Patent Publication Number: US-9429867-B2

Title: Semiconductor apparatus, exposing head, and image forming apparatus

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a semiconductor apparatus, an exposing apparatus, and an image forming apparatus, and may be advantageously applied to a print head in which a plurality of light emitting element array chips are aligned on a circuit board. 
     2. Description of the Related Art 
     A conventional light emitting diode (LED) print head, which is used in an LED printer, employs a configuration in which a plurality of semiconductor light emitting element array chips are mounted on a print wiring board and are aligned in a straight line. Each array chip has a plurality of light emitting portions formed in its surface, the light emitting portions being aligned in one dimension at predetermined intervals. 
     This type of LED print head is assembled by first applying an adhesive to a print wiring board, and then semiconductor light emitting element array chips are pressed on the adhesive against the print wiring board, and finally allowing the adhesive to cure. In this manner, the semiconductor light emitting element array chips secured on the print wiring board. 
     If the space between adjacent chips is too narrow, the adhesive is drawn into the space by capillary action up to the same level as the surface of the array chips, soiling the light emitting portions and causing usable light power to decrease. The adhesive may also contaminate the wire bonding pads formed on the end portion of the surface of the array chips, reducing the mechanical strength of the wire bonded portions. 
     Japanese Patent Laid-Open No. 2011-131475 discloses a print head in which the adhesive is applied only to a limited surface area on the print wiring board, e.g., a middle portion of the back surface of the array chip, thereby preventing the adhesive from being drawn into the space between adjacent array chips up to the upper surface of the array chips. 
     However, applying the adhesive only to a middle portion of the back surface of the array chip causes end portions of the back surface to be uplifted, so that edges of the array chip may be chipped during wire bonding or the array chip may be inclined at an angle with respect to the surface of the print wiring board. 
     If end portions of the back surface of the array chip are uplifted from the print wiring board, heat dissipation is more difficult at the end portions than in the middle portion of the back surface, so that the temperature of the end portions is higher than the middle portion. 
     Although applying the adhesive only to a limited portion of the back surface is effective in preventing the adhesive from contaminating the chip surface, but may increase the chances of the chips inclining and being damaged, impairing the reliability of the LED print head. 
     SUMMARY OF THE INVENTION 
     The present invention was made to solve the aforementioned drawbacks. 
     An object of the present invention is to provide a semiconductor apparatus in which semiconductor array chips are protected against contamination by an adhesive, an exposing head that employs the semiconductor apparatus, and an image forming apparatus that employs the exposing head. 
     A semiconductor apparatus includes a rectangular plate-like body including a major surface. A plurality of light emitting portions is formed in the major surface, and aligned in a straight line. A first terraced portion and a second terraced portion are formed in the major surface except areas in which the plurality of light emitting portions are formed. 
     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and wherein: 
         FIG. 1  illustrates the outline of an LED printer according to the present invention; 
         FIG. 2A  is a perspective view of an LED print head in its entirety; 
         FIG. 2B  is a perspective cross-sectional view taken along a line A-A in  FIG. 2A ; 
         FIG. 3A  illustrates the appearance of a chip-on-board module (COB); 
         FIG. 3B  is a partial expanded view of a pertinent portion of the COB shown in  FIG. 3A ; 
         FIG. 4A  is a further expanded perspective view of the portion P shown in  FIG. 3B ; 
         FIG. 4B  illustrates the positional relationship among two consecutive odd-numbered array chips and an even numbered array chip between the two consecutive odd-numbered array chips; 
         FIG. 5A  is a perspective view of a comparison COB in which semiconductor light emitting element array chips with no terraced portion are mounted on a print wiring board; 
         FIG. 5B  is an expanded view of a portion P shown in  FIG. 5A ; 
         FIG. 6  illustrates how an adhesive climbs up the gap between adjacent array chips and flows on the surface of the adjacent array chips of the comparison COB shown in  FIG. 5B ; 
         FIG. 7  illustrates how the adhesive climbs up the gap between the adjacent array chips and flows on the surface of the adjacent array chips of the COB according to the first embodiment; 
         FIG. 8  is a partial top view of the array chip, illustrating the distance between an endmost light emitting portion and a longitudinal end of the array chip; 
         FIG. 9  illustrates a modification of the first embodiment; 
         FIG. 10  illustrates how the adhesive flows on modified array chips; 
         FIGS. 11A and 11B  illustrate a wall that connects the top surface of the array chip and a recessed surface of the terraced portion; 
         FIG. 12A  is a partial perspective view of the COB according to a second embodiment; 
         FIG. 12B  is an expanded view of a relevant portion P of the COB shown in  FIG. 12A ; 
         FIG. 13A  is another expanded view of the relevant part of the COB shown in  FIG. 12B ; 
         FIG. 13B  illustrates the positional relationship among two consecutive odd-numbered array chips and an even numbered array chip between the two consecutive odd-numbered array chips; 
         FIG. 14A  illustrates the appearance of a comparison COB of the second embodiment; 
         FIG. 14B  is an expanded view of a portion P shown in  FIG. 14A ; 
         FIG. 15  illustrates how the adhesive climbs up the gap between adjacent array chips of the comparison COB and flows on the array chips; 
         FIG. 16  illustrates how the adhesive climbs up and flows on the surface of the adjacent array chips of the COB according to the second embodiment; 
         FIG. 17A  is a partial perspective view of a COB according to a third embodiment; 
         FIG. 17B  is a cross-sectional view taken along a line B-B in  FIG. 17A ; 
         FIG. 17C  illustrates the positional relationship among two consecutive odd-numbered array chips and an even numbered array chip between the two consecutive odd-numbered array chips; 
         FIG. 17D  is an expanded view of a projection; 
         FIG. 17E  is an expanded view of rounded corners; 
         FIG. 18  illustrates that a ball is out of an angular range in which the light emitting portions emit light; 
         FIG. 19A  is a partial perspective view of a COB according to the third embodiment; 
         FIG. 19B  is a cross-sectional view taken along a line C-C in  FIG. 19A ; 
         FIG. 19C  illustrates the positional relationship among two adjacent odd-numbered array chips and an even numbered array chip between the two adjacent odd-numbered array chips; 
         FIG. 20A  is a perspective view of the comparison COB on which semiconductor light emitting element array chips are mounted; 
         FIG. 20B  is a cross-sectional view taken along a line D-D in  FIG. 20A ; and 
         FIG. 21  illustrates an angular range R in which the light emitting portions emit light. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention will be described in detail by way of preferred embodiments with reference to the accompanying drawings. 
     First Embodiment 
     Overall Configuration of LED Printer 
       FIG. 1  illustrates the outline of an LED printer  1  according to the present invention. The LED printer  1  includes a generally box-shaped chassis  2 . 
     The chassis  2  accommodates four process units  3 A,  3 B,  3 C, and  3 D for forming yellow (Y), magenta (M), cyan (C), and black (K) images, respectively, by electrophotography. The four process units  3 A,  3 B,  3 C, and  3 D are aligned along a transport path  4  of a recording medium P. 
     Each of the process units  3 A,  3 B,  3 C, and  3 D includes a photoconductive drum  5  as an image bearing body, a charging unit  6 , an exposing unit  7 , a developing unit  9 , and a cleaning unit  10 , which are disposed to surround the photoconductive drum  5 . The charging unit  6  uniformly charges the surface of the photoconductive drum  5 . The exposing unit  7  selectively illuminates the charged surface of the photoconductive drum  5  to form an electrostatic latent image on the photoconductive drum  5 . The developing unit  9  supplies toner to the electrostatic latent image to develop the electrostatic latent image into a toner image. The cleaning unit  10  removes the residual toner from the photoconductive drum  5  after transferring the toner image onto the recording medium P. The photoconductive drum  5  is driven by a drive source through a train of gears (not shown), and rotates in a clockwise direction in  FIG. 1 . 
     The chassis  2  accommodates a paper cassette  11 , which holds a stack of sheets of the recording medium P. A hopping roller  12  feeds the recording medium P into the transport path  4  on a sheet-by-sheet basis from the paper cassette  11 . Pinch rollers  13  and  14  and registration rollers  15  and  16  are disposed between the hopping roller  12  and the process unit  3 D. The registration rollers  15  and  16  cooperate with the pinch rollers  13  and  14 , respectively, to hold the sheet of the recording medium P in a sandwiched relation. The registration rollers  15  and  16  cooperate to remove skew of the recording medium P. The hopping roller  12  and registration rollers  15  and  16  are driven in rotation in an interlocking manner by a drive source (not shown). 
