Abstract:
Sub-50-nm structures are formed using sequential top-down and bottom up lithographies in conjunction with selective etching. The preferred rendition of the method involves: (a) rough lithographic patterning, (b) size/shape selected nanostructure deposition, (c) resist reflow around the nanostructures, and (d) selective removal/etching of the nanostructure.

Description:
REFERENCE TO RELATED APPLICATION 
       [0001]    The present application claims the benefit of U.S. Provisional Patent Application No. 61/845,201, filed Jul. 11, 2013, whose disclosure is hereby incorporated by reference in its entirety into the present disclosure. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention is directed to a method for patterning very small (preferably smaller than 50 nanometers) structures and more particularly to such a method using resist and nanostructures in the fabrication of semiconductors or other articles of manufacture requiring such small structures. The method allows removal of shot noise and line-edge roughness. 
       DESCRIPTION OF RELATED ART 
       [0003]    The ability of the semiconductor industry to march on the path set up by Moore&#39;s power law has been paved through major breakthroughs in lithography. Currently, modern lithography faces severe challenges in patterning on ever decreasing nanometer-level features; these include the optical resolution limit, nanometer scale linewidth roughness, and shot noise. To overcome the optical resolution limit, excimer lasers with phase shifted masks and even immersion (borrowed from optical microscopy) in liquids have been employed. The problem of linewidth roughness arises in part from the finite molecular size of the polymers used in photoresists (imaging emulsions), which could, in principle, be overcome through the use of lower molecular weight polymers and ultimately through the use of molecular resists. An additional contribution to line-edge roughness is the shot noise, caused by statistical fluctuation in the small number of photons/particles needed to pattern on a nanometer scale. Shot noise leads to fluctuation in pattern dimensions, especially for extreme ultraviolet (EUV) and electron-beam (E-beam) lithography. Furthermore, the current practice of using supersensitive patterning emulsions, i.e., chemically amplified (quantum efficiency&gt;1) resists, introduces a chemical shot noise defined by the finite number of photo-reactive molecules in a given exposure region. 
         [0004]    A complementary approach to patterning is through the so-called bottom up methods. An example of bottom-up method, developed at IBM, relies on specific directed self-assembly to build structures on a nanometer scale using phase separation of the blocks in di-block polymers. Uncertainties in predicting topical nucleation centers and ultimate size distribution of di-block polymer molecular components also limit the scale and yield of such patterning. Yet another class of methods developed at Harvard, known as soft lithography, uses a nano-scale patterned soft stamp to print an inked pattern on a surface (of wafer/glass). The ink used is made up of self-assembling surfactant molecules which then act as a resist. In this approach, the pattern reproducibility fades due to the mechanical wear of the stamp material. 
       SUMMARY OF THE INVENTION 
       [0005]    There thus exists a need in the art to overcome the above difficulties of the prior art. 
         [0006]    It is therefore an object of the invention to form structures at a smaller scale than has traditionally been possible. 
         [0007]    It is another object of the invention to be able to do so while avoiding shot noise and line-edge roughness. 
         [0008]    To achieve the above and other objects, the present invention is directed to a hybrid approach to craft sub 50 nm dimension contact holes exploiting controllable (1) narrower size distribution in nanoparticle dimensions compared to lithographically patterned holes and (2) ability to deposit nanoparticles at the center of such holes (r nanoparticles &lt;r Hole , where r is the radius) using electrostatic funneling (EF). EF is generated by an exposed and charged self-assembled monolayer underneath a photo-lithographically patterned resist film. The lithographic fluctuations due to shot noise in the hole pattern dimensions are removed by a resist reflow around predeposited nanoparticles. It also exploits the fundamental surface-tension-based forces to smooth out line edge roughness as well. Conceptually, as long as σ Nanoparticle &lt;σ Hole , where σ is the standard deviation of the radius, such a technique should be able to remove shot noise, line-edge roughness and shrink the dimension of the hole to match the dimension of the deposited nano-objects. 
         [0009]    In at least one embodiment, negatively charged nanoparticles formed of gold or another suitable substance that can be etched away separately from the resist are deposited into positively charged holes in the resist. Since the negatively charged nanoparticles are attracted to the positively charged hole and repel one another, a suitable density of deposited nanoparticles can be achieved. 
         [0010]    The present invention provides a method to pattern sub-50-nm structures using sequential top-down and bottom up lithographies in conjunction with selective etching. The preferred rendition of the method involves: (a) rough lithographic patterning, (b) size/shape selected nanostructure deposition, (c) resist reflow around the nanostructures, and (d) selective removal/etching of the nano structure. 
         [0011]    The present invention overcomes the existing limitations of shot noise (observed in optical, e-beam, X-ray, and EUV lithographies) and line edge roughness noted in the current generation of lithographic nano-patterning methods. 
         [0012]    Any reflowable resist can be used in this type of patterning. 
         [0013]    Patterning (depositing) nanostructures can be based on different materials such as metals, insulators and semiconductors, as long as they can be selectively etched. 
         [0014]    One application is Back end of line (BEL) processing involving patterning of interconnect structures where via (to connect from one layer to another) and trenches are needed to wire electrical circuit patterns on semiconductor chips, such as microprocessors, memory cells etc. Size controlled nanoparticles and nanowires can be used in the fabrication of via and trench structures respectively. 
