voltage above a predetermined limit). If an under-erased cell is detected, an additional erase pulse can be applied to the entire array until all cells are sufficiently erased. With such an erase procedure, however, some cells may become “over-erased” before other cells are sufficiently erased. A memory cell having a threshold voltage erased below a predetermined limit, for example, may be commonly referred to as being over-erased. For several reasons it is undesirable for a memory cell to remain in an over-erased condition.  
         [0010]     Regardless of the flash architecture employed, accurately erasing and programming multiple level flash cells can be particularly sensitive with the attendant complications of maintaining narrow Vt distributions in order to accurately read and determine a data state from a corresponding Vt level. In addition, even if such narrow distributions are attained for the various multiple levels, unless sectors of memory cells can be erased to within the acceptable limits quickly, efficiently, and reliably, little competitive advantage may be gained.  
         [0011]     In view of the foregoing, a need exists for an improved method of erasing a sector or array of multi-level flash memory cells.  
       SUMMARY OF THE INVENTION  
       [0012]     The following presents a simplified summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not an extensive overview of the invention. It is intended neither to identify key or critical elements of the invention nor to delineate the scope of the invention. Rather, its primary purpose is merely to present one or more concepts of the invention in a simplified form as a prelude to the more detailed description that is presented later.  
         [0013]     The present invention erases a sector or array of multi-level bit flash memory cells (MLB) to a single bit state (data state) by applying an interactive sector erase algorithm that erases, verifies, soft programs, and programs the sector in two or more erase phases (or tiers) to achieve highly compact Vt distributions. The algorithm provides Vt distributions, which have an improved sigma over that obtainable with some conventional single phase methods. In one example, the algorithm erases all the MLB cells of the sector to an intermediate state corresponding to an intermediate threshold voltage value, using interactive erasing, soft programming and programming pulses in a first phase. The first phase brings the memory cells to the same logical state and closer to the same threshold voltage. Then in a second phase, the algorithm is used to further compact the distribution of the threshold voltage levels about a final data state. In the second phase of the present invention the algorithm again erases all the MLB cells of the sector using additional interactive erasing, and soft programming pulses until the desired final threshold voltage value of the cells corresponding to a final data state is achieved.  
         [0014]     The multi-level bit MLB flash memory cell of the present invention may comprise a single physical bit that can be programmed to three or more levels corresponding to three or more data states. Alternately, the MLB cell may comprise a dual-bit or mirror-bit cell having two physically distinct bits that may each be programmed to multiple levels such as four, wherein 16 possible states are then available. The method may be suitably implemented in a variety of flash memory architectures including single and dual bit EEPROM, and other such single or multi-bit memory architectures that may be electrically erased, and any such cell or variant is contemplated as falling within the scope of the present invention.  
         [0015]     A further aspect of the algorithm of the present invention includes an additional phase of similar erase and soft program operations that may be used to further compact the threshold voltage distributions of the memory cells. In the additional phase, a second intermediate logical state is chosen between the intermediate state and the final state. After the cells are erased, programmed and soft programmed to the intermediate state, the cells are erased and soft programmed to the second intermediate state and then finally to the final data state in a similar manner. Any number of such intermediate phases may be applied to the method including the total number of data states utilized by the sector or array of MLB memory cells.  
         [0016]     In this way, the memory cells are erased to a common erased state having a narrow Vt distribution in preparation for subsequent programming and read operations. In one aspect of the method, the actual threshold values and data states chosen for these phases may be predetermined and input to the memory device by a user.  
         [0017]     In one method and implementation of the present invention, the memory cells of the sector have four data states L 1 , L 2 , L 3 , and L 4  corresponding to four respective threshold voltage values, for example. The method erases the memory cells to an intermediate threshold voltage value corresponding to the L 2  data state in a first phase operation, and then in a second phase operation erases the memory cells to a final threshold voltage value corresponding to the L 1  data state.  
