Patent Publication Number: US-2022216142-A1

Title: Semiconductor memory structure and interconnect structure of semiconductor memory structure

Description:
BACKGROUND 
     Recently, great progress has been achieved in development of semiconductor memory devices. Due to continuously increasing requirements for memory devices with mass capacity, integration density of memory cells in a memory device keeps increasing. Scaling the memory cell size down and realizing high-density memory are eagerly needed for various applications such as internet of things (IoT) and mechanism learning. As shrinking critical dimensions of devices in integrated circuits approach the limits of common memory cell technologies, designers seek techniques for stacking multiple planes of memory cells to achieve greater storage capacity and lower per-bit costs. A three-dimensional (3D) memory array architecture has been realized by vertically stacking multiple cells on each cell region in a two-dimensional (2D) array matrix. There is a continuous demand for improved semiconductor memory structures for satisfying the above requirements. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Aspects of the present disclosure are best understood from the following detailed description w % ben read with the accompanying figures. It should be noted that, in accordance with the standard practice in the industry, various features are not drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion. 
         FIGS. 1, 2, 3, 4, 5, 6, 7, 8, 9 and 10A  are perspective drawings illustrating various stages in a method for forming a semiconductor memory structure according to aspects of one or more embodiments of the present disclosure. 
         FIG. 10B  is a cross-sectional view taken along line I-I′ in  FIG. 10A . 
         FIGS. 11, 12, 13, 14 and 15  are perspective drawings of a portion of a semiconductor memory structure in various stages subsequent to  FIGS. 10A and 10B  in the method for forming a semiconductor memory structure according to aspects of one or more embodiments of the present disclosure. 
         FIG. 16  is a schematic drawing illustrating a layout structure of a portion of a semiconductor memory structure according to aspects of one or more embodiments of the present disclosure. 
         FIGS. 17A and 17B  are exemplary cross-sectional views taken along line AI-AI′ and ling BI-BI′ in  FIG. 16  and schematically illustrate vertical layer arrangements of different portions of an interconnect structure of the semiconductor memory structure according to aspects of one or more embodiments of the present disclosure. 
         FIG. 18  is a schematic drawing illustrating a portion of a semiconductor memory structure according to aspects of one or more embodiments of the present disclosure. 
         FIG. 19  is a schematic drawing illustrating a layout structure of a portion of a semiconductor memory structure according to aspects of one or more embodiments of the present disclosure. 
         FIGS. 20A and 20B  are exemplary cross-sectional views taken along line AII-AII′ and line BII-BII′ in  FIG. 19  and schematically illustrate vertical layer arrangements of different portions of an interconnect structure of the semiconductor memory structure according to aspects of one or more embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The following disclosure provides many different embodiments, or examples, for implementing different features of the provided subject matter. Specific examples of elements and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. For example, the formation of a first feature over or on a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features may be formed between the first and second features, such that the first and second features may not be in direct contact. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed. 
     Further, spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “upper,” “on” and the like, may be used herein for ease of description to describe one element or feature&#39;s relationship to another element(s) or feature(s) as illustrated in the figures. The spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. The device may be otherwise oriented (rotated 100 degrees or at other orientations) and the spatially relative descriptors used herein may likewise be interpreted accordingly. 
     As used herein, the terms such as “first,” “second” and “third” describe various elements, components, regions, layers and/or sections, but these elements, components, regions, layers and/or sections should not be limited by these terms. These terms may be only used to distinguish one element, component, region, layer or section from another. The terms such as “first,” “second” and “third” when used herein do not imply a sequence or order unless clearly indicated by the context. 
     Many modern day electronic devices contain electronic memory configured to store data. Electronic memory may be volatile memory or non-volatile memory. Volatile memory stores data when it is powered, while non-volatile memory (NVM) is able to store data when power is removed. Unlike conventional NVM devices (e.g., flash, EEPROM, etc.), many promising candidates for next-generation NVM technology use memory cells that are located within a back-end-of-the-line (BEOL) of an integrated chip (e.g., located between metal interconnect layers overlying a semiconductor substrate). In some comparative approaches, the memory cells may have a memory array architecture realized in the BEOL interconnect structure. In some comparative approaches, the memory cells may have a memory array architecture connected by the BEOL interconnect structure. 
     For example, source lines (SLs) and bit lines (BLs) of the memory cells can be formed in the BEOL interconnect structure. In some embodiments, a BEOL interconnect structure can include a plurality of metal layers referred to, in ascending order, as the first metal layer M 1 , the second metal layer M 2 , and the nth metal layer Mn; and a plurality of connecting vias referred to as the zeroth via V 0 , the first via V 1 , and the (n−1)th via V(n−1), wherein n is a positive integer. The (n−1)th metal layer M(n−1) and the Nth metal layer Mn can be electrically connected by the (n−1)th via V(n−1). Further, the metal layer Mn and the connecting via V(n−1) can be formed in an inter-metal dielectric layer IMDn, which provides mechanical support and electrical isolation for the metal layer Mn and the connecting via V(n−1). In some embodiments, the source lines and the bit lines can be formed in a common metal layer, such as the first metal lines ML, which can be electrically connected to source structures and drain structures of the memory cells by the zeroth via V 0 . In such embodiments, it is found that a SL/BL routing pattern in a common metal layer may suffer from uneven line pitch issue, which results in an asymmetric SL/BL layout structure. Further, the uneven line pitch is not conducive to ease of manufacture, in that the SL/BL routing patterns may not be able to be manufactured given the processes and/or processing/lithography equipment used by an IC manufacturer, or the SL/BL routing patterns may be manufacturable but require too much time for mask creation. In addition, the asymmetric layout structure is not conductive to ease of design due to the uneven capacitance and non-uniform access time. 
