# Patent

## Title
NAND with back biased operation

## Abstract

Methods of programming, reading and erasing memory cells are disclosed. In at least one embodiment, program, sense, and erase operations in a memory are performed with back biased operation, such as to improve high voltage device isolation and cutoff in string drivers and bit line drivers, and no nodes of the circuitry are biased at zero volts.

## Background
RELATED APPLICATIONS
This Patent Application is a continuation of U.S. patent application Ser. No. 12/117,314, filed May 8, 2008, entitled “NAND WITH BACK BIASED OPERATION,” (now U.S. Pat. No. 7,724,577, issued May 25, 2010), which is incorporated by reference herein in its entirety.


FIELD
The present disclosure relates generally to semiconductor, memories, and in particular, the present disclosure relates to NAND memories.
BACKGROUND
Memory devices are typically provided as internal, semiconductor, integrated circuits in computers or other electronic devices. There are many different types of memory including random-access memory (RAM), read only memory (ROM), dynamic random access memory (DRAM), synchronous dynamic random access memory (SDRAM), and flash memory.
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 nodes, such as floating gates or trapping layers or other physical phenomena, determine the data value of each cell. Common uses for flash memory include personal computers, personal digital assistants (PDAs), digital cameras, digital media players, digital recorders, games, appliances, vehicles, wireless devices, cellular telephones, and removable memory modules, and the uses for flash memory continue to expand.
Flash memory typically utilizes one of two basic architectures known as NOR flash and NAND flash. The designation is derived from the logic used to read the devices. In NOR flash architecture, a column of memory cells are coupled in parallel with each memory cell coupled to a data line, such as that commonly referred to as a bit line. In NAND flash architecture, a string of memory cells are coupled in series with only the first memory cell of the column coupled to a bit line.
As the performance of electronic systems employing flash memory devices increases, flash memory device performance should also increase. A performance increase includes reducing power consumption, increasing speed, and increasing the memory density. Further, supply voltages are dropping, and there is an increased likelihood that leakage will occur given the tight controls on allowed voltages. This leads to problems with cutoff in string drivers, which drive the access line (e.g., word line) voltages of memories, and in cutoff of cells during boosting to decrease bit line to bit line leakage.
For the reasons stated above, and for other reasons stated below 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 memory devices having improved device isolation and cutoff performance.



BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is a functional block diagram of an electrical system having at least one memory device with a memory array configuration according to one embodiment of the present invention;
FIG. 2 is a diagram of a portion of a memory to be programmed according to an embodiment of the present invention;
FIG. 3 is a diagram of a string driver of a memory to be programmed according to the embodiment of FIG. 2;
FIG. 4 is a diagram of a bit line driver of a memory to be programmed according to the embodiment of FIG. 2;
FIG. 5 is a diagram of a portion of a memory to be read according to an embodiment of the present invention;
FIG. 6 is a diagram of a portion of a memory to be erased according to an embodiment of the present invention;
FIG. 7 is a diagram of a string driver of a memory to be erased according to the embodiment of FIG. 6; and
FIG. 8 is a functional block diagram of a memory module having at least one memory device in accordance with an embodiment of the invention.



