WO2001063614A1 - Mixed mode multi-level memory - Google Patents

Mixed mode multi-level memory Download PDF

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Publication number
WO2001063614A1
WO2001063614A1 PCT/US2001/003858 US0103858W WO0163614A1 WO 2001063614 A1 WO2001063614 A1 WO 2001063614A1 US 0103858 W US0103858 W US 0103858W WO 0163614 A1 WO0163614 A1 WO 0163614A1
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WO
WIPO (PCT)
Prior art keywords
mode
bit
indicator
cells
access
Prior art date
Application number
PCT/US2001/003858
Other languages
French (fr)
Inventor
Allan Parker
Joseph Kucera
Original Assignee
Advanced Micro Devices, Inc.
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Advanced Micro Devices, Inc. filed Critical Advanced Micro Devices, Inc.
Publication of WO2001063614A1 publication Critical patent/WO2001063614A1/en

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Classifications

    • G—PHYSICS
    • G11—INFORMATION STORAGE
    • G11C—STATIC STORES
    • G11C11/00—Digital stores characterised by the use of particular electric or magnetic storage elements; Storage elements therefor
    • G11C11/56—Digital stores characterised by the use of particular electric or magnetic storage elements; Storage elements therefor using storage elements with more than two stable states represented by steps, e.g. of voltage, current, phase, frequency
    • G11C11/5621—Digital stores characterised by the use of particular electric or magnetic storage elements; Storage elements therefor using storage elements with more than two stable states represented by steps, e.g. of voltage, current, phase, frequency using charge storage in a floating gate
    • G—PHYSICS
    • G11—INFORMATION STORAGE
    • G11C—STATIC STORES
    • G11C2211/00—Indexing scheme relating to digital stores characterized by the use of particular electric or magnetic storage elements; Storage elements therefor
    • G11C2211/56—Indexing scheme relating to G11C11/56 and sub-groups for features not covered by these groups
    • G11C2211/564—Miscellaneous aspects
    • G11C2211/5641—Multilevel memory having cells with different number of storage levels

