Patent Publication Number: US-11037821-B2

Title: Multiple patterning with self-alignment provided by spacers

Description:
BACKGROUND 
     The present invention relates to semiconductor device fabrication and integrated circuits and, more specifically, to methods of forming interconnects and structures for interconnects. 
     A back-end-of-line interconnect structure may be used to connect device structures, which are fabricated on a chip during front-end-of-line processing, with each other and with the environment external to the chip. Multiple patterning techniques may be used to form the interconnects of the back-end-of-line interconnect structure. For example, a double-patterning technique based on a series of lithography and etching steps may be utilized. In litho-etch double patterning (i.e., LELE), a pattern is divided into two parts or colors, each of which may be conventionally processed, that are combined in the final patterned interlayer dielectric layer. Trenches may be patterned in an interlayer dielectric layer using a sequence of two separate exposures and etchings of independent subpatterns into the same interlayer dielectric layer. For each subpattern, a unique photoresist coating is required. When the sequence is completed, the pattern is a superimposed, composite of the previously etched subpatterns. By interleaving the subpatterns, the pattern density can be increased with the half-pitch being inversely proportional to the number of subpatterns used. For example, a twenty-five (25) nm pitch pattern can be generated from interleaving a pair of fifty (50) nm half-pitch patterns using litho-etch double patterning. However, litho-etch double patterning has limits on the pitch of the pattern that can be generated. Specifically, interconnects formed in an interlayer dielectric layer by conventional litho-etch double patterning have a pitch that is limited by lithography resolution to greater than or equal to sixty-four (64) nanometers. 
     Improved methods of forming interconnects and structures for interconnects are therefore needed. 
     SUMMARY 
     In an embodiment of the invention, a method includes patterning a hardmask layer to form a plurality of first trenches arranged with a first pattern, forming sidewall spacers inside the first trenches on respective sidewalls of the hardmask layer bordering the first trenches, and forming an etch mask over the hardmask layer. The etch mask includes an opening exposing a portion of the hardmask layer between a pair of the sidewall spacers. The method further includes removing the portion of the hardmask layer exposed by the opening in the etch mask to define a second trench in the hardmask layer. 
     In an embodiment of the invention, a method includes forming a first etch mask with a first lithography process over a hardmask layer that includes a plurality of openings exposing first portions of the first hardmask layer, removing the first portions of the hardmask layer with a first etching process to form the first trenches at respective locations of the openings in the second etch mask, and forming sidewall spacers inside the first trenches on respective sidewalls of the hardmask layer bordering the first trenches. The method further includes forming, with a second lithography process, a second etch mask over the hardmask layer. The second etch mask includes an opening exposing a second portion of the hardmask layer between a pair of the sidewall spacers. The second portion of the first hardmask layer exposed by the opening in the second etch mask is removed with a second etching process to define a second trench in the hardmask layer. 
     In an embodiment of the invention, a structure includes a metallization level having an interlayer dielectric layer, a first plurality of interconnects in the interlayer dielectric layer, and a second plurality of interconnects in the interlayer dielectric layer. The first plurality of interconnects have a first pitch, and the second plurality of interconnects have a second pitch that is different from the first pitch. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate various embodiments of the invention and, together with a general description of the invention given above and the detailed description of the embodiments given below, serve to explain the embodiments of the invention. In the drawings, like reference numerals are used to indicate like features in the various views. 
         FIGS. 1-5  are cross-sectional views of a structure at successive fabrication stages of a processing method in accordance with embodiments of the invention. 
         FIG. 6  is a cross-sectional view of a structure at a fabrication stage of the processing method subsequent to  FIG. 5 . 
         FIG. 6A  is a top view of the structure in which  FIG. 6  is taken generally along line  6 - 6 . 
         FIG. 7  is a cross-sectional view of a structure at a fabrication stage of the processing method subsequent to  FIG. 6 . 
         FIG. 8  is a cross-sectional view of a structure at a fabrication stage of the processing method subsequent to  FIG. 7 . 
         FIG. 8A  is a top view of the structure in which  FIG. 8  is taken generally along line  8 - 8 . 
