Abstract:
This invention provides a structurally-simple LED light source that is capable of preventing temperature variations among its multiple LED elements arranged densely on its LED-mounting substrate and also improving the heat release capabilities of the substrate by comprising an LED light source with: a plurality of LED elements each of which is formed by connecting an LED chip to electrodes formed on a ceramic substrate; an LED-mounting substrate on which to mount the plurality of LED elements, the LED-mounting substrate having through holes therein; and a heat sink plate for releasing heat from the LED-mounting substrate, wherein a thermally conductive resin is present between the LED-mounting substrate and the heat sink plate and wherein part of the thermally conductive resin protrudes from the through holes of the LED-mounting substrate and covers the top surface of the LED-mounting substrate on which the plurality of LED elements are mounted, so that the part of the thermally conductive resin is in contact with the plurality of LED elements.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    As an example of a conventional LED (light-emitting diode) substrate, Japanese Unexamined Patent Application Publication No. 2004-311791 (Patent Document 1) discloses such an LED substrate as illustrated in  FIG. 5 . 
         [0002]    The LED substrate  10  of  FIG. 5  has a concave portion  11   a  formed on the top surface of its ceramic substrate  11 . Conductive wiring patterns  12  are formed on the concave portion  11   a , and a single LED chip  13  is glued to the conductive wiring patterns  12  with its bottom surface facing the concave portion  11   a  (the bottom surface of the LED chip  13  is the surface through which light is not emitted). The electrodes on the light-emitting-surface side of the LED chip  13  are connected by wires  14 , made of gold or the like, to particular locations of the conductive wiring patterns  12  on the concave portion  11   a  of the ceramic substrate  11 . The concave portion  11   a  is sealed with resin  15  (e.g., silicone resin), or inert gas such as nitrogen or the like is sealed inside the concave portion  11   a . Part of the side faces and part of the bottom face of the ceramic substrate  11  are covered with outer terminals  16  so that the LED substrate  10  can be electrically connected to other external devices. Note that multiple LED chips  13  with different luminescent colors can also be mounted on the concave portion  11   a  in place of the single LED chip  13 . 
         [0003]    As illustrated in  FIG. 6 , Patent Document 1 also discloses an LED light source that involves the use of multiple LED substrates  10 , each of which is the one shown in  FIG. 5 . In this LED light source, the multiple LED substrates  10  are attached by solder  17   a  to the top surface of a substrate  17 , made of glass epoxy resin or the like, in the form of a single or multiple rows. Also, an LED chip  13  ( 13   a - 13   c ) is mounted on each of the concave portions  11   a - 11   c  the LED substrates  10  with the bottom surfaces of the LED chips  13  facing the LED substrates  10 . 
         [0004]    When the LED light source of Patent Document 1 is used for a photolithography apparatus or an illumination device, sufficient heat release is required because the LED light source is driven by a high electric current. Thus, as illustrated in  FIG. 6 , a heat sink plate  18  is attached to the bottom surface of the substrate  17 , i.e., the surface opposite the surface to which the substrates  10  are attached. This LED light source is arranged, as illustrated in  FIG. 7 , across from a light guide plate  19  of a backlight so that the light from the LED substrates  10  can travel into the light guide plate  19 . 
         [0005]    As another heat release structure, Patent Document 1 also discloses such an LED light source structure as illustrated in  FIG. 8 . In this LED light source, an LED chip  13  is glued to the conductive wiring patterns on each of concave portions  11   a  of ceramic substrates  11 , with the bottom surfaces of the LED chips  13  facing the concave portions  11   a . Also, a heat sink plate  18  is glued directly to the bottom faces of the ceramic substrates  11 , i.e., the surface opposite the light emitting surface of the ceramic substrate. This LED light source also has a substrate  17  placed on the ceramic substrates  11 . The substrate  17  has windows  21  so that the light emitted from the ceramic substrates  11  can travel there through. The conductive wiring patterns on the substrate  17  (not illustrated) which are used for power supply to the LED chips  13  are connected by solder  20  to the conductive wiring patterns on the top surfaces of the ceramic substrates  11  (not illustrated). 
