Patent Publication Number: US-2023163196-A1

Title: Semiconductor device with flexible sheet structure

Description:
PRIORITY DATA 
     This application claims priority to U.S. Provisional Patent Application No. 63/283,108, filed on Nov. 24, 2021, entitled “Semiconductor Device with Flexible Sheet Structure”, the entire disclosure of which is hereby incorporated herein by reference. 
    
    
     BACKGROUND 
     The semiconductor integrated circuit (IC) industry has experienced exponential growth. Technological advances in IC materials and design have produced generations of ICs where each generation has smaller and more complex circuits than the previous generation. 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. 
     For example, as IC technologies progress towards smaller technology nodes, multi-gate metal-oxide-semiconductor field effect transistors (multi-gate MOSFETs, or multi-gate devices) have been introduced to improve gate control by increasing gate-channel coupling, reducing off-state current, and reducing short-channel effects (SCEs). A multi-gate device generally refers to a device having a gate structure (also known as gate stack), or portion thereof, disposed over more than one side of a channel region. Fin-like field effect transistors (FinFETs) and multi-bridge-channel (MBC) transistors are examples of multi-gate devices that have become popular and promising candidates for high performance and low leakage applications. A FinFET has an elevated channel wrapped by a gate structure on more than one side (for example, the gate wraps a top and sidewalls of a “fin” of semiconductor material extending from a substrate). An MBC transistor has a gate structure that can extend, partially or fully, around a channel region to provide access to the channel region on two or more sides. Because its gate structure surrounds the channel regions, an MBC transistor may also be referred to as a surrounding gate transistor (SGT) or a gate-all-around (GAA) transistor. 
     In demand of lower power and faster speed, it is driven to continuous improvement of transistor architectures. However, advanced FinFET structure is no longer suitable to control short channel effect (SCE) and other device limitation of compact logic circuits beyond sub-A scales. MBC structure provides a relatively lower subthreshold swing (SS) values in comparison to FinFET and becomes a next generation spotlight. However, there are challenges to implementing such IC fabrication flow, especially with scaled down IC features in advanced process nodes. One challenge is how to make the best use of the sheet structure in the MBC transistors to create more flexible device design, such as providing extra performance-power-area (PPA) benefits. Therefore, while existing MBC transistors are generally adequate for their intended purposes, they are not satisfactory in all aspects. 
    
    
     
       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 emphasized 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 perspective view of a semiconductor device, according to one or more aspects of the present disclosure. 
         FIG.  2    is a layout or a plan view of the semiconductor device in  FIG.  1   , according to one or more aspects of the present disclosure. 
         FIGS.  3 ,  4 ,  5 , and  6    are exemplary layouts using a standard cell in  FIG.  2    as a building block, according to one or more aspects of the present disclosure. 
         FIGS.  7 A,  7 B,  7 C,  7 D,  7 E,  7 F,  7 G,  7 H,  7 I,  7 J and  7 K  illustrate some embodiments of flexible sheet structure as active regions, according to one or more aspects of the present disclosure. 
         FIG.  8    is a diagram of an integrated circuit manufacturing system, according to one or more aspects of the present disclosure. 
         FIG.  9    is a diagram of a mask house as shown in  FIG.  8   , according to one or more aspects of the present disclosure. 
         FIG.  10    illustrate a flow chart of a method for modifying layouts to provide a flexible sheet structure for active regions of a semiconductor device, according to one or more aspects of the present disclosure. 
         FIG.  11    illustrate a flow chart of a method for forming a semiconductor device with a flexible sheet structure, according to one or more aspects of the present disclosure. 
         FIGS.  12 ,  13 ,  14 ,  15 ,  16 ,  17 ,  18 ,  19 ,  20 ,  21 , and  22    illustrate perspective views of a workpiece during a fabrication process according to the method of  FIG.  11   , according to one or more aspects of the present disclosure. 
         FIGS.  23 A,  23 B,  24 A, and  24 B  illustrate fragmentary cross-sectional views of a workpiece during a fabrication process according to the method of  FIG.  11   , according to one or more aspects of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The following disclosure provides many different embodiments, or examples, for implementing different features of the disclosure. 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. 
     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. Moreover, the formation of a feature on, connected to, and/or coupled to another feature in the present disclosure that follows may include embodiments in which the features are formed in direct contact, and may also include embodiments in which additional features may be formed interposing the features, such that the features may not be in direct contact. In addition, spatially relative terms, for example, “lower,” “upper,” “horizontal,” “vertical,” “above,” “over,” “below,” “beneath,” “up,” “down,” “top,” “bottom,” etc. as well as derivatives thereof (e.g., “horizontally,” “downwardly,” “upwardly,” etc.) are used for ease of the present disclosure of one features relationship to another feature. The spatially relative terms are intended to cover different orientations of the device including the features. Still further, when a number or a range of numbers is described with “about,” “approximate,” and the like, the term is intended to encompass numbers that are within a reasonable range including the number described, such as within +/−10% of the number described or other values as understood by person skilled in the art. For example, the term “about 5 nm” encompasses the dimension range from 4.5 nm to 5.5 nm. 
     The present disclosure is generally related to semiconductor devices and the fabrication thereof, and more particularly to providing a flexible sheet structure in multi-bridge-channel (MBC) transistors. The term “flexible” refers to a sheet structure having different sheet widths at different segments of a continuous sheet other than a constant sheet width, and thus providing different channel region widths to the MBC transistors. For example, gate structures are disposed on the segments to form a plurality of MBC transistors with different channel region widths, as well as different source/drain feature widths. The larger the channel region width, the stronger current drive performance is provided to the respective transistors, and accordingly faster operational speed. The smaller the channel region width, the less leakage current is associated with the respective transistors, and accordingly better power efficiency. Therefore, one flexible sheet structure may suit different needs of forming different transistors thereon to provide extra performance-power-area (PPA) benefits. 
     Embodiments of the present disclosure may be implemented to advance semiconductor devices that may include multi-gate devices. A multi-gate device generally refers to a device having a gate structure, or portion thereof, disposed over more than one side of a channel region. Fin-like field effect transistors (FinFETs) and multi-bridge-channel (MBC) transistors are examples of multi-gate devices that have become popular and promising candidates for high performance and low leakage applications. A FinFET has an elevated channel wrapped by a gate on more than one side (for example, the gate wraps a top and sidewalls of a “fin” of semiconductor material extending from a substrate). An MBC transistor has a gate structure that can extend, partially or fully, around a channel region to provide access to the channel region on two or more sides. Because its gate structure surrounds the channel regions, an MBC transistor may also be referred to as a surrounding gate transistor (SGT) or a gate-all-around (GAA) transistor. The channel region of an MBC transistor may be formed from nanowires, nanosheets, other nanostructures, and/or other suitable structures. The shapes of the channel region have also given an MBC transistor alternative names such as a nanosheet transistor or a nanowire transistor. Embodiments of the present disclosure are described using an MBC transistor structure with a channel region in a sheet-like shape (e.g., nanosheets), which is for illustration purpose only and should not be construed as limiting the scope of the present disclosure; for example, the present disclosure may also be applicable to channel region in other shapes, such as bar shape or other nanostructures. 
