Patent Publication Number: US-2022216340-A1

Title: Contact structure for stacked multi-gate device

Description:
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 integrated circuit (IC) technologies progress towards smaller technology nodes, 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, 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. 
     As the semiconductor industry further progresses into sub-10 nanometer (nm) technology process nodes in pursuit of higher device density, higher performance, and lower costs, challenges from both fabrication and design issues have led to stacked device structure configurations, such as complementary field effect transistors (C-FET) where an n-type multi-gate transistor and a p-type multi-gate transistor are stacked vertically, one over the other. Formation of a local interconnect feature in such a C-FET may involve forming an opening through an epitaxial source/drain feature or certain dielectric isolation features adjacent the epitaxial source/drain feature. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure is 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 and are used for illustration purposes only. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion. 
         FIG. 1  illustrates a flow chart of a method for forming a semiconductor device having a vertical C-FET structure, according to one or more aspects of the present disclosure. 
         FIGS. 2-20  illustrate fragmentary cross-sectional views of a workpiece undergoing various fabrication processes in the method of  FIG. 1 , 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 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. 
     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. 
     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 considering variations that inherently arise during manufacturing as understood by one of ordinary skill in the art. For example, the number or range of numbers encompasses a reasonable range including the number described, such as within +/−10% of the number described, based on known manufacturing tolerances associated with manufacturing a feature having a characteristic associated with the number. For example, a material layer having a thickness of “about 5 nm” can encompass a dimension range from 4.25 nm to 5.75 nm where manufacturing tolerances associated with depositing the material layer are known to be +/−15% by one of ordinary skill in the art. Still further, 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. 
     This application generally relates to stacked multi-gate devices and fabrication processes thereof, and more particularly to local contact structures of stacked multi-gate devices. 
     A stacked multi-gate device refers to a semiconductor device that includes a first multi-gate device and a second multi-gate device stacked over the first multi-gate device. When the first multi-gate device and the second multi-gate device are of different conductivity types, the stacked multi-gate device may be a complementary field effect transistor (C-FET). The multi-gate devices in a C-FET may be FinFETs or MBC transistors. For some logic operations, a local electrical connection between a source/drain feature of the first multi-gate device and a source/drain feature of the second multi-gate device is needed. In some existing technology, such a local electrical connection may be formed through dielectric features around the source/drain features. In some other existing technology, a through opening is etched through the source/drain feature of the second multi-gate device, which may result in damages to the source/drain feature in the process. 
     The present disclosure provides a local contact feature that directly and vertically couples a source/drain feature of a top multi-gate device to a source/drain feature of a bottom multi-gate device. The local contact feature is disposed in a local contact opening that is formed in a self-aligned manner. In an example process, a fin-shaped structure is formed from a stack that includes a plurality of channel layers. A bottom source/drain feature is deposited to be in contact with the channel layers in a bottom portion of the stack. A separation layer is deposited over the bottom source/drain feature to disable channel layers in a middle portion of the stack. Then, a top source/drain feature is deposited over the separation layer to be in contact with the channel layers in a top portion of the stack. The top source/drain feature includes a gap that exposes the separation layer. A filler layer is deposited in the gap. A local contact opening is formed through the filler layer and the separation layer to expose the bottom source/drain feature. A local contact feature is formed in the contact opening to couple the bottom source/drain feature and the top source/drain feature. For illustration purposes, the details of the structure and fabrication methods of the present disclosure are described below with respect to C-FETs having stacked MBC transistors. However, the present disclosure is not so limited and similar structures and processes may be applicable to C-FET having other types of stacked multi-gate transistors. 
     The various aspects of the present disclosure will now be described in more detail with reference to the figures. In that regard,  FIG. 1  is a flowchart illustrating method  100  for forming a semiconductor device having a vertical C-FET structure according to various aspects of the present disclosure. Method  100  is merely an example and is not intended to limit the present disclosure to what is explicitly illustrated in method  100 . Additional steps may be provided before, during and after method  100 , 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. Method  100  is described below in conjunction with  FIGS. 2-20 , which are fragmentary cross-sectional views of a workpiece  200  at different stages of fabrication according to embodiments of method  100 . Because the workpiece  200  will be fabricated into a semiconductor device  200  upon conclusion of the fabrication processes, the workpiece  200  may be referred to as the semiconductor device  200  as the context requires. Additionally, throughout the present application, like reference numerals denote like features, unless otherwise excepted. 
     Referring to  FIGS. 1 and 2 , method  100  includes a block  102  where a workpiece  200  is provided. The workpiece  200  may include a substrate  202  and a stack  204  disposed over the substrate  202 . In one embodiment, the substrate  202  may be a silicon (Si) substrate. In some other embodiments, the substrate  202  may include other semiconductors such as germanium (Ge), silicon germanium (SiGe), or a III-V semiconductor material. Example III-V semiconductor materials may include gallium arsenide (GaAs), indium phosphide (InP), gallium phosphide (GaP), gallium nitride (GaN), gallium arsenide phosphide (GaAsP), aluminum indium arsenide (AlInAs), aluminum gallium arsenide (AlGaAs), gallium indium phosphide (GaInP), and indium gallium arsenide (InGaAs). The substrate  202  may also include an insulating layer, such as a silicon oxide layer, to have a silicon-on-insulator ( 50 I) structure. Although not explicitly shown in the figures, the substrate  202  may include an n-type well region and a p-type well region for fabrication of transistors of different conductivity types. When present, each of the n-type well and the p-type well is formed in the substrate  202  and includes a doping profile. An n-type well may include a doping profile of an n-type dopant, such as phosphorus (P) or arsenic (As). A p-type well may include a doping profile of a p-type dopant, such as boron (B). The doping in the n-type well and the p-type well may be formed using ion implantation or thermal diffusion and may be considered portions of the substrate  202 . 