         [0006]    The LED display device of Patent Document 1 that involves the use of such LED light sources as above is capable of releasing the heat of the LED chips  13  efficiently because the heat can be transferred to the heat sink plate  18  only through the ceramic substrates  11  and the adhesive that glues the LED chips  13  to the ceramic substrates  11 . 
         [0007]    As a modification example of the LED substrate  10  of  FIG. 5 , Japanese Unexamined Patent Application Publication No. 2008-172177 (Patent Document 2) discloses, as illustrated in  FIG. 9 , a heat release structure in which a metal housing  22  the houses a liquid-to-vapor heat release device  23  is attached to the bottom surface of an LED chip  13 , i.e., the surface opposite the light emitting surface of the chip  13 . 
         [0008]    In the heat release structure of Patent Document 2, the heat of the LED chip  13  can be quickly released through the metal housing  22  and the liquid-to-vapor heat release device  23 , which is high in thermal conductivity. Thus, temperature rises of the LED chip  13  can be prevented. 
         [0009]    Further, Japanese Unexamined Patent Application Publication No. 2006-250982 (Patent Document 3) discloses a maskless photolithography apparatus. 
       SUMMARY OF THE INVENTION 
       [0010]    As stated above, in the LED light source of Patent Document 1 as illustrated in  FIG. 5 , the heat sink plate  18  is attached to the bottom surface of the substrate  17  on the top surface of which are mounted the multiple LED substrates  10 . Thus, the heat of the LED chips  13  is transferred to the heat sink plate  18  through the ceramic substrates  11  of the LED substrates  10  and through the substrate  17 . 
         [0011]    If multiple LED substrates  10   a ,  10   b , and  10   c  are closely arranged on the substrate  17  as illustrated in  FIG. 10 , the heat of an LED element of the LED substrate  10   a  located adjacent to the LED substrate  10   b  is transferred to the heat sink plate  18  through the solder  17   a  and through the substrate  17 . This increases the temperature of a portion of the heat sink plate  18  which is located right below the LED substrate  10   a , and that temperature is thus higher than those at the periphery of the heat sink plate  18 . 
         [0012]    The heat of the LED substrate  10   b  is also transferred to the heat sink plate  18  through the solder  17   a  and through the substrate  17 . However, if the heat sink plate  18  is already raised in its temperature, the heat of the LED substrate  10   b  cannot be dispersed inside the heat sink plate  18  as with the heat of the LED substrate  10   a . This will increase the temperatures of a substrate area  17   b  and a heat sink plate area  18   b  that are located below the LED substrate  10   b . As a result, the temperature of the LED substrate  10   b  will also increase because the heat of the LED substrate  10   b  cannot be transferred sufficiently to the substrate  17 . This results in the temperature difference between the LED substrates  10   a  and  10   b  as well as the temperature variations among the LED substrates  10   a  to  10   c.    
         [0013]    The LED temperature variations will increase as the heat of the LED chips  13  increases (e.g., when the LED light source is used for a photolithography apparatus or an illumination device which consumes a large amount of electric power). The LED temperature variations will also increase when the LED substrates  10   a  to  10   c  are more closely arranged on the substrate  17 . When the temperature of the LED substrate  10   a  increases, the temperature of its LED chip  13   a  will also increase. A temperature increase of an LED chip causes its illumination efficiency and light intensity to decrease. When the LED chip is constantly subjected to a high-temperature environment, the LED chip will deteriorate faster, shortening its mechanical life. For these reasons, increased temperature variations among LED chips lead to adverse consequences such as light intensity decreases, light intensity variations, and shortened LED life. 
         [0014]    While the LED display device of Patent Document 1 has the improved capabilities of transferring heat from its LED-mounting substrate to its heat sink plate, no consideration is given to LED temperature variations, making the LED display device susceptible to the LED temperature variations. 
         [0015]    Those disclosed in Patent Document 2 are also susceptible to the LED temperature variations. 
         [0016]    Patent Document 3 relates to a maskless photolithography apparatus that involves the use of a semiconductor laser or a discharge lamp such as a mercury lamp and a xenon lamp as its light source, but no mention is made of an LED light source. 
