Abstract:
A method of making a rough substrate includes: (a) forming a first oxide layer; (b) coating a photoresist layer; (c) exposing and developing the photoresist layer; (d) etching parts of the first oxide layer such that parts of the first oxide layer are formed into a plurality of sacrificial protrusions; (e) removing the photoresist regions; (f) depositing on the substrate layer and the sacrificial protrusions a second oxide layer; (g) etching the second oxide layer so as to leave portions of the second oxide layer; and (h) etching additionally the sacrificial protrusions, the substrate layer, and the portions of the second oxide layer, thereby producing a plurality of flat recess bottom faces, and substrate protrusions.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority of Taiwanese application No. 098100689, filed on Jan. 9, 2009. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    This invention relates to a method of making a substrate for a semiconductor device, more particularly to a method of making a substrate with a rough surface for growth of a semiconductor device thereon. 
         [0004]    2. Description of the Related Art 
         [0005]    A light-emitting device usually includes a substrate, an n-type semiconductor layer, a light-emitting layer, a p-type semiconductor layer, and electrodes. Light generated from recombination of electrons and holes is emitted in the light-emitting layer. 
         [0006]    When light enters an interface between the p-type semiconductor layer and the electrodes at an angle larger than a critical angle, the light is reflected to propagate laterally in the semiconductor layers. However, the light loses its energy during the propagation, thereby lowering the external quantum efficiency. An existing method is generally carried out by processing a light-emitting diode chip to be of a hemispherical form or of a pyramidal form such that light enters the interface at an angle less than the critical angle so as to reduce light reflection. However, such processing is difficult and may damage the chip. 
         [0007]    Another existing method includes roughening the surface of the light-emitting diode. However, the p-n junction may be damaged and the light-emitting efficiency may be adversely affected. 
         [0008]    A conventional semiconductor device includes a substrate having recesses or protrusions for scattering light generated in the light-emitting layer, thereby increasing the external quantum efficiency. The recesses or protrusions in the substrate are created by mechanical polishing or etching. Since the recesses or protrusions are randomly generated, the crystallinity of the grown nitride semiconductor structure is lowered, which adversely affects the light-emitting efficiency. In addition, the method of making the substrate is complicated and incurs high labor and manufacturing costs. 
         [0009]    U.S. Pat. No. 6,870,191 discloses a substrate provided with recesses/protrusions with a specific shape so as to increase crystallinity of the grown nitride semiconductor layers by virtue of the different growth rates of lateral and vertical growth of crystals. However, defects are easily produced at the interfaces of the nitride layers. 
         [0010]    U.S. Patent Application Publication No. 2005/0179130 discloses a semiconductor device and a method of making the same. The semiconductor device includes a substrate formed with recesses/protrusions each of which includes at least two surfaces having different inclination angles. However, the method is complicated and incurs high manufacturing costs. 
       SUMMARY OF THE INVENTION 
       [0011]    Therefore, an object of the present invention is to provide a method of making a rough substrate for growth of a semiconductor device that can address the problems encountered in the aforesaid prior art. 
