US20220076744A1 - Memory device - Google Patents
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- US20220076744A1 US20220076744A1 US17/012,077 US202017012077A US2022076744A1 US 20220076744 A1 US20220076744 A1 US 20220076744A1 US 202017012077 A US202017012077 A US 202017012077A US 2022076744 A1 US2022076744 A1 US 2022076744A1
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- G—PHYSICS
- G11—INFORMATION STORAGE
- G11C—STATIC STORES
- G11C13/00—Digital stores characterised by the use of storage elements not covered by groups G11C11/00, G11C23/00, or G11C25/00
- G11C13/0002—Digital stores characterised by the use of storage elements not covered by groups G11C11/00, G11C23/00, or G11C25/00 using resistive RAM [RRAM] elements
- G11C13/0021—Auxiliary circuits
- G11C13/0023—Address circuits or decoders
- G11C13/0026—Bit-line or column circuits
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- G—PHYSICS
- G11—INFORMATION STORAGE
- G11C—STATIC STORES
- G11C13/00—Digital stores characterised by the use of storage elements not covered by groups G11C11/00, G11C23/00, or G11C25/00
- G11C13/0002—Digital stores characterised by the use of storage elements not covered by groups G11C11/00, G11C23/00, or G11C25/00 using resistive RAM [RRAM] elements
- G11C13/0021—Auxiliary circuits
- G11C13/0033—Disturbance prevention or evaluation; Refreshing of disturbed memory data
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- H—ELECTRICITY
- H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
- H10B—ELECTRONIC MEMORY DEVICES
- H10B63/00—Resistance change memory devices, e.g. resistive RAM [ReRAM] devices
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- G—PHYSICS
- G11—INFORMATION STORAGE
- G11C—STATIC STORES
- G11C13/00—Digital stores characterised by the use of storage elements not covered by groups G11C11/00, G11C23/00, or G11C25/00
- G11C13/0002—Digital stores characterised by the use of storage elements not covered by groups G11C11/00, G11C23/00, or G11C25/00 using resistive RAM [RRAM] elements
- G11C13/0007—Digital stores characterised by the use of storage elements not covered by groups G11C11/00, G11C23/00, or G11C25/00 using resistive RAM [RRAM] elements comprising metal oxide memory material, e.g. perovskites
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- G—PHYSICS
- G11—INFORMATION STORAGE
- G11C—STATIC STORES
- G11C13/00—Digital stores characterised by the use of storage elements not covered by groups G11C11/00, G11C23/00, or G11C25/00
- G11C13/0002—Digital stores characterised by the use of storage elements not covered by groups G11C11/00, G11C23/00, or G11C25/00 using resistive RAM [RRAM] elements
- G11C13/0021—Auxiliary circuits
- G11C13/0023—Address circuits or decoders
- G11C13/0028—Word-line or row circuits
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- G—PHYSICS
- G11—INFORMATION STORAGE
- G11C—STATIC STORES
- G11C13/00—Digital stores characterised by the use of storage elements not covered by groups G11C11/00, G11C23/00, or G11C25/00
- G11C13/0002—Digital stores characterised by the use of storage elements not covered by groups G11C11/00, G11C23/00, or G11C25/00 using resistive RAM [RRAM] elements
- G11C13/0021—Auxiliary circuits
- G11C13/003—Cell access
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- H01L45/146—
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- H—ELECTRICITY
- H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
- H10B—ELECTRONIC MEMORY DEVICES
- H10B63/00—Resistance change memory devices, e.g. resistive RAM [ReRAM] devices
- H10B63/30—Resistance change memory devices, e.g. resistive RAM [ReRAM] devices comprising selection components having three or more electrodes, e.g. transistors
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- H—ELECTRICITY
- H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
- H10B—ELECTRONIC MEMORY DEVICES
- H10B63/00—Resistance change memory devices, e.g. resistive RAM [ReRAM] devices
- H10B63/80—Arrangements comprising multiple bistable or multi-stable switching components of the same type on a plane parallel to the substrate, e.g. cross-point arrays
- H10B63/84—Arrangements comprising multiple bistable or multi-stable switching components of the same type on a plane parallel to the substrate, e.g. cross-point arrays arranged in a direction perpendicular to the substrate, e.g. 3D cell arrays
- H10B63/845—Arrangements comprising multiple bistable or multi-stable switching components of the same type on a plane parallel to the substrate, e.g. cross-point arrays arranged in a direction perpendicular to the substrate, e.g. 3D cell arrays the switching components being connected to a common vertical conductor
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- H—ELECTRICITY
- H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
- H10N—ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
- H10N70/00—Solid-state devices without a potential-jump barrier or surface barrier, and specially adapted for rectifying, amplifying, oscillating or switching
- H10N70/20—Multistable switching devices, e.g. memristors
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- H—ELECTRICITY
- H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
- H10N—ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
- H10N70/00—Solid-state devices without a potential-jump barrier or surface barrier, and specially adapted for rectifying, amplifying, oscillating or switching
- H10N70/801—Constructional details of multistable switching devices
- H10N70/821—Device geometry
- H10N70/823—Device geometry adapted for essentially horizontal current flow, e.g. bridge type devices
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- H—ELECTRICITY
- H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
- H10N—ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
- H10N70/00—Solid-state devices without a potential-jump barrier or surface barrier, and specially adapted for rectifying, amplifying, oscillating or switching
- H10N70/801—Constructional details of multistable switching devices
- H10N70/881—Switching materials
- H10N70/883—Oxides or nitrides
- H10N70/8833—Binary metal oxides, e.g. TaOx
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- G—PHYSICS
- G11—INFORMATION STORAGE
- G11C—STATIC STORES
- G11C2213/00—Indexing scheme relating to G11C13/00 for features not covered by this group
- G11C2213/50—Resistive cell structure aspects
- G11C2213/51—Structure including a barrier layer preventing or limiting migration, diffusion of ions or charges or formation of electrolytes near an electrode
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- G—PHYSICS
- G11—INFORMATION STORAGE
- G11C—STATIC STORES
- G11C2213/00—Indexing scheme relating to G11C13/00 for features not covered by this group
- G11C2213/70—Resistive array aspects
- G11C2213/77—Array wherein the memory element being directly connected to the bit lines and word lines without any access device being used
Definitions
- the disclosure relates to a memory device.
