Patent Publication Number: US-9418750-B2

Title: Single ended word line and bit line time constant measurement

Description:
BACKGROUND 
     This application relates to the operation of re-programmable non-volatile memory systems such as semiconductor flash memory that record data using charge stored in charge storage elements of memory cells. 
     Solid-state memory capable of nonvolatile storage of charge, particularly in the form of EEPROM and flash EEPROM packaged as a small form factor card, has recently become the storage of choice in a variety of mobile and handheld devices, notably information appliances and consumer electronics products. Unlike RAM (random access memory) that is also solid-state memory, flash memory is non-volatile, and retains its stored data even after power is turned off. Also, unlike ROM (read only memory), flash memory is rewritable similar to a disk storage device. In spite of the higher cost, flash memory is increasingly being used in mass storage applications. 
     Flash EEPROM is similar to EEPROM (electrically erasable and programmable read-only memory) in that it is a non-volatile memory that can be erased and have new data written or “programmed” into their memory cells. Both utilize a floating (unconnected) conductive gate, in a field effect transistor structure, positioned over a channel region in a semiconductor substrate, between source and drain regions. A control gate is then provided over the floating gate. The threshold voltage characteristic of the transistor is controlled by the amount of charge that is retained on the floating gate. That is, for a given level of charge on the floating gate, there is a corresponding voltage (threshold) that must be applied to the control gate before the transistor is turned “on” to permit conduction between its source and drain regions. Flash memory such as Flash EEPROM allows entire blocks of memory cells to be erased at the same time. 
     The floating gate can hold a range of charges and therefore can be programmed to any threshold voltage level within a threshold voltage window. The size of the threshold voltage window is delimited by the minimum and maximum threshold levels of the device, which in turn correspond to the range of the charges that can be programmed onto the floating gate. The threshold window generally depends on the memory device&#39;s characteristics, operating conditions and history. Each distinct, resolvable threshold voltage level range within the window may, in principle, be used to designate a definite memory state of the cell. 
     In order to improve read and program performance, multiple charge storage elements or memory transistors in an array are read or programmed in parallel. Thus, a “page” of memory elements are read or programmed together. In existing memory architectures, a row typically contains several interleaved pages or it may constitute one page. All memory elements of a page are read or programmed together. 
     Nonvolatile memory devices are also manufactured from memory cells with a dielectric layer for storing charge. Instead of the conductive floating gate elements described earlier, a dielectric layer is used. An ONO dielectric layer extends across the channel between source and drain diffusions. The charge for one data bit is localized in the dielectric layer adjacent to the drain, and the charge for the other data bit is localized in the dielectric layer adjacent to the source. For example, a nonvolatile memory cell may have a trapping dielectric sandwiched between two silicon dioxide layers. Multi-state data storage is implemented by separately reading the binary states of the spatially separated charge storage regions within the dielectric. 
     SUMMARY 
     A non-volatile memory circuit includes an array of non-volatile memory cells formed along word lines as multiple multi-cell erase blocks. Word line driver circuitry provides word line voltage levels and word line decoding circuitry is connected to the array and to the word driver circuitry by which the word line voltage levels are selectively applied to the word lines. Logic circuitry connected to the word line decoding circuitry, whereby a timing for applying a word line voltage level during a first memory operation of a selected word line is varied based upon the location of the selected word line within the block to which it belongs. 
     A method is presented for operating a non-volatile memory array formed of multiple memory cells formed along multiple access lines that, during memory operations, are driven from a near end. The method determines a time constant for charging the far end of the access lines and includes driving the near end of a selected access line with a first voltage from a voltage supply. While driving the near end of a selected access line in this way, the method determines a number of clock cycles required for the amount of current supplied by the voltage supply in driving the near end of a selected access line to decrease from a first level to a second level. Based upon the number of clock cycles required, estimating the time constant for charging the far end of the selected access line is estimated. 
     A method is also presented for operating a non-volatile memory array formed of multiple memory cells formed along multiple access lines that, during memory operations, are driven from a near end. The method determines a time constant for charging the far end of the access lines and includes charging the far end of a selected access line to a first voltage. While discharging the far end of the selected access line to its the near end, the method determines the number of clock cycles required for the current discharging the far end of the selected access line to decrease from a first level to a second level. Based upon the number of clock cycles required, the time constant for charging the far end of the selected access line is estimated. 
     Various aspects, advantages, features and embodiments are included in the following description of exemplary examples thereof, which description should be taken in conjunction with the accompanying drawings. All patents, patent applications, articles, other publications, documents and things referenced herein are hereby incorporated herein by this reference in their entirety for all purposes. To the extent of any inconsistency or conflict in the definition or use of terms between any of the incorporated publications, documents or things and the present application, those of the present application shall prevail. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates schematically the main hardware components of a memory system suitable for implementing various aspects described in the following. 
         FIG. 2  illustrates schematically a non-volatile memory cell. 
         FIG. 3  illustrates the relation between the source-drain current I D  and the control gate voltage V CG  for four different charges Q 1 -Q 4  that the floating gate may be selectively storing at any one time at fixed drain voltage. 
         FIG. 4  illustrates schematically a string of memory cells organized into a NAND string. 
