Patent Publication Number: US-2023138121-A1

Title: Three-dimensional semiconductor device and method of fabricating the same

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This U.S. nonprovisional application claims priority under 35 U.S.C. § 119 to Korean Patent Application No. 10-2021-0145770, filed on Oct. 28, 2021, in the Korean Intellectual Property Office, the disclosure of which is incorporated by reference herein in its entirety. 
     TECHNICAL FIELD 
     The present inventive concept relates to a three-dimensional semiconductor device and a method of fabricating the same, and more particularly, to a three-dimensional semiconductor device including a field effect transistor and a method of fabricating the same. 
     DISCUSSION OF RELATED ART 
     A semiconductor device includes an integrated circuit including metal oxide semiconductor field effect transistors (MOSFETs). As design rules of the semiconductor device are reduced in accordance with an increasing demand for the semiconductor device to have a smaller pattern size and higher performance, sizes of the MOSFETs are also being aggressively scaled down. The scale down of MOSFETs may deteriorate operating characteristics of the semiconductor device. Accordingly, various studies have been conducted to develop methods of fabricating semiconductor devices having superior performances while overcoming limitations caused by high integration of the semiconductor devices. 
     SUMMARY 
     Embodiments of the present inventive concept provide a three-dimensional semiconductor memory device with increased integration. 
     Embodiments of the present inventive concept provide a method of fabricating a semiconductor device with increased integration. 
     According to an embodiment of the present inventive concept, a three-dimensional semiconductor device may include: a first active region on a substrate, the first active region including a plurality of lower channel patterns and a plurality of lower source/drain patterns that are alternately arranged along a first direction; a second active region on the first active region, the second active region including a plurality of upper channel patterns and a plurality of upper source/drain patterns that are alternately arranged along the first direction; a first gate electrode on a first lower channel pattern of the lower channel patterns and on a first upper channel pattern of the upper channel patterns; and a second gate electrode on a second lower channel pattern of the lower channel patterns and on a second upper channel pattern of the upper channel patterns. The first gate electrode may include: a first lower gate electrode on the first lower channel pattern; and a first upper gate electrode on the first upper channel pattern. The first lower gate electrode and the first upper gate electrode may be connected to each other. The second gate electrode may include: a second lower gate electrode on the second lower channel pattern; a second upper gate electrode on the second upper channel pattern; and an isolation pattern between the second lower gate electrode and the second upper gate electrode. The second lower gate electrode and the second upper gate electrode may be separated from each other by the isolation pattern. 
     According to an embodiment of the present inventive concept, a three-dimensional semiconductor device may include: a first active region on a substrate and a second active region on the first active region; a first gate electrode and a second gate electrode on the first active region and the second active region, in which the first gate electrode includes a first lower gate electrode on the first active region, a first upper gate electrode on the second active region, and a first isolation pattern between the first lower gate electrode and the first upper gate electrode, and in which the second gate electrode includes a second lower gate electrode on the first active region, a second upper gate electrode on the second active region, and a second isolation pattern between the second lower gate electrode and the second upper gate electrode; a first upper gate contact coupled to the first upper gate electrode; a first lower gate contact coupled to the first lower gate electrode; a second upper gate contact coupled to the second upper gate electrode; a second lower gate contact coupled to the second lower gate electrode; a first wiring line electrically connected to the first upper gate contact and the second lower gate contact; and a second wiring line electrically connected to the first lower gate contact and the second upper gate contact. 
     According to an embodiment of the present inventive concept, a three-dimensional semiconductor device may include: a first active region on a substrate, the first active region including a lower channel pattern and a lower source/drain pattern connected to the lower channel pattern; a second active region on the first active region, the second active region including an upper channel pattern and an upper source/drain pattern connected to the upper channel pattern; a dummy channel pattern between the lower channel pattern and the upper channel pattern; and a gate electrode on the lower channel pattern, the dummy channel pattern, and the upper channel pattern. The gate electrode may include: a lower gate electrode on the lower channel pattern; an upper gate electrode on the upper channel pattern; and an isolation pattern between the lower gate electrode and the upper gate electrode. 
     According to an embodiment of the present inventive concept, a method of fabricating a three-dimensional device may include: forming on a substrate a stack layer in which a first active layer, a first sacrificial layer, a second sacrificial layer, a third sacrificial layer, and a second active layer are sequentially stacked; patterning the stack layer to form a stack pattern that extends in a first direction, the stack pattern including a lower stack pattern and an upper stack pattern, the lower stack pattern including the first active layer and the first sacrificial layer, and the upper stack pattern including the third sacrificial layer and the second active layer; forming on the stack pattern a sacrificial pattern that extends in a second direction; forming an interlayer dielectric layer that covers the stack pattern and the sacrificial pattern; selectively removing the sacrificial pattern to form an outer region that exposes the stack pattern; using the outer region to replace the second sacrificial layer of the stack pattern with a dummy channel pattern; removing the first sacrificial layer and the third sacrificial layer of the stack pattern through the outer region to respectively form a first inner region and a second inner region; forming a lower gate electrode that fills the first inner region; forming an isolation pattern on the lower gate electrode; and forming on the isolation pattern an upper gate electrode that fills the second inner region. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other aspects and features of the present inventive concept will be more clearly understood from the following detailed description, taken in conjunction with the accompanying drawings, in which: 
         FIG.  1    illustrates a conceptual diagram showing a logic cell of a semiconductor device according to a comparative example of the present inventive concepts.; 
         FIG.  2    illustrates a conceptual view showing a logic cell of a semiconductor device according to an embodiment of the present inventive concept; 
         FIG.  3    illustrates a logic circuit diagram showing a flip-flop of a semiconductor device according to an embodiment of the present inventive concept; 
         FIG.  4    illustrates a logic circuit that constitutes the flip-flop of  FIG.  3   ; 
         FIG.  5    illustrates a plan view showing a three-dimensional semiconductor device according to an embodiment of the present inventive concept; 
         FIGS.  6 A,  6 B,  6 C,  6 D, and  6 E  illustrate cross-sectional views respectively taken along lines A-A′, B-B′, C-C′, D-D′, and E-E′ of  FIG.  5   ; 
         FIGS.  7 A,  7 B,  8 A,  8 B,  9 A,  9 B,  10 A,  10 B,  11 A,  11 B,  12 A,  12 B,  13 A,  13 B,  14 A,  14 B, and  15 A to  15 C  illustrate cross-sectional views showing a method of fabricating a semiconductor device according to an embodiment of the present inventive concept; 
         FIGS.  16  to  20    illustrate cross-sectional views taken along line C-C′ of  FIG.  5   , showing a method of forming a second gate electrode according to an embodiment of the present inventive concept; and 
         FIGS.  21  and  22    illustrate cross-sectional views taken along line D-D′ of  FIG.  5   , showing a method of forming a first gate electrode according to an embodiment of the present inventive concept. 
     
    
    
     Since the drawings in  FIGS.  1 - 22    are intended for illustrative purposes, the elements in the drawings are not necessarily drawn to scale. For example, some of the elements may be enlarged or exaggerated for clarity purpose. 
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       FIG.  1    illustrates a conceptual diagram showing a logic cell of a semiconductor device according to a comparative example of the present inventive concept.  FIG.  1    shows a logic cell of a two-dimensional device according to a comparative example of the present inventive concept. 
     Referring to  FIG.  1   , a single height cell SHC′ may be provided. For example, a substrate  100  may be provided thereon with a first power line POR 1  and a second power line POR 2 . One of the first and second power lines POR 1  and POR 2  may be provided with a drain voltage (VDD) or a power voltage. The other of the first and second power lines POR 1  and POR 2  may be provided with a source voltage (VSS) or a ground voltage. For example, the source voltage (VSS) may be applied to the first power line POR 1 , and the drain voltage (VDD) may be applied to the second power line POR 2 . 
     The single height cell SHC′ may be defined between the first power line POR 1  and the second power line POR 2 . The single height cell SHC′ may include a first active region AR 1  and a second active region AR 2 . One of the first and second active regions AR 1  and AR 2  may be a p-channel metal oxide semiconductor field effect transistor (PMOSFET) region, and the other of the first and second active regions AR 1  and AR 2  may be an n-channel metal oxide semiconductor field effect transistor (NMOSFET) region. For example, the first active region AR 1  may be an NMOSFET region, and the second active region AR 2  may be a PMOSFET region. For example, the single height cell SHC′ may have a complementary metal oxide semiconductor (CMOS) structure provided between the first power line POR 1  and the second power line POR 2  so as to use complementary and symmetrical pairs of PMOSFET and NMOSFET for logic functions. 
     A semiconductor device according to a comparative example may be a two-dimensional device in which transistors of a front-end-of-line (FEOL) layer are arranged two-dimensionally. For example, NMOSFETs on the first active region AR 1  may be spaced apart in a first direction D 1  from PMOSFETs on the second active region AR 2 . 
     The first and second active regions AR 1  and AR 2  may each have a first width W 1  in the first direction D 1 . A first height HE 1  may be defined to indicate a length in the first direction D 1  of the single height cell SHC′ according to a comparative example. The first height HE 1  may be substantially the same as a distance (e.g., pitch) between the first power line POR 1  and the second power line POR 2 . 
     The single height cell SHC′ may constitute one logic cell. In this description, the logic cell may mean a logic device, such as, for example, AND, OR, XOR, XNOR, or inverter, that performs a specific function. For example, the logic cell may include transistors for constituting a logic device, and may also include wiring lines that connect the transistors to each other. 
     Because a two-dimensional device is included in the single height cell SHC′ according to a comparative example, the first active region AR 1  and the second active region AR 2  may be disposed spaced apart from each other in the first direction D 1  without overlapping each other. Therefore, it may be required that the first height HE 1  of the single height cell SHC′ be defined to include all of the first and second active regions AR 1  and AR 2  that are spaced apart from each other in the first direction D 1 . As a result, the first height HE 1  of the single height cell SHC′ according to a comparative example may be required to become relatively large. Therefore, the single height cell SHC′ according to a comparative example may have a relatively large area. 
       FIG.  2    illustrates a conceptual view showing a logic cell of a semiconductor device according to an embodiment of the present inventive concept.  FIG.  2    depicts a logic cell of a three-dimensional device according to an embodiment of the present inventive concept. 
     Referring to  FIG.  2   , a single height cell SHC may be provided to include a three-dimensional device such as a stacked transistor. For example, a substrate  100  may be provided thereon with a first power line POR 1  and a second power line POR 2 . The single height cell SHC may be defined between the first power line POR 1  and the second power line POR 2 . 
     The single height cell SHC may include a first active region AR 1  and a second active region AR 2 . One of the first and second active regions AR 1  and AR 2  may be a PMOSFET region, and the other of the first and second active regions AR 1  and AR 2  may be an NMOSFET region. 
