Patent Publication Number: US-11036411-B2

Title: Yield improvement through block budget optimization by using a transient pool of multi-level blocks

Description:
BACKGROUND 
     The present technology relates to the operation of storage and memory devices. 
     Semiconductor memory devices have become more popular for use in various electronic devices. For example, non-volatile semiconductor memory is used in cellular telephones, digital cameras, personal digital assistants, mobile computing devices, non-mobile computing devices and other devices. 
     A charge-storing material such as a floating gate or a charge-trapping material can be used in such memory devices to store a charge which represents a data state. A charge-trapping material can be arranged vertically in a three-dimensional (3D) stacked memory structure, or horizontally in a two-dimensional (2D) memory structure. One example of a 3D memory structure is the Bit Cost Scalable (BiCS) architecture which comprises a stack of alternating conductive and dielectric layers. 
     A memory device includes memory cells which may be arranged in series, in NAND strings (e.g., NAND chains), for instance, where select gate transistors are provided at the ends of a NAND string to selectively connect a channel of the NAND string to a source line or bit line. However, various challenges are presented in operating such memory devices. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of an example storage device. 
         FIG. 2  is a block diagram of the example storage device  100  of  FIG. 1 , depicting additional details of the controller  122 . 
         FIG. 3  is a perspective view of a memory device  500  comprising a set of blocks in an example 3D configuration of the memory structure  126  of  FIG. 1 . 
         FIG. 4  depicts an example transistor  420 . 
         FIG. 5  depicts an example view of NAND strings in a block BLK 0  which is consistent with  FIG. 3 . 
         FIG. 6A  depicts an example Vth distribution of memory cells, where eight data states are used in an example of MLC programming. 
         FIG. 6B  depicts an example Vth distribution of memory cells, where two data states are used in SLC programming. 
         FIG. 7A  depicts an example of file system at a host and at a memory device. 
         FIG. 7B  depicts a first example of a block budget consistent with  FIG. 7A . 
         FIG. 7C  depicts a second example of a block budget consistent with  FIG. 7A . 
         FIG. 7D  depicts an example table of multi-mode block status consistent with  FIG. 7C . 
         FIG. 8A  depicts an example process for programming data to SLC blocks in the pool  721  or  723  of  FIG. 7A , then copying the data to MLC blocks in the pool  722 . 
         FIG. 8B  depicts the example host file system table  701  of  FIG. 7A  which is updated consistent with step  800  of  FIG. 8A . 
         FIG. 8C  depicts the example memory device file system table  711  of  FIG. 7A  which is updated consistent with step  802  of  FIG. 8A . 
         FIG. 8D  depicts the example memory device file system table  711  of  FIG. 7A  which is updated consistent with step  805  of  FIG. 8A . 
         FIG. 8E  depicts an example of copying data from SLC blocks to an MLC block, consistent with  FIG. 8A   
         FIG. 9A  depicts an example process for allocating blocks in a memory device. 
         FIG. 9B  depicts an example of a first set of single-mode blocks  910  and a second set of multi-mode blocks  912 , consistent with  FIG. 9A . 
         FIG. 9C  depicts an example process for transitioning multi-mode blocks between MLC (N bits per cell) mode and SLC (or M bits per cell) mode, consistent with  FIG. 9A . 
         FIG. 9D  depicts an example process for selecting a multi-mode block to transition from the MLC (N bits per cell) mode to the SLC (or M bits per cell) mode, consistent with  FIG. 9A . 
         FIG. 9E  depicts an example process for selecting a multi-mode block to program in the SLC (or M bits per cell) mode, consistent with  FIG. 9A . 
         FIG. 9F  depicts an example process for selecting a multi-mode block to program in the MLC (or N bits per cell) mode, consistent with  FIG. 9A . 
         FIG. 9G  depicts an example process for selecting a single-mode block or a multi-mode block to program in the SLC (or M bits per cell) mode, consistent with  FIG. 9A . 
         FIG. 9H  depicts an example process for maintaining first and second P-E cycle counters, consistent with step  904  of  FIG. 9A . 
         FIG. 9I  depicts an example process for a single P-E cycle counter, consistent with step  905  of  FIG. 9A . 
         FIG. 10A  depicts a plot of an error rate versus a number of P-E cycles for blocks in an SLC mode. 
         FIG. 10B  depicts a plot of an error rate versus a number of P-E cycles for blocks in an MLC mode. 
         FIG. 10C  depicts a plot of an increment of a P-E cycle counter versus a number of P-E cycles for blocks in an SLC or MLC mode. 
     
    
    
     DETAILED DESCRIPTION 
     Apparatuses and techniques are described for more efficiently allocating blocks of data in a memory device. 
     In a memory device, memory cells can be arranged in blocks such as depicted in  FIG. 3 . The memory cells can be joined to one another, e.g., in NAND strings, such as depicted in  FIG. 5 . Further, the memory cells can be arranged in a 2D or 3D structure. In a 3D memory structure, the memory cells may be arranged in vertical NAND strings in a stack, where the stack comprises alternating conductive and dielectric layers. The conductive layers act as word lines which are connected to the memory cells. Each NAND string may have the shape of a pillar which intersects with the word lines to form the memory cells. In a 2D memory structure, the memory cells may be arranged in horizontal NAND strings on a substrate. 
     The memory cells in a block can be subject to program, read and erase operations. A programming operation may include one or more sets of increasing program voltages or pulses which are applied to a word line in respective program loops or program-verify iterations. Verify tests may be performed after each program voltage to determine whether the memory cells have completed programming. When programming is completed for a memory cell, it can be locked out from further programming while programming continues for other memory cells in subsequent program loops. 
     Each memory cell may be associated with a data state according to write data in a program command. Based on its data state, a memory cell will either remain in the erased (Er) state or be programmed to a programmed data state. For example, in a one bit per cell block, also referred to as a SLC (single level cell) block, there are two data states including the erased state and the programmed state, as depicted in  FIG. 6B . In a two-bit per cell block, there are four data states including the erased state and three programmed data states referred to as the A, B and C data states. In a three-bit per cell block, there are eight data states including the erased state and seven programmed data states referred to as the A, B, C, D, E, F and G data states, as depicted in  FIG. 6A . In a four-bit per cell block, there are sixteen data states including the erased state S0 and fifteen programmed data states S1-S15. Each data state can be represented by a range of threshold voltages (Vth) in the memory cells. A block with two or more bits per cell is referred to as an MLC (multi-level cell) block. 
     After the memory cells are programmed, the data can be read back in a read operation. A read operation can involve applying a series of read voltages to a word line while sensing circuitry determines whether cells connected to the word line are in a conductive (turned on) or non-conductive (turned off) state. If a cell is in a non-conductive state, the Vth of the memory cell exceeds the read voltage. The read voltages are set at levels which are expected to be between the threshold voltage levels of adjacent data states. Moreover, during the read operation, the voltages of the unselected word lines are ramped up to a read pass level or turn on level which is high enough to place the unselected memory cells in a strongly conductive state, to avoid interfering with the sensing of the selected memory cells. A word line which is being programmed or read is referred to as a selected word line, WLn. 
     The various blocks in a memory device can be allocated between SLC and MLC blocks according to the intended application of the memory device. SLC blocks have a higher reliability and endurance while MLC blocks have a higher data density but lower endurance. SLC blocks can be used, e.g., as control blocks, which typically require high reliability, or random data blocks or backup blocks, which typically require high endurance. MLC blocks are used primarily as capacity blocks, e.g., blocks which provide the primary long-term storage capacity of the memory device. 
     The MLC blocks are allocated to achieve a desired storage capacity. However, the requirement to provide a specified number of SLC blocks with a specified endurance, along with the loss of blocks which are determined to be bad at the time of manufacture, is problematic. For products with high product endurance requirements, the maximum number of bad blocks is limited by the SLC endurance. For example, in systems that support highly random write cycling or have to support a data snapshot or a 100% data backup to counter defects, the number of SLC blocks allocated is relatively high and limits the number of blocks which can be allocated to the MLC mode. One example of such a system is an automotive storage device product. 
     Techniques provided herein address the above and other issues. In one approach, the number of dedicated SLC blocks which are allocated at the time of manufacture of a memory device can be reduced by transitioning a portion of the MLC blocks to an SLC mode at various times in the lifetime of the memory device. Dedicated SLC blocks are blocks which operate in the SLC mode only throughout their lifetime. The program-erase (P-E) cycles of the MLC mode are therefore re-allocated to the SLC mode. Moreover, the re-allocation is controlled so that a uniform number of P-E cycles are re-allocated from each MLC block. This results in a deterministic reduction in the number of allocated SLC blocks. 
