# Patent

## Title
Sensing of memory cells in a solid state memory device by fixed discharge of a bit line

## Abstract

In one or more of the disclosed embodiments, a memory device is provided that reads a target memory cell by first charging the series string of memory cells to which the target memory cell is coupled. A fixed unit of charge is removed from the charged bit line. The bit line is sensed by sense amplifiers to determine the read voltage (i.e., threshold voltage) applied to a word line coupled to the target cell in order to turn on the target cell. The threshold voltage is indicative of the analog voltage stored on the target memory cell.

## Background
TECHNICAL FIELD
The present disclosure relates generally to semiconductor memory and more particularly to non-volatile memory devices.
BACKGROUND
Electronic devices commonly have some type of bulk storage device available to them. A common example is a hard disk drive (HDD). HDDs are capable of large amounts of storage at relatively low cost, with current consumer HDDs available with over one terabyte of capacity.
HDDs generally store data on rotating magnetic media or platters. Data is typically stored as a pattern of magnetic flux reversals on the platters. To write data to a typical HDD, the platter is rotated at high speed while a write head floating above the platter generates a series of magnetic pulses to align magnetic particles on the platter to represent the data. To read data from a typical HDD, resistance changes are induced in a magnetoresistive read head as it floats above the platter rotated at high speed. In practice, the resulting data signal is an analog signal whose peaks and valleys are the result of the magnetic flux reversals of the data pattern. Digital signal processing techniques called partial response maximum likelihood (PRML) are then used to sample the analog data signal to determine the likely data pattern responsible for generating the data signal.
HDDs have certain drawbacks due to their mechanical nature. HDDs are susceptible to damage or excessive read/write errors due to shock, vibration or strong magnetic fields. In addition, they are relatively large users of power in portable electronic devices.
Another example of a bulk storage device is a solid state drive (SSD). Instead of storing data on rotating media, SSDs utilize semiconductor memory devices to store their data, but include an interface and form factor making them appear to their host system as if they are a typical HDD. The memory devices of SSDs are typically non-volatile flash memory devices.
Flash memory devices have developed into a popular source of non-volatile memory for a wide range of electronic applications. Flash memory devices typically use a one-transistor memory cell that allows for high memory densities, high reliability, and low power consumption. Changes in threshold voltage of the cells, through programming of charge storage or trapping layers or other physical phenomena, determine the data value of each cell. Common uses for flash memory and other non-volatile memory include personal computers, personal digital assistants (PDAs), digital cameras, digital media players, digital recorders, games, appliances, vehicles, wireless devices, mobile telephones, and removable memory modules, and the uses for non-volatile memory continue to expand.
Unlike HDDs, the operation of SSDs is generally not subject to vibration, shock or magnetic field concerns due to their solid state nature. Similarly, without moving parts, SSDs have lower power requirements than HDDs. However, SSDs currently have much lower storage capacities compared to HDDs of the same form factor and a significantly higher cost per bit.
For the reasons stated above, and for other reasons which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for alternative bulk storage options.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a simplified block diagram of a memory device according to an embodiment of the disclosure.
FIG. 2 is a schematic of a portion of an example NAND memory array as might be found in the memory device of FIG. 1.
FIG. 3 is a block schematic of a solid state bulk storage system in accordance with one embodiment of the present disclosure.
FIG. 4 is a depiction of a wave form showing conceptually a data signal as might be received from the memory device by a read/write channel in accordance with an embodiment of the disclosure.
FIG. 5 is a block schematic of an electronic system in accordance with an embodiment of the disclosure.
FIG. 6 is a diagram of one embodiment of threshold voltage distributions for a memory device.
FIG. 7 is a schematic diagram of one embodiment of a representation of a NAND string of memory cells in accordance with the NAND memory array of FIG. 2.
FIG. 8 is a timing diagram of the select gate drain and source clocks with the resulting charge/discharge cycle of the bit line.
FIG. 9 is a flowchart of one embodiment of a method for operating a memory device utilizing a fixed discharge sensing scheme.



