Abstract:
A method for preparing a periodically poled structure comprises the steps of providing a ferroelectric substrate having an upper surface and a bottom surface, forming an upper electrode including at least one first block and at least one second block on the upper surface, forming a bottom electrode including at least one third block and at least one fourth block on the bottom surface and performing a plurality of poling processes to form at least one first domain and at least one second domain in the ferroelectric substrate, wherein the first domain is formed between the first block and the third block, and the second domain is formed between the second block and the fourth block.

Description:
[0001]    This application is a continuation application of and claims priority to application Ser. No. 11/619,021, filed Jan. 2, 2007 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    (A) Field of the Invention 
         [0003]    The present invention relates to a method for preparing a periodically poled structure, and more particularly, to a method for preparing a periodically poled structure by performing a plurality of poling processes on two opposite surfaces of a ferroelectric substrate. 
         [0004]    (B) Description of the Related Art 
         [0005]    The periodically poled structure having poled domains in a ferroelectric single crystal such as lithium niobate (LiNbO 3 ), lithium tantalite (LiTaO 3 ) and potassium titanyl phosphate (KTiOPO 4 ) may be widely used in the optical fields such as optical storage and optical measurement. There are several methods for preparing the periodically poled structure such as the proton-exchanging method, the electron beam-scanning method, the electric voltage applying method, etc. 
         [0006]    U.S. Pat. No. 6,002,515 discloses a method for manufacturing a polarization inversion part on a ferroelectric crystal substrate. The polarization inversion part is prepared by steps of applying a voltage in the polarization direction of the ferroelectric crystal substrate to form a polarization inversion part, conducting a heat treatment for reducing an internal electric field generated in the substrate by the applied voltage, and then reinverting polarization in a portion of the polarization inversion part by applying a reverse direction voltage against the voltage that was previously applied. In other words, the method for preparing a polarization inversion part disclosed in U.S. Pat. No. 6,002,515 requires performing the application of electric voltage twice. 
         [0007]    One of the major factors for the realization of the above example applications depends upon the ability to pattern and fabricate the desired microstructures with the proper materials. The prior art provides a basic patterning and fabrication approach such as ferroelectric domain reversals via electrical field poling or thermal poling. However, as the desired patterned structures require finer microstructures such as shorter ferroelectric domain periods or pattern structures with aperiodic periods, the challenge of achieving the desired pattern structures becomes greater. Moreover, the conventional methods may not be applicable to the use of some materials. In addition, these methods also might encounter scalability and yield issues in the fabrication of large area patterned microstructures. 
         [0008]    One of the key challenges in the poling of dielectric microstructures is the electric field and electric dipole interference within the body of dielectric materials during the electric field poling process. Such electric field and electric dipole interference results in non-uniform domain structures and difficulties in generating domains with short pitch (period). Additional challenges in poling of dielectric microstructures come from the scalability of the poling area. As the poling area increases, the total required poling time will also increase. The large ratio between the total amount of poling time for large area structures and the optimized poling time for each individual microstructure enhances the fabrication difficulty for generating large area and uniform microstructures. 
         [0009]    However, as the period of the poled domains of the periodically poled structure becomes smaller, the above-mentioned conventional methods for preparing the poled domains cannot meet precision requirements. 
       SUMMARY OF THE INVENTION 
       [0010]    One aspect of the present invention provides a segmenting method for preparing a periodically poled structure 
         [0011]    A method for preparing a periodically poled structure according to this aspect of the present invention comprises the steps of providing a ferroelectric substrate having an upper surface and a bottom surface, forming an upper electrode including at least one first block and at least one second block on the upper surface, forming a bottom electrode including at least one third block and at least one fourth block on the bottom surface and performing a plurality of poling processes to form at least one first domain and at least one second domain in the ferroelectric substrate, wherein the first domain is formed between the first block and the third block, and the second domain is formed between the second block and the fourth block. 
         [0012]    Another aspect of the present invention provides a method for preparing a periodically poled structure comprising the steps of providing a ferroelectric substrate including an upper surface and a bottom surface, forming an upper electrode including at least one first block and at least one second block on the upper surface, forming a plurality of insulation blocks on the bottom surface, dipping the bottom surface in a conductive solution is and performing a plurality of poling processes to form at least one first domain and at least one second domain in the ferroelectric substrate, wherein the first domain contacts the first block and the second domain contacts the second block. 