         [0017]    To address the above problems, an object of the present invention is thus to provide a structurally-simple or easily-manufacturable LED light source that enables uniform illumination by reducing temperature variations among its LED elements arranged densely on its substrate and to provide an photolithography apparatus that involves the use of the LED light source. 
         [0018]    To achieve the above object, an LED light source according to the invention comprises: a plurality of LED elements each of which is formed by connecting an LED chip to electrodes formed on a ceramic substrate; an LED-mounting substrate on which to mount the plurality of LED elements, the LED-mounting substrate having through holes therein; and a heat sink plate for releasing heat from the LED-mounting substrate, wherein a thermally conductive resin is present between the LED-mounting substrate and the heat sink plate and wherein part of the thermally conductive resin protrudes from the through holes of the LED-mounting substrate and covers the top surface of the LED-mounting substrate on which the plurality of LED elements are mounted, so that the part of the thermally conductive resin is in contact with the plurality of LED elements. 
         [0019]    Moreover, a method for manufacturing an LED light source according to the invention comprises the steps of: applying a thermally conductive resin on a heat sink plate; pressing a bottom surface of an LED-mounting substrate on a top surface of which a plurality of LED elements are mounted against the heat sink plate on which the thermally conductive resin has been applied until the distance between the heat sink plate and the LED-mounting substrate becomes a particular value, so that the thermally conductive resin can spread between the heat sink plate and the LED-mounting substrate and so that part of the thermally conductive resin can flow from through holes of the LED-mounting substrate onto the top surface of the LED-mounting substrate so as to fill spaces between the plurality of LED elements; and heating the thermally conductive resin for solidification after the pressing step. 
         [0020]    Further, an LED-based photolithography apparatus according to the invention comprises: a light source; a photolithography pattern generation unit for generating photolithography patterns; a table on which to place a workpiece, the table being movable at least in one direction; and a control unit for controlling the light source, the photolithography pattern generation unit, and the table, wherein the light source comprises: a plurality of LED elements each of which is formed by connecting an LED chip to electrodes formed on a ceramic substrate; an LED-mounting substrate on which to mount the plurality of LED elements, the LED-mounting substrate having through holes therein; and a heat sink plate for releasing heat from the LED-mounting substrate, the heat sink plate being attached by a thermally conductive resin to the LED-mounting substrate and wherein part of the thermally conductive resin protrudes from the through holes of the LED-mounting substrate and covers the top surface of the LED-mounting substrate on which the plurality of LED elements are mounted, so that the part of the thermally conductive resin is in contact with the plurality of LED elements. 
         [0021]    Furthermore, an LED-based photolithography method according to the invention comprises the steps of: directing light emitted from a light source to a photolithography pattern generation unit; and exposing a workpiece on which a photoresist is applied to the light that has passed through a photolithography pattern generated by the photolithography pattern generation unit; wherein the light from the light source is emitted from a plurality of LED elements of the light source and wherein the plurality of LED elements are mounted on an LED-mounting substrate with a thermally conductive resin applied therebetween, and the LED-mounting substrate is cooled by a water-cooling jacket. 
         [0022]    In accordance with the invention, it is possible to reduce temperature variations among multiple LED elements mounted on an LED-mounting substrate because the LED elements are connected to the LED-mounting substrate by thermally conductive resin. In addition, because the thermally conductive resin also connects a heat sink plate to the LED-mounting substrate, the heat of the LED-mounting substrate can be more efficiently transferred to the heat sink plate than in conventional technologies. 
         [0023]    Moreover, the temperature variations can be reduced further when a ceramic or metal substrate higher in thermal conductivity is used as the LED-mounting substrate instead of using a conventional resin substrate. This reduces thermal stress on the LED elements which tend to become high in temperature, thereby enhancing the reliability of the LED elements. 
         [0024]    Further, the above-described LED light source of the invention does not require a new manufacturing process and a new device because it can be manufactured by a conventional manufacturing process. Thus, manufacturing cost increases can be prevented. 
