Patent Publication Number: US-2023154852-A1

Title: Forming Dielectric Film With High Resistance to Tilting

Description:
PRIORITY CLAIM AND CROSS-REFERENCE 
     This application claims the benefit of the following provisionally filed U.S. Patent application: Application No. 63/264,196, filed on Nov. 17, 2021, and entitled “High Mechanical Strength IMD Layer for Air Gap Process on BEOL,” which application is hereby incorporated herein by reference. 
    
    
     BACKGROUND 
     Integrated circuit devices such as transistors are formed on semiconductor wafers. The devices are interconnected through metal lines and vias to form functional circuits, wherein the metal lines and vias are formed in back-end-of-line processes. To reduce the parasitic capacitance of the metal lines and vias, the metal lines and vias are formed in low-k dielectric layers, which typically have k values lower than 3.8, lower than 3.0, or lower than 2.5. 
     In the formation of the metal lines and vias in a low-k dielectric layer, the low-k dielectric layer is etched to form trenches and via openings. The etching of the low-k dielectric layer may involve forming a patterned hard mask over the low-k dielectric material, and form trenches using the patterned hard mask as an etching mask. Via openings are also formed underlying the trenches. The trenches and the via openings are then filled with a metallic material, which may comprise copper. A Chemical Mechanical Polish (CMP) process is then performed to remove excess portions of the metallic material over the low-k dielectric layer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Aspects of the present disclosure are best understood from the following detailed description when read with the accompanying figures. It is noted that, in accordance with the standard practice in the industry, various features are not drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion. 
         FIGS.  1 - 8 ,  9 A,  9 B,  10 - 14 , and  15 A  illustrate the cross-sectional views of intermediate stages in the formation of an interconnect structure including air spacers in accordance with some embodiments. 
         FIG.  15 B  illustrates the cross-sectional view of a dual damascene structure without being surrounded by any air spacer in accordance with some embodiments. 
         FIG.  15 C  illustrates a top view of an air spacer in accordance with some embodiments. 
         FIG.  16    illustrates some example precursors for forming a dielectric layer in accordance with some embodiments. 
         FIGS.  17  and  18    illustrate some example profiles of nitrogen atomic percentage values in accordance with some embodiments. 
         FIGS.  19 - 22    illustrate the cross-sectional views of intermediate stages in the formation of an interconnect structure in accordance with some embodiments. 
         FIG.  23    illustrates a precursor including a Si—C—Si bond in accordance with some embodiments. 
         FIG.  24    illustrates some precursors free from Si—C—Si bonds in accordance with some embodiments. 
         FIG.  25    illustrates the schematic view of layers in a wafer in accordance with some embodiments. 
         FIGS.  26  and  27    illustrate the carbon concentrations in some dielectric layers in accordance with some embodiments. 
         FIG.  28    illustrates a process flow for forming conductive features and air spacers in accordance with some embodiments. 
         FIG.  29    illustrates a process flow for forming conductive features in accordance with some embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     The following disclosure provides many different embodiments, or examples, for implementing different features of the invention. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. For example, the formation of a first feature over or on a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features may be formed between the first and second features, such that the first and second features may not be in direct contact. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed. 
     Further, spatially relative terms, such as “underlying,” “below,” “lower,” “overlying,” “upper” and the like, may be used herein for ease of description to describe one element or feature&#39;s relationship to another element(s) or feature(s) as illustrated in the figures. The spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. The apparatus may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein may likewise be interpreted accordingly. 
     An interconnect structure and the method of forming the same are provided. In accordance with some embodiments of the present disclosure, the interconnect structure includes a dielectric layer having improved mechanical strength, which may be formed by adding nitrogen in the dielectric layer. Conductive features are formed in the dielectric layer, and air spacers are formed surrounding the conductive features. With the dielectric layer having improved mechanical strength, the tilting and collapsing of parts of the dielectric layer surrounded by air spacers is reduced. In accordance with alternative embodiments of the present disclosure, the interconnect structure includes a dielectric layer including increased Si—C—Si bonds compared to conventional structures. Accordingly, the depletion of carbon from the dielectric layer is reduced. Embodiments discussed herein are to provide examples to enable making or using the subject matter of this disclosure, and a person having ordinary skill in the art will readily understand modifications that can be made while remaining within contemplated scopes of different embodiments. Throughout the various views and illustrative embodiments, like reference numbers are used to designate like elements. Although method embodiments may be discussed as being performed in a particular order, other method embodiments may be performed in any logical order. 
       FIGS.  1 - 8 ,  9 A,  9 B,  10 - 14 , and  15 A  illustrate the cross-sectional views of intermediate stages in the formation of conductive features and air spacers in accordance with some embodiments of the present disclosure. The corresponding processes are also reflected schematically in the process flow shown in  FIG.  28   . 
       FIG.  1    illustrates a cross-sectional view of package component  20 . In accordance with some embodiments of the present disclosure, package component  20  is a device wafer including active devices and possibly passive devices, which are represented by the illustrated integrated circuit devices  26 . Device wafer  20  may include a plurality of dies  22  therein, with one of dies  22  illustrated. In accordance with alternative embodiments of the present disclosure, package component  20  is an interposer wafer, which may or may not include active devices and/or passive devices. In subsequent discussion, a device wafer is discussed as an example of package component  20 . The embodiments of the present disclosure may also be applied to other types of package components such as interposer wafers, package substrates, packages, etc. 
     In accordance with some embodiments of the present disclosure, wafer  20  includes semiconductor substrate  24  and the features formed at a top surface of semiconductor substrate  24 . Semiconductor substrate  24  may be formed of crystalline silicon, crystalline germanium, silicon germanium, or a III-V compound semiconductor such as GaAsP, AlInAs, AlGaAs, GalnAs, GaInP, GaInAsP, or the like. Semiconductor substrate  24  may also be a bulk semiconductor substrate or a Semiconductor-On-Insulator (SOI) substrate. Shallow Trench Isolation (STI) regions (not shown) may be formed in semiconductor substrate  24  to isolate the active regions in semiconductor substrate  24 . Although not shown, through-vias may be (or may not be) formed to extend into semiconductor substrate  24 , wherein the through-vias are used to electrically inter-couple the features on opposite sides of wafer  20 . 
     In accordance with some embodiments of the present disclosure, integrated circuit devices  26  are formed at the top surface of semiconductor substrate  24 . Integrated circuit devices  26  may include Complementary Metal-Oxide Semiconductor (CMOS) transistors, resistors, capacitors, diodes, and the like in accordance with some embodiments. The details of integrated circuit devices  26  are not illustrated in  FIG.  1   .  FIG.  16    illustrates a schematic view of an example transistor in accordance with some embodiments, which includes gate stacks  110  and source/drain regions  112 , which are formed at the top surface of semiconductor substrate  24 . In accordance with alternative embodiments, wafer  20  is used for forming interposers, and substrate  24  may be a semiconductor substrate or a dielectric substrate. 