     A semiconductor device according to an embodiment of the present inventive concept may be a three-dimensional device in which transistors of a front-end-of-line (FEOL) layer are stacked vertically. The substrate  100  may be provided thereon with the first active region AR 1  as a bottom tier, and the first active region AR 1  may be provided thereon with the second active region AR 2  as a top tier. For example, the substrate  100  may be provided thereon with NMOSFETs of the first active region AR 1 , and the NMOSFETs may be provided thereon with PMOSFETs of the second active region AR 2 . The first and second active regions AR 1  and AR 2  may be spaced apart from each other in a vertical direction or a third direction D 3 . 
     The first and second active regions AR 1  and AR 2  may each have a first width W 1  in the first direction D 1 . A second height HE 2  may be defined to indicate a length in the first direction D 1  of the single height cell SHC according to an embodiment of the present inventive concept. The second height HE 2  of the single height cell SHC may be substantially the same as a distance (e.g., pitch) between the first power line POR 1  and the second power line POR 2 . 
     Because the single height cell SHC according to an embodiment of the present inventive concept includes a three-dimensional device or a stacked transistor, the first and second active regions AR 1  and AR 2  may overlap each other. Therefore, the second height HE 2  of the single height cell SHC may have a size enough to cover the first width W 1 . As a result, the second height HE 2  of the single height cell SHC according to an embodiment of the present inventive concept may be smaller than the first height HE 1  of the single height cell SHC′ discussed above in  FIG.  1   . For example, the single height cell SHC according to an embodiment of the present inventive concept may have a relatively small area. For a three-dimensional semiconductor device according to an embodiment of the present inventive concept, an area of a logic cell may be reduced to increase integration of the device. For example, the single height cell SHC of a three-dimensional semiconductor device according to an embodiment of the present inventive concept may dramatically increase the number of transistors in a package as each area may effectively double the number of total devices in comparison to the single height cell SHC′ of a two-dimensional device according to the comparative example. 
       FIG.  3    illustrates a logic circuit diagram showing a flip-flop of a semiconductor device according to an embodiment of the present inventive concept.  FIG.  4    illustrates a logic circuit that constitutes the flip-flop of  FIG.  3   . 
     Referring to  FIGS.  3  and  4   , a flip-flop cell FF may include a logic circuit LC of  FIG.  4   . For example, the flip-flop cell FF may include first to fourth parts PA 1  to PA 4 . The first part PA 1  may be a core circuit that performs a scan function and a flip-flop function. In response to a scan enable signal SE, the first part PA 1  may select one of an external input signal D and a scan input signal SI, and based on the selected signal, the first part PA 1  may provide a first node N 1  with an internal signal. The first part PA 1  may be called, for example, a mux, a scan mux, or a selector. 
     Each of the second and third parts PA 2  and PA 3  may be a buffer region. The second part PA 2  may include a master latch, and the third part PA 3  may include a slave latch. A second node N 2  may be provided between the second part PA 2  and the third part PA 3 . The master latch of the second part PA 2  may latch the internal signal from the first node N 1 , based on a clock signal CLK. Based on the clock signal CLK, the slave latch of the third part PA 3  may latch an output of the master latch and provide an output signal Q. A third node N 3  may be connected to the third part PA 3 . The fourth part PA 4  may include a clock circuit that connects to a flip-flop and receives an external clock signal CK. 
     Referring back to  FIGS.  3  and  4   , each of the first, second, and third parts PA 1 , PA 2 , and PA 3  may include a first element E 1  to which a first input signal SI 1  is input and a second element E 2  to which a second input signal SI 2  is input. The first and second elements E 1  and E 2  may be connected to each other. 
     The first element E 1  may include first to fourth transistors that are connected in series. The first to fourth transistors may be sequentially disposed between VDD and VSS terminals. The first and second transistors may be PMOS transistors, and the third and fourth transistors may be NMOS transistors. The first input signal SI 1  may be input to the first and fourth transistors, a first signal A may be input to the second transistor, and a second signal /A may be input to the third transistor. The second signal /A may be an inverse signal to the first signal A. 
     The second element E 2  may include first to fourth transistors that are connected in series. The first to fourth transistors may be sequentially disposed between VDD and VSS terminals. The first and second transistors may be PMOS transistors, and the third and fourth transistors may be NMOS transistors. The second input signal SI 2  may be input to the first and fourth transistors, a second signal /A may be input to the second transistor, and a first signal A may be input to the third transistor. 
     As regards the first part PA 1 , the first signal A may be a scan enable signal SE, and the second signal /A may be a scan enable inverse signal SI. As regards the second and third parts PA 2  and PA 3 , the first signal A may be a clock signal CLK, and the second signal /A may be a clock inverse signal /CLK. 
       FIG.  5    illustrates a plan view showing a three-dimensional semiconductor device according to an embodiment of the present inventive concept.  FIGS.  6 A,  6 B,  6 C,  6 D , and  6 E illustrate cross-sectional views respectively taken along lines A-A′, B-B′, C-C′, D-D′, and E-E′ of  FIG.  5   . A three-dimensional device shown in  FIGS.  5  and  6 A to  6 E  may indicate a detailed example in which the logic circuit LC of  FIG.  4    is implemented in the single height cell SHC of  FIG.  2   . 
     Referring to  FIGS.  5  and  6 A to  6 E , a substrate  100  may be provided thereon with a logic cell including the logic circuit LC of  FIG.  4   . The substrate  100  may be a compound semiconductor substrate or a semiconductor substrate including, for example, silicon (Si), germanium (Ge), or silicon-germanium (SiGe). For example, the substrate  100  may be a silicon substrate. Alternatively, the substrate  100  may include, but is not limited to, SOI (silicon-on-insulator), SGOI (silicon germanium on insulator), indium antimonide (InSb), lead tellurium (PbTe) compounds, indium arsenide (InAs), indium phosphide (InP), gallium arsenide (GaAs), gallium phosphide (GaP), or gallium antimonide (GaSb). Also, the substrate  100  may include one or more semiconductor layers or structures and may include active or operable portions of semiconductor devices. 
     The logic cell may include a first active region AR 1  and a second active region AR 2  that are sequentially stacked on the substrate  100 . One of the first and second active regions AR 1  and AR 2  may be a PMOSFET region, and the other of the first and second active regions AR 1  and AR 2  may be an NMOSFET region. The first active region AR 1  may be provided on a bottom tier of a front-end-of-line (FEOL) layer, and the second active region AR 2  may be provided on a top tier of a front-end-of-line (FEOL) layer. NMOS and PMOS field effect transistors of the first and second active regions AR 1  and AR 2  may be vertically stacked to constitute three-dimensionally stacked transistors. In an embodiment of the present inventive concept, the first active region AR 1  may be an NMOSFET region, and the second active region AR 2  may be a PMOSFET region. 
     The NMOS and PMOS field effect transistors of the first and second active regions AR 1  and AR 2  may include the first to fourth transistors that constitute the logic circuit LC of  FIG.  4   . When viewed in plan, the stacked first and second active regions AR 1  and AR 2  may be positioned between a first power line POR 1  and a second power line POR 2 . For example, the first power line POR 1 , the stacked first and second active regions AR 1  and AR 2 , and the second power line POR 2  may be sequentially arranged in the first direction D 1 . 
     An active pattern AP may be defined by a trench TR formed on an upper portion of the substrate  100 . The active pattern AP may be a vertically protruding portion of the substrate  100 . When viewed in plan, the active pattern AP may have a bar shape that extends in a second direction D 2 . The active pattern AP may be provided thereon with the first and second active regions AR 1  and AR 2  that are sequentially stacked. 
     The trench TR may be filled with a device isolation layer ST. For example, the device isolation layer ST may be formed on the substrate  100  to define the active pattern AP. The device isolation layer ST may include a silicon oxide (SiO 2 ) layer, and may have a top surface at a level the same as or lower than that of a top surface of the active pattern AP. The device isolation layer ST may not cover any of lower and upper channel patterns CH 1  and CH 2  which will be discussed below. 
     The active pattern AP may be provided thereon with the first active region AR 1  that includes lower channel patterns CH 1  and lower source/drain patterns SD 1 . Each of the lower channel patterns CH 1  may be interposed between a pair of the lower source/drain patterns SD 1 . The lower channel pattern CH 1  may connect a pair of lower source/drain patterns SD 1  to each other. Referring to  FIG.  6 A , the first active region AR 1  may include a plurality of lower channel patterns CH 1  and a plurality of lower source/drain patterns SD 1  that are alternately arranged along the second direction D 2 . 
     The lower channel pattern CH 1  may include a first semiconductor pattern SP 1  and a second semiconductor pattern SP 2  that are sequentially stacked. The first and second semiconductor patterns SP 1  and SP 2  may be spaced apart from each other in a vertical direction (or a third direction D 3 ). Each of the first and second semiconductor patterns SP 1  and SP 2  may include, for example, silicon (Si), germanium (Ge), or silicon-germanium (SiGe). In an embodiment of the present inventive concept, each of the first and second semiconductor patterns SP 1  and SP 2  may include crystalline silicon (c-Si). 
     The lower source/drain patterns SD 1  may be provided on a top surface of the active pattern AP. Each of the lower source/drain patterns SD 1  may be an epitaxial pattern formed by a selective epitaxial growth (SEG) process. For example, the lower source/drain pattern SD 1  may have a top surface higher than that of the second semiconductor pattern SP 2  included in the lower channel pattern CH 1 . For example, each pair of the lower source/drain patterns SD 1  may be connected to each other by the stacked first and second semiconductor patterns SP 1  and SP 2  of the lower channel pattern CH 1 . 
     The lower source/drain patterns SD 1  may be doped with impurities to have a first conductivity type. The first conductivity type may be an n-type or p-type. In an embodiment of the present inventive concept, the first conductivity type may be an n-type. The lower source/drain patterns SD 1  may include one or more selected from, for example, silicon (Si) and silicon-germanium (SiGe). The lower source/drain patterns SD 1  may include n-type dopants such as, for example, phosphorus (P), arsenic (As), antimony (Sb) and/or bismuth (Bi). 
     A first interlayer dielectric layer  110  may be provided on the lower source/drain patterns SD 1 , and may cover the lower source/drain patterns SD 1 . The first interlayer dielectric layer  110  may be provided thereon with a second interlayer dielectric layer  120  and the second active region AR 2 . 
     The second active region AR 2  may include upper channel patterns CH 2  and upper source/drain patterns SD 2 . The upper channel patterns CH 2  may vertically overlap corresponding lower channel patterns CH 1 . The upper source/drain patterns SD 2  may vertically overlap corresponding lower source/drain patterns SD 1 . Each of the upper channel patterns CH 2  may be interposed between a pair of the upper source/drain patterns SD 2 . The upper channel pattern CH 2  may connect a pair of upper source/drain patterns SD 2  to each other. For example, the second active region AR 2  may include a plurality of upper channel patterns CH 2  and a plurality of upper source/drain patterns SD 2  that are alternately arranged along the second direction D 2 . 