         [0015]    Yet another application of the method could involve complex geometric nanostructures such as rings, discs, cubes and helices to selective deposit and pattern structures, such as those useful for protein/DNA biochips. Any suitable nanostructure can be formed through the use of nanowires or the like. The patterning of the resist would be adjusted accordingly to provide a suitable pattern to receive the nanowires or other nanostructures used. 
         [0016]    It is contemplated that the structures formed will have a size range of 2 nm to hundreds of nm. Sizes outside of that range are also possible. Patterns of extremely fine resolution (e.g., below 20 nm) can be formed through conventional 193 nm lithography by reflowing over the nanoparticles/nanostructures. Thus, the resolution limit of the lithographic technique used is no longer a resolution limit set by the wavelength of exposure source for the formation of the nano/microstructures. 
         [0017]    The nanoparticles or nanostructures can be formed from a metal, an insulator, a semiconductor, or any other suitable material. The substrate can be a semiconductor, an insulator such as glass, a metal, or any other suitable material, including a composite patterned substrate that contains metal, semiconductor, insulator surfaces or combinations thereof. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    A preferred embodiment of the present invention will be set forth in detail with reference to the drawings, in which: 
           [0019]      FIG. 1A  is a schematic representation of the process according to the preferred embodiment; 
           [0020]      FIG. 1B  is a set of photographs showing experimental results of the process of  FIG. 1A ; 
           [0021]      FIG. 2A  is a diagram showing a theoretical maximum limit of closest packing of nanoparticles on unpatterned surface; 
           [0022]      FIG. 2B  is a graph showing particle density as a function of particle diameter; 
           [0023]      FIG. 2C  is a graphs showing particle density as a function of particle concentration; 
           [0024]      FIG. 2D  is a graph showing particle density as a function of ionic strength; 
           [0025]      FIG. 2E  is a graph showing particle density as a function of pH; 
           [0026]      FIG. 2F  is a graph showing particle density as a function of resist thickness; 
           [0027]      FIG. 2G  is a graph showing particle density as a function of time: 
           [0028]      FIG. 3  is a graph showing the average number of particles per hole as a function of hole diameter; 
           [0029]      FIGS. 4A through 4D  are photographs showing stages in the process; 
           [0030]      FIG. 5A  shows SEM images of holes with Au NP before (top) and after (bottom) resist reflow, respectively along with graphs of averaged 2D Fourier transforms of the corresponding images; 
           [0031]      FIG. 5B  depicts positions of Au NPs deposited in large partially melted PMMA resist holes; 
           [0032]      FIG. 6  is a flow chart showing an evaporative technique for deposition of the gold nanoparticles; 
           [0033]      FIG. 7  is a photograph showing a cross-sectional view of holes in the resist film; 
           [0034]      FIG. 8A  shows a 200 nm hole is prior to reflow; and 
           [0035]      FIG. 8B  depicts shrinkage of the hole after a partial reflow. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0036]    A preferred embodiment of the present invention will be set forth in detail with reference to the drawings, in which like reference numerals refer to like elements or steps throughout. 
         [0037]    The process according to the preferred embodiment will be explained with reference to  FIG. 1A  and  FIG. 1B .  FIG. 1A  shows the strategy to remove effects of shot noise and line-edge roughness for contact hole patterning using NPs of precise size. Here, the critical dimension (CD) is the desired dimension of the holes. The approach (Step S 1 ) begins with depositing a self-assembled monolayer (SAM)  106  of silane molecules bearing positively charged amine groups  104  on the oxide surface of a silicon wafer  108 . Next, E-beam lithography is used to pattern holes (Steps S 2  and S 3 ) in a poly-methyl methacrylate (PMMA) photoresist film  102 , which generates shot noise as illustrated (in the SEM pattern). The lithography exposes amine groups at the bottom of the holes  110 . Step S 4  entails aqueous phase deposition of controlled-size, citrate-capped (negatively charged) gold nanoparticles (GNPs)  112  in the lithographically patterned holes using electrostatic funneling (EF). In step S 5 , the PMMA is reflown around pre-deposited nanoparticles by heating the wafer to 100° C. (below its glass transition temperature, 110° C.) which engulfs the nanoparticles. The hole-size corresponding to the GNP dimension is recovered by oxygen plasma etching (Step S 6 ) to expose the GNPs followed by wet etching (iodine) of the GNPs (Step S 7 ). Pattern transfer in SiO 2  (Step S 8 ) can be achieved by reactive ion etching or wet etching. 
         [0038]    With reference to  FIG. 1B , the initial structure included a photoresist (PMMA) film  102 , a positively charged surface  104  of NH 2 -terminated N-(2-aminoethyl)-11-aminoundecyltrimethoxysilane (AATMS), a layer  106  of SiO 2  (or another suitable material such as BARC), and a layer  108  of Si. The photoresist film  102  can be applied through spin-coating or in any other suitable way. We used conventional e-beam lithography in step S 11  to pattern the PMMA resist film  102  on top of the cationic self-assembled monolayer of the AATMS to form holes  110 . Pitch, resist thickness, and hole dimensions were varied. In step S 12 , negatively charged gold nanoparticles  112  (citrate capped) were deposited close to the center of each hole  110 , with 93 percent of the particles  112  lying within one standard deviation of particle size with respect to the center of the hole  110 . This electrostatic funneling immediately implies that the PMMA surface may bear a negative charge. For resist reflow in step S 13 , we used temperatures slightly below the glass transition temperature of the PMMA. The inwardly advancing polymer interface  114  engulfed the gold nanoparticles  112  within 5 minutes. Given that we used nanoparticle dimensions thinner than the resist film thickness, the overlying PMMA polymer was removed to expose gold nanoparticles by O 2  plasma, and the exposed gold nanoparticles were etched in step S 14  by an I 2  etching solution. The resulting pattern can then be etched into the SiO 2  layer  106  by reactive ion etching (RIE) or by any other suitable technique such as wet etching using buffered hydrofluoric acid based formulations. 
         [0039]    Assuming random loose packing of gold NP on the derivatized surface, it is easy to show that the packing density on the surface is given by: 
         [0000]    
       
