Abstract:
A light-emitting diode array comprising a conductive layer formed on a substrate, separate light-emitting portions formed on the conductive layer, a first electrode formed on at least part of an upper surface of each light-emitting portion, and a second electrode formed on the conductive layer adjacent to the light-emitting portions; the first electrode comprising a common switching electrode matrix; the second electrode comprising a common electrode divided such that one second electrode exists in every block; and at least one of bonding pads extending to the first common electrode and the second common electrode being formed on a bonding portion formed on the conductive layer like an island, whereby the bonding pads are separate from each other.

Description:
FIELD OF THE INVENTION 
   The present invention relates to a light-emitting diode array of high emission power, and particularly to a light-emitting diode array suitable for light sources of electrophotographic printers. 
   BACKGROUND OF THE INVENTION 
   An electrophotographic printer forms an electrostatic latent image on a photosensitive drum by light according to an image signal, develops the latent image by selectively attracting toner thereto, and then transfers the developed image onto a paper to obtain a print image. As a light source for forming the electrostatic latent image, a laser and a light-emitting diode array are widely used. In particular, since a light source constituted by the light-emitting diode array does not need a long optical path unlike the laser-type light source, it is suitable for small-sized printers and large-sized printing. Demand has been mounting recently on light-emitting diode arrays of higher precision, higher emission power and low in cost, as printing has been becoming faster with higher image quality, and as printers have been becoming smaller. 
   Usable to produce low-cost light-emitting diode arrays are not the provision of ICs for separately driving light-emitting diodes (static driving system), but the arrangement of light-emitting diodes in the number of the n-th power of 2 in one block, which are subjected to time division operation by a switching matrix wiring, thereby reducing the number of driving ICs and bonding wires (dynamic driving system). 
   In the production of such light-emitting diode arrays, Ti/Au and Mo/Au are widely used as wiring materials. The formation of these wirings may be conducted by a lift-off method, which comprises photoresist patterning, electron beam-heating vapor deposition and the removal of unnecessary portions together with the photoresist, and a sputtering/ion-milling method, which comprises depositing Ti/Au, Mo/Au, etc. onto the entire wafer by sputtering, and physically removing a patterned photoresist and an insulating mask of SiO 2 , etc. Because the lift-off method has insufficient step coverage, wiring layers in LEDs having large mesa-etched steps are easily broken at the steps. Though wiring layers formed by the sputtering/ion-milling method have good step coverage and are resistant to breakage at the steps, Au disadvantageously tends to remain at the mesa-etched steps. Particularly, LEDs of the dynamic driving system comprising common electrodes are disadvantageous in the short-circuiting of wirings by Au remaining after working, resulting in undesired light emission. 
     FIG. 4  is a plan view showing a conventional light-emitting diode array of a two-divided dynamic driving system.  FIG. 5(   a ) is a plan view showing one of blocks shown in  FIG. 4 .  FIG. 5(   b ) is a cross-sectional view taken along the line A-A in  FIG. 5(   a ), and  FIG. 5(   c ) is a cross-sectional view taken along the line B-B in  FIG. 5(   a ). A plurality of light-emitting portions  1  is arranged at predetermined intervals on a p-type GaAs conductive layer  11  formed on an n-type GaAs substrate  10 . Each light-emitting portion  1  comprises a p-type AlGaAs etching stopper layer  12 , a p-type AlGaAs clad layer  13 , a p-type AlGaAs active layer  14 , an n-type AlGaAs clad layer  15 , and an n-type GaAs cap layer  16 , which are laminated on the p-type GaAs conductive layer  11  in this order. The light-emitting area of the light-emitting portion  1  has a double-hetero structure composed of the p-type AlGaAs clad layer  13 , the p-type AlGaAs active layer  14 , and the n-type AlGaAs clad layer  15 . 
   Each light-emitting portion  1  is formed by removing part of epitaxial layers by mesa etching. The mesa-etched groove comprises a first mesa-etched groove  19  separating the light-emitting portions  1  from bonding portions  8   a ,  8   c , and second mesa-etched grooves  20  dividing blocks from each other. 
