Abstract:
Upgraded metallurgical grade silicon (UMG-Si) is fabricated by a ‘green’ (environmental protected) external gettering procedure. Impurities concentration of the fabricated UMG-Si is reduced for 100 times than its source material. The UMG-Si obtained has a purity ratio reaching 4N to 6N. Thus, substrates made of the UMG-Si can be used in solar cells and related photoelectrical applications.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to fabricating upgraded metallurgical grade silicon (UMG-Si); more particularly, relates to UMG-Si having a purity ratio between 4N and 6N by greatly reducing impurities concentration below a depth of a surface of a UMG-Si substrate. 
       DESCRIPTION OF THE RELATED ARTS 
       [0002]    Two kinds of methods are used to remove impurities of a semiconductor material. One is internal gettering methods; and the other is external gettering methods. Between them, the internal gettering methods are not fit for solar cells. 
         [0003]    There are four external gettering methods: 
         [0004]    (a) High temperature diffusion is used to directly diffuse atoms of aluminum, phosphorus, etc into the semiconductor material to form metal oxide for trapping the metal impurities. 
         [0005]    (b) Mechanical-, laser- or ion-implantation is used to obtain lattice strain on surface of the semiconductor material for forming sinks of impurities. 
         [0006]    (c) A thin film is applied on the semiconductor material. The film is made of polycrystalline silicon, silicon nitride, aluminum oxide or silicon germanium alloy. The thin film and the semiconductor material are heterogeneous and strain is thus formed to obtain sinks of impurities at the interface in between owing to lattice mismatch. 
         [0007]    (d) Porous-structural surface is used for sinks of impurities. 
         [0008]    Then, a layer having the sinks of impurities is etched off to obtain a high-quality semiconductor material from below a depth of the surface of the original semiconductor material. 
         [0009]    However, the above methods are chemical methods producing chemical wastes and thus do not provide environmental protection. In addition, no method is announced for fabricating UMG-Si having a purity ratio greater than 4N. 
       SUMMARY OF THE INVENTION 
       [0010]    The main purpose of the present invention is to fabricate upgraded metallurgical grade silicon (UMG-Si) having a purity ratio between 4N and 6N by greatly reducing impurities concentration below a depth of a surface of a UMG-Si substrate. 
         [0011]    To achieve the above purpose, the present invention is a method of fabricating upgraded metallurgical grade silicon by an external gettering procedure, comprising the steps of: 
         [0012]    (a) selecting a UMG-Si substrate having a purity ratio between 4N and 6N; 
         [0013]    (b) applying a hydrogen-riched amorphous silicon (a-Si:H) film on a surface of the UMG-Si substrate through chemical vapor deposition or physical vapor deposition; 
         [0014]    (c) thermal-annealing the UMG-Si substrate to diffuse and gather metal impurities from the UMG-Si substrate to sinks of the a-Si:H film to obtain a high metal-impurities concentration area; and 
         [0015]    (d) thermal-etching the a-Si:H film at a high temperature to fully etch out the local high metal-impurities concentration area to obtain a high quality silicon thin layer below a depth of a surface of the UMG-Si substrate. Accordingly, a novel method of fabricating upgraded metallurgical grade silicon by an external gettering procedure is obtained. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    The present invention will be better understood from the following detailed description of the preferred embodiment according to the present invention, taken in conjunction with the accompanying drawings, in which 
           [0017]      FIG. 1  is the flow view showing the preferred embodiment according to the present invention; and 
           [0018]      FIG. 2  until  FIG. 5  are the structural views showing the preferred embodiment. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0019]    The following description of the preferred embodiment is provided to understand the features and the structures of the present invention. 
         [0020]    Please refer to  FIG. 1  until  FIG. 5 , which are a flow view and structural views showing the preferred embodiment according to the present invention. As shown in the figures, the present invention is a method of fabricating upgraded metallurgical grade silicon (UMG-Si) by an external gettering procedure, comprising the following steps: 
         [0021]    (a) Selecting substrate  11 : In  FIG. 2 , a silicon substrate is selected, where the silicon substrate is a UMG-Si substrate  21  having a purity ratio greater than 4N. 