DETAILED DESCRIPTION
In the following detailed description of the present embodiments, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific embodiments in which the embodiments may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that process, electrical or mechanical changes may be made without departing from the scope of the present disclosure. The following detailed description is, therefore, not to be taken in a limiting sense.
Traditional solid-state memory devices pass data in the form of binary signals. Typically, a ground potential represents a first logic level of a bit of data, e.g., a ‘0’ data value, while a supply potential represents a second logic level of a bit of data, e.g., a “1” data value. A multi-level cell (MLC) may be assigned, for example, four different threshold voltage (Vt) ranges of 200 mV for each range, with each range corresponding to a distinct data state, thereby representing four data values or bit patterns. Typically, a dead space or margin of 0.2V to 0.4V is between each range to keep the Vt distributions from overlapping. If the Vt of the cell is within the first range, the cell may be deemed to store a logical 11 state and is typically considered the erased state of the cell. If the Vt is within the second range, the cell may be deemed to store a logical 10 state. If the Vt is within the third range, the cell may be deemed to store a logical 00 state. And if the Vt is within the fourth range, the cell may be deemed to store a logical 01 state.
When programming a traditional MLC device as described above, cells are generally first erased, as a block, to correspond to the erased state. Following erasure of a block of cells, the least-significant bit (LSB) of each cell is first programmed, if necessary. For example, if the LSB is a 1, then no programming is necessary, but if the LSB is a 0, then the Vt of the target memory cell is moved from the Vt range corresponding to the 11 logic state to the Vt range corresponding to the 10 logic state. Following programming of the LSBs, the most-significant bit (MSB) of each cell is programmed in a similar manner, shifting the Vt where necessary. When reading an MLC of a traditional memory device, one or more read operations determine generally into which of the ranges the Vt of the cell voltage falls. For example, a first read operation may determine whether the Vt of the target memory cell is indicative of the MSB being a 1 or a 0 while a second read operation may determine whether the Vt of the target memory cell in indicative of the LSB being a 1 or a 0. In each case, however, a single bit is returned from a read operation of a target memory cell, regardless of how many bits are stored on each cell. This problem of multiple program and read operations becomes increasingly troublesome as more bits are stored on each MLC. Because each such program or read operation is a binary operation, i.e., each programs or returns a single bit of information per cell, storing more bits on each MLC leads to longer operation times.
The memory devices of an illustrative embodiment store data as Vt ranges on the memory cells. In contrast to traditional memory devices, however, program and read operations are capable of utilizing data signals not as discrete bits of MLC data values, but as full representations of MLC data values, such as their complete bit patterns. For example, in a two-bit MLC device, instead of programming a cell's LSB and subsequently programming that cell's MSB, a target threshold voltage may be programmed representing the bit pattern of those two bits. That is, a series of program and verify operations would be applied to a memory cell until that memory cell obtained its target threshold voltage rather than programming to a first threshold voltage for a first bit, shifting to a second threshold voltage for a second bit, etc. Similarly, instead of utilizing multiple read operations to determine each bit stored on a cell, the threshold voltage of the cell may be determined and passed as a single signal representing the complete data value or bit pattern of the cell. The memory devices of the various embodiments do not merely look to whether a memory cell has a threshold voltage above or below some nominal threshold voltage as is done in traditional memory devices. Instead, a voltage signal is generated that is representative of the actual threshold voltage of that memory cell across the continuum of possible threshold voltages. An advantage of this approach becomes more significant as the bits per cell count is increased. For example, if the memory cell were to store eight bits of information, a single read operation would return a single analog data signal representative of eight bits of information.
FIG. 1 is a simplified block diagram of a memory device 101 according to an embodiment of the disclosure. Memory device 101 includes an array of memory cells 104 arranged in rows and columns. Although the various embodiments will be described primarily with reference to NAND memory arrays, the various embodiments are not limited to a specific architecture of the memory array 104. Some examples of other array architectures suitable for the present embodiments include NOR arrays, AND arrays, and virtual ground arrays. In general, however, the embodiments described herein are adaptable to any array architecture permitting generation of a data signal indicative of the threshold voltage of each memory cell.
A row decode circuitry 108 and a column decode circuitry 110 are provided to decode address signals provided to the memory device 101. Address signals are received and decoded to access memory array 104. Memory device 101 also includes input/output (I/O) control circuitry 112 to manage input of commands, addresses and data to the memory device 101 as well as output of data and status information from the memory device 101. An address register 114 is coupled between I/O control circuitry 112 and row decode circuitry 108 and column decode circuitry 110 to latch the address signals prior to decoding. A command register 124 is coupled between I/O control circuitry 112 and control logic 116 to latch incoming commands. Control logic 116 controls access to the memory array 104 in response to the commands and generates status information for the external processor 130. The control logic 116 is coupled to row decode circuitry 108 and column decode circuitry 110 to control the row decode circuitry 108 and column decode circuitry 110 in response to the addresses.
Control logic 116 is also coupled to a sample and hold circuitry 118. The sample and hold circuitry 118 latches data, either incoming or outgoing, in the form of analog voltage levels. For example, the sample and hold circuitry could contain capacitors or other analog storage devices for sampling either an incoming voltage signal representing data to be written to a memory cell or an outgoing voltage signal indicative of the threshold voltage sensed from a memory cell. The sample and hold circuitry 118 may further provide for amplification and/or buffering of the sampled voltage to provide a stronger data signal to an external device.
