Patent Publication Number: US-2022223474-A1

Title: Methods for Forming Self-Aligned Interconnect Structures

Description:
PRIORITY 
     This is a continuation of U.S. patent application Ser. No. 16/892,984, filed on Jun. 4, 2020, which claims priority to U.S. Prov. Pat. App. Ser. No. 62/881,071 filed on Jul. 31, 2019, the entire disclosure of which is incorporated herein by reference. 
    
    
     BACKGROUND 
     The semiconductor integrated circuit (IC) industry has experienced exponential growth. Technological advances in IC materials and design have produced generations of ICs where each generation has smaller and more complex circuits than the previous generation. In the course of IC evolution, functional density (i.e., the number of interconnected devices per chip area) has generally increased while geometry size (i.e., the smallest component (or line) that can be created using a fabrication process) has decreased. This scaling down process generally provides benefits by increasing production efficiency and lowering associated costs. Such scaling down has also increased the complexity of processing and manufacturing ICs. 
     ICs are commonly formed by depositing a sequence of material layers, some of which are patterned by a lithography process. It is important that the patterned layers properly align or overlay with adjacent layers. Proper alignment and overlay becomes more difficult in light of the decreasing geometry sizes of modern ICs. For interconnect structures, overlay errors may reduce contact areas (i.e., between vias and metal lines) and introduce electrical resist drifting. In addition, overlay errors may lead to short circuitry that results in chip malfunction. Furthermore, lithography processes are a significant contributor to the overall cost of manufacturing, including processing time and the cost of masks (also referred to as photomasks or reticles) used in the process. Therefore, what is needed is a lithography method to reduce the impact of overlay errors as the process overlay margins shrink with the advancement of technology nodes. 
    
    
     
       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 emphasized that, in accordance with the standard practice in the industry, various features are not drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion. 
         FIG. 1  is a cross-sectional view of a semiconductor structure during a semiconductor fabrication process according to some embodiments of the present disclosure. 
         FIG. 2  is a top view of a photomask having an integrated circuit (IC) design pattern according to some embodiments of the present disclosure. 
         FIGS. 3 and 4  are diagrammatical views of various exposure dose curves during lithography exposure processes according to various embodiments of the present disclosure. 
         FIGS. 5A, 5B, and 5C  are cross-sectional views of conductive features with high reflectivity according to some embodiments of the present disclosure. 
         FIG. 6  is a diagrammatical view of reflected exposure dose intensity distribution in a region above a semiconductor device during a lithography exposure process according to some embodiments of the present disclosure. 
         FIG. 7  is a flowchart of a method for making a semiconductor structure with self-aligned interconnect structures according to some embodiments of the present disclosure. 
         FIGS. 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, and 18  are cross-sectional views of a semiconductor structure using a single lithography patterning process according to some embodiments of the present disclosure. 
         FIGS. 19A and 19B  illustrate a flowchart of another method for making a semiconductor structure with self-aligned interconnect structures according to some embodiments of the present disclosure. 
         FIGS. 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, and 35  are cross-sectional views of a semiconductor structure using double lithography patterning processes according to some embodiments of the present disclosure. 
         FIGS. 36A, 36B, 36C, and 36D  are various cross-sectional views of interfaces between conductive features and interconnect features according to some embodiments of the present disclosure. 
     
    
    
     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. Still further, when a number or a range of numbers is described with “about,” “approximate,” and the like, the term is intended to encompass numbers that are within +/−10% of the number described, unless otherwise specified. For example, the term “about 5 nm” encompasses the dimension range from 4.5 nm to 5.5 nm. 
     The present disclosure is generally related to lithography processes, and more particularly to lithography patterning using self-aligned methods to form interconnect features in a semiconductor structure. Various embodiments discussed herein allow for forming interconnect features having a reduced size and pitch, and allow for reducing or avoiding effects caused by overlay shift during lithography, such as via-induced-metal bridge (VIMB) and via-to-via leakage effects. In some embodiments, the lithography patterning includes exposing a resist layer (also referred to as a photoresist layer) with an exposure dose configured to be less than an exposure threshold of the resist layer such that latent patterns would not be formed by the direct exposure itself. Meanwhile, underneath conductive features reflect a portion of the incident radiation (also referred to as reflected exposure dose) back to the resist layer. The resist layer absorbs both the direct incident exposure dose and the reflected exposure dose. The sum of the incident exposure dose and the reflected exposure dose is configured to be larger than the exposure threshold of the resist layer. Since the reflection happens in a proximate region directly above the underneath conductive features, the latent patterns are self-aligned with the positions of the underneath conductive features. In some embodiments, the underneath conductive features may use high reflective metallic materials or be coated with a reflective layer to increase the reflection strength, which in turn increases the exposure contrast at the edges of the underneath conductive features. It should be noted that various embodiments discussed herein are not limited to forming interconnect features in a semiconductor structure, but may also be used to form other structures having alignment and overlay shift issues. 
     Referring to  FIG. 1 , a portion of the semiconductor structure  10  is illustrated. The semiconductor structure  10  includes a semiconductor substrate  102 , a conductive feature  104  formed in a top portion of the semiconductor substrate  102 , and a dielectric layer  106  over the semiconductor substrate  102 . 
     The semiconductor substrate  102  may include silicon (Si). Alternatively or additionally, the substrate  102  may include other elementary semiconductor such as germanium (Ge). The substrate  102  may also include a compound semiconductor such as silicon carbide, gallium arsenic, indium arsenide, and indium phosphide. The substrate  102  may include an alloy semiconductor such as silicon germanium, silicon germanium carbide, gallium arsenic phosphide, and gallium indium phosphide. In some embodiments, the substrate  102  includes an epitaxial layer. For example, the substrate  102  may have an epitaxial layer overlying a bulk semiconductor. In some embodiments, the substrate  102  may include a semiconductor-on-insulator (SOI) structure. For example, the substrate  102  may include a buried oxide layer formed by a process such as separation by implanted oxygen or other suitable technique, such as wafer bonding and grinding. 
     The substrate  102  may also include various p-type doped regions and/or n-type doped regions, implemented by a process such as ion implantation and/or diffusion. Those doped regions include n-well, p-well, light doped region (LDD), heavily doped source and drain (S/D), and various channel doping profiles configured to form various integrated circuit (IC) devices, such as a complimentary metal-oxide-semiconductor field-effect transistor (CMOSFET), imaging sensor, and/or light emitting diode (LED). The substrate  102  may further include other functional features such as a resistor or a capacitor formed in and on the substrate. In some embodiments, the substrate  102  may further include lateral isolation features provided to separate various devices formed in the substrate  102 . The isolation features may include shallow trench isolation (STI) features to define and electrically isolate the functional features. In some examples, the isolation regions may include silicon oxide, silicon nitride, silicon oxynitride, an air gap, other suitable materials, or combinations thereof. The isolation regions may be formed by any suitable process. The various IC devices may further include other features, such as silicide disposed on S/D and gate stacks overlying channels. 
     The semiconductor structure  10  may also include a plurality of dielectric layers and conductive features integrated to form interconnect structures configured to couple the various p-type and n-type doped regions and the other functional features (such as gate electrodes), resulting a functional integrated circuit. In some embodiments, the substrate  102  may include a portion of the interconnect structures and is collectively referred to as the substrate  102 . 
     As noted above, the semiconductor structure  10  may include an interconnect structure. The interconnect structure includes a multi-layer interconnect (MLI) structure and an inter-level dielectric (ILD) integrated with the MLI structure, providing an electrical routing to couple various devices in the substrate  102  to the input/output power and signals. The interconnect structure includes various metal lines, contacts and via features (or via plugs). The metal lines provide horizontal electrical routing. The contacts provide vertical connection between the substrate  102  and metal lines. The via features provide vertical connection between metal lines in different metal layers. 
     Still referring to  FIG. 1 , the semiconductor structure  10  includes a conductive feature  104 . In some embodiments, the conductive feature  104  may include a metal contact, a metal via, or a metal line. In some embodiments, the conductive feature  104  may be further surrounded by a barrier layer (not shown) to prevent diffusion and/or provide material adhesion. In some examples, the conductive feature  104  includes aluminum (Al), copper (Cu) or tungsten (W). The barrier layer may include titanium nitride (TiN), tantalum nitride (TaN), tungsten nitride (WN), titanium silicon nitride (TiSiN) or tantalum silicon nitride (TaSiN). The conductive feature  104  and the barrier layer may be formed by a procedure including lithography, etching and deposition. In some embodiments, the conductive feature  104  includes an electrode of a capacitor, a resistor, or a portion of a resistor. Alternatively, the conductive feature  104  includes a doped region (such as a source or a drain), or a gate electrode (such as a metal gate of a FinFET). In some embodiments, the conductive feature  104  includes a silicide feature disposed on respective source, drain, or gate electrode. The silicide feature may be formed by a self-aligned silicide (salicide) technique. 
     Still referring to  FIG. 1 , the semiconductor structure  10  includes a dielectric layer  106  deposited over the semiconductor substrate  102 . The dielectric layer  106  may have various material layers, such as an etch stop layer (ESL)  108 , a low-k dielectric layer  110 , and a hard mask layer ( 112 ) formed successively along a direction away from the substrate  102 . In some embodiments, the ESL  108  is formed on the conductive feature  104 . The ESL  108  may include a dielectric material similar to the dielectric material in the low-k dielectric layer  110 . However, the dielectric constant of the ESL  108  may be greater than that of the low-k dielectric layer  110  deposited on the ESL  108 . The dielectric material in the ESL  108  may be chosen to have a higher etching selectivity over the low-k dielectric layer  110  for proper subsequent etching process to form via (or contact) trenches. For example, the ESL  108  may have a lower etching rate in comparison with the low-k dielectric layer  110  on the ESL  108  in an etching process. In some embodiments, the ESL  108  may be deposited using any suitable technique, such as chemical vapor deposition (CVD), physical vapor deposition (PVD), atomic layer deposition (ALD), or an epitaxial growing process. In some embodiments, the ESL  108  includes an oxide layer including carbon, oxygen, silicon, and/or other suitable materials, or combinations thereof. 
