Abstract:
A non-volatile memory device capable of reading and writing a large number of memory cells with multiple read/write circuits in parallel has an architecture that reduces redundancy in the multiple read/write circuits to a minimum. The multiple read/write circuits are organized into a bank of similar stacks of components. Redundant circuits such as a processor for processing data among stacks each associated with multiple memory cells are factored out. The processor is implemented with an input logic, a latch and an output logic. The input logic can transform the data received from either the sense amplifier or the data latches. The output logic further processes the transformed data to send to either the sense amplifier or the data latches or to a controller. This provides an infrastructure with maximum versatility and a minimum of components for sophisticated processing of the data sensed and the data to be input or output.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a continuation of application Ser. No. 11/026,536, filed on Dec. 29, 2004, which applications are incorporated herein in their entirety by this reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]     This invention relates generally to non-volatile semiconductor memory such as electrically erasable programmable read-only memory (EEPROM) and flash EEPROM, and specifically ones having an aggregate of highly compact and high performance read/write circuits sharing a common processor.  
       BACKGROUND OF THE INVENTION  
       [0003]     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, retaining its stored data even after power is turned off. In spite of the higher cost, flash memory is increasingly being used in mass storage applications. Conventional mass storage, based on rotating magnetic medium such as hard drives and floppy disks, is unsuitable for the mobile and handheld environment. This is because disk drives tend to be bulky, are prone to mechanical failure and have high latency and high power requirements. These undesirable attributes make disk-based storage impractical in most mobile and portable applications. On the other hand, flash memory, both embedded and in the form of a removable card is ideally suited in the mobile and handheld environment because of its small size, low power consumption, high speed and high reliability features.  
         [0004]     EEPROM and electrically programmable read-only memory (EPROM) are 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.  
         [0005]     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.  
         [0006]     The transistor serving as a memory cell is typically programmed to a “programmed” state by one of two mechanisms. In “hot electron injection,” a high voltage applied to the drain accelerates electrons across the substrate channel region. At the same time a high voltage applied to the control gate pulls the hot electrons through a thin gate dielectric onto the floating gate. In “tunneling injection,” a high voltage is applied to the control gate relative to the substrate. In this way, electrons are pulled from the substrate to the intervening floating gate.  
         [0007]     The memory device may be erased by a number of mechanisms. For EPROM, the memory is bulk erasable by removing the charge from the floating gate by ultraviolet radiation. For EEPROM, a memory cell is electrically erasable, by applying a high voltage to the substrate relative to the control gate so as to induce electrons in the floating gate to tunnel through a thin oxide to the substrate channel region (i.e., Fowler-Nordheim tunneling.) Typically, the EEPROM is erasable byte by byte. For flash EEPROM, the memory is electrically erasable either all at once or one or more blocks at a time, where a block may consist of 512 bytes or more of memory.  
       EXAMPLES OF NON-VOLATILE MEMORY CELLS  
       [0008]     The memory devices typically comprise one or more memory chips that may be mounted on a card. Each memory chip comprises an array of memory cells supported by peripheral circuits such as decoders and erase, write and read circuits. The more sophisticated memory devices also come with a controller that performs intelligent and higher level memory operations and interfacing. 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.  
         [0009]      FIGS. 1A-1E  illustrate schematically different examples of non-volatile memory cells.  
         [0010]      FIG. 1A  illustrates schematically a non-volatile memory in the form of an EEPROM cell with a floating gate for storing charge. An electrically erasable and programmable read-only memory (EEPROM) has a similar structure to EPROM, but additionally provides a mechanism for loading and removing charge electrically from its floating gate upon application of proper voltages without the need for exposure to UV radiation. Examples of such cells and methods of manufacturing them are given in U.S. Pat. No. 5,595,924.  
         [0011]      FIG. 1B  illustrates schematically a flash EEPROM cell having both a select gate and a control or steering gate. The memory cell  10  has a “split-channel”  12  between source  14  and drain  16  diffusions. A cell is formed effectively with two transistors T 1  and T 2  in series. T 1  serves as a memory transistor having a floating gate  20  and a control gate  30 . The floating gate is capable of storing a selectable amount of charge. The amount of current that can flow through the T 1 &#39;s portion of the channel depends on the voltage on the control gate  30  and the amount of charge residing on the intervening floating gate  20 . T 2  serves as a select transistor having a select gate  40 . When T 2  is turned on by a voltage at the select gate  40 , it allows the current in the T 1 &#39;s portion of the channel to pass between the source and drain. The select transistor provides a switch along the source-drain channel independent of the voltage at the control gate. One advantage is that it can be used to turn off those cells that are still conducting at zero control gate voltage due to their charge depletion (positive) at their floating gates. The other advantage is that it allows source side injection programming to be more easily implemented.  
         [0012]     One simple embodiment of the split-channel memory cell is where the select gate and the control gate are connected to the same word line as indicated schematically by a dotted line shown in  FIG. 1B . This is accomplished by having a charge storage element (floating gate) positioned over one portion of the channel and a control gate structure (which is part of a word line) positioned over the other channel portion as well as over the charge storage element. This effectively forms a cell with two transistors in series, one (the memory transistor) with a combination of the amount of charge on the charge storage element and the voltage on the word line controlling the amount of current that can flow through its portion of the channel, and the other (the select transistor) having the word line alone serving as its gate. Examples of such cells, their uses in memory systems and methods of manufacturing them are given in U.S. Pat. Nos. 5,070,032, 5,095,344, 5,315,541, 5,343,063, and 5,661,053.  
