Patent Publication Number: US-6984927-B2

Title: Display unit

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of U.S. patent application Ser. No. 10/192,309, filed on Jul. 10, 2002 now U.S. Pat. No. 6,873,092, the disclosure of which is herein incorporated by reference. The present application claims priority to Japanese Patent Application No. P2001-211255, filed on Jul. 11, 2001, the disclosure of which is herein incorporate by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates generally, to a display unit including light emitting devices arrayed in such a manner as to be spaced from each other and, more particularly, to a display unit in which a light emission region of each of the light emitting devices is enlarged by giving a light diffusion function to a sealing material for covering the light emitting devices. 
     The assembly of an image display unit by arraying light emitting devices in a matrix is performed in two manners. For a liquid crystal display (LCD) or a plasma display panel (PDP), the light emitting devices are directly formed on a substrate, and for a light emitting diode display (LED display), single LED packages are arrayed on a substrate. In particular, for an image display unit such as an LCD or PDP, device isolation cannot be performed. Accordingly, in general, at the beginning of the production process, devices are formed in such a manner as to be spaced from each other with a pitch equivalent to a pixel pitch of the image display unit. 
     On the other hand, for an image display unit such as an LED display, LED chips are packaged by taking out LED chips after dicing, and individually connecting the LED chips to external electrodes by wire-bonding or bump-connection using flip-chip. In this case, before or after packaging, the LED chips are arrayed with a pixel pitch of the image display unit. However, such a pixel pitch is independent from an array pitch of the devices at the time of formation of the devices. 
     Since an LED (Light Emitting Diode) as a light emitting device is expensive, an image display unit using such LEDs can be produced at a low cost by producing a large number of LED chips from one wafer. Specifically, the cost of an image display unit can be lowered by reducing the size of an LED chip from an ordinary size, about 300 μm square to several ten μm square, and producing an image display unit by connecting such small-sized LED chips to each other. From this viewpoint, various techniques are known of forming devices at a high density, and transferring the devices to a wide region in such a manner that the devices are enlargedly spaced from each other, to produce a relatively large display unit such as an image display unit. For example, U.S. Pat. No. 5,438,241 discloses a thin film transfer method, and Japanese Patent Laid-open No. Hei 11-142878 discloses a method of forming a transistor array panel for display. 
     It should be noted, in the above-described display unit in which light emitting diodes are arrayed in such a manner as to be spaced from each other, since a light emission region of each of the light emitting devices forming pixels is significantly smaller than an array pitch of the light emitting devices, the light emission devices become conspicuous as luminous spots on a screen, thereby causing a problem in significantly degrading the image quality. Even if an average brightness over the screen has a suitable value, each light emitting diodes having a small-size glares as a luminous spot on the screen, thereby failing to obtain an image display excellent in viewability. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a display unit capable of enlarging a light emission region of each of the light emitting devices, thereby allowing an image display excellent in viewability. 
     Another object of the present invention is to provide a display unit capable of moderating a light emission characteristic due to the shape of each of the light emitting devices, thereby improving uniformity of viewing angle. 
     A further object of the present invention is to provide a display unit capable of improving the light emergence efficiency, and freely setting a size of a light emission region of a pixel. 
     To achieve the above objects, according to an embodiment of the present invention, there is provided a display unit including light emitting devices arrayed in such a manner as to be spaced from each other, and a sealing material for covering the surfaces of the light emitting devices, wherein the sealing material has a light diffusion function. 
     The light diffusion function may be given to the sealing material by providing a reflection mirror and a half mirror in the sealing material; dispersing, in the sealing material, fine particles having a refractive index different from that of the sealing material, or dispersing bubbles in the sealing material. 
     Since the light diffusion function is given to the sealing material, the light emission region of each of the light emitting devices is substantially enlarged to a size nearly equal to an array pitch of the light emitting devices. As a result, the light emitting devices are not conspicuous as luminous spots on the screen. Therefore, the screen is luminous as a whole, thereby obtaining image display excellent in viewability. Each of the methods of giving the light diffusion function has an advantage inherent thereto. For example, in the case of giving the light diffusion function by the combination of the reflection mirror and the half mirror, it is possible to freely set the size of the light emission region of each of the light emitting devices forming a pixel by adjusting the reflectance or transmittance of the half mirror. In the case of giving the light emission function by dispersing fine particles or bubbles having a refractive index different from that of the sealing material, it is possible to moderate the light emission directional characteristic due to the shape of each of the light emitting devices, and hence to improve the uniformity of viewing angle. In the case of giving the light diffusion function by dispersing fine particles or bubbles with a specific periodicity, it is possible to give the photonic crystal characteristic to the sealing material, and hence to improve the light emergence efficiency. 
     Additional features and advantages of the present invention are described in, and will be apparent from, the following Detailed Description of the Invention and the Figures. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  is a typical view showing an embodiment and display unit in which a light diffusion function is given by a combination of a reflection mirror and a half mirror. 
         FIG. 2  is a typical view showing a light emission region of each of the light emitting devices of the display unit shown in  FIG. 1 , in which the light diffusion function is given. 
         FIG. 3  is a typical view showing a light emission region of each of the light emitting devices of a comparative display unit in which any light diffusion function is not given. 
         FIG. 4  is a graph showing a light intensity distribution for the case where the reflectance of a half mirror is small (the transmittance thereof is large). 
         FIG. 5  is a graph showing a light intensity distribution for the case where the reflectance of the half mirror is large (the transmittance thereof is small). 
         FIG. 6  is a typical view of a variation of the embodiment shown in  FIG. 1 , in which irregularities are formed on the half mirror. 
         FIG. 7  is a typical view of another variation of the embodiment shown in  FIG. 1 , in which irregularities are formed on the half mirror. 
         FIG. 8  is a typical view of another embodiment in which light diffusion beads are dispersed in a sealing material. 
         FIG. 9  is a typical diagram showing a diffused state of the light-diffusion beads shown in  FIG. 8 . 
         FIG. 10  is a typical view showing a further embodiment in which bubbles are dispersed in a sealing material. 
         FIG. 11  is a typical view showing still a further embodiment in which three principal colors are mixed with each other. 