     Additionally, the P-E cycles of the MLC blocks while operating in the SLC mode are accounted for so that the endurance of the MLC blocks is not unnecessarily reduced. In one approach, separate counts are maintained for an MLC block in the SLC and MLC modes. The separate counts can be used to select an MLC block to transition to the SLC mode, or to select an MLC block to program. In another approach, a single count is maintained, where the SLC cycles are weighted less heavily than the MLC cycles. The techniques can be extended generally to M bit per cell blocks (which can include SLC blocks) and N bit per cell blocks (which include MLC blocks), where N and M are integers and N&gt;M. 
     These and other features are discussed further below. 
       FIG. 1  is a block diagram of an example storage device. The storage device  100 , such as a non-volatile storage system, may include one or more memory die  108 . The memory die  108 , or chip, includes a memory structure  126  of memory cells, such as an array of memory cells, control circuitry  110 , and read/write circuits  128 . The memory structure  126  is addressable by word lines via a row decoder  124  and by bit lines via a column decoder  132 . The read/write circuits  128  include multiple sense blocks  51 ,  52 , . . .  53  (sensing circuitry) and allow a page of memory cells to be read or programmed in parallel. Typically a controller  122  is included in the same storage device  100  (e.g., a removable storage card) as the one or more memory die  108 . The controller may be separate from the memory die. Commands and data are transferred between the host  140  and controller  122  via a data bus  120 , and between the controller and the one or more memory die  108  via lines  118 . 
     The memory structure can be 2D or 3D. The memory structure may comprise one or more array of memory cells including a 3D array. The memory structure may comprise a monolithic 3D memory structure in which multiple memory levels are formed above (and not in) a single substrate, such as a wafer, with no intervening substrates. The memory structure may comprise any type of non-volatile memory that is monolithically formed in one or more physical levels of arrays of memory cells having an active area disposed above a silicon substrate. The memory structure may be in a non-volatile memory device having circuitry associated with the operation of the memory cells, whether the associated circuitry is above or within the substrate. 
     The control circuitry  110  cooperates with the read/write circuits  128  to perform memory operations on the memory structure  126 , and includes a state machine, an on-chip address decoder  114 , a power control module  115  (power control circuit), an SLC P-E cycle tracking circuit  116 , an MLC P-E cycle tracking circuit (for MLC mode)  117 , an MLC P-E cycle tracking circuit (for SLC mode)  119 , and an MLC P-E cycle tracking circuit (for SLC and MLC modes)  119 . A storage region  113  may be provided, e.g., for operational parameters and software/code. In one embodiment, the state machine is programmable by the software. In other embodiments, the state machine does not use software and is completely implemented in hardware (e.g., electrical circuits). 
     The on-chip address decoder  114  provides an address interface between that used by the host or a memory controller to the hardware address used by the decoders  124  and  132 . The power control module  115  controls the power and voltages supplied to the word lines, select gate lines, bit lines and source lines during memory operations. It can include drivers for word lines, SGS and SGD transistors and source lines. The sense blocks can include bit line drivers, in one approach. The SLC P-E cycle tracking circuit  116  is provided for each dedicated SLC block. In one option, the MLC P-E cycle tracking circuit (for MLC mode)  117  and the MLC P-E cycle tracking circuit (for SLC mode)  119  are separate counters which are provided for each MLC block. In another option, the MLC P-E cycle tracking circuit (for SLC and MLC modes)  119  is a single counter which is provided for each MLC block. 
     In some implementations, some of the components can be combined. In various designs, one or more of the components (alone or in combination), other than memory structure  126 , can be thought of as at least one control circuit which is configured to perform the techniques described herein including the steps of the processes described herein. For example, a control circuit may include any one of, or a combination of, control circuitry  110 , state machine  112 , decoders  114  and  132 , power control module  115 , tracking circuits  116 ,  117 ,  119  and  121 , sense blocks  51 ,  52 , . . . ,  53 , read/write circuits  128 , controller  122 , and so forth. 
     The off-chip controller  122  (which in one embodiment is an electrical circuit) may comprise a processor  122   e , memory such as ROM  122   a  and RAM  122   b  and an error-correction code (ECC) engine  245 . The ECC engine can correct a number of read errors. The RAM  122   b  can be a DRAM, for instance. A copy of data to be programmed is received from the host and stored temporarily in the RAM until the programming is successfully completed to blocks in the memory device. The RAM may store one or more word lines of data. In one approach, as discussed further in connection with  FIG. 8A , the data is first stored in SLC blocks (e.g., dedicated SLC blocks or MLC blocks operating in SLC mode) and then transferred to MLC blocks operating MLC mode. The write time of the host device is minimized by first storing the data in the SLC blocks which can be written in less time than MLC blocks operating MLC mode. 
     A memory interface  122   d  may also be provided. The memory interface, in communication with ROM, RAM and processor, is an electrical circuit that provides an electrical interface between controller and memory die. For example, the memory interface can change the format or timing of signals, provide a buffer, isolate from surges, latch I/O and so forth. The processor can issue commands to the control circuitry  110  (or any other component of the memory die) via the memory interface  122   d.    
     The memory in the controller  122 , such as such as ROM  122   a  and RAM  122   b , comprises code such as a set of instructions, and the processor is operable to execute the set of instructions to provide the functionality described herein. Alternatively or additionally, the processor can access code from a subset  126   a  of the memory structure  126 , such as a reserved area of memory cells in one or more word lines. 
     For example, code can be used by the controller to access the memory structure such as for programming, read and erase operations. The code can include boot code and control code (e.g., a set of instructions). The boot code is software that initializes the controller during a booting or startup process and enables the controller to access the memory structure. The code can be used by the controller to control one or more memory structures. Upon being powered up, the processor  122   e  fetches the boot code from the ROM  122   a  or the subset  126   a  of the memory structure for execution, and the boot code initializes the system components and loads the control code into the RAM  122   b . Once the control code is loaded into the RAM, it is executed by the processor. The control code includes drivers to perform basic tasks such as controlling and allocating memory, prioritizing the processing of instructions, and controlling input and output ports. 
     Generally, the control code can include instructions to perform the functions described herein including the steps of the flowcharts discussed further below, and provide the voltage waveforms including those discussed further below. A control circuit can be configured to execute the instructions to perform the functions described herein. 
     In one embodiment, the host is a computing device (e.g., laptop, desktop, smartphone, tablet, digital camera) that includes one or more processors, one or more processor readable memory devices (RAM, ROM, flash memory, hard disk drive, solid-state memory) that store processor readable code (e.g., software) for programming the one or more processors to perform the methods described herein. The host may also include additional system memory, one or more input/output interfaces and/or one or more input/output devices in communication with the one or more processors. 
     Other types of non-volatile memory in addition to NAND flash memory can also be used. 
     Semiconductor memory devices include volatile memory devices, such as dynamic random access memory (DRAM) or static random access memory (SRAM) devices, non-volatile memory devices, such as resistive random access memory (ReRAM), electrically erasable programmable read-only memory (EEPROM), flash memory (which can also be considered a subset of EEPROM), ferroelectric random access memory (FRAM), and magnetoresistive random access memory (MRAM), and other semiconductor elements capable of storing information. Each type of memory device may have different configurations. For example, flash memory devices may be configured in a NAND or a NOR configuration. 
     The memory devices can be formed from passive and/or active elements, in any combinations. By way of non-limiting example, passive semiconductor memory elements include ReRAM device elements, which in some embodiments include a resistivity switching storage element, such as an anti-fuse or phase change material, and optionally a steering element, such as a diode or transistor. Further by way of non-limiting example, active semiconductor memory elements include EEPROM and flash memory device elements, which in some embodiments include elements containing a charge storage region, such as a floating gate, conductive nanoparticles, or a charge storage dielectric material. 
     Multiple memory elements may be configured so that they are connected in series or so that each element is individually accessible. By way of non-limiting example, flash memory devices in a NAND configuration (NAND memory) typically contain memory elements connected in series. A NAND string is an example of a set of series-connected transistors comprising memory cells and SG transistors. 
