Patent ID: 12237396

DETAILED DESCRIPTION

The present disclosure relates generally to integrated circuit devices, and more particularly, to multigate devices, such as gate-all-around (GAA) devices.

The following disclosure provides many different embodiments, or examples, for implementing different features. Reference numerals and/or letters may be repeated in the various examples described herein. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various disclosed embodiments and/or configurations. Further, 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. Moreover, the formation of a feature on, connected to, and/or coupled to another feature in the present disclosure 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.

Further, 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 herein for ease of description to describe one element or feature's relationship to another element(s) or feature(s). The spatially relative terms are intended to encompass different orientations than as depicted of a device (or system or apparatus) including the element(s) or feature(s), including orientations associated with the device's use or operation. 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.

FIG.1AandFIG.1Bare a flow chart of a method100for fabricating a multigate device according to various aspects of the present disclosure. In some embodiments, method100fabricates a multi-gate device that includes p-type GAA transistors and n-type GAA transistors. At block102, a first semiconductor layer stack and a second semiconductor layer stack are formed over a substrate. The first semiconductor layer stack and the second semiconductor layer stack include first semiconductor layers and second semiconductor layers stacked vertically in an alternating configuration. At block104, a gate structure is formed over a first region of the first semiconductor layer stack and a first region of the second semiconductor layer stack. The gate structure includes a dummy gate stack and gate spacers. At block106, portions of the first semiconductor layer stack in second regions and portions of the second semiconductor layer stack in second regions are removed to form source/drain recesses. At block108, inner spacers are formed along sidewalls of the first semiconductor layers in the first semiconductor layer stack and the second semiconductor layer stack. At block110, epitaxial source/drain features are formed in the source/drain recesses. At block112, an interlayer dielectric (ILD) layer is formed over the epitaxial source/drain features. At block114, the dummy gate stack is removed, thereby forming a gate trench that exposes the first semiconductor layer stack in a p-type gate region and the second semiconductor layer stack in n-type gate region. At block116, the first semiconductor layers are removed from the first semiconductor layer stack and the second semiconductor layer stack exposed by the gate trench, thereby forming gaps between the second semiconductor layers. At block118, a gate dielectric layer is formed in the gate trench around the second semiconductor layers in the p-type gate region and the n-type gate region. The gate dielectric layer partially fills the gaps between the second semiconductor layers.

At block120, a sacrificial layer is formed over the gate dielectric layer in the gate trench in the p-type gate region and the n-type gate region. The sacrificial layer fills any remaining portion of the gaps between the second semiconductor layers. At block122, the sacrificial layer is patterned to form sacrificial features between the second semiconductor layers in the p-type gate region and the n-type gate region. At block124, the sacrificial features from are removed from between the second semiconductor layers in the p-type gate region. At block126, a p-type work function layer is formed in the gate trench over the gate dielectric in the p-type gate region and the n-type gate region. At block128, the p-type work function layer is removed from the gate trench in the n-type gate region. At block130, a determination is made whether the gate dielectric layer and the p-type work function layer fill the gate trench in the p-type gate region along a gate length direction. If no, block128and block130are repeated. If yes, method100proceeds to block132, where the sacrificial features are removed from between the second semiconductor layers in the n-type gate region. At block134, an n-type work function layer is formed in the gate trench over the gate dielectric layer in the n-type gate region and over the p-type work function layer in the p-type gate region. At block136, the n-type work function layer is removed from the gate trench in the p-type gate region. At block138, a metal bulk layer is formed in the gate trench over the n-type work function layer in the n-type gate region and over the p-type work function layer in the p-type gate region. At block140, a planarization process is performed on the metal bulk layer, the n-type work function layer, the p-type work function layer, and the gate dielectric layer, thereby forming a p-metal gate in the p-type gate region and an n-metal gate in the n-type gate region. Method100then proceeds to block142where contacts are formed. Additional processing is contemplated by the present disclosure. Additional steps can be provided before, during, and after method100, and some of the steps described can be moved, replaced, or eliminated for additional embodiments of method100. The discussion that follows illustrates various embodiments of nanowire-based integrated circuit devices that can be fabricated according to method100.

FIGS.2A-23A,FIGS.2B-23B,FIGS.2C-23C, andFIGS.2D-23Dare fragmentary diagrammatic views of a multigate device200, in portion or entirety, at various fabrication stages (such as those associated with method100inFIG.1AandFIG.1B) according to various aspects of the present disclosure. In particular,FIGS.2A-23Aare top views of multigate device200in an X-Y plane;FIGS.2B-23Bare diagrammatic cross-sectional views of multigate device200in an X-Z plane along lines B-B′ respectively ofFIGS.2A-23A,FIGS.2C-23Care diagrammatic cross-sectional views of multigate device200in a Y-Z plane along lines C-C′ respectively ofFIGS.2A-23A; andFIGS.2D-23Dare diagrammatic cross-sectional views of multigate device200in the Y-Z plane along lines D-D′ respectively ofFIGS.2A-23A. Multigate device200may be included in a microprocessor, a memory, and/or other IC device. In some embodiments, multigate device200is a portion of an IC chip, a system on chip (SoC), or portion thereof, that includes various passive and active microelectronic devices such as resistors, capacitors, inductors, diodes, p-type field effect transistors (PFETs), n-type field effect transistors (NFETs), metal-oxide semiconductor field effect transistors (MOSFETs), complementary metal-oxide semiconductor (CMOS) transistors, bipolar junction transistors (BJTs), laterally diffused MOS (LDMOS) transistors, high voltage transistors, high frequency transistors, other suitable components, or combinations thereof. In some embodiments, multigate device200is included in a non-volatile memory, such as a non-volatile random access memory (NVRAM), a flash memory, an electrically erasable programmable read only memory (EEPROM), an electrically programmable read-only memory (EPROM), other suitable memory type, or combinations thereof.FIGS.2A-23A,FIGS.2B-23B,FIGS.2C-23C, andFIGS.2D-23Dhave been simplified for the sake of clarity to better understand the inventive concepts of the present disclosure. Additional features can be added in multigate device200, and some of the features described below can be replaced, modified, or eliminated in other embodiments of multigate device200.

Turning toFIGS.2A-2D, multigate device200includes a substrate (wafer)202. In the depicted embodiment, substrate202includes silicon. Alternatively or additionally, substrate202includes another elementary semiconductor, 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 silicon germanium (SiGe), GaAsP, AlInAs, AlGaAs, GaInAs, GaInP, and/or GaInAsP; or combinations thereof. Alternatively, substrate202is a semiconductor-on-insulator substrate, such as a silicon-on-insulator (SOI) substrate, a silicon germanium-on-insulator (SGOI) substrate, or a germanium-on-insulator (GOI) substrate. Semiconductor-on-insulator substrates can be fabricated using separation by implantation of oxygen (SIMOX), wafer bonding, and/or other suitable methods. Substrate202can include various doped regions depending on design requirements of multigate device200. In the depicted embodiment, substrate202includes a p-type doped region204A (referred to hereinafter as a p-well), which can be configured for n-type GAA transistors, and an n-type doped region204B (referred to hereinafter as an n-well), which can be configured for p-type GAA transistors. N-type doped regions, such as n-well204B, are doped with n-type dopants, such as phosphorus, arsenic, other n-type dopant, or combinations thereof. P-type doped regions, such as p-well204A, are doped with p-type dopants, such as boron, indium, other p-type dopant, or combinations thereof. In some implementations, substrate202includes doped regions formed with a combination of p-type dopants and n-type dopants. The various doped regions can be formed directly on and/or in substrate202, for example, providing a p-well structure, an n-well structure, a dual-well structure, a raised structure, or combinations thereof. An ion implantation process, a diffusion process, and/or other suitable doping process can be performed to form the various doped regions.

A semiconductor layer stack205is formed over substrate202, where semiconductor layer stack205includes semiconductor layers210and semiconductor layers215stacked vertically (e.g., along the z-direction) in an interleaving or alternating configuration from a surface of substrate202. In some embodiments, semiconductor layers210and semiconductor layers215are epitaxially grown in the depicted interleaving and alternating configuration. For example, a first one of semiconductor layers210is epitaxially grown on substrate, a first one of semiconductor layers215is epitaxially grown on the first one of semiconductor layers215, a second one of semiconductor layers210is epitaxially grown on the first one of semiconductor layers215, and so on until semiconductor layers stack205has a desired number of semiconductor layers210and semiconductor layers215. In such embodiments, semiconductor layers210and semiconductor layers215can be referred to as epitaxial layers. In some embodiments, epitaxial growth of semiconductor layers210and semiconductor layers215is achieved by a molecular beam epitaxy (MBE) process, a chemical vapor deposition (CVD) process, a metalorganic chemical vapor deposition (MOCVD) process, other suitable epitaxial growth process, or combinations thereof.

A composition of semiconductor layers210is different than a composition of semiconductor layers215to achieve etching selectivity and/or different oxidation rates during subsequent processing. In some embodiments, semiconductor layers210have a first etch rate to an etchant and semiconductor layers215have a second etch rate to the etchant, where the second etch rate is less than the first etch rate. In some embodiments, semiconductor layers210have a first oxidation rate and semiconductor layers215have a second oxidation rate, where the second oxidation rate is less than the first oxidation rate. In the depicted embodiment, semiconductor layers210and semiconductor layers215include different materials, constituent atomic percentages, constituent weight percentages, thicknesses, and/or characteristics to achieve desired etching selectivity during an etching process, such as an etching process implemented to form suspended channel layers in channel regions of multigate device200. For example, where semiconductor layers210include silicon germanium and semiconductor layers215include silicon, a silicon etch rate of semiconductor layers215is less than a silicon germanium etch rate of semiconductor layers210. In some embodiments, semiconductor layers210and semiconductor layers215can include the same material but with different constituent atomic percentages to achieve the etching selectivity and/or different oxidation rates. For example, semiconductor layers210and semiconductor layers215can include silicon germanium, where semiconductor layers210have a first silicon atomic percent and/or a first germanium atomic percent and semiconductor layers215have a second, different silicon atomic percent and/or a second, different germanium atomic percent. The present disclosure contemplates that semiconductor layers210and semiconductor layers215include any combination of semiconductor materials that can provide desired etching selectivity, desired oxidation rate differences, and/or desired performance characteristics (e.g., materials that maximize current flow), including any of the semiconductor materials disclosed herein.

