Patent Publication Number: US-2023164981-A1

Title: Semiconductor memory device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority of Korean Patent Application No. 10-2021-0160546, filed on Nov. 19, 2021, which is incorporated herein by reference in its entirety. 
     BACKGROUND 
     1. Field 
     Embodiments of the present invention relate to a semiconductor device, and more particularly, to a semiconductor memory device including three-dimensional (3D) memory cells. 
     2. Description of the Related Art 
     In order to increase the net die of a memory device, the size of memory cells has decreased. As the size of memory cells is miniaturized, it is necessary to reduce the parasitic capacitance (Cb) and increase the capacitance. However, it is difficult to increase the net die due to the structural limitation of the memory cells. 
     Recently, three-dimensional semiconductor memory devices including three-dimensionally arranged memory cells are being proposed. 
     SUMMARY 
     Embodiments of the present invention are directed to a semiconductor memory device including highly integrated memory cells. 
     In accordance with an embodiment of the present invention, a semiconductor memory device, includes: a conductive line stack including a plurality of first conductive lines that are stacked over a substrate in a direction perpendicular to a surface of the substrate; conductive pads extending laterally from edge portions of the first conductive lines, respectively; and contact plugs coupled to the conductive pads, respectively. 
     In accordance with another embodiment of the present invention, a semiconductor memory device includes: a plurality of active layers vertically stacked over a substrate; a bit line commonly coupled to first ends of the active layers and extending in a direction perpendicular to a surface of the substrate; capacitors coupled to second ends of the active layers, respectively; a word line stack including double word lines extending laterally in a direction intersecting with each of the active layers are stacked; word line pads laterally extending from edge portions of the double word lines, respectively; and contact plugs coupled to the word line pads, respectively. 
     In accordance with yet another embodiment of the present invention, a semiconductor memory device includes: a plurality of bit lines vertically stacked in a first direction and laterally oriented in a second direction intersecting with the first direction; bit line pads extending laterally from edge portions of the bit lines; contact plugs coupled to the bit line pads, respectively; active layers laterally oriented in a third direction intersecting with the first and second directions from the bit lines; word lines vertically oriented in the first direction with the active layers interposed therebetween; and capacitors coupled to ends of the active layers, respectively. 
     In accordance with an embodiment of the present invention, a semiconductor memory device may include: double conductive lines stacked over a substrate in a first direction, each of the double conductive lines including first and second sub-conductive lines parallel to and spaced apart from each other; conductive pads extending laterally in a second direction from a first end of each of the first sub-conductive lines; contact plugs extending in the first direction from landing surfaces of the conductive pads, respectively; and active layers interposed between first and second sub-conductive lines of each of the double conductive lines, wherein a length of the conductive pads gradually changes in the first direction. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a schematic perspective view illustrating a semiconductor memory device in accordance with an embodiment of the present invention. 
         FIG.  2    is a schematic plan view illustrating a semiconductor memory device in accordance with the embodiment of the present invention. 
         FIG.  3    is a cross-sectional view taken along a line A-A′ shown in  FIG.  2   . 
         FIG.  4    is a cross-sectional view taken along a line B-B′ shown in  FIG.  2   . 
         FIGS.  5  to  7    are schematic cross-sectional views illustrating semiconductor memory devices in accordance with other embodiments of the present invention. 
         FIG.  8    is a schematic plan view illustrating a semiconductor memory device in accordance with another embodiment of the present invention. 
         FIGS.  9  and  10    are schematic cross-sectional views illustrating a semiconductor memory device in accordance with another embodiment of the present invention. 
         FIGS.  11 A and  11 B  are schematic perspective views illustrating a semiconductor memory device in accordance with another embodiment of the present invention. 
         FIGS.  12 A and  12 B  are schematic perspective views illustrating a semiconductor memory device in accordance with another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present invention will be described below in more detail with reference to the accompanying drawings. The present invention may, however, be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the present invention to those skilled in the art. Throughout the disclosure, like reference numerals refer to like parts throughout the various figures and embodiments of the present invention. 
     The drawings are not necessarily to scale and, in some instances, proportions may have been exaggerated in order to clearly illustrate features of the embodiments. When a first layer is referred to as being “on” a second layer or “on” a substrate, it not only refers to a case where the first layer is formed directly on the second layer or the substrate but also a case where a third layer exists between the first layer and the second layer or the substrate. 
     According to the embodiments of the present invention described below, memory cell density may be increased and parasitic capacitance may be reduced by vertically stacking memory cells. 
       FIG.  1    is a schematic perspective view illustrating a semiconductor memory device in accordance with an embodiment of the present invention.  FIG.  2    is a schematic plan view illustrating a semiconductor memory device in accordance with the embodiment of the present invention.  FIG.  3    is a cross-sectional view taken along a line A-A′ shown in  FIG.  2   .  FIG.  4    is a cross-sectional view taken along a line B-B′ shown in  FIG.  2   . 
     Referring to  FIGS.  1  to  4   , the three-dimensional semiconductor memory device  100  in accordance with the embodiment of the present invention may include a conductive line stack WLS including a plurality of first conductive lines DWL 1  to DWL 4  that are stacked over a substrate SUB in a direction perpendicular to the surface of the substrate SUB, conductive pads WLP extending laterally from edge portions of the first conductive lines DWL 1  to DWL 4 , respectively, contact plugs WC coupled to the conductive pads WLP, respectively, a second conductive line BL extending in the direction perpendicular to the surface of the substrate SUB over the substrate SUB, and active layers ACT laterally oriented in a direction intersecting with the first conductive lines DWL 1  to DWL 4 . Each of the first conductive lines DWL 1  to DWL 4  may include a pair of a first sub-conductive line WL 1  and a second sub-conductive line WL 2 . 