The handling of analog voltage signals may take an approach similar to an approach well known in the area of CMOS imager technology, where charge levels generated at pixels of the imager in response to incident illumination are stored on capacitors. These charge levels are then converted to voltage signals using a differential amplifier with a reference capacitor as a second input to the differential amplifier. The output of the differential amplifier is then passed to analog-to-digital conversion (ADC) devices to obtain a digital value representative of an intensity of the illumination. In the present embodiments, a charge may be stored on a capacitor in response to subjecting it to a voltage level indicative of an actual or target threshold voltage of a memory cell for reading or programming, respectively, the memory cell. This charge could then be converted to an analog voltage using a differential amplifier having a grounded input or other reference signal as a second input. The output of the differential amplifier could then be passed to the I/O control circuitry 112 for output from the memory device, in the case of a read operation, or used for comparison during one or more verify operations in programming the memory device. It is noted that the V/O control circuitry 112 could optionally include analog-to-digital conversion functionality and digital-to-analog conversion (DAC) functionality to convert read data from an analog signal to a digital bit pattern and to convert write data from a digital bit pattern to an analog signal such that the memory device 101 could be adapted for communication with either an analog or digital data interface.
During a write operation, target memory cells of the memory array 104 are programmed until voltages indicative of their Vt levels match the levels held in the sample and hold circuitry 118. This can be accomplished, as one example, using differential sensing devices to compare the held voltage level to a threshold voltage of the target memory cell. Much like traditional memory programming, programming pulses could be applied to a target memory cell to increase its threshold voltage until reaching or exceeding the desired value. In a read operation, the Vt levels of the target memory cells are passed to the sample and hold circuitry 118 for transfer to an external processor (not shown in FIG. 1) either directly as analog signals or as digitized representations of the analog signals depending upon whether ADC/DAC functionality is provided external to, or within, the memory device.
Threshold voltages of cells may be determined in a variety of manners. For example, a word line voltage could be sampled at the point when the target memory cell becomes activated. Alternatively, a boosted voltage could be applied to a first source/drain side of a target memory cell, and the threshold voltage could be taken as a difference between its control gate voltage and the voltage at its other source/drain side. By coupling the voltage to a capacitor, charge would be shared with the capacitor to store the sampled voltage. Note that the sampled voltage need not be equal to the threshold voltage, but merely indicative of that voltage. For example, in the case of applying a boosted voltage to a first source/drain side of the memory cell and a known voltage to its control gate, the voltage developed at the second source/drain side of the memory cell could be taken as the data signal as the developed voltage is indicative of the threshold voltage of the memory cell.
Sample and hold circuitry 118 may include caching, i.e., multiple storage locations for each data value, such that the memory device 101 may be reading a next data value while passing a first data value to the external processor, or receiving a next data value while writing a first data value to the memory array 104. A status register 122 is coupled between I/O control circuitry 112 and control logic 116 to latch the status information for output to the external processor.
Memory device 101 receives control signals at control logic 116 over a control link 132. The control signals may include a chip enable CE#, a command latch enable CLE, an address latch enable ALE, and a write enable WE#. Memory device 101 may receive commands (in the form of command signals), addresses (in the form of address signals), and data (in the form of data signals) from an external processor over a multiplexed input/output (I/O) bus 134 and output data to the external processor over I/O bus 134.
In a specific example, commands are received over input/output (I/O) pins [7:0] of I/O bus 134 at I/O control circuitry 112 and are written into command register 124. The addresses are received over input/output (I/O) pins [7:0] of bus 134 at I/O control circuitry 112 and are written into address register 114. The data may be received over input/output (I/O) pins [7:0] for a device capable of receiving eight parallel signals, or input/output (I/O) pins [15:0] for a device capable of receiving sixteen parallel signals, at I/O control circuitry 112 and are transferred to sample and hold circuitry 118. Data also may be output over input/output (I/O) pins [7:0] for a device capable of transmitting eight parallel signals or input/output (I/O) pins [15:0] for a device capable of transmitting sixteen parallel signals. It will be appreciated by those skilled in the art that additional circuitry and signals can be provided, and that the memory device of FIG. 1 has been simplified to help focus on the embodiments of the disclosure. Additionally, while the memory device of FIG. 1 has been described in accordance with popular conventions for receipt and output of the various signals, it is noted that the various embodiments are not limited by the specific signals and I/O configurations described unless expressly noted herein. For example, command and address signals could be received at inputs separate from those receiving the data signals, or data signals could be transmitted serially over a single I/O line of I/O bus 134. Because the data signals represent bit patterns instead of individual bits, serial communication of an 8-bit data signal could be as efficient as parallel communication of eight signals representing individual bits.
FIG. 2 is a schematic of a portion of an example NAND memory array 200 as might be found in the memory array 104 of FIG. 1. As shown in FIG. 2, the memory array 200 includes word lines 2021 to 202N and intersecting bit lines 2041 to 204M. For ease of addressing in the digital environment, the number of word lines 202 and the number of bit lines 204 are generally each some power of two.