     The various aspects of the present disclosure will now be described in more details with reference to the figures. Referring to  FIG.  1   , a perspective view of an example semiconductor device  100  (or device  100 ) is illustrated. The device  100  includes n-type FETs (NFETs) and/or p-type FETs (PFETs). The device  100  includes a substrate  102 . The substrate  102  may be made of silicon or other semiconductor materials. Alternatively or additionally, the substrate  102  may include other elementary semiconductor materials such as germanium. In some embodiments, the substrate  102  is made of a compound semiconductor such as silicon carbide, gallium arsenic, indium arsenide, or indium phosphide. In some embodiments, the substrate  102  is made of an alloy semiconductor such as silicon germanium, silicon germanium carbide, gallium arsenic phosphide, or gallium indium phosphide. In some embodiments, the substrate  102  includes an epitaxial layer. For example, the substrate  102  may include an epitaxial layer overlying a bulk semiconductor. 
     The device  100  also includes one or more fin structures  104  (e.g., Si fins) that extend from the substrate  102  in the Z-direction. The fin structures  104  are elongated in the X-direction and may optionally include germanium (Ge). The fin structure  104  may be formed by using suitable processes such as photolithography and etching processes. In some embodiments, the fin structure  104  is etched from the substrate  102  using dry etch or plasma processes. In some other embodiments, the fin structure  104  can be formed in a fin formation process, such as a double-patterning lithography (DPL) process. DPL is a method of constructing a pattern on a substrate by dividing the pattern into two interleaved patterns. DPL allows enhanced feature (e.g., fin) density. The fin structures  104  define active regions of the device  100 , which include channel regions  130  under the gate stacks  110  and source/drain regions  140  on both sides of the gate stacks  110 . The source/drain regions  140  include epitaxially-grown material  142  above the fin structures  104 . In some embodiments, for an NFET, the epitaxially-grown material may include SiP, SiC, SiPC, SiAs, Si, or combinations thereof. In some embodiments, for a PFET, the epitaxially-grown material may include SiGe, SiGeC, Ge, Si, a boron-doped material, or combinations thereof. The epitaxially-grown material  142  is also referred to as epitaxial S/D features  142 . 
     An isolation structure  108 , such as a shallow trench isolation (STI) structure, is formed to surround the fin structure  104 . In some embodiments, a lower portion of the fin structure  104  is surrounded by the isolation structure  108 , and an upper portion of the fin structure  104  protrudes from the isolation structure  108 . In other words, a portion of the fin structure  104  is embedded in the isolation structure  108 . The isolation structure  108  prevents electrical interference or crosstalk. 
     The device  100  further includes gate stacks  110  elongated in the Y-direction. The gate stack  110  includes a gate electrode and a gate dielectric layer (not shown) below the gate electrode. The gate dielectric layer includes a dielectric material, such as silicon oxide, germanium oxide, high-k dielectric material layer or a combination thereof. Examples of high-k dielectric material include hafnium oxide, zirconium oxide, aluminum oxide, hafnium dioxide-alumina alloy, hafnium silicon oxide, hafnium silicon oxynitride, hafnium tantalum oxide, hafnium titanium oxide, hafnium zirconium oxide, the like, or combinations thereof In another embodiment, the gate dielectric layer includes an interfacial layer (such as a silicon oxide or germanium oxide layer) and a high-k dielectric material layer on the interfacial layer. The gate electrode includes a conductive material layer, such as doped polycrystalline silicon (polysilicon), metal, metal alloy or combinations thereof. 
     The gate stacks  110  may be formed by a procedure that includes forming a gate dielectric layer, forming a gate electrode layer on the gate dielectric layer, and patterning the gate electrode layer and the gate dielectric layer. The formation of the gate stacks  110  may further include a gate replacement procedure to replace the previously formed gate stack with high-k dielectric and metal. The gate replacement may include a gate last operation or a high-k last operation where both gate dielectric and gate electrode are replaced at a later fabrication stage. The gate stacks  110  may also include gate spacers  114  formed on sidewalls of the gate stacks  110  by a procedure that includes deposition and anisotropic etch. During the forming of the gate spacers  114 , the dielectric material of the gate spacers  114  may also be deposited on sidewalls of the fin structure  104  and surround the fin structure  104  in the X-direction. 
     Each of the gate stacks  110  defines a channel region  130  underneath. The channel region  130  is sandwiched by the two source/drain regions  140  on both sides of the gate stacks  110 . Not depicted in  FIG.  1    is that the channel region  130  includes vertically stacked channel members (or referred to as channel layers) connecting the two adjacent epitaxial S/D features  142  ( FIG.  2    shows a cross-sectional view in X-Y plane that cuts through one of the channel members). Each of the channel members is surrounded by a gate stack  110  on its 4-sides. The channel members may include single crystalline silicon. Alternatively, the channel members may include germanium, silicon germanium, or another suitable semiconductor material(s). Initially, the channel members are formed as part of a semiconductor layer stack that includes the channel members and other semiconductor layers of a different material. The semiconductor layer stack is formed above the substrate  202  and patterned into a fin-like structure above the fin structure  104  during the fin formation process. The fin structure  104  is also referred to as fin base  104 . During the gate replacement process, the semiconductor layer stack is selectively etched to remove the other semiconductor layers, leaving the channel members suspended over the fin base  104  and connecting the respective epitaxial S/D features  142 . The channel members are separated from each other with vertical spacing (or referred to as gaps) therebetween. The gate stack  110  is subsequently filled in the vertical spacing and wrapping around the channel members. 
       FIG.  2    is a layout or a plan view along the A-A line of the device  100  in  FIG.  1   .  FIG.  2    illustrates that the device  100  includes active regions  150  elongated in the X-direction and gate regions  120  elongated in the Y-direction which are generally perpendicular to the active regions  150 . The active regions  150  are spaced apart from each other along the Y-direction with an active region pitch POD. The gate regions  120  are spaced apart from each other along the X-direction with a gate pitch PG. Each intersection of the active regions  150  and the gate regions  120  defines a channel region  130  between a pair of S/D regions  140 . Each gate region  120  engages the channel region  130  of the respective active region  150  and is to form a transistor. In the illustrated embodiment, the S/D regions  140  are common S/D regions shared by two adjacent transistors. The gate stacks  110  (and gate spacers  114 ) are formed in the gate regions  120 . The epitaxial S/D features  142  are formed in the S/D regions  140 . The vertically stacked channel members are formed in the channel regions  130 . In the depicted embodiment, the channel member is a sheet structure, also referred to as nanosheet structure for its nano scale. In various embodiments, the channel member may include cylindrical shape (e.g., nanowire), rectangular shape (e.g., nanobar), sheet shape (e.g., nanosheet), or have other suitable shapes. In the plane view of  FIG.  2   , the active region  150  has a rectangular shape elongated in the X-direction with generally constant width W 0  along the Y-direction. 
     Also illustrated in  FIG.  2    is a standard cell represented by the rectangular box  160  (also referred to as standard cell  160 ). The standard cell  160  represents a unit region, and multiples (e.g., replicates) of the standard cell  160  can be placed and routed such that different areas of an integrated circuit can have identical or similar configurations. That is, the standard cell  160  serves as a building block or tile to form a larger layout. For example, the standard cell  160  may be replicated in the X-direction and in the Y-direction to form more active regions  150  spaced apart along the Y-direction and continuously extending in the X-direction through a series of standard cells  160 . The standard cell  160  has a cell height Hc along the Y-direction and a cell width Wc along the X-direction. In the illustrated embodiments, the cell height Hc is twice of the active region pitch POD, and the cell width Wc is twice of the gate pitch PG, which is illustrative and not intended for limiting. In some embodiments, the cell height Hc may be about 2 to 30 times of the active region pitch POD, and the cell width Wc may be about 2 to 30 times of the gate pitch PG. In some embodiments, the cell height Hc ranges from about 60 nm to about 180 nm, and the cell width Wc ranges from about 80 nm to about 1800 nm. In some other embodiments, the cell height Hc ranges from about 60 nm to about 500 nm. 