     The upper channel pattern CH 2  may include a third semiconductor pattern SP 3  and a fourth semiconductor pattern SP 4  that are sequentially stacked. The third and fourth semiconductor patterns SP 3  and SP 4  may be spaced apart from each other in the third direction D 3 . The third and fourth semiconductor patterns SP 3  and SP 4  included in the upper channel pattern CH 2  may have a material the same as that of the first and second semiconductor patterns SP 1  and SP 2  included in the lower channel pattern CH 1 . 
     At least one dummy channel pattern DSP may be interposed between the lower channel pattern CH 1  and the upper channel pattern CH 2  that overlies the lower channel pattern CH 1 . The dummy channel pattern DSP may be spaced apart from the lower source/drain patterns SD 1 . Also, the dummy channel pattern DSP may be spaced apart from the upper source/drain patterns SD 2 . For example, the dummy channel pattern DSP may not be connected to any of source/drain patterns. The dummy channel pattern DSP may include a semiconductor material such as, for example, silicon (Si), germanium (Ge), or silicon-germanium (SiGe), or a dielectric material such as, for example, silicon oxide (SiO 2 ) or silicon nitride (Si 3 N4). 
     The upper source/drain patterns SD 2  may be provided on a top surface of the first interlayer dielectric layer  110 . Each of the upper source/drain patterns SD 2  may be an epitaxial pattern formed by a selective epitaxial growth (SEG) process. For example, the upper source/drain pattern SD 2  may have a top surface higher than that of the fourth semiconductor pattern SP 4  included in the upper channel pattern CH 2 . For example, each pair of the upper source/drain pattern SD 2  may be connected to each other by the stacked third and fourth semiconductor patterns SP 3  and SP 4  of the upper channel pattern CH 2 . 
     The upper source/drain patterns SD 2  may be doped with impurities to have a second conductivity type. The second conductivity type may be different from the first conductivity type of the lower source/drain patterns SD 1 . The second conductivity type may be a p-type. The upper source/drain patterns SD 2  may include one or more selected from, for example, silicon (Si) and silicon-germanium (SiGe). The upper source/drain patterns SD 2  may include p-type dopants such as, for example, boron (B), aluminum (Al), gallium (Ga), and/or indium (In). 
     The second interlayer dielectric layer  120  may cover the upper source/drain patterns SD 2 . The second interlayer dielectric layer  120  may have a top surface coplanar with that of a gate capping pattern GP which will be discussed below. 
     A gate electrode GE may be provided on the stacked lower and upper channel patterns CH 1  and CH 2 . When viewed in plan, the gate electrode GE may have a bar shape that extends in the first direction D 1 . A plurality of gate electrodes GE may be provided on the substrate  100 , and the plurality of gate electrodes GE may be arranged at a first pitch along the second direction D 2 . Each of the gate electrodes GE may vertically overlap the stacked lower and upper channel patterns CH 1  and CH 2 . 
     The plurality of gate electrodes GE on the logic circuit LC according to an embodiment of the present inventive concept may include first gate electrodes GE 1  and second gate electrodes GE 2 . The second gate electrodes GE 2  may be disposed between the first gate electrodes GE 1 . Each of the first gate electrodes GE 1  may be a common gate electrode in which a lower gate electrode LGE and an upper gate electrode UGE are connected to each other. Each of the second gate electrodes GE 2  may be a split gate electrode in which a lower gate electrode LGE and an upper gate electrode UGE are separated from each other. 
     The first gate electrode GE 1  will be further discussed in detail with reference to  FIGS.  6 A,  6 B, and  6 D . The first gate electrode GE 1  may extend in a vertical direction (or the third direction D 3 ) from a top surface of the device isolation layer ST (or of the active pattern AP) to a gate capping pattern GP which will be discussed below. The first gate electrode GE 1  may extend in the third direction D 3  from the lower channel pattern CH 1  of the first active region AR 1  to the upper channel pattern CH 2  of the second active region AR 2 . The first gate electrode GE 1  may extend in the third direction D 3  from the first semiconductor pattern SP 1  to the fourth semiconductor pattern SP 4 . In an embodiment of the present inventive concept, the first gate electrode GE 1  may be disposed on a first lower channel pattern of the lower channel patterns CH 1  and on a first upper channel pattern of the upper channel patterns CH 2 , and the second gate electrode GE 2  may be disposed on a second lower channel pattern of the lower channel patterns CH 1  and on a second upper channel pattern of the upper channel patterns CH 2 . 
     The first gate electrode GE 1  may be provided on a top surface, a bottom surface, and opposite sidewalls of each of the first to fourth semiconductor patterns SP 1  to SP 4 . In this sense, a transistor according to an embodiment of the present inventive concept may be a three-dimensional field effect transistor (e.g., multi-bridge channel field effect transistor (MBCFET) or gate all around field effect transistor (GAAFET)) in which the first gate electrode GE 1  three-dimensionally surrounds a channel 
     The first gate electrode GE 1  may include a lower gate electrode LGE provided on a bottom tier of a front-end-of-line (FEOL) layer or on the first active region AR 1 , and may also include an upper gate electrode UGE provided on a top tier of a front-end-of-line (FEOL) layer or on the second active region AR 2 . The lower and upper gate electrodes LGE and UGE may vertically overlap each other. The lower and upper gate electrodes LGE and UGE of the first gate electrode GE 1  may be connected to each other. For example, the first gate electrode GE 1  may be a common gate electrode in which the lower gate electrode LGE of the first active region AR 1  is connected to the upper gate electrode UGE of the second active region AR 2 . 
     The lower gate electrode LGE of the first gate electrode GE 1  may include a first portion PO 1  interposed between the active pattern AP and the first semiconductor pattern SP 1 , a second portion PO 2  interposed between the first semiconductor pattern SP 1  and the second semiconductor pattern SP 2 , and a third portion PO 3  interposed between the second semiconductor pattern SP 2  and the dummy channel pattern DSP. 
     The upper gate electrode UGE of the first gate electrode GE 1  may include a fourth portion PO 4  interposed between the dummy channel pattern DSP and the third semiconductor pattern SP 3 , a fifth portion PO 5  interposed between the third semiconductor pattern SP 3  and the fourth semiconductor pattern SP 4 , and a sixth portion PO 6  on the fourth semiconductor pattern SP 4 . 
     A pair of gate spacers GS may be disposed on opposite sidewalls of the first gate electrodes GE 1 . Referring to  FIG.  6 A , a pair of gate spacers GS may be disposed on opposite sidewalls of the sixth portion PO 6 . The gate spacers GS may extend in the first direction D 1  along the first gate electrode GE 1 , and may have their top surfaces higher than that of the first gate electrode GE 1 . The top surfaces of the gate spacers GS may be coplanar with that of the second interlayer dielectric layer  120 . The gate spacers GS may include one or more selected from, for example, silicon carbonitride (SiCN), silicon carbon oxynitride (SiCON), and silicon nitride (Si 3 N 4 ). Alternatively, the gate spacers GS may each include multiple layers formed of two or more selected from, for example, a silicon carbonitride (SiCN) layer, a silicon carbon oxynitride (SiCON) layer, and a silicon nitride (Si 3 N 4 ) layer. A pair of liner layers LIN may be provided on opposite sidewalls of each of the third and fourth portions PO 3  and PO 4  of the first gate electrode GE 1 . 
     A gate capping pattern GP may be provided on the top surface of the first gate electrode GE 1 . The gate capping pattern GP may extend in the first direction D 1  along the first gate electrode GE 1 . For example, the gate capping pattern GP may include one or more selected from, for example, silicon oxynitride (SiON), silicon carbonitride (SiCN), silicon carbon oxynitride (SiCON), and silicon nitride (Si 3 N 4 ). 
     Gate dielectric layers UGI and LGI may be interposed between the first gat electrode GE 1  and the first to fourth semiconductor patterns SP 1  to SP 4 . For example, a lower gate dielectric layer LGI may be interposed between the lower gate electrode LGE and each of the first and second semiconductor patterns SP 1  and SP 2 . An upper gate dielectric layer UGI may be interposed between the upper gate electrode UGE and each of the third and fourth semiconductor patterns SP 3  and SP 4 . 
     Each of the lower and upper gate dielectric layers LGI and UGI may include one or more selected from, for example, a silicon oxide (SiO 2 ) layer, a silicon oxynitride (SiON) layer, and a high-k dielectric layer. In an embodiment of the present inventive concept, each of the lower and upper gate dielectric layers LGI and UGI may include a silicon oxide (SiO 2 ) layer that directly covers a corresponding one of the first to fourth semiconductor patterns SP 1  to SP 4 , and may also include a high-k dielectric layer on the silicon oxide (SiO 2 ) layer. For example, each of the lower and upper gate dielectric layers LGI and UGI may include multiple layers. 
     The high-k dielectric layer may include a high-k dielectric material whose dielectric constant is greater than that of a silicon oxide (SiO 2 ) layer. For example, the high-k dielectric material may include one or more selected from, for example, hafnium oxide (HfO 2 ), hafnium silicon oxide (HfSiO 4 ), hafnium zirconium oxide (HfZrO 4 ), hafnium tantalum oxide (Hf 2 Ta 2 O 9 ), hafnium aluminum oxide (HfAlO 3 ), lanthanum oxide (La 2 O 3 ), lanthanum aluminum oxide (LaAlO 3 ), zirconium oxide (ZrO 2 ), zirconium silicon oxide (ZrSiO 4 ), tantalum oxide (Ta 2 O 5 ), titanium oxide (TiO 2 ), barium strontium titanium oxide (BaSrTi 2 O 6 ), barium titanium oxide (BaTiO 3 ), strontium titanium oxide (SrTiO 3 ), lithium oxide (Li 2 O), aluminum oxide (Al 2 O 3 ), lead scandium tantalum oxide (Pb(Sc,Ta)O 3 ), and lead zinc niobate [Pb(Zn 1/3 Nb 2/3 )O 3 ]. 
     In an embodiment of the present inventive concept, the lower gate dielectric layer LGI may include a first dipole element. The first dipole element may include, for example, lanthanum (La), aluminum (Al), or a combination thereof. For example, lanthanum (La), aluminum (Al), or a combination thereof may be contained as impurities in the lower gate dielectric layer LGI. The lower gate dielectric layer LGI may include a dipole-interface that the first dipole element creates between the high-k dielectric layer and the silicon oxide (SiO 2 ) layer. 