- resistive random access memories characterized by advantages including simple crossbar array structures and low-temperature manufacturing processes, have been widely applied in the technical field of non-volatile memories.
- the crossbar RRAM is designed according to the concept of a resistive switching device (i.e., 1R), compared to a 1-transistor-1-resistor (1T1R) or a 1-selector-1-resistor (1S1R) structure, the crossbar RRAM structure may theoretically have not only a smaller cell size but also a lower operation voltage. In other words, the crossbar RRAM may have high integration density and may effectively reduce the operating voltage.
- crossbar RRAM still encounters certain issues, e.g., sneak current and snapback during memory operation.
- the disclosure provides a memory device where resistive layers having a specified resistance value are disposed between a bit line and a resistive switching layer, so as to solve a snapback problem (shorting of the bit line to an intersecting line by the resistive switching layer) while the resistive switching layer is transformed from an initially insulating layer into a switching layer with much less resistance.
- An embodiment of the disclosure provides a memory device including a resistive switching layer, a conductive pillar, a barrier layer, a word line, a plurality of resistive layers, and a plurality of bit lines.
- the resistive switching layer is shaped as a cup and has an inner surface to define an opening.
- the conductive pillar is disposed in the opening.
- the barrier layer is disposed between the resistive switching layer and the conductive pillar.
- the word line is electrically connected to the conductive pillar.
- the resistive layers are respectively distributed on an outer surface of the resistive switching layer.
- the bit lines are electrically connected to the resistive layers, respectively.
- FIG. 1 is a schematic view illustrating a theoretical formation state of a crossbar RRAM device.
- FIG. 2 is a schematic view illustrating a formation state of a crossbar RRAM device according to an embodiment of the disclosure.
- FIG. 3A to FIG. 3I are schematic three-dimensional views of a manufacturing process of a memory device according to a first embodiment of the disclosure.
- FIG. 4A is a schematic three-dimensional view illustrating the memory cell in FIG. 3I .
- FIG. 4B is a schematic cross-sectional view illustrating the memory cell in FIG. 4A .
- FIG. 5 is a schematic three-dimensional view of a memory device according to a second embodiment of the disclosure.
- FIG. 6A to FIG. 6H are schematic three-dimensional views of a manufacturing process of a memory device according to a third embodiment of the disclosure.
- FIG. 7A is a schematic three-dimensional view of a memory device according to a fourth embodiment of the disclosure.
- FIG. 7B is a schematic top view of FIG. 7A .
- FIG. 8 is a schematic view illustrating an operation of the memory device of FIG. 7A .
- FIG. 1 is a schematic view illustrating a theoretical formation state of a crossbar RRAM device.
- FIG. 2 is a schematic view illustrating a formation state of a crossbar RRAM device according to an embodiment of the disclosure.
- a forming voltage of 1 unit (1 Vf) is applied to a first bit line BL 1
- 0 voltage is applied to a second bit line BL 2 , a first word line WL 1 , and a second word line WL 2 (e.g., grounded).
- the memory cell M 1 is turned on and thus leads to a snapback, which is a sudden and significant voltage drop between the intersecting first bit line BL 1 and first word line WL 1 .
- An embodiment of the disclosure provides a crossbar RRAM device 10 including a memory array.
- the memory array includes: a first bit line BL 1 , a second bit line BL 2 , a first word line WL 1 , a second word line WL 2 , a plurality of memory cells M 1 , M 2 , M 3 , and M 4 , and a plurality of contact resistors Rc 1 , Rc 2 , Rc 3 , and Rc 4 .
- the memory cells M 1 , M 2 , M 3 , and M 4 are located at overlapping regions of the first bit line BL 1 , the second bit line BL 2 , the first word line WL 1 , and the second word line WL 2 , respectively.
- the contact resistors Rc 1 , Rc 2 , Rc 3 , and Rc 4 are respectively located between the memory cells M 1 , M 2 , M 3 , and M 4 and the first bit line BL 1 and the second bit line BL 2 .
- a voltage of 1 unit (1 Vf) is applied to the first bit line BL 1
- 0 voltage is applied to the second bit line BL 2 , the first word line WL 1 , and the second word line WL 2 (e.g., grounded).
- the memory cell M 1 is turned on, and almost all voltages are loaded to the contact resistor Rc 1 without being transferred to the first word line WL 1 , thereby solving the issue of possible disturb of memory cell M 2 caused by snapback.
- FIG. 3A to FIG. 3I are schematic three-dimensional views of a manufacturing process of a memory device according to a first embodiment of the disclosure.
- the memory device described in the following embodiments may be a crossbar RRAM device, but the disclosure is not limited thereto.
- a substrate 102 is provided.
- the substrate 102 includes a semiconductor substrate, e.g., silicon substrate.
- a conductive layer 104 is formed on the substrate 102 .
- a method for forming the conductive layer 104 includes forming a conductive material layer and then patterning the conductive material layer to form an opening 103 exposing the substrate 102 .
- the conductive layer 104 may be shaped as a letter H, but the disclosure is not limited thereto.
- the material of the conductive layer 104 includes Ta, Ti W, Al, or a combination thereof.
- a thickness 104 t of the conductive layer 104 thickness may be within a range from 20 nm to 50 nm.
- a dielectric layer 106 is formed on the conductive layer 104 .
- the dielectric layer 106 fills the opening 103 and extends to cover a top surface of the dielectric layer 106 .
- a method for forming the dielectric layer 106 includes forming a dielectric material layer and then planarizing the dielectric material layer.
- the material of the dielectric layer 106 may include silicon oxide, silicon nitride, silicon oxynitride, or a combination thereof.
- a thickness 106 t of the dielectric layer 106 on the conductive layer 104 may be within a range from 10 nm to 50 nm.
- the dielectric layer 106 may also be selectively patterned to increase the design flexibility of the memory device.
- a conductive layer 108 is formed on the dielectric layer 106 .
- a method for forming the conductive layer 108 includes forming a conductive material layer and then patterning the conductive material layer to form an opening 107 that exposes the dielectric layer 106 .
- the conductive layer 108 may be shaped as a letter H and correspond to the pattern of the conductive layer 104 .
- the material of the conductive layer 108 includes Ta, Ti, W, Al, or a combination thereof.
- a thickness 108 t of the conductive layer 108 may be within a range from 20 nm to 50 nm.