         [0013]    A further aspect of the present invention provides a method for preparing a periodically poled structure comprising the steps of providing a ferroelectric substrate including an upper surface and a bottom surface, forming a plurality of insulation blocks on the bottom surface, forming a first insulation layer having at least one first aperture on the upper surface, performing a first poling process to form at least one first domain in the ferroelectric substrate, removing the first insulation layer from the upper surface, forming a second insulation layer having at least one second aperture on the upper surface and performing a second poling process to form at least one second domain in the ferroelectric substrate, wherein the first aperture exposes the first domain and the second aperture exposes the second domain. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    The objectives and advantages of the present invention will become apparent upon reading the following description and upon reference to the accompanying drawings in which: 
           [0015]      FIG. 1  to  FIG. 9  illustrate a method for preparing a periodically poled structure according to a first embodiment of the present invention; 
           [0016]      FIG. 10  to  FIG. 18  illustrate a method for preparing a periodically poled structure according to a second embodiment of the present invention; and 
           [0017]      FIG. 19  to  FIG. 25  illustrate a method for preparing a periodically poled structure according to a third embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0018]      FIG. 1  to  FIG. 9  illustrate a method for preparing a periodically poled structure  10  according to a first embodiment of the present invention. First, a ferroelectric substrate  12  having an upper surface  12 A and a bottom surface  12 B is provided, and an upper electrode  14  is formed on the upper surface  12 A and a bottom electrode  16  is formed on the bottom surface  12 B. The upper electrode  14  and the bottom electrode  16  can be made of metallic material. The upper electrode  14  includes first blocks  14 A, second blocks  14 B and fifth blocks  14 C, and bottom electrode  16  includes third blocks  16 A, fourth blocks  16 B and sixth blocks  16 C. The original polarization direction of the ferroelectric substrate  12  is from −Z to +Z, as shown by the arrows in  FIG. 1 . 
         [0019]    Referring to  FIG. 2 , a first poling process is performed by applying a predetermined voltage difference (V) between the first block  14 A and the third block  16 A to form at least one first domain  18 A having a polarization direction opposite to the original polarization direction of the ferroelectric substrate  12 . In other words, the poling process reverses the polarization direction of the first domain  18 A. Subsequently, a second poling process is performed by applying the predetermined voltage difference (V) between the second block  14 B and the fourth block  16 B to form at least one second domain  18 B having a polarization direction opposite to the original polarization direction of the ferroelectric substrate  12 , as shown in  FIG. 3 . 
         [0020]    Referring to  FIG. 4 , a third poling process is performed by applying a predetermined voltage difference (V) between the fifth block  14 C and the sixth block  16 C to form at least one third domain  18 C having a polarization direction opposite to the original polarization direction of the ferroelectric substrate  12  and complete the periodically poled structure  10 . The first domains  18 A, the second domains  18 B and the third domains  18 C are separated by fourth domains  18 D having a polarization direction the same as the original polarization direction of the ferroelectric substrate  12 . 
         [0021]    Referring to  FIG. 5 , the ferroelectric substrate  12  is consisting essentially of a plurality of first regions  12 C and second regions  12 D, and the first regions  12 C are positioned between the upper electrode  14  and the bottom electrode  16 . Before the poling processes are performed, the present invention may perform a doping process such as a proton exchange process to form at least one doped region such as heavy proton exchange region  20  in the upper portion of the second region  12 D of the ferroelectric substrate  12 , and the doped region  20  is formed between the first block  14 A and the second block  14 B, between the second block  14 B and the fifth block  14 C or between the first block  14 A and the fifth block  14 C. In particular, the crystal structure of the doped region  20  is different from that of the ferroelectric substrate  12 . The purpose of the doping process is to change the crystal structure of the ferroelectric substrate  12 , whose polarization direction cannot be reversed by the subsequent poling process so that the enlarging of the poled domains  18 A,  18 B and  18 C due to over-poling can be inhibited. 
