Patent Publication Number: US-2023137806-A1

Title: Semiconductor device

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the priority benefit of U.S. provisional applications serial no. 63/275,934, filed on Nov. 4, 2021. The entirety of the above-mentioned patent application is hereby incorporated by reference herein and made a part of this specification. 
    
    
     BACKGROUND 
     New semiconductor applications are ever changing our lives, from new smartphones, to healthcare, factory automation and artificial intelligence. Memory working in background plays an important role in enabling these technologies, and has drawn considerable interest along with advances in computing architectures and semiconductor technologies. Static random access memory (SRAM) is widely used as on-chip cache for microprocessors, as it is inherently fast and highly compatible with complementary metal-oxide-semiconductor (CMOS) process. However, SRAM occupies an inordinate amount of real estate on a semiconductor chip. Therefore, an ability to scale down the SRAM is critical at each generation node. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Aspects of the present disclosure are best understood from the following detailed description when read with the accompanying figures. It is 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. 
         FIG.  1    is a three-dimensional view of a semiconductor device according to some embodiments of the present disclosure. 
         FIG.  2 A  and  FIG.  2 B  are three-dimensional views of a memory array in a semiconductor device according to various embodiments of the present disclosure. 
         FIG.  3    is a circuit diagram of a memory cell in a semiconductor device according to some embodiments of the present disclosure. 
         FIG.  4 A  is a top view illustrating a first section of memory cell located at a second level over a semiconductor substrate according to some embodiments of the present disclosure. 
         FIG.  4 B  is a top view illustrating a second section of memory cell located at a second level over a semiconductor substrate according to some embodiments of the present disclosure. 
         FIG.  5 A  is a schematic cross-sectional view of a semiconductor device taken along line A-A′ shown in  FIG.  4 A  and  FIG.  4 B . 
         FIG.  5 B  is a schematic cross-sectional view of a semiconductor device taken along line B-B′ shown in  FIG.  4 A  and  FIG.  4 B . 
         FIG.  6 A  to  FIG.  6 E  are cross-sectional views illustrating structures at various stages of manufacturing a transistor in a SRAM region according to some embodiments of the present disclosure. 
         FIG.  7    is a schematic cross-sectional view of a semiconductor device taken along line A-A′ shown in  FIG.  4 A  and  FIG.  4 B  in accordance with an alternative embodiment 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 components, values, operations, materials, arrangements, or the like, are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. Other components, values, operations, materials, arrangements, or the like, are contemplated. 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” 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 apparatus may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein may likewise be interpreted accordingly. 
     In conventional semiconductor devices, such as SRAM memory devices, the SRAM array are usually fabricated at the front-end along with the logic circuits. As the SRAM array are formed aside the logic circuits on the semiconductor substrate, a large area would be occupied, and a footprint area of the semiconductor device will be increased. In accordance with some embodiments of the present disclosure, means of scaling down the footprint area of the semiconductor device is described. 
       FIG.  1    is a three-dimensional view of a semiconductor device according to some embodiments of the present disclosure. Referring to  FIG.  1   , a semiconductor device  100  includes a logic region LR formed on a semiconductor substrate  102 . In some embodiments, the semiconductor device  100  is a three-dimensional SRAM semiconductor device involving a front-end-of-line (FEOL) device stacking and a back-end-of-line (BEOL) device stacking applications. The semiconductor substrate  102  may be a semiconductor wafer, such as a silicon wafer, or the like. The logic region LR may include logic circuits  104  having active structures and gate structures formed on the semiconductor substrate  102 . For example, the logic circuits  104  having active structures and gate structures are disposed at a first level LX 1  (ground level) on the semiconductor substrate  102 . The first level LX 1  described above may be a front-end-of-line (FEOL) structure of a semiconductor device (semiconductor chip). 
     In some embodiments, a static random access memory (SRAM) region SR is formed on the logic region LR within an area overlapped with the logic region LR. For example, in one embodiment, the SRAM region SR is formed on the logic region LR and does not extend beyond a periphery of the logic region LR. In some embodiments, the SRAM region SR includes a first SRAM memory array MA 1 , a second SRAM memory array MA 2  and a third SRAM memory array MA 3  stacked up over the semiconductor substrate  102 . The first SRAM memory array MA 1  includes a plurality of first memory cells MCI disposed at a second level LX 2  over the semiconductor substrate  102 . The second SRAM memory array MA 2  includes a plurality of second memory cells MC 2  disposed at a third level LX 3  over the semiconductor substrate  102 . The third SRAM memory array MA 3  includes a plurality of third memory cells MC 3  disposed at a fourth level LX 4  over the semiconductor substrate  102 . In the exemplary embodiment, the second level LX 2  is stacked on top and overlapped with the first level LX 1 . The third level LX 3  is stacked on top and overlapped with the second level LX 2 . The fourth level LX 4  is stacked on top and overlapped with the third level LX 3 . In some embodiments, the second level LX 2 , the third level LX 3  and the fourth level LX 4  described above may be embedded in a back-end-of-line (BEOL) structure of a semiconductor device (semiconductor chip), and is formed over the FEOL structure. 
     Although three layers of stacked memory arrays are illustrated in the SRAM region SR of  FIG.  1   , it is noted that the disclosure is not limited thereto. For example, in some embodiments, there is only one layer of memory array in the SRAM region SR that is located on the logic region LR. In some alternative embodiments, there are two layers, three layers, four layers or more layers of memory array located on the logic region LR. In other words, the number of layers of stacked memory arrays may be adjusted based on design requirement. 
