Abstract:
A wavy line interconnect structure that accommodates small metal lines and large vias is disclosed. A lithography mask design used to pattern metal line trenches uses optical proximity correction (OPC) techniques to approximate wavy lines using rectangular opaque features. The large vias can be formed using a self-aligned dual damascene process without the need for a separate via lithography mask. Instead, a sacrificial layer allows etching of an underlying thick dielectric block, while protecting narrow features of the trenches that correspond to the metal line interconnects. The resulting vias have an aspect ratio that is relatively easy to fill, while the larger via footprint provides low via resistance. By lifting the shrink constraint for vias, thereby allowing the via footprint to exceed the minimum size of the metal line width, a path is cleared for further process generations to continue shrinking metal lines to dimensions below 10 nm.

Description:
BACKGROUND 
     Technical Field 
     The present disclosure generally relates to high speed integrated circuits, and in particular, to the fabrication of interconnect structures using a self-aligned dual damascene process. 
     Description of the Related Art 
     As integrated circuit technology advances, building metal interconnect structures that are used to wire transistors together becomes more and more challenging. Metal lines can assume a variety of different shapes, from straight wires to cells made up of intertwined C-shapes. Regardless of their shapes, design rules for metal lines are typically based on scaling a pitch dimension that assumes a regular pattern of equal line widths and spacings between the metal lines at each metal layer. Metal line widths are generally expected to shrink with every new process generation to further improve integrated circuit performance. 
     Depending on the type of process integration scheme used, vias connecting stacked metal lines vertically to one another are constrained to be smaller than the metal lines in order for the via footprint to be surrounded by metal. Such a constraint exists, for example, when forming vias according to a self-aligned dual damascene process that avoids a separate via lithography step. Thus, as the metal line design widths shrink with each process generation, the self-aligned vias shrink as well. However, smaller vias incur higher via resistance, causing RC delays to worsen. In addition, smaller via footprints cause the via aspect ratio, i.e., the ratio of height to width, to increase, making the vias taller and narrower, and therefore more difficult to fill with metal. Incomplete via fill in turn degrades reliability by causing open circuit failures, for example. Thus, for multiple reasons, it is advantageous for via footprints to remain large, while metal lines continue to shrink with each technology generation. 
     BRIEF SUMMARY 
     An interconnect structure having wavy metal lines allows via footprints to exceed the nominal width of metal line portions connecting them. In one embodiment, the resulting metal line profile shape resembles a bicycle chain. In another embodiment, the metal line resembles a linear series of T-shaped unit cells, in which the width of the metal line alternates between a narrow value, an intermediate value, and a wide value. The wide value corresponds to a large via landing pad for vias forming an electrical connection to the next highest metal layer. To achieve greater lateral packing density, adjacent wavy metal lines can be staggered so that the via landing pads on one metal line are next to the narrow metal connecting portions of neighboring metal lines. By lifting the shrink constraint for vias, thereby allowing the via footprint to exceed the minimum size of the metal line width, a path is cleared for further process generations to continue shrinking metal lines to dimensions below 10 nm. 
     A wavy line interconnect structure can be formed using a dual damascene process in which vias are self-aligned without the need for a dedicated lithography mask. Instead, metal line trenches are lined with a sacrificial layer that completely fills the narrow spaces while under-filling the wide spaces that correspond to via locations. The sacrificial layer thus acts as a via hard mask, allowing etching of the underlying thick dielectric block, while protecting narrow features of the trenches that correspond to the metal line interconnects. The resulting vias have an aspect ratio that is relatively easy to fill, while the larger footprint provides low via resistance. 
