Abstract:
A manufacturing method of a wire grid polarizer includes the steps of: preparing a mold; sequentially forming a metal foil and a polymer on a substrate; molding a polymer by using the mold; etching the metal foil by using the molded polymer, and forming a wire grid pattern; and removing the polymer.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates in, general to a wire grid polarizer and a manufacturing method thereof, more particularly, to a wire grid polarizer for visible light and a manufacturing method thereof.  
         [0003]     2. Discussion of the Background Art  
         [0004]     The use of an array of parallel conducting wires to polarize specific light of radio waves dates back more than 100 years.  
         [0005]     The array of parallel conducting wires is generally called a wire grid. The wire grid, formed on a transparent substrate, is also used as a polarizer for the infrared portion of the electromagnetic spectrum.  
         [0006]     The key factor that determines the performance of a wire grid polarizer is the relationship between the wire-to-wire spacing, namely period of the parallel grid elements and the wavelength of the incident light.  
         [0007]     If the period of the wire grid is longer than the wavelength of the incident light, the wire grid functions as a diffraction grating, rather than as a polarizer, and diffracts the polarized incident light;  
         [0008]     Then, according to well-known principles, a diffraction and interference pattern is formed.  
         [0009]     However, if the period or the grid spacing is shorter than the wavelength, the wire grid functions as a polarizer that reflects electromagnetic radiation polarized parallel to the grid, and transmits radiation of the orthogonal polarization.  
         [0010]     Quality criteria for the manufacture of a wire grid polarizer beam splitter are period, line width, characteristics of grid material, substrate features (index of refraction), and wavelength and incidence angle of the incident light.  
         [0011]     Here, many studies show that the characteristics of grid material have the least effect on the performance features of the polarization beam splitter.  
         [0012]      FIG. 1  illustrates a related art wire grid.  
         [0013]     As shown in  FIG. 1 , the wire grid  100  is composed of a plurality of parallel conductive wires  110  supported by an insulating substrate  120 .  
         [0014]     The period of the conductive wire  110  is denoted as ‘Λ’, the width of the conductive wire  110  is denoted as ‘w’, and the thickness of the conductive wire is denoted as ‘t’.  
         [0015]     Based on the general definitions of the S-polarization and the P-polarization, the S polarized light has a polarization vector orthogonal to the plane of incidence and thus, it is parallel to the conductive elements.  
         [0016]     In contrast, the P polarized light has a polarization vector parallel to the incidence plane and thus, it is orthogonal to the conductive elements.  
         [0017]     If the period (or the center-to-center spacing) of the conductive wires  110  is shorter than the wavelength of the electromagnetic radiation, the wire grid reflects the polarization element (s-polarization) parallel to the conductive wires  110 , and transmits the polarization element (p-polarization) orthogonal to the conductive wires  110 .  
         [0018]     Usually, the wire grid polarizer reflects light with its electric field vector parallel to the conductive wires, and transmits light with its electric field vector perpendicular to the conductive wires. Meanwhile, the plane of incidence may or may not be perpendicular to the wires of the grid. The geometric notations used here are for information clarification.  
         [0019]     An ideal wire grid will function as a perfect mirror for one polarization of light, the S polarized light, and will be perfectly transparent for the other polarization, the P polarized light for example.  
         [0020]     In practice, however, reflective metals used as mirrors absorb some fraction of the incident light and reflect only 90-95%, and plain glass does not transmit 100% of the incident light because of surface reflections.  
         [0021]     Referring back to  FIG. 1 , the performance of the wire grid polarizer can be characterized by the polarization extinction ratio and the transmittance.  
         [0022]     Here, the polarization extinction ratio and the transmittance are expressed by the following equations. 
 
Polarization extinction ratio: (Si/St)| Pi=0  
 
Transmittance: (Pt/Pz)| Si=0  
 
         [0023]     In the equations, the polarization extinction ratio indicates the ratio of the optical power of the incidented S wave (Si) to the transmitted S wave St) when the S polarized light incidents; and the transmittance indicates the ratio of the optical power of the incidented P wave (Pt) to the incidented P wave (Pi) when the P polarized light incidents.  
         [0024]     For the wire grid polarizer to have a high polarization extinction ratio, the period of the wire grid should be much shorter than the wavelength of the incident light.  