     When the lower gate dielectric layer LGI contains lanthanum (La), there may be a reduction in work function of the lower gate electrode LGE. Consequently, there may be a reduction in threshold voltage of a transistor (e.g., NMOS transistor) on the first active region AR 1 . Alternatively, when the lower gate dielectric layer LGI contains aluminum (Al), there may be an increase in effective work function of the lower gate electrode LGE. Accordingly, there may be an increase in threshold voltage of a transistor (e.g., NMOS transistor) on the first active region AR 1 . For example, the threshold voltage of the NMOS transistor may be adjusted by incorporating a predetermined amount of the first dipole element such as, for example, lanthanum (La), aluminum (Al), or a combination thereof into the lower gate dielectric layer LGI. 
     In an embodiment of the present inventive concept, the upper gate dielectric layer UGI may include no dipole element. For example, a maximum concentration of a dipole element in the upper gate dielectric layer UGI may be smaller than that of the first dipole element in the lower gate dielectric layer LGI. In an embodiment of the present inventive concept, the upper gate dielectric layer UGI may include a second dipole element. The second dipole element may be the same as or different from the first dipole element. 
     The lower gate electrode LGE of the first gate electrode GE 1  may include a first metal pattern MP 1  on the first and second semiconductor patterns SP 1  and SP 2 , and may also include a second metal pattern MP 2  on the first metal pattern MP 1 . The first metal pattern MP 1  may include a first work-function metal, and the second metal pattern MP 2  may include a second work-function metal. A composition of the first work-function metal and the second work-function metal may be adjusted to achieve a threshold voltage of a transistor on the first active region AR 1 . 
     The first work-function metal of the first metal pattern MP 1  may be a p-type work-function metal whose work function is relatively high. The first metal pattern MP 1  may include a metal nitride layer. The first metal pattern MP 1  may include nitrogen (N) and at least one selected from metals such as, for example, titanium (Ti), tantalum (Ta), aluminum (Al), tungsten (W), and molybdenum (Mo). For example, the first metal pattern MP 1  may include, for example, titanium nitride (TiN), tantalum nitride (TaN), titanium oxynitride (TiON), titanium silicon nitride (TiSiN), titanium aluminum nitride (TiAlN), tungsten carbon nitride (WCN), or molybdenum nitride (MoN). 
     The second work-function metal of the second metal pattern MP 2  may be an n-type work-function metal whose work function is relatively low. The second metal pattern MP 2  may include metal carbide. The second metal pattern MP 2  may include metal carbide doped with (or containing) one or more selected from, for example, silicon (Si) and aluminum (Al). In an embodiment of the present inventive concept, the second metal pattern MP 2  may include, for example, aluminum-doped titanium carbide (TiAlC), aluminum-doped tantalum carbide (TaAlC), aluminum-doped vanadium carbide (VAlC), silicon-doped titanium carbide (TiSiC), or silicon-doped tantalum carbide (TaSiC). In an embodiment of the present inventive concept, the second metal pattern MP 2  may include, for example, aluminum-silicon-doped titanium carbide (TiAlSiC) or aluminum-silicon-doped tantalum carbide (TaAlSiC). In an embodiment of the present inventive concept, the second metal pattern MP 2  may include, for example, aluminum-doped titanium (TiAl). In an embodiment of the present inventive concept, the second metal pattern MP 2  may include metal nitride doped with one or more selected from, for example, silicon and aluminum, such as aluminum-doped titanium nitride (TiAlN). 
     The work-function of the second metal pattern MP 2  may be controlled by adjusting a concentration of dopants (or impurities) such as silicon or aluminum. For example, impurities (e.g., silicon (Si) or aluminum (Al)) in the second metal pattern MP 2  may have a concentration ranging from about 0.1 at % to about 25 at %. The term “about” as used herein is inclusive of the stated value and means within an acceptable range of deviation for the particular value as determined by one of ordinary skill in the art, considering the measurement in question and the error associated with measurement of the particular quantity (i.e., the limitations of the measurement system). For example, “about” may mean within one or more standard deviations, or within ±30%, 20%, 10%, 5% of the stated value. 
     Each of the first, second, and third portions PO 1 , PO 2 , and PO 3  included in the lower gate electrode LGE may include a second metal pattern MP 2  and a first metal pattern MP 1  that surrounds a circumference of the second metal pattern MP 2 . For example, the second metal pattern MP 2  may have a thickness greater than that of the first metal pattern MP 1 . By adjusting the first work function of the first metal pattern MP 1  and the second work function of the second metal pattern MP 2  included in the first, second, and third portions PO 1 , PO 2 , and PO 3  which are interposed among the active pattern AP, the first semiconductor pattern SP 1 , the second semiconductor pattern SP 2  and the dummy channel pattern DSP, an operating threshold voltage of a transistor on the first active region AR 1  with good distribution may be achieved. For example, by properly adjusting the work function of the lower gate electrode LGE around the lower channel pattern CH 1 , a threshold voltage suitable for a transistor on the first active region AR 1  may be achieved. 
     The lower gate electrode LGE may further include a sixth metal pattern MP 6  at its remaining portion other than the first, second, and third portions PO 1 , PO 2 , and PO 3  (see  FIG.  6 D ). The sixth metal pattern MP 6  may have resistance lower than those of the first and second metal patterns MP 1  and MP 2 . For example, the sixth metal pattern MP 6  may include at least one selected from metals, whose resistances are low, such as, for example, tungsten (W), ruthenium (Ru), aluminum (Al), titanium (Ti), and tantalum (Ta). 
     Referring to  FIG.  6 D , the sixth metal pattern MP 6  of the first gate electrode GE 1  may have a top surface in contact with a bottom surface of the upper gate electrode UGE. The top surface of the sixth metal pattern MP 6  may be located at a level between those of top and bottom surfaces of the dummy channel pattern DSP. The sixth metal pattern MP 6  may have a first thickness TK 1  in the third direction D 3 . 
     The upper gate electrode UGE of the first gate electrode GE 1  may include a third metal pattern MP 3  on the third and fourth semiconductor patterns SP 3  and SP 4 . The third metal pattern MP 3  may surround the third and fourth semiconductor patterns SP 3  and SP 4 . The upper gate electrode UGE may further include a fourth metal pattern MP 4  and a fifth metal pattern MP 5  on the third metal pattern MP 3 . 
     The third metal pattern MP 3  may include the first work-function metal, and the fourth metal pattern MP 4  may include the second work-function metal. A composition of the first work-function metal and the second work-function metal may be adjusted to achieve a threshold voltage of a transistor on the second active region AR 2 . 
     Similar to the first metal pattern MP 1 , the first work-function metal of the third metal pattern MP 3  may be a p-type work-function metal whose work function is relatively high. The third metal pattern MP 3  may include a metal nitride layer. The metal nitride layer of the third metal pattern MP 3  may be the same as or different from that of the first metal pattern MP 1 . 
     Similar to the second metal pattern MP 2 , the second work-function metal of the fourth metal pattern MP 4  may be an n-type work-function metal whose work function is relatively low. The fourth metal pattern MP 4  may include metal carbide doped with (or containing) one or more selected from, for example, silicon (Si) and aluminum (Al). The fourth metal pattern MP 4  may include a material the same as or different from that of the second metal pattern MP 2 . 
     The fourth and fifth portions PO 4  and PO 5  of the upper gate electrode UGE may be formed of a third metal pattern MP 3 . The sixth portion PO 6  of the upper gate electrode UGE may include a third metal pattern MP 3 , a fourth metal pattern MP 4 , and a fifth metal pattern MP 5  that are sequentially stacked. 
     In an embodiment of the present inventive concept, the fifth metal pattern MP 5  may include the first work-function metal. For example, the fifth metal pattern MP 5  may include the same metal nitride layer as that of the third metal pattern MP 3 . In an embodiment of the present inventive concept, the fifth metal pattern MP 5  may include metal whose resistance is low. For example, the fifth metal pattern MP 5  may include a metal the same as that of the sixth metal pattern MP 6 . 
     The second gate electrode GE 2  will be further discussed in detail with reference to  FIGS.  6 A,  6 B, and  6 C . In the description of the second gate electrode GE 2 , the discussion of features the same as those in the first gate electrode GE 1  is omitted in order to avoid repetition and differences thereof will be mainly described. 
     The second gate electrode GE 2  may include a lower gate electrode LGE provided in the first active region AR 1  and an upper gate electrode UGE provided in the second active region AR 2 . An isolation pattern ISP may be interposed between the lower and upper gate electrodes LGE and UGE of the second gate electrode GE 2 . The isolation pattern ISP may cause the second gate electrode GE 2  to become a split gat electrode in which the lower gate electrode LGE of the first active region AR 1  is separated from the upper gate electrode UGE of the second active region AR 2 . In an embodiment of the present inventive concept, the first gate electrode GE 1  may include a first lower gate electrode of the lower gate electrodes LGE on the first lower channel pattern of the lower channel patterns CH 1 , and a first upper gate electrode of upper gate electrodes UGE on the first upper channel pattern of the upper channel patterns CH 2 , in which the first lower gate electrode and the first upper gate electrode are connected to each other. The second gate electrode may include a second lower gate electrode of the lower gate electrodes LGE on the second lower channel pattern of the lower channel patterns CH 1 , a second upper gate electrode of the upper gate electrodes UGE on the second upper channel pattern of the upper channel patterns CH 2 , and an isolation pattern ISP between the second lower gate electrode and the second upper gate electrode, in which the second lower gate electrode and the second upper gate electrode are separated from each other by the isolation pattern ISP. In an embodiment of the present inventive concept, each of the first and second lower gate electrodes may surround the first and second semiconductor patterns SP 1  and SP 2 , and each of the first and second upper gate electrodes may surround the third and fourth semiconductor patterns SP 3  and SP 4 . 
     Identical to the first, second, and third portions PO 1 , PO 2 , and PO 3  of the first gate electrode GE 1 , each of the first, second, and third portions PO 1 , PO 2 , and PO 3  of the second gate electrode GE 2  may include first and second metal patterns MP 1  and MP 2 . The lower gate electrode LGE of the second gate electrode GE 2  may be recessed at a remaining portion other than the first, second, and third portions PO 1 , PO 2 , and PO 3 . The isolation pattern ISP may fill the recessed region of the lower gate electrode LGE. 
     Referring to  FIG.  6 C , the sixth metal pattern MP 6  of the second gate electrode GE 2  may have a top surface in contact with a bottom surface of the isolation pattern ISP. The isolation pattern ISP may have a top surface in contact with a bottom surface of the upper gate electrode UGE. The top surface of the isolation pattern ISP may be located at a level between levels of top and bottom surfaces of the dummy channel pattern DSP. The top surface of the sixth metal pattern MP 6  may be located at a level between that of a top surface of the first semiconductor pattern SP 1  and that of a bottom surface of the second semiconductor pattern SP 2 . 
     The sixth metal pattern MP 6  may have a second thickness TK 2  in the third direction D 3 . The isolation pattern ISP may have a third thickness TK 3  in the third direction D 3 . Each of the second and third thicknesses TK 2  and TK 3  may be smaller than the first thickness TK 1  of  FIG.  6 D . For example, a sum of the second and third thicknesses TK 2  and TK 3  may be substantially the same as the first thickness TK 1 . In an embodiment of the present inventive concept, the third thickness TK 3  may be greater than the second thickness TK 2 . 
