Abstract:
A nanoplasmonic resonator (NPR) includes a substrate, a first metallic member disposed on the substrate, a second metallic member spaced apart from the first metallic member, and a first insulation layer at least partially disposed between the first and second metallic members. The first insulation layer includes at least one of a notch formed laterally therein such that there is an open gap separating outer edge portions of the first and second metallic members, at least a portion thereof having a toroid shape, and a length extending between the first and second metallic members which are laterally adjacent to each other.

Description:
RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 61/847,970, filed Jul. 18, 2013. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to Surface Enhanced Roman Spectroscopy (SERS) for characterizing molecular properties, and more particularly to tunable nanoplasmonic resonators (NPRs) and methods of making NPRs. 
     BACKGROUND OF THE INVENTION 
     As shown in  FIG. 1 , it is presently known to use a nanoplasmonic resonators (NPR)  2  in the form of a thin dielectric layer  4  (e.g. SiO 2 ) sandwiched between two metallic nanodisks  6  on a quartz substrate  8  to enhance SERS (Surface Enhanced Raman Scattering) Raman intensity for the detection of protease and enzyme activity. The NPR  2  results in SERS hot-spots at desired locations and in small dimensions, allowing for multiplexed high-throughput detection and lab-on-chip applications. The resonance frequency of the NPR can be precisely tuned by varying the dielectric layer thickness and the aspect ratio of the NPR. Such NPR and SERS techniques are disclosed in U.S. Pat. No. 8,685,743, which is incorporated herein by reference. 
     The nanodisk NPRs have been patterned on quartz substrates by electron beam lithography (EBL). However, EBL techniques are time consuming and expensive. Moreover, while disk shaped NPRs have shown to enhance the Raman intensity by a factor of over 6×10 6 , further Raman intensity enhancement is needed. 
     There is a need for improved techniques in forming NPRs, and there is a need for different NPR structures that further enhance the Raman intensity. 
     BRIEF SUMMARY OF THE INVENTION 
     A nanoplasmonic resonator (NPR) includes a substrate, a first metallic member disposed on the substrate, a second metallic member spaced apart from the first metallic member, and a first insulation layer at least partially disposed between the first and second metallic members. The first insulation layer includes at least one of a notch formed laterally therein such that there is an open gap separating outer edge portions of the first and second metallic members, at least a portion thereof having a toroid shape, and a length extending between the first and second metallic members which are laterally adjacent to each other. 
     A method of fabricating a nanoplasmonic resonator (NPR) include forming a first metallic member on a substrate, forming a second metallic member spaced apart from the first metallic member, and forming a first insulation layer at least partially disposed between the first and second metallic members. The first insulation layer includes at least one of a notch formed laterally therein such that there is an open gap separating outer edge portions of the first and second metallic members, at least a portion thereof having a toroid shape, and a length extending between the first and second metallic members which are laterally adjacent to each other. 
     Other objects and features of the present invention will become apparent by a review of the specification, claims and appended figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a prior art NPR structure. 
         FIGS. 2-4  are side views showing the fabrication of an NPR structure according to a first embodiment. 
         FIG. 5  is a top view of the NPR structure according to the first embodiment. 
         FIGS. 6-8  are side views showing the fabrication of an NPR structure according to a second embodiment. 
         FIGS. 9A-12A  and  FIGS. 9B-12B  are side views and top views, respectively, showing the fabrication of an NPR structure according to a third embodiment. 
         FIGS. 13A-16A  and  FIGS. 13B-16B  are side views and top views, respectively, showing the fabrication of an NPR structure according to a fourth embodiment. 
         FIGS. 17A-21A  and  FIGS. 17B-21B  are side views and top views, respectively, showing the fabrication of an NPR structure according to a fifth embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention includes improved NPR structures and fabrication techniques that enhance hot spot formation and performance, and enable fabrication of structures with smaller dimensions. 
