Patent Publication Number: US-11652155-B2

Title: Air spacer and method of forming same

Description:
PRIORITY CLAIM AND CROSS-REFERENCE 
     This application is a continuation of U.S. patent application Ser. No. 16/917,577, filed on Jun. 30, 2020, entitled “Air Spacer and Method of Forming Same,” which application is hereby incorporated herein by reference. 
    
    
     BACKGROUND 
     Semiconductor devices are used in a variety of electronic applications, such as, for example, personal computers, cell phones, digital cameras, and other electronic equipment. Semiconductor devices are typically fabricated by sequentially depositing insulating or dielectric layers, conductive layers, and semiconductor layers of material over a semiconductor substrate, and patterning the various material layers using lithography to form circuit components and elements thereon. 
     The semiconductor industry continues to improve the integration density of various electronic components (e.g., transistors, diodes, resistors, capacitors, etc.) by continual reductions in minimum feature size, which allow more components to be integrated into a given area. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Aspects of the present disclosure are understood from the following detailed description when read with the accompanying figures. It is noted that, in accordance with the standard practice in the industry, various features are not drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion. 
         FIG.  1    illustrates an example of a Fin field-effect transistor (FinFET) in a three-dimensional view of an intermediate stage, in accordance with some embodiments. 
         FIGS.  2 ,  3 ,  4 ,  5 ,  6 ,  7 ,  8 A,  8 B,  8 C,  8 D,  9 A,  9 B,  9 C,  9 D,  10 A,  10 B,  10 C,  10 D,  11 A,  11 B,  11 C ,  11 D,  12 A,  12 B,  12 C,  12 D,  13 A,  13 B,  13 C,  13 D,  14 A,  14 B,  14 C,  14 D,  15 A,  15 B,  15 C,  15 D,  16 A,  16 B,  16 C,  16 D,  17 A,  17 B,  17 C,  17 D,  18 A,  18 B,  18 C,  18 D,  19 A,  19 B,  19 C,  19 D,  20 A,  20 B,  20 C,  20 D,  21 A,  21 B,  21 C,  21 D,  21 E,  22 A,  22 B,  22 C,  22 D,  23 A,  23 B,  23 C,  23 D,  24 A,  24 B,  24 C,  24 D, and  24 E are cross-sectional views of intermediate stages in the manufacturing of FinFETs, in accordance with some embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     The following disclosure provides many different embodiments, or examples, for implementing different features of the invention. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. For example, the formation of a first feature over or on a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features may be formed between the first and second features, such that the first and second features may not be in direct contact. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed. 
     Further, spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “upper” and the like, may be used herein for ease of description to describe one element or feature&#39;s relationship to another element(s) or feature(s) as illustrated in the figures. The spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. The apparatus may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein may likewise be interpreted accordingly. 
     Embodiments will now be described with respect to particular examples including integrated circuits comprising transistors, such as Fin field-effect transistor (FinFET) devices. However, embodiments are not limited to the examples provided herein, and embodiments may be implemented in a wide array of applications. As discussed in greater detail below, spacers may be formed adjacent to the gate electrode of a FinFET device to provide insulation and improve the performance of the device. The spacers may be subsequently removed, thereby replacing the spacers with air spacers or air gaps to provide better insulation than a deposited material layer. 
       FIG.  1    illustrates an example of an intermediate stage in the formation of a FinFET in a three-dimensional view, in accordance with some embodiments. The FinFET comprises a fin  52  on a substrate  50  (e.g., a semiconductor substrate). Isolation regions  56  are disposed in the substrate  50 , and the fin  52  protrudes above and from between neighboring isolation regions  56 . Although the isolation regions  56  are described/illustrated as being separate from the substrate  50 , as used herein the term “substrate” may be used to refer to just the semiconductor substrate or a semiconductor substrate inclusive of isolation regions. Additionally, although the fin  52  is illustrated as a single, continuous material as the substrate  50 , the fin  52  and/or the substrate  50  may comprise a single material or a plurality of materials. In this context, the fin  52  refers to the portion extending between and protruding above the neighboring isolation regions  56 . 
     A dummy dielectric  66  is along sidewalls and over a top surface of the fin  52 , and a dummy gate  68  and a mask  70  are over the dummy dielectric  66 . Collectively, the dummy dielectric  66 , the dummy gate  68 , and the mask  70  form a dummy gate stack  72 . Source/drain regions  96  are disposed in opposite sides of the fin  52  with respect to the dummy dielectric  66  and dummy gate  68 .  FIG.  1    further illustrates reference cross-sections that are used in later figures. Cross-section A-A is along a longitudinal axis of the dummy gate  68  and in a direction, for example, perpendicular to the direction of current flow between the source/drain regions  96  of the FinFET. Cross-section B-B is parallel to cross-section A-A and extends through a source/drain region of the FinFET. Cross-section C-C is perpendicular to cross-sections A-A and B-B and is along a longitudinal axis of the fin  52  and in a direction of, for example, a current flow between the source/drain regions  96  of the FinFET. Cross-section D-D is parallel to cross-section C-C and extends through a shallow trench isolation (STI) region and perpendicularly through the longitudinal axis of the dummy gate  68  of the FinFET. Subsequent figures refer to these reference cross-sections for clarity. 
     Some embodiments discussed herein are discussed in the context of FinFETs formed using a gate-last process. In other embodiments, a gate-first process may be used. Also, some embodiments contemplate aspects used in planar devices, such as planar FETs, nanostructure (e.g., nanosheet, nanowire, gate-all-around, or the like) field effect transistors (NSFETs), or the like. 
       FIGS.  2  through  24 D  are cross-sectional views of intermediate stages in the manufacturing of FinFETs, in accordance with some embodiments.  FIGS.  2  through  7    and  FIGS.  8 A,  9 A,  10 A,  11 A,  12 A,  13 A,  14 A,  15 A,  16 A,  17 A,  18 A,  19 A,  20 A,  21 A,  22 A,  23 A, and  24 A  illustrate reference cross-section A-A illustrated in  FIG.  1   , except for multiple fins/FinFETs.  FIGS.  8 B,  9 B,  10 B,  11 B,  12 B,  13 B,  14 B,  15 B,  16 B,  17 B,  18 B,  19 B,  20 B,  21 B,  22 B,  23 B, and  24 B  illustrate reference cross-section B-B illustrated in  FIG.  1   , except for multiple fins/FinFETs.  FIGS.  8 C,  9 C,  10 C,  11 C,  12 C,  13 C,  14 C,  15 C,  16 C,  17 C,  18 C,  19 C,  20 C,  21 C,  22 C,  23 C, and  24 C  illustrate reference cross-section C-C illustrated in  FIG.  1   , except for multiple fins/FinFETs.  FIGS.  8 D,  9 D,  10 D,  11 D,  12 D,  13 D,  14 D,  15 D,  16 D,  17 D,  18 D,  19 D,  20 D,  21 D,  22 D,  23 D, and  24 D  illustrate reference cross-section D-D illustrated in  FIG.  1   , except for multiple fins/FinFETs.  FIGS.  21 E and  24 E  illustrate reference cross-section E-E illustrated in  FIGS.  21 B and  24 B , respectively. 