     The low-k dielectric layer  110  is formed on the ESL  108 . The low-k dielectric layer  110  may be an inter-layer dielectric (ILD) layer or an inter-metal dielectric (IMD) layer. The low-k dielectric layer may include low-k dielectric materials having k values, for example, lower than about 4.0 or even 2.0. In some embodiments, the low-k dielectric layer  110  may be made of, for example, phosphosilicate glass (PSG), borophosphosilicate glass (BPSG), fluorosilicate glass (FSG), SiO x C y , Spin-On-Glass, Spin-On-Polymers, silicon carbon material, compounds thereof, composites thereof, combinations thereof, or the like, formed by any suitable method, such as spin-on coating, CVD, plasma-enhanced CVD (PECVD), PVD, or the like. 
     The hard mask (HM) layer  112  is formed on the low-k dielectric layer  110 . The HM layer  112  may include a single material layer, or a plurality of material layers. In some embodiments, the HM layer  112  includes a lower HM layer and an upper HM layer (not shown). The lower HM layer may include a dielectric material similar to the dielectric material of the low-k dielectric layer  110 , but with a greater dielectric constant than that of the low-k dielectric layer  110 . In some embodiments, the lower HM layer includes an oxide layer including carbon, oxygen, silicon, and/or other suitable materials, and combinations thereof. For example, the lower HM layer includes a silicon oxide (SiO 2 ) layer. The lower HM layer may be formed by a deposition process, such as a CVD process. In some embodiments, the lower HM layer may have a greater hardness than the low-k dielectric layer  110 . In some embodiments, the lower HM layer may have a higher polish rate than that of the low-k dielectric layer  110 , so that the lower HM layer can be used as a buffer layer in subsequent polishing processes. The upper HM layer is formed on the lower HM layer. In some embodiments, the upper HM layer includes titanium nitride (TiN), titanium oxide (TiO 2 ), and/or other suitable oxide materials, or combinations thereof. In some embodiments, the upper HM layer is formed using any suitable technique, such as CVD, PECVD, or PVD. The upper HM layer may be used to transfer the IC design pattern from a photomask (e.g. photomask  200  in  FIG. 2 ) to the low-k dielectric layer  110 . It is to be understood that HM layer  112  may also include a single material layer or alternatively more than two material layers that can be used to transfer one or more IC design patterns from a photomask to the low-k dielectric layer  110 . 
     Still referring to  FIG. 1 , a photoresist layer  120  is formed on the dielectric layer  106 . The photoresist layer  120  may be formed by depositing a photoresist composition over the dielectric layer  106 . The photoresist layer  120  may include a photoresist material and a solvent. In some embodiments, the photoresist material includes a polymer. In one example, the molecular weight of the photoresist material may be controlled for the quality of the lithography exposure process. In another example, the molecular weight of the photoresist material is between about 1000 and 20000. In some embodiments, the photoresist layer  120  may further include a quencher and/or other additives. 
     In some embodiments, the photoresist material of the photoresist layer  120  includes a chemically amplified (CA) resist material. The CA resist material may be a positive CA resist material, which includes an acid cleavable polymer that turns soluble in a developer such as a base solution after the acid cleavable polymer is cleaved by an acid (e.g., an acid generated by photo-acid generator (PAG)). In an example, the acid cleavable polymer cleaved by the acid becomes more hydrophilic, and may be soluble in a base solution. For example, the acid cleavable polymer cleaved by the acid may be soluble in a tetramethylammonium hydroxide (TMAH) developer. In another example, the TMAH developer includes a TMAH solution with a proper concentration ranging about between 0 and 15% by weight. In yet another example, the TMAH developer includes a TMAH solution with a concentration of about 2.38% by weight. In furtherance of the embodiments, when the CA resist material is used, the photoresist material of the photoresist layer  120  may include a photo-acid generator (PAG) distributed in the photoresist layer  120 . When absorbing radiation energy, the PAG decomposes and forms a small amount of acid. The PAG may have a concentration ranging between about 1% and 30% by weight of the photoresist layer  120 . In some embodiments, the PAG can be ionic type (onium salt), such as metallic or sulfonate. The PAG may alternatively be non-ionic, such as sulfonate ester, 2 nitrobenzyl ester, organohalide, aromatic sulfonate, oxime sulfonate, N-sulfonyloxyimide, sulfonloxy ketone, or diazonaphthoquinone (DNQ) 4 sulfonate. The photoresist layer  120  may additionally include other components, such as a quencher. In an example, the quencher is base type and is capable of neutralizing acid. Collectively or alternatively, the quencher may inhibit other active components of the photoresist layer  120 , such as inhibiting photoacid from reaction. Examples of optional additives further include photo decomposable quencher (PDQ), photo base generator (PBG) that may be used to inactivate acid generated by exposure, thermal base generator, thermal acid generator, acid amplifier, chromophore, other suitable materials, and/or a combination thereof. 
     The photoresist layer  120  may be deposited by spin-on coating or other suitable technique. Other steps, such as baking, may follow the coating of the photoresist layer  120 . In some embodiments, the solvent of the photoresist layer  120  may be partially evaporated by a soft baking process. 
       FIG. 2  is a top view of a photomask (also referred to as mask or reticle)  200  having a photomask substrate  202  and an IC design pattern  204  according to some embodiments of the present disclosure. It is to be understood that the photomask  200  and the included IC design pattern  204  may in fact be part of a larger and more complicated photomask (not shown). The photomask  200  may be used to pattern one or more layers during the lithography patterning process. In some embodiments, the IC design pattern  204  may be used for forming interconnect features (e.g., vias, contacts, or plugs) using a lithography process. In the illustrated embodiment, the IC design pattern  204  has an oval shape in the top view, which may be associated with a via feature defined by the IC design pattern  204 . The oval shape has a width of dl in the X direction. In furtherance of the embodiment, the oval shape is associated with a via feature that is formed to connect and electrically coupled to a conductive feature (e.g., a metal line) in a layer below the via, such as the conductive feature  104  formed in the top portion of the semiconductor substrate  102  ( FIG. 1 ). 
     The photomask  200  includes the photomask substrate  202  and the IC design pattern  204  formed thereon. In some embodiments, when the lithography technique, such as ultraviolet (UV) or deep ultraviolet (DUV), is used for patterning features on the wafer, the photomask substrate  202  includes a transparent substrate, such as fused quartz. The IC design pattern  204  is formed on the photomask substrate  202  and is defined in an opaque material layer, such as chromium (Cr). The photomask  200  allows UV or DUV radiation to penetrate though transparent portions defined by the IC design pattern  204 . In some alternative embodiments, when extreme ultraviolet (EUV) lithography technology is used, the photomask  200  is a reflective photomask that is different from the one illustrated in  FIG. 2 . In an exemplary reflective photomask, the photomask substrate  202  is made of a low thermal expansion material (LTEM), a reflective multilayer is deposited on the substrate  202 , and an absorber layer is deposited over the reflective multilayer and is further patterned to define the IC design pattern  204 . In EUV lithography, the reflective multilayer reflects EUV radiation and the light path is different from that of UV or DUV lithography. It is to be understood that other configurations and inclusion or omission of various items may be possible. For example, a capping layer may be formed between the reflective multilayer and absorber layer. In another example, a protection layer may be formed on the absorber layer. In yet some alternative embodiments, the photomask  200  may be a phase shift mask (PSM), such as attenuating PSM or alternating PSM, for enhanced imaging resolution. 
     Alternatively, a lithography technique may be free from using a photomask, such as the photomask  200  in  FIG. 2 , which is termed as a maskless lithography technique. In an exemplary maskless lithography technique, IC design patterns may be defined in a lithography patterning data file and be transferred to material (e.g., photoresist) layers by other exposing systems, such as a charged particle beam (including electron-beam (E-beam)), in a suitable mode (such as direct writing in raster mode or vector mode, or using a digital pattern generator). In electron-beam lithography, the photoresist layers are often referred to as electron-beam sensitive resist layers. 
     Referring to  FIG. 3 , a conventional lithography exposure process  300  is illustrated, which is to be compared with an embodiment of a self-aligned lithography exposure process that will be discussed below with respect to  FIG. 4 . The lithography exposure process  300  uses the photomask  200  having the IC design pattern  204  to expose the photoresist layer  120 , thereby forming a latent pattern in a region  304  of the photoresist layer  120 . A latent pattern is referred to as a portion of the photoresist layer that is exposed but not developed yet. In the illustrated example, an overlay shift occurs in the X direction, causing a misalignment between the photomask  200  and the conductive feature  104 , such that an edge of the IC design pattern  204  is offset from an edge of the conductive feature  104  for a distance of Δx. 
     Regarding the photoresist material in the photoresist layer  120 , it has an exposure threshold to radiation (e.g., UV, DUV, EUV, or E-beam radiation), denoted as T. When the exposing intensity (also referred to as exposure dosage or exposure dose) is equal to or greater than the exposure threshold T, the corresponding portion of the photoresist is chemically changed such that a latent pattern is formed, and the latent pattern will be developed (e.g., it is removable by a developer) in a developing process. When the exposing intensity is less than the exposure threshold T, the corresponding portion of the photoresist is not chemically changed to be developed (e.g., no latent pattern is formed, and it remains during the developing process). It is understood that the term “chemically changed” means that the photoresist has sufficiently changed to respond differently, e.g., as exposed photoresist responds in the development process. In one example where the photoresist is positive tone, only portions of the photoresist exposed with exposing intensity equal to or greater than the exposure threshold T are removed by a suitable developer during the developing process. Other portions of the photoresist unexposed or exposed with exposing intensity less than the exposure threshold T remain after the developing process. In another example where the photoresist is negative tone, the portions of the photoresist unexposed or exposed with exposing intensity less than the exposure threshold T are removed by a suitable developer during the developing process. Other portions of the photoresist exposed with exposing intensity equal to or greater than the exposure threshold T remain after the developing process. 