     One way to pattern wavy metal lines using conventional optical lithography uses a mask cell design that includes a series of rectangular opaque features diagonally offset from one another along an axis. Via landing pads are then added at one end of the mask cell by placing two rectangular features adjacent to one another. The method disclosed takes advantage of the shortcomings of optical lithography to achieve the desired wavy line pattern by anticipating diffraction errors that will occur during exposure of the rectangular features. Such a technique is generally known in the art as optical proximity correction (OPC). Typically, OPC is used to produce square corners where the standard lithography tends to round off the corners of a circuit pattern. In this case, however, the rounding effect is intentional. During the patterning process, corners of the rectangular features will transfer to the photoresist and the metal trench as rounded features. Meanwhile, the offset rectangular features, instead of remaining distinct, will join together to approximate the desired undulating wavy line shape. In similar fashion, each rectangle pair will transfer to the metal trench layer as a combined shape, forming a single large landing pad to accommodate the vias. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       In the drawings, identical reference numbers identify similar elements. The sizes and relative positions of elements in the drawings are not necessarily drawn to scale. 
         FIG. 1A  is derived from a top-down scanning electron micrograph of a conventional array of metal lines, according to the prior art. 
         FIG. 1B  is a layout diagram of adjacent narrow metal lines encompassing large vias according to a first embodiment in which the metal lines resemble a bicycle chain. 
         FIG. 2A  is a layout diagram of an optical lithography mask design showing regions of chrome on a glass mask plate that can be used to form wavy metal lines according to a second embodiment. 
         FIG. 2B  is derived from a top-down scanning electron micrograph of the structure as made when the mask has an array of rectangular-shaped vias that create wavy metal lines having narrow, intermediate, and wide regions, the wide regions accommodating large vias. 
         FIG. 2C  is a schematic of a template corresponding to the unit cell that describing the via shape arrangement shown in  FIG. 2B . 
         FIG. 3  is a high-level flow diagram of a method of fabricating large self-aligned vias in a dual damascene interconnect process, according to one embodiment. 
         FIG. 4A  is a process flow diagram showing a detailed sequence of processing steps that can be used to form wavy trenches using a wavy line mask, according to one embodiment. 
         FIG. 4B  is a top plan view of wavy trenches after carrying out processing steps shown in  FIG. 4A . 
         FIGS. 4C and 4D  are cross-sectional views, along the cut lines shown, of wavy trenches after carrying out processing steps shown in  FIG. 4A . 
         FIG. 5A  is a process flow diagram showing a detailed sequence of processing steps that can be used to form a sacrificial layer for use as a hard mask, according to one embodiment. 
         FIG. 5B  is a top plan view of wavy trenches after carrying out processing steps shown in  FIG. 5A . 
         FIGS. 5C and 5D  are cross-sectional views, along the cut lines shown, of the hard mask after carrying out processing steps shown in  FIG. 5A . 
         FIG. 6A  is a process flow diagram showing a detailed sequence of processing steps that can be used to form large self-aligned vias, according to one embodiment. 
         FIG. 6B  is a top plan view of wavy trenches after carrying out processing steps shown in  FIG. 6A . 
         FIGS. 6C and 6D  are cross-sectional views, along the cut lines shown, of the large self-aligned vias after carrying out processing steps shown in  FIG. 6A . 
         FIG. 7A  is a process flow diagram showing a detailed sequence of processing steps that can be used to fill the wavy line trenches and the large vias with metal, according to one embodiment. 
         FIG. 7B  is a top plan view of wavy trenches after carrying out processing steps shown in  FIG. 7A . 
         FIGS. 7C and 7D  are cross-sectional views, along the cut lines shown, of wavy metal lines and filled vias after carrying out processing steps shown in  FIG. 7A . 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, certain specific details are set forth in order to provide a thorough understanding of various aspects of the disclosed subject matter. However, the disclosed subject matter may be practiced without these specific details. In some instances, well-known structures and methods of semiconductor processing comprising embodiments of the subject matter disclosed herein have not been described in detail to avoid obscuring the descriptions of other aspects of the present disclosure. 
     Unless the context requires otherwise, throughout the specification and claims that follow, the word “comprise” and variations thereof, such as “comprises” and “comprising” are to be construed in an open, inclusive sense, that is, as “including, but not limited to.” 
     Reference throughout the specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, the appearance of the phrases “in one embodiment” or “in an embodiment” in various places throughout the specification are not necessarily all referring to the same aspect. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more aspects of the present disclosure. 