         [0018]     In accordance with another aspect of the sector erase algorithm of the present invention, the memory cells are initially programmed to various states. Another aspect of the invention provides a method of erasing multiple sectors of the array including the entire array. The present invention provides a method of erasing a sector of an array of MLB memory cells that yields a well controlled low sigma Vt distribution using a minimal erase time, while maintaining a device endurance and reliability status.  
         [0019]     To the accomplishment of the foregoing and related ends, the following description and annexed drawings set forth in detail certain illustrative aspects and implementations of the invention. These are indicative of but a few of the various ways in which one or more aspects of the present invention may be employed. Other aspects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the annexed drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]      FIG. 1  is a Vt distribution of a four level multi-level cell in accordance with an aspect of the present invention.  
         [0021]      FIG. 2  is a Vt distribution of an eight level multi-level cell in accordance with an aspect of the present invention.  
         [0022]      FIG. 3  is a Vt distribution of one exemplary level of a cell, the distribution centered around a target Vt and having upper and lower population boundary levels in accordance with an aspect of the present invention and as depicted in  FIGS. 1 and 2 .  
         [0023]      FIG. 4A  is flow diagram illustrating an exemplary method of erasing a sector or array of MLB memory cells that includes a two-phase algorithm for interactively erasing memory cells in accordance with an aspect of the present invention.  
         [0024]      FIGS. 4B-4F  are flow diagrams illustrating further details of various portions of the exemplary method of erasing a sector or array of MLB memory cells in accordance with the MLB sector erase method of  FIG. 4A .  
         [0025]      FIG. 5  is a plot of the Vt levels of several exemplary memory cells of a sector or array of MLB memory cells programmed to various initial logical states and corresponding Vt levels, such as may be suitable for sector erasing by the method of the present invention.  
         [0026]      FIGS. 6-10  are plots of the Vt levels of the exemplary memory cells of  FIG. 5  resulting from the processing steps of the MLB sector erase method of the present invention using the two-phase algorithm of  FIG. 4A .  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0027]     One or more aspects of the present invention are described with reference to the drawings, wherein like reference numerals are generally utilized to refer to like elements throughout, and wherein the various structures are not necessarily drawn to scale. In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of one or more aspects of the present invention. It may be evident, however, to one skilled in the art that one or more aspects of the present invention may be practiced with a lesser degree of these specific details. In other instances, well-known structures and devices are shown in block diagram form in order to facilitate describing one or more aspects of the present invention.  
         [0028]     Improvements in the density of memory devices translate to increased memory capacity. Density and thus capacity is a major consideration in the cost of fabrication and the marketability of memory devices, and is directly related to the amount of real estate used to store a bit of information on a semiconductor chip. Density may be increased, for example, by scaling down feature size to accommodate more memory cell transistors on a given size chip thereby achieving higher density. Another technique to increase density and reduce manufacturing costs is thru the use of multi-level cell technology.  
         [0029]     Multi-level cells increase cell density by increasing the number of possible logical states or data states associated with a cell, thereby allowing a single memory cell to store information corresponding to more than one data bit. One way this has been done is by using multiple (three or more, in the context of cell levels and states) threshold voltage (Vt) levels, which correspond to multiple data states per cell. This contrasts to the two states and levels used in conventional flash memory cells. Thus, in one example, a single mirror bit cell may store two physical bits of data each at four Vt levels corresponding to four logical states. Cells, having multiple levels, however, present many new problems attempting to maintain well controlled or tight distributions of the Vt levels, particularly as larger data bit quantities are considered.  
         [0030]     As a result of these trends, accurate erasure and the determination of the levels of such multi-level cells becomes increasingly demanding, particularly as higher bit capacities are expected of a single cell. Accordingly, multi-level memory cells need to be erased quickly and efficiently to save erase operations time and power consumption. In addition, the cells need to be erased to a well controlled narrow erase distribution (tight bit compacting) to insure subsequent program operations also will produce narrow program Vt distributions. Such device requirements and issues are likely to increase as device features continue to shrink and the density of memory cells increases.  