       FIG.  3    illustrates using the standard cell  160  as a building block or tile to implement a larger layout  180  with many more transistors. In the depicted embodiment, the standard cell  160  is repeated three times along the X-direction and two times along the Y-direction, which is illustrative and not intended for limiting. The gate regions  120  in different standard cells  160  connect with each other along the Y-direction and form a continuous taller gate region  120 . The partial gate regions  120  on edges of the standard cells  160  also unite to form an integral gate region  120 . The active regions  150  in different standard cells  160  also connect with each other along the X-direction and form a continuous longer active region  150 . The width W 0  of the longer active region  150  still keeps constant throughout the X-direction. Accordingly, the channel regions  130  (and thus channel members) have the same width W 0  everywhere in the layout  180 . Since the channel region width is a major factor determining a transistor&#39;s power handling capability and channel resistance and in turn device speed, the transistors everywhere in the layout  180  are substantially identical or very similar. Yet, for transistors in different circuits, or transistors in different portions of the same circuit, device performance requirements are different. For example, transistors in high-speed circuits may have requirements for high power handling capability and high device speed, and transistors in input/output (I/O) circuits may have requirements for low power leakage and higher power efficiency. A constant active region width comes along with limited flexibility in achieving transistor performance variation, and the performance-power-area (PPA) criteria is not optimized. 
       FIG.  4    illustrates an adjusted layout  180 ′ based on a layout adjustment performed on the layout  180  in  FIG.  3   . The layout adjustment increases (or reduces) the width of the active region  150  in some of the standard cells  160 . In the depicted embodiment, the width of the active region  150  in the middle standard cells  160  is reduced, denoted as W 1 . The standard cells  160  with adjusted active region width is denoted as adjusted cells  160 ′. Channel regions and S/D regions in the adjusted cells  160 ′ are denoted as channel regions  130 ′ and S/D regions  140 ′, respectively. The distance between adjacent active regions along the Y-direction is also increased in the adjusted cells  160 ′ in the depicted embodiment, spared from the reduced active region width. In some embodiments, a ratio of W 1 /W 0  is from about 30% to about 90%. If the ratio is less than about 40%, the channel region width may become too small and violate certain design rules. If the ratio is larger than about 90%, benefits from the width difference is not obvious and may be outweighed by the risk of overlaying inaccuracy during active region patterning. After the layout adjustment, the active region  150  still continuously extends through the layout  180 ′ but with portions (or segments) with different widths. The transistors formed over the portions with width W 0  benefit from less channel resistance along with the larger channel region width and accordingly higher power handling capability, less parasitic capacitance (e.g., STI capacitance), and higher device speed. The transistors formed over the portion with width W 1  benefit from less leakage along with the smaller channel region width and accordingly higher power efficiency. 
     Regarding the transistor formed across the boundary between a standard cell  160  and an adjusted cell  160 ′, such as the transistor highlighted in a dashed box  182 , its channel region has a first portion with the width W 0  and a second portion with the reduced width W 1 , and its S/D region locates in the standard cell  160  has the width W 0  and the other S/D region locates in the adjusted cell  160 ′ has the reduced width W 1 . That is, the channel member formed in the region represented by the dashed box  182  has a sheet structure with two portions (segments) of different widths. The transition between the two portions may be a step-profile (e.g., a notch) as shown in  FIG.  4   , or a tapering sidewall that gradually narrows down between the two portions. Also, the epitaxial S/D features  142  in the standard cell  160  have a larger volume than the ones in the adjusted cell  160 ′. The performance of this type of transistors may reside between the ones in the standard cell  160  and the ones in the adjusted cell  160 ′ and may be suitable for general applications. 
       FIG.  5    illustrates another adjusted layout  180 ″ based on a different layout adjustment performed on the layout  180  in  FIG.  3   . The layout adjustment offsets the active regions  150  in some of the standard cells  160  along the Y-direction, creating adjusted cells  160 ″. Channel regions and S/D regions in the adjusted cells  160 ″ are denoted as channel region  130 ″ and S/D regions  140 ″, respectively. In the depicted embodiment, the active regions in the standard cells  160  and adjusted cells  160 ″ along the X-direction still connects. Further, the two active regions in the center of the adjusted layout  180 ″ connect to each other due to the offset adjustment. The connection unites two active regions otherwise spaced apart in the Y-direction and form one larger X-shape active region. The center portion of the X-shape active region has a width W 2 , which is about twice of the width W 0 . The transistors formed over the portions with width W 2  benefit from less channel resistance along with the larger channel region width and accordingly higher power handling capability, less parasitic capacitance (e.g., STI capacitance), and higher device speed. The transistors formed over the portion with width W 0  benefit from less leakage along with the smaller channel width and accordingly higher power efficiency. 
     Regarding the transistor formed across the boundary between a standard cell  160  and an adjusted cell  160 ′, such as the transistor highlighted in a dashed box  184 , its channel region has two parallel portions with the width W 0  and a third portion with the enlarged width W 2 , and its S/D region locates in the standard cell  160  has two parallel portions with the width W 0  and an S/D region locates in the adjusted cell  160 ″ has the enlarged width W 2 . Aslo, the channel member formed in the region represented by the dashed box  182  has a sheet structure with two parallel portions (segments) of a smaller width W 0  and another portion with an enlarged width W 2 . The transition between the different portions may be a step-profile (e.g., a notch) as shown in  FIG.  5   , or a tapering sidewall that gradually narrows down between the different portions. Also, there are two epitaxial S/D features  142  in the standard cell  160  having a smaller volume than the one epitaxial S/D feature  142  in the adjusted cell  160 ″. The performance of this type of transistors may reside between the ones in the standard cell  160  and the ones in the adjusted cell  160 ′ and may be suitable for general applications. 
     Further, the layout adjustment as shown in  FIG.  4    and  FIG.  5    can be applied in the same layout to create different adjusted cells  160 ′ and  160 ″.  FIG.  6    shows such an example. Reference numerals are repeated in  FIG.  6    for ease of understanding, and similar aspects are not repeated in the interest of conciseness. In  FIG.  6   , by layout adjustments to standard cells  160  to create both adjusted cells  160 ′ and adjusted cells  160 ″, transistors with channel width of W 2 , W 1 , W 0  (W 2 &gt;W 0 &gt;W 1 ), a combination of W 0  and W 2  (e.g., in region  184 ), and a combination of W 0  and W 1  (e.g., in region  182 ), can be implemented in different regions of the integrated circuit to suite for different circuit application needs. 
     There are numeral other ways to implement layout adjustments. Some embodiments are illustrated in  FIGS.  7 A- 7 K . The boundaries of standard cells and adjusted cells are omitted in  FIGS.  7 A- 7 K  for the sake of simplicity. One difference among  FIGS.  7 A- 7 K  is the contour of the active region  150 . In  FIG.  7 A , the active region  150  has an L-shape with a first portion of a larger width and a second portion of a smaller width. The gate regions  120  over the second portion may be shorter as well. In  FIG.  7 B , the active region  150  has a C-shape with a first portion of a larger width and second and third portions of a smaller width. In  FIG.  7 C , the active region  150  has an F-shape, with a first portion of a larger width and second and third portions of a smaller width. The gate regions  120  over the second and third portions may be shorter as well. In  FIG.  7 D , the active region  150  has a T-shape with a first portion with a larger width in the middle and second and third portions with a smaller width on both sides. The gate regions  120  over the second and third portions may be shorter as well. In  FIG.  7 E , the active region  150  has a U-shape with a first portion with a smaller width in the middle and second and third portions with a larger width on both sides. The gate region  120  over the first portion may be shorter as well. In  FIG.  7 F , the active region  150  has an H-shape with a first portion with a smaller width in the middle and second and third portions with a larger width on both sides. The gate regions  120  over the first portion my be shorter as well. In  FIG.  7 G , the active region  150  has an X-shape with a first portion with a larger width in the middle and four other narrower portions on four corners. In  FIG.  7 H , the active region  150  has an O-shape with first and second portions with a larger width on the sides and third and fourth portion with a smaller width in the middle. In  FIG.  7 I , the active region  150  has a P-shape with a first portion with a larger width on one side, a second portion with a medium width on another side, and third and fourth portions with a smaller width in the middle. The gate regions  120  over the second, third, fourth portions may be shorter as well. In  FIG.  7 J , the active region  150  has a C-shape with a first portion of a larger width and second and third portions of a smaller width. In  FIG.  7 K , the active region  150  has an E-shape with a first portion with a larger width and three parallel portions with a smaller width. 