Definitions

  • a flash memory cell can be a field effect transistor (FET) that includes a select gate, a floating gate, a drain, and a source.
  • FET field effect transistor
  • a cell can be read by grounding the source, and applying a voltage to a bitiine connected with the drain. By applying a voltage to the wordline connected to the select gate, the cell can be switched on and off.
  • Programming a cell includes trapping excess electrons in the floating gate to increase voltage. This reduces the current conducted by the memory cell when the select voltage is applied to the select gate.
  • the cell is programmed when the cell current is less than a reference current when the select voltage is applied.
  • the cell is erased when the cell current is greater than the reference current and the select voltage is applied.
  • Memory cells with only two programmable states contain only a single bit of information, such as a "0" or a " 1 ".
  • a multi-level cell is a cell that can be programmed with more than one voltage level. Each voltage level is mapped to corresponding bits of information. For example, a single multi-level cell can be programmed with one of four voltage levels, e.g.
  • a cell that is programmable at more voltage levels can store more bits of data based on Eqn. 1.
  • B is the number of bits of data stored
  • N is the number of voltage levels.
  • a 1 bit cell requires 2 voltage levels
  • a 2 bit cell requires 4 voltage levels
  • a 3 bit cell requires 8 voltage levels
  • a 4 bit cell requires 16 voltage levels.
  • Figure 1 shows a representation of a single bit programming voltage level diagram 100.
  • the "erase state” program distribution 102 ("erase state")
  • the "programmed state” program distribution 104 ("programmed state")represent a single bit being either a "0" or a " 1 ". respectively.
  • the voltage between the upper end 106 of the erase state 102 and ground 108 (0.0V) is the “erase margin.”
  • the erase state program distribution (erase state)
  • the read point can be at ground 108 or anywhere between lines 106 and 1 10, preferably near the mid-point between lines 106 and 1 10.
  • Figure 2 shows a representation of a four level multilevel cell program voltage diagram 200.
  • the program distribution (“distribution") of the four levels are shown at 210. 212, 214, and 216 respectively.
  • the programming distributions are located at approximately -2.5V, 0.0V, 0.8V, and 2.0V and the width of the programming distributions are approximately 100 to 600mV.
  • a four level multilevel memory cell can be programmed with any one of these voltage levels. Because the cell can store one of four binary values it can store 2 bits of information.
  • the program margin also called “margin”, “data margin”, or “guard band” is the voltage levels between distributions that is not normally used.
  • the program margins between program distributions 212, 214. and 216 are approximately 800mV to l OOmV wide.
  • the program margin between program distributions 210 and 212 is approximately 2.0V.
  • a memory device having a plurality of memory cells that are group into at least two group of cells Each cell is capable of being programmed in at least two modes
  • a mode indicator is associated with each group of cells The mode indicator indicates which programming mode is used to access the cells
  • the mode indicator is one or more bits and optionally is user selectable
  • Figure 2 is a representation of a two bit programming voltage level
  • Figure 3 is a representation of an embodiment of a memory device with mode indicators
  • Figure 4 is a flow diagram of a method ot access two segments ol memory in two different access modes
  • a user selectable mode indicator is associated with each page of mem ory
  • the mode indicator indicates which mode of the page (or group) That is, the multi-level mode or the single bit mode If more than two modes are used, the mode indicator will be more than one bit
  • a two bit mode indicator indicates four modes and a three bit indicator indicates eight modes
  • the modes are "single bit” and "two bit” and only a single bit is required tor the mode indicator for each page
  • the mode indicator is a single bit
  • the default state preferably indicates the single bit state
  • Multi-level cells while holding more information than a single bit cell can take substantially longer to program, have smaller program margins, and have lower reliability Further, NAND based Multi-level cells are usually addressed on a page-by-page basis
  • a 64Mbit (8Mbyte) memory array can contain 4096 pages where each page contains 512 bytes
  • a page can be a 4K. bit page that has 4096 bits
  • Interlaced NAND based multi-level cells are read sequent ⁇ all> l or example, if the byte at location 10 is to be read, the bj tes at locations 0 through 9 must be read first That is, interlaced NAND based multi-level cells do not support partial page writes or reads
  • the first read access can be performed at the same voltage level, e g 206 ( Figure 2)
  • the mode indicator is in the erase state (e g zeio)
  • the reference level will be lowered to 108 ( Figure 1 ) without significant impact on access times
  • the mode indicator is in the programmed state, indicating the multi-bit mode the memory is accessed with normal MLC access times
  • Figure 3 is an embodiment of a memor> device 300 with two groups of memory cells 302, 304 and two mode indicators 306, 308 associated with the respective groups of memory cells T he groups of memorv cells 302 304 can be physically separate memop, or, preferably , they are subsets of one physical memory
  • the mode indicators 306, 308 can be locations in memory, including the being part of the same memory as the first and second groups of memory 302, 304
  • the mode indicators 306, 308 can be a register, non-volatile memory, or volatile memory
  • the memory device 300 can include a storage means for storing data capable of storing data in a plurality of storage formats, an indication means for indicating which of the plurality of different formats is to be used to access the data, and an input/output means for accessing the data stored in the storage means
  • the storage means can include memory cells, memory arrays, flash memory, NAND flash memory, or any other memory storage device
  • the plurality of storage formats can preferably include a single-bit and a multi-bit format Multiple multi- bit formats can also be used
  • the indicator means can include a single memory cell, multiple memory cells, a latch or other device for storing a value
  • the storage means can include a plurality of storage areas and the indicator means can include a separate indicator for each storage area
  • the indicator means with separate indicators for each storage area can be a group of memory cells, a group of latches, or other device for storing the indicators
  • a storage area can be a page of memory including a 4096 bit page of memory
  • the storage areas can be of varying
  • the method 400 includes determining a second mode value from a second mode indicator, then accessing the second segment of multi-bit cells using an access mode corresponding to the second mode value
  • the second mode indicator being associated with a second segment of multi-bit memory cells and being capable of indicating at least a first and second mode
  • the first mode including single bit access and the second mode including multi-bit access

Abstract

A memory device (300) having a plurality of memory cells (302, 304) that are grouped into at least two groups of cells (302, 304). Each cell (302, 304) is capable of being programmed in at least two modes. A mode indicator is associated with each group of cells (302, 304). The mode indicator (306, 308) indicates which programming mode is used to access the cells (302, 304). The mode indicator (306, 308) is one or more bits and optionally is user selectable.