         [0031]     Memory device manufactures often guarantee specific values of speed, endurance, reliability, and the power consumed during a variety of operating modes. These device specifications or operating parameters are beneficial to the user to insure a device performs as intended. Accordingly, it is a goal of the present invention to provide a method of erasing a sector or array of suitable MLB flash memory cells, which achieves the aforementioned requirements while also providing the specific values of speed, endurance, reliability, and the power consumed during the applicable operating modes.  
         [0032]     A sector erase algorithm may be used to erase a specified sector or multiple sectors of an array of multi-level flash memory cells MLB to a single data state. The erase algorithm of the present invention may be applied in two or more erase phases (or tiers), for example. The algorithm achieves well controlled compact Vt distributions compared to those of some conventional single phase methods. The method may be suitably implemented in a variety of flash memory architectures including single and dual bit EEPROM, and other such single or multi-bit memory architectures that may be electrically erased, and any such cell or variant is contemplated as falling within the scope of the present invention.  
         [0033]     In a first phase, the algorithm of the present invention erases all the MLB cells of the sector or cell group to an intermediate state corresponding to an intermediate threshold voltage value, and then applies interactive erase, soft program and program pulses. The first phase brings the memory cells to the same logical state and closer to the same threshold voltage. Then, in a second phase, the algorithm is used to further compact the distribution of the threshold voltage levels about a final data state. In the second phase of the present invention the algorithm again erases all the MLB cells of the sector using additional interactive erasing, and soft programming pulses until the cells achieve a desired final threshold voltage value corresponding to a final data state.  
         [0034]     Referring initially to  FIG. 1 , an unsigned Vt distribution  100  of a four level MLB cell in accordance with an aspect of the present invention is illustrated. Vt distribution  100  represents a population of memory cell threshold voltages centered about four discrete target threshold voltages. Each target threshold voltage occupies a range of Vt values designated by levels L 1 , L 2 , L 3 , and L 4 , respectively. Ideally, each level would be centered between upper and lower Vt limits, for example, Vt 0 , Vt 1 , Vt 2 , Vt 3 , and Vt 4 . The various levels may be arbitrarily assigned corresponding binary states (e.g., L 1 =11, L 2 =10, L 3 =01, and L 4 =00, or L 1 =00, L 2 =01, L 3 =10, and L 4 =11) as desired by the user. The four level MLB cell associated with the distribution  100  may comprise a single physical bit that can be programmed to four levels, or alternatively, may comprise a dual-bit or mirror-bit cell having two physically distinct bits that may each be programmed to multiple levels such as four, wherein 16 possible states are then available.  
         [0035]     The method of the present invention is suitably implemented in MLB memory devices having any combination of both positive and negative Vt distributions. In  FIG. 1 , for example, the method of the present invention is equally applicable whether Vt 0 , Vt 4 , or another such Vt limit is used as a zero voltage potential or another reference potential of the memory cells. Considering the four level, single-bit memory cell example of  FIG. 1 , the method of the present invention may be used to erase all memory cells initially programmed to L 3  and L 4  data states to an intermediate threshold voltage value corresponding to the L 2  data state in a first phase operation. Then in a second phase operation, the method is used to erase the memory cells to a final threshold voltage value corresponding to the L 1  data state, as will be discussed in greater detail infra. Although the example would seem to imply that the L 1  level corresponds to the erased condition, the L 1 , L 4 , or any other level may represent the erased state.  
