Abstract:
A semiconductor device comprises four SRAM cells in four quadrants of a region of the semiconductor device, wherein the four SRAM cells include FinFET transistors comprising gate features engaging fin active lines, and the fin active lines of the four SRAM cells have reflection symmetry with respect to an imaginary line dividing the four quadrants along a first direction.

Description:
PRIORITY 
       [0001]    This is a continuation of U.S. application Ser. No. 14/991,526, filed Jan. 8, 2016, which is a continuation of U.S. application Ser. No. 14/191,789, filed Feb. 27, 2014, the entire disclosure of both is incorporated herein by reference. 
     
    
     BACKGROUND 
       [0002]    The semiconductor integrated circuit (IC) industry has experienced rapid growth. In the course of IC evolution, functional density (i.e., the number of interconnected devices per chip area) has generally increased while geometry size (i.e., the smallest component (or line) that can be created using a fabrication process) has decreased. This scaling down process generally provides benefits by increasing production efficiency and lowering associated costs. Such scaling down has also increased the complexity of processing and manufacturing ICs and, for these advances to be realized, similar developments in IC manufacturing are needed. 
         [0003]    For example, logic circuits and embedded static random-access memory (SRAM) cells are frequently integrated into semiconductor devices for increased functional density. Such applications range from industrial and scientific subsystems, automotive electronics, cell phones, digital cameras, microprocessors, and so on. To meet the demand for higher SRAM density, simply scalding down the semiconductor feature size is no longer enough. For example, traditional SRAM cell structure with planar transistors has experienced degraded device performance and higher leakage when manufactured with smaller semiconductor geometries. One of the techniques for meeting such a challenge is to use three-dimensional transistors having a fin or multi-fin structure (e.g., FinFETs). For example, FinFETs can be implemented for controlling short channel effect for metal-oxide-semiconductor field-effect transistors (MOSFETs). To achieve optimal short channel control and area reduction, the fin structures are desired to be as thin as possible. One of the techniques for manufacturing very thin fin structures is spacer lithography. For example, spacers are formed on sidewalls of mandrel patterns. After the mandrel patterns are removed, the spacers become an etch mask for etching a silicon substrate in forming the fin structures. The dimensions of the mandrel patterns and spacers control the width and pitch of the fin structures. A tight control of critical dimension (CD) uniformity of the mandrel patterns and spacers is a design challenge for embedded FinFET SRAM. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0004]    Aspects of the present disclosure are best understood from the following detailed description when they are 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. 
           [0005]      FIG. 1  is a simplified block diagram of an integrated circuit (IC) with embedded SRAM cells, according to various aspects of the present disclosure. 
           [0006]      FIG. 2  shows embedded SRAM cells with peripheral logic circuits, according to various aspects of the present disclosure. 
           [0007]      FIG. 3  illustrates some components of the peripheral logic circuit of  FIG. 2 , in accordance with an embodiment. 
           [0008]      FIGS. 4A and 4B  show schematic views of a six-transistor (6T) single-port (SP) SRAM cell, in accordance with an embodiment. 
           [0009]      FIGS. 5, 6, and 7  show a portion of a layout of the 6T SP SRAM cell of the  FIG. 4A , in accordance with some embodiments. 
           [0010]      FIG. 8  shows a schematic view of a two-port (TP) SRAM cell, in accordance with an embodiment. 
           [0011]      FIG. 9  shows a portion of a layout of the TP SRAM cell of  FIG. 8 , in accordance with an embodiment. 
           [0012]      FIGS. 10A and 10B  illustrate metal layer routing of embedded SRAM designs, according to various aspects of the present disclosure. 
           [0013]      FIG. 11  is a simplified block diagram of an integrated circuit (IC) with embedded SRAM cells, according to various aspects of the present disclosure. 
           [0014]      FIG. 12A  illustrates a layout of fin active lines of four SRAM cells, according to various aspects of the present disclosure. 
           [0015]      FIG. 12B  illustrates a three-layer partition of the fin active line layout of  FIG. 12A , in accordance with an embodiment. 
           [0016]      FIG. 12C  illustrates gate features of the four SRAM cells of  FIG. 12A  overlapping with the fin active line thereof, in accordance with an embodiment. 
           [0017]      FIG. 13  shows a method of forming an IC with embedded SRAM cells, according to various aspects of the present disclosure. 
           [0018]      FIGS. 14, 15A, 15B, 16A, 16B, 17A, 17B, 18A, 18B, 18C, 19A, 19B, 20A , and  20 B illustrate top and/or cross-sectional views of a portion of embedded SRAM cells manufactured with the method in  FIG. 13 , in accordance with an embodiment. 
           [0019]      FIG. 21  shows a method of forming an IC with embedded SRAM cells, according to various aspects of the present disclosure. 
           [0020]      FIGS. 22A, 22B, 22C, 23A, 23B, 24A, 24B, and 24C  illustrate top and/or cross-sectional views of a portion of embedded SRAM cells manufactured with the method in  FIG. 21 , in accordance with an embodiment. 
           [0021]      FIG. 25A  illustrates a layout of fin active lines of four SRAM cells, according to various aspects of the present disclosure. 
           [0022]      FIG. 25B  illustrates a three-layer partition of the fin active line layout of  FIG. 25A , in accordance with an embodiment. 
           [0023]      FIG. 25C  illustrates gate features of the four SRAM cells of  FIG. 25A  overlapping with the fin active line thereof, in accordance with an embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0024]    The following disclosure provides many different embodiments, or examples, for implementing different features of the provided subject matter. Specific examples of components 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. 
         [0025]    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. 