         FIGS. 12A to 12D  are typical views showing a method of arraying light emitting devices, which is used for a specific embodiment of the present invention regarding a display unit in which a light diffusion function is given to a sealing material. 
         FIG. 13  is a schematic perspective view showing a resin-covered chip. 
         FIG. 14  is a schematic plan view showing the resin-covered chip shown in  FIG. 13 . 
         FIGS. 15A and 15B  are a sectional view and a plan view showing one example of a light emitting device, respectively. 
         FIG. 16  is a schematic sectional view showing a first transfer step of a method of arraying the light emitting devices shown in  FIGS. 15A and 15B  by a two-step enlarged transfer method. 
         FIG. 17  is a schematic sectional view showing an electrode pad formation step subsequent to the step shown in  FIG. 16 . 
         FIG. 18  is a schematic sectional view showing an electrode pad formation step and a dicing step after transfer to a second temporary holding member, which steps are subsequent to the step shown in  FIG. 17 . 
         FIG. 19  is a schematic sectional view showing an attracting step subsequent to the steps shown in  FIG. 18 . 
         FIG. 20  is a schematic sectional view showing a second transfer step subsequent to the step shown in  FIG. 19 . 
         FIG. 21  is a schematic sectional view showing an insulating layer formation step subsequent to the step shown in  FIG. 20 . 
         FIG. 22  is a schematic sectional view showing a wiring formation step subsequent to the step shown in  FIG. 21 . 
         FIG. 23  is a schematic sectional view showing the specific embodiment in which a reflection mirror and a half mirror are formed in the display unit produced by the two-step enlarged transfer method. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A display unit of the present invention includes light emitting devices such as light emitting diodes arrayed on a substrate in a matrix, wherein the light emitting devices are selectively driven so as to display an image. The light emitting devices are arrayed in a state being enlargedly spaced from each other with a pitch larger than the size of each of the devices. The surfaces of the light emitting devices are covered with a sealing material for protecting the devices. 
     The display unit of this type in which the light emitting devices are arrayed in such a manner as to be enlargedly spaced from each other, however, has an inconvenience that each of the devices is conspicuous as a luminescent spot on a screen. To cope with such an inconvenience, according to the present invention, a light diffusion function is given to the sealing material covering the light emitting devices. 
     The light diffusion function given to the sealing material will be described below by way of preferred embodiments of the present invention. 
       FIG. 1  is a schematic view showing an embodiment that the light diffusion function is given by providing a combination of a reflection mirror and a half mirror. A light emitting device  1  is mounted on a substrate  2 , and the surface of the light emitting device  1  is covered with a sealing material  3 . In this embodiment, a reflection mirror  4  is disposed in the sealing material  3  at a position near the light emergence side of the light emitting device  1 , and a half mirror  5  is formed on the surface of the sealing material  3 . With this configuration, light rays emitted from the light emitting device  1  are repeatedly reflected between the reflection mirror  4  and the half mirror  5 , to be thereby spread in an in-plane direction. 
     The reflection mirror  4  may be provided at a position offset slightly, forwardly from a light emergence plane  1   a  of the light emitting device  1 , and the reflection mirror  4  has an opening  4   a  at a position corresponding to the light emitting device  1 . It is to be noted that the reflection mirror  4  is not necessarily provided in front of the light emitting device  1  but may be provided at the same height position (denoted by reference numeral  4 ′ in the  FIG. 1 ) as that of the light emitting device  1  or provided at a position (denoted by reference numeral  4 ″ in the  FIG. 1 ) rearwardly from the light emitting device  1 . In particular, if the reflection mirror  4  is provided rearwardly from the light emitting device  1 , it is possible to eliminate the need of provision of the opening in the reflection mirror  4 . In this case, the reflection mirror  4  with no opening contributes to partial emergence of light rays spread on the back surface side of the light emitting device  1 , to thereby improve the light emergence efficiency. The half mirror  5 , which is formed on the outermost surface of the sealing material  3  in such a manner as to face to the reflection mirror  4 , has no opening, unlike the reflection mirror  4 . With this configuration, light rays emitted from the light emitting device  1  travel to the half mirror  5 , where part of the light rays pass through the half mirror  5  and the remaining light rays are reflected from the half mirror  5 . The light rays having been reflected from the half mirror  5  are reflected from the reflection mirror  4 , to travel again to the half mirror  5 , and part of the light rays pass through the half mirror  5 , and the remaining light rays are reflected from the half mirror  5 . Such reflection of the light rays is repeated. As a result, the light rays emitted from the light emitting device  1  is enlargedly spread in an in-plane direction, to thereby enlarge a light emission region of each of the light emitting devices  1  forming a pixel. 
     The enlarged light emission region of the light emitting device  1  is shown in  FIG. 2 . In  FIG. 2 , character R denotes the light emitting device  1  for emission of red light, G is the light emitting device  1  for emission of green light, and B is the light emitting device  1  for emission of blue light. By enlargedly spreading light rays emitted from the light emitting device  1  by the reflection mirror  4  and the half mirror  5 , the light emission region shown by hatching becomes significantly larger than the size of the light emitting device  1 . As a result, the entire screen becomes uniformly luminous, to thereby obtain image display excellent in viewability. In the case where the light diffusion function by the reflection mirror  4  and the half mirror  5  is not given, as shown in  FIG. 3 , the light emission region of each light emitting device  1  is limited to the size of the light emitting device  1 . Accordingly, since the size of each light emitting device  1  is smaller than the array pitch of the light emitting devices  1 , each light emitting device  1  is conspicuous as a luminous spot, thereby failing to obtain an image display excellent in viewability. 
     In the case of enlargedly spreading light rays emitted from the light emitting device  1  by the combination of the reflection mirror  4  and the half mirror  5 , the size of the light emission region of each of the light emitting devices  1  forming a pixel can be freely controlled by adjusting the reflectance or transmittance of the half mirror  5 . For example, when the reflectance of the half mirror  5  becomes smaller (the transmittance becomes larger), most of the light rays pass through the half mirror  5  after being less reflected from the half mirror  5 , so that the distribution of the light rays passing through the half mirror  5  becomes narrower as shown in  FIG. 4 , with a result that the light emission region of each of the light emitting devices  1  forming a pixel becomes smaller. On the contrary, when the reflectance of the half mirror  5  becomes larger (transmittance becomes smaller), the quantity of the light rays repeatedly reflected from the half mirror  5  becomes larger, so that the distribution of the light rays passing through the half mirror  5  becomes wider as shown in  FIG. 5 , with a result that the light emission region of the light emitting device  1  becomes larger. 