     Because there is a separation between the lower and upper gate electrodes LGE and UGE of the second gate electrode GE 2 , a signal may be individually applied to each of the lower and upper gate electrodes LGE and UGE. Therefore, as discussed below, the second gate electrode GE 2  may be provided with a lower gate contact LGC connected to the lower gate electrode LGE and with an upper gate contact UGC connected to the upper gate electrode UGE. Since the lower gate contact LGC and the upper gate contact UGC may be formed to have different depths from each other, and may be correspondingly connected to the lower gate electrode LGE and the upper gate electrode UGC, a cross-couple structure may be achieved with this configuration. As a result, according to an embodiment of the present inventive concept, a semiconductor device may increase in integration. 
     Referring back to  FIG.  5   , a first cell boundary CB 1  may be defined to extend in the second direction D 2  on the logic cell of the logic circuit LC according to an embodiment of the present inventive concept. On a location opposite to that of the first cell boundary CB 1 , a second cell boundary CB 2  may be defined to extend in the second direction D 2 . Gate cutting patterns CT may be disposed on the first and second cell boundaries CB 1  and CB 2 , and arranged in the second direction D 2 . When viewed in plan, the gate cutting patterns CT may be arranged at a first pitch along the first cell boundary CBE The gate cutting patterns CT may be arranged at the first pitch along the second cell boundary CB 2 . When viewed in plan, the gate cutting patterns CT on the first and second cell boundaries CB 1  and CB 2  may be disposed to correspondingly overlap the first and second gate electrodes GE 1  and GE 2 . For example, the portions of the first and second gate electrodes GE 1  and GE 2  overlapped by the gate cutting patterns CT may be removed and replaced with the gate cutting patterns CT (see  FIGS.  5  and  6 D ). 
     The gate cutting pattern CT may penetrate the first gate electrode GE 1  or the second gate electrode GE 2 . The gate cutting pattern CT may separate the first gate electrode GE 1  or the second gate electrode GE 2  from another gate electrode that is adjacent in the first direction D 1 . That is, the gate cutting pattern CT may cut one gate electrode into two gate electrodes. For example, referring to  FIG.  6 D , a pair of gate cutting patterns CT may be provided on opposite ends of the first gate electrode GE 1 . The gate cutting patterns CT may include a dielectric material, such as, for example, a silicon oxide (SiO 2 ) layer, a silicon nitride (Si 3 N 4 ) layer, or a combination thereof. 
     A third cell boundary CB 3  may be defined to extend in the first direction D 1  on the logic cell of the logic circuit LC according to an embodiment of the present inventive concept. A fourth cell boundary CB 4  may be defined to extend in the first direction D 1  on a location opposite to that of the third cell boundary CB 3 . Cell isolation patterns DB may be correspondingly disposed on the third and fourth cell boundaries CB 3  and CB 4 . Accordingly, the pair of cell isolation patterns DB, which are opposite to each other in the second direction D 2 , may be parallel to the first and second gate electrodes GE 1  and GE 2 , and may be provided at both sides of the logic cell of  FIG.  5   . The cell isolation patterns DB may separate the logic cell of  FIG.  5    from an adjacent another logic cell, while extending in the first direction D 1 . 
     Gate contacts UGC and LGC may be provided to penetrate a following described fourth interlayer dielectric layer  140  and the gate capping pattern GP and to electrically connect to the first and second gate electrodes GE 1  and GE 2 . For example, an upper gate contact UGC may be coupled to the upper gate electrode UGE of each of the first and second gate electrodes GE 1  and GE 2 . A lower gate contact LGC may be coupled to the lower gate electrode LGE of the second gate electrode GE 2 . For example, a bottom surface of the lower gate contact LGC may be lower than that of the upper gate contact UGC. 
     Each of the upper and lower gate contacts UGC and LGC may include at least one selected from metals such as, for example, copper (Cu), aluminum (Al), ruthenium (Ru), cobalt (Co), tungsten (W), and molybdenum (Mo). Each of the upper and lower gate contacts UGC and LGC may have a pillar shape that extends in the third direction D 3 . A contact spacer CSP may be provided to surround an outer sidewall of each of the upper and lower gate contacts UGC and LGC. The contact spacer CSP on the lower gate contact LGC may insulate the lower gate contact LGC from the upper gate electrode UGE. 
     A first active contact AC 1  may be provided on at least one lower source/drain pattern SD 1  (see  FIG.  6 E ). The first active contact AC 1  may include a vertical extension VEP and a horizontal extension HEP. The vertical extension VEP may have a pillar shape that vertically extends while penetrating the first, second, and third interlayer dielectric layers  110 ,  120 , and  130 . The vertical extension VEP of the first active contact AC 1  may be horizontally offset from the stacked lower and upper source/drain patterns SD 1  and SD 2 . For example, the vertical extension VEP may be spaced apart from the lower and upper source/drain patterns SD 1  and SD 2  in the first direction D 1 . The horizontal extension HEP may be provided on a bottom tier of a front-end-of-line (FEOL) layer. The horizontal extension HEP may extend in the first direction D 1  from the vertical extension VEP, thereby being coupled to the lower source/drain pattern SD 1 . 
     The horizontal and vertical extensions HEP and VEP may be connected to constitute a single first active contact AC 1 . For example, the first active contact AC 1  may include one or more selected from doped semiconductor and metal. The metal may include one or more selected from, for example, copper (Cu), aluminum (Al), ruthenium (Ru), cobalt (Co), tungsten (W), and molybdenum (Mo). 
     A second active contact AC 2  may be provided on at least one upper source/drain pattern SD 2  (see  FIG.  6 E ). The second active contact AC 2  may be spaced apart in the first direction D 1  from the first active contact AC 1 . The second active contact AC 2  may vertically overlap the upper source/drain pattern SD 2 . 
     The second active contact AC 2  may be provided on a top tier of a front-end-of-line (FEOL) layer. The second active contact AC 2  may have a pillar shape that vertically extends, and may be directly coupled to the upper source/drain pattern SD 2 . For example, the second active contact AC 2  may include a material the same as that of the first active contact AC 1 . When the first active contact AC 1  and the second active contact AC 2  are formed of metal, during the formation of the first active contact AC 1  and the second active contact AC 2 , silicide patterns may be respectively formed between the first active contact AC 1  and the lower source/drain pattern SD 1 , and between second active contact AC 2  and the upper source/drain pattern SD 2  to provide reliable metal-semiconductor contact. 
     A fourth interlayer dielectric layer  140  may be provided on a third interlayer dielectric layer  130 . A first metal layer M 1  may be provided in the fourth interlayer dielectric layer  140 . The first metal layer M 1  may include a first power line POR 1 , a second power line POR 2 , and first to fourth wiring lines MI 1  to MI 4 . 
     When viewed in plan, the first power line POR 1  may be provided on the first cell boundary CB 1 , the second power line POR 2  may be provided on the second cell boundary CB 2 . The gate cutting patterns CT may vertically overlap the first and second power lines POR 1  and POR 2 . A drain voltage (VDD) may be applied to one of the first and second power lines POR 1  and POR 2 , and a source voltage (VSS) may be applied to the other of the first and second power lines POR 1  and POR 2 . In an embodiment of the present inventive concept, the source voltage (VSS) may be applied to the first power line POR 1 , and the drain voltage (VDD) may be applied to the second power lines POR 2 . 
     The first to fourth wiring lines MI 1  to MI 4  may be disposed between the first power line POR 1  and the second power line POR 2 . Each of the first to fourth wiring lines MI 1  to MI 4  may have a linear or bar shape that extends in the second direction D 2 . The first and second power lines POR 1  and POR 2  and the first to fourth wiring lines MI 1  to MI 4  may include at least one selected from metals such as, for example, copper (Cu), aluminum (Al), ruthenium (Ru), cobalt (Co), tungsten (W), and molybdenum (Mo). 
     The first metal layer M 1  may further include vias VI provided thereunder. One of the vias VI may connect one of the first and second active contacts AC 1  and AC 2  to one of the first and second power lines POR 1  and POR 2 . Another of the vias VI may connect to each other the first and second active contacts AC 1  and AC 2  that are adjacent to each other (see  FIG.  6 E ). Still another of the vias VI may connect one of the lower and upper contacts UGC and LGC to one of the first to fourth wiring lines MI 1  to MI 4  (see  FIG.  6 D ). 
     Additional metal layers may be stacked on the first metal layer M 1 . The first metal layer M 1  and other metal layers on the first metal layer M 1  may constitute a back-end-of-line (BEOL) layer of a semiconductor device. The other metal layers on the first metal layer M 1  may include routing lines for connecting logic cells to each other. 
     A semiconductor device according to the present inventive concept may have a three-dimensional cell structure including NMOS and PMOS field effect transistors. Among the second gate electrodes GE 2  of  FIG.  5   , one may be a first split gate SGE 1  and another may be a second split gate SGE 2 . 
     Referring to the logic circuit LC of  FIG.  4   , the first signal A may be input to the first wiring line MI 1 . The first signal A may be input through the upper gate contact UGC beneath the first wiring line MI 1  to the upper gate electrode UGE of the first split gate SGE 1 . The first signal A may be input through the lower gate contact LGC beneath the first wiring line MI 1  to the lower gate electrode LGE of the second split gate SGE 2  (See  FIGS.  5  and  6 B ). 
     The second signal /A may be input to the second wiring line MI 2 . The second signal /A may be input through the lower gate contact LGC beneath the second wiring line MI 2  to the lower gate electrode LGE of the first split gate SGE 1 . The second signal /A may be input through the upper gate contact UGC beneath the second wiring line MI 2  to the upper gate electrode UGE of the second split gate SGE 2  (see  FIG.  6 C ). For example, the first wiring line MI 1  may be electrically connected to the upper gate contact UGC which is coupled to the upper gate electrode UGE of the first split gate SGE 1 , and the second wiring line MI 2  may be electrically connected to the lower gate contact LGC which is coupled to the lower gate electrode LGE of the first split gate SGE 1  (See  FIG.  5   ). 
     A three-dimensional device according to the present inventive concept may have a split gate structure in which the lower gate electrode LGE and the upper gate electrode UGE are separated from each other, and thus a cross-couple structure of  FIG.  4    may be three-dimensionally accomplished. 
     The first input signal SI 1  may be input to the third wiring line MI 3 , and the second input signal SI 2  may be input to the fourth wiring line MI 4 . Therefore, the logic circuit LC of  FIG.  4    may be achieved into a three-dimensional cell structure according to an embodiment of the present inventive concept. 