         FIG. 5  illustrates an example of a NAND array  210  of memory cells, constituted from NAND strings  50  such as that shown in  FIG. 4 . 
         FIG. 6  illustrates a page of memory cells, organized in the NAND configuration, being sensed or programmed in parallel. 
         FIGS. 7A-7C  illustrate an example of programming a population of memory cells. 
         FIG. 8  shows an example of a physical structure of a 3-D NAND string. 
         FIGS. 9-12  look at a particular monolithic three dimensional (3D) memory array of the NAND type (more specifically of the “BiCS” type). 
         FIG. 13  shows an array of blocks spanned by set of bit lines each connected to a corresponding sense amp. 
         FIG. 14  looks at a problematical bit line pattern with respect to settling times. 
         FIG. 15  is a more detailed view of a portion of a finger structure seen from above. 
         FIG. 16  illustrates the relative times for pre-charging a bit line for a near block, TBL(near), and a bit line for a far block, TBL(far). 
         FIG. 17  illustrates the blocks of an array split up into a number of segments, each of one or more adjacent blocks. 
         FIG. 18  is a simplified flow of some aspects of the process involved. 
         FIG. 19  is a detail of  FIG. 11  to illustrate how the RC time constant associated with word lines may also vary based upon the word line&#39;s location within a block. 
         FIG. 20  represents a top view of a portion of an example of a memory chip. 
         FIG. 21  schematically represents some of the elements involved for the single ended time constant measurement of a word line. 
         FIG. 22  is a circuit functionality timing diagram for detecting the charging up of the far end of a word line. 
         FIG. 23  is a block diagram of an exemplary RC measurement circuit. 
         FIG. 24  illustrates an alternate embodiment using three detection points determines the time constant by discharging, rather than charging, the far end of the line. 
         FIG. 25  is similar to  FIG. 21 , but with a current detector at the near end for the implementation of the embodiment of  FIG. 24 . 
         FIG. 26  illustrates the splitting of word lines into multiple zones. 
         FIG. 27A  is a schematic representation of the relative rates are which differing word lines charge up. 
         FIG. 27B  is a schematic representation of the time savings available by using differing timings for word lines with a faster RC constant. 
         FIG. 28  is a block diagram of some of the circuit elements used for performance optimization based on the different settling times. 
     
    
    
     DETAILED DESCRIPTION 
     Memory System 
       FIG. 1  illustrates schematically the main hardware components of a memory system suitable for implementing the following. The memory system  90  typically operates with a host  80  through a host interface. The memory system may be in the form of a removable memory such as a memory card, or may be in the form of an embedded memory system. The memory system  90  includes a memory  102  whose operations are controlled by a controller  100 . The memory  102  comprises one or more array of non-volatile memory cells distributed over one or more integrated circuit chip. The controller  100  may include interface circuits  110 , a processor  120 , ROM (read-only-memory)  122 , RAM (random access memory)  130 , programmable nonvolatile memory  124 , and additional components. The controller is typically formed as an ASIC (application specific integrated circuit) and the components included in such an ASIC generally depend on the particular application. 
     With respect to the memory section  102 , 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, phase change material, etc., and optionally a steering element, such as a diode, etc. 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 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 exemplary, 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 two dimensional memory structure or a three dimensional memory structure. 
     In a two dimensional memory structure, the semiconductor memory elements are arranged in a single plane or a single memory device level. Typically, in a two dimensional memory structure, memory elements are arranged in a plane (e.g., in an x-z 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 three dimensional 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 y direction is substantially perpendicular and the x and z directions are substantially parallel to the major surface of the substrate). 
     As a non-limiting example, a three dimensional memory structure may be vertically arranged as a stack of multiple two dimensional memory device levels. As another non-limiting example, a three dimensional 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 in each column. The columns may be arranged in a two dimensional configuration, e.g., in an x-z plane, resulting in a three dimensional arrangement of memory elements with elements on multiple vertically stacked memory planes. Other configurations of memory elements in three dimensions can also constitute a three dimensional memory array. 
     By way of non-limiting example, in a three dimensional NAND memory array, the memory elements may be coupled together to form a NAND string within a single horizontal (e.g., x-z) memory device levels. Alternatively, the memory elements may be coupled together to form a vertical NAND string that traverses across multiple horizontal memory device levels. Other three dimensional 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. Three dimensional memory arrays may also be designed in a NOR configuration and in a ReRAM configuration. 
     Typically, in a monolithic three dimensional memory array, one or more memory device levels are formed above a single substrate. Optionally, the monolithic three dimensional 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 three dimensional 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 three dimensional memory array may be shared or have intervening layers between memory device levels. 
     Then again, two dimensional 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 three dimensional memory arrays. Further, multiple two dimensional memory arrays or three dimensional 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. 
     It will be recognized that the following is not limited to the two dimensional and three dimensional exemplary structures described but cover all relevant memory structures within the spirit and scope as described herein 
     Physical Memory Structure 
       FIG. 2  illustrates schematically a non-volatile memory cell. The memory cell  10  can be implemented by a field-effect transistor having a charge storage unit  20 , such as a floating gate or a charge trapping (dielectric) layer. The memory cell  10  also includes a source  14 , a drain  16 , and a control gate  30 . 
