Patent Publication Number: US-2022216371-A1

Title: Micro led ultraviolet radiation source

Description:
TECHNICAL FIELD 
     The present disclosure relates to a micro-LED ultraviolet radiation source. 
     BACKGROUND ART 
     A device has been proposed which uses, as a light source for radiating ultraviolet light, a deep-UV LED (Light Emitting Diode) in substitution for a fluorescent lamp and a mercury lamp. 
     Patent Document No. 1 discloses a sterilizer in which deep-UV LED groups arrayed on a metal heat dissipation plate are overmolded with a quartz glass package. 
     CITATION LIST 
     Patent Literature 
     Patent Document No. 1: Japanese Laid-Open Patent Publication No. 2015-91582 
     SUMMARY OF INVENTION 
     Technical Problem 
     It is difficult to mount a large number of deep-UV LEDs to a substrate with high density. 
     The present disclosure provides a novel ultraviolet radiation source which can solve the above-described problem. 
     Solution to Problem 
     A micro-LED ultraviolet radiation source of the present disclosure includes, in an exemplary embodiment: a sapphire substrate; a frontplane supported by the sapphire substrate, the frontplane including a plurality of micro-LEDs, each of which includes a first semiconductor layer of a first conductivity type and a second semiconductor layer of a second conductivity type and is capable of radiating ultraviolet light, and a device isolation region located between the plurality of micro-LEDs, the device isolation region including at least one metal plug electrically coupled with the second semiconductor layer; a middle layer supported by the frontplane, the middle layer including a plurality of first contact electrodes respectively electrically coupled with the first semiconductor layer of the plurality of micro-LEDs and at least one second contact electrode coupled with the metal plug; and a backplane supported by the middle layer, the backplane including an electric circuit electrically coupled with the plurality of micro-LEDs via the plurality of first contact electrodes and the at least one second contact electrode. The device isolation region includes a reflector capable of reflecting ultraviolet light radiated from each of the plurality of micro-LEDs such that the reflected ultraviolet light travels toward the sapphire substrate. 
     In one embodiment, at least a reflecting surface of the reflector is made of aluminum (Al) or rhodium (Rh). 
     In one embodiment, a wavelength of the ultraviolet light is not less than 200 nm and not more than 380 nm. 
     In one embodiment, at least part of the at least one metal plug functions as the reflector. 
     In one embodiment, each of the plurality of micro-LEDs has a forwardly-tapered side surface, and the at least one metal plug is in contact with the side surface of each of the plurality of micro-LEDs. 
     In one embodiment, each of the plurality of micro-LEDs has a forwardly-tapered side surface, and the reflector has a reflecting surface which is in contact with the side surface of each of the plurality of micro-LEDs. 
     In one embodiment, each of the plurality of micro-LEDs has a forwardly-tapered side surface, and the reflector is made of a dielectric which is in contact with the side surface of each of the plurality of micro-LEDs. 
     In one embodiment, the reflector is a dielectric multilayer film. 
     In one embodiment, the electric circuit includes a plurality of thin film transistors, the plurality of thin film transistors including a semiconductor layer deposited on the frontplane supported by the sapphire substrate and/or the middle layer. 
     In one embodiment, the device isolation region of the frontplane includes an embedded insulator filling a gap between the plurality of micro-LEDs, the embedded insulator having at least one through hole for the metal plug. 
     In one embodiment, the device isolation region of the frontplane includes a plurality of insulating layers covering a side surface of the plurality of micro-LEDs, and the metal plug fills a space in the device isolation region which is surrounded by the plurality of insulating layers. 
     In one embodiment, the metal plug includes a metal surface layer which is in contact with the first semiconductor layer and the second semiconductor layer of each of the micro-LEDs, an ohmic contact is formed between the second semiconductor layer and the metal surface layer, and a portion of the first semiconductor layer which is in contact with the metal surface layer is resistive or insulative. 
     In one embodiment, the device isolation region of the frontplane is filled with the metal plug. 
     In one embodiment, the metal surface layer of the metal plug which is in contact with the first semiconductor layer and the metal surface layer of the metal plug which is in contact with the second semiconductor layer are made of different metal materials. 
     Advantageous Effects of Invention 
     According to an embodiment of the present invention, a micro-LED ultraviolet radiation source is provided which can solve the previously-described problem. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1A  is a cross-sectional view showing part of a μLED UV source  1000  according to an embodiment of the present disclosure. 
         FIG. 1B  is a plan view showing an arrangement example of μLEDs  220  in the PLED UV source  1000 . 
         FIG. 2  is a cross-sectional view showing another configuration example of the frontplane in the μLED UV source  1000 . 
         FIG. 3  is a cross-sectional view showing still another configuration example of the frontplane in the PLED UV source  1000 . 
         FIG. 4A  is a cross-sectional view schematically showing reflection of ultraviolet light by a metal plug  24 . 
         FIG. 4B  is another cross-sectional view schematically showing reflection of ultraviolet light by a metal plug  24 . 
         FIG. 4C  provides graphs showing the relationship between the reflectance of a reflector metal and the wavelength in an embodiment of the present disclosure. 
         FIG. 4D  provides graphs showing the relationship between the reflectance of a reflector metal and the wavelength in an embodiment of the present disclosure. 
         FIG. 5  is a perspective view showing an example where the side surface  220 S is formed by a lateral surface of a truncated cone. 
         FIG. 6  is a cross-sectional view showing still another configuration example of the μLED UV source  1000 . 
         FIG. 7  is a cross-sectional view showing still another configuration example of the μLED UV source  1000 . 
         FIG. 8  is a perspective view showing an arrangement example of the first contact electrodes  31  and the second contact electrodes  32  in the μLED UV source  1000 . 
         FIG. 9  is a circuit diagram showing an example of part of an electric circuit in the μLED UV source  1000 . 
         FIG. 10A  is a perspective view schematically showing a production step of the μLED UV source  1000 . 
         FIG. 10B  is a perspective view schematically showing a production step of the μLED UV source  1000 . 
         FIG. 10C  is a perspective view schematically showing a production step of the μLED UV source  1000 . 
         FIG. 10D  is a perspective view schematically showing a production step of the μLED UV source  1000 . 
         FIG. 11  is a cross-sectional view of the μLED UV source  1000  in an embodiment of the present disclosure. 
         FIG. 12A  is a cross-sectional view schematically showing a production step of the μLED UV source  1000 . 
         FIG. 12B  is a cross-sectional view schematically showing a production step of the μLED UV source  1000 . 
         FIG. 12C  is a cross-sectional view schematically showing a production step of the μLED UV source  1000 . 
         FIG. 12D  is a cross-sectional view schematically showing a production step of the μLED UV source  1000 . 
         FIG. 12E  is a cross-sectional view schematically showing a production step of the μLED UV source  1000 . 
         FIG. 12F  is a cross-sectional view schematically showing a production step of the μLED UV source  1000 . 
         FIG. 13  is a cross-sectional view showing another configuration example of the μLED UV source  1000  in an embodiment of the present disclosure. 
         FIG. 14  is a cross-sectional view showing still another configuration example of the μLED UV source  1000  in an embodiment of the present disclosure. 
         FIG. 15  is a cross-sectional view showing still another configuration example of the μLED UV source  1000  in an embodiment of the present disclosure. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Definitions 
     In the present disclosure, “micro-LED” means a light emitting diode (LED) whose occupation region can be included within an area of 1000 μm×1000 μm or within a stripe region of not more than 1000 μm in width. In the present disclosure, the electromagnetic wave radiated by the micro-LED is ultraviolet light at the wavelength of not more than 380 nm. Hereinafter, “micro-LED” is also referred to as “μLED”. 
     μLEDs have a first semiconductor layer of the first conductivity type and a second semiconductor layer of the second conductivity type. The first conductivity type is one of p-type and n-type. The second conductivity type is the other of p-type and n-type. For example, if the first conductivity type is p-type, the second conductivity type is n-type. If, on the contrary, the first conductivity type is n-type, the second conductivity type is p-type. Each of the first semiconductor layer and the second semiconductor layer can have a single-layer structure or a multilayer structure. Typically, an emission layer which has at least one quantum well (or double heterostructure) is provided between the first semiconductor layer and the second semiconductor layer. 