     Still referring to  FIG. 3 , the exposing intensity under radiation  302 , which is emitted from the radiation source directly to the photoresist layer  120 , is referred to as incident exposure dose, denoted as E incident . E incident  has a distribution profile, also known as an exposure dose curve. Also shown in  FIG. 3  is an example of the exposure dose curve of E incident  to which the photoresist layer  120  is exposed during the lithography exposure process  300 . In an example, the exposure dose curve of E incident  illustrates the exposure intensity at the bottom surface of the photoresist layer  120 . Regions of the photoresist layer  120  closer to the center of the IC design pattern  204  may receive higher exposure dose (e.g., E1) than regions further from the center of the IC design pattern  204  (e.g., E2 near the edge of the IC design pattern  204 ). Conventionally, the lithography exposure process  300  applies an incident exposure dose larger than the exposure threshold T of the photoresist layer  120 . According to the exposure dose curve of E incident , the lowest exposure dose received by the region  304  of the photoresist layer  120  is E2. In some embodiments, E2 is greater than or equal to the exposure threshold T of the photoresist layer  120 , such that a latent pattern is formed in the region  304  under the radiation  302 . However, since the exposure dose curve is mainly defined by the radiation intensity from the source together with the geometry of the IC design pattern  204  and substantially irrelevant to the position of the underneath conductive feature  104 , the overlay error with the offset Δx is transferred to the position of the latent pattern in the region  304  and subsequently would be transferred to an interconnect feature to-be-formed in the dielectric layer  106 . The offset Δx may cause the interconnect feature electrically short to other adjacent conductive features in the dielectric layer  106 . 
     Referring to  FIG. 4 , an embodiment of a self-aligned lithography exposure process of the present disclosure is illustrated. The lithography exposure process  400  uses the photomask  200  having the IC design pattern  204  to expose the photoresist layer  120 , thereby forming a latent pattern in the photoresist layer  120 . Compared with the latent pattern illustrated in  FIG. 3 , the position of the latent pattern in  FIG. 4  is defined by the position of the underneath conductive feature  104 , such that the latent pattern is confined in a region  404  that is substantially directly above the conductive feature  104 . Compared with the conventional lithography exposure process  300  in  FIG. 3 , the position of the latent pattern formed in the photoresist layer  120  in  FIG. 4  is insensitive to overlay errors, regardless overlay shifts occurred in either positive or negative X direction. To illustrate this, in  FIG. 4 , the IC design pattern  204  in the photomask  200  is drawn to be larger than the underneath conductive feature  104 , such that both edges of the IC design pattern  204  are offset from respective edges of the conductive feature  104  for a distance of Δx. 
     The photoresist material in the photoresist layer  120  has an exposure threshold T′. The exposing intensity under radiation  402 , which is emitted from the radiation source directly to the photoresist layer  120 , is denoted as E incident . An exposure dose curve of E incident  represents its distribution profile. Also shown in  FIG. 4  is an example of the exposure dose curve of E incident  to which the photoresist layer  120  is exposed during the lithography exposure process  400 . In an example, the exposure dose curve of E incident  illustrates an exposure intensity at the bottom surface of the photoresist layer  120 . One difference compared with the lithography exposure process  300  illustrated in  FIG. 3  is that the exposure dose used in the lithography exposure process  400  in  FIG. 4  is configured to be less than the exposure threshold T′. Even though regions of the photoresist layer  120  closer to the center of the IC design pattern  204  may receive higher exposure dose (e.g., E1 near the edge of the conductive feature  104 ) than regions further from the center of the IC design pattern  204  (e.g., E2 near the edge of the IC design pattern  204 ), the whole exposure dose curve of E incident  is below the exposure threshold T′ (i.e., E1&lt;T′ and E2&lt;T′). Accordingly, the exposure dose emitted from the radiation source directly to the photoresist layer  120  is not strong enough to form a latent pattern. 
     Yet another difference compared with the lithography exposure process  300  in  FIG. 3  is that the underneath conductive feature  104  in  FIG. 4  has a higher reflectivity with respect to the incident radiation  402  at its top surface, which has substantial impact on the total radiation received by the photoresist layer  120 . The term “reflectivity” means a fraction of incident radiation that is reflected at an interface, denoted as R. With respect to the lithography exposure process  300  illustrated in  FIG. 3 , the reflectivity R may be small, such as less than about 5%, and the reflected radiation can be ignored. For example, under a DUV radiation at a wavelength of 193 nm, copper (Cu) has a reflectivity of about 2% (R 2%). That is, if the conductive feature  104  is made of Cu, about 2% of the incident DUV radiation arriving at the top surface of the conductive feature  104  would be reflected back to above layers. The total radiation received by the photoresist layer  120  is the sum of the incident radiation and the reflected radiation. But for copper, since the reflected radiation is merely 2% of the incident radiation, a small fraction that often can be ignored, the total radiation remains dominated by the strength of the incident radiation. 
     While metallic materials like copper may have poor reflectivity under certain radiation (e.g., under 193 nm DUV radiation, for Au, R&lt;1%; for Ni, R 2%; for Cr, R 1%), some other metallic materials or alloys may exhibit stronger reflectivity. For example, under a DUV radiation at a wavelength of 193 nm, aluminum (Al) has a reflectivity of about 65%, and an alloy of Al and Cu (AlCu) may reach a reflectivity of about 71%. That is, if the conductive feature  104  is made of Al, about 65% of the incident DUV radiation arriving the top surface of the conductive feature  104  would be reflected back to above layers. In the illustrated embodiment in  FIG. 4 , the conductive feature  104  has a reflectivity at least about 5%. As discussed above, for a reflectivity less than about 5%, the reflected radiation is hard to have substantial impact on the strength of the total radiation. 
     Still referring to  FIG. 4 , the intensity of the reflected radiation  408  from the top surface of the conductive feature  104  back to the photoresist layer  120  is denoted as E reflective . E reflective  has a distribution profile, also referred to as reflective exposure dose curve. Also shown in the example of  FIG. 4  is the reflective exposure dose curve of E reflective  to which the photoresist layer  120  is exposed from the underneath conductive feature  104 . In an example, the reflective exposure curve of E reflective  illustrates an exposure intensity at the bottom surface of the photoresist layer  120 , which is distant from the conductive feature by the thickness of the dielectric layer  106  (denoted as H). In various embodiments, the thickness H may range from about 1 nm to about 100 nm, such as about 10 nm. 
     According to the illustrated reflective exposure dose curve of E reflective  in  FIG. 4 , regions of the photoresist layer  120  directly above the conductive feature  104  (e.g., E1′ at the edges of the conductive feature  104 ) receive higher reflective exposure dose than regions offset from the edges of the conductive feature  104  (e.g., E2′ at the edges of IC design pattern  204 ). Since E reflective  is a fraction of E incident , the reflective exposure dose curve of E reflective  is below the exposure dose curve of E incident , which is further below the exposure threshold T′. 
     The total exposure dose received by the photoresist layer  120 , denoted as E total , is a sum of the incident exposure dose E incident  and reflected exposure dose E reflective  (E total =E incident  E reflective ). The total exposure dose curve of E total  is also shown in  FIG. 4 . At the edges of the conductive feature  104 , the total exposure dose is about E1+E1′. At the edges of the region  404 , which are offset from the edges of the conductive feature  104 , the total exposure dose is about E2+E2′. 
     In the lithography exposure process  400 , the incident radiation directly from the radiation source is configured such that E1+E1′ is larger than or equal to the exposure threshold T′, such that a latent pattern is formed in the region  404  directly above the conductive feature  104 , while E2+E2′ is less than the exposure threshold T′, such that latent pattern would not be formed in transitional regions  406  that are offset from the conductive feature  104 . By taking the reflected exposure dose into effect to shift the total exposure dose from otherwise below the exposure threshold T′ to above the exposure threshold T′, the latent pattern formed in the region  404  is self-aligned with the underneath conductive feature  104 . Accordingly, by defining the position of the latent pattern, the interconnect features to be formed subsequently in the dielectric layer  106  will be substantially self-aligned with the underneath conductive feature  104  as well. 
     A greater exposure contrast provides more design flexibility in a lithography process. The exposure contrast, denoted as y, refers to a slope of an exposure dose curve in a transitional region of a photoresist layer. The exposure contrast γ describes the ability of the resist to distinguish between light and dark areas. Regarding the exposure dose curve of E total , the exposure contrast γ at the transitional region  406  can be proximately expressed as the difference between the total exposure doses at the edge of the conductive feature  104  (about E1+E1′) and at the edge of the IC design pattern  204  (about E2+E2′) divided by the offset distance Δx, which is γ≈(E1+E1′−E2−E2′)/Δx. Since the difference between the incident exposure doses E1 and E2 is small (E1≈E2) as both E1 and E2 are exposure doses directly under the IC design pattern  204 , the expression of exposure contrast γ can be further simplified as γ≈(E1′−E2′)/Δx. In other words, the exposure contrast γ is mainly defined by the slope of the reflective exposure dose curve of E reflective . 
     To increase the slope of the reflective exposure dose curve of E reflective  in transitional regions  406  in order to enhance the exposure contrast γ, one way is to increase the reflectivity R at the top surface of the conductive feature  104 , such as by forming the conductive feature  104  with a metallic material with high reflectivity. With higher reflectivity, the reflected exposure dose curve in center regions directly above the conducive feature  104  will be shifted further up and thus rolling off faster outside the edges of the conductive feature  104 . 