     There are many commercially successful non-volatile solid-state memory devices being used today. These memory devices may employ different types of memory cells, each type having one or more charge storage element. 
     Typical non-volatile memory cells include EEPROM and flash EEPROM. Also, examples of memory devices utilizing dielectric storage elements. 
     In practice, the memory state of a cell is usually read by sensing the conduction current across the source and drain electrodes of the cell when a reference voltage is applied to the control gate. Thus, for each given charge on the floating gate of a cell, a corresponding conduction current with respect to a fixed reference control gate voltage may be detected. Similarly, the range of charge programmable onto the floating gate defines a corresponding threshold voltage window or a corresponding conduction current window. 
     Alternatively, instead of detecting the conduction current among a partitioned current window, it is possible to set the threshold voltage for a given memory state under test at the control gate and detect if the conduction current is lower or higher than a threshold current (cell-read reference current). In one implementation the detection of the conduction current relative to a threshold current is accomplished by examining the rate the conduction current is discharging through the capacitance of the bit line. 
       FIG. 3  illustrates the relation between the source-drain current I D  and the control gate voltage V CG  for four different charges Q 1 -Q 4  that the floating gate may be selectively storing at any one time. With fixed drain voltage bias, the four solid I D  versus V CG  curves represent four of seven possible charge levels that can be programmed on a floating gate of a memory cell, respectively corresponding to four possible memory states. As an example, the threshold voltage window of a population of cells may range from 0.5V to 3.5V. Seven possible programmed memory states “0”, “1”, “2”, “3”, “4”, “5”, “6”, and an erased state (not shown) may be demarcated by partitioning the threshold window into regions in intervals of 0.5V each. For example, if a reference current, IREF of 2 μA is used as shown, then the cell programmed with Q 1  may be considered to be in a memory state “1” since its curve intersects with I REF  in the region of the threshold window demarcated by VCG=0.5V and 1.0V. Similarly, Q 4  is in a memory state “5”. 
     As can be seen from the description above, the more states a memory cell is made to store, the more finely divided is its threshold window. For example, a memory device may have memory cells having a threshold window that ranges from −1.5V to 5V. This provides a maximum width of 6.5V. If the memory cell is to store 16 states, each state may occupy from 200 mV to 300 mV in the threshold window. This will require higher precision in programming and reading operations in order to be able to achieve the required resolution. 
     NAND Structure 
       FIG. 4  illustrates schematically a string of memory cells organized into a NAND string. A NAND string  50  comprises a series of memory transistors M 1 , M 2 , . . . Mn (e.g., n=4, 8, 16 or higher) daisy-chained by their sources and drains. A pair of select transistors S 1 , S 2  controls the memory transistor chain&#39;s connection to the external world via the NAND string&#39;s source terminal  54  and drain terminal  56  respectively. In a memory array, when the source select transistor S 1  is turned on, the source terminal is coupled to a source line (see  FIG. 5 ). Similarly, when the drain select transistor S 2  is turned on, the drain terminal of the NAND string is coupled to a bit line of the memory array. Each memory transistor  10  in the chain acts as a memory cell. It has a charge storage element  20  to store a given amount of charge so as to represent an intended memory state. A control gate  30  of each memory transistor allows control over read and write operations. As will be seen in  FIG. 5 , the control gates  30  of corresponding memory transistors of a row of NAND string are all connected to the same word line. Similarly, a control gate  32  of each of the select transistors S 1 , S 2  provides control access to the NAND string via its source terminal  54  and drain terminal  56  respectively. Likewise, the control gates  32  of corresponding select transistors of a row of NAND string are all connected to the same select line. 
     When an addressed memory transistor  10  within a NAND string is read or is verified during programming, its control gate  30  is supplied with an appropriate voltage. At the same time, the rest of the non-addressed memory transistors in the NAND string  50  are fully turned on by application of sufficient voltage on their control gates. In this way, a conductive path is effectively created from the source of the individual memory transistor to the source terminal  54  of the NAND string and likewise for the drain of the individual memory transistor to the drain terminal  56  of the cell. 
       FIG. 5  illustrates an example of a NAND array  210  of memory cells, constituted from NAND strings  50  such as that shown in  FIG. 4 . Along each column of NAND strings, a bit line such as bit line  36  is coupled to the drain terminal  56  of each NAND string. Along each bank of NAND strings, a source line such as source line  34  is coupled to the source terminals  54  of each NAND string. Also the control gates along a row of memory cells in a bank of NAND strings are connected to a word line such as word line  42 . The control gates along a row of select transistors in a bank of NAND strings are connected to a select line such as select line  44 . An entire row of memory cells in a bank of NAND strings can be addressed by appropriate voltages on the word lines and select lines of the bank of NAND strings. 
       FIG. 6  illustrates a page of memory cells, organized in the NAND configuration, being sensed or programmed in parallel.  FIG. 6  essentially shows a bank of NAND strings  50  in the memory array  210  of  FIG. 5 , where the detail of each NAND string is shown explicitly as in  FIG. 4 . A physical page, such as the page  60 , is a group of memory cells enabled to be sensed or programmed in parallel. This is accomplished by a corresponding page of sense amplifiers  212 . The sensed results are latched in a corresponding set of latches  214 . Each sense amplifier can be coupled to a NAND string via a bit line. The page is enabled by the control gates of the cells of the page connected in common to a word line  42  and each cell accessible by a sense amplifier accessible via a bit line  36 . As an example, when respectively sensing or programming the page of cells  60 , a sensing voltage or a programming voltage is respectively applied to the common word line WL 3  together with appropriate voltages on the bit lines. 