     In the present disclosure, “micro-LED ultraviolet radiation source (PLED UV source)” refers to a device which includes a plurality of μLEDs each capable of radiating ultraviolet light. The plurality of μLEDs in the μLED UV source are also referred to as “μLED array”. The μLED UV source can be used in various uses which require irradiation with ultraviolet light, including curing of a resin with ultraviolet light, exposure of a resist to light, and sterilization. Particularly, the μLED UV source of the present disclosure can realize an arbitrary irradiation pattern in a maskless lithography. 
     &lt;Basic Configuration&gt; 
     A basic configuration example of a μLED UV source of the present disclosure is described with reference to  FIG. 1A  and  FIG. 1B .  FIG. 1A  is a cross-sectional view showing part of a μLED UV source  1000 .  FIG. 1B  is a plan view showing an arrangement example of a μLED array in the μLED UV source  1000 . The cross section of the μLED UV source  1000  shown in  FIG. 1A  is identical with the cross section taken along line A-A of  FIG. 1B . 
     The μLED UV source  1000  can include a large number of μLEDs, for example, several hundreds to several thousands, or more than 10,000.  FIG. 1A  and  FIG. 1B  show only a part of the PLED UV source  1000  which includes several μLEDs. The entirety of the μLED UV source  1000  has a configuration where the shown part is, for example, periodically repeated or repeated in a particular pattern. 
     In the μLED UV source  1000 , ultraviolet light is radiated from a plurality of μLEDs divided into small pieces rather than radiated from a single continuous emission layer included in a conventional, single large LED device. Due to this feature, how to use ultraviolet radiation from the end face of the emission layer included in each μLED is important. This is because, as the size of the μLEDs decreases and the number of μLEDs included in the μLED UV source  1000  increases, the proportion of the area of the end face of the emission layer to an area of the emission layer which is perpendicular to the layer-stacking direction of semiconductor layers increases. In an embodiment of the present disclosure, a reflector which will be described later is provided in a region between respective μLEDs (device isolation region) such that ultraviolet light radiated in a horizontal direction from the emission layer can also be effectively used. 
     The μLED UV source  1000  includes a crystal growth substrate  100 , a frontplane  200  supported by the crystal growth substrate  100 , a middle layer  300  supported by the frontplane  200 , and a backplane  400  supported by the middle layer. 
     In the attached drawings, the proportion of the transverse size to the longitudinal size of respective components such as μLEDs is not necessarily equal to the actual proportion in an embodiment. In the drawings, clarity takes precedence in determining the proportion of the depicted components. The orientation of respective components in the drawings does not limit at all the orientation in actual production of the μLED UV source and the orientation in actual use of the μLED UV source. In  FIG. 1A  and  FIG. 1B , a coordinate system of X-axis, Y-axis, and Z-axis, which are mutually orthogonal, is shown for reference. 
     &lt;Crystal Growth Substrate&gt; 
     The crystal growth substrate  100  is a substrate on which semiconductor crystals, which are constituents of the μLEDs, are to epitaxially grow. In the present disclosure, the crystal growth substrate  100  is a sapphire substrate. Hereinafter, the crystal growth substrate  100  that is made of sapphire is simply referred to as “substrate”. A surface  100 T of the substrate  100  on which crystal growth occurs is referred to as “upper surface” or “crystal growth surface”. Another surface  100 B of the substrate  100  which is opposite to the surface  100 T is referred to as “lower surface”. In this specification, the terms “upper surface” and “lower surface” do not depend on the actual orientation of the substrate  100  when they are used. 
     A typical example of semiconductor crystals which can be used in embodiments of the present disclosure is a gallium nitride based compound semiconductor. Hereinafter, the gallium nitride based compound semiconductor is also referred to as “GaN”. Some of gallium (Ga) atoms in GaN may be substituted with aluminum (Al) atoms or indium (In) atoms. GaN in which some of Ga atoms are substituted with Al atoms is also referred to as “AlGaN”. GaN in which some of Ga atoms are substituted with In atoms is also referred to as “InGaN”. GaN in which some of Ga atoms are substituted with Al atoms and In atoms is also referred to as “AlInGaN” or “InAlGaN”. The bandgap of GaN is smaller than the bandgap of AlGaN but greater than the bandgap of InGaN. In the present disclosure, gallium nitride based compound semiconductors in which some of constituent atoms are substituted with other atoms are also generically referred to as “GaN”. “GaN” can be doped with an n-type impurity and/or a p-type impurity as impurity ion. GaN whose conductivity type is n-type is referred to as “n-GaN”. GaN whose conductivity type is p-type is referred to as “p-GaN”. Details of the method of growing semiconductor crystals will be described later. In the embodiments of the present disclosure, semiconductor crystals which are constituents of the μLED are not limited to GaN-based semiconductors but may be made of a nitride semiconductor such as AlN, InN, or AlInN, or any other type of semiconductor. 
     In an embodiment of the present disclosure, the substrate  100  is a constituent of a final μLED UV source  1000 . The thickness of the substrate  100  can be, for example, not less than 30 μm and not more than 1000 μm, preferably not more than 500 μm. The roles of the substrate  100  are the base for crystal growth and the optical member for improving the ultraviolet light extraction efficiency during operation. To these ends, the rigidity of the μLED UV source  1000  may be compensated for with any other rigid member than the substrate  100 . Such a rigid member can be fixed to the backplane  400 , for example. During the production process, a supporting substrate (not shown) for compensating for the rigidity of the substrate  100  may be secured to the lower surface  100 B of the substrate  100 . Such a supporting substrate may be removed from a final μLED UV source  1000 . 
     The upper surface (crystal growth surface)  100 T of the substrate  100  may have a structure for relieving the crystal lattice mismatch, such as grooves or ridges. Also, a buffer layer for reducing the crystal lattice mismatch may be provided at the upper surface  100 T of the substrate  100 . The lower surface  100 B of the substrate  100  may have microscopic irregularities for further improving the extraction efficiency of ultraviolet light radiated from a μLED array and then transmitted through the substrate  100  or for diffusing the ultraviolet light. Examples of the microscopic irregularities include a moth-eye structure. The moth-eye structure continuously changes the effective refractive index across the lower surface  100 B of the substrate  100  and, therefore, the proportion of light reflected by the lower surface  100 B of the substrate  100  to the inside of the substrate  100  (reflectance) can be greatly reduced (to substantially zero). 
     In the present disclosure, the positive direction of Z axis shown in  FIG. 1A  (the direction of the arrow) is also referred to as “crystal growth direction” or “semiconductor layering direction”. The lower surface  100 B and the upper surface  100 T of the substrate  100  may be referred to as “front surface” and “rear surface”, respectively, of the substrate  100 . 
     &lt;Frontplane&gt; 
     The frontplane  200  includes a plurality of μLEDs  220  and a device isolation region  240  located between the plurality of μLEDs  220 . The plurality of μLEDs  220  can be arrayed in rows and columns in a two-dimensional plane (XY plane) which is parallel to the upper surface  100 T of the substrate  100 . Each of the plurality of μLEDs  220  includes a first semiconductor layer  21  of the first conductivity type and a second semiconductor layer  22  of the second conductivity type as shown in  FIG. 1A . The second semiconductor layer  22  is closer to the substrate  100  than the first semiconductor layer  21 . 
     In an embodiment of the present disclosure, each of the μLEDs  220  includes an emission layer  23  which can emit light independently of the other μLEDs  220 . The emission layer  23  is present between the first semiconductor layer  21  and the second semiconductor layer  22 . The device isolation region  240  includes at least one metal plug  24  electrically coupled with the second semiconductor layer  22 . The metal plug  24  functions as a substrate-side electrode of the μLEDs  220 . 
     A typical example of the first semiconductor layer  21  of the first conductivity type is a p-GaN layer. A typical example of the second semiconductor layer  22  of the second conductivity type is an n-GaN layer. Each of the p-GaN layer and the n-GaN layer does not need to have a homogeneous composition along a direction perpendicular to the upper surface  100 T of the substrate  100  (semiconductor layering direction: positive direction of Z axis) but can have a multilayer structure. As previously described, Ga of GaN can be at least partially substituted with Al and/or In. Such substitution can be carried out for adjusting the bandgap and/or the refractive index of GaN. The concentration of the n-type impurity and the p-type impurity, i.e., the doping level, also does not need to be constant along the semiconductor layering direction (positive direction of Z axis). 
