Patent Publication Number: US-2021183996-A1

Title: Self-Aligned Contact Air Gap Formation

Description:
PRIORITY DATA 
     The present application is a divisional application of U.S. application Ser. No. 16/144,642, filed Sep. 27, 2018, which is hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND 
     The semiconductor integrated circuit (IC) industry has experienced rapid growth. Technological advances in IC materials and design have produced generations of IC devices 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. But these advances have also increased the complexity of processing and manufacturing IC devices. 
     For example, as device geometry shrinks, coupling capacitance tends to increase between interconnects such as source/drain (S/D) contact plugs and nearby gates. The increased coupling capacitance degrades device performance. To lower coupling capacitance, insulating materials with a relatively low dielectric constant (k), such as low-k dielectrics and air gaps, have been used between S/D features and nearby gates. But these materials have proven difficult to fabricate. In some instances, low-k dielectric materials are brittle, unstable, difficult to deposit, or sensitive to processes such as etching, annealing, and polishing, and air gap formations are difficult to control. For these reasons and others, it is desirable to improve the fabrication techniques of dielectrics between interconnects in order to reduce the coupling capacitance while maintaining a high overall transistor density in IC. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure is best understood from the following detailed description when read with the accompanying figures. It is emphasized that, in accordance with the standard practice in the industry, various features are not drawn to scale and are used for illustration purposes only. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion. 
         FIG. 1  is a flow chart showing a method for forming an IC device, according to various embodiments of the present disclosure. 
         FIGS. 2A, 2B, 2C, 2D, 2E, 2F, 2G, 2H, 2I, and 2J  are schematic diagrams illustrating cross-sectional views of an IC device during various fabrication stages, according to various embodiments of the present disclosure. 
         FIGS. 3A, 3B, and 3C  are schematic diagrams illustrating the capability of vertical depth control of air gaps, according to various embodiments of the present disclosure. 
         FIGS. 4A and 4B  are schematic diagrams illustrating the capability of lateral width control of air gaps, according to various embodiments of the present disclosure. 
         FIGS. 5A, 5B, and 5C  are schematic diagrams illustrating overlay shift adaptability of air gaps, according to various embodiments of the present disclosure. 
     
    
    
     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 sake of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed. Moreover, various features may be arbitrarily drawn in different scales for the sake of simplicity and clarity. 
     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. For example, if the device in the figures is turned over, elements described as being “below” or “beneath” other elements or features would then be oriented “above” the other elements or features. Thus, the exemplary term “below” can encompass both an orientation of above and below. 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. 
     The present disclosure is generally related to IC devices and fabrication methods, and more particularly to the formation of air gaps between source/drain (S/D) contact plugs and nearby metal gates. As FinFET technologies progress towards smaller technology nodes (such as 16 nm, 10 nm, 7 nm, 5 nm, and below), decreasing fin pitch is placing significant constraints on materials that can be used between metal gates and neighboring contact plugs that are connected to S/D features. To minimize coupling capacitance between the metal gates and contact plugs, air gaps can help reduce coupling capacitance because air has lower dielectric constant (k=1) than other dielectric materials. But, when air gaps are formed before contact plugs, the air gaps are prone to be damaged by the subsequent formation of the contact plugs. For example, when forming a contact plug, overlay shift may occur if a mask for patterning the contact plug is not aligned perfectly with lower layer components. With overlay shift, the position of a contact hole may be very close to, if not touching, a neighboring metal gate. In this case, etching the contact hole would expose an already-sealed air gap, and the exposed air gap may be partially or completely filled by a nitride liner, which is formed after the etching of the contact hole. The air gap then loses its purpose of reducing couple capacitance. 
     The present disclosure avoids such issues by forming air gaps after (not before or simultaneous with) the formation of contact plugs. For example, air gaps are formed by selectively removing dummy features, which are disposed next to contact plugs. Selective removal of the dummy features is realized by etch selectivity of dummy feature material(s) compared to other materials in direct contact with the dummy features. The post-plug formation of air gaps disclosed herein leads to self-aligned air gaps because their locations are determined by the locations of dummy features. Further, such air gaps have precisely controllable profiles. The height of air gaps extends above top surfaces of metal gates. As a result, coupling capacitance between metal stacks and contact plugs can be effectively reduced. Device reliability is improved, and optimal AC/DC gain may be achieved without potential air gap damages. 
       FIG. 1  is a flow chart of method  10  for fabricating an IC device (or device structure) according to various aspects of the present disclosure. Method  10  is merely an example, and is not intended to limit the present disclosure beyond what is explicitly recited in the claims. Additional operations can be provided before, during, and after method  10 , and some operations described can be replaced, eliminated, or moved around for additional embodiments of method  10 . In the following discussion, method  10  is described with reference to  FIGS. 2A-2J , which are fragmentary diagrammatic cross-sectional views of an IC device  100 , in portion or entirety, at various fabrication stages according to various embodiments of the present disclosure. 
     IC device  100  may be or include a FinFET device (a fin-based transistor), which can be included in a microprocessor, memory cell, and/or other IC device. IC device  100  may be an intermediate device fabricated during processing of an IC chip, a system on chip (SoC), or portion thereof, that includes various passive and active microelectronic devices such as resistors, capacitors, inductors, diodes, p-type field effect transistors (PFETs), n-type field effect transistors (NFETs), metal-oxide semiconductor field effect transistors (MOSFETs), complementary metal-oxide semiconductor (CMOSs) transistors, bipolar transistors, high voltage transistors, high frequency transistors, other suitable components, or combinations thereof.  FIGS. 2A-2J  have been simplified for the sake of clarity to better understand the inventive concepts of the present disclosure. Additional features can be added in IC device  100 , and some of the features described below can be replaced, modified, or eliminated in other embodiments of IC device  100 . 
     At operation  12 , method  10  provides, or is provided with, a starting IC device  100 . As shown in  FIG. 2A , the starting IC device  100  includes a substrate  102 , a source or drain (S/D) feature  106 , an ILD layer  110 , gate spacers  112 , gate stacks  116   a  and  116   b , an etch stop layer (ESL)  117 , a contact etch stop layer (CESL)  118 , an ILD layer  120 , as well as a contact hole  130 , which are formed across multiple layers of the IC device  100 . IC device  100  may include various other features not shown in  FIG. 2A . IC device  100 &#39;s components are described below. 
