Patent Publication Number: US-2021183646-A1

Title: Low-k Dielectric and Processes for Forming Same

Description:
PRIORITY CLAIM AND CROSS-REFERENCE 
     This application is a continuation of U.S. patent application Ser. No. 16/569,791, filed on Sep. 13, 2019, entitled “Low-k Dielectric and Processes for Forming Same,” which is a divisional of U.S. patent application Ser. No. 15/944,627, filed on Apr. 3, 2018, entitled “Low-k Dielectric and Processes for Forming Same,” now U.S. Pat. No. 10,910,216 issued on Feb. 2, 2021, which claims the benefit of and priority to U.S. Provisional Patent Application No. 62/591,536, filed on Nov. 28, 2017, entitled “Low-k Dielectric and Processes for Forming Same,” each is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     In the current process of miniaturizing semiconductor devices, low-k dielectric materials are desired as the intermetallization dielectric (IMD) and/or interlayer dielectric (ILD) between conductive interconnects in order to reduce the resistive-capacitive (RC) delay in signal propagation due to capacitive effects. As such, the lower the dielectric constant of the dielectric layer, the lower the parasitic capacitance of adjacent conductive lines and the lower the RC delay of the integrated circuit (IC). 
    
    
     
       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 through 4  are cross-sectional views of respective intermediate structures at respective stages during an example method for forming one or more dielectric layers with a conductive feature formed therein in accordance with some embodiments. 
         FIG. 5  is a graph illustrating an atomic force microscope infrared spectroscopy (AFM-IR) analysis of a sample for Si—CH 3  bonding in accordance with some embodiments. 
         FIG. 6  is a graph illustrating an AFM-IR analysis of a sample for Si—C—Si bonding in accordance with some embodiments. 
         FIG. 7  is a graph illustrating areal densities of various elements in a low-k dielectric layer in accordance with some embodiments. 
         FIG. 8  is a graph illustrating capacitance per distance as a function of inverse resistance for a number of samples of a low-k dielectric in accordance with some embodiments. 
         FIG. 9  is a graph illustrating failure rates of samples as a function of voltage breakdown (VBD) in accordance with some embodiments. 
         FIG. 10  is a graph illustrating a time to failure of samples as a function of electric field to illustrate time-dependent dielectric breakdown (TDDB) of the samples in accordance with some embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     The following disclosure provides many different embodiments, or examples, for implementing different features of the provided subject matter. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. For example, the formation of a first feature over or on a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features may be formed between the first and second features, such that the first and second features may not be in direct contact. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed. 
     Further, spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “upper” and the like, may be used herein for ease of description to describe one element or feature&#39;s relationship to another element(s) or feature(s) as illustrated in the figures. The spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. The apparatus may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein may likewise be interpreted accordingly. 
     Embodiments described herein relate generally to one or more methods for forming a dielectric structure, such as a low-k dielectric layer (e.g., an extreme low-k dielectric layer), in semiconductor processing. Generally, a low-k dielectric layer may be implemented having a dielectric constant value (k-value) equal to or less than about 3.9 (e.g., in a range from about 2.0 to about 3.9), and more particularly, such as equal to or less than about 3.5 (e.g., in a range from about 2.0 to about 3.5), and even more particularly, such as equal to or less than about 3.3 (e.g., in a range from about 2.0 to about 3.3). The low-k dielectric layer may be implemented as a variety of structures, and in illustrated examples herein, a low-k dielectric layer is implemented as an interlayer dielectric (ILD) and/or an intermetallization dielectric (IMD) in which a conductive feature is formed. A low-k dielectric layer can be deposited using a chemical vapor deposition (CVD), such as a plasma enhanced CVD (PECVD), in which a diethoxymethylsilane (mDEOS, C 5 H 4 O 2 Si) precursor gas and oxygen (O 2 ) precursor gas are used, along with a carrier gas. Flow rates of the mDEOS and oxygen can be relatively low, while a flow rate of the carrier gas can be high. The low-k dielectric layer can have increased Si—C—Si bonding, which can increase properties of the low-k dielectric layer. Various advantages may be achieved by implementing the low-k dielectric layer, some of which are described below. 