     In  FIG.  2   , a substrate  50  is provided. The substrate  50  may be a semiconductor substrate, such as a bulk semiconductor, a semiconductor-on-insulator (SOI) substrate, or the like, which may be doped (e.g., with a p-type or an n-type dopant) or undoped. The substrate  50  may be a wafer, such as a silicon wafer. Generally, an SOI substrate is a layer of a semiconductor material formed on an insulator layer. The insulator layer may be, for example, a buried oxide (BOX) layer, a silicon oxide layer, or the like. The insulator layer is provided on a substrate, typically a silicon or glass substrate. Other substrates, such as a multi-layered or gradient substrate may also be used. In some embodiments, the semiconductor material of the substrate  50  may include silicon; germanium; a compound semiconductor including silicon carbide, gallium arsenide, gallium phosphide, indium phosphide, indium arsenide, and/or indium antimonide; an alloy semiconductor including silicon-germanium, gallium arsenide phosphide, aluminum indium arsenide, aluminum gallium arsenide, gallium indium arsenide, gallium indium phosphide, and/or gallium indium arsenide phosphide; or combinations thereof. 
     The substrate  50  has an n-type region  50 N and a p-type region  50 P. The n-type region  50 N can be for forming n-type devices, such as NMOS transistors, e.g., n-type FinFETs. The p-type region  50 P can be for forming p-type devices, such as PMOS transistors, e.g., p-type FinFETs. The n-type region  50 N may be physically separated from the p-type region  50 P (as illustrated by divider  51 ), and any number of device features (e.g., other active devices, doped regions, isolation structures, etc.) may be disposed between the n-type region  50 N and the p-type region  50 P. 
     In  FIG.  3   , fins  52  are formed in the substrate  50 . The fins  52  are semiconductor strips. In some embodiments, the fins  52  may be formed in the substrate  50  by etching trenches in the substrate  50 . The etching may be any acceptable etch process, such as a reactive ion etch (RIE), neutral beam etch (NBE), the like, or a combination thereof. The etch may be anisotropic. 
     The fins may be patterned by any suitable method. For example, the fins  52  may be patterned using one or more photolithography processes, including double-patterning or multi-patterning processes. Generally, double-patterning or multi-patterning processes combine photolithography and self-aligned processes, allowing patterns to be created that have, for example, pitches smaller than what is otherwise obtainable using a single, direct photolithography process. For example, in one embodiment, a sacrificial layer is formed over a substrate and patterned using a photolithography process. Spacers are formed alongside the patterned sacrificial layer using a self-aligned process. The sacrificial layer is then removed, and the remaining spacers may then be used to pattern the fins. In some embodiments, the mask (or other layer) may remain on the fins  52 . 
     In  FIG.  4   , an insulation material  54  is formed over the substrate  50  and between neighboring fins  52 . The insulation material  54  may be an oxide, such as silicon oxide, a nitride, the like, or a combination thereof, and may be formed by a high density plasma chemical vapor deposition (HDP-CVD), a flowable CVD (FCVD) (e.g., a CVD-based material deposition in a remote plasma system and post curing to make it convert to another material, such as an oxide), the like, or a combination thereof. Other insulation materials formed by any acceptable process may be used. In the illustrated embodiment, the insulation material  54  is silicon oxide formed by a FCVD process. An anneal process may be performed once the insulation material is formed. In an embodiment, the insulation material  54  is formed such that excess insulation material  54  covers the fins  52 . Although the insulation material  54  is illustrated as a single layer, some embodiments may utilize multiple layers. For example, in some embodiments a liner (not shown) may first be formed along a surface of the substrate  50  and the fins  52 . Thereafter, a fill material, such as those discussed above may be formed over the liner. 
     In  FIG.  5   , a removal process is applied to the insulation material  54  to remove excess insulation material  54  over the fins  52 . In some embodiments, a planarization process such as a chemical mechanical polish (CMP), an etch-back process, combinations thereof, or the like may be utilized. The planarization process exposes the fins  52  such that top surfaces of the fins  52  and the insulation material  54  are level after the planarization process is complete. In embodiments in which a mask remains on the fins  52 , the planarization process may expose the mask or remove the mask such that top surfaces of the mask or the fins  52 , respectively, and the insulation material  54  are level after the planarization process is complete. 
     In  FIG.  6   , the insulation material  54  is recessed to form Shallow Trench Isolation (STI) regions  56 . The insulation material  54  is recessed such that upper portions of fins  52  in the n-type region  50 N and in the p-type region  50 P protrude from between neighboring STI regions  56 . Further, the top surfaces of the STI regions  56  may have a flat surface as illustrated, a convex surface, a concave surface (such as dishing), or a combination thereof. The top surfaces of the STI regions  56  may be formed flat, convex, and/or concave by an appropriate etch. The STI regions  56  may be recessed using an acceptable etching process, such as one that is selective to the material of the insulation material  54  (e.g., etches the material of the insulation material  54  at a faster rate than the material of the fins  52 ). For example, an oxide removal using, for example, dilute hydrofluoric (dHF) acid may be used. 
     The process described with respect to  FIGS.  2  through  6    is just one example of how the fins  52  may be formed. In some embodiments, the fins may be formed by an epitaxial growth process. For example, a dielectric layer can be formed over a top surface of the substrate  50 , and trenches can be etched through the dielectric layer to expose the underlying substrate  50 . Homoepitaxial structures can be epitaxially grown in the trenches, and the dielectric layer can be recessed such that the homoepitaxial structures protrude from the dielectric layer to form fins. Additionally, in some embodiments, heteroepitaxial structures can be used for the fins  52 . For example, the fins  52  in  FIG.  5    can be recessed, and a material different from the fins  52  may be epitaxially grown over the recessed fins  52 . In such embodiments, the fins  52  comprise the recessed material as well as the epitaxially grown material disposed over the recessed material. In an even further embodiment, a dielectric layer can be formed over a top surface of the substrate  50 , and trenches can be etched through the dielectric layer. Heteroepitaxial structures can then be epitaxially grown in the trenches using a material different from the substrate  50 , and the dielectric layer can be recessed such that the heteroepitaxial structures protrude from the dielectric layer to form the fins  52 . In some embodiments where homoepitaxial or heteroepitaxial structures are epitaxially grown, the epitaxially grown materials may be in situ doped during growth, which may obviate prior and subsequent implantations although in situ and implantation doping may be used together. 
     Still further, it may be advantageous to epitaxially grow a material in n-type region  50 N (e.g., an NMOS region) different from the material in p-type region  50 P (e.g., a PMOS region). In various embodiments, upper portions of the fins  52  may be formed from silicon-germanium (Si x Ge 1−x , where x can be in the range of 0 to 1), silicon carbide, pure or substantially pure germanium, a III-V compound semiconductor, a II-VI compound semiconductor, or the like. For example, the available materials for forming III-V compound semiconductor include, but are not limited to, indium arsenide, aluminum arsenide, gallium arsenide, indium phosphide, gallium nitride, indium gallium arsenide, indium aluminum arsenide, gallium antimonide, aluminum antimonide, aluminum phosphide, gallium phosphide, and the like. 
     Further in  FIG.  6   , appropriate wells (not shown) may be formed in the fins  52  and/or the substrate  50 . In some embodiments, a P well may be formed in the n-type region  50 N, and an N well may be formed in the p-type region  50 P. In some embodiments, a P well or an N well are formed in both the n-type region  50 N and the p-type region  50 P. 