     Inter-Layer Dielectric (ILD)  28  is formed over semiconductor substrate  24 , and fills the space between the gate stacks of transistors (not shown) in integrated circuit devices  26 . In accordance with some embodiments, ILD  28  is formed of or comprises Phospho Silicate Glass (PSG), Boro Silicate Glass (BSG), Boron-doped Phospho Silicate Glass (BPSG), Fluorine-doped Silicate Glass (FSG), silicon oxide, or the like. ILD  28  may be formed using spin coating, Flowable Chemical Vapor Deposition (FCVD), or the like. In accordance with some embodiments of the present disclosure, ILD  28  is formed using a deposition process such as Plasma Enhanced Chemical Vapor Deposition (PECVD), Low Pressure Chemical Vapor Deposition (LPCVD), or the like. 
     Contact plugs  30  are formed in ILD  28 , and are used to electrically connect integrated circuit devices  26  to overlying metal lines and vias. In accordance with some embodiments of the present disclosure, contact plugs  30  are formed of a conductive material selected from tungsten, aluminum, copper, titanium, tantalum, titanium nitride, tantalum nitride, alloys thereof, and/or multi-layers thereof. The formation of contact plugs  30  may include forming contact openings in ILD  28 , filling a conductive material(s) into the contact openings, and performing a planarization process such as a Chemical Mechanical Polish (CMP) process or a mechanical grinding process to level the top surfaces of contact plugs  30  with the top surface of ILD  28 . 
     Referring to  FIG.  2   , etch stop layer  32  is formed over ILD  28  and contact plugs  30 . The respective process is illustrated as process  202  in the process flow  200  as shown in  FIG.  28   . In accordance with some embodiments, etch stop layer  32  is in contact with the top surfaces of ILD  28  and contact plugs  30 . In accordance with alternative embodiments, there is one or a plurality of layers and the corresponding features located between ILD  28  and etch stop layer  32 . For example, there may be an additional etch stop layer(s), an additional ILD, low-k dielectric layers, etc., between ILD  28  and etch stop layer  32 . Correspondingly, there may be contact plugs, vias, metal lines, etc., in the dielectric layers. 
     Etch stop layer  32  may include silicon nitride (SiN), silicon carbide (SiC), silicon oxy-nitride (SiON), silicon oxy-carbide (SiOC), silicon Carbo-nitride (SiCN), or the like. Etch stop layer  32  may also include a metal oxide, a metal nitride, or the like. Etch stop layer  32  may be a single layer formed of a homogeneous material, or a composite layer including a plurality of dielectric sub-layers formed of different martials. In accordance with some embodiments of the present disclosure, etch stop layer  32  includes an aluminum nitride (AlN) layer, a silicon oxy-carbide layer over the aluminum nitride layer, and an aluminum oxide layer over the silicon oxy-carbide layer. 
     Further referring to  FIG.  2   , dielectric layer  34  is deposited over etch stop layer  32 . The respective process is illustrated as process  204  in the process flow  200  as shown in  FIG.  28   . In accordance with some embodiments, dielectric layer  34  is also an ILD layer. In accordance with alternative embodiments, dielectric layer  34  is an Inter-Metal Dielectric (IMD) layer for forming metal lines. 
     In accordance with some embodiments, dielectric layer  34  is formed to have a great hardness, for example, greater than about 10 GPa, and may be in the range between about 15 GPa and about 35 GPa, which hardness value may be measured by using a nanoindenter. As will be discussed in subsequent paragraphs, forming dielectric layer  34  with high hardness value may improve the reliability of the interconnect structure and reduces the RC delay (by incorporating wide air spacers) of the features formed in dielectric layer  34 . The dielectric constant (k value) of dielectric layer  34  may be in the range between about 3.0 and about 4.5. Accordingly, dielectric layer  34  may be a low-k dielectric layer or a high-k dielectric layer, or the k value of dielectric layer  34  may be equal to the k value of silicon oxide, whose k value is the dividing value of high-k and low-k. 
     In accordance with some embodiments, to increase the hardness value of dielectric layer  34 , dielectric layer  34  is formed of or comprises a nitrogen-doped dielectric such as a nitrogen-doped silicon oxide (SiON) or nitrogen-doped silicon carbide (SiCN). Adding nitrogen into dielectric layer  34  results in the increase in its hardness value, so that the reliability and RC delay performance can be improved. On the other hand, the k value of dielectric layer  34  is adversely increased due to the adding of nitrogen. The increase in the k value, however, may be compensated for by the formation of wide air spacers. 
     In accordance with some embodiments, the formation of dielectric layer  34  may be performed using Plasma Enhance Chemical Vapor Deposition (PECVD), Sub Atmospheric Chemical Vapor Deposition (SACVD), Atomic Layer deposition (ALD), or the like. The precursors may include a first precursor that is free from nitrogen therein, and a second precursor that includes nitrogen. The first precursor provides the main elements and main structures for dielectric layer  34 . Doping nitrogen through a second precursor has the advantageous feature of adding nitrogen, and hence improves hardness, without causing significant increase in k value. This is because by adding nitrogen through the second precursor, the bonds of nitrogen to the elements in the main materials are fewer. 
     Some examples of the first precursor are listed in  FIG.  16   . In accordance with some embodiments, the first precursor may include a Si atom(s), and carbon atoms and hydrogen atoms in addition to the silicon atom(s). The carbon atoms and hydrogen atoms may be in the form of one or more functional group denoted as R or “OR” (with oxygen (“O”) bonded with functional group R). Functional group R may be selected from CH 3 , C 2 H 5 , or the like. The second precursor, which is nitrogen-comprising, may include NH 3 , N 2 , N 2 O, or the like, or combinations thereof. The second precursor may further include CO 2  in addition to the nitrogen-containing precursor. In accordance with some embodiments, the formation process includes PECVD, wherein the flow rate of the precursors may be in the range between about 30 sccm and about 1,500 sccm, and/or in the range between about 200 mgm and about 2,000 mgm. The deposition temperature may be in the range between about 200° C. and about 450° C. The deposition rate may be in the range between about 2 Å/second and about 20 Å/second. In the resulting dielectric layer  34 , nitrogen atomic percentage may be in the range between about 0.5 percent and about 12 percent, which may be measured using X-ray Photoelectron Spectroscopy (XPS). 
