Abstract:
A method for making a hollow-structure metal grating is provided. The method includes the following steps. First, a substrate is provided. Second, a metal layer is located on a surface of the substrate. Third, a patterned mask layer is formed on a surface of the metal layer. The patterned mask layer is made of a chemical amplified photoresist. Fourth, the surface of the metal layer exposed out of the patterned mask layer is plasma etched. Lastly, the patterned mask layer on the surface of the metal layer is dissolved.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims all benefits accruing under 35 U.S.C. §119 from China Patent Application No. 201310429906.0 filed on Sep. 22, 2013 in the China Intellectual Property Office, the disclosure of which is incorporated herein by reference. 
         [0002]    BACKGROUND 
         [0003]    1. Technical Field 
         [0004]    The disclosure relates to a method of manufacturing metal grating. 
         [0005]    2. Description of Related Art 
         [0006]    A sub-wavelength grating is a common optical component in the semiconductor industry. The size of the sub-wavelength grating is similar to or less than the active wavelength of the sub-wavelength grating. It is difficult to make a quartz grating with high density, sub-wavelength, and mark-space ratio. The sub-wavelength grating may be made by electron beam lithography, focused ion beam lithography, deep-ultraviolet lithography, holographic lithography, and nano-imprint lithography. 
         [0007]    Currently there is no method for making the sub-wavelength grating with a hollow structure made of metal. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    The parts in the drawings are not necessarily drawn to scale, the emphasis instead being placed upon clearly illustrating the principles of at least one embodiment. In the drawings, like reference numerals designate corresponding parts throughout the various diagrams, and all the diagrams are schematic. 
           [0009]      FIG. 1  is a schematic diagram showing one embodiment of a method of manufacturing a hollow-structure metal grating. 
           [0010]      FIG. 2  is a schematic diagram showing the method for making a patterned mask layer used in the method of  FIG. 1 . 
           [0011]      FIG. 3  is a schematic diagram showing the hollow-structure metal grating obtained by the method in  FIG. 1 . 
           [0012]      FIG. 4  is a cross-sectional diagram of the hollow-structure metal grating shown in  FIG. 3 . 
           [0013]      FIG. 5  is an image taken by a scanning electron microscope, of the hollow-structure metal grating by the method in  FIG. 1 . 
           [0014]      FIG. 6  is a schematic diagram showing a hollow-structure metal grating obtained by the method in  FIG. 1  in one embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    The disclosure is illustrated by way of example and not by way of limitation in the figures of the accompanying drawings in which like references indicate similar elements. It should be noted that references to “another,” “an,” or “one” embodiment in this disclosure are not necessarily to the same embodiment, and such references mean at least one. 
         [0016]    Referring to  FIG. 1 , one embodiment of a method of manufacturing a hollow-structure metal grating  30  comprises the following steps: 
         [0017]    S 10 , providing a substrate  10 ; 
         [0018]    S 20 , applying a metal layer  12  on a surface of the substrate  10 ; 
         [0019]    S 30 , forming a patterned mask layer  140  on a surface of the metal layer  12 , wherein the patterned mask layer  140  is made of a chemical amplified photoresist; 
         [0020]    S 40 , plasma etching part of the surface of the metal layer  12  exposed out of the patterned mask layer  140 ; and 
         [0021]    S 50 , dissolving the patterned mask layer  140  on the surface of the metal layer  12 . 
         [0022]    In step S 10 , the substrate  10  can be any shape such as a circular plate and a square plate. The substrate  10  can be a semiconductor substrate or a silicon substrate. The material of the substrate  10  can be gallium nitride (GaN), gallium arsenide (GaAs), sapphire, aluminum oxide, magnesium oxide, silicon, silica, silicon nitride, or silicon carbide. The silica can form a quartz substrate or a glass substrate. In one embodiment, the substrate  10  is a quartz substrate. The material of the substrate  10  can also be a P-type semiconductor or an N-type semiconductor, e.g. a P-type GaN or N-type GaN. Furthermore, the size, the thickness, and the shape of the substrate can be set as desired. The substrate  10  can be cleaned in a clean room. 
         [0023]    In step S 20 , the metal layer  12  can be formed on the surface of the substrate  10  via electron beam evaporation or ion beam sputtering. The metal layer  12  can be made of gold, silver, copper, or aluminum. The thickness of the metal layer  12  should be greater than 10 nanometers to form a very sturdy self-supporting metal layer  12 . The thickness of the metal layer  12  can be in a range from about 20 nanometers to about 200 nanometers. In one embodiment, the thickness of the metal layer  12  is about 100 nanometers. 
         [0024]    In step S 30 , the patterned mask layer  140  can be made by a chemical amplified photoresist, and comprises a plurality of first protruding structures  142  and a plurality of first cavities  144  arranged in intervals. A part of the surface of the substrate  10  can be exposed out of the patterned mask layer  140  through the first cavities  144 . The patterned mask layer  140  can be a continuous pattern or a discontinuous pattern. In one embodiment, the plurality of first protruding structures  142  are strip structures oriented along a same direction, and the first cavities  144  are defined between adjacent protruding structures  142 . In one embodiment, the plurality of first protruding structures  142  are square-shaped bars spaced with each other. A thickness of the plurality of first protruding structures  142  can be in a range from about 150 nanometers to about 420 nanometers. A width of the plurality of first protruding structures  142  can be in a range from about 20 nanometers to about 500 nanometers. A width of the first cavities  144  can be in a range from about 80 nanometers to about 500 nanometers. 
         [0025]    The chemical amplified photoresist of the patterned mask layer  140  can be ZEP520 which is developed by Zeon Corp of Japan, PMMA (Polymethylmethacrylate), PS (Polystyrene), JEP-520, SAL601, or ARZ720. The chemical amplified photoresist can be expanded in the condition of being dissolved by organic solvent. In one embodiment, the chemical amplified photoresist of the patterned mask layer  140  is ZEP520, the first protruding structures  142  are strip structures, the first protruding structures  142  and the first cavities  144  are arranged with regular intervals, the width of each first protruding structure  142  is about 100 nm, and the depth of each cavity  144  is about 40 nm. 
         [0026]    Referring to  FIG. 2 , the step S 30  comprises the sub-steps of: 
         [0027]    S 31 , forming a first resist layer  14  and then a second resist layer  16  on the surface of the metal layer  12 ; 
         [0028]    S 32 , providing a master stamp  18  with a first nanopattern  180  defined therein; 
         [0029]    S 33 , pressing the first nanopattern  180  of the master stamp  18  into the second resist layer  16  to form a second nanopattern  160 ; 
         [0030]    S 34 , etching the second resist layer  16  to expose part of the first resist layer  14  out of the second nanopattern  160 ; 
         [0031]    S 35 , etching the part of the first resist layer  14  exposed out of the second nanopattern  160  to expose part of the metal layer  12 ; and 
         [0032]    S 36 , removing a remaining material of the second resist layer  16  to obtain the patterned mask layer  140  on the surface of the metal layer  12 . 
         [0033]    In step S 31 , the first resist layer  14  is firstly formed on the surface of the metal layer  12 , and the second resist layer  16  is formed on a surface of the first resist layer  14 . The first resist layer  14  is sandwiched between the second resist layer  16  and the metal layer  12 . The first resist layer  14  can be made of the same material as the patterned mask layer  140 . In one embodiment, the first resist layer  14  is made of ZEP520 resist. The ZEP520 resist can be spin-coated on the surface of the metal layer  12  at a speed of about 500 rounds per minute to about 6000 rounds per minute, for about 0.5 minutes to about 1.5 minutes. Further, the ZEP520 resist coated on the surface of the metal layer  12  can be dried at a temperature of about 140 degrees centigrade to 180 degrees centigrade, for about 3 minutes to about 5 minutes. Thus, the first resist layer  14  is formed on the surface of the metal layer  12  located on the substrate  10 . The thickness of the first resist layer  14  can be in a range of about 150 nanometers to about 420 nanometers. 
         [0034]    In step S 31 , the second resist layer  16  can be a layer of hydrogen silsesquioxane (HSQ) or silicon on glass (SOG), which can be deposited on the first resist layer  14  through a bead coating method or a spin-coating method. In one embodiment, the HSQ can be spin-coated on the first resist layer  14  under high pressure at a speed of about 2500 rounds per minute to about 7000 rounds per minute, for about 0.5 minutes to about 2 minutes. The thickness of the second resist layer  16  can be in a range of about 100 nanometers to about 220 nanometers. 
         [0035]    In step S 32 , the master stamp  18  can be made of rigid materials, such as nickel, silica, silicon, and carbon dioxide. The master stamp  18  can also be made of flexible materials, such as PET, PMMA, polystyrene (PS), and polydimethylsiloxane (PDMS). The master stamp  18  can be fabricated through an electron beam lithography method with the first nanopattern  180  formed therein. The first nanopattern  180  can be designed according to the actual application. In one embodiment shown in  FIG. 2 , the master stamp  18  is made of silica, and the first nanopattern  180  comprises a plurality of first ribs  182  and a plurality of first grooves  184 . 
         [0036]    In step S 33 , the master stamp  18  is first placed on the second resist layer  16  with the first nanopattern  180  contacting the second resist layer  16 . The master stamp  18  is then pressed towards the second resist layer  16  at normal temperature. During this process, the first ribs  182  are pressed into the second resist layer  16 , and some material of the second resist layer  16  are pressed into the first grooves  184 . Finally, the master stamp  18  is removed from the second resist layer  16  with the first nanopattern  180  to form a second nanopattern  160  in the second resist layer  16 . The second nanopattern  160  of the second resist layer  16  comprises a plurality of second ribs  162  and a plurality of second grooves  164 . The second ribs  162  correspond to the first grooves  184 . The second grooves  164  correspond to the first ribs  182 . 
         [0037]    In one embodiment, the master stamp  18  is pressed towards the second resist layer  16  at normal temperature in a vacuum environment of about 1×10 −1  millibars to about 1×10 −5  millibars. The pressure applied on the master stamp  18  is about 2 pounds per square foot to about 100 pounds per square foot. The pressure is applied on the master stamp  18  for about 2 minutes to about 30 minutes. After step S 33 , there may be remaining material of the second resist layer  16  at the bottom of the second grooves  164 . 
         [0038]    In step S 34 , the remaining material of the second resist layer  16  at the bottom of the second grooves  164  is removed by a plasma etching method, and part of the first resist layer  14  is exposed out of the second nanopattern  160  via the second grooves  164 . 
         [0039]    In one embodiment, a CF 4  reactive plasma etching method can be used to remove the remaining material of the second resist layer  16  at the bottom of the second grooves  164 . For example, the second resist layer  16  with the second ribs  162  and the second grooves  164  formed therein can be placed in a CF 4  reactive plasma etching system. The CF 4  reactive plasma etching system generates CF 4  plasma, and the CF 4  plasma moves towards the second resist layer  16  to etch away the remaining material of the second resist layer  16  at the bottom of the second grooves  164 , to expose part of the first resist layer  14  out of the second grooves  164 . 
         [0040]    The power of the CF 4  reactive plasma etching system can be in a range of about 10 watts to about 150 watts. The speed of the CF 4  plasma can be about 2 standard cubic centimeters per minute (sccm) to about 100 sccm. The partial pressure of the CF 4  plasma can be about 1 pascal (Pa) to about 15 Pa. The etching time can be about 2 seconds to about 4 minutes. 
         [0041]    In step S 35 , part of the first resist layer  14  exposed by the second grooves  164  can be removed by oxygen plasma etching, and part of the first resist layer  14  covered by the second ribs  162  is protected by the second ribs  162  during the etching process. For example, the first resist layer  14  can be placed in an oxygen plasma etching system. The power of the oxygen plasma etching system can be in a range of about  10  watts to about 150 watts. The speed of the oxygen plasma can be about 2 sccm to about 100 sccm. The partial pressure of the oxygen plasma can be about 0.5 Pa to about 15 Pa. The etching time can be about 5 seconds to about 1 minute. During the process of etching the part of the first resist layer  14  exposed out of the second nanopattern  160 , the part of the first resist layer  14  exposed out of the second grooves  164  is removed, and the plurality of first cavities  144  is obtained in the first resist layer  14 . The part of the first resist layer  14  covered by the second ribs  162  is remained, and the plurality of first protruding structures  142  is obtained under the second ribs  162 . Each of the plurality of first protruding structures  142  corresponds to one of the second ribs  162 . The plurality of first cavities  144  and the plurality of first protruding structures  142  form the patterned mask layer  140 . 
         [0042]    In step S 36 , a remaining material of the second resist layer  16  (such as the second ribs  162 ) can be removed by a washing method via organic solvent. The organic solvent selected can only dissolve the material of the second resist layer  16 , and the material of the first resist layer  16  is maintained. The patterned mask layer  140  would not be dissolved in the washing process, and is maintained. After step S 36 , the patterned mask layer  140  is obtained on the surface of the metal layer  12 . Part of the metal layer  12  is exposed out of the patterned mask layer  140 . 
         [0043]    In step S 40 , the metal layer  12  covered with the patterned mask layer  140  can be set in an etching system, so that the part of the metal layer  12  exposed out of the patterned mask layer  140  can be removed by etching gas. 
         [0044]    Referring to  FIG. 1 , each first protrusion  142  comprises sides  1420  protruded out of the surface of the metal layer  12 , and are substantially perpendicular to the surface of the metal layer  12 . During the plasma etching process, metal particles or metal powders are produced from the part of the metal layer  12  exposed out of the first cavities  144 . The metal particles or the metal powders would deposit on the side  1420  of each first protrusion  142 , and a sub-metal layer  122  can be formed on the side  1420  of each first protrusion  142  thereby. The sub-metal layer  122  can be perpendicular to the substrate  10 , and the parts of the substrate  10  between the sub-metal layers  122  of adjacent first protruding structures  142  is exposed. 
         [0045]    In step S 40 , the etching gas can be argon (Ar) or helium (He). The power of the gas etching system can be in a range of about 50 watts to about 150 watts. The speed of the etching gas can be about 20 sccm to about 80 sccm. The partial pressure of the etching gas can be about 10 Pa to about 50 Pa. The etching time can be about 5 seconds to about 4 minutes. In one embodiment, the metal layer  12  is made of gold, the etching gas is Ar, the speed of the Ar is about 48 sccm, the partial pressure of the Ar is about 26 Pa, the power of the gas etching system is about 70 watts, and the etching time is about 50 seconds. 
         [0046]    In step S 50 , the patterned mask layer  140  is removed by dissolving in an organic solvent. The patterned mask layer  140  can be made of a chemical amplified photoresist. The chemical amplified photoresist would expand when being dissolved by organic solvent. The first protrusions  142  would expand and push the sub-metal layers  122  tipping upwards, the two sub-metal layers  122  between adjacent two first protrusions  142  would connect to each other, and a space would be defined between the connected two adjacent sub-metal layers  122  and the substrate  10 . 
         [0047]    Referring to  FIGS. 3-5 , the hollow-structure metal grating  30  comprises a substrate  10 , a plurality of connecting metal layers  36 , and a plurality of hollow metal protrusions  34  located on the substrate  10 . The plurality of connecting metal layers  36  is attached on the substrate  10 . Each of the connecting metal layers  36  connects adjacent hollow metal protrusions  34 . Each two adjacent hollow metal protrusions  34  are connected to each other by one connecting metal layer  36 . A space  342  is defined between each of the hollow metal protrusions  34  and the substrate  10 . 
         [0048]    The hollow metal protrusions  34  can be substantially equally spaced, and the metal connecting layers  36  can be substantially equally spaced. Each of the hollow metal protrusions  34  has the same size and shape. In addition, hollow metal protrusions  34  and the connecting metal layers  36  have the same extension direction. Each of the hollow metal protrusions  34  has opposite sidewalls, which are substantially perpendicular to the surface of the substrate  110 . In addition, the hollow metal protrusions  34  and the connecting metal layers  36  are integrated to form a whole structure. A width between the adjacent hollow metal protrusions  34  can be in a range from about 80 nanometers to about 500 nanometers. A width of the hollow metal protrusions  34  can be in a range from about 70 nanometers to about 400 nanometers. A thickness of the metal connecting layers can be in a range from 20 nanometers to about 200 nanometers. 
         [0049]    In one embodiment, the plurality of hollow metal protrusions  34  and the connecting metal layers  36  are strip shaped structures, and are arranged at regular intervals, the width of each hollow metal protrusion  34  is about 100 nm, and the height of the hollow metal protrusions  34  is about 40 nm. 
         [0050]    Referring to  FIG. 6 , in one embodiment, the hollow metal protrusions  34  are closed square shaped structures regularly dispersed. A space is defined in each of the hollow metal protrusions  34 . 
         [0051]    Even though numerous characteristics and advantages of certain inventive embodiments have been set out in the foregoing description, together with details of the structures and functions of the embodiments, the disclosure is illustrative only. Changes may be made in detail, especially in matters of arrangement of parts, within the principles of the present disclosure to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.