     Various modifications are discussed with respect to disclosed embodiments; however, other modifications may be made to disclosed embodiments while remaining within the scope of the subject matter. A person having ordinary skill in the art will readily understand other modifications that may be made that are contemplated within the scope of other embodiments. Although method embodiments may be described in a particular order, various other method embodiments may be performed in any logical order and may include fewer or more steps than what is described herein. 
       FIGS. 1 through 4  illustrate cross-sectional views of respective intermediate structures at respective stages during an example method for forming one or more dielectric layers with a conductive feature formed therein in accordance with some embodiments.  FIG. 1  illustrates a semiconductor substrate  20 . The semiconductor substrate  20  may be or include a bulk semiconductor, a semiconductor-on-insulator (SOI) substrate, or the like, which may be doped (e.g., with a p-type or an n-type dopant) or undoped. Generally, an SOI substrate comprises a layer of a semiconductor material formed on an insulator layer. The insulator layer may be, for example, a buried oxide (BOX) layer, a silicon oxide layer, or the like. The insulator layer is provided on or is a substrate, typically a silicon or glass substrate. Other substrates, such as a multi-layered or gradient substrate may also be used. In some embodiments, the semiconductor material of the semiconductor substrate  20  may include an elemental semiconductor such as silicon (Si) or germanium (Ge); a compound semiconductor including silicon carbide, gallium arsenic, gallium phosphide, indium phosphide, indium arsenide, and/or indium antimonide; an alloy semiconductor including SiGe, GaAsP, AlInAs, AlGaAs, GaInAs, GaInP, and/or GaInAsP; or a combination thereof. 
     Various devices may be on the semiconductor substrate  20 . For example, the semiconductor substrate  20  may include Field Effect Transistors (FETs), such as Fin FETs (FinFETs), planar FETs, vertical gate all around FETs (VGAA FETs), or the like; diodes; capacitors; inductors; and other devices. Devices may be formed wholly within the semiconductor substrate  20 , in a portion of the semiconductor substrate  20  and a portion of one or more overlying layers, and/or wholly in one or more overlying layers, for example. Processing described herein may be used to form and/or to interconnect the devices to form an integrated circuit (IC). The integrated circuit can be any circuit, such as for an application specific integrated circuit (ASIC), a processor, memory, or other circuit. 
       FIG. 1  illustrates at least a portion of a structure  22  and a conductive feature  24  in the structure  22  as an example. The structure  22  can be, for example, a portion of the semiconductor substrate  20 , and the conductive feature  24  can be a source/drain region of a transistor (e.g., FinFET) in the semiconductor substrate  20  or another conductive region of the semiconductor substrate  20 , such as a p-type or n-type doped region of the semiconductor substrate  20 . In another example, the structure  22  can include respective portions of a first interlayer dielectric (ILD), contact etch stop layer (CESL), and gate spacers over the semiconductor substrate  20 , and the conductive feature  24  is a gate electrode of a transistor that is over the semiconductor substrate  20 . In another example, the structure  22  includes or is a second ILD over the semiconductor substrate  20 , and the conductive feature  24  is, e.g., a contact or a plug to a source/drain region in the semiconductor substrate  20  and/or gate structure on the semiconductor substrate  20 . In a further example, the structure  22  includes or is an intermetallization dielectric (IMD) over the semiconductor substrate  20 , and the conductive feature  24  is a metallization pattern, e.g., a metal line and/or via. Hence, a low-k dielectric layer  32  described herein below can be implemented as an ILD and/or as an IMD. Further, processing described below may be implemented in Front End Of the Line (FEOL), Middle End Of the Line (MEOL), and/or Back End Of the Line (BEOL). 
     As will become apparent from subsequent description, the dielectric structure  22  may be or include a low-k dielectric layer or structure, or even further, may include an extreme low-k dielectric layer or structure. By implementing low-k dielectrics, and further, extreme low-k dielectrics, in various layers or structures from adjacent or proximate to the semiconductor substrate  20  through a topmost IMD, parasitic capacitances of interconnections that form part of an IC can be decreased, and hence, resistance-capacitance (RC) delay of the IC can be decreased. A decreased RC delay can improve device operational speed. 
