Patent Publication Number: US-2003221608-A1

Title: Method of making photonic crystal

Description:
BACKGROUND OF THE INVENTION  
       [0001] The present invention relates to a method of making a photonic crystal.  
       [0002] The photonic crystal is a man-made crystal of a structure in which two kinds of media significantly different in refractive index are systematically arranged at periodic intervals substantially equal to the light wavelength, and the photonic crystal is called a one-, two- or three-dimensional photonic crystal according to the number of dimensions in which the photonic crystal is periodic.  
       [0003]FIG. 1 schematically shows an example of the periodic structure of a two-dimensional photonic crystal  7 , in which columnar crystallites (or crystalline rods)  6  are arranged at periodic intervals nearly equal to the light wavelength in a medium  5  of a refractive index different from that of the columnar crystallites  6 . The arrows indicate the directions in which to open photonic band gaps (i.e. the directions in which no light travels). The medium  5  is a material filling gaps between the columnar crystallites  6 ; in FIG. 1 the medium is air.  
       [0004]FIG. 2 depicts a sequence of steps S1 to S3 involved in a prior art method of manufacturing a photonic crystal of such a structure as shown in FIG. 1. The conventional manufacturing method will be described below with reference to FIG. 2.  
       [0005] Step S1: Prepare an SOI (Silicon On Insulator) substrate  14  of a three-layered stricture composed of a single-crystal silicon (Si) layer  11 , a silicon dioxide layer  12  and a single-crystal silicon layer  13 .  
       [0006] Step S2: Coat the one single-crystal silicon layer  13  over its entire surface area with a thin film of gold (Au) by evaporation or the like, and photoengrave the gold thin film (by photolithography and etching) to leave the desired pattern of circular island regions of gold  15  arranged at a predetermined pitch.  
       [0007] Step S3: Subject the single-crystal silicon layer  13  to anisotropic dry etching using the gold islands  15  as masks to form silicon monocrystalline rods  16 .  
       [0008] In this way, a two-dimensional photonic crystal  17  composed of the silicon monocrystalline rods  16  and air  5  is formed on the silicon dioxide layer  12 .  
       [0009] Incidentally, the problem attendant to the above-described conventional method using anisotropic silicon dry etching to form silicon monocrystalline rods in a predetermined pattern is the technical difficulty in controlling or reducing the roughness of the peripheral surface of each monocrystalline rod—this makes the fabrication of practicable photonic crystals a formidable task.  
       SUMMARY OF THE INVENTION  
       [0010] It is therefore an object of the present invention to provide a photonic crystal manufacturing method which greatly improves the smoothness of the peripheral surface of each columnar crystallite ns hence permits easy fabrication of practicable photonic crystals.  
       [0011] The manufacturing method according to the present invention comprises the steps of: (a) forming a desired periodic pattern of thin-film metal island regions all over the surface of a (111) single-crystal silicon layer overlying a layer of a material different in refractive index from silicon; and (b) growing silicon monocrystalline rods beneath the thin-film metal island regions in a silicon tetrachloride gas atmosphere to form a two-dimensional photonic crystal having columnar crystallites arranged in a predetermined periodic patter.  
       [0012] In the above, the step (b) may also be a step of forming the desired periodic pattern of thin-film metal island regions, then selectively etching away the (111) single-crystal silicon layer except the island regions, and growing the silicon monocrystalline rods. Alternatively, it is possible to form the pattern of island regions first, then grow the silicon monocrystalline rods, and selectively etch away the (111) single-crystal silicon layer except the island regions. In the above, gold, for instance, is used to form the thin-film metal island regions. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0013]FIG. 1 is a schematic showing of an example of the periodic structure of a two-dimensional photonic crystal;  
     [0014]FIG. 2 is a sequence of steps involved in a prior art method of making the photonic crystal shown in FIG. 1;  
     [0015]FIG. 3 is a sequence of steps involved in the photonic crystal manufacturing method according to an embodiment of the present invention; and  
     [0016]FIG. 4 is a plan view showing an example of a pattern of thin-film metal (gold) island regions. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
     [0017]FIG. 3 illustrates a sequence of steps involved in the photonic crystal manufacturing method according to an embodiment of the present invention, which will be described below together with concrete examples of numerical values.  
