Patent Publication Number: US-2022223684-A1

Title: Gate Air Spacer Protection During Source/Drain Via Hole Etching

Description:
BACKGROUND 
     The semiconductor integrated circuit (IC) industry has experienced exponential growth. Technological advances in IC materials and design have produced generations of ICs where each generation has smaller and more complex circuits than the previous generation. In the course of IC evolution, functional density (i.e., the number of interconnected devices per chip area) has generally increased while geometry size (i.e., the smallest component (or line) that can be created using a fabrication process) has decreased. This scaling down process generally provides benefits by increasing production efficiency and lowering associated costs. Such scaling down has also increased the complexity of processing and manufacturing ICs. 
     For example, as semiconductor devices continue to get scaled down, accurate alignment between the various components of the IC is more difficult to achieve. In some cases, even a slight misalignment between a source/drain contact and a source/drain via (formed over the source/drain contact) may lead to electrical shorting between the source/drain via and a nearby gate structure. Electrical shorting is undesirable because it may degrade device performance or even cause device failures. 
     Therefore, although conventional methods of fabricating semiconductor devices have generally been adequate, they have not been satisfactory in all aspects. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure is best understood from the following detailed description when read with the accompanying figures. It is emphasized that, in accordance with the standard practice in the industry, various features are not drawn to scale and are used for illustration purposes only. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion. 
         FIG. 1A  is a perspective view of an IC device in the form of a FinFET according to various aspects of the present disclosure. 
         FIG. 1B  is a planar top view of an IC device in the form of a FinFET according to various aspects of the present disclosure. 
         FIG. 1C  is a perspective view of an IC device in the form of a GAA device according to various aspects of the present disclosure. 
         FIGS. 2-13  are cross-sectional side views of various embodiments of IC devices at various stages of fabrication according to various aspects of the present disclosure. 
         FIG. 14  is a circuit schematic of an SRAM cell according to various aspects of the present disclosure. 
         FIG. 15  is a block diagram of a manufacturing system according to various aspects of the present disclosure. 
         FIG. 16  is a flowchart illustrating a method of fabricating a semiconductor device according to various aspects of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The following disclosure provides many different embodiments, or examples, for implementing different features of the disclosure. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. For example, the formation of a first feature over or on a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features may be formed between the first and second features, such that the first and second features may not be in direct contact. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed. 
     In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed. Moreover, the formation of a feature on, connected to, and/or coupled to another feature in the present disclosure that follows may include embodiments in which the features are formed in direct contact, and may also include embodiments in which additional features may be formed interposing the features, such that the features may not be in direct contact. In addition, spatially relative terms, for example, “lower,” “upper,” “horizontal,” “vertical,” “above,” “over,” “below,” “beneath,” “up,” “down,” “top,” “bottom,” etc., as well as derivatives thereof (e.g., “horizontally,” “downwardly,” “upwardly,” etc.) are used for ease of the present disclosure of one features relationship to another feature. The spatially relative terms are intended to cover different orientations of the device including the features. Still further, when a number or a range of numbers is described with “about,” “approximate,” and the like, the term is intended to encompass numbers that are within a reasonable range including the number described, such as within +/−10% of the number described or other values as understood by person skilled in the art. For example, the term “about 5 nm” encompasses the dimension range from 4.5 nm to 5.5 nm. 
     The present disclosure is generally related to semiconductor devices, and more particularly to field-effect transistors (FETs), such as planar FETs, three-dimensional fin-line FETs (FinFETs), or gate-all-around (GAA) devices. 
     One aspect of the present disclosure involves forming a unique etching-stop layer to protect gate air spacers below during certain fabrication processes. In that regard, air spacers (as a part of the gate spacer structure) may help reduce the parasitic capacitance of ICs due to their low dielectric constant. However, conventional methods of semiconductor fabrication may lead to the air spacers being inadvertently punched through from above. This is also known as a tiger-tooth issue, which occurs when the source/drain via is misaligned with the source/drain contact. For example, when the source/drain via-hole is laterally shifted to a point where it is formed directly above the air spacer, the subsequent step of filling the source/drain via-hole with a conductive material may cause the conductive material to also at least partially fill the air spacer. In other words, due to the portion of the conductive material that protrudes into the air spacer, the source/drain via now has a downwardly-protruding “tiger-tooth”-like portion. Such a tiger-tooth-like profile may make the source/drain via more prone to come into direct contact with a nearby gate, which causes electrical shorting between the gate and the source/drain via (and the source/drain). When this occurs, device yield, reliability, and/or performance may be degraded. 
     The present disclosure overcomes this problem by reconfiguring the shape or profile of a protective layer, such as an etching-stop layer, so that the protective layer may block the air spacer during the source/drain via hole etching. As such, the air spacers can be preserved. In addition, such an implementation also allows the source/drain via to be self-aligned with the source/drain contact, which optimizes the process window, as discussed below in more detail. 
       FIGS. 1A and 1B  illustrate a three-dimensional perspective view and a top view, respectively, of a portion of an Integrated Circuit (IC) device  90 . The IC device  90  may be an intermediate device fabricated during processing of an IC, or a portion thereof, that may comprise static random-access memory (SRAM) and/or other logic circuits, passive components such as resistors, capacitors, and inductors, and active components such as p-type FETs (PFETs), n-type FETs (NFETs), FinFETs, metal-oxide semiconductor field effect transistors (MOSFET), complementary metal-oxide semiconductor (CMOS) transistors, bipolar transistors, high voltage transistors, high frequency transistors, and/or other memory cells. The present disclosure is not limited to any particular number of devices or device regions, or to any particular device configurations, unless otherwise claimed. For example, although the IC device  90  as illustrated is a three-dimensional FinFET device, the concepts of the present disclosure may also apply to planar FET devices or GAA devices. 