         [0013]     A more refined embodiment of the split-channel cell shown in  FIG. 1B  is when the select gate and the control gate are independent and not connected by the dotted line between them. One implementation has the control gates of one column in an array of cells connected to a control (or steering) line perpendicular to the word line. The effect is to relieve the word line from having to perform two functions at the same time when reading or programming a selected cell. Those two functions are (1) to serve as a gate of a select transistor, thus requiring a proper voltage to turn the select transistor on and off, and (2) to drive the voltage of the charge storage element to a desired level through an electric field (capacitive) coupling between the word line and the charge storage element. It is often difficult to perform both of these functions in an optimum manner with a single voltage. With the separate control of the control gate and the select gate, the word line need only perform function (1), while the added control line performs function (2). This capability allows for design of higher performance programming where the programming voltage is geared to the targeted data. The use of independent control (or steering) gates in a flash EEPROM array is described, for example, in U.S. Pat. Nos. 5,313,421 and 6,222,762.  
         [0014]      FIG. 1C  illustrates schematically another flash EEPROM cell having dual floating gates and independent select and control gates. The memory cell  10  is similar to that of  FIG. 1B  except it effectively has three transistors in series. In this type of cell, two storage elements (i.e., that of T 1 -left and T 1 -right) are included over its channel between source and drain diffusions with a select transistor T 1  in between them. The memory transistors have floating gates  20  and  20 ′, and control gates  30  and  30 ′, respectively. The select transistor T 2  is controlled by a select gate  40 . At any one time, only one of the pair of memory transistors is accessed for read or write. When the storage unit T 1 -left is being accessed, both the T 2  and T 1 -right are turned on to allow the current in the T 1 -left&#39;s portion of the channel to pass between the source and the drain. Similarly, when the storage unit T 1 -right is being accessed, T 2  and T 1 -left are turned on. Erase is effected by having a portion of the select gate polysilicon in close proximity to the floating gate and applying a substantial positive voltage (e.g. 20V) to the select gate so that the electrons stored within the floating gate can tunnel to the select gate polysilicon.  
         [0015]      FIG. 1D  illustrates schematically a string of memory cells organized into an NAND cell. An NAND cell  50  consists of a series of memory transistors M 1 , M 2 , . . . Mn (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 transistors chain&#39;s connection to the external via the NAND cell&#39;s source terminal  54  and drain terminal  56 . In a memory array, when the source select transistor S 1  is turned on, the source terminal is coupled to a source line. Similarly, when the drain select transistor S 2  is turned on, the drain terminal of the NAND cell is coupled to a bit line of the memory array. Each memory transistor in the chain has a charge storage element to store a given amount of charge so as to represent an intended memory state. A control gate of each memory transistor provides control over read and write operations. A control gate of each of the select transistors S 1 , S 2  provides control access to the NAND cell via its source terminal  54  and drain terminal  56  respectively.  
         [0016]     When an addressed memory transistor within an NAND cell is read and verified during programming, its control gate is supplied with an appropriate voltage. At the same time, the rest of the non-addressed memory transistors in the NAND cell  50  are fully turned on by application of sufficient voltage on their control gates. In this way, a conductive path is effective created from the source of the individual memory transistor to the source terminal  54  of the NAND cell and likewise for the drain of the individual memory transistor to the drain terminal  56  of the cell. Memory devices with such NAND cell structures are described in U.S. Pat. Nos. 5,570,315, 5,903,495, 6,046,935.  
         [0017]      FIG. 1E  illustrates schematically a non-volatile memory with a dielectric layer for storing charge. Instead of the conductive floating gate elements described earlier, a dielectric layer is used. Such memory devices utilizing dielectric storage element have been described by Eitan et al., “NROM: A Novel Localized Trapping, 2-Bit Nonvolatile Memory Cell,” IEEE Electron Device Letters, vol. 21, no. 11, November 2000, pp. 543-545. 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, U.S. Pat. Nos. 5,768,192 and 6,011,725 disclose a nonvolatile memory cell having 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.  
         [0000]     Memory Array  
         [0018]     A memory device typically comprises of a two-dimensional array of memory cells arranged in rows and columns and addressable by word lines and bit lines. The array can be formed according to an NOR type or an NAND type architecture.  
         [0000]     NOR Array  
         [0019]      FIG. 2  illustrates an example of an NOR array of memory cells. Memory devices with an NOR type architecture have been implemented with cells of the type illustrated in  FIGS. 1B  or  1 C. Each row of memory cells are connected by their sources and drains in a daisy-chain manner. This design is sometimes referred to as a virtual ground design. Each memory cell  10  has a source  14 , a drain  16 , a control gate  30  and a select gate  40 . The cells in a row have their select gates connected to word line  42 . The cells in a column have their sources and drains respectively connected to selected bit lines  34  and  36 . In some embodiments where the memory cells have their control gate and select gate controlled independently, a steering line  36  also connects the control gates of the cells in a column.  
         [0020]     Many flash EEPROM devices are implemented with memory cells where each is formed with its control gate and select gate connected together. In this case, there is no need for steering lines and a word line simply connects all the control gates and select gates of cells along each row. Examples of these designs are disclosed in U.S. Pat. Nos. 5,172,338 and 5,418,752. In these designs, the word line essentially performed two functions: row selection and supplying control gate voltage to all cells in the row for reading or programming.  