     
    
    
     DETAILED DESCRIPTION 
     With reference to  FIG. 1  and in accordance with embodiments of the invention, an interlayer dielectric layer  10  may be composed of one or more electrically-insulating dielectric materials, such as a low-k dielectric material formed using a siloxane such as octamethylcyclotetrasiloxane (OMCTS). The interlayer dielectric layer  10  may be located over an etch stop layer  12  that is composed of a dielectric material having etch selectivity relative to the dielectric material of the interlayer dielectric layer  10 . For example, the etch stop layer  12  may be composed of a carbon-containing silicon nitride, such as silicon-carbon-nitride (SiCN). The interlayer dielectric layer  10  and etch stop layer  12  may be components of a metallization level of a back-end-of-line interconnect structure that is arranged over a substrate of a chip including device structures of an integrated circuit fabricated by front-end-of-line (FEOL) processing. 
     Hardmask layers  14 ,  16 ,  18  are arranged in a layer stack over the interlayer dielectric layer  10  and are sequentially deposited on the interlayer dielectric layer  10 . In the layer stack, the hardmask layer  18  is the topmost layer, the hardmask layer  14  is the bottommost layer, and the hardmask layer  16  is arranged between the hardmask layer  18  and the hardmask layer  16 . The hardmask layer  14  is arranged on the interlayer dielectric layer  10 , and may be composed of an oxygen-containing silicon nitride, such as silicon-oxynitride (SiON). The hardmask layer  16  is arranged on the hardmask layer  14 , and may be composed of a metal nitride, such as titanium nitride (TiN). The hardmask layer  18  is arranged on the hardmask layer  14 , and may be composed an oxygen-containing silicon nitride, such as silicon-oxynitride (SiON). In an embodiment, the hardmask layer  14  and the hardmask layer  18  may be composed of the same dielectric material. 
     The hardmask layer  18  is patterned with lithography and etching processes. To that end, an etch mask  20  is formed by applying a lithography stack that includes a spin-on hardmask  19  and an overlying hardmask  21 , and patterning the lithography stack with lithography and etching processes to expose selected portions of the hardmask layer  18  and to mask selected portions of the hardmask layer  18  with shapes. After forming the etch mask  20 , the hardmask layer  18  is etched with an etching process that removes the exposed portions of the hardmask layer  18  to define trenches  22 . The trenches  22  penetrate fully through the hardmask layer  18  to the hardmask layer  16 , which may function as an etch stop. The trenches  22  provide a pattern of a first color that is memorized in the hardmask layer  18  as residual portions of the hardmask layer  18  that cover and mask portions of the hardmask layer  16  and removed portions of the hardmask layer  18  that expose other portions of the hardmask layer  16 . 
     With reference to  FIG. 2  in which like reference numerals refer to like features in  FIG. 1  and at a subsequent fabrication stage of the processing method, the etch mask  20  is removed, and a dielectric layer  26  is conformally deposited that coats the residual portions of the patterned hardmask layer  18  and portions of the hardmask layer  16  that are exposed by the patterning of the hardmask layer  18 . The dielectric layer  26  has a thickness that is selected to avoid pinch off during deposition within the trenches  22 . The dielectric layer  26  is a conformal layer that may be composed of a dielectric material, such as silicon nitride, aluminum oxide, or titanium oxide, deposited by atomic layer deposition. 
     With reference to  FIG. 3  in which like reference numerals refer to like features in  FIG. 2  and at a subsequent fabrication stage of the processing method, the dielectric layer  26  is etched by an etching process to form sidewall spacers  28  inside the trenches  22 . The sidewall spacers  28  are arranged on the sidewalls of the hardmask layer  18  bordering each trench  22 . The dielectric layer  26  is removed from the top surface of the portions of the hardmask layer  18  and from the portions of the hardmask layer  16  between the sidewall spacers  28 . The etching process may be an anisotropic etching process, such as reactive ion etching (ME), that removes the material of the dielectric layer  26  selective to the materials of the hardmask layers  16  and  18 . As used herein, the terms “selective” and “selectivity” in reference to a material removal process (e.g., etching) denotes that the material removal rate (i.e., etch rate) for the targeted material is higher than the material removal rate (i.e., etch rate) for at least another material exposed to the material removal process. The sidewall spacers  28  narrow the width dimension of each trench  22  by an amount equal to the twice the thickness of the sidewall spacers  28 , which effectively reduces the width dimension of the trenches  22  in the pattern of the first color that is memorized in the hardmask layer  18 . 