     In accordance with some embodiments, to improve the hardness of dielectric layer  34 , in the deposition of dielectric layer  34 , no porogen is added, and accordingly, the resulting dielectric layer  34  do not include pores. Furthermore, the tilting and collapsing of stripped portions of dielectric layer  34  occurs on narrow and tall dielectric strips (as can be seem in  FIG.  9 A ). To minimize the k value while still maintain the mechanical strength of dielectric layer  34 , a first portion (such as the upper portion  34 B, for example, the upper half or upper ¾) of dielectric layer  34  may have an increased nitrogen atomic percentage than a second portion (such as the lower portion  34 A, for example, the lower half or lower ¼). This may be achieved by increasing the flow rate of the second precursor (nitrogen-containing) when depositing the second portion. By increasing the nitrogen atomic percentage of a portion, but not all, of dielectric layer  34 , the narrow strips are less likely to tilt or collapse, while the overall RC delay is increased less. 
     Conversely, the first portion (with the increased nitrogen atomic percentage) may be a lower portion, while the second portion may be an upper. In accordance with alternative embodiments, the first portion is the middle portion, while the second portion includes a top portion and a bottom portion. 
     Furthermore, assuming the first portion is a bottom portion, in accordance with some embodiments, starting at a certain point of the deposition of dielectric layer  34 , the flow rate of the nitrogen-containing precursor may be increased continuously or increased by stages. The certain point may be the beginning of the deposition of dielectric layer  34 , or may be an intermediate point (such as after ¼ or a half of dielectric layer  34  has been deposited). Before the certain point, the flow rate of the nitrogen-containing precursor may be a constant. Furthermore, to increase the nitrogen atomic percentage of the upper portions, instead of increasing the flow rate of the nitrogen-containing precursor, the first precursor (non-nitrogen-containing) may be reduced. 
     In accordance with alternative embodiments, after the deposition of dielectric layer  34 , which may have a uniformly or non-uniformly nitrogen atomic percentage, the nitrogen atoms may be implanted into dielectric layer  34 . The implanted species may also include NH 3 , N 2 , N 2 O, or the like, or combinations thereof. The implanted nitrogen atoms have a smaller chance of being bonded to atoms such as silicon or carbon, and hence the hardness of dielectric layer  34  is improved without significantly increasing its k value. Whether to implant nitrogen into the top portion, the middle portion, or the bottom portion of dielectric layer  34  may be achieved by adjusting the implantation energy. 
     Pad layer  36  and hard mask  38  are then formed on dielectric layer  34 . The respective process is illustrated as process  206  in the process flow  200  as shown in  FIG.  28   . Pad layer  36  may be a thin film formed of or comprising silicon oxide. In accordance with some embodiments of the present disclosure, pad layer  36  is formed using Tetraethyl orthosilicate (TEOS) as a precursor, and the deposition process may include PECVD, CVD, or the like. Pad layer  36  acts as an adhesion layer between dielectric layer  34  and hard mask  38 . Pad layer  36  may also act as an etch stop layer for etching hard mask  38 . In accordance with some embodiments of the present disclosure, hard mask  38  is formed of silicon nitride, for example, using Low-Pressure Chemical Vapor Deposition (LPCVD). In accordance with other embodiments of the present disclosure, hard mask  38  is formed of or comprises tungsten doped carbide (WDC), titanium nitride, boron nitride, or the like, which may be formed through PECVD, for example. Hard mask  38  is used as a hard mask during subsequent photolithography processes. 
     Further referring to  FIG.  2   , photo resist  40  is formed on hard mask  38  and is then patterned, forming openings  42  in photo resist  40 . In a subsequent process, photo resist  40  is used to etch hard mask layer  36 . The respective process is illustrated as process  208  in the process flow  200  as shown in  FIG.  28   . Pad layer  36  may act as the etch stop layer for the etching process. Accordingly, pad layer  36  is exposed. After the etching process, photo resist  40  is removed, for example, in an ashing process. 
     Next, referring to  FIG.  3   , pad layer  36  and dielectric layer  34  are etched using hard mask  38  as an etching mask, and openings  42  extend into dielectric layer  34 . The respective process is illustrated as process  210  in the process flow  200  as shown in  FIG.  28   . The pad layer  36  may be etched through a dry etching process by using a mixture of NF 3  and NH 3  gases, the mixture of HF and NH 3  gases, or the like. Alternatively, pad layer  36  may be etched through a wet etching process by using, for example, an HF solution. In accordance with some embodiments of the present disclosure, the etching of dielectric layer  34  is performed using a process gas comprising fluorine and carbon, wherein fluorine is used for etching, with carbon having the effect of protecting the sidewalls of the dielectric layer  34 , with the sidewalls facing via openings and trenches. For example, the process gases for the etching include a fluorine and carbon containing gas(es) such as C 4 F 8 , CH 2 F 2 , CH 4 , CH 3 F, and/or CF 4 , and a carrier gas such as Ar, N 2 , or the like. The etching is anisotropic. 
     The etching of dielectric layer  34  stops on etch stop layer  32 . Next, etch stop layer  32  is etched-through, and openings  42  further penetrate through etch stop layer  32 . The respective process is illustrated as process  212  in the process flow  200  as shown in  FIG.  28   . The etching chemical is selected according to the materials and the layers of etch stop layer  32 . For example, when etch stop layer  32  comprises aluminum oxide, silicon oxycarbide, aluminum nitride, etc., etching gases such as BCl 3 , Cl 2 , CF 4 , CHF 3 , etc. may be used, and oxygen (O 2 ) may be added. After the etching of dielectric layer  34 , the underlying conductive features (such as contact plugs  30  when etch stop layer  32  is immediately over contact plugs  30 ) are revealed. 
       FIG.  4    illustrates the deposition of sacrificial spacer layer  44 . The respective process is illustrated as process  214  in the process flow  200  as shown in  FIG.  28   . In accordance with some embodiments, sacrificial spacer layer  44  is formed of or comprises a semiconductor such as Si, or a dielectric material, which may be an oxide (such as titanium oxide (TiOx) or aluminum oxide (AlOx)), a nitride (such as silicon nitride), or the like. The thickness T 1  of sacrificial spacer layer  44  is able to be increased due to the greater hardness value of dielectric layer  34 . In accordance with some embodiments, thickness T 1  is greater than about 5 Å, and may be in the range between about 5 Å and about 30 Å. With nitrogen added into dielectric layer  34 , dielectric layer  34  is stronger, and thickness T 1  may be greater than about 30 Å. The deposition may be performed through a conformal deposition process such as CVD, ALD, PECVD, Physical Vapor Deposition (PVD), or the like. 
     It is also appreciated that the thickness T 1  is related to the position of dielectric layer  34 . For example, when dielectric layer  34  is a lower IMD layer such as the layer for metallization layer M 0 , M 1 , etc., thickness T 1  may be smaller, and when dielectric layer  34  is a higher IMD layer such as the layer for metallization layer M 8 , M 9  or higher, thickness T 1  may be greater. 
       FIG.  5    illustrates an anisotropic etching process  43  for patterning sacrificial spacer layer  44 . The respective process is illustrated as process  216  in the process flow  200  as shown in  FIG.  28   . In accordance with some embodiments, the etching is performed through a dry etching process, wherein the etching gas may include Cl 2 , CF 4 , CHF 3 , CH 4 , HBr, O 2 , etc., depending on the material of the sacrificial spacer layer  44 . As a result of the anisotropic etching process, the horizontal portions of sacrificial spacer layer  44  are removed. Furthermore, at the bottoms of openings  42 , contact plugs  30  are exposed. The vertical portions of sacrificial spacer layer  44  are left in openings  42 , and are on the sidewalls of etch stop layer  32 , dielectric layer  34 , pad layer  36 , and hard mask  38 . 
       FIGS.  6  through  8    illustrate the formation of conductive features  50  ( FIG.  8   ). Referring to  FIG.  6   , barrier layer  46  is formed. The respective process is illustrated as process  218  in the process flow  200  as shown in  FIG.  28   . In accordance with some embodiments, barrier layer  46  is formed of or comprises titanium, titanium nitride, tantalum, tantalum nitride, or the like. There may also be a metal seed layer (not shown) formed over barrier layer  46 . The seed layer may include copper doped with Mn, for example, with an atomic percentage of Mn in the range between about 0.1 percent and about 2 percent. Barrier layer  46  and the seed layer may be formed as conformal layers, which may be formed using PVD, CVD, ALD, or the like. The metal seed layer may be formed of or comprise copper, and may be formed, for example, using PVD. The barrier thickness and the metal seed thickness may be in the range between about 5 Å and about 20 Å. 
       FIG.  7    illustrates the deposition of conductive material  48 . The respective process is illustrated as process  220  in the process flow  200  as shown in  FIG.  28   . In accordance with some embodiments, conductive material  48  comprises copper or a copper alloy, cobalt, tungsten, aluminum, or the like, or combinations thereof. The deposition process may include Electro Chemical Plating (ECP), electroless plating, CVD, PVD, ALD, or the like. Conductive material  48  fully fills openings  42 . 
     Next, a planarization process such as a Chemical Mechanical Polish (CMP) process or a mechanical grinding process is performed to remove excess portions of the conductive material  48  and barrier layer  46 . The respective process is illustrated as process  222  in the process flow  200  as shown in  FIG.  28   . The planarization process may be stopped on the top surface of dielectric layer  34 , or on the top surface of pad layer  36 . The planarization process may also be performed to remove a top portion of dielectric layer  34 . The resulting structure is shown in  FIG.  8   . Throughout the description, the remaining portions of conductive material  48  and barrier layer  46  are collectively referred to as conductive features  50 , which may be metal lines, metal vias, contact plugs, etc. Spacer rings  44  surround the corresponding conductive features  50 . 
       FIG.  9 A  illustrates the removal of sacrificial spacer layers  44  to form air spacers  52 . The respective process is illustrated as process  224  in the process flow  200  as shown in  FIG.  28   . Air spacers  52  have substantially uniform thickness (lateral dimension) due to the conformity of sacrificial spacer layer  44 , for example, with the thicknesses of most parts of an air spacer having a variation smaller than about 20 percent. In accordance with some embodiments, sacrificial spacer layer  44  is etched using an isotropic etching process, which may include a dry etching process and/or a wet etching process. For example, when a dry etching process is performed, the etching gas may include HF, NF 3 , O 2 , H 2 , NH 3 , Cl 2 , CF 4 , CHF 3 , CH 4 , HBr, or the like, or combinations thereof, depending on the material of the sacrificial spacer layer  44 . When a wet etching process is performed, the etching chemical may include a HF solution, ammonia water (NH 4 OH), or the like. The etching may also be performed using a wet etching process combined with a dry etching process. 
     In accordance with some embodiments, sacrificial spacer layer  44  is fully removed, and air spacers  52  extend to the top surface of the underlying dielectric layer (such as ILD  28 , depending on the position of dielectric layer  34 ). It is also possible that the process variation and the high aspect ratio of air spacers  52  may cause sacrificial spacer layer  44  to be partially removed. For example, the bottom portions of sacrificial spacer layer  44  may remain un-removed after the removal process, and dashed lines  44 T represent the top surfaces of the residue of sacrificial spacer layer  44 . The residue portions of sacrificial spacer layer  44  may form a full ring encircling conductive features  50 . Also due to process variation and the non-uniformity in the removal, the sacrificial spacer layer  44  surrounding some of the conductive features  50  may be fully removed, while the sacrificial spacer layer  44  surrounding some other conductive features  50  may have residues left. Furthermore, there may be some portions of the corresponding spacer layer  44  fully removed, and the underlying ILD  28  exposed, while some other portions of the same spacer layer  44  surrounding the same conductive feature  50  are left as residue sacrificial spacer layer. 
     An example is schematically illustrated in  FIG.  9 A , which shows that residue sacrificial spacer layer  44  exists on the left side of the rightmost conductive feature  50 , while the portion of sacrificial spacer layer  44  on the right side of the rightmost conductive feature  50  is fully removed. Also, the different portions of residue sacrificial spacer layer  44  surrounding the same conductive feature  50  or different conductive features  50  may have their top surfaces at different levels, as indicated in the examples as shown in  FIG.  9 A . It is appreciated that the above-discussed air spacers  52  and residue sacrificial spacer layer  44  may exist on the same wafer and same die  22 . 
     It is appreciated that the strips of dielectric layer  34  between air spacers  52  may be narrow and tall, and hence may suffer from tilting and collapsing. In accordance with some embodiments of the present disclosure, by increasing the hardness value of dielectric layer  34 , the tilting and collapsing may be reduced and possibly eliminated. Furthermore, with the dielectric layer  34  have a greater hardness value, the widths of air spacers  52  may be increased without the concern of dielectric tilting and collapsing, resulting in further reduction in the RC delay. In the resulting structure, the RC delay may be equal to or smaller than the RC delay of conventional structures that adopt low-k dielectric materials, but no air spacers. 