     A NAND memory array may be configured so that the array is composed of multiple strings of memory in which a string is composed of multiple memory elements sharing a single bit line and accessed as a group. Alternatively, memory elements may be configured so that each element is individually accessible, e.g., a NOR memory array. NAND and NOR memory configurations are examples, and memory elements may be otherwise configured. 
     The semiconductor memory elements located within and/or over a substrate may be arranged in two or three dimensions, such as a 2D memory structure or a 3D memory structure. 
     In a 2D memory structure, the semiconductor memory elements are arranged in a single plane or a single memory device level. Typically, in a 2D memory structure, memory elements are arranged in a plane (e.g., in an x-y direction plane) which extends substantially parallel to a major surface of a substrate that supports the memory elements. The substrate may be a wafer over or in which the layer of the memory elements are formed or it may be a carrier substrate which is attached to the memory elements after they are formed. As a non-limiting example, the substrate may include a semiconductor such as silicon. 
     The memory elements may be arranged in the single memory device level in an ordered array, such as in a plurality of rows and/or columns. However, the memory elements may be arrayed in non-regular or non-orthogonal configurations. The memory elements may each have two or more electrodes or contact lines, such as bit lines and word lines. 
     A 3D memory array is arranged so that memory elements occupy multiple planes or multiple memory device levels, thereby forming a structure in three dimensions (i.e., in the x, y and z directions, where the z direction is substantially perpendicular and the x and y directions are substantially parallel to the major surface of the substrate). 
     As a non-limiting example, a 3D memory structure may be vertically arranged as a stack of multiple 2D memory device levels. As another non-limiting example, a 3D memory array may be arranged as multiple vertical columns (e.g., columns extending substantially perpendicular to the major surface of the substrate, i.e., in the y direction) with each column having multiple memory elements. The columns may be arranged in a 2D configuration, e.g., in an x-y plane, resulting in a 3D arrangement of memory elements with elements on multiple vertically stacked memory planes. Other configurations of memory elements in three dimensions can also constitute a 3D memory array. 
     By way of non-limiting example, in a 3D NAND memory array, the memory elements may be coupled together to form a NAND string within a single horizontal (e.g., x-y) memory device level. Alternatively, the memory elements may be coupled together to form a vertical NAND string that traverses across multiple horizontal memory device levels. Other 3D configurations can be envisioned wherein some NAND strings contain memory elements in a single memory level while other strings contain memory elements which span through multiple memory levels. 3D memory arrays may also be designed in a NOR configuration and in a ReRAM configuration. 
     Typically, in a monolithic 3D memory array, one or more memory device levels are formed above a single substrate. Optionally, the monolithic 3D memory array may also have one or more memory layers at least partially within the single substrate. As a non-limiting example, the substrate may include a semiconductor such as silicon. In a monolithic 3D array, the layers constituting each memory device level of the array are typically formed on the layers of the underlying memory device levels of the array. However, layers of adjacent memory device levels of a monolithic 3D memory array may be shared or have intervening layers between memory device levels. 
     2D arrays may be formed separately and then packaged together to form a non-monolithic memory device having multiple layers of memory. For example, non-monolithic stacked memories can be constructed by forming memory levels on separate substrates and then stacking the memory levels atop each other. The substrates may be thinned or removed from the memory device levels before stacking, but as the memory device levels are initially formed over separate substrates, the resulting memory arrays are not monolithic 3D memory arrays. Further, multiple 2D memory arrays or 3D memory arrays (monolithic or non-monolithic) may be formed on separate chips and then packaged together to form a stacked-chip memory device. 
     Associated circuitry is typically required for operation of the memory elements and for communication with the memory elements. As non-limiting examples, memory devices may have circuitry used for controlling and driving memory elements to accomplish functions such as programming and reading. This associated circuitry may be on the same substrate as the memory elements and/or on a separate substrate. For example, a controller for memory read-write operations may be located on a separate controller chip and/or on the same substrate as the memory elements. 
     One of skill in the art will recognize that this technology is not limited to the 2D and 3D exemplary structures described but covers all relevant memory structures within the spirit and scope of the technology as described herein and as understood by one of skill in the art. 
       FIG. 2  is a block diagram of the example storage device  100  of  FIG. 1 , depicting additional details of the controller  122 . As used herein, a flash memory controller is a device that manages data stored on flash memory and communicates with a host, such as a computer or electronic device. A flash memory controller can have various functionality in addition to the specific functionality described herein. For example, the flash memory controller can format the flash memory to ensure the memory is operating properly, map out bad flash memory cells, and allocate spare memory cells to be substituted for future failed cells. Some part of the spare cells can be used to hold firmware to operate the flash memory controller and implement other features. In operation, when a host needs to read data from or write data to the flash memory, it will communicate with the flash memory controller. If the host provides a logical address to which data is to be read/written, the flash memory controller can convert the logical address received from the host to a physical address in the flash memory. (Alternatively, the host can provide the physical address). The flash memory controller can also perform various memory management functions, such as, but not limited to, wear leveling (distributing writes to avoid wearing out specific blocks of memory that would otherwise be repeatedly written to) and garbage collection (after a block is full, moving only the valid pages of data to a new block, so the full block can be erased and reused). 
     The interface between the controller  122  and non-volatile memory die  108  may be any suitable flash interface. In one embodiment, the storage device  100  may be a card based system, such as a secure digital (SD) or a micro secure digital (micro-SD) card. In an alternate embodiment, the memory system may be part of an embedded memory system. For example, the flash memory may be embedded within the host, such as in the form of a solid-state disk (SSD) drive installed in a personal computer. 
     In some embodiments, the storage device  100  includes a single channel between the controller  122  and the non-volatile memory die  108 , the subject matter described herein is not limited to having a single memory channel. 
     The controller  122  includes a front end module  208  that interfaces with a host, a back end module  210  that interfaces with the one or more non-volatile memory die  108 , and various other modules that perform functions which will now be described in detail. 
     The components of the controller may take the form of a packaged functional hardware unit (e.g., an electrical circuit) designed for use with other components, a portion of a program code (e.g., software or firmware) executable by a processor, e.g., microprocessor, or processing circuitry that usually performs a particular function of related functions, or a self-contained hardware or software component that interfaces with a larger system, for example. For example, each module may include an Application Specific Integrated Circuit (ASIC), a Field Programmable Gate Array (FPGA), a circuit, a digital logic circuit, an analog circuit, a combination of discrete circuits, gates, or any other type of hardware or combination thereof. Alternatively or in addition, each module may include software stored in a processor readable device (e.g., memory) to program a processor for the controller to perform the functions described herein. The architecture depicted in  FIG. 2  is one example implementation that may (or may not) use the components of the controller  122  depicted in  FIG. 1  (e.g., RAM, ROM, processor, interface). 
     The controller  122  may include recondition circuitry  212 , which is used for reconditioning memory cells or blocks of memory. The reconditioning may include refreshing data in its current location or reprogramming data into a new word line or block as part of performing erratic word line maintenance, as described below. 
     Referring again to modules of the controller  122 , a buffer manager/bus controller  214  manages buffers in random access memory (RAM)  216  and controls the internal bus arbitration of Controller  122 . The RAM may include DRAM and/or SRAM. DRAM or Dynamic Random Access Memory is a type of semiconductor memory in which the memory is stored in the form of a charge. Each memory cell in a DRAM is made of a transistor and a capacitor. The data is stored in the capacitor. Capacitors lose charge due to leakage and hence DRAMs are volatile devices. To keep the data in the memory, the device must be regularly refreshed. In contrast, SRAM or Static Random Access Memory will retain a value as long as power is supplied. 
     A read only memory (ROM)  218  stores system boot code. Although illustrated in  FIG. 2  as being located separately from the controller, in other embodiments, one or both of the RAM  216  and ROM  218  may be located within the controller. In yet other embodiments, portions of RAM and ROM may be located both within the controller  122  and outside the controller. Further, in some implementations, the controller  122 , RAM  216 , and ROM  218  may be located on separate semiconductor die. 
     Front end module  208  includes a host interface  220  and a physical layer interface (PHY)  222  that provide the electrical interface with the host or next level storage controller. The choice of the type of host interface  220  can depend on the type of memory being used. Examples of host interfaces  220  include, but are not limited to, SATA, SATA Express, SAS, Fibre Channel, USB, PCIe, and NVMe. The host interface  220  typically facilitates transfer for data, control signals, and timing signals. 