As described further below, semiconductor layers215or portions thereof form channel regions of multigate device200. In the depicted embodiment, semiconductor layer stack205includes four semiconductor layers210and four semiconductor layers215configured to form four semiconductor layer pairs disposed over substrate202, each semiconductor layer pair having a respective first semiconductor layer210and a respective second semiconductor layer215. After undergoing subsequent processing, such configuration will result in multigate device200having four channels. However, the present disclosure contemplates embodiments where semiconductor layer stack205includes more or less semiconductor layers, for example, depending on a number of channels desired for multigate device200(e.g., a GAA transistor) and/or design requirements of multigate device200. For example, semiconductor layer stack205can include two to ten semiconductor layers210and two to ten semiconductor layers215. In furtherance of the depicted embodiment, semiconductor layers210have a thickness t1and semiconductor layers215have a thickness t2, where thickness t1and thickness t2are chosen based on fabrication and/or device performance considerations for multigate device200. For example, thickness t1can be configured to define a desired distance (or gap) between adjacent channels of multigate device200(e.g., between semiconductor layers215), thickness t2can be configured to achieve desired thickness of channels of multigate device200, and both thickness t1and thickness t2can be configured to achieve desired performance of multigate device200. In some embodiments, thickness t1and thickness t2are about 1 nm to about 10 nm.

Turning toFIGS.3A-3D, semiconductor layer stack205is patterned to form a fin218A and a fin218B (also referred to as fin structures, fin elements, etc.). Fins218,218B include a substrate portion (i.e., a portion of substrate202) and a semiconductor layer stack portion (i.e., a remaining portion of semiconductor layer stack205including semiconductor layers210and semiconductor layers215). Fins218A,218B extend substantially parallel to one another along a y-direction, having a length defined in the y-direction, a width defined in an x-direction, and a height defined in a z-direction. In some implementations, a lithography and/or etching process is performed to pattern semiconductor layer stack205to form fins218A,218B. The lithography process can include forming a resist layer over semiconductor layer stack205(for example, by spin coating), performing a pre-exposure baking process, performing an exposure process using a mask, performing a post-exposure baking process, and performing a developing process. During the exposure process, the resist layer is exposed to radiation energy (such as ultraviolet (UV) light, deep UV (DUV) light, or extreme UV (EUV) light), where the mask blocks, transmits, and/or reflects radiation to the resist layer depending on a mask pattern of the mask and/or mask type (for example, binary mask, phase shift mask, or EUV mask), such that an image is projected onto the resist layer that corresponds with the mask pattern. Since the resist layer is sensitive to radiation energy, exposed portions of the resist layer chemically change, and exposed (or non-exposed) portions of the resist layer are dissolved during the developing process depending on characteristics of the resist layer and characteristics of a developing solution used in the developing process. After development, the patterned resist layer includes a resist pattern that corresponds with the mask. The etching process removes portions of semiconductor layer stack205using the patterned resist layer as an etch mask. In some embodiments, the patterned resist layer is formed over a hard mask layer disposed over semiconductor layer stack205, a first etching process removes portions of the hard mask layer to form a patterned hard mask layer, and a second etching process removes portions of semiconductor layer stack205using the patterned hard mask layer as an etch mask. The etching process can include a dry etching process, a wet etching process, other suitable etching process, or combinations thereof. In some embodiments, the etching process is a reactive ion etching (RIE) process. After the etching process, the patterned resist layer (and, in some embodiments, a hard mask layer) is removed, for example, by a resist stripping process or other suitable process. Alternatively, fins218A,218B are formed by a multiple patterning process, such as a double patterning lithography (DPL) process (for example, a lithography-etch-lithography-etch (LELE) process, a self-aligned double patterning (SADP) process, a spacer-is-dielectric (SID) SADP process, other double patterning process, or combinations thereof), a triple patterning process (for example, a lithography-etch-lithography-etch-lithography-etch (LELELE) process, a self-aligned triple patterning (SATP) process, other triple patterning process, or combinations thereof), other multiple patterning process (for example, self-aligned quadruple patterning (SAQP) process), or combinations thereof. In some embodiments, directed self-assembly (DSA) techniques are implemented while patterning semiconductor layer stack205. Further, in some embodiments, the exposure process can implement maskless lithography, electron-beam (e-beam) writing, and/or ion-beam writing for patterning the resist layer.

An isolation feature(s)230is formed over and/or in substrate202to isolate various regions, such as various device regions, of multigate device200. For example, isolation features230surround a bottom portion of fins218A,218B, such that isolation features230separate and isolate fins218A,218B from each other. In the depicted embodiment, isolation features230surround the substrate portion of fins218A,218B (e.g., doped regions204A,204B of substrate202) and partially surround the semiconductor layer stack portion of fins218A,218B (e.g., a portion of bottommost semiconductor layer210). However, the present disclosure contemplates different configurations of isolation features230relative to fins218A,218B. Isolation features230include silicon oxide, silicon nitride, silicon oxynitride, other suitable isolation material (for example, including silicon, oxygen, nitrogen, carbon, or other suitable isolation constituent), or combinations thereof. Isolation features230can include different structures, such as shallow trench isolation (STI) structures, deep trench isolation (DTI) structures, and/or local oxidation of silicon (LOCOS) structures. For example, isolation features230can include STI features that define and electrically isolate fins218A,218B from other active device regions (such as fins) and/or passive device regions. STI features can be formed by etching a trench in substrate202(for example, by using a dry etching process and/or a wet etching process) and filling the trench with insulator material (for example, by using a CVD process or a spin-on glass process). A chemical mechanical polishing (CMP) process may be performed to remove excessive insulator material and/or planarize a top surface of isolation features230. In another example, STI features can be formed by depositing an insulator material over substrate202after forming fins218A,218B (in some implementations, such that the insulator material layer fills gaps (trenches) between fins218A,218B) and etching back the insulator material layer to form isolation features230. In some embodiments, STI features include a multi-layer structure that fills the trenches, such as a silicon nitride comprising layer disposed over a thermal oxide comprising liner layer. In another example, STI features include a dielectric layer disposed over a doped liner layer (including, for example, boron silicate glass (BSG) or phosphosilicate glass (PSG)). In yet another example, STI features include a bulk dielectric layer disposed over a liner dielectric layer, where the bulk dielectric layer and the liner dielectric layer include materials depending on design requirements.

Turning toFIGS.4A-4D, gate structures240are formed over portions of fins218A,218B and over isolation features230. Gate structures240extend lengthwise in a direction that is different than (e.g., orthogonal to) the lengthwise direction of fins218A,218B. For example, gate structures240extend substantially parallel to one another along the x-direction, having a length defined in the y-direction, a width defined in the x-direction, and a height defined in the z-direction. Gate structures240are disposed on portions of fins218A,218B and define source/drain regions242and channel regions244of fins218A,218B. In the X-Z plane, gate structures240wrap top surfaces and sidewall surfaces of fins218A,218B. In the Y-Z plane, gate structures240are disposed over top surfaces of respective channel regions244of fins218A,218B, such that gate structures240interpose respective source/drain regions242. Each gate structure240includes a gate region240-1that corresponds with a portion of the respective gate structure240that will be configured for an n-type GAA transistor (and thus corresponds with a portion spanning an n-type GAA transistor region) and a gate region240-2that corresponds with a portion of the respective gate structure240that will be configured for a p-type GAA transistor (and thus corresponds with a portion spanning a p-type GAA transistor region). Gate structures240are configured differently in gate region240-1and gate region240-2. For example, as described further below, each metal gate stack of gate structures240spans gate region240-1and gate region240-2and is configured differently in gate region240-1and gate region240-2to optimize performance of the n-type GAA transistors (having n-gate electrodes in gate regions240-1) and the p-type GAA transistors (having p-gate electrodes in gate regions240-2). Accordingly, gate regions240-1will be referred to as n-type gate regions240-1and gate regions240-2will be referred to as p-type gate regions240-2hereinafter.

InFIGS.4A-4D, each gate structure240includes a dummy gate stack245. In the depicted embodiment, a width of dummy gate stacks245defines a gate length (Lg) of gate structures240(here, in the y-direction), where the gate length defines a distance (or length) that current (e.g., carriers, such as electrons or holes) travels between source/drain regions242when the n-type GAA transistor and/or the p-type GAA transistor are switched (turned) on. In some embodiments, the gate length is about 5 nm to about 250 nm. Gate length can be tuned to achieve desired operation speeds of the GAA transistors and/or desired packing density of the GAA transistors. For example, when a GAA transistor is switched on, current flows between source/drain regions of the GAA transistor. Increasing the gate length increases a distance required for current to travel between the source/drain regions, increasing a time it takes for the GAA transistor to switch fully on. Conversely, decreasing the gate length decreases the distance required for current to travel between the source/drain regions, decreasing a time it takes for the GAA transistor to switch fully on. Smaller gate lengths provide GAA transistors that switch on/off more quickly, facilitating faster, high speed operations. Smaller gate lengths also facilitate tighter packing density (i.e., more GAA transistors can be fabricated in a given area of an IC chip), increasing a number of functions and applications that can be fabricated on the IC chip. In the depicted embodiment, the gate length of one or more of gate structures240is configured to provide GAA transistors having short-length (SC) channels. For example, the gate length of SC GAA transistors is about 5 nm to about 20 nm. In some embodiments, multigate device200can include GAA transistors having different gate lengths. For example, a gate length of one or more of gate structures240can be configured to provide GAA transistors having mid-length or long-length channels (M/LC). In some embodiments, the gate length of M/LC GAA transistors is about 20 nm to about 250 nm.