       FIG.  10    illustrates the formation of metal caps  54 . The respective process is illustrated as process  226  in the process flow  200  as shown in  FIG.  28   . In accordance with some embodiments, metal caps  54  are formed through a selective depositing process, so that metal caps  54  are selectively deposited on the exposed surfaces of conductive features  50 , and not on the exposed surfaces of dielectric materials such as sacrificial spacer layer  44  and dielectric layer  34 . In accordance with some embodiments, the selective deposition process may be performed through ALD or CVD. In accordance with some embodiments, metal caps  54  are formed of or comprise cobalt (Co), tungsten (W), CoWP, CoB, tantalum (Ta), nickel (Ni), molybdenum (Mo), titanium (Ti), iron (Fe), or combinations thereof. When metal caps  54  are deposited, the precursor may include a metal halide (such as WC 1   5 ) or a metal organic material and a reducing agent such as H 2 . The deposition process may be a thermal process performed at an elevated temperature, such as in the range between about 275° C. and about 500° C. The deposition may also be performed with plasma turned on. 
     In accordance with some embodiments, metal caps  54  are limited in the regions directly over conductive features  50 . Metal caps  54  may (or may) not include extension portions extending sideways slightly to form overhangs, especially when air spacers  52  are wide. The extension portions contact the top portions of the sidewalls of conductive features  50 , which sidewalls face air spacers  52 . For example,  FIG.  10    schematically illustrates dashed lines  54 ′, which represent the extension portions of metal caps  54 . The extension portions  54 ′ of metal caps  54  may extend into the top portions of air spacers  52  and extend below the top surface of dielectric layer  34 , especially when air spacers  52  are wide. Furthermore, extension portions  54 ′ may be spaced part from dielectric layer  34 , or may extend far enough to contact the nearest portion of dielectric layer  34 . Accordingly, metal caps  54  may leave air spacers  52  opened, or may partially or fully seal air spacers  52 . 
       FIGS.  9 A and  10    in combination disclose an embodiment in which air spacers  52  are formed first, followed by the formation of metal caps  54 . In accordance with alternative embodiments, metal caps  54  are formed first, followed by the removal of sacrificial spacer layer  44  to form air spacers  52 . This embodiment is shown in  FIGS.  9 B and  10    in combination. Referring to  FIG.  9 B , metal caps  54  are deposited. The deposition process is controlled, for example, by controlling the thickness of metal caps  54 , so that the lateral extensions of metal caps  54  do not extend on the top of sacrificial spacer layer  44  excessively. After the formation of metal caps  54 , there are enough parts of the top surface of sacrificial spacer layer  44  remaining exposed. After the formation of metal caps  54 , sacrificial spacer layer  44  is removed. The resulting structure is also shown in  FIG.  10   . In accordance with these embodiments, however, the entirety of metal caps  54  is higher than the top surface of dielectric layer  34 , and metal caps  54  do not extend into air spacers  52 . 
     Air spacers  52  have k values equal to  1 . 0 , which is smaller than other dielectric materials, even low-k dielectric materials. With the formation of air spacers, the parasitic capacitance between neighboring conductive features  50  is reduced. 
       FIG.  11    illustrates the formation of etch stop layer  56 . The respective process is illustrated as process  228  in the process flow  200  as shown in  FIG.  28   . Etch stop layer  56  contacts metal caps  54 , and seals air spacer  52  (if not sealed already). In accordance with some embodiments, etch stop layer  56  may be formed of a material selected from SiN, SiC, SiON, SiOC, SiCN, or combinations thereof. Etch stop layer  56  may also include a metal oxide, a metal nitride, or the like. Etch stop layer  56  may be a single layer formed of a homogeneous material, or a composite layer including a plurality of dielectric sub-layers. In accordance with some embodiments of the present disclosure, etch stop layer  56  includes an AlN layer, a SiOC layer over the AlN layer, and an AlO layer over the SiOC layer. 
       FIGS.  12  through  14    illustrate the formation of a dual damascene structure in accordance with some embodiments. Referring to  FIG.  12   , dielectric layer  58  is deposited. The respective process is also illustrated as process  228  in the process flow  200  as shown in  FIG.  28   . Dielectric layer  58  may be formed of a material selected from the same group of candidate materials for forming dielectric layer  34 , and the structures and materials for forming dielectric layer  34  also apply to dielectric layer  58 . The details are thus not repeated herein. For example, dielectric layer  58  may have an upper portion and a lower portion with different nitrogen atomic percentage values. 
     Trench  62  and via opening  60  are formed in dielectric layer  58 . The respective process is illustrated as process  230  in the process flow  200  as shown in  FIG.  28   . In accordance with some embodiments of the present disclosure, a metal hard mask (not shown) is formed and patterned to define the patterns of trench  62 . A photo lithography process is performed to etching dielectric layer  58  in order to form a via opening. The via opening extends from the top surface of dielectric layer  58  to an intermediate level between the top surface and the bottom surface of dielectric layer  58 . An anisotropic etching is then performed to etch dielectric layer  58  and to form trench  62  using the metal hard mask as an etching mask. At the same time trench  62  are formed, the via opening extends downwardly to metal cap  54 , hence forming via opening  60 . The etching for forming trench  62  may be performed using a time-mode. In accordance with alternative embodiments, via openings  60  and trenches  62  are formed in separate photo lithography processes. For example, in a first photo lithography process, via openings  60  are formed extending down to metal cap  54 . In a second lithography process, trench  62  is formed. Metal cap  54  is then exposed to via opening  60 . 
     Referring to  FIG.  13   , a patterned sacrificial spacer layer  64  is formed. The respective process is illustrated as process  232  in the process flow  200  as shown in  FIG.  28   . The formation process of sacrificial spacer layer  64  includes depositing a conformal layer, and then performing an anisotropic etching process to remove horizontal portions of the conformal layer. The materials and the process details are similar to what have been discussed referring to  FIGS.  4  and  5   , and are not repeated herein. Sacrificial spacer layer  64  includes first vertical portions in trench  62  to form a first ring, and second vertical portions in via opening  60  to form a second ring. The first ring is larger than the second ring, and is disconnected from the second ring. 
     Next, referring to  FIG.  14   , diffusion barrier  66  and metallic material  68  are deposited. The materials and the formation processes are similar to what have been discussed referring to  FIGS.  6  through  8   , and the details are not repeated herein. After the deposition of diffusion barrier  66  and metallic material  68 , a planarization process is performed, forming via  70  and metal line  72 . The respective process is illustrated as process  234  in the process flow  200  as shown in  FIG.  28   . Each of via  70  and metal line  72  is encircled by a spacer ring formed of a part of sacrificial spacer layer  64 . 
       FIG.  15 A  illustrates the removal of sacrificial spacer layer  64  from the sidewall of metal line  72 , forming air spacers  74 , which form a ring when viewed from top of wafer  20 . The respective process is illustrated as process  236  in the process flow  200  as shown in  FIG.  28   . The removal may also be performed through an isotropic etching process. The resulting air spacers  74  may extend to, and may be exposed to, the top surface of the underlying portion of dielectric layer  58 . There may be, and may not be, residue sacrificial spacer layer  64  left, wherein the top surfaces of the example residue sacrificial spacer layer  64  are shown as  64 T. Furthermore, due to process variation and loading effect, the residue sacrificial spacer layer  64  may have similar situations as residue sacrificial spacer layer  44  as discussed preceding paragraphs. For example, some parts of sacrificial spacer layer  44  may have more residues left than other parts, and some parts of sacrificial spacer layer  44  may not have residue left. The possible scenarios may be found referring to the discussion of air spacers  52 . 