     A typical example of the emission layer  23  includes at least one AlGaN or InAlGaN well layer for emitting ultraviolet light. When the emission layer  23  includes a plurality of well layers, a barrier layer which has a greater bandgap than the well layer can be provided between the respective well layers. The bandgap of the well layer defines the emission wavelength. Specifically, λ×Eg=1240 holds where λ [nm] is the emission wavelength in vacuum and Eg [electron volt: eV] is the bandgap. Therefore, for example, ultraviolet light at λ=350 nm can be radiated by adjusting the bandgap Eg of the well layer to about 3.54 eV. For example, the bandgap of the AlGaN well layer can be adjusted according to the Al molar fraction in the AlGaN well layer. 
     Each of the plurality of semiconductor layers which are constituents of each μLED  220  is a monocrystalline layer epitaxially grown on the substrate  100  (epitaxial layer). The device isolation region  240  is defined by a trench-like recessed portion (hereinafter, referred to as “trench”) which is formed by partially etching the plurality of semiconductor layers epitaxially grown on the substrate  100 . The occupation region of each of the μLEDs  220  isolated by the trench has a size which can be included within an area of 1000 μm×1000 μm (e.g., area of 100 μm×100 μm or smaller). The occupation region of the μLED  220  is defined by the contour of the first semiconductor layer  21  and/or the emission layer  23  demarcated by the device isolation region  240 . 
     As shown in  FIG. 1B , the device isolation region  240  surrounds each of the μLEDs  220  and isolates each of the μLEDs  220  from the other μLEDs  220 . More specifically, the device isolation region  240  electrically and spatially isolate the first semiconductor layer  21  and the emission layer  23  of each of the μLEDs  220  from the first semiconductor layer  21  and the emission layer  23  of the other μLEDs  220 . 
     As shown in  FIG. 1A , the second semiconductor layer  22  does not need to be completely isolated in each of the μLEDs  220 . In the example shown in  FIG. 1A , the second semiconductor layer  22  included in respective ones of the plurality of μLEDs  220  is formed by a single continuous semiconductor layer and is shared among the plurality of μLEDs  220 . When the single continuous second semiconductor layer  22  is shared among the plurality of μLEDs  220 , this second semiconductor layer  22  functions as a common electrode on the second conductivity side for the plurality of μLEDs  220 . If the second semiconductor layers  22  of respective ones of the μLEDs  220  are mutually isolated and each of the second semiconductor layers  22  is coupled with an electrode (interconnection) on the second conductivity side at the backplane  400 , occurrence of a disconnection failure in some of the electrodes or interconnections on the second conductivity side will cause an electrical communication failure in some of the μLEDs  220 . However, when the second semiconductor layers  22  of respective ones of the plurality of μLEDs  220  are formed by a single continuous semiconductor layer, occurrence of such a failure can be suppressed. Embodiments of the present disclosure are not limited to such an example. The second semiconductor layer  22  of each of the μLEDs  220  may be isolated from the second semiconductor layers  22  of the other μLEDs  220  so long as it is appropriately coupled with a metal plug  24  or a TiN buffer layer which will be described later. 
     In this example, the device isolation region  240  includes an embedded insulator  25  which fills the gap between the plurality of μLEDs  220 . The embedded insulator  25  has one or a plurality of through holes for the metal plugs  24 . The through holes are filled with the metal material which forms the metal plugs  24 . The metal plugs  24  may have a structure formed by stacking layers of different metals. 
     In an embodiment of the present disclosure, the upper surface of the frontplane  200  is preferably planarized as shown in  FIG. 1A . Such planarization is realized by making the level of the upper surfaces of the metal plug  24  and the embedded insulator  25  in the device isolation region  240  generally coincident with the level of the upper surface of the first semiconductor layer  21  in the μLEDs  220 . 
     &lt;Reflector&gt; 
     In an embodiment of the present disclosure, the device isolation region  240  of the μLED UV source  1000  includes a reflector  260  which is capable of reflecting ultraviolet light radiated from each of the plurality of μLEDs  220  such that the reflected ultraviolet light travels toward the crystal growth substrate  100 . More specifically, the device isolation region  240  includes an embedded insulator  25  which fills the gap between the plurality of μLEDs  220 . The embedded insulator  25  has a V-shape trench (through hole) for the metal plug  24 . The embedded insulator  25  is made of a material which is capable of transmitting ultraviolet light radiated from the μLED  220 . 
     The metal plug  24  is in contact with the second semiconductor layer  22  at the bottom of the V-shape trench. This metal plug  24  not only functions as a conductor for electrically coupling each of the μLEDs  220  with the backplane  400  but also functions as the reflector  260 . As shown in the drawing, the side surface (reflecting surface  260 S) of the metal plug  24  is not perpendicular to, but inclined with respect to, the upper surface  100 T of the crystal growth substrate  100 . It is desirable that at least part of the metal plug  24  which is in contact with the second semiconductor layer  22  is made of a material which can realize an ohmic contact. However, the other part can be made of various metal materials. For example, it can be made of at least one type of metal selected from the group consisting of Al, Ag, Rh, Au, Cu, Pd, Pt, Ti, Ni, Mo, and W. According to the research by the present inventor, from the viewpoint of reflecting ultraviolet light radiated from the μLED  220  with high reflectance (e.g., 90% or higher), it is desirable that at least the side surface of the metal plug  24  (the reflecting surface  260 S of the reflector  260 ) is made of Al. When the reflectance of 70% or higher is to be secured as a practical reflectance, it is preferred that the reflecting surface  260 S of the reflector  260  is made of Al, Ag, or Rh. Particularly for ultraviolet light at the wavelength of not more than 300 nm, it is desirable that the reflecting surface  260 S of the reflector  260  is made of Al or Rh. As a result of simulations which will be described later, it was found that for example the reflectance for ultraviolet light at the wavelength of 350 nm has the relationship of Al&gt;Ag&gt;Rh&gt;&gt;Cu≈Ti. The reflectance for ultraviolet light at the wavelength of not more than 300 nm has the relationship of Al&gt;Rh&gt;&gt;Ti&gt;Cu&gt;Ag. 
     The metal plug  24 , which functions as the reflector  260 , surrounds each of the μLEDs  220  as shown in  FIG. 1B . Therefore, ultraviolet light radiated in all directions from the μLED  220  is reflected by the inclined side surface (reflecting surface  260 S) of the metal plug  24  in a direction toward the crystal growth substrate  100 . The metal plug  24  does not need to be a single electrical conductor which has a grid shape but may be separated into a plurality of parts. 
     &lt;Other Forms of Reflector&gt; 
     Next, refer to  FIG. 2 . In the example shown in  FIG. 2 , the metal plug  24  includes a reflecting layer  28  over the side surface. The reflecting layer  28  functions as the reflector  260 . The reflecting layer  28  can be made of a different material from that of the metal plug  24 , for example, Al or Rh. The thickness of the reflecting layer  28  is, for example, not less than 30 nm and not more than 50 nm. The reflecting layer  28  may be made of a non-metal material. The reflecting layer  28  can be made of, for example, a dielectric material which has a different refractive index from that of the embedded insulator  25 . The difference in refractive index at the interface between the reflecting layer  28  and the embedded insulator  25  can realize reflection of ultraviolet light radiated from the μLED  220 . Ultraviolet light which has been transmitted through this interface so as to arrive at the metal plug  24  can be reflected by the metal plug  24  itself. 
       FIG. 3  is a cross-sectional view showing another configuration example of the μLED UV source  1000 . In this example, each of the plurality of μLEDs  220  has an inclined side surface  220 S. The metal plug  24  is in contact with the side surface  220 S of each of the μLEDs  220 . In this example, the metal plug  24  has a reflecting surface  260 S which is in contact with the side surface  220 S of each of the μLEDs  220  and functions as the reflector  260 . In this example, the inclination angle θ of the reflecting surface  260 S defines the inclination angle of the side surface  220 S of each of the μLEDs  220 . In the example shown in  FIG. 3 , the inclination angle θ of the reflecting surface  260 S is smaller than 90° (e.g., 30-60°). The side surface  220 S of the μLED  220  forms a forward taper. 