     Substrate  102  is a semiconductor substrate (e.g., a silicon wafer) in the present embodiment. Alternatively, substrate  102  may comprise another elementary semiconductor, such as germanium; a compound semiconductor including silicon carbide, gallium nitride, gallium arsenide, gallium phosphide, indium phosphide, indium arsenide, and indium antimonide; an alloy semiconductor including silicon germanium (SiGe), gallium arsenide phosphide, aluminum indium phosphide, aluminum gallium arsenide, gallium indium arsenide, gallium indium phosphide, and gallium indium arsenide phosphide; or combinations thereof. Substrate  102  may be a semiconductor-on-insulator substrate, such as a silicon-on-insulator (SOI) substrate, a silicon germanium-on-insulator (SGOI) substrate, or a germanium-on-insulator (GOI) substrate. Semiconductor-on-insulator substrates can be fabricated using separation by implantation of oxygen (SIMOX), wafer bonding, and/or other suitable methods. Substrate  102  can include various doped regions (not shown) depending on design requirements of IC device  100 . In some implementations, substrate  102  includes p-type doped regions (for example, p-type wells) doped with p-type dopants, such as boron, indium, other p-type dopant, or combinations thereof. In some implementations, substrate  102  includes n-type doped regions (for example, n-type wells) doped with n-type dopants, such as phosphorus, arsenic, other n-type dopant, or combinations thereof. In some implementations, substrate  102  includes doped regions formed with a combination of p-type dopants and n-type dopants. The various doped regions can be formed directly on and/or in substrate  102 , for example, providing a p-well structure, an n-well structure, a dual-well structure, a raised structure, or combinations thereof. An ion implantation process, a diffusion process, and/or other suitable doping process can be performed to form the various doped regions in substrate  102 . 
     S/D feature  106  is disposed in substrate  102  and may include n-type doped silicon for NFETs, p-type doped silicon germanium for PFETs, or other suitable materials. S/D feature  106  may be formed by etching depressions in an active region adjacent to gate spacers  112 , and then epitaxially growing semiconductor materials in the depressions. The epitaxially grown semiconductor materials may be doped with proper dopants in-situ or ex-situ. S/D feature  106  may have any suitable shape and may be wholly or partially embedded in the active region. For example, depending on the amount of epitaxial growth, S/D feature  106  may rise above, at, or below the top surface of a fin. 
     ILD layer  110  is disposed on substrate  102 . ILD layer  110  may comprise tetraethylorthosilicate (TEOS) oxide, un-doped silicate glass, or doped silicon oxide such as borophosphosilicate glass (BPSG), fused silica glass (FSG), phosphosilicate glass (PSG), boron doped silicon glass (BSG), and/or other suitable dielectric materials. Each ILD layer may be formed by plasma enhanced chemical vapor deposition (PECVD), flowable CVD (FCVD), or other suitable methods. 
     Gate stacks  116   a  and  116   b  may each include a gate dielectric layer at the bottom and a gate electrode layer disposed on the gate dielectric layer. The gate dielectric layer may include SiO 2  or a high-k dielectric material such as hafnium silicon oxide (HfSiO), hafnium oxide (HfO 2 ), alumina (Al 2 O 3 ), zirconium oxide (ZrO 2 ), lanthanum oxide (La 2 O 3 ), titanium oxide (TiO 2 ), yttrium oxide (Y 2 O 3 ), strontium titanate (SrTiO 3 ), or a combination thereof. The gate dielectric layer may be deposited using CVD, physical vapor deposition (PVD), atomic layer deposition (ALD), and/or other suitable methods. The gate electrode layer of gate stack  116   a  or  116   b  may include polysilicon and/or one or more metal layers. For example, the gate electrode layer may include work function metal layer(s), conductive barrier layer(s), and metal fill layer(s). The work function metal layer may be a p-type or an n-type work function layer depending on device type. The p-type work function layer may comprise titanium aluminum nitride (TiAlN), titanium nitride (TiN), tantalum nitride (TaN), ruthenium (Ru), molybdenum (Mo), tungsten (W), platinum (Pt), another suitable metal, or combinations thereof. The n-type work function layer may comprise titanium (Ti), aluminum (Al), tantalum carbide (TaC), tantalum carbide nitride (TaCN), tantalum silicon nitride (TaSiN), titanium aluminum nitride (TiAlN), titanium silicon nitride (TiSiN), another suitable metal, or combinations thereof. The metal fill layer may include aluminum (Al), tungsten (W), cobalt (Co), and/or other suitable materials. The gate electrode layer may be deposited using methods such as CVD, PVD, plating, and/or other suitable processes. Gate stack  116   a  or  116   b  may further include an interfacial layer under the gate dielectric layer. The interfacial layer may include a dielectric material such as SiO 2  or SiON, and may be formed by chemical oxidation, thermal oxidation, ALD, CVD, and/or other suitable methods. 
     Each gate spacer  112  may be considered as a sidewall of its neighboring gate stack, or alternatively as coupled to its neighboring gate stack. Each gate spacer  112  may be a single layer or multi-layer structure. For example, gate spacer  112  may include a dielectric material, such as silicon oxide, silicon nitride (SiN), silicon oxynitride, other dielectric material, or combination thereof. Gate spacer  112  may be formed by deposition (e.g., CVD or PVD) and etching processes. 
     Gate stacks  116   a  and  116   b  may be formed by any suitable processes such as a gate-first process and a gate-last process. In an example gate-first process, various material layers are deposited and patterned to become gate stacks  116   a  and  116   b  before S/D feature  106  is formed. In an example gate-last process (also called a gate replacement process), temporary gate structures (sometimes called “dummy” gates) are formed first. Then, after transistor S/D feature  106  is formed, the temporary gate structures are removed and replaced with gate stacks  116   a  and  116   b . In the embodiment shown in  FIG. 2A , gate stack  116   a  or  116   b  may be disposed over a channel region of a transistor to function as a gate terminal. Although not shown in this cross-sectional view, a metal plug may be disposed over such a gate stack, for example, to apply an adjustable voltage to the gate stack in order to control a channel region between S/D feature  106  and another S/D feature not shown in  FIG. 2A . 