     In the embodiments with different well types, the different implant steps for the n-type region  50 N and the p-type region  50 P may be achieved using a photoresist and/or other masks (not shown). For example, a photoresist may be formed over the fins  52  and the STI regions  56  in the n-type region  50 N. The photoresist is patterned to expose the p-type region  50 P of the substrate  50 . The photoresist can be formed by using a spin-on technique and can be patterned using acceptable photolithography techniques. Once the photoresist is patterned, an n-type impurity implant is performed in the p-type region  50 P, and the photoresist may act as a mask to substantially prevent n-type impurities from being implanted into the n-type region  50 N. The n-type impurities may be phosphorus, arsenic, antimony, or the like implanted in the region to a concentration of equal to or less than 10 18  cm −3 , such as between about 10 16  cm −3  and about 10 18  cm −3 . After the implant, the photoresist is removed, such as by an acceptable ashing process. 
     Following the implanting of the p-type region  50 P, a photoresist is formed over the fins  52  and the STI regions  56  in the p-type region  50 P. The photoresist is patterned to expose the n-type region  50 N of the substrate  50 . The photoresist can be formed by using a spin-on technique and can be patterned using acceptable photolithography techniques. Once the photoresist is patterned, a p-type impurity implant may be performed in the n-type region  50 N, and the photoresist may act as a mask to substantially prevent p-type impurities from being implanted into the p-type region  50 P. The p-type impurities may be boron, boron fluoride, indium, or the like implanted in the region to a concentration of equal to or less than 10 18  cm −3 , such as between about 10 16  cm −3  and about 10 18  cm −3 . After the implant, the photoresist may be removed, such as by an acceptable ashing process. 
     After the implants of the n-type region  50 N and the p-type region  50 P, an anneal may be performed to repair implant damage and to activate the p-type and/or n-type impurities that were implanted. In some embodiments, the grown materials of epitaxial fins may be in situ doped during growth, which may obviate the implantations, although in situ and implantation doping may be used together. 
     In  FIG.  7   , a dummy dielectric layer  60  is formed on the fins  52 . The dummy dielectric layer  60  may be, for example, silicon oxide, silicon nitride, a combination thereof, or the like, and may be deposited or thermally grown according to acceptable techniques. A dummy gate layer  62  is formed over the dummy dielectric layer  60 , and a mask layer  64  is formed over the dummy gate layer  62 . The dummy gate layer  62  may be deposited over the dummy dielectric layer  60  and then planarized, such as by a CMP. The mask layer  64  may be deposited over the dummy gate layer  62 . The dummy gate layer  62  may be a conductive or non-conductive material and may be selected from a group including amorphous silicon, polycrystalline-silicon (polysilicon), poly-crystalline silicon-germanium (poly-SiGe), metallic nitrides, metallic silicides, metallic oxides, and metals. The dummy gate layer  62  may be deposited by physical vapor deposition (PVD), CVD, sputter deposition, or other techniques for depositing the selected material. The dummy gate layer  62  may be made of other materials that have a high etching selectivity from the etching of isolation regions, e.g., the STI regions  56  and/or the dummy dielectric layer  60 . The mask layer  64  may include one or more layers of, for example, silicon nitride, silicon oxynitride, or the like. In this example, a single dummy gate layer  62  and a single mask layer  64  are formed across the n-type region  50 N and the p-type region  50 P. It is noted that the dummy dielectric layer  60  is shown covering only the fins  52  for illustrative purposes only. In some embodiments, the dummy dielectric layer  60  may be deposited such that the dummy dielectric layer  60  covers the STI regions  56 , extending over the STI regions and between the dummy gate layer  62  and the STI regions  56 . 
       FIGS.  8 A through  24 D  illustrate various additional steps in the manufacturing of embodiment devices.  FIGS.  8 A through  24 D  illustrate features in either of the n-type region  50 N, the p-type region  50 P, or both (note separately illustrated). For example, the structures illustrated in  FIGS.  8 A through  24 D  may be applicable to both the n-type region  50 N and the p-type region  50 P. Differences (if any) in the structures of the n-type region  50 N and the p-type region  50 P are described in the text accompanying each figure. 
     In  FIGS.  8 A- 8 D , the mask layer  64  (see  FIG.  7   ) may be patterned using acceptable photolithography and etching techniques to form masks  70 . The pattern of the masks  70  may then be transferred to the dummy gate layer  62  to form dummy gates  68 . In some embodiments, the pattern of the masks  70  may also be transferred to the dummy dielectric layer  60  by an acceptable etching technique to form dummy dielectrics  66 . Collectively, each dummy gate stack  72  comprises the dummy dielectric  66 , the dummy gate  68 , and the mask  70 . The dummy gate stacks  72  cover respective channel regions of the fins  52 . The pattern of the masks  70  may be used to physically separate each of the dummy gate stacks  72  from adjacent dummy gate stacks  72 . The dummy gate stacks  72  may also have a lengthwise direction substantially perpendicular to the lengthwise direction of respective epitaxial fins  52 . 
     In  FIGS.  9 A- 9 D , first spacer layer  80  is formed on exposed surfaces of the dummy gate stacks  72 , STI regions  56 , and/or the fins  52 . A deposition may form the first spacer layer  80 . The first spacer layer  80  may be formed of silicon nitride (SiN), silicon carbonitride (SiCN), silicon oxycarbonitride (SiOCN), any combinations thereof, or the like, using, for example, CVD, PVD, PECVD, ALD, the like, or any suitable process. The first spacer layer  80  may be formed to a thickness of between about 1 nm and about 10 nm. 
     In  FIGS.  10 A- 10 D , sacrificial liner  81  is formed on the first spacer layer  80 . In some embodiments, the sacrificial liner  81  is formed by oxidizing an upper surface of the first spacer layer  80 , which consequently converts the upper surface of the first spacer layer  80  to the sacrificial liner  81 . The sacrificial liner  81  may be formed by CVD, PECVD, PVD, ALD the like, or any suitable process, wherein O 2 , Ar, He, silane (Si—N—C—H), etc. precursor is flowed over the first spacer layer  80  to deposit an oxide layer and oxidize the upper surface. The precursor may be supplied at a flowrate of between about 1 slm and about 6 slm. The process may be performed at temperatures between about 200° C. and about 400° C. and at pressures between about 1 Torr and about 3 Torr. However, any suitable parameters may be utilized. The resulting sacrificial liner  81  is formed to have a high etch selectivity with the first spacer layer  80 . For example, in embodiments in which the first spacer layer  80  comprises silicon nitride (SiN), silicon carbonitride (SiCN), or silicon oxycarbonitride (SiOCN), the sacrificial liner  81  may comprise silicon dioxide (SiO 2 ). Although not specifically illustrated, in some embodiments, during the process a thin layer of, for example, silicon oxide, may form over the sacrificial liner  81 . In those cases, the thin layer disposed over the sacrificial liner  81  may be removed by a wet isotropic etch using hydrogen fluoride (HF), water, or a dry etch using HF, NH 3 , the like, any combinations thereof, or any suitable etchants. Following the etch, the sacrificial liner  81  remains along an upper surface of the remaining portion of the first spacer layer  80 . The sacrificial liner  81  may have a thickness of between about 2 nm and about 5 nm. In addition, after the upper surface of the first spacer layer  80  is converted, the first spacer layer  80  may have a thickness of between about 1 nm and about 10 nm. 