         [0025]     So far, it has been very difficult to manufacture wire grid polarizers with a shorter period of the wire grid, so wire grid polarizers were developed only for use in the infrared or microwave regions. Primarily, this is because the period of the wire grid needs to be shortened as the wavelength of the polarized light has the short wavelength.  
         [0026]     However, with recent advances in semiconductor fabrication equipment and exposure technologies, including the fine pattern generation technology, it is now possible to produce wire grid polarizers for visible light.  
         [0027]     The visible light resides in the electromagnetic spectrum which is visible to human eyes. The visible spectrum consists of wavelengths between 400 nm to 700 nm  
         [0028]     That is, for the wire grid polarizer to have the high ERs (Extinction Ranges) for three primary colors (R, G, and B), the period of the wire grid should be at least 200 nm to obtain somewhat desired polarization characteristics. To improve the polarization performance of existing polarizers, a wire grid with its period shorter than 0.1 μm is required.  
         [0029]     The line width of a recently developed semiconductor processing is approximately 0.1 μm When drawing lines periodically, the spacing between the lines should be the same with the line width, which means that the period of the wire grid is 0.2 μm.  
         [0030]     Here, if the interference effect can be generated by using an argon laser having a short wavelength, it is possible to make the period of the wire grid as short as 200 nm.  
         [0031]     Also, if the period of the related art wire grid polarizer is reduced from 200 nm to 100 nm, the performance of the wire grid polarizer will be noticeably improved. Therefore, there is a need to develop a wire grid polarizer with a short period.  
       SUMMARY OF THE INVENTION  
       [0032]     An object of the invention is to solve at least the above problems and/or disadvantages and to provide at least the advantages described hereinafter.  
         [0033]     Accordingly, one object of the present invention is to solve the foregoing problems by providing a wire grid polarizer for visible light and a manufacturing method thereof using embossing technique, whereby wire grid polarizers can be more easily and repeatedly manufactured.  
         [0034]     Another object of the present invention is to provide a wire grid polarizer having excellent polarizing performance at the R, G, and B wavelengths in the visible spectrum  
         [0035]     The foregoing and other objects and advantages are realized by providing a manufacturing method of a wire grid polarizer, the method including the steps of: preparing a mold; sequentially forming a metal foil and a polymer on a substrate; molding a polymer by using the mold; etching the metal foil by using the molded polymer, and forming a wire grid pattern; and removing the polymer.  
         [0036]     According to another aspect of the invention, a manufacturing method of a wire grid polarizer includes the steps of: preparing a mold; coating a substrate with a polymer; forming a polymer pattern by using the mold; etching the polymer pattern and exposing part of the substrate; depositing a metal foil onto the polymer pattern and the exposed substrate; and removing the polymer pattern.  
         [0037]     Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objects and advantages of the invention may be realized and attained as particularly pointed out in the appended claims. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0038]     The invention will be described in detail with reference to the following drawings in which like reference numerals refer to like elements wherein:  
         [0039]      FIG. 1  illustrates a related art wire grid;  
         [0040]      FIG. 2  is a graph showing the relationship between the period of a wire grid and the polarization extinction ratio in the visible light band;  
         [0041]      FIGS. 3A through 3E  diagrammatically illustrate a process for producing a mold for manufacturing a wire grid according to the present invention;  
         [0042]      FIGS. 4A  to  4 H illustrate a sequence of a manufacturing process of a wire grid polarizer, according to a first embodiment of the present invention; and  
         [0043]      FIGS. 5A  to  5 G illustrate a sequence of a manufacturing process of a wire grid polarizer, according to a second embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0044]     The following detailed description will present a wire grid polarizer according to a preferred embodiment of the invention in reference to the accompanying drawings.  
         [0045]      FIG. 2  is a graph showing the relationship between the period of a wire grid and the polarization extinction ratio in the visible light band.  
         [0046]     As shown in  FIG. 2 , the polarization efficiency of the wire grid polarizer is in a close relationship with the period of the wire grid.  
         [0047]     The material of the wire grid is aluminum (Al), and the height of the wire grid is 140 mL  
         [0048]     And, the line width of the wires of the grid is 60 nm, the periods of the R, the G, and the B light are 450 nm, 550 nm, and 650 nm, respectively.  
         [0049]     To obtain the polarization extinction ratio higher than 10,000:1, the grid period should be shorter than 120 nm.  
         [0050]     Before manufacturing the wire grid polarizer using the embossing technique, a mold should be prepared first.  
         [0051]      FIGS. 3A through 3E  diagrammatically illustrate a process for producing a mold for manufacturing the wire grid according to the present invention.  
         [0052]     Preferably, the mold is made from silicon, SiO 2 , quartz glass, Ni, Pt, Cr, and polymers.  
         [0053]     The embossing technique for use in the manufacture of the wire grid is largely divided into two types: hot embossing technique that applies heat for molding polymer, and UV embossing technique that presses the mold, and solidifies the polymer by using ultra violet light.  
         [0054]     All of the above described materials can be used with the hot embossing technique. Particularly, quartz glass and transparent polymers which are transparent materials can also be used with the UV embossing technique.  
         [0055]     Referring to  FIG. 3A , a polymer layer  210  is sprayed or spin coated on a mold substrate  200 , such as silicon.  
         [0056]     Preferably, the polymer layer  210  is made from an electron beam sensitive material, PMMA (polymethylmethacryiate) for example.  
         [0057]     Multiplexing usually occurs in the electron beam sensitive part of the polymer, and using this nature, it is possible to obtain a desired pattern through electron beam irradiation and developing processes.  
         [0058]     If the polymer is a positive photosensitizer, an electron beam irradiated part melts in the developer, while if the polymer is a negative photosensitizer, the rest of the polymer except for the electron beam irradiated part melt in the developer.  
         [0059]     As shown in  FIG. 3B , after the polymer layer  210  is formed on the mold substrate  200   a , a grid pattern is formed on the polymer layer  210  through the electron beam irradiation.  
         [0060]     Next, as shown in  FIG. 3C , the mold substrate  200   a  and the polymer layer  210  are dipped in the developer to ensure the grid pattern is developed as it is.  
         [0061]     As shown in  FIG. 3D , the grid pattern is used as an etching mask and the mold substrate is dry etched or wet etched.  
         [0062]     Lastly, the polymer layer used as the etching mask is removed, and as shown in  FIG. 3E , the mold  200   b  with a desired pattern for manufacturing the wire grid is produced.  
         [0063]     Here, the surface of the mold is treated with a silane-containing chemical to facilitate the separation of the polymer and the mold.  
         [0064]     Thusly prepared mold is then used for manufacturing the wire grid polarizer operating in the visible band.  
         [0065]      FIGS. 4A  to  4 H illustrate a sequence of a manufacturing process of a wire grid polarizer, according to a first embodiment of the present invention.  
         [0066]     As described before, the wire grid polarizer is manufactured by using the pre-made mold. To this end, a transparent glass substrate  300  with both surfaces polished is first prepared (refer to  FIG. 4A ).  
         [0067]     Then, as shown in  FIG. 4B , a thin metal foil  310   a  is deposited on the glass substrate  300 .  
         [0068]     The metal foil  310   a  can be made from Al, Ag, or Cr.  
         [0069]     Later, the metal foil  310   a  is coated with a polymer  320   a , as shown in  FIG. 4C   
         [0070]     The polymer  320   a  is pressed by the mold  330 , and as a result, the pattern from the mold is printed onto the polymer  320   a.    
         [0071]     Here, if the polymer  320   a  is a thermosetting material, a metal mold is employed, and if the polymer  320   a  is a UV cure material, a transparent polymer mold is employed.  
         [0072]     In the former case where the polymer  320   a  is a thermosetting material, the hot embossing technique is used to pre-bake the polymer. In the later case where the polymer  320   a  is a UV cure material, the UV embossing technique is used, so that the coated polymer is not cured and a transparent mold is used.  
         [0073]     As shown in  FIG. 4D , by applying heat or irradiating ultraviolet light onto the mold  330 , the polymer  320   b  is cured or solidified.  
         [0074]     Afterwards, as shown in  FIG. 4E , the mold  330  is separated from the polymer  320   b.    
         [0075]     Then, the pattern from the mold  330  is printed onto the polymer  320   b , that is, the polymer has an opposite pattern to the pattern from the mold  330 .  
         [0076]     In case of using the hot embossing technique, the mold  330  has to be separated from the polymer  320   b  after the temperature of the substrate is sufficiently cooled down.  
         [0077]     In case of using the UV embossing technique, the mold  330  is separated from the polymer  320   b  after the UR curing is finished.  
         [0078]     Next, the front surface of the polymer  320   b  is dry etched to exposure the surface of the metal foil  310   a , as shown in  FIG. 4F .  
         [0079]     Since part of the polymer  320   c  is recessed by the pattern from the mold  330 , a relatively thin part of the polymer  320   c  is removed by the etching process, thereby exposing the metal foil  310   a  to the surface.  
         [0080]     Afterwards, the exposed metal foil  310   a  is dry etched or wet etched, and as a result, a wire grid pattern  310   b  is formed as shown in  FIG. 4G .  
         [0081]     Finally, as shown in  FIG. 4H  the polymer  320   c  remaining on the wire grid pattern  310   b  is removed.  
         [0082]     In this procedure, the wire grid polarizer with a desired grin pattern on the substrate  300  is manufactured.  
         [0083]      FIGS. 5A  to  5 G illustrate a sequence of a manufacturing process of a wire grid polarizer, according to a second embodiment of the present invention.  
         [0084]     As explained before, the wire grid polarizer is manufactured by using the pre-made mold. To this end, a transparent glass substrate with both surfaces polished is first prepared (refer to  FIG. 5A ).  
         [0085]     Later, as shown in  FIG. 5B , the glass substrate  400  is coated with a polymer  410   a , and the mold  430  is prepared.  
         [0086]     Then, the polymer  410   a  is pressed by the mold  430 , and as a result, the pattern from the mold  430  is printed onto the polymer  410   b , as shown in  FIG. 5G   
         [0087]     The pattern printed onto the polymer  410   b  is opposite to the pattern from the mold  430 .  
         [0088]     As shown in  FIG. 5D , by applying heat or irradiating ultraviolet light onto the mold  430 , the polymer  410   b  is cured or solidified  
         [0089]     In case of using the hot embossing technique, the mold  430  has to be separated from the polymer  410   b  after the temperature of the substrate  400  is sufficiently cooled down.  
         [0090]     In case of using the UV embossing technique, the mold  430  is separated from the polymer  410   b  after the UR curing is finished.  
         [0091]     Here, if the polymer is a thermosetting material, a metal mold is employed, and if the polymer is a UV cure material, a transparent polymer mold is employed.  
         [0092]     In the former case where the polymer is a thermosetting material, the hot embossing technique is used to pre-bake the polymer. In the later case where the polymer is a UV cure material, the UV embossing technique is used, so that the coated polymer is not cured and a transparent mold is used.  
         [0093]     Afterwards, the front surface of the polymer  41   cb  is dry etched to exposure the surface of the substrate  400 , as shown in  FIG. 5E .  
         [0094]     Since part of the polymer  410   c  is recessed by the pattern from the mold  430 , a relatively thin part of the polymer  410   c  is removed by the etching process, thereby exposing the substrate  400  to the surface.  
         [0095]     Next, a metal foil  420   a  is vacuum deposited on the glass substrate  400 , as shown in  FIG. 5F .  
         [0096]     The metal foil  420   a  can be made from Al, Ag, or Cr.  
         [0097]     Later, the polymer  410   c  with the deposited metal foil  420   a  is dipped into an etchant and is removed. At the end, a wire grid pattern  420   b  shown in  FIG. 5G  is obtained.  
         [0098]     In this procedure, the wire grid polarizer with a desired grin pattern on the substrate  400  is manufactured.  
         [0099]     In conclusion, the wire grid polarizer of the present invention is advantageous for reducing the manufacture cost in that it can be mass produced by using a mold over and over.  
         [0100]     Also, the manufacturing method of the wire grid polarizer of the present invention does not require additional equipment, and its process takes a short time, consequently increasing yield.  
         [0101]     Moreover, the wire grid polarizer has the high polarization extinction ratio at visible wavelengths, so that it can be broadly used in diverse applications such as flat displays, projection displays, optical equipment, and so on.  
         [0102]     While the invention has been shown and described with reference to certain preferred embodiments thereof, it will be understood by those skied in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.  
         [0103]     The foregoing embodiments and advantages are merely exemplary and are not to be construed as limiting the present invention. The present teaching can be readily applied to other types of apparatuses. The description of the present invention is intended to be illustrative, and not to limit the scope of the claims. Many alternatives, modifications, and variations will be apparent to those skilled in the art. In the claims, means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents but also equivalent structures.