         [0022]    Referring to  FIG. 6 , the doping process may form at least one doped region  22  in the bottom portion of the second region  12 D of the ferroelectric substrate  12 , i.e., between the third block  16 A and the fourth block  16 B, between the fourth block  16 B and the sixth block  16 C or between the third block  16 A and the sixth block  16 C. In addition, the doping process may form at least one doped region  20  in the upper portion of the second region  12 D of the ferroelectric substrate  12  and at least one doped region  22  in the bottom portion of the second region  12 D of the ferroelectric substrate  12 , as shown in  FIG. 7 . 
         [0023]    Referring to  FIG. 8 , before the poling processes are performed, the present invention may perform a doping process such as a proton exchange process to form at least one doped region such as a light proton exchange region- 24  in the bottom portion of the first region  12 C of the ferroelectric substrate  12 , and the bottom electrode  16  contacts the doped region  24 . The doped region  24  can increase the internal electrical field as the voltage difference (V) is applied between the upper electrode  14  and the bottom electrode  16  during the subsequent poling process, and the increased internal electrical field is contributory to the formation of the poled domains  18 A,  18 B and  18 C. In particular, the internal electrical field generated by the doped region  24  can increase the intensity difference of the overall electrical field between the domain  12 C right below the upper electrode  14  and the domain  12 D between the domains  12 C. In addition, before the poling processes are performed, the present invention may use the doping process to form the doped regions  20  in the upper portion of in the second region  12 D of the ferroelectric substrate  12 , and to the doped regions  24  in the bottom portion of the first region  12 C of the ferroelectric substrate  12 , as shown in  FIG. 9 . 
         [0024]      FIG. 10  to  FIG. 18  illustrate a method for preparing a periodically poled structure  30  according to a second embodiment of the present invention. First, a ferroelectric substrate  12  having an upper surface  12 A and a bottom surface  12 B is provided, and an upper electrode  14  is formed on the upper surface  12 A and a plurality of insulation blocks  32  is formed on the bottom surface  12 B. The insulation blocks  32  can be made of silicon oxide. The upper electrode  14  includes first blocks  14 A, second blocks  14 B and fifth blocks  14 C. The original polarization direction of the ferroelectric substrate  12  is from −Z to +Z, as shown by the arrows in  FIG. 10 . 
         [0025]    Referring to  FIG. 11 , the bottom surface  12 B is dipped in a conductive solution  34 , and a first poling process is performed by applying a predetermined voltage difference (V) between the first block  14 A and the conductive solution  34  to form at least one first domain  18 A contacting the first block  14 A. The first domain  18 A has a polarization direction opposite to the original polarization direction of the ferroelectric substrate  12 . In other words, the poling process reverses the polarization direction of the first domain  18 A. Subsequently, a second poling process is performed by applying the predetermined voltage difference (V) between the second block  14 B and the conductive solution  34  to form at least one second domain  18 B having a polarization direction opposite to the original polarization direction of the ferroelectric substrate  12 , as shown in  FIG. 12 . 
         [0026]    Referring to  FIG. 13 , a third poling process is performed by applying a predetermined voltage difference (V) between the fifth block  14 C and the conductive solution  34  to form at least one third domain  18 C having a polarization direction opposite to the original polarization direction of the ferroelectric substrate  12  to complete the periodically poled structure  30 . The first domains  18 A, the second domains  18 B and the third domains  18 C are separated by fourth domains  18 D having a polarization direction the same as the original polarization direction of the ferroelectric substrate  12 . 
         [0027]    Referring to  FIG. 14 , before the poling processes are performed, the present invention may perform a doping process such as a proton exchange process to form at least one doped region (heavy proton exchange region)  20  in the upper portion of the second region  12 D of the ferroelectric substrate  12 , i.e., the doped region  20  can be formed between the first block  14 A and the second block  14 B, between the second block  14 B and the fifth block  14 C or between the first block  14 A and the fifth block  14 C. The purpose of the doping process is to change the crystal structure of the ferroelectric substrate  12  and the polarization direction of the doped region  20  cannot be reversed by the subsequent poling process so that the enlarging of the poled domains  18 A,  18 B and  18 C due to over-poling can be inhibited. In addition, the doping process may form at least one doped region  22  in the bottom portion of the second region  12 D, i.e., the doped region  22  contacts the insulation blocks  32 , as shown in  FIG. 15 . Furthermore, the doping process may form at least one doped region  20  in the upper portion of the second region  12 D of the ferroelectric substrate  12  and at least one doped region  22  in the bottom portion of the second region  12 D of the ferroelectric substrate  12 , as shown in  FIG. 16 . 