         [0025]    Furthermore, application of the LED light source of the invention to a photolithography apparatus leads to less power consumption during light exposure. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]      FIG. 1  is a cross section illustrating the structure of an LED light source according to Embodiment 1 of the invention; 
           [0027]      FIG. 2A  illustrates a process for bonding a heat sink plate to a substrate according to Embodiment 1, particularly showing when a given amount of thermally conductive resin is applied to the heat sink plate; 
           [0028]      FIG. 2B  illustrates the bonding process of Embodiment 1, particularly showing when the substrate is pressed against the heat sink plate so that the thermally conductive resin can be flattened by the heat sink plate and the substrate; 
           [0029]      FIG. 2C  illustrates the bonding process of Embodiment 1, particularly showing when the substrate is pressed further downward so that part of the thermally conductive resin can flow upward from the through holes of the substrate onto the top surface of the substrate; 
           [0030]      FIG. 3  is a cross section illustrating the structure of an LED light source according to Embodiment 2 of the invention; 
           [0031]      FIG. 4  is a cross section of an LED light source according to Embodiment 3 of the invention, which light source is intended for use in a photolithography apparatus; 
           [0032]      FIG. 5  is a perspective view of a conventional LED substrate; 
           [0033]      FIG. 6  is a cross section of a conventional LED light source; 
           [0034]      FIG. 7  is a cross section of a conventional LED backlight; 
           [0035]      FIG. 8  is a cross section of another conventional LED light source; 
           [0036]      FIG. 9  is a cross section of still another conventional LED light source; 
           [0037]      FIG. 10  is a cross section illustrating heat flows in a conventional LED light source; 
           [0038]      FIG. 11A  illustrates the entire configuration of a photolithography apparatus according to Embodiment 4 of the invention; and 
           [0039]      FIG. 11B  is a graph showing light exposure conditions for the photolithography apparatus of Embodiment 4. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0040]    Embodiments of the present invention will now be described with reference to the accompanying drawings. 
       Embodiment 1 
       [0041]      FIG. 1  is a cross section of an LED light source according to Embodiment 1 of the invention. The LED light source  1  of  FIG. 1  includes the following components: a substrate  3 ; multiple LED substrates  2   a  to  2   c ; and a heat sink plate  4 . The LED substrates  2   a  to  2   c , that is, substrates on which to mount LEDs, are arranged on the substrate  3  at particular intervals in the form of a single row or multiple rows. The heat sink plate  4  is attached to the bottom surface of the substrate  3 . The connection among the LED substrates  2   a  to  2   c , the substrates  3 , and the heat sink plate  4  is made by thermally conductive resin  5  (e.g., silicone adhesive). 
         [0042]    Each of the LED substrates  2   a  to  2   c  is structurally the same as the LED substrate  10  of  FIG. 5 . The LED substrates  2   a  to  2   c  each include a ceramic substrate  201 , an LED chip  202 , and wires  203 . The ceramic substrate  201  has a square shape with the size of from 2 by 2 mm to 7 by 7 mm and with the thickness of 1.5 to 3 mm. The LED chip  202 , that is, a light-emitting element, has a square shape with the size of from 0.2 by 0.2 mm to 2 by 2 mm and placed on the ceramic substrate  201 . The wires  203  are used to connect some electrodes of the LED chip  202  to particular locations of the conductive wiring patterns on the ceramic substrate  201  (not illustrated in  FIG. 1 ), which wiring patterns are made of more than 0.3-μm-high Au, Ag, Al, or other highly reflective metal. 
         [0043]    The ceramic substrate  201  is high in thermal conductivity because it is made of alumina or aluminum nitride. Also, the ceramic substrate  201  has a concave portion on its top surface. The LED chip  202  is placed inside the concave portion with its bottom surface facing the ceramic substrate  201  (the bottom surface is the surface through which light is not emitted). In this case, the LED chip  202  is bonded to a particular portion of the conductive wiring patterns on the concave portion (see  FIG. 5 ). The electrodes on the light-emitting-surface side of the LED chip  202  are connected by the wires  203  to particular locations of the conductive wiring patterns on the concave portion. Note that multiple LED chips  202  with different luminescent colors can also be placed on a single ceramic substrate  201  in place of a single LED chip  202 . 
