Abstract:
The present invention polishes a lithium aluminum oxide (LiAlo 2 ) crystal several times with three different materials and then the LiAlo 2  crystal are soaked into an acid solution to be washed for obtaining a LiAlo 2  crystal of film-free, scratch-free with smooth surface.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a method for polishing a crystal; more particularly, relates to obtaining a smooth surface of a film-free and scratch-free lithium aluminum oxide crystal with a roughness below 1.0 nanometer (nm) root-mean square (rms). 
       DESCRIPTION OF THE RELATED ART 
       [0002]    GaN-based nitride semiconductors not only have wide bandgaps (1.2˜6.2 eV) but also are grown epitaxially over a number of substrates. 
         [0003]    For a heteroepitaxy, the quality of GaN film lies much on the properties of substrate—both the inherent properties, such as lattice constants and thermal expansion coefficients; and process induced properties, such as surface roughness, step height, terrace width and wetting behavior. Thus, substrates capable of supporting better quality GaN epitaxial layers are in need of realizing the full potential of GaN-based devices. 
         [0004]    It is particularly surprising at present that sapphire still remains as the most common choice for GaN-based LEDs. Nevertheless, its structure is unsuitable to be chosen as a substrate for epitaxy according to general assumptions. It has large lattice constant (˜15%) mismatch and thermal expansion coefficient mismatches with GaN. Besides, the sapphire substrate has a roughness typically between 0.8 and 2.1 nm rms over 1 mm 2 . Hence, the prior art does not fulfill users&#39; requests on actual use. 
       SUMMARY OF THE INVENTION 
       [0005]    The main purpose of the present invention is to obtain a film-free and scratch-free LiAlO 2  crystal having a roughness below 1.0 nm rms. 
         [0006]    To achieve the above purpose, the present invention is a method for polishing LiAlO 2  crystal, where a LiAlO 2  crystal is polished on a surface by using siliconcarbides having various size of grains for the first time; Al 2 O 3  powders (having various size of grains) mixed with deionized water for the second time; and a colloidal silica suspension for the third time; and then the LiAlO 2  crystal obtained after the polishings is soaked into a phosphoric acid (H 3 PO 4 ) solution for etching to obtain a smooth surface of the LiAlO 2  crystal having a roughness below 1.0 nm rms. Accordingly, a novel method for polishing lithium aluminum oxide crystal is obtained. 
     
     
       BRIEF DESCRIPTIONS OF THE DRAWINGS 
         [0007]    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 
           [0008]      FIG. 1  is a view showing a work flow according to a preferred embodiment of the present invention; 
           [0009]      FIG. 2  is a view showing a first polishing according to the preferred embodiment of the present invention; 
           [0010]      FIG. 3  is a view showing a second polishing according to the preferred embodiment of the present invention; 
           [0011]      FIG. 4  is a view showing a third polishing according to the preferred embodiment of the present invention; and 
           [0012]      FIG. 5  is a view showing a washing according to the preferred embodiment of the present invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0013]    The following description of the preferred embodiment is provided to understand the features and the structures of the present invention. 
         [0014]    Please refer to  FIG. 1 , which is a view showing a work flow according to a preferred embodiment of the present invention. As shown in the figure, the present invention is a method for polishing a lithium aluminum oxide (LiAlO 2 ) crystal, comprising the following steps: 
         [0015]    Step 1—First polishing: A LiAlO 2  crystal is obtained to be polished on a surface of the LiAlO 2  crystal for the first time  1  by sequentially using four silicon carbides of four respective grain-size sequences, coordinated with a water. 
         [0016]    Step 2—Second polishing: After some aluminum oxide (Al 2 O 3 ) powders having various grain sizes of Al 2 O 3  are mixed with deionized waters to obtain Al 2 O 3  powder solutions, the LiAlO 2  crystal is polished on the surface for the second time  2  by a grinder/polisher machine coordinated with the Al 2 O 3  powder solutions. 
         [0017]    Step 3—Third polishing: The LiAlO 2  crystal is polished on the surface for the third time  3  by the grinder/polisher machine coordinated with a colloidal silica (SiO 2 ) suspension. 
         [0018]    And, Step 4—Washing: The LiAlO 2  crystal is soaked into a phosphoric (H 3 PO 4 ) acid solution at a room temperature, and then the LiAlO 2  crystal is washed with an acetone followed with a deionized water to be washed away contaminations on the surface. 
