Abstract:
The invention provides a method for fabricating a nano-array comprising the following steps. A template with a plurality of nano-holes is provided. A polymer is embossed by the template to integrally form a plurality of nano-protrusions thereon, and demolding to reveal the nano-protrusions. The nano-protrusion has a concave or convex top surface.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application is a Divisional of co-pending application Ser. No. 11/509,660, filed on Aug. 25, 2006, the entire contents of which are hereby incorporated by reference and for which priority is claimed under 35 U.S.C. §120. This nonprovisional application also claims priority under 35 U.S.C. §119(a) on Patent Application No. 94146967 filed in Taiwan on Dec. 28, 2005, the entirety of which is herein incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a fabrication method of a modification nano-array, and in particular to a fabrication method of a modifiable nano-array by embossing. 
     2. Description of the Related Art 
     Regular nano-structures having a diameter of about 100 nm has special functions, such as low-reflectivity of insect&#39;s ommateum, anti-adhering effect of inset&#39;s wing, lotus effect. In present nano-technology, the method of fabricating a polymer substrate with hydrophobic surface, anti-oil surface or low-reflective surface, comprises twice surface treatments, and is complicated and expensive. Using lithography as an example, the process of fabricating a surface with regular nano-structures is difficult and expensive as the nano-structures geometries scale down to 90 nm. 
     In Acta Physiol Scand, insect&#39;s ommateum with high-sensitiveness to light at night has been observed by C. G Bernhard et al (1962). Nano-structures of insect&#39;s ommateum having a diameter of less than 250 nm exhibit super-low reflectivity in wide wavelength range of light. Moth Eye Principle has been proposed in Nature (1973) by Clapham and Hutley. In Planta, Lotus effect has been observed by W. barthlott et al. There is a plurality of nano-protrusions having a diameter of less than 50 nm on the surface of lotus to obtain a self-cleaning surface due to small contact area of the nano-protrusions. 
     Using anodic alumina oxidation (AAO) as a template has been published in Science (2002) by M. Steinhard et al. The anodic alumina oxidation with a plurality of regular nano-holes is used to be a template for forming hollow polymer nano-tubes. In US published application (20030089899), Lieber et al provide nanometer-scale articles, including nanoscale wires which can be selectively doped at various locations and at various levels. In some cases, the articles may be single crystals. The nanoscale wires can be doped, for example, differentially along their length, or radially, and either in terms of identity of dopant, concentration of dopant, or both. In US published applications (2004/0126305), Chen et al provide methods of fabricating one-dimensional composite nano-fiber on a template membrane with porous array by chemical or physical process. The whole procedures are established under a base concept of “secondary template”. First of all, tubular first nano-fibers are grown up in the pores of the template membrane. Next, by using the hollow first nano-fibers as the secondary templates, second nano-fibers are produced therein. Finally, the template membrane is removed to obtain composite nano-fibers. In US published applications (20040013873), Wendorff et al provide porous fibers comprising a polymeric material. The fibers have a diameter of 20 nm to 4000 nm and pores in the form of channels extending at least to the core of said fiber and/or through said fiber. The process for producing the porous fiber comprises electrospinning a 5 to 20% by weight solution of at least one polymer in an organic solvent using an electric field above 10.sup.5 V/m to obtain a fiber having a diameter of 20 nm to 4000 nm and pores in the form of channels extending at least to the core of said fiber and/or through said fiber. The porous fiber may be used as a carrier for a catalyst, as an adsorbent or absorbent or as a biomaterial, may be chemically modified or functionalized or may be used as a template for producing highly porous solids. In above-mentioned patents, the template with a plurality of nano-holes must be removed by etching after the formation of the nano-tubes or nano-fibers in the nano-holes. 
