Abstract:
Aspects of the invention are directed to a method for forming a semiconductor device. A dielectric layer is formed on a semiconductor substrate. Subsequently, a metallic contact is formed in the dielectric layer such that it lands on the semiconductor substrate. A masking layer comprising a block copolymer is then formed on the dielectric layer. This block copolymer is caused to separate into two phases. One of the two phases is selectively removed to leave a patterned masking layer. The patterned masking layer is used to etch the dielectric layer. The patterned air gaps reduce the interconnect capacitance of the semiconductor device while leaving the dielectric layer with enough mechanical strength to serve as a middle-of-line dielectric.

Description:
BACKGROUND 
       [0001]    The present invention relates to the electrical, electronic, and computer arts, and, more particularly, to methods for introducing air gaps into dielectric materials with metal contacts in semiconductor devices. 
         [0002]    Dielectric materials in semiconductor devices must be of sufficient mechanical strength to withstand the many processing steps that go into forming these devices. These steps may include, for example, lithography, deposition, wet and dry etching, and chemical mechanical polishing (CMP). At the same time, many of these dielectric layers must be able to accommodate metal features that may be tensily or compressively stressed. Without sufficient mechanical strength, a dielectric layer may simply buckle or collapse. 
         [0003]    While introducing air gaps into dielectric layers is an effective means for decreasing the dielectric constants of these layers and reducing interconnect capacitance, the air gaps also have the undesirable effect of reducing the mechanical strengths of the dielectric layers into which they are introduced. As a result, conventional dielectric materials that contain air gaps may not be suitable for many of the dielectric features in a given integrated circuit. Dielectric layers in the middle-of-line (MOL), which, in a planar MOSFET, overlie the source and drain diffusions and contain the diffusion contacts, may be good examples. The diffusion contacts in the MOL tend to be significantly smaller than the metal interconnects in the back-end-of-line (BEOL), and tend to come in a wide range of sizes and shapes (e.g., 1×1 vias, and 1×2 and 1×4 structures). As a result, these MOL metal features may not provide a lot of mechanical stability to the MOL dielectric, and more reliance must be placed on the mechanical stability of the MOL dielectric itself. The metal features in the MOL are also often formed of tungsten deposited by chemical vapor deposition (CVD), which tends to be tensile stressed. Accordingly, dielectric materials with air gaps are typically not well suited for use as MOL dielectrics because of the mechanical weaknesses induced by the presence of the air gaps. 
       SUMMARY 
       [0004]    Embodiments of the invention provide a means for forming dielectric layers with air gaps for use in semiconductor devices. Advantageously, the air gaps are shaped and arranged so as to leave the remaining dielectric layers with substantial mechanical strength. This mechanical strength allows the dielectric layers to be used in demanding applications, for example, as MOL dielectrics. 
         [0005]    Aspects of the invention are directed to a method for forming a semiconductor device. A dielectric layer is formed on a semiconductor substrate. Subsequently, a metallic contact is formed in the dielectric layer such that it lands on the semiconductor substrate. A masking layer comprising a block copolymer is then formed on the dielectric layer. This block copolymer is caused to separate into two phases. One of the two phases is next selectively removed to leave a patterned masking layer. The patterned masking layer is used to etch the dielectric layer. 
         [0006]    Additional aspects of the invention are directed to a semiconductor device formed using the method set forth in the previous paragraph. 
         [0007]    Lastly, even additional aspects of the invention are directed to a semiconductor device. A dielectric layer is disposed on a semiconductor substrate. A metallic layer is disposed in the dielectric layer and lands on the semiconductor substrate. Lastly, a plurality of cylindrical air gaps are disposed in the dielectric layer. The plurality of cylindrical air gaps are oriented substantially normal to a surface of the dielectric layer. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0008]    These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings where: 
           [0009]      FIGS. 1A and 1B  show a plan view and a sectional view, respectively, of a portion of a film stack, in accordance with an illustrative embodiment of the invention; 
           [0010]      FIG. 2  shows a flow diagram of an illustrative method for forming the film stack in  FIGS. 1A and 1B ; 
           [0011]      FIGS. 3A-8B  show plan and sectional views of intermediate film stacks formed while performing the  FIG. 2  method, where the “A” figures show plan views, and the “B” figures show sectional views; and 
           [0012]      FIG. 9A and 9B  show a plan view and a sectional view, respectively, of a portion of an intermediate film stack, in accordance with an alternative illustrative embodiment of the invention. 
       
    
    
       [0013]    In the sectional views included herein, features present behind the sectional planes are not shown to reduce clutter and enhance clarity. 