         [0000]     NAND Array  
         [0021]      FIG. 3  illustrates an example of an NAND array of memory cells, such as that shown in  FIG. 1D . Along each column of NAND cells, a bit line is coupled to the drain terminal  56  of each NAND cell. Along each row of NAND cells, a source line may connect all their source terminals  54 . Also the control gates of the NAND cells along a row are connected to a series of corresponding word lines. An entire row of NAND cells can be addressed by turning on the pair of select transistors (see  FIG. 1D ) with appropriate voltages on their control gates via the connected word lines. When a memory transistor within the chain of a NAND cell is being read, the remaining memory transistors in the chain are turned on hard via their associated word lines so that the current flowing through the chain is essentially dependent upon the level of charge stored in the cell being read. An example of an NAND architecture array and its operation as part of a memory system is found in U.S. Pat. Nos. 5,570,315, 5,774,397 and 6,046,935.  
         [0000]     Block Erase  
         [0022]     Programming of charge storage memory devices can only result in adding more charge to its charge storage elements. Therefore, prior to a program operation, existing charge in a charge storage element must be removed (or erased). Erase circuits (not shown) are provided to erase one or more blocks of memory cells. A non-volatile memory such as EEPROM is referred to as a “Flash” EEPROM when an entire array of cells, or significant groups of cells of the array, is electrically erased together (i.e., in a flash). Once erased, the group of cells can then be reprogrammed. The group of cells erasable together may consist one or more addressable erase unit. The erase unit or block typically stores one or more pages of data, the page being the unit of programming and reading, although more than one page may be programmed or read in a single operation. Each page typically stores one or more sectors of data, the size of the sector being defined by the host system. An example is a sector of 512 bytes of user data, following a standard established with magnetic disk drives, plus some number of bytes of overhead information about the user data and/or the block in with it is stored.  
         [0000]     Read/Write Circuits  
         [0023]     In the usual two-state EEPROM cell, at least one current breakpoint level is established so as to partition the conduction window into two regions. When a cell is read by applying predetermined, fixed voltages, its source/drain current is resolved into a memory state by comparing with the breakpoint level (or reference current I REF ). If the current read is higher than that of the breakpoint level, the cell is determined to be in one logical state (e.g., a “zero” state). On the other hand, if the current is less than that of the breakpoint level, the cell is determined to be in the other logical state (e.g., a “one” state). Thus, such a two-state cell stores one bit of digital information. A reference current source, which may be externally programmable, is often provided as part of a memory system to generate the breakpoint level current.  
         [0024]     In order to increase memory capacity, flash EEPROM devices are being fabricated with higher and higher density as the state of the semiconductor technology advances. Another method for increasing storage capacity is to have each memory cell store more than two states.  
         [0025]     For a multi-state or multi-level EEPROM memory cell, the conduction window is partitioned into more than two regions by more than one breakpoint such that each cell is capable of storing more than one bit of data. The information that a given EEPROM array can store is thus increased with the number of states that each cell can store. EEPROM or flash EEPROM with multi-state or multi-level memory cells have been described in U.S. Pat. No. 5,172,338.  
         [0026]     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.  
         [0027]     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. 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.  
         [0028]      FIG. 4  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. The four solid I D  versus V CG  curves represent four 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. Six memory states may be demarcated by partitioning the threshold window into five regions in interval of 0.5V each. For example, if a reference current, I REF  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 V CG =0.5V and 1.0V. Similarly, Q 4  is in a memory state “ 5 ”.  
         [0029]     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. This will require higher precision in programming and reading operations in order to be able to achieve the required resolution.  
         [0030]     U.S. Pat. No. 4,357,685 discloses a method of programming a 2-state EPROM in which when a cell is programmed to a given state, it is subject to successive programming voltage pulses, each time adding incremental charge to the floating gate. In between pulses, the cell is read back or verified to determine its source-drain current relative to the breakpoint level. Programming stops when the current state has been verified to reach the desired state. The programming pulse train used may have increasing period or amplitude.  
         [0031]     Prior art programming circuits simply apply programming pulses to step through the threshold window from the erased or ground state until the target state is reached. Practically, to allow for adequate resolution, each partitioned or demarcated region would require at least about five programming steps to transverse. The performance is acceptable for 2-state memory cells. However, for multi-state cells, the number of steps required increases with the number of partitions and therefore, the programming precision or resolution must be increased. For example, a 16-state cell may require on average at least 40 programming pulses to program to a target state.  
         [0032]      FIG. 5  illustrates schematically a memory device with a typical arrangement of a memory array  100  accessible by read/write circuits  170  via row decoder  130  and column decoder  160 . As described in connection with  FIGS. 2 and 3 , a memory transistor of a memory cell in the memory array  100  is addressable via a set of selected word line(s) and bit line(s). The row decoder  130  selects one or more word lines and the column decoder  160  selects one or more bit lines in order to apply appropriate voltages to the respective gates of the addressed memory transistor. Read/write circuits  170  are provided to read or write (program) the memory states of addressed memory transistors. The read/write circuits  170  comprise a number of read/write modules connectable via bit lines to memory elements in the array.  
         [0033]      FIG. 6A  is a schematic block diagram of an individual read/write module  190 . Essentially, during read or verify, a sense amplifier determines the current flowing through the drain of an addressed memory transistor connected via a selected bit line. The current depends on the charge stored in the memory transistor and its control gate voltage. For example, in a multi-state EEPROM cell, its floating gate can be charged to one of several different levels. For a 4-level cell, it may be used to store two bits of data. The level detected by the sense amplifier is converted by a level-to-bits conversion logic to a set of data bits to be stored in a data latch.  