     Reference throughout the specification to integrated circuits is generally intended to include integrated circuit components built on semiconducting substrates, whether or not the components are coupled together into a circuit or able to be interconnected. Throughout the specification, the term “layer” is used in its broadest sense to include a thin film, a cap, or the like and one layer may be composed of multiple sub-layers. 
     Reference throughout the specification to conventional thin film deposition techniques for depositing silicon nitride, silicon dioxide, metals, or similar materials include such processes as chemical vapor deposition (CVD), low-pressure chemical vapor deposition (LPCVD), metal organic chemical vapor deposition (MOCVD), plasma-enhanced chemical vapor deposition (PECVD), plasma vapor deposition (PVD), atomic layer deposition (ALD), molecular beam epitaxy (MBE), electroplating, electro-less plating, and the like. Specific embodiments are described herein with reference to examples of such processes. However, the present disclosure and the reference to certain deposition techniques should not be limited to those described. For example, in some circumstances, a description that references CVD may alternatively be done using PVD, or a description that specifies electroplating may alternatively be accomplished using electro-less plating. Furthermore, reference to conventional techniques of thin film formation may include growing a film in situ. For example, in some embodiments, controlled growth of an oxide to a desired thickness can be achieved by exposing a silicon surface to oxygen gas or to moisture in a heated chamber. 
     Reference throughout the specification to conventional photolithography techniques, known in the art of semiconductor fabrication for patterning various thin films, includes a spin-expose-develop process sequence typically followed by an etch process. Alternatively or additionally, photoresist can be used to pattern a hard mask (e.g., a silicon nitride hard mask), which, in turn, can be used to pattern an underlying film. 
     Reference throughout the specification to conventional etching techniques known in the art of semiconductor fabrication for selective removal of polysilicon, silicon nitride, silicon dioxide, metals, photoresist, polyimide, or similar materials includes such processes as wet chemical etching, reactive ion (plasma) etching (RIE), washing, wet cleaning, pre-cleaning, spray cleaning, chemical-mechanical planarization (CMP) and the like. Specific embodiments are described herein with reference to examples of such processes. However, the present disclosure and the reference to certain deposition techniques should not be limited to those described. In some instances, two such techniques may be interchangeable. For example, stripping photoresist may entail immersing a sample in a wet chemical bath or, alternatively, spraying wet chemicals directly onto the sample. 
     Specific embodiments are described herein with reference to interconnect structures that have been produced; however, the present disclosure and the reference to certain materials, dimensions, and the details and ordering of processing steps are exemplary and should not be limited to those shown. 
     Turning now to the Figures,  FIG. 1A  shows a top plan view of a conventional array  90  of metal lines  92  inlaid in a dielectric block  94 . Such an array characterized by a pitch  96  of 40 nm or less typically is fabricated of copper or a copper alloy using a dual damascene process, as is well known in the art [see, for example, “Silicon Processing for the VLSI Era,” Vol. 4, p. 674-679, by Stanley Wolf]. In short, a dual damascene process entails first forming a trench in the dielectric block  94 , and then forming a via that extends from the bottom of the trench to an underlying metal layer. The trench and the via are then filled with metal together in one step. Thus, each interconnect layer entails performing two lithography steps and one metal deposition step. One of the two lithography steps can be eliminated by employing a self-aligned via process in which a sacrificial layer is used as a hard mask. Self-aligned dual damascene vias are also well known in the art and can be understood by referencing the Wolf article cited above. Vias underlying the metal lines  92  are not shown in  FIG. 1A  because they have equal or smaller widths than the metal lines and they are therefore obscured by the metal lines. 