     As regards a three-dimensional device according to an embodiment of the present inventive concept, only two gates, or the first split gate SGE 1  and the second split gate SGE 2 , may realize a cross-couple structure. Therefore, there may be a reduction in cell area for constructing the cross-couple structure of  FIG.  4   . As a result, according to an embodiment of the present inventive concept, a semiconductor device may increase in integration. 
       FIGS.  7 A to  15 C  illustrate cross-sectional views showing a method of fabricating a semiconductor device according to an embodiment of the present inventive concept.  FIGS.  7 A,  8 A,  9 A,  10 A,  11 A,  12 A,  13 A,  14 A, and  15 A  depict cross-sectional views taken along line A-A′ of  FIG.  5   .  FIGS.  9 B,  10 B,  11 B, and  12 B  depict cross-sectional views taken along line E-E′ of  FIG.  5   .  FIGS.  7 B,  8 B,  13 B,  14 B, and  15 B  depict cross-sectional views taken along line C-C′ of  FIG.  5   .  FIG.  15 C  depicts a cross-sectional view taken along line D-D′ of  FIG.  5   . 
     Referring to  FIGS.  7 A and  7 B , first sacrificial layers SAL 1  and first active layers ACL 1  may be alternately stacked on a substrate  100 . The substrate  100  may be a semiconductor substrate. For example, the substrate  100  may be a single crystal semiconductor substrate that may be formed of a semiconductor material such as silicon (Si). The first sacrificial layers SAL 1  may include one of, for example, silicon (Si), germanium (Ge), or silicon-germanium (SiGe), and the first active layers ACL 1  may include another of, for example, silicon (Si), germanium (Ge), or silicon-germanium (SiGe). For example, the first sacrificial layers SAL 1  may include silicon-germanium (SiGe) formed by an epitaxial growth process, and the first active layers ACL 1  may include silicon (Si) formed by an epitaxial growth process. Each of the first sacrificial layers SAL 1  may have a germanium (Ge) concentration of about 10 at % to about 30 at %. 
     A second sacrificial layer SAL 2  may be formed on an uppermost first active layer ACL 1 . In an embodiment of the present inventive concept, the second sacrificial layer SAL 2  may have a thickness substantially the same as that of the first sacrificial layer SAL 1 . In an embodiment of the present inventive concept, the second sacrificial layer SAL 2  may have a thickness greater than that of the first active layer ACL 1  and that of the first sacrificial layer SAL 1 . The second sacrificial layer SAL 2  may include silicon (Si) or silicon-germanium (SiGe). When the second sacrificial layer SAL 2  includes silicon-germanium (SiGe), the second sacrificial layer SAL 2  may have a germanium (Ge) concentration greater than that of the first sacrificial layer SAL 1 . For example, the second sacrificial layer SAL 2  may have a germanium (Ge) concentration of about 40 at % to about 90 at %. 
     Third sacrificial layers SAL 3  and second active layers ACL 2  may be alternately staked on the second sacrificial layer SAL 2 . Each of the third sacrificial layers SAL 3  may include a material the same as that of the first sacrificial layers SAL 1 , and each of the second active layers ACL 2  may include a material the same as that of the first active layer ACL 1 . The second sacrificial layer SAL 2  may be interposed between the first sacrificial layer SAL 1  and the third sacrificial layer SAL 3 . 
     A stack pattern STP may be formed by patterning the stacked first, second, and third sacrificial layers SAL 1 , SAL 2 , and SAL 3  and the stacked first and second active layers ACL 1  and ACL 2 . The formation of the stack pattern STP may include forming a hardmask pattern on an uppermost second active layer ACL 2 , and using the hardmask pattern as an etching mask to etch the layers SAL 1 , SAL 2 , SAL 3 , ACL 1 , and ACL 2  that are stacked on the substrate  100 . During the formation of the stack pattern STP, an upper portion of the substrate  100  may be patterned to form a trench TR that defines an active pattern AP. The stack pattern STP may have a bar shape that extends in a second direction D 2 . 
     The stack pattern STP may include a lower stack pattern STP 1  on the active pattern AP, an upper stack pattern STP 2  on the lower stack pattern STP 1 , and the second sacrificial layer SAL 2  between the lower and upper stack patterns STP 1  and STP 2 . The lower stack pattern STP 1  may include the first sacrificial layers SAL 1  and the first active layers ACL 1  that are alternately stacked. The upper stack pattern STP 2  may include the third sacrificial layers SAL 3  and the second active layers ACL 2  that are alternately stacked. 
     A device isolation layer ST may be formed on the substrate  100 , filling the trench TR. For example, a dielectric layer may be formed on an entire surface of the substrate  100 , covering the active pattern AP and the stack pattern STP. The dielectric layer may be recessed to form the device isolation layer ST until the stack pattern STP is exposed. The device isolation layer ST may have a top surface at a level the same as or lower than that of a top surface of the active pattern AP. 
     Referring to  FIGS.  8 A and  8 B , sacrificial patterns PP may be formed to run across the stack pattern STP. Each of the sacrificial patterns PP may be formed to have a linear shape that extends in the first direction D 1 . The sacrificial patterns PP may be arranged at a first pitch along the second direction D 2 . 
     The formation of the sacrificial patterns PP may include forming a sacrificial layer on the entire surface of the substrate  100 , forming hardmask patterns MP on the sacrificial layer, and using the hardmask patterns MP as an etching mask to pattern the sacrificial layer. The sacrificial layer may include one or more selected from, for example, amorphous silicon (a-Si) and polysilicon (p-Si). 
     A spacer layer GSL may be conformally formed on the entire surface of the substrate  100 . The spacer layer GSL may cover the sacrificial patterns PP and the hardmask patterns MP. For example, the spacer layer GSL may include one or more selected from, for example, silicon carbonitride (SiCN), silicon carbon oxynitride (SiCON), and silicon (Si 3 N 4 ) 
     Referring to  FIGS.  9 A and  9 B , the spacer layer GSL and the hardmask patterns MP may be used as an etching mask to perform a first etching process on the stack pattern STP. The first etching process may form a first recess RS 1  in the stack pattern STP between the sacrificial patterns PP. The first recess RS 1  may be formed between a pair of sacrificial patterns PP. 
     The first etching process may be an anisotropic etching process. The first etching process may convert the spacer layer GSL into gate spacers GS that cover sidewalls of the sacrificial patterns PP. The first etching process may continue until an uppermost first sacrificial layer SAL 1  of the lower stack pattern STP 1  is exposed. For example, the first recess RS 1  may expose the lower stack pattern STP 1 . 
     A liner layer LIN may be conformally formed on the entire surface of the substrate  100 , and may cover the gate spacers GS and the hardmask patterns MP. The liner layer LIN may cover an inner wall of the first recess RS 1 , and the exposed lower stack pattern STP 1 . For example, the liner layer LIN may include silicon nitride (Si 3 N 4 ). 
     Referring to  FIGS.  10 A and  10 B , the liner layer LIN, the gate spacers GS, and the hardmask patterns MP may be used as an etching mask to perform a second etching process on the stack pattern STP. The second etching process may remove the lower stack pattern STP 1  between the sacrificial patterns PP, thereby forming a second recess RS 2 . The second recess RS 2  may further extend downwardly from the first recess RS 1 . 
     The second etching process may be an anisotropic etching process, and may be performed until a top surface of the active pattern AP is exposed. For example, the second recess RS 2  may expose the top surface of the active pattern AP. 
     Referring to  FIGS.  11 A and  11 B , lower source/drain patterns SD 1  may be correspondingly formed in the second recesses RS 2 . For example, the lower source/drain pattern SD 1  may be formed by performing a first selective epitaxial growth (SEG) process in which an inner wall of the second recess RS 2  is used as a seed layer. The lower source/drain pattern SD 1  may be grown from a seed, or the first active layers ACL 1  and the substrate  100  that are exposed to the second recess RS 2 . For example, the first SEG process may include chemical vapor deposition (CVD) or molecular beam epitaxy (MBE). 
     The lower source/drain pattern SD 1  may be in-situ doped with impurities during the first SEG process. Alternatively, after the formation of the lower source/drain pattern SD 1 , impurities may be implanted into the lower source/drain pattern SD 1 . The lower source/drain pattern SD 1  may be doped to have a first conductivity type (e.g., n-type). In an embodiment of the present inventive concept, the lower source/drain pattern SD 1  may include a silicon (Si) layer doped with n-type dopants such as, for example, phosphorus (P), arsenic (As), antimony (Sb) and/or bismuth (Bi). 
     A lower channel pattern CH 1  may be constituted by the first active layers ACL 1  interposed between a pair of lower source/drain patterns SD 1 . For example, the first active layers ACL 1  may be formed into first and second semiconductor patterns SP 1  and SP 2  of the lower channel pattern CH 1 . The first and second semiconductor patterns SP 1  and SP 2  may be connected to the pair of lower source/drain patterns SD 1 . The lower channel patterns CH 1  and the lower source/drain patterns SD 1  may constitute a first active region AR 1  as a bottom tier of a three-dimensional device. 
     Because the liner layer LIN covers the inner wall of the first recess RS 1 , no semiconductor layer may be separately formed in the first recess RS 1  during the first SEG process. 
     Referring to  FIGS.  12 A and  12 B , a first interlayer dielectric layer  110  may be formed to cover the lower source/drain patterns SD 1 . The first interlayer dielectric layer  110  may be recessed to have a top surface lower than a bottom surface of a lowermost second active layer ACL 2 . 
     The liner layer LIN exposed to the first recess RS 1  may be partially removed. The liner layer LIN covered with the first interlayer dielectric layer  110  may cover a sidewall of the second sacrificial layer SAL 2 . For example, the portion of the liner layer LIN covered by the first interlayer dielectric layer  110  may not be removed in this removal process. The removal of the liner layer LIN may allow the first recess RS 1  to expose the second active layers ACL 2 . 
     Upper source/drain patterns SD 2  may be correspondingly formed in the first recesses RS 1 . For example, the upper source/drain pattern SD 2  may be formed by performing a second selective epitaxial growth (SEG) process in which the inner wall of the first recess RS 1  is used as a seed layer. The upper source/drain pattern SD 2  may be grown from a seed, or the second active layers ACL 2  exposed to the first recess RS 1 . The upper source/drain pattern SD 2  may be in-situ doped with impurities during the second SEG process. Alternatively, after the formation of the upper source/drain pattern SD 2 , impurities may be implanted into the upper source/drain pattern SD 2 . The upper source/drain patterns SD 2  may be doped to have a second conductivity type (e.g., p-type) different from the first conductivity type. In an embodiment of the present inventive concept, the upper source/drain pattern SD 2  may include a silicon germanium (SiGe) layer doped with p-type dopants such as, for example, boron (B), aluminum (Al), gallium (Ga), and/or indium (In). Alternatively, the upper source/drain pattern SD 2  may include a silicon (Si) layer doped with p-type dopants such as, for example, boron (B), aluminum (Al), gallium (Ga), and/or indium (In). 