     As shown in  FIG. 2 , the stack  204  includes a plurality of channel layers  208  interleaved by a plurality of sacrificial layers  206 . The channel layers  208  and the sacrificial layers  206  may have different semiconductor compositions. In some implementations, the channel layers  208  are formed of silicon (Si) and sacrificial layers  206  are formed of silicon germanium (SiGe). In these implementations, the additional germanium content in the sacrificial layers  206  allow selective removal or recess of the sacrificial layers  206  without substantial damages to the channel layers  208 . In some embodiments, the sacrificial layers  206  and channel layers  208  are epitaxy layers and may be deposited using an epitaxy process. Suitable epitaxy processes include vapor-phase epitaxy (VPE), ultra-high vacuum chemical vapor deposition (UHV-CVD), molecular beam epitaxy (MBE), and/or other suitable processes. The sacrificial layers  206  and the channel layers  208  are deposited alternatingly, one-after-another, to form the stack  204 . 
     As will be explained in greater detail below, the channel layers  208  in the bottom portion of the stack  204  will provide channel members of a bottom MBC transistor, and the channel layers  208  in the top portion of the stack  204  will provide channel members of a top MBC transistor. The term “channel member(s)” is used herein to designate any material portion for channel(s) in a transistor with nanoscale dimensions, and having an elongate shape, regardless of the cross-sectional shape of this portion. Channel members may come in forms of nanowires, nanosheet, or other nanostructures and may have cross-sections that are circular, oval, race-track shaped, rectangular, or square. For ease of references, the stack  204  may be vertically divided into a bottom portion  204 B, a middle portion  204 M over the bottom portion  204 B, and a top portion  204 T over the middle portion  204 M. As shown in  FIG. 2 , each of the bottom portion  204 B, the middle portion  204 M, and the top portion  204 T include one or more channel layers  208  and one or more sacrificial layers  206 . 
     It is noted that the stack  204  in  FIG. 2  includes eight (8) layers of the channel layers  208  interleaved by seven (7) layers of sacrificial layers  206 , which is 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 the channel layers  208  can be included in the stack  204  and distributed among the bottom portion  204 B, the middle portion  204 M, and the top portion  204 T. The number of layers depends on the desired number of channels members for the top MBC transistor and the bottom MBC transistor. In some embodiments, the number of the channel layers  208  in the stack  204  may be between 3 and 12. The thicknesses of the channel layers  208  and the sacrificial layers  206  may be selected based on device performance considerations of the bottom MBC transistor, the top MBC transistor, and the C-FET as a whole. Additionally, channel layers  208  and the sacrificial layers  206  may be intentionally made thinner to form one or more disabled channel members. For example, a channel layer  208  that is thinner than the rest of the channel layers  208  may be severed or compromised during the channel release process. For another example, a sacrificial layer  206  that is thinner than the rest of the sacrificial layers  206  may result in a reduced channel-channel spacing that may be substantially filled up by gate dielectric material. The gate dielectric material in the reduced channel-channel spacing will prevent gate electrode material from being inserted into the reduced channel-channel spacing. 
     Referring to  FIGS. 1 and 3 , method  100  includes a block  104  where a fin-shaped structure  210  is formed from the stack  204 . In some embodiments, the stack  204  and a portion of the substrate  202  are patterned to form the fin-shaped structure  210 . For patterning purposes, a hard mask layer may be deposited over the stack  204 . The hard mask layer may be a single layer or a multilayer. In one example, the hard mask layer includes a silicon oxide layer and a silicon nitride layer over the silicon oxide layer. As shown in  FIG. 3 , the fin-shaped structure  210  extends vertically along the Z direction from the substrate  202  and extends lengthwise along the X direction. The fin-shaped structure  210  may be patterned using suitable processes including double-patterning or multi-patterning processes. Generally, double-patterning or multi-patterning processes combine photolithography and self-aligned processes, allowing patterns to be created that have, for example, pitches smaller than what is otherwise obtainable using a single, direct photolithography process. For example, in one embodiment, a material layer is formed over a substrate and patterned using a photolithography process. Spacers are formed alongside the patterned material layer using a self-aligned process. The material layer is then removed, and the remaining spacers, or mandrels, may then be used as an etch mask to etch the stack  204  and the substrate  202  to form the fin-shaped structure  210 . The etching process may include dry etching, wet etching, reactive ion etching (RIE), and/or other suitable processes. 
     While not explicitly shown in  FIG. 3 , after the fin-shaped structure  210  is formed, an isolation feature is formed to separate the fin-shaped structure  210  from an adjacent fin-shaped structure (not explicitly shown). The isolation feature may also be referred to as a shallow trench isolation (STI) feature. In an example process, a dielectric material for the isolation feature is deposited over the workpiece  200 , including the fin-shaped structure  210 , using CVD, subatmospheric CVD (SACVD), flowable CVD, spin-on coating, and/or other suitable process. Then the deposited dielectric material is planarized and recessed until the fin-shaped structure  210  rises above the isolation feature. The dielectric material for the isolation feature may include silicon oxide, silicon oxynitride, fluorine-doped silicate glass (FSG), a low-k dielectric, combinations thereof, and/or other suitable materials. 
     Referring still to  FIGS. 1 and 3 , method  100  includes a block  106  where a dummy gate stack  214  is formed over the stack  204 . In some embodiments, a gate replacement process (or gate-last process) is adopted where the dummy gate stack  214  serves as a placeholder for a functional gate structure. Other processes and configuration are possible. To form the dummy gate stack  214 , a dummy dielectric layer  216 , a dummy gate electrode layer  218 , and a gate-top hard mask layer (not explicitly shown) are deposited over the workpiece  200 . The deposition of these layers may include use of low-pressure CVD (LPCVD), CVD, plasma-enhanced CVD (PECVD), PVD, ALD, thermal oxidation, e-beam evaporation, or other suitable deposition techniques, or combinations thereof. The dummy dielectric layer  216  may include silicon oxide, the dummy gate electrode layer  218  may include polysilicon, and the gate-top hard mask layer may be a multi-layer that includes silicon oxide and silicon nitride. Using photolithography and etching processes, the gate-top hard mask layer is patterned. The photolithography process may include photoresist coating (e.g., spin-on coating), soft baking, mask aligning, exposure, post-exposure baking, photoresist developing, rinsing, drying (e.g., spin-drying and/or hard baking), other suitable lithography techniques, and/or combinations thereof. The etching process may include dry etching (e.g., RIE etching), wet etching, and/or other etching methods. Thereafter, using the patterned gate-top hard mask as the etch mask, the dummy dielectric layer  216  and the dummy gate electrode layer  218  are then etched to form the dummy gate stack  214 . The dummy gate stack  214  extends lengthwise along the Y direction to wrap over the fin-shaped structure  210  and lands on the isolation feature. The portion of the fin-shaped structure  210  underlying the dummy gate stack  214  defines a channel region  210 C. The channel region  210  and the dummy gate stack  214  also define source/drain regions  210 SD that are not vertically overlapped by the dummy gate stack  214 . The channel region  210 C is disposed between two source/drain regions  210 SD along the X direction. 