     With reference to  FIG. 1B , for the reasons explained above, it is desirable to form wavy metal lines  102  shaped like bicycle chain structures  100  in which wide portions  104  accommodate via landing pads  106  spaced periodically between narrow portions  105 . Such a configuration allows the via landing pads  106  to exceed the width of the narrow portions  105 . Adjacent metal lines  102   a  and  102   b  can be staggered relative to one another so that the via landing pads located at the wide portions  104  along  102   a  are aligned with the narrow portions  105  of neighboring wavy metal line  102   b . Thus, the wavy metal lines can maintain a constant spacing  108 . The bicycle chain structures  100  are thus one exemplary embodiment of a wavy metal line design that accommodates large vias. 
     A second exemplary embodiment of such a wavy metal line design is shown in  FIGS. 2A, 2B, and 2C .  FIG. 2A  shows an optical lithography wavy line mask design  110  that results in an array of wavy metal lines structures  120  shown in  FIG. 2B . The wavy metal line structures  120  include a plurality of narrow portions  122 , intermediate width portions  123 , and wide portions  124 , along each wavy metal line. In one embodiment, the narrow portions  122  are in the range of about 5-30 nm wide, with a nominal pitch of 48 nm. Dimensions of the wide portions  124  are within the range of about 10-40 nm. Wavy metal lines having about three or more narrow portions  122  for every wide portion  124  maintain an average line width that is slightly larger than the width of the narrow portion  122 . Again, adjacent metal lines can be staggered so the wide portions  124  of one line are placed next to the intermediate or narrow portions  123 ,  122  of neighboring lines. 
     The wavy metal line structures  120  are achieved by approximating the portions of varying widths using rectangular mask features in the wavy line mask design  110 . For example, the wavy line mask design  110  can be implemented by forming rectangular opaque features  112  and  114  made of chrome on a glass plate  113 . When the mask is exposed to light in an optical lithography stepper or scanner, the light is blocked by the rectangular opaque features  112  and  114 . However, diffraction effects cause the pattern of rectangles to become blurred at the target surface, thereby producing the wavy-line patterns shown in  FIG. 2B . 
     The optical lithography mask design  110  ( FIG. 2A ) can be thought of as an optical lithography mask cell  130  ( FIG. 2C ) replicated throughout a region of a larger optical lithography mask that may include many different mask designs. The optical lithography mask cell  130 , shown in  FIG. 2C , is then a fundamental building block of the mask design  110  shown in  FIG. 2A . The optical lithography mask cell  130  shows more clearly the relative orientations of the rectangular opaque features  112  and  114 . The rectangular opaque features  112  are isolated rectangles, offset from one another by a first offset distance  116 . Pairs of rectangular opaque features  114  touch one another at least at a contact point  118 , and the pairs are then offset from the rectangular opaque features  112  by a second offset distance  117 . Furthermore, the contact point  118  and center points  131  of the rectangular opaque features  112  are aligned along an axis  132 . When a template  134  of the mask cell  130  is overlaid onto the wavy metal line structures  120 , the various elements of the metal line can more clearly be associated with corresponding mask features that define them. It is noted that the via landing pads that correspond to the wide portions  124  are formed by the pairs of rectangular opaque features  114 , while the narrow portions  122  of the metal lines correspond to locations where adjacent rectangular features blur together. 
     Intentional manipulation of mask features to compensate for shortcomings in lithographic capability is well known in the art of photolithography as Optical Proximity Correction (OPC). OPC is often used to pattern square corners that would otherwise become rounded by diffraction effects. In the present case, however, the rounded features forming the wavy metal line structures  120  are intentionally created by leveraging distortion that occurs in the lithography process, as opposed to correcting for the distortion. While it is true that conventional metal lines are not all as uniform and straight as those shown in  FIG. 1A , the resulting wavy metal line structures  120  are unique compared with known existing metal line shapes as shown in documentation describing SRAM cells associated with various microprocessor chips, e.g., those of Texas Instruments OMAP™, TMX320x, D6298x, and the like. 
       FIG. 3  shows general steps in a method  150  of fabricating wavy metal lines having large vias. The method  150  is one embodiment of a self-aligned dual damascene process for fabricating an interconnect structure, i.e., a network of wavy metal lines and large vias, that provides electrical connections for transistors within integrated circuits. Metal lines within the interconnect structure have the wavy line pattern of the wavy metal line structures  120  shown in  FIG. 2B . 