     The semiconductor memory device  100  may include a Dynamic Random Access Memory (DRAM) memory cell array. Hereinafter, the first conductive lines DWL 1  to DWL 4  may correspond to double word lines DWL 1  to DWL 4 , and the second conductive line BL may correspond to a bit line BL. The conductive line stack WLS may be simply referred to as a word line stack WLS, and the conductive pads WLP may be simply referred to as word line pads WLP. The first sub-conductive line WL 1  and the second sub-conductive line WL 2  may be referred to as a first word line WL 1  and a second word line WL 2 . Accordingly, each of the double word lines DWL 1  to DWL 4  may include a pair of a first word line WL 1  and a second word line WL 2 . 
     The semiconductor memory device  100  may include a bit line BL, a transistor TR, and a capacitor CAP. The transistor TR may include an active layer ACT and a double word line DWL, where the double word line DWL may include a first word line WL 1  and a second word line WL 2  that are opposite to each other with the active layer ACT interposed therebetween. The capacitor CAP may include a storage node SN, a dielectric layer DE, and a plate node PN. 
     The bit line BL may have a pillar shape extending in a first direction D 1 . The active layer ACT may have a bar shape extending long in a second direction D 2  which intersects with the first direction D 1 . The double word line DWL may have a line shape extending in a third direction D 3  which intersects with the first and second directions D 1  and D 2 . The plate node PN of the capacitor CAP may be coupled to the plate line PL. 
     The bit line BL may be vertically oriented in the first direction D 1 . The bit line BL may be referred to as a vertically oriented bit line or a pillar-shaped bit line. The bit line BL may include a conductive material. The bit line BL may include a silicon-based material, a metal-based material, or a combination thereof. The bit line BL may include silicon, a metal, a metal nitride, a metal silicide, or a combination thereof. The bit line BL may include polysilicon, titanium nitride, tungsten, or a combination thereof. For example, the bit line BL may include polysilicon or titanium nitride (TiN) which is doped with an N-type impurity. The bit line BL may include a TiN/W stack which includes titanium nitride and tungsten over the titanium nitride. 
     The double word line DWL may extend long in the third direction D 3 , and the active layer ACT may extend in the second direction D 2 . The active layer ACT may be laterally arranged in the second direction D 2  from the bit line BL. The double word line DWL may include a pair of word lines, that is, the first word line WL 1  and the second word line WL 2 . The first word line WL 1  and the second word line WL 2  may face each other in the first direction D 1  with the active layer ACT interposed therebetween. A gate dielectric layer GD may be formed over the upper and lower surfaces of the active layer ACT. 
     The active layer ACT may include a semiconductor material or an oxide semiconductor material. For example, the active layer ACT may include silicon, germanium, silicon-germanium, or indium gallium zinc oxide (IGZO). The active layer ACT may include polysilicon or monocrystalline silicon. The active layer ACT may include a channel CH, a first source/drain region SR between the channel CH and the bit line BL, and a second source/drain region DR between the channel CH and the capacitor CAP. The channel CH may be defined between the first source/drain region SR and the second source/drain region DR. 
     The first source/drain region SR and the second source/drain region DR may be doped with impurities of the same conductivity type. The first source/drain region SR and the second source/drain region DR may be doped with an N-type impurity or a P-type impurity. The first source/drain region SR and the second source/drain region DR may include at least one impurity selected among arsenic (As), phosphorus (P), boron (B), indium (In), and combinations thereof. 
     The transistor TR may be a cell transistor and may have a double word line DWL. In the double word line DWL, the first word line WL 1  and the second word line WL 2  may have the same potential. For example, the first word line WL 1  and the second word line WL 2  may form a pair, and the same word line driving voltage may be applied to the first word line WL 1  and the second word line WL 2 . As described above, according to the embodiment of the present invention, a transistor TR may have a double word line DWL in which two first and second word lines WL 1  and WL 2  are disposed adjacent to one channel CH. 
     According to another embodiment of the present invention, the first word line WL 1  and the second word line WL 2  may have different potentials. For example, a word line driving voltage may be applied to the first word line WL 1 , and a ground voltage may be applied to the second word line WL 2 . The second word line WL 2  may be referred to as a back word line or a shield word line. According to another embodiment of the present invention, a ground voltage may be applied to the first word line WL 1  and a word line driving voltage may be applied to the second word line WL 2 . 
     The active layer ACT may have a thickness which is smaller than those of the first and second word lines WL 1  and WL 2 . In other words, a vertical thickness of the active layer ACT in the first direction D 1  may be thinner than a vertical thickness of each of the first and second word lines WL 1  and WL 2  in the first direction D 1 . The thin active layer ACT may be referred to as a thin-body active layer. The thin active layer ACT may include a thin-body channel CH, and the thin-body channel CH may have a thickness of approximately 10 nm or less. According to another embodiment of the present invention, the channel CH may have the same vertical thickness as the thicknesses of the first and second word lines WL 1  and WL 2 . 
     The upper and lower surfaces of the active layer ACT may have a flat-surface. In other word, the upper surface and the lower surface of the active layer ACT may be parallel to each other in the second direction D 2 . 
     The gate dielectric layer GD may include silicon oxide, silicon nitride, a metal oxide, a metal oxynitride, a metal silicate, a high-k material, a ferroelectric material, an anti-ferroelectric material, or a combination thereof. The gate dielectric layer GD may include SiO 2 , Si 3 N 4 , HfO 2 , Al 2 O 3 , ZrO 2 , AlON, HfON, HfSiO, HfSiON, or HfZrO. 
     The double word line DWL may include a metal, a metal mixture, a metal alloy, or a semiconductor material. The double word line DWL may include titanium nitride, tungsten, polysilicon, or a combination thereof. For example, the double word line DWL may include a TiN/W stack in which titanium nitride and tungsten are sequentially stacked. The double word line DWL may include an N-type work function material or a P-type work function material. The N-type work function material may have a low work function of approximately 4.5 eV or less, and the P-type work function material may have a high work function of approximately 4.5 eV or more. 
