Patent Publication Number: US-10770156-B2

Title: Memory devices and methods for read disturb mitigation involving word line scans to detect localized read disturb effects and to determine error count in tracked sub sets of memory addresses

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 15/633,377, filed Jun. 26, 2017, which is incorporated herein by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     The present disclosure generally relates to semiconductor memory devices, and more particularly relates to methods of read disturb mitigation and memory devices employing the same. 
     BACKGROUND 
     Memory devices are frequently provided as internal, semiconductor, integrated circuits and/or external removable devices in computers or other electronic devices. There are many different types of memory, including volatile and non-volatile memory. Volatile memory, including random-access memory (RAM), static random access memory (SRAM), dynamic random access memory (DRAM), and synchronous dynamic random access memory (SDRAM), among others, may require a source of applied power to maintain its data. Non-volatile memory, by contrast, can retain its stored data even when not externally powered. Non-volatile memory is available in a wide variety of technologies, including flash memory (e.g., NAND and NOR) phase change memory (PCM), resistive random access memory (RRAM), and magnetic random access memory (MRAM), among others. 
     Flash memory devices can include an array of memory cells that each store data in a charge storage structure, such as a conductive floating gate or a dielectric charge trap. Flash memory devices frequently use a one-transistor memory cell that allows for high memory densities, high reliability, and low power consumption during use. Memory cells in a flash memory device can be programmed to a target state to represent information. For instance, electric charge can be placed on, or removed from, the charge storage structure (e.g., the charge trap or the floating gate) of a memory cell to program the cell to a particular data state. The data state can subsequently be read back from the memory cell by measuring a resistance of a circuit including the cell to determine the amount of charge on the charge storage structure. 
     Given the high density of flash memory cells, operations (e.g., reads, writes, erasures) on one memory cell can impact the charge stored on adjacent or nearby cells. One such effect is known as “read disturb,” in which a read operation performed on memory cells connected to one word line can change the amount of charge stored in memory cells on other word lines (e.g., the adjacent or nearby word lines) in the same memory block. 
     One approach to address this problem involves tracking the number of read operations that have occurred in a memory block (spanning multiple word lines) so that the data therein can be pre-emptively relocated to a different memory block before the number of read operations reaches levels that can cause data loss. This approach requires characterizing a number of read operations which can be safely performed on a memory block. This characterization poses a challenge, however, as the number of read operations that can be safely performed on a memory block depends upon the distribution of the read operations within the memory block. For example, in a memory block in which the read operations are evenly distributed across word lines, the number of read operations that could safely be performed would be fairly high, whereas in a memory block in which the read operations are concentrated on one or a few word lines, the number of read operations that could safely be performed would be fairly low. Choosing a single threshold value of read operations for every memory block in a device, therefore, will provide poor performance where some memory blocks experience more evenly distributed read disturb effects and others experience more localized read disturb effects on just a few word lines. Accordingly, a way to more efficiently address the localized effects of read disturb is required. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a system having a memory device configured in accordance with an embodiment of the present technology. 
         FIG. 2  is a plot of localized read disturb effects experienced by a memory device configured in accordance with an embodiment of the present technology. 
         FIG. 3  is a flow chart illustrating a method of managing a memory device in accordance with an embodiment of the present technology. 
         FIG. 4  is a flow chart illustrating a method of managing a memory device in accordance with an embodiment of the present technology. 
         FIG. 5  is a flow chart illustrating a method of managing a memory device in accordance with an embodiment of the present technology. 
         FIG. 6  is a schematic representation of various tracked subsets of memory addresses of a memory device in accordance with embodiments of the present technology. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, numerous specific details are discussed to provide a thorough and enabling description for embodiments of the present technology. One skilled in the relevant art, however, will recognize that the disclosure can be practiced without one or more of the specific details. In other instances, well-known structures or operations often associated with memory devices are not shown, or are not described in detail, to avoid obscuring other aspects of the technology. For example, several functional components of memory devices and/or memory systems that are well-known to those skilled in the art are not discussed in detail below (e.g., circuit components such as multiplexers and decoders, data structures such as address registers and data registers, etc.). In general, it should be understood that various other devices, systems, and methods in addition to those specific embodiments disclosed herein may be within the scope of the present technology. 