     An upper channel pattern CH 2  may be constituted by the second active layers ACL 2  interposed between a pair of upper source/drain patterns SD 2 . For example, the second active layers ACL 2  may be formed into third and fourth semiconductor patterns SP 3  and SP 4  of the upper channel pattern CH 2 . The third and fourth semiconductor patterns SP 3  and SP 4  may be connected to the pair of upper source/drain patterns SD 2 . The upper channel patterns CH 2  and the upper source/drain patterns SD 2  may constitute a second active region AR 2  as a top tier of a three-dimensional device. 
     Referring to  FIGS.  13 A and  13 B , a second interlayer dielectric layer  120  may be formed to cover the hardmask patterns MP, the gate spacers GS, and the upper source/drain patterns SD 2 . For example, the second interlayer dielectric layer  120  may include a silicon oxide (SiO 2 ) layer. 
     The second interlayer dielectric layer  120  may be planarized until top surfaces of the sacrificial patterns PP are exposed. An etch-back or chemical mechanical polishing (CMP) process may be employed to planarize the second interlayer dielectric layer  120 . The hardmask patterns MP may all be removed during the planarization process. As a result, the second interlayer dielectric layer  120  may have a top surface coplanar with those of the sacrificial patterns PP and those of the gate spacers GS. 
     Gate cutting patterns CT may be formed to penetrate the sacrificial patterns PP, and may be formed on first and second cell boundaries CB 1  and CB 2  of a logic cell. The gate cutting patterns CT may include one or more selected from, for example, a silicon oxide (SiO 2 ) layer and a silicon nitride (Si 3 N 4 ) layer. 
     The exposed sacrificial patterns PP may be selectively removed. The removal of the sacrificial patterns PP may form an outer region ORG that exposes the lower and upper channel patterns CH 1  and CH 2  (see  FIG.  13 B ). The removal of the sacrificial patterns PP may include performing a wet etching process using an etchant that selectively etches polysilicon (p-Si). 
     A dummy channel pattern DSP may replace the second sacrificial layer SAL 2  exposed to the outer region ORG. For example, the formation of the dummy channel pattern DSP may include using the outer region ORG to selectively remove the second sacrificial layer SAL 2 , and forming the dummy channel pattern DSP in an area where the second sacrificial layer SAL 2  is removed. Since the second sacrificial layer SAL 2  may have a high germanium (Ge) concentration of about 40 at % to about 90 at %, it may have high etch selectivity with respect to other components and may be selectively removed with an etching process. For example, the removal of the second sacrificial layer SAL 2  may include performing a wet etching process using an etchant that selectively etches silicon germanium (SiGe) having high germanium (Ge) concentration. According to an embodiment of the present inventive concept, the dummy channel pattern DSP may include a dielectric material, such as, for example, a silicon oxide (SiO 2 ) layer or a silicon nitride (Si 3 N 4 ) layer. 
     Referring to  FIGS.  14 A and  14 B , the first and third sacrificial layers SAL 1  and SAL 3  exposed to the outer region ORG may be selectively removed to form first to fifth inner regions IRG 1  to IRG 5  (see  FIG.  14 B ). For example, an etching process may be performed in which the first and third sacrificial layers SAL 1  and SAL 3  are selectively etched, such that the first and third sacrificial layers SAL 1  and SAL 3  may be removed while leaving the first to fourth semiconductor patterns SP 1  to SP 4 . The etching process may have a high etch rate with respect to silicon-germanium (SiGe) whose germanium (Ge) concentration is relatively high. For example, the etching process may have a high etch rate with respect to silicon-germanium (SiGe) whose germanium (Ge) concentration is greater than about 10 at %. For example, to form the first to fifth inner regions IRG 1  to IRG 5 , the first and third sacrificial layers SAL 1  and SAL 3  may be selectively etched by using an etching selectivity difference between the first sacrificial layers SAL 1  and the first active layers ACL 1  (i.e., the first and second semiconductor patterns SP 1  and SP 2 ) and between the third sacrificial layers SAL 3  and the second active layers ACL 2  (i.e., the third and fourth semiconductor patterns SP 3  and SP 4 ). In an embodiment of the present inventive concept, the first to fifth inner regions IRG 1  to IRG 5  may be formed by a wet etching process. 
     The selective removal of the first and third sacrificial layers SAL 1  and SAL 3  may cause the first and second semiconductor patterns SP 1  and SP 2  to remain on the first active region AR 1 , and may also cause the third and fourth semiconductor patterns SP 3  and SP 4  to remain on the second active region AR 2 . A first inner region IRG 1  may be defined to indicate an empty space between the active pattern AP and the first semiconductor pattern SP 1 , a second inner region IRG 2  may be defined to indicate an empty space between the first semiconductor pattern SP 1  and the second semiconductor pattern SP 2 , and a third inner region IRG 3  may be defined to indicate an empty space between the second semiconductor pattern SP 2  and the dummy channel pattern DSP. A fourth inner region IRG 4  may be defined to indicate an empty space between the dummy channel pattern DSP and the third semiconductor pattern SP 3 , and a fifth inner region IRG 5  may be defined to indicate an empty space between the third semiconductor pattern SP 3  and the fourth semiconductor pattern SP 4 . 
     Referring to  FIGS.  15 A,  15 B, and  15 C , gate dielectric layers UGI and LGI may be conformally formed on the exposed first to fourth semiconductor patterns SP 1  to SP 4 . For example, a lower gate dielectric layer LGI may be formed on the first and second semiconductor patterns SP 1  and SP 2 , and an upper gate dielectric layer UGI may be formed on the third and fourth semiconductor patterns SP 3  and SP 4 . 
     A lower gate electrode LGE may be formed on the lower gate dielectric layer LGI. The formation of the lower gate electrode LGE may include forming first, second, and third portions PO 1 , PO 2  and PO 3  in the first, second, and third inner regions IRG 1 , IRG 2 , and IRG 3 , respectively. An upper gate electrode UGE may be formed on the upper gate dielectric layer UGI. The formation of the upper gate electrode UGE may include respectively forming fourth and fifth portions PO 4  and PO 5  in the fourth and fifth inner regions IRG 4  and IRG 5 , and forming a sixth portion PO 6  in the outer region ORG. The lower gate electrode LGE and the upper gate electrode UGE may be connected to each other to form a single first gate electrode GE 1 . 
     The formation of the second gate electrode GE 2  may further include forming an isolation pattern ISP between the lower gate electrode LGE and the upper gate electrode UGE. The isolation pattern ISP may separate the lower and upper gate electrodes LGE and UGE from each other. For example, in the formation of the second gate electrode, the lower gate electrode LGE that fills the first, second, and third inner regions IRG 1 , IRG 2 , and IRG 3  may be formed first, then the isolation pattern ISP may be formed on the lower gate electrode LGE and recessed to allow a top surface of the isolation pattern ISP to rest at a level between levels of top and bottom surfaces of the dummy channel pattern DSP, and the upper gate electrode UGE that fills the fourth and fifth inner regions IRG 4  and IRG 5  may be formed on the isolation pattern ISP. 
     The first and second gate electrodes GE 1  and GE 2  may be recessed to have their reduced heights. Gate capping patterns GP may be formed on the recessed first and second gate electrodes GE 1  and GE 2 . The gate capping pattern GP may undergo a planarization process to allow the gate capping pattern GP to have a top surface coplanar with that of the second interlayer dielectric layer  120 . 
     Referring back to  FIGS.  5  and  6 A to  6 E , a third interlayer dielectric layer  130  may be formed on the second interlayer dielectric layer  120 . A first active contact AC 1  may be formed to penetrate the first, second, and third interlayer dielectric layers  110 ,  120 , and  130  and may be coupled to the lower source/drain pattern SD 1 . A second active contact AC 2  may be formed to penetrate the second and third interlayer dielectric layers  120  and  130  and may be coupled to the upper source/drain pattern SD 2 . 
     An upper gate contact UGC may be formed to penetrate the third interlayer dielectric layer  130  and the gate capping pattern GP, and may be coupled to the upper gate electrode UGE. A lower gate contact LGC may be formed to penetrate the third interlayer dielectric layer  130 , the gate capping pattern GP, and the upper gate electrode UGE, and may be coupled to the lower gate electrode LGE. 
     A fourth interlayer dielectric layer  140  may be formed on the third interlayer dielectric layer  130 . A first metal layer M 1  may be formed in the fourth interlayer dielectric layer  140 . The formation of the first metal layer M 1  may include forming first and second power lines POR 1  and POR 2  and first to fourth wiring lines MI 1  to MI 4  on an upper portion of the fourth interlayer dielectric layer  140 . Each of the first and second power lines POR 1  and POR 2  and first to fourth wiring lines MI 1  to MI 4  may extend in the second direction D 2  on the logic cell of the logic circuit LC. 
     A via VI may be formed beneath each of the first and second power lines POR 1  and POR 2  and the first to fourth wiring lines MI 1  to MI 4 . The active contacts AC 1  and AC 2  and the gate contacts UGC and LGC may be electrically connected through the vias VI to the first metal layer M 1 . 
     In an embodiment of the present inventive concept, the formation of the vias VI may be followed by the formation of the first and second power lines POR 1  and POR 2  and the first to fourth wiring lines MI 1  to MI 4 . In an embodiment of the present inventive concept, a dual damascene process may be adopted to form the vias VI together with the first and second power lines POR 1  and POR 2  and the first to fourth wiring lines MI 1  to MI 4 . 
     Additional metal layers may be formed on the first metal layer M 1 . The first metal layer M 1  and other additional metal layers on the first metal layer M 1  may constitute a back-end-of-line (BEOL) layer of a semiconductor device. 
       FIGS.  16  to  20    illustrate cross-sectional views taken along line C-C′ of  FIG.  5   , showing a method of forming a second gate electrode according to an embodiment of the present inventive concept. 
     Referring to  FIG.  16   , gate dielectric layers UGI and LGI may be conformally formed on a result structure of  FIG.  14 B . The gate dielectric layers UGI and LGI may include a lower gate dielectric layer LGI on the first and second semiconductor patterns SP 1  and SP 2  and the dummy channel pattern DSP, and may also include an upper gate dielectric layer UGI on the third and fourth semiconductor patterns SP 3  and SP 4 . For example, the lower gate dielectric layer LGI may cover top, bottom, and opposite side surfaces of each of the first and second semiconductor patterns SP 1  and SP 2  and the dummy channel pattern DSP, and the upper gate dielectric layer UGI may cover top, bottom, and opposite side surfaces of each of the third and fourth semiconductor patterns SP 3  and SP 4 . 
     The formation of the lower and upper gate dielectric layers LGI and UGI may include forming a silicon oxide (SiO 2 ) layer on surfaces of the first to fourth semiconductor patterns SP 1  to SP 4 , and forming a high-k dielectric layer on the silicon oxide (SiO 2 ) layer. 