     The directivity of light rays passing through the half mirror  5  can be controlled by providing irregularities on the surface of the half mirror  5  as shown in  FIG. 6 . For example, the sealing material  3  is treated to have a specific texture for giving irregularities allowing light rays to emerge in the vertical direction to the half mirror  5 , and the half mirror  5  is formed on the surface of the sealing material  3 . Since the texture of the sealing material  3  is transferred to the half mirror  5 , the irregularities are given to the half mirror  5 . With this configuration, the light rays emerge from the half mirror  5  in the direction perpendicular to the screen, thereby significantly improving the visibility as seen from the front side of the screen. 
     The above texture is not limited to a random texture. For example, in the case where light rays intensively emerge in a specific direction due to the shape of the light emitting device  1 , a texture having a periodicity to cancel the light rays in the specific direction may be given as shown in  FIG. 7 . In a variation of the embodiment, shown in  FIG. 7 , the light emitting device  1  has a tapered shape converged in one direction (for example, conical or polygonal shape), wherein light rays emerge from the bottom surface of the tapered device  1 . For this light emitting device  1 , light rays in a region hatched in the  FIG. 7  intensively emerge. To cope with such an inconvenience, a region  5   a , which is easier to diffuse light or has a high reflectance, is formed on the half mirror  5  at a position corresponding to the hatched region by transfer of the texture formed on the sealing material  3  to the half mirror  5 . With this configuration, it is possible to equalize the intensity of light rays passing through the half mirror  5 , and to improve the light emission directional characteristic due to the shape of the light emitting device  1  or the like and hence to improve the uniformity of viewing angle. 
     In the above-described embodiment, the light diffusion function is given to the sealing material by using the combination of the reflection mirror and the half mirror. However, the present invention is not limited thereto. 
     Another embodiment in which the light diffusion function is given by dispersing light-diffusion beads  6  in the sealing material  3  will be described below with reference to  FIGS. 8 and 9 . The light-diffusion beads  6  are fine particles, which have a refractive index different from that of the sealing material  3  and have spherical shapes or similar shapes thereto, although they may have other shapes. Since the light-diffusion beads  6  are dispersed in the sealing material  3 , as shown in  FIG. 9 , light rays emerging from the light emitting device  1  are irregularly reflected from the light-diffusion beads  6 . Accordingly, the intensity of the light rays having passed through the sealing material  3  is equalized, so that it is possible to improve the light emission directional characteristic due to the shape of the light emitting device  1  or the like, and hence to improve the uniformity of viewing angle. 
     A further embodiment in which the light diffusion function is given on the basis of the principle of a photonic crystal will be described below with reference to  FIG. 10 . The principle of a photonic crystal is that if a periodical refractive index distribution is given to a material, light having a wavelength in a specific range determined by the periodicity cannot travel in the material in a direction having the periodicity. For example, if in place of the above-described light-diffusion beads, bubbles  7  (refractive index: 1) having a size of sub-micron order are periodically dispersed in the sealing material having a large refractive index, the photonic crystal characteristic appears. In such a sealing material, emergence of light in the lateral direction, which is different from diffusion or scattering of light, can be obtained, so that the light emergence efficiency can be improved. The photonic crystal characteristic also can be obtained by periodically dispersing fine particles, having a refractive index different from that of the sealing material, in the sealing material. 
     It is to be noted that the photonic crystal characteristic used in the present invention is not particularly limited insofar as the photonic crystal characteristic appears by regularly disposing two kinds of transparent media being greatly different from each other in refractive index (dielectric constant) with a period of about a wavelength of light. 
     Various techniques of improving the light emission efficiency and the light emergence efficiency of a light emitting device by making use of a photonic crystal already have been studied. However, either of these techniques is generally configured such that openings are formed in a light emitting device; that is, a semiconductor device for example, in a triangular lattice shape, to produce a photonic crystal. In this case, the opened portions of the semiconductor device are wasted. On the contrary, according to this embodiment, since the light emission efficiency and the light emergence efficiency of the light emitting device  1  is improved by using the sealing material covering the light emitting device  1  as a photonic crystal, any portion of an expensive semiconductor device is not wasted. Accordingly, it becomes apparent that the improvement of the light emission efficiency and the light emergence efficiency of a light emitting device using a photonic crystal according to the present invention is based on a new technical thought. 
     As described above, the light emission region of each light emitting device can be enlarged to a size nearly equal to an array pitch of the light emitting devices. On the basis of the same technical thought, for example, three principal colors can be mixed with each other as described below. Such ideal mixture of three principal colors cannot be achieved by any other related art display unit. 
     A further embodiment in which three principal colors are mixed with each other by using a reflection mirror  11  and a half mirror  12  provided on a sealing material  10  will be described below with reference to  FIG. 11 . In this embodiment, a light emitting device  13  for emission of red color, a light emitting device  14  for emission of green color, and a light emitting device  15  for emission of blue color are disposed in the sealing material  10  in such a manner as to be spaced from each other in the horizontal direction, and the reflection mirror  11  having openings  11 A,  11 B and  11 C located at positions corresponding to those of the light emitting devices  13 ,  14  and  15  respectively is also disposed in the sealing material  10  in such a manner as to be located over the light emitting devices  13 ,  14  and  15 . The half mirror  12  having no opening, provided for each pixel, is separated from the adjacent half mirror  12  having no opening, provided for the next pixel, with an opening  12 A put therebetween to prevent mixture of colors between the pixels. With this configuration, light rays emitted from each of the light emitting devices  13 ,  14  and  15  are repeatedly reflected from the reflection mirror  11  and the half mirror  12 , so that a light emission region of the light emitting device is enlarged in an in-plane direction. As a result, a region in which three principal colors (R, G, and B) are mixed with each other appears as a light emission region of one pixel, to thereby obtain image display excellent in viewability. 