     Referring to  FIG. 1A , the IC device  90  includes a substrate  110 . The substrate  110  may comprise an elementary (single element) semiconductor, such as silicon, germanium, and/or other suitable materials; a compound semiconductor, such as silicon carbide, gallium arsenic, gallium phosphide, indium phosphide, indium arsenide, indium antimonide, and/or other suitable materials; an alloy semiconductor such as SiGe, GaAsP, AlInAs, AlGaAs, GaInAs, GaInP, GaInAsP, and/or other suitable materials. The substrate  110  may be a single-layer material having a uniform composition. Alternatively, the substrate  110  may include multiple material layers having similar or different compositions suitable for IC device manufacturing. In one example, the substrate  110  may be a silicon-on-insulator (SOI) substrate having a semiconductor silicon layer formed on a silicon oxide layer. In another example, the substrate  110  may include a conductive layer, a semiconductor layer, a dielectric layer, other layers, or combinations thereof. Various doped regions, such as source/drain regions, may be formed in or on the substrate  110 . The doped regions may be doped with n-type dopants, such as phosphorus or arsenic, and/or p-type dopants, such as boron, depending on design requirements. The doped regions may be formed directly on the substrate  110 , in a p-well structure, in an n-well structure, in a dual-well structure, or using a raised structure. Doped regions may be formed by implantation of dopant atoms, in-situ doped epitaxial growth, and/or other suitable techniques. 
     Three-dimensional active regions  120  are formed on the substrate  110 . The active regions  120  are elongated fin-like structures that protrude upwardly out of the substrate  110 . As such, the active regions  120  may be interchangeably referred to as fin structures  120  or fin structures  120  hereinafter. The fin structures  120  may be fabricated using suitable processes including photolithography and etch processes. The photolithography process may include forming a photoresist layer overlying the substrate  110 , exposing the photoresist to a pattern, performing post-exposure bake processes, and developing the photoresist to form a masking element (not shown) including the resist. The masking element is then used for etching recesses into the substrate  110 , leaving the fin structures  120  on the substrate  110 . The etching process may include dry etching, wet etching, reactive ion etching (RIE), and/or other suitable processes. In some embodiments, the fin structure  120  may be formed by 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. As an example, a layer may be formed over a substrate and patterned using a photolithography process. Spacers are formed alongside the patterned layer using a self-aligned process. The layer is then removed, and the remaining spacers, or mandrels, may then be used to pattern the fin structures  120 . The IC device  90  also includes source/drain features  122  formed over the fin structures  120 . The source/drain features  122  may include epi-layers that are epitaxially grown on the fin structures  120 . 
     The IC device  90  further includes isolation structures  130  formed over the substrate  110 . The isolation structures  130  electrically separate various components of the IC device  90 . The isolation structures  130  may include silicon oxide, silicon nitride, silicon oxynitride, fluoride-doped silicate glass (FSG), a low-k dielectric material, and/or other suitable materials. In some embodiments, the isolation structures  130  may include shallow trench isolation (STI) features. In one embodiment, the isolation structures  130  are formed by etching trenches in the substrate  110  during the formation of the fin structures  120 . The trenches may then be filled with an isolating material described above, followed by a chemical mechanical planarization (CMP) process. Other isolation structure such as field oxide, local oxidation of silicon (LOCOS), and/or other suitable structures may also be implemented as the isolation structures  130 . Alternatively, the isolation structures  130  may include a multi-layer structure, for example, having one or more thermal oxide liner layers. 
     The IC device  90  also includes gate structures  140  formed over and engaging the fin structures  120  on three sides in a channel region of each fin  120 . The gate structures  140  may be dummy gate structures (e.g., containing an oxide gate dielectric and a polysilicon gate electrode), or they may be HKMG structures that contain a high-k gate dielectric and a metal gate electrode, where the HKMG structures are formed by replacing the dummy gate structures. Though not depicted herein, the gate structure  140  may include additional material layers, such as an interfacial layer over the fin structures  120 , a capping layer, other suitable layers, or combinations thereof. 
     Referring to  FIG. 1B , multiple fin structures  120  are oriented lengthwise along the X-direction, and multiple gate structure  140  are oriented lengthwise along the Y-direction, i.e., generally perpendicular to the fin structures  120 . In many embodiments, the IC device  90  includes additional features such as gate spacers (including air spacers) disposed along sidewalls of the gate structures  140 , hard mask layer(s) disposed over the gate structures  140 , and numerous other features. 
     It is also understood that the various aspects of the present disclosure discussed below may apply to multi-channel devices such as Gate-All-Around (GAA) devices.  FIG. 1C  illustrates a three-dimensional perspective view of an example GAA device  150 . For reasons of consistency and clarity, similar components in  FIG. 1C  and  FIGS. 1A-1B  will be labeled the same. For example, active regions such as fin structures  120  rise vertically upwards out of the substrate  110  in the Z-direction. The isolation structures  130  provide electrical separation between the fin structures  120 . The gate structure  140  is located over the fin structures  120  and over the isolation structures  130 . A mask  155  is located over the gate structure  140 , and gate spacer structures  160  (which may include air spacers) are located on sidewalls of the gate structure  140 . A capping layer  165  is formed over the fin structures  120  to protect the fin structures  120  from oxidation during the forming of the isolation structures  130 . 