     The present disclosure therefore provides a semiconductor memory structure and an interconnect structure of the semiconductor memory structure. In some embodiments, the semiconductor memory structure includes an even SL/BL line pitch and a symmetric layout structure. In some embodiments, by introducing an intervening conductive layer, the metal line routing can be arranged such that the line pitch can be made even. In some embodiments, by separating even metal lines from odd metal lines in different layers, the line pitch can be made even and increased. Accordingly, the uneven line pitch issue is mitigated and a symmetric interconnect layout structure can be obtained. 
       FIGS. 1, 2, 3, 4, 5, 6, 7, and 8A  are perspective drawings illustrating various stages in a method for forming a semiconductor memory structure according to aspects of one or more embodiments of the present disclosure, and  FIG. 8B  is a cross-sectional view taken along line I-I′ in  FIG. 8A . Referring to  FIG. 1 , in some embodiments, a substrate  102  is received or provided. In some embodiments, the substrate  102  is a silicon substrate. Alternatively or additionally, the substrate  102  includes germanium, an alloy semiconductor (for example, SiGe), another suitable semiconductor material, or a combination thereof. Alternatively, the substrate  102  is a semiconductor-on-insulator substrate, such as a silicon-on-insulator (SOI) substrate, a silicon germanium-on-insulator (SGOI) substrate, or a germanium-on-insulator (GOI) substrate. Semiconductor-on-insulator substrates can be fabricated using separation by implantation of oxygen (SIMOX), wafer bonding, and/or other suitable methods. In some embodiments, the substrate  102  can include various devices, such as CMOS devices. For example, the substrate  102  can include CMOS devices under array (CuA), but the disclosure is not limited thereto. 
     As shown in  FIG. 1 , a stack  110  can be formed over the substrate  102 . In some embodiments, the stack  110  includes a plurality of first layers  112  and a plurality of second layers  114 . Further, the first layers  112  and the second layers  114  are alternately arranged. The number of the alternating layers included in the stack  110  can be as great as the number of layers needed for the semiconductor memory device. Further, in some embodiments, the topmost layer and the bottommost layer can both be the first layers  112 , as shown in  FIG. 1 , but the disclosure is not limited thereto. Thicknesses of the first layers  112  and thicknesses of the second layers  114  can be similar or different, depending on different product requirements. In some embodiments, the first layers  112  include an insulating material while the second layers  114  include a conductive material. For example, in some embodiments, the first layers  112  include silicon oxide, and the second layers  114  include metals, but the disclosure is not limited thereto. 
     Referring to  FIG. 2 , portions of the first layers  112  and portions of the second layers  114  are removed, such that remaining first layers  112  and remaining second layers  114  form a staircase structure over the substrate  102 . In some embodiments, portions of the second layers  114  are exposed, and areas of the exposed portions of the second layers  114  can be similar. In some embodiments, the remaining portion of the topmost first layer  112  can be used to define a location and a dimension of a cell array region CA, which is described below. 
     Referring to  FIG. 3 , in some embodiments, a dielectric structure  120  can be formed over the substrate  102 . Further, a top surface of the dielectric structure  120  can be aligned with a top surface of the topmost first layer  112 , as shown in  FIG. 3 . Consequently, an even and flush surface can be obtained. 
     Referring to  FIG. 4 , in some embodiments, a plurality of trenches  121  are formed in the dielectric structure  120  and the stack  110 . In some embodiments, each of the trenches  121  extends along a first direction D 1  and the trenches  121  are arranged along a second direction D 2 , which is different from the first direction D 1 . In some embodiments, the first direction D 1  and the second direction D 2  are perpendicular to each other. Further, depths of the trenches  121  are similar to each other. In some embodiments, the substrate  102  can be exposed through a bottom of each trench  121 , but the disclosure is not limited thereto. In some embodiments, the first layers  112  and the second layers  114  can be exposed through two opposite sidewalls of each trench  121 . 
     Referring to  FIG. 5 , in some embodiments, a charge-trapping layer  132  and a channel layer  134  are formed in each trench  121 . In some embodiments, the charge-trapping layer  132  can be referred to as a memory layer. The charge-trapping layer  132  and the channel layer  134  can be conformally formed in each trench  121  by, for example but not limited thereto, a deposition. Therefore, the charge-trapping layer  132  and the channel layer  134  cover the sidewalls and the bottom of each trench  121 . The charge-trapping layer  132  can include an insulating structure or a ferroelectric material. For example, in some embodiments, the charge-trapping layer  132  can include an insulating structure such as an oxide-nitride-oxide (ONO) structure, a nitride-oxide-nitride (NON) structure, a silicon oxynitride layer, or a silicon nitride layer. In some alternative embodiments, the charge-trapping layer  132  can include a ferroelectric material such as hafnium silicate (HfSiO), hafnium zirconium oxide (HfZrO, also referred to as HZO), or the like. The channel layer  134  can include semiconductor materials, oxide semiconductor or 2D materials. In some embodiments, the channel layer  134  can include oxide semiconductor materials such as indium gallium zinc oxide (IGZO), zinc oxide (ZnO), indium tungsten oxide (IWO), or the like. 
