Abstract:
An optical apparatus uses an array of light-emitting elements (LEDs or ELs) and a focusing optical system. Light emitted from the light-emitting elements of the array is directed to the focusing optical system within narrowed solid angles, thereby raising overall light utilization efficiency, with the help of recesses, each shaped as an inverted conical frustum, centered at respective light-emitting elements, to collect light from the light-emitting elements and deliver it to the focusing system within narrower solid angles, as both direct light and indirect light reflected from the slanting side walls of the recesses. The side walls of the recesses can have a reflecting film to further enhance overall light utilization efficiency. Lenses can be formed integrally with the array, to further help focus the light delivered to the focusing optical system.

Description:
BACKGROUND  
         [0001]    1. Field  
           [0002]    The apparatus and method described in this patent specification relate to an optical apparatus, in particular, an optical apparatus using a light source in the form of a micro-light source array such as an LED (light emission diode) array or an EL (electroluminescence) array, etc., employed in the optical writing-in unit of a scanner, etc., or in an electrophotographic printer, a digital copying machine, a facsimile device, etc.  
           [0003]    2. Background Technology  
           [0004]    In recent years, there has been an increase in the use at home or in small businesses of office equipment that previously was found mainly in larger firms. As a result, there has been an increase in the demand for compact and low-cost office equipment such as electrophotographic printers, etc., that still provide high resolution and high print or copy quality.  
           [0005]    One example of such apparatus is an LED printer, which is an electrophotographic printer employing an LED (light emitting diode) array comprising a large number of LEDs. Because a printer of such type uses a fixed writing light source incorporating the LED array, the apparatus itself can be more compact than a comparable raster scanning printer employing a semiconductor laser (laser diode) and a mirror scanning system. In addition, the LEDs in an LED printer can write in parallel (simultaneously) and thereby make it simpler to increase writing speed.  
           [0006]    When the light source is an LED array, the light from the individual LEDs needs to be delivered onto the light-receiving surface (e.g., photosensitive or photoconductive surface) at high resolution and high efficiency. Furthermore, in order to make the apparatus more compact, the distance between the light source (LED array) and the light-receiving surface needs to be minimized. For this reason, a suitable focusing optical system is required. A rod lens array composed of bundled plural rod lenses has been used for such focusing in many LED printers.  
           [0007]    [0007]FIG. 5 illustrates a structure discussed in the published specification of Japanese Laid-open Patent Publication No. 7-108709/1995, and is an example of an optical apparatus employing such a rod lens array in which light rays emitted from each of LEDs  102  in an LED array  101  (comprising a number of LEDs  102  arranged in a row extending in a direction perpendicular to the drawing sheet) are projected onto a photosensitive surface  105  by the focusing action of a corresponding rod lens  104  in the rod lens array  103  (which also comprises a number of lenses  104  arranged in a row extending in a direction perpendicular to the drawing sheet). As a result, a fine spot image is focused on the photosensitive surface  105 . The rod lens array  103  forms the focusing optical system  106 .  
           [0008]    [0008]FIG. 22 is a similar cross-sectional view of the proposal discussed in the same Patent Publication, and shows that the light rays emitted from an LED  302  in an LED array  300  are projected onto a photosensitive surface  306  by the focusing action of a corresponding rod lens  304  in a rod lens array  103  to thereby produce a finely focused light spot at photosensitive material  306 .  
           [0009]    Another use of an LED array in an optical apparatus is illustrated in FIGS. 6 and 23, and is discussed in the published specification of Japanese Laid-open Patent Publication 8-1998/1996. In FIG. 6, light rays emitted from an LED  112  in an LED array chip (LED array)  111  are guided to a photosensitive surface facing or contacting an optically opaque block  114  through a corresponding light guiding path  113  in block  114 , which is mounted on the LED array chip  111 . In FIG. 23, light rays emitted from an LED  312  in an LED array  310  are guided to a photosensitive surface facing or contacting an optically opaque layer  314  through a corresponding guiding path  316  in the form of a light pipe formed in layer  314 .  
           [0010]    The rod lenses of a rod lens array system of the type illustrated in FIGS. 5 and 22, transmit light relatively efficiently to the photosensitive surface  105 . However, because the light emission angle of an LED  102  in the LED array  101  is inherently wide and includes much more than the facing area of the corresponding rod lens  104 , much of the light energy emitted from an LED does not reach its rod lens  104 . As a result, there is poor utilization efficiency of the light energy that an LED emits. Consequently, if a predetermined amount of light energy or intensity is required at the photosensitive surface  105 , it is necessary to emit much more energy or intensity from the LED, with a corresponding need for high drive electric current to the LED  102  and a corresponding undesirable heating of the LED  102 .  