     Referring to  FIGS. 1 and 4 , method  100  includes a block  108  where source/drain regions  210 SD of the fin-shaped structure  210  are recessed to a first source/drain recess  224 - 1  and a second source/drain recess  224 - 2 . Operations at block  108  may include formation of a gate spacer layer  220  over the sidewalls of the dummy gate stack  214  before the source/drain regions  210 SD are recessed. In some embodiments, the formation of the gate spacer layer  220  includes deposition of one or more dielectric layers over the workpiece  200 . In an example process, the one or more dielectric layers are deposited using CVD, SACVD, or ALD. The one or more dielectric layers may include silicon oxide, silicon nitride, silicon carbide, silicon oxynitride, silicon carbonitride, silicon oxycarbide, silicon oxycarbonitride, and/or combinations thereof. After the deposition of the gate spacer layer  220 , the workpiece  200  is etched in an anisotropic etch process to form the first source/drain recess  224 - 1  and the second source/drain recess  224 - 2 . The etch process at block  108  may be a dry etch process or a suitable etch process. An example dry etch process may implement an oxygen-containing gas, hydrogen, a fluorine-containing gas (e.g., CF 4 , SF 6 , NF 3 , 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. As shown in  FIG. 4 , sidewalls of the sacrificial layers  206  and the channel layers  208  in the channel regions  210 C are exposed in the first source/drain recess  224 - 1  and the second source/drain recess  224 - 2 . 
     Referring to  FIGS. 1 and 5 , method  100  includes a block  110  where inner spacer features  226  are formed. At block  110 , the sacrificial layers  206  in the bottom portion  204 B, the middle portion  204 M, and the top portion  204 T exposed in the first source/drain recess  224 - 1  and the second source/drain recess  224 - 2  are selectively and partially recessed to form inner spacer recesses, while the exposed channel layers  208  are substantially unetched. In an embodiment where the channel layers  208  consist essentially of silicon (Si) and sacrificial layers  206  consist essentially of silicon germanium (SiGe), the selective and partial recess of the sacrificial layers  206  may include a SiGe oxidation process followed by a SiGe oxide removal. In that embodiments, the SiGe oxidation process may include use of ozone (O 3 ). In some other embodiments, the selective recess may be a selective isotropic etching process (e.g., a selective dry etching process or a selective wet etching process), and the extent at which the sacrificial layers  206  are recessed is controlled by duration of the etching process. The selective dry etching process may include use of one or more fluorine-based etchants, such as fluorine gas or hydrofluorocarbons. The selective wet etching process may include use of hydrogen fluoride (HF) or ammonium hydroxide (NH 4 OH). After the formation of the inner spacer recesses, an inner spacer material layer is deposited over the workpiece  200 , including in the inner spacer recesses. The inner spacer material layer may include silicon oxide, silicon nitride, silicon oxycarbide, silicon oxycarbonitride, silicon carbonitride, metal nitride, or a suitable dielectric material. The deposited inner spacer material layer is then etched back to remove excess inner spacer material layer over the gate spacer layer and sidewalls of the channel layers  208 , thereby forming the inner spacer features  226  as shown in  FIG. 5 . In some embodiments, the etch back process at block  110  may be a dry etch process that includes use of an oxygen-containing gas, hydrogen, nitrogen, a fluorine-containing gas (e.g., NF 3 , 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 (e.g., CF 3 I), other suitable gases and/or plasmas, and/or combinations thereof. 
     Referring to  FIGS. 1, 6, and 7 , method  100  includes a block  112  where a first bottom source/drain feature  228 - 1  and a second bottom source/drain feature  228 - 2  are formed in the first source/drain recess  224 - 1  and the second source/drain recess  224 - 2 , respectively. For ease of reference, the first bottom source/drain feature  228 - 1  and the second bottom source/drain feature  228 - 2  may be collectively referred to as bottom source/drain features  228 . Reference is first made to  FIG. 6 . In some embodiments, the bottom source/drain features  228  may be formed using an epitaxial process, such as VPE, UHV-CVD, MBE, and/or other suitable processes. The epitaxial growth process may use gaseous and/or liquid precursors, which interact with the composition of the substrate  202  as well as the channel layers  208 . The epitaxial growth of bottom source/drain features  228  may take place from both the top surface of the substrate  202  and the exposed sidewalls of the channel layers  208 . As illustrated in  FIG. 6 , the deposited bottom source/drain features  228  are in physical contact with (or adjoining) the channel layers  208 . Although the epitaxial growth of bottom source/drain features  228  is less likely to take place on surfaces of the inner spacer features  226 , overgrowth of the bottom source/drain features  228  allow the bottom source/drain features  228  to merge over the inner spacer features  226 . Depending on the design, the bottom source/drain features  228  may be n-type or p-type. In the depicted embodiments, the bottom source/drain features  228  are p-type source/drain features and may include silicon germanium (SiGe) doped with a p-type dopant, such as boron (B). In these depicted embodiments, the bottom source/drain features  228  may include boron doped silicon germanium (SiGe:B). Having the bottom source/drain features  228  as p-type source/drain features has some advantages. For example, its silicon germanium (SiGe) composition may facilitate selective removal of the substrate  202  during formation of backside contacts and backside dielectric layers. For another example, the exposed substrate  202  provides more epitaxial seed surfaces to form a less defective silicon germanium source/drain crystal structure to provide needed strain on the channel members. It is observed that strained silicon channels provide better hole mobility than unstrained silicon channels. 