     The capacitor CAP may be disposed laterally in the second direction D 2  from the transistor TR. The capacitor CAP may include a storage node SN extending laterally from the active layer ACT in the second direction D 2 . The capacitor CAP may further include a dielectric layer DE and a plate node PN over the storage node SN. The storage node SN, the dielectric layer DE, and the plate node PN may be arranged laterally in the second direction D 2 . The storage node SN may have a laterally oriented cylinder shape. The dielectric layer DE may conformally cover the inner wall and the outer wall of the cylinder shape of the storage node SN. The plate node PN may have a shape extending to the inner wall and the outer wall of the cylinder shape of the storage node SN over the dielectric layer DE. The plate nodes PN of the capacitors CAP may be coupled to each other, and the plate nodes PN may be coupled to the plate line PL. The plate nodes PN and the plate line PL may be collectively referred to as a shared plate node. The storage node SN may be electrically connected to the second source/drain region DR. 
     The storage node SN may have a three-dimensional structure, and the storage node SN of the three-dimensional structure may have a lateral three-dimensional structure which is oriented in the second direction D 2 . As an example of the three-dimensional structure, the storage node SN may have a cylinder shape. According to another embodiment of the present invention, the storage node SN may have a pillar shape or a pylinder shape. The pylinder shape may refer to a structure in which a pillar shape and a cylinder shape are merged. 
     The plate node PN may include an internal node and external nodes. The internal node and the external nodes may be coupled to each other. The internal node may be positioned inside the cylinder shape of the storage node SN. The external nodes may be positioned outside the cylinder shape of the storage node SN with the dielectric layer DE interposed therebetween. The external nodes may be positioned to surround the outer wall of the cylinder shape of the storage node SN. 
     The storage node SN and the plate node PN may include a metal, a noble metal, a metal nitride, a conductive metal oxide, a conductive noble metal oxide, a metal carbide, a metal silicide, or a combination thereof. For example, the storage node SN and the plate node PN may include titanium (Ti), titanium nitride (TiN), tantalum (Ta), tantalum nitride (TaN), tungsten (W), tungsten nitride (WN), ruthenium (Ru), ruthenium oxide (RuO 2 ), iridium (Ir), iridium oxide (IrO 2 ), platinum (Pt), molybdenum (Mo), molybdenum oxide (MoO), titanium nitride/tungsten (TiN/W) stack, or tungsten nitride/tungsten (WN/W) stack. The plate node PN may include a combination of a metal-based material and a silicon-based material. For example, the plate node PN may be a stack of titanium nitride/silicon germanium/tungsten nitride (TiN/SiGe/WN). In the titanium nitride/silicon germanium/tungsten nitride (TiN/SiGe/WN) stack, silicon germanium may be a gap-fill material that fills the inside of the cylinder shape of the storage node SN over titanium nitride, and titanium nitride (TiN) may serve as a plate node PN of a capacitor CAP, and tungsten nitride may be a low-resistance material. 
     The dielectric layer DE may be referred to as a capacitor dielectric layer. The dielectric layer DE may include silicon oxide, silicon nitride, a high-k material, or a combination thereof. The high-k material may have a higher dielectric constant than silicon oxide. Silicon oxide (SiO 2 ) may have a dielectric constant of approximately 3.9, and the dielectric layer DE may include a high-k material having a dielectric constant of approximately 4 or more. The high-k material may have a dielectric constant of approximately 20 or more. The high-k material may include hafnium oxide (HfO 2 ), zirconium oxide (ZrO 2 ), aluminum oxide (Al 2 O 3 ), lanthanum oxide (La 2 O 3 ), titanium oxide (TiO 2 ), tantalum oxide (Ta 2 O 5 ), niobium oxide (Nb 2 O 5 ) or strontium titanium oxide (SrTiO 3 ). According to another embodiment of the present invention, the dielectric layer DE may be formed of a composite layer including two or more layers of the aforementioned high-k material. 
     The dielectric layer DE may be formed of a zirconium (Zr)-based oxide. The dielectric layer DE may have a stack structure including at least zirconium oxide (ZrO 2 ). The stack structure including zirconium oxide (ZrO 2 ) may include a ZA (ZrO 2 /Al 2 O 3 ) stack or a ZAZ (ZrO 2 /Al 2 O 3 /ZrO 2 ) stack. The ZA stack may have a structure in which aluminum oxide (Al 2 O 3 ) is stacked over zirconium oxide (ZrO 2 ). The ZAZ stack may have a structure in which zirconium oxide (ZrO 2 ), aluminum oxide (Al 2 O 3 ), and zirconium oxide (ZrO 2 ) are sequentially stacked. The ZA stack and the ZAZ stack may be referred to as a zirconium oxide (ZrO 2 )-based layer. According to another embodiment of the present invention, the dielectric layer DE may be formed of hafnium (Hf)-based oxide. The dielectric layer DE may have a stack structure including at least hafnium oxide (HfO 2 ). The stack structure including hafnium oxide (HfO 2 ) may include an HA (HfO 2 /Al 2 O 3 ) stack or an HAH (HfO 2 /Al 2 O 3 /HfO 2 ) stack. The HA stack may have a structure in which aluminum oxide (Al 2 O 3 ) is stacked over hafnium oxide (HfO 2 ). The HAH stack may have a structure in which hafnium oxide (HfO 2 ), aluminum oxide (Al 2 O 3 ), and hafnium oxide (HfO 2 ) are sequentially stacked. The HA stack and the HAH stack may be referred to as a hafnium oxide (HfO 2 )-based layer. In the ZA stack, ZAZ stack, HA stack, and HAH stack, aluminum oxide (Al 2 O 3 ) may have a greater band gap energy than zirconium oxide (ZrO 2 ) and hafnium oxide (HfO 2 ) (hereinafter referred to as band gap). Aluminum oxide (Al 2 O 3 ) may have a lower dielectric constant than zirconium oxide (ZrO 2 ) and hafnium oxide (HfO 2 ). Accordingly, the dielectric layer DE may include a stack of a high-k material and a high band gap material having a greater band gap than the high-k material. The dielectric layer DE may include silicon oxide (SiO 2 ) as a high band gap material other than aluminum oxide (Al 2 O 3 ). Since the dielectric layer DE includes a high band gap material, leakage current may be suppressed. The high band gap material may be thinner than the high-k material. According to another embodiment of the present invention, the dielectric layer DE may include a laminated structure in which a high-k material and a high band gap material are alternately stacked. For example, ZAZA (ZrO 2 /Al 2 O 3 /ZrO 2 /Al 2 O 3 ) stack, ZAZAZ (ZrO 2 /Al 2 O 3 /ZrO 2 /Al 2 O 3 /ZrO 2 ) stack, HAHA (HfO 2 /Al 2 O 3 /HfO 2 /Al 2 O 3 ) stack, or HAHAH (HfO 2 /Al 2 O 3 /HfO 2 /Al 2 O 3 /HfO 2 ) stack. In the above laminated structure, aluminum oxide (Al 2 O 3 ) may be thinner than zirconium oxide (ZrO 2 ) and hafnium oxide (HfO 2 ). 
