Abstract:
In improved nonoplasmonic resonator (NPR) structure includes a silicon substrate having an upper surface, and a plurality of columns of silicon extending up from the substrate upper surface. Each of the columns includes a sidewall and terminates at an upper end. An insulation material is disposed on the sidewalls and upper ends of the columns. For each of the columns, the insulation material terminates in a bulge at an upper end of the column. A conductive layer is disposed on the insulation material along the column sidewalls and upper ends.

Description:
RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 62/207,778, filed Aug. 20, 2015, which is incorporated herein by reference. 
    
    
     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 for all purposes. 
     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 known (see U.S. Pat. No. 8,685,743). 
     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 
     In improved nonoplasmonic resonator (NPR) structure includes a silicon substrate having an upper surface, a plurality of columns of silicon extending up from the substrate upper surface, each of the columns including a sidewall and terminating at an upper end, an insulation material disposed on the sidewalls and upper ends of the columns, wherein for each of the columns, the insulation material terminates in a bulge at an upper end of the column, and a conductive layer disposed on the insulation material along the column sidewalls and upper ends. 
     A method of forming nonoplasmonic resonator (NPR) structure includes providing a silicon substrate with an upper surface, forming a plurality of columns of silicon extending up from the substrate upper surface, each of the columns including a sidewall and terminating at an upper end, forming an insulation material on the sidewalls and upper ends of the columns, wherein for each of the columns, the insulation material terminates in a bulge at an upper end of the column, and forming a conductive layer on the insulation material along the column sidewalls and upper ends. 
     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. 2A-4A  are side cross sectional views showing the fabrication of an NPR structure according to a first embodiment. 
         FIGS. 2B-4B  are top views showing the fabrication of the NPR structure according to the first embodiment. 
         FIG. 5  is a side cross sectional view of an alternate embodiment for the NPR silicon column. 
         FIG. 6A  is a side cross sectional view showing the completion of the fabrication of the NPR structure according to the first embodiment. 
         FIG. 6B  is a top view showing the completion of the fabrication of the NPR structure according to the first embodiment. 
         FIG. 7  is a top view showing an NPR structure according to a second embodiment. 
         FIG. 8A  is a side cross sectional view showing an NPR structure according to a third embodiment. 
         FIG. 8B  is a top view showing the NPR structure according to the third embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention includes improved NPR structures and fabrication techniques that enhance hot spot formation and performance without having to utilize expensive and complex nano-meter scale lithography or techniques (i.e. E-Beam, iDuv, double patterning, etc.), and enable fabrication of structures with smaller dimensions, which are enabled by forming bulges elevated above a substrate surface as described below. 
     The fabrication of an elevated NPR structure begins with the flat silicon Si wafer substrate  10 . Cylinders of photoresist PR  12  are formed on the wafer substrate  10 , as shown in  FIGS. 2A and 2B . Photo resist cylinders  12  are formed by depositing photo resist over the surface of the substrate  10 , and patterning the photo resist 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 while leaving other portions of the photo resist intact. Each cylinder  12  can be round as shown, or oval. Each row of PR cylinders is preferably offset from the adjacent row in the column direction as shown. 
     A silicon anisotropic etch is performed to remove exposed surface portions of substrate  10 , leaving columns of silicon  14  underneath the cylinders of photoresist  12 , as shown in  FIGS. 3A and 3B  (after photoresist removal). The silicon etch effectively lowers or recesses the upper surface of the substrate, or of that portion of the substrate, except for those portions thereof protected by the photo resist  12 , resulting in columns of silicon  14  extending up from the now recessed upper surface of the silicon substrate. 
     A layer of non-conformal LTO or PECVD silicon dioxide  16  is deposited on the structure in a manner that accentuates the “bread-loafing” phenomena, as shown in  FIGS. 4A and 4B . The oxide  16  coats the sides of each of the silicon columns  14 , and includes a bulge  18  at the top of the column  14  (i.e. a bulb or spherical shape of the oxide  16  at the top of the column  14  having a lateral dimension greater than the portions of the oxide  16  disposed along the sidewall of the column  14 ). Control of the oxide thickness and deposition parameters allow for achieving the desired shape and diameter of the bulge  18 . 
     Alternately, the bulge  18  can be achieved by forming a hard mask layer over the silicon substrate  10  before photo resist cylinders  12  are formed (so that a disk  20  of the hard mask layer remains at the top of each silicon column  14  when columns  14  are formed). Then, an undercut silicon column etch is used to reduce the width of the silicon column  14  under each disk  20  (i.e. resulting in a T-Top formation). Silicon dioxide deposition and trim etch then follows to form the oxide  16  along the contour of the structure, and resulting in bulge  18 , as shown in  FIG. 5 . 