     Still referring to  FIG. 6 , in some embodiments, an isolation  136  is formed in each trench  121  after the forming of the charge-trapping layer  132  and the channel layer  134 . In some embodiments, the isolation  136  can be formed by filling each trench  121  with a dielectric material, such as silicon oxide, silicon oxycarbide, silicon oxynitride, silicon carbonitride, or the like. The dielectric material can be used to fill each trench  121  and covers a top surface of the stack  110  and the dielectric structure  120 . A planarization operation such as a chemical mechanical polishing (CMP) operation can be performed to remove superfluous dielectric material to form the isolations  136 . Accordingly, a top surface of the isolation  136 , a top surface of the dielectric structure  120 , the top surface of the topmost first layer  112 , a topmost surface of the charge-trapping layer  132 , and a topmost surface of the channel layer  134  can be aligned with each other. In some embodiments, the charge-trapping layer  132 , the channel layer  134  and the isolation  136  can be referred to as a hybrid structure  130  that extends along the first direction D 1 . Further, the hybrid structure  130  passes through the cell array region CA, as shown in  FIG. 6 . 
     Referring to  FIG. 7 , in some embodiments, portions of the isolation  136  and portions of the channel layer  134  are removed. Accordingly, a plurality of first recesses  137  are formed in the cell array region CA. In some embodiments, dimensions of the first recesses  137  are the same. The first recesses  137  may be arranged in the hybrid structure  130  along the first direction D 1 . Further, distances between adjacent first recesses  137  in each hybrid structure  130  are the same. Additionally, the channel layer  134  in each hybrid structure  130  may be interrupted by the first recesses  137 , as shown in  FIG. 7 . In the cell array region CA, the first recesses  137  are arranged to form a staggered pattern, but the disclosure is not limited thereto. 
     Referring to  FIG. 8 , an isolation  138  is formed in each first recess  137 . In some embodiments, the first recesses  137  are filled with a dielectric material, and a planarization operation such as a CMP operation is performed to remove superfluous dielectric material and form the isolation  138 . In some embodiments, materials for forming the isolation  138  can be similar to those mentioned above; therefore, repeated descriptions of such details are omitted for brevity. Accordingly, a top surface of the isolation  138 , the top surface of the isolation  136 , the top surface of the hybrid structure  130 , and the top surface of the stack  110  (i.e., the top surface of the topmost first layer  112 ) can be aligned with each other. 
     Referring to  FIG. 9 , in some embodiments, a plurality of second recesses  139  are formed at two sides of each isolation  138 . In such embodiments, portions of the isolations  136  at the two sides of each isolation  138  are removed, thus forming the second recesses  139  at the two sides of the isolation  138 . In some embodiments, the second recesses  139  have similar dimensions and similar depths. Further, the channel layer  134  can be exposed through two opposite sidewalls of each second recess  139 . 
     Referring to  FIGS. 10A and 10B , a source structure  140 S and a drain structure  140 D are respectively formed in the second recesses  139 . In some embodiments, a barrier layer (not shown) can be formed to cover sidewalls and a bottom of each second recess  138 , and then the second recesses  139  are filled with a conductive material. A planarization operation such as a CMP operation is performed to remove superfluous portions of the barrier layer and the conductive material. Consequently, the source structure  140 S and the drain structure  140 D are formed at the two sides of the isolation structure  138 . In some embodiments, the conductive material can include doped polysilicon, doped amorphous silicon, tungsten, copper, or the like. In some embodiments, a plurality of stacked memory cells can be formed in the cell array region CA after the forming of the source structure  140 S and the drain structure  140 D. 
     Please refer to  FIGS. 11, 12, 13, 14 and 15 , which are perspective drawings of a portion of a semiconductor memory structure in various stages subsequent to  FIGS. 10A and 10B . In some embodiments,  FIGS. 12 to 15  are perspective drawings of the cell array region CA of the semiconductor memory structure  100 . In some embodiments, MEOL and/or BEOL manufacturing operations can be performed to form an interconnect structure  200  or  300  over the memory cells. Referring to  FIG. 12 , in some embodiments, a plurality of vias such as the zeroth vias V 0  can be formed on and coupled to each source structure  140 S and each drain structure  140 D. Referring to  FIG. 13 , in some embodiments, a plurality of conductive lines such as the first metal layers M 1  can be formed on and coupled to the zeroth vias V 0 . It should be noted that the zeroth vias V 0  and the first metal layers M 1  can be formed by dual damascene operations. Therefore, the operation shown in  FIGS. 11 and 12  can be omitted and, by using the dual damascene operations, the zeroth vias V 0  and the first metal layers M 1  can be directly formed as shown in  FIG. 13 . 
     In addition, the zeroth vias V 0  can also be formed to couple to the second layers  114  in the stack  110 , as shown in  FIG. 11 . In such embodiments, the zeroth vias V 0  in the same column along the first direction D 1  may have different depths, while the zeroth vias V 0  in the same row along the second direction may have same depths. In some embodiments, the zeroth vias V 0  coupled to the second layers  114  help to electrically connect the second layers  114  to overlying metal layer, though not shown. 
     Referring to  FIG. 14 , in some embodiments, a plurality of vias such as the first vias V 1  can be formed on and coupled to the first metal layers M 1 . Referring to  FIG. 15 , in some embodiments, a plurality of conductive lines such as the second metal layers M 2  can be formed on and coupled to the first vias V 1 . It should be noted that the first vias V 1  and the second metal layers M 2  can be formed by dual damascene operations. Therefore, the operation shown in  FIG. 14  can be omitted and, by using the dual damascene operations, the first vias V 1  and the second metal lines M 2  can be directly formed as shown in  FIG. 15 . In some embodiments, more vias such as second vias V 2 , third vias V 3 , and (n−1)th vias V(n−1), and metal layers such as third metal layers M 3 , fourth metal layers M 4  and nth metal layers Mn can be formed over the substrate  102 . Consequently, a semiconductor memory structure  100  including an interconnect structure  200  or  300  can be obtained. 