         [0000]     Factors Affecting Read/Write Performance and Accuracy  
         [0034]     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 logical “page” of memory elements are read or programmed together. In existing memory architectures, a row typically contains several interleaved pages. All memory elements of a page will be read or programmed together. The column decoder will selectively connect each one of the interleaved pages to a corresponding number of read/write modules. For example, in one implementation, the memory array is designed to have a page size of 532 bytes (512 bytes plus 20 bytes of overheads.) If each column contains a drain bit line and there are two interleaved pages per row, this amounts to 8512 columns with each page being associated with 4256 columns. There will be 4256 sense modules connectable to read or write in parallel either all the even bit lines or the odd bit lines. In this way, a page of 4256 bits (i.e., 532 bytes) of data in parallel are read from or programmed into the page of memory elements. The read/write modules forming the read/write circuits  170  can be arranged into various architectures.  
         [0035]     Referring to  FIG. 5 , the read/write circuits  170  is organized into banks of read/write stacks  180 . Each read/write stack  180  is a stack of read/write modules  190 . In a memory array, the column spacing is determined by the size of the one or two transistors that occupy it. However, as can be seen from  FIG. 6A , the circuitry of a read/write module will likely be implemented with many more transistors and circuit elements and therefore will occupy a space over many columns. In order to service more than one column among the occupied columns, multiple modules are stacked up on top of each other.  
         [0036]      FIG. 6B  shows the read/write stack of  FIG. 5  implemented conventionally by a stack of read/write modules  190 . For example, a read/write module may extend over sixteen columns, then a read/write stack  180  with a stack of eight read/write modules can be used to service eight columns in parallel. The read/write stack can be coupled via a column decoder to either the eight odd ( 1 ,  3 ,  5 ,  7 ,  9 ,  11 ,  13 ,  15 ) columns or the eight even ( 2 ,  4 ,  6 ,  8 ,  10 ,  12 ,  14 ,  16 ) columns among the bank.  
         [0037]     As mentioned before, conventional memory devices improve read/write operations by operating in a massively parallel manner on all even or all odd bit lines at a time. This architecture of a row consisting of two interleaved pages will help to alleviate the problem of fitting the block of read/write circuits. It is also dictated by consideration of controlling bit-line to bit-line capacitive coupling. A block decoder is used to multiplex the set of read/write modules to either the even page or the odd page. In this way, whenever one set bit lines are being read or programmed, the interleaving set can be grounded to minimize immediate neighbor coupling.  
         [0038]     However, the interleaving page architecture is disadvantageous in at least three respects. First, it requires additional multiplexing circuitry. Secondly, it is slow in performance. To finish read or program of memory cells connected by a word line or in a row, two read or two program operations are required. Thirdly, it is also not optimum in addressing other disturb effects such as field coupling between neighboring charge storage elements at the floating gate level when the two neighbors are programmed at different times, such as separately in odd and even pages.  
         [0039]     The problem of neighboring field coupling becomes more pronounced with ever closer spacing between memory transistors. In a memory transistor, a charge storage element is sandwiched between a channel region and a control gate. The current that flows in the channel region is a function of the resultant electric field contributed by the field at the control gate and the charge storage element. With ever increasing density, memory transistors are formed closer and closer together. The field from neighboring charge elements then becomes significant contributor to the resultant field of an affected cell. The neighboring field depends on the charge programmed into the charge storage elements of the neighbors. This perturbing field is dynamic in nature as it changes with the programmed states of the neighbors. Thus, an affected cell may read differently at different time depending on the changing states of the neighbors.  
         [0040]     The conventional architecture of interleaving page exacerbates the error caused by neighboring floating gate coupling. Since the even page and the odd page are programmed and read independently of each other, a page may be programmed under one set of condition but read back under an entirely different set of condition, depending on what has happened to the intervening page in the meantime. The read errors will become more severe with increasing density, requiring a more accurate read operation and coarser partitioning of the threshold window for multi-state implementation. Performance will suffer and the potential capacity in a multi-state implementation is limited.  
         [0041]     U.S. Patent Publication No. US-2004-0060031-A1 discloses a high performance yet compact non-volatile memory device having a large block of read/write circuits to read and write a corresponding block of memory cells in parallel. In particular, the memory device has an architecture that reduces redundancy in the block of read/write circuits to a minimum. Significant saving in space as well as power is accomplished by redistributing the block of read/write modules into a block read/write module core portions that operate in parallel while interacting with a substantially smaller sets of common portions in a time-multiplexing manner. In particular, data processing among read/write circuits between a plurality of sense amplifiers and data latches is performed by a shared processor.  
         [0042]     Therefore there is a general need for high performance and high capacity non-volatile memory. In particular, there is a need for a compact non-volatile memory with enhanced read and program performance having an improved processor that is compact and efficient, yet highly versatile for processing data among the read/writing circuits.  
       SUMMARY OF INVENTION  
       [0043]     According to one aspect of the invention, a processor for processing data between a plurality of sense amplifiers and data latches comprises an input logic, a latch and an output logic. The input logic can transform the data received from either the sense amplifier or the data latches. The output logic further processes the transformed data to send to either the sense amplifier or the data latches or to a controller. This provides an infrastructure with maximum versatility and a minimum of components for sophisticated processing of the data sensed and the data to be input or output.  
         [0044]     The saving in space by the various aspects of the present invention allows for a more compact chip design. The saving in circuits and therefore in space and power consumption can amount to as much as fifty percent as compared to existing read/write circuits. In particular, the read/write modules can be densely packed so that they can simultaneously serve a contiguous row of memory cells of the memory array.  