         [0044]    As illustrated in  FIG. 1 , multiple ceramic substrates  201  are connected to the top surface of the substrate  3  by solder  32  and electrically conductive paste. This connection is made between the conductive wiring patterns on the substrate  3  (not illustrated in  FIG. 1 ) that are used for supply of electric power to LED chips  202  and the conductive wiring patterns located on the bottom surfaces of the ceramic substrates  201  (not illustrated). 
         [0045]    The substrate  3 , which is 0.5 to 5 mm thick, is provided with multiple through holes  30 , each of which is 0.5 to 2 mm in diameter. The through holes  30  are located near the ceramic substrates  201  so that the side faces of the ceramic substrates  201  can be sufficiently covered with the thermally conductive resin  5 . 
         [0046]    The heat sink plate  4 , made of thermally conductive metal such as aluminum and copper, is glued by the thermally conductive resin  5  to the substrate  3  located above the heat sink plate  4 . 
         [0047]    The thermally conductive resin  5  is a thermoplastic resin or photo-plastic resin and made by mixing an insulating filler material, such as alumina, carbon, titanic oxide, and silica, with silicone resin or with epoxy-based resin. The thermal conductivity of the thermally conductive resin  5  is from 0.5 to 10 W/mK, its coefficient of thermal expansion is from 2 to 100 ppm/° C., and its viscosity rate is from 10 to 100 Pa·s. The thermally conductive resin  5  seals the bottom surfaces or part of the side surfaces of the ceramic substrates  201   a  to  201   c  and the through holes  30  of the substrate  3 . Also, the thermally conductive resin  5  connects the substrate  3  and the heat sink plate  4  together. 
         [0048]    Since the thermally conductive resin  5  is in contact with the bottom surfaces or part of the side surfaces of the ceramic substrates  201   a  to  201   c , the thermally conductive resin  5  can transfer the heat of the ceramic substrate  201   b , which tends to be high, to the ceramic substrates  201   a  and  201   c , which are low in temperature. This reduces the temperature differences among the ceramic substrates  201   a ,  201   b , and  201   c . Thus, the temperature variations among the ceramic substrates  201   a ,  201   b , and  201   c  also decrease. In fact, the structure of  FIG. 1  resulted in a temperature difference of 3° C. or lower between the ceramic substrates  201   a  and  201   b  when a power of 2 W was applied to each LED, with the height h of the thermally conductive resin  5  from the top surface of the substrate  3  being set to about 1 mm. This temperature difference is lower than the difference obtained with the conventional structure of  FIG. 6 , which difference was 7° C. 
         [0049]    When the above-mentioned height h is increased, the thermally conductive resin  5  can cover larger areas of the ceramic substrates  201   a  to  201   c , which would result in achieving high efficiency of thermal transfer in transferring heat from the ceramic substrates  201   a  to  201   c  to the thermally conductive resin  5 . As a result, the temperature difference between the ceramic substrates  201   a  and  201   b  can be reduced more. 
         [0050]    Next, a manufacturing process of the LED light source  1  is described with reference to  FIGS. 2A to 2C , which collectively illustrate a process for bonding the heat sink plate  4  to the substrate  3 . 
         [0051]    As illustrated in  FIG. 2A , the LED substrates  2   a  to  2   c  are placed on the substrate  3 . The LED substrates  2   a  to  2   c  are self-aligned by reflow soldering. An adjusted amount of thermally conductive resin  5  is then applied onto the heat sink plate  4  with the use of a dispenser or the like. 
         [0052]    Then in  FIG. 2B , after the substrate  3  is aligned with the heat sink plate  4 , the substrate  3  is pressed against the heat sink plate  4  from above so as to connect the substrate  3  and the heat sink  4  by the thermally conductive resin  5 . The pressure from above horizontally flattens the thermally conductive resin  5  located between the substrate  3  and the heat sink plate  4 , with the conductive resin  5  spreading in the horizontal plane. The pressing is stopped after the distance between the substrate  3  and the heat sink plate  4  becomes a particular value. 