     A transfer roller  17  is disposed to face a corresponding photoconductive drum  5  with the transport path  4  sandwiched between the photoconductive drum  5  and the transfer roller  17 . The transfer roller  17  is formed of, for example, a semi-conductive rubber material. The potentials of the photoconductive drum  5  and the transfer roller  17  are selected so that the toner image formed on the photoconductive drum  5  can be reliably transferred onto the recording medium P. 
     A fixing unit  18  is located downstream of the process unit  3 A with respect to the transport path  4 . A pair of discharge rollers  20  and  21  and a pair of discharge rollers  22  and  23  are disposed downstream of the fixing unit  18 , and discharge the recording medium P onto a stacker  19  formed on the upper surface of the chassis  2 . 
     The sheet of the recording medium P is fed by the hopping roller  12  into the transport path  4 , and is further transported by the pinch rollers  13  and  14  and the registration rollers  15  and  16 . The sheet of the recording medium P then passes through the four process units  3 A,  3 B,  3 C, and  3 D, in that stated order. While the recording medium P passes through the process units  3 A,  3 B,  3 C, and  3 D, the toner images of the respective colors are transferred onto the recording medium P one over the other in registration. The recording medium P is then fed into the fixing unit  18  where the toner images of the respective colors on the recording medium P are fixed under heat and pressure. After fixing, the recording medium P is discharged onto the stacker  19  by the discharging rollers  20  to  23 . 
     {Configuration of LED Print Head} 
     A description will be given of the configuration of an LED head  8  mounted on the exposing unit  7  of each of the process units  3 A,  3 B,  3 C, and  3 D.  FIG. 2A  is a perspective view of the LED print head  8  in its entirety and a perspective cross-sectional view.  FIG. 2B  is a cross-sectional view taken along a line A-A in  FIG. 2A . The LED print head  8  extends in a longitudinal direction thereof and includes a frame  30  having a generally U-shaped cross-section. The frame  30  is made of, for example, an aluminum block, a metal plate, or a resin (e.g., liquid crystal polymer). 
     The frame has a longitudinal opening  31  that extends in the longitudinal direction. A chip-on-board (COB) module  133  fits into the opening  31 . A narrow opening  32  is formed in the bottom of the U-shaped frame  30 , and extends in the longitudinal direction of the frame  30 . The COB  133  includes a rectangular wiring board  134 . A plurality of array chips  135  of semiconductor light emitting portions are aligned in a straight line on the print wiring board  134 . The COB  1133  is attached to the frame  30  such that the plurality of array chips  135  face the opening  32 . As shown in  FIG. 2A , the LED print head  8  extends in the longitudinal direction thereof shown by arrow S, which is parallel to a main scanning direction of the printer  1  perpendicular to a direction in which the recording medium P is transported. 
     A rod lens array  36  generally in the shape of a rectangular parallelepiped fits into the opening  32 . The rod lens array  36  forms an erect image of unity magnification of the light emitted from the array chips  135  on the charged surface of the photoconductive drum  5 . One of the lens surfaces,  36   b , of the rod lens array  36  is a distance L from the light emitting surface of the array chips and the other of the lens surfaces extends outwardly through the opening  32 , so that the image of the light emitting portions is formed on the surface of the photoconductive drum  5 , which is a distance L from the other lens surface of the rod lens array,  36   a.    
     Each LED print head  8  is assembled to a corresponding exposing unit  7  so that the lens surface projecting outwardly from the frame  30  faces the surface of the corresponding photoconductive drum  5 . 
     A further description will be given of the print wiring board  134  and the array chips  135  mounted on the print wiring board  134 .  FIG. 3A  illustrates the appearance of the chip-on-board module  133 .  FIG. 3B  is a partial expanded view of a portion P of the chip-on-board (COB)  133  shown in  FIG. 3A .  FIG. 4A  is a further perspective expanded view of a pertinent portion.  FIG. 4B  illustrates the positional relationship among two consecutive odd-numbered array chips and an even numbered array chip between the two consecutive odd-numbered array chips. For simplicity&#39;s sake,  FIG. 4A  does not show Au wires  140  ( FIG. 3B ), which connect between the print wiring board  134  and the array chips  135 . 
     Referring to  FIGS. 3A, 3B, 4A, and 4B , a plurality of rectangular plate-like array chips  135  are mounted on the surface of the print wiring board  134  using an electrically conductive or an electrically non-conducive adhesive  141 , the array chips  135  being aligned in the main scanning direction shown by arrow S so that the first short side of an even-numbered semiconductor apparatus directly faces the second short side of an odd numbered semiconductor apparatus and the light emitting portions of adjacent array chips are in a single straight line ( FIG. 4B ). Light emitting portions  143 , which are light emitting diodes, are formed in the top surface  142  of each array chip  135 . The light emitting portions  143  are formed of a GaAs compound semiconductor, and are aligned in a one dimension at intervals of 42.3 μm resolution of 600 dpi) or at intervals of 21.2 μm (i.e., a resolution of 1200 dpi). Each light emitting portion  143  may be implemented as a light emitting diode (LED) by achieving a PN junction of a P-type semiconductor and an N type semiconductor. Alternatively, the light emitting portions  143  may be Thyristors that take the form of a PNPN junction or an NPNP junction. 
     The array chip  135  is rectangular, and has a longitudinal center line CL, long sides, and short sides. A straight line of the light emitting portions  143  extends in a direction parallel to the long sides, and is closer to one of the long sides of the array chip  135  than the longitudinal center line CL ( FIG. 4A ), while a straight line of the wire bonding pads  144  extends in a direction parallel to the long sides, and is closer to the other of the long sides than the longitudinal center line CL ( FIG. 4A ). The wire bonding pads  144  are aligned at predetermined intervals. 
     Just as in the light emitting portions  143 , the array chips  135  may be fabricated from a GaAs substrate. In addition to the light emitting portions  143 , the array chip  135  may have shift registers (not shown) that sequentially shifts a signal received from an external drive circuit. Alternatively, the array chips  135  may also be fabricated from an Si substrate on which IC driver circuits are fabricated for driving the light emitting portions  143 . In other words, thin films of light emitting layer with a thickness of less than 5 μm are grown on a GaAs substrate formed of a compound semiconductor, and are integrated on an IC driver circuit substrate using intermolecular force, and then the light emitting portions  143  are fabricated using thin film wiring that can be formed using photolithography and metal thin-film forming technology, and the driver circuits and light emitting portions  143  are integrated on the GaAs substrate through electrical interconnection. The thickness of the array chips  135  can be selected to be in the range of, for example, from 200 μm to 600 μm. 
     Referring to  FIG. 4A , the array chip  135  has stepped portions or terraced portions  146  at each of the longitudinal end portions of the array chip  135 . At the terraced portion  146 , the array chip  135  has a transition or wall  146   c  from the top surface  142  to a recessed surface  145  that is recessed from the top surface  142  and is substantially parallel to the top surface. In other words, the recessed surface is lower than the top surface by a predetermined distance. 
     More specifically, the top surface  142  includes a small terraced portion  146   a  and a large terraced portion  146   b  formed at each longitudinal end portion of the array chip  135 , the small and large terraced portions  146   a  and  146   b  defining a peninsula-shaped portion  142   a  in which an endmost light emitting portion  143   x  is formed. The peninsula-shaped portion  142   a  extends to the longitudinal end of the array chip  135 . The recessed surface  145  is lower than the top surface  142  by 20 μm to 200 μm depending on the thickness of the array chip, and extends at least 20 μm in the longitudinal direction of the array chip  135  from the longitudinal end of the array chip. 
     If the array chip  135  is formed on a GaAs substrate, the terraced portions  146   a  and  146   b  may be formed as follows: A wafer is diced into individual rectangular array chips  135 . A photoresist material is applied to areas of the array chip  135  except for an area in which the terraced portion  146  is to be formed. The passivation film or interlayer dielectric film is removed by CF4 dry etching from the area in which the terraced portion  146  is to be formed, so that the GaAs substrate is exposed. The exposed GaAs substrate is subjected to wet etching using an etchant which is a mixed solution of sulfuric acid, hydrogen peroxide water, and water. The array chip  135  is etched to a depth of 20 μm to 200 μm. The photoresist material is then removed from the array chip  135 , thereby forming the terraced portions  146   a  and  146   b  in the array chip  135 . 