         
           
             
               ρ 
               = 
               
                 α 
                 
                   
                     ( 
                     
                       D 
                       + 
                       
                         β 
                          
                         
                             
                         
                          
                         
                           l 
                           Debye 
                         
                       
                     
                     ) 
                   
                   2 
                 
               
             
             , 
           
         
       
     
         [0040]    where D is the nanoparticle diameter. l Debye  is the Debye/screening length (inversely proportional to the square root of the ionic strength), and α and β are adjustable parameters with β approximately equal to one for the closest square planar packing, as shown in  FIG. 2A . 
         [0041]      FIG. 2B  is a plot of particle density in N/μm 2  as a function of particle diameter varying between 10 to 100 nms. The fitted line is a best fit to equation of type described above (ρ=α/(D+β.1 Debye ) 2 ) with α=(3.11±0.03)×10 4  and β=0.44±0.02, and l Debye  is the Debye length in the colloidal solution of gold NPs (nanoparticles). 
         [0042]      FIG. 2C  is a plot of particle density in N/μm 2  as a function of particle concentration in particles/ml. Particle density follows the Langmuir type of adsorption equation ρ=a[C]/(1+b[C]) with best fit values of a=(1.4±0.1)×10 −9  and b=4.6±0.7×10 −13 . However, the inset shows that when particle density is converted to fractional area occupied by Au NPs (gold nanoparticles), it shows a remarkably constant value of 0.22±0.02 regardless of particle size or concentration. 
         [0043]    Because of the relationship between the Debye length and the ionic strength, increased ionic strength allows increased particle density, as shown in  FIG. 2D . The change in ionic strength affects the Debye length, which in turn controls the packing density, as shown in scanning electron micrographs of  FIG. 2D . Similarly, we varied the Debye length through variation of pH to assess its effect on particle density. The results are shown in  FIG. 2E . Below pH 4, nanoparticles begin to coagulate and precipitate out of the solution. Near pH 4, gold NPs show a tendency to flocculate. Above pH 4, the particle density increases with increase in ionic strength as predicted by the equation above. Particle density appears to follow the simple behavior given in  FIG. 2B  with β=0.7±0.1 and a=1.1±0.6×10 5 . The best fit line is shown in the inset of  FIG. 2E , where deposited particle density, ρ, is plotted as a function of l Debye . 
         [0044]      FIG. 2F  shows the effect of resist thickness z for both a hyperbolic model and an exponential model. In the hyperbolic model, ρ(z)=a/z, where a=2737±5. In the exponential model, ρ(z)=a exp (−bz), where a=1430±14, and b=0.181±0.002. 
         [0045]      FIG. 2G  shows the time dependence of deposition for 20 nm gold nanoparticles for both an irreversible-adsorption model and a simple-diffusion model. For irreversible adsorption, ρ(t)=ρ(∞)(1−exp(−bt)), where ρ(∞)=523±2, and b=0.0204±0.0002. For simple diffusion, ρ(t)=at 1/2 , where a=40.74±0.06, which was used to determine the particle diffusion coefficients using Fick&#39;s law of diffusion to be about 6.4×10 −8  cm 2 /sec. The plot shows that 2-3 hours of deposition time is sufficient. 
         [0046]      FIG. 3  shows the average number of particles deposited in holes. As seen, that average number has a linear dependence on the diameter of hole, regardless of the NP particle size. The fill percentage of holes follows Poisson statistics; fill percentage, f=100(1−e −&lt;N&gt; ) where N is the average number particles per hole. For 4 nm thick AATMS film, 110 nm thick PMMA resist film, and 10 nm Au NPs, the hole filling fraction is 29% for 40 nm holes, 64% for 50 nm holes, and 76% for 60 nm holes. Gold nanoparticles do not deposit in 30 nm and smaller holes in the currently developed embodiments of the invention because of surface charge neutrality, although that may change in other embodiments falling within the scope of the invention. The PMMA thickness is also relevant; for 15 nm Au NPs in 70 nm holes and a 175 nm layer of SiO 2 , the filling fraction within 48 hours is 100% for PMMA=60 nm and 95% for PMMA=85 nm. 
