Patent Publication Number: US-2023135219-A1

Title: Resistor structures of integrated circuit devices including stacked transistors and methods of forming the same

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to U.S. Provisional Application Ser. No. 63/273,246, entitled RESISTORS OF 3D INTEGRATED CIRCUIT DEVICES AND METHODS OF FORMING THE SAME, filed in the USPTO on Oct. 29, 2021, the disclosure of which is hereby incorporated by reference herein in its entirety. 
    
    
     FIELD 
     The present disclosure generally relates to the field of electronics and, more particularly, to integrated circuit devices including stacked transistors. 
     BACKGROUND 
     Integrated circuit devices may include resistors having various resistance values for different purposes. For example, a DC bias circuit may include a resistor having a high resistance value (e.g., about 10 kiloohms), and a feedback path for reducing an IR drop or a load may include a resistor having a low resistance value (e.g., about 100 ohms or lower). Integrated circuit devices including stacked transistors, such as a complementary field effect transistor (CFET) stack, were introduced to reduce the area of integrated circuit devices, thereby increasing the integration density. Accordingly, resistor structures that can be formed by manufacturing processes compatible with manufacturing processes of stacked transistors and can have various resistance values may be desirable. 
     SUMMARY 
     According to some embodiments of the present invention, resistor structures may include a substrate, an upper semiconductor layer that may be spaced apart from the substrate in a vertical direction, a lower semiconductor layer that may be between the substrate and the upper semiconductor layer, and first and second resistor contacts that may be spaced apart from each other in a horizontal direction. At least one of the upper semiconductor layer, the lower semiconductor layer, and a portion of the substrate may contact the first and second resistor contacts. In some embodiments, an integrated circuit device may include the resistor structure and a stacked transistor structure. The stacked transistor structure may include an upper transistor comprising an upper source/drain region and a lower transistor that may be between the substrate and the upper transistor and may include a lower source/drain region. The the upper semiconductor layer and the upper source/drain region may include the same material and may have an equal thickness in the vertical direction, and the lower semiconductor layer and the lower source/drain region may include the same material and may have an equal thickness in the vertical direction. 
     According to some embodiments of the present invention, resistor structures may include a substrate, an upper semiconductor layer that may be spaced apart from the substrate in a vertical direction, and a lower semiconductor layer that may be between the substrate and the upper semiconductor layer. At least two of the upper semiconductor layer, the lower semiconductor layer, and a portion of the substrate may be configured to function as respective resistors that are connected in parallel. 
     According to some embodiments of the present invention, methods of forming an integrated circuit device may include forming a resistor structure. Forming the resistor structure may include forming a lower thin semiconductor layer and an upper thin semiconductor layer on a substrate. The lower thin semiconductor layer may be spaced apart from the substrate in a vertical direction and may be between the substrate and the upper thin semiconductor layer. Forming the resistor structure may also include forming a lower semiconductor layer by performing a first epitaxial growth process using the lower thin semiconductor layer as a first seed layer, forming an upper semiconductor layer by performing a second epitaxial growth process using the upper thin semiconductor layer as a second seed layer, and forming first and second resistor contacts that may be spaced apart from each other in a horizontal direction and may contact at least one of the upper semiconductor layer, the lower semiconductor layer, and a portion of the substrate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS.  1  to  3    are schematic illustrations of first resistor structures according to some embodiments of the present invention. 
         FIG.  4    is a plan view of an integrated circuit device according to some embodiments of the present invention. 
         FIGS.  5  and  6    are side views of the integrated circuit device from viewpoints V and W in  FIG.  4   , respectively, according to some embodiments of the present invention. 
         FIGS.  7  and  8    are cross-sectional views of the integrated circuit device taken along the line A-A′ and the line B-B′ in  FIG.  4   , respectively, according to some embodiments of the present invention. 
         FIGS.  9  through  12    are side views of the integrated circuit device from the viewpoint V in  FIG.  4    according to some embodiments of the present invention. 
         FIG.  13    is a plan view of a resistor structure according to some embodiments of the present invention. 
         FIG.  14    is a schematic illustration of the resistor structure in  FIG.  13    according to some embodiments of the present invention. 
         FIG.  15    is a side view of the resistor structure from a viewpoint V in  FIG.  13    according to some embodiments of the present invention. 
         FIG.  16    is a plan view of an integrated circuit device according to some embodiments of the present invention. 
         FIGS.  17  to  19    are cross-sectional views of the integrated circuit device taken along the line D-D′ in  FIG.  16    according to some embodiments of the present invention. 
         FIG.  20    is a flow chart of methods of forming an integrated circuit device according to some embodiments of the present invention. 
         FIGS.  21 - 23    are cross-sectional views taken along the line B-B′ and the line C-C′ in  FIG.  4    illustrating methods of forming an integrated circuit device according to some embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Resistor structures and methods of forming the same pursuant to embodiments of the present invention can simplify manufacturing processes of an integrated circuit device that includes both resistor structures and stacked transistors, as common manufacturing processes can be used to form elements of the resistor structures and stacked transistors. 
     Processes of forming stacked transistors may form multiple semiconductor elements (e.g., a first upper semiconductor layer  26 U_ 1 , a first lower semiconductor layer  26 L_ 1  and a first portion  12 _ 1  of a substrate  10  in  FIG.  5   ), each of which can be used as a resistor, and may also form conductive contacts (e.g., a first resistor contact  32 _ 1  and a second resistor contact  32 _ 2  in  FIG.  5   ) that are electrically connected to at least one of those three semiconductor elements and thus can be used as resistor contacts. Accordingly, a resistor structure according to some embodiments of the present invention may be formed by processes of forming stacked transistors without additional processes. 
     According to some embodiments of the present invention, resistor elements may be formed to have different resistance values by adjusting, for example, a material and/or an impurity concentration, and resistor structures having various resistance values may be formed by electrically connecting different sets of those resistor elements. 
     According to some embodiments of the present invention, a resistor structure may include multiple resistor elements that are electrically connected in parallel and have different respective resistance values, and thus it may be easier to reduce a deviation of a resistance value of the resistor structure from a pre-determined resistance value. 
     As used herein the term “and/or” includes any and all combinations of one or more of the associated listed items. Further, as used herein, “formed concurrently” refers to being formed by the same fabrication process(es), at approximately (but not necessarily exactly) the same time. 
       FIGS.  1  to  3    are schematic illustrations of various first resistor structures RS 1 . Referring to  FIG.  1   , a first resistor structure RS 1 _ 1  may include a first resistor element R 1 , a second resistor element R 2  and a third resistor element R 3  that are all electrically connected to a first resistor contact RC 1  and a second resistor contact RC 2 . The first, second and third resistor elements R 1 , R 2 , and R 3  may be electrically connected to each other in parallel. 