         [0019]    Through the above steps, a roughness between 0.4 and 0.9 nanometer (nm) root-mean square (rms) for the surface of the LiAlO 2  crystal is obtained. Thus, a novel method for polishing LiAlO 2  crystal is obtained. 
         [0020]    The steps of the method for polishing LiAlO 2  crystal according to the present invention are shown in  FIG. 1 ; and the followings are the detailed implementation descrptions of the steps. 
         [0021]    Please further refer to  FIG. 2 , which is a view showing the first polishing according to the preferred embodiment of the present invention. As shown in the figure, in the first polishing  1 , a LiAlO 2  crystal  5  is obtained to be polished on a surface for the first time  1  by sequentially using a first siliconcarbide  11  of grain-size sequence  800 , a second siliconcarbide  12  of grain-size sequence  1000 , a third silicon carbide  13  of grain-size sequence  2000 , and a fourth siliconcarbide  14  of grain-size sequence  4000 , coordinated with a water  15 . A time for each polishing using one of the siliconcarbides is between 40 and 50 minutes (min). Therein, after polishing with the second silicon carbide  12  of grain-size sequence  1000 , the surface is examined with naked eyes to find scratch if there is any. It is because a scratch made dunring a polishing by the second silicon carbide  12  of grain-size sequence  1000  can not be eliminated by the polishing using the third siliconcarbide  13  of grain-size sequence  2000 . Moreover, each of the directions for the polishings using the siliconcarbides are continuously changed to prevent from leaving scratch. 
         [0022]    Please further refer to  FIG. 3 , which is a view showing the second polishing according to the preferred embodiment of the present invention. As shown in the figure, in the second polishing  2 , various Al 2 O 3  powders  21  having various sizes of 1 micrometer (μm), 0.3 μm and 0.05 μm respectively are mixed with a deionized water  22 . Then the grinder/polisher machine  6  is used to polish the LiAlO 2  crystal  5  on the surface for the second time  2  with a rotation speed of 150 to 200 revolutions per minute (rpm). The polishings which are fone by using the Al 2 O 3  powder solutions  21  having various grain sizes of Al 2 O 3  spend time about 30 to 40 min for each polishing. In addition, directions for the polishings are continuously changed to prevent from any scratch. 
         [0023]    Please further refer to  FIG. 4 , which is a view showing the third polishing according to the preferred embodiment of the present invention. As shown in the figure, in the third polishing  3 , the grinder/polisher machine  6  polishes the LiAlO 2  crystal  5  on the surface with a polishing fabric for the third time  3  coordinated with a colloidal silica (SiO 2 ) suspension  31  having a grain size of 0.04 μm (produced by Precision Surfaces International Co.) under a rotation speed of 150 to 200 rpm. A time spent for the polishing is about 20 to 30 min. In addition, a direction for the polishing is continuously changed. 
         [0024]    Please further refer to  FIG. 5 , which is a view showing the washing according to the preferred embodiment of the present invention. As shown in the figure, in the step of washing  4 , the LiAlO 2  crystal  5  obtained after the three polishings  1 , 2 , 3  is soaked in a phosphoric acid (H 3 PO 4 ) solution (SHOWA, Chemical Co., LDT) for etching; an d then the LiAlO 2  crystal  5  is taken out to be washed away contaminations left on the surface with acetone  42  at first and with deionized water  43  later on. Thus, a roughness between 0.4 and 0.9 nm rms for the surface of the LiAlO 2  crystal is obtained where the phosphoric acid (H 3 PO 4 ) solution  41  can be replaced with hydrochloric (HCl) acid, nitric (HNO 3 ) acid, sulfuric (H 2 SO 4 ) acid, acetic (HCOOH) acid ir hydrofluoric (HF) acid to obtain a different roughness range. 
         [0025]    LiAlO 2  crystal is the most closely lattice-matched (1.4%) substrate currently being considered for Ga N heteroeptiaxy. The c-parameter of lattice constant for LiAlO 2  is close to two times of a-parameter (0.6378 nm) of lattice constant for GaN, while the a-parameter of lattice constant for LiAlO 2  is basically a perfect match to c-parameter (0.5165 nm) of lattice constant for GaN. The a-c (100) plane of LiAlO 2  has the same atomic arrangement as the (10-10) prismatic face plane of