         [0047]      FIG. 4A  shows 20 nm gold NP in 80 nm holes patterned in a 100 nm thick PMMA resist. It shows that 93% of the holes contain one nanoparticle, that 81% of the nanoparticles are within 20 nm from the center of the holes, and that 73% of the nanoparticles are within 10 nm from the center of the holes.  FIGS. 4B and 4C  show reflow at 100° C. for 2 and 5 minutes, respectively, followed by O 2  plasma to remove the top layer of the resist to expose the gold NPs, which were etched using I 2 /KI solution, as shown in  FIG. 4D . This illustrates how any hole size fluctuations between 30 nm to 80 nm can be removed to make uniform 20 nm size holes using 20 nm size NPs. 
         [0048]      FIG. 5A  shows photographs and Fourier analyses of the particles before and after reflowing. As shown, the nanoparticles do not move during the reflowing step as reflected in identical periodicity of the Fourier transforms. This is more clearly and qualitatively illustrated in the partially resist renown sample shown in  FIG. 5B . 
         [0049]      FIG. 6  shows a different technique for deposition of the nanoparticles, namely, evaporative deposition. An initial droplet  600  of solvent containing negatively charged gold nanoparticles  112  is deposited on a positively charged pattern hole  110 . The particles  112  are far apart because of their negative charges. In step S 21 , evaporation of the solvent at 39 degrees C. commences. As the droplet  600  evaporates, the interparticle distance decreases. As the evaporation continues in step S 22 , the particles  112  are forced closer together. The repulsive force between the particles  112  increases, pushing some of the particles  112  into the hole  110 . As the evaporation continues in step S 23 , particles  112  are deposited on the positively charged surface  602  of the hole  110 . Step S 24  includes ultrasonication in deionized water and drying in a stream of nitrogen gas. One gold nanoparticle is left on the positively charged surface  602  of the hole  110 . 
         [0050]      FIG. 7  is a cross-sectional view of the holes. Because of the back-scattering of electrons, each hole has a non-uniform radius; that is, the top and bottom radii r Top  and r Bottom  differ. That difference allows more particles to be packed in the large holes; please note  FIG. 5B . However, the holes merge when Pitch length≦2r Top +0.5r Bottom , requiring lithographic optimization for smaller pitch lengths. Optimal electrostatic funneling will occur when the hole side-wall angle with respect to the substrate is 90 degrees or more when measured from the substrate side. 
         [0051]      FIGS. 8A and 8B  show holes in photoresist thin films before and after reflow. The glass transition temperature (Tg) of PMMA is 110° C. The PMMA resist reflow below Tg is made possible by further reduction in Tg, resulting from polymer chain cleavage during the exposure of the samples to the electron beam. The optimal resist reflow temperature is 100° C. The hole size before reflow is 285.50 nm; after 200 s, the hole size decreases to 113.73 nm. The rate of reflow of 91 nm PMMA film at 100° C. is 1.67±0.14 nm/s. 
         [0052]    While a preferred embodiment has been set forth above, those skilled in the art who have reviewed the present disclosure will readily appreciate that other embodiments can be realized within the scope of the invention. For example, numerical values are illustrative rather than limiting. Also, any suitable type of lithography (e.g., optical, EUV, x-ray) can be used. Moreover, materials are illustrative rather than limiting, as long as reflow and selective etching are possible. The invention can be extended to such areas as reduction of trench line-width roughness, the use of cheaper nanoparticles based on materials such as silica, the use of DUV resist and optical lithography, electrical bias/photobase generators for improved nanoparticle centering. Other variations have been noted above. Therefore, the present invention should be construed as limited only by the applied claims.