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United States Patent Application 20160204783
Kind Code A1
Rajendra; Srinivas ;   et al. July 14, 2016

Bulk Driven Low Swing Driver

Abstract

A circuit is presented to reduce power while transmitting high speed signals across a long length of wire on an integrated circuit. A PMOS is used as a low swing driver, where the PMOS is connected between the driver's output and ground. The gate of the PMOS is also set to ground, while the input signal is connected to the bulk. The output is then transmitted over the signal path to an analogue receiver, where both single ended and differential embodiments are presented. For a single ended version, a reference voltage for the receiver can be provided by a second, similarly connected PMOS whose bulk has an input signal at an intermediate level of input swing at the transmitter PMOS.


Inventors: Rajendra; Srinivas; (Milpitas, CA) ; Ramachandra; Venkatesh Prasad; (San Jose, CA)
Applicant:
Name City State Country Type

SanDisk Technologies Inc.

Plano

TX

US
Family ID: 1000001970881
Appl. No.: 14/596772
Filed: January 14, 2015


Current U.S. Class: 365/185.2 ; 327/109; 365/185.18
Current CPC Class: H03M 1/34 20130101; G11C 16/06 20130101; H03K 19/018514 20130101
International Class: H03K 19/0185 20060101 H03K019/0185; H03M 1/34 20060101 H03M001/34; G11C 16/06 20060101 G11C016/06

Claims



1. A circuit for the high speed transfer of a digital signal over a data path, comprising: an analog receiver circuit connected to receive a first intermediate analog signal and generate therefrom a digital output signal; an intermediate signal path connected to the analog receiver circuit to supply the first intermediate analog signal thereto; and a first transmitter circuit connected to receive a digital input signal and having a first output node connected to the intermediate signal path to provide the first intermediate analog signal thereto, the first transmitter circuit including: a first current source connected between a voltage supply and the first output node to supply a fixed current level thereto; and a first PMOS device connected between the first output node and ground, wherein a gate of the first PMOS device is set to ground and the digital input signal is connected to a bulk input of the first PMOS device.

2. The circuit of claim 1, wherein the analog receiver circuit includes a difference amplifier having a first input connected to receive the first intermediate analog signal and a second input connected to receive a first reference voltage.

3. The circuit of claim 2, further comprising: a reference voltage generation circuit to generate the first reference voltage, including: a second current source connected between the voltage supply and a node from which the first reference voltage is provided to supply a fixed current level thereto; and a second PMOS device connected between the node from which the first reference voltage is provided and ground, wherein a gate of the second PMOS device is set to ground and a bulk input of the second PMOS device is connected to receive a voltage level intermediate between the level of the voltage supply and ground.

4. The circuit of claim 3, wherein the voltage level intermediate between the level of the voltage supply and ground is set to be half the level of the voltage supply.

5. The circuit of claim 1, further comprising: a second transmitter circuit connected to receive the digital input signal in inverted form and having a second output node connected to the intermediate signal path to provide a second intermediate analog signal thereto, the second transmitter circuit including: a second current source connected between the voltage supply and the second output node to supply a fixed current level thereto; and a second PMOS device connected between the second output node and ground, wherein a gate of the second PMOS device is set to ground and the digital input signal in inverted form is connected to a bulk input of the second PMOS device, wherein the intermediate signal path connected to the analog receiver circuit to supply the second intermediate analog signal thereto, and wherein the analog receiver circuit includes a difference amplifier having a first input connected to receive the first intermediate analog signal and a second input connected to receive the second intermediate analog signal.

6. The circuit of claim 1, wherein the first current source comprises: a second PMOS device connected between the voltage supply and the first output node and having a gate connected to receive a fixed voltage level.

7. The circuit of claim 1, wherein the first intermediate analog signal is of lower swing than the digital input signal.

8. The circuit of claim 1, wherein the circuit is formed on an integrated circuit, wherein a guard ring formed around the first PMOS device on the substrate of the integrated circuit.

9. The circuit of claim 1, wherein circuit is formed on a non-volatile memory circuit.

10. The circuit of claim 9, wherein the non-volatile memory circuit is a monolithic three-dimensional semiconductor memory device where the memory cells are arranged in multiple physical levels above a silicon substrate and comprise a charge storage medium.

11. An integrated non-volatile memory circuit, comprising: an input receiver connectable to receive an external signal; logic circuitry connected to the input receiver to generate a digital input signal derived from a received external signal; a signal transfer circuit, comprising: an analog receiver circuit connected to receive a first intermediate analog signal and generate therefrom a digital output signal; an intermediate signal path connected to the analog receiver circuit to supply the first intermediate analog signal thereto; and a first transmitter circuit connected to receive the digital input signal and having a first output node connected to the intermediate signal path to provide the first intermediate analog signal thereto, the first transmitter circuit including: a first current source connected between a voltage supply and the first output node to supply a fixed current level thereto; and a first PMOS device connected between the first output node and ground, wherein a gate of the first PMOS device is set to ground and the digital input signal is connected to a bulk input of the first PMOS device.