     After the formation of the first spacer layer  80  and the sacrificial liner  81 , implants for lightly doped source/drain (LDD) regions (not explicitly illustrated) may be performed. In the embodiments with different device types, similar to the implants discussed above in  FIG.  6   , a mask, such as a photoresist, may be formed over the n-type region  50 N, while exposing the p-type region  50 P, and appropriate type (e.g., p-type) impurities may be implanted into the exposed fins  52  in the p-type region  50 P. The mask may then be removed. Subsequently, a mask, such as a photoresist, may be formed over the p-type region  50 P while exposing the n-type region  50 N, and appropriate type impurities (e.g., n-type) may be implanted into the exposed fins  52  in the n-type region  50 N. The mask may then be removed. The n-type impurities may be the any of the n-type impurities previously discussed, and the p-type impurities may be the any of the p-type impurities previously discussed. The lightly doped source/drain regions may have a concentration of impurities from about 10 15  cm −3  to about 10 19  cm −3 . An anneal may be used to repair implant damage and to activate the implanted impurities. 
     In  FIGS.  11 A- 11 D , a second spacer layer  86  is formed by conformally depositing an insulating material over the sacrificial liner  81  along sidewalls of the dummy gate stacks  72 . The insulating material of the second spacer layer  86  may be silicon nitride (SiN), silicon dioxide (SiO 2 ), silicon oxynitride (SiON), silicon carbonitride (SiCN), silicon oxycarbonitride (SiOCN), any combinations thereof, or the like. The second spacer layer  86  may be formed by CVD, PVD, PECVD, ALD, the like, or any suitable process. In some embodiments, the second spacer layer  86  comprises at least one of the foregoing oxygen-containing substances, such as SiOCN. The second spacer layer  86  may have a thickness in a range between about 1 nm and about 10 nm. The second spacer layer  86  is formed to have a similar etch selectivity as the sacrificial liner  81  as well as a high etch selectivity with the first spacer layer  80  and subsequently formed overlying layers. For example, in embodiments in which the first spacer layer comprises silicon nitride (SiN), silicon carbonitride (SiCN), or silicon oxycarbonitride (SiOCN) and the sacrificial liner  81  comprises silicon dioxide (SiO 2 ), the second spacer layer  86  may comprise silicon nitride (SiN), silicon dioxide (SiO 2 ), silicon oxynitride (SiON), silicon carbonitride (SiCN), or silicon oxycarbonitride (SiOCN). As discussed in detail below, the second spacer layer  86  and the sacrificial liner  81  will be selectively etched to form air spacers. 
     It is noted that the above disclosure generally describes a process of forming spacers and LDD regions. Other processes and sequences may be used. For example, although not specifically illustrated, fewer or additional spacers may be utilized, a different sequence of steps may be utilized (e.g., the first spacer layer  80  and the sacrificial liner  81  may be anisotropically etched prior to forming the second spacer layer  86 ), spacers may be formed and removed, and/or the like. Furthermore, the n-type and p-type devices may be formed using different structures and steps. For example, LDD regions for n-type devices may be formed prior to forming the second spacer layer  86  while the LDD regions for p-type devices may be formed after forming the second spacer layer  86 . 
       FIGS.  12 A- 12 D  thru  FIGS.  14 A- 14 D  illustrated formation of source/drain regions in the p-type region  50 P and the n-type region  50 N. Note that the process that is described may be performed first on only the p-type region  50 P or the n-type region  50 N at a time while the second region is masked (not specifically illustrated). Afterward, the mask is removed, and another mask (also not specifically illustrated) is formed over the first region. At which point, the process that is described may be performed on the exposed second region. Also note that various aspects of the process may be the same or different between the p-type region  50 P and the n-type region  50 N. 
     In  FIGS.  12 A- 12 D , a third spacer layer  90  is formed over the second spacer layer  86 , the STI regions  56 , and exposed portions of the fins  52 . The third spacer layer  90  controls the spacing between the subsequently formed source/drain regions and gate electrode. A p-type third spacer layer  90  may be formed in the p-type region  50 P and an n-type third spacer layer  90  may be formed in the n-type region  50 N simultaneously or separately by masking each region while forming the layer in the other region. The third spacer layer  90  may be formed with the same or different materials and at the same or different thicknesses. The third spacer layer  90  may also be formed separately for semiconductor devices within the same region (e.g., the p-type region  50 P or the n-type region  50 N). For example, the third spacer layer  90  may comprise silicon nitride (SiN), silicon carbonitride (SiCN), silicon oxycarbonitride (SiOCN), the like, or any combinations thereof and may be formed by CVD, PVD, PECVD, ALD, the like, or any suitable process. During subsequent processing steps, oxygen may diffuse into and oxidize (or further oxidize) the third spacer layer  90 . The third spacer layer  90  (e.g., silicon oxynitride) may have an etch selectivity that is the same or similar to that of the sacrificial liner  81  and/or the second spacer layer  86 . In some embodiments, during subsequent steps, the third spacer layer  90  may be oxidized or chemically changed in a way to give the third spacer layer  90  an etch selectivity that is the same or closer to that of the sacrificial liner  81  and the second spacer layer  86 . For example, in embodiments in which the first spacer layer  80  comprises silicon nitride (SiN), silicon carbonitride (SiCN), or silicon oxycarbonitride (SiOCN); the sacrificial liner  81  comprises silicon dioxide (SiO 2 ); and the second spacer layer  86  comprises silicon nitride (SiN), silicon dioxide (SiO 2 ), silicon oxynitride (SiON), silicon carbonitride (SiCN), or silicon oxycarbonitride (SiOCN); the third spacer layer  90  may comprise silicon nitride (SiN), silicon carbonitride (SiCN), or silicon oxycarbonitride (SiOCN). 
     In  FIGS.  13 A- 13 D , an etching process may be performed to form recesses  94  in the fins  52  on lateral sides of the dummy gate stacks  72 . The recesses  94  may be formed using anisotropic etching processes, such as reactive-ion etching (RIE), neutral beam etching (NBE), the like, or any suitable process. As illustrated in  FIGS.  13 B and  13 C , portions of the first spacer layer  80 , the sacrificial liner  81 , the second spacer layer  86 , and the third spacer layer  90  overlying portions of the fins  52  may be removed by the etching process. In addition, as illustrated in  FIGS.  13 B and  13 D , other portions of the first spacer layer  80 , the sacrificial liner  81 , the second spacer layer  86 , and the third spacer layer  90  overlying portions of the STI regions  56  may be removed by the etching process. 
     In  FIGS.  14 A- 14 D , epitaxial source/drain regions  96  are formed in the recesses  94  of the fins  52 . The epitaxial source/drain regions  96  are formed in the fins  52  such that each dummy gate stack  72  is disposed between respective neighboring pairs of the epitaxial source/drain regions  96 . In some embodiments the epitaxial source/drain regions  96  may extend into, and may also penetrate through, the fins  52 . In some embodiments, the third spacer layer  90  serves to control short channel effects, threshold voltage, and other performance characteristics of the FinFET by controlling a distance between the source/drain regions  96  and subsequently formed gate electrodes. A material of the epitaxial source/drain regions  96  may be selected to exert stress in the respective channel regions, thereby improving performance. 
     As alluded to above, some regions of the substrate may be masked (not specifically illustrated) while one or more of the above steps (e.g., with respect to  FIGS.  12 A- 14 D ) may be performed on unmasked regions. Thereafter, those masks may be removed and new masks (also not specifically illustrated) may be placed over other regions of the substrate, and one or more of the above steps may be repeated for the unmasked regions, thereby allowing different configurations and/or materials to be used for different transistors, such as p-type transistors and n-type transistors. 
     The epitaxial source/drain regions  96  in the p-type region  50 P may include any acceptable material, such as appropriate for p-type FinFETs. For example, if the fin  52  is silicon, the epitaxial source/drain regions  96  in the p-type region  50 P may comprise materials exerting a compressive strain in the channel region, such as silicon-germanium, germanium, germanium tin, or the like. The epitaxial source/drain regions  96  in the p-type region  50 P may have surfaces raised from respective surfaces of the fins  52  and may have facets. 