     First Embodiment 
       FIG. 2  illustrates an NPR structure  20  according to a first embodiment of the present invention. NPR structure is fabricated using a photolithographic process that begins by forming a first dielectric layer  22  (e.g. silicon dioxide—SiO 2 ) over a substrate  24  (e.g. Si), followed by forming a first metal layer  26  (e.g. gold or silver) over the first dielectric layer  22 , followed by forming a second dielectric layer  28  (e.g. silicon dioxide) over the first metal layer  26 , followed by forming a second metal layer  30  (e.g. gold or silver) over the second dielectric layer  28 . The metal layers  26  and  30  can be formed by PVD (physical vapor deposition) or ALD (atomic layer deposition). The dielectric layers  22  and  28  can be formed by CVD (chemical vapor deposition) or ALD. The first dielectric layer  22  could instead be formed by thermal oxidation. Photo resist  32  is formed over the second metal layer  30 , and patterned using conventional photolithographic masking techniques by selectively exposing portions of the photo resist using a mask, followed by a photo resist etch, which removes some portions of the photo resist (exposing portions of the underlying second metal layer) while leaving other portions of the photo resist intact. An optional photo resist trim etch can be performed to reduce the dimensions of the remaining photo resist below that defined by the masking step. Anisotropic metal, dielectric, metal and dielectric etches are used to remove the exposed portions of the first and second metal layers  26 ,  30  and the first and second dielectric layers  22 ,  28 . The first dielectric etch can be isotropic or wet, to form inward notches or undercuts  34  in the side edges of second dielectric layer  28 , such that there is an open gap  35  separating the outer edge portions of first and second metal layers  26 ,  30 . Exemplary, non-limiting dimension examples include  25  nm thickness for the metal layers  26 ,  30 , a 5 nm thickness of the second dielectric layer  28 , and 1 μm or less thickness for the first dielectric layer  22 . 
     Alternately, an optional carbon layer  36  can be formed on the second metal layer  30 , whereby the photo resist  32  is formed on the carbon layer, as shown in  FIG. 3 . The carbon layer  36  acts as an organic anti-reflective layer so the photo resist is well patterned on exposure. This carbon layer  36  can be used underneath the photo resist for any of the embodiments herein. An additional insulation layer  38  may be formed on the top metal layer  30  (and under the carbon layer  36  if one is used) as a hard mask layer for better etching. One or both of these hard mask layers can be implemented using any appropriate organic, inorganic and/or metallic material(s). 
       FIG. 4  shows a single disc stack NPR  20  from  FIG. 2  after removal of photo resist  32 . Preferably, the NPR  20  disks are oval shaped in the horizontal direction, with the top surface  30   a  of the second metal layer  30  serving as the surface on which the biomolecule target is applied. Hot spots having the highest electric fields are located at the disk edges at or near the ends of the oval shape (i.e. those portions with the smallest radius of curvature). The notches  34  in the second dielectric layer  28  forming open gaps  35  enhance the electric field hot spots. Using a photolithographic process simplifies the manufacturing process (making it less expensive), and provides accurate and repeatable control over the formation of the NPRs  20  and their critical dimensions. The photolithographic mask could be a mask with discrete apertures (one for each NPR  20 ). Alternately, the mask could be a series of parallel elongated slits, whereby the photo resist  32  is exposed through the mask, the mask is rotated  90  degrees, and the photo resist  32  is exposed again through the mask, whereby the only portions of the photoresist  32  not exposed by either exposure is the overlap of the two orientations of the elongated slits. While only two metal layers  26 ,  30  are shown, additional alternating layers of metal and dielectric materials can be formed in the same manner as disclosed above. Preferably, the NPRs are formed together on a single wafer  40  as an array of NPRs  20  of uniform or varying density as shown in  FIG. 5 . 
     Second Embodiment 
       FIGS. 6-8  illustrate the photolithographic process used to form the NPR structure  44  according to a second embodiment. The process begins by performing a pre-oxidation clean on the substrate  24 , followed by forming a first dielectric layer  46  (e.g. silicon dioxide—SiO 2 ) over substrate  24  (e.g. Si), for example by thermal oxidation (e.g. 1 μm thick). This is followed by forming a sacrificial layer  48  over the structure (e.g. polymer that is 0.2 μm to 0.3 μm thick). A BARC layer  50  (Bottom Anti-Reflective Coating, e.g. carbon or inorganic material) is formed over the polymer layer  48 . Photo resist  52  is formed over the BARC layer  50 , and patterned with holes  54  of approximately 50 nm-250 nm in size (patterning includes photolithograph exposure and photo resist etch). BARC and polymer etches are used to extend the holes  54  down to the first dielectric layer  46 . The resulting structure is shown in  FIG. 6 . 