     Physical Organization of the Memory 
     One difference between flash memory and other of types of memory is that a cell is programmed from the erased state. That is, the floating gate is first emptied of charge. Programming then adds a desired amount of charge back to the floating gate. It does not support removing a portion of the charge from the floating gate to go from a more programmed state to a lesser one. This means that updated data cannot overwrite existing data and is written to a previous unwritten location. 
     Furthermore erasing is to empty all the charges from the floating gate and generally takes appreciable time. For that reason, it will be cumbersome and very slow to erase cell by cell or even page by page. In practice, the array of memory cells is divided into a large number of blocks of memory cells. As is common for flash EEPROM systems, the block is the unit of erase. That is, each block contains the minimum number of memory cells that are erased together. While aggregating a large number of cells in a block to be erased in parallel will improve erase performance, a large size block also entails dealing with a larger number of update and obsolete data. 
     Each block is typically divided into a number of physical pages. A logical page is a unit of programming or reading that contains a number of bits equal to the number of cells in a physical page. In a memory that stores one bit per cell, one physical page stores one logical page of data. In memories that store two bits per cell, a physical page stores two logical pages. The number of logical pages stored in a physical page thus reflects the number of bits stored per cell. In one embodiment, the individual pages may be divided into segments and the segments may contain the fewest number of cells that are written at one time as a basic programming operation. One or more logical pages of data are typically stored in one row of memory cells. A page can store one or more sectors. A sector includes user data and overhead data. 
     All-bit, Full-Sequence MLC Programming 
       FIG. 7A-7C  illustrate an example of programming a population of 4-state memory cells.  FIG. 7A  illustrates the population of memory cells programmable into four distinct distributions of threshold voltages respectively representing memory states “0”, “1”, “2” and “3”.  FIG. 7B  illustrates the initial distribution of “erased” threshold voltages for an erased memory.  FIG. 6C  illustrates an example of the memory after many of the memory cells have been programmed. Essentially, a cell initially has an “erased” threshold voltage and programming will move it to a higher value into one of the three zones demarcated by verify levels vV 1 , vV 2  and vV 3 . In this way, each memory cell can be programmed to one of the three programmed states “1”, “2” and “3” or remain un-programmed in the “erased” state. As the memory gets more programming, the initial distribution of the “erased” state as shown in  FIG. 7B  will become narrower and the erased state is represented by the “0” state. 
     A 2-bit code having a lower bit and an upper bit can be used to represent each of the four memory states. For example, the “0”, “1”, “2” and “3” states are respectively represented by “11”, “01”, “00” and “10”. The 2-bit data may be read from the memory by sensing in “full-sequence” mode where the two bits are sensed together by sensing relative to the read demarcation threshold values rV 1 , rV 2  and rV 3  in three sub-passes respectively. 
     3-D NAND Structures 
     An alternative arrangement to a conventional two-dimensional (2-D) NAND array is a three-dimensional (3-D) array. In contrast to 2-D NAND arrays, which are formed along a planar surface of a semiconductor wafer, 3-D arrays extend up from the wafer surface and generally include stacks, or columns, of memory cells extending upwards. Various 3-D arrangements are possible. In one arrangement a NAND string is formed vertically with one end (e.g. source) at the wafer surface and the other end (e.g. drain) on top. In another arrangement a NAND string is formed in a U-shape so that both ends of the NAND string are accessible on top, thus facilitating connections between such strings. 
       FIG. 8  shows a first example of a NAND string  701  that extends in a vertical direction, i.e. extending in the z-direction, perpendicular to the x-y plane of the substrate. Memory cells are formed where a vertical bit line (local bit line)  703  passes through a word line (e.g. WL 0 , WL 1 , etc.). A charge trapping layer between the local bit line and the word line stores charge, which affects the threshold voltage of the transistor formed by the word line (gate) coupled to the vertical bit line (channel) that it encircles. Such memory cells may be formed by forming stacks of word lines and then etching memory holes where memory cells are to be formed. Memory holes are then lined with a charge trapping layer and filled with a suitable local bit line/channel material (with suitable dielectric layers for isolation). 
     As with planar NAND strings, select gates  705 ,  707 , are located at either end of the string to allow the NAND string to be selectively connected to, or isolated from, external elements  709 ,  711 . Such external elements are generally conductive lines such as common source lines or bit lines that serve large numbers of NAND strings. Vertical NAND strings may be operated in a similar manner to planar NAND strings and both SLC and MLC operation is possible. While  FIG. 8  shows an example of a NAND string that has 32 cells ( 0 - 31 ) connected in series, the number of cells in a NAND string may be any suitable number. Not all cells are shown for clarity. It will be understood that additional cells are formed where word lines  3 - 29  (not shown) intersect the local vertical bit line. 