     The epitaxial source/drain regions  96  in the n-type region  50 N may include any acceptable material, such as appropriate for n-type FinFETs. For example, if the fin  52  is silicon, the epitaxial source/drain regions  96  in the n-type region  50 N may comprise materials exerting a tensile strain in the channel region, such as silicon, silicon carbide, silicon phosphide, or the like. The epitaxial source/drain regions  96  in the n-type region  50 N may have surfaces raised from respective surfaces of the fins  52  and may have facets. 
     The epitaxial source/drain regions  96  and/or the fins  52  may be implanted with dopants similarly to the process previously discussed for forming lightly-doped source/drain regions, followed by an anneal. The source/drain regions may have an impurity concentration of between about 10 19  cm −3  and about 10 21  cm −3 . The n-type and/or p-type impurities for source/drain regions may be any of the impurities previously discussed. In some embodiments, the epitaxial source/drain regions  96  may be in situ doped during growth. 
     As a result of the epitaxy processes used to form the epitaxial source/drain regions  96  in the n-type region  50 N and the p-type region  50 P, upper surfaces of the epitaxial source/drain regions  96  have facets which expand laterally outward beyond sidewalls of the fins  52 . In some embodiments, these facets cause adjacent source/drain regions  96  of a same FinFET to merge as illustrated by  FIG.  14 B . In other embodiments, adjacent source/drain regions  96  remain separated after the epitaxy process is completed. 
     Following formation of the epitaxial source/drain regions  96 , a cleaning process may be performed. In some embodiments, the cleaning process may remove exposed portions of the third spacer layer  90 , such as from over and around the dummy gate stacks  72 . Note that other portions of the third spacer layer  90  may remain in less exposed locations, such as portions underlying the epitaxial source/drain regions  96  as illustrated in  FIG.  14 B  and portions sandwiched between the epitaxial source/drain regions  96  and the dummy gate stacks  72  (or, specifically, portions of the second spacer layer  86 , the sacrificial liner  81 , and the first spacer layer  80  extending along sidewalls of the dummy gate stack  72 ) as illustrated in  FIG.  14 D . In other embodiments, the third spacer layer  90  may not be removed during the cleaning process and may, therefore, remain in underlying the source/drain regions  96 . 
     In  FIGS.  15 A- 15 D , a fourth spacer layer  98  is formed over the source/drain regions  96  and the dummy gate stacks  72 . The fourth spacer layer  98  may serve as a contact etch stop layer, such as when forming openings for contact plugs. The fourth spacer layer  98  may be formed by conformally depositing an insulating material. The insulating material of the fourth spacer layer  98  may be silicon nitride (SiN), silicon carbonitride (SiCN), silicon oxycarbonitride (SiOCN), any combinations thereof, or the like. The fourth spacer layer  98  may be formed of a material to exhibit a high etch selectivity as compared with certain overlying layers thereby allowing an etching process to form an opening through the overlying layers to be effectively stopped. 
     In addition, the fourth spacer layer  98  may be formed of a material to exhibit a high etch selectivity as compared with certain underlying layers, such as the sacrificial liner  81 , the second spacer layer  86 , and the third spacer layer  90 . As discussed in greater detail below, the sacrificial liner  81 , the second spacer layer  86 , and the third spacer layer  90  may be removed, thereby creating an air gap having a lower dielectric constant. As such, the first spacer layer  80  and the fourth spacer layer  98  may be formed of materials having a low etch rate as compared with the materials of the sacrificial liner  81 , the second spacer layer  86 , and the third spacer layer  90 , thereby allowing the sacrificial liner  81 , the second spacer layer  86 , and the third spacer layer  90  to be removed with little or no etching of the first spacer layer  80  and the fourth spacer layer  98 . 
     For example, in embodiments in which the first spacer layer  80  comprises silicon nitride (SiN), silicon carbonitride (SiCN), or silicon oxycarbonitride (SiOCN); the sacrificial liner  81  comprises silicon dioxide (SiO 2 ); the second spacer layer  86  comprises silicon nitride (SiN), silicon dioxide (SiO 2 ), silicon oxynitride (SiON), silicon carbonitride (SiCN), or silicon oxycarbonitride (SiOCN); the third spacer layer  90  comprises silicon nitride (SiN), silicon carbonitride (SiCN), or silicon oxycarbonitride (SiOCN); the fourth spacer layer  98  may comprise silicon nitride (SiN), silicon carbonitride (SiCN), or silicon oxycarbonitride (SiOCN). The fourth spacer layer  98  may be formed by CVD, PVD, PECVD, ALD, the like, or any suitable process. The fourth spacer layer  98  may have a thickness of between about 1 nm and about 10 nm. 
     In  FIGS.  16 A- 16 D , a first interlayer dielectric (ILD)  100  is deposited over the dummy gate stacks  72 , the STI regions  56 , and the source/drain regions  96 . The first ILD  100  may be formed of a dielectric material, and may be deposited by any suitable method, such as CVD, plasma-enhanced CVD (PECVD), or FCVD. Dielectric materials may include phospho-silicate glass (PSG), boro-silicate glass (BSG), boron-doped phospho-silicate glass (BPSG), undoped silicate glass (USG), or the like. Other insulation materials formed by any acceptable process may be used. 
     Further, a planarization process, such as a CMP, may be performed to level the top surface of the first ILD  100  with the top surfaces of the dummy gate stacks  72 . The planarization process may remove the masks  70  on the dummy gates  68 , and portions of the first spacer layer  80 , the sacrificial liner  81 , the second spacer layer  86 , and the fourth spacer layer  98  that may be along sidewalls of the masks  70  (as well as the third spacer layer  90  if portions remain over and around the dummy gate stacks  72 ). After the planarization process, top surfaces of the dummy gates  68 , the first spacer layer  80 , the sacrificial liner  81 , the second spacer layer  86 , and the fourth spacer layer  98  are level. Accordingly, the top surfaces of the dummy gates  68  are exposed through the first ILD  100 . In some embodiments not specifically illustrated, the masks  70  may remain, in which case the planarization process levels the top surface of the first ILD  100  with the masks  70 . 
     In  FIGS.  17 A- 17 D , the dummy gates  68 , and the masks  70  if present, are removed in an etching step(s), so that recesses  102  are formed. Portions of the dummy dielectrics  66  in the recesses  102  may also be removed as illustrated in  FIGS.  17 A- 17 D . In some embodiments, only the dummy gates  68  are removed and the dummy dielectrics  66  remain and are exposed by the recesses  102 . In some embodiments, the dummy dielectrics  66  are removed from recesses  102  in a first region of a die (e.g., a core logic region) and remain in recesses  102  in a second region of the die (e.g., an input/output region). In some embodiments, the dummy gates  68  are removed by an anisotropic dry etch process. For example, the etching process may include a dry etch process using reaction gas(es) that selectively etch the dummy gates  68  with little or no etching of the first ILD  100  or the first spacer layer  80 , the sacrificial liner  81 , the second spacer layer  86 , and the fourth spacer layer  98 . Each recess  102  exposes and/or overlies a channel region of a respective fin  52 . Each channel region is disposed between neighboring pairs of the epitaxial source/drain regions  96 . During the removal, the dummy dielectrics  66  may be used as etch stop layers when the dummy gates  68  are etched. The dummy dielectrics  66  may then be optionally removed after the removal of the dummy gates  68  by a suitable etching process. 