     [0018] Step S1: Prepare an SOI substrate  24  of a three-layer structure composed a 400-μm-thick single-crystal silicon layer  21 , a 55-μm-thick silicon dioxide layer  22  and a 50-μm-thick (111) single-crystal silicon layer  23 .  
     [0019] Step S2: Evaporate a gold (Au) film 0.2 μm thick onto the surface of a (111) single-crystal silicon layer  23  and photoengrave the gold film (by photolithography and etching) to leave a predetermined periodic pattern of gold island regions  25  having the desired shape and arranged at predetermined periodic intervals. In this example, as shown in FIG. 4, the gold island regions  25  are circular ones each having a 0.3-μm diameter D and formed at points of intersection of first parallel straight lines equally spaced apart by d and second parallel straight lines also equally spaced apart by d but forming an angle of 60° with respect to the first straight lines. The length L of each side of a regular triangle with vertexes at the centers of three adjacent circles is, for instance, 0.6 μm.  
     [0020] Step S3: Selectively etch away the (111) single-crystal silicon layer  23  except the gold island regions  25  by means of ICP (Inductive Coupled Plasma) etching, for instance.  
     [0021] Step S4: Place a substrate  24 ′, obtained by step S3, in a silicon tetrachloride (SiCl 4 ) gas atmosphere held at 950° C. to grow silicon monocrystals beneath the gold island regions  25  to form silicon monocrystalline rods  26 . The silicon monocrystalline rods  26  are grown to a height of, for example, 200 μm or so.  
     [0022] By the above-described manufacturing steps, a structure having the silicon monocrystalline rods  26  arranged as desired is obtained, that is, a two-dimensional photonic crystal  27  composed of the silicon monocrystalline rods  26  and air is formed on the silicon dioxide layer  22 .  
     [0023] The mechanism of the phenomenon in which silicon monocrystals, for example, whiskers, are grown beneath gold island regions in the (111) plane of silicon crystal in a silicon vapor growth atmosphere was reported by W. S. Wagner and W. C. Ellis (Applied Physics Letters, 4, 84 (1964)). Afterward, a method of growing silicon whiskers at low temperatures by use of gallium (Ga) instead of using gold was reported by S. Sharma et al. (Material Research Society Spring Meeting, Apr. 17, 2001).  
     [0024] According to the method described above, the silicon monocrystalline rods  26  forming the columnar crystallites are formed by vapor phase epitaxy, not by the conventional anisotropic dry etching; this greatly improves the surface smoothness of the silicon monocrystalline rods.  
     [0025] More specifically, it is possible to obtain crystalline rods of surfaces (peripheral surfaces) having a center-line average roughness Ra of 10 nm or less. Accordingly, the above-described method of the present invention permits fabrication of practicable photonic crystals.  
     [0026] While the above embodiment has been described to use the SOI substrate  21  and form the silicon monocrystalline rods  26  on the silicon dioxide layer  22  of the SOI structure  21 , the silicon dioxide layer  22  may be replaced with a layer of a different material, which has a refractive index different from that of silicon. Further, the thin-film gold island regions  25  may also be replaced with island regions of a different metal which forms a eutectic melt with silicon at high temperatures. In such an instance, the temperature for the vapor phase epitaxy of silicon varies with the metal used.  
     [0027] The above embodiment has been described to grow the silicon monocrystalline rods  26  after selectively etching away the (111) single-crystal silicon layer  23  except the gold island regions  25 , but the order of these steps may also be reversed since the silicon monocrystalline rods  26  can be grown beneath the gold island regions  25  in the silicon vapor growth atmosphere prior to the selective etching-away of the single-crystal silicon layer  23 . From the viewpoint of the configuration/position accuracy of the silicon monocrystalline rods  26 , however, the selective etching-away of the (111) single-crystal silicon layer  23  may preferably be followed by the growth of the silicon monocrystalline rods  26  as depicted in FIG. 3.  
     EFFECT OF THE INVENTION  
     [0028] As described above, the present invention enables the columnar crystallites of the photonic crystal to be formed by silicon monocrystalline rods of improved surface smoothness, and hence it permits fabrication of practicable photonic crystals.