12. The integrated non-volatile memory circuit of claim 11, wherein the analog receiver circuit includes a difference amplifier having a first input connected to receive the first intermediate analog signal and a second input connected to receive a first reference voltage.

13. The integrated non-volatile memory circuit of claim 12, further comprising: a reference voltage generation circuit to generate the first reference voltage, including: a second current source connected between the voltage supply and a node from which the first reference voltage is provided to supply a fixed current level thereto; and a second PMOS device connected between the node from which the first reference voltage is provided and ground, wherein a gate of the second PMOS device is set to ground and a bulk input of the second PMOS device is connected to receive a voltage level intermediate between the level of the voltage supply and ground.

14. The integrated non-volatile memory circuit of claim 13, wherein the voltage level intermediate between the level of the voltage supply and ground is set to be half the level of the voltage supply.

15. The integrated non-volatile memory circuit of claim 11, further comprising: a second transmitter circuit connected to receive the digital input signal in inverted form and having a second output node connected to the intermediate signal path to provide a second intermediate analog signal thereto, the second transmitter circuit including: a second current source connected between the voltage supply and the second output node to supply a fixed current level thereto; and a second PMOS device connected between the second output node and ground, wherein a gate of the second PMOS device is set to ground and the digital input signal in inverted form is connected to a bulk input of the second PMOS device, wherein the intermediate signal path connected to the analog receiver circuit to supply the second intermediate analog signal thereto, and wherein the analog receiver circuit includes a difference amplifier having a first input connected to receive the first intermediate analog signal and a second input connected to receive the second intermediate analog signal.

16. The integrated non-volatile memory circuit of claim 11, wherein the first current source comprises: a second PMOS device connected between the voltage supply and the first output node and having a gate connected to receive a fixed voltage level.

17. The integrated non-volatile memory circuit of claim 11, wherein the first intermediate analog signal is of lower swing than the digital input signal.

18. The integrated non-volatile memory circuit of claim 11, wherein the circuit is formed on an integrated circuit, wherein a guard ring formed around the first PMOS device on the substrate of the integrated circuit.

19. The integrated non-volatile memory circuit of claim 11, wherein the non-volatile memory circuit is a monolithic three-dimensional semiconductor memory device where the memory cells are arranged in multiple physical levels above a silicon substrate and comprise a charge storage medium.

20. The integrated non-volatile memory circuit of claim 11, wherein the integrated non-volatile memory circuit is part of a memory system further including a controller circuit and the external signal is received from the controller circuit.
Description



BACKGROUND

[0001] This application relates to driver circuits for transmitting signals, such as digital data signals, on integrated circuits.

[0002] Integrated circuit need to accurately and rapidly transmit data and command signal between elements. To take an example, solid-state non-volatile memory circuits receive data and instructions received from a controller at an input receiver, after which the signals are transmitted by various transmitters, repeater circuits, and receivers as these signals are passed to various registers and other elements on the circuit. Given the large number of such signals, reducing the amount of power consumer in this process is of particular importance, a situation becoming more acute as signal rates increase.

SUMMARY

[0003] A circuit for the high speed transfer of a digital signal over a data path includes: an analog receiver circuit, connected to receive a first intermediate analog signal and generate from it a digital output signal; an intermediate signal path connected to the analog receiver circuit to supply it with the first intermediate analog signal; and a transmitter circuit connected to receive a digital input signal and having a first output node connected to the intermediate signal path to provide the first intermediate analog signal. The transmitter circuit includes a current source connected between a voltage supply and the output node to supply a fixed current level and a PMOS device connected between the output node and ground. The gate of the PMOS device is set to ground and the digital input signal is connected to a bulk input of the PMOS device.

[0004] An integrated non-volatile memory circuit includes an input receiver connectable to receive an external signal; logic circuitry connected to the input receiver to generate a digital input signal derived from a received external signal; and a signal transfer circuit. The signal transfer circuit includes: an analog receiver circuit, connected to receive a first intermediate analog signal and generate from it a digital output signal; an intermediate signal path connected to the analog receiver circuit to supply it with the first intermediate analog signal; and a transmitter circuit connected to receive a digital input signal and having a first output node connected to the intermediate signal path to provide the first intermediate analog signal. The transmitter circuit includes a current source connected between a voltage supply and the output node to supply a fixed current level and a PMOS device connected between the output node and ground. The gate of the PMOS device is set to ground and the digital input signal is connected to a bulk input of the PMOS device.