     In  FIGS.  18 A- 18 D , gate dielectric layers  106  and gate electrodes  108  are formed for replacement gates  112 . Gate dielectric layers  106  may comprise one or more layers deposited in the recesses  102 , such as on the top surfaces and the sidewalls of the fins  52  and on sidewalls of the first spacer layer  80 . The gate dielectric layers  106  may also be formed on the top surface of the first ILD  100 . In some embodiments, the gate dielectric layers  106  comprise one or more dielectric layers, such as one or more layers of silicon oxide, silicon nitride, metal oxide, metal silicate, or the like. For example, in some embodiments, the gate dielectric layers  106  include an interfacial layer  104  (separately illustrated) of silicon oxide formed by thermal or chemical oxidation and an overlying high-k dielectric material, such as a metal oxide or a silicate of hafnium, aluminum, zirconium, lanthanum, manganese, barium, titanium, lead, and combinations thereof. The gate dielectric layers  106  may include a dielectric layer having a k value greater than about 7.0. The formation methods of the gate dielectric layers  106  may include Molecular-Beam Deposition (MBD), ALD, PECVD, and the like. In embodiments where portions of the dummy dielectrics  66  remain in the recesses  102 , the gate dielectric layers  106  (such as the interfacial oxide layer  104 ) include a material of the dummy dielectrics  66  (e.g., SiO 2 ). 
     The gate electrodes  108  are deposited over the gate dielectric layers  106 , respectively, and fill the remaining portions of the recesses  102 . The gate electrodes  108  may include a metal-containing material such as titanium nitride, titanium oxide, tantalum nitride, tantalum carbide, cobalt, ruthenium, aluminum, tungsten, combinations thereof, or multi-layers thereof. For example, although a single layer gate electrode  108  is illustrated, the gate electrode  108  may comprise any number of liner layers, any number of work function tuning layers, and a fill material (none of which specifically illustrated). After the filling of the recesses  102 , a planarization process, such as a CMP, may be performed to remove the excess portions of the gate dielectric layers  106  and the material of the gate electrodes  108 , which excess portions are over the top surface of the first ILD  100 . The remaining portions of material of the gate electrodes  108  and the gate dielectric layers  106  thus form replacement gates  112  of the resulting FinFETs. The gate electrodes  108  and the gate dielectric layers  106  may be collectively referred to as a “gate stack.” The gate stacks may further extend along sidewalls of a channel region of the fins  52 . 
     The formation of the gate dielectric layers  106  in the n-type region  50 N and the p-type region  50 P may occur simultaneously such that the gate dielectric layers  106  in each region are formed from the same materials, and the formation of the gate electrodes  108  may occur simultaneously such that the gate electrodes  108  in each region are formed from the same materials. In some embodiments, the gate dielectric layers  106  in each region may be formed by distinct processes, such that the gate dielectric layers  106  may be different materials/thicknesses, and/or the gate electrodes  108  in each region may be formed by distinct processes, such that the gate electrodes  108  may be different materials/thicknesses. Various masking steps may be used to mask and expose appropriate regions when using distinct processes. 
     In  FIGS.  19 A- 19 D , the gate electrodes  108  may be cut to form distinct gate electrodes  108  for different fins  52  or groups of fins  52 . For example, portions of the gate electrodes  108  may be masked, and the exposed regions may be etched, such as anisotropically etched. A first refill layer  116  may then be formed in the cut regions. The first refill layer  116  may comprise one or more layers of dielectric material, including silicon nitride (SiN), the like, or any combinations thereof. During deposition of the first refill layer  116 , an upper region may get pinched off and form a seam  117  as illustrated. 
     In  FIGS.  20 A- 20 D , the first ILD layer  100  is recessed, and a capping layer  118  is formed over the first ILD layer  100  and the gate electrodes  108 . The capping layer  118  serves to protect the first ILD layer  100  during subsequent etching steps. For example, as discussed in greater detail below, the capping layer  118  protects the first ILD layer  100  while etching the sacrificial liner  81 , the second spacer layer  86 , and the third spacer layer  90 . As such, it may be desirable to select a material for the capping layer  118  that exhibits a low etch rate as compared to the etch rate of the sacrificial liner  81 , the second spacer layer  86 , and the third spacer layer  90 , thereby allowing removal of the sacrificial liner  81 , the second spacer layer  86 , and the third spacer layer  90  with little or no etching of the capping layer  118 . For example, in embodiments in which the first spacer layer  80  comprises silicon nitride (SiN), silicon carbonitride (SiCN), or silicon oxycarbonitride (SiOCN); the sacrificial liner  81  comprises silicon dioxide (SiO 2 ); the second spacer layer  86  comprises silicon nitride (SiN), silicon dioxide (SiO 2 ), silicon oxynitride (SiON), silicon carbonitride (SiCN), or silicon oxycarbonitride (SiOCN); the third spacer layer  90  comprises silicon nitride (SiN), silicon carbonitride (SiCN), or silicon oxycarbonitride (SiOCN); the fourth spacer layer  98  comprises silicon nitride (SiN), silicon carbonitride (SiCN), or silicon oxycarbonitride (SiOCN); the capping layer  118  may comprise silicon oxycarbide (SiOC), silicon carbonitride (SiCN), or silicon nitride (SiN). The capping layer  118  may be formed by CVD, PVD, ALD, PECVD, the like, or any suitable process. A planarization process (e.g., CMP) is performed to remove portions of the capping layer  118  over the gate electrodes  108 . 
     In  FIGS.  21 A- 21 E , air spacers (or air gaps)  120  are formed by etching the second spacer layer  86  and the sacrificial liner  81 . In addition, portions or all of the third spacer layer  90  may also be etched. The figures illustrate a complete etching of entireties of the sacrificial liner  81 , the second spacer layer  86 , and the third spacer layer  90 . However, as described in greater detail below, in accordance with some embodiments, the etching may be stopped before reaching full completion (not specifically illustrated). As a reminder,  FIG.  21 C  illustrates the C-C cross-section from  FIG.  1    and also labeled in  FIG.  21 A . Additionally,  FIG.  21 D  illustrates the D-D cross-section from  FIG.  1    and also labeled in  FIG.  21 B . Further,  FIG.  21 E  illustrates an E-E cross-section, which is not labeled in  FIG.  1    but is labeled in  FIG.  21 B . Referring briefly to  FIG.  21 E , although the epitaxial source/drain region  96  is illustrated as having a lower edge being substantially flat and parallel to upper surfaces of the STI regions  56  and/or the first spacer layer  80 , the lower edge of the epitaxial source/drain region  96  may instead have a convex U-shape such that a central portion is more proximal than an outer portion to the underlying STI region  56 . 
     The etch to form the air spacers  120  may be an isotropic etch and may comprise any suitable wet or dry etchant. For example, in embodiments in which the first spacer layer  80  comprises silicon nitride (SiN), silicon carbonitride (SiCN), silicon oxycarbonitride (SiOCN); the sacrificial liner  81  comprises silicon dioxide (SiO 2 ); the second spacer layer  86  comprises silicon nitride (SiN), silicon dioxide (SiO 2 ), silicon oxynitride (SiON), silicon carbonitride (SiCN), or silicon oxycarbonitride (SiOCN); the third spacer layer  90  comprises silicon nitride (SiN), silicon carbonitride (SiCN), or silicon oxycarbonitride (SiOCN); the fourth spacer layer  98  comprises silicon nitride (SiN), silicon carbonitride (SiCN), or silicon oxycarbonitride (SiOCN); and the capping layer  118  comprises silicon oxycarbide (SiOC), silicon carbonitride (SiCN), or silicon nitride (SiN); a dry etch process using HF, H 2 O (g)  may be used to selectively etch the sacrificial liner  81 , the second spacer layer  86 , and the third spacer layer  90 , with little or no etching of first spacer layer  80 , the fourth spacer layer  98 , and the capping layer  118 . Consequently, the first spacer layer  80  and the fourth spacer layer  98  remain after the wet etch. Although not specifically illustrated, portions of one or more of the gate dielectric layers  106  (e.g., oxides such as hafnium oxide) may be etched during such a wet etch process. 