     As further illustrated in  FIG.  1   , in some embodiments, through vias  106  are electrically connecting the first SRAM memory array MA 1 , the second SRAM memory array MA 2  and the third SRAM memory array MA 3  to the logic region LR. For example, the through vias  106  are located in a central region CX of the first SRAM memory array MA 1  extending from the second level LX 2  to the first level LX 1 , and provides a vertical conduction path electrically connecting the first SRAM memory array MAI to the logic region LR. Similarly, the through vias  106  are further located in a central region CX of the second SRAM memory array MA 2  extending from the third level LX 3  to the second level LX 2 , and provides the vertical conduction path electrically connecting the second SRAM memory array MA 2  to the first memory array MA 1  and the logic region LR. In a similar way, the through vias  106  are further located in a central region CX of the third SRAM memory array MA 3  extending from the fourth level LX 4  to the third level LX 3 , and provides the vertical conduction path electrically connecting the third SRAM memory array MA 3  to the second SRAM memory array MA 2 , the first SRAM memory array MA 1  and the logic region LR. 
     In some embodiments, auxiliary through vias  108  may be further provided on a peripheral region PX of each of the first SRAM memory array MA 1 , the second SRAM memory array MA 1  and the third SRAM memory array MA 3  to electrically connect these memory arrays (MA 1 , MA 2 , MA 3 ) to the logic region LR. In other words, the auxiliary through vias  108  also extend from the fourth level LX 4  to the third level LX 3 , from the third level LX 3  to the second level LX 2 , and from the second level LX 2  to the first level LX 1 , to provide a second vertical conduction path of the these memory arrays (MA 1 , MA 2 , MA 3 ) to the logic region LR. 
       FIG.  2 A  and  FIG.  2 B  are three-dimensional views of a memory array in a semiconductor device according to various embodiments of the present disclosure. In the embodiment illustrated in  FIG.  1   , each of the memory arrays (MA 1 , MA 2 , MA 3 ) are connected to the logic region LR by through vias  106  located in the central region CX and auxiliary through vias  108  located in the peripheral region PX. However, the disclosure is not limited thereto. For example, as illustrated in  FIG.  2 A , in one embodiment, each of the memory arrays (MA 1 , MA 2 , MA 3 ) may be connected to the logic region LR by the through vias  106  in the central region CX, while the auxiliary through vias  108  in the peripheral region PX are omitted. As illustrated in  FIG.  2 B , in an alternative embodiment, each of the memory arrays (MA 1 , MA 2 , MA 3 ) may be connected to the logic region LR by the through vias  106  in the central region CX. Furthermore, each of the memory arrays (MA 1 , MA 2 , MA 3 ) may be connected to the logic region LR by the auxiliary through vias  108  that are located in a peripheral region PX that surrounds (or encircles) the memory cells (MC 1 , MC 2 , MC 3 ) and the central region CX. From these embodiments, it is noted that the routing arrangements of the memory arrays (MA 1 , MA 2 , MA 3 ) for electrical connection to the logic region LR located below is not particularly limited, and may be adjusted based on design requirements. 
       FIG.  3    is a circuit diagram of a memory cell in a semiconductor device according to some embodiments of the present disclosure. The circuit diagram may correspond to each of the memory cells (MC 1 , MC 2 , MC 3 ) located in the memory arrays (MA 1 , MA 2 , MA 3 ). In some embodiments, the memory cells (MC 1 , MC 2 , MC 3 ) may be a  6 T (six transistor) SRAM cell. As illustrated in  FIG.  3   , the memory cells (MC 1 , MC 2 , MC 3 ) includes a latch circuit  210 . 
     The latch circuit  210  is configured to retain stored data without being periodically refreshed. In some embodiments, the latch circuit  210  includes two inverters. A first inverter may include a pull up transistor  212  and a pull down transistor  214 . The pull up transistor  212  may be a P-type field effect transistor (PFET), while the pull down transistor  214  may be an N-type field effect transistor (NFET). The pull up transistor  212  and the pull down transistor  214  share a common source/drain terminal, and such common source/drain terminal may be referred as a storage node SN 1  of the memory cell (MC 1 , MC 2 , MC 3 ). In addition, the other source/drain terminal of the pull up transistor  212  is coupled to a working voltage V DD . On the other hand, the other source/drain terminal of the pull down transistor  214  is coupled to a reference voltage Vss, such as a ground voltage. Furthermore, gate terminals of the pull up transistor  212  and the pull down transistor  214  are connected with each other. A node N 1  coupled to the gate terminals of the pull up transistor  212  and the pull down transistor  214  may be an input terminal of the first inverter, and the storage node SN 1  may be an output terminal of the first inverter. 
     Similarly, a second inverter in the latch circuit  210  may include a pull up transistor  216  and a pull down transistor  218 . The pull up transistor  216  may be a PFET, while the pull down transistor  218  may be an NFET. The pull up transistor  216  and the pull down transistor  218  share a common source/drain terminal, which may be referred as a storage node SN 2  of the memory cell (MC 1 , MC 2 , MC 3 ). The other source/drain terminal of the pull up transistor  216  is coupled to the working voltage V DD , while the other source/drain terminal of the pull down transistor  218  is coupled to the reference voltage Vss. In addition, gate terminals of the pull up transistor  216  and the pull down transistor  218  are connected with each other. A node N 2  coupled to the gate terminals of the pull up transistor  216  and the pull down transistor  218  may be an input terminal of the second inverter, while the storage node SN 2  may be an output terminal of the second inverter. 