DETAILED DESCRIPTION
In the following detailed description of the embodiments, reference is made to the accompanying drawings that form a part hereof. In the drawings, like numerals describe substantially similar components throughout the several views. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. Other embodiments may be utilized and structural, logical, and electrical changes may be made without departing from the scope of the present invention.
The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present disclosure is defined only by the appended claims, along with the full scope of equivalents to which such claims are entitled.
FIG. 1 is a functional block diagram of a memory device 100, such as a flash memory device, of one embodiment of the present invention, and on which various method embodiments may be practiced, which is coupled to a processor 110. The memory device 100 and the processor 110 may form part of an electronic system 120. The memory device 100 has been simplified to focus on features of the memory that are helpful in understanding the present invention. The memory device includes an array of memory cells 130 having bias conditions for program, read, and erase operations such as those shown in FIGS. 2-8 and described below. The memory array 130 is arranged in banks of rows and columns.
An address buffer circuit 140 is provided to latch address signals provided over I/O connections 162 through I/O circuitry 160. Address signals are received and decoded by row decoder 144 and a column decoder 146 to access the memory array 130. It will be appreciated by those skilled in the art, with the benefit of the present description, that the number of address input connections depends upon the density and architecture of the memory array. That is, the number of addresses increases with both increased memory cell counts and increased bank and block counts.
The memory device reads data in the array 130 by sensing voltage or current changes in the memory array columns using sense/latch circuitry 150. The sense/latch circuitry, in one embodiment, is coupled to read and latch a row of data from the memory array. Data input and output buffer circuitry 160 is included for bi-directional data communication over a plurality of data (DQ) connections 162 with the processor 110, and is connected to write circuitry 155 and sense/latch circuitry 150 for performing read and write operations on the memory 100.
Command control circuit 170 decodes signals provided on control connections 172 from the processor 110. These signals are used to control the operations on the memory array 130, including data read, data write, and erase operations. The flash memory device has been simplified to facilitate a basic understanding of the features of the memory. A more detailed understanding of internal circuitry and functions of flash memories are known to those skilled in the art.
Bias conditions placed on NAND strings 200 during programming of a selected block that is to be programmed are shown in more detail in FIG. 2. NAND strings 200 have a source select gate line connected to source select gates 204. A programming voltage Vpgm (typically 15 to 25 volts) is applied to program the selected word line 218. Program inhibit voltages are applied to all other word lines in the NAND string. Word line 206 is used to cut off the channel voltage to improve boosting efficiency. Non-zero bias Viso (typically 0.5 to 1.5 volts) is applied to the isolation word line 206. A program unselected bit line 212 is biased at Vcc (approximately 2.3 volts in one embodiment) to cut off select gate 215. Source select gate (SGS) line 202 and drain select gate (SGD) line 214 are biased at Vsgs (typically 0.5-1.0 volts) and Vsgd (typically 2.0-3.0 volts) respectively. Word lines 222 and 224 are biased at Vbias (approximately 5.0 volts). In this embodiment, program selected bit line 210 is biased at a non-zero bias Vb1_sel (typically 0.5-1.5 volts). By the application of non-zero bias on the program selected bit line 210, field isolation between memory cells is back biased. As one result, field leakage is reduced. In one embodiment, Viso is greater than 0 volts, and less than the program unselected bit line voltage (Vcc). The biases are maintained in one embodiment from the first program pulse of a programming operation to the last program pulse in the programming operation. No ground node exists, that is, no bit line, word line, or select gate is at 0 volts during the programming operation from the first programming pulse to the last programming pulse.
An associated string driver 300 for the program operation on the selected block of FIG. 2 is shown in FIG. 3. The string driver 300 is the device that drives the word lines (e.g., applies voltages to the word lines). Transistors 302, 304, 306, 308, and 310 of the string driver 300 are shown. The transistors 302, 304, 306, 308, and 310 share a common gate 312. Transistors 302, 304, 306, 308, and 310 correlate to those transistors shown on word lines 202, 214, 222, 206, and 224, respectively. In this embodiment for programming, a positive voltage is applied to the active area of the transistors, and a pass voltage of 25 volts is applied to the gate 312. If transistors 302 and 308 had atypical bias of 0 volts, then there would be a problem with potential leakage and field isolation issues. However, Vsgs and Viso bias (0.5-1.5 V) of transistors 302 and 308 results in a back biasing of the transistors, improving field isolation.
An associated bit line driver (wmux) 400 for the program operation on the selected block of FIG. 2 is shown in FIG. 4. The bit line driver (wmux) 400 is the device that drives the bit lines. Two transistors 402 and 404 are shown in FIG. 4, which is a top view. Each of the transistors 402 and 404 drives a bit line. In this embodiment, transistor 402 drives bit line n (both even and odd) and transistor 404 drives bit line n+1 (both even and odd). The gates 406 and 408 of the transistors are biased to 0 volts since during programming the transistors are in the off state. The voltage at the sources 410 and 412 is bit line select voltage Vb1_sel (0.5-1.5 volts) in one embodiment. In typical previous designs, a source bias of 0 volts was typical, and with the gate to source voltage differential being 0 volts, leakage could occur. In the present embodiments, with the sources biased to 0.5-1.5 volts, there is an effective back bias, which is often referred to as reverse bias, of −0.5 volts on each gate to source connection. This can improve cutoff of the transistors of the bit line driver during programming.
A program operation of one embodiment provides positive biases, for the block to be programmed, on the select gate, the selected and unselected bit lines, the selected word line and the unselected word lines in the NAND string, and the common source. These positive biases result in no word line or bit line that is biased at ground. More specifically, programming according to one embodiment includes applying a positive bias to a select gate, applying a positive bias to a bit line of the string of the cell to be programmed, applying a positive bias to unselected program word lines, and applying a programming voltage to a cell to be programmed.