         [0012]    According to the present invention, a method of making a rough substrate comprises: (a) forming a first oxide layer on a substrate layer; (b) coating a photoresist layer on the first oxide layer; (c) exposing and developing the photoresist layer to form a plurality of spaced-apart photoresist regions; (d) etching parts of the first oxide layer uncovered by the photoresist regions such that portions of the substrate layer are exposed and such that parts of the first oxide layer shielded by the photoresist regions are formed into a plurality of spaced-apart sacrificial protrusions on the substrate layer; (e) removing the photoresist regions on the sacrificial protrusions; (f) depositing on the substrate layer and the sacrificial protrusions a second oxide layer; (g) etching the second oxide layer so as to expose the sacrificial protrusions and portions of the substrate layer and so as to leave rounded lateral portions of the second oxide layer which surround the sacrificial protrusions, respectively, and which have a rounded surface profile; and (h) etching additionally the sacrificial protrusions and the substrate layer which have been exposed, and the rounded lateral portions of the second oxide layer which respectively surround the sacrificial protrusions until a plurality of flat recess bottom faces are formed in the substrate layer, thereby producing substrate protrusions protruding from the flat recess bottom faces. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    Other features and advantages of the present invention will become apparent in the following detailed description of the preferred embodiments of this invention, with reference to the accompanying drawings, in which: 
           [0014]      FIGS. 1   a  to  1   h  are sectional views to illustrate consecutive steps of the first preferred embodiment of a method of making a rough substrate according to this invention; 
           [0015]      FIG. 2  is a sectional view of the rough substrate made by the first preferred embodiment; 
           [0016]      FIGS. 3   a  to  3   f  are sectional views to illustrate consecutive steps of the second preferred embodiment of a method of making a rough substrate according to this invention; 
           [0017]      FIG. 4  is a sectional view of the rough substrate made by the second preferred embodiment; 
           [0018]      FIG. 5   a  is a top view of protrusions arranged in a matrix array; and 
           [0019]      FIG. 5   b  is a top view of the protrusions arranged in a random pattern. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0020]    Before the present invention is described in greater detail with reference to the accompanying preferred embodiments, it should be noted herein that like elements are denoted by the same reference numerals throughout the disclosure. 
         [0021]      FIGS. 1   a  to  1   h  illustrate the consecutive steps of a method of making a rough substrate for growth of a semiconductor device according to the first preferred embodiment of this invention. The semiconductor device includes a plurality of semiconductor layers. 
         [0022]    Referring to  FIGS. 1   a  and  1   b,  a first oxide layer  11  is formed on a substrate layer  10 . 
         [0023]    Referring to  FIG. 1   c,  a photoresist layer  12  is coated on the first oxide layer  11 , and is exposed and developed to form a plurality of spaced-apart photoresist regions  121 . 
         [0024]    Referring to  FIG. 1   d  in combination with  FIG. 1   c,  parts  111  of the first oxide layer  11  uncovered by the photoresist regions  121  are etched such that portions  101  of the substrate layer  10  are exposed and such that parts of the first oxide layer  11  shielded by the photoresist regions  121  are formed into a plurality of spaced-apart sacrificial protrusions  112  protruding from the substrate layer  10 . 
         [0025]    Referring to  FIG. 1   e,  the photoresist regions  121  on the sacrificial protrusions  112  are removed. 
         [0026]    Referring to  FIG. 1   f,  a second oxide layer  12  is deposited on the substrate layer  10  and the sacrificial protrusions  112 . 
         [0027]    Referring to  FIG. 1   g,  the second oxide layer  12  is etched so as to expose the sacrificial protrusions  112  and portions  105  of the substrate layer  10  and so as to leave rounded lateral portions  122  of the second oxide layer  12  which surround the sacrificial protrusions  112 , respectively, and which have a rounded surface profile. The rounded lateral portions  122  of the second oxide layer  12  are spaced apart from each other. The etching in this step may be wet etching or dry etching. 
         [0028]    Referring to  FIG. 1   h  in combination with  FIGS. 1   f  and  1   g,  the sacrificial protrusions  112  and the portions  105  of the substrate layer  10  which have been exposed, and the rounded lateral portions  122  of the second oxide layer  12  which respectively surround the sacrificial protrusions  112  are additionally etched until a plurality of flat recess bottom faces  100  are formed in the substrate layer  10 , thereby producing substrate protrusions  102  protruding from the flat recess bottom faces  100 . The etching in this step may be dry etching. 
         [0029]    The substrate layer  10  may be made from a suitable transparent or non-transparent material, or a conductive or nonconductive material. In this embodiment, the first and second oxide layers  11 ,  12  are made from silicon dioxide (SiO 2 ) or silicon nitride (SiN). The substrate layer  10  is made from a material selected from the group consisting of silicon (Si), sapphire, silicon carbide (SiC), spinel (MgAl 2 O 4 ), aluminum nitride (AlN), copper tungsten (CuW), and combinations thereof. 