           [0011]    The LED arrays of the type shown in FIGS. 6 and 23 also have a relatively poor light energy utilization and, in addition, fail to provide a light focusing function and, therefore, unless the photosensitive surface is brought very close to or in contact with the optical system (the layer  114 ), the light image formed on the photosensitive surface is out of focus, resulting in poor resolution.  
         SUMMARY OF THE DISCLOSURE  
         [0012]    The system and method disclosed in this patent specification are designed to overcome these and other deficiencies in known approaches and to provide improvements in delivering light energy to a light receiving surface efficiently and effectively.  
           [0013]    To this end, the disclosed system and method use a light source such as an LED or an EL array in an arrangement that increases the light energy utilization as compared with known systems and methods, while retaining significant benefits of such known systems and methods. One aspect of the disclosed approach is to use technology similar to that used in the integrated circuit (IC) technology to form an LED at the bottom of a recess whose walls serve to direct much more of the light energy from the LED to an element such as a rod lens that guides or some other focusing or light guiding system that in turn delivers the light energy to a light receiving surface.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    A more complete appreciation of the system and method disclosed in this patent specification and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:  
         [0015]    FIGS.  1 ( a ) through  1 ( f ) are elevational cross-sections illustrating an LED array head of a first embodiment disclosed herein, in the order of process steps of manufacturing the LED array head;  
         [0016]    [0016]FIG. 2 is a schematic view of an optical apparatus using the LED array head of the first embodiment;  
         [0017]    FIGS.  3 ( a ) through  3 ( d ) are elevational cross-section illustrating an LED array head of a second embodiment disclosed herein, in the order of process of manufacturing the LED array head;  
         [0018]    [0018]FIG. 4 is a schematic view of an optical apparatus using the LED array head of the second embodiment;  
         [0019]    [0019]FIG. 5 is an elevational cross-section illustrating a known optical array apparatus utilizing a rod lens array;  
         [0020]    [0020]FIG. 6 is a perspective view illustrating a known optical apparatus using a light guide array to deliver light from LEDs;  
         [0021]    [0021]FIG. 7 is a schematic view of an optical apparatus using a third embodiment disclosed herein;  
         [0022]    [0022]FIG. 8 is a cross-section illustrating an LED array of the third embodiment;  
         [0023]    [0023]FIG. 9 is a plan view of the LED array shown in FIG. 8;  
         [0024]    [0024]FIG. 10 is a cross-section illustrating a first step in the manufacture of the LED array of FIG. 8;  
         [0025]    [0025]FIG. 11 is a cross-section illustrating a second step in the manufacture of the LED array of FIG. 8;  
         [0026]    [0026]FIG. 12 is a cross-section illustrating a third step in the manufacture of the LED array of FIG. 8;  
         [0027]    [0027]FIG. 13 is a cross-section illustrating a fourth step in the manufacture of the LED array of FIG. 8;  
         [0028]    [0028]FIG. 14 is a cross-section of an LED array illustrating a fourth disclosed embodiment;  
         [0029]    [0029]FIG. 15 is a cross-section illustrating a first step in the manufacture of the LED array of FIG. 14;  
         [0030]    [0030]FIG. 16 is a cross-section illustrating a second step in the manufacture of the LED array of FIG. 14;  
         [0031]    [0031]FIG. 17 is a cross-section illustrating a third step in the manufacture of the LED array of FIG. 14.  
         [0032]    [0032]FIG. 18 is a cross-section illustrating an LED array of a fifth disclosed embodiment;  
         [0033]    [0033]FIG. 19 is a cross-section illustrating a first step in the manufacture of the LED array of FIG. 18;  
         [0034]    [0034]FIG. 20 is a cross-section illustrating a second step in the manufacture of the LED array of FIG. 18;  
         [0035]    [0035]FIG. 21 is a cross-section illustrating a third step in the manufacture of the LED array of FIG. 18;  
         [0036]    [0036]FIG. 22 is a cross-section illustrating a known apparatus employing the rod lens array; and  
         [0037]    [0037]FIG. 23 is a perspective view illustrating a known optical apparatus using light guides.  
     
    
     DETAILED DESCRIPTION  
       [0038]    Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, a first through fifth embodiments are disclosed below, and methods of manufacturing them also are disclosed. The first embodiment is illustrated in FIGS.  1 ( a ) through  1 ( f ) and  2 , the second embodiment in FIGS.  3 ( a ) through  3 ( d ) and  4 , the third embodiment in FIGS. 7 through 13, the fourth embodiment in FIGS. 14 through 17, and the fifth embodiment in FIGS. 18 through 21.  