     As discussed above, read disturb effects in flash memory can cause data loss if not addressed in a timely fashion, such that ever more efficient methods of mitigating read disturb effects are desired. Accordingly, several embodiments of memory devices in accordance with the present technology can perform word line scans to detect localized read disturb effects before they become problematic (e.g., before uncorrectable bit errors occur). The word line scan can determine an error count (e.g., an RBER) for each word line in a tracked subset of memory addresses (e.g., a single memory block, a group of memory blocks, a memory superblock, a group of word lines corresponding to less than a memory block, etc.) and outputs a value corresponding to the error count of the word line with the highest error count (e.g. the word line that has experienced the largest amount of read disturb effects). If the word line scan reveals an amount of errors that indicates that the data in the tracked subset is experiencing levels of read disturb effects that can cause errors, then a relocation operation can be performed or scheduled for future performance. 
     Several embodiments of the present technology are directed to memory devices, systems including memory devices, and methods of operating memory devices. In one embodiment, a memory device comprises a main memory and a controller operably connected to the main memory. The main memory can comprise a plurality of memory addresses, each corresponding to a single one of a plurality of word lines. Each memory address can be included in a tracked subset of the plurality of memory addresses. Each tracked subset can include memory addresses corresponding to more than one of the plurality of word lines. The controller is configured to track a number of read operations for each tracked subset, and to scan, in response to the number of read operations for a first tracked subset exceeding a first threshold value, a portion of data corresponding to each word line of the first tracked subset to determine an error count corresponding to each word line of the first tracked subset. 
       FIG. 1  is a block diagram of a system  101  having a memory device  100  configured in accordance with an embodiment of the present technology. As shown, the memory device  100  includes a main memory  102  (e.g., NAND flash, NOR flash, chalcogenide PCM, etc.) and a controller  106  operably coupling the main memory  102  to a host device  108  (e.g., an upstream central processor (CPU)). The main memory  102  includes a plurality of memory regions, or memory units  120 , which each include a plurality of memory cells  122 . Memory units  120  can be individual memory dies, memory planes in a single memory die, a stack of memory dies vertically connected with through-silicon vias (TSVs), or the like. For example, in one embodiment, each of the memory units  120  can be formed from a semiconductor die and arranged with other memory unit dies in a single device package (not shown). In other embodiments, multiple memory units  120  can be co-located on a single die and/or distributed across multiple device packages. The memory cells  122  can include, for example, floating gate, charge trap, phase change, ferroelectric, magnetoresistive, and/or other suitable storage elements configured to store data persistently or semi-persistently. The main memory  102  and/or the individual memory units  120  can also include other circuit components (not shown), such as multiplexers, decoders, buffers, read/write drivers, address registers, data out/data in registers, etc., for accessing and/or programming (e.g., writing) the memory cells  122  and other functionality, such as for processing information and/or communicating with the controller  106 . 
     Memory cells  122  can be arranged in rows  124  (e.g., each corresponding to a word line) and columns  126  (e.g., each corresponding to a bit line). Each word line can include one or more memory pages, depending upon the number of data states the memory cells  122  of that word line are configured to store. For example, a single word line of memory cells  122  in which each memory cell  122  is configured to store one of two data states (e.g., SLC memory cells configured to store one bit each) can include a single memory page. Alternatively, a single word line of memory cells  122  in which each memory cell  122  is configured to store one of four data states (e.g., MLC memory cells configured to store two bits each) can include two memory pages. Moreover, memory pages can be interleaved so that a word line comprised of memory cells  122  in which each memory cell  122  is configured to store one of two data states (e.g., SLC memory cells) can span two memory pages, in an “even-odd bit line architecture,” where all the memory cells  122  in odd-numbered columns  126  of a single word line are grouped as a first memory page, and all the memory cells  122  in even-numbered columns  126  of the same word line are grouped as a second memory page. When even-odd bit line architecture is utilized in a word line of memory cells  122  in which each memory cell  122  is configured to store larger numbers of data states (e.g., memory cells configured as MLC, TLC, QLC, etc.), the number of memory pages per word line can be even higher (e.g., 4, 6, 8, etc.). 