         [0026]      FIG. 1  shows a semiconductor device  100  with an SRAM macro  102 . The semiconductor device can be, e.g., a microprocessor, an application specific integrated circuit (ASIC), a field programmable gate array (FPGA), or a digital signal processor (DSP). The exact functionality of the semiconductor device  100  is not a limitation to the provided subject matter. 
         [0027]      FIG. 2  shows a more detailed view of a portion of the SRAM macro  102 , according to various aspects of the present disclosure. Referring to  FIG. 2 , the SRAM macro  102  includes a plurality of SRAM cells  202  and a plurality of peripheral logic circuits  210 . Each SRAM cell  202  is used to store one memory bit, while the peripheral logic circuits  210  are used to implement various logic functions, such as write and/or read address decoder, word/bit selector, data drivers, memory self-testing, etc. The logic circuits  210  include a plurality of FinFETs having gate features  218  and fin active lines  212 . Although not shown, each of the SRAM cells  202  also includes a plurality of FinFETs having gate features and fin active lines. In addition, even though  FIG. 2  shows only 16 SRAM cells  202 , the SRAM macro  102  may include a large number of SRAM cells  202  for a given semiconductor device  100 . For example, the SRAM macro  102  may include thousands or millions of the SRAM cells  202 . 
         [0028]    As shown in  FIG. 2 , the SRAM cells  202  are formed over a plurality of P-wells or P-diffusions (e.g., for n-type FinFETs or N-FinFETs) and N-wells or N-diffusions (e.g., for p-type FinFETs or P-FinFETs) wherein the P-wells and the N-wells are rectangular semiconductor regions arranged in alternating order in an X direction. As will be shown in later, each of the SRAM cells  202  includes a plurality of N-FinFETs and a plurality of P-FinFETs. Furthermore, the SRAM cells  202  are arranged in an array with one SRAM cell abutting another. Each of the SRAM cells  202  occupies a rectangular region of the SRAM macro  102  wherein the rectangular region has a first dimension  204  in the X direction and a second dimension  206  in a Y direction that is orthogonal to the X direction. In the following discussion, the first dimension  204  is also referred to as the SRAM cell  202 &#39;s X-pitch, and the second dimension  206  the SRAM cell  202 &#39;s Y-pitch. 
         [0029]    Furthermore, each of the SRAM cells  202  is configured in one of four orientations. As shown in  FIG. 2 , a group  203  includes four SRAM cells  202  in a two-by-two array, denoted as Cell-R 0 , Cell-Mx, Cell-My, and Cell-R 180  for the convenience of discussion. In an embodiment, the gate features and fin active lines of the Cell-R 0  are mirror images (or reflection) of those respective features of the Cell-Mx with respect to an imaginary lines A-A through a geometric center of the group  203  in the X direction. Similarly, the gate features and fin active lines of the Cell-R 0  are mirror images of those respective features of the Cell-My with respect to an imaginary lines B-B through the geometric center of the group  203  in the Y direction. Similarly, the Cell-R 180  is a mirror image of the Cell-Mx with respect to the imaginary lines B-B, and a mirror image of the Cell-My with respect to the imaginary lines A-A. 
         [0030]    As semiconductor technology has progressed into small feature sizes, such as 32 nanometer (nm), 20 nm, and beyond, restricted design rules are often followed so as to improve design manufacturability. The configuration of the SRAM macro  102 , as shown in  FIG. 2 , allows alignment of the features of the peripheral logic circuits  210  (e.g., the gate features  218  and fin active lines  212 ) with those respective features of the SRAM cells  202 . This can be accomplished by careful consideration of ratios between the X-pitch  204  and fin pitch  214 , and between the Y-pitch  206  and gate pitch  216 . Such alignment enables dense fin active line definition and formation thereby providing many benefits, such as higher SRAM cell density, higher manufacturing reliability in view of optical proximity effect, etc. Furthermore, having a fixed ratio between the Y-pitch  206  and gate pitch  216  allows certain peripheral logic circuits (e.g., word-line drivers, decoders, etc.) to be automatically generated as a circuit block which is then repetitively placed along the SRAM cells. Similarly, having a fixed ratio between the X-pitch  204  and fin pitch  214  allows certain peripheral logic circuits (e.g., column selector, bit-line pre-charge circuit, decoders, etc.) to be automatically generated and placed. 
         [0031]      FIG. 3  illustrates a top view of a portion of the peripheral logic circuit  210 . Each of the fin active lines  212  has a rectangular shape with its long edge extending in the Y direction and its short edge extending in the X direction. In the present embodiment, the fin pitch  214  is defined as the edge-to-edge spacing between two adjacent fin active lines  212 . Alternatively, the fin pitch  214  may be defined as the center-line-to-center-line spacing between two adjacent fin active lines  212 . The gate features  218  are oriented orthogonally with respect to the fin active lines  212 . Each of the gate features  218  has a rectangular shape with its long edge extending in the X direction and its short edge extending in the Y direction. In the present embodiment, the gate pitch  216  is defined as the edge-to-edge spacing between two adjacent gate features  218 . Alternatively, the gate pitch  216  may be defined as the center-line-to-center-line spacing between two adjacent gate features  218 . The peripheral logic circuit  210  further includes a plurality of active contacts  220  that couple multiple fin active lines  212  to form common drains/sources for respective FinFETs. 
         [0032]      FIG. 4A  shows a schematic view of a six-transistor (6T) single port (SP) SRAM cell that may be implemented as the SRAM cell  202  of  FIG. 2 . Referring to  FIG. 4A , the 6T SP SRAM cell  202 , includes two P-FinFETs as pull-up transistors, PU- 1  and PU- 2 ; two N-FinFETs as pull-down transistors, PD- 1  and PD- 2 ; and two N-FinFETs as pass-gate transistors, PG- 1  and PG- 2 . The PU- 1  and PD- 1  are coupled to form an inverter (Inverter- 1  in  FIG. 4B ). The PU- 2  and PD- 2  are coupled to form another inverter (Inverter- 2  in  FIG. 4B ). The inverters, Inverter- 1  and Inverter- 2 , are cross-coupled to form a storage unit of the SRAM cell  202 .  FIG. 4A  further shows word line (WL), bit line (BL), and bit line bar (BL) for accessing the storage unit of the SRAM cell  202 . 