     Back end module  210  includes an error correction controller (ECC) engine  224  that encodes the data bytes received from the host, and decodes and error corrects the data bytes read from the non-volatile memory. A command sequencer  226  generates command sequences, such as program and erase command sequences, to be transmitted to non-volatile memory die  108 . A RAID (Redundant Array of Independent Dies) module  228  manages generation of RAID parity and recovery of failed data. The RAID parity may be used as an additional level of integrity protection for the data being written into the storage device  100 . In some cases, the RAID module  228  may be a part of the ECC engine  224 . Note that the RAID parity may be added as an extra die or dies as implied by the common name, but it may also be added within the existing die, e.g. as an extra plane, or extra block, or extra word lines within a block. A memory interface  230  provides the command sequences to non-volatile memory die  108  and receives status information from the non-volatile memory die. A flash control layer  232  controls the overall operation of back end module  210 . 
     Additional components of storage device  100  include media management layer  238 , which performs wear leveling of memory cells of non-volatile memory die  108 . The memory system also includes other discrete components  240 , such as external electrical interfaces, external RAM, resistors, capacitors, or other components that may interface with Controller  122 . In alternative embodiments, one or more of the physical layer interface  222 , RAID module  228 , media management layer  238  and buffer management/bus controller  214  are optional components that are not necessary in the Controller  122 . 
     The Flash Translation Layer (FTL) or Media Management Layer (MML)  238  may be integrated as part of the flash management that may handle flash errors and interfacing with the host. In particular, MML may be a module in flash management and may be responsible for the internals of NAND management. In particular, the MML  238  may include an algorithm in the memory device firmware which translates writes from the host into writes to the memory structure  126 , e.g., flash memory, of die  108 . The FTL may implement the file system tables discussed further below. The MML  238  may be needed because: 1) the flash memory may have limited endurance; 2) the flash memory may only be written in multiples of pages; and/or 3) the flash memory may not be written unless it is erased as a block. The MML  238  understands these potential limitations of the flash memory which may not be visible to the host. Accordingly, the MML  238  attempts to translate the writes from host into writes into the flash memory. Erratic bits may be identified and recorded using the MML  238 . This recording of erratic bits can be used for evaluating the health of blocks and/or word lines (the memory cells on the word lines). 
     The controller  122  may interface with one or more memory dies  108 . In in one embodiment, the controller and multiple memory dies (together comprising the storage device  100 ) implement a solid-state drive (SSD), which can emulate, replace or be used instead of a hard disk drive inside a host, as a network-attached storage (NAS) device, and so forth. Additionally, the SSD need not be made to work as a hard drive. 
       FIG. 3  is a perspective view of a memory device  300  comprising a set of blocks in an example 3D configuration of the memory structure  126  of  FIG. 1 . On the substrate are example blocks BLK 0 , BLK 1 , BLK 2  and BLK 3  of memory cells (storage elements) and peripheral areas with circuitry for use by the blocks. The peripheral area  304  runs along an edge of each block while the peripheral area  305  is at an end of the set of blocks. The circuitry can include voltage drivers which can be connected to control gate layers, bit lines and source lines of the blocks. In one approach, control gate layers at a common height in the blocks are commonly driven. The substrate  301  can also carry circuitry under the blocks, and one or more lower metal layers which are patterned in conductive paths to carry signals of the circuitry. The blocks are formed in an intermediate region  302  of the memory device. In an upper region  303  of the memory device, one or more upper metal layers are patterned in conductive paths to carry signals of the circuitry. Each block comprises a stacked area of memory cells, where alternating levels of the stack represent word lines. In one possible approach, each block has opposing tiered sides from which vertical contacts extend upward to an upper metal layer to form connections to conductive paths. While four blocks are depicted as an example, two or more blocks can be used, extending in the x- and/or y-directions. 
     In one possible approach, the blocks are in a plane, and the length of the plane, in the x-direction, represents a direction in which signal paths to word lines extend in the one or more upper metal layers (a word line or SGD line direction), and the width of the plane, in the y-direction, represents a direction in which signal paths to bit lines extend in the one or more upper metal layers (a bit line direction). The z-direction represents a height of the memory device. The blocks could also be arranged in multiple planes. 
       FIG. 4  depicts an example transistor  420 . The transistor comprises a control gate CG, a drain D, a source S and a channel CH and may represent a memory cell or a select gate transistor, for example. The drain end of the transistor is connected to a bit line BL optionally via one or more other transistors in a NAND string, and the source end of the transistor is connected to a source line SL optionally via one or more other transistors in a NAND string, 
     In one approach, the block of memory cells comprises a stack of alternating control gate and dielectric layers, and the memory cells are arranged in vertically extending memory holes in the stack. 
     A number of layers can be deposited along sidewalls of the memory holes and/or within each word line layer to form memory cells. The layers can include a blocking oxide layer, a charge-trapping layer or film such as silicon nitride (Si3N4) or other nitride, a tunneling layer (e.g., a gate oxide) and a channel (e.g., comprising polysilicon). A dielectric core (e.g., comprising silicon dioxide) may fill a central core of each memory hole. 
     While the above example is directed to a 3D memory device with vertically extending NAND strings, the techniques provided herein are also applicable to a 2D memory device in which the NAND strings extend horizontally on a substrate. 
       FIG. 5  depicts an example view of NAND strings in a block BLK 0  which is consistent with  FIG. 3 . The NAND strings are arranged in sub-blocks of the block in a 3D configuration. Each sub-block includes multiple NAND strings, where one example NAND string is depicted. For example, SB 0 , SB 1 , SB 2  and SB 3  comprise example NAND strings  500   n ,  510   n ,  520   n  and  530   n , respectively. The NAND strings have data word lines, dummy word lines and select gate lines. Each sub-block comprises a set of NAND strings which extend in the x direction and which have a common SGD line or control gate layer. The NAND strings  500   n ,  510   n ,  520   n  and  530   n  are in sub-blocks SB 0 , SB 1 , SB 2  and SB 3 , respectively. Programming of the block may occur based on a word line programming order. One option is to program the memory cells in different portions of a word line which are in the different sub-blocks, one sub-block at a time, before programming the memory cells of the next word line. For example, this can involve programming WL 0  in SB 0 , SB 1 , SB 2  and then SB 2 , then programming WL 1  in SB 0 , SB 1 , SB 2  and then SB 2 , and so forth. The word line programming order may start at WL 0 , the source-end word line and end at WL 95 , the drain-end word line, for example. 
     The NAND strings  500   n ,  510   n ,  520   n  and  530   n  have channels  500   a ,  510   a ,  520   a  and  530   a , respectively. Additionally, NAND string  500   n  includes SGS transistor  501 , dummy memory cell  502 , data memory cells  503 - 514 , dummy memory cell  515  and SGD transistor  516 . NAND string  510   n  includes SGS transistor  521 , dummy memory cell  522 , data memory cells  523 - 534 , dummy memory cell  535  and SGD transistor  536 . NAND string  520   n  includes SGS transistor  541 , dummy memory cell  542 , data memory cells  543 - 554 , dummy memory cell  555  and SGD transistor  556 . NAND string  530   n  includes SGS transistor  561 , dummy memory cell  562 , data memory cells  563 - 574 , dummy memory cell  575  and SGD transistor  576 . 
     This example depicts one SGD transistor at the drain-end of each NAND string, and one SGS transistor at the source-end of each NAND string. The SGD transistors in SB 0 , SB 1 , SB 2  and SB 3  may be driven by separate control lines SGD( 0 ), SGD( 1 ), SGD( 2 ) and SGD( 3 ), respectively, in one approach. In another approach, multiple SGD and/or SGS transistors can be provided in a NAND string. 
       FIG. 6A  depicts an example Vth distribution of memory cells, where eight data states are used in an example of MLC programming. In  FIGS. 6A and 6B , the vertical axis depicts a number of memory cells on a logarithmic scale, and the horizontal axis depicts a Vth of the memory cells on a linear scale. In one approach, at a start of a program operation, the memory cells are all initially in the erased (Er) state, as represented by the Vth distribution  600 . The memory cells assigned to the Er state continue to be represented by the Vth distribution  600  after programming is completed. The memory cells assigned to the A-G states are programmed to the Vth distributions  601 - 607 , respectively, using verify voltages of VvA-VvG, respectively, in program-verify tests. Read voltages VrA-VrG can be used for reading the states of the memory cells in a read operation. 