       FIGS. 5A-C  illustrate various embodiments of the conductive feature  104 . In some embodiments, the conductive feature  104  is a uniform layer of a bulk metallic material that has high reflectivity, such as Aluminum (Al), Tantalum (Ta), Titanium (Ti), or a metallic alloy, such as AlCu, as shown in  FIG. 5A . 
     In some other embodiments, the conductive feature  104  has a bilayer arrangement with a bulk metal layer  114   a  at the bottom portion of the conductive feature  104  and a reflective layer  114   b  coated on the bulk metal layer  114   a , as shown in  FIG. 5B . The bulk metal layer  114   a  may include a metallic material with relatively high conductivity but low reflectivity, such as Copper (Cu), Gold (Au), Tungsten (W), Chromium (Cr), Cobalt (Co), Nickel (Ni). The reflective layer  114   b  is formed of materials with high reflectivity, such as Al, Ta, Ti, AlCu, TiN, CrSi 2 , or other suitable material. The reflective layer  114   b  may be conductive, or alternatively non-conductive. As will be explained in further detail below, when the reflective layer  114   b  is non-conductive, an extra etching step may be applied to the reflective layer  114   b  to expose the underneath bulk metal layer  114   a  during lithography processes. 
     In some other embodiments, the reflective layer  114   b  coated on the bulk metal layer  114   a  is a reflective multilayer, such as a plurality of alternating layers of a first material layer  116   a  and a second material layer  116   b , as shown in  FIG. 5C . The reflective multilayer is configured to effectively reflect radiation at a predetermined range of wavelength, such as deep ultraviolet (DUV) (from about 100 nm to about 300 nm) or EUV (from about 13.2 nm to about 13.8 nm). For example, the reflective layer  114   b  may include a plurality of alternating layers of a relatively high refractive index for radiation scattering and a relatively low refractive index for radiation transmitting. Pairing these two type materials together provides a resonant reflectivity. In some embodiments, the reflective multilayer configuration includes multiple molybdenum/silicon (Mo/Si) pairs (e.g., a layer of Mo above or below a layer of Si in each pair) or multiple molybdenum compound/silicon compound pairs. In some embodiments, the reflective multilayer  114   b  includes multiple molybdenum/beryllium (Mo/Be) pairs or other appropriate material pairs (e.g., Ru/Si pairs, Pd/Si pairs, or Rh/Si pairs) that have refractive index difference to cause a high reflectivity (e.g., from about 10% to about 60%) to a selected radiation. In some embodiments, each layer of the reflective multilayer has a thickness from about 2 nm to about 5 nm. The thickness is adjusted to achieve a maximum constructive interference of the selected radiation diffracted at each interface and a minimum absorption of the selected radiation thereof. In furtherance of some embodiments where the reflective multiplayer  114   b  includes Mo/Si pairs, a layer of silicon or silicon compound may have a thickness about 4 nm and a layer of molybdenum or molybdenum compound may have a thickness about 3 nm. In some embodiments, the reflective multilayer  114   b  includes a number of pairs from about 3 to about 20. A number of pairs fewer than 3 decreases a reflectivity, in some instances. A number of pairs greater than 20 increases a likelihood of contacting foreign particles and/or an occurrence of defects, in some instances. In some embodiments where the reflective multilayer  114   b  includes Mo/Si pairs, a number of pairs is from about 3 to 10, such as 4. In at least one embodiment, the conductive feature  104  further includes a backside coating layer (not shown) stacked between the reflective multilayer  114   b  and the bulk metal layer  114   a . In some instances, the backside coating layer is a metallic film or a polycrystalline silicon film. 
     Referring back to  FIG. 4 , to enhance the exposure contrast γ, besides increasing the reflectivity of the underneath conductive feature  104 , the inventors of the present disclosure have also observed that a distance between the photoresist layer  120  and the conductive feature  104  (i.e., defined by the thickness H of the dielectric layer  106 ) is a tuning factor. 
     Referring to  FIG. 6 , a diagrammatical view of reflected exposure dose intensity distribution in a region above the conductive feature  104  according to some embodiments of the present disclosure is illustrated. The coordination is set at the top surface of the conductive feature  104  (Z=0). In the illustrated embodiment, the top surface of the conductive feature  104  has a reflectivity R about 25%.  FIG. 6  also includes a reflected exposure dose intensity scale marked with various intensity levels represented by various grey scales. In the present example, for simplicity, the unit for the exposing intensity is a relative unit in percentage, ranging from 0 to 100%. In this case, “25%” stands for the reflected exposure dose is at 25% of the incident exposure dose from the exposing system. Along line L1 which is perpendicular to the top surface of the substrate  102  and aligned with an edge of the conductive feature  104 , the reflected exposure dose generally decreases at a larger vertical distance from the conductive feature  104 . In the illustrated embodiment in  FIG. 6 , for example, at Z=0, the reflected exposure dose is about the same as the reflectivity R, at about 25%; at Z=H1, the reflected exposure dose (E1′) becomes smaller, at about 23%; at Z=H2, which is further away from the conductive feature  104 , the reflected exposure dose (E1″) further decreases to about 22%. However, a decreasing relationship between reflected exposure dose and a vertical distance may not be the case at other locations, because the energy of the reflected radiation generally is not linearly distributed in space considering diffraction and interference effects. For example, along line L2 which is also perpendicular to the top surface of the substrate  102  but offset from the conductive feature  104  at a distance of Δx, when the distance from the conductive feature  104  increases, the reflected exposure dose does not decrease monotonically. Contrarily, the reflected exposure dose may inversely increase at a larger distance. For example, along line L2, at Z=0, the reflected exposure dose is at its minimum, about 12%; at Z=H1, the reflected exposing dose intensity (E2′) increases to about 18%; at Z=H2, which is further away from the conductive feature  104 , the reflected exposure dose (E2″) further increases to about 21%. 
     As discussed above, an expression of the exposure contrast γ can be simplified as γ≈(E1′−E2′)/Δx. At Z=H1, γ≈(23%−18%)/Δx=5%/Δx; at Z=H2, γ≈(22%−21%)/Δx=1%/Δx, which is significantly smaller than y at Z=H1. Accordingly, to increase the slope of the reflective exposure dose curve of E reflective , a distance between the photoresist layer  120  and the conductive feature  104  (i.e., defined by the thickness H of the dielectric layer  106 ) is a tuning factor and often needs to be less than a threshold distance H th . That is, in a self-aligned lithography process, the photoresist layer may need to be spaced from the underneath reflective conductive feature for a distance smaller than a threshold distance H th . The threshold distance H th  is affected by multiple factors, such as radiation wavelength, reflectivity of the conductive features, geometry of the photomask. In some embodiments, under a DUV radiation, a threshold distance H th  may range from about 5 nm to about 20 nm, such as about 10 nm. In some embodiments, under an EUV radiation, a threshold distance H th  may range from about 1 nm to about 10 nm, such as about 5 nm. 
     In various embodiments, by properly choosing the incident exposure dose, adjusting position of the photoresist layer relative to the underneath conductive features, choosing reflectivity of underneath conductive features, adjusting exposure threshold through tuning the composition of the photoresist materials, or a combination thereof, latent patterns that are self-aligned to underneath conductive features can be formed as illustrated in the present disclosure. Further, in some embodiments, the lithography exposure process uses photons, such as UV, DUV or EUV radiation. In an alternative embodiment, charged particles are used as radiation beam during the lithography exposure process. In this case, the IC design pattern may be defined in a data file and the sensitive resist material is chosen to be sensitive to the charged particles, such as E-beam. 
     The methods of forming self-aligned interconnect features and the semiconductor structures made thereby are further described below according to various embodiments. 
       FIG. 7  is a flowchart of a method  700  for making a semiconductor structure using a self-aligned lithography exposure process according to one or more embodiments of the present disclosure. The method  700  is merely an example, and is not intended to limit the present disclosure beyond what is explicitly recited in the claims. Additional steps can be provided before, during, and after method  700 , and some of the steps described can be moved, replaced, or eliminated for additional embodiments of method  700 .  FIG. 7  will be described below in conjunction with  FIGS. 8-18 , which are cross sectional views of a semiconductor structure  20  at various fabrication stages according to method  700 . 
     The semiconductor structure  20  may be an intermediate device fabricated during processing of an integrated circuit (IC), or a portion thereof, that may comprise static random access memory (SRAM) and/or logic circuits, passive components such as resistors, capacitors, and inductors, and active components such as p-type FETs (pFETs), n-type FETs (nFETs), FinFETs, metal-oxide semiconductor field effect transistors (MOSFET), and complementary metal-oxide semiconductor (CMOS) transistors, bipolar transistors, high voltage transistors, high frequency transistors, other memory cells, and combinations thereof. Furthermore, the various features including transistors, gate stacks, active regions, isolation structures, and other features in various embodiments of the present disclosure are provided for simplification and ease of understanding and do not necessarily limit the embodiments to any types of devices, any number of devices, any number of regions, or any configuration of structures or regions. 
     At operation  702 , the method  700  ( FIG. 7 ) provides the semiconductor structure  20 , as shown in  FIG. 8 . The various compositions and material layers of the semiconductor structure  20  are similar to what have been discussed above with reference to the semiconductor structure  10  in  FIG. 1  and will be briefly discussed below for the sake of convenience. Reference numerals are repeated for ease of understanding. The semiconductor structure  20  includes a semiconductor substrate  102 , a plurality of conductive features  104   a - c  (collectively as conductive features  104 ) formed in a top portion of the semiconductor substrate  102 , and a dielectric layer  106  over the semiconductor substrate  102 . As will be discussed below, in the illustrated embodiment, interconnect features (e.g., vias, contacts, or plugs) will be formed to contact and electrically couple with the conductive features  104   a  and  104   b , but not on the conductive feature  104   c . The conductive feature  104   c  may be part of an intra-layer routing or have interconnect features elsewhere. 