         [0033]    In practice, the SRAM cell  202  of  FIG. 4A  can be implemented physically (e.g., layout) in many ways. The following discussion will describe some layout designs of three embodiments of the SRAM cell  202 , namely, SRAM cells  202 A,  202 B, and  202 C, according to various aspects of the present disclosure. A person having ordinary skill in the art should appreciate that these three embodiments are merely examples and are not intended to limit the inventive scope of the provided subject matter. 
         [0034]      FIG. 5  shows a top view of a portion of the SRAM macro  102 &#39;s layout including the SRAM cell  202 A. Referring to  FIG. 5 , the SRAM cell  202 A is indicated with a rectangular boundary (a dotted line) with a first dimension (X-pitch)  204 A and a second dimension (Y-pitch)  206 A. The layout includes one N-well active region and two P-well active regions, one on each side of the N-well active region in the X direction. The layout further includes two fin active lines,  222 A and  224 A, with one in each of the P-well active regions, extending lengthwise in the Y direction and overlapping the SRAM cell  202 A. The layout further includes two fin active lines,  226 A and  228 A, in the N-well active region, extending lengthwise in the Y direction and partially overlapping the SRAM cell  202 A. The fin active lines  226 A and  228 A are shortened for reducing cell area. The four fin active lines,  222 A,  226 A,  228 A, and  224 A, are spaced edge-to-edge by about twice of the fin pitch  214 . In some embodiments, the spacing between these fin active lines are set to between about 2 and about 2.5 times of the fin pitch  214  to allow enough design margin and process margin when forming the SRAM cell fin lines. In such cases, the X-pitch  204 A can still be maintained as an integer multiple of the fin pitch  214 . Furthermore, the layout includes two gate features,  232 A and  234 A, extending lengthwise in the X direction and partially overlapping the SRAM cell  202 A and being shared between the SRAM cell  202 A and adjacent SRAM cells (not shown), and two gate features,  236 A and  238 A, extending lengthwise in the X direction within the SRAM cell  222 A. The above gate features and the fin active lines collectively define the six transistors, PU- 1 / 2 , PD- 1 / 2 , and PG- 1 / 2  of  FIG. 4A . The Y-pitch  206 A is substantially equal to the sum of the pass-gate transistor (PG- 1  or PG- 2 ) pitch and the pull-down transistor (PD- 1  or PD- 2 ) pitch, wherein a transistor&#39;s pitch refers to a distance between the transistor&#39;s source and drain. 
         [0035]    In an embodiment, the Y-pitch  206 A is set to be about twice of the gate pitch  216  ( FIG. 3 ), while the X-pitch  204 A is set to be about 8, 8.5, or 9 times of the fin pitch  214  ( FIG. 3 ). Such settings take into account the fact that proper alignment of respective features between the SRAM cells  202 A and the peripheral logic circuits  210  improves overall manufacturability of the semiconductor device  100  having the SRAM macro  102  ( FIGS. 1 and 2 ). For example, having a single fin pitch rule among the SRAM cells  202 A and the peripheral circuits  210  helps improve fin active lines&#39; critical dimension uniformity during lithography process. Due to its compact layout, the SRAM cell  202 A is well-suited for high density embedded SRAM applications. In an embodiment where high memory cell density is desired, the SRAM macro  102  ( FIG. 2 ) includes only this type of SRAM cell and the X-pitch  204 A is set to about 8 times of the fin pitch  214  ( FIG. 3 ). In another embodiment, the X-pitch  204 A is set to about 9 times of the fin pitch  214 . In some embodiments, the X-pitch  204 A is set to a non-integer multiple of the fin pitch  214 , such as 8.5 times. That is made possible by the configuration of the SRAM cells  202 A in the SRAM macro  102  ( FIG. 2 ) wherein four adjacent SRAM cells  202 A will collectively have an X-dimension that is an integer multiple (e.g., 34×) of the fin pitch  214 . Such flexibility in placing the SRAM cells  202 A yet still maintaining proper alignment of fin active lines between the SRAM cells  202 A and the peripheral logic circuits  210  is one of the many benefits provided by the present disclosure. 
         [0036]      FIG. 6  shows a portion of the SRAM cell  202 B&#39;s layout, while  FIG. 7  shows a portion of the SRAM cell  202 C&#39;s layout. Many aspects of the SRAM cells  202 B and  202 C are similar to those of the SRAM cell  202 A, and are hereby omitted from discussion for brevity. 
         [0037]    Referring to  FIG. 6 , the SRAM cell  202 B is indicated with a rectangular boundary (a dotted line) with a first dimension (X-pitch)  204 B and a second dimension (Y-pitch)  206 B. One difference between the SRAM cells  202 B and  202 A is that the SRAM cell  202 B includes two fin active lines in each of the two P-well active regions,  222 B- 1 / 2  and  224 B- 1 / 2 . In effect, the transistors PG- 1 / 2  and PD- 1 / 2  of the SRAM cell  202 B have dual-fin active lines for increased current sourcing capability. The two fins  222 B- 1  and  222 B- 2  are spaced edge-to-edge by one fin pitch  214 , so are the two fins  224 B- 1  and  224 B- 2 . In the present embodiment, the X-pitch  204 B is greater than the X-pitch  204 A ( FIG. 5 ) by about twice of the fin pitch  214  ( FIG. 3 ). For similar reasons stated above with respect to  FIG. 5 , the Y-pitch  206 B is about twice of the gate pitch  216 . In an embodiment, a ratio between the X-pitch  204 B and the Y-pitch  206 B is in a range of about 2.7 to about 2.9. 