     For example, the first, second and third resistor elements R 1 , R 2 , and R 3  may be an upper semiconductor layer (e.g., a first upper semiconductor layer  26 U_ 1  in  FIG.  5   ), a lower semiconductor layer (e.g., a first lower semiconductor layer  26 L_ 1  in  FIG.  5   ) and a portion of a substrate (e.g., a first portion  12 _ 1  of a substrate  10  in  FIG.  5   ), respectively. 
     Referring to  FIG.  2   , the first and second resistor contacts RC 1  and RC 2  of each of the first resistor structures RS 1 _ 2 , RS 1 _ 3  and RS 1 _ 4  may be electrically connected to two of the first, second and third resistor elements R 1 , R 2 , and R 3 , and may be electrically isolated from the remaining one of the first, second and third resistor elements R 1 , R 2 , and R 3 . 
     Referring to  FIG.  3   , the first and second resistor contacts RC 1  and RC 2  of each of the first resistor structures RS 1 _ 5 , RS 1 _ 6  and RS 1 _ 7  may be electrically connected to one of the first, second and third resistor elements R 1 , R 2 , and R 3  and may be electrically isolated from the remaining two of the first, second and third resistor elements R 1 , R 2 , and R 3 . The first resistor structures RS 1  illustrated in  FIGS.  1  to  3    may have various resistance values, as different sets of the first, second and third resistor elements R 1 , R 2 , and R 3  are electrically connected to each other. 
       FIG.  4    is a plan view of an integrated circuit device according to some embodiments of the present invention.  FIGS.  5  and  6    are side views of the integrated circuit device from viewpoints V and W in  FIG.  4   , respectively, according to some embodiments of the present invention. Specifically,  FIG.  5    shows an example configuration of elements of the first resistor structure RS 1 _ 1  in  FIG.  1   .  FIGS.  7  and  8    are cross-sectional views of the integrated circuit device taken along the line A-A′ and the line B-B′ in  FIG.  4   , respectively, according to some embodiments of the present invention. For simplicity of illustration,  FIG.  4    does not show some elements (e.g., conductive wires  36  and an interlayer insulating layer  42 ) in  FIGS.  5 - 8   . 
     Referring to  FIGS.  4  through  8   , the integrated circuit device may include a stacked transistor structure ST on a substrate  10  and a first resistor structure RS 1 . The substrate  10  may include a first surface S 1  and a second surface S 2 . The first surface S 1  and the second surface S 2  may be opposite and parallel to each other. The first surface S 1  may be a front side of the substrate  10 , and the second surface S 2  may be a backside of the substrate  10 . 
       FIG.  4    illustrates that the stacked transistor structure ST is spaced apart from the first resistor structure RS 1  in a first direction D 1 , but the present invention is not limited thereto. In some embodiments, the stacked transistor structure ST may be spaced apart from the first resistor structure RS 1  in any direction. The first direction D 1  may be parallel to the first surface S 1  and the second surface S 2  of the substrate  10  and may be a first horizontal direction. 
     The first resistor structure RS 1  may include a first upper semiconductor layer  26 U_ 1  and a second upper semiconductor layer  26 U_ 2  that may be spaced apart from the first upper semiconductor layer  26 U_ 1  in a second direction D 2 . The second direction D 2  may be parallel to the first surface S 1  and the second surface S 2  of the substrate  10  and may be a second horizontal direction. The second direction D 2  may be different from the first direction D 1 , and in some embodiments, the second direction D 2  may be perpendicular to the first direction D 1 . The first upper semiconductor layer  26 U_ 1  and the second upper semiconductor layer  26 U_ 2  may have a first conductivity type (e.g., an N-type conductivity or a P-type conductivity). 
     Each of the first upper semiconductor layer  26 U_ 1  and the second upper semiconductor layer  26 U_ 2  may have a first thickness T 1  in a third direction D 3 . The third direction D 3  may be perpendicular to the first and second directions D 1  and D 2  and may be a vertical direction. 
     A first upper thin semiconductor layer  22 U_ 1  may be provided between the first upper semiconductor layer  26 U_ 1  and the second upper semiconductor layer  26 U_ 2  and may contact both the first upper semiconductor layer  26 U_ 1  and the second upper semiconductor layer  26 U_ 2 . In some embodiments, the first upper thin semiconductor layer  22 U_ 1  may contact side surfaces of the first upper semiconductor layer  26 U_ 1  and the second upper semiconductor layer  26 U_ 2  as illustrated in  FIG.  8   . The first upper thin semiconductor layer  22 U_ 1  may have a conductivity type that is different from the conductivity type of the first upper semiconductor layer  26 U_ 1  and the second upper semiconductor layer  26 U_ 2 . In some embodiments, the first upper thin semiconductor layer  22 U_ 1  may include a material different from the first upper semiconductor layer  26 U_ 1  and the second upper semiconductor layer  26 U_ 2 . For example, the first upper thin semiconductor layer  22 U_ 1  may include a silicon layer, and each of the first upper semiconductor layer  26 U_ 1  and the second upper semiconductor layer  26 U_ 2  may include a silicon germanium layer. 
     The first upper thin semiconductor layer  22 U_ 1  may have a third thickness T 3  in the third direction D 3 , and the third thickness T 3  may be thinner than the first thickness T 1 . For example, the third thickness T 3  may be in a range of from 1 nm to 100 nm, and the first thickness T 1  may be at least 1.5 times the third thickness T 3 . 
     The first resistor structure RS 1  may also include a first lower semiconductor layer  26 L_ 1  and a second lower semiconductor layer  26 L_ 2  that may be spaced apart from the first lower semiconductor layer  26 L_ 1  in the second direction D 2 . A first lower thin semiconductor layer  22 L_ 1  may be provided between the first lower semiconductor layer  26 L_ 1  and the second lower semiconductor layer  26 L_ 2  and may contact both the first lower semiconductor layer  26 L_ 1  and the second lower semiconductor layer  26 L_ 2 . In some embodiments, the first lower thin semiconductor layer  22 L_ 1  may contact side surfaces of the first lower semiconductor layer  26 L_ 1  and the second lower semiconductor layer  26 L_ 2  as illustrated in  FIG.  8   . 
