Abstract:
A method for fabricating a semiconductor lighting chip includes steps of: providing a substrate with a first block layer dividing an upper surface of the substrate into a plurality of epitaxial regions; forming a first semiconductor layer on the epitaxial regions; forming a second block layer partly covering the first semiconductor layer; forming a lighting structure on an uncovered portion of the first semiconductor layer; removing the first and the second block layers thereby defining clearances at the bottom surfaces of the first semiconductor layer and the lighting structure; and permeating etching solution into the first and second clearances to etch the first semiconductor layer and the lighting structure, thereby to form each of the first semiconductor layer and the lighting structure with an inverted frustum-shaped structure.

Description:
TECHNICAL FIELD 
       [0001]    The disclosure generally relates to a method for fabricating a semiconductor lighting chip. 
       DESCRIPTION OF RELATED ART 
       [0002]    In recent years, due to excellent light quality and high luminous efficiency, light emitting diodes (LEDs) have increasingly been used as substitutes for incandescent bulbs, compact fluorescent lamps and fluorescent tubes as light sources of illumination devices. 
         [0003]    The LED generally includes a lighting chip, which includes an n-type semiconductor layer, an active layer and a p-type semiconductor layer sequentially formed on a substrate. When a voltage is applied between the n-type semiconductor layer and the p-type semiconductor layer, hole-electron recombination will happen at the active layer, and energy is released in the form of light. 
         [0004]    In order to improve luminescent efficiency of the lighting chip, the lighting chip is etched to form an inverted frustum-shaped structure, in which a width of the lighting chip gradually decreases from an upper surface to a bottom surface thereof. Therefore, more light will travel to the external environment through inclined sidewalls of the lighting chip. However, the lighting chip is generally etched at a temperature higher than 170° C.; such a high temperature may affect lighting properties of the lighting chip. 
         [0005]    Therefore, a method for fabricating a semiconductor lighting chip is desired to overcome the above described shortcomings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    Many aspects of the disclosure can be better understood with reference to the following drawings. The components in the drawings are not necessarily drawn to scale, the emphasis instead being placed upon clearly illustrating the principles of the disclosure. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views. 
           [0007]      FIG. 1  is a top view of a substrate and a first block layer in a first step of a method for fabricating a semiconductor lighting chip according to an embodiment of the present disclosure. 
           [0008]      FIG. 2  is a side view of the substrate and the first block layer in  FIG. 1 . 
           [0009]      FIG. 3  is a schematic view showing a semiconductor structure formed by a second step of the method for fabricating the semiconductor lighting chip. 
           [0010]      FIG. 4  is a schematic view showing the semiconductor structure formed by a third step of the method for fabricating the semiconductor lighting chip. 
           [0011]      FIG. 5  is a schematic view showing the semiconductor structure formed by a fourth step of the method for fabricating the semiconductor lighting chip. 
           [0012]      FIG. 6  is a schematic view showing the semiconductor structure formed by a fifth step of the method for fabricating the semiconductor lighting chip. 
           [0013]      FIG. 7  is a schematic view showing the semiconductor structure formed by a sixth step of the method for fabricating the semiconductor lighting chip. 
           [0014]      FIG. 8  is a schematic view showing the semiconductor structure formed by a seventh step of the method for fabricating the semiconductor lighting chip. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    An embodiment of a method for fabricating a semiconductor lighting chip will now be described in detail below and with reference to the drawings. 
         [0016]    Referring to  FIGS. 1-2 , a substrate  10  is firstly provided. Material of the substrate  10  can be selected from a group consisting of Si, SiC, GaN and sapphire. In this embodiment, the substrate  10  is made of sapphire. And then, a first block layer  20  is formed on an upper surface of the substrate  10  by vacuum evaporation or sputtering. The first block layer  20  is in the form of a two-dimensional grid. The first block layer  20  is made of SiO 2  or Si 3 N 4  to prevent semiconductor layers from growing thereon. In this embodiment, the first block layer  20  is made of SiO 2  and divides the upper surface of the substrate  10  into a number of epitaxial regions  12  for growth of semiconductor layers. The epitaxial regions  12  each are rectangular. A width of the rectangular epitaxial region  12  is about 300 μm and a distance between two neighboring epitaxial regions  12  is about 20 μm. In other words, a width of each line of the grid formed by the first block layer  20  is about 20 μm. 
         [0017]    Referring to  FIG. 3 , a first semiconductor layer  30  is formed on the epitaxial regions  12  by metal organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE) or hydride vapor phase epitaxy (HYPE). The first semiconductor layer  30  can be an n-type GaN layer, an AlGaN layer, an AlInGaN layer or a GaAs layer, and a thickness thereof is about 2 μm. The first block layer  20  prevents the first semiconductor layer  30  from directly growing thereon and divides the first semiconductor layer  30  into a number of individual islands. To a certainty, the first semiconductor layer  30  growing on the epitaxial regions  12  will cover a part of the first block layer  20  due to lateral growth thereof. By controlling the growth condition, the first semiconductor layer  30  can be limited to cover only a peripheral edge of the first block layer  20 , thereby exposing a central part of the first block layer  20 . For reducing defects caused by the lattice mismatch between the first semiconductor layer  30  and the substrate  10 , a buffer layer  40  is grown on the epitaxial regions  12  of the substrate  10  before forming the first semiconductor layer  30 . The buffer layer  40  is made of GaN or AlN, and a thickness thereof is about 20 nm. 