     In korea Patent (Pat. No.KR20030084279), Woo Lee et al provide a method of fabricating nano-structures by AAO templates with different sizes of nano-holes or by a twice anodic alumina oxidation template. In this fabricating process, polymer dissolved in an organic solvent fills the nano-holes of the AAO template, and removing the organic solvent after the nano-holes is filled with polymer. After consolidation of polymer in the nano-holes, the template must be removed by etching to reveal nano-structures formed by the nano-holes. During the etching process, the nano-structures would be deformed and lose the shape of the nano-hole. 
     In Germany Patent (Pat. No. DE10154756) Sawitowski Thomas uses an oxide coating as a template to have an embossing process for forming nano-columns. In this fabricating process, the shape of the nano-columns cannot change with different process conditions, and the nano-columns are too weak to demolding without surface treatment. 
     BRIEF SUMMARY OF THE INVENTION 
     The invention provides a modification nano-array comprising a plurality of nano-protrusions formed integrally on a substrate. The nano-protrusion has a concave or convex top surface. 
     The invention further provides a fabricating method of a nano-array. A template with a plurality of nano-holes is provided. A polymer substrate is embossed by the template, and a plurality of nano-protrusions are revealed by demolding. The nano-protrusions can be further coated with a layer of organic or inorganic coating to enhance scratch resistance, toughness and hydrophile/hydrophobicity, and to reduce the reflectivity of the nano-array or increase the affinity of the surface thereof. 
     A detailed description is given in the following with reference to the accompanying drawing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
         FIG. 1  to  FIG. 3  are cross-sectional diagrams of embossing process; 
         FIG. 4   a  and  FIG. 4   b  are cross-sectional diagrams of nano-arrays with organic or inorganic coating thereon of the invention; 
         FIG. 5  and  FIG. 6  are cross-sectional diagrams of nano-arrays formed by embossing process in different conditions; 
         FIG. 7   a  is a cross-sectional diagram of concave head nano-arrays observed by an electron microscope; 
         FIG. 7   b  is a top view of  FIG. 7   a;    
         FIG. 8   a  is a tilt diagram of convex head nano-arrays observed by an electron microscope; 
         FIG. 8   b  is a top view of  FIG. 8   a;    
         FIG. 9   a  shows the contact angle between a drop of water and a thermoplastic polymer substrate; 
         FIG. 9   b  shows the contact angle between a drop of water and a thermoplastic polymer substrate having a plurality of nano-protrusions thereon formed by embossing process of the invention; 
         FIG. 10   a  shows the gecko&#39;s effect of the nano-protrusion of the invention; 
         FIG. 10   b  shows sweat adhering to a surface having a plurality of nano-protrusions; 
         FIG. 11  shows the reflectivity of visible spectrum; 
         FIG. 12   a  shows the adhesion test result of 100 nm thick Au sputtered on a polymer substrate with and without nano-protrusions; and 
         FIG. 12   b  shows the adhesion test result of 200 nm thick Au sputtered on a polymer substrate with and without nano-protrusions. 
     