     The first lower semiconductor layer  26 L_ 1  and the second lower semiconductor layer  26 L_ 2  may have a second conductivity type that is different from the first conductivity type. The first lower thin semiconductor layer  22 L_ 1  may have a conductivity type that is different from the conductivity type of the first lower semiconductor layer  26 L_ 1  and the second lower semiconductor layer  26 L_ 2 . In some embodiments, the first lower thin semiconductor layer  22 L_ 1  may include a material different from the first lower semiconductor layer  26 L_ 1  and the second lower semiconductor layer  26 L_ 2 . For example, the first lower thin semiconductor layer  22 L_ 1  may include a silicon layer, and each of the first lower semiconductor layer  26 L_ 1  and the second lower semiconductor layer  26 L_ 2  may include a silicon germanium layer. 
     Each of the first lower semiconductor layer  26 L_ 1  and the second lower semiconductor layer  26 L_ 2  may have a second thickness T 2  in the third direction D 3 . The first lower thin semiconductor layer  22 L_ 1  may have a fourth thickness T 4  in the third direction D 3 , and the fourth thickness T 4  may be thinner than the second thickness T 2 . For example, the fourth thickness T 4  may be in a range of, for example, from 1 nm to 100 nm, and the second thickness T 2  may be at least 1.5 times the fourth thickness T 4 . 
     Further, the first resistor structure RS 1  may include a first portion  12 _ 1  of the substrate  10 . The first portion  12 _ 1  of the substrate  10  may include impurities (e.g., boron, aluminum, gallium, indium, phosphorus or arsenic) and may have the first conductivity type or the second conductivity type. The first portion  12 _ 1  of the substrate  10  may have an impurity concentration in a range of from 10 11  cm −3  to 10 15  cm −3  (e.g., from 10 12  cm −3  to 10 14  cm −3 ). 
     The first resistor structure RS 1  may additionally include a first contact  32 _ 1  (also referred to as a first resistor contact) and a second contact  32 _ 2  (also referred to as a second resistor contact). The first contact  32 _ 1  and the second contact  32 _ 2  may be spaced apart from each other in the first direction D 1 . Each of the first contact  32 _ 1  and the second contact  32 _ 2  may contact the first upper semiconductor layer  26 U_ 1 , the first lower semiconductor layer  26 L_ 1  and the first portion  12 _ 1  of the substrate  10 , and thus the first upper semiconductor layer  26 U_ 1 , the first lower semiconductor layer  26 L_ 1  and the first portion  12 _ 1  of the substrate  10  may be electrically connected in parallel. 
     Although  FIG.  4    illustrates that the first contact  32 _ 1  and the second contact  32 _ 2  respectively contact opposing ends of the first upper semiconductor layer  26 U_ 1  and the first lower semiconductor layer  26 L_ 1 , the present invention is not limited thereto. In some embodiments, at least one of the first contact  32 _ 1  and the second contact  32 _ 2  may contact a side surface of the the first upper semiconductor layer  26 U_ 1 , which extends in the first direction D 1 , or a side surface of the first lower semiconductor layer  26 L_ 1 , which extends in the first direction D 1 . 
     In some embodiments, a first metal layer  24 _ 1  may be provided between the first lower semiconductor layer  26 L_ 1  and the second lower semiconductor layer  26 L_ 2  and between the first upper semiconductor layer  26 U_ 1  and the second upper semiconductor layer  26 U_ 2 . The first metal layer  24 _ 1  may be spaced apart from the first lower semiconductor layer  26 L_ 1 , the second lower semiconductor layer  26 L_ 2 , the first upper semiconductor layer  26 U_ 1  and the second upper semiconductor layer  26 U_ 2  in the second direction D 2  as illustrated in  FIGS.  4  and  8   . A portion of the first lower thin semiconductor layer  22 L_ 1  and a portion of the first upper thin semiconductor layer  22 U_ 1  may be in the first metal layer  24 _ 1  as illustrated in  FIGS.  4 ,  7  and  8   . 
     Still referring to  FIGS.  4  through  8   , the stacked transistor structure ST may include a third lower semiconductor layer  26 L_ 3  and a fourth lower semiconductor layer  26 L_ 4  that may be spaced apart from the third lower semiconductor layer  26 L_ 3  in the second direction D 2 . A second lower thin semiconductor layer  22 L_ 2  may be provided between the third lower semiconductor layer  26 L_ 3  and the fourth lower semiconductor layer  26 L_ 4  and may contact both the third lower semiconductor layer  26 L_ 3  and the fourth lower semiconductor layer  26 L_ 4 . In some embodiments, the second lower thin semiconductor layer  22 L_ 2  may contact side surfaces of the third lower semiconductor layer  26 L_ 3  and the fourth lower semiconductor layer  26 L_ 4 . The third lower semiconductor layer  26 L_ 3  and the fourth lower semiconductor layer  26 L_ 4  may have the second conductivity type. The second lower thin semiconductor layer  22 L_ 2  may be a lower active layer of a lower transistor, and the third lower semiconductor layer  26 L_ 3  and the fourth lower semiconductor layer  26 L_ 4  may be lower source/drain regions of the lower transistor. 
     Each of the third lower semiconductor layer  26 L_ 3  and the fourth lower semiconductor layer  26 L_ 4  may have the second thickness T 2  in the third direction D 3 . The second lower thin semiconductor layer  22 L_ 2  may have the fourth thickness T 4  in the third direction D 3 . 
     The stacked transistor structure ST may also include a third upper semiconductor layer  26 U_ 3  and a fourth upper semiconductor layer  26 U_ 4  that may be spaced apart from the third upper semiconductor layer  26 U_ 3  in the second direction D 2 . A second upper thin semiconductor layer  22 U_ 2  may be provided between the third upper semiconductor layer  26 U_ 3  and the fourth upper semiconductor layer  26 U_ 4  and may contact both the third upper semiconductor layer  26 U_ 3  and the fourth upper semiconductor layer  26 U_ 4 . In some embodiments, the second upper thin semiconductor layer  22 U_ 2  may contact side surfaces of the third upper semiconductor layer  26 U_ 3  and the fourth upper semiconductor layer  26 U_ 4 . The third upper semiconductor layer  26 U_ 3  and the fourth upper semiconductor layer  26 U_ 4  may have the first conductivity type. The second upper thin semiconductor layer  22 U_ 2  may be an upper active layer of an upper transistor, and the third upper semiconductor layer  26 U_ 3  and the fourth upper semiconductor layer  26 U_ 4  may be upper source/drain regions of the upper transistor. 
     Each of the third upper semiconductor layer  26 U_ 3  and the fourth upper semiconductor layer  26 U_ 4  may have the first thickness T 1  in the third direction D 3 . The second upper thin semiconductor layer  22 U_ 2  may have the third thickness T 3  in the third direction D 3 . 