     The above-described configuration of the present invention can be applied to a display unit in which light emitting devices formed on an original substrate are transferred to and arrayed on a final substrate in such a manner as to be enlargedly spaced from each other by an enlarged transfer method. 
     A specific embodiment in which a display unit is produced by a two-step enlarged transfer method, wherein the above-described configuration regarding the light diffusion function is applied to the display unit will be described below. 
     The two-step enlarged transfer method, used for producing the display unit according to the present invention, is carried out by transferring devices formed on a first substrate at a high density to a temporary holding member in such a manner that the devices are enlargedly spaced from each other with a pitch larger than a pitch of the devices arrayed on the first substrate, and further transferring the devices held on the temporary holding member to a second substrate in such a manner that the devices are enlargedly spaced from each other with a pitch larger than the pitch of the devices held on the temporary holding member. It is to be noted that two-step transfer is adopted in this embodiment, multi-step transfer such as three or more-step transfer can be adopted in accordance with a required enlargement ratio between the pitch of the devices arrayed on the first substrate and the pitch of the devices mounted on the second substrate. 
       FIGS. 12A to 12D  show basic steps of the two-step enlarged transfer method. First, devices  22  such as light emitting devices are densely formed on a first substrate  20  shown in  FIG. 12A . By densely forming devices on a substrate, the number of devices formed per each substrate can be increased to reduce a production cost thereof. The first substrate  20  may be a substrate on which devices can be formed; for example, a semiconductor wafer, a glass substrate, a quartz glass substrate, a sapphire substrate, or a plastic substrate. The devices  22  may be directly formed on the first substrate  20 , or may be formed once on a substrate different from the first substrate  20 , and then transferred and arrayed on the first substrate  20 . 
     The devices  22  are subjected to a first transfer step shown in  FIG. 12B , in which the devices  22  are transferred from the first substrate  20  to a temporary holding member  21  shown by a broken line in  FIG. 12B , and held on the temporary holding member  21 . Here, as shown in  FIG. 12B , the devices  22  are arrayed in a matrix in which the adjacent each of the devices  22  is enlargedly spaced from each other. Specifically, to array the devices  22  in the matrix shown in  FIG. 12B , the transfer of the devices  22  is made such that the devices  22  are enlargedly spaced from each other not only in the X direction but also in the Y direction perpendicular to the X direction. The enlarged distance between the adjacent each of the devices  22  may be set, while not limited thereto, in consideration of production of resin-covered chips and formation of electrode pads thereon in the subsequent steps. In addition, when the devices  22  are transferred from the first substrate  20  to the temporary holding member  22 , the devices  22  on the first substrate  20  all can be transferred to the temporary holding member  21  in such a manner as to be enlargedly spaced from each other. In this case, a size of the temporary holding member  21  in each of the X direction and the Y direction may be equal to or more than a value obtained by multiplying the enlarged distance by the number of those, arrayed in each of the X direction and the Y direction, of the devices  22  arrayed in the matrix. Alternatively, part of the devices  22  on the first substrate  20  may be transferred to the temporary holding member  21  in such a manner as to be enlargedly spaced from each other. 
     After the first transfer step, each of the devices  22  enlargedly spaced from each other on the temporary holding member  21  is, as shown in  FIG. 12C , covered with a resin, and electrode pads are formed on the resin covering the device  22 . The covering of each device  22  with the resin is made so as to facilitate the formation of the electrode pads for the device  22  and to facilitate the handling of the device  22  in a second transfer step subsequent thereto. To prevent occurrence of a wiring failure in a final wiring step, which is performed after the second transfer step as will be described later, the electrode pads are formed into relatively large sizes. It is to be noted that the electrode pads are not shown in  FIG. 12C . As enlargedly shown in  FIG. 12C , a resin-covered chip  24  (equivalent to a display device of the present invention) is thus formed by covering each of the devices  22  with a resin  23 . The device  22  is located at an approximately central portion of the resin-covered chip  24  in a plan view according to this embodiment. However, the device  22  may be located at a position offset to one side or a corner of the resin-covered chip  24 . 
     The devices  22  are then subjected to the second transfer step shown in  FIG. 12D . In this second transfer step, the devices  22  arrayed in the matrix on the temporary holding member  21  in the form of the resin-covered chips  24  are transferred on a second substrate  25  in such a manner as to be more enlargedly spaced from each other. 
     Even in the second transfer step, the devices  22  are arrayed in a matrix shown in  FIG. 12D , in which adjacent each of the devices  22  in the form of the resin-covered chips  24  are more enlargedly spaced from each other. Specifically, to array the devices  22  in the matrix shown in  FIG. 12D , the devices  22  are transferred in such a manner as to be more enlargedly spaced from each other not only in the X direction but also in the Y direction perpendicular to the X direction. If positions of the devices  22  arrayed by the second transfer step correspond to positions of pixels of a final product such as an image display unit, a pitch of the devices  22  arrayed by the second transfer step becomes about integer times an original pitch of the devices  22  arrayed on the first substrate  20 . It is now assumed that an enlargement ratio between the pitch of the devices  22  held on the temporary holding member  21  and the pitch of the devices  22  arrayed on the first substrate  20  is taken as “n” and an enlargement ratio between the pitch of the devices  22  arrayed on the second substrate  25  and the pitch of the devices  22  held on the temporary holding member  21  is taken as “m”. On this assumption, a value E of the above-described about integer times is expressed by E=n×m. 
     The devices  22  in the form of the resin-covered chips  24 , which have been transferred in the second substrate  25  in such a manner as to be sufficiently enlargedly spaced from each other, are then subjected to wiring. The wiring is performed with care taken not to cause a connection failure by making use of the previously-formed electrode pads and the like. If the devices  22  are light emitting devices such as light emitting diodes, the wiring includes wiring to p-electrodes and n-electrodes. If the devices  22  are liquid crystal control devices, the wiring includes wiring to selective signal lines, voltage lines, alignment electrode films, and the like. 