         [0036]      FIG. 2  illustrates another exemplary unsigned Vt distribution  200  of a single-bit eight level cell in accordance with an aspect of the present invention. Vt distribution  200  of  FIG. 2  represents a population of memory cell threshold voltages centered about eight discrete target threshold voltages. Each target threshold voltage occupies a range of Vt values designated levels L 1  thru L 8 . Ideally, each level would be centered between upper and lower Vt limits, for example, Vt 0  thru Vt 8 . The various levels may be arbitrarily assigned corresponding binary states (e.g., L 1 =111, L 2 =110, L 3 =101, thru L 8 =000, or L 1 =000, L 2 =001, L 3 =010, thru L 8 =111) as desired by the user. Again, no polarity is assigned to Vt distribution  200 , as the method of the present invention is suitably implemented in MLB memory devices having any combination of both positive and negative Vt distributions. In the event a dual-bit cell is employed (having two physically distinct bit locations), an eight level cell would correspond to 64 available data states.  
         [0037]      FIG. 3  illustrates a Vt distribution  300  of a population of Vt values at one exemplary level of a multi-level cell in accordance with the present invention and as depicted in  FIGS. 1 and 2 . The exemplary level LX of Vt distribution  300  is ideally centered around a target Vt(target) having upper and lower population boundary levels L U  and L L , respectively. The population of Vt values is further ideally centered between upper and lower Vt limits VtX−1 and VtX, but can vary. An objective of the method of the present invention is to narrow, or “compact” the population boundary levels L U  and L L  closer to one another. Sigma is often used to symbolize the standard deviation of such a population, which is a measure of the variability of the population. Accordingly, a smaller sigma indicates a narrower Gaussian distribution of the population, indicating that more cell threshold voltages are populated closer to the target Vt(target).  
         [0038]     The contributors of the present invention have realized that to get a predictable and well-controlled programmed Vt distribution from MLB cells, one solution is to first bring all cells within a group to a common erased state, which has a predictably well controlled erased Vt distribution. The contributors of the present invention have further observed and appreciated that each time an operation is performed on a particular group of memory cells that the group progressively tends to compact or filter themselves closer to the same Vt potential. Accordingly, the inventors have devised an interactive method of iteratively erasing, programming, and soft programming the cells in two or more phases between two or more discrete Vt level values. These iterative operations tend to progressively cause the cell distributions to narrow and compact the cells to a common data state.  
         [0039]     For example, in one aspect of the present invention, erase operations are used to insure that the slowest erasing bits conform to Vt&lt;=L U  level, while programming and soft programming operations are used to insure that the lowest Vt bits conform to Vt&gt;=L L  level. Stated another way, in one aspect of the present invention, erase operations may be used to reduce the Vt population of cells from the L U  boundary toward the target Vt(target), while in another method aspect, programming and soft programming may be used to raise the Vt population of cells from the L L  boundary toward the target Vt(target) value. As a result, the inventors have observed that, beneficially, the sigma of the Vt population decreases and compacts the bits with each successive alternate operation attempting to move the Vt in opposite directions, for example, an erase operation followed by a soft program operation, then an erase, etc. The method of the present invention has similarities to the technique of successive approximation, in that the Vt distribution population of the cells is progressively fine-tuned toward the target Vt(target) value with each successive operation. As a result, this method may be faster and more energy efficient than some other conventional single phase methods.  
         [0040]     A sector or group erase algorithm implemented in one or more sectors or groups of an array of suitable flash MLB flash memory cells facilitates the method of the present invention. The algorithm of the present invention may be applied in two or more erase phases (or tiers), for example, and erases the selected sector(s) to a single data state for improved subsequent programming operations. The algorithm achieves well controlled compact Vt distributions of the multi-level memory cells, thereby effectively allowing for higher device density and memory capacity with improved erase speed and efficiency. The erase algorithm and method of the invention is interactively applied and uniformly distributed across each sector of the array in the two phases.  
         [0041]     Although the methodology is illustrated and described hereinafter as a series of acts or events, it will be appreciated that the present invention is not limited by the illustrated ordering of such acts or events. For example, some acts may occur in different orders and/or concurrently with other acts or events apart from those illustrated and/or described herein. In addition, not all illustrated steps may be required to implement a one or more of the acts may be carried out in one or more separate acts or phases.  