- the opening 107 is filled with the dielectric layer 110 , and then a planarization process, e.g., a chemical-mechanical planarization (CMP) process, is performed, so that the top surface of the dielectric layer 110 and a top surface of the conductive layer 108 are coplanar.
- a planarization process e.g., a chemical-mechanical planarization (CMP) process
- CMP chemical-mechanical planarization
- the material of the dielectric layer 110 may include silicon oxide, silicon nitride, silicon oxynitride, or a combination thereof.
- a thickness 110 t of the dielectric layer 110 may be within a range from 10 nm to 30 nm.
- a mask pattern 112 is formed on the conductive layer 108 and the dielectric layer 110 .
- a portion of the dielectric layer 110 , a portion of the conductive layer 108 , a portion of the dielectric layer 106 , and a portion of the conductive layer 104 are removed with use of a mask pattern 112 as a mask to form a trench 114 .
- the trench 114 exposes a top surface of the substrate 102 .
- the mask pattern 116 has an opening 118 .
- the opening 118 exposes the trench 114 , the top surface of the dielectric layer 110 , and the conductive layers 108 and 104 protruding between the dielectric layers 106 and 110 .
- An etching process is performed with use of the mask pattern 116 as a mask to remove the conductive layers 108 and 104 protruding between the dielectric layers 106 and 110 and to further form a gap 120 .
- the etching process includes a wet etching process or an isotropic etching process.
- a resistive material 122 is formed in the gap 120 , so that the resistive material 122 completely fills the gap 120 and extends to cover a sidewall of the trench 114 .
- the resistive material 122 includes a material of a high resistance value, such as TaN, TiN, or a combination thereof.
- a method of forming the resistive material 122 includes atomic layer deposition (ALD), chemical vapor deposition (CVD), or a combination thereof.
- a mask pattern 124 is formed on the conductive layer 108 and the dielectric layer 110 .
- a portion of the resistive material 122 exposed outside the trench 114 is removed with use of the mask pattern 124 as a mask. Under such circumstances, the remaining portion of the resistive material 122 is sandwiched between the dielectric layers 106 and 110 and is hereinafter referred to as resistive layers 126 .
- a dielectric layer 128 is formed in the trench 114 .
- the material of the dielectric layer 128 may include silicon oxide, silicon nitride, silicon oxynitride, or a combination thereof.
- the dielectric layers 128 , 110 , and 106 have the same dielectric material, such as silicon oxide.
- a memory structure 130 is formed in the dielectric layer 128 between two adjacent resistive layers 126 , thereby completing the memory device 100 provided in the first embodiment of the disclosure.
- the memory structure 130 includes a conductive pillar 132 , a barrier layer 134 , and a resistive switching layer 136 .
- a method for forming the memory structure 130 includes: forming an opening 131 in the dielectric layer 128 , wherein the opening 131 exposes the top surface of substrate 102 ; conformally forming the resistive switching layer 136 in the opening 131 ; conformally forming the barrier layer 134 on the resistive switching layer 136 ; forming the conductive pillar 132 on the barrier layer 134 .
- the resistive switching layer 136 is shaped as a cup and has an inner surface 136 s 1 to define an opening 135 .
- the conductive pillar 132 is disposed in the opening 135 .
- the barrier layer 134 is also shaped as a cup and is disposed between the resistive switching layer 136 and the conductive pillar 132 .
- the material of the conductive pillar 132 includes Ti, Ta, or a combination thereof
- the material of the barrier layer 134 includes Al 2 O 3 , TiO x or a combination thereof
- the material of the resistive switching layer 136 includes HfO 2 , ZrO 2 , Ta 2 O 5 , TiO 2 , or a combination thereof.
- the resistive layers 126 are respectively distributed on an outer surface 136 s 2 of the resistive switching layer 136 .
- a length 126 l of the resistive layers 126 is 10 nm to 100 nm.
- the conductive layers 104 and 108 are shaped as bars and are respectively connected to the resistive layers 126 .
- the bar-shaped conductive layers 104 and 108 may be collectively referred to as the bit lines BL.
- the conductive pillar 132 may also be electrically connected to the word line WL.
- each portion of the memory structure 130 connected to one of the resistive layers 126 may be regarded as a memory cell MC.
- the memory structure 130 is respectively connected to four resistive layers 126 , thereby forming four memory cells MC.
- a resistance value of the resistive layers 126 are greater than a resistance value of the conductive pillar 132 , a resistance value of the bit lines BL, and a resistance value of the word line WL.
- the resistive layers 126 may serve as contact resistors (as shown in FIG. 2 ) to hold the voltage when the memory cells MC are turned on, so as to prevent the damages to the caused by snapback.
- the resistance value of the resistive layers 126 may be expressed by the following formula:
- Rc is the resistance value of the resistive layers 126
- Rp is the resistance value of the peripheral circuit 20 (as shown in FIG. 2 )
- Vr is the reset voltage
- Vf is the forming voltage. This is to ensure that Rc rather than Rp takes the bulk of the voltage as a result of snapback.
- the resistance value of the resistive layer 126 is 1 KOhm to 3 KOhm; and the resistance value of the resistance switching layer 136 is 5 KOhm to 100 KOhm. In some embodiments, the a ratio of the resistance value of the resistive switching layer 136 to the resistance value of the resistive layer 126 is about 2 to 100.
- FIG. 5 is a schematic three-dimensional view of a memory device according to a second embodiment of the disclosure.
- the memory device 200 provided in the second embodiment of the disclosure is similar to the memory device 100 provided in the first embodiment of the disclosure, and the main difference lies in that the memory device 200 has a memory array.
- the memory array includes four memory structures 130 a , 130 b , 130 c , and 130 d , and the four memory structures 130 a , 130 b , 130 c , and 130 d are electrically connected to the bar-shaped conductive layers 104 and 108 (also referred to as the bit lines BL) through the resistive layers 126 , respectively.
- FIG. 6A to FIG. 6H are schematic three-dimensional views of a manufacturing process of a memory device according to a third embodiment of the disclosure.
- a conductive layer 204 , a dielectric layer 206 , and a conductive layer 208 are sequentially formed on the substrate 102 .