     In some embodiments, a lower surface of the first upper semiconductor layer  26 U_ 1  and a lower surface of the third upper semiconductor layer  26 U_ 3  may be coplanar with each other, and a lower surface of the first lower semiconductor layer  26 L_ 1  and a lower surface of the third lower semiconductor layer  26 L_ 3  may be coplanar with each other as illustrated in  FIG.  5   . As used herein, “a lower surface of an element A” (or similar language) means a surface of the element A facing the substrate  10 . 
     The stacked transistor structure ST may further include a fifth contact  32 _ 5 , a sixth contact  32 _ 6 , a seventh contact  32 _ 7 , an eighth contact  32 _ 8  and a second metal layer  24 _ 2 . The fifth contact  32 _ 5  may contact the third lower semiconductor layer  26 L_ 3  and the third upper semiconductor layer  26 U_ 3 , and the sixth contact  32 _ 6  may contact the second metal layer  24 _ 2 . The seventh contact  32 _ 7  and the eight contact  32 _ 8  may contact the fourth lower semiconductor layer  26 L_ 4  and the fourth upper semiconductor layer  26 U_ 4 , respectively. In some embodiments, the first, second, fifth, seventh and eight contacts  32 _ 1 ,  32 _ 2 ,  32 _ 5 ,  32 _ 7  and  32 _ 8  may have upper surfaces coplanar with each other and may be at an equal height from the first surface S 1  of the substrate  10 . 
     The second metal layer  24 _ 2  may be provided between the third lower semiconductor layer  26 L_ 3  and the fourth lower semiconductor layer  26 L_ 4  and between the third upper semiconductor layer  26 U_ 3  and the fourth upper semiconductor layer  26 U_ 4 . The second metal layer  24 _ 2  may be spaced apart from the third lower semiconductor layer  26 L_ 3 , the fourth lower semiconductor layer  26 L_ 4 , the third upper semiconductor layer  26 U_ 3  and the fourth upper semiconductor layer  26 U_ 4  in the second direction D 2  as illustrated in  FIG.  4   . A portion of the second lower thin semiconductor layer  22 L_ 2  and a portion of the second upper thin semiconductor layer  22 U_ 2  may be in the second metal layer  24 _ 2  as illustrated in  FIG.  7   . The second metal layer  24 _ 2  may be a layer of a gate electrode. Although not shown, an insulating layer (i.e., a gate insulating layer) may be provided between the portion of the second lower thin semiconductor layer  22 L_ 2  and the second metal layer  24 _ 2  and between the portion of the second upper thin semiconductor layer  22 U_ 2  and the second metal layer  24 _ 2 . 
     In some embodiments, a first upper surface US 1  of the first upper thin semiconductor layer  22 U_ 1  may be coplanar with a second upper surface US  2  of the second upper thin semiconductor layer  22 U_ 2  as illustrated in  FIG.  7   , and the first upper thin semiconductor layer  22 U_ 1  and the second upper thin semiconductor layer  22 U_ 2  may have an equal thickness in the third direction D 3 . In some embodiments, a third upper surface US 3  of the first lower thin semiconductor layer  22 L_ 1  may be coplanar with a fourth upper surface US 4  of the second lower thin semiconductor layer  22 L_ 2  as illustrated in  FIG.  7   , and the first lower thin semiconductor layer  22 L_ 1  and the second lower thin semiconductor layer  22 L_ 2  may have an equal thickness in the third direction D 3 . 
     In  FIG.  7   , the first upper semiconductor layer  26 U_ 1 , the first lower semiconductor layer  26 L_ 1 , the third upper semiconductor layer  26 U_ 3 , the third lower semiconductor layer  26 L_ 3  are represented by dotted boxes to show spatial relationships of those elements with other elements. In some embodiments, each of the first upper semiconductor layer  26 U_ 1 , the first lower semiconductor layer  26 L_ 1 , the third upper semiconductor layer  26 U_ 3 , and the third lower semiconductor layer  26 L_ 3  may overlap the entirety of a corresponding thin semiconductor layer in the second direction D 2  as illustrated in  FIG.  7   . For example, the first upper semiconductor layer  26 U_ 1  may overlap the entirety of the first upper thin semiconductor layer  22 U_ 1 . As used herein, “an element A overlapping an element B in a direction X” (or similar language) means that there is at least one line that extends in the direction X and intersects both the elements A and B. 
     Although each of the first metal layer  24 _ 1  and the second metal layer  24 _ 2  is illustrated as a single layer, the present invention is not limited thereto. In some embodiments, each of the first metal layer  24 _ 1  and the second metal layer  24 _ 2  may include multiple conductive layers. For example, the first and second metal layers  24 _ 1  and  24 _ 2  may include a semiconductor layer (e.g., a poly silicon layer), a work function layer (e.g., TiC layer, TiAl layer, TiAlC layer or TiN layer) and/or a metal layer (e.g., a tungsten layer, an aluminum layer or a copper layer). 
     Further, in some embodiments, a lower portion of each of the first metal layer  24 _ 1  and the second metal layer  24 _ 2  may include material(s) different from an upper portion of each of the first metal layer  24 _ 1  and the second metal layer  24 _ 2 . Further, in some embodiments, an isolation layer may be provided between the lower portion and the upper portion of each of the first metal layer  24 _ 1  and the second metal layer  24 _ 2 , and the lower portion may be electrically isolated from the upper portion. 
     The integrated circuit device may further include first to third and fifth to eighth conductive vias  34 _ 1 ,  34 _ 2 ,  34 _ 3 ,  34 _ 5 ,  34 _ 6 ,  34 _ 7  and  34 _ 8  that may contact the first contact  32 _ 1 , the second contact  32 _ 2 , the first metal layer  24 _ 1 , the fifth contact  32 _ 5 , the second metal layer  24 _ 2 , the seventh contact  32 _ 7 , and the eighth contact  32 _ 8 , respectively. The conductive vias  34 _ 1 ,  34 _ 2 ,  34 _ 3 ,  34 _ 5 ,  34 _ 6 ,  34 _ 7  and  34 _ 8  may be electrically connected to conductive wires  36 , respectively. In some embodiments, the conductive vias  34 _ 1 ,  34 _ 2 ,  34 _ 3 ,  34 _ 5 ,  34 _ 6 ,  34 _ 7  and  34 _ 8  may contact the conductive wires  36 , respectively, as illustrated in  FIGS.  5  and  6   . 