         [0053]    Since the substrate  3  has the through holes  30  near the LED substrates  2   a  to  2   c  are placed, adding pressure causes the thermally conductive resin  5  to flow upward from the through holes  30 . 
         [0054]    Referring to  FIG. 2C , by continuing to add pressure, the thermally conductive resin  5  existing between the substrate  3  and heat sink plate  4  flows through the through holes  30  to underneath the bottom surfaces or around the side surfaces of the LED substrates  2   a  to  2   c  to fill their associated spaces. This provides connection of the LED substrates  2   a  to  2   c  by the bottom surfaces and/or the side surfaces with the thermally conductive resin  5 . An amount of thermally conductive resin flows through the through holes  30  to underneath the bottom surfaces or around the side surfaces of the LED substrates  2   a  to  2   c  shall be determined based on the following: the height of the thermally conductive resin  5  that flows upward from the through holes  30  is set so as not to exceed the heights of the LED substrates  2   a  to  2   c.    
         [0055]    Thereafter, the thermally conductive resin  5  is heated at about 150° C. for an hour for solidification. In solidifying, the thermally conductive resin  5  hardly changes in volume. Thus, the thermally conductive resin  5  will neither come off nor develop cracks. While we assume here that the thermally conductive resin  5  is a thermoplastic resin, a different solidification method (light curing or normal temperature leaving) has to be employed if the thermally conductive resin  5  is made of a different material such as a photo-plastic resin or the like. Because the manufacturing method described above is the same as conventional methods, there is no need to add a different bonding process and a different device. Thus, manufacturing cost increases can be prevented. 
         [0056]    Further, since the thermally conductive resin  5  is allowed to flow upward from the through holes  30 , there is no chance of the thermally conductive resin  5  fouling the light-emitting surfaces of the LED substrates  2   a  to  2   c . This prevents the light intensity of their LED chips from dropping. 
         [0057]    In addition, because the thermally conductive resin  5  is liquid, it can flow into small spaces. Thus, the thermally conductive resin  5  can be filled in the spaces between the LED substrates  2   a  to  2   c  on the substrate  3  even if the LED substrates  2   a  to  2   c  are closely spaced. Even in that case, high thermal conductivity is maintained because the thermally conductive resin  5  can sufficiently surround the LED substrates  2   a  to  2   c , and the temperature differences among the LED substrates  2   a  to  2   c  can also be reduced. 
       Embodiment 2 
       [0058]      FIG. 3  is a cross section of an LED light source according to Embodiment 2 of the invention. As illustrated, protrusions  307  are formed on the top surface of a heat sink plate  304 , and alignment holes  308  are formed in a substrate  303  so that each of the protrusions  307  can be inserted into one of the alignment holes  308 . By aligning the protrusions  307  with the alignment holes  308 , the position of the heat sink plate  304  relative to the substrate  303  can be determined. 
         [0059]    The height of each of the protrusions  307  is greater than the thickness of the substrate  303 . Thus, the top ends  307   a  of the protrusions  307  protrude from the top surface of the substrate  303  on which ceramic substrates  301   a  and  301   b  (i.e., LED substrates  300   a  and  300   b ) are mounted. 
         [0060]    By applying the same bonding process as illustrated in  FIGS. 2A to 2C , thermally conductive resin  305  applied between the heat sink plate  304  and the substrate  303  flows upward from the through holes  309  of the substrate  303  and then flows onto the top surface of the substrate  303  on which the LED substrates  300   a  and  300   b  are mounted, as illustrated in  FIG. 3 . Thus, the thermally conductive resin  305  flowed onto the top surface of the substrate  303  contacts the bottom surfaces and/or the side surfaces of the LED substrates  300   a  and  300   b  and also the protrusions  307  of the heat sink plate  304 . 