- the conductive layer 204 , the dielectric layer 206 , and the conductive layer 208 are all shaped as a letter H, and a forming method includes: forming a stacked layer structure composed of a conductive material layer, a dielectric material layer, and another conductive material layer, and then patterning the stacked layer structure to form an opening 203 exposing the substrate 102 .
- the opening 203 is backfilled with a filling material 210 .
- the protruding portions of the conductive layers 204 and 208 are etched with an isotropic etch, forming a slot 214 while the line-shaped portions of 208 are covered with a mask pattern 212 .
- another mask pattern 216 is formed on the conductive layer 208 .
- the mask pattern 216 has an opening 28 .
- the opening 218 exposes the slot 214 , the top surface of the dielectric layer 210 , and the protruding portions of the conductive layers 208 and 204 .
- An etching process is performed with use of the mask pattern 216 as a mask to remove the protruding portions of the conductive layers 208 and 204 and to further form a gap 220 .
- a resistive material 222 is formed in the gap 220 , so that the resistive material 222 completely fills the gap 220 and extends to cover a sidewall of the slot 214 .
- a mask pattern 224 is formed on the conductive layer 208 and the dielectric layer 210 . A portion of the resistive material 222 exposed outside the slot 214 is removed with use of the mask pattern 224 as a mask.
- resistive layers 226 the remaining portion of the resistive material 222 is sandwiched between the dielectric layers 206 and 210 and is hereinafter referred to as resistive layers 226 .
- a dielectric layer 228 is formed in the slot 214 .
- a memory structure 130 is formed in the dielectric layer 228 between two adjacent resistive layers 226 , thereby completing the memory device 300 provided in the third embodiment of the disclosure.
- FIG. 7A is a schematic three-dimensional view of a memory device according to a fourth embodiment of the disclosure.
- FIG. 7B is a schematic top view of FIG. 7A .
- the memory structure described in the following embodiments may be the memory structure 130 shown in FIG. 4A , but the disclosure is not limited thereto.
- a memory device 400 of the fourth embodiment of the disclosure includes a select transistor 410 , a memory structure 130 , a plurality of resistive layers 126 , and a plurality of bit lines BL.
- the memory structure 130 includes a conductive pillar 132 , a barrier layer 134 , and a resistive switching layer 136 .
- the resistive switching layer 136 is shaped as a cup and has an inner surface to define an opening.
- the conductive pillar 132 is disposed in the opening.
- the barrier layer 134 is disposed between the resistive switching layer 136 and the conductive pillar 132 .
- the select transistor 410 is disposed over the conductive pillar 132 .
- the select transistor 410 may also be disposed below the conductive pillar 132 .
- the select transistor 410 may be a metal oxide semiconductor field effect transistor (MOSFET) including a gate, a source, and a drain.
- MOSFET metal oxide semiconductor field effect transistor
- the gate of the select transistor 410 is electrically connected to the word line WL, and the word line WL may extend along a Y direction.
- the source of the select transistor 410 is electrically connected to the source line SL, and the source line SL may extend along a X direction.
- the drain of the select transistor 410 is electrically connected to the conductive pillar 132 .
- the resistive layers 126 are respectively distributed on the outer surface of the resistive switching layer 136 .
- the plurality of bit lines BL include a first bit line (odd bit line) BL 1 and a second bit line (even bit line) BL 2 .
- the first bit line BL 1 includes sub bit lines BL 1 a and BL 1 b vertically arranged along the memory structure 130 .
- the sub bit lines BL 1 a and BL 1 b are connected to some resistive layers 126 a 1 and 126 a 2 at a first side S 1 of the memory structure 130 .
- the sub bit lines BL 1 a and BL 1 b are electrically connected to each other.
- the second bit line BL 2 includes sub bit lines BL 2 a and BL 2 b vertically arranged along the memory structure 130 .
- the sub bit lines BL 2 a and BL 2 b are connected to some resistive layers 126 b 1 and 126 b 2 at a second side S 2 of the memory structure 130 .
- the sub bit lines BL 2 a and BL 2 b are electrically connected to each other.
- the memory device 400 may include a plurality of memory structures 130 , a plurality of bit lines BL, a plurality of select transistors 410 , a plurality of source lines SL, and a plurality of word lines WL.
- the plurality of memory structures 130 are arranged as an array.
- the plurality of bit lines BL includes a plurality of odd bit lines BL 1 , BL 3 , BL 5 and a plurality of even bit lines BL 2 , BL 4 .
- the odd bit lines BL 1 , BL 3 , BL 5 are respectively disposed at the first side S 1 of the memory structures 130
- the even bit lines BL 2 , BL 4 are respectively disposed at the second side S 2 of the memory structures 130 .
- the odd bit lines BL 1 , BL 3 , BL 5 are electrically connected to each other.
- the even bit lines BL 2 , BL 4 are electrically connected to each other.
- the plurality of select transistors 410 are respectively disposed over the memory structures 130 and have the drains electrically connected to the corresponding conductive pillars 132 .
- the select transistors 410 have the sources electrically connected to the plurality of source lines SL including the source lines SL 0 , SL 1 , SL 2 , SL 3 , respectively.
- the source lines SL 0 , SL 1 , SL 2 , SL 3 are extending along the X direction and arranged along the Y direction.
- the select transistors 410 have the gates electrically connected to the plurality of word lines WL including the word lines WL 0 , WL 1 , WL 2 , WL 3 , respectively.
- the word lines WL 0 , WL 1 , WL 2 , WL 3 are extending along the Y direction and arranged along the X direction.
- FIG. 8 is a schematic view illustrating an operation of the memory device of FIG. 7A .
- a gate voltage (Vg) is applied to the word line WL
- a reset voltage (Vr) is applied to the source line SL
- 0 voltage is applied to the bit line BL 1 a
- an open circuit voltage is applied to other bit lines BL 1 b , BL 2 a , BL 2 b .
- the accumulated resistance across three unselected memory cells such as M 2 , M 3 , M 4 is able to minimize the sneak currents and accidental cell disturbances.
- the accumulated resistance is the total resistance of three contact resistors plus the corresponding resistive switching layers in series. The total resistance is approximately 5-6 times the resistance of the contact resistor Rc 1 of the selected memory cell M 1 .
- a gate voltage (Vg) is applied to the word line WL
- 0 voltage is applied to the source line SL
- a set voltage is applied to the bit line BL 1 a
- an open circuit voltage is applied to other bit lines BL 1 b , BL 2 a , BL 2 b .