     Referring to  FIG. 7 , the deposited bottom source/drain features  228  are then etched back/pulled back until the bottom source/drain features  228  only cover channel layers  208  in the bottom portion  204 B but not the channel layers  208  in the middle portion  204 M or the top portion  204 T. The etch back or pull back process may include dry etching, wet etching, and/or other suitable processes. Because the bottom source/drain features  228  and the sacrificial layers  206  are all formed of silicon germanium (SiGe), the etch process used to form the inner spacer recesses may be used to etch back the deposited bottom source/drain features  228 . An example selective dry etching process at block  112  may include use of one or more fluorine-based etchants, such as fluorine gas or hydrofluorocarbons. An example selective wet etching process at block  112  may include use of hydrogen fluoride (HF) or ammonium hydroxide (NH 4 OH). 
     Referring to  FIGS. 1 and 8 , method  100  includes a block  114  where a separation layer  242  is formed over the bottom source/drain features  228 . The separation layer  242  may also be referred to as an isolation layer  242  as it functions to separate or isolate the bottom source/drain features  228  from top source/drain features (to be described below). In some embodiments, the separation layer  242  may include silicon oxide or a silicon oxide containing material, such as 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 one embodiment, the separation layer  242  includes silicon oxide. In some implementations, the separation layer  242  may be formed by depositing a dielectric material over the workpiece  200 , including over the first bottom source/drain feature  228 - 1  and the second bottom source/drain feature  228 - 2 , by flowable CVD (FCVD), CVD, high density plasma CVD (HDPCVD), or a suitable deposition technique. In one embodiment, the dielectric material for the separation layer  242  may be deposited using FCVD. The deposited dielectric material is then etched back until the separation layer  242  only covers channel layers  208  in the middle portion  204 M, but the channel layers  208  in the top portion  204 T. 
     Referring to  FIGS. 1 and 9 , method  100  includes a block  116  where a first top source/drain feature  248 - 1  is formed in the first source/drain recess  224 - 1  while the second source/drain recess  224 - 2  is covered by a first masking layer  250 . At block  116 , the first masking layer  250  is deposited over the workpiece  200  and is then patterned to cover the second source/drain recess  224 - 2  while the first source/drain recess  224 - 1  is exposed. In one embodiment, the first masking layer  250  is a bottom antireflective coating (BARC) layer that may include polysulfones, polyureas, polyurea sulfones, polyacrylates, poly(vinyl pyridine), or a silicon-containing polymer. In another embodiment, the first masking layer  250  is a hard mask layer that may be a single layer or a multi-layer. For example, when the first masking layer  250  is a hard mask layer, the first masking layer  250  may include silicon oxide, silicon nitride, silicon carbide, silicon oxycarbide, silicon oxynitride, or silicon oxycarbonitride. Photolithography and etching processes may be used to pattern the first masking layer  250  shown in  FIG. 9 . The photolithography process may include photoresist coating (e.g., spin-on coating), soft baking, mask aligning, exposure, post-exposure baking, photoresist developing, rinsing, drying (e.g., spin-drying and/or hard baking), other suitable lithography techniques, and/or combinations thereof. The resulting patterned photoresist layer is then applied as an etch mask to pattern the first masking layer  250 . 
     With the first masking layer  250  covering the second source/drain recess  224 - 2 , the first top source/drain feature  248 - 1  is deposited over the separation layer  242  in the first source/drain recess  224 - 1 . Similar to the first bottom source/drain feature  228 - 1  and the second bottom source/drain feature  228 - 2 , the first top source/drain feature  248 - 1  may be formed using an epitaxial process, such as VPE, UHV-CVD, MBE, and/or other suitable processes. The epitaxial growth process may use gaseous and/or liquid precursors, which interact with the channel layers  208  in the top portion  204 T. Therefore, the epitaxial growth of the first top source/drain feature  248 - 1  may take place from the exposed sidewalls of the channel layers  208  in the top portion  204 T, but not from surfaces of the inner spacer features  226  in the top portion  204 T and the separation layer  242 . As shown in  FIG. 9 , overgrowth of the first top source/drain feature  248 - 1  may merge over the inner spacer features  226  and the separation layer  242 . In embodiments represented in  FIG. 9 , the deposition of the first top source/drain feature  248 - 1  is performed for a duration to substantially fill the space between two adjacent top portions  204 T of two adjacent channel regions  210 C. Depending on the design, the first top source/drain feature  248 - 1  may be n-type or p-type. In the depicted embodiments, the first top source/drain feature  248 - 1  is an n-type source/drain feature and may include silicon (Si) doped with an n-type dopant, such as phosphorus (P) or arsenic (As). In these depicted embodiments, the first top source/drain feature  248 - 1  may include phosphorus doped silicon (Si:P). After the formation of the first top source/drain feature  248 - 1 , the first masking layer  250  is remover by, for example, ashing, stripping, or selective etching. 
     Referring to  FIGS. 1 and 10 , method  100  includes a block  118  where a second top source/drain feature  248 - 2  is formed in the second source/drain recess  224 - 2  while the first source/drain recess  224 - 1  is covered by a second masking layer  252 . At block  118 , the second masking layer  252  is deposited over the workpiece  200  and is then patterned to cover the first top source/drain feature  248 - 1  while the second source/drain recess  224 - 2  is exposed. In one embodiment, the second masking layer  252  is a bottom antireflective coating (BARC) layer that may include polysulfones, polyureas, polyurea sulfones, polyacrylates, poly(vinyl pyridine), or a silicon-containing polymer. In another embodiment, the second masking layer  252  is a hard mask layer that may be a single layer or a multi-layer. For example, when the second masking layer  252  is a hard mask layer, the second masking layer  252  may include silicon oxide, silicon nitride, silicon carbide, silicon oxycarbide, silicon oxynitride, or silicon oxycarbonitride. Photolithography and etching processes may be used to pattern the second masking layer  252  shown in  FIG. 10 . The photolithography process may include photoresist coating (e.g., spin-on coating), soft baking, mask aligning, exposure, post-exposure baking, photoresist developing, rinsing, drying (e.g., spin-drying and/or hard baking), other suitable lithography techniques, and/or combinations thereof. The resulting patterned photoresist layer is then applied as an etch mask to pattern the second masking layer  252 . 