Memory array 200 includes NAND strings 2061 to 206M. Each NAND string includes transistors 2081 to 208N, each located at an intersection of a word line 202 and a bit line 204. The transistors 208, depicted as floating-gate transistors in FIG. 2, represent non volatile memory cells for storage of data. The floating-gate transistors 208 of each NAND string 206 are connected in series source to drain between one or more source select gates 210, e.g., a field-effect transistor (FET), and one or more drain select gates 212, e.g., an FET. Each source select gate 210 is located at an intersection of a local bit line 204 and a source select line 214, while each drain select gate 212 is located at an intersection of a local bit line 204 and a drain select line 215.
A source of each source select gate 210 is connected to a common source line 216. The drain of each source select gate 210 is connected to the source of the first floating-gate transistor 208 of the corresponding NAND string 206. For example, the drain of source select gate 2101 is connected to the source of floating-gate transistor 2081 of the corresponding NAND string 2061. A control gate of each source select gate 210 is connected to source select line 214. If multiple source select gates 210 are utilized for a given NAND string 206, they would be coupled in series between the common source line 216 and the first floating-gate transistor 208 of that NAND string 206.
The drain of each drain select gate 212 is connected to a local bit line 204 for the corresponding NAND string at a drain contact. For example, the drain of drain select gate 2121 is connected to the local bit line 2041 for the corresponding NAND string 2061 at a drain contact. The source of each drain select gate 212 is connected to the drain of the last floating-gate transistor 208 of the corresponding NAND string 206. For example, the source of drain select gate 2121 is connected to the drain of floating-gate transistor 208N of the corresponding NAND string 2061. If multiple drain select gates 212 are utilized for a given NAND string 206, they would be coupled in series between the corresponding bit line 204 and the last floating-gate transistor 208N of that NAND string 206.
Typical construction of floating-gate transistors 208 includes a source 230 and a drain 232, a floating gate 234, and a control gate 236, as shown in FIG. 2. Floating-gate transistors 208 have their control gates 236 coupled to a word line 202. A column of the floating-gate transistors 208 are those NAND strings 206 coupled to a given local bit line 204. A row of the floating-gate transistors 208 are those transistors commonly coupled to a given word line 202. Other forms of transistors 208 may also be utilized with embodiments of the disclosure, such as NROM, magnetic or ferroelectric transistors and other transistors capable of being programmed to assume one of two or more threshold voltage ranges.
Memory devices of the various embodiments may be advantageously used in bulk storage devices. For various embodiments, these bulk storage devices may take on the same form factor and communication bus interface of traditional HDDs, thus allowing them to replace such drives in a variety of applications. Some common form factors for HDDs include the 3.5′′, 2.5′′ and PCMCIA (Personal Computer Memory Card International Association) form factors commonly used with current personal computers and larger digital media recorders, as well as 1.8′′ and 1′′ form factors commonly used in smaller personal appliances, such as mobile telephones, personal digital assistants (PDAs) and digital media players. Some common bus interfaces include universal serial bus (USB), AT attachment interface (ATA) [also known as integrated drive electronics or IDE], serial ATA (SATA), small computer systems interface (SCSI) and the Institute of Electrical and Electronics Engineers (IEEE) 1394 standard. While a variety of form factors and communication interfaces were listed, the embodiments are not limited to a specific form factor or communication standard. Furthermore, the embodiments need not conform to a HDD form factor or communication interface. FIG. 3 is a block schematic of a solid state bulk storage device 300 in accordance with one embodiment of the present disclosure.
The bulk storage device 300 includes a memory device 301 in accordance with an embodiment of the disclosure, a read/write channel 305 and a controller 310. The read/write channel 305 provides for analog-to-digital conversion of data signals received from the memory device 301 as well as digital-to-analog conversion of data signals received from the controller 310. The controller 310 provides for communication between the bulk storage device 300 and an external processor (not shown in FIG. 3) through bus interface 315. It is noted that the read/write channel 305 could service one or more additional memory devices, as depicted by memory device 301′ in dashed lines. Selection of a single memory device 301 for communication can be handled through a multi-bit chip enable signal or other multiplexing scheme.
The memory device 301 is coupled to a read/write channel 305 through an analog interface 320 and a digital interface 325. The analog interface 320 provides for the passage of analog data signals between the memory device 301 and the read/write channel 305 while the digital interface 325 provides for the passage of control signals, command signals and address signals from the read/write channel 305 to the memory device 301. The digital interface 325 may further provide for the passage of status signals from the memory device 301 to the read/write channel 305. The analog interface 320 and the digital interface 325 may share signal lines as noted with respect to the memory device 101 of FIG. 1. Although the embodiment of FIG. 3 depicts a dual analog/digital interface to the memory device, functionality of the read/write channel 305 could optionally be incorporated into the memory device 301 as discussed with respect to FIG. 1 such that the memory device 301 communicates directly with the controller 310 using only a digital interface for passage of control signals, command signals, status signals, address signals and data signals.
The read/write channel 305 is coupled to the controller 310 through one or more interfaces, such as a data interface 330 and a control interface 335. The data interface 330 provides for the passage of digital data signals between the read/write channel 305 and the controller 310. The control interface 335 provides for the passage of control signals, command signals and address signals from the controller 310 to the read/write channel 305. The control interface 335 may further provide for the passage of status signals from the read/write channel 305 to the controller 310. Status and command/control signals may also be passed directly between the controller 310 and the memory device 301 as depicted by the dashed line connecting the control interface 335 to the digital interface 325.