     Since the part of sacrificial spacer layer  64  encircling via  70  cannot be removed, these parts of sacrificial spacer layer  64  will be left in the final structure. It is appreciated that these parts of sacrificial spacer layer  64  will result in increased parasitic capacitance compared to air spacers and low-k dielectric material. Vias  70 , however, are laterally short, and are most likely to have longer distance from neighboring vias. Accordingly, the adverse increase in the parasitic capacitance is small compared to the reduction in parasitic capacitance due to the formation of air spacers  74 . Alternatively stated, the reduction in the parasitic capacitance more than offsets the increase in the parasitic capacitance. 
       FIG.  15 A  further illustrates the formation of metal cap  76 , which may be formed of a material and a method selected from the same group of candidate materials and candidate methods, respectively, for forming metal caps  54 . Metal cap  76  may be formed before or after the formation of air spacers  74 , which is similar to the embodiments as shown in  FIGS.  9 A and  9 B . Also, when metal cap  76  is formed after the formation of air spacers  74 , extension portions  76 ′ may be formed and extend below the top surface of dielectric layer  58 . Alternatively, when metal cap  76  is formed before the formation of air spacers  74 , an entirety of sacrificial spacer layer  64 , including the extension portions  76 ′ that is directly over air spacers  74 , will not extend below the top surface of dielectric layer  58 . Etch stop layer  78  may then be deposited. 
       FIG.  15 B  illustrates the formation of air-gap-free via  70 ′ and metal line  72 ′ in accordance with alternative embodiments. These embodiments are similar to the embodiments as shown in  FIG.  15 A , except that no sacrificial spacer layer is formed, and accordingly, no air spacers are formed. Accordingly, dielectric layer  58  may have a lower k-value since it does not have the concern of dielectric tilting and collapsing. For example, dielectric layer  58  may have a k value lower than about 3.5 or lower than about 3.0, for example, between about 2.5 and about 3.0. Dielectric layer  58  may also have pores therein, which may be achieved by incorporating porogen in the deposition of dielectric layer  58 , and removing the porogen through annealing. 
     In the embodiments shown in  FIG.  15 B , via  70 ′ and metal line  72 ′ are in physical contact with the sidewalls of the surrounding dielectric layer  58 . It is appreciated that although  FIG.  15 B  illustrates that air-gap-free via  70 ′ and metal line  72 ′ are formed immediately over conductive features, in accordance with alternative embodiments, the air spacer  74  as shown in  FIG.  15 A  may be formed in a dielectric layer immediately over dielectric layer  34 , while the air-gap-free via  70 ′ and metal line  72 ′ are formed in layers over the layer in which air spacer  74  is formed since parasitic capacitance problem is less severe in upper metal layers than in lower metal layers. 
       FIG.  15 C  illustrates a top view of air spacer  52  or  74  in accordance with some embodiments. As shown in  FIG.  15 C , each of air spacers  52  and  74  may form a full ring encircling the corresponding conductive feature  50 / 72 . Each of air spacers  52  may have a substantially uniform width W 1  (for example, with variation being smaller than about 10 percent). The width W 1  may be measured at the middle height of the corresponding air spacers  52  and  74 . The width W 1  of air spacer  52  may be the same as or different from the width W 1  of air spacer  74 . Furthermore, when viewed from a cross-sectional view, air spacer  52  has a substantially uniform width (for example, with variation smaller than about 20 percent or smaller than about 10 percent) from top to bottom, and air spacer  74  has a substantially uniform width (for example, with variation smaller than about 20 percent or smaller than about 10 percent) from top to bottom. 
       FIGS.  17  and  18    illustrate some example profiles of nitrogen in dielectric layer  34  in accordance with some embodiments. It is assumed that etch stop layers  28  and  56  have high nitrogen atomic percentage values. In  FIG.  17   , dielectric layer  34  has increasingly greater nitrogen atomic percentage values from bottom to top. In  FIG.  18   , a lower portion of dielectric layer  34  has a constant nitrogen atomic percentage, while an upper portion of dielectric layer  34  has increasingly higher atomic percentage values.  FIGS.  17  and  18    may also reflect the schematic flow rates of the nitrogen-comprising precursor used for depositing dielectric layer  34 . 
       FIGS.  19  through  22    illustrate the cross-sectional views of intermediate stages in the formation of an interconnect structure in accordance with some embodiments of the present disclosure. Unless specified otherwise, the materials and the formation processes of the components in these embodiments are essentially the same as the like components, which are denoted by like reference numerals in the preceding embodiments shown in  FIGS.  1 - 14 ,  15 A,  15 B, and  15 C . The details regarding the formation process and the materials of the components shown in  FIGS.  19  through  22    may thus be found in the discussion of the preceding embodiments. The initial steps of these embodiments are essentially the same as shown in preceding embodiments. 
       FIG.  19    illustrates the formation of an initial structure. This structure is essentially the same as the structure shown in  FIG.  2   , except that dielectric layer  34  is formed differently than in the preceding embodiments, and no air spacer is formed. First, etch stop layer  32  is deposited. The respective process is illustrated as process  302  in the process flow  300  as shown in  FIG.  29   . Dielectric layer  34  is then deposited. The respective process is illustrated as process  304  in the process flow  300  as shown in  FIG.  29   . In accordance with some embodiments, dielectric layer  34  is formed to have a high concentration of Si—C—Si bonds. As will be discussed in subsequent paragraphs, forming dielectric layer  34  with a high concentration of Si—C—Si bonds may reduce the carbon-depletion problem, which may incur TDDB performance degradation and the increase in RC delay. The dielectric constant (k value) of dielectric layer  34  may be lower than about 3.5, and may be in the range between about 3.0 and about 3.5. Accordingly, dielectric layer  34  may be a low-k dielectric layer. 