     A plurality of nano-structures  170  are disposed over each of the fin structures  120 . The nano-structures  170  may include nano-sheets, nano-tubes, or nano-wires, or some other type of nano-structure that extends horizontally in the X-direction. Portions of the nano-structures  170  under the gate structure  140  may serve as the channels of the GAA device  150 . Dielectric inner spacers  175  may be disposed between the nano-structures  170 . In addition, although not illustrated for reasons of simplicity, each of the nano-structures  170  may be wrapped around circumferentially by a gate dielectric as well as a gate electrode. In the illustrated embodiment, the portions of the nano-structures  170  outside the gate structure  140  may serve as the source/drain features of the GAA device  150 . However, in some embodiments, continuous source/drain features may be epitaxially grown over portions of the fin structures  120  outside of the gate structure  140 . Regardless, conductive source/drain contacts  180  may be formed over the source/drain features to provide electrical connectivity thereto. An interlayer dielectric (ILD)  185  is formed over the isolation structures  130  and around the gate structure  140  and the source/drain contacts  180 . 
     Additional details pertaining to the fabrication of GAA devices are disclosed in U.S. Pat. No. 10,164,012, titled “Semiconductor Device and Manufacturing Method Thereof” and issued on Dec. 25, 2018, as well as in U.S. Pat. No. 10,361,278, titled “Method of Manufacturing a Semiconductor Device and a Semiconductor Device” and issued on Jul. 23, 2019, and also in U.S. Pat. No. 9,887,269, titled “Multi-Gate Device and Method of Fabrication Thereof” and issued on Feb. 6, 2018, the disclosures of each which are hereby incorporated by reference in their respective entireties. To the extent that the present disclosure refers to a fin structure or FinFET devices, such discussions may apply equally to the GAA devices. 
       FIGS. 2-13  illustrate the cross-sectional side views of an IC device  200  at different stages of fabrication.  FIGS. 2-13  correspond to the cross-sectional cuts taken along an X-direction, for example along the cutline A-A′ in  FIG. 1A . As such,  FIGS. 2-13  may be referred to as X-cut Figures. For reasons of consistency and clarity, similar components appearing in  FIGS. 1A-1C  may be labeled the same in  FIGS. 2-13 . 
     Referring to  FIG. 2 , the IC device  200  includes the substrate  110  discussed above with reference to  FIGS. 1A-1C , for example a silicon substrate. The substrate includes a plurality of active regions, for example the fin structures  120  discussed above with reference to  FIGS. 1A-1B . The fin structures  120  each extend laterally in the X-direction, and they are separated from one another in the Y-direction by the isolation structures  130 . Source/drain components  122  may be formed in or over the substrate  110 , which may be formed using an epitaxial growth process. In some embodiments, the source/drain components  122  are grown on/over the fin structures  120  (as shown in  FIG. 2B ). These source/drain components  122  may belong to different transistors. 
     The interlayer dielectric (ILD)  185  (also shown in  FIG. 1C ) is formed over the substrate  110 . The ILD  185  may be formed by plasma enhanced chemical vapor deposition (PECVD), flowable CVD (FCVD), or other suitable methods. In some embodiments, the ILD  185  may include silicon oxide. In some other embodiments, the ILD  185  may include a low-k dielectric material having a dielectric constant that is less than the dielectric constant of silicon oxide (which is about 3.9). The ILD  185  may also be referred to as an ILD- 0 . 
     The IC device  200  also includes gate structures  140  (also shown in  FIGS. 1A-1C ) that are formed over the substrate  110 . In some embodiments, the gate structures  140  include high-k metal gate (HKMG) gate structures, which may include a high-k gate dielectric and a metal gate electrode. The gate structures  140  may be performed using a gate replacement process. In more detail, dummy gate structures may be formed first over the substrate  110  and between the source/drain components  122 . The dummy gate structures may each include a dummy gate electrode (e.g., a polysilicon gate electrode) and either a dummy gate dielectric (e.g., a silicon oxide gate dielectric) or a high-k gate dielectric (e.g., a dielectric material having a dielectric constant greater than that of silicon oxide). 
     After the formation of the dummy gate structures, gate spacer structures  160  (also shown in  FIG. 1C ) may be formed on the sidewalls of the dummy gate structures. In some embodiments, the gate spacer structures  160  may each include multiple gate spacers, for example a gate spacer  250 , a gate spacer  260 , and a gate spacer  270 . The gate spacer  250  is disposed directly on the sidewall of the dummy gate structure and may include a low-k dielectric material. The gate spacer  260  is disposed on the sidewall of the gate spacer  250  and may include a silicon material. Note that this silicon material will be removed in a later process to form air spacers, and therefore the gate spacer  250  may also be referred to as a dummy silicon spacer. The gate spacer  270  is disposed on the sidewall of the gate spacer  260  and may include a dielectric material such as silicon nitride. In this configuration, the gate spacer  260  is formed between the gate spacers  250  and  270 . It is understood that the ILD  185  may be formed after the formation of the gate spacer structures  160 . 