     According to another embodiment of the present invention, the dielectric layer DE may include a stack structure, a laminated structure, or a mixed structure including zirconium oxide, hafnium oxide, and aluminum oxide. 
     According to another embodiment of the present invention, the dielectric layer DE may include a ferroelectric material or an antiferroelectric material. 
     According to another embodiment of the present invention, an interface control layer (not shown) for improving leakage current may be further formed between the storage node SN and the dielectric layer DE. The interface control layer may include titanium oxide (TiO 2 ), niobium oxide, or niobium nitride. The interface control layer may also be formed between the plate node PN and the dielectric layer DE. 
     The capacitor CAP may include a MIM (metal-insulator-metal) capacitor. The storage node SN and the plate node PN may include a metal-based material. 
     The capacitor CAP may be replaced with another data storage material. For example, the data storage material may be a phase change material, a magnetic tunnel junction (MTJ), or a variable resistance material. 
     Referring back to  FIGS.  1 ,  2 , and  3   , the semiconductor memory device  100  may include a word line stack WLS, and the word line stack WLS may include a plurality of double word lines DWL 1  to DWL 4 . The double word lines DWL 1  to DWL 4  may be vertically stacked in the first direction D 1 . The double word lines DWL 1  to DWL 4  may extend laterally in the third direction D 3 . 
     The semiconductor memory device  100  may include a memory cell array MCA and a contact area CA. A word line stack WLS may be positioned in the memory cell array MCA, and a word line pad portion WLP may be positioned in the contact area CA. The memory cell array MCA may include a plurality of memory cells, and the memory cells may be vertically stacked in the first direction D 1 . Each memory cell may include a bit line BL, a transistor TR, and a capacitor CAP. In the memory cell array MCA, a plurality of transistors TR may be stacked in the first direction D 1 . In the memory cell array MCA, a plurality of capacitors CAP may be stacked in the first direction D 1 . In the memory cell array MCA, the bit line BL may extend vertically in the first direction D 1 . In the memory cell array MCA, the double word lines DWL 1  to DWL 4  may extend laterally in the third direction D 3 . 
     Referring back to  FIG.  3   , the word line pad portion WLP may include a first edge portion VL and a second edge portion ST. The first edge portion VL of the word line pad portion WLP may be coupled to an end of the word line stack WLS, and the second edge portion ST of the word line pad portion WLP may have a stepwise shape. A plurality of word line pads WLP 1  to WLP 4  may be positioned in the contact area CA. The word line pads WLP 1  to WLP 4  may extend laterally in the third direction D 3 . The lateral length may gradually increase as it goes from the lowermost word line pad WLP 1  to the uppermost word line pad WLP 4 . Contact plugs WC may be respectively coupled to the word line pads WLP 1  to WLP 4 , and metal lines ML may be respectively coupled to the contact plugs WC. The edge portions of the double word lines DWL 1  to DWL 4  may be self-aligned at the same vertical level. Here, the same vertical level may correspond to the first edge portion VL of the word line pad portion WLP. The first edge portions VL of the word line pads WLP 1  to WLP 4  may be self-aligned at the same vertical level. Inter-layer dielectric layers ILD may be positioned between the double word lines DWL 1  to DWL 4 . The double word lines DWL 1  to DWL 4  may have the same lateral length in the third direction D 3 . The word line pads WLP 1  to WLP 4  may have different lateral lengths in the third direction D 3 . The first word line WL 1  and the second word line WL 2  of each of the double word lines DWL 1  to DWL 4  may be thinner than the word line pads WLP 1  to WLP 4 . For example, when vertical heights in the first direction D 1  are compared, the first word line WL 1  and the second word line WL 2  may have a thickness which is smaller than those of the word line pads WLP 1  to WLP 4 . Each of the word line pads WLP 1  to WLP 4  may have a height that couples the first word line WL 1  and the second word line WL 2  to each other. The inter-layer dielectric layers ILD may be positioned between the vertically stacked word line pads WLP 1  to WLP 4 . 
     The word line pads WLP 1  to WLP 4  may be respectively coupled to ends of the double word lines DWL 1  to DWL 4 . For example, the first double word line DWL 1  may be coupled to the first word line pad WLP 1 , and the second double word line DWL 2  may be coupled to the second word line pad WLP 2 . The third double word line DWL 3  may be coupled to the third word line pad WLP 3 , and the fourth double word line DWL 4  may be coupled to the fourth word line pad WLP 4 . The word line pads WLP 1  to WLP 4  may be simultaneously coupled to the first word line WL 1  and the second word line WL 2 , respectively. 
     As described above, since thick word line pads WLP are formed, word line resistance may be improved. 