- the memory cell M 1 is selected for performing the set operation, and adjacent memory cells M 2 , M 3 , M 4 are able to decrease the sneak currents and accidental cell disturbances.
- the transistor may suffer nonlinear increases in current.
- the contact resistor of the present embodiment is able to mitigate any adverse effects from this nonlinearity.
- the contact resistors with appropriate resistance value are disposed between the bit lines and the memory cells to solve the problem of the peripheral circuit damages caused by snapback during the formation operation, thereby improving the reliability of the memory device.
- one or more embodiments of the disclosure provides the set operation and the reset operation of the memory device to reduce the sneak current, thereby preventing interference with the operation or interpretation of the memory device.
Abstract
A memory device includes: a resistive switching layer, a conductive pillar, a barrier layer, a word line, a plurality of resistive layers, and a plurality of bit lines. The resistive switching layer is shaped as a cup and has an inner surface to define an opening. The conductive pillar is disposed in the opening. The barrier layer is disposed between the resistive switching layer and the conductive pillar. The word line is electrically connected to the conductive pillar. The resistive layers are respectively distributed on an outer surface of the resistive switching layer. The bit lines are electrically connected to the resistive layers, respectively.
Description
- The disclosure relates to a memory device.
- Recently, resistive random access memories (RRAMs), characterized by advantages including simple crossbar array structures and low-temperature manufacturing processes, have been widely applied in the technical field of non-volatile memories. Since the crossbar RRAM is designed according to the concept of a resistive switching device (i.e., 1R), compared to a 1-transistor-1-resistor (1T1R) or a 1-selector-1-resistor (1S1R) structure, the crossbar RRAM structure may theoretically have not only a smaller cell size but also a lower operation voltage. In other words, the crossbar RRAM may have high integration density and may effectively reduce the operating voltage.
- However, the crossbar RRAM still encounters certain issues, e.g., sneak current and snapback during memory operation.
- The disclosure provides a memory device where resistive layers having a specified resistance value are disposed between a bit line and a resistive switching layer, so as to solve a snapback problem (shorting of the bit line to an intersecting line by the resistive switching layer) while the resistive switching layer is transformed from an initially insulating layer into a switching layer with much less resistance.
- An embodiment of the disclosure provides a memory device including a resistive switching layer, a conductive pillar, a barrier layer, a word line, a plurality of resistive layers, and a plurality of bit lines. The resistive switching layer is shaped as a cup and has an inner surface to define an opening. The conductive pillar is disposed in the opening. The barrier layer is disposed between the resistive switching layer and the conductive pillar. The word line is electrically connected to the conductive pillar. The resistive layers are respectively distributed on an outer surface of the resistive switching layer. The bit lines are electrically connected to the resistive layers, respectively.
- Several exemplary embodiments accompanied with figures are described in detail below to further describe the disclosure in details.
- The accompanying drawings are included to provide a further understanding of the disclosure, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the disclosure and, together with the description, serve to explain the principles of the disclosure.
-
FIG. 1 is a schematic view illustrating a theoretical formation state of a crossbar RRAM device. -
FIG. 2 is a schematic view illustrating a formation state of a crossbar RRAM device according to an embodiment of the disclosure. -
FIG. 3A toFIG. 3I are schematic three-dimensional views of a manufacturing process of a memory device according to a first embodiment of the disclosure. -
FIG. 4A is a schematic three-dimensional view illustrating the memory cell inFIG. 3I . -
FIG. 4B is a schematic cross-sectional view illustrating the memory cell inFIG. 4A . -
FIG. 5 is a schematic three-dimensional view of a memory device according to a second embodiment of the disclosure. -
FIG. 6A toFIG. 6H are schematic three-dimensional views of a manufacturing process of a memory device according to a third embodiment of the disclosure. -
FIG. 7A is a schematic three-dimensional view of a memory device according to a fourth embodiment of the disclosure. -
FIG. 7B is a schematic top view ofFIG. 7A . -
FIG. 8 is a schematic view illustrating an operation of the memory device ofFIG. 7A . -
FIG. 1 is a schematic view illustrating a theoretical formation state of a crossbar RRAM device.FIG. 2 is a schematic view illustrating a formation state of a crossbar RRAM device according to an embodiment of the disclosure. - With reference to
FIG. 1 , when a memory cell M1 is selected for performing a forming operation, a forming voltage of 1 unit (1 Vf) is applied to a first bit line BL1, and 0 voltage is applied to a second bit line BL2, a first word line WL1, and a second word line WL2 (e.g., grounded). Under such circumstances, the memory cell M1 is turned on and thus leads to a snapback, which is a sudden and significant voltage drop between the intersecting first bit line BL1 and first word line WL1. This pulls the voltage of the first word line WL1 to be the same as the voltage of the first bit line BL1, which in turn can cause the improper disturb or switching of memory cell M2 with an opposite polarity voltage of 1 Vf. - An embodiment of the disclosure provides a
crossbar RRAM device 10 including a memory array. The memory array includes: a first bit line BL1, a second bit line BL2, a first word line WL1, a second word line WL2, a plurality of memory cells M1, M2, M3, and M4, and a plurality of contact resistors Rc1, Rc2, Rc3, and Rc4. Specifically, the memory cells M1, M2, M3, and M4 are located at overlapping regions of the first bit line BL1, the second bit line BL2, the first word line WL1, and the second word line WL2, respectively. The contact resistors Rc1, Rc2, Rc3, and Rc4 are respectively located between the memory cells M1, M2, M3, and M4 and the first bit line BL1 and the second bit line BL2. When the memory cell M1 is selected for performing a formation operation, a voltage of 1 unit (1 Vf) is applied to the first bit line BL1, and 0 voltage is applied to the second bit line BL2, the first word line WL1, and the second word line WL2 (e.g., grounded). Under such circumstances, the memory cell M1 is turned on, and almost all voltages are loaded to the contact resistor Rc1 without being transferred to the first word line WL1, thereby solving the issue of possible disturb of memory cell M2 caused by snapback. -