     Please refer to  FIGS. 16, 17A, and 17B , wherein  FIG. 16  is a schematic drawing illustrating a layout structure of a portion of a semiconductor memory structure according to aspects of one or more embodiments of the present disclosure, and  FIGS. 17A and 17B  are exemplary cross-sectional views schematically illustrating vertical layer arrangements of different portions of an interconnect structure  200  of a semiconductor memory structure  100  according to aspects of one or more embodiments of the present disclosure. In some embodiments, the interconnect structure  200  can be disposed over memory cells as mentioned above, but the disclosure is not limited thereto. 
     In some embodiments, semiconductor memory structure  100  includes a first cell C 1  and a second cell C 2 . In some embodiments, the first cell C 1  and the second cell C 2  can be disposed in the cell array region CA. The first cell C 1  and the second cell C 2  both extend along the first direction D 1 . Further, the first cell C 1  and the second cell C 2  are separated from each other along the second direction D 2 . In some embodiments, the first cell C 1  and the second cell C 2  are offset from each other along the first direction D 1 . In such embodiments, the first cell C 1  and the second cell C 2  can be arranged in a repeating manner in the cell array region CA to form a staggered pattern as shown in  FIG. 16 . The first cell C 1  includes a first source structure  140 S 1  and a first drain structure  140 D 1 . The first source structure  140 S 1  is separated from the first drain structure  140 D 1  by the isolation  138 . The second cell C 2  includes a second source structure  140 S 2  and a second drain structure  140 D 2 . The second source structure  140 S 2  is separated from the second drain structure  140 D 2  by the isolation  138 . It should be noted that the second layers  114  in the stack  110  serve as the gate layers (also referred to as the word lines) in the cell array region CA; therefore, the locations of the gate layers of each memory cell should be easily understood, although they are not shown in  FIGS. 16, 17A and 17B . 
     In some embodiments, the semiconductor memory structure  100  includes a first source line SL 1  coupled to the first source structure  140 S 1  and a first bit line BL 1  coupled to the first drain structure  140 D 1 . The semiconductor memory structure  100  includes a second bit line BL 2  coupled to the second drain structure  140 D 2  and a second source line SL 2  coupled to the second source structure  140 S 2 . It should be noted that the first source line SL 1 , the first bit line BL 1 , the second source line SL 2  and the second bit line BL 2  extend along the second direction D 2 . Further, the first source line SL 1 , the first bit line BL 1 , the second source line SL 2  and the second bit line BL 2  are arranged in an order of SL 1 , BL 2 , SL 2  and BL 1 . In some embodiments, the second bit line BL 2  and the second source line SL 2  are between the first source line SL 1  and the first bit line BL 1 , and the second bit line BL 2  is between the first source line SL 1  and the second source line SL 2 , as shown in  FIG. 15 . In some embodiments, a width of a feature can be defined as a distance between two opposite sides measured along the first direction D 1 . In such embodiments, a width of the first source line SL 1 , a width of the first bit line BL 1 , a width of the second source line SL 2 , and a width of the second bit line BL 2  are similar. It should be noted that, a distance between the first source line SL 1  and the second bit line BL 2 , a distance between the second bit line BL 2  and the second source line SL 2 , and a distance between the second source line SL 2  and the first bit line BL 1  are similar. In some embodiments, pitches PT, which are defined as the center-to-center or edge-to-edge distance between features, between adjacent source and bit lines are substantially the same. 
     In some embodiments, the first source line SL 1 , the first bit line BL 1 , the second source line SL 2 , and the second bit line BL 2  are in a common layer of the interconnect structure  200 . For example, the first source line SL 1 , the first bit line BL 1 , the second source line SL 2 , and the second bit line BL 2  can all be referred to as the second metal layers M 2  of the interconnect structure  200  of the semiconductor memory structure  100 , but the disclosure is not limited thereto. 
     The semiconductor memory structure  100  includes a first conductive line  150 - 1  between the first source structure  140 S 1  and the first source line SL 1 , a second conductive line  150 - 2  between the first drain structure  140 D 1  and the first bit line BL 1 , a third conductive line  150 - 3  between the second drain structure  140 D 2  and the second bit line BL 2 , and a fourth conductive line  150 - 4  between the second source structure  140 S 2  and the second source line SL 2 . In some embodiments, the first conductive line  150 - 1  is disposed over the first source structure  140 S 1 . The second conductive line  150 - 2  is disposed over the first drain structure  140 D 1  and extends to cover a portion of the isolation  136 . The third conductive line  150 - 3  is disposed over the second drain structure  140 D 2 . The fourth conductive line  150 - 4  is disposed over the second source structure  140 S 2  and extends to cover a portion of the isolation  138 . In some embodiments, the first conductive line  150 - 1 , the second conductive line  150 - 2 , the third conductive line  150 - 3 , and the fourth conductive line  150 - 4  are in a common layer of the interconnect structure  200 . For example, the first conductive line  150 - 1 , the second conductive line  150 - 2 , the third conductive line  150 - 3 , and the fourth conductive line  150 - 4  can all be referred to as the first metal layers M 1  of the interconnect structure  200  of the semiconductor memory structure  100 , but the disclosure is not limited thereto. 
     As mentioned above, a width of a feature can be defined as a distance between two opposite sides measured along the first direction D 1 . Therefore, a width of the first conductive line  150 - 1  and a width of the second conductive line  150 - 2  are different from each other, as shown in  FIG. 17A . In some embodiments, the width of the second conductive line  150 - 2  is greater than the width of the first conductive line  150 - 1 . A width of the third conductive line  150 - 3  and a width of the fourth conductive line  150 - 4  are also different from each other, as shown in  FIG. 17B . In some embodiments, the width of the fourth conductive line  150 - 4  is greater than the width of the third conductive line  150 - 3 . In some embodiments, the first conductive line  150 - 1  and the second conductive line  150 - 2  are separated from each other by a first distance while the third conductive line  150 - 3  and the fourth conductive line  150 - 4  are separated from each other by a second distance different from the first distance. 