         [0045]     Additional features and advantages of the present invention will be understood from the following description of its preferred embodiments, which description should be taken in conjunction with the accompanying drawings.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0046]      FIGS. 1A-1E  illustrate schematically different examples of non-volatile memory cells.  
         [0047]      FIG. 2  illustrates an example of an NOR array of memory cells.  
         [0048]      FIG. 3  illustrates an example of an NAND array of memory cells, such as that shown in  FIG. 1D .  
         [0049]      FIG. 4  illustrates the relation between the source-drain current and the control gate voltage for four different charges Q 1 -Q 4  that the floating gate may be storing at any one time.  
         [0050]      FIG. 5  illustrates schematically a typical arrangement of a memory array accessible by read/write circuits via row and column decoders.  
         [0051]      FIG. 6A  is a schematic block diagram of an individual read/write module.  
         [0052]      FIG. 6B  shows the read/write stack of  FIG. 5  implemented conventionally by a stack of read/write modules.  
         [0053]      FIG. 7A  illustrates schematically a compact memory device having a bank of partitioned read/write stacks, in which the improved processor of the present invention is implemented.  
         [0054]      FIG. 7B  illustrates a preferred arrangement of the compact memory device shown in  FIG. 7A .  
         [0055]      FIG. 8  illustrates schematically a general arrangement of the basic components in a read/write stack shown in  FIG. 7A .  
         [0056]      FIG. 9  illustrates one preferred arrangement of the read/write stacks among the read/write circuits shown in  FIGS. 7A and 7B .  
         [0057]      FIG. 10  illustrates an improved embodiment of the common processor shown in FIG  9 .  
         [0058]      FIG. 11A  illustrates a preferred embodiment of the input logic of the common processor shown in  FIG. 10 .  
         [0059]      FIG. 11B  illustrates the truth table of the input logic of  FIG. 11A .  
         [0060]      FIG. 12A  illustrates a preferred embodiment of the output logic of the common processor shown in  FIG. 10 .  
         [0061]      FIG. 12B  illustrates the truth table of the output logic of  FIG. 12A .  
         [0062]      FIG. 13  illustrates the basic functional steps in the operation of the common processor shown in  FIG. 10 .  
         [0063]      FIG. 14  illustrates an example of read operation by the common processor.  
         [0064]      FIG. 15  illustrates an example of program verify (2 bits program) by the common processor.  
         [0065]      FIG. 16  illustrates an example of program inhibit (2 bits program) by the common processor.  
         [0066]      FIG. 17  illustrates an example of error detection (2 bits program) by the common processor.  
         [0067]      FIG. 18  illustrates the full cycle of in including the auxiliary steps into the basic steps of  FIG. 13  for transferring data from the sense amplifier to the data latches.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0068]      FIG. 7A  illustrates schematically a compact memory device having a bank of partitioned read/write stacks, in which the improved processor of the present invention is implemented. The memory device includes a two-dimensional array of memory cells  300 , control circuitry  310 , and read/write circuits  370 . The memory array  300  is addressable by word lines via a row decoder  330  and by bit lines via a column decoder  360 . The read/write circuits  370  is implemented as a bank of partitioned read/write stacks  400  and allows a block (also referred to as a “page”) of memory cells to be read or programmed in parallel. In a preferred embodiment, a page is constituted from a contiguous row of memory cells. In another embodiment, where a row of memory cells are partitioned into multiple blocks or pages, a block multiplexer  350  is provided to multiplex the read/write circuits  370  to the individual blocks.  
         [0069]     The control circuitry  310  cooperates with the read/write circuits  370  to perform memory operations on the memory array  300 . The control circuitry  310  includes a state machine  312 , an on-chip address decoder  314  and a power control module  316 . The state machine  312  provides chip level control of memory operations. The on-chip address decoder  314  provides an address interface between that used by the host or a memory controller to the hardware address used by the decoders  330  and  370 . The power control module  316  controls the power and voltages supplied to the word lines and bit lines during memory operations.  
         [0070]      FIG. 7B  illustrates a preferred arrangement of the compact memory device shown in  FIG. 7A . Access to the memory array  300  by the various peripheral circuits is implemented in a symmetric fashion, on opposite sides of the array so that access lines and circuitry on each side are reduced in half. Thus, the row decoder is split into row decoders  330 A and  330 B and the column decoder into column decoders  360 A and  360 B. In the embodiment where a row of memory cells are partitioned into multiple blocks, the block multiplexer  350  is split into block multiplexers  350 A and  350 B. Similarly, the read/write circuits are split into read/write circuits  370 A connecting to bit lines from the bottom and read/write circuits  370 B connecting to bit lines from the top of the array  300 . In this way, the density of the read/write modules, and therefore that of the partitioned read/write stacks  400 , is essentially reduced by one half.  
         [0071]      FIG. 8  illustrates schematically a general arrangement of the basic components in a read/write stack shown in  FIG. 7A . According to a general architecture of the invention, the read/write stack  400  comprises a stack of sense amplifiers  212  for sensing k bit lines, an I/O module  440  for input or output of data via an I/O bus  231 , a stack of data latches  430  for storing input or output data, a common processor  500  to process and store data among the read/write stack  400 , and a stack bus  421  for communication among the stack components. A stack bus controller among the read/write circuits  370  provides control and timing signals via lines  411  for controlling the various components among the read/write stacks.  
         [0072]      FIG. 9  illustrates one preferred arrangement of the read/write stacks among the read/write circuits shown in  FIGS. 7A and 7B . Each read/write stack  400  operates on a group of k bit lines in parallel. If a page has p=r*k bit lines, there will be r read/write stacks,  400 - 1 , . . . ,  400 -r.  