     Two source/drain contacts  186  are depicted in  FIGS.  7 J and  7 K , respectively, for illustrative purpose and omitted from other figures. Yet those skilled in the art may understand that source/drain contacts also exist in circuits represented by layouts illustrated in  FIGS.  2 - 7 J  to implement a functional circuit. In one transistor, the source/drain contact  186  electrically couples the S/D regions  140  with smaller width together. The parasitic capacitance between the source/drain contacts  186  and STI structure  108  ( FIG.  1   ) may be a design consideration. The active region  150  in  FIG.  7 K  with an E-shape provides less contact area between the source/drain contacts  186  and the STI structure  108  than the active region  150  in  FIG.  7 J  with a C-shape. Therefore, when the parasitic capacitance between the source/drain contacts and STI structure is a weighted factor, active region with an E-shape may be selected during layout design over a C-shape. Further, the flexible active regions illustrated in  FIGS.  7 A- 7 K  can serve as building blocks to implement even larger active regions with more complicate contours. 
     An integrated circuit manufacturing flow in an integrated circuit manufacturing system in modifying and/or optimizing sheet structures for active regions is depicted below.  FIG.  8    is a simplified block diagram of an embodiment of an integrated circuit manufacturing system  800  and an integrated circuit manufacturing flow associated therewith, which may benefit from various aspects of the provided subject matter. The integrated circuit manufacturing system  800  includes a plurality of entities, such as a design house  820 , a mask house  840 , and an integrated circuit manufacturer  860  (i.e., a fab), that interact with one another in the design, development, and manufacturing cycles and/or services related to manufacturing an integrated circuit device  862 . The plurality of entities are connected by a communications network, which may be a single network or a variety of different networks, such as an intranet and the Internet, and may include wired and/or wireless communication channels. Each entity may interact with other entities and may provide services to and/or receive services from the other entities. One or more of the design house  820 , mask house  840 , and integrated circuit manufacturer  860  may be owned by a single larger company, and may even coexist in a common facility and use common resources. 
     The design house (or design team)  820  generates an IC design layout  802 . The integrated circuit design layout  802  includes various geometrical patterns designed for the integrated circuit device  862 . The various geometrical patterns in the IC design layout  802 , such as rectangles, may correspond to patterns of active regions and gate regions to be fabricated. The design house  820  implements a proper design procedure to form the integrated circuit design layout  802 . The design procedure may include logic design, physical design, and/or place and route. The integrated circuit design layout  802  is presented in one or more data files having information of the geometrical patterns. For example, the integrated circuit design layout  802  can be expressed in a GDSII file format, a DFII file format, or another suitable computer-readable data format. 
     The mask house  840  uses the design layout  802  to manufacture one or more masks to be used for fabricating various layers of the integrated circuit device  862 , particularly a layout of a flexible sheet structure for active regions. The mask house  840  performs mask data preparation  832 , mask fabrication  834 , and other suitable tasks. The mask data preparation  832  turns the sheet structure layout with standard cells into flexible sheet structure layout, and also translates the layout into a form that can be physically written by a mask writer. The mask fabrication  834  then fabricates a plurality of masks that are used for patterning a substrate (e.g., a wafer). In the present embodiment, the mask data preparation  832  and mask fabrication  834  are illustrated as separate elements. However, the mask data preparation  832  and mask fabrication  834  can be collectively referred to as mask data preparation. 
     In the present embodiment, the mask data preparation  832  includes layout optimization and simulation to adjust sheet structures for active regions, which will be described in details later. The mask data preparation  832  may further include optical proximity correction (OPC) which uses lithography enhancement techniques to compensate for image errors, such as those that can arise from diffraction, interference, or other process effects. The mask data preparation  832  may further include a mask rule checker (MRC) that checks the integrated circuit design layout with a set of mask creation rules which may contain certain geometric and connectivity restrictions to ensure sufficient margins, to account for variability in semiconductor manufacturing processes, etc. The mask data preparation  832  may further include lithography process checking (LPC) that simulates processing that will be implemented by the integrated circuit manufacturer  860  to fabricate wafers and further diced into integrated circuit device  862 . The processing parameters may include parameters associated with various processes of the integrated circuit manufacturing cycle, parameters associated with tools used for manufacturing the integrated circuit, and/or other aspects of the manufacturing process. 
     It should be understood that the above description of the mask data preparation  832  has been simplified for the purposes of clarity, and data preparation may include additional features such as a logic operation (LOP) to modify the integrated circuit design layout according to manufacturing rules, particularly a hybrid bonding layer design rule. Additionally, the processes applied to the integrated circuit design layout  802  during mask data preparation  832  may be executed in a variety of different orders. 
     After mask data preparation  832  and during mask fabrication  834 , a mask or a group of masks are fabricated based on the modified (flexible) sheet structure design layout. For example, an electron-beam (e-beam) or a mechanism of multiple e-beams is used to form a pattern on a mask (photomask or reticle) based on the modified layout. The mask can be formed in various technologies such as a transmissive mask or a reflective mask. In an embodiment, the mask is formed using binary technology, where a mask pattern includes opaque regions and transparent regions. A radiation beam, such as an ultraviolet (UV) beam, used to expose the image sensitive material layer (e.g., photoresist) coated on a wafer, is blocked by the opaque region and transmits through the transparent regions. In one example, a binary mask includes a transparent substrate (e.g., fused quartz) and an opaque material (e.g., chromium) coated in the opaque regions of the mask. In another example, the mask is formed using a phase shift technology. In the phase shift mask (PSM), various features in the pattern formed on the mask are configured to have proper phase difference to enhance the resolution and imaging quality. In various examples, the phase shift mask can be attenuated PSM or alternating PSM. 
     The integrated circuit manufacturer  860 , such as a semiconductor foundry, uses the mask (or masks) fabricated by the mask house  840  to fabricate the integrated circuit device  862 . The integrated circuit manufacturer  860  is an integrated circuit fabrication business that can include a myriad of manufacturing facilities for the fabrication of a variety of different integrated circuit products. For example, there may be a manufacturing facility for the front-end fabrication of a plurality of integrated circuit products (i.e., front-end-of-line (FEOL) fabrication), while a second manufacturing facility may provide the back-end fabrication for the interconnection and packaging of the integrated circuit products (i.e., back-end-of-line (BEOL) fabrication), and a third manufacturing facility may provide other services for the foundry business. 