     As a part of the gate replacement process, the dummy polysilicon gate electrodes of the dummy gate structures may be removed using one or more etching processes, which leaves openings (trenches) defined by the gate spacer structures  160  and the ILD  185 . These openings or trenches are subsequently filled by the metal gate electrodes of the gate structures  140 . In some embodiments, if the dummy gate structures include a dummy gate dielectric (e.g., a silicon oxide gate dielectric), then the dummy gate dielectric will also be replaced by a high-k gate dielectric as a part of the gate structures  140 . Example materials of the high-gate k dielectric include hafnium oxide, zirconium oxide, aluminum oxide, hafnium dioxide-alumina alloy, hafnium silicon oxide, hafnium silicon oxynitride, hafnium tantalum oxide, hafnium titanium oxide, hafnium zirconium oxide, or combinations thereof. The metal gate electrode is formed on the high-k gate dielectric and may include one or more work function metal layers and one or more fill metal layers. The work function metal layers may be configured to tune a work function of the respective transistor. Example materials for the work function metal layers may include titanium nitride (TiN), Titanium aluminide (TiAl), tantalum nitride (TaN), titanium carbide (Tic), tantalum carbide (TaC), tungsten carbide (WC), aluminum titanium nitride (TiAlN), zirconium aluminide (ZrAl), tungsten aluminide (WAl), tantalum aluminide (TaAl), hafnium aluminide (HfAl), or combinations thereof. The fill metal layer may serve as a main conductive portion of the gate electrode. In some embodiments, the HKMG structures  140  may include additional layers, such as interfacial layers, capping layers, diffusion/barrier layers, or other applicable layers. 
     As semiconductor fabrication progresses to smaller technology nodes, the sizes or dimensions of the various components of the IC device will shrink, including the gate spacer structure  160 . This may increase difficulties in fabrication, such as alignment issues. For example, the gate spacers  250 ,  260 , and  270  may have lateral dimensions (e.g., widths)  255 ,  265 , and  275 , respectively, that are measured in the X-direction. The lateral dimensions  255 ,  265 , and  275  may shrink as semiconductor fabrication progresses to smaller technology nodes. In some embodiments, the lateral dimensions  255 ,  265 , and  275  may each be in a range between about 0.1 nanometer (nm) and about 2.5 nm. Such small sizes may bring certain fabrication challenges. 
     For example, in a later fabrication process discussed below, the portions of the ILD  185  disposed directly over the source/drain components  122  may be removed and replaced by source/drain contacts, and a source/drain via may be formed over the source/drain contact. In another fabrication process discussed below, the gate spacer  260  will be removed and replaced by an air spacer. Due to the small lateral dimension  265  of the gate spacer  260 , even a slight lateral shift of the location of the source/drain via could cause the source/drain via to leak into the air spacer. This is referred to as a tiger-tooth problem and is undesirable, since the presence of the metal materials of the source/drain via inside what is supposed to be air compromises the intended functionality of the air spacer. 
     The tiger-tooth problem could also lead to the unintended electrical shorting between the source/drain via and one of the nearby gate structures  140 . For example, due to their small lateral dimensions  255  and  275 , the gate spacers  250  and  270  may be inadvertently consumed in one or more later etching processes that are meant to etch other components of the IC device  200 . Again, the inadvertent consumption of the gate spacers  250  and/or  270  may allow the metal materials of the source/drain via to leak into what is supposed to be the gate spacer structure  160 . In some cases, the leakage of the source/drain via (e.g., the downwardly-protruding tiger-tooth) may come into direct physical contact with one of the nearby gate structures  140 , which results in electrical shorting between the gate structure  140  and the source/drain via (and therefore the source/drain component  122 ). To overcome these problems, the present disclosure prevents the occurrence of the tiger-tooth source/drain via by forming a uniquely shaped etching-stop layer, as discussed below in more detail. 
     Referring now to  FIG. 3 , one or more etching processes  300  are performed to the IC device  200 . In some embodiments, the one or more etching processes  300  may include etch-back processes to reduce the height of the gate structures  140  and the height of the gate spacer structures  160 . As a result of the one or more etching processes  300  being performed, the gate structures  140  have been reduced to a height  310 . In some embodiments, the height  310  is in a range between about 5 nm and about 20 nm. The gate spacer structures  160  also have a reduced height  320 , which is still taller than the height  310  of the gate structures  140 . The ILD  185  is substantially unremoved, or at least not removed as much as the gate spacer structures  160  or the gate structures  140  by the etching processes  300 . As such, the partial removal of the gate spacer structures  160  and the gate structures  140  form openings  330  (or recesses  330 ). 
     Referring now to  FIG. 4 , a deposition process  350  is performed to form a metal layer  360  over the upper surface of each of the gate structures  140 . The deposition process  350  may include atomic layer deposition (ALD), physical vapor deposition (PVD), or CVD. In some embodiments, the metal layer  360  is formed to contain ruthenium (Ru), cobalt (Co), copper (Cu), aluminum (Al), or another suitable pure metal. In some embodiments, the metal layer  360  is formed to have a thickness in a range between about 1 nm and about 7 nm. 
     Referring now to  FIG. 5 , a deposition process  400  is performed to the IC device  200  to form a hard mask layer  410  over the metal layer  360  and over the gate structures  160 . The deposition process  400  may include CVD, PVD, ALD, or combinations thereof. In some embodiments, the hard mask layer  410  may include a metal oxide, such as tantalum oxide (Ta 2 O 5 ), aluminum oxide (Al 2 O 3 ), or zirconium oxide (ZrO 2 ). In some embodiments, the hard mask layer  410  may include silicon nitride (SiN) or silicon oxynitride (SiON). In some embodiments, the hard mask layer  410  may include a nitrogen content oxide, a nitrided oxide, a metal oxide dielectric, a hafnium content oxide (e.g., an oxide material having a hafnium content), a tantalum content oxide, a titanium content oxide, a zirconium content oxide, an aluminum content oxide, a lanthanum content oxide, or a high-K dielectric material, or combinations thereof. The hard mask layer  410  fills the openings  330 . It is understood that a planarization process such as a chemical mechanical polishing (CMP) process may also be performed to planarize the upper surfaces of the hard mask layer  410  and the upper surfaces of the ILD  185 . 