FIG. 3A toFIG. 3I are schematic three-dimensional views of a manufacturing process of a memory device according to a first embodiment of the disclosure. The memory device described in the following embodiments may be a crossbar RRAM device, but the disclosure is not limited thereto. - As shown in
FIG. 3A , asubstrate 102 is provided. In an embodiment, thesubstrate 102 includes a semiconductor substrate, e.g., silicon substrate. Aconductive layer 104 is formed on thesubstrate 102. In an embodiment, a method for forming theconductive layer 104 includes forming a conductive material layer and then patterning the conductive material layer to form anopening 103 exposing thesubstrate 102. In this embodiment, as shown inFIG. 3A , theconductive layer 104 may be shaped as a letter H, but the disclosure is not limited thereto. In an embodiment, the material of theconductive layer 104 includes Ta, Ti W, Al, or a combination thereof. Athickness 104 t of theconductive layer 104 thickness may be within a range from 20 nm to 50 nm. - With reference to
FIG. 3B , adielectric layer 106 is formed on theconductive layer 104. Thedielectric layer 106 fills theopening 103 and extends to cover a top surface of thedielectric layer 106. In an embodiment, a method for forming thedielectric layer 106 includes forming a dielectric material layer and then planarizing the dielectric material layer. The material of thedielectric layer 106 may include silicon oxide, silicon nitride, silicon oxynitride, or a combination thereof. Athickness 106 t of thedielectric layer 106 on theconductive layer 104 may be within a range from 10 nm to 50 nm. In alternative embodiments, thedielectric layer 106 may also be selectively patterned to increase the design flexibility of the memory device. - With reference to
FIG. 3C , aconductive layer 108 is formed on thedielectric layer 106. In an embodiment, a method for forming theconductive layer 108 includes forming a conductive material layer and then patterning the conductive material layer to form anopening 107 that exposes thedielectric layer 106. In this embodiment, as shown inFIG. 3C , theconductive layer 108 may be shaped as a letter H and correspond to the pattern of theconductive layer 104. In an embodiment, the material of theconductive layer 108 includes Ta, Ti, W, Al, or a combination thereof. Athickness 108 t of theconductive layer 108 may be within a range from 20 nm to 50 nm. - Next, the
opening 107 is filled with thedielectric layer 110, and then a planarization process, e.g., a chemical-mechanical planarization (CMP) process, is performed, so that the top surface of thedielectric layer 110 and a top surface of theconductive layer 108 are coplanar. In an embodiment, the material of thedielectric layer 110 may include silicon oxide, silicon nitride, silicon oxynitride, or a combination thereof. Athickness 110 t of thedielectric layer 110 may be within a range from 10 nm to 30 nm. - With reference to
FIG. 3C andFIG. 3D , amask pattern 112 is formed on theconductive layer 108 and thedielectric layer 110. A portion of thedielectric layer 110, a portion of theconductive layer 108, a portion of thedielectric layer 106, and a portion of theconductive layer 104 are removed with use of amask pattern 112 as a mask to form atrench 114. Thetrench 114 exposes a top surface of thesubstrate 102. - With reference to
FIG. 3D andFIG. 3E , after themask pattern 112 is removed, anothermask pattern 116 is formed on theconductive layer 108. Themask pattern 116 has anopening 118. Theopening 118 exposes thetrench 114, the top surface of thedielectric layer 110, and theconductive layers dielectric layers mask pattern 116 as a mask to remove theconductive layers dielectric layers gap 120. In an embodiment, the etching process includes a wet etching process or an isotropic etching process. - With reference to
FIG. 3E andFIG. 3F , after themask pattern 116 is removed, aresistive material 122 is formed in thegap 120, so that theresistive material 122 completely fills thegap 120 and extends to cover a sidewall of thetrench 114. In an embodiment, theresistive material 122 includes a material of a high resistance value, such as TaN, TiN, or a combination thereof. A method of forming theresistive material 122 includes atomic layer deposition (ALD), chemical vapor deposition (CVD), or a combination thereof. - With reference to
FIG. 3F andFIG. 3G , amask pattern 124 is formed on theconductive layer 108 and thedielectric layer 110. A portion of theresistive material 122 exposed outside thetrench 114 is removed with use of themask pattern 124 as a mask. Under such circumstances, the remaining portion of theresistive material 122 is sandwiched between thedielectric layers resistive layers 126. - With reference to
FIG. 3G andFIG. 3H , after themask pattern 124 is removed, adielectric layer 128 is formed in thetrench 114. In an embodiment, the material of thedielectric layer 128 may include silicon oxide, silicon nitride, silicon oxynitride, or a combination thereof. In this embodiment, thedielectric layers - With reference to
FIG. 3H andFIG. 3I , amemory structure 130 is formed in thedielectric layer 128 between two adjacentresistive layers 126, thereby completing thememory device 100 provided in the first embodiment of the disclosure. Specifically, thememory structure 130 includes aconductive pillar 132, abarrier layer 134, and aresistive switching layer 136. In an embodiment, a method for forming thememory structure 130 includes: forming anopening 131 in thedielectric layer 128, wherein theopening 131 exposes the top surface ofsubstrate 102; conformally forming theresistive switching layer 136 in theopening 131; conformally forming thebarrier layer 134 on theresistive switching layer 136; forming theconductive pillar 132 on thebarrier layer 134. It can be known fromFIG. 4B that theresistive switching layer 136 is shaped as a cup and has an inner surface 136s 1 to define anopening 135. Theconductive pillar 132 is disposed in theopening 135. Thebarrier layer 134 is also shaped as a cup and is disposed between theresistive switching layer 136 and theconductive pillar 132. In an embodiment, the material of theconductive pillar 132 includes Ti, Ta, or a combination thereof, the material of thebarrier layer 134 includes Al2O3, TiOx or a combination thereof, and the material of theresistive switching layer 136 includes HfO2, ZrO2, Ta2O5, TiO2, or a combination thereof. - With reference to
FIG. 4A andFIG. 4B , theresistive layers 126 are respectively distributed on an outer surface 136 s 2 of theresistive switching layer 136. In an embodiment, a length 126 l of theresistive layers 126 is 10 nm to 100 nm. Theconductive layers resistive layers 126. In this embodiment, the bar-shapedconductive layers conductive pillar 132 may also be electrically connected to the word line WL. Here, each portion of thememory structure 130 connected to one of theresistive layers 126 may be regarded as a memory cell MC. As shown inFIG. 4A , thememory structure 130 is respectively connected to fourresistive layers 126, thereby forming four memory cells MC. - In an embodiment, a resistance value of the
