Patent Publication Number: US-2022223767-A1

Title: Ultraviolet light emitting device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2021-003828 filed on Jan. 13, 2021. 
     TECHNICAL FIELD 
     The technical field of the present specification relates to an ultraviolet light emitting device including an ultraviolet light emitting element. 
     BACKGROUND ART 
     In a light emitting device which emits visible light, a semiconductor light emitting element mounted on a substrate is sealed with a resin. The sealing resin is, for example, a silicone resin or an epoxy resin. Refractive index of these resins are greater than a refractive index of the atmosphere. Therefore, reflection at an interface between the semiconductor light emitting element and the sealing resin is prevented. That is, light extraction efficiency is high. 
     In recent years, a light emitting device including an ultraviolet light emitting element has been researched and developed. Ultraviolet light modifies the silicone resin and the epoxy resin. Accordingly, cracks may occur in the sealing resin. Therefore, a light emitting device in which a periphery of an ultraviolet light emitting element is not sealed with a resin has been developed. The ultraviolet light emitting element has a refractive index of about 1.7, which is greatly different from the refractive index of the atmosphere. Therefore, total reflection is likely to occur at an interface between the ultraviolet light emitting element and the atmosphere. That is, light extraction efficiency of such an air emitting type light emitting device tends to be low. 
     On the other hand, there is a technique sealing a light emitting device including an ultraviolet light emitting element. For example, JP-A-2016-207754 discloses a technique of sealing with a liquid organic halide (paragraphs [0042] to [0044] in JP-A-2016-207754). However, the liquid is less likely to be deteriorated by ultraviolet light, and may react with an adhesive. In this case, a substrate and a lens or the like may be separated from each other. In addition, the liquid may leak from a slight gap of the adhesive. 
     Therefore, it is preferable to prevent peeling between the substrate and the lens or the like. In addition, it is preferable to prevent leakage of the liquid. Of course, it is preferable that an output of the light emitting device is high. 
     SUMMARY OF INVENTION 
     An object of the present specification is to provide an ultraviolet light emitting device having a high output and capable of preventing peeling between a substrate and a lens or the like. 
     An ultraviolet light emitting device according to a first aspect includes a substrate, an ultraviolet light emitting element, a bonding layer, a light transmitting member, an adhesive layer, and a fluorocarbon compound. The substrate has a mounting surface mounting the ultraviolet light emitting element. The ultraviolet light emitting element has a first surface having an electrode, a second surface opposite to the first surface, and a side surface. The bonding layer bonds the electrode on the first surface of the ultraviolet light emitting element and a part of the mounting surface of the substrate. The light transmitting member is configured to transmit ultraviolet light. The adhesive layer adheres the substrate to the light transmitting member. The substrate and the light transmitting member are disposed in a state where the ultraviolet light emitting element is sandwiched therebetween. The fluorocarbon compound is a liquid at normal temperature and pressure. The fluorocarbon compound fills a gap between the second surface of the ultraviolet light emitting element and the light transmitting member in a state of being in contact with the second surface and the light transmitting member. The fluorocarbon compound is in contact with the side surface of the ultraviolet light emitting element. The fluorocarbon compound is not in contact with the adhesive layer. 
     In the ultraviolet light emitting device, light extraction efficiency from the ultraviolet light emitting element is high. In addition, there is almost no possibility that the adhesive layer adhering the substrate to the light transmitting member is adversely affected by the fluorocarbon compound. Therefore, the ultraviolet light emitting device has a high output. In addition, peeling between the substrate and the light transmitting member is less likely to occur. 
     In the present specification, it is possible to provide an ultraviolet light emitting device having a high output and capable of preventing peeling between the substrate and the lens or the like. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic configuration diagram of a light emitting device  100  according to a first embodiment. 
         FIG. 2  is a cross-sectional view illustrating a lens  140  of the light emitting device  100  according to the first embodiment. 
         FIG. 3  is a plan view illustrating the lens  140  of the light emitting device  100  according to the first embodiment. 
         FIG. 4  is diagram illustrating a positional relationship between an air layer  170  and a fluorocarbon compound  160  of the light emitting device  100  according to the first embodiment. 
         FIG. 5  is diagram illustrating a region of the air layer  170  of the light emitting device  100  according to the first embodiment. 
         FIG. 6  is diagram illustrating a distance between a second surface  120   b  of an ultraviolet light emitting element  120  and a ceiling surface  144  of a lens  140  in the light emitting device  100  according to the first embodiment. 
         FIG. 7  is a diagram (part  1 ) illustrating a method for producing the light emitting device  100  according to the first embodiment. 
         FIG. 8  is a diagram (part  2 ) illustrating the method for producing the light emitting device  100  according to the first embodiment. 
         FIG. 9  is a diagram (part  3 ) illustrating a method for producing the light emitting device  100  according to the first embodiment. 
         FIG. 10  is a schematic configuration diagram of a light emitting device  200  according to a modification of the first embodiment. 
         FIG. 11  is a schematic configuration diagram of a light emitting device  300  according to a modification of the first embodiment. 
         FIG. 12  is a diagram illustrating a lens  440  of an light emitting device  400  according to a modification of the first embodiment. 
         FIG. 13  is a diagram illustrating a lens  540  of an light emitting device  500  according to a modification of the first embodiment. 
         FIG. 14  is a diagram illustrating a region of an air layer  670  of an light emitting device  600  according to a modification of the first embodiment. 
         FIG. 15  is a schematic configuration diagram of a light emitting device  700  according to a second embodiment. 
