Abstract:
A semiconductor device and a method of fabricating a contact to interface with an interconnect in a semiconductor device are described. The device includes a dielectric layer formed on a semiconductor layer, and a contact fabricated in a via formed within the dielectric layer. An interconnect formed above the contact interfaces with an exposed surface of the contact opposite a surface closest to the semiconductor layer. The contact includes a contact material in a first portion of the contact and an interface metal in a second portion of the contact.

Description:
DOMESTIC PRIORITY 
     This application is a continuation of U.S. application Ser. No. 14/749,811 filed Jun. 25, 2015, the disclosure of which is incorporated by reference herein in its entirety. 
    
    
     BACKGROUND 
     The present invention relates to contacts in semiconductor devices, and more specifically, to low resistance metal contacts to interconnects. 
     In semiconductor devices, a contact acts as a bridge between the semiconductor (e.g., silicon (Si)) and a low-resistance metal interconnect (e.g., copper (Cu)). The contact material is typically disposed in an opening (via) in a dielectric layer and is needed as a buffer because the metal interconnect is typically a material that reacts with the semiconductor. For example, Cu dissolves in Si such that a Cu interconnect requires a contact (e.g., tungsten (W)) between the Cu and the Si. As semiconductor devices are made smaller, the contacts are also smaller. Because less area results in more resistance, the smaller contacts have higher resistance. 
     SUMMARY 
     According to one embodiment, a semiconductor device includes a dielectric layer formed on a semiconductor layer; a contact fabricated in a via formed within the dielectric layer; and an interconnect formed above the contact and interfacing with an exposed surface of the contact, wherein the contact includes a contact material in a first portion of the contact and a metal in a second portion of the contact, and at least a part of the second portion is at the exposed surface. 
     According to another embodiment of the present invention, a method of fabricating a contact to interface with an interconnect in a semiconductor device includes forming a via in a dielectric layer formed above a semiconductor layer; filling a first portion of the via with a contact material; and filling a second portion of the via with a metal, wherein the contact includes the first portion and the second portion, and an exposed surface of the contact interfaces with the interconnect. 
     Additional features and advantages are realized through the techniques of the present invention. Other embodiments and aspects of the invention are described in detail herein and are considered a part of the claimed invention. For a better understanding of the invention with the advantages and the features, refer to the description and to the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The forgoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
         FIG. 1  illustrates top and side cross sectional views of a semiconductor device according to an embodiment of the invention; 
         FIGS. 2-5  are cross-sectional views illustrating the formation of a contact according to an embodiment of the invention, in which: 
         FIG. 2  illustrates a via formed in a dielectric layer; 
         FIG. 3  shows the result of a W fill in the via shown in  FIG. 2 ; 
         FIG. 4  shows the slit opening resulting from a CMP process on the structure shown in  FIG. 3 ; 
         FIG. 5  shows the result of a metal fill in the slit opening shown in  FIG. 4 ; and 
         FIG. 6  shows the contact resulting from another CMP process; 
         FIG. 7-9  are cross sectional views illustrating the formation of a contact according to another embodiment of the invention, in which: 
         FIG. 7  shows W conformally lining a via; 
         FIG. 8  shows the result of a metal fill within the W lining; and 
         FIG. 9  shows the structure resulting from a CMP process on the structure shown in  FIG. 8 ; 
         FIGS. 10-13  are cross sectional views illustrating the formation of a contact according to yet another embodiment of the invention, in which: 
         FIG. 10  shows a W fill in a via with a seam formed in the W fill; 
         FIG. 11  shows an enlargement of the seam; 
         FIG. 12  shows the result of creating an opening within the seam; and 
         FIG. 13  shows the structure resulting from metal fill of the opening followed by a CMP process; 
         FIGS. 14-18  are cross sectional views illustrating the formation of a contact according to yet another embodiment of the invention, in which: 
         FIG. 14  shows a W fill in a via formed in a dielectric; 
         FIG. 15  shows the result of performing a CMP process; 
         FIG. 16  results from oxidation of some of the W fill; 
         FIG. 17  shows the structure resulting from metal fill of the portions where W was oxidized as shown in  FIG. 16 ; and 
         FIG. 18  shows the result of a CMP process on the structure of  FIG. 17 ; and 
         FIG. 19  shows a cross sectional view of a portion of a semiconductor device according to an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     As noted above, contacts are required as a buffer between a semiconductor such as Si and a metal interconnect such as Cu, but smaller contacts result in higher resistance. Because a given semiconductor device may have a number of contacts on the order of millions, the increased resistance of these smaller contacts can limit the performance of the semiconductor chip. Embodiments of the systems and methods detailed herein relate to reducing contact resistance by including an interface metal in addition to a contact material (e.g., tungsten (W)) in the contact. Several different embodiments are discussed but each reduces resistance without increasing the diameter of the via in which the contact is formed. The different embodiments all include the contact comprising both the material and an interface metal. Each embodiment may be seen as a different tradeoff between reducing contact material to interconnect material interface area (by replacement with interface metal to interconnect material interface area) and increasing surface area of contact material to interface metal interaction within the contact. 