     ESL  117  is situated adjacent to and surrounding gate spacers  112 . ESL  117  may comprise silicon nitride, silicon oxide, silicon oxynitride (SiON), and/or other materials. During fabrication, before forming ILD layer  110  and gate stacks  116   a  and  116   b , ESL  117  is formed over gate spacers  112 . ESL  117  may be formed by one or more methods such as PECVD, ALD, and/or other suitable methods. CESL  118  is situated over and surrounding ILD layer  110  and gate stacks  116   a  and  116   b . CESL  118  may comprise silicon nitride, silicon oxide, silicon oxynitride (SiON), and/or other materials. Unlike ESL  117  which is formed before ILD layer  110  and gate stacks  116   a  and  116   b , CESL  118  is formed after ILD layer  110  and gate stacks  116   a  and  116   b . CESL  118  may be formed by one or more methods including PECVD, ALD, and/or other suitable methods. 
     In some embodiments, ILD layer  120  is formed over CESL  118 . ILD layer  120  may include materials such as tetraethylorthosilicate (TEOS) oxide, un-doped silicate glass, or doped silicon oxide such as borophosphosilicate glass (BPSG), fused silica glass (FSG), phosphosilicate glass (PSG), boron doped silicon glass (BSG), a low-k dielectric material, and/or other suitable dielectric materials. ILD layer  120  may be formed by FCVD, PECVD, or other suitable methods. ILD layer  120  may have the same or different thicknesses as ILD layer  110 . 
     Contact hole  130  is situated between gate stacks  116   a  and  116   b . Contact hole  130  penetrates, from top to bottom, ILD layer  120 , CESL  118 , ILD layer  110 . As shown in  FIG. 2A , contact hole  130  exposes a top portion of S/D feature  106 . Contact hole  130  comprises a sidewall surface  132  and a bottom surface  134 , where bottom surface  134  is effectively the same as the top surface of S/D feature  106 . Method  10  then forms a contact plug  136  in contact hole  130 . This involves a variety of processes, as discussed below. 
     At operation  14 , method  10  ( FIG. 1 ) sequentially deposits multiple layers—including a dummy layer  140  and a nitride liner layer  142 —over IC device  100  ( FIG. 2B ). Dummy layer  140  covers at least bottom surface  134  and sidewall surface  132  of contact hole  130 , but may also cover the topmost surface of IC device  100  (as shown in  FIG. 2B ). In an embodiment, dummy layer  140  is deposited uniformly across the top surface of IC device  100 . Dummy layer  140  includes silicon, germanium, silicon germanium (SiGe), low density silicon nitride, low density silicon oxide, and/or other suitable materials. Since dummy layer  140  is to be selectively etched later to form air gaps (at operation  26 ), the composition of dummy layer  140  may be tailored or optimized for such a selective etching process. Dummy layer  140  may be formed by one or more methods such as PECVD, ALD, and/or other suitable deposition or oxidation processes. In some embodiments, the dimensions of dummy layer  140 , including its height (H) and width (W 1  or W 2 ) as shown in  FIG. 2B , are tailored to control the dimensions of air gaps formed in IC device  100  (described below). As  FIG. 2B  indicates, dummy layer  140  may extend above top surfaces of gate stacks  116   a  and  116   b , and also extend below bottom surfaces of gate stacks  116   a  and  116   b . In an embodiment, the height of dummy layer  140  is about 20 to about 50 nm, and the width of dummy layer  140  is about 1 to about 5 nm. The suitable width of dummy layer  140  may relate to a width of contact hole  130  (as shown in  FIG. 2A ). In an embodiment, a ratio between the width of dummy layer  140  and the width of contact hole  130  is about 1:10 to about 1:5. For example, if contact hole  130  is 15 nm wide, dummy layer  140  may be about 1.5 nm to about 3 nm wide (on each side of contact hole  130 ). The range is determined because dummy layer  140  should be wide enough to create sufficient air gap width (described below) but narrow enough to allow sufficient volume to form a reliable contact feature within contact hole  130 . For example, if dummy layer  140  on each side of contact hole  130  takes up 40% of the width of contact hole  130 , there would be less than 20% of space left to fill the contact feature (since there is also nitride liner layer  142  which also has a width). 
     Nitride liner layer  142  may comprise various material(s) such as carbon-doped SiN, high density SiN, and/or other suitable materials. Nitride liner layer  142  may be formed by one or more methods such as PECVD, ALD, and/or other suitable deposition or oxidation processes. In some embodiments, nitride liner layer  142  is a thin layer with a generally conformal thickness across the top of IC device  100 . The conformal quality of nitride liner layer  142  through sidewall surface  132  helps avoid a current leakage path from contact plug  136  (formed at operation  24 ) to gate stacks  116   a  and  116   b , or vice versa. In some embodiments, operation  14  may be repeatedly executed to reach a target thickness of nitride liner layer  142 . 
     At operation  16 , method  10  ( FIG. 1 ) performs a selective etching process to remove parts of dummy layer  140  and nitride liner layer  142 , thereby generating dummy features  140   a  and  140   b  as well as nitride liners  142   a  and  142   b , respectively, on sidewall surface  132  ( FIG. 2C ). Note that dummy features  140   a  and  140   b  may represent the same dummy feature in the three-dimensional IC device  100 , but they are labeled separately for clarity in the cross-sectional views herein. The same consideration applies to other labels such as nitride liners  142   a  and  142   b  (and air gaps  150   a  and  150   b  described further below). Since the top surface of S/D feature  106  is to be exposed, the selective etching process is performed so as to etch through nitride liner layer  142  and dummy layer  140  on bottom surface  134  and on the topmost surface of IC device  100 . But the selective etching process does not etch through nitride liners  142   a  and  142   b , which are the sidewall segments of the nitride layer  142 . Further, in the selective etching process, operation  16  may “thin” (remove a thickness portion of) nitride liners  142   a  and  142   b . Indeed, if nitride liners  142   a  and  142   b  are too thick, they may block lateral space for subsequent processes. Therefore, such thinning opens up more space for deposition of contact plug  136 . But operation  16  is controlled such that it stops before nitride liners  142   a  and  142   b  are penetrated through. The remaining thickness of nitride liners  142   a  and  142   b  remains in the final product. In some embodiments, nitride liners  142   a  and  142   b  are each about 1-5 nm thick. As described above, a suitable thickness or width of nitride liners  142   a  and  142   b  and a suitable thickness or width of nitride liners  142   a  and  142   b  are determined to allow sufficient space or volume for a reliable contact feature to form inside contact hole  130 . Additionally, dummy features  140   a  and  140   b  (located on sidewall surface  132  and on the edge of bottom surface  134 ) are shielded from the selective etching process by nitride liners  142   a  and  142   b . For example, as shown in the cross-sectional view of  FIG. 2C , dummy features  140   a  and  140   b  may each take the form of an “L” shape at the bottom of contact hole  130 . 