     A 3D NAND array can, loosely speaking, be formed tilting up the respective structures  50  and  210  of  FIGS. 5 and 6  to be perpendicular to the x-y plane. In this example, each y-z plane corresponds to the page structure of  FIG. 6 , with m such plane at differing x locations. The (global) bit lines, BL 1 -m, each run across the top to an associated sense amp SA 1 -m. The word lines, WL 1 -n, and source and select lines SSL 1 -n and DSL 1 -n, then run in x direction, with the NAND string connected at bottom to a common source line CSL. 
       FIGS. 9-12  look at a particular monolithic three dimensional (3D) memory array of the NAND type (more specifically of the “BiCS” type), where one or more memory device levels are formed above a single substrate, in more detail.  FIG. 9  is an oblique projection of part of such a structure, showing a portion corresponding to two of the page structures in  FIG. 5 , where, depending on the embodiment, each of these could correspond to a separate block or be different “fingers” of the same block. Here, instead to the NAND strings lying in a common y-z plane, they are squashed together in the y direction, so that the NAND strings are somewhat staggered in the x direction. On the top, the NAND strings are connected along global bit lines (BL) spanning multiple such sub-divisions of the array that run in the x direction. Here, global common source lines (SL) also run across multiple such structures in the x direction and are connect to the sources at the bottoms of the NAND string, which are connected by a local interconnect (LI) that serves as the local common source line of the individual finger. Depending on the embodiment, the global source lines can span the whole, or just a portion, of the array structure. Rather than use the local interconnect (LI), variations can include the NAND string being formed in a U type structure, where part of the string itself runs back up. 
     To the right of  FIG. 9  is a representation of the elements of one of the vertical NAND strings from the structure to the left. Multiple memory cells are connected through a drain select gate SGD to the associated bit line BL at the top and connected through the associated source select gate SDS to the associated local source line LI to a global source line SL. It is often useful to have a select gate with a greater length than that of memory cells, where this can alternately be achieved by having several select gates in series (as described in U.S. patent application Ser. No. 13/925,662, filed on Jun. 24, 2013), making for more uniform processing of layers. Additionally, the select gates are programmable to have their threshold levels adjusted. This exemplary embodiment also includes several dummy cells at the ends that are not used to store user data, as their proximity to the select gates makes them more prone to disturbs. 
       FIG. 10  shows a top view of the structure for two blocks in the exemplary embodiment. Two blocks (BLK 0  above, BLK 1  below) are shown, each having four fingers that run left to right. The word lines and select gate lines of each level also run left to right, with the word lines of the different fingers of the same block being commonly connected at a “terrace” and then on to receive their various voltage level through the word line select gates at WLTr. The word lines of a given layer in a block can also be commonly connected on the far side from the terrace. The selected gate lines can be individual for each level, rather common, allowing the fingers to be individually selected. The bit lines are shown running up and down the page and connect on to the sense amp circuits, where, depending on the embodiment, each sense amp can correspond to a single bit line or be multiplexed to several bit lines. 
       FIG. 11  shows a side view of one block, again with four fingers. In this exemplary embodiment, the select gates SGD and SGS at either end of the NAND strings are formed of four layers, with the word lines WL in-between, all formed over a CPWELL. A given finger is selected by setting its select gates to a level VSG and the word lines are biased according to the operation, such as a read voltage (VCGRV) for the selected word lines and the read-pass voltage (VREAD) for the non-selected word lines. The non-selected fingers can then be cut off by setting their select gates accordingly. 
       FIG. 12  illustrates some detail of an individual cell. A dielectric core runs in the vertical direction and is surrounded by a channel silicon layer, that is in turn surrounded a tunnel dielectric (TNL) and then the charge trapping dielectric layer (CTL). The gate of the cell is here formed of tungsten with which is surrounded by a metal barrier and is separated from the charge trapping layer by blocking (BLK) oxide and a high K layer. 
     Block Segmentation for Faster Bit Line Settling/Recovery 
     In memory arrays, such as those described above, memory cells are typically formed along word lines and bit lines.  FIGS. 5 and 10  show some examples of array structures where the NAND strings of memory cells are connected along global bit lines running across the array. These bit lines span multiple blocks or, typically, an entire plane. Sensing and other operations often require that bit lines be set to a particular voltage level. Blocks nearer to the sense amplifier circuits see a faster bit line settling/recover time compared to blocks further from the sense amps. This is because the far side blocks sees a huge bit line resistance and have an IR drop across it. Thus, the far side bit line level will be less compared to near side. As the far side bit line level is less compared to near blocks, it strongly couples its neighbor bit lines, taking more time to settle/recover. Consequently, far side blocks have and thus we see slower performance and also burn more current as current is flowing for a longer time until the BL settles/recovers. To avoid error, the memory device would set timings based on the slowest blocks, requiring more time and current than needed for the faster, nearer blocks. 
     The situation can be illustrated by  FIG. 13  that shows an array of N+1 blocks, where N can run into the hundreds or thousands. A set of bit lines BL 0  to BLM,  303 - 0  to  303 -M spans the blocks and are each connected to a corresponding sense amp SA  305 - 0  to  305 -M.  FIG. 13  shows one bit line per sense amp, but in other arrangements where less than all of the bit lines are selected concurrently (as where only every other or every fourth bit line is sensed together). The resistance along the bit lines between the near (to the sense amps) and far blocks is represented schematically as R_BL and the inter-bit line capacitance is also shown. This resistance along the bit lines and the inter-bit line capacitance can result to significant differences in settling times between the nearest block  301 -N and the farthest block  301 - 0 .  FIG. 14  considers this situation further. 