       First Embodiment  
       [0039]    The first embodiment, illustrated in FIGS.  1 ( a ) through  1 ( f ) and  2 , is described, as a non-limiting example, as used for writing-in optical information through an image focusing optical system  1  comprising rod lens array, etc., that can be used in place of the focusing optical system  106  of FIG. 5.  
         [0040]    As seen in FIG. 2, such optical apparatus employs a structure in which the light emitted from an LED array head  2  is focused on a light receiving (e.g., photosensitive or photoconductive) surface  3 .  
         [0041]    The LED array head  2  seen in FIG. 1( f ) comprises LED array (light emitting element array)  5  in which plural LEDs  4  are employed as the light emitting element and are arranged in a row extending to the left and to the right. A reflection mirror  7  for each LED  4  serves as a respective optical system  6  narrowing the solid angle within which LED array  5  emits light from the LED  4 .  
         [0042]    The optical system  6  narrowing the solid angle of the emitted light is integratedly formed with the LED array  5 . An exemplary and non-limiting method of manufacturing the optical system  6  is described below, referring to the elevational cross-sections of FIGS.  1 ( a ) through  1 ( f ) illustrating steps in the process of manufacturing the LED array  5 .  
         [0043]    At first, after LEDs  4  are formed in a substrate  8 , a layer of polyamide  9  is formed over substrate  8  and LEDs  4 , for use in forming the mirror structure. Preferably, layer  9  is approximately 25 μm thick. Refer to FIGS.  1 ( a ) and  1 ( b ).  
         [0044]    Next, openings are formed, e.g., by etching, in layer  9  for a mirror structure  10 . Each opening is in the shape of an inverted frustum (truncated cone) and is centered at a respective LED  4 . Refer to FIG. 1( c ). Preferably, the diameter of each frustum-shaped opening at the LED side (bottom side) of the mirror structure  10  is approximately 10 μ m and the diameter of the light emission side thereof (top) is approximately 26 μm.  
         [0045]    An aluminum reflection film  11  is formed over the mirror structure  10  to serve as a mirror reflection film. Refer to FIG. 1( d ). The aluminum reflection film  11  which is over the LEDs  4  is removed but the film  11  is left in over the sides of the frustum-shaped openings of the mirror structure  10 . Refer to FIG. 1( e ). Thus, the reflection mirror  7  is formed over the surface surrounding and extending up from each respective LED  4 . A protective film, such as an SiO 2  film  12 , is formed over the entire surface of the mirror structure  10  to complete the LED array head  2 .  
         [0046]    Referring to FIG. 2, some of the light emitted from an LED  4  goes directly to the focusing optical system  1 , as incident direct light L 1 . In addition, some of the light emitted from the same LED  4  reaches the focusing optical system  1  indirectly—after being reflected by the reflection mirror  7  surrounding and extending up from the LED  4 —as incident indirect light L 2 . The focusing optical system  1  focuses the light it receives from an LED  4  onto the light receiving surface  3  as earlier discussed.  
         [0047]    In this non-limiting example, if the focusing optical system  1  is 1 mm from the LED array head  2 , the half-power width of the light spot from an LED onto the system  1  is approximately 550 μm, corresponding to a solid angle at the half-power level of approximately 31°. The term half-power refers to light intensity distribution level at which the power above and below that level is the same. The term half-power width refers to the width (diameter) of the light beam at that level.  
         [0048]    If without using the reflection mirror  7  the solid angle at the half power level of the light from an LED is 65°, the use of the mirror  7  reduces that angle to about half, thus bringing about a substantial improvement. Furthermore, the efficiency of illuminating the light receiving surface  3  with the use of the reflection mirror  7  can be increased almost five-fold compared with the case of not using the reflection mirror  7 , given that the focusing optical system  1  typically has an incident angle of nearly 20°, thus substantially improving the overall efficiency of the light from the light from the LED array  5 .  
         [0049]    Consequently, if the same light intensity is required at the light receiving surface  3  as in the known systems discussed above, the LED driving electric current that is required in the first disclosed embodiment can be reduced significantly because of the improved light utilization efficiency. As a result, the heating up of the LEDs  4  can be reduced. Conversely, if the LED drive current in the first embodiment is the same as in the known systems discussed above, the light intensity at the light receiving surface  3  can be significantly higher and, as a result, the exposure time can be significantly shortened and, thus, the scanning speed (writing-in speed) can be significantly decreased to thereby realize much higher-speed printing.  
         [0050]    Furthermore, it is possible to provide the first embodiment structure, with its integration of the LEDs  4  and the reflecting mirror  7 , using well developed thin film technology, thus realizing low cost production of the LED array  2 .  