         FIG. 16  is a schematic configuration diagram of a light emitting device  800  according to a modification of the second embodiment. 
         FIG. 17  is a schematic configuration diagram of a light emitting device  900  according to a modification of the second embodiment. 
         FIG. 18  is a schematic configuration diagram of a light emitting device  1000  according to a modification of the second embodiment. 
         FIG. 19  is a diagram illustrating a sample of Comparative Example 2 in an evaluation test. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, specific embodiments will be described with reference to the drawings, taking an ultraviolet light emitting device as an example. However, the technique of the present specification is not limited to the embodiments. A structure different from that of the embodiment may be included. The thickness ratio of each layer in each figure is conceptually illustrated, and does not indicate the actual thickness ratio. 
     First Embodiment 
     1. Light Emitting Device 
       FIG. 1  is a schematic configuration diagram of a light emitting device  100  according to a first embodiment. As illustrated in  FIG. 1 , the light emitting device  100  includes a substrate  110 , an ultraviolet light emitting element  120 , a bonding layer  130 , a lens  140 , an adhesive layer  150 , a fluorocarbon compound  160 , and an air layer  170 . The light emitting device  100  is an ultraviolet light emitting device which emits ultraviolet light. 
     The substrate  110  is a substrate for mounting the ultraviolet light emitting element  120 . The substrate  110  has a mounting surface  110   a . The mounting surface  110   a  is a surface for mounting the ultraviolet light emitting element  120 . The mounting surface  110   a  is a surface of a circuit pattern. 
     The ultraviolet light emitting element  120  is a semiconductor light emitting element which emits ultraviolet light. An emission wavelength of the ultraviolet light emitting element  120  is, for example, 200 nm or more and 320 nm or less. The ultraviolet light emitting element  120  is mounted on the substrate  110  via the bonding layer  130 . The ultraviolet light emitting element  120  has a first surface  120   a , a second surface  120   b , and a side surface  120   c . The first surface  120   a  has an electrode thereon. The first surface  120   a  faces the mounting surface  110   a  of the substrate  110 . The second surface  120   b  is a surface opposite to the first surface  120   a . The second surface  120   b  is a light extraction surface which extracts light to the outside of the ultraviolet light emitting element  120 . The second surface  120   b  faces the lens  140 . The side surface  120   c  is a surface other than the first surface  120   a  and the second surface  120   b.    
     The bonding layer  130  is a layer for mounting the ultraviolet light emitting element  120  on the substrate  110 . The bonding layer  130  bonds the electrode on the first surface  120   a  of the ultraviolet light emitting element  120  and the circuit pattern of the mounting surface  110   a  of the substrate  110 . The material of the bonding layer  130  is, for example, an Au—Sn solder. 
     The lens  140  is a light transmitting member for suitably extracting ultraviolet light emitted from the ultraviolet light emitting element  120  to the outside. Of course, the lens  140  transmits ultraviolet light. The lens  140  is made of, for example, quartz glass. The lens  140  is has a refractive index larger than the refractive index of the atmosphere. The refractive index of the lens  140  is, for example, 1.2 or more and 1.6 or less. 
       FIG. 2  is a cross-sectional view illustrating the lens  140  of the light emitting device  100  according to the first embodiment. As illustrated in  FIG. 2 , the lens  140  includes a convex surface  141 , a flat surface  142 , an inner wall  143 , a ceiling surface  144 , and a base portion  145 . The lens  140  includes a concave portion U 1  on a mounting surface  110   a  side of the substrate  110 . The concave portion U 1  has the inner wall  143  and the ceiling surface  144 . The convex surface  141  faces the outside of the light emitting device  100 . The convex surface  141  has a rotary paraboloid. In addition, the light emitting device  100  emits ultraviolet light from the convex surface  141  (see an arrow L 1  in  FIG. 1 ). The flat surface  142  is an adhesive surface to the substrate  110 . The inner wall  143  faces the substrate  110  together with the ceiling surface  144 . The inner wall  143  extends toward the substrate  110 . The ceiling surface  144  is a surface facing the substrate  110  and the ultraviolet light emitting element  120  after being bonded to the substrate  110 . The base portion  145  is a portion located at a base of the lens  140 . 
       FIG. 3  is a plan view illustrating the lens  140  of the light emitting device  100  according to the first embodiment. As illustrated in  FIG. 3 , since the convex surface  141  is a rotary paraboloid, the convex surface  141  is circular in the plan view. The base portion  145  is square in the plan view. 
     The adhesive layer  150  adheres the substrate  110  to the lens  140 . The adhesive layer  150  adheres the substrate  110  to the lens  140  in a state where the substrate  110  and the lens  140  sandwich the ultraviolet light emitting element  120  therebetween. Therefore, the substrate  110  and the lens  140  are disposed in a state where the ultraviolet light emitting element  120  is sandwiched between the substrate  110  and the lens  140 . 
     The fluorocarbon compound  160  is located between the ultraviolet light emitting element  120  and the lens  140 . 
     The air layer  170  is a closed space located between the substrate  110  and the lens  140 . 
     2. Fluorocarbon Compound 
     2-1. Region of Fluorocarbon Compound 
     The fluorocarbon compound  160  is a polymer having a CF bond. The fluorocarbon compound  160  is a liquid at normal temperature and pressure. The number of carbon atoms in the fluorocarbon compound  160  is 1.9 times or less the number of fluorine atoms in the fluorocarbon compound  160 . The fluorocarbon compound  160  is, for example, perfluoropolyether (PFPE). It is sufficient that the refractive index of the fluorocarbon compound  160  is greater than the refractive index of the atmosphere and equal to or less than the refractive index of the ultraviolet light emitting element  120 . The refractive index of the fluorocarbon compound  160  is, for example, 1.2 or more and 1.6 or less. 
     As illustrated in  FIG. 1 , the fluorocarbon compound  160  fills a gap between the second surface  120   b  of the ultraviolet light emitting element  120  and the lens  140 . Therefore, the fluorocarbon compound  160  is in contact with the second surface  120   b  of the ultraviolet light emitting element  120  and the lens  140 . 