     In the two-step enlarged transfer method shown in  FIGS. 12A to 12D , the covering of each of the devices  22  with a resin and the formation of electrode pads on the resin covering the device  22  (that is, the resin-covered chip  24 ) can be performed by making use of the enlarged distance between adjacent each of the devices  22  after the first transfer, and wiring can be performed after the second transfer without occurrence of any connection failure by making use of the previously-formed electrode pads and the like. As a result, it is possible to improve a production yield of the image display unit. Also, in the two-step enlarged transfer method according to this embodiment, two enlarged transfer steps in each of which the devices are enlargedly spaced from each other are carried out. In general, by performing a number of such enlarged transfer steps in each of which the devices are enlargedly spaced from each other, the number of transfers can be reduced. For example, it is now assumed that an enlargement ratio between the pitch of the devices  22  on the temporary holding member  21  and the pitch of the devices  22  on the first substrate  20  is taken as 2 (n=2) and an enlargement ratio between the pitch of the devices  22  on the second substrate  25  and the pitch of the devices  22  on the temporary holding member  21  is taken as 2 (m=2). In this case, the total enlargement ratio becomes 2×2=4. To achieve the total enlargement ratio (=4), according to a one-step transfer method, the number of transfers (alignment) of the devices  22  from the first substrate  20  to the second substrate  25  becomes 16 times as a result of squaring the total enlargement ratio (=4). On the contrary, to achieve the same total enlargement ratio (=4) according to the two-step enlarged transfer method of this embodiment, the number of transfers (alignment) is obtained by simply adding a square of the enlargement ratio (=2) in the second transfer step to a square of the enlargement ratio (=2) in the first transfer step, and therefore, the number of transfers becomes eight times as a result of adding 4 ( 22 ) to 4 ( 22 ). Generally, to achieve the transfer magnification of n×m, according to the two-step enlarged transfer method, the number of transfer becomes (n2+m2) times, whereas according to the one-step transfer method, the number of transfers becomes (n+m)2=n2+2 nm+m2. Consequently, according to the two-step enlarged transfer method, the number of transfers can be reduced by 2 nm times, and the time and the cost required for the production step can be correspondingly saved. This is particularly advantageous in the case of transfer at a larger transfer magnification (enlargement ratio). 
     The resin-covered chip  24  used as the display device in the above-described two-step enlarged transfer method will be described below. As shown in  FIGS. 13 and 14 , the resin-covered chip  24  is formed into an approximately flat plate shape with an approximately square shaped principal plane. The resin-covered chip  24  is formed by covering the device  22  with a cured resin  23 . More specifically, a number of the resin-covered chips  24  are obtained by coating the overall surface of the temporary holding member  21  so as to cover the devices  22  with a non-cured resin, curing the resin, and cutting the cured resin  23  into approximately square chips by dicing. 
     Electrode pads  26  and  27  are formed on front and back surfaces of the approximately flat plate-like resin  23  of the resin-covered chip  24 , respectively. These electrode pads  26  and  27  are each produced by forming a conductive layer made from a metal or polysilicon as a material of each of the electrode pads  26  and  27  overall on each of the front and back surfaces of the resin  23 , and patterning the conductive layer into specific electrode shapes by photolithography. These electrode pads  26  and  27  are formed so as to be connected to a p-electrode and an n-electrode of the device  22  as the light emitting device, respectively. If needed, via-holes may be formed in the resin  23  of the resin-covered chip  24 . 
     The electrode pads  26  and  27  are formed, in this embodiment, on the front and back surface sides of the resin-covered chip  24 , respectively. However, they may be formed on either of the front and back surface sides of the resin-covered chip  24 . It is to be noted that the electrode pads  26  and  27  are offset from each other in a plan view in order to prevent the electrode pads  26  and  27  from being overlapped to each other when a contact hole is formed from above at the time of formation of final wiring. The shape of each of the electrode pads  26  and  27  is not limited to a square shape but may be any other shape. 
     The formation of such a resin-covered chip  24  is advantageous in that, since the device  22  is covered with the flattened resin  23 , the electrode pads  26  and  27  can be accurately formed on the flattened front and back surfaces of the resin  23 , and the electrode pads  26  and  27  can be formed so as to extend to a region wider than the size of the device  22 , thereby facilitating the handling of the device  22  by an attracting jig in the second transfer step. As will be described later, since final wiring is performed after the second transfer step, a wiring failure can be prevented by performing wiring using the electrode pads  26  and  27  having relatively large sizes. 
       FIGS. 15A and 15B  show a structure of a light emitting device as one example of the device used in the above-described two-step enlarged transfer method, wherein  FIG. 15A  is a sectional view of the device and  FIG. 15B  is a plan view of the device. The light emitting device shown is exemplified by a GaN based light emitting diode formed, for example, on a sapphire substrate by crystal growth. In this GaN based light emitting diode, when the light emitting diode is irradiated with a laser beam passing through the substrate, laser abrasion occurs, to evaporate nitrogen of GaN, thereby causing film peeling at an interface between the sapphire substrate and a GaN based growth layer. As a result, device peeling can be performed easily. 
     The structure of the GaN based light emitting diode will be described below. A hexagonal pyramid shaped GaN layer  32  is formed by selective growth on an under growth layer  31  composed of a GaN based semiconductor layer. An insulating film (not shown) is formed on the under growth layer  31 , and the hexagonal pyramid shaped GaN layer  32  is grown from an opening formed in the insulating film by a MOCVD process or the like. The GaN layer  32  is a growth layer having a pyramid shape covered with a S-plane; that is, ( 1 – 101 ) plane when a principal plane of a sapphire substrate used for growth is taken as a C-plane. The GaN layer  32  is a region doped with silicon. The tilt S-plane portion of the GaN layer  32  functions as a cladding portion of a double-hetero structure. An InGaN layer  33  functioning as an active layer is formed in such a manner as to cover the tilt S-plane of the GaN layer  32 . A GaN layer  34  doped with magnesium is formed on the InGaN layer  33 . The GaN layer  34  doped with magnesium also functions as a cladding portion. 
     The light emitting diode has a p-electrode  35  and an n-electrode  36 . A metal material such as Ni/Pt/Au or Ni(Pd)/Pt/Au is vapor-deposited on the GaN layer  34  doped with magnesium to form the p-electrode  35 . A metal material such as Ti/Al/Pt/Au is vapor-deposited in an opening formed in the above-described insulating film (not shown) to form the n-electrode  36 . In the case of extracting an n-electrode from a back surface side of the under growth layer  31 , the n-electrode  36  is not required to be formed on the front surface side of the under growth layer  31 . 