     The semiconductor memory device  100  may further include a substrate SUB, and the substrate SUB may include a peripheral circuit portion. The bit line BL of the memory cell array MCA may be oriented perpendicular to the surface of the substrate SUB in the first direction D 1 , and the double word line DWL may be oriented parallel to the surface of the substrate SUB in the third direction D 3 . 
     The peripheral circuit portion may be positioned at a lower level than the memory cell array MCA and the contact area CA. This may be referred to as a COP (Cell over Peri) structure. The peripheral circuit portion may include at least one control circuit for driving the memory cell array MCA. The at least one control circuit of the peripheral circuit portion may include an N-channel transistor, a P-channel transistor, a CMOS circuit, or a combination thereof. The at least one control circuit of the peripheral circuit portion may include an address decoder circuit, a read circuit, a write circuit, and the like. The at least one control circuit of the peripheral circuit portion may include a planar channel transistor, a recess channel transistor, a buried gate transistor, a fin channel transistor (FinFET) and the like. 
     According to another embodiment of the present invention, in the semiconductor memory device  100 , the peripheral circuit portion may be positioned at a higher level than the memory cell array MCA. This may be referred to as a POC (Peri over Cell) structure. 
       FIG.  5    is a schematic cross-sectional view illustrating a semiconductor memory device in accordance with another embodiment of the present invention. In  FIG.  5   , detailed description on the constituent elements which also appear in  FIGS.  1  to  4    will be omitted. 
     Referring to  FIG.  5   , the semiconductor memory device  110  may include a word line stack WLS, and the word line stack WLS may include a plurality of double word lines DWL 1  to DWL 4 . The double word lines DWL 1  to DWL 4  may be vertically stacked in the first direction D 1 . The double word lines DWL 1  to DWL 4  may extend laterally in the third direction D 3 . 
     The semiconductor memory device  110  may include a memory cell array MCA and a contact area CA. The word line stack WLS may be positioned in the memory cell array MCA, and a word line pad portion WLP 10  may be positioned in the contact area CA. 
     The word line pad portion WLP 10  may include a first edge portion VL and a second edge portion ST. The first edge portion VL of the word line pad portion WLP 10  may be coupled to an end of the word line stack WLS, and the second edge portion ST of the word line pad portion WLP 10  may have a stepwise shape. A plurality of word line pads WLP 1  to WLP 4  may be positioned in the contact area CA. The word line pads WLP 1  to WLP 4  may extend laterally in the third direction D 3 . The lateral length of the word line pads WLP 1  to WLP 4  may gradually decrease as it goes from the lowermost word line pad WLP 1  to the uppermost word line pad WLP 4 . Contact plugs WC may be respectively coupled to the word line pads WLP 1  to WLP 4 , and the metal lines ML may be respectively coupled to the contact plugs WC. The edge portions of the double word lines DWL 1  to DWL 4  may be self-aligned at the same vertical level. Inter-layer dielectric layers ILD may be positioned between the double word lines DWL 1  to DWL 4 . The double word lines DWL 1  to DWL 4  may have the same lateral length in the third direction D 3 . The word line pads WLP 1  to WLP 4  may have different lateral lengths in the third direction D 3 . The first word line WL 1  and the second word line WL 2  of each of the double word lines DWL 1  to DWL 4  may be thinner than the word line pads WLP 1  to WLP 4 . For example, when the vertical heights in the first direction D 1  are compared, the first word line WL 1  and the second word line WL 2  may have a thickness which is smaller than those of the word line pads WLP 1  to WLP 4 . Each of the word line pads WLP 1  to WLP 4  may have a height that couples the first word line WL 1  and the second word line WL 2  to each other. The inter-layer dielectric layers ILD may be positioned between the vertically stacked word line pads WLP 1  to WLP 4 . 
     The word line pads WLP 1  to WLP 4  may be coupled to the ends of the double word lines DWL 1  to DWL 4 , respectively. For example, the first double word line DWL 1  may be coupled to the first word line pad WLP 1 , and the second double word line DWL 2  may be coupled to the second word line pad WLP 2 . The third double word line DWL 3  may be coupled to the third word line pad WLP 3 , and the fourth double word line DWL 4  may be coupled to the fourth word line pad WLP 4 . Each of the word line pads WLP 1  to WLP 4  may be simultaneously coupled to the first word line WL 1  and the second word line WL 2  of the corresponding one of the double word lines DWL 1  to DWL 4 . 
     Each of the word line pads WLP 1  to WLP 4  may further include a metallic liner TL and a metallic bulk WB. The metallic liner TL and the metallic bulk WB may be electrically coupled to edge portions of the double word lines DWL 1  to DWL 4 . The metallic liner TL may be directly coupled to edge portions of the double word lines DWL 1  to DWL 4 . The metallic liner TL may include titanium nitride, and the metallic bulk WB may include tungsten. The contact plugs WC may directly contact the metallic liner TL. The first and second word lines WL 1  and WL 2  of the double word lines DWL 1  to DWL 4  may directly contact the metallic liner TL. 
       FIG.  6    is a schematic cross-sectional view illustrating a semiconductor memory device  120  in accordance with another embodiment of the present invention. In  FIG.  6   , detailed descriptions on the constituent elements which also appear in  FIGS.  1  to  5    will be omitted. 
     Referring to  FIG.  6   , the semiconductor memory device  120  may include a word line stack WLS, and the word line stack WLS may include a plurality of double word lines DWL 1  to DWL 4 . The double word lines DWL 1  to DWL 4  may be vertically stacked in the first direction D 1 . The double word lines DWL 1  to DWL 4  may extend laterally in the third direction D 3 . 
     The semiconductor memory device  120  may include a memory cell array MCA and a contact area CA. A word line stack WLS may be positioned in the memory cell array MCA, and a word line pad portion WLP 20  may be positioned in the contact area CA. 