     If the array chip  135  takes the form of an Si IC driver circuit substrate, the terraced portions  146   a  and  146   b  may be formed as follows: The driver circuit is designed such that no circuit occupies an area in which the terraced portions  146   a  and  146   b  are to be formed. Just as in the GaAs substrate, a photoresist material is applied to areas of the array chip except for the areas in which the terraced portions  146   a  and  146   b  are to be formed. The passivation film or interlayer dielectric film in the areas in which the portions  146   a  and  146   b  are to be formed is removed by CF4 dry etching, so that the Si substrate is exposed. The exposed Si substrate is subjected to chemical dry etching that uses a gas, for example, SF6, thereby etching the array chip to a depth of 20 μm to 200 μm. The photoresist material is then removed from the array chip  135 , thereby forming the terraced portions  146   a  and  146   b  in the array chip  135 . 
     Using an adhesive  141 , the array chips having the terraced portion  146  formed therein are mounted on the wiring board  134  formed of, for example, composite epoxy material (CEM3) or flame retardant 4 (FR4), being aligned in the longitudinal direction of the array chip  135 . The center-to-center distance D 2  between the respective endmost light emitting portions of adjacent array chips is equal to the center-to-center distance D 1  between adjacent light emitting portions  143  on the array chip  135 . When the light emitting portions  143  are arranged at intervals of about 42.3 μm (equivalent to 600 dpi), the distance between adjacent array chips  135  is selected to be about 10 μm, which is shorter than the distance D 1 , and when the light emitting portions are arranged at intervals of about 42.3 μm (equivalent to 1200 dpi), the distance between adjacent array chips  135  is selected to be about 5 μm, which is shorter than the distance D 1 . 
     The adhesive  141  may be transferred onto the print wiring board  134  using the stamp function of a die bonder (not shown). Alternatively, a dispenser (not shown) is used to form a layer of the adhesive  141  on predetermined areas on the print wiring board  134 . The layer of the adhesive  141  is formed so that the entire back surface of the array chip  135  can be in contact with the layer of the adhesive  141 . In this manner, the array chip  135  is pressed against the layer of the adhesive  141 . The adhesive  141  then cures, thereby fixedly bonding the entire back surface of the array chip  135  to the print wiring board  134 . 
     As described above, the array chip  135  carries wire bonding pads  144  through which data can be inputted from and outputted to an external driver circuit, and the print wiring board  134  carries wire bonding pads  147  through which data can be inputted from and outputted to the array chip  135 . A bonding wire connects a wire bonding pad  147  to a corresponding wire bonding pad  144 . A line of wire bonding pads  144  and a line of wire bonding pads  147  extend in directions substantially parallel to the longitudinal direction or the center line CL of the array chip  135 , and are spaced apart by a predetermined distance. 
     Referring to  FIG. 3B , the wire bonding pads  144  and the wire bonding pads  147  are electrically connected by means of Au wires  140 . A ROM, chip capacitors, connectors through which the data is communicated between the LED printer  1  and the LED print head  8  are mounted on the print wiring board  134 , thereby configuring the COB  133  for the LED print head  8 . This COB  133  is assembled to the frame  30  of the LED print head  8 . 
     Effects of First Embodiment 
     Effects of forming the terraced portions  146  in the array chip  135  will be described. The effects will be described by comparing the COB  133  according to the first embodiment with a comparison COB  152  whose array chips  150  have no terraced portions. 
       FIG. 5A  is a perspective view of the comparison COB  152  in which semiconductor light emitting element array chips  150  with no terraced portions  146  are mounted on a print wiring board  151 .  FIG. 5B  is an expanded view of a portion P shown in  FIG. 5A . The comparison COB  152  has the same configuration as the COB  133  except that the array chips  50  have not the terraced portions  146  formed therein. 
     The comparison COB  152  includes a plurality of array chips  150  bonded to the print wiring board  151  using an adhesive  153  and aligned in a straight line. The adjacent array chips  150  are disposed so that the lines of the light emitting portions of the adjacent array chips  150  are in line with each other and the distance D 2  between the respective endmost light emitting portions  154   x  of the adjacent array chips  152  is equal to the distance D 1  between adjacent light emitting portions  154  in the array chips  150 . When the light emitting portions  154  are disposed at intervals of about 42.3 μm (equivalent to 600 dpi), the distance between adjacent array chips  150  is selected to be about 10 μm or less, and when the light emitting portions  154   x  are disposed at intervals of about 42.3 μm (equivalent to 1200 dpi), the distance between adjacent array chips  150  is selected to be about 5 μm. 
     The array chip  150  has wire bonding pads  155  formed thereon and the print wiring board  151  has wire bonding pads  156  formed thereon. Au wires  157  connect between the wire bonding pads  155  and corresponding wire bonding pads  156 . 
       FIG. 6  illustrates how the adhesive climbs up and flows on the surface of the adjacent array chips of the comparison COB  152 . The adhesive  153  is drawn into the space between the adjacent array chips  150  by capillary action up to the same level as the surface of the array chips  150 , soiling the light emitting portions  154  and causing usable light power to decrease. The adhesive  153  may also contaminate the wire bonding pads  155  formed on the end portion of the surface of the array chips  150 , reducing the mechanical strength of the wire bonding. 
       FIG. 7  illustrates how the adhesive climbs up and flows on the surface of the adjacent array chips  135  of the COB  133  according to the first embodiment. While the adjacent array chips  135  on the COB  133  are aligned at the same intervals as the array chips  150  on the comparison COB, and the entire back surface of the array chips  135  are in contact with the layer of the adhesive  141 , the recessed surface  145  lower than the top surface  142  bypasses the adhesive  141  that would otherwise climb up the gap between the adjacent array chips to the top surface  142 . 
     The COB  133  according to the first embodiment does not place any limitation on the area on the back surface of the array chip  135  in which the adhesive  141  may be applied. In other words, the adhesive  141  can be applied to the entire back surface of the array chip  135  so that the array chip  135  can be bonded in its entire back surface to the print wiring board  134 . This prevents the end portions of the back surface of the array chip  135  from being uplifted, thereby reducing the chances of the array chip  135  inclining and the chip&#39;s end portions being damaged. 
     Since the entire back surface of the array chip  135  is bonded to the print wiring board  134  using the adhesive  141 , the heat generated by the array chip  135  can be evenly conducted to the COB  133 . In other words, heat dissipation can be uniform across the entire array chip  135 . 
       FIG. 8  is a partial top view of the array chip  135 , illustrating the distance between the endmost light emitting portion  142   x  and the longitudinal end of the array chip  135 . The terraced portion  146  of the COB  133  according to the first embodiment effectively receives or bypasses the adhesive  141 , which climbs up the narrow gap between the adjacent array chips  135 , even if the COB  133  employs a shorter distance D 3  between adjacent array chips  135  than the comparison COB  152 . Thus, employing the shorter distance D 3  in the first embodiment provides a longer distance D 4  ( FIG. 8 ) between the endmost light emitting portion  143   x  and the end of the array chip  135  shown in  FIG. 8  as compared to the comparison COB  152 . The longer distance D 4  increases mechanical strength of the array chip  135 , reducing the chances of the edge portions of the array chip  135  being chipped. 
     As described above, the COB  133  has a configuration in which the array chips  135  are mounted in a straight line on the print wiring board  134  using the adhesive  141  and the entire back surface of the array chips  135  may be bonded to the print wiring board  134 . In addition, the terraced portion  146  of the COB  133  bypasses the adhesive  141  which would otherwise climb up the narrow gap, thereby preventing the adhesive  141  from climbing onto the top surface of the array chips  135 . 
     As described above, without sacrificing the reliability, the COB  133  effectively prevents the array chips  135  from being damaged, being inclined, increasing in temperature, and being contaminated by the adhesive  141 . 
     Modification to First Embodiment 
       FIG. 9  illustrates a modification of the first embodiment. The modification differs from the first embodiment in that the terraced portions have arrises or rounded portions  160 ,  161 ,  62 , and  163 . 