     An interlayer insulating layer  40  may be provided on the substrate  10 . Although the interlayer insulating layer  40  is illustrated as a single layer, the interlayer insulating layer  40  may include multiple layers stacked on the substrate  10 . The first resistor structure RS 1  and the stacked transistor structure ST may be provided in the interlayer insulating layer  40 . The interlayer insulating layer  40  may include an insulating material (e.g., silicon oxide, silicon nitride, silicon oxynitride, silicon carbide and/or low-k material). The low k material may include, for example, fluorine-doped silicon dioxide, organosilicate glass, carbon-doped oxide, porous silicon dioxide, porous organosilicate glass, spin-on organic polymeric dielectric, or spin-on silicon based polymeric dielectric. 
     The substrate  10  may include one or more semiconductor materials, for example, Si, Ge, SiGe, GaP, GaAs, SiC, SiGeC and/or InP. In some embodiments, the substrate  10  may be a bulk substrate (e.g., a bulk silicon substrate) or a semiconductor on insulator (SOI) substrate. 
     Each of the first lower thin semiconductor layer  22 L_ 1 , the first upper thin semiconductor layer  22 U_ 1 , the second lower thin semiconductor layer  22 L_ 2  and the second upper thin semiconductor layer  22 U_ 2  may include multiple thin semiconductor layers stacked in the third direction D 3 . For example, the first upper thin semiconductor layer  22 U_ 1  may include two thin semiconductor layers as illustrated in  FIGS.  7  and  8    but the present invention is not limited thereto. Each of the stacked thin semiconductor layers may be, for example, a nanosheet. The nanosheet may include semiconductor material(s) (e.g., silicon, germanium, silicon-germanium, and/or III-V semiconductor compound). For example, each of nanosheets may have a thickness in a range of, for example, from 1 nm to 100 nm in the third direction D 3 . 
     Each of the first to fourth lower semiconductor layers  26 L_ 1  ,  26 L_ 2 ,  26 L_ 3  and  26 L_ 4  and the first to fourth upper semiconductor layers  26 U_ 1 ,  26 U_ 2 ,  26 U_ 3  and  26 U_ 4  may include semiconductor material(s) (e.g., silicon, germanium, silicon-germanium) and may also include impurities (e.g., boron, aluminum, gallium, indium, phosphorus, and/or arsenic). In some embodiments, the first lower semiconductor layer  26 L_ 1  and the first upper semiconductor layer  26 U_ 1  may be a silicon layer (e.g., an amorphous silicon layer). 
     The first contact  32 _ 1 , the second contact  32 _ 2 , the fifth contact  32 _ 5 , the sixth contact  32 _ 6 , the seventh contact  32 _ 7 , the eighth contact  32 _ 8 , the first to third and fifth to eighth conductive vias  34 _ 1 ,  34 _ 2 ,  34 _ 3 ,  34 _ 5 ,  34 _ 6 ,  34 _ 7  and  34 _ 8 , and the conductive wires  36  may include a metal layer (e.g., a ruthenium layer, a molybdenum layer, a copper layer, a cobalt layer, an aluminum layer and/or a tungsten layer) and/or a metal nitride layer (e.g., a titanium nitride layer and/or a tantalum nitride layer). 
       FIGS.  9  through  12    are side views of the integrated circuit device from the viewpoint V in  FIG.  4    according to some embodiments of the present invention.  FIG.  9    shows an example configuration of elements of the first resistor structure RS 1 _ 2  in  FIG.  2    according to some embodiments of the present invention. Referring to  FIG.  9   , the first and second contacts  32 _ 1  and  32 _ 2  may be spaced apart from the substrate  10  in the third direction D 3 , and thus the first portion  12 _ 1  of the substrate  10  may be electrically isolated from the first and second contacts  32 _ 1  and  32 _ 2 . 
       FIG.  10    shows an example configuration of elements of the first resistor structure RS 1 _ 5  in  FIG.  3    according to some embodiments of the present invention. Referring to  FIG.  10   , lower surfaces of the first and second contacts  32 _ 1  and  32 _ 2  may be above an upper surface of the first lower semiconductor layer  26 L_ 1  such that the first lower semiconductor layer  26 L_ 1  may be electrically isolated from the first and second contacts  32 _ 1  and  32 _ 2 . 
     Each of  FIGS.  11  and  12    shows an example configuration of elements of the first resistor structure RS 1 _ 3  in  FIG.  2    according to some embodiments of the present invention. In some embodiments, the lower semiconductor layer  26 L_ 1  may be formed to have a width in the first direction D 1  narrower than a width of the upper semiconductor layer  26 U_ 1  in the first direction D 1 , and opposing sides, in the first direction D 1 , of the lower semiconductor layer  26 L_ 1  may be spaced apart from the first and second contacts  32 _ 1  and  32 _ 2  as illustrated in  FIG.  11   . Accordingly, the lower semiconductor layer  26 L_ 1  may be electrically isolated from the first and second contacts  32 _ 1  and  32 _ 2 . In some embodiments, one of the opposing sides, in the first direction D 1 , of the lower semiconductor layer  26 L_ 1  may contact the first contact  32 _ 1  or the second contact  32 _ 2 . 
     Referring to  FIG.  12   , in some embodiments, the lower semiconductor layer  26 L_ 1  may have a width in the first direction D 1  equal to a width of the upper semiconductor layer  26 U_ 1  in the first direction D 1 , and each of the first and second contacts  32 _ 1  and  32 _ 2  may have a lower portion having a narrower width in the first direction D 1  compared to a width of its upper portion. Opposing sides, in the first direction D 1 , of the lower semiconductor layer  26 L_ 1  may be spaced apart from the first and second contacts  32 _ 1  and  32 _ 2  as illustrated in  FIG.  12    and may be electrically isolated from the first and second contact  32 _ 1  and  32 _ 2 . In some embodiments, only one of the first and second contacts  32 _ 1  and  32 _ 2  may have the narrower lower portion, and the lower semiconductor layer  26 L_ 1  may contact the first contact  32 _ 1  or the second contact  32 _ 2 . In some embodiments, the upper semiconductor layer  26 U_ 1  may not be used as a resistor element, as in the first resistor structure RS 1 _ 4  in  FIG.  2   , and at least one of the first and second contacts  32 _ 1  and  32 _ 2  may have an upper portion having a narrow width in the first direction D 1  compared to a width of its lower portion such that the upper semiconductor layer  26 U_ 1  is spaced apart from the at least one of the first and second contacts  32 _ 1  and  32 _ 2 . 
       FIG.  13    is a plan view of a resistor structure according to some embodiments of the present invention. The resistor structure may include two resistor structures (i.e., a first resistor structure RS 1  and a second resistor structure RS 2 ) that may be connected in series and may share a common resistor contact (e.g., a second contact  32 _ 2 ). Each of the first and second resistor structures RS 1  and RS 2  may have any configuration and structure described with reference to  FIGS.  1  to  12   . For example, the first resistor structure RS 1  may be the first resistor structure RS 1 _ 7  in  FIG.  3   , and the second resistor structure RS 2  may be the first resistor structure RS 1 _ 2  in  FIG.  2   . 