Although depicted as two distinct devices in FIG. 3, the functionality of the read/write channel 305 and the controller 310 could alternatively be performed by a single integrated circuit device. And while maintaining the memory device 301 as a separate device would provide more flexibility in adapting the embodiments to different form factors and communication interfaces, because it is also an integrated circuit device, the entire bulk storage device 300 could be fabricated as a single integrated circuit device.
The read/write channel 305 is a signal processor adapted to at least provide for conversion of a digital data stream to an analog data stream and vice versa. A digital data stream provides data signals in the form of binary voltage levels, i.e., a first voltage level indicative of a bit having a first binary data value, e.g., 0, and a second voltage level indicative of a bit having a second binary data value, e.g., 1. An analog data stream provides data signals in the form of analog voltages having more than two levels, with different voltage levels or ranges corresponding to different bit patterns of two or more bits. For example, in a system adapted to store two bits per memory cell, a first voltage level or range of voltage levels of an analog data stream could correspond to a bit pattern of 11, a second voltage level or range of voltage levels of an analog data stream could correspond to a bit pattern of 10, a third voltage level or range of voltage levels of an analog data stream could correspond to a bit pattern of 00 and a fourth voltage level or range of voltage levels of an analog data stream could correspond to a bit pattern of 01. Thus, one analog data signal in accordance with the various embodiments would be converted to two or more digital data signals, and vice versa.
In practice, control and command signals are received at the bus interface 315 for access of the memory device 301 through the controller 310. Addresses and data values may also be received at the bus interface 315 depending upon what type of access is desired, e.g., write, read, format, etc. In a shared bus system, the bus interface 315 would be coupled to a bus along with a variety of other devices. To direct communications to a specific device, an identification value may be placed on the bus indicating which device on the bus is to act upon a subsequent command. If the identification value matches the value taken on by the bulk storage device 300, the controller 310 would then accept the subsequent command at the bus interface 315. If the identification value did not match, the controller 310 would ignore the subsequent communication. Similarly, to avoid collisions on the bus, the various devices on a shared bus may instruct other devices to cease outbound communication while they individually take control of the bus. Protocols for bus sharing and collision avoidance are well known and will not be detailed herein. The controller 310 then passes the command, address and data signals on to the read/write channel 305 for processing. Note that the command, address and data signals passed from the controller 310 to the read/write channel 305 need not be the same signals received at the bus interface 315. For example, the communication standard for the bus interface 315 may differ from the communication standard of the read/write channel 305 or the memory device 301. In this situation, the controller 310 may translate the commands and/or addressing scheme prior to accessing the memory device 301. In addition, the controller 310 may provide for load leveling within the one or more memory devices 301, such that physical addresses of the memory devices 301 may change over time for a given logical address. Thus, the controller 310 would map the logical address from the external device to a physical address of a target memory device 301.
For write requests, in addition to the command and address signals, the controller 310 would pass digital data signals to the read/write channel 305. For example, for a 16-bit data word, the controller 310 would pass 16 individual signals having a first or second binary logic level. The read/write channel 305 would then convert the digital data signals to an analog data signal representative of the bit pattern of the digital data signals. To continue with the foregoing example, the read/write channel 305 would use a digital-to-analog conversion to convert the 16 individual digital data signals to a single analog signal having a potential level indicative of the desired 16-bit data pattern. For one embodiment, the analog data signal representative of the bit pattern of the digital data signals is indicative of a desired threshold voltage of the target memory cell. However, in programming of a one-transistor memory cells, it is often the case that programming of neighboring memory cells will increase the threshold voltage of previously programmed memory cells. Thus, for another embodiment, the read/write channel 305 can take into account these types of expected changes in the threshold voltage, and adjust the analog data signal to be indicative of a threshold voltage lower than the final desired threshold voltage. After conversion of the digital data signals from the controller 310, the read/write channel 305 would then pass the write command and address signals to the memory device 301 along with the analog data signals for use in programming the individual memory cells. Programming can occur on a cell-by-cell basis, but is generally performed for a page of data per operation. For a typical memory array architecture, a page of data includes every other memory cell coupled to a word line.
For read requests, the controller would pass command and address signals to the read/write channel 305. The read/write channel 305 would pass the read command and address signals to the memory device 301. In response, after performing the read operation, the memory device 301 would return the analog data signals indicative of the threshold voltages of the memory cells defined by the address signals and the read command. The memory device 301 may transfer its analog data signals in parallel or serial fashion.
The analog data signals may also be transferred not as discrete voltage pulses, but as a substantially continuous stream of analog signals. In this situation, the read/write channel 305 may employ signal processing similar to that used in HDD accessing called PRML or partial response, maximum likelihood. In PRML processing of a traditional HDD, the read head of the HDD outputs a stream of analog signals representative of flux reversals encountered during a read operation of the HDD platter. Rather than attempting to capture the true peaks and valleys of this analog signal generated in response to flux reversals encountered by the read head, the signal is periodically sampled to create a digital representation of the signal pattern. This digital representation can then be analyzed to determine the likely pattern of flux reversals responsible for generation of the analog signal pattern. This same type of processing can be utilized with embodiments of the present disclosure. By sampling the analog signal from the memory device 301, PRML processing can be employed to determine the likely pattern of threshold voltages responsible for generation of the analog signal.