       DETAILED DESCRIPTION 
       [0014]    The present invention will be described with reference to illustrative embodiments. For this reason, numerous modifications can be made to these embodiments and the results will still come within the scope of the invention. No limitations with respect to the specific embodiments described herein are intended or should be inferred. 
         [0015]    As the term is used herein, “substantially” means within plus or minus ten percent. A first element “directly contacts” or “directly overlies” a second element when the first element contacts or overlies, respectively, the second element without any intermediate elements therebetween. 
         [0016]      FIGS. 1A and 1B  show a plan view and a sectional view, respectively, of a portion of a film stack  100 , in accordance with an illustrative embodiment of the invention. The film stack  100  comprises a semiconductor substrate  105 . A dielectric layer  110  is disposed on the semiconductor substrate  105  and is capped by a capping layer  115 . The dielectric layer  110  defines a plurality of air gaps  120  therein. The dielectric layer  110  further encompasses two metallic contacts  125  that pass vertically through the dielectric layer  110  and land on the semiconductor substrate  105 . Each of the metallic contacts  125  comprises a respective liner  130  and a respective core  135 . Even though not directly visible, the positioning of the air gaps  120  and the metallic contacts  125  are shown by broken lines in the plan view in  FIG. 1A . 
         [0017]    In one or more embodiments, the semiconductor substrate  105  may comprise crystalline silicon, and the dielectric layer  110  and capping layer  115  may comprise silicon dioxide. The liners  130  may comprise a combination of titanium and titanium nitride (hereinafter “Ti/TiN”), while the cores  135  may comprise tungsten. 
         [0018]    While not limiting, it is contemplated that the film stack in  FIGS. 1A and 1B  may constitute a portion of the MOL region of a complementary metal-oxide-semiconductor (CMOS) integrated circuit. Accordingly, the semiconductor substrate  105  may include source and drain diffusions, and the dielectric layer  110  may constitute the MOL dielectric. The metallic contacts  125  may contact the source and drain diffusions in the semiconductor substrate  105 , making these metallic contacts “diffusion contacts” or “CA contacts.” Gate features, not visible, would also be incorporated into this MOL region and at least partially surrounded by the dielectric layer  110 . 
         [0019]    Each of the air gaps  120  in the dielectric layer  110  is shaped as an open cylinder that is oriented substantially normal to an uppermost surface  140  of the dielectric layer  110 . Viewed from above, the cylindrical air gaps  120  are arranged in a hexagonal, honeycomb pattern relative to one another (i.e., the cylindrical air gaps  120  are arranged on the nodes of a hexagonal lattice, as illustrated by the dashed hexagonal shape  143  in  FIG. 1A ). Advantageously, the air gaps  120  instill the dielectric layer  110  with an decreased effective dielectric constant. At the same time, the cylindrical shape and regular hexagonal arrangement of the air gaps  120  leaves the remaining dielectric layer  110  with a skeleton that is mechanically robust. The dielectric layer  110  with the air gaps  120  is strong enough to survive subsequent processing steps, such as lithography, deposition, dry and wet etching, and CMP without buckling or collapsing. The dielectric layer  110  is therefore a suitable candidate for demanding applications, including for use as a MOL dielectric as set forth herein. 
         [0020]      FIG. 2  shows a flow diagram of an illustrative method  200  for forming the film stack  100  in  FIGS. 1A and 1B .  FIGS. 3A-8B  show plan and sectional views of intermediate film stacks formed while performing the method, where the “A” figures show plan views, and the “B” figures show corresponding sectional views. Although the method  200  and the structures formed thereby are entirely novel, many of the individual processing steps required to implement the method  200  may utilize conventional semiconductor fabrication techniques and conventional semiconductor fabrication tooling. These techniques and tooling will already be familiar to one having ordinary skill in the relevant arts given the teachings herein. Moreover, details of the individual processing steps used to fabricate semiconductor devices described herein may be found in a number of publications, for example, S. Wolf and R. N. Tauber,  Silicon Processing for the VLSI Era, Volume  1, Lattice Press, 1986; S. Wolf,  Silicon Processing for the VLSI Era, Vol.  4:  Deep - Submicron Process Technology , Lattice Press, 2003; and S. M. Sze,  VLSI Technology, Second Edition , McGraw-Hill, 1988, all of which are incorporated by reference herein. It is also emphasized that the descriptions provided herein are not intended to encompass all of the processing steps that may be required to successfully form a functional device. Rather, certain processing steps that are conventionally used in forming integrated circuit devices, such as, for example, wet cleaning steps, are purposefully not described herein for economy of description. However, one skilled in the art will readily recognize those processing steps omitted from this more generalized description. 