       FIG.  14    is a schematic illustration of a resistor structure in  FIG.  13   , and  FIG.  15    is a side view of the resistor structure from a viewpoint V in  FIG.  13    when each of the first and second resistor structures RS 1  and RS 2  has a configuration and structure of the first resistor structure RS 1 _ 1  in  FIG.  1    according to some embodiments of the present invention. The second resistor structure RS 2  may also include a first resistor element R 1 ′ (e.g., a fifth upper semiconductor layer  26 U_ 5  in  FIG.  15   ), a second resistor element R 2 ′ (e.g., a fifth lower semiconductor layer  26 L_ 5  in  FIG.  15   ) and a third resistor element R 3 ′ (e.g., a second portion  12 _ 2  of the substrate  10  in  FIG.  15   ), which are connected in parallel, as illustrated in  FIG.  14   . The second resistor structure RS 2  may have any configuration and structure described with reference to  FIGS.  1  to  12   . 
     In some embodiments, a resistor element (e.g., the first resistor element R 1 ) of the first resistor structure RS 1  and a resistor element (e.g., the first resistor element R 1 ′) of the second resistor structure RS 2  may be electrically connected in series through the second contact  32 _ 2 . 
     Referring to  FIGS.  13  to  15   , the second resistor structure RS 2  may include a fifth upper semiconductor layer  26 U_ 5 , a sixth upper semiconductor layer  26 U_ 6 , a fifth lower semiconductor layer  26 L_ 5 , a sixth lower semiconductor layer  26 L_ 6 , which may include a material and/or may have a shape and spatial relationship with other elements the same as or similar to the first upper semiconductor layer  26 U_ 1 , the second upper semiconductor layer  26 U_ 2 , the first lower semiconductor layer  26 L_ 1 , the second lower semiconductor layer  26 L_ 2 , respectively. The fifth upper semiconductor layer  26 U_ 5  and the sixth upper semiconductor layer  26 U_ 6  may have the first conductivity type, and the fifth lower semiconductor layer  26 L_ 5  and the sixth lower semiconductor layer  26 L_ 6  may have the second conductivity type. 
     The second resistor structure RS 2  may also include a second portion  12 _ 2  of the substrate  10 . The second portion  12 _ 2  of the substrate  10  may include impurities (e.g., boron, aluminum, gallium, indium, phosphorus, and/or arsenic) and may have the first conductivity type or the second conductivity type. The second portion  12 _ 2  of the substrate  10  may have an impurity concentration in a range of from 10 11  cm −3  to 10 15  cm −3  (e.g., from 10 12  cm −3  to 10 14  cm −3 ). 
     A third upper thin semiconductor layer  22 U_ 3  may be provided between the fifth upper semiconductor layer  26 U_ 5  and the sixth upper semiconductor layer  26 U_ 6  and may contact both the fifth upper semiconductor layer  26 U_ 5  and the sixth upper semiconductor layer  26 U_ 6 . A third lower thin semiconductor layer  22 L_ 3  may be provided between the fifth lower semiconductor layer  26 L_ 5  and the sixth lower semiconductor layer  26 L_ 6  and may contact both the fifth lower semiconductor layer  26 L_ 5  and the sixth lower semiconductor layer  26 L_ 6 . The third upper thin semiconductor layer  22 U_ 3  and the third lower thin semiconductor layer  22 L_ 3  may include a material and/or may have a shape and spatial relationship with other elements the same as or similar to the first upper thin semiconductor layer  22 U_ 1  and the first lower thin semiconductor layer  22 L_ 1 , respectively. 
     The second resistor structure RS 2  may also include a ninth contact  32 _ 9  (also referred to as a third resistor contact). In some embodiments, the ninth contact  32 _ 9  may contact the fifth upper semiconductor layer  26 U_ 5 , the fifth lower semiconductor layer  26 L_ 5 , and the second portion  12 _ 2  of the substrate  10  as illustrated in  FIG.  15   . Additionally, the second resistor structure RS 2  may include a third metal layer  24 _ 3 , conductive vias  34 _ 9  and  34 _ 10 , and conductive wires  36  as illustrated in  FIGS.  13  and  15   . 
       FIG.  16    is a plan view of an integrated circuit device according to some embodiments of the present invention, and  FIGS.  17  to  19    are cross-sectional views of the integrated circuit device taken along the line D-D′ in  FIG.  16    according to some embodiments of the present invention. The integrated circuit devices illustrated in  FIGS.  16 - 19    are the same as or similar to the integrated circuit devices illustrated in  FIGS.  4 - 8    with primary differences being that a first contact  32 _ 1  and a second contact  32 _ 2 ′ of a third resistor structure RS 3  are on opposing sides of the first metal layer  24 _ 1 , respectively, and the first upper thin semiconductor layer  22 U_ 1 ′ and the first lower thin semiconductor layer  22 L_ 1 ′ of the third resistor structure RS 3  include impurities for electrical connection between the first contact  32 _ 1  and the second contact  32 _ 2 ′ through the first upper thin semiconductor layer  22 U_ 1 ′ and the first lower thin semiconductor layer  22 L_ 1 ′. 
     The third resistor structure RS 3  may also include three resistor elements, a first resistor element R 1 , a second resistor element R 2 , and a third resistor element R 3 , which can be connected in various ways as illustrated in  FIGS.  1  and  3   . The first resistor element R 1  of the third resistor structure RS 3  may include a first upper semiconductor layer  26 U_ 1 , a second upper semiconductor layer  26 U_ 2  and a first upper thin semiconductor layer  22 U_ 1 ′ that contacts the first upper semiconductor layer  26 U_ 1  and the second upper semiconductor layer  26 U_ 2 . The first upper thin semiconductor layer  22 U_ 1 ′ may include impurities (e.g., boron, aluminum, gallium, indium, phosphorus, and/or arsenic) to have the same conductivity type as the first upper semiconductor layer  26 U_ 1  and the second upper semiconductor layer  26 U_ 2 . 
     The second resistor element R 2  of the third resistor structure RS 3  may include a first lower semiconductor layer  26 L_ 1 , a second lower semiconductor layer  26 L_ 2  and a first lower thin semiconductor layer  22 L_ 1 ′ that contacts the first lower semiconductor layer  26 L_ 1  and the second lower semiconductor layer  26 L_ 2 . The first lower thin semiconductor layer  22 L_ 1 ′ may include impurities (e.g., boron, aluminum, gallium, indium, phosphorus, and/or arsenic) to have the same conductivity type as the first lower semiconductor layer  26 L_ 1  and the second lower semiconductor layer  26 L_ 2 . 