         [0030]    It is worth mentioning that the sacrificial protrusions  112  and the rounded lateral portions  122  can serve as a mask for buffering the action of etching. Accordingly, when etching is conducted in step ( 1   g ) to etch the substrate layer  10 , the portions  105  of the substrate layer  10  uncovered by the sacrificial protrusions  112  and the rounded lateral portions  122  are etched first and recessed. Portions of the substrate layer  10  below the sacrificial protrusions  112  and the rounded lateral portions  122  are etched next and formed into the substrate protrusions  102 . The substrate protrusions  102  have a rounded surface profile corresponding in shape to the rounded lateral portions of the second oxide layer  12 . 
         [0031]    Preferably, the substrate protrusions  102  have the shape of a circle, an oval, a triangle, a quadrangle, a hexagon, a rhombus, or a polygon, when viewed from a top side of the substrate protrusions  102 . 
         [0032]    It is worth mentioning that each of the sacrificial protrusions  112  of the first oxide layer  11  and the rounded lateral portions  122  of the second oxide layer  12  can be varied in shape according to a desired light emitting power of the semiconductor device. 
         [0033]    Referring to  FIG. 2 , the rough substrate made by the first preferred embodiment of the method includes a plurality of the substrate protrusions  102  protruding from the flat recess bottom faces  100 . Each of the substrate protrusions  102  has a planar top surface  104 , and a rounded sidewall  103  that extends annularly and downwardly from the planar top surface  102  to a contiguous one of the flat recess bottom faces  100 . The substrate protrusions  102  are spaced apart from each other by a distance (A) ranging from 0.5 μm to 5 μm. The planar top surface  104  has a largest width (C) ranging from 0.5 μm to 5 μm. The rounded sidewall  103  has a top end  1031  meeting the planar top surface  104  and a bottom end  1032  meeting an adjacent one of the flat recess bottom faces  100 . The rounded sidewall  103  has a length from the top end  1031  to the bottom end  1032  that produces a projected length (B) when projected onto a projection plane parallel to the flat recess bottom face  100 . The projected length (B) is about 1-2 times a distance (A) between adjacent ones of the substrate protrusions  102 . Moreover, the rounded sidewall  103  has a tangent line intersecting the bottom end  1032  of the rounded sidewall  103 . The tangent line is inclined with a plane coplanar with the flat recess bottom faces  100  by an angle (θ) of about 25°-75°. The rounded sidewall  103  has a chordal line interconnecting the top and bottom ends  1031 ,  1032  thereof. The chordal line is inclined with a plane coplanar with the flat recess bottom faces  100  by an angle (θ m ) which is smaller than 45°. 
         [0034]    By virtue of the substrate protrusions  102 , defects of the semiconductor device can be reduced, thereby enhancing the external quantum efficiency and the light extraction efficiency. 
         [0035]      FIGS. 3   a  to  3   f  illustrate the consecutive steps of a method of making the rough substrate according to the second preferred embodiment of this invention. 
         [0036]    Referring to  FIGS. 3   a  and  3   b , a photoresist layer  12 ′ is coated on a substrate layer  10 ′. 
         [0037]    Referring to  FIG. 3   c , the photoresist layer  12 ′ is exposed and developed to form a plurality of spaced-apart photoresist regions  121 ′ on the substrate layer  10 ′. 
         [0038]    Referring to  FIG. 3   d , a reflective layer  13  is deposited on portions of the substrate layer  10 ′ uncovered by the photoresist regions  121 ′ and on the photoresist regions  121 ′. 