     In an erase operation, the data memory cells transition from the Vth distributions of the programmed data states, e.g., states A-G, to the erased state. The erase operation includes an erase phase in which the memory cells are biased for erasing followed by an erase-verify test. The erase-verify test can use an erase-verify voltage, VvEr, which is applied to the word lines. 
     The Er-G states are examples of assigned data states, and the A-G states are examples of programmed data states, in this eight state example. The number of data states could be higher or low than eight data states. 
       FIG. 6B  depicts an example Vth distribution of memory cells, where two data states are used in SLC programming. An erased state Vth distribution  620  and a programmed state Vth distribution  621  are depicted. The verify voltage is Vv and the read voltage is Vr. 
       FIG. 7A  depicts an example of file system at a host and at a memory device. When the host reads data from the memory device, the host does not know where the data is stored in the memory device. Similarly, in a program operation, the host does not know the physical location in the memory device in which data will be written. Instead, the data is identified by the host using one or more logical addresses which are mapped to physical or virtual locations, e.g., block and word lines within the block, by a file system. 
     A host file system  700  includes one or more tables  701  which cross reference files of data to logical block addresses (LBAs). For example, the file may be divided into sectors. A file can be a video, image, word processing document, application or other data. See, e.g.,  FIG. 8B . 
     The memory device file system  710  includes one or more tables  711  which cross reference the LBAs to physical or virtual blocks and pages in the memory structure  126 . There can be a direct or indirect mapping from the LBAs to physical addresses. In an example of indirect mapping, the LBA of the host is cross-referenced to a virtual address in the memory device, and the virtual address in the memory device is cross-referenced to a physical address. An address which is mapped to or cross references another address is said to point to the other address. 
     The memory structure  126  can include an SLC pool  721 , an MLC pool  722  and a transient MLC-SLC pool  723 . The SLC pool includes dedicated SLC blocks which operate in the SLC mode throughout their lifetime. The lifetime of a block can be measured by the number of program-erase cycles in which it is expected to operate. The MLC pool includes blocks which operate in the MLC mode. The transient MLC-SLC pool includes MLC blocks operating in the SLC mode. As a generalization, the SLC pool can include blocks which are dedicated to operating in an M bit per cell mode, the MLC pool can include blocks which operate in an N bit per cell mode, and the transient MLC-SLC pool can include blocks operating in the M bit per cell mode, where N&gt;M. The pools  721  and  723  therefore comprise blocks operating in the SLC or M bit per cell mode. 
     In one approach, a fixed number of MLC blocks are allocated to the transient MLC-SLC pool so that a fixed number of blocks are in the SLC or M bit per cell mode throughout the lifetime of the memory device. The MLC blocks can cycle into and out of the transient MLC-SLC pool so that each MLC block is allocated to the MLC-SLC pool for an equal number (e.g., equal, or within a specified plus/minus tolerance) of P-E cycles in the SLC or M bit per cell mode. The MLC blocks can be operated so that the number of SLC or M bit per cell P-E cycles is equal (e.g., equal, or within a specified plus/minus tolerance) among the MLC blocks over the lifetime of the memory device. The MLC blocks can also be operated so that the number of MLC or N bit per cell P-E cycles is equal (e.g., equal, or within a specified plus/minus tolerance) among the MLC blocks over the lifetime of the memory device. The transient MLC-SLC pool can be relatively small in size, compared to the MLC pool and the SLC pool. These and other implementation details are discussed further below. 
       FIG. 7B  depicts a first example of a block budget consistent with  FIG. 7A . In this example, the memory structure has a total of 1,478 blocks. There are 115 dedicated SLC blocks, and a specified memory endurance of 100 k or 100,000 P-E cycles. The memory endurance refers to the maximum number of P-E cycles in which the block is expected to operate reliably, e.g., without uncorrectable read errors. The product endurance refers to the maximum number of P-E cycles in which the block is required to operate in a particular product, and this can be less than the memory endurance. The endurance of a memory cell is limited because the blocking oxide is degraded from the stress of the P-E cycles. This leads to an increase in a leakage current and a reduction in data retention. 
     There are 1,283 MLC capacity blocks which have a memory endurance of 5 k cycles and a product endurance of 3 k cycles. The endurance of MLC blocks is typically much lower than that of SLC blocks due to factors such as the use of higher program voltages and a longer programming time. There are 41 other blocks, such as spare blocks and control blocks, and the bad block limit is 39 blocks. 
       FIG. 7C  depicts a second example of a block budget consistent with  FIG. 7A . This example adds a category for the transient MLC-SLC pool  723  with two blocks. Advantageously, the number of dedicated SLC blocks can be decreased by 48 blocks to 67 blocks, and the bad block limit is increased by 46 blocks to 85 blocks. There are 41 other blocks as in  FIG. 7B . 
     Essentially, the P-E cycles which the MLC blocks would have performed in the MLC or N bits per cell mode are re-allocated as P-E cycles in the SLC or M bit per cell mode, so that the number of dedicated SLC blocks can be reduced. Moreover, this re-allocation can come from the extra P-E cycles which are based on the difference between the memory endurance and the product endurance. The difference is 5 k−3 k=2 k P-E cycles per block in this example. Moreover, a P-E cycle in MLC mode correlates with more than one P-E cycle in SLC mode since the amount of deterioration of the memory cells due to a P-E cycle in the SLC mode is less than the amount due to a P-E cycle in the MLC mode. As a result, the 2 k P-E cycles which are re-allocated from the MLC mode to the SLC mode can translate to perhaps 4 k SLC cycles. See, e.g.,  FIG. 10A-10C . By increasing the bad block limit, the yield of the memory device manufacturing process can be improved. 
       FIG. 7D  depicts an example table of multi-mode block status consistent with  FIG. 7C . A bit or other flag can be maintained in a table such as an inverted global address table (IGAT) by the controller or circuitry  110 , to identify whether an MLC block is in the SLC or MLC mode. In this example, the MLC blocks are numbered from 0 to 1283, one bit indicates whether the block is in the SLC or MLC mode (with a bit value of 0 or 1, respectively) and another bit indicates whether the block is retired (where a bit value of 0 or 1 corresponds to no or yes, respectively). A block is retired when it is at the end of its lifetime and is no longer used. A block may be retired when it reaches a threshold number of P-E cycles, for instance. In one approach, the retiring involves moving an identifier of the block from a list of active blocks to a list of retired blocks, which are not available for storing data. A pointer to the block can also be removed from the memory device file system table  711 . 
       FIG. 8A  depicts an example process for programming data to SLC blocks in the pool  721  or  723  of  FIG. 7A , then copying the data to MLC blocks in the pool  722 . As mentioned, one approach to using the SLC and MLC blocks is for data from the host to be written to the SLC blocks initially and then copied to the MLC blocks for long term storage, in a folding operation. It is desirable to store data which is received from a host in SLC blocks because the data can be written more quickly to these blocks. This allows for a high speed data transfer from the host such as in a burst mode. After the data is stored in one or more SLC blocks, it can be transferred to one or more MLC blocks. Typically, the data of multiple SLC blocks is transferred to one MLC block, such as depicted in  FIG. 8E . 
     Step  800  includes updating a host file system table to identify data to be written to a memory device. See  FIG. 8B . Step  801  includes transferring data from the host device to the memory device for writing in the SLC blocks of the memory device. This involves programming memory cells of one or more word lines of the SLC blocks. In one approach, the data is stored in one SLC block until it become full and then in a next SLC block, and so forth. Step  802  includes updating the memory device file system table, e.g., to cause the logical block addresses of the host to point to physical addresses of the SLC blocks. See  FIG. 8C . 
     A decision step  803  determines if the SLC blocks are full. This may involve determining if a set of SLC blocks is full, where this set contains data which can be copied to one MLC block. For example, this determination can be made by the controller  122  or circuitry  110  within the memory device ( FIG. 1 ). If decision step  803  is false, the process returns to step  801  to wait to receive additional data from the host to store in the SLC blocks. If decision step  803  is true, step  804  includes copying data from the SLC blocks to an MLC block. This involves reading memory cells of the SLC blocks and programming the read data into memory cells of the MLC block. Moreover, a page which is read from an SLC block can be stored as a lower, middle or upper page in the MLC block, assuming three bit cells are used in the MLC block. In one approach, data from three word lines of an SLC block is stored in one word line of an MLC block. This can be one physical word line or one virtual word line which comprises multiple physical word lines. Step  805  includes updating the memory device file system table to cause the logical block addresses of the host to point to physical addresses in the MLC blocks. See  FIG. 8D . 