     For example, the second distance is less than the first distance, but the disclosure is not limited thereto. 
     Still referring to  FIGS. 16, 17A, and 17B , the semiconductor memory structure  100  includes a plurality of first vias V 0   a  coupling the first source structure  140 S 1  to the first conductive line  150 - 1  and coupling the first drain structure  140 D 1  to the second conductive line  150 - 2 . In some embodiments, the first vias V 0   a  can be the zeroth vias of the interconnect structure  200 . That is, the memory device formed over the substrate  102  can be coupled to the first metal layers M 1  of the interconnect structure  200 . The semiconductor memory structure  100  includes a plurality of second vias Via coupling the first conductive line  150 - 1  to the first source line SL 1  and coupling the second conductive line  150 - 2  to the first bit line BL 1 . In some embodiments, the second vias Via can be the first vias of the interconnect structure  200 . That is, the first metal layers M 1  of the interconnect structure  200  can be coupled to the second metal layers M 2  of the interconnect structure  200 . 
     Accordingly, the first vias V 0   a  and the second vias Via are in different layers of the interconnect structure  200 . 
     The semiconductor memory structure  100  further includes a plurality of third vias V 0   b  coupling the second drain structure  140 D 2  to the third conductive line  150 - 3  and coupling the second source structure  140 S 2  to the fourth conductive line  150 - 4 . As mentioned above, the third vias V 0   b  can be the zeroth vias of the interconnect structure  200 . That is, the memory device formed over the substrate  102  can be coupled to the first metal layers M 1  of the interconnect structure  200 . The semiconductor memory structure  100  further includes a plurality of fourth vias V 1   b  coupling the third conductive line  150 - 3  to the second bit line BL 2  and coupling the fourth conductive line  150 - 4  to the second source line SL 2 . In some embodiments, the fourth vias V 1   b  can be the first vias of the interconnect structure  200 . That is, the first metal layers M 1  of the interconnect structure  200  can be coupled to the second metal layers M 2  of the interconnect structure  200 . Accordingly, the third vias V 0   b  and the fourth vias V 1   b  are in different layers of the interconnect structure  200 . 
     It should be noted that the first vias V 0   a  and the third vias V 0   b  can be in a same layer or a same level, while the second vias Via and the fourth vias V 1   b  can be in a same layer or a same level over that of the first vias V 0   a  and the third vias V 0   b . In some embodiments, the first vias V 0   a , the third vias V 0   b , the first conductive layer  150 - 1 , the second conductive layer  150 - 2 , the third conductive layer  150 - 3  and the fourth conductive layer  150 - 4  can all be disposed in a same layer, such as a first inter-metal dielectric layer (IMD 1 ). The second vias Via, the fourth vias V 1   b , the first source line SL 1 , the second bit line BL 2 , the second source line SL 2 , and the first bit line BL 1  can all be disposed in a same layer, such as a second inter-metal dielectric layer (IMD 2 ) over the IMD 1 . However, the IMD 1  and the IMD 2  are omitted from  FIGS. 17A and 17B . In some embodiments, the first vias V 0   a  and the third vias V 0   b  can be in a same layer, while the first conductive layer  150 - 1 , the second conductive layer  150 - 2 , the third conductive layer  150 - 3  and the fourth conductive layer  150 - 4  can be in a same layer. Further, the second vias V 1   a  and the fourth vias V 1   b  are in a same layer while the first source line SL 1 , the second bit line BL 2 , the second source line SL 2 , and the first bit line BL 1  can be in a same layer. In such embodiments, the layer accommodating the first vias V 0   a  and the third vias V 0   b , and the layer accommodating the first conductive layer  150 - 1 , the second conductive layer  150 - 2 , the third conductive layer  150 - 3  and the fourth conductive layer  150 - 4  can be referred to as the IMD 1 . The layer accommodating the second vias V 1   a  and the fourth vias V 1   b , and the layer accommodating the first source line SL 1 , the second bit line BL 2 , the second source line SL 2 , and the first bit line BL 1  can be referred to as the IMD 2  over the IMD 1 . 
     In some embodiments, the interconnect structure  200  of the semiconductor memory structure  100  includes the first to fourth conductive lines  150 - 1 ,  150 - 2 ,  150 - 3  and  150 - 4  between the source/bit lines SL 1 , BL 2 , SL 2 , and BL 1  and the source/drain structures  140 S 1 .  140 D 2 ,  140 S 2 , and  140 B 1  in order to obtain even pitches between the source lines and the bit lines SL 1 , BL 2 , SL 2  and BL 1 . In some embodiments, the width of the fourth conductive line  150 - 4  is made greater than that of the third conductive line  150 - 3 , and the width of the second conductive line  150 - 2  is made greater than that of the first conductive line  150 - 1 . Further, the second conductive line  150 - 2  extends to overlap a portion of the isolation  136  while the fourth conductive line  140 - 4  extends to overlap a portion of the isolation  138  between the second source structure  140 S 2  and the second drain structure  140 D 2 . In some embodiments, the wider second and fourth conductive lines  150 - 2  and  150 - 4  and the different overlapping relationships help to optically arrange the routing of the overlying metal lines. Therefore, even pitches between the source lines and the bit lines SL 1 , BL 2 , SL 2  and BL 1  and a symmetric pattern formed by the source lines and the bit lines SL 1 , BL 2 , SL 2  and BL 1  can be obtained. It is found that the even pitches help to mitigate the issue of difficulty of manufacturing, while the symmetric pattern helps to mitigate the issue of difficulty of design. 