       FIG. 4  is diagram illustrating a positional relationship between the air layer  170  and the fluorocarbon compound  160  of the light emitting device  100  according to the first embodiment. As illustrated in  FIG. 4 , the substrate  110  includes a base material  111  and a circuit pattern  112 . The mounting surface  110   a  is a surface on the circuit pattern  112 . 
     The fluorocarbon compound  160  forms a fillet shape. The fluorocarbon compound  160  is in contact with the side surface  120   c  of the ultraviolet light emitting element  120  and the inner wall  143  of the lens  140 . However, the fluorocarbon compound  160  is not in contact with the mounting surface  110   a  of the substrate  110 . 
     In addition, the fluorocarbon compound  160  covers the second surface  120   b  and the side surface  120   c  of the ultraviolet light emitting element  120 , but does not cover the first surface  120   a . Therefore, the fluorocarbon compound  160  does not cover the bonding layer  130 . 
     The fluorocarbon compound  160  is not in contact with the adhesive layer  150 . 
     The fluorocarbon compound  160  includes a central portion  161  and an outer edge portion  162 . The central portion  161  and the outer edge portion  162  are connected to each other. The central portion  161  is a region sandwiched between the ultraviolet light emitting element  120  and the lens  140 . The central portion  161  has a rectangular parallelepiped shape. Vertical and horizontal lengths of the rectangular parallelepiped shape are equal to vertical and horizontal lengths of the ultraviolet light emitting element  120 . 
     The outer edge portion  162  is a region sandwiched between the lens  140  and the air layer  170 . The outer edge portion  162  is in contact with the side surface  120   c  of the ultraviolet light emitting element  120  and the inner wall  143  of the lens  140 . Therefore, in a region immediately outside the ultraviolet light emitting element  120 , the substrate  110 , an annular portion  171  of the air layer  170 , the outer edge portion  162  of the fluorocarbon compound  160 , and the lens  140  are stacked in this order. 
     A thickness of the fluorocarbon compound  160  in contact with the side surface  120   c  of the ultraviolet light emitting element  120  increases as a distance from the mounting surface  110   a  of the substrate  110  increases. 
     As illustrated in  FIG. 4 , a point B 1  or a point C 1  is located at a position in the fluorocarbon compound  160  closest to the mounting surface  110   a  of the substrate  110 . The point B 1  is a point at which the flat surface  142  of the lens  140  and the inner wall  143  intersect each other. The point C 1  is a point at which the side surface  120   c  of the ultraviolet light emitting element  120  and the first surface  120   a  intersect each other. 
     That is, a portion of the fluorocarbon compound  160  which is located closest to the mounting surface  110   a  of the substrate  110  is located at a position along the inner wall  143  of the lens  140  or at a position along the side surface  120   c  of the ultraviolet light emitting element  120 . 
     Particularly, the portion of the fluorocarbon compound  160  which is located closest to the mounting surface  110   a  of the substrate  110  is located at a position (point B 1 ) closest to the mounting surface  110   a  among points on the inner wall  143  of the lens  140 , or at a position (point C 1 ) closest to the mounting surface  110   a  among points on the side surface  120   c  of the ultraviolet light emitting element  120 . 
     2-2. Effect of Fluorocarbon Compound 
     The fluorocarbon compound  160  covers the second surface  120   b  and the side surface  120   c  of the ultraviolet light emitting element  120 . The refractive index of the fluorocarbon compound  160  is greater than the refractive index of the atmosphere and equal to or less than the refractive index of the ultraviolet light emitting element  120 . Therefore, the light to be emitted from the second surface  120   b  and the side surface  120   c  of the ultraviolet light emitting element  120  is less likely to be totally reflected at a boundary surface with the fluorocarbon compound  160 . That is, the light extraction efficiency on the second surface  120   b  and the side surface  120   c  of the ultraviolet light emitting element  120  is high. 
     The fluorocarbon compound  160  does not cover the first surface  120   a  of the ultraviolet light emitting element  120 . Here, the refractive index of the ultraviolet light emitting element  120  is about 1.7, and the refractive index of the atmosphere is about 1. Therefore, the light to be emitted from the first surface  120   a  of the ultraviolet light emitting element  120  is likely to be totally reflected. If the ultraviolet light emitting element  120  in which ultraviolet light is emitted to some extent also to a first surface  120   a  side is used, the light to be emitted to the first surface  120   a  side can be reflected to a second surface  120   b  side. 
     3. Air Layer 
     3-1. Region of Air Layer 
     The air layer  170  is filled with a gas. The gas is, for example, atmosphere. The air layer  170  is located between the substrate  110  and the lens  140 . The air layer  170  is in contact with the fluorocarbon compound  160  and the adhesive layer  150 . 
     The air layer  170  includes an annular portion  171  annularly surrounding a periphery of the ultraviolet light emitting element  120 , and a coupling portion  172 , which is a gap between the ultraviolet light emitting element  120  and the substrate  110 . The annular portion  171  is an annular space based on a quadrangle conforming to the shape of the ultraviolet light emitting element  120 . The coupling portion  172  is a gap which couples to the annular portion  171 . 
     The annular portion  171  is located between the mounting surface  110   a  of the substrate  110  and the outer edge portion  162  of the fluorocarbon compound  160 . The annular portion  171  surrounds a periphery of the side surface  120   c  of the ultraviolet light emitting element  120 . The fluorocarbon compound  160  is present between the annular portion  171  and the side surface  120   c  of the ultraviolet light emitting element  120 . 
     The coupling portion  172  is located between the mounting surface  110   a  of the substrate  110  and the first surface  120   a  of the ultraviolet light emitting element  120 . Therefore, the first surface  120   a  of the ultraviolet light emitting element  120  is not in contact with the fluorocarbon compound  160 . 