         [0042]      FIG. 4A  illustrates a flow diagram of an exemplary method  400  of erasing a sector or array of MLB flash memory cells in accordance with the present invention. While the term “sector: is use throughout, it should be understood that such term is not to be construed as limiting to one specified grouping of cells, but rather may apply to any grouping of MLB cells.  FIGS. 4B-4F  further illustrate flow diagrams of details of the various operations of the MLB sector erase method  400  of  FIG. 4A . Assume for the sake of the following method discussion and the examples of  FIGS. 4B-4F , a four-level flash memory cell similar to that of  FIG. 1  with L 1  representing a blank or erased state, and L 4  representing the highest level. Although L 1  will represent the erased state in this example, it should be appreciated that method  400  will work for any level assignments and Vt distribution polarities of MLB memory cells, and such variations are contemplated as falling with the scope of the present invention.  
         [0043]     For example, method  400  of  FIG. 4A  includes a two-phase algorithm for interactively erasing the memory cells in accordance with the present invention. The first phase of method  400 , for example, comprises steps  402 ,  410 ,  420 , and  430 , while the second phase comprises steps  440 ,  450 , and  460 . In the first phase of method  400 , cells are essentially brought to an intermediate threshold voltage value (IV) (e.g., L 2  of the four levels of  FIG. 1 ), while in the second phase the cells are further erased to a final threshold voltage value (FV) (e.g., L 1  of the four levels of  FIG. 1 , assuming L 1  is used as the erased state).  
         [0044]     For example, the first phase of the MLB sector erase method  400  begins at  402 , wherein different portions of the sector or array may be initially programmed to different levels (e.g., some to L 1 , L 2 , L 3 , or L 4  levels of  FIG. 1 ). At  410  all the memory cells within the selected sector or sectors of the array are erased to the intermediate value IV. Act  410  of  FIG. 4B  illustrates one implementation of interactively erasing and testing all the memory cells of a sector by iteratively applying erase pulses to the sector at  414  until it is determined at  416  that all the memory cells of a sector have been at least erased to the intermediate threshold voltage value IV (e.g., Vt&lt;=IV, erasing all cells to L 2  in  FIG. 1 ).  
         [0045]     At  420 , the over-erased cells from the erase operation of  410  are soft-programmed to the final value FV (e.g., soft-programming over-erased cells to L 1  in  FIG. 1 ). Act  420  of  FIG. 4C  illustrates one example of soft programming over-erased cells to the final value by iteratively verifying at  424  whether a selected cell is over-erased (Vt&lt;FV), then applying a soft program pulse at  426  to the yet over-erased cell, and re-verifying the cell again at  424 . This soft-programming and verifying process continues iteratively with each over-erased cell until at  428  it is determined that all over-erased are restored to the final value FV (e.g., L 1  of  FIG. 1 ).  
         [0046]     At  430  all remaining final value FV (e.g., at L 1 ) cells are programmed to the intermediate value (e.g., to L 2  of  FIG. 1 ) to bring all cells of the sector to a single state. Act  430  of  FIG. 4D  illustrates one example of programming all the final value FV cells to the intermediate value IV by iteratively verifying at  434  whether a selected cell is still at FV (Vt=FV), then applying a program pulse at  436  if the cell remains at FV (e.g., at L 1 ), and re-verifying the cell again at  434 . This programming and verifying process continues iteratively with each FV level cell until at  438  it is determined that all FV level cells are programmed to the intermediate value IV (e.g., L 2  of  FIG. 1 ). At this point, and the end of the first phase, all the cells have been brought to the same intermediate value state, and have a modest Vt distribution sigma. The term final value cells in this example refers to cells that were initially programmed to the state that the entire sector will ultimately be erased to. In this example, that corresponds to cells that were already programmed to L 1  at the beginning of the erase method  400 .  