     Each column  126  can include a string of series-coupled memory cells  122  connected to a common source. The memory cells  122  of each string can be connected in series between a source select transistor (e.g., a field-effect transistor) and a drain select transistor (e.g., a field-effect transistor). Source select transistors can be commonly coupled to a source select line, and drain select transistors can be commonly coupled to a drain select line. 
     In other embodiments, the memory cells  122  can be arranged in different types of hierarchies and/or groups than those shown in the illustrated embodiments. Further, although shown in the illustrated embodiments with a certain number of memory cells, rows, columns, blocks, and memory units for purposes of illustration, the number of memory cells, rows, columns, blocks, and memory units can vary, and can, in other embodiments, be larger or smaller in scale than shown in the illustrated examples. For example, in some embodiments, the memory device  100  can include only one memory unit  120 . Alternatively, memory device  100  can include two, three, four, eight, ten, or more (e.g., 16, 32, 64, or more) memory units  120 . Although the memory units  120  are shown in  FIG. 1  as including two memory blocks  128  each, in other embodiments, each memory unit  120  can include one, three, four eight, or more (e.g., 16, 32, 64, 100, 128, 256 or more memory blocks). In some embodiments, each memory block  128  can include, e.g., 2 15  memory pages, and each memory page within a block can include, e.g., 2 12  memory cells  122  (e.g., a “4 k” page). 
     The controller  106  can be a microcontroller, special purpose logic circuitry (e.g., a field programmable gate array (FPGA), an application specific integrated circuit (ASIC), etc.), or other suitable processor. The controller  106  can include a processor  130  configured to execute instructions stored in memory. In the illustrated example, the memory of the controller  106  includes an embedded memory  132  configured to perform various processes, logic flows, and routines for controlling operation of the memory device  100 , including managing the main memory  102  and handling communications between the memory device  100  and the host device  108 . In some embodiments, the embedded memory  132  can include memory registers storing, e.g., memory pointers, fetched data, etc. The embedded memory  132  can include volatile and/or non-volatile memory (e.g., DRAM, SRAM, NAND, NOR, PCM) for storing the memory registers, and can also include read-only memory (ROM) (e.g., for storing micro-code). Although in the example set forth in  FIG. 1 , memory device  100  has been illustrated as including a controller  106 , in another embodiment of the present technology, a memory device may not include a controller, and may instead rely upon external control (e.g., provided by an external host, or by a processor or controller separate from the memory device). 
     In operation, the controller  106  can directly write or otherwise program (e.g., erase) the various memory regions of the main memory  102 , such as by writing to groups of memory pages and/or memory blocks  128 . In NAND-based memory, a write operation often includes programming the memory cells  122  in selected memory pages with specific data values (e.g., a string of data bits having a value of either logic 0 or logic 1). An erase operation is similar to a write operation, except that the erase operation re-programs an entire memory block  128  or multiple memory blocks  128  to the same data state (e.g., logic 1). 
     The controller  106  communicates with the host device  108  over a host-device interface  110 . In some embodiments, the host device  108  and the controller  106  can communicate over a serial interface, such as a serial attached SCSI (SAS), a serial AT attachment (SATA) interface, a peripheral component interconnect express (PCIe), or other suitable interface (e.g., a parallel interface). The host device  108  can send various requests (in the form of, e.g., a packet or stream of packets) to the controller  106 . A request can include a command to write, erase, return information, and/or to perform a particular operation (e.g., a TRIM operation). A request can also include an interrupt or another command that indicates a change in condition (e.g., a power loss event), which can trigger the implementation of a power loss algorithm. 
     Host device  108  can be any one of a number of electronic devices capable of utilizing memory for the temporary or persistent storage of information, or a component thereof. For example, host device may be a computing device such as a desktop or portable computer, a server, a hand-held device (e.g., a mobile phone, a tablet, a digital reader, a digital media player), or some component thereof (e.g., a central processing unit, a co-processor, a dedicated memory controller, etc.). Host device  108  may be a networking device (e.g., a switch, a router, etc.) or a recorder of digital images, audio and/or video, a vehicle, an appliance, a toy, or any one of a number of other products. In one embodiment, host device  108  may be connected directly to memory device  100 , although in other embodiments, host device  108  may be indirectly connected to memory device (e.g., over a networked connection or through intermediary devices). 