       FIG. 1  illustrates top and side cross sectional views of a semiconductor device according to an embodiment of the invention. The cross sectional top view  101  shows six of the contacts ( 120 / 130 ) and, specifically, the interface metal  130  of the semiconductor device. A pitch a may define the distance between adjacent contacts  120  along one axis and a pitch p may define the distance between adjacent contacts  120  along another (perpendicular) axis. The two pitches (a and p) may be equal or a may be less than p, as shown in  FIG. 1 , for example. Distance between the wires  102  may be S. An exemplary targeted technology may be a 7 nanometer (nm) node, meaning that the minimum dimension (targeted gate size) that is printed lithographically is 7 nm. In a 7 nm node, a pitch between contacts (e.g., a, p, or both) may be 44 nm and each contact may be 18 nm in diameter. The wire  102  width may be 22 nm, and the distance S between wires  102  may also be 22 nm. In alternate embodiments, the pitches (a and p) may be on the order of 66 nm, S may be on the order of 40 nm, and width of the wire may be on the order of 26 nm. While exemplary values are provided for explanatory purposes, embodiments detailed herein contemplate other values for the wire  102  width, distance S, and pitches (a and p). The cross-sectional view along A-A  103  details the contact ( 120 / 130 ) to interconnect  135  interface according to the embodiment. According to the present embodiment, the contact material or W  120  fill is in vias ( 115  ( FIG. 2 ) formed in a dielectric layer  110  above a semiconductor layer  105 . The semiconductor layer  105  may be Si, carbon (C), or gallium arsenide (GaAs) or a different III-V semiconductor layer, or an alloy of Si such as silicon germanium (SiGe), or SiC with a transistor fabricated therein. The contact ( 120 / 130 ) is formed with the dielectric layer  110 , as detailed below, and includes the W  120  fill and the interface metal  130  fill. In the embodiment shown in  FIG. 1 , the interface metal  130  fill forms the entire contact-to-interconnect interface in this embodiment. That is, W  120  fill does not directly interface with the interconnect  135  according to the embodiment shown in  FIG. 1 . Instead, the interface metal  130  fill acts as a cap on the W  120  fill and forms the interface with the interconnect  135 . The interface metal fill  130  may comprise cobalt (Co) and the interconnect  135  may comprise Cu, for example. In alternate embodiments, the interface metal  130  may be nickel (Ni), palladium (Pd), platinum (Pt), or ruthenium (Ru). A low-k dielectric layer  145  is formed above the dielectric layer  110 , and a nitride cap  140  is conformally formed over the interconnects  135 . The copper may be additively patterned in a known damascene process or may be subtractively patterned. That is, the lowest level (M 1 ) line may be subtractively patterned such that the interconnect  135  (e.g., Cu) is blanket deposited and then lithographically etched. 