     A sensing operation can be of the lock out type or no lock out type. When reading multi-state data, such as illustrated above with respect to  FIG. 7A-7C , the memory typically starts with the lowest state and works its way up through the higher states. Once a cell is read and determined to be in, say, the 1 state, it does not need to be checked for the 2 and higher states; and if the cell is read for these higher states, they will be conducting, wasting current while providing no additional information. To avoid this, the memory device can use a “lock out read” where, once a cell&#39;s state is determined, that bit line is locked out from further reading for other, higher states until that page is finished and the memory moves on to a new page. Consequently, a lock out read uses less current, but at the cost greater complexity and lower performance so that “no lock out” (NLO) operations are also commonly used, depending on the application. (More information on no lock out reads and also sense amplifier circuitry applicable here can be found in US patent publication number 2014-0003157.) The consequences of the near block-far block variation is more pronounced in the no lock out case, as can be illustrated with respect to FIG.  14 . 
       FIG. 14  looks at the most problematical bit line pattern for the no lock out case. With respect to the center g bit line, the second-most adjacent bit lines are highly conductive (as if the corresponding selected memory cells were in, for example, the 0 state and the sensing operation is for the 3 state, pulling, say, 200 nA) can couple down the immediate neighbor non-conducting bit lines, making their ramp up slower and resulting in a longer bit line settling time. The amount of coupling is dependent on the final level of the highly conducting bit lines. Due to bit line resistance, when a far side block is selected the bit line will see a significant IR drop and the highly conductive bit line level at the far side block will be much lower, thus coupling even more strongly, leading to longer bit line settling time. 
     The following discussion is mainly given the context of a 3D NAND structure of the BiCS type since the situation is often more acute for this architecture, but the techniques described are applicable to other types of array architectures, including the 2D NAND. The reason why the problem is more acute for the BiCS types structure can be seen by comparing  FIG. 6  with  FIG. 15 . In the 2D NAND block of  FIG. 6 , each bit line (aside from the end bit lines) has one neighbor on each side.  FIG. 15  is a more detailed view of a portion of a finger structure seen from above, where the NAND strings run down into the page. Looking at NAND string PL26, this has a number of near neighbors with which it may couple and the off-setting of these columns places the global bit lines across the top in close proximity, aggravating the problem relative to the 2D structure. Whether for 2D or 3D, the continued in decreasing device scales and increasing of place size will continue to make this problem prominent. 
       FIG. 16  illustrates the effect, showing the relative times for pre-charging a bit line for a near block, TBL(near), and a bit line for a far block, TBL(far). The near block bit line has a shorting settling time, ramping up mover quickly than for the far side, consequently having faster performance. If the same timing is used for both the near and far blocks, the additional time allotted for the far blocks will burn current not needed to the settling process. To be able to improve performance and power consumption for the block nearer the sense amp circuits, these can use different timing. In the exemplary embodiment, the total number of blocks is divided into N number of segments, where the closer a segments is to the sense amps can, short the time allotted, improving performance and using less current.  FIG. 17  represents this schematically. 
     As shown in  FIG. 17 , the blocks spanned by the bit lines are split up into a number of segments, each of one or more adjacent blocks. In the example, Segment  0  is nearest the sense amplifier circuits and each of the N additional segments, where N≧1, use a different timing depending on proximity to the sense amps. Being closest to the sense amps, the blocks in Segment  0  will have the best performance, use the least system current ICC, and can use the fasting timing, Tbl. Each of the other segments will have different timing offset Δi for Segment i, the offsets being longer as the block segment is further away: Δ 1 &lt;Δ 2 &lt;Δ 3  . . . &lt;ΔN. (Equivalently, the base timing could be for the furthest or an intermediate Segment, the offsets being decreases or increases as appropriate.) The setting of the time can be based on a control signal (here represented as “cont”) to the sense amp circuitry from the on-chip control logic, for example, based on the physical address to be accessed. For example, a sensing operation will typically have a number of sub-operations or phases to establish the needed bias conditions, such as setting levels on a bit line clamp, turning on select gates, and so on. The total read or verify time while the combined clocks from these phases. Although the other phases&#39; clocks signals can be the same, the control signal here will set the time allotted for bit lines to settle after a bit line select gate is turned on to pre-charge the bit line form the sense amp. 
     In  FIG. 17  the segments are represented as being of the same size, but the number of blocks can also vary from segment to segment if the timing variations along the bit lines is particularly non-linear, with, for example, segments having fewer blocks where timing variations change more quickly. Similarly, the difference in the Δi may be a uniform step size (i.e., number of clock cycles) or not, depending upon device behavior. 