     The GaN based light emitting diode having such a structure enables light emission of blue. In particular, such a light emitting diode can be relatively simply peeled from the sapphire substrate by laser abrasion. In other words, the diode can be selectively peeled by selective irradiation of the diode with a laser beam. In addition, the GaN based light emitting diode may have a structure that an active layer be formed into a planar or strip shape, or may be a pyramid structure with a C-plane formed on an upper end portion of the pyramid. The GaN light emitting diode also may be replaced with any other nitride based light emitting device or a compound semiconductor device. 
     A method of arraying the light emitting devices shown in  FIGS. 15A and 15B  will be described below with reference to  FIGS. 16 to 22 . First, as shown in  FIG. 16 , a number of light emitting diodes  42  are formed in a matrix on a principal plane of a first substrate  41 . A size of the light emitting device  42  is set to about 20 μm. The first substrate  41  is made from a material; for example, sapphire substrate having a high transmittance for a laser beam having a specific wavelength suitably used for irradiation of the light emitting diode  42 . The light emitting diodes  42  are in the state that electrodes such as p-electrodes already have been formed, but final wiring is not yet performed, and that device isolation grooves  42   g  are formed, whereby the light emitting diodes  42  are isolatable from each other. The grooves  42   g  are formed, for example, by reactive ion etching. As shown in  FIG. 16 , such a first substrate  41  is placed opposite to a temporary holding member  43  for selective transfer of the light emitting diodes  42  therebetween. 
     Both a release layer  44  and an adhesive layer  45  are formed on a surface, opposed to the first substrate  41 , of the temporary holding member  43 . The temporary holding member  43  may be made from glass substrate, quartz glass substrate, or plastic substrate. The release layer  44  on the temporary holding member  43  can be made from a fluorine coat material, a silicone resin, a water soluble adhesive (for example, polyvinyl alcohol: PVA), or polyimide. The adhesive layer  45  on the temporary holding member  43  can be made from an ultraviolet (UV)-curing type adhesive, a thermosetting type adhesive, or a thermoplastic type adhesive. As one example, a quartz glass substrate is used as the temporary holding member  43 , and a polyimide film is formed as the release layer  44  on the temporary holding member  43  to a thickness of 4 μm and an UV-curing type adhesive layer is formed as the adhesive layer  45  on the release layer  44  to a thickness of about 20 μm. 
     The adhesive layer  45  on the temporary holding member  43  is adjusted such that cured regions  45   s  and non-cured regions  45   y  are mixed. The first substrate  41  is positioned to the temporary holding member  43  such that the light emitting diodes  42  to be selectively transferred are located at the non-cured regions  45   y . The adjustment of the adhesive layer  45  in such a manner that the cured regions  45   s  and the non-cured regions  45   y  are mixed may be performed by selectively exposing a UV-curing type adhesive with a pitch of 200 μm by an exposure system, so that portions of the adhesive layer  45  to which the light emitting diodes  42  are to be transferred remain non-cured and the other portions are cured. After such selective curing of the adhesive layer  45 , each of the light emitting diodes  42  to be transferred is irradiated with a laser beam from the back surface of the first substrate  41 , and is then peeled from the first substrate  41  by laser abrasion. Since the GaN based light emitting diode  42  is decomposed into gallium and nitrogen at the interface between the GaN layer and sapphire, the light emitting diode  42  can be relatively simply peeled from the first substrate  41 . The laser beam used for irradiation is exemplified by an excimer laser beam or a harmonic YAG laser beam. 
     The light emitting diode  42 , which has been selectively irradiated with a laser beam, is peeled from the first substrate  41  at the interface between the GaN layer and the first substrate  41  by laser abrasion, and is transferred to the opposed temporary holding member  43  in such a manner that the p-electrode portion of the light emitting diode  42  is pieced in the corresponding non-cured region  45   y  of the adhesive layer  45 . The other light emitting diodes  42 , which are not irradiated with laser beams and located at positions corresponding to the cured region  45   s  of the adhesive layer  45 , are not transferred to the temporary holding member  43 . It is to be noted that, in the example shown in  FIG. 16 , only one light emitting diode  42  is selectively irradiated with a laser beam. However, in actuality, the light emitting diodes  42  spaced from each other with an n-pitch are similarly irradiated with laser beams. With such selective transfer, the light emitting diodes  42  are arrayed on the temporary holding member  43  in such a manner as to be enlargedly spaced from each other with a pitch larger than an original pitch of the light emitting diodes  42  arrayed on the first substrate  41 . 
     In the state that the light emitting diode  42  is held by the adhesive layer  45  of the temporary holding member  43 , the back surface of the light emitting diode  42 , which is taken as an n-electrode side (cathode electrode side), is cleaned for removal of the resin (adhesive) therefrom. Accordingly, when an electrode pad  46  is formed on the back surface of the light emitting diode  42  as shown in  FIG. 17 , it can be electrically connected thereto. 
     The cleaning of the adhesive layer  45  may be performed, for example, by etching the resin used as the adhesive with oxygen plasma and cleaning it by irradiation of UV ozone. When the GaN based light emitting diode is peeled from the first substrate  41  made from sapphire substrate by laser irradiation, gallium is deposited on the peeling plane. Such an element Ga must be etched, for example, by using an NaOH containing water solution or dilute nitric acid. The electrode pad  46  is then formed by patterning. The electrode pad  46  on the cathode side can be formed into a size of about 60 μm square. As the electrode pad  46 , there can be used a transparent electrode (ITO or ZnO based electrode) or a Ti/Al/Pt/Au electrode. In the case of using such a transparent electrode, even if the electrode largely covers the back surface of the light emitting diode, it does not shield light emission. Accordingly, a patterning accuracy of the transparent electrode may be rough and the size of the electrode can be made large, to thereby facilitate the patterning process. 