     Specifically, the wall  146   c , which connects the top surface  142  and the recessed surface  145  of the terraced portions  146   a  and  146   b , has rounded arrises or rounded portions  161  that surround the peninsula-shaped portion  142   a . Due to the rounded portions  162  and  163 , the surfaces  145  of the small and large terraced portions  146   a  and  146   b  are wider nearer the long sides of the array chip  135 . 
     The rounded portions  160  and  161  are effective in widening the gap between the peninsula portions  142   a  of the adjacent array chips  135 , reducing the chances of the adhesive  141  climbing up in the gap as well as guiding the adhesive  141  to the surfaces  145 . 
       FIG. 10  illustrates how the adhesive flows on the array chips on modified array chips. The array chips  135 A are disposed on the print wiring board  134  so that one of the small and large terraced portions  146   a  and  146   b  of the adjacent array chips  135 A is a mirror image of the other. Some of the adhesive  141  climbs up the gap to the surfaces  145 , and then flows away from the peninsula portions  142   a  to the surface of the print wiring board  134 , which is lower than the surfaces  145 . 
     The two rounded portions  162  and  163  of the wall  146   c  effectively widen the path in which the adhesive  141  flows to the print wiring board  134 , prompting the excess adhesive  141  to quickly flow out of the terraced portions  146 . Other corners of the walls  146   c  may also be rounded as required. 
     In the first embodiment, the small and large terraced portions  146   a  and  146   b  are disposed on both sides of the peninsula-shaped portion  142   a . The present invention is not limited to this. The array chip  135  may have only the large terraced portion  146   b  rather than the small and large terraced portions  146   a  and  146   b.    
       FIGS. 11A and 11B  illustrate a wall  146   c  that connects the top surface  142  and the recessed surface  145 . The wall  146   c  may be perpendicular to the top surface  142  and the recessed surface  145  as shown in  FIG. 11A . Alternatively, the array chip  135  may be undercut such that the wall  65   b  extends obliquely over the recessed surface  145  to form an acute angle θ with the recessed surface  145  as shown in  FIG. 11B . In other words, the wall  65   b  makes an acute angle with the recessed surface  145 . The wall  146   c  is more effective in preventing the adhesive  141  from climbing up the gap between the adjacent array chips  135 . 
     Second Embodiment 
     Configuration of COB 
     A second embodiment differs from the first embodiment in the configuration of semiconductor light emitting element array chip. The second embodiment will be described mainly in terms of chip-on-board (COB). 
       FIGS. 12A, 12B and 13A  illustrate the appearance of a COB  200  according to a second embodiment.  FIG. 12A  is a partial perspective view of the COB  200 .  FIG. 12B  is an expanded view of a relevant portion P of the COB  200 .  FIG. 13A  is another expanded view of the relevant part of the COB  200 .  FIG. 13B  illustrates the positional relationship among two consecutive odd-numbered array chips and an even numbered array chip between the two consecutive odd-numbered array chips. For simplicity&#39;s sake, Au wires  203 , which connect between a print wiring board  201  and semiconductor light emitting element array chips  202 , are omitted from  FIG. 13A . 
     A plurality rectangular plate-shaped array chips  202  of light emitting portions are aligned in a main scanning direction S of an LED printer  1  so that a line of even-numbered array chips  202  extends parallel to a line of odd-numbered array chips  202 , the even-numbered array chips  202  are staggered with respect to the odd-numbered array chips  202 , and long sides of adjacent array chips face each other in an overlapped relation. 
     Just as in the first embodiment, each array chip  202  can be fabricated on a GaAs substrate or an IC driver circuit substrate. The array chip  202  has a thickness of, for example, 200 μm to 600 μm. A straight line of a plurality of light emitting portions  206  is fabricated in the top surface  205  of the array chip  202  at intervals D 1  of about 42.3 μm (600 dpi) or about 21.2 μm (1200 dpi). The light emitting portions  206  are formed mainly of, for example, a GaAs compound semiconductor material. 
     The light emitting portions  206  are located closer to one of the long sides than a longitudinal center line CL ( FIG. 13 ) and wire bonding pads  208  are arranged closer to the other of the long sides than the longitudinal center line CL. Adjacent array chips  202  are arranged so that the lines of even-numbered and odd numbered light emitting portions  206  are parallel to each other and the endmost light emitting portions  206   x  in the two lines are spaced apart by the distance D 2 . Each array chip  202  includes stepped portions or terraced portions  210  ( FIG. 13 ) at each of the longitudinal end portions of the array chip  202 , the terraced portions  110  defining a peninsula-shaped portion  207  ( FIG. 13 ) in which no light emitting portion  206  is formed. The terraced portion  210  extends in the line of the light emitting portions  206  further than the endmost light emitting portion  106   x  to the longitudinal end of the array chip  202 . The terraced portion  210  has a transition or an L-shaped wall  211  that connects the top surface  205  and a recessed surface  209 , which is recessed from the top surface  205  and is substantially parallel to the top surface  205 . In other words, the recessed surface  209  is lower than the top surface  205  by a predetermined distance. The recessed surface  209  is lower than the top surface  205  by, for example, 20 μm to 200 μm, and extends over 20 μm or longer in a direction substantially perpendicular to the longitudinal center line CL. The terraced portion  210  and the peninsula-shaped portion  207  are aligned side by side in a direction perpendicular to the longitudinal center line CL. The peninsula-shaped portion  207  extends a distance of 100 μm to 500 μm from the top surface  205  to the end of the array chip  102 . The peninsula-shaped portion  207  has a plurality of wire bonding pads  108  formed therein and aligned in a direction parallel to the longitudinal center line CL with an interval of a predetermined distance. Since the wire bonding pads  208  are formed in the peninsula-shaped portion  214 , thereby providing more efficient utilization of space so that a larger number of array chips  202  can be diced from a single wafer. 
     If the array chip  202  is formed on a GaAs substrate, the terraced portion  210  may be formed as follows: A wafer is diced into individual rectangular array chips  202 . A photoresist material is applied to areas of the array chip  202  except for an area in which the terraced portion  210  is to be formed. The passivation film or interlayer dielectric film in the area, which will be the terraced portion  210 , is removed by, for example, CF4 dry etching, so that the GaAs substrate is exposed. The exposed GaAs substrate is subjected to chemical dry etching that uses, for example, chlorinated gas, or wet etching that uses an etchant which is a mixed solution of sulfuric acid, hydrogen peroxide water, and water. The array chip  202  is etched to a depth of 20 μm to 200 μm. The photoresist material is then removed from the array chip  202 , thereby forming the terraced portion  210  in the array chip  202 . 
     If the array chip  202  takes the form of an Si IC driver circuit substrate, the terraced portion  210  may be formed as follows: The driver circuit is designed such that no circuit occupies an area in which the terraced portion  210  is to be formed. Just as in the GaAs substrate, a photoresist material is applied to areas of the array chip  202  except for the area in which the terraced portion  210  is to be formed. The passivation film or interlayer dielectric film in the area, which will be the terraced portion  210 , is removed by CF4 dry etching, so that the Si substrate is exposed. The exposed Si substrate is subjected to chemical dry etching that uses a gas, for example, SF6, thereby etching the array chip to a depth of 20 μm to 200 μm. The photoresist material is then removed from the array chip  202 , thereby forming the terraced portion  210  in the array chip  202 . 
     As shown in  FIG. 13A , the wall  211 , which connects the top surface  205  and the recessed surface  209  of the terraced portion  210 , has rounded arrises or rounded portions  211   a  at the longitudinal end of the array chip  202  and rounded portions  211   b  near the endmost light emitting portion  206   x  such that the surface  205  is wider nearer the short side of the array chip  202  and is wider nearer the long side of the array chip  202 . 
     Referring to  FIG. 13B , the array chips  202  are aligned generally in two directions parallel to the center line CL of the array chips, so that even-numbered array chips  202  lie in one of two directions and odd-numbered array chips  202  lie in the other of the two directions. The even-numbered array chips  202  are staggered with respect to the odd-numbered array chips  202 , so that the even-numbered array chips  202  and the odd-numbered array chips  202  are overlapped with each other. One of two parallel lines perpendicular to the center line CL passes through the center of the endmost light emitting portion  206   x  of one of the adjacent array chips  202 , and the other of the two parallel lines passes through the center of the endmost light emitting portion  206   x  of the other of the adjacent array chips  202  such that the distance D 2  between the two parallel lines is equal to a center-to-center distance D 1  between adjacent light emitting portions  206  in each array chip  202 . 