     As illustrated in  FIG. 4 , a distance T 1   a  is smaller than a distance T 2  and larger than distances T 3  and T 4 . The distance T 1   a  is a distance from a point Q 1   a  in the air layer  170  located farthest from the mounting surface  110   a  of the substrate  110  to the mounting surface  110   a  of the substrate  110 . The distance T 2  is a distance from the ceiling surface  144  of the lens  140  to the mounting surface  110   a  of the substrate  110 . The distance T 3  is a distance from the second surface  120   b  of the ultraviolet light emitting element  120  to the mounting surface  110   a  of the substrate  110 . The distance T 4  is a distance from the first surface  120   a  of the ultraviolet light emitting element  120  to the mounting surface  110   a  of the substrate  110 . 
     That is, the distance T 1   a  from the point Q 1   a  in the air layer  170  located farthest from the mounting surface  110   a  of the substrate  110  to the mounting surface  110   a  of the substrate  110  is smaller than the distance T 2  from the ceiling surface  144  of the lens  140  to the mounting surface  110   a  of the substrate  110 . In addition, the distance T 1   a  from the point Q 1   a  in the air layer  170  located farthest from the mounting surface  110   a  of the substrate  110  to the mounting surface  110   a  of the substrate  110  is larger than the distance T 3  from the second surface  120   b  of the ultraviolet light emitting element  120  to the mounting surface  110   a  of the substrate  110 . 
     The point Q 1   a  in the air layer  170  located farthest from the mounting surface  110   a  of the substrate  110  is located at an outer side of the side surface  120   c  of the ultraviolet light emitting element  120  and at an inner side of the point B 1  at which the flat surface  142  and the inner wall  143  of the lens  140  intersect each other. 
     In addition, as illustrated in  FIG. 4 , there is no air layer between the second surface  120   b  of the ultraviolet light emitting element  120  and the ceiling surface  144  of the lens  140 . 
       FIG. 5  is diagram illustrating a region of the air layer  170  of the light emitting device  100  according to the first embodiment. In  FIG. 5 , the fluorocarbon compound  160  around a periphery of the ultraviolet light emitting element  120  is virtually drawn. The air layer  170  surrounds the periphery of the ultraviolet light emitting element  120  with the fluorocarbon compound  160  sandwiched therebetween. An outer edge of the air layer  170  on the side opposite to the ultraviolet light emitting element  120  has a quadrangular shape. 
     3-2. Effect of Air Layer 
     When the annular portion  171  of the air layer  170  is present between the outer edge portion  162  of the fluorocarbon compound  160  and the mounting surface  110   a  of the substrate  110 , the fluorocarbon compound  160  is not in contact with the adhesive layer  150 . Therefore, there is almost no possibility that the fluorocarbon compound  160  and the adhesive layer  150  react with each other. In addition, since the fluorocarbon compound  160  does not reach the adhesive layer  150 , even when there is a slight gap in the adhesive layer  150 , there is almost no possibility that the fluorocarbon compound  160  spills out from the gap to the outside of the light emitting device  100 . 
     4. Distance between Ultraviolet Light Emitting Element and Lens 
     4-1. Distance 
       FIG. 6  is diagram illustrating a distance between the second surface  120   b  of the ultraviolet light emitting element  120  and the ceiling surface  144  of the lens  140  in the light emitting device  100  according to the first embodiment. The second surface  120   b  of the ultraviolet light emitting element  120  faces a side of the ultraviolet light emitting element  120  opposite to the substrate  110 . As illustrated in  FIG. 6 , a distance H 1  between the second surface  120   b  of the ultraviolet light emitting element  120  and the ceiling surface  144  of the lens  140  is 0.1 μm or more and 500 μm or less. Preferably, the distance H 1  is 0.1 μm or more and 400 μm or less. More preferably, the distance H 1  is 0.1 μm or more and 300 μm or less. 
     4-2. Effect of Distance 
     The fluorocarbon compound  160  absorbs ultraviolet light to some extent. 
     Therefore, the distance H 1  between the second surface  120   b  of the ultraviolet light emitting element  120  and the ceiling surface  144  of the lens  140  is preferably small. As the distance H 1  decreases, the ultraviolet light is not absorbed by the fluorocarbon compound  160 , and the light output increases. 
     5. Refractive Index 
     The ultraviolet light emitting element  120  has a refractive index of about 1.7. The fluorocarbon compound  160  has a refractive index of about 1.2 or more and 1.6 or less. The lens  140  has a refractive index of about 1.2 or more and 1.6 or less. The atmosphere has a refractive index of 1. 
     In the first embodiment, the second surface  120   b  and the side surface  120   c  of the ultraviolet light emitting element  120  are not in contact with the air layer  170 . As described above, the refractive index of the ultraviolet light emitting element  120  is sufficiently greater than the refractive index of the air layer  170 . Since in the ultraviolet light emitting element  120 , the second surface  120   b  and the side surface  120   c  from which the light is extracted to the outside are not in contact with the air layer  170  having a lower refractive index, the light from the ultraviolet light emitting element  120  is likely to be emitted to the outside of the element. 
     6. Production Method 
     6-1. Element Mounting Step 
     As illustrated in  FIG. 7 , the ultraviolet light emitting element  120  is mounted on the mounting surface  110   a  of the substrate  110 . A solder is placed on the mounting surface  110   a  of the substrate  110 . The ultraviolet light emitting element  120  is placed on the solder such that the electrode on the first surface  120   a  of the ultraviolet light emitting element  120  is in contact with the solder. Then, the ultraviolet light emitting element  120  is mounted on the substrate  110  by, for example, reflow. 
     6-2. Dropping Step 
     As illustrated in  FIG. 8 , the fluorocarbon compound  160  is dropped onto the second surface  120   b  of the ultraviolet light emitting element  120 . Accordingly, the fluorocarbon compound  160  spreads on the second surface  120   b  of the ultraviolet light emitting element  120 . At this stage, the fluorocarbon compound  160  is not in contact with the side surface  120   c  of the ultraviolet light emitting element  120 . 
     6-3. Lens Adhering Step 