     For example, each of the first upper thin semiconductor layer  22 U_ 1 ′ and the first lower thin semiconductor layer  22 L_ 1 ′ may be a silicon layer or a silicon germanium layer and may have an impurity concentration in a range of from 10 15  cm −3  to 10 20  cm −3  (e.g., about from 10 16  cm −3  to 10 19  cm −3  or about from 10 17  cm −3  to 10 18  cm −3 ). 
     The third resistor element R 3  of the third resistor structure RS 3  may include a third portion  12 _ 3  of the substrate  10 . The third portion  12 _ 3  of the substrate  10  may include impurities (e.g., boron, aluminum, gallium, indium, phosphorus, and/or arsenic) and may have the first conductivity type or the second conductivity type. The third portion  12 _ 3  of the substrate  10  may have an impurity concentration in a range of from 10 11  cm −3  to 10 15  cm −3  (e.g., from 10 12  cm −3  to 10 14  cm −3 ). 
     The first, second and third resistor elements R 1 , R 2  and R 3  of the third resistor structure RS 3  may be connected to the first contact  32 _ 1  and the second contact  32 _ 2 ′ in various ways as illustrated in  FIGS.  1  and  3    to make the third resistor structure RS 3  have a pre-determined resistance value. 
     As examples,  FIGS.  17  to  19    show cross-sectional views of the third resistor structures RS 3  that have electrical connections the same as the first resistor structures RS 1 _ 1  in  FIG.  1   , RS 1 _ 2  in  FIG.  2    and RS 1 _ 5  in  FIG.  3   , respectively. 
     In some embodiments, the first, second and third resistor elements R 1 , R 2  and R 3  of the third resistor structure RS 3  all may contact the first contact  32 _ 1  and the second contact  32 _ 2 ′ and may be electrically connected in parallel as illustrated in  FIG.  17   . In some embodiments, only two resistor elements (e.g., the first and second resistor elements R 1 , R 2 , the first and third resistor elements R 1 , R 3  or the second and third resistor elements R 2 , R 3 ) among the three resistor elements R 1 , R 2  and R 3  may contact the first contact  32 _ 1  and the second contact  32 _ 2 ′ and may be electrically connected in parallel. For example, the first and second resistor elements R 1 , R 2  may contact the first contact  32 _ 1  and the second contact  32 _ 2 ′, and the third resistor element R 3  may not contact the first contact  32 _ 1  and the second contact  32 _ 2 ′ as illustrated in  FIG.  18   . In some embodiments, only single resistor element (e.g., the first resistor element R 1 , the second resistor element R 2 , or the third resistor element R 3 ) among the three resistor elements R 1 , R 2  and R 3  may contact and may be electrically connected to the first contact  32 _ 1  and the second contact  32 _ 2 ′. For example, the first resistor element R 1  may contact the first contact  32 _ 1  and the second contact  32 _ 2 ′, and the second and third resistor elements R 2 , R 3  may not contact the first contact  32 _ 1  and the second contact  32 _ 2 ′ as illustrated in  FIG.  19   . 
       FIG.  20    is a flow chart of methods of forming an integrated circuit device according to some embodiments of the present invention.  FIGS.  21 - 23    are cross-sectional views taken along the line B-B′ and the line C-C′ in  FIG.  4    illustrating methods of forming an integrated circuit device according to some embodiments of the present invention. 
     Referring to  FIGS.  20  and  21   , the methods may include forming a doped portion (e.g., the first portion  12 _ 1  of the substrate  10  in  FIG.  5   ) in a substrate  10  (Block  1000 ) by adding impurities (e.g., boron, aluminum, gallium, indium, phosphorus, and/or arsenic) to the substrate  10 . For example, an ion implantation process and/or a diffusion process may be used to form the doped portion. The methods may also include forming a lower thin semiconductor layer  22 L_ 1  (also referred to as a first lower thin semiconductor layer) and a lower active layer  22 L_ 2  (also referred to as a second lower thin semiconductor layer) on the substrate  10  (Block  1100 ). 
     In some embodiments, each of the lower thin semiconductor layer  22 L_ 1  and the lower active layer  22 L_ 2  may include multiple layers (e.g., two layers as illustrated in  FIG.  21   ) stacked in the third direction D 3 . In some embodiments, one of the lower thin semiconductor layers  22 L_ 1  and one of the lower active layers  22 L_ 2  may be formed concurrently at the same height from the first surface S 1  of the substrate  10  as illustrated in  FIG.  21   . For example, preliminary lower semiconductor layers may be formed on the substrate  10  and then the preliminary lower semiconductor layers may be patterned, thereby forming the lower thin semiconductor layers  22 L_ 1  and the lower active layers  22 L_ 2 . One of the lower thin semiconductor layers  22 L_ 1  and one of the lower active layers  22 L_ 2 , which have lower surfaces coplanar with each other, may be respective portions of a single preliminary lower semiconductor layer. First interlayer insulating layers  42 _ 1  may be formed to be stacked alternately with the lower thin semiconductor layers  22 L_ 1  and the lower active layers  22 L_ 2  and may expose opposing side surfaces of the lower thin semiconductor layers  22 L_ 1  and the lower active layers  22 L_ 2 . 
     An upper thin semiconductor layer  22 U_ 1  (also referred to as a first upper thin semiconductor layer) and an upper active layer  22 U_ 2  (also referred to as a second upper thin semiconductor layer) may be formed on the lower thin semiconductor layer  22 L_ 1  and the lower active layer  22 L_ 2 , respectively (Block  1200 ). In some embodiments, each of the upper thin semiconductor layer  22 U_ 1  and the upper active layer  22 U_ 2  may include multiple layers (e.g., two layers as illustrated in  FIG.  21   ) stacked in the third direction D 3 . In some embodiments, one of the upper thin semiconductor layers  22 U_ 1  and one of the upper active layers  22 U_ 2  may be formed concurrently at the same height from the first surface S 1  of the substrate  10  as illustrated in  FIG.  21   . For example, preliminary upper semiconductor layers may be formed on the substrate  10  and then the preliminary upper semiconductor layers may be patterned, thereby forming the upper thin semiconductor layers  22 U_ 1  and the upper active layers  22 U_ 2 . One of the upper thin semiconductor layers  22 U_ 1  and one of the upper active layers  22 U_ 2 , which have lower surfaces coplanar with each other, may be respective portions of a single preliminary upper semiconductor layer. Second interlayer insulating layer  42 _ 2  may be formed to be stacked alternately with the upper thin semiconductor layer  22 U_ 1  and the upper active layer  22 U_ 2  and may expose opposing side surfaces of the upper thin semiconductor layer  22 U_ 1  and the upper active layer  22 U_ 2 . 