         [0039]    Referring to  FIG. 3   e  in combination with  FIG. 3   d , the photoresist regions  121 ′ are lifted-off such that the reflective layer  13  on the photoresist regions  121 ′ is removed and the reflective layer  13  left on the substrate layer  10 ′ is formed into a plurality of space-apart protrusions  131  protruding from a surface  101 ′ of the substrate layer  10 ′. 
         [0040]    Referring to  FIG. 3   f , the protrusions  131  are oxidized to produce oxidized skin layers  14  on the protrusions  131 , respectively. 
         [0041]    Preferably, the protrusions  131  have the shape of a circle, an oval, a triangle, a quadrangle, a hexagon, a rhombus, or a polygon, when viewed from above the protrusions  131 . 
         [0042]    Preferably, the reflective layer  13  is made of a material selected from the group consisting of aluminum (Al), silver (Ag), and combinations thereof. Alternatively, the reflective layer  13  can be a distributed Bragg reflector. 
         [0043]    Preferably, the substrate layer  10 ′ is made from a material selected from the group consisting of silicon (Si), sapphire, carbon silicon (SiC), spinel (MgAl 2 O 4 ), aluminum nitride (AlN), copper tungsten (CuW), and combinations thereof. 
         [0044]    Referring to  FIG. 4 , the rough substrate made by the second preferred embodiment of the method includes a plurality of the protrusions  131 . The protrusions  131  are spaced apart from each other by a distance (A′) ranging from 0.5 μm to 5 μm. Each of the protrusions  131  has a planar top surface  211 , and a truncated cone-shaped sidewall  212  extending annularly and downwardly from the planar top surface  211 . The planar top surface  211  has a width (C′) ranging from 0.5 μm to 5 μm. The truncated cone-shaped sidewall  212  has a top end  2121  meeting the planar top surface  211  and a bottom end  2122  meeting the surface  101 ′ of the substrate layer  10 ′. The truncated cone-shaped sidewall  212  has a length from the top end  2121  to the bottom end  2122  thereof, that produces a projected length (B′) on a projection plane coplanar with the surface  101 ′ of the substrate layer  10 ′. The projected length (B′) is 1-2 times a distance (A′) between adjacent ones of the protrusions  131 . 
         [0045]    In this embodiment, an inclining angle (θ m ′) of the truncated cone-shaped sidewall  212  with respect to the surface  101 ′ of the substrate layer  10 ′ is smaller than 45°. 
         [0046]    Likewise, by virtue of the protrusions  131 , defects of the semiconductor device can be reduced, thereby enhancing the external quantum efficiency and the light extraction efficiency. 
         [0047]    In addition, by oxidizing the protrusions  131 , the oxidized skin layers  14  on the protrusions  131  can be the same material as the substrate layer  10 ′. 
         [0048]    For example, the substrate layer  10 ′ is sapphire (Al 2 O 3 ) and the reflective layer  13  is made of aluminum (Al). When the reflective layer  13  is oxidized to produce the oxidized skin layer  14 , the oxidized skin layer  14  is aluminum oxide (Al 2 O 3 ) which is identical to the material of the sapphire substrate layer  10 ′. Therefore, the rough sapphire substrate has a surface layer that contains aluminum oxide (Al 2 O 3 ) like the remaining part of the rough sapphire substrate. 
         [0049]    Referring to  FIG. 5   a , the substrate protrusions  102  made by the first preferred embodiment, or the protrusions  131  made by the second preferred embodiment have a circular profile when viewed from a top side and are arranged in a matrix array. 
         [0050]    Referring to  FIG. 5   b , the substrate protrusions  102  or the protrusions  131  can be arranged in a random pattern. 
         [0051]    It is worth mentioning that when the substrate protrusions  102  or protrusions  131  are regularly formed, external extraction efficiency of the light-emitting device can be increased and crystal defects in the semiconductor layers can be prevented when grown on the substrate of this invention. 
         [0052]    With the invention thus explained, it is apparent that various modifications and variations can be made without departing from the spirit of the present invention. It is therefore intended that the invention be limited only as recited in the appended claims.