         [0038]    Similar observations are made with respect to the SRAM cell  202 C in  FIG. 7 : the transistors PG- 1 / 2  and PD- 1 / 2  of the SRAM cell  202 C have triple-fin active lines  222 C- 1 / 2 / 3  and  224 C- 1 / 2 / 3  respectively for increased current sourcing capability; the X-pitch  204 C is greater than the X-pitch  204 A ( FIG. 5 ) by about four times of the fin pitch  214  ( FIG. 3 ); and the Y-pitch  206 C is about twice of the gate pitch  216  ( FIG. 3 ). The three fins  222 C- 1 ,  222 C- 2 , and  222 C- 3  are spaced edge-to-edge by one fin pitch  214 , so are the three fins  224 C- 1 ,  2224 - 2 , and  2224 - 3 . 
         [0039]      FIG. 8  shows a schematic view of a two-port (TP) SRAM cell  202 D that may be implemented as the SRAM cell  202  of  FIG. 2 . The SRAM cell  202 D, as shown in  FIG. 8 , includes a write-port portion  802  and a read-port portion  804 . The write-port portion  802  is effectively a 6T SP SRAM cell as shown in  FIG. 4A . The read-port portion  804  includes a read pull-down transistor R_PD and read pass-gate transistor R_PG. 
         [0040]    In practice, the SRAM cell  202 D of  FIG. 8  can be implemented physically (e.g., layout) in many ways.  FIG. 9  shows a top view of a portion of the SRAM cell  202 D&#39;s layout, in accordance with an embodiment. Referring to  FIG. 9 , the layout of the write-port portion  802  is substantially the same as that of the SRAM cell  202 B ( FIG. 6 ), while the layout of the read-port portion  804  includes the transistors R_PD and R_PG, each as a dual-fin FinFET. Two fin active lines  902 - 1  and  902 - 2  are spaced edge-to-edge by one fin pitch  214 . Many aspects of the SRAM cells  202 D are similar to those discussed above with respect to  FIGS. 5-7 , and are hereby omitted from discussion for brevity. In an embodiment, to improve manufacturability and circuit density of the SRAM macro  102  having the SRAM cells  202 D, the Y-pitch  206 D is set to about twice of the gate pitch  216 , while the X-pitch  204 D is an integer multiple, e.g., 15 times, of the fin pitch  214 . 
         [0041]      FIGS. 10A and 10B  show metal routing of the SRAM cells thus far discussed, in accordance with some embodiments.  FIG. 10A  shows that the power supply lines (CVdd), bit lines (BL), and bit bar lines (BL) are routed in a first metal layer, while the word lines (WL) and the ground lines (Vss) are routed in a second metal layer.  FIG. 10B  shows that the word lines (WL) are routed in the first metal layer; and the power supply lines (CVdd), bit lines (BL), bit bar lines (BL), and the ground lines (Vss) are routed in the second metal layer. In an embodiment, the first metal layer is located in between the second metal layer and the active regions of the respective SRAM cells. In an embodiment, the first and second metal layers are coupled through inter-layer vias. 
         [0042]    In some applications, a semiconductor device may include more than one SRAM macros. Careful considerations must be taken to ensure manufacturability and circuit density of each of the SRAM macros as well as that at the device level. The present disclosure is well adapted to solving such a problem.  FIG. 11  shows that the semiconductor device  100  includes another SRAM macro  104  in addition to the SRAM macro  102 . Although they are shown side by side in  FIG. 11 , in practice, the two SRAM macros may be placed anywhere in the semiconductor device  100 . Furthermore, the two SRAM macros  102  and  104  may include the same or different types of SRAM cells. For example, the SRAM macro  102  includes an array of the SRAM cells  202 A, while the SRAM macro  104  includes an array of the SRAM cells  202 A,  202 B,  202 C, or  202 D. Following are some embodiments of the semiconductor  100  wherein various dimensions of the SRAM macros and the peripheral logic circuits are designed so as to improve full-chip layout automation, fin active line critical dimension uniformity, and overall device manufacturability. 
         [0043]    In an embodiment, the SRAM macro  102  includes an array of the SRAM cells  202 A ( FIG. 5 ) while the SRAM macro  104  includes an array of the SRAM cells  202 B ( FIG. 6 ). The X-pitch  204 B is set to be about equal to the X-pitch  204 A plus twice of the fin pitch  214  ( FIG. 3 ). In an embodiment, the X-pitch  204 A is set to be about 8 times of the fin pitch  214  and the X-pitch  204 B is set to be about 10 times of the fin pitch  214 . In another embodiment, the X-pitch  204 A is set to be about 8.5 times of the fin pitch  214  and the X-pitch  204 B is set to be about 10.5 times of the fin pitch  214 . In yet another embodiment, the X-pitch  204 A is set to be about 9 times of the fin pitch  214  and the X-pitch  204 B is set to be about 11 times of the fin pitch  214 . Both the Y-pitch  206 A and the Y-pitch  206 B are set to be about twice of the gate pitch  216 . Furthermore, the ratio of the X-pitch  204 B to the Y-pitch  206 B is in a range of about 2.7 to about 2.9, such as 2.8; and the ratio of the X-pitch  204 A to the Y-pitch  206 A is in a range of about 2.25 to about 2.28, such as 2.2667. 
         [0044]    In an embodiment, the SRAM macro  102  includes an array of the SRAM cells  202 B ( FIG. 6 ) while the SRAM macro  104  includes an array of the SRAM cells  202 D ( FIG. 8 ). The X-pitch  204 B is set to be about 10.5 times of the fin pitch  214  ( FIG. 3 ) and the X-pitch  204 D is set to be about 15 times of the fin pitch  214 . Both the Y-pitch  206 B and the Y-pitch  206 D are set to be about twice of the gate pitch  216 . 