         [0022]    (b) Depositing  12 : In  FIG. 3 , a hydrogen-riched amorphous silicon (a-Si:H) film  22  is applied on a surface of the UMG-Si substrate  21  through physical vapor deposition (PVD) or chemical vapor deposition (CVD). 
         [0023]    (c) Thermal-annealing  13 : In  FIG. 4 , the UMG-Si substrate  21  applied with the a-Si:H film  22  is processed through thermal-annealing at a temperature between 1100 and 1300 Celsius degrees (° C.) for a period between 1 and 30 minutes (min). Thus, metal impurities below a depth of the surface of the UMG-Si substrate  21  are diffused and gathered to sinks of the a-Si:H film  22  to obtain a local high metal-impurities concentration area. 
         [0024]    (d) Thermal-etching  14 : in  FIG. 5 , the a-Si:H film  22  is processed through thermal-etching with a gas of HCl at a temperature between 1100° C. and 1300° C. for a period between 1 min and 30 min to fully etch off the local high metal-impurities concentration area for obtaining a high quality silicon thin layer  23  from below the depth of the surface of the UMG-Si substrate  21 . 
         [0025]    On using the present invention, a UMG-Si substrate  21  having a purity ratio greater than 4N is selected. Plasma-enhanced chemical vapor deposition (PECVD) is used to deposit an a-Si:H film  22  on a surface of the UMG-Si substrate  21 , where the a-Si:H film  22  has a thickness between 500 Å and 2000 Å. Then, thermal-annealing is processed to expel hydrogen from the a-Si:H film  22  through evaporation to leave sinks in the a-Si:H film  22 , where each sink has a shape of a point, line or area. At the same time, metal impurities below a depth of the surface of the UMG-Si substrate  21  are rapidly diffused to the a-Si:H film  22  and are firmly trapped by the sinks in the a-Si:H film  22 . Finally, the metal impurities are gathered in the a-Si:H film  22  to form a high metal-impurities concentration area. 
         [0026]    In  FIG. 3 , a first distribution curve of impurities concentration before thermal treatment  3   a  and a second distribution curve of impurities concentration after thermal treatment  3   b  show that thermal-annealing process accelerates impurities below a surface of the UMG-Si substrate  21  to diffuse and gather in sinks. Thus, impurities concentration below a depth of the surface of the UMG-Si substrate  21  is reduced by 100 times to form a high-quality silicon thin layer. HCl gas is used for thermal-etching. Under a high temperature, the a-Si:H film  22  having the high impurities concentration area is totally etched off by the HCl gas to obtain a high quality silicon thin layer  23  from below the depth of the surface of the UMG-Si substrate  21  as shown in a third distribution curve of impurities concentration  3   c.    
         [0027]    After the above processes, the impurities concentration under the depth of the surface of the UMG-Si substrate  21  is reduced by 100 times the original impurities concentration. Hence, the high quality silicon thin layer  23  thus obtained can be applied to solar cells and related photoelectrical applications. Besides, no chemical solvent is used and thickness of the UMG-Si substrate is not affected; and the present invention is a green procedure for the HCl used is recyclable. 
         [0028]    To sum up, the present invention is a method of fabricating upgraded metallurgical grade silicon by an external gettering procedure, where impurities concentration below a depth of a surface of a UMG-Si substrate is reduced for 100 times than original impurities concentration; and the present invention is a green procedure while the impurities concentration below the depth of the surface of the UMG-Si substrate is reduced by an external gettering method to obtain a high quality silicon thin layer for solar cells and related photoelectrical applications. 
         [0029]    The preferred embodiment herein disclosed is not intended to unnecessarily limit the scope of the invention. Therefore, simple modifications or variations belonging to the equivalent of the scope of the claims and the instructions disclosed herein for a patent are all within the scope of the present invention.