     Referring now to  FIG. 6 , one or more etching processes  450  may be performed to the IC device  200  to remove the ILD  185 . The one or more etching processes  450  may include a wet etching process, a dry etching process, or a combination thereof. The removal of the ILD  185  forms openings  460  in place of the removed ILD  185 , which expose the upper surfaces of the source/drain components  122 . 
     Referring now to  FIG. 7 , a deposition process  470  is performed to the IC device  200  to form conductive contacts  480  in the openings  460 . Since the conductive contacts  480  are formed on the source/drain components  122 , they may also be referred to as source/drain contacts  480 . In some embodiments, the deposition process  470  includes a suitable deposition process such as CVD, PVD, or ALD. In some embodiments, the source/drain contacts  480  may contain a combination of titanium/titanium nitride and another metal such as cobalt, tantalum, tantalum nitride, ruthenium, copper, aluminum, or another pure metal. It is understood that a CMP process may also be performed to planarize the upper surfaces of the source/drain contacts  480  and the hard mask layer  410 . The source/drain contacts  480  provide electrical connectivity to the source/drain components  122 . 
     Referring now to  FIG. 8 , an etching process  500  is performed to the IC device  200  to partially remove the source/drain contacts  480 . In some embodiments, the etching process  500  may include an etching-back process. The process parameters of the etching process  500  may be configured such that the remaining portions of the source/drain contacts  480  have substantially similar heights as the combination of the metal layer  360  and the gate structures  140 . Alternatively stated, at the end of the etching process  500 , the upper surfaces of the source/drain contacts  480  may be substantially co-planar with the upper surfaces of the metal layers  360 . 
     Referring now to  FIG. 9 , an etching process  550  is performed to the IC device. In some embodiments, the etching process  550  includes a wet etching process. The etching process  550  reduces the lateral dimension (e.g., in the X-direction) of the hard mask layer  410 , until at least the dummy silicon gate spacers  260  are exposed. The etching process  550  (or a subsequent etching process) then etches away the dummy silicon gate spacers  260 , such that air spacers  560  are formed in place of the removed dummy silicon gate spacers  260 . 
     As shown in  FIG. 9 , the air spacers  560  are located between the gate spacers  250  and  270 . The air spacers  560  have a low dielectric constant (e.g., approximately 1) and therefore will help lower the parasitic capacitance of the transistor. The air spacers  560  each have a lateral dimension  565  (e.g., measured in the X-direction). In some embodiments, the lateral dimension  565  is in a range between about 0.5 nm and about 5 nm. The range for the dimension  565  is not randomly chosen but rather specifically configured to optimize the parasitic capacitance reduction without removing too much of the gate spacers  250  and/or  270 , which are still needed to provide electrical isolation. 
     It is understood that the etching process  550  not only partially removes the hard mask layer  410 , but it also partially removes the gate spacer structure  160 . In other words, the remaining portions of the gate spacers  250  and  270  may have heights that are substantially equal to or lower than the heights of the source/drain contacts  480 . It is also understood that while the illustrated embodiment shows that the sidewalls of the remaining portion of the hard mask layer  410  are approximately aligned with the sidewalls of the gate structure  140  below, this need not be the case in other embodiments. In some embodiments, the hard mask layer  410  may be etched a little bit more laterally, such that portions of the gate structures  140  below are exposed. In yet other embodiments, the hard mask layer  410  may be etched a little bit less laterally, such that the remaining portion of the hard mask layer  410  may partially cover up the upper surfaces of the gate spacers  270 , so long as the dummy silicon gate spacers  260  can be exposed and removed to form the air spacers  560 . 
     Referring now to  FIG. 10 , a plurality of deposition-etching cycles  600  are performed to the IC device  200  to form an etching-stop layer (ESL)  610 . In a first step of each cycle of the deposition-etching cycles  600 , a deposition process is performed to deposit a dielectric material of the ESL  610 . In some embodiments, the deposition process includes ALD, which can be configured to precisely control a thickness of the deposited dielectric material. In some embodiments, the deposited dielectric material may have a different material composition than the hard mask layer  410 . For example, in some embodiments, the deposited material of the ESL  610  includes silicon nitride, while the hard mask layer  410  includes a metal oxide material. In a second step of each cycle of the deposition-etching cycles  600 , a dry etching process is performed to etch the deposited material of the ESL  610 , mostly in a downward vertical direction (e.g., in the downward Z-direction). This is because the dry etching process can be performed anisotropically, for example it may etch in the downward vertical direction much more than in lateral directions (in the X-directions). 
     As a result of the performance of the deposition-etching cycles  600 , the deposited ESL  610  is formed to have substantially thicker dimensions laterally than vertically. In some embodiments, the lateral dimension of the ESL  610  is at least 33% thicker than its vertical dimension. This characteristic allows the ESL  610  to cover up the air spacer  560  without unduly increasing the thickness of the portion of the ESL  610  disposed above the source/drain contact  480 . Therefore, the relatively wide portion of the ESL  610  can adequately protect the air spacers  560  below, while the relatively thin portion of the ESL  610  can be easily etched through in a later process to expose the source/drain contact  480 . 