    
    
     DETAILED DESCRIPTION OF INVENTION 
     The invention provides a controllable embossing process to forming a plurality of nano-protrusions on a thermoplastic polymer substrate instead of melting polymer in an organic solvent, avoiding environmental protection problems and deformed nano-protrusions caused while removing the solvent and demolding. 
       FIG. 1  to  FIG. 3  are cross-sectional diagrams of a embossing process of the invention. As shown in  FIG. 1 , an anodic alumina oxidation template (AAO)  101  with a plurality of nano-holes  105  is fabricated by anodizing an aluminum substrate, and further anodizing again to increase the uniformity of the nano-holes for reducing the difference of diameter between each nano-protrusion formed by the anodic alumina oxidation template  101  in sequent process. The diameter of the nano-holes  105  is less than about 200 nm, preferably between about 20 nm and 150 nm. 
     As shown in  FIG. 2 , a polymer substrate  103  is softened by heating. The heating temperature depends on the material of the polymer substrate  103 . The polymer substrate  103  comprises thermoplastic polymer, thermoset polymer or UV curing polymer, preferably thermoplastic polymer such as PMMA (polymethyl Methacrylate), PC (polycarbonate), COC (cyclo-olefin copolymers), PP (polypropylene), PE (polyethylene), PVC (polyvinyl chloride), PET (polyethylene terephthalate) or PI (polymide), or thermoset polymer such as PI (polyimide) or Epoxy. As shown in  FIG. 2 , the polymer substrate  103  is extruded by the AAO template  101 . During the extrusion process, the AAO template  101  sinks into the polymer substrate  103  while the polymer substrate  103  is extruded into the nano-holes  105 . After the extrusion process, the polymer substrate  103  is cooled to consolidate, and a surfactant is introduced to separate the polymer substrate  103  and anodic alumina oxidation template  101 . As shown in  FIG. 3 , a plurality of nano-protrusions  107  is formed on the polymer substrate  103 . The diameter of the nano-protrusions  107  is between about 20 nm and 150 nm. The height of the nano-protrusions  107  is smaller than about 400 nm. The distance between adjacent nano-protrusions is less than about 50 nm. The aspect ratio of the nano-protrusions  107  is smaller than about 3, preferably smaller than about 2. 
     Still referring to  FIG. 3 , during demolding process, there is no organic solvent remaining on the polymer substrate  103 , thus avoiding environmental protection problem and deformed nano-protrusion caused by organic solvent. The embossing process by using AAO template of the invention costs less than the conventional lithography process per unit process area. The nano-protrusions  107  formed on the shallow portion of the polymer substrate  103  are revealed by demolding through the difference of affinity between the polymer substrate  103  and AAO template  101  instead of removing the template by etching. The top portion of the nano-protrusions  107  can be formed in different shapes by adhesive force on the inner wall of the nano-holes  105 , cohesion of polymer as heating and shrink by consolidation. 
       FIG. 5  and  FIG. 6  show nano-arrays formed by embossing process of the invention at different conditions. Cyclo-olefin polymer (Tg=130° C.) substrate is embossed by an AAO template having a plurality of nano-holes with a diameter of less than about 100 nm, at conditions of 150° C., vacuum less than 1 atm and pressure less than 5 bar, forming a plurality of nano-protrusions  107   a  with a convex top surface  108   a  as shown in  FIG. 5 . In addition, Cyclo-olefin polymer(Tg=130° C.) substrate is embossed by an AAO template having a plurality of nano-holes with a diameter of between about 100 nm and 200 nm, at conditions of 152° C., vacuum more than 1 atm and pressure less than 5 bar, forming a plurality of nano-protrusion  107   b  having a concave top surface  108   b . The height of the nano-protrusions  107   a  and  107   b  are less than 400 nm. The function of the nano-protrusion depends on the shape of the top surface thereof. The nano-protrusions  107  of the invention have high contact angle, hydrophobicity and Van de Waals force due to smaller contact area of the top surface thereof. In addition, the polymer substrate  103  with a plurality of nano-protrusions may be transparent and covered by a coating to reduce the reflectivity in wavelength range of visible light, thus improving the utility rate of light. 
     A layer of organic or inorganic coating  109  may be further formed conformally on the nano-protrusions  107  to enhance the strength thereof, as shown in  FIG. 4   a  and  FIG. 4   b . The thickness of the organic or inorganic coating  107  is less than about 100 nm preferably. The inorganic coating may be metal such as Zr, Ti, Cu, Ag, Au, Al, Ni, W, Fe or Pt, oxide such as SiO2, TiO2 or ITO, GaAs, InGaAs, polysilicon or amorphous silicon. The organic coating may be polysiloxane, silicon, conductive polymer, OLED (organic light emitting diode), PLED (polymer light emitting diode) or PEDOT (polyethylenethioxythiophene) to enhance the toughness of the nano-protrusions  107 . The organic or inorganic coating can reveal the strength thereof directly on the nano-protrusions  107 , just like a strengthening sugar coating. The organic or inorganic coating has the same shape with the nano-protrusions  107  by conformal formation of coatings on the nano-protrusions. 