     The surface of the metal plug  24  is preferably made of a material which can realize an ohmic contact with the second semiconductor layer  22 . When the second semiconductor layer  22  is made of n-GaN, a metal which has a smaller work function Φm than the work function Φn of the n-GaN (for example, Ti) is used, whereby a selective ohmic contact is realized between the second semiconductor layer  22  and the metal plug  24  and, meanwhile, a high-resistance layer can be formed between the first semiconductor layer  21  that is made of p-GaN and the metal plug  24 . According to the configuration example of  FIG. 3 , the step of forming the embedded insulator  25  in the device isolation region  240  and the step of forming a through hole in the embedded insulator  25  can be omitted. 
     In the configuration example of  FIG. 3 , the configuration of the metal plug  24  is not limited to the above-described example but may have a multilayer structure (upper layer metal and lower layer metal). The material of the upper layer metal is selected such that a highly resistive or insulative interface is formed between the upper layer metal and the first semiconductor layer  21 . The material of the lower layer metal is selected such that a low-resistance ohmic contact is formed between the lower layer metal and the second semiconductor layer  22 . Herein, as will be described in the following section, it is preferred that a material which has high reflectance for ultraviolet light is in contact with at least the emission layer  23 . 
     When the first semiconductor layer  21  is made of p-GaN, formation of an ohmic contact is difficult in general, and the damage by the etching for device isolation forms the resistance between the p-GaN and the metal plug  24 . Therefore, as shown in  FIG. 3 , the problem of an electrical short circuit between the first semiconductor layer  21  and the second semiconductor layer  22  due to the metal plug  24  is avoided. 
     &lt;Inclination Angle and Material of Reflector&gt; 
       FIG. 4A  and  FIG. 4B  schematically show reflection by the metal plug  24  of ultraviolet light produced at the emission layer  23 . In the examples shown in the drawings, part of the metal plug  24  which is to reflect ultraviolet light includes a high-reflectance layer  24 R. The high-reflectance layer  24 R functions as the reflector  260 . The ultraviolet light produced at the emission layer  23  in the μLED is basically isotropically radiated but is likely to be guided horizontally along the emission layer  23  that has a relatively-large bandgap and a high refractive index. Thus, for realizing high reflectance and enabling the ultraviolet light to be incident on the substrate  100  at an appropriate angle, the inclination angle of the reflector  260  with respect to the emission layer  23  is important. 
     The reflecting surface  260 S of the reflector  260  shown in  FIG. 4A  and  FIG. 4B  reflects the ultraviolet light received from the emission layer  23  such that the reflected ultraviolet light travels downward (in the negative direction of Z axis). 
     In the present disclosure, the angle between the reflecting surface  260 S of the reflector  260  and the XY plane is defined as “inclination angle θ of reflecting surface”. The angle of the ultraviolet light transmitted through the emission layer  23  with respect to the normal N of the reflecting surface  260 S is α. Herein, θ+α=90° holds. The ultraviolet light reflected by the reflecting surface  260 S travels in a direction which forms an angle of |2θ−90| degrees with respect to the negative direction of Z axis. The angle represented by |2θ−90| degrees with respect to the negative direction of Z axis is herein referred to as “substrate incidence angle”. 
     In the example shown in  FIG. 4B , the side surface of the semiconductor layers  21 ,  22 ,  23  (the side surface  220 S of the μLED  220 ) is inclined at inclination angle θ and forms a forward taper. The present inventor conducted a simulation based on the configuration example shown in  FIG. 4B  and found that, for realizing extraction of the ultraviolet light, the substrate incidence angle needs to be 25° or smaller, and is preferably 15° or smaller, more preferably 100 or smaller. Therefore, the inclination angle of the reflecting surface  260 S of the reflector  260 , θ, needs to be in the range of 32.5° to 57.5°. The angle θ is preferably in the range of 37.5° to 52.5°, more preferably in the range of 40° to 50°. 
     When the angle θ is in the range of 40° to 50°, a high light extraction efficiency of about 90% is realized. However, such a high light extraction efficiency is achieved when the substrate  100  is made of sapphire, but is not achieved when the substrate  100  is made of any other material, for example, GaN. Specifically, when the substrate  100  is a GaN substrate, ultraviolet light at the wavelength of not more than 375 nm cannot be extracted even if the substrate incidence angle is 0°. 
     As will be described later, a titanium nitride (TiN) layer may be provided on the upper surface  100 T of the substrate  100 . The TiN layer contributes to crystal growth but affects transmission of the ultraviolet light. According to the research by the present inventor, when the substrate incidence angle is 23′ or smaller, extraction of the ultraviolet light is possible. When there is a TiN layer of 5-15 nm in thickness, the substrate incidence angle is preferably 10° or smaller. So long as the substrate incidence angle is 10° or smaller, the light extraction efficiency of not less than 60% can be realized. 
     The material of the reflecting surface  260 S of the metal plug  24  (reflector  260 ) is preferably Al or Rh, which has high reflectance for ultraviolet light. When the reflecting surface  260 S is realized by an Al or Rh layer, the inside of the metal plug  24  (reflector  260 ) may be made of any other metal, for example, Cu, Ag, Ti, TiN, or the like. It was also found that as the thickness of the Al or Rh layer increases up to about 50 nm, the ultraviolet reflectance has a tendency to increase. A preferred thickness of the Al or Rh layer that functions as the reflector is, for example, not less than 30 nm. 
     According to simulations by the present inventor, even metals which exhibit relatively high reflectances in the wavelength range of visible light, exclusive of Al and Rh, exhibit significantly-decreased reflectances in the wavelength range of, for example, not less than 200 nm and not more than 300 nm, which is used for sterilization. For example, if in the example of  FIG. 4B  θ=45° and the thickness of the metal film that forms the reflecting surface  260 S of the reflector  260  is 30 nm, the reflectance of Al is generally 90% or higher over a wide wavelength range of 200-380 nm as shown in  FIG. 4C . The reflectance of Rh is also generally 68% or higher over a wide wavelength range of 200-380 nm. On the other hand, for example, as shown in  FIG. 4D , the reflectance of Ag is about 85% for the wavelength of 350 nm but decreases to 37% for the wavelength of 295 nm. The reflectance of Cu is 50% or higher for the wavelength of 380 nm and about 40% for the wavelength of around 260-280 nm. Specifically comparing Cu and Ag, the reflectances in the wavelength range of 200 nm to 280 nm are in the relationship of Cu≈Ag. However, in the wavelength range of 280 nm to 305 nm, they are in the relationship of Cu&gt;Ag. When the wavelength exceeds 305 nm, they are in the relationship of Ag&gt;&gt;Cu. Thus, Ag is preferred in the wavelength range of more than 305 nm, and Cu is preferred at the wavelength of around 300 nm. Note that Al always exhibits higher reflectance than Ag and Cu in all of these wavelength ranges. Rh exhibits lower reflectance than Ag at the wavelength of not less than 313 nm but exhibits higher reflectance than Ag and Cu in the other wavelength ranges, particularly in the range of not more than 300 nm. 
     As clearly seen from the foregoing, when the μLED ultraviolet radiation source of the present disclosure is used for sterilization purposes (wavelength: not less than 200 nm and not more than 300 nm, typically not less than 250 nm and not more than 300 nm), it is desirable that at least the reflecting surface  260 S of the reflector  260  is made of Al or Rh. 
     For achieving high reflectance in the range of ultraviolet light, it is desirable that the thickness of the Al or Rh layer that forms the reflecting surface  260 S is not less than about 30 nm. Even if the thickness of this layer is increased to 50 nm or greater, the increase of the reflectance saturates. Therefore, the thickness of the Al or Rh layer in a portion which functions as the reflector  260  is preferably 30-50 nm. The metal plug  24 , exclusive of the superficial region of about 30-50 nm in thickness from the side surface, can be made of a material selected from the other metals from the viewpoint of reducing the electrical resistivity or contact resistance without consideration of the ultraviolet reflectance. 
     From the viewpoint of improving the ohmic contact property with respect to the semiconductor layer, it is preferred to use TiN in the contact portion of the metal plug  24 . However, if there is a TiN layer at the reflecting surface  260 S, the reflectance for ultraviolet light will decrease. When there is a metal layer other than the Al layer such as TiN layer at the reflecting surface  260 S, it is preferred that the angle θ is 40° or smaller. As the angle θ decreases, the reflectance improves. 
     The size in X-axis direction or Y-axis direction (width W) of the metal plug  24  (reflector  260 ) may be greater than the size in Z-axis direction (height h) of the metal plug  24 . A typical example of the proportion of the width to the height (W/h) of the metal plug  24  can be not less than 0.5 and not more than 10. 