[0005] Various aspects, advantages, features and embodiments are included in the following description of exemplary examples thereof, which description should be taken in conjunction with the accompanying drawings. All patents, patent applications, articles, other publications, documents and things referenced herein are hereby incorporated herein by this reference in their entirety for all purposes. To the extent of any inconsistency or conflict in the definition or use of terms between any of the incorporated publications, documents or things and the present application, those of the present application shall prevail.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006] FIG. 1 illustrates schematically the main hardware components of a memory system suitable for implementing various aspects described in the following.

[0007] FIG. 2 illustrates schematically a non-volatile memory cell.

[0008] FIG. 3 illustrates the relation between the source-drain current I.sub.D and the control gate voltage V.sub.CG for four different charges Q1-Q4 that the floating gate may be selectively storing at any one time at fixed drain voltage.

[0009] FIG. 4 illustrates schematically a string of memory cells organized into a NAND string.

[0010] FIG. 5 illustrates an example of a NAND array 210 of memory cells, constituted from NAND strings 50 such as that shown in FIG. 4.

[0011] FIG. 6 illustrates a page of memory cells, organized in the NAND configuration, being sensed or programmed in parallel.

[0012] FIGS. 7A-7C illustrate an example of programming a population of memory cells.

[0013] FIG. 8 shows an example of a physical structure of a 3-D NAND string.

[0014] FIGS. 9-12 look at a particular monolithic three dimensional (3D) memory array of the NAND type (more specifically of the "BiCS" type).

[0015] FIG. 13 illustrates some of the elements typically found on a memory circuit.

[0016] FIG. 14 is a schematic representation of a signal path using a transmitter, repeater, and receiver.

[0017] FIG. 15 is an exemplary embodiment of a low swing driver circuit.

[0018] FIG. 16 illustrates an exemplary transmitter arrangement.

[0019] FIG. 17 looks at input and output waveforms of the transmitter of FIG. 16.

[0020] FIG. 18 illustrates a technique for generating a receiver reference voltage.

[0021] FIG. 19 illustrates an alternate, differential embodiment.

[0022] FIG. 20 shows a PMOS device connected as in FIG. 16.

DETAILED DESCRIPTION

Memory System

[0023] FIG. 1 illustrates schematically the main hardware components of a memory system suitable for implementing the following. The memory system 90 typically operates with a host 80 through a host interface. The memory system may be in the form of a removable memory such as a memory card, or may be in the form of an embedded memory system. The memory system 90 includes a memory 102 whose operations are controlled by a controller 100. The memory 102 comprises one or more array of non-volatile memory cells distributed over one or more integrated circuit chip. The controller 100 may include interface circuits 110, a processor 120, ROM (read-only-memory) 122, RAM (random access memory) 130, programmable nonvolatile memory 124, and additional components. The controller is typically formed as an ASIC (application specific integrated circuit) and the components included in such an ASIC generally depend on the particular application.

[0024] With respect to the memory section 102, semiconductor memory devices include volatile memory devices, such as dynamic random access memory ("DRAM") or static random access memory ("SRAM") devices, non-volatile memory devices, such as resistive random access memory ("ReRAM"), electrically erasable programmable read only memory ("EEPROM"), flash memory (which can also be considered a subset of EEPROM), ferroelectric random access memory ("FRAM"), and magnetoresistive random access memory ("MRAM"), and other semiconductor elements capable of storing information. Each type of memory'device may have different configurations. For example, flash memory devices may be configured in a NAND or a NOR configuration.

[0025] The memory devices can be formed from passive and/or active elements, in any combinations. By way of non-limiting example, passive semiconductor memory elements include ReRAM device elements, which in some embodiments include a resistivity switching storage element, such as an anti-fuse, phase change material, etc., and optionally a steering element, such as a diode, etc. Further by way of non-limiting example, active semiconductor memory elements include EEPROM and flash memory device elements, which in some embodiments include elements containing a charge storage region, such as a floating gate, conductive nanoparticles, or a charge storage dielectric material.

[0026] Multiple memory elements may be configured so that they are connected in series or so that each element is individually accessible. By way of non-limiting example, flash memory devices in a NAND configuration (NAND memory) typically contain memory elements connected in series. A NAND memory array may be configured so that the array is composed of multiple strings of memory in which a string is composed of multiple memory elements sharing a single bit line and accessed as a group. Alternatively, memory elements may be configured so that each element is individually accessible, e.g., a NOR memory array. NAND and NOR memory configurations are exemplary, and memory elements may be otherwise configured.

[0027] The semiconductor memory elements located within and/or over a substrate may be arranged in two or three dimensions, such as a two dimensional memory structure or a three dimensional memory structure.

[0028] In a two dimensional memory structure, the semiconductor memory elements are arranged in a single plane or a single memory device level. Typically, in a two dimensional memory structure, memory elements are arranged in a plane (e.g., in an x-z direction plane) which extends substantially parallel to a major surface of a substrate that supports the memory elements. The substrate may be a wafer over or in which the layer of the memory elements are formed or it may be a carrier substrate which is attached to the memory elements after they are formed. As a non-limiting example, the substrate may include a semiconductor such as silicon.