         [0047]     In the second phase of the MLB sector erase method  400 , the sigma of the Vt distribution will be further improved. At  440  of  FIG. 4A  all the memory cells within the selected sector or sectors of the array are again erased, but this time to the final value FV (e.g., L 1  of  FIG. 1 ). Act  440  of  FIG. 4E  illustrates one example of interactively erasing and testing all the memory cells of a sector by iteratively applying erase pulses to the sector at  444  until it is determined at  446  that all the memory cells of a sector have been at least erased to the final value FV (e.g., Vt&lt;=FV, erasing all cells to L 1  in  FIG. 1 ).  
         [0048]     At  450 , once again, the over-erased cells (e.g., Vt&lt;FV) from the erase operation of  440  are soft-programmed to restore them to the final value FV (e.g., soft-programming over-erased cells to L 1  in  FIG. 1 ). Act  450  of  FIG. 4F  illustrates one example of soft programming over-erased cells to the final value by iteratively verifying at  454  whether a selected cell is over-erased (Vt&lt;FV), then applying a soft program pulse at  456  to the yet over-erased cell, and re-verifying the cell again at  454 . This soft-programming and verifying process continues iteratively with each over-erased cell until at  458  it is determined that all over-erased are restored to the final value FV (e.g., L 1  of  FIG. 1 ). Thereafter, method  400  ends at  460 , wherein all MLB flash memory cells of the one or more sectors of the array have been brought to the same data state and to within a narrow Vt population distribution at an erased state centered about the final value FV.  
         [0049]     In accordance with another aspect of the present invention, various voltages used in the erase, program, and soft-program operations of method  400  may be adjusted to further optimize and speed the algorithm and compacting of the bits.  
         [0050]      FIG. 5  illustrates a plot of the Vt levels of several exemplary memory cells of a sector or array of MLB flash memory cells programmed to various initial logical states and corresponding Vt levels, such as may be suitable for sector erasing by the method of the present invention. Subsequently,  FIGS. 6-10  will then illustrate plots of the Vt levels of the exemplary memory cells of  FIG. 5  resulting from the various processing steps of the MLB sector erase method of the present invention, for example, using the two-phase algorithm and method  400  of  FIG. 4A .  
         [0051]     For example,  FIG. 5  illustrates a random selection and programming  500  of six memory cells Cell 1 , Cell 2 , Cell 3 , Cell 4 , Cell 5 , and Cell 6  from different portions of a sector or array of memory cells. Cells  1 - 6  are initially programmed to various Vt levels (e.g., L 1 , L 2 , L 3 , and L 4  of  FIG. 1 ) corresponding to one of four logical states of, for example, a single-bit four-level MLB memory cell. For the example of  FIG. 5 , Cell 1  is programmed to L 4 , Cell 2  to L 4 , Cell 3  to L 3 , Cell 4  to L 2 , Cell 5  to L 1 , and Cell 6  to L 1  as illustrated.  
         [0052]      FIG. 6  illustrates the results  600  of the erase operation  410  of the first phase of method  400 , wherein all the cells of the one or more sectors are erased to an intermediate threshold voltage value IV (e.g., the L 2  level). The inventors have observed that cells generally erase somewhat proportionately to the level they originate from. For example, cells originating from the “higher” L 4  level will generally change more by an erase pulse and the potential applied to the cell than will those cells originating from “lower” levels such as the L 3 , L 2 , and L 1  levels. Thus, subsequent to one or more erase pulses,  FIG. 6  shows that the cells have been shifted “down”, and that the cells originating from the L 3  and L 4  have shifted a greater percentage than those originating from the L 1  and L 2  levels.  
         [0053]     Invariably, one or more cells (e.g., Cell 2 ) may have a marginally greater post-erase threshold and require more erase pulses (or more erasing potential) to finally arrive at the data state corresponding to the L 2  level. However, as the erase operation  410  is a bulk operation, Cell 5  and Cell 6  originating from L 1  may have, as a consequence, become over-erased (e.g., Vt&lt;FV) as illustrated in  FIG. 6  before Cell 2  is sufficiently erased to the L 2  level. Such cells require some level of programming and/or soft programming to correct the over-erased condition, as is addressed next by  420 .  