     In other examples, processes for forming a low-k dielectric as described below can be implemented to form other dielectric structures. For example, the processes described below can be implemented in forming a layer used to form gate spacers or a CESL. Other dielectric structures may be formed using an implementation of a process described below. 
     A first etch stop sub-layer  26  is formed over the structure  22  and conductive feature  24 , and a second etch stop sub-layer  28  is formed over the first etch stop sub-layer  26 . The first etch stop sub-layer  26  and the second etch stop sub-layer  28  form an etch stop for subsequent etching to form an interconnect, such as including a via and/or line, in an overlying dielectric layer. Generally, an etch stop can provide a mechanism to stop an etch process when forming, e.g., vias. An etch stop may be formed of a dielectric material having a different etch selectivity from adjacent layers or components. In an example, the first etch stop sub-layer  26  is or includes aluminum nitride (AlN), aluminum oxide (AlO x  or Al 2 O 3 ), or another material, and the second etch stop sub-layer  28  is or includes oxygen-doped silicon carbide (SiC:O), silicon oxycarbide (SiOC), or another material. Each of the first etch stop sub-layer  26  and the second etch stop sub-layer  28  can be deposited by chemical vapor deposition (CVD), physical vapor deposition (PVD), or another deposition technique. A thickness of the first etch stop sub-layer  26  can be in a range from about 1 nm to about 5 nm, and a thickness of the second etch stop sub-layer  28  can be in a range from about 3 nm to about 10 nm. Different etch stop schemes may be implemented in the place of or in addition to the etch stop illustrated. For example, a single layer etch stop may be implemented, or more sub-layers may be implemented for an etch stop. Further, other example materials may be implemented as an etch stop, such as silicon nitride, silicon carbon nitride, silicon oxynitride, and other materials. 
     An adhesion layer  30  is formed over the second etch stop sub-layer  28 . In some examples, the adhesion layer  30  can be omitted. The adhesion layer  30  can be formed to provide an interface between the etch stop and an overlying dielectric layer that permits good adhesion with the etch stop and the overlying dielectric layer. The adhesion layer  30  can be an oxygen-containing dielectric layer, such as silicon oxide (SiO), silicon oxycarbide (SiOC), or another oxide-like material. In some examples, the adhesion layer  30  can be deposited using CVD, plasma enhanced CVD (PECVD), or another deposition technique. A thickness of the adhesion layer  30  can be in a range from about 0.1 nm to about 10 nm. Other materials and/or layers may be implemented as an adhesion layer. 
     In an example, the adhesion layer  30  can be silicon dioxide (SiO 2 ) and can be deposited using CVD. The CVD may use precursors including silane (SiH 4 ) or tetraethoxysilane (TEOS, SiC 8 H 20 O 4 ), and oxygen (O 2 ). The CVD may also use a carrier gas, such as helium (He). A flow rate of the silane precursor gas or TEOS precursor gas can be in a range from about 10 sccm to about 300 sccm; a flow rate of the oxygen precursor gas can be in a range from about 50 sccm to about 200 sccm; and a flow rate of the carrier gas can be in a range from about 500 sccm to about 5,000 sccm. A pressure of the CVD can be in a range from about 5 Torr to about 10 Torr. A temperature of the CVD process can be in a range from about 300° C. to about 400° C. 
     In another example, the adhesion layer  30  can be silicon oxycarbide (SiOC) and can be deposited using PECVD. The PECVD may use precursors including diethoxymethylsilane (mDEOS, C 5 H 14 O 2 Si) and oxygen (O 2 ). The PECVD may also use a carrier gas, such as helium (He). A flow rate of the mDEOS precursor gas can be equal to or less than about 600 sccm; a flow rate of the carrier gas (e.g., He) can be greater than or equal to about 4,000 sccm; and a flow rate of the oxygen precursor gas can be in a range from about 10 sccm to about 200 sccm, such as about 50 sccm. A ratio of the flow rate of the mDEOS precursor gas to the flow rate of the carrier gas can be equal to or less than about 0.5. A ratio of the flow rate of the mDEOS precursor gas to the flow rate of the oxygen precursor gas can be equal to or less than about 20. A pressure of the PECVD can be less than 12 Torr, such as equal to or less than about 9 Torr. A power of the plasma generator of the PECVD can be greater than 300 W, such as in a range from about 300 W to about 1,200 W, such as about 750 W. A temperature of the PECVD process can be greater than or equal to about 200° C., such as in a range from about 200° C. to about 400° C., such as about 260° C. 