     The difference in segment timings can be based on using a different number of clock cycles. The values can be settable parameters determined by the behavior of the device or similar devices during device characterization tests. Depending on the embodiment, the initial values set prior the devices being supplied to users can be fixed or changed as the device ages. A wide range of values for the parameter can be available, from which values can be selected, where these values may change as a device ages, depending on the change of sheet resistance of the metal and also bit line lengths. The settling times involved can be for sensing (both data read and verify) operations, including pre-charge and other phases where a bit line level needs to be set, or other operations using the sense amps to set bit line levels. 
       FIG. 18  is a simplified flow of some aspects of the process involved. The memory circuit receives an access request for a specified physical address at  501 . At  503  the memory circuit then determines the block segment, or more generally just the block, to which the specified address corresponds. At  505 , the core timing is then set for the corresponding operation based on the determined block segment to reflect the proximity to the sense amps. In this way, the memory system can provide high performance while reducing current consumption. 
     Single Ended Word Line/Bit Line Time Constant Measurement 
     The preceding section considered block segmentation for faster bit line settling and recovery. The following section considers word line settling times. In both cases, time constants related to the rate at which the bit lines or word lines charge or discharge are used. In memory circuits such as those described above, each of the word lines and bit lines are frequently driven from just a single end. This section looks at techniques for determining these sorts of time constants. In particular, it presents techniques for determining at the driving end the rate at which the far end of an access line (such as bit line or word line) charges or discharge in order to determining the time constants used. 
     The preceding section discussed how the RC constant along a bit line can vary based upon a block&#39;s location. The RC time constant associated with word lines may also vary based upon the word line&#39;s location within a block.  FIG. 19  illustrates the situation in a BiCS context.  FIG. 19  is a detail of  FIG. 11  and shows a portion of one finger and the adjacent local interconnect for the common source line. Here 9 horizontal word lines, such as  601 , are shown along with 4 memory holes, such as  603 , along which the NAND strings reside. To the left is a local interconnect LI  605  for the source line. Due to processing differences, the word lines are not uniform, varying from one another down the stack. As illustrated, the word lines tend to get wider, and the memory holes narrower, as they are deeper in the stack. For example, variations in the spacing from LI to the different word lines can arise from the etching process. This can result in the RC time constant for the top word line differing noticeably from that of the bottom word line, perhaps by a factor of two or more. This sort of variation in word line and bit line time constants makes it useful to measure them. 
     The determination of the time constant at the far end of a control line when driving it at the near could be done by actually measuring at the far end; however, a typical memory structure generally lacks the needed connections and circuitry on the far end.  FIG. 20  illustrates some of the constraints involved in RC time constant detection.  FIG. 20  represents a top view of a portion of an example of a memory chip. In the shown arrangement, there are two planes (Plane  0 , Plane  1 ), each with two memory arrays. Along the bottom of each array are the sense amplifier circuits SA, as in  FIG. 13 , and below these are the column related logic YLOG and other peripheral circuitry PERI for the memory&#39;s operation. The row decoders used in decoding and driving the word lines are formed between the pair of arrays of each plane, so that the circuitry is connected along only the one side of the word lines. Under this arrangement, there is only the structure to perform a time constant measurement using just one input/output source for word lines (from the row decoders) and bit lines (from the sense amp end). 
     When a bit line or word line is charged up from the supply, the current is supplied by an RC network. To the first order, the equation for the current i from the supply vsup can be expressed as
 
 i ( v sup)= A  exp(− t /τ).
 
If values can be obtained for i(vsup) for two time values, this can be solved for the constants A and τ 32  RC. The output at voltage v(WL) at far side of the WL can be expressed as
 
 v (WL)= V SUP×(1−exp(− t /τ)).
 
       FIG. 21  schematically represents some of the elements involved for the single ended time constant measurement of a word line. A supply  701  is connected through decoding circuitry, here a switch  703 , to the word line, whose intermediate portion is here represented as an RC network. To perform a time constant measurement, the word line can initially be grounded, after which VSUP is connected by flipping the switch  703 . The number of clock pulses until the current i(VSUP) crosses certain points can be counted. These crossing points can then be used to extract the time and i(VSUP) relationship with a curve fit to the exponential equation to obtain time constant. 
       FIG. 22  is a circuit functionality timing diagram for detecting the charging up of the word line. At top is the voltage level at the far side, starting at ground and asymptotically approaching the supply level. In the middle is the current i(vsup) from the supply, initially high when the switch is closed and then dropping past first and second detection points. The first detection point is a first current trip point that start the clock CLK and the second detection point is the trip point to turn the clock off. The number of clock pulses times the clock period is then the time when i(vsup) equals to a predefined current trip point. 
     A fast RC measurement can be done by logic on the circuit. Returning to the equations, as i(t)=A exp(−t/RC), then t=RC×ln(A/i(t), so that 
                 Δ   ⁢           ⁢   t     -     t   1     -     t   0     -     RC   ×     (       ln   ⁡     (     A     i   ⁡     (     t   1     )         )       -     ln   ⁡     (     A     i   ⁡     (     t   0     )         )         )         =     RC   ×     ln   ⁡     (       i   ⁡     (     t   0     )         i   ⁡     (     t   1     )         )               
The difference in time can be measured by the number of clocks n between t 1  and t 0 , Δt=n×t clk , so that RC=n×{t clk /ln(i(t 0 )/i(t j ))}. The detection current can be set at convenient values for easier calculation. For example, setting i(t 0 )/i(t 1 ) to 2.718 (e) or 7.389 (e 2 ) respectively gives ln(i(t 0 )/i(t 1 )=1 or 2. For example, i(t 0 )/i(t 1 )=2.718 yields RC=n×t clk , simplifying calculations.