Description

MIXED MODE MULTI -LEVEL MEMORY
BACKGROUND ART
A flash memory cell can be a field effect transistor (FET) that includes a select gate, a floating gate, a drain, and a source. A cell can be read by grounding the source, and applying a voltage to a bitiine connected with the drain. By applying a voltage to the wordline connected to the select gate, the cell can be switched on and off.
Programming a cell includes trapping excess electrons in the floating gate to increase voltage. This reduces the current conducted by the memory cell when the select voltage is applied to the select gate. The cell is programmed when the cell current is less than a reference current when the select voltage is applied. The cell is erased when the cell current is greater than the reference current and the select voltage is applied. Memory cells with only two programmable states contain only a single bit of information, such as a "0" or a " 1 ". A multi-level cell ("MLC") is a cell that can be programmed with more than one voltage level. Each voltage level is mapped to corresponding bits of information. For example, a single multi-level cell can be programmed with one of four voltage levels, e.g. -2.5V, 0.0V, +1.0V. +2.0V that correspond to binary bits "00", "01 ", " 10", and " 1 1 ", respectively. A cell that is programmable at more voltage levels can store more bits of data based on Eqn. 1.
N = 2ΛB Eqn. 1
B is the number of bits of data stored
N is the number of voltage levels.
Thus, a 1 bit cell requires 2 voltage levels, a 2 bit cell requires 4 voltage levels, a 3 bit cell requires 8 voltage levels, and a 4 bit cell requires 16 voltage levels.
Figure 1 shows a representation of a single bit programming voltage level diagram 100. The "erase state" program distribution 102 ("erase state"), and the "programmed state" program distribution 104 ("programmed state")represent a single bit being either a "0" or a " 1 ". respectively. The voltage between the upper end 106 of the erase state 102 and ground 108 (0.0V) is the "erase margin." The voltage between ground 108 (0.0V) and the lower end 1 10 of the programmed state 102 and is the "program margin." The erase state program distribution
102 is center near -2.5V and the programmed state program distribution 104 is centered around 2.25V. The read point can be at ground 108 or anywhere between lines 106 and 1 10, preferably near the mid-point between lines 106 and 1 10.
Figure 2 shows a representation of a four level multilevel cell program voltage diagram 200. The program distribution ("distribution") of the four levels are shown at 210. 212, 214, and 216 respectively. For example, the programming distributions are located at approximately -2.5V, 0.0V, 0.8V, and 2.0V and the width of the programming distributions are approximately 100 to 600mV. A four level multilevel memory cell can be programmed with any one of these voltage levels. Because the cell can store one of four binary values it can store 2 bits of information. The program margin (also called "margin", "data margin", or "guard band") is the voltage levels between distributions that is not normally used. For example, the program margins between program distributions 212, 214. and 216 are approximately 800mV to l OOmV wide. The program margin between program distributions 210 and 212 is approximately 2.0V. DISCLOSURE OF THE INVENTION
A memory device having a plurality of memory cells that are group into at least two group of cells Each cell is capable of being programmed in at least two modes A mode indicator is associated with each group of cells The mode indicator indicates which programming mode is used to access the cells The mode indicator is one or more bits and optionally is user selectable
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention is described with reference to the accompanying figures In the figures, like reference numbers indicate identical or functionally similar elements Additionally, the left-most dιgιt(s) of a reference number identifies the figure in which the reterence number first appears Figure 1 is a representation of a single bit programming voltage level,
Figure 2 is a representation of a two bit programming voltage level,
Figure 3 is a representation of an embodiment of a memory device with mode indicators, and Figure 4 is a flow diagram of a method ot access two segments ol memory in two different access modes
MODES OF CARRYING OUT THE INVENTION Since multi-level cells and single bit cells each have advantages and disadvantages it is desirable to access memory cells using both modes A user selectable mode indicator is associated with each page of mem ory The mode indicator indicates which mode of the page (or group) That is, the multi-level mode or the single bit mode If more than two modes are used, the mode indicator will be more than one bit A two bit mode indicator indicates four modes and a three bit indicator indicates eight modes Preferably, the modes are "single bit" and "two bit" and only a single bit is required tor the mode indicator for each page When the mode indicator is a single bit, the default state (erase state) preferably indicates the single bit state
Multi-level cells while holding more information than a single bit cell can take substantially longer to program, have smaller program margins, and have lower reliability Further, NAND based Multi-level cells are usually addressed on a page-by-page basis A 64Mbit (8Mbyte) memory array can contain 4096 pages where each page contains 512 bytes A page can be a 4K. bit page that has 4096 bits Interlaced NAND based multi-level cells are read sequentιall> l or example, if the byte at location 10 is to be read, the bj tes at locations 0 through 9 must be read first That is, interlaced NAND based multi-level cells do not support partial page writes or reads
Tor interlaced MLC accesses and single bit accesses, the first read access can be performed at the same voltage level, e g 206 (Figure 2) When the mode indicator is in the erase state (e g zeio), indicating the single bit mode, the reference level will be lowered to 108 (Figure 1 ) without significant impact on access times When the mode indicator is in the programmed state, indicating the multi-bit mode the memory is accessed with normal MLC access times
Regardless of the mode selected the initial read access is similar tor both modes After the initial access if the mode indicator is set to the multi-level mode a second access would occur Figure 3 is an embodiment of a memor> device 300 with two groups of memory cells 302, 304 and two mode indicators 306, 308 associated with the respective groups of memory cells T he groups of memorv cells 302 304 can be physically separate memop, or, preferably , they are subsets of one physical memory The mode indicators 306, 308 can be locations in memory, including the being part of the same memory as the first and second groups of memory 302, 304 The mode indicators 306, 308 can be a register, non-volatile memory, or volatile memory
The memory device 300 can include a storage means for storing data capable of storing data in a plurality of storage formats, an indication means for indicating which of the plurality of different formats is to be used to access the data, and an input/output means for accessing the data stored in the storage means The storage means can include memory cells, memory arrays, flash memory, NAND flash memory, or any other memory storage device The plurality of storage formats can preferably include a single-bit and a multi-bit format Multiple multi- bit formats can also be used The indicator means can include a single memory cell, multiple memory cells, a latch or other device for storing a value The storage means can include a plurality of storage areas and the indicator means can include a separate indicator for each storage area The indicator means with separate indicators for each storage area can be a group of memory cells, a group of latches, or other device for storing the indicators A storage area can be a page of memory including a 4096 bit page of memory The storage areas can be of varying sizes and need not each be of equal size Figure 4 is a flow diagram of an embodiment of a method 400 of accessing multi-bit memory cells The method includes determining a first mode value from a first mode indicator 402 then accessing the first segment of multi-bit cells using an access mode corresponding to the first mode value 404 The first mode indicator being associated with a first segment of multi-bit memory cells The first mode indicator being capable of indicating at least a first and second mode The first mode including single bit access and the second mode including multi-bit access
Optionally , the method 400 includes determining a second mode value from a second mode indicator, then accessing the second segment of multi-bit cells using an access mode corresponding to the second mode value The second mode indicator being associated with a second segment of multi-bit memory cells and being capable of indicating at least a first and second mode The first mode including single bit access and the second mode including multi-bit access
While preferred embodiments have been shown and described, it will be understood that they are not intended to limit the disclosure, but rather it is intended to cov er all modifications and alternative methods and apparatuses falling within the spirit and scope of the invention as defined in the appended claims or their equivalents