         [0073]     The entire bank of partitioned read/write stacks  400  operating in parallel allows a block (or page) of p cells along a row to be read or programmed in parallel. Thus, there will be p read/write modules for the entire row of cells. As each stack is serving k memory cells, the total number of read/write stacks in the bank is therefore given by r=p/k. For example, if r is the number of stacks in the bank, then p =r*k. One example memory array may have p=512 bytes (512×8 bits), k=8, and therefore r=512. In the preferred embodiment, the block is a run of the entire row of cells. In another embodiment, the block is a subset of cells in the row. For example, the subset of cells could be one half of the entire row or one quarter of the entire row. The subset of cells could be a run of contiguous cells or one every other cell, or one every predetermined number of cells.  
         [0074]     Each read/write stack, such as  400 - 1 , essentially contains a stack of sense amplifiers  212 - 1  to  212 -k servicing a segment of k memory cells in parallel. A preferred sense amplifier is disclosed in U.S. Patent Publication No. 2004-0109357-A1, the entire disclosure of which is hereby incorporated herein by reference.  
         [0075]     The stack bus controller  410  provides control and timing signals to the read/write circuit  370  via lines  411 . The stack bus controller is itself dependent on the memory controller  310  via lines  311 . Communication among each read/write stack  400  is effected by an interconnecting stack bus  431  and controlled by the stack bus controller  410 . Control lines  411  provide control and clock signals from the stack bus controller  410  to the components of the read/write stacks  400 - 1 .  
         [0076]     In the preferred arrangement, the stack bus is partitioned into a SABus  422  for communication between the common processor  500  and the stack of sense amplifiers  212 , and a DBus  423  for communication between the processor and the stack of data latches  430 .  
         [0077]     The stack of data latches  430  comprises of data latches  430 - 1  to  430 -k, one for each memory cell associated with the stack The I/O module  440  enables the data latches to exchange data with the external via an I/O bus  231 .  
         [0078]     The common processor also includes an output  507  for output of a status signal indicating a status of the memory operation, such as an error condition. The status signal is used to drive the gate of an n-transistor  550  that is tied to a FLAG BUS  509  in a Wired-Or configuration. The FLAG BUS is preferably precharged by the controller  310  and will be pulled down when a status signal is asserted by any of the read/write stacks.  
         [0079]      FIG. 10  illustrates an improved embodiment of the common processor shown in  FIG. 9 . The common processor  500  comprises a processor bus, PBUS  505  for communication with external circuits, an input logic  510 , a processor latch PLatch  520  and an output logic  530 .  
         [0080]     The input logic  510  receives data from the PBUS and outputs to a BSI node as a transformed data in one of logical states “1”, “0”, or “Z” (float) depending on the control signals from the stack bus controller  410  via signal lines  411 . A Set/Reset latch, PLatch  520  then latches BSI, resulting in a pair of complementary output signals as MTCH and MTCH*.  
         [0081]     The output logic  530  receives the MTCH and MTCH* signals and outputs on the PBUS  505  a transformed data in one of logical states “1”, “0”, or “Z” (float) depending on the control signals from the stack bus controller  410  via signal lines  411 .  
         [0082]     At any one time the common processor  500  processes the data related to a given memory cell. For example,  FIG. 10  illustrates the case for the memory cell coupled to bit line  1 . The corresponding sense amplifier  212 - 1  comprises a node where the sense amplifier data appears. In the preferred embodiment, the node assumes the form of a SA Latch,  214 - 1  that stores data. Similarly, the corresponding set of data latches  430 - 1  stores input or output data associated with the memory cell coupled to bit line  1 . In the preferred embodiment, the set of data latches  430 - 1  comprises sufficient data latches,  434 - 1 , . . . ,  434 -n for storing n-bits of data.  
         [0083]     The PBUS  505  of the common processor  500  has access to the SA latch  214 - 1  via the SBUS  422  when a transfer gate  501  is enabled by a pair of complementary signals SAP and SAN. Similarly, the PBUS  505  has access to the set of data latches  430 - 1  via the DBUS  423  when a transfer gate  502  is enabled by a pair of complementary signals DTP and DTN. The signals SAP, SAN, DTP and DTN are illustrated explicitly as part of the control signals from the stack bus controller  410 .  
         [0084]      FIG. 11A  illustrates a preferred embodiment of the input logic of the common processor shown in  FIG. 10 . The input logic  520  receives the data on the PBUS  505  and depending on the control signals, either has the output BSI being the same, or inverted, or floated. The output BSI node is essentially affected by either the output of a transfer gate  522  or a pull-up circuit comprising p-transistors  524  and  525  in series to Vdd, or a pull-down circuit comprising n-transistors  526  and  527  in series to ground. The pull-up circuit has the gates to the p-transistor  524  and  525  respectively controlled by the signals PBUS and ONE. The pull-down circuit has the gates to the n-transistors  526  and  527  respectively controlled by the signals ONEB&lt;1&gt; and PBUS.  
         [0085]      FIG. 11B  illustrates the truth table of the input logic of  FIG. 11A . The logic is controlled by PBUS and the control signals ONE, ONEB&lt;0&gt;, ONEB&lt;1&gt; which are part of the control signals from the stack bus controller  410 . Essentially, three transfer modes, PASSTHROUGH, INVERTED, and FLOATED, are supported.  