       FIG.  9    is a more detailed block diagram of the mask house  840  shown in  FIG.  8    according to various aspects of the present disclosure. In the illustrated embodiment, the mask house  840  includes a mask design system  880  that is tailored to perform the functionality described in association with mask data preparation  832  of  FIG.  8   . The mask design system  880  is an information handling system such as a computer, server, workstation, or other suitable device. The mask design system  880  includes a processor  882  that is communicatively coupled to a system memory  884 , a mass storage device  886 , and a communication module  888 . The system memory  884  provides the processor  882  with non-transitory, computer-readable storage to facilitate execution of computer instructions by the processor. Examples of system memory may include random access memory (RAM) devices such as dynamic RAM (DRAM), synchronous DRAM (SDRAM), solid state memory devices, and/or a variety of other memory devices known in the art. Computer programs, instructions, and data are stored on the mass storage device  886 . Examples of mass storage devices may include hard drives, optical drives, magneto-optical drives, solid-state storage devices, and/or a variety of other mass storage devices known in the art. The communication module  888  is operable to communicate information such as integrated circuit design layout files with the other components in the integrated circuit manufacturing system  800 , such as the design house  820 . Examples of communication modules may include Ethernet cards, 802.11 WiFi devices, cellular data radios, and/or other suitable devices. 
     In operation, the mask design system  880  is configured to manipulate the standard cell design layout before it is transferred to a mask  890  by the mask fabrication  834 . In an embodiment, the mask data preparation  832  is implemented as software instructions executing on the mask design system  880 . To further this embodiment, the mask design system  880  receives a first GDSII file  892  containing the standard cell design layout from the design house  820 , and modifies the standard cell design layout, for example, to adjust widths of active regions or offset locations of active regions in some standard cells. After the mask data preparation  832  is complete, the mask design system  880  transmits a second GDSII file  894  containing a modified design layout to the mask fabrication  834 . In alternative embodiments, the integrated design layout may be transmitted between the components in integrated circuit manufacturing system  800  in alternate file formats such as DFII, CIF, OASIS, or any other suitable file type. Further, the mask design system  880  and the mask house  840  may include additional and/or different components in alternative embodiments. 
       FIG.  10    is a flowchart of a method  1000  of modifying sheet structures for active regions and generating associated photomask for manufacturing, according to various aspects of the present disclosure. The method  1000  may be performed at the mask house  840  as depicted in  FIGS.  8  and  9   . The method  1000  includes operations at blocks  1002 ,  1004 ,  1008 ,  1010 ,  1012 ,  1014 , and  1016 . The operations at block  1002  receives a defined device structure, such as a schematic representing a circuit design. The operations at block  1004  receives layout database, such as standard cell definition, and implement layout based on the defined device structure. The operations at block  1006  identify device requirements, such as transistors for high-speed applications and transistors for low-power applications. The operations at block  1008  modifies the layout to adjust some of the standard cells to implement flexible sheet structures for active regions. The operations at block  1008  also runs performance-power-area (PPA) simulations to optimize PPA benefits based on tweaking the standard cells. The operations at block  1010  evaluates process risks. For example, if width of some portions of the active region becomes too small, it would be flagged to avoid line broken during manufacturing. The operations at block  1012  forbids risk patterns from the layout adjustment and regenerates a modified layout. The operations at block  1014  performs Automatic Placement and Routing (APR) constrain and impact assessment. If the assessment fails, the method  1000  may loop back to operations at block  1008  to readjust the layout until the assessment at block  1014  is successful. If the assessment successfully completes, the operation  1016  manufactures the corresponding photomask for forming flexible sheet structures for active regions. 
       FIG.  11    is a flowchart of a method  1100  of forming a semiconductor device. The method  1100  may be performed at the integrated circuit manufacturer  860  as depicted in  FIG.  8   . The method  1100  is merely an example and is not intended to limit the present disclosure to what is explicitly illustrated in the method  1100 . Additional steps may be provided before, during and after the method  1100 , and some steps described can be replaced, eliminated, or moved around for additional embodiments of the method. Not all steps are described herein in detail for reasons of simplicity. The method  1100  is described below in conjunction with  FIGS.  12 - 24 B , which illustrate perspective and cross-sectional views of a workpiece  200  (or device  200 ) at different stages of fabrication according to embodiments of the method  1100 . Embodiments of the present disclosure are described in forming an MBC transistor with an X-shape active region as in  FIG.  7 G , which is for illustration purpose only and should not be construed as limiting the scope of the present disclosure; for example, the present disclosure may also be applicable to forming active regions with other shapes, including the ones illustrated in  FIGS.  7 A- 7 K . 
     Referring to  FIGS.  11  and  12   , the method  1100  includes a block  1102  where a workpiece  200  is received. The workpiece  200  includes a substrate  202 . In some embodiments, the substrate  202  may be a semiconductor substrate such as a silicon (Si) substrate. In some embodiments, the substrate  202  includes a single crystalline semiconductor layer on at least its surface portion. The substrate  202  may comprise a single crystalline semiconductor material such as, but not limited to Si, Ge, SiGe, GaAs, InSb, GaP, GaSb, InAlAs, InGaAs, GaSbP, GaAsSb and InP. Alternatively, the substrate  202  may include a compound semiconductor and/or an alloy semiconductor. The substrate  202  may include various layers, including conductive or insulating layers formed on a semiconductor substrate. 
     Referring to  FIGS.  11  and  13   , the method  1100  includes a block  1104  where one or more epitaxial layers are formed over the substrate  202 . In some embodiments, an epitaxial stack  212  is formed over the substrate  202 . The epitaxial stack  212  includes epitaxial layers  214  of a first composition interposed by epitaxial layers  216  of a second composition. The first and second compositions can be different. In an embodiment, the epitaxial layers  214  are silicon germanium (SiGe) and the epitaxial layers  216  are silicon (Si). However, other embodiments are possible including those that provide for a first composition and a second composition having different oxidation rates and/or etch selectivity. The numbers of the epitaxial layers  214  and  216  as illustrated in  FIG.  13    are for illustrative purposes only and not intended to be limiting beyond what is specifically recited in the claims. It can be appreciated that any number of epitaxial layers can be formed in the epitaxial stack  212 ; the number of epitaxial layers depending on the desired number of channel layers for forming transistors. In some embodiments, the numbers of epitaxial layers  214  and  216  are between 2 and 10, respectively. 
     In some embodiments, the epitaxial layer  214  has a thickness ranging from about 8 nm to about 12 nm. The epitaxial layers  214  may be substantially uniform in thickness. In some embodiments, the epitaxial layer  216  has a thickness ranging from about 8 nm to about 10 nm. In some embodiments, the epitaxial layers  216  may be substantially uniform in thickness. As described in more detail below, the epitaxial layers  216  may serve as channel members (or channel layers) for subsequently-formed MBC transistors and its thickness is chosen based on device performance considerations. The epitaxial layers  214  may serve to reserve a spacing (or referred to as a gap) between adjacent channel layers and its thickness is chosen based on device performance considerations. The epitaxial layers  214  would be subsequently removed and may also be referred to as the sacrificial layers  214 . 
     By way of example, epitaxial growth of the epitaxial stack  212  may be performed by a molecular beam epitaxy (MBE) process, a metalorganic chemical vapor deposition (MOCVD) process, and/or other suitable epitaxial growth processes. In some embodiments, the epitaxially grown layers, such as the epitaxial layers  216 , include the same material as the substrate  202 , such as silicon (Si). In some embodiments, the epitaxial layers  214  and  216  include a different material than the substrate  202 . As stated above, in at least some examples, the epitaxial layer  214  includes an epitaxially grown Si 1-x Ge x  layer (e.g., x is about 25˜55%) and the epitaxial layer  216  includes an epitaxially grown Si layer. Alternatively, in some embodiments, either of the epitaxial layers  214  and  216  may include other materials such as germanium, a compound semiconductor such as silicon carbide, gallium arsenide, gallium phosphide, indium phosphide, indium arsenide, and/or indium antimonide, an alloy semiconductor such as SiGe, GaAsP, AlInAs, AlGaAs, InGaAs, GaInP, and/or GaInAsP, or combinations thereof. As discussed, the materials of the epitaxial layers  214  and  216  may be chosen based on providing differing oxidation and etch selectivity properties. In various embodiments, the epitaxial layers  214  and  216  are substantially dopant-free (i.e., having an extrinsic dopant concentration from about 0 cm −3  to about 1×10 17  cm −3 ), where for example, no intentional doping is performed during the epitaxial growth process. 