     At operation  18 , method  10  ( FIG. 1 ) forms a metal layer  146  over IC device  100  ( FIG. 2D ). Metal layer  146  covers at least bottom surface  134  of contact hole  130 , but may also cover sidewall surface  132  and the topmost surface of IC device  100  (as shown in  FIG. 2D ). For example, metal layer  146  may be uniformly deposited over IC device  100  using an ALD process. Metal layer  146  may comprise various material(s) such as nickel (Ni), cobalt (Co), tungsten (W), tantalum (Ta), or titanium (Ti), combinations thereof, or other suitable material. 
     At operation  20 , method  10  ( FIG. 1 ) forms a metal silicide  148  on bottom surface  134  of contact hole  130  by selectively etching and converting metal layer  146  ( FIG. 2E ). In an embodiment of forming metal silicide  148 , metal layer  146  is first annealed at an elevated temperature such that metal layer  146  reacts with semiconductor material(s) in S/D feature  106  to form metal silicide. Then, non-reacted portions of metal layer  146  (on sidewall surface  132  and topmost surface of IC device  100 ) are removed, thereby leaving metal silicide  148  on bottom surface  134 . Metal silicide  148  may include nickel silicide, cobalt silicide, titanium silicide, or other suitable silicidation or germanosilicidation. Metal silicide  148  may cover a heavily doped region of S/D feature  106  and in some cases may be considered part of S/D feature  106 . For example, in a p-type S/D feature  106 , its heavily doped region may comprise SiGe, and therefore metal silicide  148  may comprise SiGeNi, SiGeCo, SiGeW, SiGeTa, or SiGeTi. In an n-type S/D feature  106 , its heavily doped region may comprise SiP, and therefore metal silicide  148  may comprise SiPNi, SiPCo, SiPW, SiPTa, or SiPTi. 
     At operation  22 , method  10  ( FIG. 1 ) forms a contact layer  149  over IC device  100  ( FIG. 2F ). Contact layer  149  may include aluminum (Al), tungsten (W), copper (Cu), cobalt (Co), titanium (Ti), combinations thereof, or other suitable material. Note that contact or metal layers disclosed herein, such as metal layer  146  and contact layer  149 , may also contain non-metal material(s). For instance, contact layer  149  may include a barrier layer made of conductive nitrides such as TaN or TiN. Contact layer  149  may be formed by PVD, CVD, ALD, plating, or other suitable methods. As shown in  FIG. 2F , contact layer  149  penetrates ILD layer  120 , CESL  118 , and ILD layer  110 . Further, contact layer  149  is electrically coupled to S/D feature  106  through metal silicide  148 . In an alternative embodiment, contact layer  149  may be directly connected to S/D feature  106  without an intermediate silicide feature. 
     At operation  24 , method  10  ( FIG. 1 ) forms contact plug  136  from contact layer  149  ( FIG. 2G ). In an embodiment, a chemical mechanical planarization (CMP) process is used to remove a top thickness of IC device  100 , including top portions of contact layer  149 , ILD layer  120 , dummy features  140   a  and  140   b , and nitride liners  142   a  and  142   b . A contact plug is sometimes also called a via, a via plug, a metal contact, or a metal plug. To facilitate operation  28 , the CMP process is sufficiently long to ensure exposure of dummy features  140   a  and  140   b.    
     At operation  26 , method  10  ( FIG. 1 ) removes by etching the remaining portions of dummy features  140   a  and  140   b  to form air gaps  150   a  and  150   b  ( FIG. 2H ). Specifically, air gap  150   a  is formed between contact plug  136  and gate stack  116   a  to reduce a first capacitance therebetween, and air gap  150   b  is formed between contact plug  136  and gate stack  116   b  to reduce a second capacitance therebetween. Capacitances are reduced because air has a dielectric constant (k) of about one, which is lower than other dielectric materials. In some embodiments (e.g., when there is no overlay shift), air gaps  150   a  and  150   b  have about the same dimensions, and the first and second capacitances are about equal. But if there is overlay shift (as described below in  FIGS. 5A-5C ), air gaps  150   a  and  150   b  may have different dimensions, and the first and second capacitances may be different. Unequal capacitances on two sides of contact plug  136  may impact related circuitry unequally, but since both the first and second capacitances are reduced herein, their overall impact on circuitry is reduced. 
     It should be noted that method  10  disclosed herein forms air gaps  150   a  and  150   b  after forming contact plug  136 . This differs from conventional air gap formation approaches, which formed air gaps before forming their corresponding contact hole (and contact plug). Such a change in sequence is counter-intuitive, for example, because post-plug formation of air gaps brings unique etch selectivity considerations (discussed below), and conventional approaches were unable to achieve such etch selectivity. But post-plug formation of air gaps, as disclosed herein, brings various benefits. For instance, conventional air gaps formed before a contact plug had high risks of short circuit between the contact plug and a neighboring gate stack. That is because, when etching a contact hole between two sealed air gaps, the etching may expose such sealed air gaps. As a result, in the next step of forming nitride liners in the contact hole, the nitride liners were prone to fill the now-exposed air gaps (“punch through”), especially if there was an overlay shift. The volume of the air gaps was significantly reduced, and worse, the nitride liners could lead to a short circuit between the contact plug (formed after the nitride liners) and the neighboring gate stack, which may cause device failure. 