         [0045]    In an embodiment, the SRAM macro  102  includes an array of the SRAM cells  202 B ( FIG. 6 ) while the SRAM macro  104  includes an array of the SRAM cells  202 C ( FIG. 7 ). The X-pitch  204 C is set to be about the X-pitch  204 B plus twice of the fin pitch  214  ( FIG. 3 ). For example, the X-pitch  204 B is set to be about 10 times of the fin pitch  214  and the X-pitch  204 C is set to be about 12 times of the fin pitch  214 . For another example, the X-pitch  204 B is set to be about 10.5 times of the fin pitch  214  and the X-pitch  204 C is set to be about 12.5 times of the fin pitch  214 . 
         [0046]      FIG. 12A  shows fin active lines of the group  203  ( FIG. 2 ) that includes four adjacent SRAM cells  202 A ( FIG. 5 ), Cell-R 0 , Cell-My, Cell-Mx, and Cell-R 180 . The four cells are arranged in two rows and two columns. The imaginary line A-A denotes their boundary along the X direction, and the imaginary line B-B denotes their boundary along the Y direction. With respect to fin active line configuration (shape, size, and position of the active lines within a cell), Cell-R 0  and Cell-My are mirror images of Cell-Mx and Cell-R 180  along the line A-A, while Cell-R 0  and Cell-Mx are mirror images of Cell-My and Cell-R 180  along the line B-B. In the present disclosure, these fin active lines are formed using spacer lithography with three masks (or reticles),  1202 ,  1204 , and  1206 , as shown in  FIG. 12B . 
         [0047]    Referring to  FIG. 12B , the three masks,  1202 ,  1204 , and  1206 , are three layers of the design layout of the SRAM macro  102  (and of the semiconductor device  100 ). The mask  1202  defines mandrel patterns for spacer formation, the mask  1204  defines dummy-fin cut patterns for removing dummy spacers (or dummy fin lines), and the mask  1206  defines fin-end cut patterns, e.g., for shortening fin lines for the pull-up transistors (e.g. PU- 1  and PU- 2  in  FIG. 5 ). Each mandrel pattern has a rectangular shape (top view) extending lengthwise in the Y direction. In an embodiment, although not shown, each mandrel pattern extends over at least four SRAM cells  202 A (see  FIG. 2 ). In an embodiment, there are four mandrel patterns extending over each SRAM cell  202 A. With respect to mandrel pattern configuration (shape, size, and position of the mandrel patterns within each cell), Cell-R 0  and Cell-My are mirror images of Cell-Mx and Cell-R 180  along the line A-A, while Cell-R 0  and Cell-Mx are translations of Cell-My and Cell-R 180 , i.e., shifted by one X-pitch  204 A in the X direction. Each dummy-fin cut pattern  1204  is also a rectangular shape (top view) extending lengthwise in the Y direction. The fin-end cut patterns  1206  are located at the boundaries of the SRAM cells in the Y direction for cutting fin lines, e.g., for reducing active areas for the PU- 1  and PU- 2  transistors. Partitioning the layout of  FIG. 12A  into three masks of  FIG. 12B  allows dense and/or regular patterns to be created with each of the masks  1202 ,  1204  and  1206 , which greatly improves pattern critical dimension uniformity during photolithography. 
         [0048]      FIG. 12C  shows the gate features of the group  203  superimposed onto the fin active lines of the same group. Each gate feature is a rectangular shape extending lengthwise in the X direction. The gate features are spaced in the Y direction having a pitch about half of the Y-pitch  206 A. The gate features extend over the fin active lines for forming various P-FinFETs and N-FinFETs. With respect to gate feature configuration (shape, size, and position of gate features within each cell), Cell-R 0  and Cell-My are mirror images of Cell-Mx and Cell-R 180  along the line A-A, while Cell-R 0  and Cell-Mx are mirror images of Cell-My and Cell-R 180  along the line B-B. 
         [0049]      FIG. 13  shows a method  1300  of forming the fin active lines of the group  203  ( FIG. 12A ) using the masks  1202 ,  1204 , and  1206  ( FIG. 12B ), in accordance with an embodiment. Additional operations can be provided before, during, and after the method  1300 , and some operations described can be replaced, eliminated, or moved around for additional embodiments of the method. The method  1300  will be described in conjunction with  FIGS. 14-24C . 
         [0050]    At operation  1302 , the method  1300  ( FIG. 13 ) deposits dielectric layers  1404  and  1406  over a silicon substrate  1402  (e.g., semiconductor wafer). Referring to  FIG. 14 , shown therein is the silicon substrate  1402  with the first dielectric layer  1404  (such as silicon oxide) and the second dielectric layer  1406  (such as silicon nitride) formed thereon. Materials suitable for the dielectric layers  1404  and  1406  include, but not limited to, silicon oxide, silicon nitride, poly-silicon, Si 3 N 4 , SiON, TEOS, nitrogen-containing oxide, nitride oxide, high K material (K&gt;5), or combinations thereof. The dielectric layers  1404  and  1406  are formed by a procedure that includes deposition. For example, the first dielectric layer  1404  of silicon oxide is formed by thermal oxidation. The second dielectric layer  1406  of silicon nitride (SiN) is formed by chemical vapor deposition (CVD). For example, the SiN layer is formed by CVD using chemicals including Hexachlorodisilane (HCD or Si 2 C 16 ), Dichlorosilane (DCS or SiH 2 C 12 ), Bis(TertiaryButylAmino) Silane (BTBAS or C 8 H 22 N 2 Si) and Disilane (DS or Si 2 H 6 ). In an embodiment, the dielectric layer  1406  is about 20 nm to about 200 nm thick. 