     With the second masking layer  252  covering the first top source/drain feature  248 - 1 , the second top source/drain feature  248 - 2  is deposited in the second source/drain recess  224 - 2 . The second top source/drain feature  248 - 2  may be formed using an epitaxial process, such as VPE, UHV-CVD, MBE, and/or other suitable processes. The epitaxial growth process may use gaseous and/or liquid precursors, which interact with the channel layers  208  in the top portion  204 T exposed in the second source/drain recess  242 - 2 . Therefore, the epitaxial growth of the second top source/drain feature  248 - 2  may take place from the exposed sidewalls of the channel layers  208  in the top portion  204 T, but not from surfaces of the inner spacer features  226  in the top portion  204 T and the separation layer  242 . As shown in  FIG. 10 , overgrowth of the second top source/drain feature  248 - 2  may merge over the inner spacer features  226 . Unlike the first top source/drain feature  248 - 1 , the deposition of the second top source/drain feature  248 - 2  lasts for a shorter duration such that the second top source/drain feature  248 - 2  does merge over the separation layer  242 . In some instances, the second top source/drain feature  248 - 2  may have a thickness, along the X direction, between 2 nm and about 10 nm. That is, the second top source/drain feature  248 - 2  is allowed to merge over the inner spacer features  226  along the Z direction but is not allowed to grow the thickness to merge over the separation layer  242  along the X direction. As shown in  FIG. 10 , after the second top source/drain feature  248 - 2  is fully formed at block  118 , a gap  249  remains in the second top source/drain feature  248 - 2  and the separation layer  242  is exposed in the gap  249 . Depending on the design, the second top source/drain feature  248 - 2  may be n-type or p-type. In the depicted embodiments, the second top source/drain feature  248 - 2 , like the first top source/drain feature  248 - 1 , is an n-type source/drain feature and may include silicon (Si) doped with an n-type dopant, such as phosphorus (P) or arsenic (As). In these depicted embodiments, the second top source/drain feature  248 - 2  may include phosphorus doped silicon (Si:P). After the formation of the second top source/drain feature  248 - 2 , the second masking layer  252  is remover by, for example, ashing, stripping, or selective etching. 
     Referring to  FIGS. 1, 11 and 12 , method  100  includes a block  120  where a filler layer  254  is formed in the gap  249  in the second top source/drain feature  248 - 2 . Operations at block  120  may include depositing a filler material  253  over the workpiece  200  (as shown in  FIG. 11 ) and etching back the filler material  253  to form the filler layer  254  in the gap  249  (as shown in  FIG. 12 ). In some embodiments, the filler material  253  may be deposited using ALD, FCVD or HDPCVD. In one embodiment, the filler material  253  may be deposited using ALD. ALD-deposited filler material  253  may fill the gap  249  efficiently and requires little etching back to form the filler layer  254 . As shown in  FIG. 11 , the deposited filler material  253  not only fills in the gap  249  but also is disposed over the first top source/drain feature  248 - 1  and sidewalls of the gate spacer layer  220 . The filler material  253  may include a dielectric material. In some implementations, a composition of the filler material  253  may be similar to a composition of the separation layer  242 . In some instances, the filler material  253  may include silicon oxide or a silicon oxide containing material, such as 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 one embodiment, the filler material  253  may include silicon oxide. In some embodiments, the filler material  253  is etched back until the first top source/drain feature  248 - 1  and the second top source/drain feature  248 - 2  are exposed in the first source/drain recess  224 - 1  and the second source/drain recess  224 - 2 . At the conclusion of the operations of block  120 , the filler layer  254  is formed, as shown in  FIG. 12 . The filler layer  254  serves as a plug to plug the gap  249  shown in  FIG. 10 . 
     Referring to  FIGS. 1 and 13 , method  100  includes a block  122  where a contact etch stop layer (CESL)  256  and an interlayer dielectric (ILD) layer  258  are deposited on the first top source/drain feature  248 - 1  and the second top source/drain feature  248 - 2 . The CESL  256  may include silicon nitride, silicon oxynitride, and/or other materials known in the art and may be formed by CVD, ALD, plasma-enhanced chemical vapor deposition (PECVD) process and/or other suitable deposition or oxidation processes. In some embodiments, the CESL  256  is first conformally deposited on the workpiece  200  and the ILD layer  258  is deposited over the CESL  256  by spin-on coating, FCVD, CVD, or other suitable deposition technique. The ILD layer  258  may include materials such as 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 some embodiments, after formation of the ILD layer  258 , the workpiece  200  may be annealed to improve integrity of the ILD layer  258 . To remove excess materials and to expose top surfaces of the dummy gate stacks  214 , a planarization process, such a chemical mechanical polishing (CMP) process may be performed. 
     Referring to  FIGS. 1 and 14 , method  100  includes a block  124  where the dummy gate stack  214  is replaced with a gate structure  260 . Operations at block  124  may include removal of the dummy gate stacks  214 , release of the channel layers  208  as channel members  2080 , formation of gate structures  260  to wrap around the channel members  2080 , and formation of a self-aligned capping (SAC) layer  267  over the gate structures  260 . The removal of the dummy gate stacks  214  may include one or more etching processes that are selective to the material in the dummy gate stacks  214 . For example, the removal of the dummy gate stacks  214  may be performed using as a selective wet etch, a selective dry etch, or a combination thereof. After the removal of the dummy gate stacks  214 , sidewalls of the channel layers  208  and sacrificial layers  206  in the channel regions  210 C are exposed. Thereafter, the sacrificial layers  206  in the channel regions  210 C are selectively removed to release the channel layers  208  as the channel members  2080 . Here, because the dimensions of the channel members  2080  are nanoscale, the channel members  2080  may also be referred to as nanostructures. The selective removal of the sacrificial layers  206  may be implemented by selective dry etch, selective wet etch, or other selective etch processes. In some embodiments, the selective wet etching includes an APM etch (e.g., ammonia hydroxide-hydrogen peroxide-water mixture). In some other embodiments, the selective removal includes SiGe 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. 