         [0018]    Referring to  FIG. 4 , a second block layer  50  is formed on an upper surface of the first semiconductor layer  30  by vacuum evaporation or sputtering. The material of the second block layer  50  is similar to that of the first block layer  20 , such as SiO 2  or Si 3 N 4 . In this embodiment, the second block layer  50  is made of SiO 2  and is partly etched away to cover a right portion of an upper surface of each island of the first semiconductor layer  30 , whereby a left portion of the upper surface of each island of the first semiconductor layer  30  is exposed. An area of the second block layer  50  on each island is smaller than a half of the upper surface of the island. 
         [0019]    Referring to  FIG. 5 , a lighting structure  60  is formed on the exposed left portion of the upper surface of each island of the first semiconductor layer  30  with a width gradually decreased from the first semiconductor layer  30  toward a top of the lighting structure  60 . The lighting structure  60  includes a second semiconductor layer  62 , an active layer  64  and a third semiconductor layer  66  formed on the first semiconductor layer  30  in sequence by MOCVD, MBE or HYPE. The second semiconductor layer  62  not only covers the exposed left portion of the upper surface of the corresponding island, but also partly covers the second block layer  50  on the corresponding island. In this embodiment, the second semiconductor layer  62  is an n-type GaN layer, the active layer  64  is a multiple quantum well (MQW) GaN layer, and the third semiconductor layer  66  is a p-type GaN layer. A thickness of the lighting structure  60  is the same as the thickness of the first semiconductor layer  30 . In this embodiment, the thickness of the lighting structure  60  is about 2 μm. 
         [0020]    Referring to  FIG. 6 , the first block layer  20  and the second block layer  50  are removed by a buffered oxide etch (BOE) solution, which is a mixture of hydrofluoric acid (HF) and ammonium fluoride (NH 4 F). The BOE solution can effectively etch away the SiO 2  layer without damaging other semiconductor layers. After the first block layer  20  and the second block layer  50  are removed, a number of clearances  200  are formed at the position of the first block layer  20 , thereby exposing a part of a bottom of the first semiconductor layer  30 , and a number of clearances  500  are formed at the position of the second block layer  50 , thereby exposing a part of a bottom of the second semiconductor layer  62  of the lighting structure  60 . 
         [0021]    And then, the first semiconductor layer  30  and the lighting structure  60  on the substrate  10  each are etched by chemical etching to form an inverted frustum-shaped structure. In this embodiment, KOH is adopted for etching the first semiconductor layer  30  and the lighting structure  60 . A concentration of the KOH is between 2 mol/L and 7 mol/L, an etching time is between 5 minutes and 30 minutes, and an etching temperature is lower than 100° C. In this embodiment, the first semiconductor layer  30  and the lighting structure  60  are etched at a temperature of 75° C. for 15 minutes by a 2 mol/L KOH solution, thereby to obtain an acquired shape of the semiconductor lighting chip. 
         [0022]    Because the first block layer  20  and the second block layer  50  are removed before etching, the KOH solution can permeate into the clearances  200  and  500 , thereby to etch the first semiconductor layer  30  and the lighting structure  60  from the bottom surfaces thereof simultaneously. During the etching, the etching solution removes sidewalls of the chip. Therefore, the etching can be accelerated at a temperature lower than 100° C. to achieve the desired shapes of the lighting structure  60  and the first semiconductor layer  30 , without the necessity of higher etching temperature. Therefore, an inverted frustum-shaped structure of the first semiconductor layer  30  and of the lighting structure  60  is obtained in a relatively low temperature. In addition, the required etching time is reduced in accordance with the present disclosure. 
         [0023]    Referring to  FIG. 7 , after the etching process, a width of the first semiconductor layer  30  gradually decreases from an upper surface to a bottom surface thereof, and a width of the lighting structure  60  gradually decreases from an upper surface to a bottom surface thereof. Because planes (10-1-1) and (11-2-2) of GaN structure are hard to be etched by the KOH solution, the two planes (10-1-1) and (11-2-2) will be left after the etching process. Therefore, an included angle α between sidewalls of the first semiconductor layer  30  and the bottom surface thereof is in a range from 57 degrees to 62 degrees. Similarly, an included angle between sidewalls of the lighting structure  60  and the bottom surface thereof is in a range from 57 degrees to 62 degrees. 
         [0024]    Finally, a transparent conductive layer  70  is formed on an upper surface of the third semiconductor layer  66  by vacuum evaporation, sputtering, chemical deposition, or E-gun evaporation. A second electrode  82  is formed on an upper surface of the transparent conductive layer  70  by E-gun evaporation, vacuum evaporation or sputtering. A first electrode  80  is formed on the first semiconductor layer  30  by E-gun evaporation, vacuum evaporation or sputtering. The transparent conductive layer  70  can be made of ITO or Ni/Au alloy; therefore current will spread in the third semiconductor layer  66  uniformly. In this embodiment, the first electrode  80  and the second electrode  82  are made of metal to connect the semiconductor lighting chip with an external power source. Finally, the substrate  10  can be cut to form a plurality of semiconductor lighting chips. 
         [0025]    It is believed that the present embodiments and their advantages will be understood from the foregoing description, and it will be apparent that various changes may be made thereto without departing from the spirit and scope of the disclosure or sacrificing all of its material advantages, the examples hereinbefore described merely being preferred or exemplary embodiments of the disclosure.