     The present disclosure avoids such issues by forming air gaps  150   a  and  150   b  after the formation of contact plug  136  therebetween. First, air gaps  150   a  and  150   b  are self-aligned because their lateral locations and profiles are determined by the lateral locations and profiles of dummy features  140   a  and  140   b , which are disposed close to contact plug  136 . Second, there is no etching at the vicinity of air gaps  150   a  and  150   b , and thus any punch-through issues are avoided. This in turn improves device reliability and enables higher breakdown voltage. Third, since the volume of air gaps is precisely controllable by adjusting heights and/or widths of dummy features, the coupling capacitance between gate stack  116   a  or  116   b  and contact plug  136  can be effectively controlled. Optimal AC/DC gain may be achieved without potential air gap damages. Fourth, unlike conventional approaches where air gaps were lower than top surfaces of gate stacks, air gaps  150   a  and  150   b  disclosed herein extend above the top surfaces of gate stacks  116   a  and  116   b . Therefore, more reduction in the coupling capacitance is allowed between the upper portions of gate stack  116   a  and contact plug  136 , and between the upper portions of gate stack  116   b  and contact plug  136 . In addition, as shown in the cross-sectional view of  FIG. 2H , air gaps  150   a  and  150   b  (like dummy features  140   a  and  140   b  from which air gaps  150   a  and  150   b  are formed) may each take the form of an “L” shape at the bottom of contact hole  130 . The horizontal portion of the “L” shape of air gap  150   a  may have a width that roughly equals a total width of dummy feature  140   a  and nitride liner  142   a . The horizontal portion of the “L” shape of air gap  150   b  may have a width that roughly equals a total width of dummy feature  140   b  and nitride liner  142   b . Since air gaps  150   a  and  150   b &#39;s horizontal portions underneath nitride liners  142   a  and  142   b  are relatively small, nitride liners  142   a  and  142   b  (which are attached to contact plug  136 ) have no structural support issues. 
     In an embodiment, the material of dummy features  140   a  and  140   b  has etch selectivity with respect to nitride liner layer  142  and ILD layer  120  such that dummy features  140   a  and  140   b  can be fully removed without substantially impacting either ILD layer  120 , or nitride liners  142   a  and  142   b , or gate stacks  116   a  and  116   b . In an embodiment, dummy features  140   a  and  140   b  are selectively removed by an etching process that etches dummy features  140   a  and  140   b  at least 10 times (or 20 times, or 50 times) faster than other materials in contact with dummy features  140   a  and  140   b . Such etch selectivity depends on the different choices of materials for dummy layer  140 , nitride liner layer  142 , and ILD layer  120 , and gate stacks  116   a  and  116   b . Thus, the material makeup of these layers is considered in a combined fashion. For example, dummy layer  140  may use material(s) selected from the group of silicon, germanium, silicon germanium (SiGe), low density nitride such as silicon nitride, and low density oxide such as silicon oxide. At the same time, nitride liner layer  142  uses material(s) selected from the group of carbon-doped nitride such as silicon nitride and high density nitride such as silicon nitride. At the same time, ILD layer  120  uses either an oxide formed by FCVD or a dopant-doped oxide (e.g., silicon oxide doped with boron at a doping concentration of 10 19 -10 20 ). Gate stacks  116   a  and  116   b  may use cobalt (Co) and/or other suitable metals. The etch selectivity is based on different reactivity to the same etchant. For instance, when dummy layer  140  uses low density Si 3 N 4 , and nitride liner layer  142  uses high density Si 3 N 4 , dummy layer  140  has a faster etch rate because low density Si 3 N 4  is easier to be oxidized by the etchant than high density Si 3 N 4 . Further, it should be understood that “low density” and “high density” are relative terms to signify differences in doping concentrations. For example, dummy layer  140  is doped with an appropriate dopant (e.g., fluorine) at a doping concentration of 1-9*10 13  (unit is per square centimeter), while nitride liner layer  142  is doped with an appropriate dopant (e.g., carbon) at a doping concentration no less than 1*10 15 . 
     The selective etching process at operation  26  may include dry etching, wet etching, reactive ion etching (RIE), and/or other suitable processes. For example, a dry etching process may implement an oxygen-containing gas, a fluorine-containing gas (e.g., CF 4 , SF 6 , CH 2 F 2 , CHF 3 , and/or C 2 F 6 ), a chlorine-containing gas (e.g., Cl 2 , CHCl 3 , CCl 4 , and/or BCl 3 ), a bromine-containing gas (e.g., HBr and/or CHBR 3 ), an iodine-containing gas, other suitable gases and/or plasmas, and/or combinations thereof. In an embodiment, a plasma etching process is conducted at a flow rate of about 500 standard cubic centimeters per minute (sccm) to about 2000 sccm. For another example, a wet etching process may comprise etching in diluted hydrofluoric acid (DHF); potassium hydroxide (KOH) solution; ammonia; a solution containing hydrofluoric acid (HF), nitric acid (HNO 3 ), and/or acetic acid (CH 3 COOH); or other suitable wet etchant. The wet etching process may be conducted in any suitable manner such as by immersing IC device  100  into the wet etchant for a time period (e.g., less than 1 hour). 
     At operation  28 , method  10  ( FIG. 1 ) seals air gaps  150   a  and  150   b  by forming a seal layer  152  ( FIG. 2I ) that covers air gaps  150   a  and  150   b . Seal layer  152  may be deposited using CVD, PVD, ALD, and/or other suitable methods. Seal layer  152  may use any suitable material as long as it ensures full enclosure of air gaps  150   a  and  150   b  to prevent other materials from getting into air gaps  150   a  and  150   b . Upon formation of seal layer  152 , the volumes of air gaps  150   a  and  150   b  are finalized. As shown in  FIG. 2I , seal layer  152  interfaces air gaps  150   a  and  150   b  (via respective interfaces  154   a  and  154   b ) at a height that is above the top surfaces of gate stacks  116   a  and  116   b . In some embodiments, a height difference between interface  154   a  (or  154   b ) and the top surfaces of gate stacks  116   a  and  116   b  is about 20% to about 40% of the height of contact hole  130 . For example, when the height of contact hole  130  is about 30 nm, the height difference is about 6 to about 12 nm. The interfaces  154   a  and  154   b  may also be slightly lower than the top surface of ILD layer  120  because during their formation seal layer  152  penetrates slightly into air gaps  150   a  and  150   b  (e.g., for 1-4 nm). In some embodiments, air gaps  150   a  and  150   b  have very small width(s) (e.g., no more than 10 nm, 5 nm, 3 nm, or 2 nm), thus there is no risk of seal layer  152  penetrating deep into air gaps  150   a  and  150   b.    