     A low-k dielectric layer  32  is formed over the adhesion layer  30 . The low-k dielectric layer  32  may be an extreme low-k (ELK) dielectric layer. For example, the low-k dielectric layer  32  can have a k-value equal to or less than about 3.9 (e.g., in a range from about 2.0 to about 3.9), and more particularly, such as equal to or less than about 3.5 (e.g., in a range from about 2.0 to about 3.5), and even more particularly, such as equal to or less than about 3.3 (e.g., in a range from about 2.0 to about 3.3). In some examples, the low-k dielectric layer  32  is or includes silicon oxycarbide (SiOC). The low-k dielectric layer  32  can be deposited using plasma-enhanced CVD (PECVD), for example. The PECVD may use precursors including diethoxymethylsilane (mDEOS, C 5 H 14 O 2 Si) and oxygen (O 2 ). The PECVD may also use a carrier gas, such as helium (He). A flow rate of the mDEOS precursor gas can be equal to or less than about 600 sccm; a flow rate of the carrier gas (e.g., He) can be greater than or equal to about 3,000 sccm; and a flow rate of the oxygen precursor gas can be in a range from about 10 sccm to about 100 sccm, such as about 50 sccm. A ratio of the flow rate of the mDEOS precursor gas to the flow rate of the carrier gas can be equal to or less than about 0.2. A ratio of the flow rate of the mDEOS precursor gas to the flow rate of the oxygen precursor gas can be equal to or less than about 30. A pressure of the PECVD can be equal to or less than 9.5 Torr, such as equal to or less than about 9 Torr. A power of the plasma generator of the PECVD can be equal to or greater than 350 W, such as in a range from about 400 W to about 1,200 W, such as about 750 W. A temperature of the PECVD process can be greater than or equal to about 260° C., such as in a range from about 260° C. to about 400° C. A thickness of the low-k dielectric layer  32  can be in a range from about 20 nm to about 100 nm. 
     The low-k dielectric layer  32  formed according to the PECVD process described above can be silicon oxycarbide (SiOC) with a k-value equal to or less than about 3.9 (e.g., in a range from about 2.0 to about 3.9), and more particularly, such as equal to or less than about 3.5 (e.g., in a range from about 2.0 to about 3.5), and even more particularly, such as equal to or less than about 3.3 (e.g., in a range from about 2.0 to about 3.3). The low-k dielectric layer  32  formed according to the PECVD process described above can have a concentration of carbon in a range from about 5 atomic percent (at. %) to about 30 at. %, a concentration of oxygen in a range from about 40 at. % to about 55 at. %, and a concentration of silicon in a range from about 30 at. % to about 40 at. %. In some examples, the concentration of oxygen can be greater than the concentration of silicon, and the concentration of silicon can be greater than the concentration of carbon. 
     The low-k dielectric layer  32  formed according to the PECVD process described above can have enhanced properties. For example, a hardness of the low-k dielectric layer  32  can be improved relative to previous low-k dielectrics. A hardness of the low-k dielectric layer  32  can be in a range from about 3 GPa to about 10 GPa. The improved hardness may result from increased Si—C—Si bonding and decreased Si—CH 3  bonding in the low-k dielectric layer  32 . It is believed that Si—C—Si bonding is stronger than Si—CH 3  bonding. For example, Si—C—Si may form a bridge between atoms and/or molecules, whereas Si—CH 3  bonding may form a terminal to which another atom and/or molecule may not bond. By forming more bridges (e.g., increasing Si—C—Si bonding) and/or forming fewer terminals (e.g., decreasing Si—CH 3  bonding) can result in the improved hardness because the material contains more bonds between atoms and/or molecules. A concentration of Si—C—Si bonds in the low-k dielectric layer  32  can be in a range from about 8% to about 50%, such as about 30%, and a concentration of Si—CH 3  bonds in the low-k dielectric layer  32  can be less than about 20% or in a range from about 5% to about 40%, which may be measured by atomic force microscope infrared spectroscopy (AFM-IR).  FIGS. 5 and 6  are graphs illustrating AFM-IR analyses of a sample for Si—CH 3  bonding and Si—C—Si bonding, respectively, in accordance with some embodiments. The Si—CH 3  bonding in  FIG. 5  illustrates a 66.67% or more decrease in Si—CH 3  bonding in a low-k dielectric layer  32  as described herein relative to another low-k dielectric layer. The Si—C—Si bonding in  FIG. 6  illustrates an approximately 600% increase in Si—C—Si bonding in a low-k dielectric layer  32  as described herein relative to another low-k dielectric layer. 