     Further, a mask layer  218  is formed over the epitaxial stack  212 . In some embodiments, the mask layer  218  includes a first mask layer  218 A and a second mask layer  218 B. The first mask layer  218 A is a pad oxide layer made of silicon oxide, which can be formed by a thermal oxidation process. The second mask layer  218 B is made of silicon nitride (SiN), which is formed by chemical vapor deposition (CVD), including low pressure CVD (LPCVD) and plasma enhanced CVD (PECVD), physical vapor deposition (PVD), atomic layer deposition (ALD), or other suitable process. 
     Referring to  FIGS.  11  and  14   , the method  1100  includes a block  1106  where the mask layer  218  is patterned to form an X-shape by using one of the photomasks manufactured by the method  1000  as depicted in  FIG.  10   . In some embodiments, the mask layer  218  is patterned using any suitable methods such as a photolithography process, which may include forming a resist layer on the mask layer  218 , exposing the resist by a lithography exposure process, performing a post-exposure bake process, developing the photoresist layer to form the patterned photoresist layer that exposes part of the mask layer  218 , patterning the mask layer  218 , and finally removing the patterned resist layer. The lithography process may be alternatively replaced by other suitable techniques, such as e-beam writing, ion-beam writing, maskless patterning or molecular printing. As depicted in  FIG.  14   , the mask layer  218  has an X-shape with a middle portion having a larger width W 2  (measured along the Y-direction) and four corner portions having a smaller width W 0 . 
     Referring to  FIGS.  11  and  15   , the method  1100  includes a block  1108  where a top portion of the substrate  202  is patterned to form a fin structure (also referred to as fin base)  204  and the stack  212  is patterned to form a fin-shaped structure  220  above the fin base  204 . To pattern the stack  212 , the patterned mask layer  218  may be used as an etch mask to etch the stack  212  through openings in the etch mask to form the fin-shaped structure  220 . The etching process can include dry etching, wet etching, reactive ion etching (RIE), and/or other suitable processes. The second mask layer  218 B may be removed during the etching process, while the first mask layer  218 A may still remain over the fin-shaped structure  220 . 
     Referring to  FIGS.  11  and  16   , the method  1100  includes a block  1110  where an isolation feature  208  is formed. The isolation feature  208  may be referred to as a shallow trench isolation (STI) feature  208 . In an example process to form the isolation feature  208 , a dielectric material is deposited over the workpiece  200 , filling the fin trenches with the dielectric material. In some embodiments, the dielectric material may include tetraethylorthosilicate (TEOS) oxide, un-doped silicate glass, or doped silicon oxide such as borophosphosilicate glass (BPSG), fused silica glass (FSG), phosphosilicate glass (PSG), boron doped silicon glass (BSG), and/or other suitable dielectric materials. In various examples, at block  1110 , the dielectric material may be deposited by flowable CVD (FCVD), spin-on coating, and/or other suitable process. The deposited dielectric material is then thinned and planarized, for example by a chemical mechanical polishing (CMP) process, until the first mask layer  218 A is exposed. After the planarization, the deposited dielectric material is recessed in an etching-back process until the fin-shaped structures  220 , particularly the sacrificial layers  214  and the channel layers  216 , rise above the isolation feature  208 . The first mask layer  218 A may also be removed in the etching-back process. At this point, the fin base  204  is surrounded by the isolation feature  208 . The fin-like structure  220  rises above the isolation feature  208 . As depicted in  FIG.  16   , the fin-like structure  220  has an X-shape with a middle portion  220   a  having a larger width W 2  (measured along the Y-direction) and four corner portions  220   b  having a smaller width W 0 . 
     Referring to  FIGS.  11  and  17   , the method  1100  includes a block  1112  where dummy gate stacks  240  are formed over the channel regions of the fin-like structure  220 . In some embodiments, a gate replacement process (or gate-last process) is adopted where the dummy gate stacks  240  serve as a placeholder for functional gate structures. Other processes and configuration are possible. Although in  FIG.  7 G , eight gate regions  120  are illustrated, only one dummy gate stack  240  formed in one gate region is illustrated in  FIG.  17    for the sake of simplicity. The dummy gate stack  240  is directly above the boundary between the middle portion  220   a  and the corner portions  220   b  of the fin-like structure  220 . Yet those skilled in the art may understand that there may be other dummy gate stacks  240  formed directly above the middle portion  220   a  and other dummy gate stacks  240  formed directly above the corner portions  220   b , as shown in  FIG.  7 G . 
     In the illustrated embodiment, the dummy gate stack  240  includes a dummy dielectric layer  242  and a dummy electrode  244  disposed over the dummy dielectric layer  242 . For patterning purposes, a gate top hard mask  246  is deposited over the dummy gate stacks  240 . The gate top hard mask  246  may be a multi-layer and include a silicon nitride mask layer  248  and a silicon oxide mask layer  250  over the silicon nitride mask layer  248 . The regions of the fin-like structure  220  underlying the dummy gate stack  240  may be referred to as the channel region. The channel region in the fin-like structure  220  is sandwiched between two source/drain regions for source/drain formation. In an example process, the dummy dielectric layer  242  is blanketly deposited over the workpiece  200  by CVD. A material layer for the dummy electrode  244  is then deposited over the dummy dielectric layer  242 . The dummy dielectric layer  242  and the material layer for the dummy electrode  244  are then patterned using photolithography processes to form the dummy gate stack  240 . In some embodiments, the dummy dielectric layer  242  may include silicon oxide and the dummy electrode  244  may include polycrystalline silicon (polysilicon). 
     Referring to  FIGS.  11  and  18   , the method  1100  includes a block  1114  where the source/drain regions of the fin-shaped structures  210  are recessed to form source recesses and drain recesses, collectively as source/drain recesses  254  (or source/drain trenches  254 ). The method  1100  may first form gate spacers  252  on sidewalls of the dummy gate stack  240 . With the dummy gate stack  240  and the gate spacers  252  serving as an etch mask, the workpiece  200  is anisotropically etched to form the source/drain recesses  254  over the source/drain regions of the fin-like structure  220 . The anisotropic etch may include a dry etching process. For example, the dry etching process may implement hydrogen, a fluorine-containing gas (e.g., CF 4 , SF 6 , CH 2 F 2 , CHF 3 , and/or C 2 F 6 ), a chlorine-containing gas (e.g., Cl 2 , CHCl 3 , CCl 4 , and/or BCl 3 ), a bromine-containing gas (e.g., HBr and/or CHBr 3 ), an iodine-containing gas, other suitable gases and/or plasmas, and/or combinations thereof. 