     At  152 , wavy trenches are formed in a dielectric block. 
     At  156 , self-aligned vias are formed in the dielectric block, extending away from the wavy trenches. 
     At  158 , the wavy trenches and the vias are filled with metal in the same process step. 
       FIGS. 4A-7D  show and describe in further detail steps in the method  150 . In each set of Figures A-D, A is a detailed flow diagram; B is a top plan view showing at least a wide portion  104  and a narrow portion  105  of a wavy metal line  102  after performing the steps shown in A; C is a cross-sectional schematic view through the wide portion  104  that accommodates a via; and D is a cross-sectional view through the narrow portion  105 . In accordance with convention, arrows on each cut line represent the direction of an observer&#39;s eye looking at the corresponding cut plane. 
       FIGS. 4A-4D  show and describe patterning the wavy line trenches in a dielectric block, at step  152 , according to one embodiment. Formation of the wavy line trenches is the only process step in the method  150  that entails use of photolithography. The trench formation begins with a dielectric block  104  that has been deposited onto an underlying metal layer, e.g., metal 1. The dielectric block  157  is made of silicon dioxide or an ultra-low-k (ULK) dielectric material, as is well known in the art. The underlying metal layer includes a metal liner  151  made of, for example, Ti, TiN, or TaN; a layer of bulk metal  153 , e.g., copper; and a metal cap layer  155 , e.g., silicon nitride carbide (SiNC), as are well known in the art. It is noted that the trench extends in a direction perpendicular to the underlying metal layer. 
     At  160 , photoresist is applied to the thick inter-layer dielectric (ILD) block  157 . Applying the photoresist entails first applying an optical planarization layer (OPL)  159 , followed by an anti-reflection coating (ARC)  161 , and finally the actual photoresist  163 . The OPL  159  is used to fine-tune planarization of the surface of the dielectric block  157  prior to exposure, in order to reduce distortion of the pattern. The OPL  159  is made of, for example, a spin-on glass (SOG), as is known in the art. 
     At  162 , the photoresist  163  is patterned by exposure to light through a mask fabricated according to the wavy line mask design  110 , followed by treatment with a developer that removes portions of the photoresist  163 . 
     At  164 , a wavy line trench  166  is etched in the dielectric block  157  down to a trench depth  167 . The wavy line trench  166  includes a wide portion of width D where the via will be formed, as shown in  FIG. 4C , and a narrow portion of width d, that corresponds to the nominal metal linewidth, e.g., 24 nm, as shown in  FIG. 4D . 
       FIGS. 5A-5D  show and describe the formation of a sacrificial, or disposable, layer, according to one embodiment. 
     At  170 , a sacrificial layer  171  is deposited into the wavy line trench  166 . In one embodiment, the sacrificial layer  171  is made of a material such as polysilicon, low-temperature silicon dioxide, or SiN. The sacrificial layer  171  overfills the narrow portion of the trench (d) to a narrow portion thickness  172  having a notch  178  therein, while conformally underfilling the wide portion of the trench (D) to a wide portion thickness  175 . Thus, as shown in  FIGS. 5C and 5D , the narrow portion d of the trench is entirely blocked, while the wide portion D contains a hard mask that will define a large via having a via width  176 . 
     The via width  176  is much greater than the narrow portion, d, of the metal line, which eases formation of the overall interconnect structure. The significance of the independence of the via width from the nominal metal linewidth d is that a path is cleared for the nominal metal linewidth in future technology generations to shrink without causing via failures. The metal linewidths can continue getting smaller and smaller with each technology generation while the via size remains the same. The narrow portion thickness  172  is larger than d/2, but smaller than D/2. By using the sacrificial layer  171  as a hard mask, a via lithography step is avoided. Via formation without the need for a separate mask layer is referred to by those skilled in the art as a self-aligned via. 