     In one embodiment, the semiconductor substrate  102  is a silicon substrate. The semiconductor substrate  102  may comprise another elementary semiconductor, such as germanium; a compound semiconductor including silicon carbide, gallium arsenide, gallium phosphide, indium phosphide, indium arsenide, and/or indium antimonide; an alloy semiconductor including SiGe, GaAsP, AlInAs, AlGaAs, GaInAs, GaInP, and/or GaInAsP; or combinations thereof. In yet another alternative, the semiconductor substrate  102  is a semiconductor on insulator (SOI). 
     The semiconductor substrate  102  includes a plurality of conductive features  104 . The conductive features  104  may be IC features such as metal lines, metal contacts, or metal vias. In some embodiments, the conductive features  104  include electrodes of capacitors or resistors. Alternatively, the conductive features  104  may include doped regions (such as source or drain), or gate electrodes (such as metal gates of FinFETs). 
     The conductive features  104  comprise conductive material compositions, such as highly-conductive metal, low-resistive metal, elemental metal, transition metal, or the like. In some embodiments, the conductive features  104  may be further surrounded by a barrier layer to prevent diffusion and/or provide material adhesion. The conductive features  104  may be deposited by electroplating techniques, although any method of formation could alternatively be used. In an embodiment, the conductive features  104  includes metallic material of relatively high reflectivity, such as Al, Ta, Ti, or metallic alloy, such as AlCu, which is similar to what have been discussed above with reference to the conductive feature  104  in  FIG. 5A . In another embodiment, the conductive features  104  have a bilayer arrangement with a bulk metal layer  114   a  and a reflective layer  114   b  coated on the bulk metal layer  114   a , which is similar to what have been discussed above with reference to the conductive feature  104  in  FIG. 5B . The bulk metal layer  114   a  may be formed of Cu, although other materials, such as W, Al, Au, or the like, could alternatively be utilized. The reflective layer  114   b  is formed of materials with relatively high reflectivity, such as Al, Ta, Ti, AlCu, TiN, CrSi 2 , or other suitable material. The reflective layer  114   b  may be conductive, or alternatively non-conductive. In yet another embodiment, the reflective layer  114   b  may be a reflective multilayer, such as a plurality of alternating layers of a first material layer and a second material layer, which is similar to what have been discussed above with reference to the reflective layer  114   b  in  FIG. 5C . 
     The dielectric layer  106  may have various material layers formed on the substrate  102 , such as an etch stop layer (ESL), a low-k dielectric layer (e.g., ILD layer or IMD layer), and a hard mask layer formed successively along a direction away from the substrate  102 , which is similar to what have been discussed above with reference to the ESL  108 , low-k dielectric layer  110 , and hard mask layer  112  in  FIG. 1 . In various embodiments, the dielectric layer  106  has a thickness H that is smaller than the threshold distance H th  determined in the subsequent lithography exposure process. For a lithography exposure process using a DUV radiation, the thickness H th  may range from about 5 nm to about 20 nm, such as about 10 nm. In an example that H th  is about 10 nm, H may range from about 5 nm to about 10 nm, such as about 8 nm. For a lithography exposure process using an EUV radiation, the thickness H th  may range from about 1 nm to about 10 nm, such as about 5 nm. In an example that H th  is about 5 nm, H may range from about 1 nm to about 5 nm, such as about 3 nm. 
     At operation  704 , the method  700  ( FIG. 7 ) forms a photoresist layer  120  over the dielectric layer  106 , as shown in  FIG. 9 . Forming of the photoresist layer  120  includes coating the photoresist solution on the dielectric layer  106  by a suitable technique, such as spin-on coating. Other manufacturing steps, such as soft baking may be further applied to the photoresist layer  120 . The photoresist layer  120  may include a positive photoresist material that may become dissolvable to the developer solution after exposing to the radiation source. In some alternative embodiments, the photoresist layer  120  may include a negative photoresist material that becomes indissolvable to the developer solution after exposing to the radiation source. The photoresist layer  120  has a predetermined exposure threshold. The composition of the photoresist layer  120  may be adjusted, for example, by changing the ratio of carbon, hydrogen and oxygen, to have a suitable exposure threshold for the lithography exposure process as discussed later in the present disclosure. The photoresist layer  120  may have a thickness in a range from about 200 Å to about 800 Å. 
     At operation  706 , the method  700  ( FIG. 7 ) performs a lithography exposure process  900  to form latent patterns in the photoresist layer  120 , as shown in  FIG. 10 . The lithography exposure process  900  uses a photomask  200  having a first IC design pattern  204   a  and a second IC design pattern  204   b  (collectively as IC design patterns  204 ) to expose the photoresist layer  120  with a radiation  920 , thereby forming latent patterns in the photoresist layer  120 . The IC design patterns  204   a  and  204   b  are used for forming interconnect features (e.g., vias, contacts, or plugs) in the dielectric layer  106 . In the illustrated embodiment, the IC design pattern  204   a  aligns with the underneath conductive feature  104   a , while overlay errors occur in the IC design pattern  204   b  such that an edge of the IC design pattern  204   b  is offset from an edge of the underneath conductive feature  104   b  for a distance of Δx. The overlay errors may be due to misalignment between the photomask  200  and the semiconductor structure  20 , inaccuracy in the geometry of the IC design pattern  204   b  during fabrication of the photomask  200 , calibration inaccuracy occurred in the optical apparatus used in the lithography exposure process  900 , or other reasons. 
     The exposing source used in the lithography exposure to generate radiation  920  may include any suitable source such as UV, DUV, EUV, or charged particles, such as E-beam. In some alternative embodiments, the IC design pattern is defined in a data file and is transferred to the photoresist layers by direct writing or other suitable technique, such as digital pattern generator. Other steps may be implemented before, during, or after the exposure process. In some embodiments, a post exposure baking process may be applied to the photoresist layer  120  after the lithography exposure process. 
     In the illustrated embodiment, the radiation  920  is configured such that the incident exposure dose directly from the radiation  920  to the photoresist layer  120  is less than the exposure threshold of the photoresist layer  120 , thereby no latent pattern will be formed in the photoresist layer  120  by absorbing the incident exposure dose alone. A portion of the radiation  920  reaches the top surface of the conductive features  104   a  and  104   b  and is reflected as reflected radiation  922  back to the photoresist layer  120 . The reflected exposure dose is mainly controlled by the reflectivity R of the top surface of the conductive feature  104  and the strength of the incident exposure dose. In some embodiments, the thickness H of the dielectric layer  106  is adjusted to control the reflected exposure dose. A sum of the incident exposure dose and the reflected exposure dose is configured to be larger than or at least equal to the exposure threshold of the photoresist layer  120 . Therefore, latent patterns will be formed in portions of the photoresist layer  120  that receive both the incident exposure dose and the reflected exposure dose. The reflected radiation happens at portions of the top surface of the conductive features  104  where incident radiation  920  reaches. At other portions of the top surface of the conductive features  104  where incident radiation  920  does not reach or offset from the top surface of the conductive features  104 , the strength of the reflected exposure dose decreases sharply. In other words, only portions of the photoresist layer  120 , such as regions  904   a  and  904   b  that are directly under the respective IC patterns  204  in the photomask  200  and also directly above the conductive features  104 , receive the sum of the incident exposure dose and the reflected exposure dose, which causes chemical changes to form latent patterns. Regarding the region  904   c  adjacent to the region  904   b , which is offset from an edge of the conductive feature  104   b , it receives substantially only the incident radiation  920  but no reflected radiation  922 , which is not strong enough to expose the region  904   c . Therefore, the latent pattern formed in the region  904   b  does not extend into the region  904   c . Accordingly, the latent pattern formed in the region  904   b  is self-aligned with the underneath conducive feature  104   b.    
     At operation  708 , the method  700  ( FIG. 7 ) develops the photoresist layer  120  to form a patterned photoresist layer  122 , as shown in  FIG. 11 . In the illustrated embodiment, the photoresist layer  120  is positive, so the portions of the photoresist layer  120  associated with the latent patterns in the regions  904   a  and  904   b  are removed by the corresponding developer to form openings in regions  904   a  and  904   b . The region  904   c , which is offset from the conductive feature  104   b , does not receive exposure doses higher than the exposure threshold and remains undeveloped. 
     At operation  710 , the method  700  ( FIG. 7 ) etches the dielectric layer  106  by using the patterned photoresist layer  122  as an etch mask, thereby transferring a pattern in the patterned photoresist layer  122  to the dielectric layer  106 , as shown in  FIG. 12 . Operation  710  may include one or more etching processes to remove different portions of the dielectric layer  106 , such as a hard mask layer, a low-k dielectric layer, and an etch stop layer to extend the openings in the regions  904   a  and  904   b  downwardly to the top surface of the conductive features  104   a  and  104   b . In some embodiments, the conductive features  104  are metal lines and the openings in the regions  904   a  and  904   b  are referred to as via trenches. The top surface of the conductive features  104   a  and  104   b  are exposed in the respective via trenches. The etching process may include any suitable etching technique, such as dry etching, wet etching, or a combination thereof. Other operations may be subsequently implemented. For example, the patterned photoresist layer  122  may be removed by wet stripping or plasma ashing process, as shown in  FIG. 13 . The plasma ashing process may include using oxygen ( 02 ) plasma or carbon dioxide (CO 2 ) plasma. 
     In some embodiments, the conductive features  104  are formed of continuous metallic material or the reflective layer  114   b  is conductive, the method  700  ( FIG. 7 ) may optionally proceed from operation  710  to operation  714  by depositing a metal layer  130  to fill the via trenches in regions  904   a  and  904   b  and covers the dielectric layer  106 , as shown in  FIG. 14 . The metal layer  130  is in direct contact with the top surfaces of the conductive features  104   a  and  104   b . In some embodiments, the metal layer  130  includes Cu, Al, W, or other suitable conductive material. In some embodiments, the metal layer  130  includes Cu alloy, such as copper magnesium (CuMn), copper aluminum (CuAl) or copper silicon (CuSi). In some embodiments, the metal layer  130  is deposited by PVD. In some examples, the metal layer  130  is formed by depositing a corresponding metal seed layer using PVD, and then forming a bulk metal layer by plating. 