     After the formation of the electrode pad  46 , the cured adhesive layer  45  is cut for each light emitting diode  42  into each resin-covered chip containing the light emitting diode  42 . The dicing process is made by mechanical dicing or laser dicing. A cut-in width by dicing is dependent on the size of the light emitting diode  42  covered with the adhesive layer  45  within a pixel of an image display unit. If a cut-in width being as narrow as 20 μm or less is needed, the dicing is required to be performed by laser dicing using a laser beam such as an excimer laser beam, a harmonic YAG laser beam, or a carbon dioxide laser beam. 
     Referring to  FIG. 18 , after the light emitting diode  42  is transferred from the temporary holding member  43  to a second temporary holding member  47 , a via-hole  50  on an anode electrode (p-electrode) side is formed in the adhesive layer  45  and an anode side electrode pad  49  is formed so as to be buried in the via-hole  50 . The adhesive layer  45  made from the resin is then diced. As a result of dicing, device isolation grooves  51  are formed to isolate the light emitting diode  42  from the adjacent light emitting diodes  42 . To isolate the light emitting diodes  42  arrayed in a matrix from each other, the device isolation grooves  51  have a planar pattern composed of a number of parallel lines extending in the vertical and horizontal directions. From the bottom of the device isolation groove  51 , the front surface of the second temporary holding member  47  is exposed. The second temporary holding member  47  is exemplified by a so-called dicing sheet composed of a plastic substrate coated with an UV adhesive whose adhesive strength becomes weak by irradiation of ultraviolet rays. 
     In the above-described transfer, the release layer  44  is irradiated with an excimer laser from the back surface, opposed to the release layer  44  side, of the temporary holding member  43 . If the release layer  44  is made from polyimide, the release layer  44  is peeled by abrasion of polyimide, with a result that each light emitting diode  42  is transferred to the second temporary holding member  47 . In one example of the process of forming the anode electrode pad  49 , the surface of the adhesive layer  45  is etched with oxygen plasma until the surface of the light emitting diode  42  is exposed. The via-hole  50  may be formed by using an excimer laser, a harmonic YAG laser beam, or a carbon dioxide laser beam. The diameter of the via-hole  50  is set to about 3 to 7 μm. The anode side electrode pad  49  is made from Ni/Pt/Au. 
     The light emitting diode  42  is peeled from the second temporary holding member  47  via a mechanical process. A release layer  48  has been formed on the second temporary holding member  47 . The release layer  48  can be made from a fluorine coat material, a silicone resin, a water soluble resin (for example, PVA), polyimide, or the like. The release layer  48  is irradiated with, for example, a YAG third harmonic laser beam from the back surface, opposed to the release layer  48  side, of the second temporary holding member  47 . If the release layer  48  is made from polyimide, peeling occurs at the interface between polyimide and the quartz substrate by abrasion of polyimide. As a result, the light emitting diode  42  can be peeled easily from the second temporary holding member  47  via the mechanical process. 
       FIG. 19  shows a state that each of the light emitting diodes  42  arrayed on the second temporary holding member  47  is picked up via an attracting system  53 . The attracting system  53  has attracting holes  55  opened in a matrix having a pitch corresponding to a pixel pitch of an image display unit in order to collectively attract a number of the light emitting diodes  42 . The attracting holes  55 , for example, each having an opening diameter of about 100 μm, are arranged into a matrix with a pitch of 600 μm, and the attracting system  53  can collectively attract 300 pieces of the light emitting diodes  42 . The attracting hole  55  portion is obtained by forming a hole in a metal plate  52 , which is made from Ni or stainless steel (SUS), by etching. It is to be noted that the metal plate  52  made from Ni is produced by electrocasting. An attracting chamber  54  is formed at the depth of the attracting hole  55  formed in the metal plate  52 . By controlling the pressure in the attracting chamber  54  into a negative pressure, the attracting system  53  can attract the light emitting diode  42 . Since each light emitting diode  42  is in a state covered with the resin  23  whose surface is nearly flattened, the selective attraction of the light emitting diode  42  by the attracting system  53  can be performed easily. 
       FIG. 20  is a view showing a state that the light emitting diode  42  is transferred to a second substrate  60 . An adhesive layer  56  is previously formed on the second substrate  60  before the light emitting diode  42  is transferred to the second substrate  60 . By curing a portion, located on the back surface of the light emitting diode  42 , of the adhesive layer  56 , the light emitting diode  42  can be fixedly placed on the second substrate  60 . Upon this mounting, the pressure of the attracting chamber  54  of the attracting system  53  becomes high, to release the coupling state between the light emitting diodes  42  and the attracting system  53  by attraction. 
     The adhesive layer  56  is made from an UV-curing type adhesive, a thermosetting adhesive, or a thermoplastic adhesive. Here, the adhesive layer  56  becomes a sealing material covering the surface of the light emitting diode  42 . Therefore, the adhesive layer  56  is given the above-described light diffusion function. For example, light-diffusion beads are diffused in the adhesive layer  56 , or bubbles are periodically formed in the adhesive layer  56 . Alternatively, a combination of a reflection mirror and a half mirror is formed in the adhesive layer  56 . 
     The light emitting diodes  42  thus arrayed on the second substrate  60  are enlargedly spaced from each other with a pitch larger than each of the pitch of the light emitting diodes  42  held on the first temporary holding member  43  and the pitch of the light emitting diodes  42  held on the second temporary holding member  47 . An energy (a beam  73 ) for curing the resin of the adhesive layer  56  is given from the back surface of the second substrate  60 . A portion, located on the back surface of the light emitting diode  42 , of the adhesive layer  56  may be cured by using ultraviolet rays if the adhesive layer  56  is made from a UV-curing type adhesive or cured by using a laser beam if the adhesive layer  56  is made from a thermosetting adhesive, to be adhesively bonded to the light emitting device  42 . Alternatively, a portion, located on the back surface of the light emitting diode  42 , of the adhesive layer  56  may be melted by laser irradiation if the adhesive layer  56  is made from a thermoplastic adhesive, to be adhesively bonded to the light emitting diode  42 . 