     In accordance with some embodiments, the formation of dielectric layer  34  may be performed using PECVD, SACVD, ALD, or the like. The precursors may include a first precursor that includes Si—C—Si bonds, and a second precursor that provides carbon.  FIG.  23    illustrates an example precursor comprising Si—C—Si bonds, while other precursors comprising Si—C—Si bonds may also be used. The functional group “R” in  FIG.  23    and the subsequently discussed  FIG.  24    may include CH 3 , C 2 H 5 , or the like. The second precursor, may include a carbon-containing precursor such as CO 2 , CO, or the like, and may or may not include oxygen (O 2 ). By adopting the precursor comprising Si—C—Si bonds, the resulting dielectric layer  34  has a higher concentration of Si—C—Si bonds, which may be determined using Fourier-Transform Infrared Spectroscopy (FTIR), with the peak of the Si—C—Si bonds being in the range between about 1,342/cm and about 1381/cm. Otherwise, although Si—C—Si bonds may also be generated from the precursors that do not include Si—C—Si bonds directly, the concentration of Si—C—Si bonds in dielectric layer  34  will be low. Si—C—Si bonds may improve the resistance of dielectric layer  34  to carbon depletion, and may improve the TDDB performance and reduce RC delay. 
     The precursors for forming dielectric layer may, or may not, include a third precursor, which do not include Si—C—Si bonds.  FIG.  24    illustrates some examples of the third precursor. The third precursor may include silicon and carbon atoms, and possibly hydrogen atoms. The silicon and carbon atoms may also form functional groups such as CH 3 , C 2 H 5 , or the like. 
     In accordance with some embodiments, the deposition process of dielectric layer  34  includes PECVD, wherein the flow rate of the precursors may be in the range between about 30 sccm and about 1,500 sccm, and/or in the range between about 200 mgm and about 2,000 mgm. The temperature of wafer  20  during the deposition process cannot be too high or too low. If the temperature is too high, the deposition rate will be reduced and the deposition time is increased, resulting in too-high thermal budget. If the temperature is too low, there will be too much OH bonds formed, resulting in a high k value, which leads to high RC delay. Accordingly, the deposition temperature may be in the range between about 200° C. and about 420° C. In the resulting dielectric layer  34 , carbon atomic percentage may be in the range between about 10 percent and about 30 percent. The deposition time may be in the range between about 30 seconds and about 100 seconds. The resulting dielectric layer  34  may include SiOCH, SiOC, or the like. 
     In accordance with some embodiments, in the deposition of dielectric layer  34 , porogen may be (or may not be) added, and accordingly, dielectric layer  34  may or may not include pores. Since the carbon-depletion problem is more severe in upper portions of dielectric layer  34  (as revealed by  FIG.  28   ), to minimize the carbon-depletion problem, the upper portions (such as the upper half or upper ¾) of dielectric layer  34  may be different from the lower portions, for example, with the upper portions having greater concentrations of Si—C—Si bonds than the respective lower portions (such as the lower half or lower ¼). This may be achieved by increasing the flow rate of the Si—C—Si-comprising precursor when depositing the upper portions. The lower portion may be formed using a precursor selected from  FIG.  24    without using any Si—C—Si-comprising precursor. Alternatively, the lower portion may be formed using a first precursor selected from  FIG.  24    and a second precursor comprising Si—C—Si bonds, with the second precursor having a low flow rate than for forming the upper portions. When the upper portions are formed, the Si—C—Si-comprising precursor may be added, and/or the flow rate may be increased. 
     Furthermore, in accordance with some embodiments, starting at a certain point of the deposition of dielectric layer  34 , the flow rate of the Si—C—Si-comprising precursor may be increased continuously or increased by stages. The certain point may be the beginning of the deposition of dielectric layer  34 , or may be an intermediate point (such as after ¼ or a half of dielectric layer  34  has been deposited). Before the certain point, there may be (or may not be) Si—C—Si-comprising precursor, and the flow rate of the non-Si—C—Si-comprising precursor may be kept unchanged. Alternatively, to increase the concentration of Si—C—Si in the upper portions, instead of increasing the flow rate of the Si—C—Si-comprising precursor, the non-Si—C—Si-comprising precursor may be reduced. 
     Next, as also shown in  FIG.  19   , pad layer  36  and hard mask  38  are formed. The respective process is illustrated as process  306  in the process flow  300  as shown in  FIG.  29   . Hard mask  38  is patterned to form openings  42 . The respective process is illustrated as process  308  in the process flow  300  as shown in  FIG.  29   . In a subsequent process, as shown in  FIG.  20   , openings  42  are extended downwardly through etching processes until openings  42  penetrate through dielectric layer  34  and etch stop layer  32 . The respective process is illustrated as process  310  in the process flow  300  as shown in  FIG.  29   .  FIG.  21    illustrates the formation of conductive features  50 . The respective process is illustrated as process  312  in the process flow  300  as shown in  FIG.  29   . The details of conductive feature  50  are discussed in preceding embodiments, and are not repeated herein. 
       FIG.  22    illustrates the formation of metal caps  54 , etch stop layer  56 , dielectric layer  58 , and etch stop layer  78 . Via  70 , metal line  72 , and metal cap  76  are also formed. The respective process is illustrated as process  314  in the process flow  300  as shown in  FIG.  29   . The details are discussed in preceding embodiments, and are not repeated herein. 
     In accordance with some embodiments, the metal lines and vias in dielectric layer  58  are spaced apart farther from each other than the conductive features  50  in dielectric layer  34 . Accordingly, the carbon depletion problem is less severe in dielectric layer  58  than in dielectric layer  34 . Dielectric  58  may thus be formed adopting precursors that are free from the Si—C—Si bonds. For example, dielectric layer  58  may be formed using the precursors shown in  FIG.  24   . Alternatively, dielectric layer  58  may be formed using a lower flow rate of the Si—C—Si-comprising precursor than for forming dielectric layer  34 . As a result, dielectric layer  58  has a lower Si—C—Si concentration than in dielectric layer  34 . 
       FIG.  25    illustrates the scheme of the possible layers of conductive features in wafer  20  (die  22 ). Transistor  114  is formed at the top surface of semiconductor substrate  24 , and transistor  114  represents the integrated circuit devices  26  as shown in  FIGS.  15 A,  15 B , or  FIG.  22   . Transistor  114  includes gate stacks  110  and source/drain regions  112 . Over transistor  114 , there is a contact (CT) layer, in which contact plugs  30  ( FIGS.  15 A / 15 B and  22 ) are formed. Metal layer MO, which may include metal lines therein, are formed over contact layer. There are also a plurality of metal layers such as M 1  through M 14  and via layers such as V 1  through V 13  formed. These layers may be formed using single damascene processes or dual damascene processes. 
     Air spacers may be formed aside the metal features in any of these layers in any combination. When air spacers are formed in the layers that are formed using single damascene processes, the processes as shown in  FIGS.  4 - 8 ,  9 A,  9 B and  10 - 11    may be adopted. When air spacers are formed in the layers that are formed using dual damascene processes, the processes as shown in  FIGS.  12  through  15 A  may be adopted. In accordance with some embodiments, the dielectric layers of lower layers such as metal layers M 0 , M 1 , M 2 , etc., may have higher nitrogen atomic percentage than the dielectric layers of the upper metal layers. Also, the Si—C—Si bond concentration in the dielectric layers of the lower metal layers may be greater than in the dielectric layers of the upper metal layers. 