     As set forth in greater detail above, read operations on a memory array can cause read disturb effects to memory cells on word lines adjacent to the memory cell that is the target of a read operation.  FIG. 2  is a plot  200  of localized read disturb effects experienced by a memory device configured in accordance with an embodiment of the present technology. As can be seen with reference to  FIG. 2 , a memory cell (or memory page)  201  targeted for a read operation does not itself experience read disturb effects. Rather, due to the read-pass voltage applied to other the word lines in the same memory block as the word line being read, the read disturb effects accrue to the other word lines. The effects are not evenly distributed across the other word lines in the same memory block, however. Due in part to the increasing density of flash memory arrays, the difference between the read pass voltage applied to non-selected word lines and the read voltage applied to a selected word line increases the read disturb effect for unselected word lines that are in close proximity to the selected word line. In the plot  200  of  FIG. 2 , this can be observed in the amount of read disturb experienced at an unselected word line  202  immediately adjacent to the selected word line  201 , when compared to more distant word lines  203  and  204 . Accordingly, if a large number of read operations target memory cells on the same word line of a memory block, the cumulative read disturb effects on nearby or adjacent word lines can cause uncorrectable bit errors sooner than if read operations were distributed more evenly across word lines in the memory block. 
     One approach to address this challenge involves lowering the number of read count operations permitted on a memory block before a copy-and-erase or other relocation operation is triggered. This approach can be inefficient, as setting the threshold value for every memory block based on a worst-case scenario of highly localized read disturb effects would trigger premature relocation operations on blocks with more evenly distributed read disturb effects. Frequent erase operations (e.g., as part of relocation operations) can degrade both the performance (e.g., due to more frequent garbage collection operations) and useful life (e.g., as flash devices frequently have a lifespan limited by a maximum number of erase operations per block) of the memory device so configured. 
     Another approach involves tracking read operations with greater granularity (e.g., at the word line level). Given the large number of word lines that each memory block can span (e.g., 128, 256, 512, 1024, etc.), increasing the tracking granularity in this way can be cost-prohibitive. For example, in a memory device in which each memory block spans  128  word lines, tracking read counts for each word line rather than for each memory block would increase the space required to store the tracked read counts by about two orders of magnitude. 
     Accordingly, the present technology provides a method for performing word line scans to detect localized read disturb effects before they become problematic (e.g., before uncorrectable bit errors occur). The word line scan determines an error count (e.g., an RBER) for each word line in a tracked subset of memory addresses (e.g., a single memory block, a group of memory blocks, a group of word lines corresponding to less than a memory block, etc.) and outputs a value corresponding to the error count of the word line or lines with the highest count (e.g. the word line that has experienced the largest amount of read disturb effects). If the word line scan reveals an amount of errors that indicates that the data in the tracked subset is experiencing levels of read disturb effects that can cause errors, then a relocation operation can be performed or scheduled for future performance. 
     In accordance with one aspect of the present technology, a word line scan does not have to read all of the data on each word line in the tracked subset to provide an error count corresponding to each word line. Rather, the word line scan can be configured to exploit the architectural characteristics of the memory device on which the scan is performed. For example, where multiple pages are stored on the same word line (e.g., in an even-odd architecture, or in an MLC, TLC, QLC, or even higher number of bits-per-cell architecture), a single page can be read from each word line. In other embodiments, in which the memory device is capable of performing read operations at a sub-page level (e.g., reading a single allocation unit from a page), then the word line scan can be performed at that sub-page level. Reducing the amount of data that is read from each word line during the word line scan can improve the speed of the scanning. 