     As illustrated, the air spacers  120  may extend along the first spacer layer  80  adjacent to the replacement gates  112  as well as under and between source/drain regions  96 . As illustrated in  FIGS.  21 C- 21 E , portions of the air spacers  120  at various depths D 1 , D 2 , and D 3  (measured from an upper surface of the first spacer layer  80 ) in each cross-section may have thicknesses T 1 , T 2 , and T 3  (measured in lateral directions). For example, depth D 1  may be between about 15 nm and about 50 nm, depth D 2  may be between about 20 nm and about 60 nm, and depth D 3  may be between about 60 nm and about 120 nm. In addition, thickness T 1  may be between about 1 nm and about 4 nm, thickness T 2  may be between about 2 nm and about 8 nm, and thickness T 3  may be between about 15 nm and about 30 nm. As illustrated in  FIG.  21 B , the air spacers may have a depth D 4  (measured from below outer portions of the epitaxial source/drain regions  96  to the first spacer layer  80 ), the depth D 4  being between about 10 nm and about 40 nm. As illustrated in  FIG.  21 E  and as described above, a lower edge of the epitaxial source/drain region  96  may be substantially flat or have a convex U-shape, such that the air spacers  120  may also have a depth D 5  (measured from below central portions of the epitaxial source/drain regions  96  to the first spacer layer  80 ), the depth D 5  being between about 5 nm and about 40 nm. 
     In some embodiments, the FinFET devices in the n-type region  50 N and in the p-type region  50 P are etched at the same time. The combination of the second spacer layer  86  and the sacrificial liner  81  provide a greater total thickness and composition for etching than if only the second spacer layer  86  were being etched. The greater total thickness and composition benefits the process of forming the air spacers  120  in multiple ways. The second spacer layer  86  may be formed to a lesser thickness, which reduces the effects that the second spacer layer  86  may have on neighboring or nearby layers during subsequent processing steps. For example, the greater total thickness and composition increases the etch rate and yield in both regions (e.g., the n-type region  50 N and the p-type region  50 P) by providing more space for the etchant to reach lower portions of the second spacer layer  86  and the sacrificial liner  81  along the first spacer layer  80  and the replacement gates  112 . For example, the etch rate of the second spacer layer  86  and the sacrificial liner  81  may be between about 100% and about 200% faster than the etch rate would be for only the second spacer layer  86 . 
     In addition, although the second spacer layer  86  and the sacrificial liner  81  may not etch at the exact same rate in the p-type region  50 P as compared to the n-type region due to differences in thicknesses and compositions of some layers as well as other treatment processes, the greater total thickness and corresponding increased etch rates reduce the disparity. As discussed above, the second spacer layer  86  in the p-type region  50 P may be doped with boron, while the second spacer layer  86  in the n-type region  50 N may be doped with phosphorous. The etchants (e.g., hydrogen fluoride and water) may more efficiently etch the phosphorous-doped regions of the second spacer layer  86  as compared to the boron-doped regions of the second spacer layer  86 . Due to one or more of these differences between the regions, the etch rate to form the air spacers  120  in the p-type region  50 P may be some degree slower than the etch rate to form the air spacers  120  in the n-type region  50 N. However, the greater total thickness for etching reduces the differences between the etch rates between the p-type region  50 P and the n-type region  50 N—that is, an N-P loading. For example, when the air spacer  120  in the p-type region  50 P reaches a depth of about 65 nm, the air spacer  120  in the n-type region  50 N may have etched about 17 nm further. In addition, when the air spacer  120  in the p-type region  50 P reaches a depth of about 88 nm (or almost the full height of the replacement gate  112 , such as to the first spacer layer  80 ), the air spacer  120  in the n-type region  50 N may have etched about 1 nm further. 
     Further, the increased etch rates in forming the air spacers  120  reduce unwanted etching of other exposed features, such as the gate dielectric layers  106  and/or the gate electrodes  108 . For example, as alluded to above, one or more of the gate dielectric layers  106  (e.g., a high-k dielectric layer) may lose about 5% less, or up to about 10% less, when etching the second spacer layer  86  and the sacrificial liner  81  as compared to what may be lost from the one or more gate dielectric layers  106  if etching only the second spacer layer  86  as a result of less etching time. 
     In  FIGS.  22 A- 22 D , a second refill layer  124  is deposited over the gate electrodes  108 , over the capping layer  118 , and into the upper portions of the air spacers  120 . As illustrated, the second refill layer  124  accumulates in the upper portions of the air spacers  120  without depositing throughout other portions of the air spacers  120 . The second refill layer  124  may comprise silicon nitride (SiN), silicon carbonitride (SiCN), silicon oxycarbonitride (SiOCN) the like, or any combinations thereof. The second refill layer  124  may be formed by CVD, PVD, ALD, PECVD, the like, or any suitable process. Following the deposition of the second refill layer  124 , a planarization process, such as CMP, may be performed to remove excess portions of the second refill layer  124  as well as the capping layer  118 . Portions of the second refill layer  124  remain within the upper portions of the air spacers  120  and serve to cap the air spacers  120  to prevent or reduce other non-gaseous materials from getting into the air spacers  120  during subsequent processing steps. The second refill layer  124  may have a thickness in a range between about 5 nm and about 20 nm. 
     Although not specifically illustrated, a gate mask may be formed over the replacement gate  112  (e.g., the gate dielectric layers  106  and the gate electrode  108 ), and the gate mask may be disposed between opposing portions of the first spacer layer  80 . In some embodiments, forming the gate mask includes recessing the replacement gate so that a recess is formed directly over the replacement gate  112  and between opposing portions of the first spacer layer  80 . A gate mask comprising one or more layers of dielectric material, such as silicon nitride, silicon oxynitride, or the like, is filled in the recess, followed by a planarization process to remove excess portions of the dielectric material extending over the first ILD  100 . 
     In  FIGS.  23 A- 23 D , a second ILD  130  is deposited over the first ILD  100 . In some embodiments, the second ILD  130  is a flowable film formed by a flowable CVD method. In some embodiments, the second ILD  130  is formed of a dielectric material such as PSG, BSG, BPSG, USG, or the like, and may be deposited by any suitable method, such as CVD and PECVD. The subsequently formed gate contacts  134  ( FIGS.  24 A- 24 E ) penetrate through the second ILD  130  to contact the top surface of the gate electrode  108 . In some embodiments, a contact etch stop layer (CESL) (not specifically illustrated) is disposed between the second ILD  130  and the replacement gates  112  and the first ILD  100 . The CESL may comprise a dielectric material, such as, silicon nitride, silicon oxide, silicon oxynitride, or the like, having a lower etch rate than the material of the overlying second ILD  130 . 