     With the channel members  2080  released, the gate structure  260  is deposited to wrap around each of the channel members  2080 , thereby forming a bottom MBC transistor  300  and a top MBC transistor  400  disposed over the bottom MBC transistor  300 . In some instances, the gate structure  260  may be a common gate structure to engage channel members  2080  of the bottom MBC transistor  300  and the top MBC transistor  400 , the gate structures  260 . In some other instances, the gate structure  260  may include a bottom gate portion to engage channel members  2080  of the bottom MBC transistor  300  and a top gate portion to engage channel members  2080  of the top MBC transistor  400 . In those other instances, the bottom gate portion is electrically isolated from the top gate portion. Each of the gate structures  260  includes an interfacial layer  262  to interface the channel members  2080 , a gate dielectric layer  264 , a p-type electrode layer  266 P, and an n-type electrode layer  266 N. In some embodiments, the interfacial layer  262  includes silicon oxide and may be formed in 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 gate dielectric layer  264  is then deposited over the interfacial layer  262  using ALD, CVD, and/or other suitable methods. The gate dielectric layer  264  is formed of high-K dielectric materials. As used and described herein, high-k dielectric materials include dielectric materials having a high dielectric constant, for example, greater than that of thermal silicon oxide (˜3.9). The gate dielectric layer  264  may include hafnium oxide. Alternatively, the gate dielectric layer  264  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 (HfSiO4), zirconium oxide (ZrO 2 ), zirconium silicon oxide (ZrSiO2), 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 (HfTaO), hafnium titanium oxide (HfTiO), (Ba,Sr)TiO 3  (BST), silicon nitride (SiN), silicon oxynitride (SiON), combinations thereof, or other suitable material. In some embodiments illustrated in  FIG. 14 , small channel-channel spacing resulted from thinner sacrificial layers may result in completely space filling by the gate dielectric layer  264  to selectively disable certain channel members as a measure to isolate the bottom MBC transistor  300  from the top MBC transistor  400 . 
     After the deposition of the gate dielectric layer  264 , the p-type electrode layer  266 P and the n-type electrode layer  266 N are sequentially deposited over the channel regions  210 C. The p-type electrode layer  266 P and the n-type electrode layer  266 N may include a single layer or alternatively a multi-layer structure, such as various combinations of a metal layer with a selected work function to enhance the device performance (work function metal layer), a liner layer, a wetting layer, an adhesion layer, a metal alloy or a metal silicide. By way of example, the p-type electrode layer  266 P may include titanium nitride (TiN), tantalum nitride (TaN), ruthenium (Ru), molybdenum (Mo), aluminum (Al), tungsten nitride (WN), zirconium silicide (ZrSi 2 ), molybdenum silicide (MoSi 2 ), tantalum silicide (TaSi 2 ), nickel silicide (NiSi 2 ), other p-type work function material, or combinations thereof. The n-type electrode layer  266 N may include titanium (Ti), aluminum (Al), silver (Ag), manganese (Mn), zirconium (Zr), titanium aluminum (TiAl), titanium aluminum carbide (TiAlC), tantalum carbide (TaC), tantalum carbonitride (TaCN), tantalum silicide nitride (TaSiN), tantalum aluminum (TaAl), tantalum aluminum carbide (TaAlC), titanium aluminum nitride (TiAlN), other n-type work function material, or combinations thereof. To form the n-type electrode layer  266 N over the p-type electrode layer  266 P, the p-type electrode layer  266 P is deposited first and then selectively etched back to expose the channel members  2080  in the top portion  204 T. Then the n-type electrode layer  266 N is deposited to wrap around each of the channel members in the top portion  204 T. 
     Operations at block  124  may also include forming the self-aligned capping (SAC) layer  267  over the gate structures  260 . In some embodiments, the SAC layer  267  may include silicon nitride (SiN), silicon carbide (SiC), silicon carbonitride (SiCN), silicon oxycarbonitride (SiOCN), silicon oxycarbide (SiOC), zirconium nitride (ZrN), aluminum oxynitride (AlON), tantalum carbonitride (TaCN), zirconium silicide (ZrSi), or a metal oxide. Example metal oxides may include lanthanum oxide (La 2 O 3 ), aluminum oxide (Al 2 O 3 ), zinc oxide (ZnO), zinc aluminum oxide (Zr 2 Al 3 O 9 ), titanium oxide (TiO 2 ), tantalum oxide (TaO 2 ), zirconium oxide (ZrO 2 ), hafnium oxide (HfO 2 ), yittrium oxide (Y 2 O 3 ). The SAC layer  267  protects the gate structures  260  from etching processes that are used for etching frontside source/drain contact openings (such as the first frontside contact opening  268  and the second frontside contact opening  270  to be described below). The SAC layer  267  may be formed by recessing the gate structure, depositing one or more dielectric materials over the recessed gate structure, and performing a CMP process to the one or more dielectric materials. 