     As illustrated in  FIG. 9 , the adhesive layer  150  is coated onto the mounting surface  110   a  of the substrate  110 . Thereafter, the lens  140  is adhered to the mounting surface  110   a  of the substrate  110 . At this time, the ceiling surface  144  of the lens  140  presses the fluorocarbon compound  160 . With this pressure, the fluorocarbon compound  160  spreads outward from above the second surface  120   b  of the ultraviolet light emitting element  120 . Then, the fluorocarbon compound  160  spreads over the side surface  120   c  of the ultraviolet light emitting element  120  and also spreads over the inner wall  143  of the lens  140 . Accordingly, the light emitting device  100  is produced. 
     7. Effect of First Embodiment 
     7-1. Fluorocarbon Compound 
     The fluorocarbon compound  160  covers the second surface  120   b  and the side surface  120   c  of the ultraviolet light emitting element  120 . Therefore, the light to be emitted to the outside from the second surface  120   b  and the side surface  120   c  of the ultraviolet light emitting element  120  is less likely to be totally reflected at the boundary surface with the fluorocarbon compound  160 . That is, the light extraction efficiency on the second surface  120   b  and the side surface  120   c  of the ultraviolet light emitting element  120  is high. 
     7-2. Air Layer 
     When the annular portion  171  of the air layer  170  is present between the outer edge portion  162  of the fluorocarbon compound  160  and the mounting surface  110   a  of the substrate  110 , the fluorocarbon compound  160  is not in contact with the adhesive layer  150 . 
     Therefore, there is almost no possibility that the fluorocarbon compound  160  and the adhesive layer  150  react with each other. In addition, the fluorocarbon compound  160  is less likely to spill out to the outside of the light emitting device  100 . 
     7-3. Distance Between Ultraviolet Light Emitting Element and Lens 
     The fluorocarbon compound  160  absorbs ultraviolet light to some extent. 
     Therefore, the distance H 1  between the second surface  120   b  of the ultraviolet light emitting element  120  and the ceiling surface  144  of the lens  140  is preferably small. As the distance H 1  decreases, the ultraviolet light is not absorbed by the fluorocarbon compound  160 , and the light output increases. 
     8. Modifications 
     8-1. Shape of Fillet 
       FIG. 10  is a schematic configuration diagram of a light emitting device  200  according to a modification of the first embodiment. In the light emitting device  200 , a fluorocarbon compound  260  and an air layer  270  are different from those of the light emitting device  100 . As illustrated in  FIG. 10 , the fluorocarbon compound  260  covers the side surface  120   c  of the ultraviolet light emitting element  120 , but does not cover the inner wall  143  of the lens  140 . 
     In this case, the light emitted from the second surface  120   b  of the ultraviolet light emitting element  120  is emitted without being reflected so much at an interface between the ultraviolet light emitting element  120  and the fluorocarbon compound  260 . This is because a difference between the refractive index of the ultraviolet light emitting element  120  and a refractive index of the fluorocarbon compound  260  is small. 
     Similarly, the light emitted from the side surface  120   c  of the ultraviolet light emitting element  120  is not reflected so much at the interface between the ultraviolet light emitting element  120  and the fluorocarbon compound  260 . That is, a large amount of light is incident on the fluorocarbon compound  260  from the ultraviolet light emitting element  120 . The light is incident on the air layer  270  from the fluorocarbon compound  260 , and the refractive index of the fluorocarbon compound  260  is a value between the refractive index of the ultraviolet light emitting element  120  and a refractive index of the air layer  270 . Therefore, the amount of light incident on the air layer  270  from the fluorocarbon compound  260  is larger than that in the case where light is directly incident on the air layer from the ultraviolet light emitting element  120 . 
       FIG. 11  is a schematic configuration diagram of a light emitting device  300  according to a modification of the first embodiment. In the light emitting device  300 , a fluorocarbon compound  360  and an air layer  370  are different from those of the light emitting device  100 . As illustrated in  FIG. 11 , a distance T 1   b  is smaller than the distances T 2  and T 3  and larger than the distance T 4 . The distance T 1   b  is a distance from a point Q 1   b  in the air layer  370  located farthest from the mounting surface  110   a  of the substrate  110  to the mounting surface  110   a  of the substrate  110 . 
     Thus, even when the amount of the fluorocarbon compound  360  is large, a portion of the fluorocarbon compound  360  which is located closest to the mounting surface  110   a  of the substrate  110  is located at a position (point B 1 ) closest to the mounting surface  110   a  among points on the inner wall  143  of the lens  140 , or at a position (point C 1 ) closest to the mounting surface  110   a  among points on the side surface  120   c  of the ultraviolet light emitting element  120 . 
     8-2. Filler 
     The fluorocarbon compound  160  may contain a filler which transmits ultraviolet light. The material of the filler is, for example, fluorine powder or silica. The filler may have a refractive index about the same as the refractive index of the fluorocarbon compound  160 . The refractive index of the filler is, for example, 1.2 or more and 1.6 or less. The filler has a particle diameter of, for example, 20 nm or more and 50 μm or less. An abundance ratio of the filler in the fluorocarbon compound  160  is, for example, 0.1 wt % or more and 50 wt % or less. 
     When the filler is a material which does not contain fluorine, such as silica, the filler may absorb ultraviolet light. In this case, the particle diameter of the filler may be smaller than a peak value of the emission wavelength of the ultraviolet light emitting element  120 . The filler has a particle diameter of, for example, 20 nm or more and 100 nm or less. 
     The filler is preferably used for adjusting a static viscosity and a kinematic viscosity of the fluorocarbon compound  160 . That is, it is preferable that the kinematic viscosity is low so as to facilitate the dropping during the dropping, and the static viscosity is high such that the shape of the fillet is not easily deformed after the fillet is formed. 
     8-3. Shape of Lens 
       FIG. 12  is a diagram illustrating a lens  440  of an light emitting device  400  according to a modification of the first embodiment. As illustrated in  FIG. 12 , a concave portion  445  is present between an inner wall  443  of the lens  440  and a flat surface  442  of the lens  440 . The concave portion  445  is concave from the flat surface  442  or the inner wall  443  toward the lens  440 . The concave portion  445  is located between the fluorocarbon compound  160  and the adhesive layer  150 . Therefore, the fluorocarbon compound  160  remains at a position of an end portion B 2  of the concave portion  445  due to surface tension. That is, the fluorocarbon compound  160  is less likely to come into contact with the adhesive layer  150 . 