     An electrode layer  57 , which also functions as a shadow mask, is disposed on the second substrate  60 . In particular, a black chromium layer  58  is formed on the screen side surface; that is, on the viewer side surface of the electrode layer  57 . An advantage of provision of the black chromium layer  58  is to improve the contrast of an image. Another advantage of provision of the black chromium layer  58  is that the higher energy absorptivity of the black chromium layer  58  allows a portion of the adhesive layer  56  selectively irradiated with the beam  73  to be cured faster. If the adhesive layer  56  is made from an UV-curing type adhesive, it may be irradiated with ultraviolet rays having an energy of about 1,000 mJ/cm2. 
       FIG. 21  is a view showing a state that light emitting diodes  42 ,  61 , and  62  of three colors, RGB are arrayed on the second substrate  60  and are coated with an insulating layer  59 . By mounting the light emitting diodes  42 ,  61 , and  62  on the second substrate  60  at respective positions of the three colors by the attracting system  53  shown in  FIGS. 19 and 20 , a pixel composed of the light emitting diodes  42 ,  61  and  62  of the three colors can be formed with a pitch of the pixel fixed. The insulating layer  59  may be made from a transparent epoxy adhesive, UV-curing type adhesive, or polyimide. The shapes of the light emitting diodes  42 ,  61 , and  62  of the three colors are not necessarily identical to each other. In the example shown in  FIG. 21 , the red light emitting diode  61 , which has a structure having no hexagonal pyramid shaped GaN layer, is different in shape from each of the other light emitting diodes  42  and  62 . However, in this stage, each of the light emitting diodes  42 ,  61 , and  62  already has been covered with the resin  23  to be thus formed into a resin-covered chip. Therefore, the light emitting diodes  42 ,  61 , and  62  can be handled in the same manner irrespective of the difference in device structure. 
     As shown in  FIG. 22 , to electrically connect the electrode pads  46  and  49  of each of the light emitting diodes  42 ,  61  and  62  to the electrode layer  57  on the second substrate  60 , opening portions (via-holes)  65 ,  66 ,  67 ,  68 ,  69 , and  70  are formed in the insulating layer  59 , followed by formation of wiring portions as will be described later. The formation of the opening portions is performed, for example, by using a laser beam. Since the areas of the electrode pads  46  and  49  of each of the light emitting diodes  42 ,  61 , and  62  are large, the shapes of the opening portions, that is, via-holes  65 ,  66 ,  67 ,  68 ,  69  and  70  can be made large. As a result, the positioning accuracy of each via-hole may be made rough as compared with the case where a via-hole is directly formed in each light emitting diode. For example, for each of the electrode pads  46  and  49  having a size of about 60 μm square, the via-hole having a diameter of about 20 μm can be formed. The via-holes have three types having different depths: the first type is for connection to the second substrate (wiring substrate), the second type is for connection to the anode electrode, and the third type is for connection to the cathode electrode. The depth of each via-hole is optimized by controlling the pulse number of a laser beam depending on the type of the via-hole. 
     After the opening portions  65 ,  66 ,  67 ,  68 ,  69  and  70  are formed in the insulating layer  59 , wiring portions  63 ,  64 , and  71  are formed for connecting the electrode pads already connected to the anodes and cathodes of the light emitting diodes  42 ,  61  and  62  to the wiring electrode layer  57  formed on the second substrate  60 . A protective layer is then formed on the wiring to accomplish a panel of an image display unit. The protective layer may be made from the same transparent epoxy adhesive as that used for the insulating layer  59  shown in  FIG. 21 . The protective layer is heated to be cured to perfectly cover the wiring. After that, a driver IC is connected to the wiring at the end portion of the panel to produce a drive panel. 
     According to the method of arraying light emitting devices described above, since the light emitting diodes  42  have been held on the temporary holding member  43  in the state being enlargedly spaced from each other, the relatively large electrode pads  46  and  49  can be provided on each of the diodes  42  by making use of the large distance between adjacent each of the diodes  42 , and since the wiring is performed by making use of the relatively large electrode pads  46  and  49 , even if the size of the final unit is significantly larger than the device size, the wiring can be formed easily. Also, according to the method of arraying light emitting devices in this embodiment, since each light emitting diode  42  is covered with the flattened, cured adhesive layer  45 , the electrode pads  46  and  49  can be accurately formed on the flattened front and back surfaces of the adhesive layer  45  and also can be disposed to extend to a region wider than the device size, so that the handling of the light emitting diode  32  by the attracting jig in the second transfer step can be facilitated. 
     In the display unit produced as described above, the light emitting diodes  42  are arrayed on the second substrate  60  in such a manner as to be enlargedly spaced from each other with a pitch larger than the size of each of the diodes  42 . To improve the display quality of such a display unit, according to this embodiment, light emission regions of the light emitting devices  42  are enlarged by giving a light diffusion function to the sealing material (adhesive layer  56  in this embodiment) on the outermost surface for covering the light emitting diodes  42 .  FIG. 23  shows a state that a reflection mirror  81  having an opening  81  a is formed at a position, near the light emitting diode  42 , of the adhesive layer  56 , and a half mirror  82  is formed on the surface, on the second substrate  60  side, of the adhesive layer  56 . Light rays emitted from the light emitting diode  42  are repeatedly reflected from the half mirror  82  while partially passing therethrough, to be spread in the in-plane direction. It is to be noted that, as described above, in place of the combination of the reflection mirror  81  and the half mirror  82 , light-diffusion beads may be dispersed in the adhesive layer  56  as shown in  FIG. 8  to enlarge the light emission region of the light emitting device  42 , or bubbles may be periodically formed in the adhesive layer  56  as shown in  FIG. 10 , to enlarge the light emission region of the light emitting device  42  on the basis of the principle of a photonic crystal. 
     As described above, according to the present invention, since a light diffusion function is given to a sealing material covering light emitting devices, the light emission region of each of the light emitting devices can be enlarged to a size nearly equal to the array pitch of the devices. As a result, it is possible to provide a display unit allowing an image display excellent in viewability. Also, according to the present invention, it is possible to moderate the light emission characteristic due to the shapes of the light emitting devices and hence to improve the uniformity of viewing angle, and further to improve the light emergence efficiency and to freely set the size of each pixel of the display unit. 
     Although the present invention has been described with reference to specific embodiments, those of skill in the art will recognize that changes may be made thereto without departing from the spirit and scope of the present invention as set forth in the hereafter appended claims.