     It is preferable that the adjacent array chips  202  are mounted on the print wiring board  201  so that the distance D 5  between the long side of one of the adjacent array chips  202  and the long side of the other of the adjacent array chips  202  is as short as possible. Since the array chips  202  mounted on the print wiring board  201  includes the even-numbered array chips  202  and the odd-numbered array chips, the even-numbered array chips  202  being staggered with respect to the odd-numbered array chips  202 , if the distance D 5  can be sufficiently short, the amount of light emitted from the even-numbered array chips  202  and incident on a rod lens  36  can be substantially equal to the amount of light emitted from the odd-numbered array chips  202  and incident on the rod lens  36 . 
     The adhesive  204  is applied to a predetermined area of the print wiring board  201  so that the entire back surface of the array chip  202  may be in contact with the adhesive  204 . The array chip  202  is pressed against the layer of the adhesive  204  formed on the print wiring board  201 . The adhesive  204  is then cured, thereby securely bonding the array chip  202  to the print wiring board  201  across the entire back surface of the array chip  202 . 
     As described above, the array chip  202  has wire bonding pads  208  formed therein for communicating data with external driver circuits. Likewise, wire bonding pads  213  are formed on the print wiring board  201  in correspondence with the wire bonding pads  208 . Each wire bonding pad  208  and a corresponding wire bonding pad  213  are spaced apart by a predetermined distance. 
     An Au bonding wire  203  ( FIG. 12B ) connects each wire bonding pad  208  and a corresponding wire bonding pad  213 . The print wiring board  201  has a ROM, chip capacitors, and connectors for communicating data with the LED printer  1 , all being not shown, thereby completing the COB  200  for an LED print head  8 . The COB  200  is assembled to a frame  30  of the LED print head  8  just as in the first embodiment. 
     Effects of Second Embodiment 
     A description will be given of the effects of forming the terraced portion  210  in the array chips  202  arranged in a staggered relation. The effects will be described by comparing the COB  200  according to the second embodiment with a comparison COB  222  whose array chips  202  have no terraced portions. 
       FIG. 14A  illustrates the comparison COB  222  in which light emitting element array chips  220  are mounted on a print wiring board  221 .  FIG. 14B  is an expanded view of a portion P shown in  FIG. 14A . The comparison COB  222  differs from the comparison COB  200  according to the second embodiment in that the array chips  220  have no terraced portions  210 . 
     The comparison COB  222  includes a plurality of array chips  220  bonded to the print wiring board  221  using an adhesive  223  and aligned in such a way that even-numbered array chips  220  are staggered with respect to odd-numbered array chips  220 . The array chips  220  are aligned generally in two directions parallel to the center line CL of the array chips  220 , so that even-numbered array chips  220  lie in one of two directions and odd-numbered array chips  220  lie in the other of the two directions. The even-numbered array chips  220  are staggered with respect to the odd-numbered array chips  220 , so that the even-numbered array chips  220  and the odd-numbered array chips  220  are overlapped with each other′. One of two parallel lines perpendicular to the center line CL passes through the center of the endmost light emitting portion  224   x  of one of the adjacent array chips  220 , and the other of the two parallel lines passes through the center of the endmost light emitting portion  224   x  of the other of the adjacent array chips  202  such that the distance D 2  between the two parallel lines is equal to a center-to-center distance D 1  between adjacent light emitting portions  224  in each array chip  220 . Thus, the array chips  220  are mounted on the print wiring board  221  so that the distance D 2  is equal to the center-to-center distance D 1 . 
     It is preferable that the distance D 5  between the two parallel lines is as short as possible. For this reason, it is preferable that the distance D 6  between the long side of one of the adjacent array chips  220  and the long side of the other of the adjacent array chips  220  is as short as possible, that is, the even-numbered array chips  220  and the odd-numbered array chips  220  should be disposed as close to each other as possible. For example, the distance D 6  between the array chips  220  is selected to be in the range of 10 to 50 μm. 
     Wire bonding pads  226  are formed on longitudinal end portions  225  of the array chip  220  where no light emitting portions  224  are formed, and wire bonding pads  227  are formed on the print wiring board  221 . Au wires  228  connect between the wire bonding pads  226  and corresponding wire bonding pads  227 . 
       FIG. 15  illustrates how the adhesive climbs up the gap between adjacent array chips of the comparison COB and flows on the array chips. 
     The adhesive  223  climbs up the gap between the adjacent array chips  220  to the top surface of the adjacent array chips  220 , contaminating the light emitting portions  224  and/or the wire bonding pads  226 . Contamination of the light emitting portions  224  decreases available light power and contamination of the bonding wire pads  226  impairs mechanical strength of the wire bonding portion to decrease. 
       FIG. 16  illustrates how the adhesive  223  climbs up and flows on the surface of the adjacent array chips  202  of the COB  220 . The distance D 6  between the adjacent array chips  202  is equal to that between the adjacent array chips  202  of the comparison COB  222 , and the adhesive  204  is applied to the entire back surface of the array chip. However, as shown in  FIG. 16 , the adhesive  204  climbs up the gap between the adjacent array chips  202  due to capillary action, and is guided to flow to the recessed surface  209  of the terraced portion  210 , thereby preventing the adhesive  204  from climbing onto the top surface  205 . 
     The COB  200  of the second embodiment does not place any limitation on an area in the back surface of the array chip  202  to which the adhesive may be applied, but allows the adhesive  204  to be applied to the entire back surface of the array chip  202  so that the entire back surface of the array chip  202  can be securely bonded to the print wire board  201 . This prevents the end portion of the back surface of the array chip  202  from being uplifted from the print wiring board  201 . The COB  200  also prevents the corners and edges of the array chip  202  being chipped or the array chip  202  being inclined relative to the print wiring board  201 . 
     The fact that the back surface of the array chip  202  can be securely bonded to the print wire board  201  is advantageous in that the heat generated by the array chip  202  can be uniformly conducted to the print wiring board  201 , hence uniform heat dissipation across the array chip  202 . 
     For the COB  200  according to the second embodiment, the distance D 6  between adjacent array chips  202  is shorter than the comparison COB  222  but the adhesive  204  that climbs up the gap between the adjacent array chips  202  can be bypassed to the terraced portion  210 . Thus, the distance D 7  between the long side of the array chip  202  and the line of light emitting portions close to the long side may be longer than the comparison COB  200 , providing more freedom in arranging the light emitting portions on the array chip  202 . 
     The terraced portion  210  has the L-shaped wall  211  that partially surrounds the recessed surface  209 . The L-shaped wall  211  has a rounded portion  211   b , which is further effective in providing as wide a gap between adjacent array chips  202  as possible in the vicinity of the longitudinal end of the array chips  202 , thereby promoting the adhesive  204  to flow to the recessed surface  209 . 
     The L-shaped wall  211  also has a rounded portion  211   a , which is further effective in providing as wide a gap between adjacent array chips  202  as possible in the vicinity of the endmost light emitting portions, thereby promoting the adhesive to flow to the recessed surface  209 . 
     When the array chips  202  have been mounted on the print wiring board  201 , the terraced portion  210  of one of adjacent array chips  202  faces the light emitting portions  206  closest to the endmost light emitting portion  206   x  of the other of the adjacent array chips  202 . Thus, the adhesive  204  climbs up the gap between the adjacent array chips  202 , then flows on the recessed surface  209 , and finally flows down to the print wiring board  201  from the end of the terraced portion  210  at the longitudinal end of the array chip  202 . The rounded portion  211   a  of the L-shaped wall  211  is effective in widening the area of the recessed surface  209  at the longitudinal end of the array chip  202 , permitting the adhesive  204  to smoothly flow onto the print wiring board  201 . 
     As described above, the second embodiment still provides the same advantages as the first embodiment when the array chips are aligned in the main scanning direction S such that even-numbered array chips  202  are staggered with respect to odd-numbered array chips  202  and long sides of adjacent array chips face each other and are overlapped. In other words, the second embodiment prevents contamination of array chips  202  due to an adhesive without sacrificing reliable operation of the array chips  202 , and provides more freedom in arranging the light emitting portions  206  on the array chip  202 . 