         [0051]    The method  1300  ( FIG. 13 ) proceeds to operation  1304  to form mandrel patterns  1502  in the dielectric layer  1406 . Referring to  FIG. 15A  (top view) and  FIG. 15B  (cross-sectional view along the A-A lines of  FIG. 15A ), the mandrel patterns  1502  are evenly distributed in the X direction. The mandrel patterns  1502  are formed by patterning the dielectric layer  1406  with a procedure including a lithography process and an etching process. In the present embodiment, a photoresist layer is formed on the dielectric layer  1406  using a spin-coating process and soft baking process. Then, the photoresist layer is exposed to a radiation using the mask  1202  ( FIG. 12B ). The exposed photoresist layer is developed using post-exposure baking (PEB), developing, and hard baking thereby forming a patterned photoresist layer over the dielectric layer  1406 . Subsequently, the dielectric layer  1406  is etched through the openings of the patterned photoresist layer, forming a patterned dielectric layer  1406 . The patterned photoresist layer is removed thereafter using a suitable process, such as wet stripping or plasma ashing. In one example, the etching process includes applying a dry (or plasma) etch to remove the dielectric layer  1406  within the openings of the patterned photoresist layer. In another example, the etching process includes applying a wet etch with a hydrofluoric acid (HF) solution to remove the SiO layer  1406  within the openings. During the above photolithography process, the pattern regularity of the mandrel patterns  1502  helps improve pattern critical dimension uniformity in view of optical proximity effect. 
         [0052]    The method  1300  ( FIG. 13 ) proceeds to operation  1306  to form spacers  1602 . Referring to  FIG. 16A  (top view) and  FIG. 16B  (cross-sectional view along the A-A lines of  FIG. 16A ), shown therein are the spacers  1602  formed on the sidewalls of the mandrel patterns  1502 . The spacers  1602  include one or more material different from the mandrel patterns  1502 . In an embodiment, the spacers  1602  may include a dielectric material, such as titanium nitride, silicon nitride, or titanium oxide. Other materials suitable for the spacers  1602  include, but not limited to, poly-silicon, SiO 2 , Si 3 N 4 , SiON, TEOS, nitrogen-containing oxide, nitride oxide, high K material (K&gt; 5 ), or combinations thereof. The spacers  1602  can be formed by various processes, including a deposition process and an etching process. For example, the deposition process includes a chemical vapor deposition (CVD) process or a physical vapor deposition (PVD) process. For example, the etching process includes an anisotropic etch such as plasma etch. 
         [0053]    The method  1300  ( FIG. 13 ) proceeds to operation  1308  to remove the mandrel patterns  1502 . Referring to  FIG. 17A  (top view) and  FIG. 17B  (cross-sectional view along the A-A lines of  FIG. 17A ), the spacers  1602  remain over the dielectric layer  1404  after the mandrel patterns  1502  have been removed, e.g., by an etching process selectively tuned to remove the dielectric material  1406  but not the spacer material. The etching process can be a wet etching, a dry etching, or a combination thereof. 
         [0054]    The method  1300  ( FIG. 13 ) proceeds to operation  1310  to form fin lines  1802  in the silicon substrate  1402 . Referring to  FIG. 18B  which is cross-sectional view along the A-A lines of  FIG. 18A , the silicon substrate  1402  are etched with the spacers  1602  as an etch mask. The spacers  1602  and the dielectric layer  1404  are subsequently removed thereby forming the fin lines  1802  in the silicon substrate  1402  ( FIG. 18C ). 
         [0055]    The method  1300  ( FIG. 13 ) proceeds to operation  1312  to perform a first fin cut process with the mask  1204  ( FIG. 12B ) thereby removing dummy fin lines. Referring to  FIG. 19A  (top view) and  FIG. 19B  (cross-sectional view along the A-A lines of  FIG. 19A ), dummy fin lines  1802 D are removed thereby leaving the fin lines  1802 A on the silicon substrate  1402 . In the present embodiment, the dummy fin lines  1802 D are removed by a procedure including a lithography process and an etching process. For example, a photoresist layer is formed on the silicon substrate using a spin-coating process and soft baking process. Then, the photoresist layer is exposed to a radiation using the mask  1204  where the dotted lines of  FIG. 19A  indicate openings to be formed. The exposed photoresist layer is subsequently developed and stripped thereby forming a patterned photoresist layer. The fin lines  1802 A are protected by the patterned photoresist layer while the dummy fin lines  1802 D are not protected as such. Subsequently, the dummy fin lines  1802 D are etched through the openings of the patterned photoresist layer. The patterned photoresist layer is removed thereafter using a suitable process, such as wet stripping or plasma ashing. 
         [0056]    The method  1300  ( FIG. 13 ) proceeds to operation  1314  to perform a second fin cut process with the mask  1206  ( FIG. 12B ) thereby cutting fin lines for pull-up transistors such as PU- 1  and PU- 2  of  FIG. 5 . Referring to  FIG. 20A  (top view) and  FIG. 20B  (cross-sectional view along the A-A lines of  FIG. 20A ), portions of the fin lines  1802 A are removed across the boundaries of the SRAM cells  202 A thereby forming shortened fin lines for the pull-up transistors PU- 1  and PU- 2 . In the present embodiment, the second fin cut process is similar to the first fin cut process discussed with respect to  FIGS. 19A and 19B  except that the second fin cut process uses the mask  1206 . 
         [0057]    The method  1300  ( FIG. 13 ) proceeds to operation  1316  to form a final device with the fin lines  1802 A. For example, the operation  1316  may include implanting dopant for well and channel doping, forming gate dielectric, forming lightly doped source/drain, forming gate stacks, and so on. 