FIG. 4 is a depiction of a wave form showing conceptually a data signal 450 as might be received from the memory device 301 by the read/write channel 305 in accordance with an embodiment of the disclosure. The data signal 450 could be periodically sampled and a digital representation of the data signal 450 can be created from the amplitudes of the sampled voltage levels. For one embodiment, the sampling could be synchronized to the data output such that sampling occurs during the steady-state portions of the data signal 450. Such an embodiment is depicted by the sampling as indicated by the dashed lines at times t1, t2, t3 and t4. However, if synchronized sampling becomes misaligned, values of the data samples may be significantly different than the steady-state values. In an alternate embodiment, sampling rates could be increased to allow determination of where steady-state values likely occurred, such as by observing slope changes indicated by the data samples. Such an embodiment is depicted by the sampling as indicated by the dashed lines at times t5, t6, t7 and t8, where a slope between data samples at times t6 and t7 may indicate a steady-state condition. In such an embodiment, a trade-off is made between sampling rate and accuracy of the representation. Higher sampling rates lead to more accurate representations, but also increase processing time. Regardless of whether sampling is synchronized to the data output or more frequent sampling is used, the digital representation can then be used to predict what incoming voltage levels were likely responsible for generating the analog signal pattern. In turn, the likely data values of the individual memory cells being read can be predicted from this expected pattern of incoming voltage levels.
Recognizing that errors will occur in the reading of data values from the memory device 301, the read/write channel 305 may include error correction. Error correction is commonly used in memory devices, as well as HDDs, to recover from expected errors. Typically, a memory device will store user data in a first set of locations and error correction code (ECC) in a second set of locations. During a read operation, both the user data and the ECC are read in response to a read request of the user data. Using known algorithms, the user data returned from the read operation is compared to the ECC. If the errors are within the limits of the ECC, the errors will be corrected.
FIG. 5 is a block schematic of an electronic system in accordance with an embodiment of the disclosure. Example electronic systems may include personal computers, PDAs, digital cameras, digital media players, digital recorders, electronic games, appliances, vehicles, wireless devices, mobile telephones and the like.
The electronic system includes a host processor 500 that may include cache memory 502 to increase the efficiency of the processor 500. The processor 500 is coupled to a communication bus 504. A variety of other devices may be coupled to the communication bus 504 under control of the processor 500. For example, the electronic system may include random access memory (RAM) 506; one or more input devices 508 such as keyboards, touch pads, pointing devices, etc.; an audio controller 510; a video controller 512; and one or more bulk storage devices 514. At least one bulk storage device 514 includes a digital bus interface 515 for communication with the bus 504, one or more memory devices in accordance with an embodiment of the disclosure having an analog interface for transfer of data signals representative of data patterns of two or more bits of data, and a signal processor adapted to perform digital-to-analog conversion of digital data signals received from the bus interface 515 and analog-to-digital conversion of analog data signals received from its memory device(s).
The resistance of a series string of memory cells varies in response to the programmed pattern of the memory cells above a target cell in the series string. A change in resistance causes the bit line discharge rate to change, resulting in the appearance of a higher threshold voltage (Vt) for a read cell. This concept is illustrated with reference to FIGS. 6 and 7.
FIG. 6 illustrates a schematic diagram of an equivalent circuit for a NAND memory cell string as illustrated in the array schematic of FIG. 2. FIG. 7 illustrates one embodiment of threshold voltage distributions for a multilevel cell memory device having four different states 703-706. The widths 701 of each distribution width can change due to variations in string resistance.
The schematic of FIG. 6 is comprised of the bit line 601 that is coupled to the memory cell string equivalent 600. The string equivalent 600 is comprised of a select gate drain transistor 604 that couples the string to the bit line 601. A select gate source transistor 605 couples the string 600 to the source line 602. A target memory cell 609 is the memory cell of the string that is selected to be programmed or read. In the illustrated embodiment, the target transistor 609 is at the bottom of the string 600 closest to the source line 602.
A resistor 607 represents the sum, RS, of all of the resistances of the memory cells in the NAND string 600 between the target memory cell 609 and the select gate drain transistor 604 (i.e., above the selected cell). The capacitor 608 represents the total capacitance of the memory cells of the NAND string 600 above the target memory cell 609.
In a typical prior art sensing operation, the series string of memory cells is initially precharged from the bit line 601 to which it is coupled. An attempt is then made to discharge the series string current through the target cell to be read/verified. If the target cell is erased, the string discharges. If the cell is programmed, the string does not discharge. This scheme is used to determine the programmed state of a target cell.
When writing to the target cell 609, all of the memory cells in the string above it are normally erased since programming of a memory string typically begins at the bottom cell. In this case, RS is small resulting in a higher bit line current during the verify operation. As an example, the target cell 609 might be verified to the low point 710 of the “00” state 705 as shown in FIG. 7.
In a worst case scenario, all of the cells in the string 600 above the target cell 609 are then programmed, thus increasing RS. With an increased RS, the bit line current decreases and a read operation of the target cell 609 might appear to have a threshold voltage 712 that is outside of the programmed state 705 of FIG. 7. In any case, the increase of the resistance of the series string of memory cells when programmed will add a certain level of millivolts that widens the state Vt distribution and makes it appear that the read cell has a higher threshold voltage than what was programmed. Since the series resistance varies due to the different, unknown programmed states of the cells above the selected cell, the amount of change in Vt cannot be predicted.