       Second Embodiment  
       [0051]    The second embodiment is described hereinafter, referring to FIGS.  3 ( a ) through  3 ( d ) and  4 . The elements that are the same as in the first embodiment bear the same reference numerals, and the description thereof is not repeated here. In the second embodiment, the optical system  21  for each of the LEDs  4  comprises a reflection mirror  7  and a focusing lens  22 . The focusing lens  22  is formed integrally with the rest of the LED array  5 , as is the reflection mirror  7 .  
         [0052]    A method of manufacturing the LED array head  23  of the second embodiment is described below, referring to the elevational cross-sections of FIGS.  3 ( a ) through  3 ( d ), where the reflection mirror  7  can be manufactured as discussed in connection with FIGS.  1 ( a ) through  1 ( e ).  
         [0053]    In one non-limiting example, the diameter of the LEDs  4  is 10 μm and the diameter of the inverted frustum-shaped reflection mirror  7  also is 10 μm at its bottom, at the level of the LEDs  4 . However, in order to form the focusing lens  22  of the second embodiment, the reflection mirror  7  differs in certain respects from that of the first embodiment.  
         [0054]    In order to form the LED array head  23  of the second embodiment, a layer of a transparent resin  24 , preferably 75 μm thick, is formed over an array formed as discussed in connection with FIGS.  1 ( a ) through  1 ( e ), i.e., over the array illustrated in FIG. 1( f ) that has been completed through the formation of the protective layer of SiO 2 . Refer to FIG. 3( b ). The refractive index of the transparent resin  24  preferably is 1.42. A plurality of focusing lenses  22  is formed over the SiO 2 , e.g., by dry etching, each lens  22  centered at a respective LED  4 . Refer to FIG. 3( c ). Each focusing lens  22  is formed as an spherical lens conforming to a hyperboloid. The diameter of the lens  22  preferably is 40 μ m, the radius of curvature at the apex of the lens preferably is 25 μm, and the circular cone coefficient preferably is −1.2934. Finally, a Cr film  25 , opaque to light, is formed over the portion of the transparent resin film  24  outside the areas occupied by the focusing lenses  22 . Refer to FIG. 3( d ). This essentially completes the LED array head  23 .  
         [0055]    As seen in FIG. 4, some of the light emitted from an LED  4  goes directly to the focusing optical system  1 , as incident direct light L 1 . In addition, some of the light emitted from the same LED  4  reaches the focusing optical system  1  indirectly—after being reflected by the reflection mirror  7  surrounding the LED  4 , and after being focused by a lens  22 —as incident indirect light L 2 . The focusing optical system  1  focuses the light it receives from an LED  4  onto the light receiving surface  3  as earlier discussed.  
         [0056]    In the second embodiment, the reflection mirror  7  effects a reduction in the solid angle of the light an LED  4  emits, and the focusing lens  22  effects a further reduction in the solid angle of the light delivered to the optical system  1  that in turn further focuses the light onto the light receiving surface  3 .  
         [0057]    For instance, if the focusing optical system  1  is 1 mm from the LED array head  2 , the half-power width of the light from an LED  4  at the optical system  1  is approximately 150 μm. This width corresponds to a half-power level solid angle of approximately 9°.  
         [0058]    Thus, the addition of the focusing lens  22  further improves efficiency as compared with the first embodiment, through further harrowing the solid angle of the light from an LED that is delivered to the optical system  1  and the light receiving surface  3 . As compared with the known technology earlier discussed, that does not use a reflection mirror  7  or a lens  22 , the second embodiments provides a ten-fold increase in illumination.  
       Third Embodiment  
       [0059]    [0059]FIG. 7 schematically illustrates an optical apparatus using the third embodiment&#39;s LED array, FIG. 8 is a cross-section illustrating the third embodiment&#39;s LED array, FIG. 9 is a plan view of the LED array of FIG. 8, and FIGS. 10 through 13 are cross-sections illustrating steps in a process of manufacturing the LED array of FIG. 8.  
         [0060]    As seen in FIG. 7, the third embodiment&#39;s LED array can be used, as a non-limiting example, in a writing-in optical apparatus of an LED printer. Light emitted from an LED array  210 , in the form of direct light L 3  and indirect (reflection) light L 4 , impinges of a light focusing system  212  light, and the focused light emerging from focusing system  212  impinges on a light-receiving surface  214 , e.g., a photosensitive or a photoconductive surface or some other type of a light receiving surface.  
         [0061]    Next, the structure of the LED array  210  is described below.  