         [0028]    Referring to  FIG. 17 , before the poling processes are performed, the present invention may perform a doping process to form at least one doped region (light proton exchange region)  24  in the bottom portion of first region  12 C of the ferroelectric substrate  12 , i.e., the doped region  24  is formed between the insulation blocks  34 . The doped region  24  can increase the internal electrical field as the voltage difference (V) is applied between the upper electrode  14  and the bottom electrode  16  during the subsequent poling process, and the increased internal electrical field is contributory to the formation of the poled domains  18 A,  18 B and  18 C. In addition, before the poling processes are performed, the present invention may use the doping process to form the doped regions  20  in the upper portion of the second region  12 D of the ferroelectric substrate  12 , and to the doped regions  24  in the bottom portion of the first region  12 C of the ferroelectric substrate  12 , as shown in  FIG. 18 . 
         [0029]      FIG. 19  to  FIG. 25  illustrate a method for preparing a periodically poled structure  50  according to a third embodiment of the present invention. First, a ferroelectric substrate  12  having an upper surface  12 A and a bottom surface  12 B is provided, and a deposition process is performed to form an insulation layer  52  on the upper surface  12 A. The insulation layer  52  can be made of silicon oxide, and the original polarization direction of the ferroelectric substrate  12  is from −Z to +Z. A lithographic process is performed to form an etching mask  54  having at least one opening  56  on the insulation layer  52 , and an etching process is then performed to remove a portion of the insulation layer  52  not covered by the opening  56  to form at least one aperture  58  in the insulation layer  52 . Subsequently, the etching mask  54  is removed, and the same processes are performed to form a plurality of insulation blocks  32  on the bottom surface  12 B, as shown in FIG.  20 . 
         [0030]    Referring to  FIG. 21 , the upper surface  12 A is dipped in a conductive solution  36  and the bottom surface  12 B is dipped in a conductive solution  34 , and a predetermined voltage difference (V) is applied between the conductive solution  36  and the conductive solution  34  to perform a first poling process to form at least one first domain  18 A in the ferroelectric substrate  12 . The first domain  18 A has a polarization direction opposite to the original polarization direction of the ferroelectric substrate  12 . In other words, the poling process reverses the polarization direction of the first domain  18 A. In particular, the aperture  58  exposes the first domain  18 A. 
         [0031]    Referring to  FIG. 22 , the insulation layer  52  is removed from the upper surface  12 A, and the processes shown in  FIG. 19  are performed to form an insulation layer  60  having at least one aperture  62  on the upper surface  12 A. Subsequently, the upper surface  12 A is dipped in the conductive solution  36  and the bottom surface  12 B is dipped in the conductive solution  34 , and a predetermined voltage difference (V) is applied between the conductive solution  36  and the conductive solution  34  to perform a second poling process to form at least one second domain  18 B in the ferroelectric substrate  12 , as shown in  FIG. 23 . In particular, the aperture  62  exposes the second domain  18 B. 
         [0032]    Referring to  FIG. 24 , the insulation layer  60  is removed from the upper surface  12 A, and the processes shown in  FIG. 19  are performed to form an insulation layer  64  having at least one aperture  66  on the upper surface  12 A. Subsequently, the upper surface  12 A is dipped in the conductive solution  36  and the bottom surface  12 B is dipped in the conductive solution  34 , and a predetermined voltage difference (V) is applied between the conductive solution  36  and the conductive solution  34  to perform a third poling process to form at least one third domain  18 C in the ferroelectric substrate  12  and complete the periodically poled structure  50 , as shown in  FIG. 25 . In particular, the aperture  66  exposes the third domain  18 C, and the first domains  18 A, the second domains  18 B and the third domains  18 C are separated by fourth domains  18 D. 
         [0033]    The above-described embodiments of the present invention are intended to be illustrative only. Numerous alternative embodiments may be devised by those skilled in the art without departing from the scope of the following claims.