Dummy gate stacks245include a dummy gate electrode, and in some embodiments, a dummy gate dielectric. The dummy gate electrode includes a suitable dummy gate material, such as polysilicon layer. In embodiments where dummy gate stacks245include a dummy gate dielectric disposed between the dummy gate electrode and fins218A,218B, the dummy gate dielectric includes a dielectric material, such as silicon oxide, a high-k dielectric material, other suitable dielectric material, or combinations thereof. Examples of high-k dielectric material include HfO2, HfSiO, HfSiON, HfTaO, HfSiO, HfZrO, zirconium oxide, aluminum oxide, hafnium dioxide-alumina (HfO2—Al2O3) alloy, other suitable high-k dielectric materials, or combinations thereof. In some embodiments, the dummy gate dielectric includes an interfacial layer (including, for example, silicon oxide) disposed over fins218A,218B and a high-k dielectric layer disposed over the interfacial layer. Dummy gate stacks245can include numerous other layers, for example, capping layers, interface layers, diffusion layers, barrier layers, hard mask layers, or combinations thereof. For example, dummy gate stacks245can further include a hard mask layer disposed over the dummy gate electrode.

Dummy gate stacks245are formed by deposition processes, lithography processes, etching processes, other suitable processes, or combinations thereof. For example, a deposition process is performed to form a dummy gate electrode layer over fins218A,218B and isolation features230. In some embodiments, a deposition process is performed to form a dummy gate dielectric layer over fins218A,218B and isolation features230before forming the dummy gate electrode layer. In such embodiments, the dummy gate electrode layer is deposited over the dummy gate dielectric layer. In some embodiment, a hard mask layer is deposited over the dummy gate electrode layer. The deposition process includes CVD, physical vapor deposition (PVD), atomic layer deposition (ALD), high density plasma CVD (HDPCVD), metal organic CVD (MOCVD), remote plasma CVD (RPCVD), plasma enhanced CVD (PECVD), low-pressure CVD (LPCVD), atomic layer CVD (ALCVD), atmospheric pressure CVD (APCVD), plating, other suitable methods, or combinations thereof. A lithography patterning and etching process is then performed to pattern the dummy gate electrode layer (and, in some embodiments, the dummy gate dielectric layer and the hard mask layer) to form dummy gate stacks245, such that dummy gate stacks245(including the dummy gate electrode layer, the dummy gate dielectric layer, the hard mask layer, and/or other suitable layers) is configured as depicted inFIGS.4A-4D. The lithography patterning processes include resist coating (for example, spin-on coating), soft baking, mask aligning, exposure, post-exposure baking, developing the resist, rinsing, drying (for example, hard baking), other suitable lithography processes, or combinations thereof. The etching processes include dry etching processes, wet etching processes, other etching methods, or combinations thereof.

Each gate structure240further includes gate spacers247disposed adjacent to (i.e., along sidewalls of) respective dummy gate stacks245. Gate spacers247are formed by any suitable process and include a dielectric material. The dielectric material can include silicon, oxygen, carbon, nitrogen, other suitable material, or combinations thereof (e.g., silicon oxide, silicon nitride, silicon oxynitride (SiON), silicon carbide, silicon carbon nitride (SiCN), silicon oxycarbide (SiOC), silicon oxycarbon nitride (SiOCN)). For example, a dielectric layer including silicon and nitrogen, such as a silicon nitride layer, can be deposited over dummy gate stacks245and subsequently etched (e.g., anisotropically etched) to form gate spacers247. In some embodiments, gate spacers247include a multi-layer structure, such as a first dielectric layer that includes silicon nitride and a second dielectric layer that includes silicon oxide. In some embodiments, more than one set of spacers, such as seal spacers, offset spacers, sacrificial spacers, dummy spacers, and/or main spacers, are formed adjacent to dummy gate stacks245. In such implementations, the various sets of spacers can include materials having different etch rates. For example, a first dielectric layer including silicon and oxygen (e.g., silicon oxide) can be deposited and etched to form a first spacer set adjacent to dummy gate stacks245, and a second dielectric layer including silicon and nitrogen (e.g., silicon nitride) can be deposited and etched to form a second spacer set adjacent to the first spacer set.

Turning toFIGS.5A-5D, exposed portions of fins218A,218B (i.e., source/drain regions242of fins218A,218B that are not covered by gate structures240) are at least partially removed to form source/drain trenches (recesses)250. In the depicted embodiment, an etching process completely removes semiconductor layer stack205in source/drain regions242of fins218A,218B, thereby exposing the substrate portion of fins218A,218B in source/drain regions242(e.g., p-well204A and n-well204B). Source/drain trenches250thus have sidewalls defined by remaining portions of semiconductor layer stack205, which are disposed in channel regions244under gate structures240, and bottoms defined by substrate202, such as top surfaces of p-well204A and n-well204B in source/drain regions242. In some embodiments, the etching process removes some, but not all, of semiconductor layer stack205, such that source/drain trenches250have bottoms defined by semiconductor layer210or semiconductor layer215in source/drain regions242. In some embodiments, the etching process further removes some, but not all, of the substrate portion of fins218A,218B, such that source/drain recesses250extend below a topmost surface of substrate202. The etching process can include a dry etching process, a wet etching process, other suitable etching process, or combinations thereof. In some embodiments, the etching process is a multi-step etch process. For example, the etching process may alternate etchants to separately and alternately remove semiconductor layers210and semiconductor layers215. In some embodiments, parameters of the etching process are configured to selectively etch semiconductor layer stack with minimal (to no) etching of gate structures240(i.e., dummy gate stacks245and gate spacers247) and/or isolation features230. In some embodiments, a lithography process, such as those described herein, is performed to form a patterned mask layer that covers gate structures240and/or isolation features230, and the etching process uses the patterned mask layer as an etch mask.

Turning toFIGS.6A-6D, inner spacers255are formed in channel regions244along sidewalls of semiconductor layers210by any suitable process. For example, a first etching process is performed that selectively etches semiconductor layers210exposed by source/drain trenches250with minimal (to no) etching of semiconductor layers215, such that gaps are formed between semiconductor layers215and between semiconductor layers215and substrate202under gate spacers247. Portions (edges) of semiconductor layers215are thus suspended in the channel regions244under gate spacers247. In some embodiments, the gaps extend partially under dummy gate stacks245. The first etching process is configured to laterally etch (e.g., along the y-direction) semiconductor layers210, thereby reducing a length of semiconductor layers210along the y-direction. The first etching process is a dry etching process, a wet etching process, other suitable etching process, or combinations thereof. A deposition process then forms a spacer layer over gate structures240and over features defining source/drain trenches250(e.g., semiconductor layers215, semiconductor layers210, and substrate202), such as CVD, PVD, ALD, HDPCVD, MOCVD, RPCVD, PECVD, LPCVD, ALCVD, APCVD, plating, other suitable methods, or combinations thereof. The spacer layer partially (and, in some embodiments, completely) fills the source/drain trenches250. The deposition process is configured to ensure that the spacer layer fills the gaps between semiconductor layers215and between semiconductor layers215and substrate202under gate spacers247. A second etching process is then performed that selectively etches the spacer layer to form inner spacers255as depicted inFIGS.6A-6Dwith minimal (to no) etching of semiconductor layers215, dummy gate stacks245, and gate spacers247. In some embodiments, the spacer layer is removed from sidewalls of gate spacers247, sidewalls of semiconductor layers215, dummy gate stacks245, and substrate202. The spacer layer (and thus inner spacers255) includes a material that is different than a material of semiconductor layers215and a material of gate spacers247to achieve desired etching selectivity during the second etching process. In some embodiments, the spacer layer includes a dielectric material that includes silicon, oxygen, carbon, nitrogen, other suitable material, or combinations thereof (for example, silicon oxide, silicon nitride, silicon oxynitride, silicon carbide, or silicon oxycarbonitride). In some embodiments, the spacer layer includes a low-k dielectric material, such as those described herein. In some embodiments, dopants (for example, p-type dopants, n-type dopants, or combinations thereof) are introduced into the dielectric material, such that spacer layer includes a doped dielectric material.

Turning toFIGS.7A-7D, epitaxial source/drain features are formed in source/drain recesses250. For example, a semiconductor material is epitaxially grown from portions of substrate202and semiconductor layers215exposed by source/drain recesses250, forming epitaxial source/drain features260A in source/drain regions242that correspond with n-type GAA transistor regions and epitaxial source/drain features260B in source/drain regions242that correspond with p-type GAA transistor regions. An epitaxy process can use CVD deposition techniques (for example, VPE and/or UHV-CVD), molecular beam epitaxy, other suitable epitaxial growth processes, or combinations thereof. The epitaxy process can use gaseous and/or liquid precursors, which interact with the composition of substrate202and/or semiconductor layer stack205(in particular, semiconductor layers215). Epitaxial source/drain features260A,260B are doped with n-type dopants and/or p-type dopants. In some embodiments, for the n-type GAA transistors, epitaxial source/drain features260A include silicon. Epitaxial source/drain features260A can be doped with carbon, phosphorous, arsenic, other n-type dopant, or combinations thereof (for example, forming Si:C epitaxial source/drain features, Si:P epitaxial source/drain features, or Si:C:P epitaxial source/drain features). In some embodiments, for the p-type GAA transistors, epitaxial source/drain features260B include silicon germanium or germanium. Epitaxial source/drain features260B can be doped with boron, other p-type dopant, or combinations thereof (for example, forming Si:Ge:B epitaxial source/drain features). In some embodiments, epitaxial source/drain features260A and/or epitaxial source/drain features260B include more than one epitaxial semiconductor layer, where the epitaxial semiconductor layers can include the same or different materials and/or dopant concentrations. In some embodiments, epitaxial source/drain features260A,260B include materials and/or dopants that achieve desired tensile stress and/or compressive stress in respective channel regions244. In some embodiments, epitaxial source/drain features260A,260B are doped during deposition by adding impurities to a source material of the epitaxy process (i.e., in-situ). In some embodiments, epitaxial source/drain features260A,260B are doped by an ion implantation process subsequent to a deposition process. In some embodiments, annealing processes (e.g., rapid thermal annealing (RTA) and/or laser annealing) are performed to activate dopants in epitaxial source/drain features260A,260B and/or other source/drain regions (for example, heavily doped source/drain regions and/or lightly doped source/drain (LDD) regions). In some embodiments, epitaxial source/drain features260A,260B are formed in separate processing sequences that include, for example, masking p-type GAA transistor regions when forming epitaxial source/drain features260A in n-type GAA transistor regions and masking n-type GAA transistor regions when forming epitaxial source/drain features260B in p-type GAA transistor regions.