   Each light-emitting portion  1  has a cathode  2  formed on part of the upper surface thereof. An anode  3  provided on each p-type GaAs conductive layer  11  adjacent to the light-emitting portion  1  is formed by vapor-depositing metals on the p-type GaAs conductive layer  11  and turning the metals to an alloy. Except for contact holes  7   c ,  7   a  for the cathode  2  and the anode  3 , the light-emitting portions  1  and an exposed surface of the conductive layer  11  are covered by a first insulating film  17  made of PSG (phosphorus glass). Common electrodes  4 ,  4  are formed on the first insulating film  17  covering the bonding portion (mesa-top surface)  8   c . After covering the common electrodes  4 ,  4  with a second insulating film  18 , an Au wiring layer  5   c  is formed, such that it extends from one end connected to the cathode  2  not covered by the second insulating film  18 , to the other end formed on a bonding portion  8   c , which is connected to one of the common electrodes  4 ,  4  and constitutes a bonding pad  6   c . An Au wiring layer  5   a  is formed, such that it extends from one end connected to the anode  3  to the other end on the surface of the bonding portion (mesa-top surface)  8   a , which constitutes a bonding pad  6   a.    
   When the Au wiring layers  5  are formed on the light-emitting diode array of such a structure by a sputtering/ion-milling method, Au tends to remain on the slanting surfaces  21  of the first mesa-etched groove  19  after ion-milling, unsatisfactory to prevent the short-circuiting of Au wirings (see JP 2000-323750 A). Though this problem can be solved by removing all epitaxial layers by the first mesa-etched groove  19  except for the light-emitting portions  1 , and forming wiring layers, common electrodes and bonding pads on the bottom surface of the mesa-etched groove, another problem occurs. That is, when all epitaxial layers are etched except for the light-emitting portions  1 , there is difference in depth between large etching areas and small etching areas between the light-emitting portions. Because an etching speed depends on an area to be etched (loading effect), it is extremely difficult to control the dimension the light-emitting portions affecting the performance of the light-emitting diode array. 
   JP 10-157193 A discloses a light-emitting diode array having recesses in part of a semiconductor substrate, in which common electrodes are formed. However, because only common electrodes are formed in the recesses in this system, Au remaining on the slanting surfaces of the mesa-etched grooves tends to cause short-circuiting. 
   OBJECTS OF THE INVENTION 
   Accordingly, an object of the present invention is to provide a light-emitting diode array with a high yield and excellent mass producibility and free from the short-circuiting of wirings due to Au remaining after working on slanting surfaces of mesa-etched steps, which comprises light-emitting portions with easy dimension control. 
   Another object of the present invention is to provide a method for producing such a light-emitting diode array. 
   DISCLOSURE OF THE INVENTION 
   As a result of intense research in view of the above-mentioned objects, the inventors have found that by forming bonding pads on bonding portions (mesa-top surfaces) separated by mesa-etched grooves, the short-circuiting of wirings due to Au remaining after working can be prevented. The present invention has been completed based on this finding. 
   Thus, the light-emitting diode array of the present invention comprises a conductive layer formed on a substrate, separate light-emitting portions formed on the conductive layer, a first electrode formed on at least part of an upper surface of each light-emitting portion, and a second electrode formed on the conductive layer adjacent to the light-emitting portions; the first electrode comprising a common switching electrode matrix; the second electrode comprising a common electrode divided such that one second electrode exists in every block; and at least one of bonding pads extending to the first common electrode and the second common electrode being formed on a bonding portion formed on the conductive layer like an island, whereby the bonding pads are separate from each other. 
   One of a bonding pad on the side of the first electrode and a bonding pad on the side of the second electrode may be formed on an insulating layer formed in a mesa-etched groove. The common electrode matrix is preferably formed on an insulating layer formed in a mesa-etched groove. 
   In the light-emitting diode array of the present invention, the light-emitting portions and the bonding portions are preferably formed by separating or dividing epitaxial layers formed on the conductive layer by mesa-etched grooves. In the light-emitting diode array according to a preferred embodiment, the bonding pads are formed on separate bonding portions, so that there is no short-circuiting between the bonding pads even though Au wiring remains on slanting surfaces of the mesa-etched grooves. Also, because the bonding portions are formed on the epitaxial layers remaining after the formation of the first mesa-etched groove, an etched area need not be increased. It is thus possible to avoid a loading effect, and easy to control the dimension of the light-emitting portions, epitaxial layers left after the formation of the first mesa-etched groove. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a plan view showing the light-emitting diode array according to one embodiment of the present invention; 
       FIG. 2(   a ) is a plan view showing a unit cell constituting the light-emitting diode array shown in  FIG. 1 ; 
       FIG. 2(   b ) is a cross-sectional view taken along the line A-A in  FIG. 2(   a ); 
       FIG. 2(   c ) is a cross-sectional view taken along the line B-B in  FIG. 2(   a ); 
       FIG. 3(   a ) is a plan view showing a unit cell constituting the light-emitting diode array according to another embodiment of the present invention; 
       FIG. 3(   b ) is a cross-sectional view taken along the line A-A in  FIG. 3(   a ); 
       FIG. 4  is a plan view showing a conventional light-emitting diode array; 
       FIG. 5(   a ) is a plan view showing a unit cell constituting the light-emitting diode array shown in  FIG. 4 ; 
       FIG. 5(   b ) is a cross-sectional view taken along the line A-A in  FIG. 5(   a ); and 
       FIG. 5(   c ) is a cross-sectional view taken along the line B-B in  FIG. 5(   a ). 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   [1] Structure of Light-emitting Diode Array 
   As shown in  FIGS. 1 and 2 , the light-emitting diode array of the present invention comprises a substrate  10 , a plurality of light-emitting portions  1  formed on the substrate  10 , a first electrode  2  partially formed on a top surface of each light-emitting portion  1 , and a second electrode  3  formed on the conductive layer  11  adjacent to the light-emitting portions  1 . In the depicted embodiment, each light-emitting portion  1  is a separate epitaxial layer portion obtained by dividing an epitaxial layer uniformly formed on the substrate  10  by mesa-etched grooves  19 ,  20 . 