     The node N 1  as the input terminal of the first inverter is coupled to the storage node SN 2  as the output terminal of the second inverter, and the node N 2  as the input terminal of the second inverter is coupled to the storage node SN 1  as the output terminal of the first inverter. In other words, the first and second inverters of the latch circuit  210  are cross-coupled. As a result, the storage nodes SN 1 , SN 2  are ensured to store complementary logic data. For instance, when a logic data “ 0 ” is stored at the storage node SN 1 , the P-type pull up transistor  216  may be turned on as its gate terminal is coupled to the storage node SN 1 , and the storage node SN 2  as a source/drain terminal of the pull up transistor  216  is pulled up by the working voltage V DD  coupled to the other source/drain terminal of the pull up transistor  216 . Therefore, a logic data “ 1 ” complementary to the logic data “ 0 ” is stored at the storage node SN 2 . 
     On the other hand, the N-type pull down transistor  218  is kept in an off state as its gate terminal is also coupled to the storage node SN 1  holding at the logic data “ 0 ”, thus the storage node SN 2  as a source/drain terminal of the pull down transistor  218  would not be pulled down by the reference voltage Vss coupled to the other source/drain terminal of the pull down transistor  218 . In addition, the N-type pull down transistor  214  is turned on as its gate terminal is coupled to the storage node SN 2  holding at the logic data “ 1 ”, and the storage node SN 1  as a source/drain terminal of the pull down transistor  214  is kept discharged by the reference voltage Vss coupled to the other source/drain terminal of the pull down transistor  214 . In addition, the P-type pull up transistor  212  is kept in an off state as its gate terminal is also coupled to the storage node SN 2  holding at the logic data “ 1 ”, thus the storage node SN 1  as a source/drain terminal of the pull up transistor  212  would not be pulled up by the working voltage V DD  coupled to the other source/drain terminal of the pull up transistor  212 . Therefore, the logic data “ 0 ” can be retained at the storage node SN 1 . 
     Moreover, the memory cell (MC 1 , MC 2 , MC 3 ) may further include an access transistor  220 . A gate terminal of the access transistor  220  is connected to a word line WL. In addition, a source/drain terminal of the access transistor  220  is coupled to the storage node SN 1 , while the other source/drain terminal of the access transistor  220  is connected to a bit line BL 1 . When the access transistor  220  is turned on, the bit line BL 1  can charge/discharge the storage node SN 1 , or vice versa. Accordingly, logic data can be programmed to the storage node SN 1 , or read out from the storage node SN 1 . On the other hand, when the access transistor  220  is in an off state, the storage node SN 1  is decoupled from the bit line BL 1 , and logic data cannot be written to or read out from the storage node SN 1 . In other words, the access transistor  220  may control access of the storage node SN 1 . 
     Similarly, access of the storage node SN 2  is controlled by an access transistor  222 . The word line WL for controlling switching of the access transistor  220  may also connect to a gate terminal of the access transistor  222 . In this way, the access transistors  220 ,  222  may be switched simultaneously. In addition, a source/drain terminal of the access transistor  222  is coupled to the storage node SN 2 , while the other source/drain terminal of the access transistor  220  is connected to a bit line BL 2 . When the access transistor  222  is turned on, the bit line BL 2  can charge/discharge the storage node SN 2 , or vice versa. Accordingly, logic data can be programmed to the storage node SN 2 , or read out from the storage node SN 2 . On the other hand, when the access transistor  222  is in an off state, the storage node SN 2  is decoupled from the bit line BL 2 , and logic data cannot be written to or read out from the storage node SN 2 . During a write operation, the bit lines BL 1 , BL 2  may receive complementary logic data, in order to overwrite the logic data previously stored at the storage nodes SN 1 , SN 2 . In addition, during a read operation, both of the bit lines BL 1 , BL 2  are pre-charged, and one of them is slightly pulled down by the corresponding storage node. By comparing voltage difference of the bit lines BL 1 , BL 2 , the logic data stored at the storage nodes SN 1 , SN 2  can be read out. 
     The arrangement of the memory cell (MC 1 , MC 2 , MC 3 ) will be described in more details by referring to  FIG.  4 A  to  FIG.  5 B . The first memory cell MC 1  located at the second level LX 2  will be used as an example for description, but it should be noted that the arrangements shown in  FIG.  4 A  to  FIG.  5 B  may be applied to the memory cells (MC 2 , MC 3 ) located at other levels. 
       FIG.  4 A  is a top view illustrating a first section LX 2 -a of the first memory cell MC 1  located at a second level LX 2  over a semiconductor substrate  102  according to some embodiments of the present disclosure. As illustrated in  FIG.  4 A , NFETs including the pull down transistors  214 ,  218  and the access transistors  220 ,  222  in each first memory cell MC 1  are formed at the second level LX 2  (first section LX 2 - a ) over the logic region LR of the semiconductor substrate  102 . Similarly, PFETs including the pull up transistors  212 ,  216  in each first memory cell MC 1  may be formed at the second level LX 2  (first section LX 2 - a ) over the logic region LR of the semiconductor substrate  102 . 