     Third Embodiment 
     A third embodiment differs from the second embodiment in that an additional terraced portion is formed in a light emitting element array chip. The basic configuration of an LED print head  8  according to the third embodiment is the same as that of the first and second embodiments, and their detailed description is omitted. A description will given of only a chip-on-board (COB)  300  according to the third embodiment. 
     {Configuration of COB} 
       FIG. 17A  is a partial perspective view of the COB  300 .  FIG. 17B  is a cross-sectional view taken along a line B-B in  FIG. 17A .  FIG. 17C  illustrates the positional relationship among two consecutive odd-numbered array chips and an even numbered array chip between the two consecutive odd-numbered array chips. A plurality of rectangular plate-like array chips  303  are aligned in a main scanning direction S of an LED printer  1  or in a longitudinal direction of the LED print head  8 , and securely bonded to a print wiring board  301 . The array chips  303  are aligned in the main scanning direction S so that a line of even-numbered array chips  303  extends parallel to a line of odd-numbered array chips  303 , the even-numbered array chips are staggered with respect to the odd-numbered array chips  303 , and long sides of adjacent array chips face each other in an overlapped relation just as in the second embodiment. 
     Just as in the second embodiment, the array chip  303  may be formed on a GaAs substrate or an IC driver circuit substrate. The array chip  303  may have a thickness in the range of 300 to 600 μm. A plurality of light emitting portions  305  are aligned in a straight line on a top surface  304  of the array chip  303 , and are arranged at intervals of about 42.3 μm (600 dpi) or about 21.2 μm (1200 dpi). 
     The rectangular array chip  303  has long sides and short sides. A line of the light emitting portions  305  is disposed closer to one of the long sides than a longitudinally extending center line CL of the array chip  303 , and extends in a direction parallel to the center line CL. The array chip  303  has a small terraced portion  308  and a large terraced portion  310  formed in the top surface  304  at each longitudinal end portion of the array chip  303  where no light emitting portion is formed, the small and large terraced portions  308  and  310  defining a peninsula-shaped portion  311 . The small terraced portion  308  and the large terraced portion  310  extend in directions parallel to the center line CL further than the endmost light emitting portion  305   x  to the longitudinal end of the array chip  303 . The peninsula-shaped portion  311  extends in a direction parallel to the center line CL. A rectangular recessed surface  310   a  is recessed from the top surface  304 , and is substantially parallel to the top surface  304 . In other words, the recessed surface is lower than the top surface  304  by a predetermined distance. A surface  308   a  is also recessed from the top surface  304 . 
     Just as in the first and second embodiments, the terraced portions  308  and  310  can be formed by partially etching away the surface  304  simultaneously, thereby defining the peninsular-shaped portion  311 . 
     A wall  310   b  slopes down from the top surface  304  to the recessed surface  310   a , thereby connecting the top surface  304  and the recessed surface  310   a . In other words, the wall  310  makes an obtuse angle with the recessed surface  310   a . Thin film wiring patterns (not shown) are formed on the sloped wall  310   b , and electrically connect the wire bonding pads  313  to the light emitting portions  305 . The sloped wall  310   b  is formed of an organic insulating film, which can be formed by photolithography. Alternatively, if the array chip  303  is fabricated of a GaAs substrate, the etching rate in a vertical direction and in a horizontal direction may be independently adjusted to form a sloped surface. If the array chip  303  is fabricated from an Si substrate, the etching rate in a vertical direction and in a horizontal direction may be independently adjusted to form a sloped surface. 
       FIG. 17D  is an expanded view of a projection. FIG.  17 E is an expanded view of rounded corners. 
     Just as in the second embodiment, an L-shaped wall has rounded arrises or corners, and connects the top surface  304  and the surface  308   a . The array chip  303  may have a projection  314  at the longitudinal ends of the array chip  303 , the projection  314  projecting from the peninsula-shape portion  311  in a direction perpendicular to the center line CL. This projection  314  effectively prevents the adhesive  302  from climbing onto the surface  310   a , and prevents the wire bonding pads  313  from being contaminated. 
     As described above, the array chips  303  are mounted on the print wiring board  301  so that the even-numbered array chips  303  are staggered with respect to the odd-numbered array chips  303  and the terraced portion  308  of one of adjacent array chips  303  faces the light emitting portions  305  on the other of the adjacent array chips  303 . 
     The adjacent array chips  305  should be arranged so that the distance D 5  between a line passing through the center of the light emitting portions  305  formed on one of adjacent array and a line passing through the center the light emitting portions  305  formed on the other of the adjacent array chips.  303  is as short as possible. The distance D 6  between the long sides of the adjacent array chips  305  which directly face each other should also be as short as possible. In practice, the distance D 6  is selected to be in the range of, for example, 10 to 50 μm. 
     Just as in the first and second embodiments, the adhesive  302  may be transferred onto the print wiring board  301  using the stamp function of a die bonder (not shown). Alternatively, a dispenser (not shown) may be used to form a layer of the adhesive  302  on predetermined areas on the print wiring board  301 . The layer of the adhesive  302  is formed so that the entire back surface of the array chip  303  is in contact with the layer of the adhesive  302 . 
     As described above, the array chip  303  carries wire bonding pads  313  formed on the recessed surface  310   a  through which data can be inputted from and outputted to external driver circuits. The print wiring board  301  carries wire bonding pads  315  through which data can be inputted from and outputted to the array chips  303 . A bonding wire  316  connects a wire bonding pad  313  to a corresponding wire bonding pad  315 . A line of wire bonding pads  313  and a line of wire bonding pads  315  extend in directions substantially parallel to the longitudinal center line CL or the longitudinal direction of the array chip  303 , and are spaced apart by a predetermined distance. 
     The wire bonding pads  313  formed on the array chip  303  and the wire bonding pads  315  formed on the print wiring board  301  are electrically connected by means of Au wires  316 . Balls  317  are formed on the wire bonding pads  313  and  315 , so that the top of the balls  317  after wire bonding should be lower than the top surface of the peninsula-shaped portion  311 . 
     In practice, the surface  310   a  is lower than the top surface  304  by the sum of the thickness of the bonding pad  313  and the height of the ball  317 . 
     A ROM, chip capacitors, and connectors through which the data is communicated between the LED printer  1  and the LED print head  8 , which are not shown, are mounted on the print wiring board  301 , thereby configuring the COB  300  for the LED print head  8 . Just as in the first and second embodiment, the COB  300  is assembled to the frame  30  of the LED print head  8 . 
     Effects of Third Embodiment 
     Just as in the second embodiment, the third embodiment also provides effects of forming the terraced portion  308  on a side of the peninsula-shaped portion opposite the terraced portion  310 , and its detailed description is omitted. Effects of the third embodiment will be described. 
     The third embodiment has the terraced portions  308  and  310 , which defines the peninsula-shaped portion  311  formed between the terraced portions  308  and  310 . The wire bonding pads  313  are formed on the recessed surface  310   a.    
     The terraced portion  308  of one of adjacent array chips  303  faces the light emitting portions  305  closest to the endmost light emitting portion  305   x  of the other of the adjacent array chips  303 . 
       FIG. 18  illustrates that the ball  317  is out of an angular range R in which the light emitting portions emit light. If the ball  317  reflects the light emitted from the light emitting portions  305 , the reflected light may illuminate the charged surface of the photoconductive drum  5 , which causes streaks and lines in the print results leading to poor print quality. To avoid such adverse effects, the Au bonding wires  316  on one of adjacent array chips  303  extend from the wire bonding pads  313  in such a direction as to be away from the light emitting portions  305  on the other of the adjacent array chips  303 . As a result, the ball  317  is out of the angular range R in which light emitting portions  305  emits light as shown in  FIG. 18 , so that there is no chance of the ball  317  reflecting the light emitted from the light emitting portions  305 . 
     A recessed surface (not shown) may be formed in the longitudinal end portions of the array chip  303  in which no light emitting portions are fabricated, and the balls  317  may be formed in the recessed surface, thereby hiding the balls  317 . The recessed surface may have a depth lager than the sum of the height of the balls  317  and the bonding pad  213 . This configuration is also effective in preventing the ball  317  from reflecting the light emitted from the light emitting portions  305 . Each recessed surface  304  may accommodate a corresponding ball  317 . Alternatively, a larger recessed surface may be formed in which a plurality of wire bonding pads  313  and corresponding balls  317  are accommodated. 