     The photo resist  52  and BARC layer  50  are then removed, and a first layer of metal  56  (e.g. gold) is deposited on the structure which covers the top surface of the polymer  48  and lines the side and bottom walls of the holes  54  (e.g. 25 nm thickness). A second dielectric layer  58  (e.g. silicon based or HiK) is formed over the first gold layer  56 . A second layer of metal  60  (e.g. gold) is formed over the second dielectric layer  58  (e.g. 25 nm thickness), which results in filling the holes  54 . The resulting structure is shown in  FIG. 7 . 
     A CMP etch is used to remove portions of the gold layers  56 ,  60  not in the holes  54  and optionally a top portion of the polymer  48  (for example, down to a total polymer height of 55 nm). A polymer etch is used to remove the polymer  48 . A wet etch is then used to recess the second dielectric layer  58  in the holes  54 , resulting in the final structure of NPR  44  shown in  FIG. 8 . 
     The structure of NPR  44  increases the number of edges on which hot spots can form in each NPR. The NPR  44  is formed on dielectric layer (e.g. SiO 2 )  46 , whereby metallic layer  56  (e.g. gold) is disposed on the dielectric layer  46  in discrete blocks. A cavity  62  extends into the top surface of the metallic layer  56 . A second metallic layer  60  is disposed inside the cavity but insulated from the first metallic layer  56  by second dielectric layer  58  at the bottom of the cavity  62 . Preferably, the second dielectric layer  58  extends part way up toward the top of the cavity  62 , leaving an open gap between upper portions of metallic layers  56 ,  60 . Each NPR  44  of this second embodiment includes two metallic top surfaces  56   a  and  60   a  (one for each metallic layer), and six annular top surface edges (compared to just one top surface and one annular top surface edge for the disk shaped first embodiment in  FIG. 4 ). This increase in the number of top surfaces and the number of top surface edges provides additional hot spot locations of increased electric field strength. 
     Third Embodiment 
       FIGS. 9A-12A and 9B-12B  illustrate the photolithographic process used to form the NPR structure  66  according to a third embodiment. The process begins by performing a pre-oxidation clean of substrate  24 , following by forming a first dielectric layer  68  (e.g. silicon dioxide—SiO 2 ) over substrate  24  (e.g. Si), for example by thermal oxidation (e.g. 1 μm). This is followed by forming a sacrificial layer  70  over the structure (e.g. polymer that is 50 nm-10 μm thick). A BARC layer  72  is formed over the polymer  70 . Photo resist  74  is formed over the BARC layer  72 , and patterned with oval shaped cavities  76 , as illustrated in  FIGS. 9A and 9B  (patterning includes photolithograph exposure and resist etch). 
     BARC and polymer etches are used to extend the cavities  76  defined by the photo resist down through the BARC and polymer layers  72 ,  70  (i.e. down to and exposing the first dielectric layer  68 ). The photo resist  74  is then removed. A second photo resist is formed over the BARC layer  72 , and patterned with oval shaped holes  78  over the center of the oval shaped cavities  76 . BARC and polymer etches are used to extend the holes  78  defined by the second photo resist down through the BARC and polymer layers  72 ,  70  (i.e. down to and exposing the first dielectric layer  68 ). The second photo resist is then removed, resulting in the structure shown in  FIGS. 10A and 10B . 