     Referring to  FIGS.  11  and  19   , the method  1100  includes a block  1116  where inner spacers  256  are formed on lateral ends of the epitaxial layers  214 . In some embodiments, a lateral etching (or horizontal recessing) is performed to recess the epitaxial layers  214  to form cavities on lateral ends of the epitaxial layers  214 . The amount of etching of the epitaxial layers  214  may range from about 2 nm to about 10 nm. When the epitaxial layers  214  are SiGe, the lateral etching process may use an etchant, such as, but not limited to, ammonium hydroxide (NH 4 OH), tetramethylammonium hydroxide (TMAH), ethylenediamine pyrocatechol (EDP), or potassium hydroxide (KOH) solutions. Subsequently, an insulating layer is deposited in the source/drain recesses  254  and fill the cavities on lateral ends of the epitaxial layers  214 . The insulating layer may include a dielectric material, such as SiN, SiOC, SiOCN, SiCN, SiO 2 , and/or other suitable material. In some embodiments, the insulating layer is conformally deposited, for example, by ALD or any other suitable method. After the conformal deposition of the insulating layer, an etch-back process is performed to partially remove the insulating layer from outside of the cavities. By this etching the insulating layer remains substantially within the cavities, thereby forming the inner spacers  256 . 
     Referring to  FIGS.  11  and  20   , the method  1100  includes a block  1118  where source/drain features  258  are formed. The source/drain features  258  are selectively and epitaxially deposited in the source/drain recesses  254 . The source/drain features  258  may be deposited using an epitaxial process, such as vapor-phase epitaxy (VPE), ultra-high vacuum CVD (UHV-CVD), molecular beam epitaxy (MBE), and/or other suitable processes. Depending on the design of the workpiece  200 , the source/drain features  258  may be n-type or p-type. When the source/drain features  258  are n-type, they may include silicon (Si) doped with an n-type dopant, such as phosphorus (P) or arsenic (As). When the source/drain features  258  are p-type, they may include silicon germanium (SiGe) doped with a p-type dopant, such as boron (B) or gallium (Ga). Doping of the source/drain features  258  may be performed either in situ with their deposition or ex situ using an implantation process, such as a junction implant process. Also as shown in  FIG.  20   , the source/drain features  258  includes a first source/drain feature  258   a  formed on the middle portion  220   a  of the X-shape fin-like structure  220  and second source/drain features  258   b  formed on the corner portions  220   b  of the X-shape fin-like structure  220 . The one first source/drain feature  258   a  is on one side of the dummy gate stack  240 . The two second source/drain features  258   b  are on the other side of the dummy gate stack  240 . Since the middle portion  220   a  has a larger width, the first source/drain feature  258   a  has a larger width and a larger volume than each of the two second source/drain feature  258   b . Further, the two second source/drain feature  258   b  may be laterally merged (in the Y-direction), such as depicted in  FIG.  20   . Alternatively, the two second source/drain feature  258   b  may remain separated. 
     Referring to  FIGS.  11  and  21   , the method  1100  includes a block  1120  where a contact etch stop layer (CESL)  262  and an interlayer dielectric (ILD)  264  layer are deposited on the workpiece  200 . In an example process, the CESL  262  is first conformally deposited over the workpiece  200  and then the ILD layer  264  is deposited over the CESL  262 . The CESL  262  may include silicon nitride, silicon oxide, silicon oxynitride, and/or other materials known in the art. The CESL  262  may be deposited using ALD, plasma-enhanced chemical vapor deposition (PECVD) process and/or other suitable deposition or oxidation processes. In some embodiments, the ILD layer  264  includes materials such as SiCN, SiON, SiOCN, tetraethylorthosilicate (TEOS) oxide, un-doped silicate glass, or doped silicon oxide such as borophosphosilicate glass (BPSG), fused silica glass (FSG), phosphosilicate glass (PSG), boron doped silicon glass (BSG), and/or other suitable dielectric materials. The ILD layer  264  may be deposited by spin-on coating, an FCVD process, or other suitable deposition technique. In some embodiments, after formation of the ILD layer  264 , the workpiece  200  may be annealed to improve integrity of the ILD layer  264 . To remove excess materials (including the gate top hard mask  246 ) and to expose top surfaces of the dummy electrode  244  of the dummy gate stack  240 , a planarization process (such as a CMP process) may be performed to the workpiece  200  to provide a planar top surface. Top surfaces of the dummy electrodes  244  are exposed on the planar top surface. 
     Referring to  FIGS.  11  and  22   , the method  1100  includes a block  1122  where the dummy gate stack  240  is removed. The dummy gate stack  240  exposed at the conclusion of the block  1120  is removed from the workpiece  200  by a selective etching process. The selective etching process may be a selective wet etching process, a selective dry etching process, or a combination thereof. 
     In the depicted embodiment, the selective etching process selectively removes the dummy dielectric layer  242  and the dummy electrode  244  without substantially damaging the channel layers  216 , the sacrificial layers  214 , and the gate spacers  252 . The removal of the dummy gate stack  240  results in a gate trench  266  exposing the middle portion  220   a  and two corner portions  220   b  of the channel region in the X-shape active region. 
     Referring to  FIGS.  11  and  23 A -B, the method  1100  includes a block  1124  where the channel layers  216  is released in the gate trench  266 .  FIG.  23 A  is a cross-sectional view in the X-Z plane of the workpiece  200  along the A-A line in  FIG.  22   .  FIG.  23 B  is a plane view in the X-Y plane of the workpiece  200  along the B-B line in  FIG.  22   . In the depicted embodiment, the release of the channel layers  216  includes a selective etching process that selectively removes the sacrificial layers  214  from the gate trench  266 . The released channel layers  216  are also denoted as the channel members  216 . In the depicted embodiment where the channel members  216  resemble a sheet or a nanosheet, the channel member release process may also be referred to as a sheet formation process. The channel members  216  are vertically stacked along the Z direction. The selective removal of the sacrificial layers  214  may be implemented by selective dry etching, selective wet etching, or other selective etching processes. In some embodiments, the selective wet etching includes ammonium hydroxide (NH 4 OH), hydrogen fluoride (HF), hydrogen peroxide (H 2 O 2 ), or a combination thereof (e.g. an APM etch that includes an ammonia hydroxide-hydrogen peroxide-water mixture). In some alternative embodiments, the selective removal includes silicon germanium oxidation followed by a silicon germanium oxide removal. For example, the oxidation may be provided by ozone clean and then silicon germanium oxide removed by an etchant such as NH 4 OH. 
     Referring to  FIGS.  11  and  24 A -B, the method  1100  includes a block  1126  where gate structure  270  (also known as functional gate structure  270  or metal gate structure  270 ) are formed in the gate trenches  266  to engage each of the channel members  216 . Particularly, each of the middle portion  220   a  and the two corner portions  220   b  of the channel members  216  is wrapped around by the gate structure  270 .  FIG.  24 A  is a cross-sectional view in the X-Z plane of the workpiece  200  along the A-A line in  FIG.  22   .  FIG.  24 B  is a plane view in the X-Y plane of the workpiece  200  along the B-B line in  FIG.  22   . 