Turning toFIGS.8A-8D, an inter-level dielectric (ILD) layer270is formed over isolation features230, epitaxial source/drain features260A,260B, and gate spacers247, for example, by a deposition process (such as CVD, PVD, ALD, HDPCVD, MOCVD, RPCVD, PECVD, LPCVD, ALCVD, APCVD, plating, other suitable methods, or combinations thereof). ILD layer270is disposed between adjacent gate structures240. In some embodiments, ILD layer270is formed by a flowable CVD (FCVD) process that includes, for example, depositing a flowable material (such as a liquid compound) over multigate device200and converting the flowable material to a solid material by a suitable technique, such as thermal annealing and/or ultraviolet radiation treating. ILD layer270includes a dielectric material including, for example, silicon oxide, silicon nitride, silicon oxynitride, TEOS formed oxide, PSG, BPSG, low-k dielectric material, other suitable dielectric material, or combinations thereof. Exemplary low-k dielectric materials include FSG, carbon doped silicon oxide, Black Diamond® (Applied Materials of Santa Clara, California), Xerogel, Aerogel, amorphous fluorinated carbon, Parylene, BCB, SILK (Dow Chemical, Midland, Michigan), polyimide, other low-k dielectric material, or combinations thereof. In the depicted embodiment, ILD layer270is a dielectric layer that includes a low-k dielectric material (generally referred to as a low-k dielectric layer). ILD layer270can include a multilayer structure having multiple dielectric materials. In some embodiments, a contact etch stop layer (CESL) is disposed between ILD layer270and isolation features230, epitaxial source/drain features260A,260B, and gate spacers247. The CESL includes a material different than ILD layer270, such as a dielectric material that is different than the dielectric material of ILD layer270. For example, where ILD layer270includes a low-k dielectric material, the CESL includes silicon and nitrogen, such as silicon nitride or silicon oxynitride. Subsequent to the deposition of ILD layer270and/or the CESL, a CMP process and/or other planarization process can be performed until reaching (exposing) a top portion (or top surface) of dummy gate stacks245. In some embodiments, the planarization process removes hard mask layers of dummy gate stacks245to expose underlying dummy gate electrodes of dummy gate stacks245, such as polysilicon gate electrode layers.

ILD layer170may be a portion of a multilayer interconnect (MLI) feature disposed over substrate202. The MLI feature electrically couples various devices (for example, p-type GAA transistors and/or n-type GAA transistors of multigate device200, transistors, resistors, capacitors, and/or inductors) and/or components (for example, gate structures and/or epitaxial source/drain features of p-type GAA transistors and/or n-type GAA transistors), such that the various devices and/or components can operate as specified by design requirements of multigate device200. The MLI feature includes a combination of dielectric layers and electrically conductive layers (e.g., metal layers) configured to form various interconnect structures. The conductive layers are configured to form vertical interconnect features, such as device-level contacts and/or vias, and/or horizontal interconnect features, such as conductive lines. Vertical interconnect features typically connect horizontal interconnect features in different layers (or different planes) of the MLI feature. During operation, the interconnect features are configured to route signals between the devices and/or the components of multigate device200and/or distribute signals (for example, clock signals, voltage signals, and/or ground signals) to the devices and/or the components of multigate device200.

Turning toFIGS.9A-9D, dummy gate stacks245are removed from gate structures240, thereby exposing semiconductor layer stacks205of fins218A,218B in n-type gate regions240-1and p-type gate regions240-2. In the depicted embodiment, an etching process completely removes dummy gate stacks245to expose semiconductor layers215and semiconductor layers210in channel regions244. The etching process is a dry etching process, a wet etching process, other suitable etching process, or combinations thereof. In some embodiments, the etching process is a multi-step etch process. For example, the etching process may alternate etchants to separately remove various layers of dummy gate stacks245, such as the dummy gate electrode layers, the dummy gate dielectric layers, and/or the hard mask layers. In some embodiments, the etching process is configured to selectively etch dummy gate stacks245with minimal (to no) etching of other features of multigate device200, such as ILD layer270, gate spacers247, isolation features230, semiconductor layers215, and semiconductor layers210. In some embodiments, a lithography process, such as those described herein, is performed to form a patterned mask layer that covers ILD layer270and/or gate spacers247, and the etching process uses the patterned mask layer as an etch mask.

Turning toFIGS.10A-10D, semiconductor layers210of semiconductor layer stack205(exposed by gate trenches275) are selectively removed from channel regions244, thereby forming suspended semiconductor layers215′ in channel regions244. In the depicted embodiment, an etching process selectively etches semiconductor layers210with minimal (to no) etching of semiconductor layers215and, in some embodiments, minimal (to no) etching of gate spacers247and/or inner spacers255. Various etching parameters can be tuned to achieve selective etching of semiconductor layers210, such as etchant composition, etching temperature, etching solution concentration, etching time, etching pressure, source power, RF bias voltage, RF bias power, etchant flow rate, other suitable etching parameters, or combinations thereof. For example, an etchant is selected for the etching process that etches the material of semiconductor layers210(in the depicted embodiment, silicon germanium) at a higher rate than the material of semiconductor layers215(in the depicted embodiment, silicon) (i.e., the etchant has a high etch selectivity with respect to the material of semiconductor layers210). The etching process is a dry etching process, a wet etching process, other suitable etching process, or combinations thereof. In some embodiments, a dry etching process (such as an RIE process) utilizes a fluorine-containing gas (for example, SF6) to selectively etch semiconductor layers210. In some embodiments, a ratio of the fluorine-containing gas to an oxygen-containing gas (for example, O2), an etching temperature, and/or an RF power may be tuned to selectively etch silicon germanium or silicon. In some embodiments, a wet etching process utilizes an etching solution that includes ammonium hydroxide (NH4OH) and water (H2O) to selectively etch semiconductor layers210. In some embodiments, a chemical vapor phase etching process using hydrochloric acid (HCl) selectively etches semiconductor layers210.

At least one suspended semiconductor layer215′ is thus exposed in n-type gate regions240-1and p-type gate regions240-2by gate trenches275. In the depicted embodiment, each n-type gate region240-1and each p-type gate region240-2includes four suspended semiconductor layers215′ vertically stacked that will provide four channels through which current will flow between respective epitaxial source/drain features (epitaxial source/drain features260A or epitaxial source/drain features260B) during operation of the GAA transistors. Suspended semiconductor layers215′ are thus referred to as channel layers215′ hereinafter. Channel layers215′ in n-type gate regions240-1are separated by gaps277A, and channel layers215′ in p-type gate regions240-2are separated by gaps277B. Channel layers215′ in n-type gate regions240-1are also separated from substrate202by gaps277A, and channel layers215′ in p-type gate regions240-2are also separated by gaps277B. A spacing s1is defined between channel layers215′ along the z-direction in n-type gate regions240-1, and a spacing s2is defined between channel layers215′ along the z-direction in p-type gate regions240-2. Spacing s1and spacing s2correspond with a width of gaps277A and gaps277B, respectively. In the depicted embodiment, spacing s1is about equal to s2, though the present disclosure contemplates embodiments where spacing s1is different than spacing s2. In some embodiments, spacing s1and spacing s2are both about equal to thickness t1of semiconductor layers210. Further, channel layers215′ in n-type gate regions240-1have a length l1along the x-direction and a width w1along the y-direction, and channel layers215′ in p-type gate regions240-2have a length l2along the y-direction and a width w2along the x-direction. In the depicted embodiment, length l1is about equal to length l2, and width w1is about equal to width w2, though the present disclosure contemplates embodiments where length l1is different than length l2and/or width w1is different than width w2. In some embodiments, length l1and/or length l2is about 10 nm to about 50 nm. In some embodiments, width w1and/or width w2is about 4 nm to about 10 nm. In some embodiments, each channel layer215′ has nanometer-sized dimensions and can be referred to as a “nanowire,” which generally refers to a channel layer suspended in a manner that will allow a metal gate to physically contact at least two sides of the channel layer, and in GAA transistors, will allow the metal gate to physically contact at least four sides of the channel layer (i.e., surround the channel layer). In such embodiments, a vertical stack of suspended channel layers can be referred to as a nanostructure, and the process depicted inFIGS.10A-10Dcan be referred to as a channel nanowire release process. In some embodiments, after removing semiconductor layers210, an etching process is performed to modify a profile of channel layers215′ to achieve desired dimensions and/or desired shapes (e.g., cylindrical-shaped (e.g., nanowire), rectangular-shaped (e.g., nanobar), sheet-shaped (e.g., nanosheet), etc.). The present disclosure further contemplates embodiments where the channel layers215′ (nanowires) have sub-nanometer dimensions depending on design requirements of multigate device200.