resistive layers 126 are greater than a resistance value of theconductive pillar 132, a resistance value of the bit lines BL, and a resistance value of the word line WL. Under such circumstances, theresistive layers 126 may serve as contact resistors (as shown inFIG. 2 ) to hold the voltage when the memory cells MC are turned on, so as to prevent the damages to the caused by snapback. In some embodiments, the resistance value of theresistive layers 126 may be expressed by the following formula: -
Rc/Rp≥(Vf−Vr)/Vr=Vf/Vr−1, - where Rc is the resistance value of the
resistive layers 126, Rp is the resistance value of the peripheral circuit 20 (as shown inFIG. 2 ), Vr is the reset voltage, and Vf is the forming voltage. This is to ensure that Rc rather than Rp takes the bulk of the voltage as a result of snapback. - For instance, the resistance value of the
resistive layer 126 is 1 KOhm to 3 KOhm; and the resistance value of theresistance switching layer 136 is 5 KOhm to 100 KOhm. In some embodiments, the a ratio of the resistance value of theresistive switching layer 136 to the resistance value of theresistive layer 126 is about 2 to 100. -
FIG. 5 is a schematic three-dimensional view of a memory device according to a second embodiment of the disclosure. - With reference to
FIG. 5 , thememory device 200 provided in the second embodiment of the disclosure is similar to thememory device 100 provided in the first embodiment of the disclosure, and the main difference lies in that thememory device 200 has a memory array. The memory array includes fourmemory structures memory structures conductive layers 104 and 108 (also referred to as the bit lines BL) through theresistive layers 126, respectively. -
FIG. 6A toFIG. 6H are schematic three-dimensional views of a manufacturing process of a memory device according to a third embodiment of the disclosure. - Specifically, as shown in
FIG. 6A , aconductive layer 204, adielectric layer 206, and aconductive layer 208 are sequentially formed on thesubstrate 102. In this embodiment, theconductive layer 204, thedielectric layer 206, and theconductive layer 208 are all shaped as a letter H, and a forming method includes: forming a stacked layer structure composed of a conductive material layer, a dielectric material layer, and another conductive material layer, and then patterning the stacked layer structure to form anopening 203 exposing thesubstrate 102. - With reference to
FIG. 6B , theopening 203 is backfilled with a fillingmaterial 210. InFIG. 6C , the protruding portions of theconductive layers slot 214 while the line-shaped portions of 208 are covered with amask pattern 212. After themask pattern 212 is removed, as shown inFIG. 6D , anothermask pattern 216 is formed on theconductive layer 208. Themask pattern 216 has an opening 28. Theopening 218 exposes theslot 214, the top surface of thedielectric layer 210, and the protruding portions of theconductive layers mask pattern 216 as a mask to remove the protruding portions of theconductive layers gap 220. After themask pattern 216 is removed, aresistive material 222 is formed in thegap 220, so that theresistive material 222 completely fills thegap 220 and extends to cover a sidewall of theslot 214. Afterwards, amask pattern 224 is formed on theconductive layer 208 and thedielectric layer 210. A portion of theresistive material 222 exposed outside theslot 214 is removed with use of themask pattern 224 as a mask. In the case, the remaining portion of theresistive material 222 is sandwiched between thedielectric layers resistive layers 226. After themask pattern 224 is removed, as shown inFIG. 6G , adielectric layer 228 is formed in theslot 214. Thereafter, amemory structure 130 is formed in thedielectric layer 228 between two adjacentresistive layers 226, thereby completing thememory device 300 provided in the third embodiment of the disclosure. -
FIG. 7A is a schematic three-dimensional view of a memory device according to a fourth embodiment of the disclosure.FIG. 7B is a schematic top view ofFIG. 7A . The memory structure described in the following embodiments may be thememory structure 130 shown inFIG. 4A , but the disclosure is not limited thereto. - With reference to
FIG. 7A , amemory device 400 of the fourth embodiment of the disclosure includes aselect transistor 410, amemory structure 130, a plurality ofresistive layers 126, and a plurality of bit lines BL. Specifically, thememory structure 130 includes aconductive pillar 132, abarrier layer 134, and aresistive switching layer 136. Theresistive switching layer 136 is shaped as a cup and has an inner surface to define an opening. Theconductive pillar 132 is disposed in the opening. Thebarrier layer 134 is disposed between theresistive switching layer 136 and theconductive pillar 132. Theselect transistor 410 is disposed over theconductive pillar 132. However, the disclosure is not limited thereto, in other embodiments, theselect transistor 410 may also be disposed below theconductive pillar 132. In some embodiments, theselect transistor 410 may be a metal oxide semiconductor field effect transistor (MOSFET) including a gate, a source, and a drain. The gate of theselect transistor 410 is electrically connected to the word line WL, and the word line WL may extend along a Y direction. The source of theselect transistor 410 is electrically connected to the source line SL, and the source line SL may extend along a X direction. The drain of theselect transistor 410 is electrically connected to theconductive pillar 132. Theresistive layers 126 are respectively distributed on the outer surface of theresistive switching layer 136. The plurality of bit lines BL include a first bit line (odd bit line) BL1 and a second bit line (even bit line) BL2. As shown inFIG. 7A , the first bit line BL1 includes sub bit lines BL1 a and BL1 b vertically arranged along thememory structure 130. The sub bit lines BL1 a and BL1 b are connected to some resistive layers 126 a 1 and 126 a 2 at a first side S1 of thememory structure 130. In some embodiments, the sub bit lines BL1 a and BL1 b are electrically connected to each other. The second bit line BL2 includes sub bit lines BL2 a and BL2 b vertically arranged along thememory structure 130. The sub bit lines BL2 a and BL2 b are connected to some resistive layers 126 b 1 and 126 b 2 at a second side S2 of thememory structure 130. In some embodiments, the sub bit lines BL2 a and BL2 b are electrically connected to each other. - With reference to
FIG. 7B , thememory device 400 may include a plurality ofmemory structures 130, a plurality of bit lines BL, a plurality ofselect transistors 410, a plurality of source lines SL, and a plurality of word lines WL. The plurality ofmemory structures 130 are arranged as an array. The plurality of bit lines BL includes a plurality of odd bit lines BL1, BL3, BL5 and a plurality of even bit lines BL2, BL4. The odd bit lines BL1, BL3, BL5 are respectively disposed at the first side S1 of thememory structures 130, and the even bit lines BL2, BL4 are respectively disposed at the second side S2 of thememory structures 130. In some embodiments, the odd bit lines BL1, BL3, BL5 are electrically connected to each other. In alternative embodiments, the even bit lines BL2, BL4 are electrically connected to each other. The plurality ofselect transistors 410 are respectively disposed over thememory structures 130 and have the drains electrically connected to the correspondingconductive pillars 132. Theselect transistors 410 have the sources electrically connected to the plurality of source lines SL including the source lines SL0, SL1, SL2, SL3, respectively. In some embodiments, the source lines SL0, SL1, SL2, SL3 are extending along the X direction and arranged along the Y direction. Theselect transistors 410 have the gates electrically connected to the plurality of word lines WL including the word lines WL0, WL1, WL2, WL3, respectively. In some embodiments, the word lines WL0, WL1, WL2, WL3 are extending along the Y direction and arranged along the X direction. -