In order to reduce the effect of the increased resistance as the upper cells are programmed, for example, the sensing method of one embodiment of the present disclosure performs a charge/discharge cycle of the memory cell string during the read operation. This is accomplished by cycling the bit line, coupled to the series string, through charge/discharge cycles with a clocking signal on each select gate transistor control gate in order to remove fixed units of charge from the bit line. This reduces the effect of the variable and unknown string resistance between the target cell and the bit line during a read operation by making the bit line current appear to be a relatively fixed current. Thus, the threshold voltage of the target cell is indicated by the bias voltage on the word line that causes the target cell to start to conduct.
The charging and discharging scheme of one embodiment of the present disclosure operates by increasing the discharge rate of a bit line. A typical prior art bit line may have a set time to discharge (e.g., 5 μs) during verification. A bit line discharge without the memory cells above the target cell being programmed may take substantially less than the full time to discharge (e.g., 3 μs). Once the memory cells of the string above the target cell are programmed with an unknown pattern, thus creating an unknown and variable resistance, the discharge will take longer than the initial verification due to the increased resistance of the string. The charge/discharge method of the present disclosure reduces this increased discharge time, thus making the bit line current look more uniform without prior knowledge of how the memory cell string is programmed.
A timing diagram of the charge/discharge cycling is illustrated in FIG. 8. The cycling is comprised of first turning on the select gate drain (SGD) transistor while the select gate source (SGS) transistor is off in order to charge the series string to the supply voltage (e.g., VCC). The leading edge 800 of the SGD pulse starts the charge cycle of the representative capacitance, CS, as illustrated in FIG. 7. The time t1 is the time necessary to make sure CS is charged.
The SGD transistor is then turned off after time t1 and the SGS transistor is turned on. The leading edge 802 of this pulse starts the discharge of the NAND series string. The illustrated time t2 is the time to discharge the CS capacitance of the bit line. At the end of t2, the charge/discharge cycle is complete.
The bit line BL graph illustrates the different bit line discharge scenarios depending on the worst case programmed/erased status of the memory cells coupled to the bit line. The more vertical discharge 801 is the result of all of the memory cells being erased. Since the resistance will be lower in this scenario, the bit line discharges at a more rapid rate.
The slower discharging curve 803 is the result of all of the memory cells of the bit line being programmed. In this case, RS will be higher than the all erased scenario and, thus, taking longer for the bit line to discharge. There will be more charge removed from the bit line if memory cells above the target cell have more erase cells than programmed cells due to higher CS and channel charge
At the end of the end of the charge/discharge cycle, a fixed unit of charge has been removed from the bit line. This is repeated periodically such that, at the end of every cycle, a fixed amount of charge is removed periodically. The frequency of the charge/discharge cycle is limited only by the times t1 and t2 such that t1 is long enough to charge the series string and t2 is long enough to discharge the series string.
The charging and discharging method is initiated when a sense operation is initiated. The above-described clocking of the select gate drain and source lines may be performed only on a select number of memory cells of the NAND memory cell string. As the target memory cell gets closer to the select gate drain transistor, the representative resistance decreases due to the decreasing quantity of memory cells. By the time the top cell is the selected cell, the resistance RS no longer exists or is at least negligible. Thus, in one embodiment, the sensing by fixed discharge is not performed on the top most five memory cells. Alternate embodiments do not perform this method on other quantities of top most memory cells of the NAND string.
Coupling can occur between the select gate drain transistor and the top-most memory cells when the select gate drain clock transitions. Coupling can also occur between the select gate source transistor and the bottom-most memory cells during the transitions of the select gate source clock. If the fixed discharge sensing method is not used on a certain quantity of upper-most memory cells of the string, the coupling can be reduced or eliminated. Similarly, the coupling of the bottom-most memory cell and the select gate source transistor can be reduced or eliminated by the inclusion of a dummy cell between the last memory cell and the select gate source transistor. The dummy cell is a normal floating gate memory cell that is not used for storing data. This cell provides a distance buffer between the bottom-most used cells and the select gate source clock.
FIG. 9 illustrates a flowchart of one embodiment of a method for operating a memory device, such as the NAND memory array of FIG. 2, utilizing the fixed discharge sensing scheme described previously. The target cell is programmed to a voltage level that represents a particular bit pattern 901. The cell is then verified to this voltage level 903. If any other cells of the series string of memory cells require programming, these cells are programmed as well 905. When the original target cell is read, the reading is performed by sensing with the discharge scheme described above by removing fixed units of charge from the bit line 907.
CONCLUSION
The embodiments of the present disclosure provide fixed discharge sensing of a target memory cell by removing fixed units of charge from the bit lines of a memory array during a read operation. The removal of the fixed units of charge from the bit line causes the bit line current to appear more uniform during sensing, thus reducing the variation in sensed threshold voltages during the read operation.
Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement that is calculated to achieve the same purpose may be substituted for the specific embodiments shown. Many adaptations of the disclosure will be apparent to those of ordinary skill in the art. Accordingly, this application is intended to cover any adaptations or variations of the disclosure.