     Referring to  FIGS. 1, 15 and 16 , method  100  includes a block  126  where a first frontside contact opening  268  and a second frontside contact opening  272  are formed. As illustrated in  FIGS. 15 and 16 , the formation of the first frontside contact opening  268  and the second frontside contact opening  270  may include multiple steps. For example, the ILD layer  258  may be first removed by selective etching. Because the ILD layer  258  includes silicon oxide while the gate spacer layer  220 , the CESL  256 , and the SAC layer  267  are formed of other dielectric materials, the ILD layer  258  may be selectively etched using a wet etch process or a dry etch process that is selective to silicon oxide. An example wet etch process may include use of diluted hydrofluoric acid (DHF), buffered hydrofluoric acid (BHF, including ammonium fluoride and hydrofluoric acid). An example dry etch process may include use of carbon tetrafluoride (CF 4 ), sulfur hexafluoride (SF 6 ), or nitrogen trifluoride (NF 3 ). Then an anisotropic etch process may be used to breach the CESL  256  to expose the first top source/drain feature  248 - 1 , the second top source/drain feature  248 - 2 , and the filler layer  254 . The anisotropic etch process to breach the CESL  256  may be a dry etch process that includes use of carbon tetrafluoride (CF 4 ), sulfur hexafluoride (SF 6 ), nitrogen trifluoride (NF 3 ), or fluoroform (CHF 3 ). At this point, the first frontside contact opening  268  is formed over the first top source/drain feature  248 - 1 , as shown in  FIG. 15 . Thereafter, the filler layer  254  is selectively etched to expose the separation layer  242  below the second top source/drain feature  248 - 2 . The etch process for the filler layer  254  may be an anisotropic etch process that includes use of oxygen (O 2 ), a fluorine-containing gas (e.g., CF 4 , SF 6 , NF 3 , 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. It is noted that because the etch process for the filler layer  254  is selective to the filler layer  254 , the first top source/drain feature  248 - 1  and the second top source/drain feature  248 - 2  may remain substantially unetched or undamaged. Because the composition of the filler layer  254  may be similar to that of the separation layer  242 , the dry etch process for the filler layer  254  may extend the opening through the separation layer  242  and into the second bottom source/drain feature  228 - 2 , thereby forming the second frontside contact opening  270 . As shown in  FIG. 16 , the first frontside contact opening  268  exposes the first top source/drain feature  248 - 1 . The second frontside contact opening  270  not only exposes sidewalls of the second top source/drain feature  248 - 2  but also the top surface of the second bottom source/drain feature  228 - 2 . 
     Referring to  FIGS. 1 and 17 , method  100  includes a block  128  where a first frontside contact  276  is formed in the first frontside contact opening  268  and a second frontside contact  278  is formed in the second frontside contact opening  272 . In some embodiments, operations at block  128  may include formation of silicide layers and deposition of a metal fill layer. In an example process, a metal precursor is deposited over the workpiece  200 , including over the first frontside contact opening  268  and the second frontside contact opening  270 . The metal precursor may include titanium (Ti), tantalum (Ta), nickel (Ni), cobalt (Co), or tungsten (W). Then an anneal process is performed to bring about silicidation between the metal precursor and semiconductor material (such as the first top source/drain feature  248 - 1 , the second bottom source/drain feature  228 - 2 , and the second top source/drain feature  248 - 2 ), thereby forming the first silicide layer  273 , the second silicide layer  274 , and the third silicide layer  275 . The silicide layers may include titanium silicide, tantalum silicide, nickel silicide, cobalt silicide, or tungsten silicide. The metal precursor that does not turn into silicide may be removed. After the formation of the silicide layers, the metal fill layer may include tungsten (W), ruthenium (Ru), cobalt (Co), nickel (Ni), or copper (Cu). After the deposition of the metal fill layer, a CMP process may be performed to remove excess materials and define the first frontside contact  276  and the second frontside contact  278 . As shown in  FIG. 17 , the first frontside contact  276  is electrically coupled to the first top source/drain feature  248 - 1  via the first silicide layer  273 ; the second frontside contact  278  is electrically coupled to the second top source/drain feature  248 - 2  via the second silicide layer  274 ; and the second frontside contact  278  is electrically coupled to the second bottom source/drain feature  228 - 2  via the third silicide layer  275 . It can be seen that the second frontside contact  278  electrically couple the second bottom source/drain feature  228 - 2  of the bottom MBC transistor  300  and the second top source/drain feature  248 - 2  of the top MBC transistor  400 . In the depicted embodiment, the bottom MBC transistor  300  is p-type and the top MBC transistor  400  is n-type. 
     Referring to  FIGS. 1, 18, 19, and 20 , method  100  includes a block  130  where a first backside contact  286  and a second backside contact  288  are formed. Operations at block  130  may be performed with a back side of the workpiece  200  facing up. In some embodiments, block  130  may include thinning of the substrate, replacement of the substrate  202  with a backside dielectric layer  280  (as shown in  FIG. 18 ), formation of a first backside contact opening  281  and a second backside contact opening  282  (as shown in  FIG. 19 ), and formation of the first backside contact  286  and the second backside contact  288  (as shown in  FIG. 20 ). In an example process, the replacement of the substrate  202  starts off with thinning of the substrate  202 . In some instances, the substrate thinning may be performed using grinding or chemical mechanical polishing (CMP). In some embodiments, the substrate  202  is thinned until the isolation feature (not shown) is exposed from the back side of the workpiece  200 . After the substrate  202  is thinned, it is selectively removed by a selective etch process. An example selective wet etch process may include use of ammonia (NH 3 ) or a suitable wet etchant. Because of the selective nature, the removal of the substrate  202  does not substantially etch the first bottom source/drain feature  228 - 1 , the second bottom source/drain feature  228 - 2 , the gate structure  260 , and the bottommost inner spacer features  226 . 