     In  FIG. 4A  and  FIG. 4B , the cross section of the metal plug  24  (reflector  260 ) has the shape of an inverted trapezoid or an inverted triangle, although the cross-sectional shape of the metal plug  24  (reflector  260 ) is not limited to such examples. The side surface  220 S of each μLED  220  does not need to be a flat surface.  FIG. 5  is a perspective view showing an example where the side surface  220 S of the μLED  220  is formed by a lateral surface of a truncated cone. The shape of each μLED  220  can be formed by any frustum whose base is polygonal, circular or elliptical. 
       FIG. 6  is a cross-sectional view showing still another configuration example of the μLED UV source  1000 . Also in this example, each of the plurality of μLEDs  220  has a forwardly-tapered side surface  220 S. However, the metal plug  24  is not in contact with the side surface  220 S of each of the μLEDs. In this example, the metal plug  24  is located inside a through hole formed in the embedded insulator  25 . In this example, the reflector  260 , which is capable of reflecting ultraviolet light radiated from each of the μLEDs  220  such that the reflected ultraviolet light travels toward the crystal growth substrate  100 , is the interface between the embedded insulator  25  and the μLED  220  (side surface  220 S). Such an interface reflection is Fresnel reflection which is attributed to the difference between the refractive index of the embedded insulator  25  and the refractive index of the μLED  220 . The refractive index of a semiconductor which can be a constituent of the μLED  220  is, for example, in the range of not less than 2.1 and not more than 3.0. When the embedded insulator  25  is made of a dielectric which has a lower refractive index than these refractive indices, total reflection of ultraviolet light radiated from the emission layer  23  can be caused by adjusting the inclination angle of the reflecting surface. 
     The refractive index of the embedded insulator  25  may be higher than the refractive index of the μLED  220 . 
       FIG. 7  is a cross-sectional view showing still another configuration example of the μLED UV source  1000 . Also in this example, each of the plurality of μLEDs  220  has a forwardly-tapered side surface  220 S. However, in this example, the reflector  260  is formed by the reflecting layer  28  which is in contact with the side surface  220 S of the μLED  220 . This reflecting layer  28  can be a dielectric multilayer film realized by alternately stacking up a plurality of dielectric layers of different refractive indices. Herein, for example, SiO 2  (refractive index n=1.47) and TiO 2  (n=2.7) can be suitably used as the dielectric film. In this case, respective film thicknesses are adjusted, and the films are stacked up over five or more periods, such that the reflectance of the dielectric multilayer film can be 95% or higher. 
     The ultraviolet light reflected by the above-described reflector  260  toward the crystal growth substrate  100  is transmitted through the crystal growth substrate  100  together with ultraviolet light radiated from the μLED  220  directly toward the crystal growth substrate  100 , and goes out of the μLED UV source. Such ultraviolet light can be employed in various uses. 
     &lt;Middle Layer&gt; 
     The middle layer  300  includes a plurality of first contact electrodes  31  and second contact electrodes  32  (see  FIG. 1A ). The plurality of first contact electrodes  31  are, respectively, electrically coupled with the first semiconductor layers  21  of the plurality of μLEDs  220 . At least one second contact electrode  32  is coupled with the metal plug  24 . 
       FIG. 8  is a perspective view showing an arrangement example of the first contact electrodes  31  and the second contact electrodes  32 . In  FIG. 8 , illustration of the backplane  400  is omitted for showing the arrangement example of the contact electrodes  31 ,  32 . The structure shown in  FIG. 8  is merely a part of the μLED UV source  1000 . As previously described, an embodiment of the μLED UV source  1000  includes a large number of μLEDs  220 . 
     The second contact electrodes  32  shown in  FIG. 8  are electrically coupled with the second semiconductor layer  22  via the metal plugs  24 . The shape and size of the second contact electrodes  32  are not limited to the example shown in the drawing. Since the metal plugs  24  can have various shapes as previously described, the flexibility in arrangement of the second contact electrodes  32  is high so long as they are electrically coupled with the second semiconductor layer  22  via the metal plugs  24 . Meanwhile, respective ones of the first contact electrodes  31  are independently electrically coupled with the first semiconductor layers  21  of the plurality of μLEDs  220 . When viewed in a direction perpendicular to the upper surface  100 T of the substrate  100 , the shape and size of the first contact electrodes  31  do not need to be identical with the shape and size of the first semiconductor layers  21 . 
     Since the upper surface of the frontplane  200  is planarized as previously described, the distances from the substrate  100  to the first contact electrodes  31  and the second contact electrodes  32 , in other words, the “heights” or “levels” of the contact electrodes  31 ,  32 , are mutually equal. This feature facilitates formation of the backplane  400  (described later) with the use of a semiconductor manufacture technique. In the present disclosure, the “semiconductor manufacture technique” includes the process of depositing a thin film of a semiconductor, insulator, or conductor and the process of patterning the thin film by lithography and etching. In this specification, a “planarized surface” means a surface at which the level difference caused by raised or recessed portions at the surface is not more than 300 nm. In a preferred embodiment, this level difference is not more than 100 nm. 
     Refer again to  FIG. 1A . In the example shown in  FIG. 1A , the middle layer  300  includes an interlayer insulating layer  38  which has a flat surface. The interlayer insulating layer  38  has a plurality of contact holes for respectively coupling the first and second contact electrodes  31 ,  32  with the electric circuit of the backplane  400 . The contact holes are filled with via electrodes  36 . 
     In an embodiment of the present disclosure, it is preferred to planarize the upper surface of the interlayer insulating layer  38  prior to formation of the backplane  400 . In planarizing the insulating layer prior to, or in the middle of, formation of the backplane  400 , chemical mechanical polishing (CMP) can be preferably used instead of etch back. 
     &lt;Backplane&gt; 
     The backplane  400  includes an electric circuit which is not shown in  FIG. 1A . The electric circuit is electrically coupled with the plurality of μLEDs  220  via the plurality of first contact electrodes  31  and at least one second contact electrode  32 . In a preferred embodiment, the electric circuit includes a plurality of thin film transistors (TFTs) and other circuit components. As will be described later, each of the TFTs includes a semiconductor layer deposited on the frontplane  200  supported by the substrate  100  and/or on the middle layer  300 . Note that some uses do not require the plurality of μLEDs  220  to carry out an active matrix operation. When used for such uses, the electric circuit of the backplane  400  does not need to include TFTs. Note that even when the electric circuit of the backplane  400  does not include TFTs, this electric circuit includes a layer of a metal, semiconductor, and/or insulative material directly grown on the middle layer  300  by physical or chemical vapor deposition (grown layers or deposited layers). These layers are patterned by lithography techniques. 
       FIG. 9  is a basic equivalent circuit diagram in a case where the μLED UV source  1000  radiates ultraviolet light by the units of μLED. In the example shown in  FIG. 9 , the electric circuit of the backplane  400  includes a selection TFT element Tr 1 , a driving TFT element Tr 2 , and a holding capacitance CH. The μLED shown in  FIG. 9  is present in the frontplane  200  rather than the backplane  400 . 
     In the example of  FIG. 9 , the selection TFT element Tr 1  is coupled with an intensity signal line DL and a selection line SL. The intensity signal line DL is an interconnection for carrying signals which define the intensity of ultraviolet radiation. The intensity signal line DL is electrically coupled with the gate of the driving TFT element Tr 2  via the selection TFT element Tr 1 . The selection line SL is an interconnection for carrying signals which control the ON/OFF of the selection TFT element Tr. The driving TFT element Tr 2  controls the state of conduction between a power line PL and the μLED. When the driving TFT element Tr 2  is ON, an electric current flows from the power line PL to the ground line GL via the μLED. This electric current causes the μLED to emit light. If the selection TFT element Tr 1  is turned OFF, the ON state of the driving TFT element Tr 2  is maintained by the holding capacitance CH. 
     The backplane  400  that has the above-described configuration can control the radiation intensity of ultraviolet light by the units of μLED. This can widely expand the uses of the ultraviolet radiation source. For example, a resin to be cured with ultraviolet light can be irradiated with ultraviolet light which has an arbitrary intensity distribution in a maskless lithography. Also, the intensity distribution of ultraviolet irradiation can be easily changed according to the input intensity signal. 