       FIGS. 2-6  are cross-sectional views illustrating the formation of a contact ( 120 / 130 ) according to an embodiment of the invention.  FIG. 2  shows a via  115  or hole formed in a dielectric layer  110 . The dielectric layer  110  may be silicon dioxide (SiO 2 ), for example, and is formed above the semiconductor device. The material that ultimately fills this via  115  forms the contact ( 120 ,  130 ). A non-conformal W  120  fill in the via  115  results in the structure shown in  FIG. 3 . A non-conformal fast deposition is used to form the keyhole-shaped slit  125  within the W  120  fill. By performing a chemical mechanical planarization (CMP) process on the structure shown in  FIG. 3 , the slit  125  is opened at the top as shown in  FIG. 4 . A wet clean process may be performed to remove W oxides from the slit  125  opening. While a slit  125  is specifically shown and discussed for explanatory purposes, the opening may be of another shape, as well. The structure shown in  FIG. 5  results from an interface metal  130  fill. This interface metal  130  may be Co, Ni, Pd, Pt, or Ru for example, and another CMP process results in the structure shown in  FIG. 6 . As  FIG. 6  shows, the top surface of the via  115  in the dielectric layer  110  includes W  120  but also the metal  130 . That is, the contact surface or interface  610  to the interconnect  135  includes W  120  and metal  130 . As noted above, the interconnect  135  may comprise Cu. For the same given via  115  diameter, the W  120  to interconnect  135  interface area is reduced and is replaced with lower resistance interface metal  130  to interconnect  135  interface according to the embodiment shown in  FIGS. 2-6 . 
       FIGS. 7-9  are cross sectional views illustrating the formation of a contact according to another embodiment of the invention.  FIG. 7  shows the W  120  conformally lining the via  115 . The W  120  may be deposited using atomic layer deposition (ALD), for example. Depositing an interface metal  130  to completely fill the via  115  results in the structure shown in  FIG. 8 . Again, the interface metal  130  may be Co, Ni, Pd, Pt, or Ru for example. Polishing the W  120  and interface metal  130  (e.g., by one or more CMP processes) results in the structure shown in  FIG. 9 . The contact surface or interface  910  includes mostly interface metal  130  but also W  120 . A comparison of  FIG. 6  with  FIG. 9  (a comparison of  610  with  910 ) indicates that even more of the interface between the interconnect  135  and the via  115  surface will be with the interface metal  130  than with the W  120  in the embodiment illustrated in  FIG. 9  than in the embodiment illustrated in  FIG. 6 . Additionally, the surface area of interaction between the W  120  and interface metal  130  within the contact ( 120 / 130 ) is larger in the embodiment illustrated in  FIG. 9  than in the embodiment illustrated in  FIG. 6 . Accordingly, the resistance resulting from the embodiment shown in  FIG. 9  is even lower than the resistance resulting from the embodiment shown in  FIG. 6 . The W  120  liner acts as a buffer between the dielectric layer  110  and semiconductor layer below and the interface metal  130 , which is lower in resistance than the W  120 . 
       FIGS. 10-13  are cross sectional views illustrating the formation of a contact according to yet another embodiment of the invention.  FIG. 10  shows a W  120  fill in the via  115  formed within a dielectric layer  110 . A CMP process is performed on the W  120  fill to form a seam  1010  or narrow void in the W  120  fill. This seam  1010  is enlarged as shown in  FIG. 11 . The enlargement may be achieved through oxidation of the W  120  at 500 to 600 degrees Celsius, for example, in an oxygen or nitrous oxide (N 2 O) plasma environment. Creating an opening  1210  in the enlarged seam  1010  results in the structure shown in  FIG. 12 . The opening  1210  may be formed by dissolving tungsten (III) oxide (W 2 O 3 ) or tungsten trioxide (WO 3 ) in a high pH solution. In alternate embodiments to those shown in  FIG. 12 , the shape and depth of the opening  1210  may be different than shown in  FIG. 12 . A interface metal  130  fill in the opening  1210  followed by a CMP process results in the structure shown in  FIG. 13 . The interface metal  130  may be Co, Ni, Pd, Pt, or Ru for example. The interface metal  130  fill may be preceded by a wet clean process to clean out W oxide from the opening  1210 . The contact surface or interface  1310  with the interconnect  135  indicates that the amount of interface metal  130  replacing W  120  is more similar to that of interface  610  than that of interface  910 . However, in comparison to the embodiment shown in  FIG. 6 , the interaction between W  120  and the interface metal  130  within the contact ( 120 / 130 ) is over a larger surface area in the embodiment shown in  FIG. 13 . Thus, as a comparison of  FIGS. 6 and 9  with  FIG. 13  indicates, the resulting decrease in resistance due to the interface metal  130  replacing some of the W  120  at the interface  1310  with the interconnect  135  is likely to be more than that of the embodiment shown in  FIG. 6  (interface  610 ) and less than that of the embodiment shown in  FIG. 9  (interface  910 ). 