     A dipole-containing layer DPL may be selectively formed only on the lower gate dielectric layer LGI. For example, the dipole-containing layer DPL may be conformally formed on the lower and upper gate dielectric layers LGI and UGI. A mask layer MA may be formed to cover the lower gate dielectric layer LGI and to expose the upper gate dielectric layer UGI. The mask layer MA may have a top surface at a level similar to that of a top surface of the dummy channel pattern DSP. The mask layer MA may be used as an etching mask to selectively remove the dipole-containing layer DPL on the upper gate dielectric layer UGI. Therefore, the dipole-containing layer DPL may selectively remain only on the lower gate dielectric layer LGI, and not on the upper gate dielectric layer UGI. 
     The dipole-containing layer DPL may include a dipole element. The dipole element may include, for example, lanthanum (La), aluminum (Al), or a combination thereof. For example, the dipole-containing layer DPL may include, for example, a lanthanum oxide (La 2 O 3 ) layer, an aluminum oxide (Al 2 O 3 ) layer, or a combination thereof. 
     Afterwards, the mask layer MA may be removed, and then the dipole-containing layer DPL may undergo an annealing process to diffuse the dipole element from the dipole-containing layer DPL into the lower gate dielectric layer LGI. Therefore, a dipole-interface that may be formed between a silicon oxide (SiO 2 ) layer and a high-k dielectric layer that are included in the lower gate dielectric layer. The dipole-element diffused into the lower gate dielectric layer LGI may adjust an effective work function of a lower gate electrode LGE which will be formed subsequently. 
     During the annealing process, the dipole-containing layer DPL may discharge the dipole element and at the same time may be removed. The dipole-containing layer DPL may be formed to have an extremely small thickness such as about 1 nm, and thus may be easily removed. 
     Referring to  FIG.  17   , a first metal pattern MP 1  may be conformally formed on the lower and upper gate dielectric layers LGI and UGI. The first metal pattern MP 1  may surround the first to fourth semiconductor patterns SP 1  to SP 4 . 
     The first metal pattern MP 1  may include a first work-function metal (e.g., p-type work-function metal). The formation of the first metal pattern MP 1  may include conformally depositing a metal nitride layer on the lower and upper gate dielectric layers LGI and UGI. For example, the first metal pattern MP 1  may include, for example, titanium nitride (TiN), tantalum nitride (TaN), titanium oxynitride (TiON), titanium silicon nitride (TiSiN), titanium aluminum nitride (TiAlN), tungsten carbon nitride (WCN), or molybdenum nitride (MoN). 
     A second metal pattern MP 2  may be formed on the first metal pattern MP 1 . The second metal pattern MP 2  may be formed to completely fill the first to fifth inner regions IRG 1  to IRG 5 . The second metal pattern MP 2  may be formed also in the outer region ORG. 
     The second metal pattern MP 2  may include a second work-function metal (e.g., n-type work-function metal). The formation of the second metal pattern MP 2  may include depositing, on the first metal pattern MP 1 , a metal carbide doped with (or containing) one or more selected from, for example, silicon (Si) and aluminum (Al). For example, the second metal pattern MP 2  may include, for example, aluminum-doped titanium carbide (TiAlC), aluminum-doped tantalum carbide (TaAlC), aluminum-doped vanadium carbide (VAlC), silicon-doped titanium carbide (TiSiC), or silicon-doped tantalum carbide (TaSiC). 
     The second metal pattern MP 2  may undergo an etching process to selectively remove the second metal pattern MP 2  in the outer region ORG. For example, the etching process may remove the second metal pattern MP 2  in the outer region ORG, and may leave the second metal pattern MP 2  in the first to fifth inner regions IRG 1  to IRG 5 . 
     Referring to  FIG.  18   , a sixth metal pattern MP 6  may be formed in a lower portion of the outer region ORG. For example, the sixth metal pattern MP 6  may be formed on the first and second metal patterns MP 1  and MP 2 , filling the outer region ORG. Thereafter, the sixth metal pattern MP 6  may be recessed to allow its top surface to rest at a first level LV 1 . For example, the first level LV 1  may be the same as or higher than a top surface of the first semiconductor pattern SP 1 , and the same as or lower than a bottom surface of the second semiconductor pattern SP 2 . The sixth metal pattern MP 6  may include at least one of metals, whose resistances are low, such as, for example, tungsten (W), ruthenium (Ru), aluminum (Al), titanium (Ti), and tantalum (Ta). 
     Referring to  FIG.  19   , the sixth metal pattern MP 6  may be used as an etching mask to etch and selectively remove an exposed portion of the first metal pattern MP 1 . The second metal pattern MP 2  may not be removed, and thus the first metal pattern MP 1  between the second metal pattern MP 2  and the first to fourth semiconductor patterns SP 1  to SP 4  may not be removed. In addition, the second metal pattern MP 2  may also remain which is positioned beneath the top surface of the sixth metal pattern MP 6 . 
     An exposed portion of the first metal pattern MP 1  may be removed to form a lower gate electrode LGE. The lower gate electrode LGE may include first, second, and third portions PO 1 , PO 2 , and PO 3  that are respectively formed in the first, second, and third inner regions IRG 1 , IRG 2 , and IRG 3 . Each of the first, second, and third portions PO 1 , PO 2 , and PO 3  may include a first metal pattern MP 1  and a second metal pattern MP 2 . The lower gate electrode LGE may further include a sixth metal pattern MP 6  formed in a lower portion of the outer region ORG. 
     Referring to  FIG.  20   , an isolation pattern ISP may be formed on the sixth metal pattern MP 6 . The isolation pattern ISP may be formed to have a top surface at a level between those of top and bottom surfaces of the dummy channel pattern DSP. 
     The isolation pattern ISP may be used as an etching mask to remove the first and second metal patterns MP 1  and MP 2  formed in the fourth and fifth inner regions IRG 4  and IRG 5 . Therefore, the upper gate dielectric layer UGI may be exposed. 
     Referring back to  FIG.  15 B , a third metal pattern MP 3  may be formed on the upper gate dielectric layer UGI. The third metal pattern MP 3  may be formed to have a thickness that completely fills the fourth and fifth inner regions IRG 4  and IRG 5 . The third metal pattern MP 3  may include a metal nitride layer, as a first work-function metal, the same as or different from that of the first metal pattern MP 1 . 
     A fourth metal pattern MP 4  may be formed on the third metal pattern MP 3 , partially filling the outer region ORG. The fourth metal pattern MP 4  may include metal carbide, as a second work-function metal, the same as or different from that of the second metal pattern MP 2 . 
     A fifth metal pattern MP 5  may be formed on the fourth metal pattern MP 4 , filling an unoccupied portion of the outer region ORG. The fifth metal pattern MP 5  may include a first work-function metal (e.g., titanium nitride (TiN)) or a low-resistance metal (e.g., tungsten (W)). In an embodiment of the present inventive concept, the fifth metal pattern MP 5  may include a metal the same as that of the sixth metal pattern MP 6 . 
     The third to fifth metal patterns MP 3  to MP 5  may be formed on the isolation pattern ISP, and thus an upper gate electrode UGE may be formed. The upper gate electrode UGE may include fourth and fifth portions PO 4  and PO 5  that are respectively formed in the fourth and fifth inner regions IRG 4  and IRG 5 . Each of the fourth and fifth portions PO 4  and PO 5  may include a third metal pattern MP 3 . The upper gate electrode UGE may further include a sixth portion PO 6  formed in the outer region ORG. The sixth portion PO 6  may include third, fourth, and fifth metal patterns MP 3 , MP 4 , and MP 5  that are sequentially stacked. 
       FIGS.  21  and  22    illustrate cross-sectional views taken along line D-D′ of  FIG.  5   , showing a method of forming a first gate electrode according to an embodiment of the present inventive concept. 
     Referring to  FIG.  21   , a sixth metal pattern MP 6  may be formed on a resultant structure of  FIG.  17   . The sixth metal pattern MP 6  may be recessed to allow its top surface to rest at a second level LV 2 . The second level LV 2  may be higher than the first level LV 1 . The second level LV 2  may be positioned between top and bottom surfaces of the dummy channel pattern DSP. 
     Referring to  FIGS.  22   , the sixth metal pattern MP 6  may be used as an etching mask to remove the first and second metal patterns MP 1  and MP 2 . The upper gate dielectric layer UGI may thus be exposed. 
     The first and second metal patterns MP 1  and MP 2  may remain below the top surface of the sixth metal pattern MP 6 , and thus a lower gate electrode LGE may be formed. The lower gate electrode LGE may include first, second, and third portions PO 1 , PO 2 , and PO 3  that are respectively formed in the first, second, and third inner regions IRG 1 , IRG 2 , and IRG 3 . Each of the first, second, and third portions PO 1 , PO 2 , and PO 3  may include a first metal pattern MP 1  and a second metal pattern MP 2 . The lower gate electrode LGE may further include a sixth metal pattern MP 6  formed in a lower portion of the outer region ORG. 
     Unlike the formation of the second gate electrode GE 2  shown in  FIG.  20   , the formation of the first gate electrode GE 1  of  FIG.  22    may not include the formation of the isolation pattern ISP or may include the complete removal of the isolation pattern ISP that has been formed. For example, the first gate electrode GE 1  is a common gate electrode which does not have the isolation pattern ISP, and the second gate electrode GE 2  is a split gate electrode which has the isolation pattern ISP. 
     Referring back to  FIG.  15 C , a third metal pattern MP 3  may be formed on the upper gate dielectric layer UGI. The third metal pattern MP 3  may be formed to have a thickness that completely fills the fourth and fifth inner regions IRG 4  and IRG 5 . A fourth metal pattern MP 4  may be formed on the third metal pattern MP 3 , partially filling the outer region ORG. A fifth metal pattern MP 5  may be formed on the fourth metal pattern MP 4 , filling an unoccupied portion of the outer region ORG. The third to fifth metal patterns MP 3  to MP 5  may constitute an upper gate electrode UGE. 
     The isolation pattern ISP may be omitted, and thus the upper gate electrode UGE of the first gate electrode GE 1  may be directly formed on and connected to the lower gate electrode LGE of the first gate electrode GE 1 . 
     A three-dimensional semiconductor device according to the present inventive concept may achieve a cross-couple structure by using an isolation pattern between a lower gate electrode and an upper gate electrode. Only two split gates may accomplish the cross-couple structure, and thus there may be a reduction in area of a flip-flop cell. As a result, integration of the three-dimensional semiconductor device according to the present inventive concept may be increased. 
     Although the present inventive concept has been described in connection with some specific embodiments of the present inventive concept illustrated in the accompanying drawings, it will be understood by one of ordinary skill in the art that variations in form and detail may be made therein without departing from the spirit and scope of the present inventive concept as defined in the appended claims. The above disclosed embodiments of the present inventive concept should thus be considered illustrative and not restrictive.