       FIG. 13  is a diagram illustrating a lens  540  of an light emitting device  500  according to a modification of the first embodiment. As illustrated in  FIG. 13 , a concave portion  545  is present between an inner wall  543  of the lens  540  and a flat surface  542  of the lens  540 . The concave portion  545  is concave from the flat surface  542  or the inner wall  543  toward the lens  540 . The concave portion  545  is located between the fluorocarbon compound  160  and the adhesive layer  150 . Therefore, the fluorocarbon compound  160  remains at a position of an end portion B 3  of the concave portion  545  due to surface tension. That is, the fluorocarbon compound  160  is less likely to come into contact with the adhesive layer  150 . 
     8-4. Shape of Air Layer 
       FIG. 14  is a diagram illustrating a region of an air layer  670  of an light emitting device  600  according to a modification of the first embodiment. In  FIG. 14 , to fluorocarbon compound  660  around the periphery of the ultraviolet light emitting element  120  is virtually drawn. The air layer  670  surrounds the periphery of the ultraviolet light emitting element  120  with the fluorocarbon compound  660  sandwiched therebetween. An outer edge of the air layer  670  on the side opposite to the ultraviolet light emitting element  120  has a circular shape. An outer edge of the air layer  670  depends on the shape of the inner wall of the lens. The outer edge of the air layer  670  may have another shape. 
     8-5. Ceiling Surface 
     The ceiling surface  144  is a plane parallel to the mounting surface  110   a  of the substrate  110 . However, the ceiling surface  144  may not be parallel to the mounting surface  110   a  of the substrate  110 . The ceiling surface  144  may be a curved surface. 
     8-6. Combination 
     The above modifications may be freely combined. 
     Second Embodiment 
     A second embodiment will be described. Points different from the first embodiment will be mainly described. 
     1. Light Emitting Device 
       FIG. 15  is a schematic configuration diagram of a light emitting device  700  according to the second embodiment. As illustrated in  FIG. 15 , the light emitting device  700  includes a substrate  710 , the ultraviolet light emitting element  120 , the bonding layer  130 , a glass  740 , an adhesive layer  750 , a fluorocarbon compound  760 , and an air layer  770 . 
     The substrate  710  has a wall  780 . 
     The glass  740  is a light transmitting member which transmits ultraviolet light. The glass  740  has a flat plate shape. The glass  740  has a flat surface on a substrate  710  side. 
     The adhesive layer  750  adheres the wall  780  of the substrate  710  to the glass  740 . 
     The adhesive layer  750  adheres the wall  780  of the substrate  710  to the flat surface of the glass  740  on the substrate  710  side. The material of the adhesive layer  750  is, for example, a silicone resin. 
     The wall  780  is formed on a mounting surface  710   a  of the substrate  710 . The wall  780  surrounds four sides of the ultraviolet light emitting element  120 . The wall  780  is for partitioning individual ultraviolet light emitting elements  120 . The wall  780  surrounds the periphery of the ultraviolet light emitting element  120  in a manner not in contact with the ultraviolet light emitting element  120 . The material of the wall  780  is, for example, copper or AlN. The material of the wall  780  may be a material that does not easily absorb ultraviolet light. 
     2. Fluorocarbon Compound 
     The fluorocarbon compound  760  forms a fillet shape. Therefore, the fluorocarbon compound  760  is in contact with the side surface  120   c  of the ultraviolet light emitting element  120  and a ceiling surface  740   a  of the glass  740 . However, the fluorocarbon compound  760  is not in contact with the mounting surface  710   a  of the substrate  710 . 
     In addition, the fluorocarbon compound  760  covers the second surface  120   b  and the side surface  120   c  of the ultraviolet light emitting element  120 , but does not cover the first surface  120   a . Therefore, the fluorocarbon compound  760  does not cover the bonding layer  130 . The fluorocarbon compound  760  is not in contact with the wall  780  and the adhesive layer  750 . 
     The fluorocarbon compound  760  includes a central portion  761  and an outer edge portion  762 . The central portion  761  is a region sandwiched between the ultraviolet light emitting element  120  and the glass  740 . The central portion  761  has a rectangular parallelepiped shape. Vertical and horizontal lengths of the rectangular parallelepiped shape are equal to vertical and horizontal lengths of the ultraviolet light emitting element  120 . 
     The outer edge portion  762  is a region sandwiched between the glass  740  and the air layer  770 . The outer edge portion  762  is in contact with the side surface  120   c  of the ultraviolet light emitting element  120  and the ceiling surface  740   a  of the glass  740 . Therefore, in a region immediately outside the ultraviolet light emitting element  120 , the substrate  710 , an annular portion  771  of the air layer  770 , the outer edge portion  762  of the fluorocarbon compound  760 , and the glass  740  are stacked in this order. 
     In the central portion  761 , the fluorocarbon compound  760  fills a gap between the second surface  120   b  of the ultraviolet light emitting element  120  and the ceiling surface  740   a  of the glass  740 . 
     3. Air Layer 
     The air layer  770  includes the annular portion  771  annularly surrounding the periphery of the ultraviolet light emitting element  120 , and a coupling portion  772 , which is a gap between the ultraviolet light emitting element  120  and the substrate  710 . The annular portion  771  is an annular space based on a quadrangle conforming to the shape of the ultraviolet light emitting element  120 . The coupling portion  772  is a gap that couples to the annular portion  771 . 
     The annular portion  771  is located between the mounting surface  710   a  of the substrate  710  and the outer edge portion  762  of the fluorocarbon compound  760 . The annular portion  771  surrounds the periphery of the side surface  120   c  of the ultraviolet light emitting element  120 . The fluorocarbon compound  760  is present between the annular portion  771  and the side surface  120   c  of the ultraviolet light emitting element  120 . 