     As apparent from the above description, the low-k dielectric layer  32  does not implement a porogen material during formation and does not have pores as formed. Dielectric layers that have pores can be mechanically weak and susceptible to cracking and other failures. The low-k dielectric layer  32  can have increased mechanical properties, such as hardness, that may not be possible if the low-k dielectric layer  32  included pores. 
     A dielectric cap layer  34  is formed over the low-k dielectric layer  32 . The dielectric cap layer  34  may, for example, protect the low-k dielectric layer  32  during subsequent processing. The dielectric cap layer  34  can be a nitrogen-free anti-reflection layer (NFARL) in some examples. In some examples, the NFARL can be or include carbon-doped silicon dioxide (e.g., SiO 2 :C), wherein a concentration of the carbon can be in a range from about 1% to about 5%, such as about 2%. The carbon-doped silicon dioxide of the NFARL can be deposited using CVD with silane (SiH 4 ) and carbon dioxide (CO 2 ) precursor gases, which may further use a carrier gas, such as helium (He). The NFARL can suppress light reflections from underlying layers during subsequent patterning of a photoresist overlying the NFARL, for example. The dielectric cap layer  34  can be another oxide. For example, the oxide can be a silicon oxide deposited using PECVD or another CVD technique using tetraethoxysilane (TEOS, SiC 8 H 20 O 4 ) and oxygen (O 2 ) or ozone (O 3 ) as precursor gases, which may further use a carrier gas, such as helium (He). A thickness of the dielectric cap layer  34  can be in a range from about 10 nm to about 40 nm. Other materials, which may be formed by other processes, may be implemented as the dielectric cap layer  34 . 
     A metal cap layer  36  is formed over the dielectric cap layer  34 . The metal cap layer  36  can be titanium nitride (TiN), titanium oxide (TiO), boron nitride (BN), or another material. The metal cap layer  36  can be deposited using CVD, PVD, or another deposition technique. A thickness of the metal cap layer  36  can be in a range from about 5 nm to about 30 nm. The metal cap layer  36 , alone or with the dielectric cap layer  34 , can act as a hard mask during subsequent patterning of underlying layers. Other materials, which may be formed by other processes, may be implemented as the metal cap layer  36 . 
       FIG. 2  illustrates the formation of an opening  40  through the dielectric cap layer  34 , low-k dielectric layer  32 , adhesion layer  30 , second etch stop sub-layer  28 , and first etch stop sub-layer  26  to the conductive feature  24 . The opening  40  can include a trench and/or a via opening, for example. The opening  40  can be formed using photolithography and etching processes. For example, a photo resist can be formed on the metal cap layer  36 , such as by using spin-on coating, and patterned with a pattern corresponding to the opening  40  by exposing the photo resist to light using an appropriate photomask. Exposed or unexposed portions of the photo resist may then be removed depending on whether a positive or negative resist is used. The pattern of the photo resist may then be transferred to the metal cap layer  36  and dielectric cap layer  34 , such as by using one or more suitable etch processes. The photo resist can be removed in an ashing or wet strip process, for example. With the pattern transferred to the metal cap layer  36  and dielectric cap layer  34 , the metal cap layer  36  and dielectric cap layer  34  can be used as a hardmask to transfer the pattern to the low-k dielectric layer  32 , adhesion layer  30 , second etch stop sub-layer  28 , and first etch stop sub-layer  26  to form the opening  40 , which may be by using one or more suitable etch processes. The etch processes may include a reactive ion etch (RIE), neutral beam etch (NBE), inductive coupled plasma (ICP) etch, the like, or a combination thereof. The etch processes may be anisotropic. 