   (1) Substrate 
   The substrate  10  is not particularly restrictive as long as it may be used for light-emitting diodes, and it need only have a structure capable of electrically insulating the light-emitting parts. It may be either an n-type substrate or a p-type substrate, and a semi-insulating substrate such as a semi-insulating GaAs substrate, etc. or an insulating substrate may also be used. The substrate  10  and the conductive layer  11  may be insulated from each other by forming a high-resistance layer such as an undoped GaAs layer, etc., or by forming a semiconductor layer having an opposite polarity to that of the conductive layer  11 . 
   (2) Light-emitting Portion 
   The type and thickness of a compound semiconductor crystal layer laminated on the conductive layer  11  on the substrate  10  may be properly determined depending on the desired light emission wavelength and light-emitting power. Usable as the compound semiconductors are, for instance, AlGaAs, AlGaInP, etc. The light-emitting portion  1  preferably has a double hetero structure comprising a clad layer of a first conductor type, an active layer and a clad layer of a second conductor type, which is preferably formed by dividing an epitaxial layer formed on the conductive layer  11  by a first mesa-etched groove  19 . A second mesa-etched groove  20  divides the conductive layer  11  to each block. 
   The light-emitting diode array according to the depicted embodiment of the present invention comprises separate light-emitting portions  1  each constituted by a p-type AlGaAs etching stopper layer  12 , a p-type AlGaAs clad layer  13 , a p-type AlGaAs active layer  14 , an n-type AlGaAs clad layer  15  and an n-type GaAs cap layer  16  formed in this order on an n-type GaAs substrate  10  via a p-type GaAs conductive layer  11 . The n-type GaAs cap layer  16  is removed by etching in regions of light-emitting portions  9 . To prevent short-circuiting to an Au wiring layer  5 , insulating layers  17 ,  18  cover the entire surface of each light-emitting portion  1  except for each cathode  2 . 
   The above light-emitting portion  1  has a so-called double hetero structure in a region directly contributing to light emission, which comprises a p-type AlGaAs active layer  14  having an energy band gap corresponding to an emission wavelength sandwiched by a p-type AlGaAs clad layer  13  (clad layer of a first conductor type) and an n-type AlGaAs clad layer  15  (clad layer of a second conductor type) both having larger energy band gaps than that of the active layer  14 . 
   (3) Electrode and Wiring Layer 
   What is required is only that one of the first and second electrodes is a cathode, while the other is an anode. Accordingly, for instance, the first electrode per se may be either a cathode or an anode. Each electrode is required to have good bonding characteristics as well as good ohmic contact to underlayers. For instance, an AuZnINi/Au laminate electrode may be used as the anode, and an AuGe/Ni/Au laminate electrode may be used as the cathode. 
   Each of the common electrodes and the Au wiring layer is preferably constituted by a plurality of metal layers, because it is required to have good bonding characteristics as well as good adhesion to an upper layer and an underlayer. Each of the uppermost layer and the undermost layer is preferably a metal layer such as Ti, Mo, etc. having good bonding characteristics as well as good adhesion to insulating layers. For instance, a laminate electrode such as Ti/Au/Ti, Mo/Au/Mo and TiW/Au/TiW may be used as a common electrode. 
   In each electrode, the metal layers can be formed by a method such as a resistance-heating vapor deposition method, an electron beam-heating vapor deposition method, sputtering method, etc., and the oxide layer can be formed by various known film-forming methods. A heat treatment (alloying) is preferably performed on the metal layers, to make an ohmic electrode. 