     4. Height of Wall 
     The wall  780  has an upper surface  780   a . The upper surface  780   a  is located on a side of the wall  780  opposite to the substrate  710 . A height of the wall  780  is a distance from the mounting surface  710   a  of the substrate  710  to the upper surface  780   a  of the wall  780 . The distance from the mounting surface  710   a  of the substrate  710  to the upper surface  780   a  of the wall  780  is smaller than a distance from the mounting surface  710   a  of the substrate  710  to the second surface  120   b  of the ultraviolet light emitting element  120 . 
     A thickness of the central portion  761  of the fluorocarbon compound  760  is smaller than a thickness of the adhesive layer  750 . 
     5. Method for Producing Light Emitting Device 
     5-1. Element Mounting Step 
     The ultraviolet light emitting element  120  is mounted on the mounting surface  710   a  of the substrate  710 . 
     5-2. Wall Forming Step 
     The wall  780  is formed around the periphery of the ultraviolet light emitting element  120 . 
     5-3. Dropping Step 
     The fluorocarbon compound  760  is dropped onto the second surface  120   b  of the ultraviolet light emitting element  120 . 
     5-4. Adhering Step 
     The adhesive layer  750  is coated onto the upper surface  780   a  of the wall  780 , and the glass  740  is adhered thereto. 
     At this time, the fluorocarbon compound  760  is pressed and spread. 
     6. Effect of Second Embodiment 
     6-1. Fluorocarbon Compound 
     The fluorocarbon compound  760  covers the second surface  120   b  and the side surface  120   c  of the ultraviolet light emitting element  120 . Therefore, the light to be emitted to the outside from the second surface  120   b  and the side surface  120   c  of the ultraviolet light emitting element  120  is less likely to be totally reflected at the boundary surface with the fluorocarbon compound  760 . That is, the light extraction efficiency on the second surface  120   b  and the side surface  120   c  of the ultraviolet light emitting element  120  is high. 
     6-2. Air Layer 
     When the annular portion  771  of the air layer  770  is present between the outer edge portion  762  of the fluorocarbon compound  760  and the mounting surface  710   a  of the substrate  710 , the fluorocarbon compound  760  is not in contact with the adhesive layer  750 . Therefore, there is almost no possibility that the fluorocarbon compound  760  and the adhesive layer  750  react with each other. In addition, the fluorocarbon compound  760  is less likely to spill out to the outside of the light emitting device  700 . 
     7. Modifications 
     7-1. Fluorocarbon Compound 
       FIG. 16  is a schematic configuration diagram of a light emitting device  800  according to a modification of the second embodiment. As illustrated in  FIG. 16 , in the light emitting device  800 , a fluorocarbon compound  860  is in contact with and covers the mounting surface  710   a  of the substrate  710 . An air layer  870  is formed from an end portion of the upper surface  780   a  of the wall  780  to the ceiling surface  740   a  of the glass  740 . The fluorocarbon compound  860  is not in contact with the adhesive layer  750 . 
     7-2. Wall 
       FIG. 17  is a schematic configuration diagram of a light emitting device  900  according to a modification of the second embodiment. As illustrated in  FIG. 17 , a height of an upper surface  980   a  of a wall  980  from the substrate  710  is larger than the height of the second surface  120   b  of the ultraviolet light emitting element  120  from the substrate  710 . 
     In the light emitting device  900 , a fluorocarbon compound  960  is in contact with and covers the mounting surface of the substrate  710 . The fluorocarbon compound  960  includes a central portion  961  and an outer edge portion  962 . The central portion  961  and the outer edge portion  962  are separated from each other. 
     The central portion  961  is located between the second surface  120   b  of the ultraviolet light emitting element  120  and the ceiling surface  740   a  of the glass  740 . The outer edge portion  962  fills a gap between the wall  980  and the ultraviolet light emitting element  120 . The outer edge portion  962  is filled up to the second surface  120   b  of the ultraviolet light emitting element  120  or the upper surface  980   a  of the wall  980 . The fluorocarbon compound  960  is not in contact with an adhesive layer  950 . 
     7-3. Counterbore 
       FIG. 18  is a schematic configuration diagram of a light emitting device  1000  according to a modification of the second embodiment. As illustrated in  FIG. 18 , a wall  1080  has a counterbore  1081 . The glass  740  is placed on the counterbore  1081 . The wall  1080  and the glass  740  are adhered to each other via an adhesive layer  1050 . 
     7-4. Glass 
     A light transmitting member other than the flat plate-shaped glass may be used. The light transmitting member may be, for example, a member having a flat surface on a lower surface and a convex lens opposite to the lower surface. Even in this case, the light transmitting member has a flat surface on the substrate  710  side. The flat surface is adhered to the wall via an adhesive layer. 
     7-5. Combination 
     The above modifications may be freely combined. 
     (Combination of Embodiments) 
     The modifications of the first embodiment and the second embodiment may be combined. 
     (Evaluation Test) 
     1. Sample 
     An experiment was conducted on a light emitting device having a lens. The brightness of the light emitting device was evaluated by changing the region of the fluorocarbon compound inside the light emitting device. The same applies to all samples except for the region of the fluorocarbon compound. 
     2. Experiment Results 
     Table 1 summarizes the experiment results. Example 1 corresponds to the first embodiment. Comparative Example 1 is a sample containing no fluorocarbon compound. As illustrated in  FIG. 19 , Comparative Example 2 is a sample in which the fluorocarbon compound is filled only between the ultraviolet light emitting element and the ceiling surface of the lens. Comparative Example 3 is a sample in which the gap between the substrate and the lens is filled with the fluorocarbon compound and the air layer is not provided. 
     The brightness of Comparative Example 1 was normalized as 1. 
     As shown in Table 1, Example 1 has high brightness and excellent adhesion. In Comparative Examples 1 and 2, the adhesion is sufficient, but the brightness is slightly weak. In Comparative Example 3, the brightness is the largest but the adhesion is insufficient. That is, peeling has occurred between the substrate and the lens. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                 Fluorocarbon  
                   