         [0061]    In the above structure, the ceramic substrates  301   a  and  301   b  are connected to the heat sink plate  304  through the thermally conductive resin  305 . Thus, the heat of the LED substrates  300   a  and  300   b  is transferred not only through the heat transfer path mentioned in Embodiment 1, but also from the side surfaces  301   a  and  301   b  of the LED substrates  300   a  and  300   b  through the protrusions  307  to the heat sink plate  304 . Thus, the structure of Embodiment 2 is more effective in releasing the heat of the LED light source. In addition, the structure of Embodiment 2 is capable of reducing the temperature difference between the ceramic substrates  301   a  and  301   b  of the LED substrates  300   a  and  300   b  and reducing the temperatures of the ceramic substrates  301   a  and  301   b  as well. 
       Embodiment 3 
       [0062]      FIG. 4  is a cross section of an LED light source according to Embodiment 3 of the invention, which light source is intended for use in a photolithography apparatus. The LED light source  1 A of  FIG. 4  differs from its counterparts of Embodiments 1 and 2 in that a lens substrate  402  is placed on the ceramic substrates  401  of LED substrates  410 . The lens substrate  402  has windows  411 , and a lens  412  is fit in each of the windows  411  in order to prevent light dispersion. Each of the lenses  412  is designed to convert radial light rays emitted from the LED substrates  410  into parallel ones. 
         [0063]    Each of the lenses  412  has a transmittance of 50% or greater at the wavelength of the light from the LED substrates  410  and is molded from inorganic glass such as quartz or the like or from organic resin such as silicone resin, acrylic resin, or epoxy resin. The lenses  412  can be spherical lenses, aspherical lenses, or Fresnel lenses. 
         [0064]    The LED light source  1 A of  FIG. 4  can be fabricated by a process similar to that of  FIGS. 2A to 2C  of Embodiment 1. To fabricate the LED light source  1 A of Embodiment 3, the lens substrate  402  having the lenses  412  is first bonded to a substrate  403  on which the LED substrates  410  were mounted by reflow soldering. Then, the substrate  403  is aligned with and pressed against a heat sink plate  404  on which thermally conductive resin  405  was applied. The pressing causes the thermally conductive resin  405 , now located between the heat sink plate  404  and the substrate  403 , to flow via the through holes  409  of the substrate  403  onto the top surface of the substrate  403  (the top surface is the surface on which the LED substrates  410  are mounted). Thus, the thermally conductive resin  405  is supplied to contact with the bottom surfaces and/or the side surfaces of the ceramic substrates  401  of the LED substrates  410 . 
         [0065]    Thereafter, the assembly of the heat sink plate  404  and the substrate  403  is heated at about 150° C. for an hour for solidifying the thermally conductive resin  405 . In solidifying, the thermally conductive resin  405  hardly changes in volume. Thus, the thermally conductive resin  405  will neither come off from the heat sink plate  404 , the substrate  403 , and the LED substrates  410  nor develop cracks therein. This prevents loss of heat transfer among the heat sink plate  404 , the substrate  403 , and the LED substrates  410 . 
         [0066]    It should be noted that the above-mentioned process of bonding the lens substrate  402  to the substrate  403  can instead be performed after the process of heating the assembly of the heat sink plate  404  and the substrate  403  for solidifying the thermally conductive resin  405 . 
         [0067]    Since the LED substrates  410  and the heat sink plate  404  are connected together by the thermally conductive resin  405 , the heat of the LED substrates  410  can be released through the heat transfer path mentioned in Embodiment 1, which results in good heat release capabilities of the LED light source  1 A. This in turn leads to small temperature variations among the ceramic substrates  401  of the LED substrates  410  as well as reduction in the temperatures of the ceramic substrates  401 . 
         [0068]    Note also that the lenses  412  to be fit in the windows  411  of the lens substrate  402  can be collimating lenses. In addition, as stated above, the presence of the lenses  412  inside the windows  411  as illustrated in  FIG. 4  allows conversion of the light rays emitted from the LED substrates  410 , which tend to disperse radially, into vertical ones with respect to an area to be illuminated (not illustrated). Thus, illumination light rays with uniform intensity can be obtained for the illumination area facing the LED light source. 
       Embodiment 4 
       [0069]      FIG. 11A  illustrates the configuration of a maskless photolithography apparatus according to Embodiment 4 of the invention in which LED light sources according to the invention are used. 