Turning toFIGS.11A-11D, a gate dielectric layer is formed over multigate device200, where the gate dielectric layer partially fills gate trenches275and wraps (surrounds) channel layers215′ in n-type gate regions240-1and p-type gate regions240-2of gate structures240. In the depicted embodiment, the gate dielectric layer includes an interfacial layer280and a high-k dielectric layer282, where interfacial layer280is disposed between the high-k dielectric layer282and channel layers215′. In furtherance of the depicted embodiment, interfacial layer280and high-k dielectric layer282partially fill gaps277A between channel layers215′ and between channel layers215′ and substrate202in n-type gate regions240-1and partially fill gaps277B between channel layers215′ and between channel layers215′ and substrate202in p-type gate regions240-2. In some embodiments, interfacial layer280and/or high-k dielectric layer282are also disposed on substrate202, isolation features230, and/or gate spacers247. Interfacial layer280includes a dielectric material, such as SiO2, HfSiO, SiON, other silicon-comprising dielectric material, other suitable dielectric material, or combinations thereof. High-k dielectric layer282includes a high-k dielectric material, such as HfO2, HfSiO, HfSiO4, HfSiON, HfLaO, HfTaO, HfSiO, HfZrO, HfAlOx, ZrO, ZrO2, ZrSiO2, AlO, AlSiO, Al2O3, TiO, TiO2, LaO, LaSiO, Ta2O3, Ta2O5, Y2O3, SrTiO3, BaZrO, BaTiO3(BTO), (Ba,Sr)TiO3(BST), Si3N4, hafnium dioxide-alumina (HfO2—Al2O3) alloy, other suitable high-k dielectric material, or combinations thereof. High-k dielectric material generally refers to dielectric materials having a high dielectric constant, for example, greater than that of silicon oxide (k≈3.9). Interfacial layer280is formed by any of the processes described herein, such as thermal oxidation, chemical oxidation, ALD, CVD, other suitable process, or combinations thereof. In some embodiments, interfacial layer280has a thickness of about 0.5 nm to about 3 nm. High-k dielectric layer282is formed by any of the processes described herein, such as ALD, CVD, PVD, oxidation-based deposition process, other suitable process, or combinations thereof. In some embodiments, high-k dielectric layer282has a thickness of about 1 nm to about 2 nm.

Turning toFIGS.12A-12D, a sacrificial (dummy) layer284is deposited over multigate device200, where sacrificial layer284partially fills gate trenches275and wraps (surrounds) channel layers215′ in n-type gate regions240-1and p-type gate regions240-2. For example, sacrificial layer284is deposited on the gate dielectric layer by any of the processes described herein, such as ALD, CVD, PVD, other suitable process, or combinations thereof. In the depicted embodiment, sacrificial layer284is disposed on high-k dielectric layer282and surrounds high-k dielectric layer282, interfacial layer280, and channel layers215′. For example, sacrificial layer284is disposed along sidewalls, tops, and bottoms of channel layers215′. A thickness of sacrificial layer284is configured to fill any remaining portion of gaps277A between channel layers215′ in n-type gate regions240-1and any remaining portion of gaps277B between channel layers215′ in p-type gate regions240-2without filling gate trenches275(i.e., any portions of gaps277A,277B not filled by the gate dielectric layer). In some embodiments, the thickness of sacrificial layer284is about 0.5 nm to about 5 nm. Sacrificial layer284includes a material that is different than a high-k dielectric material to achieve etching selectivity between sacrificial layer284and high-k dielectric layer282during an etching process, such that sacrificial layer284can be selectively etched with minimal (to no) etching of high-k dielectric layer282. The material of sacrificial layer284is also different than a p-type work function material of a p-type work function layer (e.g., formed later as a portion of the gate electrodes of gate structures240) to achieve etching selectivity between sacrificial layer284and the p-type work function layer during an etching process, such that sacrificial layer284can be selectively etched with minimal (to no) etching of the p-type work function layer, and vice versa. The material of sacrificial layer284may also be different than a low-k dielectric material to achieve etching selectivity between sacrificial layer284and low-k dielectric material, such as that of ILD layer270, during an etching process, such that sacrificial layer284can be selectively etched with minimal (to no) etching of ILD layer270. In some embodiments, sacrificial layer284includes metal and oxygen (and can thus be referred to as a metal oxide layer), such as aluminum and oxygen (e.g., AlOx). In some embodiments, sacrificial layer284includes polysilicon. In some embodiments, sacrificial layer284includes silicon. In some embodiments, sacrificial layer284includes a dielectric material that includes silicon, such as SiO2, SiN, SiON, other suitable dielectric material including silicon, or combinations thereof. The present disclosure contemplates sacrificial layer284including other semiconductor materials and/or other dielectric materials that can provide the desired etching selectivity as described herein.

Turning toFIGS.13A-13D, an etching process is performed to partially remove sacrificial layer284, such that sacrificial layer284is patterned into sacrificial (dummy) features284′ between channel layers215′ and between channel layers215′ and substrate202in n-type gate regions240-1and p-type gate regions240-2. In some embodiments, the etching process is a wet etching process that uses an etching solution having a high etching selectivity with respect to sacrificial layer284relative to high-k dielectric layer282. In some embodiments, the etching solution exhibits an etching selectivity (i.e., a ratio of an etch rate of sacrificial layer284to the etching solution to an etch rate of high-k dielectric layer282to the etching solution) of about 10 to about 100. In some embodiments, the etching selectivity is greater than or equal to 100. In some embodiments, the wet etching process implements an NH4OH-based wet etching solution. Parameters of the etching process are controlled (tuned) to remove sacrificial layer284from sidewalls of channel layers215′ and from over isolation features230with minimal (to no) etching of high-k dielectric layer282, such as etching temperature, etching solution concentration, etching time, other suitable wet etching parameters, or combinations thereof. For example, an etching time (i.e., how long sacrificial layer284is exposed to the ammonia-based wet etching solution) is tuned to remove sacrificial layer284along sidewalls of channel layers215′ and along a topmost portion of high-k dielectric layer282(i.e., a portion of high-k dielectric layer282that is disposed over a top surface of a topmost channel layer215′). In furtherance of the example, the etching time is further tuned to achieve lateral etching (e.g., along the x-direction and/or the y-direction) of sacrificial layer284until a width of the sacrificial features284′ (here, along the x-direction) is less than a sum of the width of channel layers215′ and a thickness of the gate dielectric (here, a sum of the thickness of interfacial layer282and the thickness of high-k dielectric layer284). In some embodiments, a width of sacrificial features284′ is substantially equal to a width of channel layers215′. Sidewalls of sacrificial features284′ are thus recessed a distance d along the x-direction relative to sidewalls of high-k dielectric layer282. In some embodiments, distance d is greater than 0, for example, about 0.5 nm to about 5 nm. In some embodiments, sidewalls are not recessed along the x-direction relative to sidewalls of high-k dielectric layer282, such that distance d is equal to 0.

Turning toFIGS.14A-14D, sacrificial features284′ are removed from p-type gate regions240-2by implementing a lithography process and an etching process. For example, a patterned mask layer290having one or more openings292is formed over multigate device200by the lithography process. Patterned mask layer290covers n-type GAA transistor regions, which include n-type gate regions240-1. Patterned mask layer290includes a material that is different than a material of sacrificial features284′ and a material of high-k dielectric layer282to achieve etching selectivity during removal of sacrificial features284′. For example, patterned mask layer290includes a resist material (and thus may be referred to as a patterned resist layer and/or a patterned photoresist layer). In some embodiments, patterned mask layer290has a multi-layer structure, such as a resist layer disposed over an anti-reflective coating (ARC) layer. The present disclosure contemplates other materials for patterned mask layer290, so long as etching selectivity is achieved during removal of sacrificial features284′. In some embodiments, the lithography process includes forming a resist layer over multigate device200(e.g., by spin coating), performing a pre-exposure baking process, performing an exposure process using a mask, performing a post-exposure baking process, and performing a developing process. During the exposure process, the resist layer is exposed to radiation energy (e.g., UV light, DUV light, or EUV light), where the mask blocks, transmits, and/or reflects radiation to the resist layer depending on a mask pattern of the mask and/or mask type (e.g., binary mask, phase shift mask, or EUV mask), such that an image is projected onto the resist layer that corresponds with the mask pattern. Since the resist layer is sensitive to radiation energy, exposed portions of the resist layer chemically change, and exposed (or non-exposed) portions of the resist layer are dissolved during the developing process depending on characteristics of the resist layer and characteristics of a developing solution used in the developing process. After development, the patterned resist layer (e.g., patterned mask layer290) includes a resist pattern that corresponds with the mask, where the patterned resist layer covers the n-type GAA transistor regions, which include n-type gate regions240-1, and exposes the p-type GAA transistor regions, which include p-type gate regions240-2. Alternatively, the exposure process can be implemented or replaced by other methods, such as maskless lithography, e-beam writing, ion-beam writing, or combinations thereof.