[0029] The memory elements may be arranged in the single memory device level in an ordered array, such as in a plurality of rows and/or columns. However, the memory elements may be arrayed in non-regular or non-orthogonal configurations. The memory elements may each have two or more electrodes or contact lines, such as bit lines and word lines.

[0030] A three dimensional memory array is arranged so that memory elements occupy multiple planes or multiple memory device levels, thereby forming a structure in three dimensions (i.e., in the x, y and z directions, where the y direction is substantially perpendicular and the x and z directions are substantially parallel to the major surface of the substrate).

[0031] As a non-limiting example, a three dimensional memory structure may be vertically arranged as a stack of multiple two dimensional memory device levels. As another non-limiting example, a three dimensional memory array may be arranged as multiple vertical columns (e.g., columns extending substantially perpendicular to the major surface of the substrate, i.e., in the y direction) with each column having multiple memory elements in each column. The columns may be arranged in a two dimensional configuration, e.g., in an x-z plane, resulting in a three dimensional arrangement of memory elements with elements on multiple vertically stacked memory planes. Other configurations of memory elements in three dimensions can also constitute a three dimensional memory array.

[0032] By way of non-limiting example, in a three dimensional NAND memory array, the memory elements may be coupled together to form a NAND string within a single horizontal (e.g., x-z) memory device levels. Alternatively, the memory elements may be coupled together to form a vertical NAND string that traverses across multiple horizontal memory device levels. Other three dimensional configurations can be envisioned wherein some NAND strings contain memory elements in a single memory level while other strings contain memory elements which span through multiple memory levels. Three dimensional memory arrays may also be designed in a NOR configuration and in a ReRAM configuration.

[0033] Typically, in a monolithic three dimensional memory array, one or more memory device levels are formed above a single substrate. Optionally, the monolithic three dimensional memory array may also have one or more memory layers at least partially within the single substrate. As a non-limiting example, the substrate may include a semiconductor such as silicon. In a monolithic three dimensional array, the layers constituting each memory device level of the array are typically formed on the layers of the underlying memory device levels of the array. However, layers of adjacent memory device levels of a monolithic three dimensional memory array may be shared or have intervening layers between memory device levels.

[0034] Then again, two dimensional arrays may be formed separately and then packaged together to form a non-monolithic memory device having multiple layers of memory. For example, non-monolithic stacked memories can be constructed by forming memory levels on separate substrates and then stacking the memory levels atop each other. The substrates may be thinned or removed from the memory device levels before stacking, but as the memory device levels are initially formed over separate substrates, the resulting memory arrays are not monolithic three dimensional memory arrays. Further, multiple two dimensional memory arrays or three dimensional memory arrays (monolithic or non-monolithic) may be formed on separate chips and then packaged together to form a stacked-chip memory device.

[0035] Associated circuitry is typically required for operation of the memory elements and for communication with the memory elements. As non-limiting examples, memory devices may have circuitry used for controlling and driving memory elements to accomplish functions such as programming and reading. This associated circuitry may be on the same substrate as the memory elements and/or on a separate substrate. For example, a controller for memory read-write operations may be located on a separate controller chip and/or on the same substrate as the memory elements.

[0036] It will be recognized that the following is not limited to the two dimensional and three dimensional exemplary structures described but cover all relevant memory structures within the spirit and scope as described herein

Physical Memory Structure

[0037] FIG. 2 illustrates schematically a non-volatile memory cell. The memory cell 10 can be implemented by a field-effect transistor having a charge storage unit 20, such as a floating gate or a charge trapping (dielectric) layer. The memory cell 10 also includes a source 14, a drain 16, and a control gate 30.

[0038] There are many commercially successful non-volatile solid-state memory devices being used today. These memory devices may employ different types of memory cells, each type having one or more charge storage element.

[0039] Typical non-volatile memory cells include EEPROM and flash EEPROM. Also, examples of memory devices utilizing dielectric storage elements.

[0040] In practice, the memory state of a cell is usually read by sensing the conduction current across the source and drain electrodes of the cell when a reference voltage is applied to the control gate. Thus, for each given charge on the floating gate of a cell, a corresponding conduction current with respect to a fixed reference control gate voltage may be detected. Similarly, the range of charge programmable onto the floating gate defines a corresponding threshold voltage window or a corresponding conduction current window.

[0041] Alternatively, instead of detecting the conduction current among a partitioned current window, it is possible to set the threshold voltage for a given memory state under test at the control gate and detect if the conduction current is lower or higher than a threshold current (cell-read reference current). In one implementation the detection of the conduction current relative to a threshold current is accomplished by examining the rate the conduction current is discharging through the capacitance of the bit line.