     Each of the gate structures  270  includes an interfacial layer  272  disposed on the channel members  216 , a high-k dielectric layer  274  disposed on the interfacial layer  2721 , and a gate electrode layer  276  over the high-k dielectric layer  274 . The interfacial layer  272  and the high-k dielectric layer  274  are collectively referred to as a gate dielectric layer. The interfacial layer  272  may include silicon oxide and be formed as result of a pre-clean process. An example pre-clean process may include use of RCA SC- 1  (ammonia, hydrogen peroxide and water) and/or RCA SC- 2  (hydrochloric acid, hydrogen peroxide and water). The pre-clean process oxidizes the exposed semiconductive surfaces of the channel members  216  to form the interfacial layer. The high-k dielectric layer  274  is then deposited over the interfacial layer  272  using ALD, CVD, and/or other suitable methods. The high-k dielectric layer  274  includes high-K dielectric materials. In one embodiment, the high-k dielectric layer  274  may include hafnium oxide. Alternatively, the high-k dielectric layer  274  may include other high-K dielectrics, such as titanium oxide (TiO 2 ), hafnium zirconium oxide (HfZrO), tantalum oxide (Ta 2 O 5 ), hafnium silicon oxide (HfSiO 4 ), zirconium oxide (ZrO 2 ), zirconium silicon oxide (ZrSiO 2 ), lanthanum oxide (La 2 O 3 ), aluminum oxide (Al 2 O 3 ), zirconium oxide (ZrO), yttrium oxide (Y 2 O 3 ), SrTiO 3  (STO), BaTiO 3  (BTO), BaZrO, hafnium lanthanum oxide (HfLaO), lanthanum silicon oxide (LaSiO), aluminum silicon oxide (AlSiO), hafnium tantalum oxide (HMO), hafnium titanium oxide (HMO), (Ba,Sr)TiO 3  (BST), silicon nitride (SiN), silicon oxynitride (SiON), combinations thereof, or other suitable material. 
     After the formation of the high-k dielectric layer  274 , the gate electrode layer  276  is deposited over the high-k dielectric layer  274 . The gate electrode layer  276  may be a multi-layer structure that includes at least one work function layer and a metal fill layer. By way of example, the at least one work function layer may include titanium nitride (TiN), titanium aluminum (TiAl), titanium aluminum nitride (TiAlN), tantalum nitride (TaN), tantalum aluminum (TaAl), tantalum aluminum nitride (TaAlN), tantalum aluminum carbide (TaAlC), tantalum carbonitride (TaCN), or tantalum carbide (TaC). The metal fill layer may include aluminum (Al), tungsten (W), nickel (Ni), titanium (Ti), ruthenium (Ru), cobalt (Co), platinum (Pt), tantalum silicon nitride (TaSiN), copper (Cu), other refractory metals, or other suitable metal materials or a combination thereof In various embodiments, the gate electrode layer  276  may be formed by ALD, PVD, CVD, e-beam evaporation, or other suitable process. 
     The workpiece  200  may undergo further processing to form various features and regions known in the art. For example, subsequent processing may form various contacts, vias, metal lines, and multilayers interconnect features (e.g., metal layers and interlayer dielectrics) on the substrate  202 , configured to connect the various features to form a functional circuit that may include one or more multi-gate devices. In furtherance of the example, a multilayer interconnection may include vertical interconnects, such as vias or contacts, and horizontal interconnects, such as metal lines. The various interconnection features may employ various conductive materials including copper, tungsten, and/or silicide. In one example, a damascene and/or dual damascene process is used to form a copper related multilayer interconnection structure. Moreover, additional process steps may be implemented before, during, and after the method  1100 , and some process steps described above may be replaced or eliminated in accordance with various embodiments of the method  1100 . 
     Although not intended to be limiting, embodiments of the present disclosure provide one or more of the following advantages. For example, embodiments of the present disclosure form flexible sheet structures for active regions, providing channel regions with different widths. This advantageously meets various requirements of transistors for different applications. Further, embodiments of the present disclosure can be readily integrated into existing semiconductor manufacturing processes. 
     In one exemplary aspect, the present disclosure is directed to a method. The method includes patterning a top portion of a substrate, thereby forming a fin-like structure protruding from the substrate, the fin-like structure having a first portion with a first width and a second portion with a second width that is smaller than the first width, forming a dummy gate structure across the fin-like structure, the dummy gate structure being directly above the first portion and the second portion of the fin-like structure, thereby defining a channel region with a first width and a second width, recessing the fin-like structure with the dummy gate structure as an etch mask, thereby forming recesses on both sides of the dummy gate structure, growing epitaxial features in the recesses, the epitaxial features abutting the first and second portions of the fin-like structure on the both sides of the dummy gate structure, and replacing the dummy gate structure with a metal gate structure, the metal gate structure engaging both the first portion and the second portion of the fin-like structure. In some embodiments, the patterning of the top portion of the substrate includes forming a patterned mask layer over the substrate, the patterned mask layer including a first portion with the first width and a second portion with the second width, and etching the top portion of the substrate through openings in the patterned mask layer. In some embodiments, a ratio of the second width to the first width ranges from about 30% to about 90%. In some embodiments, the first width is about twice of the second width. In some embodiments, the epitaxial features include a first epitaxial feature over the first portion of the fin-like structure and a second epitaxial feature over the second portion of the fin-like structure, and the first epitaxial feature has a width larger than the second epitaxial feature. In some embodiments, the first epitaxial feature has a volume larger than the second epitaxial feature. In some embodiments, the fin-like structure has a third portion with a third width that is smaller than the first width, the third portion and the second portion both connect to the first portion, and the metal gate structure also engages the third portion. In some embodiments, the second and third portions of the fin-like structure elongate in a same direction and are parallel with each other. In some embodiments, the fin-like structure includes channel layers and sacrificial layers alternatingly disposed in a vertical direction, further includes removing the sacrificial layers, such that the metal gate structure wraps around each of the channel layers. In some embodiments, the dummy gate structure is a first dummy gate structure and the metal gate structure is a first metal gate structure, further includes forming a second dummy gate structure across the first portion of the fin-like structure, forming a third dummy gate structure across the second portion of the fin-like structure, replacing the second dummy gate structure with a second metal gate structure, the second metal gate structure engaging the first portion of the fin-like structure, and replacing the third dummy gate structure with a third metal gate structure, the third metal gate structure engaging the second portion of the fin-like structure. 
     In another exemplary aspect, the present disclosure is directed to a method. The method includes receiving a first layout of a standard cell, the first layout of the standard cell including a strip corresponding to an active region of an integrated circuit, creating a second layout, at least a portion of the second layout including repeating the first layout of the standard cell in a first direction thereby connecting multiple strips into a larger strip elongating in the first direction, performing a standard cell adjustment process, such that a width or a position of the strip in at least one of the standard cells is adjusted, resulting the larger strip having a first segment of a first width and a second segment of a second width that is different from the first width, and providing the second layout for fabricating a mask in association with the integrated circuit. In some embodiments, the standard cell adjustment process includes increasing or decreasing the width of the strip. In some embodiments, the standard cell adjustment process includes offsetting the strip such that the strip connects to another strip in an adjacent standard cell. In some embodiments, the at least one of the standard cells is selected based on a speed or power requirement of the integrated circuit. In some embodiments, the second layout includes a shape corresponding to a gate region of the integrated circuit, the shape overlapping with both the first segment and the second segment of the larger strip. In some embodiments, a ratio of the second width to the first width ranges from about 30% to about 90%. 
     In another exemplary aspect, the present disclosure is directed to a semiconductor device. The semiconductor device includes a plurality of channel members vertically stacked above a substrate, a gate structure wrapping around each of the channel members, a first epitaxial feature abutting the channel members from a first side of the gate structure, and a second epitaxial feature abutting the channel members from a second side of the gate structure. Between the first and second epitaxial features the channel members have a first width and a second width that is different from the first width in a plan view. In some embodiments, the first and second epitaxial features have different widths and volumes. In some embodiments, a transition from the first width to the second width includes a step-profile. In some embodiments, the semiconductor device further includes a third epitaxial feature abutting the channel members from the second side of the gate structure. The second epitaxial feature and the third epitaxial feature have a same width that is smaller than the first epitaxial feature. 
     The foregoing has outlined features of several embodiments so that those skilled in the art may better understand the detailed description that follows. 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.