A read circuit 500 is shown in greater detail in FIG. 5. In this embodiment, to read in a selected read block, a bias of 0.5 volts is placed on the common source 502, a bias of 0.5-2.5 volts (Vb1_unselected) is placed on the non-selected bit line(s) 504, and a bias of 1.0 volts is placed on the selected bit line 506. In prior configurations, the common source and non-selected bit lines were biased at 0 volts. A read pass voltage of approximately 5.0 volts is applied to the word lines 508 coupled to the string of cells 510. The same bias (Vb1_unselected) is applied to the source to prevent current flow on unselected bit lines. In bit line driver, 0.5 volts bias can improve isolation of the device as discussed above. During a read operation, Atub can be biased at 0 volts or at Vb1_unselected (approximately 0.5 volts).
An erase circuit configuration 600 is shown in greater detail in FIG. 6. In the prior art, word line voltages for selected erase blocks were 0 volts. In the present erase embodiment, for a selected erase block, the voltage bias on the word line is 0.5 volts. This results in a similar improvement to that of the programming operation discussed above with respect to the string driver. In erase configuration 600, a bias voltage of 0.5 volts is placed on word lines 602 coupled to the cells to be erased, and a bias of 20 volts is placed on the tub 604.
A string driver configuration 700 for an erase operation on a selected erase block is shown in greater detail in FIG. 7. A Vse1 bias of approximately 0.5 volts is applied to global word lines 702 and to local word lines 704 of the selected erase block 706. On non-selected erase blocks, such as block 710, a bias of 20 volts is applied to the local word lines 708 while the global word lines 702 are biased at 0.5 volts. In this configuration, 0.5 volts applied to the local word lines 704 and 20 volts applied to the local word lines 708 results in a positive bias on both sides of the transistors, and therefore improves field isolation.
Erasing cells in a selected erase block according to one embodiment includes applying a positive bias to selected and unselected bit lines and to a common source of the memory, and applying a positive word line voltage to all global word lines and to local word lines of the selected erase block during an erase operation.
For operations including program, read, or erase on selected blocks, all word lines or bit lines are positively biased as described above. There are no word lines or bit lines within the selected block for the operation that are biased at ground voltage, which should improve cutoff performance and field isolation. Prior art had isolation problems due to the ground biasing of the word lines and/or bit lines.
FIG. 8 is an illustration of an exemplary memory module 800. Memory module 800 is illustrated as a memory card, although the concepts discussed with reference to memory module 800 are applicable to other types of removable or portable memory, e.g., USB flash drives, and are intended to be within the scope of “memory module” as used herein. In addition, although one example form factor is depicted in FIG. 8, these concepts are applicable to other form factors as well.
In some embodiments, memory module 800 will include a housing 805 (as depicted) to enclose one or more memory devices 810, though such a housing is not essential to all devices or device applications. At least one memory device 810 is a non-volatile memory including a biasing scheme and configuration as described above with respect to FIGS. 2-7 according to various embodiments of the present invention. Where present, the housing 805 includes one or more contacts 815 for communication with a host device. Examples of host devices include digital cameras, digital'recording and playback devices, PDAs, personal computers, memory card readers, interface hubs and the like. For some embodiments, the contacts 815 are in the form of a standardized interface. For example, with a USB flash drive, the contacts 815 might be in the form of a USB Type-A male connector. For some embodiments, the contacts 815 are in the form of a semi-proprietary interface. In general, however, contacts 815 provide an interface for passing control, address and/or data signals between the memory module 800 and a host having compatible receptors for the contacts 815.
The memory module 800 may optionally include additional circuitry 820 which may be one or more integrated circuits and/or discrete components. For some embodiments, the additional circuitry 820 may include a memory controller for controlling access across multiple memory devices 810 and/or for providing a translation layer between an external host and a memory device 810. For example, there may not be a one-to-one correspondence between the number of contacts 815 and a number of I/O connections to the one or more memory devices 810. Thus, a memory controller could selectively couple an I/O connection (not shown in FIG. 8) of a memory device 810 to receive the appropriate signal at the appropriate I/O connection at the appropriate time or to provide the appropriate signal at the appropriate contact 815 at the appropriate time. Similarly, the communication protocol between a host and the memory module 800 may be different than what is required for access of a memory device 810. A memory controller could then translate the command sequences received from a host into the appropriate command sequences to achieve the desired access to the memory device 810. Such translation may further include changes in signal voltage levels in addition to command sequences.
The additional circuitry 820 may further include functionality unrelated to control of a memory device 810 such as logic functions as might be'performed by an ASIC (application specific integrated circuit). Also, the additional circuitry 820 may include circuitry to restrict read or write access to the memory module 800, such as password protection, biometrics or the like. The additional circuitry 820 may include circuitry to indicate a status of the memory module 800. For example, the additional circuitry 820 may include functionality to determine whether power is being supplied to the memory module 800 and whether the memory module 800 is currently being accessed, and to display an indication of its status, such as a solid light while powered and a flashing light while being accessed. The additional circuitry 820 may further include passive devices, such as decoupling capacitors to help regulate power requirements within the memory module 800.
CONCLUSION
Biasing methods have been described that include applying a positive bias to word lines and bit lines for program and erase operations in a specific program or erase block, such as to improve cut off of string drivers and bit line drivers. In various embodiments, this biasing method back biases transistors and field devices in string drivers, such as to improve isolation.
Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement, which is calculated to achieve the same purpose, may be substituted for the specific embodiment shown. This application is intended to cover any adaptations or variations of the present invention. Therefore, it is manifestly intended that this invention be limited only by the claims and the equivalents thereof.