     In the third embodiment, the Au wires  316  are connected to the wire bonding pads  313  by ball bonding. Instead, stitch bonding may be employed. The wire bonding pads  313  require to be larger in stitch bonding than in ball bonding. Therefore, the width of the array chips  303  also requires to be larger in stitch bonding than in ball bonding. On the other hand, the height of stitched portions may be lower than that of the balls  317 , which is effective in preventing the ball  317  from reflecting light emitted from the light emitting portions  305 . In addition, the depth of the recessed surface from the top surface  304  may be as shallow as, for example, 10 μm. 
     {Modifications} 
     Modification #1 
     The first, second, and third embodiments have been described in terms of the LED print head  8  mounted on the LED printer  1 . The invention is not limited to the LED print head  8 , and may also be applied to other exposing heads that use light emitting portions other than LEDs, for example, contact image sensors (CIS), which are used as reading heads used for, for example, scanners. 
     When a CIS is employed, the reading head may have a configuration in which light receiving element array chips, each having a plurality of light receiving portions aligned in one dimension, are securely bonded using an adhesive. The present invention may also be applicable to this type of CIS. 
     The first, second, and third embodiments have been described in terms of COB  133 , COB  200 , and COB  300 , respectively. The invention is not limited to these, and may also be applicable to apparatus having a configuration in which semiconductor chips are securely bonded to a circuit board using an adhesive. The semiconductor chip may be any type in which a plurality of semiconductor portions are fabricated in its surface. 
     The first, second, and third embodiments have been described in terms of the LED printer  1 . The invention may also be applied to printers and image forming apparatus that employ exposing heads whose light emitting portions are other than LEDs. The image forming apparatus include scanners, facsimile machines, multi-function printers (MFPs), and copying machines. The scanners employ, for example, reading heads such as compact image sensors. The first, second, and third embodiment have been described as using Au wires  140 ,  203 , and  316 , respectively. However, any electrically conductive wires including a Cu wire can be used. 
     Modification #2 
     The invention is not limited to the first, second, and third embodiments but modifications may be made by combining these embodiments or using a part of these embodiments. 
     Fourth Embodiment 
     A fourth embodiment differs from the first to third embodiments in the configuration of semiconductor light emitting element array chip. The fourth embodiment will be described mainly in terms of chip-on-board (COB). 
     A description will be given of the print wiring board  434  and the array chips  435  mounted on the print wiring board  434 .  FIG. 19A  is a partial perspective view of the chip-on-board module  433 .  FIG. 19B  is a cross-sectional view taken along a line C-C in  FIG. 19A . 
     Referring to  FIGS. 19A and 19B , a plurality of rectangular plate-like array chips  435  are mounted on the surface of the print wiring board  434  using an electrically conductive or an electrically non-conducive adhesive  440 . The array chips  435  are aligned in the main scanning direction shown by arrow S. Light emitting portions  442 , which are light emitting diodes, are formed in the surface  441  of each array chip  435 . 
     The light emitting portions  442  are formed of a GaAs compound semiconductor, and are aligned in a one dimension at intervals of 42.3 μm (i.e., a resolution of 600 dpi) or at intervals of 21.2 μm (i.e., a resolution of 1200 dpi). Each light emitting portion  442  may be implemented as a light emitting diode (LED) by forming a PN junction of a P-type semiconductor and an N type semiconductor. Alternatively, the light emitting portions  442  may be Thyristors that take the form of a PNPN junction or an NPNP junction. 
     The array chip  435  is rectangular, and has a longitudinal center line CL, long sides, and short sides. A straight line of the light emitting portions  442  extends in a direction parallel to the long sides and is closer to one of the long sides of the array chip  435  than the longitudinal center line CL, while a straight line of the wire bonding pads  447  extends in a direction parallel to the long sides and is closer to the other of the long sides than the longitudinal center line CL. The wire bonding pads  447  are aligned at predetermined intervals. 
     Referring to  FIG. 19A , the array chip  435  has an extended portion  443  at each of the longitudinal end portions of the array chip  435 , the extended portion  443  extending a distance D 8  of 100 to 500 μm from the endmost light emitting portions  442   x . The array chip  435  also has a stepped portion or terraced portion  445  formed in the extended portion  443 . The terraced portion  445  has a transition or wall  445   b  that extends from the top surface  441  to a recessed surface  445   a  that is recessed from the top surface  441 . For example, the recessed surface  445   a  is lower than the top surface  441  by a distance of 20 to 200 μm. The recessed surface  445   a  extends from the wall  445   b  to the longitudinal end of the array chip  435  and from the center line CL to one of the longitudinal sides of the array chip  435 . 
     An insulating film  446 , which is formed of, for example, Si or SiO2, is formed on the recessed surface  445   a , and a plurality of (e.g., two) wire bonding pads  447  are formed on the insulating film  446  at predetermined intervals. Forming the terraced portion  445  in the extended portion  443  leaves a peninsula-shaped portion  443   a . The terraced portion  445  and the peninsula-shaped portion  443   a  are aligned side by side in a direction perpendicular to the main scanning direction S of the LED printer  1 . The peninsula-shaped portion  442   a  extends to the longitudinal end of the array chip  435 . Forming the terraced portion  445  and forming the wire bonding pads  447  on the recessed surface  445   a  is advantageous in that the short sides of the array chip can be shorter and a larger number of array chips  435  can be diced from a single wafer. 
     If the array chip  435  is formed on a GaAs substrate, the terraced portion  445  may be formed as follows: A wafer is diced into individual rectangular array chips  435 . A photoresist material is applied to areas of the array chip  435  except for an area in which the terraced portion  445  is to be formed. The passivation film or interlayer dielectric film is removed by CF4 dry etching from the area in which the terraced portion  445  is to be formed, so that the GaAs substrate is exposed. The exposed GaAs substrate is subjected to wet etching using an etchant, which is a mixed solution of sulfuric acid, hydrogen peroxide water, and water. The array chip  435  is etched to a depth of 20 μm to 200 μm. The photoresist material is then removed from the array chip  435 , thereby forming the terraced portions  445  in the array chip  435 . 
     If the array chip  435  takes the form of an Si IC driver circuit substrate, the terraced portions  445  may be formed as follows: The driver circuit is designed such that no circuit occupies areas in which the terraced portions  445  are to be formed. Just as in the GaAs substrate, a photoresist material is applied to areas of the array chip  435  except for the areas in which the terraced portions  445  are to be formed. The passivation film or interlayer dielectric film in the areas in which the terraced portions  445  are to be formed is removed by CF4 dry etching, so that the Si substrate is exposed. The exposed Si substrate is then subjected to chemical dry etching that uses a gas, for example, SF6, thereby etching the array chip  435  to a depth of 20 μm to 200 μm. The photoresist material is then removed from the array chip  435 , thereby forming the terraced portions  445  in the array chip  435 . 
     A wall  445   b  slopes down from the top surface  441  to the recessed surface  445   a , thereby connecting the top surface  441  and the recessed surface  445   a . Thin film wiring patterns (not shown) are formed on the sloped wall  445   b , and electrically connect the wire bonding pads  447  to the light emitting portions  442 . 
     The sloped wall  445   b  is formed of an organic insulating film, which can be formed by photolithography. Alternatively, if the array chip  435  is fabricated of a GaAs substrate, the etching rate in a vertical direction and in a horizontal direction may be independently adjusted to form the sloped wall  445   b . If the array chip  435  is fabricated from an Si substrate, the etching rate in a vertical direction and in a horizontal direction may be independently adjusted to form the sloped wall  445   b.    
     Using an adhesive  440 , the array chips  435 , which have the terraced portions  445  formed thereon, are mounted on the wiring board  434  formed of, for example, composite epoxy material (CEM3) or flame retardant 4 (FR4), and are aligned in the longitudinal direction parallel to the center line CL of the array chip  435 . 
       FIG. 19C  illustrates the positional relationship among two adjacent odd-numbered array chis and an even numbered array chip between the two adjacent odd-numbered array chips. The array chips  435  mounted on the print wiring board  434  include a line of even-numbered array chips  402  and a line of odd-numbered array chips  435 , the even-numbered array chips  435  being staggered with respect to the odd-numbered array chips  435 , so that the even-numbered array chips  435  and the odd-numbered array chips  435  are overlapped with each other. 
     When the array chips  435  have been mounted on the print wiring board  434 , the peninsula-shaped portion  443   a  of one of adjacent array chips  435  faces the light emitting portions  442  closest to the endmost light emitting portion  442   x  of the other of the adjacent array chips  435 . 