                   
               
               
                   
                 Sample 
                 compound 
                 Brightness 
                 Adhesion 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 Example 1 
                 FIG. 1  
                 1.4 
                 o 
               
               
                   
                 Comparative Example 1 
                 No 
                 1 
                 o 
               
               
                   
                 Comparative Example 2 
                 FIG. 19 
                 1.3 
                 o 
               
               
                   
                 Comparative Example 3 
                 Full filling 
                 1.5 
                 x 
               
               
                   
                   
               
            
           
         
       
     
     APPENDIX 
     An ultraviolet light emitting device according to a first aspect includes a substrate, an ultraviolet light emitting element, a bonding layer, a light transmitting member, an adhesive layer, and a fluorocarbon compound. The substrate has a mounting surface for mounting the ultraviolet light emitting element. The ultraviolet light emitting element has a first surface having an electrode, a second surface opposite to the first surface, and a side surface. The bonding layer bonds the electrode on the first surface of the ultraviolet light emitting element and a part of the mounting surface of the substrate. The light transmitting member is configured to transmit ultraviolet light. The adhesive layer adheres the substrate to the light transmitting member. The substrate and the light transmitting member are disposed in a state where the ultraviolet light emitting element is sandwiched therebetween. The fluorocarbon compound is a liquid at normal temperature and pressure. The fluorocarbon compound fills a gap between the second surface of the ultraviolet light emitting element and the light transmitting member in a state of being in contact with the second surface and the light transmitting member. The fluorocarbon compound is in contact with the side surface of the ultraviolet light emitting element. The fluorocarbon compound is not in contact with the adhesive layer. 
     An ultraviolet light emitting device according to a second aspect further includes an air layer. The air layer is located between the substrate and the light transmitting member. The air layer is in contact with the fluorocarbon compound and the adhesive layer. 
     In an ultraviolet light emitting device according to a third aspect, the air layer surrounds a periphery of the ultraviolet light emitting element. The fluorocarbon compound is present between the air layer and the ultraviolet light emitting element. 
     In an ultraviolet light emitting device according to a fourth aspect, a distance from a point in the air layer located farthest from the mounting surface of the substrate to the mounting surface of the substrate is larger than a distance from the second surface of the ultraviolet light emitting element to the mounting surface of the substrate. 
     In an ultraviolet light emitting device according to a fifth aspect, a distance from a point in the air layer located farthest from the mounting surface of the substrate to the mounting surface of the substrate is smaller than a distance from the second surface of the ultraviolet light emitting element to the mounting surface of the substrate. 
     In an ultraviolet light emitting device according to a sixth aspect, the fluorocarbon compound is not in contact with the mounting surface of the substrate. 
     In an ultraviolet light emitting device according to a seventh aspect, the fluorocarbon compound is not in contact with the bonding layer. 
     In an ultraviolet light emitting device according to an eighth aspect, a thickness of the fluorocarbon compound in contact with the side surface of the ultraviolet light emitting element increases as a distance from the mounting surface of the substrate increases. 
     In an ultraviolet light emitting device according to a ninth aspect, the fluorocarbon compound contains a filler configured to transmit ultraviolet light. The filler has a refractive index of 1.2 or more and 1.6 or less. 
     In an ultraviolet light emitting device according to a tenth aspect, the filler is a material which does not contain fluorine. A particle diameter of the filler is smaller than a peak value of an emission wavelength of the ultraviolet light emitting element. 
     In an ultraviolet light emitting device according to an eleventh aspect, the light transmitting member is a lens. A concave portion is formed in the lens on a mounting surface side of the substrate. The concave portion has a ceiling surface and an inner wall. 
     In an ultraviolet light emitting device according to a twelfth aspect, the fluorocarbon compound is present on the inner wall. 
     In an ultraviolet light emitting device according to a thirteenth aspect, a distance between the second surface of the ultraviolet light emitting element and the ceiling surface of the lens is 0.1 μm or more and 500 μm or less. 
     In an ultraviolet light emitting device according to a fourteenth aspect, the substrate has a wall surrounding the periphery of the ultraviolet light emitting element in a manner not in contact with the ultraviolet light emitting element. the light transmitting member has a flat surface on a substrate side. The adhesive layer adheres the wall of the substrate to the flat surface of the light transmitting member. A height of the wall from the mounting surface is smaller than a height of the second surface of the ultraviolet light emitting element from the mounting surface.