Multi-state flash EEPROM system using incremental programing and erasing methods
First Claim
1. An electrically erasable programmable read only memory (EEPROM) system, comprising:
- a semiconductor substrate including an array of a plurality of storage cells arranged in rows and columns across said substrate, each cell including;
a source region and a drain region formed in said substrate with a channel therebetween,a floating gate positioned over a first portion of the channel in a manner that a level of charge on the floating gate establishes a threshold level of a first transistor, thereby affecting a level of conductance through the first channel portion between the source and drain regions,a control gate positioned over a second portion of the channel adjacent the first channel portion, thereby forming a second transistor connected in series with the first transistor, said control gate also extending across the floating gate in a manner that a level of electrical potential on the control gate affects the level of conductance of both of the first and second transistors through the respective first and second portions of the channel between the source and drain regions, andan erase gate positioned adjacent said floating gate;
programming means connectable to a selected cell within the array of cells for applying voltages to the first transistor of the selected cell to increase an electron charge on its floating gate, thereby to alter the threshold level of the first transistor of the selected cell,reading means connectable to an addressed cell within the array of cells for detecting a level of conductance between the source and drain regions of the addressed cell in response to a predetermined voltage being applied across the source and drain regions of the addressed cell and a predetermined potential level being applied to the control gate of the addressed cell,erasing means connectable to erase gates of storage cells within one of a plurality of distinct blocks of cells of the array for simultaneously removing electrical charge from the floating gates of the cells within said one block that is sufficient to drive the threshold levels of the first transistors of said one block of cells to an erased level, each of said plurality of blocks including cells within multiple rows and multiple columns,wherein said reading means includes means for discriminating between a plurality of more than two predetermined threshold levels of the first transistor of the addressed cell, at least one of said more than two programmable threshold levels having a net positive charge on the floating gate of the first transistor of the addressed cell, each cell having a plurality of storage states more than two, andsaid programming means includes means for applying a sequence of programming voltage pulses to the first transistor of the selected cell to increase an electron charge on its floating gate from said erased level until at least either (a) a threshold level detected by said reading means in the first transistor of the selected cell is substantially equal to a desired one of the more than two threshold levels, or (b) a preset number of pulses has been applied.
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Accused Products
Abstract
A memory system made up of electrically programmable read only memory (EPROM) or flash electrically erasable and programmable read only memory (EEPROM) cells. An intelligent programming technique allows each memory cell to store more than the usual one bit of information. More than one bit is stored in a cell by establishing more than two distinct threshold states into which the cell is programmed. A series of pulses of increasing voltage is applied to each addressed memory cell during its programming, the state of the cell being read in between pulses. The pulses are terminated upon the addressed cell reaching its desired state or a preset maximum number of pulses has been reached. An intelligent erase algorithm prolongs the useful life of the memory cells. A series of pulses is also applied to a block of cells being erased, the state of at least a sample number of cells being read in between pulses. The erasing process is stopped when the cells being read are determined to have reached a fully erased state or one of a number of other conditions has occurred. Use of these various features provides a memory having a very high storage density and a long life, making it particularly useful as a solid state memory in place of magnetic disk storage devices in computer systems.
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Citations
27 Claims
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1. An electrically erasable programmable read only memory (EEPROM) system, comprising:
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a semiconductor substrate including an array of a plurality of storage cells arranged in rows and columns across said substrate, each cell including; a source region and a drain region formed in said substrate with a channel therebetween, a floating gate positioned over a first portion of the channel in a manner that a level of charge on the floating gate establishes a threshold level of a first transistor, thereby affecting a level of conductance through the first channel portion between the source and drain regions, a control gate positioned over a second portion of the channel adjacent the first channel portion, thereby forming a second transistor connected in series with the first transistor, said control gate also extending across the floating gate in a manner that a level of electrical potential on the control gate affects the level of conductance of both of the first and second transistors through the respective first and second portions of the channel between the source and drain regions, and an erase gate positioned adjacent said floating gate; programming means connectable to a selected cell within the array of cells for applying voltages to the first transistor of the selected cell to increase an electron charge on its floating gate, thereby to alter the threshold level of the first transistor of the selected cell, reading means connectable to an addressed cell within the array of cells for detecting a level of conductance between the source and drain regions of the addressed cell in response to a predetermined voltage being applied across the source and drain regions of the addressed cell and a predetermined potential level being applied to the control gate of the addressed cell, erasing means connectable to erase gates of storage cells within one of a plurality of distinct blocks of cells of the array for simultaneously removing electrical charge from the floating gates of the cells within said one block that is sufficient to drive the threshold levels of the first transistors of said one block of cells to an erased level, each of said plurality of blocks including cells within multiple rows and multiple columns, wherein said reading means includes means for discriminating between a plurality of more than two predetermined threshold levels of the first transistor of the addressed cell, at least one of said more than two programmable threshold levels having a net positive charge on the floating gate of the first transistor of the addressed cell, each cell having a plurality of storage states more than two, and said programming means includes means for applying a sequence of programming voltage pulses to the first transistor of the selected cell to increase an electron charge on its floating gate from said erased level until at least either (a) a threshold level detected by said reading means in the first transistor of the selected cell is substantially equal to a desired one of the more than two threshold levels, or (b) a preset number of pulses has been applied. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17)
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18. An electrically erasable programmable read only memory (EEPROM) system, comprising:
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a semiconductor substrate including an array of a plurality of storage cells arranged in rows and columns across said substrate, said plurality of cells being formed to individually include; a source region and a drain region formed in a surface of said substrate with a channel therebetween, a floating gate positioned over a first portion of the channel and separated from said substrate surface by a first dielectric layer in a manner that a level of charge on the floating gate controls a level of conductance through the first channel portion, thereby forming a first transistor whose threshold level is controlled by the level of charge on the floating gate, a control gate positioned over a second portion of the channel adjacent the first channel portion and separated from said substrate surface by a second dielectric layer, thereby forming a second transistor connected in series with the first transistor, said control gate also extending across the floating gate in a manner that a level of electrical potential on the control gate affects the level of conductance of both of the first and second transistors through the respective first and second portions of the channel between the source and drain regions, and an erase gate positioned adjacent said floating gate and separated therefrom by a third dielectric layer, reading means connectable to an addressed cell within the array of cells for detecting a level of conductance between the source and drain regions of the addressed cell, said reading means includes means for discriminating between a plurality of more than two predetermined threshold levels of the first transistor of the addressed cell wherein at least one of said predetermined threshold levels results from a net positive charge on the floating gate of the first transistor of the addressed cell, the cells of the array having a predetermined number of storage states more than two, erasing means simultaneously connectable to the erase gates of storage cells within one of a plurality of distinct blocks of cells in multiple rows and columns of the array for simultaneously reducing an electron charge from the floating gates of cells within said one block by applying thereto a sequence of pulses of increasing magnitude until at least either (a) threshold levels detected by said reading means between said erase pulses in at least a preset proportion of the cells of said one block reach an erased level, or (b) a preset maximum number of erase pulses have occurred, and programming means connectable to a selected cell within the array of cells for increasing an electron charge on its floating gate by applying a sequence of programming pulses to the selected cell until at least either (a) a threshold level detected by said reading means between said programming pulses in the first transistor of the selected cell is raised from said erased level to substantially equal a desired one of said more than two predetermined threshold levels, or (b) a preset maximum number of programming pulses has been applied. - View Dependent Claims (19, 20, 21, 22, 23, 24, 25, 26, 27)
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Specification