       FIG. 3  is a flow chart illustrating a method of managing a memory device in accordance with one embodiment of the present technology. The method includes determining (block  302 ) a read count for a tracked subset of memory addresses corresponding to more than one word line (e.g., a subset of the memory addresses of the memory device corresponding to a single memory block, to multiple memory blocks, to a memory superblock, to a plurality of pages across a number of word lines comprising less than a memory block, etc.). For example, the read count for each tracked subset can be stored in a table managed (e.g., tracked) by the memory controller for the memory device implementing the method. The determining (block  302 ) can be performed in response to a read operation (e.g., in response to a read command received from a connected host), or can alternatively be performed as part of background memory management operations. The method includes comparing (box  304 ) the determined read count to a first threshold value to evaluate (box  306 ) whether the determined read count meets and/or exceeds the first threshold value. If the evaluation (box  306 ) determines that the read count does not yet meet and/or exceed the first threshold value, the method can end (e.g., if the method is triggered in response to a read operation, the read operation can continue as normal, or if the method is part of background operations, the background scanning can proceed to the next tracked subset of memory addresses). 
     If, however, the evaluation (box  306 ) determines that the read count exceeds the first threshold value, the method can include performing a word line scan (box  308 ) of the tracked subset to determine an error count (e.g., a count of correctable bit errors) corresponding to each word line in the tracked subset. The determined error count can then be compared (box  310 ) to a second threshold value (e.g., a maximum allowable error count based on an ECC capability of the memory device) to evaluate whether the determined error count meets and/or exceeds the second threshold value (box  312 ). If the evaluation (box  312 ) determines that the error count does not yet meet and/or exceed the second threshold value, the method can end (e.g., if the method is triggered in response to a read operation, the read operation can continue as normal, or if the method is part of background operations, the background scanning can proceed to the next tracked subset of memory addresses). If, however, the evaluation (box  312 ) determines that the error count exceeds the second threshold value, the method can include relocating (box  314 ) data from the tracked subset of memory addresses (e.g., with a copy-and-erase operation) to prevent the data from being corrupted by additional read disturb effects (box  314 ). The relocation (box  314 ) can occur immediately, or it can be scheduled for future performance (e.g., added to a list of blocks to be relocated in a background operation). 
     In accordance with one embodiment of the subject technology, the first threshold value (e.g., corresponding to a number of read counts that can occur before a word line scan is performed) can remain constant for a life of the memory device, and be configured at a time of manufacturing or provisioning. In accordance with another embodiment of the subject technology, the first threshold value can change over time to accommodate the increasing susceptibility of a memory device to read disturb errors over the life of the memory device. In this regard, as a memory device begins to wear out from a growing number of cumulative erase operations, the susceptibility of the memory device to read disturb effects increases, such that a first threshold value that is reduced over time (e.g., as measured by a number of erase operations) can provide improved performance. In accordance with another aspect of the subject technology, the first threshold value can be user-configurable, or otherwise adjustable by a host device. 
     Moreover, although the foregoing embodiments have been described with reference to a single first threshold value (e.g., corresponding to a number of read counts that can occur before a word line scan is performed) that corresponds to all of the tracked subsets of memory addresses of a memory device, in other embodiments a memory device can track multiple first threshold values, each corresponding to one or more tracked subsets. In a memory device employing this approach, the first threshold value corresponding to each tracked subset can also be adjusted based on a total erase count for that tracked subset, or otherwise user- or host-configurable. 
     In accordance with an aspect of the subject technology, although a word line scan does not need to read all of the data from each word line in a tracked subset (e.g., reading only a single page, a single allocation unit, or some other minimal amount of data that the memory architecture permits to be read), the scan can utilize a non-negligible amount of device resources (e.g., time, power, etc.). Therefore, in accordance with one embodiment of the present technology, a word line scan can be scheduled in any one of a variety of different ways configured to achieve a more efficient balance of costs and benefits. For example, the word line scans can be scheduled to be performed with some regular periodicity after the number of reads on a tracked subset of memory addresses has become high enough that a localized disturb effect may have begun to occur (e.g., at a threshold number of read operations on the tracked subset that could cause problematic read disturb effects if the read operations had been concentrated on a single word line). In this regard, the word line scans can be performed after every n read operations on a tracked subset (e.g., where n is an integer number of read operations, such as 100, 1000, 10000, 32768, etc.) after the tracked subset has experienced a cumulative count of read operations (e.g., since a last erase operation on the tracked subset) above a predetermined threshold (e.g., the first threshold value). This can be facilitated by increasing the first threshold value corresponding to a tracked subset of memory addresses by a fixed amount following a determination (e.g., in box  312  of  FIG. 3 ) that the determined error count for that tracked subset does not yet exceed the second threshold (and resetting the first threshold value for that tracked subset to an initial value following an erase operation on the tracked subset). 