         [0062]    As seen in FIGS. 8 and 9, an n-type GaAs layer  222 , such as an epitaxial layer, is formed over a substrate such as a GaAs substrate  220 . Recesses  224  are opened from the top of layer  222 , each shaped as an inverted frustum (truncated cone). Each recess  224  preferably has a circular bottom of radius 5 μm. The circular opening of a frustum at the top of layer  222  preferably has a radius of 10 μm, and the depth of the inverted frustum preferably is 20 μm. The side wall of the recesses  224  is a slanted surface  226 .  
         [0063]    To form LEDs  228  of the LED array  210 , the n-type GaAs at the circular bottom of each inverted frustum  224  is doped with an impurity such as Zn to p-type polarity to thereby form an LED  228  at the bottom of each recess  228 . Furthermore. electrodes (not shown) are formed for supplying electric current to the light-emitting portions  228 .  
         [0064]    Next, the operation of the LED array  210  is described. As each light-emitting portion  228  of the LED array  210  is a circle of radius 5 μm, it need not be regarded as a point source but can be considered a source that emits light from each of a number of laterally spaced point sources within a single portion  228 . The light that a light emitting portion  228  emits can be considered diffused light emitted within a solid angle of 120° at the half-power level.  
         [0065]    The recesses surrounding the light-emitting portion  228  of the LED are spreading out in the shape of the reversed circular cone frustum just like the cocktail glass from the light-emitting portion  228  of the bottom surface portion toward the opening portion, that is, the light-emitting side of the surface of the n-type GaAs epitaxial layer  222 .  
         [0066]    Thus, light from a light-emitted portion  228  reaches the focusing optical system  212  as direct incident light L 3 , and additional light from the same portion  228  reflects from different portion of the inclined surface  226  of the side wall of the recess  224  and reaches the system  212  as indirect (reflected) light L 4 . The slanted surface  226  of the side wall of each recess  224  acts as mirror reflecting light emitted from its respective light-emitting portion  228 , and the slanted surface  226  thus serves to narrow the solid angle within which light reaches the optical system  212 .  
         [0067]    Next, a method of manufacturing the third embodiment&#39;s LED array  210  is described hereinafter, referring to FIGS. 10 through 13.  
         [0068]    Using a process such as epitaxial growth, the n-type GaAs layer  222  is formed over the GaAs substrate. Thereafter, using a process such as photolithography, a mask  230  is patterned over the n-type GaAs epitaxial layer  222  to expose the layer  222  where the recesses  224  will be formed and protect it elsewhere.  
         [0069]    Using the mask  230 , the n-type GaAs epitaxial layer  222  is selectively etched to form the recesses  224 , each shaped as an inverted frustum (truncated cone) approximately 20 μm deep and with a circular bottom of a 5 μm radius. The side wall of each recess is a slanted surface  226  in vertical section. Refer to FIG. 10.  
         [0070]    Following the formation of recesses  224  (and possible removal of the mask  230 ), an insulation film  232  of a material such as SiO 2  is formed over the entire exposed surface of the layer  222 . Using a process such as photolithography and etching process, the SiO 2  insulation film  222  is selectively removed from the bottoms  234  of the recesses  224 . Refer to FIG. 11.  
         [0071]    Using a process of introducing impurities, such as a diffusion process utilizing the remaining portions of the SiO 2  insulation film  232  as a diffusion mask, the exposed portions of the layer  222  are doped to p-type polarity with dopants such as Zn, to thereby form p-type portions  228  at the bottoms of the recesses  224 .  
         [0072]    Thus, LEDs  4  having a pn junction part between the n-type GaAs layer  222  and the p-type GaAs portions  228  are formed. Refer to FIG. 12.  
         [0073]    The remaining portions of the SiO 2  insulation film  232  are removed, and electrodes (not shown) are formed for supplying electric current to the light-emitting portions  228 , essentially completing the LED array  210  seen in FIG. 8. Refer to FIG. 13.  
         [0074]    In an optical system using the third embodiment&#39;s LED array  210 , the light-emitting portions  228  of the respective LEDs are at the bottoms of the recesses  224  into the surface of the n-type GaAs epitaxial layer  222 . Each recess  224  surrounds a respective light-emitting portion  228  and is in the shape of an inverted frustum (truncated cone) centered at a corresponding light-emitting portion  228 . The slanted surface  226  of the side wall of the recesses  224  acts as a mirror reflecting light emitted from the light-emitting portion  228  of the LED toward the optical focusing system  212 , thereby narrowing the solid angle at which light from an LED reaches system  212 . The reflection optical system for narrowing this solid angle is integratedly formed as a part of the LED array  210 .  
         [0075]    When each LED light-emitting portion  228  is a circular surface and has a radius of 5 μm, the light emitted from the light-emitting portion  228  approximates diffused light emitted within a solid angle of 120° at the half-power level but the reflection from the slanted side surface of the corresponding recess  224  effectively reduces the solid angle of the light the LED delivers to the focusing system  212  as direct incident light L 3  and indirect incident light L 4 . Consequently, the third embodiment increases the overall light utilization efficiency as compared with the known systems discussed earlier.  