     With reference to  FIG. 4  in which like reference numerals refer to like features in  FIG. 3  and at a subsequent fabrication stage of the processing method, an etch mask  30  is formed by applying a lithography stack that includes a spin-on hardmask  29  and an overlying hardmask  31 , and patterning the lithography stack with lithography and etching processes to form an opening  32  that exposes a selected portion of the hardmask layer  18 . The opening  32  in the etch mask  30  has a width dimension that may be greater than the width dimension of the exposed portion of the hardmask layer  18  to increase the misalignment tolerance of the opening  32 . As a result, the opening  32  also overlaps in its width dimension with the sidewall spacers  28  positioned at the side edges of the exposed portion of the hardmask layer  18 . The etch mask  30  masks other portions of the hardmask layer  18 , the portions of the hardmask layer  16  exposed by trenches  22 , and the sidewall spacers  28  bordering the trenches  22 . The etching process removes the material of the spin-on hardmask  29  selective to the materials of the hardmask layer  18  and sidewall spacers  28 . 
     With reference to  FIG. 5  in which like reference numerals refer to like features in  FIG. 4  and at a subsequent fabrication stage of the processing method, the portion of the hardmask layer  18  exposed by the opening  32  in the etch mask  30  is removed from its position between the sidewall spacers  28  with an etching process to define a trench  24 . The etching process may be an anisotropic etching process, such as reactive ion etching, that removes the material of the hardmask layer  18  selective to the materials of the hardmask layer  16  and the sidewall spacers  28 . The sidewall spacers  28  self-align the etching process removing the exposed portion of the hardmask layer  18 . The trench  24  has a width dimension substantially equal to the width dimension of the removed portion of the hardmask layer  18 , and contributes to forming a pattern of a second color. Sidewall spacers  28  are respectively arranged between the trench  24  and its neighboring trenches  22 , and define the width of dielectric material of the interlayer dielectric layer  10  arranged between the subsequently-formed interconnects. The trench  24  may be considered to be a pattern of the second color that is superimposed on the first-color pattern provided by the trenches  22  to define a composite pattern of trenches  22 ,  24 . 
     With reference to  FIGS. 6, 6A  in which like reference numerals refer to like features in  FIG. 5  and at a subsequent fabrication stage of the processing method, the etch mask  30  is removed, and a block mask  34  is formed. The block mask  34  is formed by patterning a spin-on hardmask and an overlying hardmask with lithography and etching processes. The block mask  34  covers a rectangular section of the hardmask layer  16  and overlaps with an adjacent pair of the sidewall spacers  28 . The block mask  34  defines a region for forming a dielectric-filled cut between the ends or tips of adjacent interconnects that are subsequently formed. The block mask  34  is formed in the process flow after the trenches  22 ,  24  are patterned. 
     With reference to  FIG. 7  in which like reference numerals refer to like features in  FIG. 6  and at a subsequent fabrication stage of the processing method, the hardmask layer  14  and the hardmask layer  16  are successively etched with different etching processes to define trenches  36  in the hardmask layers  14 ,  16  having a composite pattern given by the different patterns defined by the trenches  22 ,  24  in the hardmask layer  18 . An initial etching process may be an anisotropic etching process, such as reactive ion etching, that removes the material of the hardmask layer  16  selective to the materials of the hardmask layers  14  and  18 , the sidewall spacers  28 , and the block mask  34 . A subsequent etching process may be an anisotropic etching process, such as reactive ion etching, that removes the material of the hardmask layer  14  selective to the materials of the hardmask layer  18 , the sidewall spacers  28 , and the block mask  34 . The hardmask layer  18 , the sidewall spacers  28 , and the block mask  34  may be removed after the formation of the trenches  36 . 
     With reference to  FIGS. 8, 8A  in which like reference numerals refer to like features in  FIG. 7  and at a subsequent fabrication stage of the processing method, the pattern of trenches  36  is transferred from the hardmask layers  14 ,  16  to the interlayer dielectric layer  10  and the underlying etch stop layer  12 . The hardmask layers  14 ,  16  are removed, and interconnects  40 ,  42  are then formed in the trenches  36  in the interlayer dielectric layer  10  and etch stop layer  12 . The primary conductor of the interconnects  40 ,  42  may be composed of a low-resistivity metal formed using a deposition process, such as copper or cobalt deposited by electroplating or electroless deposition or by chemical vapor deposition. A liner (not shown) composed of titanium, titanium nitride, tantalum, tantalum nitride, or a layered combination of these materials (e.g., a bilayer of tantalum nitride and tantalum) may be applied to the walls of the trenches  36  before filling with the primary conductor. The interconnects  40 ,  42  are conductive features that are located in the same metallization level in a back-end-of-line interconnect structure. In an embodiment, the interconnects  40 ,  42  may be located in the metallization level that is the closest of multiple metallization levels in the back-end-of-line interconnect structure to the front-end-of-line device structures, and in which the interconnects  40 ,  42  may be connected with the device structures by contacts in an intervening contact level. 