     The etch processes for etching different layers may have different chemistries based on the selectivity of the materials to be etched, as a person having ordinary skill in the art will readily understand. Based on the different materials and different selectivity, the second etch stop sub-layer  28  and first etch stop sub-layer  26  can be an etch stop for one or more of the etch processes. As an example, the low-k dielectric layer  32  may be etched using a dry plasma etch process using a fluorocarbon gas, such as octafluorocyclobutane (C 4 F 8 ), tetrafluoromethane (CF 4 ), hexafluorocyclobutene (C 4 F 6 ), the like, or a combination thereof. As illustrated in  FIG. 2 , the metal cap layer  36  may be consumed in one or more of the etch processes implemented to form the opening  40 . 
     In some examples, multiple photo resist patterning and etching steps may be performed to form the opening  40 , such as may be performed in a dual damascene process. For example, a photo resist can be formed on the metal cap layer  36  and patterned with a pattern corresponding to a via opening of the opening  40 . One or more etch processes may then be used to etch at least partially through the low-k dielectric layer  32  to transfer the pattern of the via opening to the low-k dielectric layer  32 . Then, a photo resist can be formed on the metal cap layer  36  and patterned with a pattern corresponding to a trench of the opening  40 , where the pattern of the trench overlays the via opening of the opening  40 . One or more etch processes may then be used to etch through the low-k dielectric layer  32 , adhesion layer  30 , second etch stop sub-layer  28 , and first etch stop sub-layer  26  to continue transferring the pattern of the via opening to those layers and to transfer the pattern of the trench to the low-k dielectric layer  32 . Other processes may be used to form the opening  40 . 
       FIG. 3  illustrates the formation of a barrier layer  50  in the opening  40  and on a top surface of the dielectric cap layer  34 , a seed layer  52  on the barrier layer  50 , and a conductive fill material  54  (e.g., a metal) on the seed layer  52 . The barrier layer  50  is formed conformally in the opening  40  and on the top surface of the dielectric cap layer  34 . The seed layer  52  is then formed conformally on the barrier layer  50 . The conductive fill material  54  is formed on the seed layer  52  (wherein the seed layer  52  can be and/or have nucleation sites for deposition of the conductive fill material  54 ) and filling the opening  40 . The barrier layer  50  may be or comprise titanium nitride, titanium oxide, tantalum nitride, tantalum oxide, the like, or a combination thereof, and may be deposited by CVD, ALD, or another deposition technique. The seed layer  52  may be or comprise copper, aluminum, silver, gold, titanium, tungsten, another suitable material, an alloy thereof, or a combination thereof, and may be deposited by PVD, CVD, or another deposition technique. The conductive fill material  54  may be or comprise copper, aluminum, tungsten, cobalt, ruthenium, gold, silver, an alloy thereof, the like, or a combination thereof, and may be deposited by electroless plating, PVD, CVD, or another deposition technique. 
       FIG. 4  illustrates the removal of excess conductive fill material  54 , seed layer  52 , and barrier layer  50 , and the removal of the dielectric cap layer  34 . A planarization process, such as a chemical mechanical polish (CMP), may be used to remove the excess conductive fill material  54 , seed layer  52 , and barrier layer  50 , and the dielectric cap layer  34 . Hence, a metallization pattern comprising the conductive fill material  54 , seed layer  52 , and barrier layer  50  can be formed in the opening  40  and with a top surface coplanar with a top surface of the low-k dielectric layer  32  as illustrated in  FIG. 4 . 
       FIGS. 3 and 4  illustrate an example process for forming a metallization pattern in the opening  40 . Other processes and/or materials may be implemented. For example, the barrier layer  50  may be omitted if the conductive fill material  54  is not likely to diffuse significantly into the low-k dielectric layer  32 , such as when the conductive fill material  54  is titanium, tungsten, cobalt, or ruthenium. Further, the seed layer  52  may be omitted based on the deposition technique implemented to deposit the conductive fill material  54 . For example, a selective deposition process may use the exposed portion of the top surface of the conductive feature  24  (e.g., a metal surface) to selectively deposit the conductive fill material  54  in the opening  40  (e.g., growing from the metal surface) without depositing the conductive fill material  54  on the top surface of the dielectric cap layer  34 . For example, a selective CVD may deposit ruthenium on the conductive feature  24  in the opening  40  and not significantly nucleate on a dielectric surface. Similarly, an electroless plating may deposit cobalt on the conductive feature  24  in the opening  40 . Hence, the metallization pattern can comprise the conductive fill material  54  with or without the barrier layer  50  and/or the seed layer  52 . 