     The electric circuit of the backplane  400  can include the selection TFT element Tr 1 , the driving TFT element Tr 2 , the intensity signal line DL, the selection line SL, and other elements, although the configuration of the electric circuit is not limited to such an example. By adjusting the shape, size, and arrangement of respective μLEDs included in the μLED UV source  1000 , various intensity distributions of ultraviolet irradiation can be realized. Even if that intensity distribution is fixed for each μLED UV source  1000 , it is sufficient for some uses. 
     &lt;Production Method&gt; 
     Next, a basic example of the method of producing the μLED UV source  1000  is described. 
     Firstly, as shown in  FIG. 10A , a substrate  100  is provided which has an upper surface (crystal growth surface)  100 T.  FIG. 10A  shows only a part of the substrate  100  extending across a plane which is parallel to the upper surface  100 T. 
     Then, a plurality of semiconductor layers, including a second semiconductor layer  22  of the second conductivity type, an emission layer  23 , and a first semiconductor layer  21  of the first conductivity type, are epitaxially grown from the upper surface  100 T of the substrate  100 . Each of the semiconductor layers is a monocrystalline epitaxially-grown layer of a gallium nitride based compound semiconductor. The epitaxial growth of the gallium nitride based compound semiconductor can be carried out by, for example, MOCVD (Metal Organic Chemical Vapor Deposition). Impurities which define each conductivity type can be introduced for doping from a gaseous phase during the crystal growth. 
     After a semiconductor multilayer structure  280  which includes the above-described semiconductor layers is formed on the substrate  100 , a mask M 1  is formed on the first semiconductor layer  21  as shown in  FIG. 10B . The mask M 1  has an opening which defines the shape and position of the device isolation region  240 . In other words, the mask M 1  defines the shape and position of the μLEDs  220 . Part of the semiconductor multilayer structure  280  which is not covered with the mask M 1  is etched from the upper surface, whereby a trench which defines the device isolation region  240  is formed as shown in  FIG. 10C . This etching (mesa etching) can be carried out by, for example, inductively coupled plasma (ICP) etching or reactive ion etching (RIE). The depth of the etching is determined such that the second semiconductor layer  22  appears at the bottom of the trench. The depth of the trench formed by etching can be, for example, not less than 0.5 μm and not more than 5 μm. The width of the trench can be, for example, not less than 5 μm and not more than 100 μm. From the viewpoint of improving the in-plane uniformity of the ultraviolet irradiation intensity, it is preferred that the width of the trench is small. The transverse dimension of each of the μLEDs  220  can be not less than 5 μm and not more than 1000 μm, for example, 10-100 μm. For selectively performing ultraviolet irradiation on an arbitrary patterned region, it is preferred to reduce the size of each of the two-dimensionally arrayed μLEDs  220  (for example, it is preferred to reduce the size so as to be within a region of 100 μm×100 μm or smaller or within a stripe region of 100 μm in width). Side surfaces  220 S of the μLEDs  220  are exposed by etching. In other words, each of the μLEDs  220  has etched side surfaces  220 S. Although in the example of  FIG. 10C  the side surface  220 S is not inclined, the previously-described forwardly-tapered side surface can be formed by adjusting the material of the mask M 1  and the etching conditions. 
     Then, after the device isolation region  240  including the metal plug  24  is formed, first contact electrodes  31  and second contact electrodes  32  are formed as shown in  FIG. 8  that has been previously referred to. Then, an interlayer insulating layer  38  (thickness: for example, 500 nm to 1500 nm) of the middle layer  300  is formed and, thereafter, a plurality of contact holes (not shown in  FIG. 8 ) are formed in the interlayer insulating layer  38  for coupling the electric circuit of the backplane  400  with the μLEDs  220  of the frontplane  200 . The contact holes are formed so as to reach the contact electrodes  31 ,  32  which are present in the underlying layer. The contact holes are filled with via electrodes. The upper surface of the interlayer insulating layer  38  can be planarized by CMP. 
     Then, as shown in  FIG. 10D , a backplane  400  is formed on the middle layer  300 . A characteristic feature of the present disclosure resides in that various electronic elements and interconnections which are constituents of the backplane  400  are directly formed by a semiconductor manufacture technique on a multilayer stack which includes the frontplane  200  and the middle layer  300 , rather than adhering the backplane  400  onto the middle layer  300 . As a result, each of a plurality of TFTs included in the backplane  400  includes semiconductor layers deposited on the multilayer stack that includes the frontplane  200  supported by the substrate  100  and the middle layer  300 . 
     As previously described, when the upper surface of the frontplane  200  and the upper surface of the middle layer  300  are planarized, it is easy to produce the backplane  400  which includes the TFTs by a semiconductor manufacture technique. In general, when TFTs are formed by a semiconductor manufacture technique, it is necessary to perform patterning of deposited semiconductor layers, insulating layers, and metal layers. The patterning is realized by a lithography process which involves exposure to light. If there is a large step in the underlayer of the deposited semiconductor layers, insulating layers, and metal layers, light will not be correctly focused in the exposure so that micropatterning with high precision cannot be realized. In an embodiment of the present disclosure, the entirety of the frontplane  200  including the device isolation region  240  is planarized and, accordingly, the middle layer  300  is also planarized, so that it is easy to form the backplane  400  by a semiconductor manufacture technique. 
     In the configuration example which has previously been described with reference to  FIG. 10A  to  FIG. 10D , the shape of the μLEDs  220  is generally rectangular parallelepipedic, although the shape of the μLEDs  220  may be the shape of a cylindrical pillar, a polygonal pillar such as hexagonal pillar, or an elliptical pillar. Also, the μLEDs  220  may have an inclined side surface as shown in  FIG. 4B . 
     Embodiment 
     Hereinafter, a basic embodiment of a μLED UV source of the present disclosure is described in more detail. 
     Refer to  FIG. 11 . The μLED UV source  1000  of the present embodiment is an ultraviolet radiation source which has the same configuration as the previously-described basic configuration example. The μLED UV source  1000  includes a substrate  100  which is made of sapphire, a frontplane  200  provided on the substrate  100 , a middle layer  300  provided on the frontplane  200 , and a backplane  400  provided on the middle layer  300 . 
     Next, an example of the configuration and production method of the μLED UV source  1000  of the present embodiment is described with reference to  FIG. 12A  through  FIG. 15 . 
     First, refer to  FIG. 12A . In the present embodiment, a substrate  100  is placed in a reactor of a MOCVD apparatus, and various gases are supplied into the reactor for carrying out epitaxial growth of a gallium nitride (GaN) based compound semiconductor. In the present embodiment, the substrate  100  is a sapphire substrate whose thickness is, for example, about 50-600 μm. The upper surface  100 T of the substrate  100  is typically a C-plane ( 0001 ), although the substrate  100  may have a nonpolar or semipolar plane, such as m-plane, a-plane, and r-plane, at the upper surface. The upper surface  100 T may be inclined by about several degrees from these crystal planes. The substrate  100  typically has the shape of a circular plate. The diameter of the substrate  100  can be, for example, from 1 inch to 8 inches. The shape and size of the substrate  100  are not limited to this example. The substrate  100  may have a rectangular shape. The production process may be carried on using a substrate  100  in the shape of a circular plate, and the substrate  100  may be processed into a rectangular shape by cutting away peripheral parts of the substrate  100  in the final steps. Alternatively, the production process may be carried on using a relatively-large substrate  100 , and the single substrate  100  may be divided into a plurality of μLED UV sources in the final steps (singulation). 
     Firstly, trimethyl gallium (TMG) or triethyl gallium (TEG), hydrogen (H 2 ) as the carrier gas, nitrogen (N 2 ), ammonia (NH 3 ), and silane (SiH 4 ) are supplied into the reactor of the MOCVD apparatus. The substrate  100  is heated to about 1100° C., and an n-GaN layer  22   n  (thickness: for example, 2 μm) is grown. Silane is a material gas for supplying Si as the n-type dopant. The doping concentration of the n-type impurity can be, for example, 5×10 17  cm −3 . 