     In some embodiments, each first memory cell MC 1  includes a plurality of gate lines GLX. The gate lines GLX includes a first gate line GL 1 , a second gate line GL 2 , a third gate line GL 3  and a fourth gate line GL 4  extending along a first direction D 1 . The firs gate line GL 1 , the second gate line GL 2 , the third gate line GL 3  and the fourth gate line GL 4  are formed at the second level LX 2  (first section LX 2 - a ) and spaced apart from one another. In some embodiments, each first memory cell MC 1  further includes a plurality of active structures ASX. The active structures ASX includes a first active structure AS 1 , a second active structure AS 2 , a third active structure AS 3  and a fourth active structure AS 4  extending along a second direction D 2 , wherein the second direction D 2  is perpendicular to the first direction D  1 . The first active structure AS 1 , the second active structure AS 2 , the third active structure AS 3  and the fourth active structure AS 4  are formed at the second level LX 2  (first section LX 2 - a ) and spaced apart from one another. Source/drain structures (not shown) may be formed in portions of the active structure ASX at opposite sides of the gate line GLX. 
     In some embodiments, the pull down transistor  214  and the access transistor  220  connected by a common source/drain terminal may be formed on the first active structure AS 1  extending along the second direction D 2 . The pull up transistor  212  may be formed on the second active structure AS 2 , while the pull up transistor  216  may be formed on the third active structure AS 3 . Furthermore, the pull down transistor  218  and the access transistor  222  connected by a common source/drain terminal may be formed on the fourth active structure AS 4 . 
     In some embodiments, the first gate line GL 1  is functioned as a gate terminal of the access transistor  220 . The first gate line GL 1  intersects and covers the first active structure AS 1 . In some embodiments, the second gate line GL 2  is functioned as a gate terminal of the pull down transistor  214  and the pull up transistor  212 . The second gate line GL 2  intersects and covers the first active structure AS 1  and the second active structure AS 2 . In some embodiments, the third gate line GL 3  is functioned as the gate terminal of the pull up transistor  216  and the pull down transistor  218 . The third gate line GL 3  intersects and covers the third active structure AS 3  and the fourth active structure AS 4 . In some embodiments, the fourth gate line GL 4  is functioned as a gate terminal of the access transistor  222 . The fourth gate line GL 4  intersects and covers the fourth active structure AS 4 . 
     In some embodiments, the pull down transistors  214 ,  218 , the pull up transistors  212 ,  216  and the access transistors  220 ,  222  are planar type field effect transistors (FETs), and the active structures ASX may be a doped region in a semiconductor material. In alternative embodiments, the pull down transistors  214 ,  218 , the pull up transistors  212 ,  216  and the access transistors  220 ,  222  are fin type FETs or gate-all-around (GAA) FETs, and the active structures ASX may be a semiconductor fin structure or a stack of semiconductor nanosheets/rods at a surface of a semiconductor material. Since the pull down transistor  214 ,  218  and the access transistor  220 ,  222  are NFETs, the active structures AS 1 , AS 4  may be formed with P-type. Since the pull up transistors  212 ,  216  are PFETs, the active structures AS 2 , AS 3  may be formed with N-type. 
     As further illustrated in  FIG.  4 A , each of the first memory cell MC 1  includes a plurality of contact structures CSX. The contact structures CSX include bit line contacts CBL 1 , CBL 2 , a first node contact CN 1 , a second node contact CN 2 , power contacts CVdd 1 , CVdd 2 , and ground contacts CVss 1 , CVss 2 . The contact structures CSX are located on the active structures ASX and located aside the gate lines GLX. The bit line contact CBL 1  is electrically coupled to a source/drain terminal of the access transistor  220 , and the bit line contact CBL 2  is electrically coupled to a source/drain terminal of the access transistor  222 . The power contact CVdd 1  is electrically coupled to a source/drain terminal of the pull-up transistor  216 , while the power contact CVdd 2  is electrically coupled a source/drain terminal of the pull-up transistor  212 . The ground contact CVss 1  is electrically coupled to a source/drain terminal of the pull-down transistor  214 , while the ground contact CVss 2  is electrically coupled to a source/drain terminal of the pull-down transistor  218 . The first node contact CN 1  electrically couples together other source/drain terminals of the access transistor  220 , the pull up transistor  212  and the pull down transistor  214 . The second node contact CN 2  electrically couples together other source/drain terminals of the access transistor  222 , the pull up transistor  216  and the pull down transistor  218 . 
     In some embodiments, a butted contact BC 1  electrically couples the first node contact CN 1  and the third gate line GL 3 , and extends above the first node contact CN 1  and the third gate line GL 3 . Similarly, another butted contact BC 2  electrically couples the second node contact CN 2  and the second gate line GL 2 , and extends above the second node contact CN 2  and the second gate line GL 2 . 
     In some embodiments, a via structure VA 1  is formed on the bit line contact CBL 1  for out routing one of the source/drain terminals of the access transistor  220 . In some embodiments, a via structure VA 2  is formed on the first gate line GL 1  for out routing the first gate line GL 1 . In some embodiments, a via structure VA 3  is disposed on the ground contact CVss 1  for out routing one of the source/drain terminals of the pull down transistor  214 . In some embodiments, a via structure VA 4  is disposed on the power contact CVdd 1  for out routing one of the source/drain terminals of the pull up transistor  216 . In some embodiments, a via structure VA 5  is disposed on the power contact CVdd 2  for out routing one of the source/drain terminals of the pull up transistor  212 . In some embodiments, a via structure VA 6  is disposed on the ground contact CVss 2  for out routing one of the source/drain terminals of the pull down transistor  218 . In some embodiments, a via structure VA 7  is formed on the fourth gate line GL 4  for out routing the fourth gate line GL 4 . In some embodiments, a via structure VA 8  is formed on the bit line contact CBL 2  for out routing one of the source/drain terminals of the access transistor  222 . 