     In accordance with another embodiment, the output from a word line scan can be utilized to more efficiently schedule a subsequent word line scan on the same tracked subset. For example, after performing a word line scan that determines an error count below the second threshold value for triggering a relocation operation (e.g., where a number of bit errors on a most-disturbed word line is still well within the ECC capability of the memory device), the number of read counts that can be performed on that particular tracked subset before the next scan occurs can be increased by an amount corresponding to the detected error count. This scheduling approach allows the next word line scan for the same tracked subset to be deferred by a large number of read operations when read disturb effects are more evenly distributed (e.g., in a tracked subset where the read operations have been distributed across the word lines), and by a smaller number of read operations when read disturb effects are more localized (e.g., in a tracked subset where the read operations have been concentrated on one or a few word lines). Although this scheduling approach involves maintaining a different first threshold value for each tracked subset of memory addresses, it provides the benefit of tailoring a word line scanning schedule more particularly suited to the activity profile of each tracked subset. 
       FIG. 4  is a flow chart illustrating a method of managing a memory device in accordance with one embodiment of the present technology, in which the output of the word line scan is used in the scheduling of the next word line scan. The method includes determining (block  402 ) a read count for a tracked subset of memory addresses corresponding to more than one word line (e.g., a subset of the memory addresses of the memory device corresponding to a single memory block, to multiple memory blocks, to a memory superblock, to a plurality of pages across a number of word lines comprising less than a memory block, etc.). For example, the read count for each tracked subset can be stored in a table managed (e.g., tracked) by the memory controller for the memory device implementing the method. The determining (block  402 ) can be performed in response to a read operation (e.g., in response to a read command received from a connected host), or can alternatively be performed as part of background memory management operations. The method includes comparing (box  404 ) the determined read count to a first threshold value that corresponds to the tracked subset to evaluate (box  406 ) whether the determined read count meets and/or exceeds the first threshold value. If the evaluation (box  406 ) determines that the read count does not yet meet and/or exceed the first threshold value, the method can end (e.g., if the method is triggered in response to a read operation, the read operation can continue as normal, or if the method is part of background operations, the background scanning can proceed to the next tracked subset of memory addresses). 
     If, however, the evaluation (box  406 ) determines that the read count exceeds the first threshold value, the method can include performing a word line scan (box  408 ) of the tracked subset to determine an error count (e.g., a count of correctable bit errors) corresponding to each word line in the tracked subset. The determined error count can then be compared (box  410 ) to a second threshold value (e.g., a maximum allowable error count based on an ECC capability of the memory device) to evaluate (box  412 ) whether the determined error count meets and/or exceeds the second threshold value. If the evaluation (box  412 ) determines that the error count exceeds the second threshold value, the method can include relocating (box  414 ) data from the tracked subset of memory addresses (e.g., with a copy-and-erase operation) to prevent the data from being corrupted by additional read disturb effects. The relocation (box  414 ) can occur immediately, or it can be scheduled for future performance (e.g., added to a list of blocks to be relocated in a background operation). 
     If, however, the evaluation (box  412 ) determines that the error count does not yet meet and/or exceed the second threshold value, the method can include adjusting (box  413 ) the first threshold value based on the determined error count. For example, if the determined error count is less than half of the second threshold value, the first threshold value may be increased by a larger amount (e.g., permitting another 10000 read operations to occur before the word line scan for the current tracked subset is performed again), whereas if the determined error count is greater than or equal to half of the second threshold value, the first threshold value may be increased by a smaller amount (e.g., only permitting another 2000 read operations to occur before the word line scan for the current tracked subset is performed again). A memory device employing this method can maintain a table correlating determined error counts to amounts by which to increase the first threshold value similar to Table 1, below: 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Determined 
                 Amount by which to Increase 
               