     The word line pad portion WLP 20  may include a first edge portion VL and a second edge portion ST. The first edge portion VL of the word line pad portion WLP 20  may be coupled to an end of the word line stack WLS, and the second edge portion ST of the word line pad portion WLP 20  may have a stepwise shape. A plurality of word line pads WLP 1  to WLP 4  may be respectively positioned in the contact area CA. The word line pads WLP 1  to WLP 4  may extend laterally in the third direction D 3 . The lateral length may gradually decrease as it goes from the lowermost word line pad WLP 1  to the uppermost word line pad WLP 4 . Contact plugs WC may be respectively coupled to the word line pads WLP 1  to WLP 4 , and metal lines ML may be coupled to the contact plugs WC, respectively. The edge portions of the double word lines DWL 1  to DWL 4  may be self-aligned at the same vertical level. Inter-layer dielectric layers ILD may be positioned between the double word lines DWL 1  to DWL 4 . The double word lines DWL 1  to DWL 4  may have the same lateral length in the third direction D 3 . The word line pads WLP 1  to WLP 4  may have different lateral lengths in the third direction D 3 . The first word line WL 1  and the second word line WL 2  of each of the double word lines DWL 1  to DWL 4  may be thinner than the word line pads WLP 1  to WLP 4 . For example, when vertical heights in the first direction D 1  are compared, the first word line WL 1  and the second word line WL 2  may have a thickness which is smaller than those of the word line pads WLP 1  to WLP 4 . Each of the word line pads WLP 1  to WLP 4  may have a height that couples the first word line WL 1  and the second word line WL 2  to each other. The inter-layer dielectric layers ILD may be positioned between the vertically stacked word line pads WLP 1  to WLP 4 . 
     Each of the word line pads WLP 1  to WLP 4  may be coupled to an end of each of the double word lines DWL 1  to DWL 4 . For example, the first double word line DWL 1  may be coupled to the first word line pad WLP 1 , and the second double word line DWL 2  may be coupled to the second word line pad WLP 2 . The third double word line DWL 3  may be coupled to the third word line pad WLP 3 , and the fourth double word line DWL 4  may be coupled to the fourth word line pad WLP 4 . Each of the word line pads WLP 1  to WLP 4  may be simultaneously coupled to the first word line WL 1  and the second word line WL 2  of the corresponding one of the double word lines DWL 1  to DWL 4 . 
     Each of the word line pads WLP 1  to WLP 4  may include a protrusion portion PP and a landing portion LP. The protrusion portions PP may be coupled to the edge portions of the double word lines DWL 1  to DWL 4 . For example, the protrusion portions PP may extend into a gap between an edge portion of the first word line WL 1  and an edge portion of the second word line WL 2 . The contact plugs WC may be coupled to the landing portions LP. The landing portions LP may have a height which is greater than the gap between the edge portion of the first word line WL 1  and the edge portion of the second word line WL 2  (i.e., a vertical thickness of the protrusion portion). 
       FIG.  7    is a schematic cross-sectional view illustrating a semiconductor memory device in accordance with another embodiment of the present invention. In  FIG.  7   , detailed description on the constituent elements which also appear in  FIGS.  1  to  6    will be omitted. 
     Referring to  FIG.  7   , the semiconductor memory device  130  may include a word line stack WLS, and the word line stack WLS may include a plurality of double word lines DWL 1  to DWL 4 . The double word lines DWL 1  to DWL 4  may be vertically stacked in the first direction D 1 . The double word lines DWL 1  to DWL 4  may extend laterally in the third direction D 3 . 
     The semiconductor memory device  130  may include a memory cell array MCA and a contact area CA. The word line stack WLS may be positioned in the memory cell array MCA, and a word line pad portion WLP 30  may be positioned in the contact area CA. 
     The word line pad portion WLP 30  may include a first edge portion VL and a second edge portion ST. The first edge portion VL of the word line pad portion WLP 30  may be coupled to an end of the word line stack WLS, and the second edge portion ST of the word line pad portion WLP 30  may have a stepwise shape. A plurality of word line pads WLP 1  to WLP 4  may be respectively positioned in the contact area CA. The word line pads WLP 1  to WLP 4  may extend laterally in the third direction D 3 . The lateral length may gradually decrease as it goes from the lowermost word line pad WLP 1  toward the uppermost word line pad WLP 4 . Contact plugs WC may be respectively coupled to the word line pads WLP 1  to WLP 4 , and metal lines ML may be coupled to the contact plugs WC, respectively. The edge portions of the double word lines DWL 1  to DWL 4  may be self-aligned at the same vertical level. Inter-layer dielectric layers ILD may be positioned between the double word lines DWL 1  to DWL 4 . The double word lines DWL 1  to DWL 4  may have the same lateral length in the third direction D 3 . The word line pads WLP 1  to WLP 4  may have different lateral lengths in the third direction D 3 . The first word line WL 1  and the second word line WL 2  of each of the double word lines DWL 1  to DWL 4  may be thinner than the word line pads WLP 1  to WLP 4 . For example, when the vertical heights in the first direction D 1  are compared, the first word line WL 1  and the second word line WL 2  may have a thickness which is smaller than those of the word line pads WLP 1  to WLP 4 . Each of the word line pads WLP 1  to WLP 4  may have a height that couples the first word line WL 1  and the second word line WL 2  to each other. Inter-layer dielectric layers ILD may be positioned between the vertically stacked word line pads WLP 1  to WLP 4 . 
     Each of the word line pads WLP 1  to WLP 4  may be coupled to an end of each of the double word lines DWL 1  to DWL 4 . For example, the first double word line DWL 1  may be coupled to the first word line pad WLP 1 , and the second double word line DWL 2  may be coupled to the second word line pad WLP 2 . The third double word line DWL 3  may be coupled to the third word line pad WLP 3 , and the fourth double word line DWL 4  may be coupled to the fourth word line pad WLP 4 . The word line pads WLP 1  to WLP 4  may be simultaneously coupled to the first word line WL 1  and the second word line WL 2 . 