## Claims

1. A method of erasing memory cells, comprising:
biasing a tub in which the cells to be erased are formed; and
biasing control gates of the memory cells at a non-zero voltage wherein biasing the control gates at a non-zero voltage comprises biasing the control gates at a positive voltage.

2. The method of claim 1, wherein the positive voltage comprises approximately 0.5V, and wherein biasing the tub comprises biasing the tub at 20V.

3. A method of erasing memory cells, comprising:
biasing global access lines and local access lines configured to be selectively coupled to memory cells in a block of memory selected to be erased at a positive voltage; and
biasing global access lines and local access lines configured to be selectively coupled to memory cells in a block of memory not selected to be erased at a positive voltage.

4. The method of claim 3, wherein biasing global access lines and local access lines configured to be selectively coupled to memory cells in a block of memory not selected to be erased with a positive voltage comprises:
biasing the global access lines with a first positive voltage; and
biasing the local access lines configured to be selectively coupled to the memory cells in the block of memory not selected to be erased with a second positive voltage, wherein the second positive voltage is greater than the first positive voltage.

5. A method of programming a memory cell in a string of memory cells, comprising:
biasing a data line selectively coupled to the string at a non-zero voltage;
applying a programming voltage to a control gate of the memory cell to be programmed.

6. The method of claim 5, further comprising:
biasing a control gate of another memory cell in the string at an isolation voltage.

7. The method of claim 6, wherein the isolation voltage comprises a non-zero voltage.

8. The method of claim 7, wherein biasing a data line selectively coupled to the string at a non-zero voltage comprises biasing a data line selectively coupled to the string at a positive voltage, and wherein the isolation voltage is less than the positive voltage.