[0042] FIG. 3 illustrates the relation between the source-drain current I.sub.D and the control gate voltage V.sub.CG for four different charges Q1-Q4 that the floating gate may be selectively storing at any one time. With fixed drain voltage bias, the four solid I.sub.D versus V.sub.CG curves represent four of seven possible charge levels that can be programmed on a floating gate of a memory cell, respectively corresponding to four possible memory states. As an example, the threshold voltage window of a population of cells may range from 0.5V to 3.5V. Seven possible programmed memory states "0", "1", "2", "3", "4", "5", "6", and an erased state (not shown) may be demarcated by partitioning the threshold window into regions in intervals of 0.5V each. For example, if a reference current, IREF of 2 .mu.A is used as shown, then the cell programmed with Q1 may be considered to be in a memory state "1" since its curve intersects with I.sub.REF in the region of the threshold window demarcated by VCG=0.5V and 1.0V. Similarly, Q4 is in a memory state "5".

[0043] As can be seen from the description above, the more states a memory cell is made to store, the more finely divided is its threshold window. For example, a memory device may have memory cells having a threshold window that ranges from -1.5V to 5V. This provides a maximum width of 6.5V. If the memory cell is to store 16 states, each state may occupy from 200 mV to 300 mV in the threshold window. This will require higher precision in programming and reading operations in order to be able to achieve the required resolution.

NAND Structure

[0044] FIG. 4 illustrates schematically a string of memory cells organized into a NAND string. A NAND string 50 comprises a series of memory transistors M1, M2, Mn (e.g., n=4, 8, 16 or higher) daisy-chained by their sources and drains. A pair of select transistors S1, S2 controls the memory transistor chain's connection to the external world via the NAND string's source terminal 54 and drain terminal 56 respectively. In a memory array, when the source select transistor S1 is turned on, the source terminal is coupled to a source line (see FIG. 5). Similarly, when the drain select transistor S2 is turned on, the drain terminal of the NAND string is coupled to a bit line of the memory array. Each memory transistor 10 in the chain acts as a memory cell. It has a charge storage element 20 to store a given amount of charge so as to represent an intended memory state. A control gate 30 of each memory transistor allows control over read and write operations. As will be seen in FIG. 5, the control gates 30 of corresponding memory transistors of a row of NAND string are all connected to the same word line. Similarly, a control gate 32 of each of the select transistors S1, S2 provides control access to the NAND string via its source terminal 54 and drain terminal 56 respectively. Likewise, the control gates 32 of corresponding select transistors of a row of NAND string are all connected to the same select line.

[0045] When an addressed memory transistor 10 within a NAND string is read or is verified during programming, its control gate 30 is supplied with an appropriate voltage. At the same time, the rest of the non-addressed memory transistors in the NAND string 50 are fully turned on by application of sufficient voltage on their control gates. In this way, a conductive path is effectively created from the source of the individual memory transistor to the source terminal 54 of the NAND string and likewise for the drain of the individual memory transistor to the drain terminal 56 of the cell.

[0046] FIG. 4B illustrates an example of a NAND array 210 of memory cells, constituted from NAND strings 50 such as that shown in FIG. 4. Along each column of NAND strings, a bit line such as bit line 36 is coupled to the drain terminal 56 of each NAND string. Along each bank of NAND strings, a source line such as source line 34 is coupled to the source terminals 54 of each NAND string. Also the control gates along a row of memory cells in a bank of NAND strings are connected to a word line such as word line 42. The control gates along a row of select transistors in a bank of NAND strings are connected to a select line such as select line 44. An entire row of memory cells in a bank of NAND strings can be addressed by appropriate voltages on the word lines and select lines of the bank of NAND strings.

[0047] FIG. 6 illustrates a page of memory cells, organized in the NAND configuration, being sensed or programmed in parallel. FIG. 6 essentially shows a bank of NAND strings 50 in the memory array 210 of FIG. 5, where the detail of each NAND string is shown explicitly as in FIG. 4. A physical page, such as the page 60, is a group of memory cells enabled to be sensed or programmed in parallel. This is accomplished by a corresponding page of sense amplifiers 212. The sensed results are latched in a corresponding set of latches 214. Each sense amplifier can be coupled to a NAND string via a bit line. The page is enabled by the control gates of the cells of the page connected in common to a word line 42 and each cell accessible by a sense amplifier accessible via a bit line 36. As an example, when respectively sensing or programming the page of cells 60, a sensing voltage or a programming voltage is respectively applied to the common word line WL3 together with appropriate voltages on the bit lines.