     A conformal layer of gold (Au)  22  is then formed on the structure, as shown in  FIG. 6A . The thicknesses of the gold and silicon layers  22 / 16  can be optimized to produce the highest SERS amplification signal. The resulting structure is a plurality of the bulges  18  disposed in a plane above the substrate surface in an ordered array. Multiple points of the bulges  18  of adjacent columns  14  are preferably separated by only a few nanometers as shown in  FIG. 6B . With the distance of these multiple points of gold layer  22  on adjacent bulges  18  being less than approximately 10 nanometers, the signal intensity from hot spots  24  there between increases significantly. In  FIG. 6B , each bulge  18  is surrounded by six neighbors, thus producing 6 hot spots for each bulge  18 . 
       FIG. 7  illustrates an alternate embodiment, in which the bulges  18  of adjacent columns  14  are touching bulges  18  of adjacent columns  14  (i.e. each bulge  18  is formed continuously with other bulges  18 ). Such a configuration would create a greater number of hot spots  24  (i.e. the number of hot spots  24  is doubled because there would be two hot spots  24  associated with each location in which the bulge  18  is touching another bulge). 
       FIGS. 8A and 8B  illustrate another alternate embodiment, where gold covered spikes  26  extend up from the substrate surface and between columns  14 , and terminate in a point proximate to the bulges  18 . The spikes  26  are formed in a similar manner as the columns  14 , but with decreasing size extending away from the substrate and no bulge at the top. Non-conformal oxide can be deposited simultaneously on the columns  14  and in spaces between the columns  14 , where optimization of the spikes  26  can be accomplished in terms of aspect ratio, pitch and non-conformal oxide thickness. The spikes  26  are coated with gold in a similar manner as are the columns  14 . With this configuration, each spike  26  can form additional hot spots  24 , or hot spots that cover a range of adjacent surfaces of the spheres, in places where adjacent bulges cannot. 
     The elevated SERS structures can be fabricated very precisely, with high repeatability and surface periodicity, with sizes and separation tightly controlled by process conditions. The elevated SERS structures can be defined with I-Line or d-UV, as well as in nanometer scale using E-Beam Lithography. However, no advanced lithography is required to achieve very small gaps between adjacent bulges  18  and/or spikes  26 . Finally, the pitch or periodicity of the structures may be optimized to contribute to constructive signal interference, thus maximizing the enhanced signal off the SERS structure surface. 
     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 any claims. For example, references to the present invention herein are not intended to limit the scope of any claim or claim term, but instead merely make reference to one or more features that may be covered by one or more of the claims. Materials, processes and numerical examples described above are exemplary only, and should not be deemed to limit the claims. Specifically, silver could be used instead of gold for layer  22 . The elevated SERS structures can have a thin nanometer layer of dielectric on top. While bulges are shown with spherical or bulb shapes, bulges  18  could be formed with other shapes, such as oval, square, rectangle, triangle, planar, or other regular or irregular shapes. While columns  14  are shown with a circular cross section, other cross sectional shapes could be used, including oval, square, rectangle, triangle, hexagon, star, etc. Square cross sectional shape has the advantage that there is a constant separation distance between columns. Further, as is apparent from the claims and specification, not all method steps need be performed in the exact order illustrated or claimed, but rather in any order that allows the proper formation of the elevated SERS structure of the present invention. Single layers of material could be formed as multiple layers of such or similar materials, and vice versa. Lastly, the terms “forming” and “formed” as used herein shall include material deposition, material growth, or any other technique in providing the material as disclosed or claimed. 
     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 there between) and “indirectly on” (intermediate materials, elements or space disposed there between). Likewise, the term “adjacent” includes “directly adjacent” (no intermediate materials, elements or space disposed there between) and “indirectly adjacent” (intermediate materials, elements or space disposed there between), “mounted to” includes “directly mounted to” (no intermediate materials, elements or space disposed there between) and “indirectly mounted to” (intermediate materials, elements or spaced disposed there between), and “electrically coupled” includes “directly electrically coupled to” (no intermediate materials or elements there between that electrically connect the elements together) and “indirectly electrically coupled to” (intermediate materials or elements there between that electrically connect the elements together). For example, forming an element “over a substrate” can include forming the element directly on the substrate with no intermediate materials/elements there between, as well as forming the element indirectly on the substrate with one or more intermediate materials/elements there between.