     Then, supply of SiH 4  is stopped, the substrate  100  is cooled to a temperature lower than 800° C., and an emission layer  23  is formed. Specifically, firstly, an Al x In y Ga z N (0≤x&lt;1, 0&lt;y&lt;1, 0&lt;z&lt;1) barrier layer is grown. Further, supply of trimethyl indium (TMI) is started, and an Al x′ In y′ Ga z′ N (0≤x′&lt;1, 0&lt;y′&lt;1, 0&lt;z′&lt;1) well layer is grown. The barrier layer and the well layer are alternately grown over two or more periods, whereby an emission layer  23  (thickness: for example, 100 nm), including a multi-quantum well which functions as the light-emitting part, can be formed. As the number of well layers is larger, the carrier density inside the well layers can be prevented from being excessively large in driving with a large electric current. A single emission layer  23  may include a single well layer interposed between two barrier layers. A well layer may be directly formed on the n-GaN layer  22   n , and a barrier layer may be formed on the well layer. 
     After the emission layer  23  is formed, supply of TMI is once stopped. Thereafter, nitrogen is added to the carrier gas (hydrogen), supply of ammonia is resumed, the growth temperature is increased to a temperature in the range of 850° C. to 1000°, and trimethyl aluminum (TMA) and biscyclopentadienyl magnesium (Cp 2 Mg) as the material for Mg as the p-type dopant are supplied, whereby an overflow suppression layer may be grown. Then, supply of TMA is stopped, and a p-GaN layer  21   p  (thickness: for example, 0.5 μm) is grown. The doping concentration of the p-type impurity can be, for example, 5×10 17  cm −3 . 
     An n-AlGaN layer may be provided between the n-GaN layer  22   n  and the emission layer  23 . The n-GaN layer  22   n  may be replaced by an n-AlGaN layer. Alternatively, a p-AlGaN layer may be provided between the emission layer  23  and the p-GaN layer  21   p.    
     Then, as shown in  FIG. 12B , photolithography and etching are performed on the substrate  100  pulled out of the reactor of the MOCVD apparatus, whereby predetermined regions of the p-GaN layer  21   p  and the emission layer  23  (portions in which the device isolation region  240  is to be formed; Depth: for example, 1.5 μm) are removed such that the n-GaN layer  22   n  is partially exposed. Etching of the gallium nitride based semiconductor can be carried out using a plasma of a chloric gas as will be described later. 
     As shown in  FIG. 12C , the spaces that define the device isolation region  240  are filled with the embedded insulator  25 . The material and formation method of the embedded insulator  25  are arbitrary so long as they are selected from materials which are capable of transmitting ultraviolet light and their formation methods. In the example shown in the drawing, the upper surface of the embedded insulator  25  is planarized and located at the same level as the upper surface of the p-GaN layer  21   p.    
     As shown in  FIG. 12D , through holes  26  are formed in part of the embedded insulator  25  so as to reach the n-GaN layer  22   n . The through holes  26  define the position and shape of the metal plugs  24 . In this example, the side surfaces of the through holes  26  are inclined such that the metal plugs  24  function as reflectors. The through holes  26  contain the metal plugs  24  which have such a shape as shown in  FIG. 1B . 
     As shown in  FIG. 12E , metal plugs  24  are formed so as to fill the through holes  26 , and the upper surface of the frontplane  200  is planarized. Thereafter, first contact electrodes  31  and second contact electrodes  32  are formed. The planarization can be carried out through various processes such as, for example, etch back, selective growth, or lift off. 
     The metal plugs  24  can be made of metal, for example, titanium (Ti) and/or aluminum (Al), such that an ohmic contact with the n-GaN layer  22   n  can be established. The metal plugs  24  preferably include a metal layer which contains Ti in a portion in contact with the n-GaN layer  22   n  (e.g., TiN layer). The presence of the TiN layer contributes to realization of a low-resistance n-type ohmic contact. The TiN layer can be formed by forming a Ti layer so as to be in contact with the n-GaN layer  22   n  and thereafter performing a heat treatment at, for example, about 600° C. for 30 seconds. In a portion which is to reflect ultraviolet light, Al or Rh is desirably present as previously described. 
     The first and second contact electrodes  31 ,  32  can be formed by deposition and patterning of a metal layer. Between the first contact electrodes  31  and the p-GaN layer  21   p  of the μLEDs  220 , a metal-semiconductor interface is formed. To realize a p-type ohmic contact, the material of the first contact electrodes  31  can be selected from metals which have large work functions, for example, platinum (Pt) and/or palladium (Pd). After a layer of Pt or Pd (thickness: about 50 nm) is formed, a heat treatment can be performed at a temperature of, for example, not less than 350° C. and not more than 400° C. for about 30 seconds. So long as a layer of Pt or Pd is present in a portion which is in direct contact with the p-GaN layer  21   p , a layer of a different metal, for example, a Ti layer (thickness: about 50 nm) and/or an Au layer (thickness: about 200 nm), may be formed on that layer. 
     In the upper part of the p-GaN layer  21   p , a region doped with the p-type impurity at a relatively-high concentration may be formed. The second contact electrodes  32  are electrically coupled with the metal plugs  24  rather than the semiconductor. Therefore, the material of the second contact electrodes  32  can be selected from a wide range. The first contact electrodes  31  and the second contact electrodes  32  may be formed by patterning a single continuous metal layer. This patterning also includes lift off. If the first contact electrodes  31  and the second contact electrodes  32  have equal thicknesses, connection with the electric circuit in the backplane  400 , such as TFT  40  which will be described later, will be easy. 
     After the first and second contact electrodes  31 ,  32  are formed, these electrodes are covered with an interlayer insulating layer  38  (thickness: for example, 1000 nm to 1500 nm). In a preferred example, the upper surface of the interlayer insulating layer  38  can be planarized by CMP or the like. The thickness of the interlayer insulating layer  38  that has the planarized upper surface means “average thickness”. 
     As shown in  FIG. 12F , contact holes  39  are formed in the interlayer insulating layer  38 . The contact holes  39  are used for electrically coupling the electric circuit, of the backplane  400  with the μLEDs  220  of the frontplane  200 . 
     Hereinafter, a configuration example and formation method of TFTs included in the electric circuit of the backplane  400  are described with again reference to  FIG. 11 . 
     In the example shown in  FIG. 11 , the TFT  40  includes a drain electrode  41  and a source electrode  42  which are provided on the interlayer insulating layer  38 , a semiconductor thin film  43  which is in contact with at least part of the upper surface of each of the drain electrode  41  and the source electrode  42 , a gate insulating film  44  provided on the semiconductor thin film  43 , and a gate electrode  45  provided on the gate insulating film  44 . In the example shown in the drawing, the drain electrode  41  and the source electrode  42  are coupled with the first contact electrode  31  and the second contact electrode  32 , respectively, via the via electrodes  36 . These constituents of the TFT  40  are formed by a known semiconductor manufacture technique. 
     The semiconductor thin film  43  can be made of polycrystalline silicon, amorphous silicon, oxide semiconductor, and/or gallium nitride based semiconductor. The polycrystalline silicon can be formed by depositing amorphous silicon on the interlayer insulating layer  38  of the middle layer  300  by, for example, a thin film deposition technique and thereafter crystallizing the amorphous silicon with a laser beam. The thus-formed polycrystalline silicon is referred to as LTPS (Low-Temperature Poly Silicon). The polycrystalline silicon is patterned into a desired shape by lithography and etching. 
     In  FIG. 11 , the TFT  40  is covered with an insulating layer  46  (thickness: for example, 500 nm to 3000 nm). The insulating layer  46  has an unshown hole which enables coupling of, for example, the gate electrode  45  of the TFT  40  with an external driver integrated circuit device or the like. Preferably, the upper surface of the insulating layer  46  is also planarized. The electric circuit of the backplane  400  can include circuit components such as unshown TFTs, capacitors, and diodes. Thus, the insulating layer  46  may have a configuration where a plurality of insulating layers are stacked up. In this case, each of the insulating layers can include a via electrode for coupling circuit components when necessary. On each of the insulating layers, interconnections can be formed when necessary. 
     In the present embodiment, the backplane  400  can have the same configuration as a known backplane for use in display devices (e.g., TFT substrate). Note that, however, the backplane  400  of the present disclosure is characterized in that it is formed on the μLEDs  220  in the underlying layer by a semiconductor manufacture technique. Therefore, for example, the drain electrode  41  and the source electrode  42  of the TFT  40  can be formed by patterning a metal layer which is deposited so as to cover the frontplane  200 . Such patterning enables high-precision aligning which is based on lithography techniques. Particularly in the present embodiment, the frontplane  200  and/or the middle layer  300  are planarized and, therefore, it is possible to increase the resolution of the lithography. As a result, it is possible to produce a device which includes a large number of μLEDs  220  aligned at a microscopic pitch of for example not more than 20 μm, in an extreme example not more than 5 μm, at a high yield and at a low cost. 