               
                   
                 Error Count 
                 First Threshold Value 
               
               
                   
                   
               
             
            
               
                   
                  0-20 
                 524,288 (i.e., 2 18)   
               
               
                   
                 21-40 
                 262,144 (i.e., 2 17)   
               
               
                   
                 41-80 
                 131,072 (i.e., 2 16)   
               
               
                   
                  81-100 
                  65,536 (i.e., 2 15)   
               
               
                   
                 101-150 
                  32,768 (i.e., 2 14)   
               
               
                   
                 151-210 
                  16,384 (i.e., 2 13)   
               
               
                   
                   
               
            
           
         
       
     
     In another embodiment, rather than increasing the first threshold value for the tracked subset by a fixed increment, the method can increase the first threshold value by a multiple or percentage of its current value. A memory device employing this approach can maintain a table correlating determined error counts to multiples by which to increase the first threshold value similar to Table 2, below: 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Determined 
                 Multiple by which to Increase 
               
               
                   
                 Error Count 
                 First Threshold Value 
               
               
                   
                   
               
             
            
               
                   
                  0-40 
                 2.00 
               
               
                   
                  41-100 
                 1.75 
               
               
                   
                 101-150 
                 1.50 
               
               
                   
                 151-210 
                 1.25 
               
               
                   
                   
               
            
           
         
       
     
     As set forth above, tracking a threshold number of read operations for each tracked subset of memory addresses requires more storage space than maintaining a single threshold value applied to all of the tracked subsets. Given the large number of read operations that may occur on some memory devices before read disturb effects become problematic (e.g., measuring in the tens of thousands, hundreds of thousands, or even millions of read operations), the number of bits required to store the threshold values can be large. In accordance with another embodiment of the present technology, rather than storing a threshold value for each tracked subset of memory addresses, a memory device can be configured to store a scaling factor for each tracked subset of memory addresses, and to calculate the threshold value from the scaling factor “on-the-fly.” In this regard, storing a scaling factor that requires fewer bits (e.g., less than 16, or less than 8, or even less than 4) reduces the storage requirements for tracking a separate threshold value for each tracked subset, at a cost of increased computational complexity (e.g., the threshold value can be calculated in response to each read operation as an offset added to a single threshold value for the memory device, where the offset is calculated by multiplying the scaling factor with the single threshold). Accordingly, rather than increasing a threshold value of read operations in response to an error count determined by the word line scan, a scaling factor corresponding to the tracked subset can be increased instead, as set forth in greater detail below. 
       FIG. 5  is a flow chart illustrating a method of managing a memory device in accordance with one embodiment of the present technology, in which the output of the word line scan is used in the scheduling of the next word line scan. The method includes determining (block  502 ) a read count for a tracked subset of memory addresses corresponding to more than one word line (e.g., a subset of the memory addresses of the memory device corresponding to a single memory block, to multiple memory blocks, to a memory superblock, to a plurality of pages across a number of word lines comprising less than a memory block, etc.). For example, the read count for each tracked subset can be stored in a table managed (e.g., tracked) by the memory controller for the memory device implementing the method. The determining (block  502 ) can be performed in response to a read operation (e.g., in response to a read command received from a connected host), or can alternatively be performed as part of background memory management operations. The method includes calculating (box  503 ) a first threshold value corresponding to the tracked subset based on a scaling factor that corresponds to the tracked subset (e.g., the threshold value can be calculated in response to each read operation as an offset added to a single threshold value for the memory device, where the offset is calculated by multiplying the scaling factor with the single threshold). The method further includes comparing (box  504 ) the determined read count to the first threshold value to evaluate (box  506 ) whether the determined read count meets and/or exceeds the first threshold value. If the evaluation (box  506 ) determines that the read count does not yet meet and/or exceed the first threshold value, the method can end (e.g., if the method is triggered in response to a read operation, the read operation can continue as normal, or if the method is part of background operations, the background scanning can proceed to the next tracked subset of memory addresses). 
     If, however, the evaluation (box  506 ) determines that the read count exceeds the first threshold value, the method can include performing a word line scan (box  508 ) of the tracked subset to determine an error count (e.g., a count of correctable bit errors) corresponding to each word line in the tracked subset. The determined error count can then be compared (box  510 ) to a second threshold value (e.g., a maximum allowable error count corresponding to a value within the ECC capability of the memory device) to evaluate (box  512 ) whether the determined error count meets and/or exceeds the second threshold value. If the evaluation (box  512 ) determines that the error count meets and/or exceeds the second threshold value, the method can include relocating data (box  514 ) from the tracked subset of memory addresses (e.g., with a copy-and-erase operation) to prevent the data from being corrupted by additional read disturb effects. The relocation (box  514 ) can occur immediately, or it can be scheduled for future performance (e.g., added to a list of blocks to be relocated in a background operation). 
     If, however, the evaluation (box  512 ) determines that the determined error count does not yet meet and/or exceed the second threshold value, the method can include adjusting (box  513 ) the scaling factor corresponding to the tracked subset based on the determined error count. For example, if the determined error count is less than half of the second threshold value, the scaling factor may be increased by a larger amount (e.g., thereby increasing the number of read operations permitted to occur before the word line scan for the current tracked subset is performed again by, e.g., 10%), whereas if the determined error count is greater than or equal to half of the second threshold value, the scaling factor may be increased by a smaller amount (e.g., thereby increasing the number of read operations permitted to occur before the word line scan for the current tracked subset is performed again by, e.g., 5%). A memory device employing this approach can maintain a table correlating determined error counts to amounts by which to increase the scaling factor similar to Table 3, below: 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                 Determined 
                 Amount by which to Increase 
               