The etching process then uses patterned mask layer290as an etch mask when removing sacrificial features284′ between channel layers215′ and between channel layers215′ and substrate202, thereby exposing high-k dielectric layer282in p-type gate regions240-2. The etching process essentially re-forms a portion of gaps277B in p-type gate regions240-2. In some embodiments, the etching process is a wet etching process that uses an etching solution having a high etching selectivity with respect to sacrificial features284′ relative to high-k dielectric layer282. In some embodiments, the etching solution exhibits an etching selectivity of about 10 to about 100. In some embodiments, the etching selectivity is greater than or equal to 100. In some embodiments, the wet etching process implements an NH4OH-based wet etching solution. Parameters of the etching process are controlled to ensure complete removal of sacrificial features284′ in p-type gate regions240-2, such as etching temperature, etching solution concentration, etching time, other suitable wet etching parameters, or combinations thereof. For example, an etching time (i.e., how long sacrificial features284′ are exposed to the ammonia-based wet etching solution) is tuned to completely remove sacrificial features284′ with minimal (to no) etching of high-k dielectric layer282. In some embodiments, an etching time for completely removing sacrificial features284′ is greater than an etching time for patterning sacrificial layer284into sacrificial features284′. In some embodiments, the etching solution further has an etching selectivity with respect to sacrificial features284′ relative to patterned mask layer290. In some embodiments, the etching process partially etches patterned mask layer290. After the etching process, the patterned mask layer290can be removed from multigate device200, for example, by a resist stripping process or other suitable process.

Turning toFIGS.15A-15D, a p-type work function layer300is formed over multigate device200, particularly over high-k dielectric layer282. For example, an ALD process conformally deposits p-type work function layer300on high-k dielectric layer282, such that p-type work function layer300has a substantially uniform thickness and partially fills gate trenches275. In p-type gate regions240-2, p-type work function layer300is disposed on high-k dielectric layer282and surrounds high-k dielectric layer282, interfacial layer280, and channel layers215′. For example, p-type work function layer300is disposed along sidewalls, tops, and bottoms of channel layers215′. A thickness of p-type work function layer300is configured to at least partially fill gaps277B between channel layers215′ and between channel layers215′ and substrate202(and, in some embodiments, without filling gate trenches275along the gate length direction (here, along the y-direction)). In some embodiments, p-type work function layer300has a thickness of about 1 nm to about 10 nm. In contrast, in n-type gate regions240-1, p-type work function layer300is disposed along sidewalls of channel layers215′ and the top surfaces of the topmost channel layers215′. Sacrificial features284′ thus function as deposition stop layers, preventing significant deposition of p-type work function layer300in gaps277A between channel layers215′ and between channel layers215′ and substrate202in n-type gate regions240-1. In the depicted embodiment, p-type work function layer300is further disposed along sidewalls of sacrificial features284′. P-type work function layer300includes any suitable p-type work function material, such as TiN, TaN, TaSN, Ru, Mo, Al, WN, WCN ZrSi2, MoSi2, TaSi2, NiSi2, other p-type work function material, or combinations thereof. In the depicted embodiment, p-type work function layer300includes titanium and nitrogen, such as TiN. P-type work function layer300can be formed using another suitable deposition process, such as CVD, PVD, HDPCVD, MOCVD, RPCVD, PECVD, LPCVD, ALCVD, APCVD, spin coating, plating, other deposition process, or combinations thereof.

Turning toFIGS.16A-16D, p-type work function layer300is removed from n-type gate regions240-1of gate structures240, for example, by implementing a lithography process and an etching process. For example, a patterned mask layer310having one or more openings312is formed over multigate device200by the lithography process, such as the lithography process described above to form patterned mask layer290. Patterned mask layer310covers p-type gate regions240-2of gate structures240. Patterned mask layer310includes a material that is different than a material of p-type work function layer300, a material of sacrificial features284,′ and a material of high-k dielectric layer282to achieve etching selectivity during removal of p-type work function layer300. For example, patterned mask layer310includes a resist material. In some embodiments, patterned mask layer310has a multi-layer structure, such as a resist layer disposed over an ARC layer. The present disclosure contemplates other materials for patterned mask layer310, so long as etching selectivity is achieved during removal of p-type work function layer300from n-type gate regions240-1.

Any suitable process is then used to completely remove p-type work function layer300from n-type gate regions240-1, thereby exposing high-k dielectric layer282in n-type gate regions240-1. In some embodiments, the etching process is a wet etching process that uses an etching solution having a high etching selectivity with respect to p-type work function layer300relative to high-k dielectric layer282and sacrificial features284′. In some embodiments, the etching solution exhibits an etching selectivity (i.e., a ratio of an etch rate of p-type work function layer300to the etching solution to an etch rate of high-k dielectric layer282(and/or sacrificial features284′) to the etching solution) of about 10 to about 100. In some embodiments, the etching selectivity is greater than or equal to 100. In some embodiments, the wet etching process implements a wet etching solution that includes NH4OH, HCl, and diazine (N2H2) (in other words, an NH4OH:HCl:N2H2solution). Parameters of the etching process are controlled (tuned) to remove p-type work function layer300with minimal (to no) etching of high-k dielectric layer282and/or sacrificial features284′, such as etching temperature, etching solution concentration, etching time, other suitable wet etching parameters, or combinations thereof. In some embodiments, the wet etching solution includes NH4OH, hydrogen peroxide (H2O2), sulfuric acid (H2SO4), tetramethylammonium hydroxide (TMAH), HCl, other suitable wet etching solution, or combinations thereof. For example, the wet etching solution can utilize an NH4OH:H2O2solution, an HCl:H2O2:H2O solution (known as an hydrochloric-peroxide mixture (HPM)), an NH4OH:H2O2:H2O solution (known as an ammonia-peroxide mixture (APM)), or an H2SO4:H2O2solution (known as a sulfuric peroxide mixture (SPM)). In some embodiments, a dry etching process or combination of a dry etching process and a wet etching process is implemented for removing p-type work function layer300. After the etching process, the patterned mask layer310can be removed from multigate device200, for example, by a resist stripping process or other suitable process.

In some embodiments, the etching process is not an over etching process, which generally refers to an etching process that is performed for a longer time than a required, expected etching time to remove a given material. In conventional GAA gate replacement processes, an over etching process is often required to completely remove p-type work function layer300from n-type gate regions240-1, particularly from between channel layers215′ and between channel layers215′ and substrate202. However, it has been observed that the over etching process may undesirably laterally etch a portion of the p-type work function layer300in p-type gate regions240-2underneath patterned mask layer310at a boundary315between n-type gate regions240-2and p-type gate regions240-2(often referred to as an n/p boundary or mixed threshold voltage boundary). One solution to mitigate the metal gate lateral loss at boundary315is to limit a thickness of the p-type work function layer300, which limits threshold voltage tuning of the p-type GAA transistors. The proposed GAA gate replacement process overcomes such problems by forming sacrificial features284′ between channel layers215′ and between channel layers215′ and substrate202in n-type gate regions240-1before forming p-type work function layers of the gate electrodes. This eliminates the need for an over etching process to remove p-type work function layers from between channel layers215′ and between channel layers215′ and substrate202in n-type gate regions240-2, such as p-type work function layer300, and allows for thicker p-type work function layers in p-type gate regions240-2, increasing threshold voltage tuning flexibility for p-type GAA transistors and avoiding unintended increases in the threshold voltages of p-type GAA transistors. For example, in some embodiments, the etching time of the etching process for removing p-type work function layer300is not configured to ensure removal of p-type work function material between channel layers215′ and between channel layers215′ and substrate202in n-type gate regions240-1. Different embodiments may have different advantages, and no particular advantage is necessarily required of any embodiment.

The processes described with reference toFIGS.15A-15DandFIGS.16A-16Dcan be referred to as a p-type work function layer deposition/etch cycle, which the present disclosure proposes repeating until the gate dielectric and the p-type work function layer completely fill the gate trenches275along the gate length direction in p-type gate regions240-2of gate structures240. For example, inFIG.15DandFIG.16D, gate trenches275in p-type gate regions240-2are not completely filled along the gate length direction by interfacial layer282, high-k dielectric layer284, and p-type work function layer300. Additional p-type work function layer deposition/etch cycles are therefore performed until gate trenches275in p-type gate regions240-2are completely filled along the gate length direction by interfacial layer282, high-k dielectric layer284, and a p-type work function layer (including p-type work function layer300). In some embodiments, p-type work function layer deposition/etch cycles are performed until a thickness T of the combined p-type work function layers is greater than or equal to about half of the gate length (i.e., T≥Lg), where thickness T is defined between sidewalls of high-k dielectric layer282and sidewalls of the p-type work function layer (both of which are disposed along sidewalls of channel layers215′). Completely filling gate trenches275in p-type gate regions240-1along the gate length direction ensures that subsequently formed n-type work function layers are formed above the gate structures240(in particular, over gate spacers247) along the gate length direction, such that p-type gate regions240-2do not include remnants of n-type work function layers along the gate length direction, which can adversely alter desired threshold voltages of p-type GAA transistors of p-type gate regions240-2. Different embodiments may have different advantages, and no particular advantage is necessarily required of any embodiment.

Turning toFIGS.17A-17D, a p-type work function layer320is formed over multigate device200, particularly over high-k dielectric layer282in n-type gate regions240-1of gate structures240and over p-type work function layer300in p-type gate regions240-2of gate structures240. For example, an ALD process conformally deposits p-type work function layer320on high-k dielectric layer282and p-type work function layer300, such that p-type work function layer320has a substantially uniform thickness and partially fills gate trenches275along the gate length direction in n-type gate regions240-1and completely fills any remaining portions of gate trenches along the gate length direction in p-type gate regions240-2. In p-type gate regions240-2, p-type work function layer320is disposed on p-type work function layer300and surrounds p-type work function layer300, high-k dielectric layer282, interfacial layer280, and channel layers215′. For example, p-type work function layer320is disposed along sidewalls, tops, and bottoms of channel layers215′. A thickness of p-type work function layer320is configured to fill any remaining portions of gaps277B between channel layers215′ and between channel layers215′ and substrate202. In some embodiments, p-type work function layer320has a thickness of about 1 nm to about 10 nm. In the depicted embodiment, thickness T (i.e., a sum of a thickness of p-type work function layer300and a thickness of p-type work function layer320) is greater than or equal to about half of the gate length (i.e., T≥Lg). In some embodiments, thickness T is about 2 nm to about 20 nm. In contrast, in n-type gate regions240-1of gate structures240, p-type work function layer320is disposed along sidewalls of channel layer215′ and the top surface of the topmost channel layer215′. Sacrificial features284′ thus again function as deposition stop layers, preventing significant deposition of p-type work function layer320between channel layers215′ and between channel layers215′ and substrate202in n-type gate regions240-1. In the depicted embodiment, p-type work function layer320is disposed along sidewalls of sacrificial features284′. P-type work function layer320includes any suitable p-type work function material, such as TiN, TaN, TaSN, Ru, Mo, Al, WN, WCN ZrSi2, MoSi2, TaSi2, NiSi2, other p-type work function material, or combinations thereof. In the depicted embodiment, p-type work function layer320includes titanium and nitrogen, such as TiN. P-type work function layer320can be formed using another suitable deposition process, such as CVD, PVD, HDPCVD, MOCVD, RPCVD, PECVD, LPCVD, ALCVD, APCVD, spin coating, plating, other deposition process, or combinations thereof.