     At operation  716 , the method  700  ( FIG. 7 ) performs a chemical mechanical polishing (CMP) process to remove the excessive metal layer  130 , thereby forming interconnect features  140   a  and  140   b , such as vias  140   a  and  140   b , as shown in  FIG. 15 . The CMP process may also remove a top portion of the dielectric layer  106 , such as a hard mask layer. A substantially coplanar top surface of the interconnect features  140   a  and  140   b  and the dielectric layer  106  may be formed after the CMP process. Due to the self-aligned lithography exposure process, both sidewalls S1 and S2 of the interconnect features  140   a  and  140   b  are landing on the top surfaces of the respective conductive features  104   a  and  104   b . The sidewall S1 of the interconnect feature  140   b  has a landing point P that is substantially at the edge of the conductive feature  104   b , such as within a lateral distance to the edge of the conductive feature  104   b  at about 20% of the interconnect feature CD size. For example, for a via feature&#39;s diameter with a CD (critical dimension) of 20 nm, the landing point P is about within 4 nm (20 nm×20%=4 nm) lateral distance to the edge of the conductive feature  104   b . Accordingly, the sidewall S1 of the interconnect features  140   b  is also referred to be substantially aligned with an edge of the conductive feature  104   b.    
     In some embodiments, the reflective layers  114   b  of the conductive features  104  are formed of high-resist material, non-conductive material, or reflective multilayers, and the method  700  ( FIG. 7 ) may optionally proceed from operation  710  to operation  712  by selectively etching the reflective layers  114   b  to expose the underneath high-conductive bulk metal layer  114   a  in the openings, as shown in  FIG. 16 . Operation  712  includes a selective etching process to remove portions of the reflective layers  114   b  exposed in the via trenches. The etch process and the etchant are properly chosen for selective etch without damage to the dielectric layer  106 . The etching process may include any suitable etching technique, such as dry etching, wet etching, or a combination thereof. The etching process stops at the bulk metal layer  114   a . In the illustrated embodiment, after operation  712 , portions of the reflective layer  114   b  remain on both ends of the conductive feature  104   a , while portions of the reflective layer  114   b  remain on only one end of the conductive feature  104   b.    
     Referring to  FIGS. 17 and 18 , the method  700  ( FIG. 7 ) then proceeds to operations  714  and  716  to deposit a metal layer  130  to fill the via trenches in regions  904   a  and  904   b  and perform a CMP process to remove the excessive metal layer  130 , thereby forming interconnect features  140   a  and  140   b . Due to the self-aligned lithography exposure process, both sidewalls S1 and S2 of the interconnect features  140   a  and  140   b  are landing on the top surfaces of the respective conductive features  104   a  and  104   b . The sidewall S1 of the interconnect features  140   b  may substantially align with an edge of the conductive feature  104   b . Bottom portions of the sidewalls S1 and S2 of the interconnect features  140   a  are covered by the reflective layer  114   b . Bottom portion of the sidewall S2 of the interconnect feature  140   b  is covered by the reflective layer  114   b . Bottom portion of the sidewall S1 of the interconnect feature  140   b  is covered by a top portion of the substrate  102 . Upper portions of the sidewalls S1 and S2 of the interconnect features  140   a  and  140   b  are covered by the dielectric layer  106 . 
       FIGS. 19A and 19B  show a flowchart of a method  1000  for making a semiconductor structure using a self-aligned lithography exposure process according to one or more alternative embodiments of the present disclosure. The method  1000  is merely an example, and is not intended to limit the present disclosure beyond what is explicitly recited in the claims. Additional steps can be provided before, during, and after method  1000 , and some of the steps described can be moved, replaced, or eliminated for additional embodiments of method  1000 .  FIGS. 19A and 19B  will be described below in conjunction with  FIGS. 20-35 , which are cross sectional views of a semiconductor structure  30  at various fabrication stages according to method  1000 . 
     At operation  1002 , the method  1000  ( FIG. 19A ) provides the semiconductor structure  30 , as shown in  FIG. 20 . The semiconductor structure  30  includes a semiconductor substrate  102 , a plurality of conductive features  104   a - c  (collectively as conductive features  104 ) formed in a top portion of the semiconductor substrate  102 , and a dielectric layer  106  over the semiconductor substrate  102 . As will be discussed below, in the illustrated embodiment, interconnect features (e.g., vias, contacts, or plugs) will be formed to contact and electrically couple with the conductive features  104   a  and  104   b , but not on the conductive feature  104   c . The conductive feature  104   c  may be part of an intra-layer routing or have interconnect features elsewhere. The various compositions and material layers of the semiconductor structure  30  are similar to what have been discussed above with reference to the semiconductor structure  20  in  FIG. 8  and the semiconductor structure  10  in  FIG. 1  and will be briefly discussed below for the sake of convenience. Reference numerals are repeated for ease of understanding. Some differences will be emphasized. 
     In various embodiments, the dielectric layer  106  has a thickness H′ that is larger than a threshold distance H th  in a subsequent lithography exposure process. For a lithography exposure process using a DUV radiation, the threshold distance H th  may range from about 5 nm to about 20 nm, such as about 10 nm. In an example that H th  is about 10 nm, H′ may range from about 15 nm to about 50 nm, such as about 20 nm. For a lithography exposure process using an EUV radiation, the threshold distance H th  may range from about 1 nm to about 10 nm, such as about 5 nm. In an example that H th  is about 5 nm, H′ may range from about 10 nm to about 50 nm, such as about 15 nm. As discussed above in association with  FIGS. 4-6 , when the distance between a photoresist layer and underneath conductive feature is larger than the threshold distance H th , the exposure contrast may be poor and further affect critical dimensions (CD) of latent patterns to-be-formed in the photoresist layer. As will be discussed below, the method  1000  uses a first lithography exposure process to partially recess the dielectric layer  106  to a thickness below the threshold distance H th , then applies a second lithography exposure process to form self-aligned interconnect features. 
     At operation  1004 , the method  1000  ( FIG. 19A ) forms a first photoresist layer  120   a  over the dielectric layer  106 , as shown in  FIG. 21 . Forming of the photoresist layer  120   a  includes coating the photoresist solution on the dielectric layer  106  by a suitable technique, such as spin-on coating. Other manufacturing steps, such as soft baking may be further applied to the first photoresist layer  120   a . The first photoresist layer  120   a  may include a positive photoresist material that may become dissolvable to the developer solution after exposing to the radiation source. In some alternative embodiments, the photoresist layer  120   a  may include a negative photoresist material that becomes indissolvable to the developer solution after exposing to the radiation source. The first photoresist layer  120   a  has a predetermined exposure threshold T1. The first photoresist layer  120   a  may have a thickness H res  in a range from about 200 Å to about 800 Å. 
     At operation  1006 , the method  1000  ( FIG. 19A ) performs a first lithography exposure process  900   a  to from latent patterns in the photoresist layer  120   a , as shown in  FIG. 22 . The lithography exposure process  900   a  uses a photomask  200  having a first IC design pattern  204   a  and a second IC design pattern  204   b  (collectively as IC design patterns  204 ) to expose the photoresist layer  120   a  in a radiation  920   a , thereby forming latent patterns in the photoresist layer  120 . The IC design patterns  204   a  and  204   b  are used for forming interconnect features (e.g., vias, contacts, or plugs) in the dielectric layer  106 . In the illustrated embodiment, the IC design pattern  204   a  aligns with the underneath conductive feature  104   a , while overlay errors occur in the IC design pattern  204   b  such that an edge of the IC design pattern  204   b  is offset from an edge of the underneath conductive feature  104   b  for a distance of Δx. The overlay errors may be due to misalignment between the photomask  200  and the semiconductor structure  30 , inaccuracy in the geometry of the IC design pattern  204   b  during fabrication of the photomask  200 , calibration inaccuracy occurred in the optical apparatus used in the lithography exposure process  900   a , or other reasons. 
     The exposing source used in the lithography exposure to generate radiation  920   a  may include any suitable source such as UV, DUV, EUV, or charged particles, such as E-beam. In some alternative embodiments, the IC design pattern is defined in a data file and is transferred to the photoresist layers by direct writing or other suitable technique, such as digital pattern generator. Other steps may be implemented before, during, or after the exposure process. In some embodiments, a post exposure baking process may be applied to the photoresist layer  120   a  after the lithography exposure process. 
     The radiation  920   a  is configured such that the incident exposure dose directly from the radiation  920  to the photoresist layer  120  is larger than the exposure threshold T1 of the photoresist layer  120   a . Accordingly, portions of the photoresist layer  120  directly under the IC design patterns  204   a  and  204   b  receive an incident exposure dose larger than the exposure threshold T1, which causes chemical changes in forming latent patterns. The latent patterns are formed in the regions  904   a  and  904   b , as well as in the adjacent region  904   c  that is offset from the underneath conductive feature  104   b.    
     At operation  1008 , the method  1000  ( FIG. 19A ) develops the photoresist layer  120   a  to form a patterned photoresist layer  122   a , as shown in  FIG. 23 . In the illustrated embodiment, the photoresist layer  120   a  is positive, so the portions of the photoresist layer  120   a  associated with the latent patterns in regions  904   a ,  904   b , and  904   c  are removed by the corresponding developer to form openings. Since the photoresist layer  120   a  in the region  904   c  receives incident exposure dose higher than the exposure threshold T1, it is also developed even though it is offset from the conductive feature  104   b.    