     Further, multiple deposition processes may be implemented to deposit different or the same conductive fill material  54  in different portions of the opening  40 . For example, a via in a via opening of the opening  40  may be, for example, cobalt or ruthenium deposited by a selective deposition process, and a line in a trench of the opening  40  may be, for example, cobalt, tungsten, or copper deposited by another deposition process. 
     Other processes and/or materials may be used to form the metallization pattern in the opening  40 . In any processes for forming the metallization pattern, any excess material and the dielectric cap layer  34  may be removed by a planarization process, such as a CMP, as described with respect to  FIG. 4  to form a top surface of the metallization pattern coplanar with the top surface of the low-k dielectric layer  32 . 
     After the metallization pattern is formed in the low-k dielectric layer  32 , the low-k dielectric layer  32  may have a consistent concentration of carbon throughout the low-k dielectric layer  32 . Other dielectric layers may be susceptible to carbon depletion as a result of plasma induced damage, such as resulting from a plasma-based etch process. For example, for some dielectric layers, a plasma-based etch process may etch an opening in the dielectric layer, and in a region of the dielectric layer at a sidewall of the opening, the dielectric layer may have a reduced carbon concentration compared to another region of the dielectric layer away from the opening as a result of plasma induced carbon depletion. In some examples, the low-k dielectric layer  32  is more robust and less susceptible to plasma induced carbon depletion as a result of the etch processes that form the opening  40 . Hence, in those examples, the low-k dielectric layer  32  can have a consistent concentration of carbon from each sidewall of the opening  40  (and metallization pattern) to a region of the low-k dielectric layer  32  disposed a distance away from the respective sidewall. 
       FIG. 7  is a graph illustrating areal densities of various elements in the low-k dielectric layer  32  in accordance with some embodiments. The graph illustrates relative areal densities as a function of position within the low-k dielectric layer  32 . The low-k dielectric layer  32  has a surface  60  that is proximate a plasma during a plasma-based process, such as a plasma-based etch process. The graph illustrates a first areal density profile  200  of oxygen, a second areal density profile  202  of silicon, a third areal density profile  204  of carbon, and a fourth areal density profile  206  of nitrogen. As illustrated, the third areal density profile  204  of carbon is consistent throughout the low-k dielectric layer  32 , and more particularly, in an exterior region  32   a  of the low-k dielectric layer  32  (e.g., from the surface  60  towards an interior of the low-k dielectric layer  32 ). As a comparison, in other dielectric layers as a result of plasma induced carbon depletion, a concentration of carbon at a surface of the dielectric layer (e.g., which would correspond to the surface  60 ) would be a lowest concentration in the dielectric layer, and the concentration of carbon would increase from the surface of the dielectric layer towards an interior of the dielectric layer (e.g., which would correspond to a region outside of the exterior region  32   a  of the low-k dielectric layer  32 ). 
     As indicated previously, the low-k dielectric layer  32  may be formed over the adhesion layer  30 , and in some examples, the low-k dielectric layer  32  and adhesion layer  30  may be deposited using PECVD that uses an mDEOS precursor gas and an oxygen precursor gas. The ratio of the flow rate of the mDEOS precursor gas to the flow rate of the oxygen precursor gas in depositing the low-k dielectric layer  32  is greater than the ratio of the flow rate of the mDEOS precursor gas to the flow rate of the oxygen precursor gas in depositing the adhesion layer  30 . Conversely, the flow rate of the oxygen precursor gas relative to the flow rate of the mDEOS precursor gas is greater in depositing the adhesion layer  30  than in depositing the low-k dielectric layer  32 . Hence, the low-k dielectric layer  32  and the adhesion layer  30  may both be silicon oxycarbide (SiOC), and a concentration of oxygen in the adhesion layer  30  is greater than a concentration of oxygen in the low-k dielectric layer  32 . In this situation, the adhesion layer  30  may provide more Si—O bonding with the underlying second etch stop sub-layer  28  and with the overlying low-k dielectric layer  32 . 