       FIGS. 14-18  are cross sectional views illustrating the formation of a contact according to yet another embodiment of the invention.  FIG. 14  shows a W  120  fill of a via  115  formed in a dielectric layer  110 , and a CMP process on the structure shown in  FIG. 14  results in a standard contact as shown in  FIG. 15 . Typically, an interconnect  135  would then be formed on this W  120  contact. According to the current embodiment, W  120  at the interface with where the interconnect  135  would be formed (around the perimeter of the via  115 ) is oxidized to result in the structure shown in  FIG. 16 . As  FIG. 16  indicates, the W  120  is recessed to have an inverted-v shape such that more W  120  is recessed along the outer portions of the via  115 . The exposed W  120  has a larger surface area than the (flat) W  120  prior to oxidation. The oxidation may be performed under gas cluster ion beam (GCIB) exposure of the reactant gas (oxidizer), for example. A wet clean process may then be performed to selectively remove oxidized W  120 . An interface metal  130  fill results in the structure shown in  FIG. 17 . As shown, the portions of the via  115  in which oxidized W  120  is removed are now filled with interface metal  130 . A further CMP process results in the structure shown in  FIG. 18 . As a result of the oxidation and interface metal  130  fill, the contact surface or interface  1810  with an interconnect  135  comprises only interface metal  130  and substantially no W  120  such that the interconnect  135  formed above the dielectric layer  110  would interface only with the interface metal  130 . Additionally, based on the inverted-v shape of the recessed W  120 , more surface area interaction between W  120  and the interface metal  130  within the contact ( 120 / 130 ) is facilitated as compared to a uniform recess of the W  120  across the diameter of the via  115 . The combination of these two factors results in decreased resistance of the contact ( 120 / 130 ) shown in  FIG. 18  (as compared with that in  FIG. 15 ). 
       FIG. 19  shows a cross sectional view of a portion of a semiconductor device according to an embodiment of the invention. Like  FIG. 1 ,  FIG. 19  illustrates cross sectional views of three contacts ( 120 / 130 ). The contacts ( 120 / 130 ) are formed in vias ( 115 ) of a dielectric layer  110  formed over a semiconductor layer  105 . The W  120  portion of the contacts ( 120 / 130 ) is recessed in a v shape (rather than in an inverted-v shape, as in the embodiment of  FIG. 18 ). The recessed portion is filled with interface metal  130 . Like the inverted-v shaped recess of the embodiment shown in  FIG. 18  (and in contrast with the uniform recess of W  120  shown in  FIG. 1 ), the v-shaped recess of W  120  results in a larger surface area for interaction between the W  120  and the interface metal  130  filled in the recessed portion. The interconnects  135  formed above the contacts ( 120 / 130 ) interface only with the interface metal  130  (rather than with any W  120 ), like the embodiments shown in  FIGS. 1 and 18 . A nitride cap  140  is formed conformally on the interconnects  135 , and a low-k dielectric  145  is formed on the dielectric layer  110 . 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one more other features, integers, steps, operations, element components, and/or groups thereof. 
     The corresponding structures, materials, acts, and equivalents of all means or step plus function elements in the claims below are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. The description of the present invention has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the invention in the form 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 invention. The embodiment was chosen and described in order to best explain the principles of the invention and the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated. 
     The flow diagrams depicted herein are just one example. There may be many variations to this diagram or the steps (or operations) described therein without departing from the spirit of the invention. For instance, the steps may be performed in a differing order or steps may be added, deleted or modified. All of these variations are considered a part of the claimed invention. 
     While the preferred embodiment to the invention had been described, it will be understood that those skilled in the art, both now and in the future, may make various improvements and enhancements which fall within the scope of the claims which follow. These claims should be construed to maintain the proper protection for the invention first described. 
     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.