         [0054]      FIG. 7  illustrates the results  700  of the soft programming operation  420  of  FIG. 4A  of the first phase of method  400 . Over-erased cells Cell 5  and Cell 6 , for example, are soft-programmed (see, e.g.,  FIG. 4C ) to restore them back to the final value FV (e.g., to L 1  of  FIG. 1 ).  
         [0055]      FIG. 8  illustrates the results  800  of the programming operation  430  of the first phase of method  400  of  FIG. 4A . At  430  all the remaining final value FV cells, for example, those at or near L 1  including Cell 3 , Cell 4 , Cell 5 , Cell 6 , are programmed to the intermediate value to bring all cells of the sector to a single state (e.g., to L 2 ). As discussed, this may be accomplished by iteratively verifying a cell and applying program pulses to a cell until the cell achieves the intermediate value IV (e.g., L 2  of  FIG. 1 ). At this point, and the end of the first phase, all the cells have been brought to the same intermediate value state, and have a modest Vt distribution sigma, as illustrated in  FIG. 8 .  
         [0056]      FIG. 9  illustrates the results  900  of the second erase operation  440  of the second phase of method  400  of  FIG. 4A . In the second phase the sigma of the Vt distribution is further improved (decreased). At  440  all the memory cells within the selected sector or sectors of the array (Cell 1 -Cell 6 ) are again erased, but this time to the final value FV (e.g., to L 1 ). All the memory cells of a sector receive an iterative application of erase pulses (see, e.g.,  FIG. 4E ) until it is determined that all the memory cells have been at least erased to the final value FV (e.g., to L 1  in  FIG. 1 ). Once again, however, this may leave a cell over-erased, as Cell 5  indicates, if it is more responsive to the applied erase voltages than other cells in the sector.  
         [0057]      FIG. 10  illustrates the results  1000  of the second soft programming operation  450  of the second phase of method  400 . At  450 , the over-erased cells (e.g., Vt&lt;FV) from the erase operation of  440 , are once again soft-programmed to restore them to the final value FV (e.g., to L 1 ). Cell 5 , for example, may be iteratively verified and soft programmed with a soft program pulse and re-verified until the cell is determined to be restored to the final value FV (e.g., L 1  of  FIG. 1 ).  
         [0058]     Thereafter, the method  400  ends having erased all the exemplary MLB flash memory cells (Cell 1 -Cell 6 ) of the one or more sectors of the array to the same data state and to within a narrow Vt population distribution at an erased state centered about the final value FV (e.g., L 1 ).  
         [0059]     Note, that further phases may be used at selected additional intermediate level values between the intermediate value IV and the final value FV. For example, if eight-level MLB memory cells had been used, a first IV may have been established at L 6 , a second IV at L 4 , a third IV at L 2 , and the final value FV at L 1 . In another example, if eight-level MLB memory cells had been used, a first IV may have been established at L 3 , a second IV at L 5 , and a final value FV at L 6  as the erased state.  
         [0060]     Although the invention has been shown and described with respect to one or more implementations, equivalent alterations and modifications will occur to others skilled in the art based upon a reading and understanding of this specification and the annexed drawings. The invention includes all such modifications and alterations and is limited only by the scope of the following claims. In particular regard to the various functions performed by the above described components (assemblies, devices, circuits, etc.), the terms (including a reference to a “means”) used to describe such components are intended to correspond, unless otherwise indicated, to any component which performs the specified function of the described component (i.e., that is functionally equivalent), even though not structurally equivalent to the disclosed structure which performs the function in the herein illustrated exemplary implementations of the invention. In addition, while a particular feature of the invention may have been disclosed with respect to only one of several implementations, such feature may be combined with one or more other features of the other implementations as may be desired and advantageous for any given or particular application. Furthermore, to the extent that the terms “includes”, “having”, “has”, “with”, or variants thereof are used in either the detailed description or the claims, such terms are intended to be inclusive in a manner similar to the term “comprising.”