         [0021]    The method starts in steps  205  and  210  with the forming of the dielectric layer  110  on a semiconductor substrate  105  (step  205 ) and the forming of the metallic contacts  125  (step  210 ) in the dielectric layer  110  to yield the film stack shown in  FIGS. 3A and 3B . If the dielectric layer  110  comprises silicon dioxide, that silicon dioxide may be deposited utilizing conventional CVD with, for example, tetraethylorthosilicate (TEOS). Formation of the metallic contacts  125  may occur by what is frequently called a “damascene” process, namely, by utilizing photolithography and reactive ion etching (RIE) to pattern contact openings in the dielectric layer  110 , depositing liner material and the core material to the point that they fill the contact openings, and then utilizing CMP to remove excess metallic material from the top of the dielectric layer  110 . Ti/TiN liners  130  and tungsten cores  135  may be deposited by conventional CVD. The liners  130  act as diffusion barriers and to enhance adhesion of the cores  135 . 
         [0022]    Step  215  involves forming a masking layer  145  on the dielectric layer  110  and the tops of the metallic contacts  125  to yield the film stack shown in  FIGS. 4A and 4B . In accordance with aspects of the invention, the masking layer  145  comprises a block copolymer (BCP). The BCP may be deposited by conventional spin coating as a diluted polymer solution in the manner of a photoresist. BCPs contain two blocks of mutually-repulsive polymers joined by a covalent bond. When the polymers are allowed to move, the different blocks will naturally self-assemble into distinct phases based on the composition and volume fractions of the polymer constituents. By heating a BCP past its glass transition temperature, the polymer chains become mobile and rearrange towards an equilibrium structure. Equilibrium structures include spheres, cylinders, gyroids, diamonds, and lamellae (i.e., thin, plate-like structures). 
         [0023]    In the present non-limiting embodiment, the BCP preferably comprises polystyrene (PS) covalently bonded to poly(methyl methacralate) (PMMA) to form what may be called a PS-b-PMMA BCP. Nevertheless, alternative embodiments may utilize different BCPs such as, but not limited to, PS and 4-(tert-butyldimehtylsilyl)oxy styrene (PS-b-PSSi), PS and dimethylsiloxane (PS-b-PDMS), and PS and vinylpyrrolidone (PS-b-PVP). For purposes of this illustrative embodiment, the volume fraction of PS to PMMA is preferably such that the PS-b-PMMA segregates into PMMA cylinders when annealed (i.e., the PS-b-PMMA is cylinder forming). This BCP may be further tuned to give the desired cylinder diameters and spacings. Research with PS-b-PMMA has shown, for example, that the diameter of cylindrical domains can be selected to be 14-50 nm, depending on the molecular weight of the BCP. Addition of PS and PMMA homopolymer to the PS-b-PMMA to form a blend can also affect the diameter of the cylinders, resulting in diameters and domain spacings that are anywhere from 10% smaller to 150% larger than the corresponding values of pure PS-b-PMMA. This latter effect depends on the relative amount and molecular weight of the homopolymers added to the BCP. 
         [0024]    Step  220  includes the step of causing the BCP in the masking layer  145  to separate into two phases (cylindrical polymer domains  150  and a surrounding polymer domain  155 ) to yield the film stack shown in  FIGS. 5A and 5B . The self-assembled masking layer is now labelled by reference numeral  145 ′. In the present embodiment, this self-assembly may be accomplished by annealing the material above its glass transition temperature (e.g., about 200-300° C.). As indicated in  FIGS. 5A and 5B , the cylindrical polymer domains  150  (comprising PMMA) of the self-assembled masking layer  145 ′ are oriented substantially normal to the uppermost surface  140  of the dielectric layer  110 . They are arranged hexagonally relative to one another.  FIGS. 5A and 5B  do not show self-assembly of the masking layer  145 ′ over the metallic contacts  125  because of the effect of the metallic contacts&#39; upper surfaces on the BCP material. Nevertheless, if self-assembly were to occur on the metallic contacts  125 , that self-assembly is ultimately immaterial so long as the etching step in step  230  (set forth below) does not substantially etch the metallic contacts  125 . 