     At operation  1010 , the method  1000  ( FIG. 19A ) partially recesses the dielectric layer  106  in an etching process, as shown in  FIG. 24 . The etching process uses the patterned photoresist layer  122   a  as an etch mask. Operation  1010  may include one or more etching processes to remove different portions of the dielectric layer  106 , such as a hard mask layer, a low-k dielectric layer, and an etch stop layer (as shown in  FIG. 1 ) to form via trenches and extend the via trench in the region  904   a  and the via trench in the regions  904   b  and  904   c  downwardly towards the conductive features  104   a  and  104   b , respectively. In some embodiments, suitable etching process, such as a plasma dry etching using CH x F y , CF x , Cl 2  or BCl 3 -based chemistries, is used. After operation  1010 , a distance between bottom surfaces  952  of the via trenches and the top surfaces of the conductive features  104 , denoted as H″, is less than the threshold distance H th  (H″&lt;H th ). In one embodiment, the etching process uses time mode to control the etching depth (H′−H″), such that the low-k dielectric layer (not shown) of the dielectric layer  106  is partially etched but not through, and the bottom surfaces  952  of the via trenches stop inside the low-k dielectric layer. In another embodiment, the etching process etches through the low-k dielectric layer and stops at the etch stop layer (not shown) of the dielectric layer  106 , such that the distance H″ is defined by the thickness of the etch stop layer. Other operations may be subsequently implemented. For example, the patterned photoresist layer  122   a  may be removed by wet stripping or plasma ashing process, as shown in  FIG. 25 . The plasma ashing process may include using oxygen (O 2 ) plasma or carbon dioxide (CO 2 ) plasma. 
     At operation  1012 , the method  1000  ( FIG. 19A ) forms a second photoresist layer  120   b  over the dielectric layer  106  and fills the via trenches in the regions  904   a - c , as shown in  FIG. 26 . Forming of the second photoresist layer  120   b  may be substantially similar to that of the first photoresist layer  120   a  as discussed with respect to operation  1004 . In some embodiments, forming of the second photoresist layer  120   b  includes coating the photoresist solution on the dielectric layer  106  by a suitable technique, such as spin-on coating. Other manufacturing steps, such as soft baking may be further applied to the second photoresist layer  120   b . In some embodiments, the second photoresist layer  120   b  includes the same composition as the first photoresist layer  120   a . In some embodiments, the second photoresist layer  120   b  includes different composition from the first photoresist layer  120   a . The composition of the second photoresist layer  120   b  may be adjusted, for example, by changing the ratio of carbon, hydrogen and oxygen, to have a different exposure threshold for the second lithography exposure process as will be discussed later in the present disclosure. The composition of the second photoresist layer  120   b  may also be adjusted to be sensitive to radiation wavelengths different from the first photoresist layer  120   a . The second photoresist layer  120   b  has a predetermined exposure threshold T2. The second photoresist layer  120   a  may have a thickness H′ res  in a range from about 200 Å to about 800 Å. 
     At operation  1014 , the method  1000  ( FIG. 19B ) performs a second lithography exposure process  900   b  to form latent patterns in the photoresist layer  120   b , as shown in  FIG. 27 . In some embodiments, the second lithography exposure process  900   b  uses the same mask as the first lithography exposure  900   a , such as the mask  200  as discussed with respect to operation  1006 . In the illustrated embodiment, the second lithography exposure process  900   b  is a blanket lithography exposure process. In other words, the second lithography exposure process  900   b  does not use a mask, which exposes the whole semiconductor structure  30 . A blanket lithography exposure process contributes to the reduction of overall cost of manufacturing, such as reducing the cost of masks and the processing time. The exposing source used in the second lithography exposure to generate radiation  920   b  may include any suitable source such as UV, DUV, EUV, or charged particles, such as E-beam. In some alternative embodiments, the IC design pattern is defined in a data file and is transferred to the photoresist layers by direct writing or other suitable technique, such as digital pattern generator. The first and second lithography exposure processes  900   a  and  900   b  may use the same radiation source. Alternatively, the first and second lithography exposure processes  900   a  and  900   b  may use different radiation sources, such as under different radiation wavelengths. In one example, the first lithography exposure process  900   a  uses a 248 nm DUV radiation, and the second lithography exposure process  900   b  uses a 193 nm DUV radiation. In another example, the first lithography exposure process  900   a  uses a DUV radiation, and the second lithography exposure process  900   b  uses an EUV radiation. Other steps may be implemented before, during, or after the exposure process. In some embodiments, a post exposure baking process may be applied to the photoresist layer  120   b  after the second lithography exposure process  900   b.    
     In the illustrated embodiment, the radiation  920   b  is configured such that the incident exposure dose directly from the radiation  920   b  to the photoresist layer  120   b  is less than the exposure threshold T2 of the photoresist layer  120   b , thereby no latent pattern will be formed in the photoresist layer  120   b  by absorbing the incident exposure dose alone. A portion of the radiation  920   b  reaches the top surface of the conductive features  104   a  and  104   b  and is reflected as reflected radiation  922  back to the photoresist layer  120   b . The reflected exposure dose is mainly controlled by the reflectivity R of the top surface of the conductive feature  104  and the strength of the incident exposure dose. In some embodiments, the thickness H″ of the dielectric layer  106  under the via trenches is adjusted to control the reflected exposure dose. A sum of the incident exposure dose and the reflected exposure dose is configured to be larger than or at least equivalent to the exposure threshold T2 of the photoresist layer  120   b . Therefore, latent patterns will be formed in portions of the photoresist layer  120   b  that receive both the incident exposure dose and the reflected exposure dose. The reflected radiation happens at portions of the top surface of the conductive features  104  where incident radiation  920   b  reaches. At other portions of the top surface of the conductive features  104  where incident radiation  920   b  does not reach or offset from the top surface of the conductive features  104 , the strength of the reflected exposure dose decreases sharply. By partially recessing the dielectric layer  106  as discussed with respect to operation  1010  to reduce a distance between the photoresist layer  120   b  and the conductive features  104  to be below the threshold distance H th  (H″&lt;H th ), the exposure contrast is increased. In other words, only portions of the photoresist layer  120   b , such as regions  904   a  and  904   b  that are directly above the conductive features  104 , receive the sum of the incident exposure dose and the reflected exposure dose, which causes chemical changes to form latent patterns. Regarding the region  904   c  adjacent to the region  904   b , which is offset from an edge of the conductive feature  104   b , it receives substantially only the incident radiation  920   b  but no reflected radiation  922 , which is not strong enough to expose the region  904   c . Therefore, the latent pattern formed in the region  904   b  does not extend into the region  904   c . Accordingly, the latent pattern formed in the region  904   b  is self-aligned with the underneath conducive feature  104   b.    
     At operation  1016 , the method  1000  ( FIG. 19B ) develops the photoresist layer  120   b  to form a patterned photoresist layer  122   b , as shown in  FIG. 28 . In the illustrated embodiment, the photoresist layer  120  is positive, so the portions of the photoresist layer  122   b  associated with the latent patterns in the regions  904   a  and  904   b  are removed by the corresponding developer to expose via trenches in the regions  904   a  and  904   b . The photoresist layer  120  in the region  904   c , which is offset from the conductive feature  104   b , does not receive exposure doses higher than the exposure threshold and remains undeveloped. In other words, the via trench above the conductive feature  104   b  is partially filled with the photoresist. 
     At operation  1018 , the method  1000  ( FIG. 19B ) etches the dielectric layer  106  by using the patterned photoresist layer  122   b  as an etch mask, thereby transferring a pattern in the patterned photoresist layer  122   b  to the dielectric layer  106 , as shown in  FIG. 29 . Operation  1018  may include one or more etching processes to remove different portions of the dielectric layer  106 , such as a low-k dielectric layer and an etch stop layer to extend the via trenches in the regions  904   a  and  904   b  downwardly to the top surface of the conductive features  104   a  and  104   b . The top surface of the conductive features  104   a  and  104   b  are exposed in the respective via trenches. The etching process may include any suitable etching technique, such as dry etching, wet etching, or a combination thereof. Other operations may be subsequently implemented. For example, the patterned photoresist layer  122   b  may be removed by wet stripping or plasma ashing process, as shown in  FIG. 30 . The plasma ashing process may include using oxygen ( 02 ) plasma or carbon dioxide (CO 2 ) plasma. 
     In some embodiments, the conductive features  104  are formed of continuous metallic material or the reflective layer  114   b  is conductive, the method  1000  ( FIG. 19B ) may optionally proceed from operation  1018  to operation  1022  by depositing a metal layer  130  to fill the via trenches in the regions  904   a - c  and covers the dielectric layer  106 , as shown in  FIG. 31 . The metal layer  130  is in direct contact with the top surfaces of the conductive features  104   a  and  104   b . In some embodiments, the metal layer  130  includes Cu, Al, W or other suitable conductive material. In some embodiments, the metal layer  130  includes Cu alloy, such as copper magnesium (CuMn), copper aluminum (CuAl) or copper silicon (CuSi). In some embodiments, the metal layer  130  is deposited by PVD. In some examples, the metal layer  130  is formed by depositing a corresponding metal seed layer using PVD, and then forming a bulk metal layer by plating. 
     At operation  1024 , the method  1000  ( FIG. 19B ) performs a chemical mechanical polishing (CMP) process to remove the excessive metal layer  130 , thereby forming interconnect features  140   a  and  140   b , such as vias  140   a  and  140   b , as shown in  FIG. 32 . The CMP process may also remove a top portion of the dielectric layer  106 , such as a hard mask layer. A substantially coplanar top surface of the interconnect features  140   a  and  140   b  and the dielectric layer  106  may be formed after the CMP process. Interconnect feature  140   a  has two opposing sidewalls S1 and S2 landing on the top surfaces of the conductive features  104   a . Both sidewalls S1 and S2 of the interconnect feature  140   a  have a straight profile. Interconnect feature  140   b  has two opposing sidewalls S1 and S2 landing on the top surfaces of the conductive features  104   b . Sidewall S2 of the interconnect feature  140   b  has a straight profile. Sidewall S1 of the interconnect feature  104   b  has a step profile. The step profile includes two vertical sidewall portions S12 and S11 and a horizontal sidewall portion joining the vertical sidewall portions S12 and S11. The upper sidewall portion S12 is offset from an edge of the conductive feature  104   b  for a distance of Δx. Due to the self-aligned lithography exposure process, the lower sidewall portion S11 substantially aligns with the edge of the conductive feature  104   b.    