     The structure is then covered with a layer of metal  80  (e.g. gold deposited by ALD) which fills the cavities  76  and the holes  78 . A metal etch (e.g. CMP or wet etch) is used to remove any of the metal on the top surface of the BARC layer  72 . BARC and polymer etches then are used to remove BARC and polymer layers  72 ,  70 , leaving an oval toroid shaped (i.e. donut shaped with a center opening) gold pillar structure  82  surrounding and spaced from an oval gold pillar structure  84  forming an annular (e.g. oval shaped) cavity  86  therebetween. A second dielectric layer  88  is formed over the structure (e.g. HiK or Si based) and in cavities  86 , as illustrated in  FIGS. 11A and 11B . An etch is used to remove the second dielectric layer  86  on the gold pillar structures  82 ,  84  and on the first dielectric layer  68 , and to recess those portions of the second dielectric layer  88  in cavities  86 , resulting in the final structure shown in  FIGS. 12A and 12B . NPR  66  includes two top surfaces  82   a  and  84   a  and six annular top surface edges, but the second dielectric layer  88  is recessed in cavities  86  and entirely removed outside of cavities  86  (i.e. leaving the first dielectric layer  68  exposed and leaving upper portions of the gold pillar structures  82  and  84  separated by an open gap). 
     Fourth Embodiment 
       FIGS. 13A-16A and 13B-16B  illustrate the photolithographic process used to form the NPR structure  100  according to a fourth embodiment. The process begins by performing a pre-oxidation clean of the substrate  24 , following by forming a first dielectric layer  102  (e.g. silicon dioxide—SiO 2 ) over substrate  24  (e.g. Si), for example by thermal oxidation (e.g. 1 μm). This is followed by forming a sacrificial layer  104  over the structure (e.g. silicon that is 100 nm thick). A BARC layer  106  is formed over the sacrificial layer  104 . Photo resist  108  is formed over the BARC layer  106 , and patterned with parallel trenches  110  approximately 5 nm in width (patterning includes photolithograph exposure and resist etch). BARC and silicon etches are used to extend the trenches  110  down to the first dielectric layer  102 . The resulting structure is shown in  FIGS. 13A and 13B . 
     The photo resist  108  and BARC layer  106  are etched away. A second layer of dielectric material  112  is formed over the structure, filling the trenches  110  with the dielectric material  112 . A CMP dielectric etch using the sacrificial silicon  104  as an etch stop is used to remove the second layer of dielectric material except for the dielectric material  112  in the trenches, as shown in  FIGS. 14A and 14B . 
     A second BARC layer  114  is formed over the structure. A second photo resist  116  is formed over the BARC layer  114  and patterned leaving columns of photoresist  116  over the dielectric material  112  in filled trenches  110 , and rows of photoresist  116  separated by approximately 100 nm. BARC and silicon etches are used to remove the BARC and silicon layers  114 ,  116  (using the first dielectric layer  102  as an etch stop). These etches result in pillars of the second dielectric  112  extending in the column direction and pillars of the silicon  104  extending in the row direction, leaving cavities  118  extending down to first dielectric  102  formed therebetween, as illustrated in  FIGS. 15A and 15B . 
     A photo resist and BARC etch is used to remove the remaining photo resist  116  and BARC layer  114 . A metal is formed over the structure (e.g. gold), which fills the cavities  118 , followed by a metal etch using the dielectric  112  in the filled trenches  110  as an etch stop. A silicon etch is then used to remove the exposed pillars of silicon  104  extending the row direction. The resulting structure is shown in  FIGS. 16A and 16B , and includes rows of metal pillars  120  separated by dielectric material  112 . The metal pillars  120  are self-aligned to the trenches  110  which were formed in sacrificial silicon layer  104  and filled with dielectric material  112 . In contrast to the previous embodiments where the NPR is stacked vertically over the substrate, here the NPR  100  is stacked horizontally over the substrate  24 , whereby laterally adjacent metal pillars  120  are separated from each other by dielectric layer  112  thus resonating off each other to form the NPR structure. A preferable non limiting configuration includes one or two rows, and 2 to 20 columns, of the gold pillars  120 . 