Physical Organization of the Memory

[0048] One difference between flash memory and other of types of memory is that a cell must be programmed from the erased state. That is the floating gate must first be emptied of charge. Programming then adds a desired amount of charge back to the floating gate. It does not support removing a portion of the charge from the floating gate to go from a more programmed state to a lesser one. This means that updated data cannot overwrite existing data and must be written to a previous unwritten location.

[0049] Furthermore erasing is to empty all the charges from the floating gate and generally takes appreciable time. For that reason, it will be cumbersome and very slow to erase cell by cell or even page by page. In practice, the array of memory cells is divided into a large number of blocks of memory cells. As is common for flash EEPROM systems, the block is the unit of erase. That is, each block contains the minimum number of memory cells that are erased together. While aggregating a large number of cells in a block to be erased in parallel will improve erase performance, a large size block also entails dealing with a larger number of update and obsolete data.

[0050] Each block is typically divided into a number of physical pages. A logical page is a unit of programming or reading that contains a number of bits equal to the number of cells in a physical page. In a memory that stores one bit per cell, one physical page stores one logical page of data. In memories that store two bits per cell, a physical page stores two logical pages. The number of logical pages stored in a physical page thus reflects the number of bits stored per cell. In one embodiment, the individual pages may be divided into segments and the segments may contain the fewest number of cells that are written at one time as a basic programming operation. One or more logical pages of data are typically stored in one row of memory cells. A page can store one or more sectors. A sector includes user data and overhead data.

All-Bit, Full-Sequence MLC Programming

[0051] FIG. 7A-7C illustrate an example of programming a population of 4-state memory cells. FIG. 7A illustrates the population of memory cells programmable into four distinct distributions of threshold voltages respectively representing memory states "0", "1", "2" and "3". FIG. 7B illustrates the initial distribution of "erased" threshold voltages for an erased memory. FIG. 6C illustrates an example of the memory after many of the memory cells have been programmed. Essentially, a cell initially has an "erased" threshold voltage and programming will move it to a higher value into one of the three zones demarcated by verify levels vV.sub.1, vV.sub.2 and vV.sub.3. In this way, each memory cell can be programmed to one of the three programmed states "1", "2" and "3" or remain un-programmed in the "erased" state. As the memory gets more programming, the initial distribution of the "erased" state as shown in FIG. 7B will become narrower and the erased state is represented by the "0" state.

[0052] A 2-bit code having a lower bit and an upper bit can be used to represent each of the four memory states. For example, the "0", "1", "2" and "3" states are respectively represented by "11", "01", "00" and "10". The 2-bit data may be read from the memory by sensing in "full-sequence" mode where the two bits are sensed together by sensing relative to the read demarcation threshold values rV.sub.1, rV.sub.2 and rV.sub.3 in three sub-passes respectively.

3-D NAND Structures

[0053] An alternative arrangement to a conventional two-dimensional (2-D) NAND array is a three-dimensional (3-D) array. In contrast to 2-D NAND arrays, which are formed along a planar surface of a semiconductor wafer, 3-D arrays extend up from the wafer surface and generally include stacks, or columns, of memory cells extending upwards. Various 3-D arrangements are possible. In one arrangement a NAND string is formed vertically with one end (e.g. source) at the wafer surface and the other end (e.g. drain) on top. In another arrangement a NAND string is formed in a U-shape so that both ends of the NAND string are accessible on top, thus facilitating connections between such strings.

[0054] FIG. 8 shows a first example of a NAND string 701 that extends in a vertical direction, i.e. extending in the z-direction, perpendicular to the x-y plane of the substrate. Memory cells are formed where a vertical bit line (local bit line) 703 passes through a word line (e.g. WL0, WL1, etc.). A charge trapping layer between the local bit line and the word line stores charge, which affects the threshold voltage of the transistor formed by the word line (gate) coupled to the vertical bit line (channel) that it encircles. Such memory cells may be formed by forming stacks of word lines and then etching memory holes where memory cells are to be formed. Memory holes are then lined with a charge trapping layer and filled with a suitable local bit line/channel material (with suitable dielectric layers for isolation).

[0055] As with planar NAND strings, select gates 705, 707, are located at either end of the string to allow the NAND string to be selectively connected to, or isolated from, external elements 709, 711. Such external elements are generally conductive lines such as common source lines or bit lines that serve large numbers of NAND strings. Vertical NAND strings may be operated in a similar manner to planar NAND strings and both SLC and MLC operation is possible. While FIG. 8 shows an example of a NAND string that has 32 cells (0-31) connected in series, the number of cells in a NAND string may be any suitable number. Not all cells are shown for clarity. It will be understood that additional cells are formed where word lines 3-29 (not shown) intersect the local vertical bit line.

