Abstract:
A method of replicating a nanoscale pattern which comprises forming the pattern on the outer surface of a cylindrical roller, providing a surface upon which the pattern is to be replicated, and transferring the nanoscale pattern from the cylindrical roller onto the surface to provide at least one replication of the pattern on that surface. The roller is adapted to carry the pattern on its outer surface and transfer the pattern to a substrate. The ultimate product may be a grating polarizer.

Description:
This Application claims benefit of Provisional No. 60/108,721 filed Nov. 17, 1998. 
    
    
     FIELD OF THE INVENTION 
     Replicating a nanoscale pattern onto a substrate from a mold, and a polarizer for UV and visible light thus produced. 
     BACKGROUND OF THE INVENTION 
     It is common practice to produce a pattern composed of depressions and/or protrusions in a mold, for example, a metal mold. The mold is then used in accordance with pressing or printing technology to reproduce the mold pattern. However, the resolution achieved with such procedure is generally limited to several microns. 
     Accordingly, lithography technology has been resorted to in forming patterns involving sub-micron dimensions. Such processes have been used in the production of integrated circuits and other semiconductor devices. However, the process tends to be complex, and to require expensive equipment. These problems are exacerbated where patterns involving nanometer dimensions are to be produced. 
     This led to the development of nanoimprint lithography for fabricating imprint patterns having nanometer dimensions. Such procedures involve producing a nanometer-scale pattern in a mold. The mold may be a rigid material, such as a metal or a dielectric. The pattern on the mold may be produced by electron beam, or x-ray lithography. The desired pattern, or a negative depending on the procedure, is then created by etching. 
     The procedure works well on articles having small surfaces upon which an individual, printing procedure can be employed. However, it is often desirable to print a larger, and/or repetitive, pattern on a large surface with precision. This may be a single article, such as a display panel, or a large sheet to be subdivided into a plurality of smaller articles. One area of potential application is polarizers, particularly grating polarizers for UV and visible light. Another application is production of structures used in biochemical, analysis procedures. 
     It is a basic purpose of the present invention to provide an improved method of producing a nanometer-scale pattern by mold imprinting. 
     Another purpose is to provide a method of producing a precision, nanometer-scale pattern on a large surface, such as a display panel. 
     A further purpose is to produce a plurality of identical, nanometer-scale patterns on a large surface. 
     A still further purpose is to provide a simple, inexpensive, rolling device for producing a nanometer-scale pattern by mold imprinting. 
     A particular purpose is to provide an improved grating polarizer for UV or visible light. 
     SUMMARY OF THE INVENTION 
     The invention resides, in part, in a method of replicating a nanoscale pattern which comprises forming the nanoscale pattern on the surface of a cylindrical roller, providing a substrate surface upon which the pattern is to be replicated, and transferring the nanoscale pattern onto the substrate surface to provide replication(s). 
     The invention further resides in a device for replicating a nanoscale pattern on a substrate, the device comprising a cylindrical roller carrying the nanoscale pattern on its cylindrical surface. 
     The invention also resides in a grating polarizer comprising a substrate having elongated, parallel lines of metal in a nanoscale pattern that are transferred to the substrate from a cylindrical roller. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanying drawings, 
     FIG. 1 is a perspective view of a grating polarizer produced in accordance with the invention. 
     FIGS. 2A-2E illustrate, step-wise, a method of replicating a nanoscale pattern to produce the polarizer of FIG.  1 . 
     FIGS. 3A-3F,  4 A- 4 F,  5 A- 5 D and  6 A- 6 D illustrate, each in a series of steps, alternative, replication methods employing a cylindrical member such as a roller or drum. 
    
    
     DESCRIPTION OF THE INVENTION 
     The present invention represents the culmination of studies designed to produce an improved, grating-type polarizer. More particularly, they were designed to meet a need for a large-area polarizer for UV and visible light. While not so limited, an immediate application was a visible light polarizer of a size to match a panel in an LCD device used in television projection. 
     A high energy load is placed on such a panel by the intense light source employed for such projection. This dictated that the polarizer embody a transparent, inorganic material, in particular, glass. The polarizer developed to meet this need was a grating-type polarizer, also known as a wire grid polarizer. Various materials and processes have been proposed to produce such polarizers. However, they have suffered from high, production costs. They have also been limited in properties, such as the width of the wavelength band over which the polarizer is effective. 
     FIG. 1 is a perspective view of a typical grating polarizer designated by the numeral  10 . Polarizer  10  comprises a substrate  12  produced from a suitable glass. Formed on substrate  12  are elongated, parallel, reflective metal lines  14 . A particularly suited metal for lines  14  is aluminum. 