9. A method of programming a memory cell in a string of memory cells, comprising:
biasing a data line selectively coupled to the string at a non-zero voltage;
applying a programming voltage to a control gate of the memory cell to be programmed;
biasing a control gate of another memory cell in the string at an isolation voltage;
wherein the isolation voltage comprises a non-zero voltage, wherein biasing a data line selectively coupled to the string at a non-zero voltage comprises biasing a data line selectively coupled to the string at a positive voltage, and wherein the isolation voltage is less than the positive voltage, and wherein the positive voltage is a first positive voltage and the isolation voltage comprises a second positive voltage, and further comprising applying a positive pass voltage to remaining memory cells in the string.

10. The method of claim 5, further comprising
biasing a source select gate coupled to the string at a positive source select gate voltage; and
biasing a drain select gate coupled to the string at a positive drain select gate voltage.

11. The method of claim 10, further comprising biasing another data line at a positive program unselected voltage, the other data line being selectively coupled to an adjacent string of memory cells.

12. A method of programming a memory cell in a string of memory cells, comprising:
biasing a data line selectively coupled to the string at a non-zero voltage;
applying a programming voltage to a control gate of the memory cell to be programmed;
wherein applying a programming voltage to a control gate of the memory cell to be programmed comprises applying a plurality of programming pulses to the control gate of the memory cell to be programmed, and wherein biasing the data line comprises maintaining the biasing from a first programming pulse of the plurality of pulses to a last programming pulse of the plurality of pulses.

13. A method of programming a memory cell using a string driver, the string driver comprising a first transistor configured to drive a source select gate line and a second transistor configured to drive an access line coupled to an isolation memory cell in a string of memory cells including the memory cell to be programmed, and wherein the first and second transistors share a common gate, the method comprising:
applying a pass voltage to the common gate; and
back biasing at least one of the first transistor and the second transistor.

14. The method of claim 13, wherein back biasing at least one of the first transistor and the second transistor comprises biasing the source select gate line at a first positive voltage and biasing the access line coupled to the isolation memory cell at a second positive voltage.

15. The method of claim 14, wherein the first positive voltage equals the second positive voltage.

16. The method of claim 14, wherein the first positive voltage comprises 0.5-1.5V and the second positive voltage comprises 0.5-1.5V.

17. A method of programming a memory cell selected for programming, comprising:
back biasing a transistor configured to drive a data line selectively coupled to the memory cell selected for programming; and
biasing a gate of the transistor to turn off the transistor during programming.

18. A method of reading a memory cell in a string of memory cells, comprising:
biasing a source selectively coupled to the string of memory cells at a non-zero voltage; and
biasing a data line selectively coupled to the string at a selected data line voltage.

19. A method of reading a memory cell in a string of memory cells, comprising:
biasing a source selectively coupled to the string of memory cells at a non-zero voltage; and
biasing a data line selectively coupled to the string at a selected data line voltage;
applying a read voltage to the memory cell to be read; and
applying a pass voltage to other memory cells in the string.

20. The method of claim 18, wherein the non-zero voltage comprises a first positive voltage, and further comprising biasing at a second positive voltage another data line, the other data line being selectively coupled to a string of memory cells that do not include a memory cell selected for reading.

21. The method of claim 20, wherein the first positive voltage comprises approximately 0.5V, the second positive voltage comprises a voltage of 0.5-1.5V, and the selected data line voltage is a higher positive voltage than the first and second positive voltages.

22. The method of claim 18, further comprising biasing a tub in which the string of memory cells is formed at 0V.

23. The method of claim 20, further comprising biasing a tub in which the string of memory cells is formed at the second positive voltage.

24. A method of reading a memory cell in a string of memory cells, comprising:
biasing a data line selectively coupled to the string at a selected data line voltage; and
biasing another data line at a non-zero voltage, wherein the other data line is selectively coupled to another string of memory cells that is adjacent to the string selectively coupled to the data line biased at the selected data line voltage.