     The interconnects  40  and the interconnects  42  are the product of the different color patterns producing the trenches  22 ,  24 , and the resulting pattern of interconnects  40 ,  42  represents a composite of those different color patterns. One of the interconnects  40  is divided by a cut  44  that is filled by the dielectric material of the interlayer dielectric layer  10 , and that results from the formation of the block mask  34 . The pitch of the interconnects  40 ,  42  is reduced by the introduction of the sidewall spacers  28  into the trenches  22 , and the thickness of the sidewall spacers  28  may in part determine the composite pitch of the interconnects  40 ,  42  by determining the width of the section of the interlayer dielectric layer  10  between the interconnect  42  and the adjacent interconnects  40 . The pitch of the interconnects  40 ,  42  is reduced without resorting to a self-aligned multiple patterning process that does not involve lithography and etching of patterns of different colors, and that is only able to form a pattern of a single pitch. The interconnects  40 ,  42  are interleaved in the interlayer dielectric layer  10  with the interconnect  42  arranged between the interconnects  40 . 
     In an embodiment, the interconnects  40  may have a width of about twenty (20) nanometers. In an embodiment, the pitch, P, of the interconnects  40 ,  42  may be as small as about forty (40) nanometers, which is considerably less than the conventional minimum pitch for LELE of 64 nanometers. In an embodiment, the pitch of the interconnects  40 ,  42  may be in a range of about forty (40) nanometers to less than sixty-four (64) nanometers. The interconnects  40 ,  42  may be formed with a different pitch by adjusting the pitch of the trenches  22  and/or by adjusting the thickness of the sidewall spacers  28 . The ability to adjust the parameters of the different color patterns permits interconnects with a different pitch to be formed in the same metallization level as the interconnects  40 ,  42  using the same process flow. For example, additional interconnects with a pitch ranging from sixty-four (64) nanometers up to about five hundred (500) nanometers may be concurrently formed by the process flow along with the interconnects  40 ,  42 . 
     The methods as described above are used in the fabrication of integrated circuit chips. The resulting integrated circuit chips can be distributed by the fabricator in raw wafer form (e.g., as a single wafer that has multiple unpackaged chips), as a bare die, or in a packaged form. The chip may be integrated with other chips, discrete circuit elements, and/or other signal processing devices as part of either an intermediate product or an end product. The end product can be any product that includes integrated circuit chips, such as computer products having a central processor or smartphones. 
     References herein to terms modified by language of approximation, such as “about”, “approximately”, and “substantially”, are not to be limited to the precise value specified. The language of approximation may correspond to the precision of an instrument used to measure the value and, unless otherwise dependent on the precision of the instrument, may indicate +/−10% of the stated value(s). 
     References herein to terms such as “vertical”, “horizontal”, etc. are made by way of example, and not by way of limitation, to establish a frame of reference. The term “horizontal” as used herein is defined as a plane parallel to a conventional plane of a semiconductor substrate, regardless of its actual three-dimensional spatial orientation. The terms “vertical” and “normal” refer to a direction perpendicular to the horizontal, as just defined. The term “lateral” refers to a direction within the horizontal plane. 
     A feature “connected” or “coupled” to or with another feature may be directly connected or coupled to or with the other feature or, instead, one or more intervening features may be present. A feature may be “directly connected” or “directly coupled” to or with another feature if intervening features are absent. A feature may be “indirectly connected” or “indirectly coupled” to or with another feature if at least one intervening feature is present. A feature “on” or “contacting” another feature may be directly on or in direct contact with the other feature or, instead, one or more intervening features may be present. A feature may be “directly on” or in “direct contact” with another feature if intervening features are absent. A feature may be “indirectly on” or in “indirect contact” with another feature if at least one intervening feature is present. 
     The descriptions of the various embodiments of the present invention have been presented for purposes of illustration, but are not intended to be exhaustive or limited to the embodiments disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the described embodiments. The terminology used herein was chosen to best explain the principles of the embodiments, the practical application or technical improvement over technologies found in the marketplace, or to enable others of ordinary skill in the art to understand the embodiments disclosed herein.