     The configuration of the TFT  40  shown in  FIG. 11  is exemplary. For the sake of clear description, in the example described herein, the drain electrode  41  of the TFT  40  is electrically coupled with the first contact electrode  31 , although the drain electrode  41  of the TFT  40  may be coupled with any other circuit component or interconnection included in the backplane  400 . The source electrode  42  of the TFT  40  does not need to be electrically coupled with the second contact electrode  32 . The second contact electrode  32  can be coupled with an interconnection which commonly gives a predetermined potential to the n-GaN layers  22   n  of the μLEDs  220  (e.g., ground interconnection). 
     In the present embodiment, the electric circuit of the backplane  400  includes a plurality of metal layers which are respectively coupled with the first contact electrode  31  and the second contact electrode  32  (metal layers which function as the drain electrode  41  and the source electrode  42 ). In the present embodiment, the plurality of first contact electrodes  31  respectively cover the p-GaN layers  21   p  of the plurality of μLEDs  220  and function as a light-blocking layer or a light-reflecting layer. Each of the first contact electrodes  31  does not need to cover the upper surface of the μLED  220 , i.e., the entirety of the upper surface of the p-GaN layer  21   p . The shape, size, and position of the first contact electrodes  31  are determined such that sufficiently-low contact resistance is realized while the first contact electrodes  31  sufficiently suppress arrival of ultraviolet light radiated from the emission layer  23  at the channel region of the TFT  40 . Prevention of arrival of ultraviolet light radiated from the emission layer  23  at the channel region of the TFT  40  can also be realized by arranging the other metal layers at appropriate positions. 
     According to an embodiment of the present disclosure, the middle layer  300  that has a planarized upper surface is formed on the frontplane  200  that has a flat upper surface which is realized by filling the device isolation region  240  with the metal plugs  24  and the embedded insulator  25 . These structures (underlying structures) function as a base on which circuit components such as TFTs are to be formed. In depositing semiconductors for TFT or in performing a heat treatment after the deposition, the above-described underlying structures are treated at, for example, 350° C. or higher. Thus, the embedded insulator  25  in the device isolation region  240  and the interlayer insulating layer  38  included in the middle layer  300  are preferably made of a material which will not be degraded even by a heat treatment at 350° C. or higher. For example, polyimide and SOG (Spin-on Glass) can be suitably used. 
     The configuration of TFTs included in the electric circuit in the backplane  400  is not limited to the above-described examples. 
       FIG. 13  is a cross-sectional view schematically showing another example of the TFT.  FIG. 14  is a cross-sectional view schematically showing still another example of the TFT. 
     In the example of  FIG. 13 , the TFT  40  includes a drain electrode  41 , a source electrode  42 , and a gate electrode  45  which are provided on the interlayer insulating layer  38 , a gate insulating film  44  which is provided on the gate electrode  45 , and a semiconductor thin film  43  which is provided on the gate insulating film  44  so as to be in contact with at least part of the upper surface of each of the drain electrode  41  and the source electrode  42 . In the example shown in the drawing, the drain electrode  41  and the source electrode  42  are coupled with the first contact electrode  31  and the second contact electrode  32 , respectively, via the via electrodes  36 . 
     In the example of  FIG. 14 , the TFT  40  includes a semiconductor thin film  43  provided on the interlayer insulating layer  38 , a drain electrode  41 , and a source electrode  42  which are provided on the interlayer insulating layer  38  so as to be in contact with part of the semiconductor thin film  43 , a gate insulating film  44  provided on the semiconductor thin film  43 , and a gate electrode  45  provided on the gate insulating film  44 . In the example shown in the drawing, the drain electrode  41  and the source electrode  42  are coupled with the first contact electrode  31  and the second contact electrode  32 , respectively, via the via electrodes  36 . 
     The configuration of the TFT  40  is not limited to the above-described examples. In an embodiment of the present disclosure, in the initial phase of the process of forming the TFT  40 , a plurality of metal layers are formed so as to be in contact with the first and second contact electrodes  31 ,  32  of the frontplane  200  via the contact holes  39  of the interlayer insulating layer  38  in the middle layer  300 . These metal layers can be the drain electrode  41  or the source electrode  42  of the TFT  40  but are not limited to such examples. 
     In the present embodiment, the drain electrode  41  and the source electrode  42  are formed by depositing a metal layer on the interlayer insulating layer  38  in the planarized middle layer  300  and thereafter patterning the metal layer by photolithography and etching. Therefore, misalignment which can cause decrease in yield will not occur between the frontplane  200  (the middle layer  300 ) and the backplane  400 . 
     &lt;TiN Buffer Layer&gt; 
       FIG. 15  is a cross-sectional view schematically showing part of a μLED UV source which includes a titanium nitride (TiN) layer  50  located between the substrate  100  and the n-GaN layer  22   n  of each of the μLEDs  220 . The thickness of the TiN layer  50  can be, for example, not more than 5 nm and not less than 20 nm. The TiN layer  50  can be suitably used in combination with a substrate  100  which is made of sapphire. 
     The TiN layer  50  is electrically conductive. In an embodiment of the present disclosure, a large number of μLEDs  220  are arrayed over a wide area, and at least one metal plug  24  couples the n-GaN layer  22   n  of the μLEDs  220  with the electric circuit of the backplane  400 . Thus, if an electrical resistance component (sheet resistance) relative to the electric current flowing from the n-GaN layer  22   n  to the metal plug  24  is excessively high, an increase in power consumption will be caused. The TiN layer  50  functions as a buffer layer which relaxes the lattice mismatch in crystal growth and contributes to reduction in density of crystallographic defects, and also contributes to reduction in the above-described electrical resistance component in the operation of the device. The thickness of the TiN layer  50  is preferably not less than 10 nm, more preferably not less than 12 nm, from the viewpoint of reducing the electrical resistance component such that it can function as the substrate-side electrode. Meanwhile, from the viewpoint of transmitting ultraviolet light radiated from the μLEDs  220 , the thickness of the TiN layer  50  is preferably, for example, not more than 20 nm, more preferably 5-15 nm. 
     In the example shown in  FIG. 15 , a single continuous n-GaN layer  22   n  (second semiconductor layer) is shared among the plurality of μLEDs  220 . However, the n-GaN layer  22   n  may be isolated for each of the μLEDs  220 . In that case, the bottom of a trench which defines the device isolation region  240  reaches the upper surface of the TiN layer  50 , and the metal plugs  24  are in contact with the TiN layer  50 . Since the single continuous TiN layer  50  is electrically coupled with the n-GaN layer  22   n  in all of the μLEDs  220 , electrical conduction between the metal plug  24  and the n-GaN layer  22   n  of each of the μLEDs  220  is secured. In this example, the TiN layer  50  functions as the n-side common electrode of the plurality of μLEDs  220 . In an embodiment of the present disclosure, the electrodes on the second conductivity side in the plurality of μLEDs  220  are realized in a common form by a semiconductor layer or a TiN layer. Thus, a problem of conduction failure in some of the μLEDs  220  due to interconnection breakage is avoided. 
     INDUSTRIAL APPLICABILITY 
     An embodiment of the present invention provides a novel micro-LED ultraviolet radiation source. The micro-LED ultraviolet radiation source can be used in various uses which require irradiation with ultraviolet light, including curing of a resin with ultraviolet light, exposure of a resist to light, lift-off of a resin film, and sterilization. Particularly, it is useful in a device which requires selectively irradiating a predetermined region with ultraviolet light. 
     REFERENCE SIGNS LIST 
     
         
           21  . . . First semiconductor layer,  22  . . . Second semiconductor layer,  23  . . . Emission layer,  24  . . . Metal plug,  25  . . . Embedded insulator,  31  . . . First contact electrode,  32  . . . Second contact electrode,  36  . . . Via electrode,  38  . . . Interlayer insulating layer,  100  . . . Crystal growth substrate,  200  . . . Frontplane,  220  . . . μLED,  240  . . . Device isolation region,  300  . . . Middle layer,  400  . . . Backplane,  1000  . . . LED LUV source