Turning toFIGS.18A-18D, p-type work function layer320is removed from n-type gate regions240-1of gate structures240, for example, by implementing a lithography process and an etching process. For example, a patterned mask layer330having one or more openings332is formed over multigate device200by the lithography process, such as the lithography process described above to form patterned mask layer290. Patterned mask layer330covers p-type gate regions240-2of gate structures240. Patterned mask layer330includes a material that is different than a material of p-type work function layer320, a material of sacrificial features284,′ and a material of high-k dielectric layer282to achieve etching selectivity during removal of p-type work function layer320. For example, patterned mask layer330includes a resist material. In some embodiments, patterned mask layer330has a multi-layer structure, such as a resist layer disposed over an ARC layer. The present disclosure contemplates other materials for patterned mask layer330, so long as etching selectivity is achieved during removal of p-type work function layer320from n-type gate regions240-1. Any suitable process, such as the etching processes described above for removing p-type work function layer300, is then used to completely remove p-type work function layer320from n-type gate regions240-1, thereby exposing high-k dielectric layer282in n-type gate regions240-1. In some embodiments, patterned mask layer330may intentionally or unintentionally (e.g., resulting from overlay shift associated with the lithography process) cover a portion of n-type gate regions240-1at boundary315as depicted inFIG.18B. This results in the p-type work function layer having different thicknesses (e.g., along the z-direction) at boundary315, such as a thickness t3and a thickness t4. In some embodiments, a difference between thickness t4(e.g., a sum of the thickness of p-type work function layer300and the thickness of p-type work function layer320) and thickness t3(substantially equal to the thickness of p-type work function layer300) is greater than or equal to about 1 nm. After the etching process, the patterned mask layer330can be removed from multigate device200, for example, by a resist stripping process or other suitable process.

Turning toFIGS.19A-19D, sacrificial features284′ are removed from n-type gate regions240-1of gate structures240by implementing an etching process. In the depicted embodiment sacrificial features284′ are removed from between channel layers215′ and between channel layers215′ and substrate202, thereby exposing high-k dielectric layer282in n-type gate regions240-1. The etching process essentially re-forms a portion of gaps277A in in n-type gate regions240-1. In some embodiments, the etching process is a wet etching process that uses an etching solution having a high etching selectivity with respect to sacrificial features284′ relative to high-k dielectric layer282and p-type work function layer330. In some embodiments, the etching solution exhibits an etching selectivity of about 10 to about 100. In some embodiments, the etching selectivity is greater than or equal to 100. In some embodiments, the wet etching process implements an NH4OH-based wet etching solution. Parameters of the etching process are controlled to ensure complete removal of sacrificial features284′ in n-type gate regions240-1, such as etching temperature, etching solution concentration, etching time, other suitable wet etching parameters, or combinations thereof. For example, an etching time (i.e., how long sacrificial features284′ are exposed to the ammonia-based wet etching solution) is tuned to completely remove sacrificial features284′ with minimal (to no) etching of high-k dielectric layer282and p-type work function layer330. In some embodiments, an etching time for completely removing sacrificial features284′ is greater than an etching time for patterning sacrificial layer284into sacrificial features284′. In some embodiments, an etching time for completely removing sacrificial features284′ from between channel layers215′ and from between channel layers215′ and substrate202is greater than an etching time that would be required to remove a material of sacrificial features284′ from over portions of the gate dielectric layer disposed over ILD layer270, gate spacers247, and/or isolation features230. The etching process can thus be configured as an over etching process to ensure complete removal of sacrificial features248. Etching solutions implemented for the over etching process are configured to have high etching selectivity to sacrificial features284′ relative to p-type work function layers300,320to reduce (or eliminate) concerns of lateral loss of p-type work function layers300,320in p-type gate regions240-2at boundary315. In some embodiments, the etching solution further has an etching selectivity with respect to sacrificial features284′ relative to a patterned mask layer. In some embodiments, the etching process partially removes (etches) patterned mask layer. In some embodiments, patterned mask layer330remains over p-type gate regions240-2during removal of sacrificial features284′ from n-type gate regions240-1, and the etching process then uses patterned mask layer330as an etch mask. In such embodiments, after the etching process, the patterned mask layer330can be removed, for example, by a resist stripping process or other suitable process.

Turning toFIGS.20A-20D, an n-type work function layer340is formed over multigate device200, particularly over high-k dielectric layer282in n-type gate regions240-1of gate structures240and over p-type work function layer320in p-type gate regions240-2of gate structures240. For example, an ALD process conformally deposits n-type work function layer340on high-k dielectric layer282and p-type work function layer320, such that n-type work function layer340has a substantially uniform thickness and partially fills gate trenches275along the gate length direction in n-type gate regions240-1. In n-type gate regions240-1, n-type work function layer340is disposed on high-k dielectric layer282and surrounds n-type work function layer340, high-k dielectric layer282, interfacial layer280, and channel layers215′. For example, n-type work function layer340is disposed along sidewalls, tops, and bottoms of channel layers215′. In contrast, in p-type gate regions240-2, because the disclosed gate replacement process ensures that gate trenches275are filled (e.g., the gate dielectric (i.e., high-k dielectric layer282and interfacial layer280) and the p-type work function layer (i.e., p-type work function layer300and p-type work function layer320)) along the gate length direction in p-type gate regions240-2, n-type work function layer340is formed over gate structures240in p-type gate regions240-2, in particular, over gate spacers247. This prevents n-type work function layer340or any remnants (residue) thereof from affecting the threshold voltage of the p-type GAA transistors, particularly for short-channel p-type GAA transistors. In the depicted embodiment, a thickness of n-type work function layer340completely fills remaining portions of gaps277B between channel layers215′ and between channel layers215′ and substrate202in n-type gate regions240-1. In some embodiments, n-type work function layer340has a thickness of about 1 nm to about 5 nm. In some embodiments, a thickness of n-type work function layer340is less than a thickness of the p-type work function layer (i.e., a combined thickness of p-type work function layer300and p-type work function layer320). N-type work function layer340includes any suitable n-type work function material, such as Ti, Al, Ag, Mn, Zr, TiAl, TiAlC, TiAlSiC, TaC, TaCN, TaSiN, TaAl, TaAlC, TaSiAlC, TiAlN, other n-type work function material, or combinations thereof. In the depicted embodiment, n-type work function layer340includes aluminum. For example, n-type work function layer340includes titanium and aluminum, such as TiAl, TiAlC, TaSiAl, or TiSiAlC. Alternatively, n-type work function layer340is formed using another suitable deposition process, such as CVD, PVD, HDPCVD, MOCVD, RPCVD, PECVD, LPCVD, ALCVD, APCVD, spin coating, plating, other deposition process, or combinations thereof.

Turning toFIGS.21A-21D, n-type work function layer340is removed from p-type gate regions240-2of gate structures240, for example, by implementing a lithography process and an etching process. For example, a patterned mask layer345having one or more openings347is formed over multigate device200by the lithography process, such as the lithography process described above to form patterned mask layer290. Patterned mask layer345covers n-type gate regions240-1of gate structures240. Patterned mask layer345includes a material that is different than a material of n-type work function layer340to achieve etching selectivity during removal of n-type work function layer340. For example, patterned mask layer345includes a resist material. In some embodiments, patterned mask layer345has a multi-layer structure, such as a resist layer disposed over an ARC layer. The present disclosure contemplates other materials for patterned mask layer345, so long as etching selectivity is achieved during removal of n-type work function layer340from n-type gate regions240-1. Any suitable process, such as the etching processes described herein, is then used to completely remove n-type work function layer340from p-type gate regions240-2, thereby exposing p-type work function layer320in p-type gate regions240-2. After the etching process, the patterned mask layer345can be removed from multigate device200, for example, by a resist stripping process or other suitable process.

Turning toFIGS.22A-22D, a metal fill (or bulk) layer350is formed over multigate device200, particularly over n-type work function layer340in n-type gate regions240-1and over p-type work function layer320in p-type gate regions240-2. For example, a CVD process or a PVD process deposits metal fill layer350on n-type work function layer340and p-type work function layer320, such that metal fill layer350fills any remaining portion of gate trenches275, including any remaining portions of gaps277A in n-type gate regions240-1. Metal fill layer350includes a suitable conductive material, such as Al, W, and/or Cu. Metal fill layer350may additionally or collectively include other metals, metal oxides, metal nitrides, other suitable materials, or combinations thereof. In some implementations, a blocking layer is optionally formed over n-type work function layer340and p-type work function layer320before forming metal fill layer350, such that metal fill layer350is disposed on the blocking layer. For example, an ALD process conformally deposits the blocking layer on n-type work function layer340and p-type work function layer320, such that the blocking layer has a substantially uniform thickness and partially fills gate trenches275. The blocking layer includes a material that blocks and/or reduces diffusion between gate layers, such as metal fill layer350and n-type work function layer340and/or p-type work function layers300,320. Alternatively, metal fill layer350and/or the blocking layer are formed using another suitable deposition process, such as ALD, CVD, PVD, HDPCVD, MOCVD, RPCVD, PECVD, LPCVD, ALCVD, APCVD, spin coating, plating, other deposition process, or combinations thereof.