   In the embodiment shown in  FIGS. 1 and 2 , the cathode  2  is formed on the mesa-top surface of each light-emitting portion  1 , and the side surfaces of the light-emitting portion  1  and the cathode  2  are covered by the first insulating layer  17 . The cathode  2  exposed in a contact hole  7  on each light-emitting portion  1  is connected to one of two common electrodes  4 ,  4  via an Au wiring layer  5   c , forming a bonding pad  6   c  on a separate bonding portion  8   c . An anode  3  is formed on the p-type GaAs conductive layer  11  at a position adjacent to each light-emitting portion  1  in an ohmic contact manner. The anode  3  is connected to a bonding pad  6   a  on a mesa-top surface of a bonding portion  8   a  via an Au wiring layer  5   a.    
   (4) Mesa-etched Groove 
   The mesa-etched groove for forming separate light-emitting portions  1  and separate bonding portions  8  comprises a first mesa-etched groove  19  reaching the conductive layer  111  or the etching stopper layer  12  for electrically separating the light-emitting portions  1  from the bonding portions  8 , and second mesa-etched grooves  20  removing the conductive layer  11  for separating blocks of the light-emitting portion  1  from each other. The first mesa-etched groove  19 , a characteristic constituent of this light-emitting diode array, forms not only the separate light-emitting portions  1  but also the separate bonding portions  8 . 
   [2] Production of Light-emitting Diode Array 
   The light-emitting diode array of the present invention can be produced by the same methods as the conventional ones except for forming separate bonding portions  8  by the first mesa-etched groove  19 . In a preferred embodiment, a p-type GaAs conductive layer  11  (carrier concentration: 4×10 19  cm −3 , thickness: 1 μm), a p-type AlGaAs etching stopper layer  12  (carrier concentration: 3×10 19  cm −3 , thickness: 0.1 μm), a p-type AlGaAs clad layer  13  (carrier concentration: 1×10 18  cm −3 , thickness: 1 μm), a p-type AlGaAs active layer  14  (carrier concentration: 1×10 18  cm −3 , thickness: 1 μm), an n-type AlGaAs clad layer  15  (carrier concentration: 2×10 18  cm −3 , thickness: 3 μm), and an n-type GaAs cap layer  16  (carrier concentration: 1×10 18  cm −3 , thickness: 0.5 μm) are grown in this order on a top surface of the n-type GaAs substrate  10  by a metal-organic vapor phase epitaxy (MOVPE) method. 
   The resultant crystal layers are subjected to selective wet etching. First, the n-type GaAs cap layer  16  is removed in the light-emitting portions  1  except its portions in contact with the cathodes  2 . Next, the first mesa-etched groove  19  is formed to such a depth that the p-type AlGaAs etching stopper layer  12  is exposed, to divide the epitaxial layers on the p-type GaAs conductive layer  11  to plural light-emitting portions  1  separated from the bonding portions  8   a ,  8   c . Either one of bonding portions  8   a ,  8   c  may be removed as shown in  FIG. 3  as long as the loading effect is avoided. Further, the p-type GaAs conductive layer  11  is removed by the second mesa-etched grooves  20  to electrically separate each block of the light-emitting portions  1 . In this case, the second mesa-etched grooves  20  preferably have such depth that the n-type GaAs substrate  10  is slightly etched, so that no conductive layer  11  remains in areas corresponding to the second mesa-etched grooves  20  even though there are etching errors. 
   The cathode  2  (AuGe/Ni/Au) and the anode  3  (AuZn/Ni/Au) are formed by a vapor deposition method and a lift-off method. Next, the first insulating layer  17  is grown on the entire upper surface of the light-emitting diode array by a chemical vapor deposition (CVD) method, and the common electrodes  4 ,  4  (Ti/Au/Ti) are then formed by a vapor deposition method and a lift-off method. Further, after the second insulating layer  18  is grown by the CVD method, contact holes  7   c ,  7   a ,  7   k  are formed in both insulating layers  17 ,  18  or only in the insulating layer  18  by etching on the cathode  2 , the anode  3  and desired one of two common electrodes in each block. Ti/Au is sputtered and etched by ion-milling to form Au wiring layers  5  extending to each bonding portion  8 . 
   Though the light-emitting diode array having a 2×2 structure comprising two common electrodes has been explained here, the present invention is also applicable to those having a 4×4 structure comprising four light-emitting portions in one block with four common electrodes, those having a 8×8 structure, a 16×16 structure, etc. 
   As described above in detail, the light-emitting diode array of the present invention comprises bonding portions separated by the mesa-etched grooves. Accordingly, no short-circuiting would occur in wiring, even though there are Au wiring layers remaining on slanting surfaces of the mesa-etched grooves, or even though the light-emitting diode array is designed such that mesa-etched slanting surfaces are covered with the Au wiring layers, and it is easy to control the dimension of the light-emitting portions, resulting in a high-yield mass production process.