     As illustrated in  FIG. 2I , seal layer  152  may interface air gaps  150   a  and  150   b  at about the same height. However, in some embodiments, interfaces at the top of air gaps  150   a  and  150   b  may have different height and/or surface profiles. As illustrated in  FIG. 2J  which represents a variation of  FIG. 2I , seal layer  152  interfaces air gap  150   a  at interface  154   a  and air gap  150   b  at interface  154   b , where interface  154   a  is higher than interface  154   b . This may occur, for example, when the width of air gap  150   a  is smaller than the width of air gap  150   b . Further, interface  154   a , interface  154   b , or both, may have a flat surface (as shown in  FIG. 2I ) or a curved surface (as shown in  FIG. 2J ). The curved surface may be formed as a natural result of seal layer  152  penetrating into the space for air gaps  150   a  and  150   b.    
     In method  10 , each component may be formed with suitable dimensions (e.g., thickness, height, depth or width). For example, in an embodiment, gate spacers  112  and gate stacks  116   a  and  116   b  each has a thickness between 15-25 nm (e.g., about 20 nm), ILD layer  120  has a thickness between 50-80 nm (e.g., about 65 nm). At operation  24 , the CMP process may reduce the thickness of ILD layer  120  to 10-20 nm (e.g., about 15 nm). At operation  28 , seal layer  152  may be several nanometers thick (e.g., 2-10) nm. 
     Although not elaborated herein, after operation  28  method  10  performs further processes to IC device  100 . For example, another contact plug may be formed over (and electrically connected) to contact plug  136 . Other etch stop layers, ILD layers, and metal wires may be formed. The metal wires are configured to interconnect upper plugs as well as other circuit features. 
     Method  10  may be used to fabricate not only IC device  100  (as shown in  FIG. 2I ) but also variations thereof. For example,  FIGS. 3A, 3B, 3C, 4A, 4B, 5A, 5B, and 5C  illustrate cross-sectional views of different IC device embodiments. Since the IC devices shown in these figures share various common features with IC device  100  discussed above, in the interest of conciseness such common features will not be described repeatedly. 
       FIGS. 3A-3C  illustrate vertical depth control of air gaps disclosed herein according to some embodiments of the present disclosure. Specifically,  FIG. 3A  illustrates a cross-sectional view of an IC device  200 ,  FIG. 3B  illustrates a cross-sectional view of an IC device  300 , and  FIG. 3C  illustrates a cross-sectional view of an IC device  350 . Compared to IC device  100 , which has air gaps  150   a  and  150   b  that vertically extend to the top surface of S/D feature  106  (or be horizontally aligned with the bottom surfaces of the gate stacks  116   a  and  116   b ), IC devices  200 ,  300 , and  350  have relatively shallower or shorter air gaps. Specifically, IC device  200  has air gaps  210   a  and  210   b  that do not vertically extend to the top surface of its S/D feature. IC device  300  has air gaps  310   a  and  310   b  that do not vertically extend to the top surface of its S/D feature. In other words, the lowest portions of air gaps  310   a  and  310   b  are higher than bottom surfaces of gate stacks  116   a  and  116   b  (e.g., with a height difference that is about 5%-10% of the height of contact hole  130 ). IC device  350  has air gaps  360   a  and  360   b  that are even shallower or shorter than air gaps  310   a  and  310   b , respectively (e.g., with a height difference that is about 20%-40% of the height of contact hole  130 ). Note that the highest portions of air gaps  310   a  and  310   b  are still higher than top surfaces of gate stacks  116   a  and  116   b . The heights of air gaps may be equal or different (e.g., the highest portions of air gaps  310   a  and  310   b  may have a different height than those of air gaps  150   a  and  150   b  as well as those of air gaps  360   a  and  360   b ). As discussed above, the ability to precisely control the depth of air gaps helps achieve optimal AC/DC gain without potential air gap damages. 
     To realize depth control of air gaps as illustrated in  FIGS. 3A-3C , the profile of dummy layer  140  is tailored or adjusted in method  10 . Various approaches may be used to control the depth of air gaps. In a first approach as shown in  FIG. 3A , dummy features  140   a  and  140   b  are removed at operation  26  in a way that their remaining height is controllable. For example, when etching dummy features  140   a  and  140   b , the time or duration of the etch process may be controlled to control the etch depth and therefore control the remaining height. An etch rate of the dummy features  140   a  and  140   b  may be constant or may vary during the etch process, but etch time is a reliable indicator of how much thickness of dummy features  140   a  and  140   b  has been etched. In the first approach, the unetched portions of dummy features  140   a  and  140   b  remain at the bottom of air gaps  210   a  and  210   b , respectively, as shown in  FIG. 3A . 
     A second approach of depth control utilizes the fact that, since the air gaps may be formed by fully removing dummy features  140   a  and  140   b  (which are formed from the dummy layer  140 ), the initial profile of dummy layer  140  may substantially determine the profiles of air gaps. Thus, the second approach forms an initial dummy layer  140  that does not reach the bottom of contact hole  130 . For example, in operation  12 , the starting IC device may already have contact hole  130  with a tiered sidewall surface  132 . Sidewall surface  132  may obtain a tiered profile where its upper tier is wider than its bottom tier (as shown in  FIG. 3B  and  FIG. 3C ) using any suitable processes (e.g., multiple masking and etching steps). Next, dummy features  140   a  and  140   b  may be formed on the upper tier(s) of sidewall surface  132 , and nitride liners  142   a  and  142   b  are formed adjacent to dummy features  140   a  and  140   b . As shown in  FIG. 3B  and  FIG. 3C , nitride liners  142   a  and  142   b  still reach S/D feature  106  at the bottom of contact hole  130 . Later, in operation  26  dummy features  140   a  and  140   b  are removed to form air gaps  310   a  and  310   b  as shown in  FIG. 3B  (or  360   a  and  360   b  as shown in  FIG. 3C ). In the second approach, since dummy features  140   a  and  140   b  are fully removed, what underlies the air gaps may be ESL  117 , as shown in  FIG. 3B  and  FIG. 3C  (instead of remaining portions of dummy features  140   a  and  140   b , as in the first approach shown in  FIG. 3A ). 