         [0086]     In the case of the PASSTHROUGH mode where BSI is the same as the input data, the signals ONE is at a logical “1”, ONEB&lt;0&gt; at “0” and ONEB&lt;1&gt; at “0”. This will disable the pull-up or pull-down but enable the transfer gate  522  to pass the data on the PBUS  505  to the output  523 . In the case of the INVERTED mode where BSI is the invert of the input data, the signals ONE is at “0”, ONEB&lt;0&gt; at “1” and ONE&lt;1&gt; at “1”. This will disable the transfer gate  522 . Also, when PBUS is at “0”, the pull-down circuit will be disabled while the pull-up circuit is enabled, resulting in BSI being at “1”. Similarly, when PBUS is at “1”, the pull-up circuit is disabled while the pull-down circuit is enabled, resulting in BSI being at “0”. Finally, in the case of the FLOATED mode, the output BSI can be floated by having the signals ONE at “1”, ONEB&lt;0&gt; at “1” and ONEB&lt;1&gt; at “0”. The FLOATED mode is listed for completeness although in practice, it is not used.  
         [0087]      FIG. 12A  illustrates a preferred embodiment of the output logic of the common processor shown in  FIG. 10 . The signal at the BSI node from the input logic  520  is latched in the processor latch, PLatch  520 . The output logic  530  receives the data MTCH and MTCH* from the output of PLatch  520  and depending on the control signals, outputs on the PBUS as either in a PASSTHROUGH, INVERTED OR FLOATED mode. In other words, the four branches act as drivers for the PBUS  505 , actively pulling it either to a HIGH, LOW or FLOATED state. This is accomplished by four branch circuits, namely two pull-up and two pull-down circuits for the PBUS  505 . A first pull-up circuit comprises p-transistors  531  and  532  in series to Vdd, and is able to pull up the PBUS when MTCH is at “0”. A second pull-up circuit comprises p-transistors  533  and  534  in series to ground and is able to pull up the PBUS when MTCH is at “1”. Similarly, a first pull-down circuit comprises n-transistors  535  and  536  in series to Vdd, and is able to pull down the PBUS when MTCH is at “0”. A second pull-up circuit comprises n-transistors  537  and  538  in series to ground and is able to pull up the PBUS when MTCH is at “1”.  
         [0088]     One feature of the invention is to constitute the pull-up circuits with PMOS transistors and the pull-down circuits with NMOS transistors. Since the pull by the NMOS is much stronger the PMOS, the pull-down will always overcome the pull-up in any contentions. In other words, the node or bus can always default to a pull-up or “1” state, and if desired, can always be flipped to a “0” state by a pull-down.  
         [0089]      FIG. 12B  illustrates the truth table of the output logic of  FIG. 12A . The logic is controlled by MTCH, MTCH* latched from the input logic and the control signals PDIR, PINV, NDIR, NINV, which are part of the control signals from the stack bus controller  410 . Four operation modes, PASSTHROUGH, INVERTED, FLOATED, and PRECHARGE are supported.  
         [0090]     In the FLOATED mode, all four branches are disabled. This is accomplished by having the signals PINV=1, NINV=0, PDIR=1, NDIR=0, which are also the default values. In the PASSTHROUGH mode, when MTCH=0, it will require PBUS=0. This is accomplished by only enabling the pull-down branch with n-transistors  535  and  536 , with all control signals at their default values except for NDIR=1. When MTCH=1, it will require PBUS=1. This is accomplished by only enabling the pull-up branch with p-transistors  533  and  534 , with all control signals at their default values except for PINV=0. In the INVERTED mode, when MTCH=0, it will require PBUS=1. This is accomplished by only enabling the pull-up branch with p-transistors  531  and  532 , with all control signals at their default values except for PDIR=0. When MTCH=1, it will require PBUS=0. This is accomplished by only enabling the pull-down branch with n-transistors  537  and  538 , with all control signals at their default values except for NINV=1. In the PRECHARGE mode, the control signals settings of PDIR=0 and PINW=0 will either enable the pull-up branch with p-transistors  531  and  531  when MTCH=1 or the pull-up branch with p-transistors  533  and  534  when MTCH=0.  
         [0000]     Common Processor Operations  
         [0091]     When the common processor  500  is configured as described, it is able to perform versatile data operation with respect to the sense amplifier and the data latches.  
         [0092]      FIG. 13  illustrates the basic functional steps in the operation of the common processor shown in  FIG. 10 . To be more specific, the data latches contains at least two latches, DL 1  (or Lower Data Latch) and DL 2  (or Upper Data Latch) for storing two bits of data  
         [0093]     RESET is always at the beginning of the processor operations. Steps can be skipped according to the need of the operations.)  
         [0094]      1 ) The common processor reading data from the sense amplifier:  
         [0095]     Sense amplifier&#39;s data will be fetched out of SA latch  214 - 1  and latched into PLatch  520  through SBUS  422  and input logic  510 . Platch could flip MTCH  524  from “1” to “0” as controlled by the control signals.  
         [0096]      2 ) The common processor reading data from the first of the data latches:  
         [0097]     Data in DL 1  of data latches  430 - 1  will be fetched from DBUS  423  going through input logic  510 , and latched into PLatch  520 . Platch could flip MTCH from “1” to “0”. If MTCH=0 from step  1 ), then MTCH will stay at 0 for the rest of steps.  
         [0098]     3) The common processor reading data from the second of the data latches:  
         [0099]     Data in DL 2  of data latches will be fetched from DBUS going through input logic  510 , and latched into PLatch. Platch could flip MTCH from “1” to “0”. If MTCH=0 from step 1), then MTCH will stay at 0 for the rest of steps. (Similarly, if there are more than two data latches with data, successive latches can be read.)  