     Fifth Embodiment 
       FIGS. 17A-21A and 17B-21B  illustrate the photolithographic process used to form the NPR structure  130  according to a fifth embodiment. The process begins by forming a first dielectric layer  132  (e.g. silicon dioxide—SiO 2 ) over substrate  24  (e.g. Si), followed by forming a first metal layer  134  (e.g. gold—25 nm thick) over the first dielectric layer  132 , followed by forming a second dielectric layer  136  (e.g. silicon dioxide—5 nm thick) over the first metal layer  134 , followed by forming a second metal layer  138  (e.g. gold—25 nm thick) over the second dielectric layer  136 . A third dielectric layer  140  (e.g. SiN) is formed over the second metal layer  138 . A sacrificial layer  142  (e.g. silicon) is formed over the third dielectric layer  140 . Photo resist  144  is formed over the sacrificial silicon layer  142 , and patterned (exposure plus photo resist etch) leaving rectangular or oval blocks of photo resist on the structure, as shown in  FIGS. 17A and 17B . 
     A silicon etch is used to remove the sacrificial silicon  142  except for blocks thereof underneath the blocks of photo resist  144 . After the photo resist  144  is removed, a fourth layer of dielectric material (e.g. silicon oxide) is formed over the structure, followed by an oxide etch that removes the silicon oxide except for spacers  146  thereof around the sacrificial silicon blocks. Formation of spacers is well known in the art, and involves the deposition of a material over the contour of a structure, followed by an anisotropic etch process, whereby the material is removed from horizontal surfaces of the structure, while the material remains largely intact on vertically  20  oriented surfaces of the structure. A layer of BARC/HM  148  is formed over the structure, along with another photo resist layer, which is patterned to form a pair of blocks for each block of sacrificial silicon  142  that extend across the width of the sacrificial silicon block. A BARC/HM etch removes those portions of BARC/HM layer  148  not protected by the blocks of photo resist.  FIGS. 18A and 18B  show the resulting structure after the photo resist  150  is removed. 
     Oxide and silicon etch(es) are performed to remove those portions of the sacrificial silicon  142  and oxide spacer  146  not protected by the BARC/HM blocks  148 , leaving discrete (separate) spacers  146   a . The BARC/HM blocks  148  are then removed, with the resulting structure shown in  FIGS. 19A and 19B . A silicon etch is used to remove the remaining portions of the sacrificial silicon layer  142 . A SiN etch is used to remove the exposed portions of the SiN layer  140 , exposing selective portions of the second metal layer  138 , as shown in  FIGS. 20A and 20B . 
     Gold and dielectric etches are performed to remove those portions of the first and second metal layers  134 ,  138  (and the dielectric  136  therebetween) not protected by the oxide spacers  146   a . The spacers  146  and SiN  140  underneath are then etched away, resulting in the final NPR  130  structure of  FIGS. 20A and 20B . With this embodiment, rectangular or oval SERS structures are formed using two lithography masks, whereby smaller NPR structural dimensions can be achieved (i.e. smaller dimensions that the lithography resolution used to make them) because the dimensions of the NPRs  130  are dictated by the size of spacers  146   a.    
     It is to be understood that the present invention is not limited to the embodiment(s) described above and illustrated herein, but encompasses any and all variations falling within the scope of the appended claims. For example, materials, processes and numerical examples described above are exemplary only, and should not be deemed limiting. Further, single layers of material could be formed as multiple layers of such or similar materials, and vice versa. The number of interleaving gold and insulation layers in each embodiment can vary from the number shown and described. Finally, while it is preferable to include insulation layer  22 ,  46 ,  68 ,  102 ,  132  between the substrate and first gold layer, the NPR&#39;s could be formed directly on a non-conducting substrate or on such a substrate using an intervening conductive layer that assists the adhesion of the gold layer. 
     It should be noted that, as used herein, the terms “over” and “on” both inclusively include “directly on” (no intermediate materials, elements or space disposed therebetween) and “indirectly on” (intermediate materials, elements or space disposed therebetween). Likewise, the term “adjacent” includes “directly adjacent” (no intermediate materials, elements or space disposed therebetween) and “indirectly adjacent” (intermediate materials, elements or space disposed there between). For example, forming an element “over a substrate” can include forming the element directly on the substrate with no intermediate materials/elements therebetween, as well as forming the element indirectly on the substrate with one or more intermediate materials/elements therebetween.