     One of two parallel lines perpendicular to the center line CL passes through the center of the endmost light emitting portion  442   x  of one of the adjacent array chips  435 , and the other of the two parallel lines passes through the center of the endmost light emitting portion  442   x  of the other of the adjacent array chips  435  such that the distance D 2  between the two parallel lines is equal to a center-to-center distance D 1  between adjacent light emitting portions  442  in each array chip  435 . 
     It is preferable that the adjacent array chips  435  are mounted on the print wiring board  434  so that the distance D 3  between a line parallel to the center line CL and passing through the light emitting portions  442  of one of the adjacent array chips  435  and a line parallel to the center line CL and passing through the light emitting portions  442  of the other of the adjacent array chips is as short as possible. If the distance D 3  can be sufficiently short, the amount of light emitted from the even-numbered array chips  435  and incident on a rod lens  36  can be substantially equal to the amount of light emitted from the odd-numbered array chips  435  and incident on the rod lens  36 . 
     Thus, it is preferable that the distance D 3  between the directly facing long sides of the adjacent array chips  435  is as short as possible. Thus, achieving the shorter distance D 3  in the fourth embodiment provides a shorter distance D 4 . As a result, the wire bonding pads  447  of one of the adjacent array chips  435  may be closer to the light emitting portions  442  of the other of the adjacent array chip  435 . The distance between a line passing through the center of the wire bonding pads  447  and a line passing through the center of the light emitting portions  442  is selected to be about 100 μm. 
     The wire bonding pads  448  are formed on the print wiring board  434 , and the wire bonding pads  447  are formed on the array chips  435  in correspondence with the wire bonding pads  448 . The wire bonding pads  447  and  448  are spaced apart by a predetermined distance in directions perpendicular to the main scanning direction S. 
     Au wires  449  connect the wire bonding pads  447  and corresponding bonding pads  448 . The Au wire  449  is bonded to the bonding pad  447  by ball bonding or stitch bonding. The bonding pads  447  can be smaller when ball bonding is employed than when stitch bonding is employed. In fact, a 50 μm square bonding pads  447  can be used in ball bonding. The ball  450  of the Au wire  449  is a substantially spherical ball  450  having a diameter of 50 μm and a height greater than 20 μm. The top surface  441  is above the top of the ball  450 . The depth of the recessed surface  445   a  from the top surface  441  is selected to be larger than the sum of the height of the ball  450  and the thickness of the bonding pads  447 . 
     If stitch bonding is employed, the size of the wire bonding pads  447  should be more than 80 μm square. The height of stitched portion of the Au wire  449  can be lower than the diameter of the Au wire  449 , and well below the top surface  441 . 
     The print wiring board  434  includes a ROM, chip capacitances, and connectors through which the data is communicated between the LED printer  1  and the LED print head  8 , thus configuring the COB  433  for the LED print head  8 . This COB  433  is assembled to a frame  30  of the LED print head  8 . 
     Effects of Fourth Embodiment 
     Effects of forming the terraced portions  445  in the array chip  435  will be described. The effects will be described by comparing the COB  433  according to the fourth embodiment with a comparison COB  462  whose array chips  460  have no terraced portions. 
       FIG. 20A  is a perspective view of the comparison COB  462  on which semiconductor light emitting element array chips  460  are mounted.  FIG. 20B  is across-sectional view taken along a line D-D in  FIG. 20A . The comparison COB  462  has the same configuration as the COB  433  except that the array chips  460  have no terraced portion formed therein. 
     The comparison COB  462  includes a plurality of array chips  460  bonded to the print wiring board  461  using an adhesive  463  and aligned generally in a line. The array chips  460  are aligned generally in two directions parallel to the center line CL of the array chips  460 , so that even-numbered array chips  460  lie in one of two directions and odd-numbered array chips  460  lie in the other of the two directions. The even-numbered array chips  460  are staggered with respect to the odd-numbered array chips  460 , so that the even-numbered array chips  460  and the odd-numbered array chips  460  are overlapped with each other. One of two parallel lines perpendicular to the center line CL passes through the center of the endmost light emitting portion  464   x  of one of the adjacent array chips  460 , and the other of the two parallel lines passes through the center of the endmost light emitting portion  464   x  of the other of the adjacent array chips  460 . The adjacent array chips  460  are arranged such that the distance D 2  between the two parallel lines is equal to a center-to-center distance D 1  between adjacent light emitting portions  464   x  in each array chip  460 . 
     The distance D 3  between the line of light emitting portions  464  of the even-numbered array chips  460  and the line of light emitting portions  464  of the odd-numbered array chips  460  should be as short as possible, and is selected to be in the range of 50 to 100 μm. Accordingly, the distance D 4  between the long sides of the adjacent array chips  460  should be as short as possible, and is selected to be in the range of 10 to 50 μm. 
     Thus, the wire bonding pads  466  formed on an extended portion  465  of one of the adjacent array chips  460  may also be closer to the light emitting portions on the other of the adjacent array chips  460 . The extended portion  465  extends a distance of D 9 . Likewise, the Au wire  467  on one of the adjacent array chips  460  may also be closer to the light emitting portions  464  on the other of the adjacent array chips  460 . 
     A plurality of bonding pads  468  are formed on the print wiring board  61  in correspondence with the wire bonding pads  466  formed on the array chips  460 . The Au wire  467  connects each wire bonding pad  466  to a corresponding wire bonding pad  468 . As described above, the Au wire  467  should be bonded by ball bonding so that the wire bonding pad  466  occupies as small an area as possible, in which case the height of the ball  469  is usually higher than 20 μm. 
     When the array chips  460  of the comparison COB  462  have been mounted on the print wiring board  461 , the extended portion  465  of one of adjacent array chips  460  faces the light emitting portions  464  closest to the endmost light emitting portion  464   x  of the other of the adjacent array chips  460 . 
     The balls  469  sit on the bonding pads  466  formed on the top surface  470  of the array chip  460 . Thus, the light emitted from the light emitting portions  464  reaches the balls  469 , which in turn reflects the light. 
       FIG. 21  illustrates an angular range R in which the light emitting portions  442  emit light. In contrast, the COB  433  according to the fourth embodiment has the following configuration. The distance D 3  and the distance D 4  are the same as the comparison COB  462 . The wire bonding pads  447  are formed on the recessed surface  445   a  such that the top of the balls  450  is below the top surface  441  as shown in  FIG. 21 , i.e., the balls  450  are out of an angular range R in which the light emitting portions  442  emit light. The difference in height between the top surface  441  and the recessed surface  445   a  is selected to be in the range of 20 to 200 μm. 
     Therefore, there is no chance of the balls  450  reflecting the light emitted from the light emitting portions  442 . 
     As described above, a light-blocking wall is not formed on the top surface  441  of the array chip  435 , which is intended to prevent the light emitted from the light emitting portions  442  from reaching the balls  450 . Instead, the recessed surface  445   a  is formed below the top surface  441  to prevent the light emitted from the light emitting portions  442  from reaching the balls  450 . This simple configuration ensures that the balls  450  are out of the angular range R in which the light emitting portions  442  emit light, and is effective in preventing the balls  450  from reflecting the light emitted from the light emitting portions  442 . 
     With the comparison COB  462 , the balls  469  reflect the light from the light emitting portions  464 , and the reflected light may illuminate the charged surface of a photoconductive drum  5  (not shown) of the LED printer  1 , causing streaks and lines in the print results, thus leading to poor print quality. In contrast, the COB  433  according to the fourth embodiment minimizes the chance of the balls  450  reflecting the light emitted from the light emitting portions  442 , and therefore minimizes streaks and lines in the print results and not leading to poor print quality. 
     In the fourth embodiment, the Au wires  449  are connected to the wire bonding pads  447  by ball bonding. Instead, stitch bonding may be employed. The wire bonding pads  447  require to be larger in stitch bonding than in ball bonding. Therefore, the array chips  435  requires to have a larger width (i.e., short sides) in stitch bonding than in ball bonding. However, the height of stitched portions may be lower than that of the balls  450 , which is effective in preventing the balls  450  from reflecting light emitted from the light emitting portions  442 . In addition, the depth of the recessed surface  445   a  from the top surface  441  may be as shallow as, for example, 10 μm. 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.