     There are several mathematical models and mathematical expressions that can be employed to determine polarizer performance with respect to transmission of the parallel and perpendicular electric fields of incident light. The mathematical expressions utilize the variables of the period (width and spacing) of the metallic lines, the refractive index of the transparent substrate, and the light wavelengths of interest. By using the mathematical expressions, one can design a polarizer to give adequate, polarizer performance over certain, light wavebands. 
     Present studies were directed to a polarizer with good contrast ratio and good transmission in the visible light waveband of 400 to 700 nm. Calculations were made using specific, mathematical expressions to determine the period of the metallic lines necessary for such a polarizer. The influence of line period on the contrast ratio of the polarizer was determined at the wavelengths of 400 nm and 700 nm with a substrate index of 1.52. High contrast ratios are obtained at shorter line periods. For example, with a 50 nm line period, the polarizer has a theoretical contrast ratio of 83:1 at the 400 nm wavelength. At the 700 nm wavelength, the theoretical contrast ratio is 256:1. A comparison of polarizer transmission versus period indicated that the highest transmission was obtained also at shorter line periods. The theoretical transmission coefficients at the 400 and 700 nm wavelengths are 0.954 and 0.966, respectively. 
     These contrast ratios are not very high for a single polarizer unit. However, by placing two like polarizers in series, the contrast ratios are multiplied, and theoretical contrast ratios at 400 and 700 nm would be 6889:1 and 65,536:1, respectively. The theoretical transmission coefficient of two polarizers in series would be ˜0.912. A visible light polarizer with such performance characteristics would be considered very good. This performance can be improved by either using a glass with a lower index of refraction, or by coating the glass with a lower index coating (e.g., magnesium fluoride) prior to placing metal lines on the substrate. 
     The challenge in fabricating a grating polarizer with a period of 50 nm is the creation of metallic wires, or “lines,” and spacings between the lines, with nanometer scale dimensions. For a polarizer with a 50 nm period, the line width and the spacing would each be 25 nm. Conventional, photolithography techniques are currently limited to making features as small as 500 nm. Electron beam lithography is capable of creating features in the tens of nanometers range. However, this method is quite expensive and very slow. 
     A new method of creating nanometer-scale features has been developed that is purportedly capable of creating features as small as 10 nm. This method is called imprint lithography. The imprint lithograph technique utilizes a mold with a designed, nanometer-scale, patterned structure. This mold can be made of metal or dielectrics. The pattern structure on the mold can be produced by electron beam lithography, or x-ray lithography, with etching to create a series of parallel lines or channels. 
     A mold having a nanoscale pattern produced in this manner, in combination with a deformable material, can be employed in various ways to replicate the nanoscale pattern of the mold. The deformable material may be an organic polymer applied as a coating or film, either to a flat substrate, or to a pattern mold. 
     FIG. 2 is a step-wise, schematic showing of a typical proceeding for replicating a nanoscale pattern in metal from a mold. 
     As shown in step one of FIG. 2, a flat glass substrate  20  is first coated with a polymer, thin film  21  by either spin coating or dip coating. A nanoscale mold  22 , prepared as described earlier, is then pressed onto the polymer, thin film  21  to create a patterned polymer  23  having a thickness contrast as shown in step  2 . During molding, heating may be required to assist the flow of polymer  21  to completely fill the channels of mold  22 . Polymer film  21  is preferably a thermoplastic material that has a low, glass transition temperature. Mold  22  can first have a low, surface energy coating (not shown) deposited on it to improve its release property. 
     After mold  22  is released from the surface of the patterned polymer  23 , that surface is subjected to anisotropic, ion etching, as shown in step  3 . The glass substrate at the troughs of the polymer pattern  23  is exposed by etching, since the polymer coating is much thinner in these areas. This produces spaced polymer grids  24 . 
     After etching, aluminum metal is deposited in the spacing channels between polymer grids  24  by sputtering, or evaporative deposition, as shown in step  4 . A thin layer of aluminum may be deposited to some extent atop the polymer grids. This is removed by the procedure commonly referred to as liftoff. Final step  5  involves the lift-off of aluminum metal and removal of polymer grids  24 . Polymer grids  24  are removed by using a chemical solvent. This leaves parallel, aluminum metal lines  26  with proper width and separation on glass substrate  20 . 
     The method illustrated in FIG. 2 is effective for small-area polarizers where development of a single pattern suffices. However, it is less effective for larger areas where the pattern must be repeated. Repetitive processing can be time consuming. Also, if accurate registry is required over a large area, the process may be less than satisfactory. 
     To avoid this situation, the present invention has devised a replicating procedure based on use of a patterned, cylindrical roller, or drum. 
     FIG. 3 is a schematic, step-wise illustration of a replicating procedure utilizing a cylindrical printing drum generally designated  30 . The surface of printing drum  30  is first coated with a release coating  31 . Then, a thin layer of polymer resist  32  is applied over release coating  31  as shown in step  1 . Drum  30  is then pressed onto, and rolled across, a pattern mold  33  to create a thickness contrast pattern  34  in polymer resist  32  as shown in step  2  of FIG.  3 . The thickness contrast pattern  34  is created in polymer film  32  on printing drum  30 . The patterned layer of polymer film  32  is later transferred to a glass substrate  35  as shown in step  3 . With a proper design of surface energies of the release coatings of both mold  33  and printing drum  30 , the transfer of the patterned, polymer film  32  between the different surfaces can be easily achieved. 
     Anisotropic, ion etching is then used to remove a thin section of the polymer film  32  to create a polymer, grid structure  36  on the glass substrate  35  as shown in step  4 . Aluminum metal  37  is then deposited in sufficient amount to fill the intervening channels  38  as shown in step  5 . The polymer grid structure  36 , and any aluminum atop that structure, are then removed as described above with reference to step  5  of FIG.  2 . This leaves parallel, aluminum lines  39  of proper width and separation on the surface of substrate  35  as shown in step  6 . 
     FIG. 4 is a schematic, step-wise illustration of an alternative, replicating procedure utilizing a cylindrical printing drum  40 . This procedure involves applying a layer of UV-curable polymer  41  to a rigid, glass mold  42  as shown in step  1  to pattern the polymer. Drum  40  is then pressed onto, and rolled across, mold  42  as shown in step  2 . At the same time, UV light  43  is applied from the back side  44  of the mold  42  to cure and solidify the patterned polymer  41 . As the surface of the printing drum  40  separates from the mold  42 , the embossed, polymer pattern  41  is transferred onto the printing drum  40  to effectively create a mold  45  on the exterior of drum  40 . This patterned mold polymer  45  can be used directly, or modified by depositing a release coating on it. Following this, a glass substrate  46  is coated with a polymer. The printing drum  40  with the embossed, polymer mold  45  is then pressed onto, and rolled across, it to create a patterned, polymer layer  47  on the glass substrate  46  as shown in step  3 . Patterned, polymer layer  47  is then etched sufficiently to remove the thin, polymer sections in the pattern channels, leaving a polymer grid  48  as shown in step  4 . Aluminum is now deposited as described with respect to FIG.  3  and shown in step  5 . Any aluminum atop the polymer, and the polymer, are then removed, as described above. This leaves parallel, aluminum lines  49  on glass substrate  45  as shown in step  6 . 
     Another, alternative procedure for replicating a nanopattern, that utilizes a cylindrical printing roll, is illustrated in the step-wise, schematic illustration of FIG.  5 . 
     This procedure involves using patterned, printing drum  40 , 45 , created as described in FIG.  4 . First, a thin layer of polymer resist  50  (with thickness in the order of 100 nm) is applied to a glass substrate  51 . Drum  40 , 45  is then brought in contact with, and rolled across, the polymer resist. The raised tips on drum  40 , 45  will pick up part of the polymer resist  50 , thereby creating a polymer pattern  52 . Drum  40 , 45  is then rolled across a second glass substrate  53  to transfer the polymer pattern  52  onto the glass substrate  53  as shown in step  2 . Aluminum metal is then deposited, as described above, in sufficient amount to cover polymer pattern  52  and fill the intervening channels as shown in step  3 . The aluminum atop polymer pattern  52 , and the polymer pattern, are then removed by lift-off and chemically, respectively, as described previously. This leaves parallel, spaced lines  54  as shown in step  4 . 
     A further procedure for replicating a nanopattern, utilizing a cylindrical printing roll, is illustrated in the step-wise, schematic illustration of FIG.  6 . 
     This procedure involves using the patterned printing drum  40 , 45  created as described above. A glass substrate  60  is first coated with a thin layer of aluminum  61  as shown in step  1 . A thin layer of alkanethiolates (HS(CH 2 ) n R, or fluorosurfactants  62 , such as Zonyl RP (Dupont Product), is applied to the pattern  45  on printing drum  40 . The printing drum  40  is pressed onto aluminum layer  61  to transfer the organic coating pattern  62  to the aluminum surface as shown in step  2 . The surface, thus pattern-coated, is then exposed to a reactive ion, chlorine, etching gas to etch away aluminum that is not protected by the organic coating. This exposes the underneath, glass substrate  60 . The aluminum, protected by the organic coating, is preserved to form a metal grid structure coated with organic, as shown in step  3 . Final step  4  involves the use of oxygen plasma to remove the organic coating. This leaves parallel, spaced, aluminum lines  64  on the surface of glass substrate  60 .