       FIGS. 4A and 4B  illustrate lateral width control of air gaps disclosed herein according to some embodiments of the present disclosure. Specifically,  FIG. 4A  illustrates a cross-sectional view of an IC device  400 , and  FIG. 4B  illustrates a cross-sectional view of an IC device  450 . Compared to IC device  100 , which has relatively narrow air gaps  150   a  and  150   b  (e.g., whose width is about 10% to about 20% of the width of contact hole  130 ) that are laterally separated from spacer  112  by ESL  117 , IC devices  400  and  450  have relatively wide air gaps (e.g., whose width is about 20% to about 25% of the width of contact hole  130 ). Specifically, IC device  400  has air gaps  410   a  and  410   b  that directly contact spacer  112  without the intervening ESL  117 . IC device  450  has air gaps  460   a  and  460   b  that are even wider (e.g., whose width is about 25% to about 35% of the width of contact hole  130 ) than air gaps  410   a  and  410   b , respectively. In an embodiment, air gaps  460   a  and  460   b  may reach conductive portions of gate stacks  116   a  and  116   b , respectively. The maximal of air gaps such as air gaps  460   a  and  460   b  may be limited by the need to have sufficient space or volume to fill a contact feature into contact hole  130 , as described. For example, when the overall width of contact hole  130  is about 15 nm, the contact feature may be about 5 nm wide, and the air gaps on each side of the contact feature may be about 5 nm wide. The ability to precisely control the width of air gaps helps achieve optimal AC/DC gain without potential air gap damages. Note that exposing gate stacks to air gaps, as shown in  FIG. 4B , carries no adverse risks such as shorting circuit because the air gaps do not contain any conductive or otherwise harmful materials. 
     To realize width control of air gaps as illustrated in  FIGS. 4A and 4B , the profile of dummy layer  140  is tailored or adjusted in method  10 . Since the air gaps are formed by removing dummy features  140   a  and  140   b  (which are formed from the dummy layer  140 ), the profile of dummy layer  140  substantially determines the profiles of air gaps. For example, in operation  12  ESL  117  may have been removed such that contact hole  130  directly contacts spacers  112 . In operation  12  upper portions of spacers  112  may have been removed such that contact hole  130  reaches upper-corner conductive portions of gate stacks  116   a  and  116   b . In operation  14  dummy layer  140  and nitride liner layer  142  are formed as described above. Later, in operation  26  dummy layer  140  is selectively removed to form air gaps  410   a  and  410   b  as shown in  FIG. 4A  (or  460   a  and  460   b  as shown in  FIG. 4B ). 
       FIGS. 5A-5C  illustrate overlay shift adaptability of air gaps disclosed herein according to some embodiments of the present disclosure. Overlay shift occurs when a mask for defining an upper layer does not match perfectly with lower layer components when such components are on the scale of nanometers (e.g., when contact hole  130  formed using the mask does not sit right in the middle of gate stacks  116   a  and  116   b ). Specifically,  FIG. 5A  illustrates a cross-sectional view of an IC device  500 ,  FIG. 5B  illustrates a cross-sectional view of an IC device  550 , and  FIG. 5C  illustrates a cross-sectional view of an IC device  580 . Compared to IC device  100 , in which contact plug  136  sits about the center point between gate stacks  116   a  and  116   b  (assuming no overlay shift), IC devices  500  and  550  have air gaps that are offset from the center point between gate stacks  116   a  and  116   b  (assuming there is overlay shift to the left side). In both IC devices  500  and  550 , contact plug  136  is closer to gate stack  116   a  than to gate stack  116   b . IC device  500  has relatively narrow air gaps  510   a  and  510   b , and IC device  550  has relatively wider air gaps  560   a  and  560   b . In  FIG. 5A , air gap  510   a  directly contacts spacer  112  (but does not laterally extend over gate stack  116   a ), while air gap  510   b  is separated from spacer  112  by at least ESL  117  (and potentially by portions of ILD layer  110 ). In  FIG. 5B , air gap  560   a  directly contacts spacer  112 , directly contacts a conductive portion of gate stack  116   a , and laterally extends over gate stack  116   a . Note that exposing gate stack  116   a  to air gap  560   a  carries no adverse risks such as shorting circuit because air gap  560   a  does not contain any conductive or otherwise harmful materials. On the other hand, air gap  560   b  directly contacts spacer  112  but does not contact or laterally extend over gate stack  116   b.    
       FIG. 5C  is similar to  FIG. 5B  in that, like air gap  560   a , air gap  590   a  directly contacts spacer  112 , directly contacts a conductive portion of gate stack  116   a , and laterally extends over gate stack  116   a . On the other hand, air gap  590   b  directly contacts spacer  112  but does not contact or laterally extend over gate stack  116   b . As shown in  FIG. 5C , in some embodiments, when gate stack  116   a  is directly exposed to air gap  590   a , a corner portion of gate stack  116   a  and its spacer  112  may get removed during a wet etching process that forms air gap  590   a , thereby creating a rounded corner profile. The shape of the rounded corner profile may depend on various factors such as materials of gate stack  116   a  and spacer  112  as well as formation conditions of air gap  590   a  (e.g., etchant, duration, etc.). 
     As discussed above, the present disclosure allows overlay shift adaptability of air gaps because the air gaps herein are formed after the formation of contact plug  136 . Had the air gaps been formed before or concurrently with the formation of contact plug  136 , the air gaps would be prone to be filled by subsequent processes (“punch through”). In the present disclosure, there is a safe margin between gate stack  116   a  or  116   b  and contact plug  136  even with overlay shift. There is no contact-etching-induced punch through of air gaps, which improves device reliability and leads to a higher breakdown voltage. 