     After the removal of the substrate  202 , the backside dielectric layer  280  may be deposited over a back side of the workpiece  200  by flowable CVD (FCVD), CVD, plasma-enhanced CVD (PECVD), spin-on coating, or a suitable process, as shown in  FIG. 18 . Referring to  FIG. 19 , the first backside contact openings  281  and the second backside contact opening  282  are formed to expose bottom surfaces of the first bottom source/drain feature  228 - 1  and the second bottom source/drain feature  228 - 2 , respectively. The first backside contact  286  and the second backside contact  288  are then formed in the first backside contact opening  281  and the second backside contact opening  282 , respectively. Each of the first backside contact  286  and the second backside contact  288  may include a barrier layer  283  and a backside silicide layer  284 . In an example process to form the first backside contact  286  and the second backside contact  288 , a metal layer is deposited using PVD or CVD over the first bottom source/drain feature  228 - 1 , the second bottom source/drain feature  228 - 2 , sidewalls of the first backside contact opening  281 , and sidewalls of the second backside contact opening  282 . The metal layer is then nitridated to form a metal nitride layer. The metal nitride layer is then anisotropically etched to expose the first bottom source/drain feature  228 - 1  and the second bottom source/drain feature  228 - 2 , thereby forming the barrier layer  283 . In some embodiments, the barrier layer  283  may include titanium nitride or tantalum nitride. A metal precursor is then deposited over the barrier layer  283 , the first bottom source/drain feature  228 - 1 , and the second bottom source/drain feature  228 - 2 . An anneal process is performed to bring about silicidation between the metal precursor and the first bottom source/drain feature  228 - 1  or the second bottom source/drain feature  228 - 2  to form the backside silicide layer  284 . Thereafter, a metal fill layer is deposited over the backside silicide layer  284 . After a CMP process to remove excess materials, the first backside contact  286  and the second backside contact  288  are formed, as shown in  FIG. 20 . In some embodiments, the backside silicide layer  284  may include titanium silicide, tantalum silicide, nickel silicide, cobalt silicide, or tungsten silicide. The metal fill layer for the first backside contact  286  and the second backside contact  288  may include tungsten (W), ruthenium (Ru), cobalt (Co), nickel (Ni), or copper (Cu). 
     Embodiments of the present disclosure provide advantages. The present disclosure provides a local interconnect structure to couple a source/drain feature of a bottom MBC transistor to a source/drain feature of a top MBC transistor stacked over the bottom MBC transistor. While the local interconnect structure of the present disclosure extends through a gap in the source/drain feature on the top, its formation does not include etching through that source/drain feature. According to the embodiments of the present disclosure, the formation of the opening for the local interconnect structure is self-aligned. 
     In one exemplary aspect, the present disclosure is directed to a semiconductor device. The semiconductor device includes a stack of first channel members, a stack of second channel members disposed directly over the stack of first channel members, a bottom source/drain feature in contact with the stack of the first channel members, a separation layer disposed over the bottom source/drain feature, a top source/drain feature in contact with the stack of second channel members and disposed over the separation layer, and a frontside contact that extends through the top source/drain feature and the separation layer to be electrically coupled to the bottom source/drain feature. 
     In some embodiments, the frontside contact is spaced apart from the stack of second channel members by the top source/drain feature. In some implementations, the semiconductor device may further include a first silicide feature disposed between the frontside contact and the bottom source/drain feature. In some instances, the semiconductor device may further include a second silicide feature disposed between the frontside contact and the top source/drain feature. In some implementations, the stack of first channel members are disposed over a backside dielectric layer. In some embodiments, the semiconductor device may further include a backside contact disposed in the backside dielectric layer and the backside contact is disposed below and in electrical contact with the bottom source/drain feature. In some implementations, the bottom source/drain feature includes silicon germanium and a p-type dopant and the top source/drain feature includes silicon and an n-type dopant. In some embodiments, the separation layer includes silicon oxide. In some embodiments, the semiconductor device may further include a gate structure wrapping around each of the stack of first channel members and each of the stack of second channel members. 
     In another exemplary aspect, the present disclosure is directed to a semiconductor structure. The semiconductor structure includes a first bottom source/drain feature and a second bottom source/drain feature, a stack of first channel members extending between the first bottom source/drain feature and the second bottom source/drain feature, a first separation layer over the first bottom source/drain feature, a second separation layer over the second bottom source/drain feature, a first top source/drain feature disposed over the first separation layer, a second top source/drain feature disposed over the second separation layer, a stack of second channel members extending between the first top source/drain feature and the second top source/drain feature, a first frontside contact disposed over and in contact with the first top source/drain feature, and a second frontside contact extending through the second top source/drain feature and the second separation layer to be in contact with the second bottom source/drain feature. 
     In some embodiments, the stack of first channel members are disposed over a backside dielectric layer. In some implementations, the semiconductor structure may further include a first backside contact below and in contact with the first bottom source/drain feature and a second backside contact below and in contact with the second bottom source/drain feature. The first backside contact and the second backside contact are disposed in the backside dielectric layer. In some implementations, the second frontside contact is spaced apart from the stack of second channel members by the second top source/drain feature. In some instances, the second bottom source/drain feature includes silicon germanium and a p-type dopant and the second top source/drain feature includes silicon and an n-type dopant. In some instances, the first separation layer and the second separation layer include silicon oxide. 
     In yet another exemplary aspect, the present disclosure is directed to a method. The method includes receiving a workpiece including a substrate and a stack over the substrate, the stack including a plurality of channel layers interleaved by a plurality of sacrificial layers, the plurality of channel layers including a plurality of bottom channel layer and a plurality of top channel layers disposed over the plurality of bottom channel layer, patterning the stack and a portion of the substrate into a fin-shaped structure, the fin-shaped structure including a source/drain region, recessing the source/drain region to form a source/drain trench, exposing sidewalls of the plurality of channel layers, depositing a bottom source/drain feature in the source/drain trench to be in contact with the plurality of bottom channel layers, depositing a separation layer over the bottom source/drain feature, depositing a top source/drain feature over the separation layer to be in contact with the plurality of top channel layers, the top source/drain feature including a gap that exposes the separation layer, forming a filler layer in the gap, forming a contact opening through the filler layer and the separation layer to expose the bottom source/drain feature, and forming a contact feature in the contact opening. In some embodiments, the forming of the filler layer that includes depositing a filler material using atomic layer deposition (ALD) and after the depositing of the filler material, etching back the deposited filler material. In some instances, the filler material includes silicon oxide. In some instances, the bottom source/drain feature includes silicon germanium and a p-type dopant and the top source/drain feature includes silicon and an n-type dopant. In some embodiments, the forming of the contact opening includes use of a selective etch process that etches the filler layer without substantially etching the top source/drain feature. 
     The foregoing outlines features of several embodiments so that those of ordinary skill in the art may better understand the aspects of the present disclosure. Those of 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 of 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.