         [0076]    Because the light-emitting portion  228  of the LED and the GaAs layer  222  have substantially the same thermal expansion coefficient, as they are both the same GaAs material doped with different dopants, the third embodiment effectively avoids undesirable thermal effects such as thermal stress that could distort the reflections from the side surface of the recesses  224 , even if the operational temperature of the light-emitting portion  228  rises.  
         [0077]    Furthermore, when the light-emitting portions  228  of p-type GaAs layer are formed by introducing dopants such as Zn into the n-type GaAs layer  222  using a diffusion process, the SiO 2  insulation film  232  covering the top surface of the n-type GaAs epitaxial layer  222  and the slanted surface  226  on the side wall of the recesses  224  can serve as the diffusion mask, and thereby accurately position the portions  228  relative to the recesses  224  and reduce manufacturing cost while improving performance.  
       Fourth Embodiment  
       [0078]    [0078]FIG. 14 is a cross-section illustrating the structure of an optical apparatus using the fourth embodiment, and FIGS. 15 through 17 are cross-sections illustrating steps in a method of manufacturing the LED array of FIG. 14.  
         [0079]    As a non-limiting example, an optical apparatus using the fourth embodiment can be employed for writing-in optical information in a device such as an LED printer, as can an optical apparatus using the third embodiment. Because of the similarities with the third embodiment illustrated in FIG. 7, the same reference numerals are used for like elements in FIGS.  14 - 17  as in FIGS. 7 through 13, and the description thereof is not repeated here.  
         [0080]    The structure of the LED array  210  used in the fourth embodiment is described hereinafter.  
         [0081]    As seen in FIG. 14, a GaAs layer  222  of one polarity, for instance, n-type GaAs, is formed, for example as an epitaxial layer, over a GaAs substrate  220 , and recesses  224  are formed into layer  222 , each in the shape of an inverted frustum (truncated cone). Each recess  224  preferably has a circular bottom whose radius is 5 μm. The top of a recess  224  preferably is the shape of a circular opening of a 10 μm radius, and the depth of a recess  224  preferably is 20 μm. Each recess  224  has a slanted side wall  226  in vertical section.  
         [0082]    Portions  228  of opposite polarity, for example p-type, are formed at the circular portions of the GaAs layer  222  exposed at the bottoms of the recesses  224 , for example by introducing a dopant such as Zn, to thereby form light-emitting portion  228  serving as LEDs. Electrodes (not shown) are formed to supply electric current to the light-emitting portions  228 .  
         [0083]    In the fourth embodiment, Au/Cr film  236  is formed on the slanted surface  226  of the side wall of each recesses  224 . A Cr film is interposed between the GaAs and the Au in order to improve bonding between the Au film, which has a high reflection coefficient, and the underlying n-type GaAs epitaxial layer  222 .  
         [0084]    Next, the operation of the LED array  210  is described hereinafter.  
         [0085]    As seen in FIG. 14, the light-emitting portion  228  of an LED emits light at its entire upper surface, which light can be considered as approximating diffused light emitted within a solid angle of 120° at the half-power width. The recess  224  surrounding the light emitting portion  228  of an LED, restricts the light to a lesser solid angle, and the Au/Cr film  236  serves as a reflection film.  
         [0086]    Due to this structure, light from an LED area  228  is directed up, toward an optical focusing system (as to system  212  in FIG. 7) as direct incident light L 5  and as indirect incident light L 6  that is reflected from the Au/Cr film  236  on the slanted side wall of the recess  224 . The recess  224  and the Au/Cr film  236  thus serve to reduce the solid angle within which light from a light-emitting portion  228  is directed up to the optical focusing system and therefrom to a light receiving surface (such as surface  214  in FIG. 7).  
         [0087]    Next, a method of manufacturing the LED array  210  is described hereinafter, referring to the cross-sections of FIGS. 15 through 17.  
         [0088]    After forming the n-type GaAs epitaxial layer  222  over the GaAs substrate  220 , a mask  230  is patterned over the n-type GaAs epitaxial layer  222 . Using the mask  230  as an etching mask, the n-type GaAs epitaxial layer  222  is selectively etched to form the recesses  224  in the shape and dimensions discussed earlier. Refer to FIG. 15.  