 
       FIG. 23  is a block diagram of an exemplary RC measurement circuit. The word line voltage CG is supplied by the current mirror  801  that mirror this level for a current detector circuit  821 . The current mirror  801  provides the word line voltage CG through a transistor  805  above a node of a voltage divider  809 , shown as a pair of resistors in this example. The gate of the supply transistor is controlled by op-amp  803  whose first input connected to a reference level VREF and whose second input is taken from the feedback network  809 . The word line voltage is taken from the first leg of the mirroring transistors  807 , whose second leg provides the mirrored current to the current detector  821  and includes the current sink  811  to cancel resistor current. 
     In the current detector section  821 , the GC current mirror value is supplied to a first leg of current mirror there through the transistors  823  when enabled by the DET_EN_A signal. A mirroring section  825  has three legs that are selectively enabled based on corresponding signals DET_1ST_A, DET_2ND_A and DET_3RD_A that are sized in the ratios of 1, 2.72, and 7.39 in order to simplify the logics computation, as discussed above. The three legs of the mirroring section  825  are connected to one input of an op-amp 827  that has its other input connected to a fixed reference current  829  for comparator offset cancellation and has a flag value FLG as the output. The logic block has two main functions: to use FLG to determine which current mirror branch of  825  to enable; and enable the counter depending on the DET_1ST/2ND/3RD_A signals. 
     The embodiment of  FIG. 22  illustrated using two detection points, but more can be used for increased accuracy, as with the three values of  FIG. 23 .  FIG. 24  illustrates an alternate embodiment using three detection points determines the time constant by discharging, rather than charging, the far end of the line. 
       FIG. 24  is a timing diagram for discharging the word line&#39;s far end as represented schematically by  FIG. 25 , that is similar to  FIG. 21  but with a current detector  709  at the near end. Initially the far side of the word lines is pre-charged. The detection process begins when the switch  703  is opened and the word line&#39;s far end begins to discharge, as shown in the top line. As shown in the second line, the current level goes high and asymptotically decreases, passing first, second, and third detection points. The clock signal s shown at bottom. The number of clock pulses time the clock period gives the time when i(VSUP) equals the predefined current trip point. 
     Performance Optimization for Word Line Settling 
     This section looks at optimizing the performance of the memory by varying timing based on the time constant for the word line&#39;s settling, either on a word line by word line basis or by grouping word lines with similar RC values into zones.  FIG. 26  illustrates the splitting of word lines into multiple zones, where the example shows three zones for the shown word lines. The techniques of the previous section can be used to measure the worst word line RC constant within each of the zones and the result can be written into the corresponding parameter values. The memory can then adjust a selected word line&#39;s ramp time according to its zone to gain performance on average. In one embodiment, the parameter measurement can be done through the controller, with the loading of the parameter based on a selected word line is then done independently of the controller. 
       FIG. 27A  looks at the case without word line RC feedback so that all words lines use the same timing. When the word lines begin to ramp up, word lines with a small RC constant will ramp up more quickly than word lines with a big RC constant. If all word lines use the same fixed word line settling time, the settling/ramp time for sensing, programming and other operations requiring a settled word line value will be limited by the worst case word line RC time constant. If instead a variable word line settling time is used, operations can start earlier on word lines with smaller time constants.  FIG. 27B  schematically illustrates the time savings. 
       FIG. 28  is a block diagram of some of the circuit elements used for performance optimization based on the different settling times. A portion of a single finger is shown at  901 , with the word lines split into three zones. A row decoder is represented at ROWDEC  903  that receives the various bias voltages from the drivers at  905 . The figure shows the selected word line as being in word line zone # 2 . NAND memory operations often require that non-selected word lines also be biased to some level to which they will also need to ramp up, and that this non-selected word lines will have a different RC constant. These non-selected word line&#39;s ramp times sill often not be limiting as the levels on these lines will have previously be established. The logic block  911  provides the word line decoding signals and includes a finite state machine  913  and can use a look up table, for example, to provide values for the ramp times for different zones, such as illustrated schematically at  915  with some numbers listed for demonstration purposes. An RC Measurement Circuit  921 , such as that in  FIG. 23 , can determine the values for the table  915 . This arrangement improves performance as all but the slowest zone can use faster ramp times than the worst case. 
     CONCLUSION 
     The forgoing has presented a set of techniques and circuitry for a singled ended determination of time constants, where the exemplary circuit used a clock counter and current comparator with relatively minor penalty in area. These time constants can then be used to improve performance on average by using them to have differing ramp for different word lines or bit lines based on their locations within the array. The determined constants can also be used to provide finer word line ramp rate control so that word line ramp time is more consistent regardless of load. The time constants can be measured on-die, without need for hooking up the die to a measurement device, and can be quickly calculated. 
     The foregoing detailed description has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the above to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. The described embodiments were chosen in order to explain the principles involved and its practical application, to thereby enable others to best utilize the various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope be defined by the claims appended hereto.