     Referring to  FIGS.  18  and  22   , lower semiconductor layers  26 L_ 1  and  26 L_ 2  (also referred to as first and second lower semiconductor layers) and lower source/drain regions  26 L_ 3  and  26 L_ 4  (also referred to as third and fourth lower semiconductor layers) may be formed (Block  1300 ). In some embodiments, the first and second lower semiconductor layers  26 L_ 1  and  26 L_ 2  may be formed by an epitaxial growth process using the lower thin semiconductor layer  22 L_ 1  as a seed layer, and the lower source/drain regions  26 L_ 3  and  26 L_ 4  may be formed by an epitaxial growth process using the lower active layer  22 L_ 2  as a seed layer. In some embodiments, the first and second lower semiconductor layers  26 L_ 1  and  26 L_ 2  and the lower source/drain regions  26 L_ 3  and  26 L_ 4  may be formed by a single epitaxial growth process (e.g., a first epitaxial growth process). 
     Opposing side surfaces of the lower thin semiconductor layer  22 L_ 1  may contact the first and second lower semiconductor layers  26 L_ 1  and  26 L_ 2 , respectively, and opposing side surfaces of the lower active layer  22 L_ 2  may contact the lower source/drain regions  26 L_ 3  and  26 L_ 4 , respectively, as illustrated in  FIG.  22   . 
     Although  FIG.  22    illustrates that the first and second lower semiconductor layers  26 L_ 1  and  26 L_ 2  and the lower source/drain regions  26 L_ 3  and  26 L_ 4  are formed after the upper thin semiconductor layers  22 U_ 1  and the upper active layers  22 U_ 2  are formed, in some embodiments, the lower source/drain regions  26 L_ 3  and  26 L_ 4  may be formed before the upper thin semiconductor layers  22 U_ 1  and the upper active layers  22 U_ 2  are formed. 
     Referring to  FIGS.  18  and  23   , upper semiconductor layers  26 U_ 1  and  26 U_ 2  (also referred to as first and second upper semiconductor layers) and upper source/drain regions  26 U_ 3  and  26 U_ 4  (also referred to as third and fourth upper semiconductor layers) may be formed (Block  1400 ). In some embodiments, the upper semiconductor layers  26 U_ 1  and  26 U_ 2  may be formed by an epitaxial growth process using the upper thin semiconductor layer  22 U_ 1  as a seed layer, and the upper source/drain regions  26 U_ 3  and  26 U_ 4  may be formed by an epitaxial growth process using the upper active layer  22 U_ 2  as a seed layer. In some embodiments, the upper semiconductor layers  26 U_ 1  and  26 U_ 2  and the upper source/drain regions  26 U_ 3  and  26 U_ 4  may be formed by a single epitaxial growth process (e.g., a second epitaxial growth process). 
     Referring to  FIGS.  4  to  6 ,  16  and  18   , the methods may include forming resistor contacts (e.g., the first and second contact  32 _ 1  and  32 _ 2 ) and source/drain contacts (e.g., fifth, seventh and eight contacts  32 _ 5 ,  32 _ 7  and  32 _ 8 ) (Block  1500 ) and forming a first metal layer  24 _ 1  and a second metal layer  24 _ 2  (Block  1600 ). In some embodiments, portions of the first and second interlayer insulating layers  42 _ 1  and  42 _ 2  may be replaced with the first metal layer  24 _ 1  and the second metal layer  24 _ 2 . In some embodiments, the resistor contacts and the source/drain contacts may be formed concurrently. For example, the resistor contacts and the source/drain contacts may include the same metal layer formed by the same deposition process. In some embodiments, the resistor contacts and the source/drain contacts may have upper surfaces coplanar with each other. 
     In some embodiments, the first metal layer  24 _ 1  and the second metal layer  24 _ 2  may be formed concurrently. For example, first metal layer  24 _ 1  and the second metal layer  24 _ 2  may include the same metal layer formed by the same deposition process. 
     It should also be noted that in some alternate implementations, the functions/acts noted in flowchart blocks herein may occur out of the order noted in the flowcharts. For example, two blocks shown in succession may in fact be executed substantially concurrently or the blocks may sometimes be executed in the reverse order, depending upon the functionality/acts involved. Moreover, the functionality of a given block of the flowcharts and/or block diagrams may be separated into multiple blocks and/or the functionality of two or more blocks of the flowcharts and/or block diagrams may be at least partially integrated. Finally, other blocks may be added/inserted between the blocks that are illustrated, and/or blocks/operations may be omitted without departing from the scope of the present inventive concepts. 
     Example embodiments are described herein with reference to the accompanying drawings. Many different forms and embodiments are possible without deviating from the scope of the present invention. Accordingly, the present invention should not be construed as limited to the example embodiments set forth herein. Rather, these example embodiments are provided so that this disclosure will be thorough and complete and will convey the scope of the present invention to those skilled in the art. In the drawings, the sizes and relative sizes of layers and regions may be exaggerated for clarity. Like reference numbers refer to like elements throughout. 
     Example embodiments of the present invention are described herein with reference to cross-sectional views that are schematic illustrations of idealized embodiments and intermediate structures of example embodiments. As such, variations from the shapes of the illustrations as a result, for example, of manufacturing techniques and/or tolerances, are to be expected. Thus, example embodiments of the present invention should not be construed as limited to the particular shapes illustrated herein but include deviations in shapes that result, for example, from manufacturing, unless the context clearly indicates otherwise. 
     Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the present invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the present invention. As used herein, the singular forms “a,” “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises,” “comprising,” “includes” and/or “including,” when used in this specification, specify the presence of the stated features, steps, operations, elements and/or components, but do not preclude the presence or addition of one or more other features, steps, operations, elements, components and/or groups thereof. As used herein the term “and/or” includes any and all combinations of one or more of the associated listed items. 
     It will be understood that although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. Thus, a first element could be termed a second element without departing from the scope of the present invention. 
     The above-disclosed subject matter is to be considered illustrative, and not restrictive, and the appended claims are intended to cover all such modifications, enhancements, and other embodiments, which fall within the scope of the invention. Thus, to the maximum extent allowed by law, the scope is to be determined by the broadest permissible interpretation of the following claims and their equivalents and shall not be restricted or limited by the foregoing detailed description.