               
                   
                 Error Count 
                 Scaling Factor 
               
               
                   
                   
               
             
            
               
                   
                  0-15 
                 1.00 
               
               
                   
                 16-31 
                 0.75 
               
               
                   
                 32-63 
                 0.50 
               
               
                   
                  64-127 
                 0.20 
               
               
                   
                 128-191 
                 0.10 
               
               
                   
                 192-256 
                 0.05 
               
               
                   
                   
               
            
           
         
       
     
     For example, a table correlating each tracked subset with a corresponding scaling factor can be initialized so that each entry has a value of 1.0. Accordingly, when calculating a specific first threshold value for a particular tracked subset of memory addresses, the calculated value will be equal to the scaling factor (1.0) multiplied with a threshold value for the memory device (e.g., and optionally updated over time with regard to the total erase count for the memory device). If the word line scan for the first subset outputs a determined error count of less than 25, then the scaling factor for that tracked subset can be increased by 0.75 (per the correlation in Table 3). Accordingly, the next word line scan for that tracked subset will not be triggered until the read count for that tracked subset meets or exceeds a threshold value equal to 1.75 multiplied with the threshold value for the memory device. If that next word line scan determines an error count of 85, then the scaling factor for the tracked subset can be increased by a further 0.20 (per the correlation in Table 3), such that the next word line scan will not be triggered until the read count for that tracked subset meets or exceeds a threshold value equal to 1.95 multiplied with the threshold value for the memory device, and so on. 
     As set forth in greater detail above, embodiments of the present technology include memory devices that can track read counts (and optionally erase counts, threshold levels, and/or scaling factors) corresponding to subsets of memory addresses at various granularities (e.g., a single memory block, a group of memory blocks, a group of word lines corresponding to less than a memory block, etc.), so that word line scans can be performed on the tracked subset based in part upon the read count for the tracked subset reaching a threshold value.  FIG. 6  is a schematic representation of the scale of various tracked subsets of memory addresses of a memory device in accordance with embodiments of the present technology. As can be seen with reference to  FIG. 6 , a memory array  600  can include a plurality of memory planes organized into logical unit numbers (LUNs), such as LUN 0  to LUN z−1 . Each memory plane can include a number of memory pages connected to a number of word lines (WLs) grouped into a number of super blocks (SBs), such as SB 0  to SB y−1 , each comprising WL 0  to WL x−1 . The scale of the subset of memory addresses that is tracked by the memory device including memory array  600  can be selected to optimize a balance of storage required (e.g., more tracked subsets consuming more storage to track the read counts thereof) and performance (e.g., more tracked subsets providing greater efficiency in delaying copy-and-erase or other relocation operations closer to a maximum read disturb endurance for the tracked subset). 
     For example, the tracked subsets can correspond to physical memory blocks, such as physical memory block  601 . Alternatively, the tracked subsets can correspond to super blocks, such as super block SB 0  (shown in outline with reference number  602 ). One of skill in the art will readily understand, however, that the scale of the tracked subsets need not be integer numbers of physical blocks, as in the foregoing two examples. Any subset of memory addresses that include memory addresses associated with multiple word lines can be tracked and subject to word line scans, as set forth in greater detail above, including sub-superblock groups of word lines such as group  603 , or sub-superblock groups of physical blocks such as group  604 . These subsets could be even further subdivided to provide more granular subsets, such as a subset of group  604  that included only those memory addresses associated with word lines WL 0  through WL 2 . 
     From the foregoing, it will be appreciated that specific embodiments of the invention have been described herein for purposes of illustration, but that various modifications may be made without deviating from the scope of the invention. Accordingly, the invention is not limited except as by the appended claims.