       FIG. 7   a  is a cross-sectional diagram of nano-arrays observed by an electron microscope according to an embodiment of the invention. A transparent thermoplastic polymer substrate is embossed by an AAO template having a plurality of nano-holes with a diameter of less than about 100 nm and an adjacent distance of about 20 nm. During embossing process, the transparent thermoplastic polymer substrate is extruded to fills part of the nano-holes. After embossing process, a plurality of nano-protrusions with aspect ratio of about 2 are revealed on the transparent thermoplastic polymer substrate by demolding, as shown in  FIG. 7   a . The nano-protrusion has a concave top surface as shown in  FIG. 6 .  FIG. 7   b  is a top view of the nano-protrusions observed by an electron microscope. As shown in  FIG. 7   b , the nano-protrusions distribute uniformly. 
       FIG. 8   a  is a cross-sectional diagram of nano-arrays observed by an electron microscope according to another embodiment of the invention. A transparent thermoplastic polymer is embossed by an AAO template having a plurality of nano-holes with a diameter of about 100 nm and an adjacent distance of about 50 nm. After embossing process, a plurality of nano-protrusions with aspect ratio about 3 are revealed on the transparent thermoplastic polymer substrate by demolding, as shown in  FIG. 8   a . The nano-protrusion has a convex top surface as shown in  FIG. 5 .  FIG. 8   b  is a top view of the nano-protrusions. As shown in  FIG. 8   b , the nano-protrusions distribute uniformly. 
       FIG. 9   a  shows the contact angle between a drop of water and a general thermoplastic polymer substrate. The contact angle therebetween in  FIG. 9   a  is about 90 degrees.  FIG. 9   b  shows the contact angle between a drop of water and a thermoplastic polymer substrate having a plurality of nano-protrusions thereon formed by embossing process of the invention. The contact angle therebetween in  FIG. 9   b  is over 140 degrees. Accordingly, the nano-arrays formed by a template with a plurality of nano-holes exhibit superhydrophobicity as lotus effect. 
       FIG. 10   a  shows the gecko&#39;s effect of nano-arrays of the invention. As shown in  FIG. 10   a , a drop of water having a volume of less than about 10 μL, are locked on a substrate having a plurality of nano-protrusions thereon.  FIG. 10   b  shows drops of water adhering to a substrate having a plurality of nano-protrusions thereon. 
       FIG. 11  shows a transparent substrate comprising region A with nano-protrusions of the invention thereon and region B having no nano-protrusions of the invention. As shown in  FIG. 11 , the reflective brightness of light in region A is lower than in region B. The transparent substrate in  FIG. 11  may be PC (Polycarbonate) or COC (Cyclo Olefin Copolymers). For PC, region A has a reflectivity of about 2-3 between visible light wavelength of about 400-700 nm. For COC, region A has a reflectivity of about 1-2 between visible light wavelength of about 400-700 nm. The reflectivity of region A for different substrate materials is similar. It is known that the nano-array of the invention can reduce glare and reflection without inducing color-shift. 
     Experimental study indicates that the nano-protrusions substantially increase the adhesion between a polymer substrate and an overlying coating.  FIGS. 12   a  and  12   b  respectively show the adhesion test results of 100 nm and 200 nm-thick Au coatings on PC polymer substrates. The adhesion test was carried out using crosshatch adhesion test, where a grid of 100 squares was cut into the coated substrate, and a 3M tape was applied over the grid, and then rapidly peeled away. The number of squares remaining on the substrate gives a relative percentage value of adhesion. As shown in  FIGS. 12   a  and  12   b , regions C with nano-protrusions passed the adhesion test, while regions D without nano-protrusions failed. Note that the nano-protrusions can be used to improve adhesion of any organic or inorganic coatings, such as Si, Au; Cu, or the like. Further, polymer substrates other than the PC substrate (COC substrates, for example) may be used to practice the invention. 
     Table 1 shows transmission of a thermoplastic polymer substrate having a plurality of nano-protrusions thereon formed by embossing process of the invention. The reflectivity of general transparent thermoplastic polymer substrate, such as plastic, measured by Hazemeter is under 3.5%. As shown in Table 1, nano-protrusions formed by embossing process of the invention can increase the reflectivity of the general transparent polymer substrate to 94%. 
     
       
         
               
               
             
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 transmission 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 conventional thermoplastic polymer 
                 92.51 
               
               
                   
                 thermoplastic polymer substrate having 
                 94.18 
               
               
                   
                 a plurality of nano-protrusions 
               
               
                   
                 thermoplastic polymer substrate having 
                 93.65 
               
               
                   
                 a plurality of nano-protrusions 
               
               
                   
                   
               
             
          
         
       
     
     Finally, while the invention has been described by way of example and in terms of preferred embodiment, it is to be understood that the invention is not limited thereto. On the contrary, it is intended to cover various modifications and similar arrangements as would be apparent to those skilled in the art. Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.