         [0058]      FIG. 21  shows a method  2100  of forming the fin active lines of the group  203  ( FIG. 12A ) with the three masks,  1202 ,  1204 , and  1206 , of  FIG. 12B , in accordance with an embodiment. Additional operations can be provided before, during, and after the method  2100 , and some operations described can be replaced, eliminated, or moved around for additional embodiments of the method. Some operations of the method  2100  are the same or similar to those respective operations of the method  1300 , and are hereby omitted from discussion for brevity. 
         [0059]    After operation  1308 , the method  2100  ( FIG. 21 ) has formed spacers  1602 A and  1602 D ( FIGS. 22A and 22B ) where the spacers  1602 A will be used for forming fin active lines while the spacers  1602 D (dummy spacers) will not. 
         [0060]    At operation  2110 , the method  2100  ( FIG. 21 ) removes the dummy spacers  1602 D with the aid of the mask  1204 , e.g., by a photolithography process and an etching process as discussed above with reference to  FIGS. 19A and 19B , wherein the etching process is selectively tuned to remove the spacer material ( FIG. 22C ). 
         [0061]    At operation  2112 , the method  2100  ( FIG. 21 ) cuts the spacers  1602 A across the boundaries of the SRAM cells  202 A with the aid of the mask  1206  ( FIGS. 23A and 23B ). This can be done with a process similar to the photolithography process and the etching process as discussed above with reference to  FIGS. 20A and 20B , wherein the etching process is selectively tuned to remove the spacer material ( FIG. 23B ). 
         [0062]    At operation  2114 , the method  2100  ( FIG. 21 ) etches the silicon substrate  1402  with the remaining spacers  1602 A as an etch mask ( FIGS. 24A and 24B ). The spacers  1602 A and the dielectric layer  1404  are subsequently removed thereby forming fin lines  1802 A in the silicon substrate  1402  for the transistors PU- 1 / 2 , PD- 1 / 2 , and PG- 1 / 2  ( FIG. 24C ). 
         [0063]    The method  2100  ( FIG. 21 ) proceeds to operation  1316  to form a final device with the fin lines  1802 A as discussed above. 
         [0064]      FIG. 25A  shows fin active lines of the group  203  ( FIG. 2 ) that includes four adjacent SRAM cells  202 B ( FIG. 6 ), Cell-R 0 , Cell-My, Cell-Mx, and Cell-R 180 . The four cells are arranged in two rows and two columns. The imaginary line A-A denotes their boundary along the X direction, and the imaginary line B-B denotes their boundary along the Y direction. With respect to fin active line configuration (shape, size, and position of the active lines within a cell), Cell-R 0  and Cell-My are mirror images of Cell-Mx and Cell-R 180  along the line A-A, while Cell-R 0  and Cell-Mx are mirror images of Cell-My and Cell-R 180  along the line B-B. In the present disclosure, these fin active lines are formed using spacer lithography with three masks,  2502 ,  2504 , and  2506 , as shown in  FIG. 25B . 
         [0065]    Referring to  FIG. 25B , similar to the three masks  1202 ,  1204 , and  1206  in  FIG. 12B , the three masks  2502 ,  2504 , and  2506  are three layers of the design layout of the SRAM macro  102  (and of the semiconductor device  100 ). The mask  2502  defines mandrel patterns for spacer formation, the mask  2504  defines dummy-fin cut patterns for removing dummy fin lines (or dummy spacers), and the mask  2506  defines fin-end cut patterns for shortening fin lines for the pull-up transistors (e.g. PU- 1  and PU- 2  in  FIG. 5 ). As shown in  FIG. 25B , the mandrel patterns are evenly distributed in the X direction. Each mandrel pattern has a rectangular shape (top view) extending lengthwise in the Y direction. In an embodiment, although not shown, each mandrel pattern extends over at least four SRAM cells  202 B (see  FIG. 2 ). In the present embodiment, the layout includes five mandrel patterns extending over each SRAM cell  202 B. With respect to mandrel pattern configuration (shape, size, and position of the mandrel patterns within each cell), Cell-R 0  and Cell-My are mirror images of Cell-Mx and Cell-R 180  along the line A-A, while Cell-R 0  and Cell-Mx are translations of Cell-My and Cell-R 180 , i.e., shifted by one X-pitch  204 B in the X direction. Each dummy-fin cut pattern is also a rectangular shape (top view) extending lengthwise in the Y direction. The fin-end cut patterns are located at the boundaries of the SRAM cells  202 B in the Y direction and are used for cutting fin lines, e.g., for reducing active areas for the PU- 1  and PU- 2  transistors. Partitioning the layout of  FIG. 25A  into three masks of  FIG. 25B  allows dense and/or regular patterns to be created with the masks, which improves pattern critical dimension uniformity during photolithography. The fin active lines of  FIG. 25A  can be formed with the masks of  FIG. 25B  using an embodiment of the method  1300  ( FIG. 13 ) or the method  2100  ( FIG. 21 ) as discussed above. 
         [0066]      FIG. 25C  shows the gate features of the group  203  superimposed onto the fin active lines of the same group ( FIG. 25A ). Each gate feature is a rectangular shape extending lengthwise in the X direction. The gate features are spaced in the Y direction having a pitch about half of the Y-pitch  206 B. The gate features extend over the fin active lines for forming various P-FinFETs and N-FinFETs. With respect to gate feature configuration (shape, size, and position of gate features within each cell), Cell-R 0  and Cell-My are mirror images of Cell-Mx and Cell-R 180  along the line A-A, while Cell-R 0  and Cell-Mx are mirror images of Cell-My and Cell-R 180  along the line B-B. 