     In some embodiments, the abovementioned interconnect structure  200  can be formed over the memory cells, as shown in  FIG. 18 . In some alternative embodiments, the abovementioned interconnect structure  200  can be formed under the memory cells, as shown in  FIG. 18 . The manufacturing operations and element arrangements of the semiconductor memory structure  100 ′ can be similar to those of the semiconductor memory structure  100 ; therefore, repeated descriptions of such details are omitted for brevity. 
     Please refer to  FIGS. 19, 20A and 20B , wherein  FIG. 19  is a schematic drawing illustrating a layout structure of a portion of a semiconductor memory structure according to aspects of one or more embodiments of the present disclosure, and  FIGS. 20A and 20B  are exemplary cross-sectional views schematically illustrating vertical layer arrangements of different portions of an interconnect structure  300  of a semiconductor memory structure  100  according to aspects of one or more embodiments of the present disclosure. In some embodiments, the interconnect structure  300  can be disposed over memory cells as mentioned above, but the disclosure is not limited thereto. 
     In some embodiments, the semiconductor memory structure  100  includes a first cell C 1  and a second cell C 2 . In some embodiments, the first cell C 1  and the second cell C 2  can be disposed in the cell array region CA. The first cell C 1  and the second cell C 2  both extend along the first direction D 1 . Further, the first cell C 1  and the second cell C 2  are separated from each other along the second direction D 2 . In some embodiments, the first cell C 1  and the second cell C 2  are offset from each other along the first direction D 1 . In some embodiments, the first cell C 1  and the second cell C 2  can be arranged in a repeating manner in the cell array region CA to form a staggered pattern as shown in  FIG. 19 . The first cell C 1  includes a first source structure  140 S 1  and a first drain structure  140 D 1 . The first source structure  140 S 1  is separated from the first drain structure  140 D 1  by the isolation  138 . The second cell C 2  includes a second source structure  140 S 2  and a second drain structure  140 D 2 . The second source structure  140 S 2  is separated from the second drain structure  140 D 2  by the isolation  138 . It should be noted that the second layers  114  in the stack  110  serve as the gate layers (also referred to as the word lines) in the cell array region CA; therefore, the locations of the gate layers of each memory cell should be easily understood though not shown in  FIGS. 19, 20A and 20B . 
     In some embodiments, the semiconductor memory structure  100  includes a first bit line BL 1  coupled to the first drain structure  140 D 1  and a first source line SL 1  coupled to the first source structure  140 S 1 . The semiconductor memory structure  100  includes a second source line SL 2  coupled to the second source structure  140 S 2  and a second bit line BL 2  coupled to the second drain structure  140 D 2 . It should be noted that the first source line SL 1 , the first bit line BL 1 , the second source line SL 2  and the second bit line BL 2  extend along the second direction D 2 . In some embodiments, the first source line SL 1  and the first bit line BL 1  are in a common layer. For example, the first source line SL 1  and the first bit line BL 1  can be referred to as the third metal layers M 3  of the interconnect structure  300  of the semiconductor structure  100 . Further, the first source line SL 1  and the first bit line BL 1  can be periodically arranged to form an SL 1 /BL 1 /SL 1 /BL 1  pattern, as shown in  FIG. 20A . The second source line SL 2  and the second bit line BL 2  are in another common layer. For example, the first source line SL 1  and the first bit line BL 1  can be referred to as the second metal layers M 2  of the interconnect structure  300  of the semiconductor structure  100 . Further, the second source line SL 2  and the second bit line BL 2  can be periodically arranged to form an SL 2 /BL 2 /SL 2 /BL 2  pattern, as shown in  FIG. 20B . However, from a top view, the first source line SL 1 , the first bit line BL 1 , the second source line SL 2  and the second bit line BL 2  are arranged to form a BL 1 /BL 2 /SL 1 /SL 2  pattern, as shown in  FIGS. 19, 20A and 20B . 
     In some embodiments, a width of a feature can be defined as a distance between two opposite sides measured along the first direction D 1 . In such embodiments, a width of the first source line SL 1  and a width of the first bit line BL 1  are similar, while a width of the second source line SL 2  and a width of the second bit line BL 2  are similar. In some embodiments, the widths of the first source line SL 1  and the first bit line BL 1  can be similar to the widths of the second source line SL 2  and the second bit line BL 2 . It should be noted that a distance between the first source line SL 1  and the first bit line BL 1  is similar to a distance between the second source line SL 2  and the second bit line BL 2 , even though the first source and bit lines SL 1 /BL 1  and the second source and/bit lines SL 2 /BL 2  are in different layers. In some embodiments, a pitch PT 1  between the adjacent first source line SL 1  and first bit line BL 1  are substantially the same as a pitch PT 2  between the adjacent second source line SL 2  and second bit line BL 2 , as shown in  FIG. 19 . 