[0056] A 3D NAND array can, loosely speaking, be formed tilting up the respective structures 50 and 210 of FIGS. 5 and 6 to be perpendicular to the x-y plane. In this example, each y-z plane corresponds to the page structure of FIG. 6, with m such plane at differing x locations. The (global) bit lines, BL1-m, each run across the top to an associated sense amp SA1-m. The word lines, WL1-n, and source and select lines SSL1-n and DSL1-n, then run in x direction, with the NAND string connected at bottom to a common source line CSL.

[0057] FIGS. 9-12 look at a particular monolithic three dimensional (3D) memory array of the NAND type (more specifically of the "BiCS" type), where one or more memory device levels are formed above a single substrate, in more detail. FIG. 9 is an oblique projection of part of such a structure, showing a portion corresponding to two of the page structures in FIG. 5, where, depending on the embodiment, each of these could correspond to a separate block or be different "fingers" of the same block. Here, instead to the NAND strings lying in a common y-z plane, they are squashed together in the y direction, so that the NAND strings are somewhat staggered in the x direction. On the top, the NAND strings are connected along global bit lines (BL) spanning multiple such sub-divisions of the array that run in the x direction. Here, global common source lines (SL) also run across multiple such structures in the x direction and are connect to the sources at the bottoms of the NAND string, which are connected by a local interconnect (LI) that serves as the local common source line of the individual finger. Depending on the embodiment, the global source lines can span the whole, or just a portion, of the array structure. Rather than use the local interconnect (LI), variations can include the NAND string being formed in a U type structure, where part of the string itself runs back up.

[0058] To the right of FIG. 9 is a representation of the elements of one of the vertical NAND strings from the structure to the left. Multiple memory cells are connected through a drain select gate SGD to the associated bit line BL at the top and connected through the associated source select gate SDS to the associated local source line LI to a global source line SL. It is often useful to have a select gate with a greater length than that of memory cells, where this can alternately be achieved by having several select gates in series (as described in U.S. patent application Ser. No. 13/925,662, filed on Jun. 24, 2013), making for more uniform processing of layers. Additionally, the select gates are programmable to have their threshold levels adjusted. This exemplary embodiment also includes several dummy cells at the ends that are not used to store user data, as their proximity to the select gates makes them more prone to disturbs.

[0059] FIG. 10 shows a top view of the structure for two blocks in the exemplary embodiment. Two blocks (BLK0 above, BLK1 below) are shown, each having four fingers that run left to right. The word lines and select gate lines of each level also run left to right, with the word lines of the different fingers of the same block being commonly connected at a "terrace" and then on to receive their various voltage level through the word line select gates at WLTr. The word lines of a given layer in a block can also be commonly connected on the far side from the terrace. The selected gate lines can be individual for each level, rather common, allowing the fingers to be individually selected. The bit lines are shown running up and down the page and connect on to the sense amp circuits, where, depending on the embodiment, each sense amp can correspond to a single bit line or be multiplexed to several bit lines.

[0060] FIG. 11 shows a side view of one block, again with four fingers. In this exemplary embodiment, the select gates SGD and SGS at either end of the NAND strings are formed of four layers, with the word lines WL in-between, all formed over a CPWELL. A given finger is selected by setting its select gates to a level VSG and the word lines are biased according to the operation, such as a read voltage (VCGRV) for the selected word lines and the read-pass voltage (VREAD) for the non-selected word lines. The non-selected fingers can then be cut off by setting their select gates accordingly.

[0061] FIG. 12 illustrates some detail of an individual cell. A dielectric core runs in the vertical direction and is surrounded by a channel silicon layer, that is in turn surrounded a tunnel dielectric (TNL) and then the charge trapping dielectric layer (CTL). The gate of the cell is here formed of tungsten with which is surrounded by a metal barrier and is separated from the charge trapping layer by blocking (BLK) oxide and a high K layer.

Bulk Driven Low Swing Driver

[0062] This section looks at the transmission of signals on integrated circuits. Although much more generally applicable, the discussion is placed in the context of non-volatile memory circuits, such as those described above, in order to provide concrete examples.

[0063] FIG. 13 is a schematic representation of a memory system to illustrate some of the elements typically found on a memory circuit and between which signals would be transmitted. To the left of the broken line in a controller 320 and to the right a memory chip 300, including an array 301 that can be one of those such as are described above. In this representation, the control, address, and I/O data lines are shown separated out. After being received on the memory chip 300, data is transmitted to the data input/output circuits 306, on to the column control circuits 302, and then on to the array 301, with the data moving in the other direction when read out to the controller. The addresses are transmitted between the controller and the column control circuits 302 and the row control circuits. Control signals are transferred between the memory's controller interface and the command circuits 307; between the command circuits 307 and state machine 308; and from the state machine to the various control circuits (302, 303, 304, 305) used in operations on the array. The particulars will vary with the embodiment, but this gives a general idea of some of the elements involved.