     In  FIGS.  24 A- 24 E , gate contacts  134  are formed through the second ILD  130 , and source/drain contacts  136  are formed through the second ILD  130  and the first ILD  100 , in accordance with some embodiments. As discussed above, the figures illustrate a complete etching of entireties of the sacrificial liner  81 , the second spacer layer  86 , and the third spacer layer  90 . However, in accordance with some embodiments, the etching may have been stopped before reaching full completion (not specifically illustrated).  FIG.  24 C  illustrates the C-C cross-section from  FIG.  1    and also labeled in  FIG.  24 A . Additionally,  FIG.  24 D  illustrates the D-D cross-section from  FIG.  1    and also labeled in  FIG.  24 B . Further,  FIG.  24 E  illustrates the E-E cross-section, which is not labeled in  FIG.  1    but is labeled in  FIG.  24 B . 
     Openings for the source/drain contacts  136  are formed through the first and second ILDs  100  and  130 , respectively, and the fourth spacer layer  98 . Openings for the gate contact  134  are formed through the second ILD  130 . The openings may be formed using acceptable photolithography and etching techniques. A liner (not shown), such as a diffusion barrier layer, an adhesion layer, or the like, and a conductive material are formed in the openings. The liner may include titanium, titanium nitride, tantalum, tantalum nitride, or the like. The conductive material may be copper, a copper alloy, silver, gold, tungsten, cobalt, aluminum, nickel, or the like. A planarization process, such as a CMP, may be performed to remove excess material from a surface of the second ILD  130 . The remaining liner and conductive material form the source/drain contacts  136  and gate contacts  134  in the openings. An anneal process may be performed to form a silicide at the interface between the epitaxial source/drain regions  96  and the source/drain contacts  136 . The source/drain contacts  136  are physically and electrically coupled to the epitaxial source/drain regions  96 , and the gate contacts  134  are physically and electrically coupled to the gate electrodes  108 . The source/drain contacts  136  and gate contacts  134  may be formed in different processes, or may be formed in the same process. Although shown as being formed in the same cross-sections, it should be appreciated that each of the source/drain contacts  136  and gate contacts  134  may be formed in different cross-sections, which may avoid shorting of the contacts. 
     As illustrated, as a result of the second refill layer  124  filling upper portions of the air spacers  120 , the air spacers  120  may extend at somewhat different dimensions along the first spacer layer  80  adjacent to the replacement gates  112  as well as under and between source/drain regions  96 . As illustrated in  FIGS.  24 C- 24 E , portions of the air spacers  120  at various depths D 1 ′, D 2 ′, and D 3 ′ may be measured from a bottom surface of the second refill layer  124 , while portions of the air spacers  120  at depths D 4  and D 5  may be unaffected by the second refill layer  124 . In addition, the thicknesses T 1 , T 2 , and T 3  may be unaffected by the second refill layer  124 . For example, depth D 1 ′ may be between about 5 nm and about 30 nm, depth D 2 ′ may be between about 10 nm and about 40 nm, and depth D 3 ′ may be between about 40 nm and about 80 nm. 
     The disclosed FinFET embodiments could also be applied to nanostructure devices such as nanostructure (e.g., nanosheet, nanowire, gate-all-around, or the like) field effect transistors (NSFETs). In an NSFET embodiment, the fins are replaced by nanostructures formed by patterning a stack of alternating layers of channel layers and sacrificial layers. Dummy gate stacks and source/drain regions are formed in a manner similar to the above-described embodiments. After the dummy gate stacks are removed, the sacrificial layers can be partially or fully removed in channel regions. The replacement gate structures are formed in a manner similar to the above-described embodiments, the replacement gate structures may partially or completely fill openings left by removing the sacrificial layers, and the replacement gate structures may partially or completely surround the channel layers in the channel regions of the NSFET devices. ILDs and contacts to the replacement gate structures and the source/drain regions may be formed in a manner similar to the above-described embodiments. A nanostructure device can be formed as disclosed in U.S. Patent Application Publication No. 2016/0365414, which is incorporated herein by reference in its entirety. 
     In an embodiment, a method of forming a semiconductor device includes forming a dummy gate stack over a substrate; forming a first spacer layer over the dummy gate stack; oxidizing a surface of the first spacer layer to form a sacrificial liner; forming one or more second spacer layers over the sacrificial liner; forming a third spacer layer over the one or more second spacer layers; forming an inter-layer dielectric (ILD) layer over the third spacer layer; etching at least a portion of the one or more second spacer layers to form an air gap, the air gap being interposed between the third spacer layer and the first spacer layer; and forming a refill layer to fill an upper portion of the air gap. In an embodiment, etching includes etching at least a portion of the sacrificial liner. In an embodiment, each of the one or more second spacer layers and the sacrificial liner includes an oxide. In an embodiment, the method further includes, after forming the one or more second spacer layers, epitaxially growing source/drain region in the substrate adjacent the third spacer layer, wherein forming the third spacer layer includes forming the third spacer layer over the source/drain regions. In an embodiment, the air gap extends along a lower surface of the source/drain region. In an embodiment, the air gap extends along opposing sidewalls of the source/drain region. In an embodiment, the method further includes before the etching step, forming a capping layer over the ILD layer; and after the etching step, performing a planarization to remove the capping layer. 
     In another embodiment, a semiconductor structure includes a first fin protruding above an isolation region; a gate structure disposed over the first fin; a source/drain region over the first fin and adjacent the gate structure; an interlayer dielectric (ILD) disposed over the source/drain region; a first spacer along a sidewall of the gate structure, the first spacer being interposed between the ILD and the gate structure; and an air gap disposed over the first spacer, the air gap being laterally interposed between the ILD and the first spacer, the air gap being vertically interposed between the source/drain region and the isolation region. In an embodiment, the first spacer contacts an upper surface of the first fin, an upper surface of the isolation region, and the sidewall of the gate structure. In an embodiment, the structure further includes a second spacer, the second spacer contacting the upper surface of the isolation region and a bottom surface of the ILD. In an embodiment, the air gap contacts a lower surface of the source/drain region and a lower surface of the second spacer, wherein the second spacer contacts an upper surface of the source/drain region. In an embodiment, the structure further includes an insulating material directly interposed between the first spacer and the second spacer, wherein the air gap is bounded by the first spacer, the second spacer, the source/drain region, and the insulating material. In an embodiment, a portion of the air gap is directly interposed between the first spacer and the source/drain region. 
     In yet another embodiment, a semiconductor structure includes a shallow trench isolation (STI) region interposed between a first fin and a second fin; a first gate structure disposed over the STI region, the first fin, and the second fin; a second gate structure disposed over the STI region, the first fin, and the second fin; a source/drain region disposed over the first fin and the second fin, the source/drain region being interposed between the first gate structure and the second gate structure; a first spacer along a sidewall of the first gate structure, a sidewall of the second gate structure, and an upper surface of the STI region; a second spacer along an upper surface of the source/drain region; and an air gap interposed between the first spacer and the second spacer, the air gap exposing a lower surface of the second spacer and a lower surface of the source/drain region. In an embodiment, the air gap extends between the source/drain region and the first gate structure. In an embodiment, the air gap extends between the source/drain region and the second gate structure. In an embodiment, the first spacer separates the air gap from the STI. In an embodiment, the air gap has a first width between the first spacer and the second spacer, wherein the air gap has a second width between the first spacer and the source/drain region, wherein the first width is less than the second width. In an embodiment, the structure further includes a refill material over the air gap and interposed between the first spacer and the second spacer, an upper surface of the refill material being level with an upper surface of the first spacer. 
     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.