     The semiconductor memory structure  100  includes a first conductive line  150 - 1  between the first drain structure  140 D 1  and the first bit line BL 1 , a first conductive pad P 1  between the first conductive line  150 - 1  and the first bit line BL 1 , a second conductive line  150 - 2  between the first source structure  140 S 1  and the first source line SL 1 , and a second conductive pad P 2  between the second conductive line  150 - 2  and the first source line SL 1 . Further, the semiconductor memory structure  100  includes a third conductive line  150 - 3  between the second drain structure  140 D 2  and the second bit line BL 2 , and a fourth conductive line  150 - 4  between the second source structure  140 S 2  and the second source line SL 2 . As mentioned above, the first conductive line  150 - 1 , the second conductive line  150 - 2 , the third conductive line  150 - 3  and the fourth conductive line  150 - 4  are in a common layer. In some embodiments, the first conductive line  150 - 1 , the second conductive line  150 - 2 , the third conductive line  150 - 3  and the fourth conductive line  150 - 4  can be referred to as the first metal layers M 1  of the interconnect structure  300 . Further, the first conductive pad P 1  and the second conductive pad P 2  are in same common layer as the second bit line BL 2  and the second source line SL 2 . For example, the first conductive pad P 1 , the second conductive pad P 2 , the second bit line BL 2 , and the second source line SL 2  can be referred to as the second metal layers M 2  of the interconnect structure  300 . Further, the second bit line BL 2 , the first pad P 1 , the second source line SL 2  and the second pad P 2  can be periodically arranged, as shown in  FIG. 20A . The first bit line BL 1  and the first source line SL 1  are in a common layer. For example, the first bit line BL 1  and the first source line SL 1  can be referred to as the third metal layers M 3  of the interconnect structure  300 . 
     In some embodiments, a width of the first conductive line  150 - 1  and a width of the second conductive line  150 - 2  are different from each other, as shown in  FIGS. 19 and 20A . In some embodiments, the width of the second conductive line  150 - 2  is greater than the width of the first conductive line  150 - 1 . A width of the third conductive line  150 - 3  and a width of the fourth conductive line  150 - 4  are also different from each other, as shown in  FIGS. 19 and 20B . In some embodiments, the width of the fourth conductive line  150 - 4  is greater than the width of the third conductive line  150 - 3 . In some embodiments, the first conductive line  150 - 1  and the second conductive line  150 - 2  are separated from each other by a first distance while the third conductive line  150 - 3  and the fourth conductive line  150 - 4  are separated from each other by a second distance different from the first distance. For example, the second distance is greater than the first distance, but the disclosure is not limited thereto. 
     In some embodiments, the second conductive line  150 - 2  extends to overlap a portion of the isolation  138 , which is between the first drain structure  140 D 1  and the first source structure  140 S 1 , and a first overlapping region may be obtained. The fourth conductive line  150 - 4  extends to overlap a portion of the isolation  138 , which is between the second drain structure  140 D 2  and the second source structure  140 S 2 , and a second overlapping region may be obtained. In some embodiments, the first overlapping region can be greater than the second overlapping region, but the disclosure is not limited thereto. 
     Still referring to  FIGS. 19, 20A and 20B , the semiconductor memory structure  100 , includes a plurality of first vias V 0   a  coupling the first drain structure  140 D 1  to the first conductive line  150 - 1  and coupling the first source structure  140 S 1  to the second conductive line  150 - 2 . In some embodiments, the first vias V 0   a  can be referred to as the zeroth vias of the interconnect structure  300 . The semiconductor memory structure  100  includes a plurality of second vias Via coupling the first conductive line  150 - 1  to the first conductive pad P 1  and coupling the second conductive line  150 - 2  to the second conductive pad P 2 . In some embodiments, the second vias Via can be referred to as the first vias of the interconnect structure  300 . The semiconductor memory structure  100  further includes a plurality of third vias V 2  coupling the first conductive pad P 1  to the first bit line BL 1  and the second conductive pad P 2  to the first source line SL 1 . In some embodiments, the third vias V 2  can be referred to as the second vias of the interconnect structure  300 . Accordingly, the first vias V 0   a , the second vias V 1   a , and the third vias V 2  are in different layers of the interconnect structure  300 . 
     The semiconductor memory structure  100  further includes a plurality of fourth vias V 0   b  coupling the second drain structure  140 D 2  to the third conductive line  150 - 3  and coupling the second source structure  140 S 2  to the fourth conductive line  150 - 4 . In some embodiments, the fourth vias V 0   b  can be referred to as the zeroth vias of the interconnect structure  300 . The semiconductor memory structure  100  further includes a plurality of fifth vias V 1   b  coupling the third conductive line  150 - 3  to the second bit line BL 2  and coupling the fourth conductive line  150 - 4  to the second source line SL 2 . In some embodiments, the fifth vias V 1   b  can be referred to as the first vias of the interconnect structure  300 . Accordingly, the fourth vias V 0   b  and the fifth vias V 1   b  are in different layers of the interconnect structure  300 . 
     It should be noted that the first vias V 0   a  and the fourth vias V 0   b  can be in a same layer or a same level, while the second vias V 1   a  and the fifth vias V 1   b  can be in a same layer or a same level over that of the first vias V 0   a  and the fourth vias V 0   b . In some embodiments, the first vias V 0   a , the fourth vias V 0   b , the first conductive layer  150 - 1 , the second conductive layer  150 - 2 , the third conductive layer  150 - 3  and the fourth conductive layer  150 - 4  can all be disposed in a same layer, such as a first inter-metal dielectric layer (IMD 1 ). The second vias Via, the fifth vias V 1   b , the first pad P 1 , the second pad P 2 , the second bit line BL 2 , and the second source line SL 2  can all be disposed in a same layer, such as a second inter-metal dielectric layer (IMD 2 ) over the IMD 1 . The third vias V 2 , the first source line SL 1  and the first bit line BL 1  can be disposed in a same layer, such as the third inter-metal dielectric layer (IMD 3 ) over the IMD 2 . However, the IMD 1 , the IMD 2  and the IMD 3  are omitted from  FIGS. 20A and 20B . In some embodiments, the first vias V 0   a  and the fourth vias V 0   b  can be in a same layer, while the first conductive layer  150 - 1 , the second conductive layer  150 - 2 , the third conductive layer  150 - 3  and the fourth conductive layer  150 - 4  can be in a same layer. The layer accommodating the first vias V 0   a  and the fourth vias V 0   b , and the layer accommodating the first conductive layer  150 - 1 , the second conductive layer  150 - 2 , the third conductive layer  150 - 3  and the fourth conductive layer  150 - 4  can be referred to as the IMD 1 . Further, the second vias Via and the fifth vias V 1   b  are in a same layer while the first pad P 1 , the second pad P 2 , the second bit line BL 2  and the second source line SL 2  can be in a same layer. The layer accommodating the second vias V 1   a  and the fifth vias V 1   b , and the layer accommodating the first pad P 1 , the second pad P 2 , the second bit line BL 2  and the second source line SL 2  can be referred to as the IMD 2  over the IMD 1 . The third vias V 2  can be in one layer while the first source line SL 1  and the first bit line BL 1  are in another layer. The layer accommodating the third via V 2  and the layer accommodating the first source line SL 1  and the first bit line BL 1  can be referred to as the IMD 3  over the IMD 2 . 