         [0089]    After forming a SiO 2  insulation layer  232  over the entire upper surface of the layer  222 , including the slanted surfaces  226  of the recesses  224  and over the n-type GaAs at the bottoms of the recesses  224  (and after possible removal of the mask  230 ), the SiO 2 , the insulation film  232  is patterned by etching to expose GaAs at the bottoms of the recesses  224 . Using the patterned SiO 2  layer  232  as a diffusion mask, the GaAs exposed at the bottoms of the recesses  224  is doped, e.g., with Zn, to thereby form light-emitting portions  228  of p-type GaAs at the bottoms of the recesses  224 . Refer to FIG. 16.  
         [0090]    The remaining SiO 2  insulation film  232  is removed, and an Au/Cr film  236  is formed over the upper surface of the layer  222 , including over the slanted surfaces  226  of the side walls of the recesses  224 , the Au/Cr film  236 , by first forming a Cr film and then an Au film over it using photolithography, the Au/Cr film  236  is patterned to leave a reflection film  236  of Au/Cr film only over the slanted surfaces  226  of the side walls of the recesses  224 . Electrodes (not shown) are formed for supplying electric current to the light-emitting portion  228 , to essentially complete the LED array  210  seen in FIG. 14.  
         [0091]    Because in the fourth embodiment the reflection coefficient of the Au/Cr film  236  (FIG. 14) is greater than that of the GaAs at the slanted surface  226  of the n-type GaAs epitaxial layer  222  in the third embodiment (FIG. 7), the overall light utilizing efficiency can be further improved.  
         [0092]    According to experimental results, it is believed that the structure of the fourth embodiment can improve overall light utilization efficiency by 50% compared with the known structures discussed earlier which use the light-emitting portion of LEDs but not recesses and a reflecting film as in the fourth embodiment described herein.  
         [0093]    Further, the LED array structure of the fourth embodiment can be manufactured efficiently and precisely by methods such as described above, resulting in low manufacturing cost.  
       Fifth Embodiment  
       [0094]    [0094]FIG. 18 is a cross-section illustrating an LED array for use in an optical apparatus in accordance with a fifth embodiment, and FIGS. 19 through 21 illustrate steps in a method of manufacturing the LED array of FIG. 18.  
         [0095]    As a non-limiting example, an optical apparatus using the fifth embodiment can be employed for writing-in optical information in a device such as an LED printer, as can an optical apparatus using the third embodiment. Because of the similarities with the third embodiment illustrated in FIG. 7, the same reference numerals are used for like elements in FIGS.  18 - 21  as in FIGS. 7 through 13, and the description thereof is not repeated here.  
         [0096]    The structure of an LED array  210  in accordance with the fifth embodiment is described hereinafter.  
         [0097]    As shown in FIG. 18, an n-type GaAs epitaxial layer  222  is formed over a GaAs substrate  220 , and recesses  224  each in the shape of an inverted frustum (truncated cone) are formed into the n-type GaAs epitaxial layer  222 . The bottom of each recess  224  preferably is circular and has a radius of 5 μm, the top of a recess  224  preferably is circular and has a radius of 10 μm, and the depth of a recess  224  preferably is 20 μm. Each recess  224  has a slanted side wall  226  in a vertical section. P-type GaAs portions (doped with Zn) are formed at the bottoms of the recesses  224  and preferably also are circular and have a radius of 5 μm. The p-type GaAs portions serve as light-emitting portions (LEDs)  228 . A metal reflection film (Au/Cr film)  238  extends onto the periphery of the light-emitting portions  228  (and makes Ohmic contact therewith) and covers the slanted side walls  226  of the recesses  224  as well as the top portions of layer  222  that are between the recesses  224 , to thereby serve both as a light reflection film and as an electrode supplying electrical current to the light-emitting portions  228 . The Cr film is between the Au film and the GaAs layer  22  to improve bonding between the Au film that has a high reflection coefficient and the underlying n-type and p-type GaAs.  
         [0098]    Next, the operation of the LED array  210  is described hereinafter.  
         [0099]    As seen in FIG. 18, a light-emitting portion  228  emits toward a focusing system (not shown, but corresponding to system  212  of FIG. 7) both direct light L 7  and indirect light L 8  reflected by the Au/Cr film  238 . Because of the size of the light-emitting portion  228 , the light emitted therefrom can be considered diffused light emitted within a solid angle of 120° at the half-power width. The recess  224  limits this solid angle and the reflections of light L 8  from the Au/Cr film  238  further concentrates the light from the corresponding light-emitting portion  228 .  
         [0100]    Next, a method of manufacturing the LED array  210  is described hereinafter, referring to the cross-sections of FIGS. 19 through 21.  