## Claims

1. A method for sensing of a target memory cell, the method comprising:
charging a series string of memory cells coupled to the target memory cell by clocking a select gate drain transistor with a clocking signal coupled to a control gate of the transistor such that the transistor turns on and off in response to the clocking signal;
discharging the series string by a fixed unit of charge after the series string is charged; and
sensing a current on the bit line in response to the target memory cell turning on.

2. The method of claim 1 wherein discharging the series string comprises clocking a select gate source transistor with a positive-going pulse coupled to a control gate of the transistor such that the transistor turns on with a rising edge of the clocking signal and the transistor turns off with a falling edge of the clocking signal.

3. The method of claim 1 wherein a length of the positive-going pulse coupled to the control gate of the select gate drain transistor and the positive-going pulse coupled to control gate of the select gate source transistor transfers charge to ground.

4. The method of claim 1 wherein a rising edge of the select gate source clocking one of either coincides with a falling edge of the select gate drain clocking or is delayed from the falling edge of the select gate drain clocking.

5. The method of claim 1 and further including determining a bias voltage on a word line of the target memory cell that causes the target memory cell to turn on.

6. The method of claim 5 wherein the bias voltage is a threshold voltage of the target memory cell.

7. The method of claim 1 wherein clocking of the select gate drain and select gate source transistors is initiated during a read operation of the target memory cell.

8. A method for operating a memory device, the method comprising:
programming, to a first voltage level, a target memory cell of a series string of memory cells coupled to a bit line;
verifying that the target memory cell is programmed to the first voltage level; and
sensing the target memory cell by:
precharging the bit line to a second voltage level;
charging the series string to the bit line;
discharging the series string by a fixed quantity of charge/discharge cycles; and
sensing a current on the bit line in response to a read voltage applied to a word line coupled to the target memory cell.

9. The method of claim 8 wherein the memory device is a NAND flash memory device.

10. The method of claim 8 wherein the second voltage level is a supply voltage for the memory device.

11. The method of claim 8 wherein the first voltage level is representative of a digital bit pattern to be stored in the target memory cell.

12. The method of claim 8 wherein charging the series string to the bit line comprises enabling a select gate drain transistor for a first time such that a charge can be transferred from the bit line to the series string of memory cells.

13. The method of claim 12 wherein discharging the series string comprises enabling a select gate source transistor for a second time such that the fixed unit of charge is removed from the series string of memory cells to a source line.

14. A solid state memory device comprising:
an array of memory cells organized in rows of memory cells coupled to word lines and series strings of memory cells coupled to bit lines;
control circuitry for the array of memory cells wherein the control circuitry is adapted to sense a current on a bit line coupled to a target memory cell, in order to determine a representative analog voltage programmed into the target memory cell, by transferring a charge from the bit line to a series string of memory cells comprising the target memory cell then removing a fixed unit of charge from the bit line; and
a read/write channel coupled between the control circuitry and the array of non-volatile memory cells, the read/write channel having digital-to-analog conversion circuitry and analog-to-digital conversion circuitry.

15. The solid state memory device of claim 14 wherein the array of non-volatile memory cells is organized in a NAND architecture.

16. The solid state memory device of claim 14 wherein the control circuitry is further adapted to receive a digital bit pattern to be programmed into a target memory cell, submit the digital bit pattern to the read/write channel for conversion to the representative analog voltage, and program the representative analog voltage into the target memory cell.

17. The solid state memory device of claim 14 wherein the control circuitry controls enablement of a select gate drain transistor to transfer charge from the bit line and controls enablement of a select gate source transistor to remove the fixed unit of charge.

18. The solid state memory device of claim 17 wherein each series string of memory cells comprises a memory cell, between the target memory cell and the select gate source transistor, that is unused.