         [0070]    As illustrated in  FIG. 11A , the photolithography apparatus of Embodiment 4 includes the following components: an illumination system  500 ; an integrator  503 ; a collimating mirror  504 ; a pattern generation unit  505 ; a drive unit  5050  for driving the pattern generation unit  505 ; a table  5060  on which to place a workpiece  506  or a substrate to be irradiated; and a control unit  510  for controlling the drive unit  5050 , the table  5060 , and the illumination system  500 . 
         [0071]    The illumination system  500  that radiates light for lithography purposes includes multiple light sources  5001 ,  5002 , and  5003 , and these light sources  5001  to  5003  are all attached to a water-cooling jacket  501  for heat release purposes. The external wires of the light sources  5001 ,  5002 , and  5003  are connected via harnesses  5021 ,  5022 , and  5023  to power units  5024 ,  5025 , and  5026 , respectively. 
         [0072]    The arrangements of the light sources  5001  to  5003  including their tilt angles with respect to the integrator  503  are designed such that the light emitted from the light sources  5001  to  5003  is incident on the integrator  503  efficiently. Although not illustrated in  FIG. 11A , the light sources  5001  to  5003  and the integrator  503  extend in a direction vertical to the page on which  FIG. 11A  is shown. 
         [0073]    The light sources  5001  to  5003  of the illumination system  500  are such LED light sources as described in Embodiments 1 to 3. Such light sources allow uniform illumination of a large area. Moreover, because the light sources  5001  to  5003  are capable of emitting light with substantially the same intensity with each other, they have substantially the same length of life, thereby extending the life of the illumination system  500 . 
         [0074]    In the above photolithography apparatus, the light emitted from the illumination system  500  passes through the integrator  503 . The collimating mirror  504  then converges the light passed through the integrator  503 , converting it into a linear shaped light ray, which is extending linearly along the lithography patterns formed on the pattern generation unit  505  (which is vertical to the page in  FIG. 11A ). The linear shaped light ray is projected on the pattern generation unit  505 . The light passes through the lithography patterns formed on the pattern generation unit  505  is projected onto the workpiece  506  on which a photosensitive material (i.e., photoresist) is coated. By that, particular portions of the photoresist are exposed to the light, thereby transferring the patterns of the pattern generation unit  505  onto the photoresist coated on the workpiece  506  (For simplification purposes,  FIG. 11A  does not illustrate the optical focusing system of the photolithography apparatus). 
         [0075]    During the pattern transfer onto the workpiece  506 , the drive unit  5050  drives the pattern generation unit  505 , and the control unit  510  moves the workpiece  506  placed on the table  5060  at a particular speed in a particular direction. The above photolithography apparatus can be the one disclosed, for example, in Patent Document 3. 
         [0076]    Since the optical converging system of the photolithography apparatus of Embodiment 4 does not use a transmissive lens but uses the collimating mirror  504 , it is free from chromatic aberration. The collimating mirror  504  also allows light exposure of smaller and shaper patterns when multi-wavelength light is used as the LED light source. 
         [0077]      FIG. 11B  illustrates light exposure conditions for the illumination system  500  of the above photolithography apparatus. The photolithography process generally includes the following steps: workpiece loading; fixation of the workpiece to a particular position; alignment of the workpiece with a mask; light exposure; unfastening of the workpiece; and unloading of the workpiece. The entire photolithography process often lasts 10 to 120 seconds, but the light exposure step lasts 5 to 60 seconds. 
         [0078]    In a conventional photolithography apparatus that involves the use of a mercury lamp as its light source, the intensity of light from the lamp is unstable right after electric power is supplied to the lamp, which is due to temperature fluctuations of the lamp. It takes about thirty minutes for the light intensity to become stable. Accordingly, in using the conventional photolithography apparatus, its mercury lamp has to be kept turned on, so that the light intensity can be stabilized for light exposure. In case of using the LED light sources according to the invention, however, the light intensity stabilizes in a few milliseconds after power supply. Therefore, the LED light sources have only to be turned on during light exposure, which greatly reduces power consumption by the photolithography apparatus.