     In some embodiments, the reflective layers  114   b  of the conductive features  104  are formed of high-resist material, non-conductive material, or reflective multilayers, and the method  1000  ( FIG. 19B ) may optionally proceed from operation  1018  to operation  1020  by selectively etching the reflective layers  114   b  to expose the underneath high-conductive bulk metal layer  114   a  in the trenches, as shown in  FIG. 33 . Operation  1018  includes a selective etching process to remove portions of the reflective layers  114   b  exposed in the trenches in the regions  904   a  and  904   b . The etch process and the etchant are properly chosen for selective etch without damage to the dielectric layer  106 . The etching process may include any suitable etching technique, such as dry etching, wet etching, or a combination thereof. The etching process stops at the bulk metal layer  114   a . In the illustrated embodiment, after operation  1020 , portions of the reflective layer  114   b  remain on both ends of the conductive feature  104   a , while portions of the reflective layer  114   b  remain on only one end of the conductive feature  104   b.    
     Referring to  FIGS. 34 and 35 , the method  1000  ( FIG. 19B ) then proceeds to operations  1022  and  1024  to deposit a metal layer  130  to fill the trenches in regions  904   a - c  and perform a CMP process to remove the excessive metal layer  130 , thereby forming interconnect features  140   a  and  140   b . Interconnect feature  140   a  has two opposing sidewalls S1 and S2 landing on the top surfaces of the conductive features  104   a . Both sidewalls S1 and S2 of the interconnect feature  140   a  have a straight profile. Interconnect feature  140   b  has two opposing sidewalls S1 and S2 landing on the top surfaces of the conductive features  104   b . Sidewall S2 of the interconnect feature  140   b  has a straight profile. Sidewall S1 of the interconnect feature  140   b  has a step profile. The step profile includes two vertical sidewall portions S12 and S11 and a horizontal sidewall portion joining the vertical sidewall portions S12 and S11. The upper sidewall portion S12 is offset from an edge of the conductive feature  104   b  for a distance of Δx. Due to the self-aligned lithography exposure process, the lower sidewall portion S11 substantially aligns with the edge of the conductive feature  104   b . Bottom portions of the sidewalls S1 and S2 of the interconnect features  140   a  are covered by the reflective layer  114   b . As shown in a region  180  surrounding the interface between the interconnect features  140   b  and the conductive feature  104   b , bottom portion of the sidewall S2 of the interconnect feature  140   b  is covered by the reflective layer  114   b . Bottom portion of the sidewall S1 of the interconnect feature  140   b  is covered by a top portion of the substrate  102 . Upper portions of the sidewalls S1 and S2 of the interconnect features  140   a  and  140   b  are covered by the dielectric layer  106 . 
     Various embodiments of the interface between the interconnect feature  140   b  and the conductive feature  104   b  in the region  180  are shown in  FIGS. 36A-D . As discussed above, the interconnect feature  140   b  has two opposing sidewalls S1 and S2, where sidewall S1 has an upper sidewall portion S12 offset from an edge of the conductive feature  104   b  and a lower sidewall portion S11. The upper sidewall portion S12 is omitted in  FIGS. 36A-D . In  FIG. 36A , both sidewalls S2 and S11 are substantially perpendicular to the top surface of the conductive feature  104   b . The sidewall S2 lands in the middle of the top surface of the conductive feature  104   b , while the sidewall S11 lands at point P, which is substantially at the edge of the conducive feature  104   b  (i.e., within ±4 nm lateral distance of the edge of the conductive feature  104   b , which is less than 20% of a via feature CD size of 20 nm). Bottom portion of the sidewall S2 is covered by the reflective layer  114   b , which may be a single reflective layer or a reflective multilayer in various embodiments. Bottom portion of the sidewall S11 is covered by a top portion of the substrate  102 . Upper portions of the sidewalls S2 and S11 are covered by the dielectric layer  106 . In  FIG. 36B , sidewalls S2 and S11 are slanted with respect to the top surface of the conductive feature  104   b , which may be due to an etching process in forming the via trench. The sidewall S11 lands at point P, which is at the edge of the conducive feature  104   b . In  FIG. 36C , the slanted sidewall S11 may land at a point P that is slightly offset from the edge of the conductive feature  104   b . The offset may range from about 1 nm to about 5 nm (depending on via feature CD sizes) in some embodiments, which leaves abundant margin to avoid the interconnect feature  140   b  shorting to adjacent conductive features. In  FIG. 36D , the slanted sidewall S11 may land at a point P that is on the top surface of the conductive feature  104   b , which is slightly offset in an opposite direction compared with the point P in  FIG. 36C . The offset may range from about 1 nm to about 5 nm in some embodiments. As shown in  FIG. 36D , the slanted sidewall S11 may chop a top portion of the reflective layer  114   b , such that the bottom surface of the sidewall S11 may be cover by a bottom portion of the reflective layer  114   b  and a top portion of the semiconductor substrate  102 . The bottom portion of the reflective layer  114   b  on the sidewall S11 is lower than the reflective layer  114   b  on the opposing sidewall S2. 
     Although not intended to be limiting, one or more embodiments of the present disclosure provide many benefits to a semiconductor device and the formation thereof. For example, embodiments of the present disclosure provide self-aligned interconnect structures that allow for reducing or avoiding effects caused by overlay shift during lithography processes. The present disclosure provides lithography methods that rely on the reflected radiation from underneath conductive features for the right amount of exposure doses in forming latent patterns in a resist layer. The latent patterns are confined in a region directly above the underneath conductive features. The self-aligned methods provide a significant contributor to the overall manufacturing cost reduction, including processing time and the cost of masks used in the lithography process. Further, the various embodiments discussed herein are not limited to forming interconnects in a semiconductor structure, but may be also used to form other structures having alignment and overlay shift issues. 
     In one exemplary aspect, the present disclosure is directed to a method for lithography patterning. The method includes providing a semiconductor structure including a substrate and a conductive feature formed in a top portion of the substrate; depositing a resist layer over the substrate, wherein the resist layer has an exposure threshold; providing a radiation with an incident exposure dose to the resist layer, wherein the incident exposure dose is configured to be less than the exposure threshold of the resist layer while a sum of the incident exposure dose and a reflected exposure dose from a top surface of the conductive feature is larger than the exposure threshold of the resist layer, thereby forming a latent pattern above the conductive feature; and developing the resist layer to form a patterned resist layer. In some embodiments, the latent pattern is directly above the conductive feature. In some embodiments, the conductive feature includes a reflective layer coated on a bulk metal. In some embodiments, the reflective layer includes a first metal that is different from the bulk metal. In some embodiments, the reflective layer includes a metallic alloy. In some embodiments, the reflective layer includes a plurality of alternating repeating layers. In some embodiments, the method further includes prior to the depositing of the resist layer, forming a dielectric layer over the substrate; after the developing of the resist layer, etching the dielectric layer using the patterned resist layer as an etch mask, thereby forming an opening exposing the top surface of the conductive feature; and depositing a conductive material in the opening, thereby forming a conductive structure landing on the conductive feature. In some embodiments, the conductive feature includes a reflective layer coated on a bulk metal, and the method further includes partially etching the reflective layer to expose the bulk metal such that the conductive structure lands on the bulk metal. In some embodiments, the method further includes prior to the depositing of the resist layer, partially recessing a portion of the resist layer to form a trench above the conductive feature, wherein the resist layer fills the trench. In some embodiments, the radiation is one of a deep ultraviolet (DUV) radiation, an extreme ultraviolet (EUV) radiation, and an electron-beam (E-beam) radiation. 
     In another exemplary aspect, the present disclosure is directed to a method for lithography patterning. The method includes forming a first conductive feature in a top portion of a substrate; forming a dielectric layer over the substrate; partially recessing the dielectric layer to form a trench above the first conductive feature; coating a resist layer over the dielectric layer, the resist layer filling the trench; exposing the resist layer in a radiation, wherein an incident exposure dose of the radiation is configured such that a latent pattern is formed in the trench; developing the resist layer to form an opening in the resist layer; etching the dielectric layer through the opening in the resist layer, thereby extending a portion of the trench through the dielectric layer; and forming a second conductive feature in the trench and in contact with the first conductive feature. In some embodiments, a top portion of the first conductive feature includes a reflective layer. In some embodiments, the reflective layer includes a plurality of alternating first material layers and second material layers. In some embodiments, the method further includes partially etching the reflective layer to expose a bottom portion of the first conducive feature. In some embodiments, the incident exposure dose of the radiation is configured to be less than an exposure threshold of the resist layer while a sum of the incident exposure dose and a reflected exposure dose from a top surface of the first conductive feature is larger than the exposure threshold of the resist layer. In some embodiments, the radiation is a blanket radiation without using a mask. In some embodiments, the radiation is an extreme ultraviolet (EUV) radiation. 
     In another exemplary aspect, the present disclosure is directed to a semiconductor structure. The semiconductor structure includes a substrate; a first conductive feature embedded in a top portion of the substrate; a dielectric layer over the substrate; and a second conductive feature surrounded by the dielectric layer and in contact with the first conductive feature, the second conductive feature having a first sidewall and a second sidewall opposing the first sidewall, wherein the first sidewall has a straight profile and is above the first conductive feature, and wherein the second sidewall has a step profile and a top portion of the step profile is horizontally offset from an edge of the first conductive feature. In some embodiments, the first conductive feature includes a reflective layer, and wherein a bottom portion of the first sidewall is covered by the reflective layer. In some embodiments, a bottom portion of the step profile substantially aligns with the edge of the first conductive feature. 
     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.