FIG. 8 is a schematic view illustrating an operation of the memory device ofFIG. 7A . - With reference to
FIG. 8 , in one embodiment, when the memory cell M1 is selected for performing a reset operation, a gate voltage (Vg) is applied to the word line WL, a reset voltage (Vr) is applied to the source line SL, 0 voltage is applied to the bit line BL1 a, and an open circuit voltage is applied to other bit lines BL1 b, BL2 a, BL2 b. The accumulated resistance across three unselected memory cells such as M2, M3, M4 is able to minimize the sneak currents and accidental cell disturbances. In the present embodiment, the accumulated resistance is the total resistance of three contact resistors plus the corresponding resistive switching layers in series. The total resistance is approximately 5-6 times the resistance of the contact resistor Rc1 of the selected memory cell M1. - On the other hands, when the memory cell M1 is selected for performing a set operation, a gate voltage (Vg) is applied to the word line WL, 0 voltage is applied to the source line SL, a set voltage (Vs) is applied to the bit line BL1 a, and an open circuit voltage is applied to other bit lines BL1 b, BL2 a, BL2 b. In this case, the memory cell M1 is selected for performing the set operation, and adjacent memory cells M2, M3, M4 are able to decrease the sneak currents and accidental cell disturbances.
- Further, when the memory cells M1, M2, M3, and M4 along the
memory structure 130 are not selected for performing the operation, 0 voltage is applied to the word line WL, thereby turning off theselect transistor 410. - Moreover, under the large bias voltage conditions used in forming, the transistor may suffer nonlinear increases in current. The contact resistor of the present embodiment is able to mitigate any adverse effects from this nonlinearity.
- To sum up, in one or more embodiments of the disclosure, the contact resistors with appropriate resistance value are disposed between the bit lines and the memory cells to solve the problem of the peripheral circuit damages caused by snapback during the formation operation, thereby improving the reliability of the memory device. In addition, one or more embodiments of the disclosure provides the set operation and the reset operation of the memory device to reduce the sneak current, thereby preventing interference with the operation or interpretation of the memory device.
- It will be apparent to those skilled in the art that various modifications and variations can be made to the disclosed embodiments without departing from the scope or spirit of the disclosure. In view of the foregoing, it is intended that the disclosure covers modifications and variations provided they fall within the scope of the following claims and their equivalents.
Claims (10)
1. A memory device, comprising:
a memory structure comprising:
a resistive switching layer, shaped as a cup and having an inner surface to define an opening;
a conductive pillar, disposed in the opening; and
a barrier layer, disposed between the resistive switching layer and the conductive pillar;
a word line, electrically connected to the conductive pillar;
a plurality of resistive layers respectively distributed on the outer surface of the resistive switching layer, wherein a portion of the memory structure connected to one of the plurality of resistive layers constitutes a memory cell; and
a plurality of bit lines, electrically connected to the plurality of resistive layers, respectively, wherein each resistive layer has a first sidewall and a second sidewall opposite to each other, the first sidewall is in direct contact with the resistive switching layer, and the second sidewall is in direct contact with a corresponding bit line.
2. The memory device according to claim 1 , wherein a resistance value of the plurality of resistive layers is 1 KOhm to 3 KOhm.
3. The memory device according to claim 1 , wherein a resistance value of the plurality of resistive layers is greater than a resistance value of the conductive pillar, a resistance value of the word line, and a resistance value of the plurality of bit lines.
4. The memory device according to claim 1 , wherein a ratio of a resistance value of the resistive switching layer to a resistance value of the plurality of resistive layers is 2 to 100.
5. The memory device according to claim 1 , wherein a length of the plurality of resistive layers is 10 nanometers to 100 nanometers.
6. The memory device according to claim 1 , wherein a material of the plurality of resistive layers comprises TaN, TiN, or a combination thereof.
7. The memory device according to claim 1 , wherein the barrier layer is conformally disposed in the opening to have a cup-shaped structure, and a material of the barrier layer comprises Al2O3, TiOx, or a combination thereof.
8. The memory device according to claim 1 , wherein a material of the conductive pillar comprises Ti, Ta, Al, W, or a combination thereof.
9. The memory device according to claim 1 , wherein a material of the resistive switching layer comprises HfO2, ZrO2, Ta2O5, TiO2, or a combination thereof.
10. A memory device, comprising:
a memory structure comprising:
a resistive switching layer, shaped as a cup and having an inner surface to define an opening;
a conductive pillar, disposed in the opening; and
a barrier layer, disposed between the resistive switching layer and the conductive pillar;
a select transistor, electrically connected to the conductive pillar;
a plurality of resistive layers respectively distributed on the outer surface of the resistive switching layer, wherein a portion of the memory structure connected to one of the plurality of resistive layers constitutes a memory cell; and
a plurality of bit lines comprising a plurality of odd bit lines and a plurality of even bit lines, wherein the plurality of odd bit lines are electrically connected to each other and connected to corresponding resistive layers at a first side of the conductive pillar, and the plurality of even bit lines are electrically connected to each other and connected to other corresponding resistive layers at a second side opposite to the first side of the conductive pillar, wherein each resistive layer has a first sidewall and a second sidewall opposite to each other, the first sidewall is in direct contact with the resistive switching layer, and the second sidewall is in direct contact with a corresponding bit line.
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