       FIG. 8B  depicts the example host file system table  701  of  FIG. 7A  which is updated consistent with step  800  of  FIG. 8A . The table includes a column for a file name and a column for a logical address. The file names identifies different files and their sectors. Each sector points to a corresponding logical address. In this simplified example, the logical addresses are consecutive numbers, 0-8. Also, each file has the same number of sectors. In other cases, the logical addresses are non-consecutive and different files can have different numbers of sectors. File 1, sectors 0-2 are associated with logical addresses 0-2, respectively. File 2, sectors 0-2 are associated with logical addresses 3-5, respectively. File 3, sectors 0-2 are associated with logical addresses 6-8, respectively. 
       FIG. 8C  depicts the example memory device file system table  711  of  FIG. 7A  which is updated consistent with step  802  of  FIG. 8A . The table includes columns for the logical address of the host, and at the memory device, an SLC physical address and an MLC physical address. The data associated with logical addresses 0-2 at the host are stored at SLC block 0, pages 0-2, respectively. The data associated with logical addresses 3-5 at the host are stored at SLC block 1, pages 0-2, respectively. The data associated with logical addresses 6-8 at the host are stored at SLC block 2, pages 0-2, respectively. The data is not yet associated with an MLC block, as represented by the notation “n/a” (not applicable). 
       FIG. 8D  depicts the example memory device file system table  711  of  FIG. 7A  which is updated consistent with step  805  of  FIG. 8A . In this case, the data is no longer associated with an SLC block, as represented by the notation “n/a” (not applicable). The data associated with logical addresses 0-8 at the host are stored at MLC block 3, pages 0-8, respectively, in this example. 
       FIG. 8E  depicts an example of copying data from SLC blocks to an MLC block, consistent with  FIG. 8A . Each SLC block includes data on three word lines, as a simplified example, where one page of data is stored in each word line, in this example. The MLC block, block 3, also includes data on three word lines, where three pages of data are stored in each word line. In the SLC block 0, pages 0, 1 and 2 are stored in word lines  851 ,  852  and  853 , respectively. In the SLC block 1, pages 0, 1 and 2 are stored in word lines  854 ,  855  and  856 , respectively. In the SLC block 2, pages 0, 1 and 2 are stored in word lines  857 ,  858  and  859 , respectively. 
     When the SLC data is copied to the MLC block, pages 0, 1 and 2 of SLC block 0 are stored in a first word line  861  of MLC block 3 as pages 0, 1 and 2, respectively. Pages 0, 1 and 2 of SLC block 1 are stored in a second word line  862  of MLC block 3 as pages 3, 4, and 5, respectively. Pages 0, 1 and 2 of SLC block 2 are stored in a third word line  863  of MLC block 3 as pages 6, 7 and 8, respectively. The three pages of an SLC block may be stored as lower, middle and upper pages, respectively of a word line in the MLC block. 
       FIG. 9A  depicts an example process for allocating blocks in a memory device. Step  900  includes testing blocks of a memory device to identify bad blocks. For example, this can include writing data to a block and reading the data back. If there are uncorrectable errors, the block is declared a bad block. Step  901  includes, among the remaining blocks, allocating a first set of single-mode blocks, a second set of multi-mode blocks and spare blocks. See  FIG. 9B . For example, the single-mode blocks can be SLC blocks or M bit per cell blocks, where M&gt;=1 bit per cell, and the multi-mode blocks can be MLC blocks with N bits per cell, where N&gt;1 and N&gt;M. Step  902  includes, during the lifetime of the memory device, using the single-mode blocks in SLC (or M bits per cell) mode, and transitioning multi-mode blocks between MLC (N bits per cell) mode and SLC (or M bits per cell) mode, where M&lt;N. Various options are possible, as discussed further below. 
     Step  903  includes tracking the P-E cycles of the single-mode blocks. Step  904  or  905  may be used for the multi-mode blocks. Step  904  includes tracking the P-E cycles of the multi-mode blocks using separate counters (e.g., first and second counters, respectively) for the SLC (or M bits per cell) mode and the MLC (N bits per cell) mode. Step  905  includes tracking P-E cycles of the multi-mode blocks using a single counter which is incremented differently when in the SLC (or M bits per cell) mode compared to the MLC (N bits per cell) mode. The counter may be incremented by a smaller amount for a P-E cycle in the SLC mode compared to a P-E cycle in the MLC mode. See  FIG. 10C . 
     Tracking a multi-mode block with separate counters provides greater control of the use of the blocks since the separate counters maintain the exact number of P-E cycles for each mode and can be used to maintain the number of SLC and MLC P-E cycles within respective ranges. Tracking a multi-mode block with a single counter requires less overhead data to maintain the count. In either approach, the increased wear of a block which occurs with an MLC P-E cycle compared to an SLC P-E cycle can be accounted for. Additionally, the single or separate counts can be used to make decisions such as selecting an MLC block to transition from the MLC mode to the SLC mode, program in the SLC mode, program in the MLC mode, or retire. 
       FIG. 9B  depicts an example of a first set of single-mode blocks  910  and a second set of multi-mode blocks  912 , consistent with  FIG. 9A . The first set of single-mode blocks  910  includes single-mode blocks in the SLC (or M bits per cell) mode  911 . This could be the 67 SLC blocks in  FIG. 7C . The second set of blocks includes multi-mode blocks in the SLC (or M bits per cell) mode  913  (this could be the 2 transient MLS-SLC blocks in  FIG. 7C ) and multi-mode blocks in the MLC (N bits per cell) mode (this could be the 1,283 MLC capacity blocks in  FIG. 7C ). In one approach, the sizes of the first and second sets are fixed, and the number of blocks  913  and  914  are also fixed throughout the lifetime of the memory device, to provide a predictable number of P-E cycles in the SLC and MLC modes. Optionally, fewer or more multi-mode blocks in the SLC (or M bits per cell) mode can be used if appropriate throughout the lifetime of the memory device. 
       FIG. 9C  depicts an example process for transitioning multi-mode blocks between MLC (N bits per cell) mode and SLC (or M bits per cell) mode, consistent with  FIG. 9A . Step  917  begins the process for transitioning multi-mode blocks between MLC (or N bits per cell) mode and SLC (or M bits per cell) mode. At step  918 , a first option involves making the MLC blocks available for transitioning to the SLC mode at the start of the lifetime of the memory device, when the memory device is fresh and has no P-E cycles. With this option, the MLC blocks are repeatedly transitioned between the MLC and SLC modes. The MLC blocks are subject to wear in MLC and SLC modes at different times. The first option involves step  920 . Step  920  includes, when the memory device is fresh, allocating a number X multi-mode blocks for SLC (or M bits per cell) mode and the remaining multi-mode blocks for MLC (N bits per cell) mode. For example X=2 blocks in  FIG. 7C . 
     At step  919 , a second option involves using the MLC blocks in the MLC mode exclusively until they reach a threshold number of P-E cycles, such as 3 k P-E cycles. Subsequently, the MLC blocks can be used exclusively in the SLC mode until an end of their lifetime. Or they can be repeatedly transitioned between the MLC and SLC modes. This approach allows the MLC blocks to be used in the MLC mode when they are at an earlier part of their lifetime, and less degraded, and in an SLC mode at a later part of their lifetime. The second option involves steps  925  and  926 . Step  925  includes, when the memory device is fresh, allocating multi-mode blocks to MLC (N bits per cell) mode. Step  926  includes, when the multi-mode blocks reach a threshold number of P-E cycles, allocating X multi-mode blocks in turn for the SLC (or M bits per cell) mode. 
     In one approach, a selected block of the second set of blocks transitions from the N bit per cell mode to the M bit per cell mode a single time in a lifetime of the selected block and remains in the M bit per cell mode in a remainder of its lifetime. 
     Step  921  includes performing SLC (or M bits per cell) P-E cycles for the X multi-mode blocks. This can be done for a specified number of one or more P-E cycles or until a lifetime limit on P-E cycles is reached. Subsequently, in one option, step  922  includes transitioning the X multi-mode blocks back to MLC (N bits per cell) mode, where the blocks are available for re-allocation to the SLC (or M bits per cell) mode. In another option, step  923  includes marking the X multi-mode blocks are being retired. The X multi-mode blocks can be transitioned into the SLC mode and back to the MLC mode individually, at different times, or concurrently, as a group. Step  924  includes allocating X additional multi-mode blocks to the SLC (or M bits per cell) mode. 