Turning toFIGS.23A-23D, a planarization process is performed to remove excess gate materials from multigate200. For example, a CMP process is performed until a top surface of ILD layer270is reached (exposed), such that a top surface of gate structures240are substantially planar with a top surface of ILD layer270after the CMP process. In the depicted embodiment, gate structures240are thus configured with two different metal gate portions—n-metal gates360A in n-type gate regions240-1and p-metal gates360B in p-type gate regions240-2. Metal gates360A include a gate dielectric (e.g., interfacial layer280and high-k dielectric layer282) and a gate electrode (e.g., n-type work function layer340and metal fill layer350). Metal gates360B include a gate dielectric (e.g., interfacial layer280and high-k dielectric layer282) and a gate electrode (e.g., a p-type work function layer362(e.g., p-type work function layer300and p-type work function layer320) and metal fill layer350). Accordingly, multigate device200includes n-type GAA transistors having metal gates360A wrapping respective channel layers215′, such that metal gates360A are disposed between respective epitaxial source/drain features260A, and p-type GAA transistors having metal gates360B wrapping respective channel layers215′, such that metal gates360B are disposed between respective epitaxial source/drain features260B.

Fabrication can proceed to continue fabrication of multigate device200. For example, various contacts can be formed to facilitate operation of the n-type GAA transistors and the p-type GAA transistors. For example, one or more ILD layers, similar to ILD layer270, and/or CESL layers can be formed over substrate202(in particular, over ILD layer270and gate structures240). Contacts can then be formed in ILD layer270and/or ILD layers disposed over ILD layer270. For example, contacts are respectively electrically and/or physically coupled with gate structures240and contacts are respectively electrically and/or physically coupled to source/drain regions of the n-type GAA transistors and the p-type GAA transistors (particularly, epitaxial source/drain features260A,260B). Contacts include a conductive material, such as metal. Metals include aluminum, aluminum alloy (such as aluminum/silicon/copper alloy), copper, copper alloy, titanium, titanium nitride, tantalum, tantalum nitride, tungsten, polysilicon, metal silicide, other suitable metals, or combinations thereof. The metal silicide may include nickel silicide, cobalt silicide, tungsten silicide, tantalum silicide, titanium silicide, platinum silicide, erbium silicide, palladium silicide, or combinations thereof. In some implementations, ILD layers disposed over ILD layer270and the contacts (for example, extending through ILD layer270and/or the other ILD layers) are a portion of the MLI feature described above.

The present disclosure provides for many different embodiments. An exemplary method forming a gate dielectric layer in a gate trench in a gate structure. The gate dielectric layer is formed around first channel layers in a p-type gate region and around second channel layers in an n-type gate region. The method further includes forming a p-type work function layer in the gate trench over the gate dielectric layer in the p-type gate region and the n-type gate region. In some embodiments, the p-type work function layer completely fills the gate trench along a gate length direction. Sacrificial features are formed between the second channel layers in the n-type gate region before forming the p-type work function layer and the sacrificial features are removed between the second channel layers in the n-type gate region after removing the p-type work function layer from the gate trench in the n-type gate region. In some embodiments, removing the sacrificial features between the second channel layers in the n-type gate region includes performing an etching process that selectively etches the sacrificial features without substantially etching the p-type work function layer. In some embodiments, p-type work function layer is removed from the gate trench in the n-type gate region by performing an etching process that selectively etches the p-type work function layer without substantially etching the gate dielectric layer and the sacrificial features. The method further includes forming an n-type work function layer in the gate trench over the gate dielectric layer in the n-type gate region. The n-type work function layer surrounds the gate dielectric layer and the second channel layers in the n-type gate region. The method further includes forming a metal fill layer in the gate trench over the p-type work function layer in the p-type gate region and the n-type work function layer in the n-type gate region. In some embodiments, the n-type work function layer is also formed in the gate trench over the p-type work function layer in the p-type gate region. In such embodiments, the n-type work function layer is disposed above gate spacers of the gate structure and the gate trench is free of the n-type work function layer along a gate length of the gate trench in the p-type gate region. In such embodiments, the n-type work function layer is removed from the gate trench in the p-type gate region before forming the metal fill layer.

In some embodiments, forming the sacrificial features in the gate trench between the second channel layers in the n-type gate region include forming a sacrificial layer over the gate dielectric layer in the gate trench. The sacrificial layer and the gate dielectric layer fill gaps between the first channel layers in the p-type gate region and the gate dielectric layer and the sacrificial layer fill gaps between the second channel layers in the n-type gate region. The sacrificial layer is then patterned to form sacrificial features between the first channel layers in the p-type gate region and between the second channel layers in the n-type gate region. The sacrificial features are removed from between the first channel layers in the p-type gate region. In some embodiments, removing the sacrificial features from between the first channel layers in the p-type gate region includes performing a lithography process to form a patterned mask layer that covers the n-type gate region and exposes the p-type gate region, and performing an etching process to remove the sacrificial features in the p-type gate region. The etching process can use the patterned mask layer as an etch mask.

In some embodiments, forming the p-type work function layer includes forming a first p-type work function layer in the gate trench over the gate dielectric layer in the p-type gate region and the n-type gate region, and forming a second p-type work function layer in the gate trench over the first p-type work function layer in the p-type gate region and over the gate dielectric layer in the n-type gate region. In such embodiments, removing the p-type work function layer from the gate trench in the n-type gate region includes removing the first p-type work function layer from the gate trench in the n-type gate region before forming the second p-type work function layer and removing the second p-type work function layer from the gate trench in the n-type gate region. In some embodiments, the first p-type work function layer is removed from the gate trench in the n-type gate region by performing a first lithography process and a first etching process, where the first lithography process forms a mask layer that covers the p-type gate region. In some embodiments, the second p-type work function layer is removed from the gate trench in the n-type gate region by performing a second lithography process and a second etching process, where the second lithography process forms a mask layer that covers the p-type gate region and a portion of the n-type gate region at a boundary of the p-type gate region and the n-type gate region.

Another exemplary method includes depositing a gate dielectric layer over first semiconductor layers in a p-type gate region and over second semiconductor layers in an n-type gate region, wherein the gate dielectric layer wraps the first semiconductor layers and the second semiconductor layers. The first semiconductor layers have first gaps therebetween and the second semiconductor layers have second gaps therebetween after depositing the gate dielectric layer. The method further includes depositing a sacrificial layer over the gate dielectric layer in the p-type gate region and the n-type gate region and etching the sacrificial layer in the p-type gate region and the n-type gate region to form first sacrificial features in the p-type gate region between the first semiconductor layers and second sacrificial features in the n-type gate region between the second semiconductor layers. The method further includes, after removing the first sacrificial features from the p-type gate region, depositing a p-type work function layer over the gate dielectric layer in the p-type gate region and the n-type gate region. In some embodiments, a material of the sacrificial layer is different than a material of the p-type work function layer. In some embodiments, a thickness (T) of the p-type work function layer is greater than or equal to half a gate length (Lg) (T≥0.5 Lg). The method further includes removing the p-type work function layer from over the gate dielectric layer in the n-type gate region, and after removing the second sacrificial features from the n-type gate region, depositing an n-type work function layer over the gate dielectric layer in the n-type gate region.

In some embodiments, etching the sacrificial layer, removing the first sacrificial features from the p-type gate region, and removing the second sacrificial features from the n-type gate region each include performing a wet etching process that selectively etches the sacrificial layer without substantially etching the gate dielectric layer. The wet etching process may utilize an etching ammonium hydroxide based etching solution. An etching time of the wet etching process for etching the sacrificial layer is less than an etching time of the wet etching process for removing the first sacrificial features from the p-type gate region and removing the second sacrificial features from the n-type gate region. In some embodiments, the etching the p-type work function layer includes performing a wet etching process that selectively etches the p-type work function layer without substantially etching the gate dielectric layer. The wet etching process can utilize an ammonium hydroxide/hydrogen chloride/diazine based etching solution.

An exemplary multigate device includes first channel layers disposed in a p-type gate region over a substrate and second channel layers disposed in an n-type gate region over the substrate. A gate stack spans the p-type gate region and the n-type gate region. The gate stack is disposed between first epitaxial source/drain features disposed in the p-type gate region and second epitaxial source/drain features disposed in the n-type gate region. The gate stack includes a p-metal gate in the p-type gate region that surrounds the first channel layers. The p-metal gate includes a gate dielectric layer, a p-type work function layer disposed over the gate dielectric layer, and a metal fill layer disposed over the p-type work function layer. The gate stack further includes an n-metal gate in the n-type gate region that surrounds the second channel layers. The n-metal gate includes the gate dielectric layer, an n-type work function layer disposed over the gate dielectric layer, and the metal fill layer disposed over the n-type work function layer. A thickness (T) of the p-type work function layer is greater than or equal to half a gate length (Lg) of the gate stack (T≥0.5 Lg). In some embodiments, the p-type work function layer has a first thickness and a second thickness in a boundary region between the p-metal gate and the n-metal gate, where a difference between the first thickness and the second thickness is greater than or equal to about 1 nm. In some embodiments, the thickness of the p-type work function layer is greater than a thickness of the n-type work function layer.

The foregoing outlines features of several embodiments so that those skilled in the art may better understand the aspects of the present disclosure. Those skilled in the art should appreciate that they may readily use the present disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiments introduced herein. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the present disclosure, and that they may make various changes, substitutions, and alterations herein without departing from the spirit and scope of the present disclosure.