         [0025]    Step  225  causes the cylindrical polymer domains  150  to be etched away to leave only the surrounding polymer domain  155  in the manner shown in  FIGS. 6A and 6B . The resultant patterned masking layer is now labelled by reference numeral  145 ″. PMMA tends to be more reactive with oxygen than PS. Accordingly, exposing the film stack in  FIGS. 5A and 5B  to an oxygen plasma is an effective way of selectively removing the cylindrical polymer domains  150  while leaving the surrounding polymer domain  155  in place. Alternative techniques may include exposing the film stack to a solvent such as one comprising an organic reagent like acetic acid. In either case, the film stack in  FIGS. 5A and 5B  may be exposed to UV light before etching to make the PMMA even more susceptible to the etchant in relation to the PS. UV light tends to de-crosslink PMMA (i.e., cause scission in the PMMA) in the manner of a positive UV photoresist. 
         [0026]    Step  230  involves using the patterned masking layer  145 ″ as a mask to etch the underlying dielectric layer  110 . The resultant film stack is shown in  FIGS. 7A and 7B . This etching step may be performed by anisotropic RIE selective to the underlying semiconductor substrate (e.g., crystalline silicon) and the metallic contacts  125  if they are exposed. For example, the RIE may utilize CF 4  with O 2  or H 2 ; CHF 3 ; or SiCl 4  as reactants. After etching, the dielectric layer  110  comprises the cylindrical air gaps  120  that span from the uppermost surface  140  of the dielectric layer  110  to the semiconductor substrate  105 . The cylindrical air gaps  120  are oriented substantially perpendicular to the uppermost surface  140  of the dielectric layer  110 . 
         [0027]    Step  235  involves the removal of the patterned masking layer  145 ″. Here, a wet etch may be utilized, again utilizing an organic solvent. A suitable solvent may comprise, for example, toluene. The resultant film stack is shown in  FIGS. 8A and 8B . Finally, step  245  involves forming the capping layer  115  on the film stack in  FIGS. 8A and 8B  to yield the film stack initially shown in  FIGS. 1A and 1B . If the capping layer  115  comprises silicon dioxide, the formation of the capping layer may be by atmospheric pressure CVD with silane and oxygen, which tends to form in a nonconformal, reentrant manner at the tops of small trench features. So formed, the capping layer  115  pinches off the air gaps  120  without substantially filling them in. With the capping layer in place additional processing may be performed on the film stack  100  to convert it into working devices. 
         [0028]    The methods 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 (that is, as a single wafer that has multiple unpackaged chips), as a bare die, or in a packaged form. In the latter case the chip is mounted in a single chip package (such as a plastic carrier, with leads that are affixed to a motherboard or other higher level carrier) or in a multichip package (such as a ceramic carrier that has either or both surface interconnections or buried interconnections). In any case, the chip is then integrated with other chips, discrete circuit elements, and/or other signal processing devices as part of either (a) an intermediate product, such as a motherboard, or (b) an end product. The end product can be any product that includes integrated circuit chips, ranging from toys and other low-end applications to advanced computer products having a display, a keyboard or other input devices, and a central processor. These integrated circuits and end products would also fall within the scope of the invention. 
         [0029]    It should again be emphasized that the above-described embodiments of the invention are intended to be illustrative only. Other embodiments may, for example, utilize different materials and processing steps from those expressly set forth above to achieve embodiments falling within the scope of the invention. 
         [0030]    As just one example, while the above-described embodiment had the BCP of the masking layer  145  be tuned to form cylindrical polymer domains upon self-assembly, the composition of the BCP may instead be tuned to segregate into lamellae. Such tuning may be accomplished by, for example, modifying the relative volume fractions of the two block copolymers and/or by choosing block copolymers with suitable interaction parameters, both of which influence the phase diagram for the chosen BCP. Processing according to the method  200  would, in turn, yield the film stack shown in  FIGS. 9A and 9B  after performing step  225  on the self-assembled masking layer (i.e., selectively removing one polymer domain from the self-assembled masking layer to form a patterned masking layer). In  FIGS. 9A and 9B , a patterned masking layer  900  includes a series of narrow linear trenches  905 . In subsequent processing, these linear trenches  905  are transferred into the underlying dielectric layer  110 . Thus rather than having a plurality of cylindrical air gaps, the resultant dielectric layer  110  would instead have a series of closely spaced linear air gap trenches. 
         [0031]    At least a portion of the features disclosed herein may be replaced by alternative features serving the same, equivalent, or similar purposes, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features. 
         [0032]    Any element in a claim that does not explicitly state “means for” performing a specified function or “step for” performing a specified function is not to be interpreted as a “means for” or “step for” clause as specified in AIA 35 U.S.C. §112(f). In particular, the use of “steps of” in the claims herein is not intended to invoke the provisions of AIA 35 U.S.C. §112(f).