[0064] FIG. 14 is a schematic representation of one such signal path. An input signal from the controller is received at the input receiver 401, also having reference voltage input, and supplied on to the logic block 403, which in turn generates the (digital) signal to be transmitted to one of the other blocks on the circuit by block 410. A transmitter 411, such as a series of inverters transmits the signal to a receiver 413, here a single inverter. The intervening path is represented as series of resistances and capacitances. To maintain the signal over the path, one or more repeaters 415 are used. Among the priorities for such a circuit are performance (jitter, skew), power consumption, and area. The main goal in this section is to reduce power consumption in this block without affecting performance. In transmitting a digital of amplitude between ground and Vdd at a frequency f is P.sub.avg=1/2 C V.sub.dd.sup.2f, were C is the capacitance along the path. As C is a factor of the path and it is not wanted to drop the frequency, two ways to reduce power consumption are to either to reduce the supply level Vdd, which is typically not practical as this level is widely used across the device, or to reduce the voltage swing of the block 401.

[0065] To reduce power consumption while transmitting high speed signals across a long length of wire, without affecting performance, the exemplary embodiments of this section reduce the voltage swing along the transmission path. More specifically, a low swing driver (for example, of 300 mV relative to a Vdd of .about.2V) is used with an analog receiver, without need an intervening repeater. This is illustrated in FIG. 15: The input driver 501 receives the input from the controller and a reference voltage Vref, which in this arrangement may differ from that of FIG. 14, and provides the signal to the logic block 503, which in turn supplies the (digital) signal to be transmitted to the block 510. The driver 511 will provide the low swing signal through the path to the analog receiver 513. The embodiment of FIG. 15 uses a one sided embodiment, where the second input of the analog receiver is a reference value Vref whose generation is discussed below. As also discussed below, a differential embodiment can be used.

[0066] FIG. 16 shows an exemplary embodiment for the transmitter of the low swing driver. A fixed value current source Idc 603 is connected between Vdd and the output node that supplies the signal wire. A PMOS 601 is connected between the out node and ground. The gate of PMOS 601 is also connected to ground. The input is connected to the bulk input of PMOS 601 and modulates the devices threshold voltage, and thus the output voltage. The digital input has values of 0V and Vdd. The output will swing between the low value Vol and the high value Voh. Under this arrangement, as Vin increases the threshold of PMOS 601 and, consequently Vout, increasing in an almost linear manner. For example, an input signal with Vdd .about.2V will lead to an output swing of about 300 mV.

[0067] FIG. 17 looks at typical input and output waveforms of the transmitter of FIG. 16. As shown, the input is taken as a series of alternate "0" (ground) and "1" (Vdd=.about.2V, in this example) values. The output values alternate between corresponding high value Voh and low value Vol. To properly differentiate these values, the receivers reference value reference value Vref' needs to be set near the center of this swing, which will typically not be centered around Vdd/2.

[0068] Consequently, the reference voltage value Vref for the analog receiver 513 in the single ended analogue input arrangement of FIG. 15 needs to be set appropriately to track with the swing across all PVT corners. This can be achieved by using a PMOS arranged similarly to the transmitter driver of FIG. 16, but with the input set to Vdd/2 (the midpoint of Vin for the transmitter of FIG. 16), providing an output at the midpoint of transmitter's swing. This is illustrated in FIG. 18 that shows the threshold voltage and output voltage of a PMOS connected as in FIG. 16. Although the threshold voltage and output voltage are not completely linear with the Vin value, they do track relatively closely; and for applications where speed and simplicity is of importance, these may provide sufficient accuracy to generate the reference voltage for the differential amplifier at receiver. One can sweep the bulk voltage, plot the output vs input transfer characteristics and decide whether the linearity is sufficient enough to meet performance. If not, one can opt for the differential version, such as illustrated in FIG. 19.

[0069] The exemplary embodiment of FIG. 15 uses a single ended transmitter to reduce complexity, area and power. For higher performance, improved accuracy or both, a differential arrangement can also be used. In a differential arrangements, a pair of transmitters as in FIG. 16 can be used, one for the input and one for the inverted input. Both signals are then transmitted to the receiver, one for each input of receiver's differential amplifier, as is illustrated in FIG. 19.

[0070] One possible concern of the arrangement illustrated with respect to FIG. 16 is forward biasing of the PMOS when the bulk is at the low input value. FIG. 20 illustrates the situation. This shows a PMOS device whose gate and drain are set to ground, the input voltage is applied to the bulk and the output is taken from the source. When Vin is at VDD, there is no forward biasing concern. However, when Vin is at ground and source is at Vol, the source-bulk P-N junction is forward biased. This issue can be mitigated by using standard latchup protection like a guard ring.

CONCLUSION

[0071] The foregoing detailed description has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the above to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. The described embodiments were chosen in order to explain the principles involved and its practical application, to thereby enable others to best utilize the various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope be defined by the claims appended hereto.

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