         [0101]    After forming the n-type GaAs epitaxial layer  222  over the GaAs substrate  220 , a mask  230  is patterned over the n-type GaAs epitaxial layer  222 . Using the mask  230 , the n-type GaAs epitaxial layer  222  is selectively etched to form the inverted frustum-shaped recesses  224  that preferably have circular bottoms of a 5 μm radius, tops of a 10 μm radius, and depth of 20 μm. Refer to FIG. 19.  
         [0102]    An Au/Cr film  238  is formed over the entire upper surface of layer  222 , by first forming the Cr film to improve bonding of the Au to GaAs. Using photolithography and selective etching, the Au/Cr film  238  is patterned to form openings  234  at the bottoms of the recesses  224  to thereby expose portions of the n-type GaAs centered with recesses  224 . The openings are somewhat smaller in area than the bottom of the inverted frustum. Refer to FIG. 20.  
         [0103]    Using the Au/Cr film  238  as a mask, the portions of layer  222  that are exposed through openings  234  are doped to p-type, e.g., through ion implantation with Zn, to thereby form light-emitting portions  228  serving as LEDs. Because the mask opening is smaller than the bottom of the recess  224 , and because the p-type doping expands laterally due to various effects including heat activation, the Au/Cr film  238  comes into ohmic contact with the periphery of the p-type regions of the completed LEDs. The Au/Cr firm  238  thus serves as an electrode, as a metal reflection film, and as a bonding pad. Refer to FIG. 21.  
         [0104]    The use of the recesses  224  and the reflection film  238  in the fifth embodiment improves the overall light utilization efficiency as compared with the known systems discussed above. In addition, the structure is simplified as compared with, for example, the fourth embodiment, because it is not necessary to provide an electrode and a bonding pad in addition to the Au/Cr film  238 .  
         [0105]    Furthermore, because the light-emitting portions  228  are formed by doping with Zn through openings  234  in the Au/Cr film at the bottoms of the recesses  224 , the light-emitting portions  228  are in effect self-aligned with the recesses  224  and with the Au/Cr film  238  that serves as an electrode, a metal reflection film, and a bonding pad, and a doping mask. As a result, the relative positioning of the light-emitting portions  228  and the Au/Cr film  238  can be made very precise. Furthermore, since the light-emitting portions  228  and an electrode thereto are achieved through the same process of ion implanting Zn using the patterned Au/Cr as a mask, a manufacturing simplification is achieved.  
         [0106]    The use of an LED array has been described in detail above, but it should be understood that an EL (electroluminescence) array can be used, such that EL devices are used in place of the LED devices.  
         [0107]    Although one particular example of use of the LED arrays described above is in a system for writing-in optical information in an LED printer, the use of the LED arrays described in detail above is not so limited. For example, they can be used for writing-in optical information in digital copying machines, in facsimile devices, etc. or in other systems that utilize light beams of the type described above.  
         [0108]    The recesses  224  have been described above as having the shape of an inverted frustum, but are not so limited. For instance, the recesses  224  can conform to the shape of a spherical or a spheroid, e.g., it can be arc-shaped in vertical section, or curved in some other way in vertical section to thereby reduce the solid angle of the light emitted from the LED or EC device. The inclination angle of the slanted side wall of the recess  224  can be selected so that the angle is different from that determined by the dimensions described above, and those dimensions can be selected to have different values, in order to optimize different aspects of the device, including light utilization efficiency.  
         [0109]    In an example of the detailed description above, the SiO 2  insulation film  232  is used as a mask for the diffusion of Zn to form the light-emitting portions  228 . It should be understood that this mask, or another mask, can be used for doping by another process, such as ion implantation.  
         [0110]    Similarly, the Au/Cr film  238  can be used as a mask for diffusion of a dopant to form the p-type areas  228  rather than for ion implantation, or a mask of SiO 2  can be used for either diffusion or ion implantation.  
         [0111]    On the contrary, in the aforementioned fifth embodiment, it is also allowed that, after forming the light-emitting portion  228  utilizing the impurities diffusion process, in which the SiO 2  insulation film, etc. is employed as the protection film, the Au/Cr film  238  is used.  
         [0112]    The Au/Cr film  238  serving as a reflection film can be formed directly on the slanted surface  226  of the side wall of the recesses  224 , or a film such as an insulation film of a material such as SiO 2  can be interposed between the GaAs layer  222  and the Au/Cr film  238 .  
         [0113]    Numerous other embodiments or modifications are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.  
         [0114]    Based on the full description provided herein, it will be apparent to one of ordinary skill in the art that many changes and modifications can be made thereto without departing from the spirit and scope of the invention claimed below.  
         [0115]    This application is based on Japanese Patent Application No. JPAP09-248,088, filed on Sep. 12, 1997, and Japanese Patent Application No. JPAP09-333,599, the entire contents of both of which are herein incorporated by reference.