     In another embodiment, the sacrificial layer  171  is made of a direct self-aligned (DSA) polymer material. A DSA polymer sacrificial layer  171  fills the narrow portions (d) of the wavy line trench so that the narrow portion thickness  172  is negligible. Likewise, the DSA polymer sacrificial layer  171  deposits on the sidewalls of the wide portions (D) to form a hard mask having a negligible wide portion thickness  175 . Consequently, use of the DSA polymer as the sacrificial layer  171  simplifies the etch process, as is shown below. 
       FIGS. 6A-6D  show and describe the formation of vias at step  156 , according to one embodiment. 
     At  180 , the sacrificial layer  171  is etched to expose a via landing pad  181  at the surface of the underlying bulk metal  153 . A first anisotropic etch step removes the sacrificial layer  171  from the bottom of the wavy line trench  166 , down to the trench depth  167 . If a DSA polymer is used as the sacrificial layer  171 , the first anisotropic etch step is not needed because the bottom of the trench is already exposed. 
     At  182 , a second anisotropic etch continues removing dielectric material in a downward direction to the underlying three-metal layer, creating a large via  169 . Finally, a third etch step removes the cap layer  155  to expose the bulk metal  153 , as shown in  FIG. 6C . Meanwhile, as shown in  FIG. 6D , the sacrificial layer  171  protects the narrow portion of the wavy line trench  166 . 
       FIGS. 7A-7D  show and describe the trench and via fill at step  158 , according to one embodiment. 
     At  190 , the sacrificial layer  171  is removed using an etch chemistry that has a high selectivity to the dielectric block  157 . For example, if the sacrificial layer  171  is SiN and the dielectric  157  is silicon dioxide, the sacrificial layer  171  can be removed using a phosphoric acid dip. 
     At  192 , the wavy line trench  166  and the large via  169  are filled with metal. The metal fill process includes first depositing a metal liner  195 , e.g., titanium (Ti), titanium nitride (TiN), tantalum nitride (TaN), titanium carbide (TiC), cobalt (Co), rubidium (Ru), and combinations thereof; followed by deposition of a bulk metal  197 , e.g., aluminum (Al), copper (Cu), tungsten (W), cobalt (Co), nickel silicide (NiSi), cobalt silicide (CoSi), and combinations thereof, wherein combinations include metal laminates, alloys, and the like. 
     At  194 , a chemical mechanical planarization (CMP) step is performed to planarize the metal liner  195  and bulk metal  197  to the height of the dielectric block  157 . 
     The resulting metal lines shown in  FIG. 7B  are the wavy metal line structures  120  shown in  FIG. 2B , including the narrow portions  122  of width d, intermediate portions  123 , and wide portions  124  of width D, accommodating the large via  169 . If, for example, the metal lines thus formed are at metal 2, the wide portions  124  serve as large via landing pads for subsequently formed vias that will later connect to upper metal lines, e.g., metal 3. It is noted that the large via  169  has an aspect ratio of about 3:1 or less, which can easily be filled with metal compared with existing vias that present more challenging aspect ratios, in the range of about 4:1-5:1. 
     The techniques presented herein are generic and can be used at any metal layer, to ease constraints on via formation, thus supporting metal structures having a variety of different linewidths. Without the benefit of the structures and processes described herein, via aspect ratios would continue to increase as the nominal metal linewidth shrinks with each new technology generation. Thus, severing the dependence of via dimensions on metal linewidths has important advantages and implications for future generations of integrated circuit development. 
     The various embodiments described above can be combined to provide further embodiments. All of the U.S. patents, U.S. patent application publications, U.S. patent applications, foreign patents, foreign patent applications and non-patent publications referred to in this specification and/or listed in the Application Data Sheet are incorporated herein by reference, in their entirety. Aspects of the embodiments can be modified, if necessary to employ concepts of the various patents, applications and publications to provide yet further embodiments. 
     It will be appreciated that, although specific embodiments of the present disclosure are described herein for purposes of illustration, various modifications may be made without departing from the spirit and scope of the present disclosure. Accordingly, the present disclosure is not limited except as by the appended claims. 
     These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure.