Abstract:
A method for preparing a photonic crystal slab waveguides is disclosed, wherein the photonic crystal slab waveguides are prepared by combining near-field phase-shifting contact lithography (NFPSCL) with interference lithography (IL). Conventional methods used for preparing the photonic crystal slab waveguides, such as electron beam lithography or direct laser writing, are time consuming. In contrast, the present method allows rapid production of many photonic crystal slab waveguides over a large area composed of microstructures.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a method for preparing photonic crystal slab waveguides and, more particularly, to a method for preparing photonic crystal slab waveguides for light transport. 
         [0003]    2. Description of Related Art 
         [0004]    Photonic crystals are composed of periodic dielectric structures, and the forms of the photonic crystals can be divided into 1-dimensional, 2-dimensional, and 3-dimensional structures. The photonic crystals can affect the light propagation through the periodic dielectric structures depending on the wavelength of the light. Besides, the photonic crystals are used in light transport, so the periodicity of photonic crystal structure has to be similar to the operating wavelength of light. 
         [0005]    Currently, progress in the semiconductor processing has enabled the photonic crystals to be realized. In addition, the photonic crystals can be applied widely in the fields of optical communication and optical light sources due to their unique characteristics. The 2-dimensional photonic crystals have been employed in waveguide elements, beam splitters, and Mach-Zehnder interferometers. The techniques have been used for manufacturing 2-dimensional photonic crystals including lithography and etching techniques, which are usually used in conventionally semiconductor processing. 
         [0006]    The 2-dimensional photonic crystal comprising the waveguide elements is called photonic crystal slab waveguides. The defect structures incorporated into the photonic crystal can confine and guide light along the defect structures.  FIG. 1  shows conventional photonic crystal slab waveguides. Photonic crystals  112  are formed on the substrate  11 , and the defect structures in the photonic crystals  112  are called waveguides  111 . The loss of photons is extremely low during light transport in the photonic crystal slab waveguides. Particularly, light can transport in the waveguides with right angles (i.e. 90°). Hence, the photonic crystal slab waveguides are suitable for connecting micro opto-electronic elements in advanced integrated circuits (ICs). 
         [0007]    Several fabrication methods for photonic crystal slab waveguide are being developed currently. For example, interference lithography (IL) is combined with electron beam lithography, focused ion beam (FIB), direct laser writing, and photolithography technique to fabricate photonic crystals containing defects. However, either electron beam or ion beam lithography for defining the waveguides in photonic crystals is very time consuming because of inherent writing speed of each is too slow especially when the waveguides are long. Furthermore, the waveguides can be defined rapidly in photonic crystals by the direct laser writing or the photolithography technique, but light diffraction may cause additional problems for waveguide patterning, limiting the line widths of waveguides. Hence, it is difficult for the aforementioned methods to fabricate the photonic crystal slab waveguide over a large area composed of photonic crystal micro structures. 
         [0008]    Therefore, it is desirable to provide a simple method to fabricate photonic crystal slab waveguide having different periods and line widths rapidly and economically. It is also desirable to form photonic crystal slab waveguide with different photonic crystal pattern and waveguide pattern, which can be applied in various kinds of communication elements, sensor elements, and opto-electronic elements. 
       SUMMARY OF THE INVENTION 
       [0009]    The object of the present invention is to provide a method for preparing photonic crystal slab waveguides, which combines Near-Field Phase-Shifting Contact Lithography (NFPSCL) and Interference Lithography (IL), to fabricate photonic crystal slab waveguides over a large area rapidly. 
         [0010]    To achieve the object, the method for preparing photonic crystal slab waveguides of the present invention includes the following steps: (A) providing a phase-shift mask on a base; (B) applying a light source over the phase-shift mask to form an etching mask having a waveguide pattern on a surface of the base, and removing the phase-shift mask; (C) applying at least two coherent light beams with predetermined configurations on the surface of the base to form an etching mask having a photonic crystal pattern on the surface of the base; and (D) etching the base to form a photonic crystal with at least one waveguide on the base, and removing the etching mask. 
         [0011]    In the process for preparing waveguides, the waveguide pattern can be adjusted through the phase-shift mask of the present invention. Besides, through adjustment in exposure doses of the light source, the waveguides with different line widths can be prepared by the same phase-shift mask. Furthermore, using two coherent light beams multiple times can form interference fringes, wherein predetermined angles can be formed between the interference fringes to prepare photonic crystal with good periodicity. Plural coherent light beams with the same incident angle can also perform interference lithography by different azimuth angles at the same time to prepare photonic crystal with good period. Hence, it is possible to fabricate photonic crystal slab waveguides rapidly and economically by the method for preparing photonic crystal slab waveguides of the present invention. 
         [0012]    In addition, in the method for preparing photonic crystal slab waveguides of the present invention, the base comprises: a substrate, an etch-mask layer covering on the substrate, and a photoresist layer covering on the etch-mask layer, wherein the etch-mask layer is disposed between the photoresist layer and the substrate. Besides, the material of the substrate is not limited. Preferably, the material of the substrate is Si, or silicon on insulator (SOI). 
         [0013]    In one aspect of the present invention, the step (C) of the method for preparing photonic crystal slab waveguides may be a step (C1): projecting an interference pattern on the photoresist layer with two coherent light beams; rotating the base with a rotation angle; projecting another interference pattern on the photoresist layer to form the photonic crystal pattern; and forming the etching mask having the photonic crystal pattern on the surface of the base. 
         [0014]    Furthermore, in another aspect of the present, the step (C) of the method for preparing photonic crystal slab waveguides may be a step (C2): projecting multiple interference patterns on the photoresist layer by using more than two coherent light beams with different azimuth angles at the same time to form the photonic crystal pattern, and forming the etching mask having the photonic crystal pattern on the surface of the base. 
         [0015]    In the method for preparing photonic crystal slab waveguides of the present invention, the waveguide pattern, which is formed on the photoresist layer by the phase-shift mask, is transferred to the etch-mask layer by removing the etch-mask layer. 
         [0016]    In the method for preparing photonic crystal slab waveguides of the present invention, the photonic crystal pattern on the photoresist layer is transferred to the etch-mask layer by removing the etch-mask layer. 
         [0017]    Particularly, in the method for preparing photonic crystal slab waveguides of the present invention, step (C) for forming the photonic crystal pattern may be performed before the step (B) for forming the waveguide pattern. 
         [0018]    In the method for preparing photonic crystal slab waveguides of the present invention, the light source is unlimited. Preferably, the light source is near-UV light. 
         [0019]    Preferably, in the method for preparing photonic crystal slab waveguides of the present invention, the material used in the etching mask is Sn, Ag, Cu, Au, Cr, Ti, Zn, Ni, Cu—Cr alloy, or Zn—Pb alloy. More preferably, the material used in the etching mask is Cr. 
         [0020]    In the method for preparing photonic crystal slab waveguides of the present invention, the material used in the phase-shift mask is a light-transmitting material, which may include a light-transmitting and rigid material, or a light-transmitting and elastomeric material. Preferably, the material used in the phase-shift mask is silicon dioxide (SiO 2 ), or polydimethylsiloxane (PDMS). Furthermore, the phase-shift mask may be a bulk having a relief pattern, and a relief depth on the surface of the bulk. 
         [0021]    In the method for preparing photonic crystal slab waveguides of the present invention, the material used in the photoresist layer is unlimited. Preferably, the material used in the photoresist layer is positive photoresist. 
         [0022]    In the method for preparing photonic crystal slab waveguides of the present invention, the line width of the waveguides may be 100 nm-1.3 μm. Besides, the line width of the waveguides may comprise one fold, or multiple folds of photonic crystal periods. 
         [0023]    Moreover, in an aspect of the present invention, plural coherent light beams at different azimuth directions may generate 2-dimensional photonic crystals. Two of the plural coherent light beams with the same incident angle generate first interference fringes, wherein the difference in the azimuth angles of the two coherence light beams is 180°. Another two of the plural coherent light beams at specific azimuth directions generate second interference fringes, wherein predetermined angles between the first interference fringes and the second interference fringes are the same as the difference angles between the azimuth directions of the coherent light beams which generate the first interference fringes and the second interference fringes. Besides, third interference fringes can be generated by the same method for generating the first interference fringes and the second interference fringes, wherein the azimuth directions for generating the third interference fringes are different from the azimuth directions for generating the first interference fringes and the second interference fringes. Hence, by using multiple interferences with different azimuth angles, 2-dimensional photonic crystals with different patterns can be formed. 
         [0024]    Preferably, the line widths of each of the first interference fringes, the second interference fringes, or the third interference fringes are the same. Besides, preferably, the gaps between the adjacent first interference fringes, the adjacent second interference fringes, and the adjacent third interference fringes are the same. Furthermore, if four coherent light beams generate the first interference fringes and the second interference fringes at the same time, the predetermined angles between the first interference fringes and the second interference fringes are about 90°, preferably. If six coherent light beams generate the first interference fringes, the second interference fringes, and the third interference fringes at the same time, the predetermined angles between the first interference fringes, the second interference fringes, and the third interference fringes are about 60°. 
         [0025]    Other objects, advantages, and novel features of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]      FIG. 1  is a perspective view of a conventional photonic crystal slab waveguide; 
           [0027]      FIG. 2A  to  FIG. 2E  are cross-sectional views showing a process for preparing a phase-shift mask of the present invention; 
           [0028]      FIG. 3A  to  FIG. 3F  are cross-sectional views showing a process for preparing another phase-shift mask of the present invention; 
           [0029]      FIG. 4A  to  FIG. 4H  are cross-sectional views showing a process for preparing a photonic crystal of the embodiment 1 of the present invention; 
           [0030]      FIG. 5  is a top view of a photonic crystal slab waveguides of the embodiment 1 of the present invention; 
           [0031]      FIG. 6A  to  FIG. 6F  are cross-sectional views showing a process for preparing a photonic crystal of the embodiment 2 of the present invention; 
           [0032]      FIG. 7  is a top view of a photonic crystal of the embodiment 2 of the present invention; and 
           [0033]      FIG. 8A  to  FIG. 8F  are perspective views of showing a process for preparing a photonic crystal of the embodiment 3 of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     1-1 A Method for Preparing a Phase-Shift Mask 
       [0034]      FIG. 2A  to  FIG. 2E  are cross-sectional views showing a process for preparing a phase-shift mask of the present invention. First, with reference to  FIG. 2A , a substrate  21  is provided, and then a photoresist film  22  is coated on the substrate  21 . The general method for coating photoresist film  22  is dip coating, roll coating, printing, laminating, or spin coating. Here, the photoresist  22  is coated on the substrate  21  by spin coating. One kind of positive photoresist, which can be used for preparing a phase-shift, is EPG510. However, the ultimate thickness of the photoresist film  22  is 700 nm with 6000 rpm spinning speed, due to the viscosity of EPG510. Hence, the material used for forming the photoresist film  22  is a positive photoresist comprising EPG510 and EPT10, wherein the weight ratio between EPG510 and EPT10 is 5:1. The thickness of the photoresist film  22 , which is prepared with the mix of EPG510 and EPT10, is 500 nm by the spinning speed 6000 rpm. 
         [0035]    With reference to  FIG. 2B , a mask  23  is placed in contact with the photoresist film  22 . Then, the photoresist film  22  is patterned by conventional photolithography. After removing the mask  23 , the photoresist film  22  is treated with a post-exposure-bake (PEB) to harden the photoresist film  22  and make the pattern formed on the photoresist film  22  clearer. After development processing, a photoresist pattern  221  is formed, as shown in  FIG. 2C . 
         [0036]    With reference to  FIG. 2D , a prepolymer of PDMS is cast on the photoresist pattern  221 . After curing the PDMS, the photoresist pattern  221  is transferred to a polymer of PDMS. As shown in  FIG. 2E , an elastomeric phase-shift mask  221  is formed. 
         [0037]    The PDMS used for the phase-shift mask is a light-transmitting and elastomeric material. 
       1-2 A Method for Preparing a Phase-Shift Mask 
       [0038]    Here, another method for preparing a phase-shift mask is provided, wherein the material used for the phase-shift mask of the present invention can further be a light-transmitting and rigid material. 
         [0039]    With reference to  FIG. 3A , a substrate  21  is provided, and then a photoresist film  22  is coated on the substrate  21 . The material of the substrate  21  can be any kind of light-transmitting materials, such as glass or quartz. The material of the substrate  21  used herein is glass. With reference to  FIG. 3B , a mask  23  is placed in contact with the photoresist film  22 . Then, the photoresist film  22  is patterned by conventional photolithography. After removing the mask  23 , the photoresist film  22  is treated with PEB to harden the photoresist film  22  and make the pattern formed on the photoresist film  22  clearer. After development processing, a photoresist pattern  221  is formed, as shown in  FIG. 3C . 
         [0040]    With reference to  FIG. 3D , after the photoresist pattern  221  is obtained, a metal mask  25  can be plated on the photoresist pattern  221  directly, due to the material of the substrate is a light-transmitting material. After removing the photoresist pattern  221  (as shown in  FIG. 3E ), the substrate  21  is etched by anisotropic etching. With reference to  FIG. 3F , a rigid phase-shift mask is formed after removing the metal mask  25 . 
       Embodiment 1  
       [0041]    The method for preparing photonic crystal slab waveguides of the present embodiment is described with reference to  FIG. 4A  to  FIG. 4H , wherein  FIG. 4A  to  FIG. 4H  are cross-sectional views showing a process for preparing a photonic crystal slab waveguides of the present embodiment. 
         [0042]    The method for preparing photonic crystal slab waveguides of the present embodiment comprises the following steps: 
         [0043]    First, with reference to  FIG. 4A , a substrate  41  is provided, wherein the material of the substrate  41  may be Si or SOI. In the present embodiment, the material of the substrate  41  is SOI. 
         [0044]    A first metal layer  42  is deposited on the substrate  41  by e-gun evaporation (as shown in  FIG. 4A ), wherein the material of the first metal layer  42  may be Sn, Ag, Cu, Au, Cr, Ti, Zn, Ni, Cu—Cr alloy, Sn—Pb alloy. In the present embodiment, the material of the first metal layer  42  is Cr. 
         [0045]    Then, a first photoresist layer  43  is coated on the first metal layer  42  by spinning coating, and the first metal layer  42  is disposed between the substrate  41  and the first photoresist layer  43  (as shown in  FIG. 4A ). In the present embodiment, the material of the first photoresist layer  43  is a positive photoresist. 
         [0046]    A phase-shift mask  44  is provided and placed on the first photoresist layer  43 , so that the first photoresist layer  43  is disposed between the phase-shift mask  44  and the first metal layer  42  (as shown in  FIG. 4A ). In the present embodiment, the material used for the phase-shift mask  44  is PDMS. 
         [0047]    A light source is applied over the phase-shift mask  44  to expose and pattern the first photoresist layer  43  (as shown in  FIG. 4B ). In the present embodiment, the light source is UV radiation. 
         [0048]    After removing the phase-shift mask and etching the first metal layer  42 , the pattern (not shown in the figure) is transferred to the first metal layer  42 , and a waveguide pattern  421  is formed in the first metal layer  42  (as shown in  FIG. 4C ). 
         [0049]    After forming the waveguide pattern  421 , the first photoresist layer  43  is removed. Then a second photoresist layer  46  is coated on the first metal layer  42  having the waveguide pattern  421  by spinning coating (as shown in  FIG. 4D ). 
         [0050]    In the present embodiment, Ar +  laser is used for IL. Plural first interference fringes (not shown in the figure) are formed on the second photoresist layer  46  by coherent light beams. Furthermore, in the present embodiment, the line widths of each first interference fringes are the same, and the gaps between the adjacent first interference fringes are the same. 
         [0051]    Plural second interference fringes (not shown in the figure) are formed on the second photoresist layer  46  by Ar +  laser to pattern the second photoresist layer  46 , wherein predetermined angles are formed between the first interference fringes and the second interference fringes (as shown in  FIG. 4E ). Besides, in the present embodiment, the line widths of each second interference fringes are the same, and the gaps between the adjacent second interference fringes are the same. Furthermore, the predetermined angles between the first interference fringes and the second interference fringes are about 90°. 
         [0052]    Plural third interference fringes (not shown in the figure) may be formed on the second photoresist layer  46  by performing IL for the third time to pattern the second photoresist layer  46 , wherein other predetermined angles are formed between the first interference fringes, the second interference fringes, and the third interference fringes. In the present embodiment, the line widths of each third interference fringes are the same, and the gaps between the adjacent third interference fringes are the same. Besides, the predetermined angles between the first interference fringes, the second interference fringes, and the third interference fringes are about 60°. 
         [0053]    Plural coherent light beams with the same incident angle may also perform interference lithography by different azimuth angles at the same time to form interference fringes on the second photoresist layer  46 , and to pattern the second photoresist layer  46 . 
         [0054]    Four beams may be used at the same time to form the first interference fringes and the second interference fringes on the second photoresist layer  46 , wherein predetermined angles are formed between the first interference fringes and the second interference fringes. In the present embodiment, the line widths of each first interference fringes are the same, and the gaps between the adjacent first interference fringes are the same. Besides, the line widths of each second interference fringes are the same, and the gaps between the adjacent second interference fringes are the same. Preferably, the predetermined angles between the first interference fringes and the second interference fringes are about 90°. 
         [0055]    Six beams may be used at the same time to form the first interference fringes, the second interference fringes, and the third interference fringes on the second photoresist layer  46 , wherein predetermined angles are formed between the first interference fringes, the second interference fringes, and the third interference fringes. In the present embodiment, the line widths of each first interference fringes are the same, and the gaps between the adjacent first interference fringes are the same. The line widths of each second interference fringes are the same, and the gaps between the adjacent second interference fringes are the same. Besides, the line widths of each third interference fringes are the same, and the gaps between the adjacent third interference fringes are the same. Preferably, the predetermined angles between the first interference fringes, the second interference fringes, and the third interference fringes are about 60°. 
         [0056]    With reference to  FIG. 4F , a second metal layer  47  is deposited on the second photoresist layer  46 . Preferably, the material of the second metal layer  47  is the same as the material of the first metal layer  42 . In the present embodiment, the material used in the second metal layer  47  is Cr. 
         [0057]    After the second photoresist layer  46  is washed away by using acetone, the pattern on the second photoresist layer  46  is transferred to the substrate  41  to form a photonic crystal pattern  471  (as shown in  FIG. 4F ). The waveguide pattern  421  and the photonic crystal pattern  471  serve together as an etching mask  48  to etch the substrate  41  (as shown in  FIG. 4G ). 
         [0058]    After the etching mask  48  is removed, a photonic crystal  412  with waveguides  411  is formed on the substrate, as shown in  FIG. 4H . 
         [0059]    In the present embodiment, after applying a light source over the phase-shift mask  44  to expose and pattern the first photoresist layer  43 , the method for preparing photonic crystal slab waveguides may further comprise a step: treating the first photoresist layer  43  with PEB to make the pattern formed on the photoresist layer  43  clear. Besides, in the present embodiment, after the second photoresist layer  46  is patterned, the method for preparing photonic crystal slab waveguides may further comprise a step: treating the second photoresist layer  46  with PEB to make the pattern formed on the photoresist layer  46  clear. 
         [0060]      FIG. 5  is a top view of a photonic crystal slab waveguides of the present embodiment, wherein the photonic crystal slab waveguides comprises waveguides  411  and photonic crystals  412 . Generally, the line widths W of the waveguides may be 100 nm-1.3 μm. In the present embodiment, the line widths W of the waveguides  411  are between 350 nm to 400 nm. 
         [0061]    On the other hand, with reference to  FIG. 5 ,  FIG. 7 , and  FIG. 2E , the number and the pattern of the waveguides  421  of the photonic crystal slab waveguides are defined by the bottom edges  2411  of the phase-shift mask  241 . The photonic crystal slab waveguides prepared in the present embodiment have  2  parallel waveguides  411 . Besides, the line widths W of the waveguides  411  are defined according to the relief depths  2412  of the phase-shift mask  241  and the exposure dose of the light source. Table 1 presents the relationship between the relief depths  2412  of the phase-shift mask  241 , the exposure dose of the light source, and the line widths W of the waveguides  411 . 
         [0000]    
       
         
               
               
               
             
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Exposure dose of light 
                 Relief depth of 
                 Line width of 
               
               
                 source (mJ/cm 2 ) 
                 phase-shift mask (nm) 
                 waveguides W (nm) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 25 
                 500 
                 150~400 
               
               
                 25 
                 700 
                 400~600 
               
               
                 25 
                 1100 
                 700~1100 
               
               
                 15 
                 500 
                 120~200 
               
               
                 15 
                 700 
                 200 
               
               
                 15 
                 1100 
                 300~400 
               
               
                   
               
             
          
         
       
     
         [0062]    With reference to Table 1,  FIG. 5 ,  FIG. 7 , and  FIG. 2E , different line widths W of the waveguides  411  can be prepared with different relief depths of the phase-shift mask  241  at the same exposure dose of the light source. Furthermore, different line widths W of the waveguides  411  can be prepared with the same relief depths of the phase-shift mask  241  by adjusting the exposure dose of the light source. Hence, the line widths W of the waveguides  411  can be defined by the exposure dose of the light source and the relief depths  2412  of the phase-shift mask  241 . 
       Embodiment 2  
       [0063]      FIG. 6A  to  FIG. 6H  are cross-sectional views showing a process for preparing a photonic crystal of the present embodiment. 
         [0064]    First, with reference to  FIG. 6A , a substrate  41  is provided. Then, a first metal layer  42 , and a first photoresist layer  43  are disposed on the substrate  41 . The first photoresist layer  43  is exposed and patterned by way of a phase-shift mask  44 . 
         [0065]    After removing the phase-shift mask  44 , the first metal layer  42  is etched to form a waveguide pattern  421 , as shown in  FIG. 6B . 
         [0066]    With reference to  FIG. 6C , a second photoresist layer  46  is coated on the first metal layer  42  after the first photoresist layer  43  is removed. 
         [0067]    With reference to  FIG. 6D , plural first interference fringes are projected on the second photoresist layer  46  by coherent light beams. In the present embodiment, the line widths of each first interference fringes are the same, and the gaps between the adjacent first interference fringes are the same. Then, plural second interference fringes are projected on the second photoresist layer  46 , wherein the first interference fringes and the second interference fringes cross each other. In the present embodiment, the line widths of each second interference fringes are the same, and the gaps between the adjacent second interference fringes are the same. Finally, plural third interference fringes are projected on the second photoresist layer  46 , wherein predetermined angles are formed between the first interference fringes, the second interference fringes, and the third interference fringes. In the present embodiment, the line widths of each third interference fringes are the same, and the gaps between the adjacent third interference fringes are the same. Besides, the predetermined angles are about 60°. 
         [0068]    With reference to  FIG. 6D  and  FIG. 6E , the second photoresist layer  46  serves as a mask to etch the first metal layer  42  to form a photonic crystal pattern  422 . After removing the second photoresist layer  46 , the waveguide pattern  421  and the photonic crystal pattern  422  serve together as an etching mask  48  to etch the substrate  41 . Finally, after the etching mask  48  is removed, a photonic crystal  412  with waveguides  411  is formed on the substrate  41 , as shown in  FIG. 6F . 
         [0069]    With reference to  FIG. 7 , the photonic crystal  412  is formed by processing IL three times in the present embodiment. Besides, the patterns of the waveguides  411  are defined by the bottom edges  2411  of the phase-shift mask  241  (with reference to  FIG. 2E ). 
       Embodiment 3  
       [0070]    In methods for preparing photonic crystal slab waveguides disclosed in the embodiment 1 and embodiment 2, a photonic crystal pattern is formed after a waveguide pattern. However, in the present embodiment, the waveguide pattern is formed after the photonic crystal pattern. 
         [0071]      FIG. 8A  to  FIG. 8C  are cross-sectional views showing a process for preparing a photonic crystal of the present embodiment. 
         [0072]    With reference to  FIG. 8A , a first metal layer  42  and a second photoresist layer  46  are formed on the substrate  41  sequentially. Then, plural first interference fringes and plural second interference fringes are projected on the second photoresist layer  46  sequentially to pattern the second photoresist layer  46 . Besides, the angles formed between the first interference fringes and the second interference fringes are 90°. 
         [0073]    Plural third interference fringes may be formed on the second photoresist layer  46  by performing IL for the third time to pattern the second photoresist layer  46 , wherein other predetermined angles are formed between the first interference fringes, the second interference fringes, and the third interference fringes. In the present embodiment, the line widths of each third interference fringes are the same, and the gaps between the adjacent third interference fringes are the same. Besides, the predetermined angles between the first interference fringes, the second interference fringes, and the third interference fringes are about 60°. 
         [0074]    Plural coherent light beams with the same incident angle may also perform interference lithography by different azimuth angles at the same time to form interference fringes on the second photoresist layer  46 , and to pattern the second photoresist layer  46 . 
         [0075]    Four beams may be used at the same time to form the first interference fringes and the second interference fringes on the second photoresist layer  46 , wherein predetermined angles are formed between the first interference fringes and the second interference fringes. In the present embodiment, the line widths of each first interference fringes are the same, and the gaps between the adjacent first interference fringes are the same. Besides, the line widths of each second interference fringes are the same, and the gaps between the adjacent second interference fringes are the same. Preferably, the predetermined angles between the first interference fringes and the second interference fringes are about 90°. 
         [0076]    Six beams may be used at the same time to form the first interference fringes, the second interference fringes, and the third interference fringes on the second photoresist layer  46 , wherein predetermined angles are formed between the first interference fringes, the second interference fringes, and the third interference fringes. In the present embodiment, the line widths of each first interference fringes are the same, and the gaps between the adjacent first interference fringes are the same. The line widths of each second interference fringes are the same, and the gaps between the adjacent second interference fringes are the same. Besides, the line widths of each third interference fringes are the same, and the gaps between the adjacent third interference fringes are the same. Preferably, the predetermined angles between the first interference fringes, the second interference fringes, and the third interference fringes are about 60°. 
         [0077]    With reference to  FIG. 8B , the second photoresist layer  46  serves as a mask to etch the first metal layer  42  to form a photonic crystal pattern  422 . 
         [0078]    After the second photoresist layer  46  is removed, a first photoresist layer  43  is coated on the photonic crystal pattern  422 , as shown in  FIG. 8C . Then, a phase-shift mask is provided on the first photoresist layer  43 . 
         [0079]    A UV radiation is applied over the phase-shift mask  44  to pattern the first photoresist layer  43 . After the phase-shift mask  44  is removed, a second metal layer  47  is formed on the first photoresist layer  43  by e-gun evaporation (as shown in  FIG. 8D ). Preferably, the material used in the second metal layer  47  is the same as the material used in the material used in the first metal layer  42 . In the present embodiment, the material used in the second metal layer  47  is Cr. When the first photoresist layer  43  is removed, the second metal layer  47  is also removed at the same time. Hence, with reference to  FIG. 8E , only the waveguide pattern  472  is kept on the substrate  41 . 
         [0080]    Finally, the waveguide pattern  472  and the photonic crystal pattern  422  are served together as an etching mask  48  to etch the substrate  41 . After removing the etching mask  48 , the photonic crystal slab waveguides is achieved, as shown in  FIG. 8F . 
         [0081]    In conclusion, in the present invention, waveguides having different line widths can be easily achieved with only one phase-shift mask by controlling the exposure dose of the light source and the relief depths of the phase-shift mask. Hence, the cost for manufacturing can be reduced. In addition, the 2-dimensional interference fringes can be fabricated at the same time or at multiple times, so it is possible to prepare photonic crystal with good period quickly. Hence, it is possible to prepare photonic crystal slab waveguides over a large area by the method for preparing photonic crystal slab waveguides of the present invention. 
         [0082]    Although the present invention has been explained in relation to its preferred embodiment, it is to be understood that many other possible modifications and variations can be made without departing from the scope of the invention as hereinafter claimed.