     In this process, a selected block of the second set of blocks may remain in the M bit per cell mode for a predetermined number of multiple M bit per cell program-erase cycles. Also, a predetermined number (e.g., X) of blocks of the second set of blocks may be in the M bit per cell mode throughout a lifetime of the plurality of blocks as different blocks of the second set of blocks transition between the M bit per cell mode and the N bit per cell mode. 
     In one approach, each MLC block is used in the SLC or M bit per cell mode for 4 k consecutive cycles and then either returned to the MLC pool  722  or retired. The block exchange between the MLC pool  722  and the transient MLC-SLC pool occurs about 640 times in this example, with a transient MLC-SLC pool size of two blocks, before all of about 1,280 MLC blocks are equally used. During every block exchange, when these two blocks are exercised for 4 k cycles, the other blocks in the SLC pool will be cycled only about 100 k/640 times. This ratio of wear leveling across the SLC and the transient pool can be maintained to ensure that the dedicated SLC blocks are used for 100 k cycles and the transient blocks are used in the SLC or M bit per cell mode for 4 k cycles. 
       FIG. 9D  depicts an example process for selecting a multi-mode block to transition from the MLC (N bits per cell) mode to the SLC (or M bits per cell) mode, consistent with  FIG. 9A . From time to time, new MLC blocks can be selected to transition from the MLC mode to the SLC mode. Step  930  begins the process to select a multi-mode block to transition from the MLC (N bits per cell) mode, to the SLC (or M bits per cell) mode. Step  931  includes, among the multi-mode blocks currently in the MLC (N bits per cell) mode, select a multi-mode block with the lowest SLC (or M bits per cell) P-E counter or single P-E counter. Step  932  represents a tie-breaking process if there is a tie among the counts of different MLC blocks in step  931 . Step  932  includes, if the SLC (or M bits per cell) P-E counters are equal, select a multi-mode block with lowest MLC (N bits per cell) P-E counter. In this way, an MLC block with a least amount of degradation is selected. Step  933  includes using the selected multi-mode block in the SLC (or M bits per cell) mode. Step  934  includes, after a specified number of SLC (or M bits per cell) P-E cycles, transitioning the multi-mode block back to MLC (N bits per cell) mode or retiring the block. 
     In this process, a selected block is selected among the second set of blocks to transition from the N bit per cell mode to the M bit per cell mode based on its respective count of M bit per cell program-erase cycles being lower than respective counts of M bit per cell program-erase cycles of other blocks in the second set of blocks. 
     In step  932 , the selected block is selected among the second set of blocks to transition from the N bit per cell mode to the M bit per cell mode based on its respective count of M bit per cell program-erase cycles being equal to respective counts of M bit per cell program-erase cycles of other blocks in the second set of blocks and its respective count of N bit per cell program-erase cycles being lower than respective counts of N bit per cell program-erase cycles of other blocks in the second set of blocks. 
       FIG. 9E  depicts an example process for selecting a multi-mode block to program in the SLC (or M bits per cell) mode, consistent with  FIG. 9A . Once one or more MLC blocks are in the SLC (or M bits per cell) mode, step  940  begins a process to select a multi-mode block to program in the SLC (or M bits per cell) mode. The selection can be among the blocks  913  in  FIG. 9B . Step  941  includes, among the multi-mode blocks currently in the SLC (or M bits per cell) mode, selecting a multi-mode block with the lowest SLC (or M bits per cell) P-E counter or single P-E counter. Step  942  represents a tie-breaking process if there is a tie among the counts of different MLC blocks in step  941 . Step  942  includes, if the SLC (or M bits per cell) P-E counters are equal among the multi-mode blocks currently in the SLC (or M bits per cell) mode, selecting a multi-mode block with lowest MLC (N bits per cell) P-E counter. In this way, an MLC block with a least amount of degradation is selected. 
     In this process, a control circuit is configured to maintain a respective count of M bit per cell program-erase cycles of each block of the second set of blocks, and maintain a respective count of N bit per cell program-erase cycles of each block of the second set of blocks. The control circuit is also configured to keep the respective counts of M bit per cell program-erase cycles of the second set of blocks uniform within a first tolerance, and keep the respective counts of N bit per cell program-erase cycles of the second set of blocks uniform within a second tolerance, as blocks in the second set of blocks repeatedly transition between the N bit per cell mode and the M bit per cell mode. 
     At step  942 , the selected block is selected among the second set of blocks to transition from the N bit per cell mode to the M bit per cell mode based on its respective count of N bit per cell program-erase cycles being lower than respective counts of N bit per cell program-erase cycles of other blocks in the second set of blocks. 
       FIG. 9E  is also an example of selecting a block to program among the second set of blocks based on its respective count of N bit per cell program-erase cycles being equal to respective counts of N bit per cell program-erase cycles of other blocks in the second set of blocks and its respective count of M bit per cell program-erase cycles being lower than respective counts of M bit per cell program-erase cycles of other blocks in the second set of blocks. 
       FIG. 9F  depicts an example process for selecting a multi-mode block to program in the MLC (or N bits per cell) mode, consistent with  FIG. 9A . Step  950  includes beginning a process to select a multi-mode block to program in the MLC (N bits per cell) mode. The selection can be among the blocks  914  in  FIG. 9B . Step  951  includes, among multi-mode blocks currently in the MLC (N bits per cell) mode, selecting a multi-mode block with the lowest MLC (N bits per cell) P-E counter or single P-E counter. Step  952  includes, if the MLC (N bits per cell) P-E counters are equal among the multi-mode blocks currently in the MLC (N bits per cell) mode, selecting a multi-mode block with lowest SLC (or M bits per cell) P-E counter. 
       FIG. 9G  depicts an example process for selecting a single-mode block or a multi-mode block to program in the SLC (or M bits per cell) mode, consistent with  FIG. 9A . Step  960  begins a process to select a block to program in the SLC (or M bits per cell) mode. A difference from  FIG. 9E  is that  FIG. 9G  includes the dedicated SLC blocks in the decision process so that the selection is among the blocks  910  and  913  in  FIG. 9B . Step  961  includes, among the single-mode blocks, selecting a block with the lowest P-E counter. Step  962  includes, among the multi-mode blocks currently in the SLC (or M bits per cell) mode, selecting a block with lowest SLC (or M bits per cell) counter. Step  963  includes, if the counters are equal for the multi-mode blocks currently in the SLC (or M bits per cell) mode at step  962 , selecting a block with the lowest MLC (N bits per cell) P-E counter. This is a tie-breaking process. Step  964  includes selecting a block with the lowest counter, among the previously-selected blocks in steps  961 - 963 . 
     This process is an example of selecting a transitioned block in the second set of blocks to program in the M bit per cell mode instead of a block in the first set of blocks based on the respective count of M bit per cell program-erase cycles of the transitioned block in the second set of blocks being lower than the respective count of program-erase cycles of the block of the first set of blocks. 
       FIG. 9H  depicts an example process for maintaining first and second P-E cycle counters, consistent with step  904  of  FIG. 9A . Step  970  includes initializing (e.g., setting to zero) a first P-E counter for SLC (or M bits per cell) mode for a multi-mode block. The first P-E counter, such as the MLC P-E cycle tracking circuit (for SLC mode)  119  in  FIG. 1 , keeps track of M bit per cell program-erase cycles. Step  971  includes initializing a second P-E counter for MLC (N bits per cell) mode for a multi-mode block. The second P-E counter, such as the MLC P-E cycle tracking circuit (for MLC mode)  117  in  FIG. 1 , keeps track of N bit per cell program-erase cycles. Step  972  includes incrementing the first P-E counter by a first amount when the multi-mode block is used in the SLC (or M bits per cell) mode. Step  973  includes incrementing a second P-E counter by a second amount when the multi-mode block is used in the MLC (N bits per cell) mode. The amount of the increment can correspond to the amount of degradation of the memory cells. The first amount can be less than the second amount since a P-E cycle in the SLC (or M bits per cell) mode causes less degradation than a P-E cycle in the MLC (or N bits per cell) mode. See also  FIG. 10C . 