Claims

WHAT IS CLAIMED IS:
1. A memory device 300, comprising:
(a) a plurality of memory cells 302, 304 comprising at least a first and second group of cells 302, 304, each cell being capable of being programmed in a plurality of programming modes; and (b) a first mode indicator 306 associated with the first group of cells 302, and a second mode indicator 308 associated with the second group of cells 304 each mode indicator 306, 308 being capable of indicating one of the plurality of program modes.
2. The memory device 300 of claim 1 , wherein the first and second mode indicators 306, 308 are stored in a mode indicator memory 300.
3. The memory device 300 of claim 1 , wherein the plurality of modes includes a first mode and a second mode, the first mode being a single bit mode and the second mode being a multi-level mode.
4. The memory device 300 of claim 8, wherein the multi-level mode includes a first, second, third, and fourth programming level and the single bit mode includes a fifth and sixth programming level, the fifth and six programming levels being approximately equal to two of the first, second, third, and fourth programming levels.
5. A method of accessing multi-bit memory cells 302, 304 , comprising:
(a) determining a first mode value from a first mode indicator 306, the first mode indicator 306 being associated with a first segment of the multi-bit memory cells 302, the first mode indicator 306 being capable of indicating at least a first and second mode, the first mode comprising single bit access and the second mode comprising multi-bit access; and (b) accessing the first segment of multi-bit cells 302 using an access mode corresponding to the first mode value.
6. The method of claim 5, further including:
(c) determining a second mode value from a second mode indicator 308, the second mode indicator 308 being associated with a second segment of the multi-bit memory cells 304, the second mode indicator 308 being capable of indicating at least a first and second mode, the first mode comprising single bit access and the second mode comprising multi-bit access; and
(d) accessing the second segment of multi-bit cells 304 using an access mode corresponding to the second mode value.
7. A programmable memory device 300 characterized in that data is stored in a storage means 302, 304 in one of a plurality of storage formats and an accessing means 300 accesses the data stored in the storage means
302, 304 as a function of an indication means 306, 308 that indicates which of the plurality of different formats is to be used to access the data.
8. The programmable memory device 300 of claim 7 characterized in that the plurality of storage formats include at least a single-bit and a multi-bit format.
9. The programmable memory device 300 of claim 7. characterized in that the storage means 300 includes a plurality of storage areas 302, 304 and the indicator means 306, 308 includes a separate indicator for each storage area 302 and 304.
10. The programmable memory device 300 of claim 19, characterized in that each storage area 302, 304 is a page of memory.
PCT/US2001/003858 2000-02-25 2001-02-06 Mixed mode multi-level memory WO2001063614A1 (en)

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US09/513,402 US6297988B1 (en) 2000-02-25 2000-02-25 Mode indicator for multi-level memory

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