     Referring now to  FIG. 11 , a deposition process  650  is performed to the IC device  200 . The deposition process  650  may include CVD, PVD, ALD, or combinations thereof. The deposition process  650  deposits a dielectric layer over the ESL  610  as the ILD  660 . The ILD  660  may also be referred to as an ILD 2 . In some embodiments, the ILD  660  includes silicon oxide. 
     Referring now to  FIG. 12 , an etching process  700  is performed to the IC device  200  to form an opening  710 . The opening  710  extends vertically through a portion of the ILD  660  and a portion of the ESL  610  to expose an upper surface of one of the source/drain contacts  480 . Note that since portions of the ESL  610  protect the air spacers  560  below during the etching process  700 . Since the opening  710  will be filled by a conductive material subsequently to establish electrical connections with the source/drain contact  480  (and therefore with the source/drain components  122 ), the opening  710  may also be referred to as a source/drain via hole. 
     Referring now to  FIG. 13 , a deposition process  740  is performed to the IC device  200  to form a source/drain via  750 . The source/drain via  750  is formed by filling the opening  710  with a conductive material, such as tungsten, cobalt, copper, or aluminum. Due to the unique fabrication processes performed to form the source/drain via  750 , the source/drain via  750  is formed in a self-aligned manner. For example, the upper portion of the opening  710  can be etched to be substantially wider than the bottom portion of the opening  710 , or even wider than the source/drain contact  480 . Since the ESL  610  protects the air spacer  560  (and also the gate spacers  250  and  270  below from being etched) during the etching process  700 , the risk of the opening  710  bridging with the air spacer  560  is substantially reduced or eliminated. As such, the opening  710  (or the source/drain via  750  subsequently filling the opening  710 ) need not be sufficiently narrow and exactly aligned with the source/drain contact  480 . In other words, the source/drain via  750  can have a cross-sectional view profile that resembles the letter “T” where it has a wider upper portion  750 A (which relaxes the processing window when the source/drain via  750  was etched) and a narrower lower portion  750 B. The upper portion  750 A has a bottom surface  755  that is disposed directly on an upper surface  756  of the ESL  610 . The side surfaces of the upper portion  750 A are also in direct contact with the ILD  660 . Meanwhile, the sidewalls of the lower portion  750 B are in direct contact with the side surfaces of the ESL  610 , and the bottom surface of the lower portion  750 B are in direct contact with the top surface of the source/drain contact  480 , so that electrical connectivity with the source/drain component  122  can be established. Even if the opening  710  shifts laterally to the “left” or “right” in the X-direction, such as a shift is unlikely to disrupt the electrical connection between the source/drain via  750  and the source/drain contact  480 , because the ESL  610  effectively “guide” the opening  710  to be formed in an aligned manner with the source/drain contact  480  below. 
     In some embodiments, a top surface of the source/drain via  750  has a lateral dimension  760  that is measured in the X-direction, a bottom surface of the source/drain via  750  has a lateral dimension  770  that is measured in the X-direction, where the lateral dimension  760  is substantially greater than the lateral dimension  770 . In some embodiments, the lateral dimension  760  is in a range between about 8 nm and about 40 nm, and the lateral dimension  770  is in a range between about 5 nm and about 30 nm. The source/drain via  750  also has a height or a vertical dimension  780  that is measured in the Z-direction. Meanwhile, the ESL  610  has a height or a vertical dimension  790  that is measured in the Z-direction. The vertical dimension  780  is substantially greater than the vertical dimension  790 . In some embodiments, the vertical dimension  780  is in a range between about 5 nm and about 25 nm, and the vertical dimension  790  is in a range between about 0.5 nm and about 20 nm. The above ranges for the dimensions  760 ,  770 ,  780 , and  790  are not randomly chosen but rather specifically configured to maximize the processing window for forming the self-aligning source/drain via  750  without creating additional unintended electrical bridging risks. 
     The IC device  200  may be implemented in a variety of IC applications, including memory devices such as Static Random-Access Memory (SRAM) devices. In that regard,  FIG. 14  illustrates an example circuit schematic for a single-port SRAM cell (e.g., 1-bit SRAM cell)  800 . The single-port SRAM cell  800  includes pull-up transistors PU 1 , PU 2 ; pull-down transistors PD 1 , PD 2 ; and pass-gate transistors PG 1 , PG 2 . As show in the circuit diagram, transistors PU 1  and PU 2  are p-type transistors, and transistors PG 1 , PG 2 , PD 1 , and PD 2  are n-type transistors. According to the various aspects of the present disclosure, the PG 1 , PG 2 , PD 1 , and PD 2  transistors are implemented with thinner spacers than the PU 1  and PU 2  transistors. Since the SRAM cell  800  includes six transistors in the illustrated embodiment, it may also be referred to as a 6T SRAM cell. 
     The drains of pull-up transistor PU 1  and pull-down transistor PD 1  are coupled together, and the drains of pull-up transistor PU 2  and pull-down transistor PD 2  are coupled together. Transistors PU 1  and PD 1  are cross-coupled with transistors PU 2  and PD 2  to form a first data latch. The gates of transistors PU 2  and PD 2  are coupled together and to the drains of transistors PU 1  and PD 1  to form a first storage node SN 1 , and the gates of transistors PU 1  and PD 1  are coupled together and to the drains of transistors PU 2  and PD 2  to form a complementary first storage node SNB 1 . Sources of the pull-up transistors PU 1  and PU 2  are coupled to power voltage Vcc (also referred to as Vdd), and the sources of the pull-down transistors PD 1  and PD 2  are coupled to a voltage Vss, which may be an electrical ground in some embodiments. 