         [0067]    Although not intended to be limiting, the present disclosure provides many benefits. For example, the present disclosure defines an embedded FinFET SRAM macro structure which enables alignment of respective features (e.g., fin active lines, gate features, etc.) between SRAM cells and peripheral logic circuits. Such alignment enables dense fin active lines formation and single fin pitch design, as an example. The embedded FinFET SRAM macro structure is flexible in that it may include high density SRAM cells, high current SRAM cells, single port SRAM cells, two-port SRAM cells, or a combination thereof. Therefore, it can be deployed in a wide range of applications, such as computing, communication, mobile phones, and automotive electronics. The present disclosure further teaches layout designs of the fin active regions for some embodiments of the SRAM cells, as well as methods for making the same. In some embodiments, the fin active region layout is partitioned into a mandrel pattern layer (mask) and two cut pattern layers (masks). The mandrel patterns are dense, parallel, and rectangular shapes, enhancing critical dimension uniformity during photolithography process. 
         [0068]    In one exemplary aspect, the present disclosure is directed to an integrated circuit (IC) layout. The IC layout includes a first rectangular region, wherein the first rectangular region has its longer sides in a first direction and its shorter sides in a second direction that is orthogonal to the first direction; and a first imaginary line through a geometric center of the first rectangular region in the first direction and a second imaginary line through the geometric center in the second direction divide the first rectangular region into a first, second, third, and fourth sub-regions in counter-clockwise order with the first sub-region located at an upper-right portion of the first rectangular region. The IC layout further includes at least eight first patterns located at a first layer of the IC layout, wherein each of the first patterns is a rectangular shape extending lengthwise in the second direction over the first rectangular region; the first patterns are spaced from each other in the first direction; a first, second, third, and fourth portions of the first patterns overlap with the first, second, third, and fourth sub-regions respectively; the first and second portions of the first patterns are mirror images of the respective fourth and third portions of the first patterns with respect to the first imaginary line; and the first and fourth portions of the first patterns are translations of the respective second and third portions of the first patterns. The IC layout further includes at least eight second patterns located at a second layer of the IC layout, wherein each of the second patterns is a rectangular shape extending lengthwise in the second direction, the second patterns are spaced from each other in the first direction, each of the second patterns partially overlaps with one of the first patterns and fully covers a longer side of the respective first pattern when the first and second layers are superimposed. The IC layout further includes a plurality of third patterns located at a third layer of the IC layout, wherein each of the third patterns is a rectangular shape, the third patterns are spaced from each other, each of the third patterns partially overlaps with one of the first patterns and covers a portion of a longer side of the respective first pattern that is not covered by the second patterns when the first, second, and third layers are superimposed. In the above IC layout, the first, second, and third patterns are used for collectively defining a plurality of active regions for forming transistors; and the plurality of active regions are defined along longer sides of the first patterns that are not covered by the second and third patterns when the first, second, and third layers are superimposed. 
         [0069]    In another exemplary aspect, the present disclosure is directed to a semiconductor device. The semiconductor device includes a first SRAM macro, wherein the first SRAM macro includes a first plurality of single-port SRAM cells and a second plurality of peripheral logic circuits, the first plurality are arranged to have a first pitch in a first direction and a second pitch in a second direction orthogonal to the first direction, the first plurality include FinFET transistors formed by first gate features and first fin active lines, the second plurality include FinFET transistors formed by second gate features and second fin active lines, the second gate features are arranged to have a third pitch in the second direction, and the second fin active lines are arranged to have a fourth pitch in the first direction. The semiconductor device further includes a second SRAM macro, wherein the second SRAM macro includes a third plurality of single-port SRAM cells and a fourth plurality of peripheral logic circuits, the third plurality are arranged to have a fifth pitch in the first direction and a sixth pitch in the second direction, the third plurality include FinFET transistors formed by third gate features and third fin active lines, the fourth plurality include FinFET transistors formed by fourth gate features and fourth fin active lines, the fourth gate features are arranged to have the third pitch in the second direction, and the fourth fin active lines are arranged to have the fourth pitch in the first direction. In the semiconductor device above, the second pitch is about twice of the third pitch; the sixth pitch is about the same as the second pitch; and the fifth pitch is greater than the first pitch by about twice of the fourth pitch. 
         [0070]    In another exemplary aspect, the present disclosure is directed to a semiconductor device. The semiconductor device includes a first SRAM macro, wherein the first SRAM macro includes a first plurality of single-port SRAM cells and a second plurality of peripheral logic circuits, the first plurality are arranged to have a first pitch in a first direction and a second pitch in a second direction orthogonal to the first direction, the first plurality include first FinFET transistors formed by first gate features and first fin active lines, the second plurality include second FinFET transistors formed by second gate features and second fin active lines, the second gate features are arranged to have a third pitch in the second direction, and the second fin active lines are arranged to have a fourth pitch in the first direction. The semiconductor device further includes a second SRAM macro, wherein the second SRAM macro includes a third plurality of two-port SRAM cells and a fourth plurality of peripheral logic circuits, the third plurality are arranged to have a fifth pitch in the first direction and a sixth pitch in the second direction, the third plurality include third FinFET transistors formed by third gate features and third fin active lines, the fourth plurality include fourth FinFET transistors formed by fourth gate features and fourth fin active lines, the fourth gate features are arranged to have the third pitch in the second direction, and the fourth fin active lines are arranged to have the fourth pitch in the first direction. In the semiconductor device above, the second pitch is about twice of the third pitch; the sixth pitch is about the same as the second pitch; a first ratio between the first pitch and the fourth pitch is not an integer; and a second ratio between the fifth pitch and the fourth pitch is an integer. 
         [0071]    The foregoing outlines features of several embodiments so that those having ordinary skill in the art may better understand the aspects of the present disclosure. Those having ordinary skill 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 having ordinary skill 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.