       FIG.  4 B  is a top view illustrating a second section LX 2 -b of the first memory cell MC 1  located at the second level LX 2  over a semiconductor substrate  102  according to some embodiments of the present disclosure. The second section LX 2 -b is located above the first section LX 2 -a. As illustrated in  FIG.  4 B , the first memory cell MC 1  further includes a plurality of conductive elements CEX extending above the pull down transistors  214 ,  218 , the pull up transistors  212 ,  216  and the access transistors  220 ,  222 . In some embodiments, the conductive elements CEX includes a first bit line BL 1 , a second bit line BL 2 , a first word line contact CWL 1 , a second word line contact CWL 2 , interconnect structures IC 1 , IC 2 , a power line Vdd, reference lines VSS, and a word line WL. 
     As illustrated in  FIG.  4 A  and  FIG.  4 B , the first bit line BL 1  extends above the first active structure AS 1 , and is electrically coupled to the access transistor  220  through the via structure VA 1  and the bit line contact CBL 1 . The second bit line BL 2  extends above the fourth active structure AS 4 , and is electrically coupled to the access transistor  222  through the via structure VA 8  and the bit line contact CBL 2 . In some embodiments, the first bit line BL 1  and the second bit line BL 2  extends towards the central region CX of the first SRAM memory array MA 1  (as illustrated in  FIG.  1   ) to be physically connected to the through vias  106 . For example, the through vias  106  extends from the second level LX 2  to the first level LX 1 , and provides a vertical conduction path electrically connecting the first bit line BL 1  and the second bit line BL 2  to the logic region LR (as illustrated in  FIG.  1   ). 
     In some embodiments, the first word line contact CWL 1  extends above the first gate line GL 1 , and is electrically coupled to the access transistor  220  through the via structure VA 2 . The second word line contact CWL 2  extends above the fourth gate line GL 4 , and is electrically coupled to the access transistor  222  through the via structure VA 7 . Furthermore, a word line WL extends above the first word line contact CWL 1  and the second word line contact CWL 2 , and is electrically coupled to the first word line contact CWL 1  and the second word line contact CWL 2  through the via structures VA 9 , VA 12 . In some embodiments, the word line WL extends towards the peripheral region PX of the first SRAM memory array MA 1  (as illustrated in  FIG.  1   ) to be physically connected to the auxiliary through vias  108 . For example, the auxiliary through vias  108  extends from the second level LX 2  to the first level LX 1 , and provides a vertical conduction path electrically connecting the word line WL to the logic region LR (as illustrated in  FIG.  1   ). 
     In some embodiments, the power line Vdd extends above the power contacts CVdd 1 , CVdd 2  and is electrically coupled to the power contacts CVdd 1 , CVdd 2  through the via structures VA 4 , VA 5 . In certain embodiments, the power line Vdd is a signal line coupled to the working voltage V DD  as described with reference to  FIG.  3   . In some embodiments, the interconnect structures IC 1 , IC 2  extend above the ground contacts CVss 1 , CVss 2  and is electrically coupled to the ground contacts CVss 1 , CVss 2  through the via structures VA 3 , VA 6 . In some embodiments, the reference lines VSS extend above the interconnect structures IC 1 , IC 2 , and is electrically coupled to the interconnect structures IC 1 , IC 2  through the via structures VA 10 , VA 11 . In certain embodiments, the reference lines VSS indicates a signal line coupled to the reference voltage Vss as described with reference to  FIG.  3   . 
       FIG.  5 A  is a schematic cross-sectional view of a semiconductor device taken along line A-A′ shown in  FIG.  4 A  and  FIG.  4 B .  FIG.  5 B  is a schematic cross-sectional view of a semiconductor device taken along line B-B′ shown in  FIG.  4 A  and  FIG.  4 B . The logic circuit  104  disposed at the first level LX 1  over the semiconductor substrate  102 , and the first memory cells MCI disposed at the second level LX 2  over the semiconductor substrate  102  will be described in more detail with reference to  FIG.  5 A  and  FIG.  5 B . The same reference numerals applied in  FIG.  1    to  FIG.  4 B  may be used to refer to the same or liked parts in  FIG.  5 A  and  FIG.  5 B . Therefore, the detailed description of each element may be omitted or simplified. 
     Referring to  FIG.  5 A  and  FIG.  5 B , at the first level LX 1  over the semiconductor substrate  102 , logic circuits  104  having logic active structures  302  and gate structures GSX are provided. The logic active structures  302  and the gate structures GSX form a plurality of transistors on the semiconductor substrate. For example, the logic active structures  302  are disposed at the first level LX 1  on the semiconductor substrate  102 , and first isolation structures  304  may be formed at the first level LX 1  on the semiconductor substrate  102  separating the logic active structures  302 . In some embodiments, when the transistors at the first level LX are planar type FETs, then the logic active structures  302  may be a doped region in the semiconductor substrate  102 . In some other embodiments where the transistors at the first level LX 1  are fin type FETs or GAA FETs, then the logic active structures  302  may be a semiconductor fin structure or a stack of semiconductor nanosheets/rods formed at a surface of the semiconductor substrate  102 . 
     In some embodiments, each transistor located at the first level LX 1  may include one gate structure GSX. For example, the gate structures GSX may include a gate line  308  disposed on a logic active structure  302 ; a gate dielectric layer  310  lying between the gate line  308  and the logic active structure  302 ; and sidewall spacers  312  covering sidewalls of the gate line  308  and the gate dielectric layer  310 . In some embodiments, each transistor located at the first level LX 1  further includes source/drain terminals  306  (source/drain regions) located at opposite sides of the gate structure GSX. Furthermore, conductive patterns  314  and conductive vias  318  are formed on the source/drain terminals  306  for out routing the transistors in the logic region. In certain embodiments, the conductive patterns  314  and the gate structures GSX are laterally surrounded by a dielectric layer  315 . Similarly, the conductive vias  318  are laterally surrounded by another dielectric layer  316 . 
     In some embodiments, at the second level LX 2  over the semiconductor substrate  102 , the first memory cell MCI described in  FIG.  4 A  and  FIG.  4 B  may be formed over the logic circuit  104 . For example, at the second level LX 2 , a stack of interlayer dielectrics  320 ,  330  may be formed on the dielectric layer  316  covering the logic circuits  104 , and interconnection patterns  322  may be embedded in the interlayer dielectric  320  for connecting to the conductive patterns  314  located at the first level LX 1 . 
     As further illustrated in  FIG.  5 A  and  FIG.  5 B , active structures ASX including the first active structure AS 1 , the second active structure AS 2 , the third active structure AS 3  and the fourth active structure AS 4  are disposed on the interlayer dielectric  330  at the second level LX 2  on the semiconductor substrate  102 . In some embodiments, second isolation structures  360  are formed at the second level LX 2  on the semiconductor substrate  102  and separating the active structures ASX (including AS 1 , AS 2 , AS 3 , AS 4 ) from one another. For example, a height of the second isolation structures  360  may be substantially equal to a height of the active structures ASX. In some embodiments, an interlayer dielectric  350  is disposed on the interlayer dielectric  330  and surrounding the active structures ASX and the second isolation structures  360 . 
     In some embodiments, the transistors (NFETs, PFETs) located at the second level LX 2  may include gate structures having gate lines GLX (including GL 1 , GL 2 , GL 3 , GL 4 ). For example, each of the gate structures may include a gate line GLX (either one of GL 1 , GL 2 , GL 3 , GL 4 ) disposed on the active structures ASX; a gate dielectric layer  404  lying between the gate line GLX and the active structure ASX; and sidewall spacers  406  covering sidewalls of the gate line GLX and the gate dielectric layer  404 . In some embodiments, each transistor located at the second level LX 2  further includes source/drain terminals  402  formed in the active structures ASX, and located at opposite sides of the gate structure. Furthermore, contact structures CSX including include bit line contacts CBL 1 , CBL 2 , a first node contact CN 1 , a second node contact CN 2 , power contacts CVdd 1 , CVdd 2 , and ground contacts CVss 1 , CVss 2  are formed over the source/drain terminals  402  for out routing the transistors (NFETs, PFETs) located at the second level LX 2  (only the ground contact CVss 2 , first node contact CN 1 , second node contact CN 2  and the bit line contact CBL 2  are illustrated). 
     As further illustrated in  FIG.  5 A  and  FIG.  5 B , via structures (including VA 1 -VA 8 ) and butted contacts (including BC 1 , BC 2 ) are disposed over the contact structures CSX for providing further interconnection. In some embodiments, a dielectric layer  408  is formed to surround the gate structures and the contact structures CSX. In certain embodiments, another dielectric layer  410  is formed over the dielectric layer  408  to surround the via structures (including VA 1 -VA 8 ) and butted contacts (including BC 1 , BC 2 ). Furthermore, conductive elements CEX including a first bit line BL 1 , a second bit line BL 2 , a first word line contact CWL 1 , a second word line contact CWL 2 , interconnect structures IC 1 , IC 2  and a power line Vdd, reference lines VSS, and a word line WL are disposed over the dielectric layer  410  for providing further interconnection. 
     In some embodiments, a dielectric layer  412  is disposed on the dielectric layer  410  to surround the first bit line BL 1 , the second bit line BL 2 , the first word line contact CWL 1 , the second word line contact CWL 2 , the interconnect structures IC 1 , IC 2  and the power line Vdd. Furthermore, dielectric layers  414 ,  416  are disposed on the dielectric layer  412 , whereby the reference lines VSS and the word line WL are embedded in the dielectric layer  416 . In the exemplary embodiment, the through vias  106  extend from the second level LX 2  to the first level LX 1 , and provides a vertical conduction path electrically connecting the bit lines (e.g. second bit line BL 2  as illustrated) to the source/drain terminal  306  of the logic circuit  104 . In some embodiments, auxiliary through vias (not shown) may extend from the second level LX 2  to the first level LX 1 , and provides a vertical conduction path electrically connecting the word line WL to the source/drain terminal  306  of the logic circuit  104 . 
       FIG.  6 A  to  FIG.  6 E  are cross-sectional views illustrating structures at various stages of manufacturing a transistor in a SRAM region according to some embodiments of the present disclosure. For example, a method of forming transistors (NFETs, PFETs) over the interlayer dielectric  330  at the second level LX 2  over the semiconductor substrate  102  will be described. 
     Referring to  FIG.  6 A , a semiconductor pattern SP 1  may be formed on the interlayer dielectric  330 . In some embodiments, a method for forming the semiconductor pattern SP 1  includes globally depositing a semiconductor layer, and patterning the semiconductor layer to form the semiconductor pattern SP 1  by a lithography process and an etching process. The semiconductor pattern SP 1  is formed of a semiconductor material. For instance, the semiconductor material is amorphous silicon. 
     Referring to  FIG.  6 B , another interlayer dielectric  350  may be formed to laterally surround the semiconductor pattern SP 1 . In some embodiments, a method for forming the interlayer dielectric  350  includes forming a dielectric layer globally covering the semiconductor pattern SP 1  and the underlying interlayer dielectric  330 , and performing a planarization process to remove portions of the dielectric layer above the semiconductor pattern SP 1 . Remained portions of the dielectric layer form the interlayer dielectric  350 . As examples, the planarization process may include a polishing process, an etching process or a combination thereof. 
     Referring to  FIG.  6 C , an annealing process is performed on the semiconductor pattern SP 1 , such that the semiconductor pattern SP 1  turns into the active structures ASX (including AS 1 , AS 2 , AS 3 , AS 4 ) for the NFETs and PFETs described above. In those embodiments where the semiconductor pattern SP 1  is formed of amorphous silicon, the amorphous silicon may be crystallized to form polycrystalline silicon or crystalline silicon during the annealing process. Accordingly, in these embodiments, the formed active structures ASX include polycrystalline silicon or crystalline silicon. In some embodiments, the annealing process is a laser annealing process or thermal annealing process, and a process temperature of the laser annealing process may be about 400° C. As a result of such annealing process, a field effect mobility and/or other characteristics of the transistors can be significantly improved. 
     Referring to  FIG.  6 D , a gate structure (including GLX,  402 ,  404 ) is formed over the active structures ASX. For example, similar to that described in  FIG.  5 A  and  FIG.  5 B , the gate structure includes a gate line GLX disposed over the active structure ASX; a gate dielectric layer  404  lying between the gate line GLX and the active structure ASX; and sidewalls spacers  406  covering the gate line GLX and the gate dielectric layer  404 . 
     In some embodiments, the gate line GLX is formed of polycrystalline silicon. In these embodiments, a method for forming the gate structure may include sequentially forming a dielectric layer and a conductive layer on the active structure ASX, and patterning the dielectric layer and the conductive layer to form the gate dielectric layer  404  the gate line GLX respectively. Subsequently, the sidewall spacer  406  may be formed on sidewalls of the gate line GLX and the gate dielectric layer  404  by a deposition process and an etching back process. In alternative embodiments where the gate line GLX is formed of a metallic material, a replacement gate process may be used for forming the gate structure. Further, although not shown, a pair of lightly doped regions may be optionally formed in the active structure ASX at opposite sides of the gate line GLX before formation of the sidewall spacers  406 . 
     Referring to  FIG.  6 E , source/drain terminals  402  formed in the active structures ASX at opposite sides of the gate structure (including GLX,  402 ,  404 ). In those embodiments where the source/drain terminals  402  are doped regions in the active structure ASX, a method for forming the source/drain terminals  402  may include an ion implantation process and an annealing process. In alternative embodiments, a method for forming the source/drain terminals  402  includes forming openings in the active structure ASX, and filling the source/drain terminals  402  into these openings by, for example, an epitaxial process. Up to here, the transistors (NFETs, PFETs) located at the second level LX 2  of the SRAM region SR are formed. As compared with transistors in the logic region LR formed using logic active structures  302  on the semiconductor substrate  102 , the transistors in the SRAM region SR uses the active structures ASX formed from a deposited semiconductor layer. Although, the transistors located at the second level LX 2  of the SRAM region SR is used as an example for description, it is noted that the transistors located at other levels (e.g. third level LX 3 , fourth level LX 3  etc.) of the SRAM region SR in the back-end-of-line (BEOL) process may be formed in a similar manner. In other words, the process described in  FIG.  6 A  to  FIG.  6 E  is a BEOL compatible process. 
       FIG.  7    is a schematic cross-sectional view of a semiconductor device taken along line A-A′ shown in  FIG.  4 A  and  FIG.  4 B  in accordance with an alternative embodiment of the present disclosure. The semiconductor device illustrated in  FIG.  7    is similar to the semiconductor device shown in  FIG.  5 A . Therefore, the same reference numerals may be used to refer to the same or liked parts, and its detailed description will be omitted herein. The difference between the embodiment is that a third level LX 3  structure is further illustrated in  FIG.  7   . 
     In the exemplary embodiment, the semiconductor device includes a plurality of first memory cells MC 1  disposed at the second level LX 2  over the semiconductor substrate  102  in a manner similar to that described in  FIG.  5 A  and  FIG.  5 B . In some embodiments, the semiconductor device further includes a plurality of second memory cells MC 2  disposed at a third level LX 3  over the semiconductor substrate  102 . For example, the third level LX 3  is stacked on top and overlapped with the second level LX 2  and the first level LX 1 , and each of the second memory cells MX 2  includes the same components and circuits as described for the first memory cell MX 1 . 
     As illustrated in  FIG.  7   , in some embodiments, an interlayer dielectric  420  may be formed on the dielectric layer  416 . Active structures ASX including the first active structure AS 1 , the second active structure AS 2 , the third active structure AS 3  and the fourth active structure AS 4  are disposed on the interlayer dielectric  420  at the third level LX 3 . In some embodiments, the transistors (NFETs, PFETs) located at the third level LX 3  may include gate structures having gate lines GLX (including GL 1 , GL 2 , GL 3 , GL 4 ). For example, each of the gate structures may include a gate line GLX (either one of GL 1 , GL 2 , GL 3 , GL 4 ) disposed on the active structures ASX; a gate dielectric layer  404  lying between the gate line GLX and the active structure ASX; and sidewall spacers  406  covering sidewalls of the gate line GLX and the gate dielectric layer  404 . In some embodiments, each transistor located at the third level LX 3  further includes source/drain terminals  402  formed in the active structures ASX, and located at opposite sides of the gate structure. Furthermore, contact structures CSX including include bit line contacts CBL 1 , CBL 2 , a first node contact CN 1 , a second node contact CN 2 , power contacts CVdd 1 , CVdd 2 , and ground contacts CVss 1 , CVss 2  are formed over the source/drain terminals  402  for out routing the transistors (NFETs, PFETs) located at the third level LX 3 . 
     In the exemplary embodiment, via structures (including VA 1 -VA 8 ) and butted contacts (including BC 1 , BC 2 ) are further disposed over the contact structures CSX for providing further interconnection (not illustrated). In some embodiments, a dielectric layer  430  is formed to surround the gate structures and the contact structures CSX in the third level LX 3 . In certain embodiments, another dielectric layer  440  is formed over the dielectric layer  430  to surround the via structures (including VA 1 -VA 8 ) and butted contacts (including BC 1 , BC 2 ). Furthermore, conductive elements CEX including a first bit line BL 1 , a second bit line BL 2 , a first word line contact CWL 1 , a second word line contact CWL 2 , interconnect structures IC 1 , IC 2  and a power line Vdd, reference lines VSS, and a word line WL are disposed over the dielectric layer  440  for providing further interconnection (only second bit line BL 2  is shown). 
     In some embodiments, a dielectric layer  442  is disposed on the dielectric layer  430  to surround the first bit line BL 1 , the second bit line BL 2 , the first word line contact CWL 1 , the second word line contact CWL 2 , the interconnect structures IC 1 , IC 2  and the power line Vdd. Furthermore, dielectric layers  444 ,  446  are disposed on the dielectric layer  442 , whereby the reference lines VSS and the word line WL are embedded in the dielectric layer  446 . In the exemplary embodiment, the through vias  106  may include a first through via  106 A, a second through via  106 B and a third through via  106 C. The first through via  106 A extend from the second level LX 2  to the first level LX 1 , and provides a vertical conduction path electrically connecting the bit lines (e.g. second bit line BL 2  as illustrated) at the second level LX 2  to the source/drain terminal  306  of the logic circuit  104 . The second through via  106 B extend from the third level LX 3  to the second level LX 2 , and provides a vertical conduction path electrically connecting the bit lines (e.g. second bit line BL 2  as illustrated) at the third level LX 3  to the first through via  106 A and to the source/drain terminal  306  of the logic circuit  104 . Similarly, the third through via  106 C may extend from the above levels (if present) to electrically connect the bit lines from the above levels to the source/drain terminal  306  of the logic circuit  104 . In a similar way, auxiliary through vias (not shown) may extend from the third level LX 3  to the second level LX 2 , and from the second level LX 2  to the first level LX 1 , to provide a vertical conduction path electrically connecting the word line WL at each level of the SRAM region SR to the source/drain terminal  306  of the logic circuit  104 . 
     According to the above embodiments, the semiconductor device includes a logic circuit disposed at a first level over the semiconductor substrate, and a SRAM region including a plurality of memory cells disposed at a second level over the semiconductor substrate. As such, by deploying the memory cells of the semiconductor device at different horizontal levels with the logic circuit, a footprint area of the semiconductor device can be significantly reduced. Furthermore, the memory cells do not occupy the front-end-of-line (FEOL) area, thus the chip area and costs of the semiconductor device can be reduced to a minimum. 
     In accordance with some embodiments of the present disclosure, a semiconductor device includes a semiconductor structure, a logic circuit, a plurality of first memory cells and through vias. The logic circuit is disposed at a first level over the semiconductor substrate. The first memory cells are disposed at a second level over the semiconductor substrate, wherein the second level is stacked on top and overlapped with the first level. Each of the first memory cells include a latch circuit and conductive elements. The latch circuit is formed of N-type field effect transistors (NFETs) and P-type field effect transistors (PFETs). The conductive elements are extending above the NFETs and the PFETs and electrically coupled to the NFETs and the PFETs. The through vias are extending from the second level to the first level and electrically connecting the conductive elements to the logic circuit by a vertical conduction path. 
     In accordance with some other embodiments of the present disclosure, a semiconductor device includes a logic circuit, an interlayer dielectric, a static random access memory (SRAM) cell and a through via. The logic circuit includes logic active structures and gate structures. The logic active structures are disposed at a first level on a semiconductor substrate. The gate structures are disposed at the first level, wherein the gate structures cover and intersect with the logic active structures. The interlayer dielectric is disposed on the semiconductor substrate covering the logic circuit. The SRAM cell is disposed on the logic circuit above the interlayer dielectric, and includes a first active structure and a second active structure, a first gate line and a second gate line, a bit line contact and a first bit line. The first active structure and the second active structure are disposed on the interlayer dielectric at a second level on the semiconductor substrate. The first gate line and the second gate line are disposed on the interlayer dielectric at the second level, wherein the first gate line covers and intersects with the first active structure, and the second gate line covers and intersects with the first active structure and the second active structure. The bit line contact is disposed on the first active structure aside the first gate line at the second level on the semiconductor substrate. The first bit line is extending above the bit line contact and electrically connected to the bit line contact. The through via is vertically connecting the first bit line to a source/drain region of the logic active structures. 
     In accordance with yet another embodiment of the present disclosure, a semiconductor device includes a logic region, a static random access memory (SRAM) region and a plurality of through vias. The logic region is formed on a semiconductor substrate. The SRAM region is formed on the logic region within an area overlapped with the logic region. The SRAM region includes a first SRAM memory array and a second SRAM memory array. The first SRAM memory array includes a plurality of first memory cells. The second SRAM memory array is stacked on top of the first SRAM memory array and includes a plurality of second memory cells. The through vias provide a vertical conduction path electrically connecting the first SRAM memory array and the second SRAM memory array to the logic region. 
     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.