     As illustrated above, in the present disclosure, the temporal change in the formation of air gaps leads to structural and positional changes of various components. For example, air gaps now extend above the top surfaces of gate stacks. In some embodiments, air gaps are formed such that their lowest portions are situated at the same height as bottom surfaces of surrounding gate stacks ( FIG. 2I ). In other embodiments, air gaps are formed such that their lowest portions are situated higher than bottom surfaces of surrounding gate stacks ( FIGS. 3B and 3C ). In some embodiments, a first air gap is separated from a first gate electrode layer by at least a gate spacer (e.g., air gap  560   b  in  FIG. 5B ), while a second air gap is in direct contact with both a second spacer and an upper portion of a second gate electrode layer (e.g., air gap  560   a  in  FIG. 5B ). In  FIG. 5B , air gap  560   a  even laterally extends over the second gate electrode layer of the second gate stack. 
     Although not intended to be limiting, one or more embodiments of the present disclosure provide many benefits to a semiconductor device and the formation thereof. For example, the air gap formation techniques disclosed herein realizes self-aligned air gaps with controllable profiles. Coupling capacitance between a gate stack and a contact plug can be effectively controlled. There is no punch through issues, so device reliability is improved with higher breakdown voltage. Therefore, optimal AC/DC gain may be achieved without potential air gap damages. Embodiments of the disclosed methods can be readily integrated into existing manufacturing processes and technologies, such as middle end of line (MEoL) and back end of line (BEoL) processes. 
     In one example aspect, the present disclosure provides a method for IC fabrication, which comprises providing a device structure including a substrate, an S/D feature on the substrate, a gate stack on the substrate, a contact hole over the S/D feature; and a dummy feature over the S/D feature and between the gate stack and the contact hole. The method further comprises forming in the contact hole a contact plug that is electrically coupled to the S/D feature, and, after forming the contact plug, selectively removing the dummy feature to form an air gap that extends higher than a top surface of the gate stack. The method further comprises forming over the contact plug a seal layer that covers the air gap. 
     In an embodiment, the device structure further includes first and second gate stacks, where the first air gap is formed between the contact plug and the first gate stack to reduce a first capacitance therebetween, and where the second air gap is formed between the contact plug and a second gate stack to reduce a second capacitance therebetween. In an embodiment, the seal layer interfaces the first and second air gaps at a height that is above top surfaces of the first and second gate stacks. In an embodiment, the first and second air gaps are formed such that bottom surfaces of the first and second air gaps are situated higher than bottom surfaces of the first and second gate stacks. In an embodiment, the first gate stack comprises a gate electrode layer and a spacer in contact with the gate electrode layer. The first air gap is separated from the spacer by at least one more dielectric layer. In an embodiment, the second gate stack comprises a gate electrode layer and a spacer in contact with the gate electrode layer, and the second air gap is in direct contact with both the spacer and an upper portion of the gate electrode layer. In an embodiment, the second air gap laterally extends over the gate electrode layer of the second gate stack. In an embodiment, forming the contact plug comprises depositing a metal layer covering the device structure, and removing a top portion of the metal layer using a CMP process. The CMP process also exposes the first and second dummy features facilitate selective removal of the first and second dummy features after the formation of the contact plug. In an embodiment, the first and second dummy features have etch selectivity in an etching process such that the first and second dummy features are selectively removed by the etching process that etches the first and second dummy features at least 10 times faster than other materials in contact with the first and second dummy features. In an embodiment, the first air gap formed from the first dummy feature is in direct contact with a first nitride liner disposed between the contact hole and the first dummy feature. 
     In another example aspect, the present disclosure provides a device structure including a substrate, first and second gate stacks on the substrate, first and second dummy features between the first and second gate stacks, and a contact plug between the first and second dummy features. A method comprises etching the first and second dummy features to form first and second air gaps, respectively, and forming a seal layer over the contact plug to seal the first and second air gaps. The seal layer interfaces the first and second air gaps at a height that is above top surfaces of the first and second gate stacks. In an embodiment, profiles of the first and second air gaps are controlled based on profiles of the first and second dummy features. In an embodiment, the first and second air gaps are formed such that bottom surfaces of the first and second air gaps are situated higher than bottom surfaces of the first and second gate stacks. In an embodiment, the first air gap is formed such that the first air gap is separated from a first gate electrode layer of the first gate stack by at least a first spacer. The second air gap is formed such that the second air gap is in direct contact with both a second spacer and a conductive portion of the second gate stack and that the second air gap laterally extends over the second gate stack. In an embodiment, the device structure further includes a first nitride liner between the contact plug and the first dummy feature, and an ILD layer in direct contact with the first dummy feature. The first and second dummy features are etched faster than both the first nitride liner and the ILD layer, as one or more materials for the first nitride liner is selected from the group consisting of carbon-doped silicon nitride and high density silicon nitride, as one or more materials for the first and second dummy features is selected from the group consisting of silicon, germanium, silicon germanium, low density silicon nitride, and low density silicon oxide, and as one or more materials for the ILD layer is either an oxide formed by flowable chemical vapor deposition (FCVD) or a dopant-doped oxide. 
     In another example aspect, the present disclosure provides an IC device comprising a substrate, an S/D feature disposed on the substrate, a contact plug disposed over the S/D feature and electrically coupled to the S/D feature, a gate stack disposed over the S/D feature and adjacent to the contact plug, an air gap disposed between the contact plug and the gate stack, and a seal layer covering the air gap. An interface between the seal layer and the air gap is higher than a top surface of the gate stack. In an embodiment, the IC device further comprises a nitride liner between the contact plug and the air gap, and the nitride liner is in direct contact with both the contact plug and the air gap without any intervening dielectric layer. In an embodiment, one or more materials for the nitride liner is selected from the group consisting of carbon-doped silicon nitride and high density silicon nitride. In an embodiment, the IC device further comprises an ILD layer in direct contact with the air gap. One or more materials for the ILD layer is an oxide formed by flowable chemical vapor deposition (FCVD) or a dopant-doped oxide. In an embodiment, the gate stack comprises a gate electrode layer and a spacer that touches the gate electrode layer. The air gap touches both the spacer and an upper portion of the gate electrode layer, and the air gap laterally extends over the gate electrode layer. 
     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.