     In one option, the amount of the increments in steps  972  and  973  changes over the lifetime of the memory device. For example, the increment in step  972  can increase progressively when the multi-mode block is used in the SLC (or M bits per cell) mode, and the increment in step  973  can increase progressively when the multi-mode block is used in the MLC (N bits per cell) mode. See also  FIG. 10C . A rate of increase of the second increment over the P-E cycles can be lower than a rate of increase of the first increment over the P-E cycles, since the memory cells tend to degrade faster when in the MLC (N bits per cell) mode compared to the SLC (or M bits per cell) mode. In this approach, the count accurately represents the amount of degradation of the memory cells. In contrast, if the count was incremented by a fixed amount for each P-E cycle throughout the lifetime of the memory device, it would tend to over-estimate the degradation when the count was lower and under-estimate the degradation when the count was higher. 
     Step  972  can be repeated or transition to step  973 , and step  973  can be repeated or transition to step  972 . Step  974  includes retiring the multi-mode block when the sum of P-E counts from the first and second P-E counters reaches a threshold. 
       FIG. 9I  depicts an example process for a single P-E cycle counter, consistent with step  905  of  FIG. 9A . Step  980  includes initializing the single P-E counter, such as the MLC P-E cycle tracking circuit (for SLC and MLC modes)  121  in  FIG. 1 , for a multi-mode block. Step  981  includes incrementing the P-E counter by a first amount when the multi-mode block is used in the SLC (or M bits per cell) mode. Step  982  includes incrementing the P-E counter by a second amount when the multi-mode block is used in the MLC (N bits per cell) mode. The first amount can be less than the second amount, and the amount of the increment can change over the lifetime of the memory device, as discussed in connection with  FIG. 9H . 
     Step  981  can be repeated or transition to step  982 , and step  982  can be repeated or transition to step  981 . Step  983  includes retiring the multi-mode block when the P-E count reaches a threshold. 
     A related process can include programming and erasing data in a first set of blocks in an M bit per cell mode, programming and erasing data in a second set of blocks in an N bit per cell mode, transitioning blocks in the second set of blocks to the M bit per cell mode from the N bit per cell mode, where M&lt;N, and maintaining a respective count of program-erase cycles for each block of the second set of blocks, where the respective count of program-erase cycles is incremented by a first amount when data is programmed and erased in the block in the M bit per cell mode and by a second amount, which is greater than the first amount, when data is programmed and erased in the block in the N bit per cell mode. 
     The process can further include increasing the first amount by progressively larger increments as data is repeatedly programmed and erased in the block in the M bit per cell mode over time; increasing the second amount in progressively larger increments which are larger than the progressively larger increments of the first amount as data is repeatedly programmed and erased in the block in the N bit per cell mode over time, and retiring each block in the second set of blocks when the respective count of program-erase cycles reaches a threshold. 
     In a further aspect, each block of the second set of blocks operates in the N bit per cell mode until its respective count of program-erase cycles reaches a threshold, after which each block of the second set of blocks transitions to the M bit per cell mode. 
     In a further aspect, after transitioning to the M bit per cell mode from the N bit per cell mode, each block of the second set of blocks remains in the M bit per cell mode in a remainder of its lifetime. 
     In a further aspect, each block of the second set of blocks transitions repeatedly between the N bit per cell mode and the M bit per cell mode based on its respective count of program-erase cycles. 
       FIG. 10A  depicts a plot of an error rate versus a number of P-E cycles for blocks in an SLC mode. The horizontal axis may represent 100 k P-E cycles, for example. As mentioned, the blocking oxide layer of a memory cell is degraded from the stress of the P-E cycles. The degradation is a function of the amount of charge stored in the memory cells. The average charge stored in a memory cell is about twice as much in MLC mode compared to SLC mode. Moreover, this damage becomes progressively larger over the lifetime of the memory cell, as the number of P-E cycles increases. The plot indicates that the error rate is close to zero for about the first half of the lifetime of the memory cell, then increases at a relatively low rate and then at a relatively high rate. The error rate is a measurement of the degradation. 
       FIG. 10B  depicts a plot of an error rate versus a number of P-E cycles for blocks in an MLC mode. The horizontal axis may represent 5 k P-E cycles, for example. As in  FIG. 10B , the damage becomes progressively larger over the lifetime of the memory cell, as the number of P-E cycles increases. The plot indicates that the error rate is close to zero for about the first half of the lifetime of the memory cell, then increases at a relatively high rate. The vertical axis in  FIG. 10B  is about ten times greater than in  FIG. 10A . 
       FIG. 10C  depicts a plot of an increment of a P-E cycle counter versus a number of P-E cycles for blocks in an SLC or MLC mode. A plot  1010  is for the MLC (N bits per cell) mode and a plot  1020  is for the SLC (or M bits per cell) mode. An increment of 1 can be used initially starting at a P-E cycle count of 0 cycles for the SLC (or M bits per cell) mode. A larger increment of 1.5, for instance, can be used initially for the MLC (or N bits per cell) mode. The increment of plot  1010  increases relatively quickly as the P-E cycles increase while the increment of plot  1020  increases relatively slowly as the P-E cycles increase. The plot  1010  may increase to 2.5, for example, while the plot  1020  increases to 1.5. Another option is for plot  1010  to begin at 1 and for plot  1020  to begin at a value between 0 and 1. 
     This is an example of the SLC cycles being weighted less heavily than the MLC cycles. 
     As mentioned, the increment or amount by which the P-E cycle count increases can increase progressively as the number of P-E cycles increases. As a result, the current count accurately represents the amount of degradation of a block. The use of the count in decisions such as selecting a multi-mode block to transition to/from the SLC mode, or selecting a block to program, can therefore be made based on an accurate assessment of the amount of degradation of the block. Moreover, the amount of degradation can be kept relatively constant among the blocks in a wear leveling process. 
     In one approach, a control circuit is configured to maintain a respective count of M bit per cell program-erase cycles of each block of the second set of blocks, and maintain a respective count of N bit per cell program-erase cycles of each block of the second set of blocks. The control circuit is also configured to increment the count of N bit per cell program-erase cycles of each block of the second set of blocks in progressively larger increments over time, and increment the count of M bit per cell program-erase cycles of each block of the second set of blocks in progressively larger increments over time at a lower rate than the increase of the increments for the count of N bit per cell program-erase cycles. Further, a selected block is selected among the second set of blocks to transition from the N bit per cell mode to the M bit per cell mode based on its respective count of N bit per cell program-erase cycles and its respective count of M bit per cell program-erase cycles. 
     In another approach, the control circuit is configured to increment the count of M bit per cell program-erase cycles of each block of the second set of blocks in progressively larger increments over time at a lower rate that the increase of the increments for the count of N bit per cell program-erase cycles, and retire each block in the second set of blocks based on a sum of its respective count of M bit per cell program-erase cycles and its respective count of N bit per cell program-erase cycles. 
     Accordingly, it can be seen that, in one implementation, an apparatus comprises a set of memory cells arranged in a plurality of blocks, the plurality of blocks comprising a first set of blocks and a second set of blocks; and a control circuit configured to store data in the first set of blocks in an M bit per cell mode, store data in the second set of blocks in the M bit per cell mode at some times and in an N bit per cell mode at other times, where M&lt;N, maintain a respective count of M bit per cell program-erase cycles of each block of the second set of blocks, and select a block of the second set of blocks to transition from the N bit per cell mode to the M bit per cell mode based on its respective count of M bit per cell program-erase cycles. 
     In another implementation, an apparatus comprises: a set of memory cells arranged in a plurality of blocks, the plurality of blocks comprising a first set of blocks and a second set of blocks; and a control circuit. The control circuit is configured to: store data in the first set of blocks in an M bit per cell mode; repeatedly transition blocks in the second set of blocks between storing data in the M bit per cell mode and storing data in an N bit per cell mode, where M&lt;N; maintain a respective count of M bit per cell program-erase cycles of each block of the second set of blocks; maintain a respective count of N bit per cell program-erase cycles of each block of the second set of blocks; and keep the respective counts of M bit per cell program-erase cycles of the second set of blocks uniform within a first tolerance, and keep the respective counts of N bit per cell program-erase cycles of the second set of blocks uniform within a second tolerance, during the repeated transition of the blocks in the second set of blocks. 
     The foregoing detailed description of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teachings. The described embodiments were chosen in order to best explain the principles of the invention and its practical application, to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto.