Abstract:
Memory devices and methods of operating memory devices are provided, such as those that involve a memory architecture that replaces typical static and/or dynamic components with emerging non-volatile memory (NV) elements. The emerging NV memory elements can replace conventional latches, can serve as a high speed interface between a flash memory array and external devices and can also be used as high performance cache memory for a flash memory array.

Description:
FIELD OF THE INVENTION 
       [0001]    Embodiments described herein relate to flash memory devices and more particularly to flash memory devices having emerging non-volatile (NV) memory elements used therewith. 
       BACKGROUND 
       [0002]    Memory can generally be characterized as either volatile or non-volatile. Volatile memory, for example, most types of random access memory (RAM), requires constant power to maintain stored information. Non-volatile memory does not require power to maintain stored information. Various types of non-volatile memories include read only memories (ROMs), erasable programmable read only memories (EPROMs), and electrically erasable programmable read only memories (EEPROMs). 
         [0003]    Flash memory is a type of EEPROM that is programmed and erased in blocks as opposed to cells. A conventional flash memory device includes a plurality of memory cells, each cell is provided with a floating gate covered with an insulating layer. There is also a control gate which overlays the insulating layer. Below the floating gate is another insulating layer sandwiched between the floating gate and the cell substrate. This insulating layer is an oxide layer and is often referred to as the tunnel oxide. The substrate contains doped source and drain regions, with a channel region disposed between the source and drain regions. In a flash memory device, a charged floating gate represents one logic state, e.g., a logic value “0”, while a non-charged floating gate represents the opposite logic state e.g., a logic value “1”. The flash memory cell is programmed by placing the floating gate into one of these charged states. Charges may be injected or written on to the floating gate by any number of methods, including e.g., avalanche injection, channel injection, Fowler-Nordheim tunneling, and channel hot electron (CHE) injection. The floating gate may be discharged or erased by any number of methods including e.g., Fowler-Nordheim tunneling. This type of flash memory element is a transistor-based non-volatile memory element. 
         [0004]    The “NAND” and “NOR” architectures are two common types of flash memory architectures. NAND flash memory has gained widespread popularity over NOR flash memory because it can pack a greater number of storage cells in a given area of silicon, providing NAND with density and cost advantages over other nonvolatile memory. A NAND flash memory device typically utilizes a NAND flash controller to write data to the NAND in a page-by-page fashion. An example NAND memory array  10  is illustrated in  FIG. 1 . Pages  12  are typically grouped into blocks  14 , where a block is the smallest erasable unit of the NAND flash memory device. For example, and without limitation, a typical NAND flash memory device contains 2,112 bytes of memory per page  12  and 64 or 128 pages of memory are contained in a block  14 .  FIG. 1  illustrates blocks  14  comprising 64 pages  12 . For a page  12  having 2,112 bytes in total, there is a 2,048-byte data area  16  and a 64-byte spare area  18 . The spare area  18  is typically used for error correction code (ECC), redundancy cells, and/or other software overhead functions. The smallest entity that can be programmed in the illustrated array  10  is a bit. 
         [0005]      FIG. 2  illustrates a NAND flash memory device  110  having a memory array  120  and sense circuitry  130  connected to the memory array  120  by data lines, which are commonly referred to as bitlines (BL). The array  120  comprises typical transistor-based non-volatile flash memory elements. When data is to be written into the NAND memory array, the data is initially loaded into the sense circuitry  130 . Once the data is latched, a programming operation is used to write a page of data into one of the pages of memory cells in the memory array  120 . The sense circuitry  130  typically comprises volatile static or dynamic memory elements. 
         [0006]    A simplified schematic of a portion of the sense circuitry  130  is illustrated in  FIG. 3 . As can be seen, there is sense operation circuitry  132  comprising three n-channel MOSFET transistors  134 ,  136 ,  138 , a data latch  140 , cache latch  150  and additional n-channel MOSFET transistors  160 ,  162 ,  164 ,  166 ,  168 . The data latch  140  is illustrated as comprising cross-coupled inverters  142 ,  144 . The cache latch  150  is illustrated as comprising cross-coupled inverters  152 ,  154 . The inverters  142 ,  144 ,  152 ,  154  may each consist of e.g., an n-channel CMOS transistor and a p-channel CMOS transistor configured such that their gates are coupled together and at least one source/drain node of the n-channel transistor is coupled to a source/drain node of the p-channel transistor. Thus, the data and cache latches  140 ,  150  in the illustrated example are implemented as static memory elements, which would lose their contents if power were removed from the circuit.  130 . Thus, a situation could arise where latched data could be lost if power to the array  110  ( FIG. 2 ) were lost before the latched data was copied into the NAND memory arrays. Accordingly, the inventor of the present application appreciated that it would be desirable to prevent latched information from being lost in the event of a power failure or similar condition. 
         [0007]    Continuing with the  FIG. 3  example, data Da, Db is input into the sense circuitry  130  through the cache latch  150  when a data load/output enable signal data_load/out_en, connected to the gates of transistors  166 ,  168 , is activated. Typically, data Da is the complement of data Db, and vice versa. A data signal Data connected at the gate of transistor  160  couples the data latch  140  to the cache latch  150 . When the data signal Data is at a level that activates transistor  160 , latched data is transferred from the cache latch  150  to the data latch  140 . A verify enable signal, verify_en, is used to activate transistor  162 , which is connected to transistor  164 . The gate of transistor  164  is connected to the data latch  140 . The same node of transistor  160  that is connected to the data latch  140  is also connected to a node of transistor  138  within the sense operation circuitry  132 . A precharge enable signal, precharge_en, controls transistor  136  while a bitline sensing signal, blsn, controls transistor  134 . A node of transistor  134  is connected to a write multiplexer (wmux) where data-to-be written, dw, based on the input data, is sent to and eventually stored in a conventional non-volatile memory array, which utilizes transistor-based memory elements. 
         [0008]    As can be seen from the illustrated example, many transistors are required to implement the sense circuitry  130 . It is desirable to reduce the circuitry used in sense circuitry  130 . It is also desirable to increase the speed of sense circuitry  130 . 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  illustrates an example NAND flash memory array. 
           [0010]      FIG. 2  illustrates an example NAND flash memory device having a memory array and sense circuitry. 
           [0011]      FIG. 3  illustrates a schematic view of the sense circuitry used in the array of  FIG. 2 . 
           [0012]      FIG. 4  illustrates an example flash memory device constructed in accordance with an embodiment described herein. 
           [0013]      FIGS. 5 and 6  illustrate schematic views of example sense circuitry with emerging NV elements used in the array of  FIG. 4 . 
           [0014]      FIG. 7  illustrates an example flash memory device constructed in accordance with another embodiment described herein. 
           [0015]      FIG. 8  illustrates an example flash memory module comprising an emerging NV cache constructed in accordance with an embodiment disclosed herein. 
           [0016]      FIG. 9  illustrates example packaging of an emerging NV cache chip stacked with a flash memory chip constructed in accordance with an embodiment disclosed herein. 
           [0017]      FIG. 10  shows a processor system incorporating at least one flash memory device constructed in accordance with an embodiment disclosed herein. 
           [0018]      FIG. 11  shows a universal serial bus (USB) memory device incorporating at least one flash memory device constructed in accordance with an embodiment disclosed herein. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    Embodiments described herein refer to emerging NV (non-volatile memory elements). As used herein, and in accordance with the general understanding of one skilled in the relevant memory art, “emerging NV memory elements” means a non-transistor-based, non-volatile memory element such as phase change random access memory (PCRAM), magnetoresistive random access memory (MRAM), resistive random access memory (RRAM), ferroelectric random access memory (FeRAM), spin-transfer-torque random access memory (STTRAM), nano-tube memory, and equivalent non-volatile memory elements. 
         [0020]    Referring to the figures, where like reference numbers designate like elements,  FIG. 4  illustrates an example NAND flash memory device  210  constructed in accordance with an embodiment described herein. The device  210  includes a memory array  120  and sense circuitry including an emerging NV memory circuit  230  connected to the memory array  120  by bitlines (BL). The array  120  comprises typical transistor-based non-volatile flash memory elements. As will be discussed below in more detail, the flash memory device  210  differs from the conventional device  110  ( FIG. 2 ) in that it includes emerging NV memory elements instead of the conventional cross-coupled inverters used in data and cache latches  140 ,  150  ( FIG. 3 ). By replacing the latches with emerging NV memory elements, the illustrated embodiment can help prevent data loss during programming of the NAND memory array if power to the device  210  (or a device incorporating the device  210 ) is interrupted. In addition, in standby mode, power to the emerging memory could be cut off to reduce standby power consumption without loss of data. Due to their construction, the emerging NV memory elements are usually smaller than the conventional latches and could possibly be implemented in metal 1 and 2 layers of the flash memory device, giving them a smaller device footprint. 
         [0021]    A simplified schematic of an example portion of the sense circuitry with emerging NV memory elements  230  is illustrated in  FIG. 5 . As can be seen, there is sense operation circuitry  132  comprising three n-channel MOSFET transistors  134 ,  136 ,  138 , which is the same as the sense operation circuitry  130  used in the conventional NAND device  110  ( FIG. 3 ). In the illustrated embodiment, however, the cross-coupled inverters of data latch  140  and cache latch  150  are replaced with emerging NV memory circuits  240 ,  250 . The first emerging NV memory circuit  240  is controlled by a first control signal (or signals) control 1  and the second emerging NV memory circuit  250  is controlled by a second control signal (or signals) control 2 . 
         [0022]    Data Da, Db is input into the sense circuitry  230  through emerging NV memory circuit  250  when control signal control 2  is activated. Typically, data Da is the complement of data Db, and vice versa. A data signal Data connected at the gate of transistor  160  couples circuit  250  to circuit  240 . When the data signal Data is at a level that activates transistor  160 , the stored data is transferred from circuit  250  to circuit  240 , which is controlled by control signal control 1 . The same node of transistor  160  that is connected to circuit  240  is also connected to a node of transistor  138  within the sense operation circuitry  132 . A precharge enable signal, precharge_en, controls transistor  136  while a bitline sensing signal, blsn, controls transistor  134 . A node of transistor  134  is connected to a write multiplexer (wmux) where data-to-be written, dw, based on the input data, is sent to and eventually stored in a NAND memory array. 
         [0023]    It should be appreciated that it may be desirable to reduce the number of emerging NV memory elements used in the sense circuitry  230 .  FIG. 6  illustrates a simplified schematic for another example of sense circuitry  230 ′. Circuitry  230 ′ differs from circuitry  230  ( FIG. 5 ) in that only one emerging NV memory circuit  255  is used to store data Da, Db before it is programmed into a NAND memory array. In the illustrated embodiment, the emerging NV memory circuit  255  is controlled by a control signal (or signals) control. It should also be appreciated that the emerging NV memory elements could be used with latches to provide additional functionality to the circuitry  230 ,  230 ′, if desired. 
         [0024]    It should also be appreciated that the illustrated control signals and input data may vary from the illustrated embodiment depending upon the type of emerging NV memory element used in the actual implementation of a device, such as device  210 . That is, for example, a PCRAM memory element may require a different control signal than the control signal, used for an RRAM memory element. As such, the illustrated embodiments are not to be limited to the example number of control signals and data bits shown in  FIGS. 5 and 6 . 
         [0025]    It should be appreciated that other benefits may be obtained by using emerging NV memory elements in other areas of, and to implement other functions in, a conventional memory device. For example, as illustrated in  FIG. 7 , one or more blocks of emerging NV memory elements  370  can be included within a device  310  that includes a conventional NAND array  120 . The NV blocks  370  can be used, for example, to achieve faster writes from an external source of data. As such, the blocks of emerging NV memory elements  370  can serve as a high speed interface to the external source of data. Once the data is within one of the blocks of emerging NV memory elements  370 , the data can be copied into other blocks  370  before it is stored in the NAND memory array  120  (either through the sense circuitry  330  as shown in  FIG. 7  or without going through the sense circuitry  330 ). This way, the blocks  370  can also or alternatively serve as a high speed cache memory for the device  310 . It could be desirable to use as many blocks of emerging NV memory elements  370  as the application design will allow. Accordingly, the illustrated embodiment is not to be limited to the example number of blocks of emerging NV memory elements  370  shown in  FIG. 7 . 
         [0026]    It should be appreciated that better block management can be achieved by storing data in the blocks of emerging NV memory elements  370  prior to programming the NAND array  120  in the device  310 . That is, fragment fixing, error correction and other data and memory cleaning operations can be performed while the data is in the faster emerging NV blocks  370 . In addition, it is also possible to use some of the blocks  370  as redundant memory as part of the bad block management that is typically performed on flash memory devices. That is, the bad block management function of the flash memory device can replace bad blocks of memory elements in the NAND array  120  with a good block of emerging NV memory elements. The device  310  can be operated to map a bad NAND memory block to a block of emerging NV memory elements and then copy data (to be stored in the array  120 ) into the block of emerging NV memory elements. 
         [0027]    It should also be appreciated that that emerging NV memory blocks could be used to initially store data so that required data adjustments can be performed before the data is stored in a NAND block. For example, there are times when an entire NAND block&#39;s data is needed to carry out adjustments to counter interference effects sometimes present in the NAND array. Once the adjustments are done in the emerging NV memory elements, then the data can be safely stored in the NAND block; thus, improving the NAND device&#39;s reliability. 
         [0028]      FIG. 8  illustrates a memory module  400  having a conventional NAND flash memory device  410  and an emerging NV cache memory  420  housed on the same circuit board  402 . Bond wire connections  404  (or printed circuit board traces) may be placed along the sides of the flash memory device  410  die to connect it to the emerging NV cache memory device  420 . The module  400  also includes pins  406  serving as an interface to the conventional device  410  and for providing ground and power to the device  410 , and pins  408  serving as an interface to the emerging NV cache memory device  420  and for providing ground and power to the device  420 . It should be appreciated that the number of pins and connections shown in  FIG. 8  is only an example number of pins and connections and that the actual implementation of the module  400  could have more or less pins and connections. In the illustrated embodiment, the emerging NV cache memory device  420  can serve as a high performance non-volatile cache for the flash device  410 , which provides the data loss prevention and other advantages described above. Although not shown, the circuit board  402  could also include a memory controller; in such a case, the module  400 /circuit board  402 , could be used as a cache for a cheaper storage device such e.g., as a hard drive. 
         [0029]      FIG. 9  illustrates a memory chip package  500  comprising an encasement  502  having a cavity  504  where in an emerging NV cache  520  is stacked with a NAND flash memory device  510 . As with other embodiments, the emerging NV cache  520  can serve as a high performance non-volatile cache for the NAND flash device  510 , which could provide the data loss prevention and other advantages described above. 
         [0030]      FIG. 10  illustrates a processor system  600  utilizing a memory device, e.g., a flash memory device  210 ,  310 ,  400 ,  500  constructed in accordance with embodiments described above. That is, the memory device  210 ,  310 ,  400 ,  500  is a NAND flash memory device incorporating one or more emerging NV memory elements as set described above. The system  600  may be a computer system, camera system, personal digital assistant (PDA), cellular telephone, smart telephone, a process control system or any system employing a processor and associated memory. The system  600  includes a central processing unit (CPU)  602 , e.g., a microprocessor, that communicates with the flash memory  210 ,  310 ,  400 ,  500  and an I/O device  612  over a bus  610 . It must be noted that the bus  610  may be a series of buses and bridges commonly used in a processor system, but for convenience purposes only, the bus  610  has been illustrated as a single bus. A second I/O device  614  is illustrated, but is not necessary to practice the embodiments described above. The system  600  also includes random access memory device  616  and may include a read-only memory device (not shown), and peripheral devices such as a floppy disk drive  604  and a compact disk (CD) ROM drive  606  that also communicate with the CPU  602  over the bus  610  as is well known in the art. 
         [0031]      FIG. 11  shows a universal serial bus (USB) memory device  700  incorporating at least one flash memory device  400 ,  500  constructed in accordance with an embodiment disclosed herein. The device  700  includes a USB connector  702  electrically and mechanically connected to a printed circuit board  710 . The connector  702  allows the device  700  to be inserted within a USB port of a computer or other device to allow data to be exchanged between the device  700  and the computer, etc. Moreover, power for the device  700  will also come from the USB port. The printed circuit board  710  comprises a USB interface (I/F) chip  712  electrically connected to the USB connector  702 . The USB interface  712  is electrically connected to and communicates with a controller  714 . The controller  714  controls and communicates with the flash memory device  400 ,  500  over a bus  720 . The controller  714  also controls a light emitting diode  718 , via the bus  720 . Typically, the light emitting diode  718  is controlled to blink when the flash memory device  400 ,  500  is being accessed.  FIG. 11  also illustrates an oscillator  716 , which is used as a clock for the device  700 . 
         [0032]    It should be appreciated that, although the embodiments have been described as using NAND flash memory arrays, other types of non-volatile flash memory could be used to practice the embodiments. For example, NOR and AND type flash memory arrays could be used in any of the illustrated embodiments. It should also be appreciated that the emerging NV memory elements can also be used to store data that has been read out of the conventional memory cells. In addition, it should be appreciated that the emerging NV memory elements can be used to store trim and fuse information as well as diagnostic data (e.g., program time, erase time, cycling information, number of failed bits or blocks) that can be acquired through out the life of the NAND chip regarding its performance and reliability. 
         [0033]    The above description and drawings illustrate various embodiments It should be appreciated that modifications, though presently unforeseeable, of these embodiments that can be made without departing from the spirit and scope of the claimed invention, which is defined by the following claims.