     Processing can continue by repeating the processes described with respect to  FIGS. 1 through 4  to create, for example, one or more IMDs over the low-k dielectric layer  32 . For example, the processing can be repeated as described above under the assumption that the low-k dielectric layer  32  is the dielectric structure  22  and that the metallization pattern (comprising at least the conductive fill material  54 ) is the conductive feature  24 . The processing of  FIGS. 1 through 4  can be repeated any number of times. 
     Some embodiments may achieve advantages. Some embodiments can obtain a decreased parasitic capacitance. The low-k dielectric layer  32  can achieve a lower k-value than other conventional dielectric layers. With a lower k-value, parasitic capacitance of devices interconnected using the metallization pattern (e.g., that includes the conductive fill material  54 ) can be decreased, such as by about 6% to 7%, for example.  FIG. 8  is a graph illustrating capacitance per distance (femtofarad per micrometer (fF/μm)) as a function of inverse resistance (1/R) for a number of samples of an example low-k dielectric in accordance with some embodiments. The samples exhibited 6.7% reduced capacitance and reduced inverse resistance relative to samples formed with another low-k dielectric. The reduced parasitic capacitance can result in a reduced RC delay and increased device speed. Further, as described above, the low-k dielectric layer  32  can have increased hardness. The increased hardness may permit the low-k dielectric layer  32  to accommodate stresses in neighboring layers with less bending or warping. Hence, via induced bending (VIB) in the low-k dielectric layer  32  may be reduced. A reduction in VIB can improve gap fill for a metallization, which can in turn improve yield. Even further, the low-k dielectric layer  32  can be more robust, which can lead to improvements in voltage breakdown (VBD) and time-dependent dielectric breakdown (TDDB).  FIG. 9  is a graph illustrating failure rates of samples as a function of voltage breakdown (VBD) in accordance with some embodiments. The samples fabricated in accordance with embodiments described herein exhibited increased VBD, such as by 1.5 V, relative to samples fabricated with another low-k dielectric layer.  FIG. 10  is a graph illustrating a time to failure of samples as a function of electric field (E-field) to illustrate TDDB of samples fabricated in accordance with some embodiments. The samples fabricated in accordance with embodiments described herein exhibited improved TDDB, such as a 10 times improvement, relative to samples fabricated with another low-k dielectric layer. A low-k dielectric layer formed according to the illustrated and described examples can be advantageous in scaling small technology nodes, such as 5 nm and smaller, even though such a low-k dielectric layer may be applied in larger technology nodes. 
     An embodiment is a method. A dielectric is formed over a semiconductor substrate. The dielectric has a k-value equal to or less than 3.9. Forming the dielectric includes using a plasma enhanced chemical vapor deposition (PECVD). The PECVD includes flowing a diethoxymethylsilane (mDEOS, C 5 H 14 O 2 Si) precursor gas, flowing an oxygen (O 2 ) precursor gas; and flowing a carrier gas. A ratio of a flow rate of the mDEOS precursor gas to a flow rate of the carrier gas is less than or equal to 0.2. 
     Another embodiment is a method. A dielectric layer is formed over a semiconductor substrate. An opening is formed through the dielectric layer. A conductive feature is formed in the opening through the dielectric layer. Forming the dielectric layer includes using a first plasma enhanced chemical vapor deposition (PECVD) that uses a first diethoxymethylsilane (mDEOS, C 5 H 14 O 2 Si) precursor gas and a first carrier gas. During the first PECVD, a flow rate of the first mDEOS precursor gas is equal to or less than 600 sccm, and a flow rate of the first carrier gas is equal to or greater than 3,000 sccm. 
     A further embodiment is a structure. The structure includes a dielectric over a semiconductor substrate, and a conductive feature along a sidewall of the dielectric. The dielectric has a k-value less than or equal to 3.9. The dielectric has a carbon concentration, an oxygen concentration, and a silicon concentration. The carbon concentration is in a range from 5 atomic percent to 30 atomic percent. The oxygen concentration is in a range from 40 atomic percent to 55 atomic percent. The silicon concentration is in a range from 30 atomic percent to 40 atomic percent. 
     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.