     In some embodiments, the interconnect structure  300  of the semiconductor memory structure  100  includes the first to fourth conductive lines  150 - 1 ,  150 - 2 ,  150 - 3 ,  150 - 4 ; the first and second pads P 1 , P 2  between the source and bit lines SL 1 , BL 2 , SL 2 , BL 1 ; and the source and drain structures  140 S 1 ,  140 D 2 ,  140 S 2 ,  140 B 1  in order to obtain even pitches between the source and bit lines BL 1 , BL 2 , SL 1 , SL 2 . In some embodiments, the first source line SL 1  and the first bit line BL 1  for connecting the first cell C 1  are formed in the layer overlying the second source line SL 2  and the second bit line BL 2  for connecting the second cell C 2 . Further, the rearrangement of routing can be achieved by introduction of the conductive lines  150 - 1  to  150 - 4  and the conductive pads P 1  and P 2 . Consequently, even pitches are obtained between the first source lines SL 1  and the first bit lines BL 1  in the same layer. Further, even pitches are obtained between the second bit line BL 2 , the first pad P 1 , the second source line SL 2  and the second pad P 2  in the same layer. Further, a symmetric pattern formed by the first source lines SL 1  and the first bit lines BL 1  in the same layer is obtained, while a symmetric pattern formed by the second bit line BL 2 , the first pad P 1 , the second source line SL 2  and the second pad P 2  in the same layer is obtained. It is found that the even pitches help to mitigate the issue of difficulty of manufacturing, while the symmetric pattern helps to mitigate the issue of difficulty of design. 
     Accordingly, the present disclosure provides a semiconductor memory structure and an interconnect structure of the semiconductor memory structure. In some embodiments, the semiconductor memory structure includes an even SL/BL line pitch and a symmetric layout structure. In some embodiments, by introducing the intervening conductive lines, such as the first to fourth conductive lines between the source/bit lines and the source/drain structures, the source/bit line (SL/BL) routing can be arranged such that the line pitch can be made even. In some embodiments, by separating even metal lines of different cells in different layers, the line pitch can be increased and made even. Accordingly, the uneven line pitch issue is mitigated and a symmetric interconnect layout structure can be obtained. 
     In some embodiments, a semiconductor memory structure is provided. The semiconductor memory structure includes a first cell, a second cell, a first bit line, a first source line, a second bit line and a second source line. The first cell and the second cell extend along a first direction. The first cell and the second cell are separated from each other along a second direction different from the first direction. The first cell includes a first source structure and a first drain structure, and the second cell includes a second source structure and a second drain structure. The first source line is coupled to the first source structure, and the first bit line is coupled to the first drain structure. The second source line is coupled to the second source structure, and the second bit line is coupled to the second drain structure. The first source line, the first bit line, the second source line, and the second bit line are in a common layer of an interconnect structure. The second bit line and the second source line are between the first source line and the first bit line. The second bit line is between the first source line and the second source line. A distance between the first source line and the second bit line, a distance between the second bit line and the second source line, and a distance between the second source line and the first bit line are similar. 
     In some embodiments, a semiconductor memory structure is provided. The semiconductor memory structure includes a first cell, a second cell, a first bit line, a first source line, a second bit line and a second source line. The first cell and the second cell extend along a first direction. The first cell and the second cell are separated from each other along a second direction different from the first direction. The first cell includes a first source structure and a first drain structure, and the second cell includes a second source structure and a second drain structure. The first bit line is coupled to the first drain structure, and the first source line is coupled to the first source structure. The second bit line is coupled to the second drain structure, and the second source line is coupled to the second source structure. The first source line and the first bit line are in a first common layer of an interconnect structure. The second source line and the second bit line are in a second common layer of the interconnect structure, and the second common layer is under the first common layer. A distance between the first source line and the first bit line is similar to a distance between the second source line and the second bit line. 
     In some embodiments, an interconnect structure is provided. The interconnect structure includes a first conductive line over a first source structure of a first cell, a second conductive line over a first drain structure of the first cell, a first source line over the first conductive line, a first bit line over the second conductive line, a third conductive line over a second drain structure of a second cell, a fourth conductive line over a second source structure of the second cell, a second bit line over the third conductive line, and a second source line over the fourth conductive line. The first conductive line and the second conductive line are separated from each other by a first distance. The third conductive line and the fourth conductive line are separated from each other by a second distance. The second distance is less than the first distance. A distance between the first source line and the second bit line, a distance between the second bit line and the second source line, and a distance between the second source line and the first bit line are similar. 
     The foregoing outlines features of several embodiments so that those skilled in the art may better understand the aspects of the present disclosure. Those skilled in the art should appreciate that they may readily use the present disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiments introduced herein. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the present disclosure, and that they may make various changes, substitutions, and alterations herein without departing from the spirit and scope of the present disclosure.