         [0100]     4) The common processor writing data to various latches:  
         [0101]     The PLatch data can be used in one of three ways:  
         [0102]     a. PLatch&#39;s data MTCH/MTCH* will be used to drive DBUS to update DL 2  of the data latches.  
         [0103]     b. PLatch&#39;s data MTCH/MTCH* will be used to drive SBUS to change the data in SA Latch  214 - 1 .  
         [0104]     c. PLatch&#39;s data MTCH/MTCH* will be used to drive the FLAG BUS to indicate the status of any error condition outside the read/write stack.  
         [0105]     5) PLatch&#39;s result MTCH/MTCH* will be used to drive DBUS to update DL 1  of the data latches.  
         [0106]      FIG. 14  illustrates an example of read operation by the common processor.  
         [0000]     Reset  
         [0000]    
       
          1) SA to PROCESSOR  
          2) Skipped  
          3) Skipped  
          4) PROCESSOR to DL 2   
          5) Skipped  
          The sensing information is transferred to DL 2 .  
       
     
         [0113]      FIG. 15  illustrates an example of program verify (2 bits program) by the common processor:  
         [0000]     Reset  
         [0000]    
       
          1) SA to PROCESSOR  
          2) DL 1  to PROCESSOR  
          3) DL 2  to PROCESSOR  
          4) PROCESSOR to DL 2   
       
     
         [0118]     Need to have program lockout if the verify passed, and therefore data to DL 2  will be changed from “0” to “1” to enforce no programming. 
    5) PROCESSOR to DL 1     
 
         [0120]     Need to have program lockout if the verify passed, and data to DL 1  will be changed from “0” to “1” to enforce no programming.  
         [0121]      FIG. 16  illustrates an example of program inhibit (2 bits program) by the common processor:  
         [0000]     Reset  
         [0000]    
       
          1 1 ) Skipped  
          2) DL 1  to PROCESSOR  
          3) DL 2  to PROCESSOR  
          4) PROCESSOR to SA  
          5) Skipped 
 
 Step 2) and 3) will ONLY match the data “ 11 ”. If it is “ 11 ” data, then step 4) will make the SA to be “ 1 ” data for program inhibit. 
 
       
     
         [0127]      FIG. 17  illustrates an example of error detection (2 bits program) by the common processor:  
         [0000]     Reset  
         [0000]    
       
          1) Skipped  
          2) DL 1  to PROCESSOR  
          3) DL 2  to PROCESSOR  
          4) PROCESSOR to PBUS to FLAG BUS  
          5) Skipped 
 
 Step 2) and 3) will check if any data is “0”. If there is, then the PBUS will be pulled to “1”. This in turn will drive the gate of the n-transistor  550  to pull down the FLAG BUS  509  (see  FIG. 9 ). 
 
       
     
         [0133]     The basic functional steps illustrated in  FIG. 13  also include some auxiliary in-between steps and considerations:  
         [0134]     1) Precharge of the DBUS or SBUS by the common processor.  
         [0135]     The DBUS interconnecting the common processor and the data latches is precharged to Vdd by virtue of being connected to the PBUS, which is precharged to Vdd by the common processor. The DBUS is normally precharged to Vdd (or logic state “1”). When reading a Data Latch (DL), if the DL has the data of “1”, the DBUS will remain at “1”, otherwise a data “0” at DL will drive the DBUS to GND.  
         [0136]     As can be seen from the truth table in  FIG. 12B , the DBUS is precharged to Vdd by the processor driver (output logic  530 ) with the control signals settings of PDIR=0 and PINV=0, regardless of the value MTCH and MTCH*. The Precharge DBUS to Vdd cycle is inserted before every cycle of DL 2  to PROCESSOR, or DL 1  to PROCESSOR.  
         [0137]     The cycle to fetch the data from the SA latch also involves charging the SBUS to Vdd by the processor driver. Similar to the data latches case, when reading the SA Latch, if it has the data of “1”, the SBUS will remain at “1”, otherwise a data “0” at the SA Latch will drive the SBUS to GND.  
         [0138]     2) Precharge of DBUS with the Data Latches, such as DL 1  and DL 2 .  
         [0139]     For the PROCESSOR to DL 1  or PROCESSOR to DL 2  cycle, the DBUS is precharged by the data in the data latch (DL) to prevent flipping the Data Latches, the PROCESSOR output (i.e., PBUS) is floated at HiZ.  
         [0140]     Also, HiZ on DBUS is needed for program verify sequence. Whenever the data fail to verify, program data at the Data Latches would be kept unchanged.  
         [0141]      FIG. 18  illustrates the full cycle of in including the auxiliary steps into the basic steps of  FIG. 13  for transferring data from the sense amplifier to the data latches. 
    1. RESET     2. Precharge SBUS to Vdd from PROCESSOR;     3. Transfer SA to PROCESSOR;     4. Precharge DBUS to Vdd;     5. DL 2  to PROCESSOR;     6. Precharge DBUS to Vdd;     7. DL 1  to PROCESSOR;     8. PROCESSOR to DL 1  (since step 7 already has updated data on DBUS, so PROCESSOR can update DL 1  directly)     9. Charge DBUS with DL 2 ;     10. PROCESSOR to DL 2 .      
         [0152]     Although the various aspects of the present invention have been described with respect to certain embodiments, it is understood that the invention is entitled to protection within the full scope of the appended claims.