     The first storage node SN 1  of the first data latch is coupled to bit line BL through pass-gate transistor PG 1 , and the complementary first storage node SNB 1  is coupled to complementary bit line BLB through pass-gate transistor PG 2 . The first storage node N 1  and the complementary first storage node SNB 1  are complementary nodes that are often at opposite logic levels (logic high or logic low). Gates of pass-gate transistors PG 1  and PG 2  are coupled to a word line WL. SRAM devices such as the SRAM cell  800  may be implemented using “planar” transistor devices, with FinFET devices, and/or with GAA devices. 
       FIG. 15  illustrates an integrated circuit fabrication system  900  according to embodiments of the present disclosure. The fabrication system  900  includes a plurality of entities  902 ,  904 ,  906 ,  908 ,  910 ,  912 ,  914 ,  916  . . . , N that are connected by a communications network  918 . The network  918  may be a single network or may be a variety of different networks, such as an intranet and the Internet, and may include both wire line and wireless communication channels. 
     In an embodiment, the entity  902  represents a service system for manufacturing collaboration; the entity  904  represents an user, such as product engineer monitoring the interested products; the entity  906  represents an engineer, such as a processing engineer to control process and the relevant recipes, or an equipment engineer to monitor or tune the conditions and setting of the processing tools; the entity  908  represents a metrology tool for IC testing and measurement; the entity  910  represents a semiconductor processing tool, such an EUV tool that is used to perform lithography processes to define the gate spacers of an SRAM device; the entity  912  represents a virtual metrology module associated with the processing tool  910 ; the entity  914  represents an advanced processing control module associated with the processing tool  910  and additionally other processing tools; and the entity  916  represents a sampling module associated with the processing tool  910 . 
     Each entity may interact with other entities and may provide integrated circuit fabrication, processing control, and/or calculating capability to and/or receive such capabilities from the other entities. Each entity may also include one or more computer systems for performing calculations and carrying out automations. For example, the advanced processing control module of the entity  914  may include a plurality of computer hardware having software instructions encoded therein. The computer hardware may include hard drives, flash drives, CD-ROMs, RAM memory, display devices (e.g., monitors), input/output device (e.g., mouse and keyboard). The software instructions may be written in any suitable programming language and may be designed to carry out specific tasks. 
     The integrated circuit fabrication system  900  enables interaction among the entities for the purpose of integrated circuit (IC) manufacturing, as well as the advanced processing control of the IC manufacturing. In an embodiment, the advanced processing control includes adjusting the processing conditions, settings, and/or recipes of one processing tool applicable to the relevant wafers according to the metrology results. 
     In another embodiment, the metrology results are measured from a subset of processed wafers according to an optimal sampling rate determined based on the process quality and/or product quality. In yet another embodiment, the metrology results are measured from chosen fields and points of the subset of processed wafers according to an optimal sampling field/point determined based on various characteristics of the process quality and/or product quality. 
     One of the capabilities provided by the IC fabrication system  900  may enable collaboration and information access in such areas as design, engineering, and processing, metrology, and advanced processing control. Another capability provided by the IC fabrication system  900  may integrate systems between facilities, such as between the metrology tool and the processing tool. Such integration enables facilities to coordinate their activities. For example, integrating the metrology tool and the processing tool may enable manufacturing information to be incorporated more efficiently into the fabrication process or the APC module, and may enable wafer data from the online or in site measurement with the metrology tool integrated in the associated processing tool. 
       FIG. 16  is a flowchart illustrating a method  1000  of fabricating a semiconductor device according to embodiments of the present disclosure. The method  1000  includes a step  1010  to form a gate spacer structure next to a gate that is disposed over a substrate. The gate spacer structure includes a dummy silicon spacer. The substrate includes a source/drain region. 
     The method  1000  includes a step  1020  to perform a first etching process to the gate and the gate spacer structure. The gate has a lower upper surface than the gate spacer structure after the first etching process. 
     The method  1000  includes a step  1030  to form a mask layer over the gate and over the gate spacer structure. 
     The method  1000  includes a step  1040  to form a source/drain contact over the source/drain region. 
     The method  1000  includes a step  1050  to perform a second etching process to reduce a height of the source/drain contact. 
     The method  1000  includes a step  1060  to perform a third etching process to shrink the mask layer laterally and to remove the dummy silicon spacer. An air spacer is formed as a result of a removal of the dummy silicon spacer. 
     The method  1000  includes a step  1070  to form a dielectric structure over the source/drain contact and over the mask layer. The dielectric structure is formed sufficiently wide to cover the air spacer. 
     In some embodiments, the forming the gate spacer structure comprises forming a gate spacer structure that includes: a first dielectric spacer, a second dielectric spacer, and the dummy silicon spacer. The dummy silicon spacer is formed between the first dielectric spacer and the second dielectric spacer. 
     In some embodiments, the forming the dielectric structure is performed using a plurality of deposition-etching cycles. In each of the deposition-etching cycles: a dielectric layer is deposited, followed by a dry etching process to etch the dielectric layer. 
     In some embodiments, the forming the mask layer comprises forming a metal oxide layer as the mask layer. In some embodiments, the depositing the dielectric layer in each of the deposition-etching cycles comprises depositing a silicon nitride layer as the dielectric layer. 
     In some embodiments, the fourth etching process is performed such that the opening has a cross-sectional profile that resembles a letter “T”. 