     Each of the word line pads WLP 1  to WLP 4  may include a protrusion portion PP and a landing portion LP. The protrusion portions PP may be coupled to the edge portions of the double word lines DWL 1  to DWL 4 . For example, the protrusion portions PP may extend into a space between an edge portion of the first word line WL 1  and an edge portion of the second word line WL 2 . Contact plugs WC may be coupled to the landing portions LP. 
     Each of the word line pads WLP 1  to WLP 4  may further include a metallic liner TL and a metallic bulk WB. The metallic liner TL and the metallic bulk WB may be electrically coupled to the edge portions of the double word lines DWL 1  to DWL 4 . The metallic liner TL may be directly coupled to the edge portions of the double word lines DWL 1  to DWL 4 . The metallic liner TL may include titanium nitride, and the metallic bulk WB may include tungsten. The contact plugs WC may directly contact the metallic liner TL. The first and second word lines WL 1  and WL 2  of the double word lines DWL 1  to DWL 4  may directly contact the metallic liner TL. 
       FIG.  8    is a schematic plan view illustrating a semiconductor memory device in accordance with another embodiment of the present invention. 
     Referring to  FIGS.  1 ,  2 ,  3 , and  8   , the semiconductor memory device  200  may include a word line stack WLS, and the word line stack WLS may include a plurality of double word lines DWL 1  to DWL 4 . The double word lines DWL 1  to DWL 4  may be vertically stacked in the first direction D 1 . The double word lines DWL 1  to DWL 4  may extend laterally in the third direction D 3 . 
     The semiconductor memory device  200  of  FIG.  8    may include a memory cell array MCA and a contact area CA. The word line stack WLS may be positioned in the memory cell array MCA, and a word line pad portion WLP′ may be positioned in the contact area CA. 
     The word line pad portion WLP′ may include a plurality of word line pads (WLP 1  to WLP 4  shown in  FIG.  3   ). From the perspective of a top view, the word line pad portion WLP′ may have a cross shape WLPE, and thus structural stability of the word line pad portion WLP′ may be secured. 
     According to the above-described embodiments, since thick word line pads are formed, the resistance of the double word lines may be improved. 
     In order to form the word line pads WLP 1  to WLP 4 , the sacrificial materials between the inter-layer dielectric layers ILD may be removed from the contact area CA to form a gap, and then the gap may be filled with a metallic material. Here, the sacrificial materials may include a nitride, monocrystalline silicon, polysilicon, or a combination thereof. The inter-layer dielectric layers ILD may include silicon oxide. For example, a method of forming the word line pads WLP 1  to WLP 4  in the contact area CA may include repeatedly forming a mold stack where a first inter-layer dielectric layer, a first sacrificial layer, a silicon layer, a second sacrificial layer, and a second inter-layer dielectric are stacked in the mentioned order, forming a slit by etching the mold stack, forming a gap by selectively removing the first sacrificial layer, the silicon layer, and the second sacrificial layer through a slit, and forming a metallic material to fill the gap. The method of forming the double word lines DWL 1  to DWL 4  in the memory cell array MCA may include repeatedly forming a mold stack where a first inter-layer dielectric layer, a first sacrificial layer, a silicon layer, a second sacrificial layer, and a second inter-layer dielectric are stacked in the mentioned order, forming a slit by etching the mold stack, forming a gap in the upper and lower portions of the silicon layer by selectively removing the first sacrificial layer and a portion of the second sacrificial layer through the slit, forming a gate dielectric layer on a surface of the silicon layer exposed by the gaps, and forming first and second word lines WL 1  and WL 2  filling the gaps over the gate dielectric layer. After the mold stack is repeatedly formed, double word lines DWL 1  to DWL 4  of the memory cell array MCA may be formed, and then word line pads WLP 1  to WLP 4  may be formed in the contact area CA. In the mold stack, the first and second sacrificial layers may include a silicon germanium layer. 
       FIG.  9    is a schematic cross-sectional view illustrating a semiconductor memory device in accordance with another embodiment of the present invention. 
     Referring to  FIGS.  1 ,  2 ,  3 , and  9   , the semiconductor memory device  300  may include a word line stack WLS, and the word line stack WLS may include a plurality of single word lines SWL 1  to SWL 4 . The single word lines SWL 1  to SWL 4  may be vertically stacked in the first direction D 1 . The single word lines SWL 1  to SWL 4  may extend laterally in the third direction D 3 . A gate dielectric layer GD may be positioned between the single word lines SWL 1  to SWL 4  and the active layers ACT. 
     The semiconductor memory device  300  may include a memory cell array MCA and a contact area CA. The word line stack WLS may be positioned in the memory cell array MCA, and the word line pad portion WLP may be positioned in the contact area CA. 
     The word line pad portion WLP may include a first edge portion VL and a second edge portion ST. The first edge portion VL of the word line pad portion WLP may be coupled to an end of the word line stack WLS, and the second edge portion ST of the word line pad portion WLP 30  may have a stepwise shape. A plurality of word line pads WLP 1  to WLP 4  may be positioned in the contact area CA. The word line pads WLP 1  to WLP 4  may extend laterally in the third direction D 3 . The lateral length may gradually decrease as it goes from the lowermost word line pad WLP 1  toward the uppermost word line pad WLP 4 . Contact plugs WC may be respectively coupled to the word line pads WLP 1  to WLP 4 , and metal lines ML may be coupled to the contact plugs WC, respectively. The edge portions of the single word lines SWL 1  to SWL 4  may be self-aligned at the same vertical level. Inter-layer dielectric layers (not shown) may be positioned between the single word lines SWL 1  to SWL 4 . The single word lines SWL 1  to SWL 4  may have the same lateral length in the third direction D 3 . The word line pads WLP 1  to WLP 4  may have different lateral lengths in the third direction D 3 . Each of the single word lines SWL 1  to SWL 4  may be thinner than the word line pads WLP 1  to WLP 4 . For example, when vertical heights in the first direction D 1  are compared, the single word lines SWL 1  to SWL 4  may have a thickness which is smaller than those of the word line pads WLP 1  to WLP 4 . 
       FIG.  10    is a schematic cross-sectional view illustrating a semiconductor memory device  400  in accordance with another embodiment of the present invention. 
     Referring to  FIGS.  1 ,  2 ,  3 , and  10   , the semiconductor memory device  400  may include a word line stack WLS, and the word line stack WLS may include a plurality of gate all-around word lines. (GAA-WL 1  to GAA-WL 4 ). The gate all-around word lines GAA-WL 1  to GAA-WL 4  may be vertically stacked in the first direction D 1 . The gate all-around word lines GAA-WL 1  to GAA-WL 4  may extend laterally in the third direction D 3 . The gate all-around word lines GAA-WL 1  to GAA-WL 4  may surround the active layers ACT, and the gate dielectric layers GD may surround the active layers ACT. 
     The semiconductor memory device  400  may include a memory cell array MCA and a contact area CA. The word line stack WLS may be positioned in the memory cell array MCA, and the word line pad portion WLP may be positioned in the contact area CA. 
     The word line pad portion WLP may include a first edge portion VL and a second edge portion ST. The first edge portion VL of the word line pad portion WLP may be coupled to an end of the word line stack WLS, and the second edge portion ST of the word line pad portion WLP may have a stepwise shape. A plurality of word line pads WLP 1  to WLP 4  may be positioned in the contact area CA. The word line pads WLP 1  to WLP 4  may extend laterally in the third direction D 3 . The lateral length may gradually decrease as it goes from the lowermost word line pad WLP 1  toward the uppermost word line pad WLP 4 . Contact plugs WC may be respectively coupled to the word line pads WLP 1  to WLP 4 , and metal lines ML may be coupled to the contact plugs WC, respectively. The edge portions of the gate-all-around word lines GAA-WL 1  to GAA-WL 4  may be self-aligned at the same vertical level. Inter-layer dielectric layers (not shown) may be positioned between the gate all-around word lines GAA-WL 1  to GAA-WL 4 . The gate all-around word lines GAA-WL 1  to GAA-WL 4  may have the same lateral length in the third direction D 3 . The word line pads WLP 1  to WLP 4  may have different lateral lengths in the third direction D 3 . Each of the gate-all-around word lines GAA-WL 1  to GAA-WL 4  may have the same vertical height as those of the word line pads WLP 1  to WLP 4 . 
       FIGS.  11 A and  11 B  are schematic perspective views illustrating a semiconductor memory device in accordance with another embodiment of the present invention.  FIG.  11 B  is a partial perspective view of the double word line DWL 1 . 
     Referring to  FIG.  11 A , the semiconductor memory device  500  may include a plurality of bit lines BL that are stacked over a substrate (not shown) In the first direction D 1  which is perpendicular to the surface of the substrate and laterally oriented in the second direction D 2  which intersects with the first direction D 1 , bit line pads BLP laterally extending from the edge portions of the bit lines BL, contact plugs coupled to the bit line pads BLP, active layers ACT laterally oriented in the third direction D 3  which intersects with the first and second directions D 1  and D 2  from the bit line BL, double word lines DWL 1  to DWL 4  vertically oriented in the first direction D 1  with the active layers ACT interposed therebetween, and a capacitor CAP coupled to an end of an active layer ACT. 
     Referring to  FIG.  11 B , the first word line WL 1  and the second word line WL 2  may extend vertically with the active layers ACT interposed therebetween. In other words, the first word line WL 1  and the second word line WL 2  may be parallel to each other to form one double word line DWL 1 . A gate dielectric layer GD surrounding the active layer ACT may be formed between the active layer ACT and the double word line DWL 1 . 
       FIGS.  12 A and  12 B  are schematic perspective views illustrating a semiconductor memory device  510  in accordance with another embodiment of the present invention.  FIG.  12 B  is a partial perspective view illustrating a gate all-around word line GAA-WL 1 . 
     Referring to  FIG.  12 A , the semiconductor memory device  510  may include a plurality of bit lines BL that are stacked over a substrate (not shown) in the first direction D 1  which is perpendicular to the surface of the substrate and laterally oriented in the second direction D 2  which intersects with the first direction D 1 , bit line pads BLP laterally extending from the edge portions of the bit lines BL, contact plugs coupled to the bit line pads BLP, active layers ACT laterally oriented in the third direction D 3  which intersects with the first and second directions D 1  and D 2  from the bit line BL, gate all-around word lines GAA-WL 1  to GAA-WL 4  vertically oriented in the first direction D 1 , and capacitors CAP coupled to the ends of the active layers ACT. The contact plugs BC may be coupled to the metal lines ML. 
     Referring to  FIG.  12 B , a portion of the first word line WL 1  may extend between the active layers ACT to contact the second word line WL 2 . In other words, the first word line WL 1  and the second word line WL 2  may be coupled to each other to be integrated so as to form one gate all-around word line GAA-WL 1 . A gate dielectric layer GD surrounding the active layer ACT may be formed between the active layer ACT and the gate all-around word lines GAA-WL 1 . 
     The bit line pads BLP of  FIGS.  11 A and  12 A  may be modified to have diverse shapes as referenced in  FIGS.  3 ,  5 ,  6 ,  7  and  8   . 
     The semiconductor memory devices  500  and  510  of  FIGS.  11 A and  12 A  may include a word line of a single structure. 
     According to the embodiment of the present invention, word line resistance may be improved by forming thick word line pads that are coupled to edge portions of double word lines. 
     The effects desired to be obtained in the embodiments of the present invention are not limited to the effects mentioned above, and other effects not mentioned above may also be clearly understood by those of ordinary skill in the art to which the present invention pertains from the description below. 
     While the present invention has been described with respect to the specific embodiments, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the invention as defined in the following claims.