     In accordance with some embodiments, there is a threshold metal layer (such as M 3 , M 4 , or M 5 ), and the dielectric layers of the lower metal layers including and lower than the threshold metal layer have higher nitrogen atomic percentages than the dielectric layers of the upper metal layers higher than the threshold metal layer. There may not be air spacer formed in any of the upper metal layers. Also, the dielectric layers of the lower metal layer may be formed with precursors comprising Si—C—Si bonds, while the dielectric layers of the upper metal layers may be formed with precursors not comprising Si—C—Si bonds. 
       FIGS.  26  and  27    illustrate the profile of carbon atomic percentage values in accordance with some embodiments. The x-axis represents the depth measured from the top surface of dielectric layer to the bottom of etch stop layer in the position marked by arrow  80  in  FIG.  22   . The Y-axis represents the relative carbon atomic percentage values.  FIG.  26    illustrates the carbon atomic percentage values in dielectric layer  34  in which no Si—C—Si-comprising precursors are used.  FIG.  27    illustrates the carbon atomic percentage values in dielectric layer  34  in which Si—C—Si-comprising precursors are used.  FIGS.  26  and  27    illustrate that the carbon depletion mainly occurs close to the top portions of dielectric layer  34  because the dielectric strips  34  are narrower than the lower portions. It is observed that in  FIG.  27   , the carbon-depletion region  84  is much narrower than the carbon-depletion region  82  in  FIG.  26   , indicating that more Si—C—Si bonds may effectively reduce carbon depletion. 
     The embodiments of the present disclosure have some advantageous features. From one aspect, when air spacers are formed in a dielectric layer, the dielectric layer is formed as having improved mechanical strength (by adopting more nitrogen). The tilting and collapsing of the narrow parts of the dielectric layer may thus be reduced. From another aspect, the interconnect structure includes a dielectric layer including increased Si—C—Si bonds compared to conventional structures. Accordingly, the depletion of carbon from the dielectric layer is reduced. 
     In accordance with some embodiments of the present disclosure, a method comprises depositing a dielectric layer over a substrate; etching the dielectric layer to form an opening, wherein a first conductive feature underlying the dielectric layer is exposed to the opening, wherein the dielectric layer is formed using a precursor comprising nitrogen therein; depositing a sacrificial spacer layer extending into the opening; patterning the sacrificial spacer layer, wherein a bottom portion of the sacrificial spacer layer at a bottom of the opening is removed to reveal the first conductive feature, and a first vertical portion of the sacrificial spacer layer in the opening and on sidewalls of the dielectric layer is left to form a first ring; forming a second conductive feature in the opening, wherein the second conductive feature is encircled by the first ring, and is over and electrically coupled to the first conductive feature; and removing at least a portion of the first ring to form an air spacer. In an embodiment, the depositing the dielectric layer comprises depositing a high-k dielectric material. 
     In an embodiment, the method further comprises forming a metal capping layer over the second conductive feature, wherein the metal capping layer comprises an extension portion extending into the air spacer. In an embodiment, the method further comprises forming a metal capping layer over the second conductive feature, wherein the first ring is removed after the metal capping layer is formed. In an embodiment, the depositing the dielectric layer is performed using a first precursor comprising silicon, carbon, and hydrogen, and a second precursor comprising nitrogen. In an embodiment, the depositing the dielectric layer is performed without using porogen. In an embodiment, the depositing the dielectric layer comprises depositing a lower portion having a first nitrogen atomic percentage; and depositing an upper portion over the lower portion, wherein the upper portion has a second nitrogen atomic percent different from the first nitrogen atomic percentage. 
     In an embodiment, the opening comprises a trench and a via opening underlying the trench, and the first ring is in the trench, and the patterning the sacrificial spacer layer leaves a second ring remaining in the via opening. In an embodiment, the method further comprises depositing an etch stop layer over the second conductive feature, wherein at a time after the etch stop layer is deposited, the second ring remains. In an embodiment, the method further comprises forming an additional dielectric layer over the sacrificial spacer layer and sealing the air spacer, wherein a residue portion of the first ring is left underlying the additional dielectric layer. In an embodiment, the first ring is fully removed. 
     In accordance with some embodiments of the present disclosure, a structure comprises a substrate; a first conductive feature over the substrate; a first etch stop layer over the first conductive feature; a dielectric layer over the first etch stop layer, wherein the dielectric layer comprises nitrogen therein, wherein the dielectric layer comprises a high-k dielectric material; a second conductive feature in the dielectric layer and the first etch stop layer, wherein the second conductive feature is over and contacting the first conductive feature; an air spacer encircling the second conductive feature, wherein sidewalls of the second conductive feature are exposed to the air spacer; and a second etch stop layer over and contacting the dielectric layer, wherein the second etch stop layer is further over the second conductive feature. 
     In an embodiment, the dielectric layer comprises a top portion and a bottom portion, wherein the top portion has a greater nitrogen atomic percentage than the lower portion. In an embodiment, the air spacer extends from a top surface of the dielectric layer to a bottom surface of the first etch stop layer. In an embodiment, the dielectric layer has a hard less in a range between about 15 GPa and about 35 GPa. In an embodiment, the structure further comprises a dielectric material contacting a sidewall of a bottom portion of the second conductive feature, wherein a top portion of the second conductive feature is exposed to the air spacer, and wherein the dielectric material and the dielectric layer are formed of different materials. In an embodiment, no dielectric material is between the second conductive feature and the air spacer. 
     In accordance with some embodiments of the present disclosure, a structure comprises a first conductive feature; a second conductive feature over and electrically coupling to the first conductive feature, wherein the second conductive feature comprises a diffusion barrier; and a metallic material in a basin formed by the diffusion barrier; an air spacer encircling a top portion of the second conductive feature; and a dielectric layer encircling the air spacer, wherein the dielectric layer comprises a high-k dielectric material that comprises nitrogen. In an embodiment, the structure further comprises a dielectric material separating a bottom portion of the second conductive feature from the dielectric layer, wherein the dielectric material is directly underlying and exposed to the air spacer. In an embodiment, the dielectric layer has a hard less in a range between about 15 GPa and about 35 GPa. 
     The foregoing outlines features of several embodiments so that those skilled in the art may better understand the aspects of the present disclosure. Those skilled in the art should appreciate that they may readily use the present disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiments introduced herein. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the present disclosure, and that they may make various changes, substitutions, and alterations herein without departing from the spirit and scope of the present disclosure.