     It is understood that the method  1000  may include further steps performed before, during, or after the steps  1010 - 1070 . For example, the method  1000  may include a step of forming a metal layer over the gate but not over the gate spacer structure. The mask layer may be formed over the metal layer. This step may be performed after the first etching process has been performed but before the mask layer has been formed. As another example, the method  1000  may include a step of forming an interlayer dielectric (ILD) over the dielectric structure, performing a fourth etching process to etch an opening through portions of the ILD and the dielectric structure disposed over the source/drain contact, wherein the fourth etching process has a substantially greater etching rate with respect to the ILD than with respect to the dielectric structure, and forming a source/drain via by filling the opening with a conductive material. The method  1000  may also include steps of forming dummy gate structures, forming additional metallization layers, etc. For reasons of simplicity, these additional steps are not discussed herein in detail. 
     The advanced lithography process, method, and materials described above can be used in many applications, including fin-type field effect transistors (FinFETs). For example, the fins may be patterned to produce a relatively close spacing between features, for which the above disclosure is well suited. In addition, spacers used in forming fins of FinFETs, also referred to as mandrels, can be processed according to the above disclosure. It is also understood that the various aspects of the present disclosure discussed above may apply to multi-channel devices such as Gate-All-Around (GAA) devices. To the extent that the present disclosure refers to a fin structure or FinFET devices, such discussions may apply equally to the GAA devices. 
     In summary, the present disclosure forms a protective layer (e.g., an ESL) to protect the gate air spacers during the etching of the source/drain via holes. Such a protective layer is formed as follows: first, source/drain components, gate structures, and gate spacer structure including air spacers are formed. A hard mask layer is formed over a gate and then etched laterally to ensure that the air spacer is exposed. A protective layer is then formed over the air spacer and the hard mask layer using a plurality of deposition-etching cycles. In each of the deposition-etching cycles, a dielectric material having a different material composition from the hard mask layer is deposited, followed by a directional dry etching process. As a result, a thick portion of the protective layer covers the air spacers, but a thin portion of the protective layer covers the source/drain contact. An ILD is then formed over the protective layer, and a source/drain via hole is etched to expose the source/drain contact. The source/drain via hole is then filled by a conductive material to form the source/drain via. 
     The present disclosure may offer advantages over conventional devices. However, it is understood that not all advantages are discussed herein, different embodiments may offer different advantages, and that no particular advantage is required for any embodiment. One advantage is that undesirable electrical shorting may be prevented. In more detail, as semiconductor feature sizes continue to shrink with each technology generation, a shift of the source/drain via may result in a tiger-tooth profile, such that a portion of the source/drain via extends into the air spacer. This could cause the gate to be electrically shorted to the source/drain via (and therefore the source/drain), which is undesirable. Here, the present disclosure forms a protective layer over the air spacer to protect the air spacer during the source/drain via hole etching. Hence, the risks of electrically shorting the gate with the source/drain are substantially reduced. Another advantage is ease of self-alignment and relaxed process window. Whereas conventional fabrication processes may require an accurate alignment between the source/drain via and the source/drain contact to minimize the risks of the electrical bridging between the gate and the source/drain via discussed above, the present disclosure allows the source/drain via hole to be etched with a much wider top opening. This is because the protective layer serves as a “guide” to automatically align the bottom portion of the source/drain via hole with the source/drain contact. As such, the present disclosure may tolerate misalignment between the source/drain via and the source/drain contact much better than conventional fabrication processes. Other advantages may include compatibility with existing fabrication processes (including for both FinFET and GAA processes) and the ease and low cost of implementation. 
     One aspect of the present disclosure pertains to a device. The device includes a gate disposed over a substrate. A source/drain is disposed in the substrate. A conductive contact is disposed over the source/drain. An air spacer is disposed between the gate and the conductive contact. A first component is disposed over the gate. A second component is disposed over the air spacer. The second component is different from the first component. 
     Another aspect of the present disclosure pertains to a device. The device includes a gate disposed over a substrate; a source/drain disposed in the substrate; a source/drain contact disposed over the source/drain; an air spacer disposed between the gate and the source/drain contact; an etching-stop component disposed over the air spacer; and a via disposed over the source/drain contact. An upper segment of the via has a bottom surface that is disposed on an upper surface of the etching-stop component. A lower segment of the via has a bottom surface that is disposed on an upper surface of the source/drain contact and a side surface that is disposed on a side surface of the etching-stop component. 
     Yet another aspect of the present disclosure pertains to a method. A gate spacer structure is formed next to a gate that is disposed over a substrate. The gate spacer structure includes a dummy silicon spacer. The substrate includes a source/drain region. A first etching process is performed to the gate and the gate spacer structure. The gate has a lower upper surface than the gate spacer structure after the first etching process. A mask layer is formed over the gate and over the gate spacer structure. A source/drain contact is formed over the source/drain region. A second etching process is performed to reduce a height of the source/drain contact. A third etching process is performed to shrink the mask layer laterally and to remove the dummy silicon spacer. Air spacer is formed as a result of a removal of the dummy silicon spacer. A dielectric structure is formed over the source/drain contact and over the mask layer, wherein the dielectric structure is formed sufficiently wide to cover the air spacer. 
     The foregoing outlines features of several embodiments so that those of ordinary skill in the art may better understand the aspects of the present disclosure. Those of ordinary skill in the art should appreciate that they may readily use the present disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiments introduced herein. Those of ordinary skill in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the present disclosure, and that they may make various changes, substitutions, and alterations herein without departing from the spirit and scope of the present disclosure.