Patent Publication Number: US-2023159367-A1

Title: Wavelength conversion element and method for manufacturing wavelength conversion element

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application is a National Phase entry based on PCT Application No. PCT/JP2021/016818 filed on Apr. 27, 2021, entitled “WAVELENGTH CONVERSION ELEMENT AND METHOD F 0 R MANUFACTURING WAVELENGTH CONVERSION ELEMENT”, which claims the benefit of Japanese Patent Application No. 2020-079470, filed on Apr. 28, 2020, entitled “WAVELENGTH CONVERSION ELEMENT AND METHOD F 0 R MANUFACTURING WAVELENGTH CONVERSION ELEMENT”. The contents of which are incorporated by reference herein in their entirety. 
    
    
     FIELD 
     The present disclosure relates to a wavelength conversion element and a method for manufacturing a wavelength conversion element. 
     BACKGROUND 
     Known wavelength conversion elements convert laser light to light with a different wavelength using a phosphor. For example, Patent Literature  1  describes a phosphor body containing phosphor particles held together with a binder of silicon dioxide (SiO 2 ). 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: Japanese Unexamined Patent Application Publication No. 2013-35953 
     SUMMARY 
     Problem to be Solved 
     After exposure to laser light for a long time or with high power, a wavelength conversion element may have its phosphor degraded under heat from the laser light and may reduce its wavelength conversion capability. The heat resistance of the wavelength conversion element is to be improved. 
     Sotlution to Problem 
     A wavelength conversion element and a method for manufacturing a wavelength conversion element are described. 
     In an aspect, a wavelength conversion element converts excitation light to light with a different wavelength. The wavelength conversion element includes a substrate including an upper surface, and a wavelength converter on the upper surface of the substrate. The wavelength converter includes a phosphor including a plurality of phosphor particles, molten glass in contact with the plurality of phosphor particles and binding the plurality of phosphor particles to one another, and voids at least between the plurality of phosphor particles, in the molten glass, or between the plurality of phosphor particles and the molten glass. A maximum area of areas of the voids is less than a maximum area of areas of the plurality of phosphor particles in a cross-sectional view of the wavelength converter. 
     In an aspect, a method for manufacturing a wavelength conversion element is a method for manufacturing a wavelength conversion element including a wavelength converter for converting excitation light to light with a different wavelength. The method includes forming a powder filler comprising phosphor powder and glass powder, heating the powder filler to form a presintered compact, applying pressure to the presintered compact maintained at a temperature higher than or equal to a melting point of the glass powder and lower than a temperature at which a phosphor loses fluorescence, and cooling the presintered compact under the pressure to form the wavelength conversion element. 
     Advantageous Effect 
     For example, the resistance to laser radiation is improved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a cross-sectional view of a wavelength conversion element according to an embodiment. 
         FIG.  2    is a cross-sectional view of a wavelength converter. 
         FIG.  3    is a graph showing the resistance to laser radiation and the total luminous flux of the wavelength converter versus the porosity of the wavelength converter. 
         FIG.  4    is a flowchart showing processing for manufacturing the wavelength converter. 
         FIG.  5    is a cross-sectional view of the wavelength converter illustrating a manufacturing process. 
         FIG.  6    is a cross-sectional view of the wavelength converter illustrating a manufacturing process. 
         FIG.  7    is a cross-sectional view of the wavelength converter illustrating a manufacturing process. 
         FIG.  8    is a cross-sectional view of the wavelength converter illustrating a manufacturing process. 
         FIG.  9    is a cross-sectional view of the wavelength converter illustrating a manufacturing process. 
         FIG.  10    is a view of the wavelength converter illustrating voids on a surface facing a substrate. 
         FIG.  11    is a partial cross-sectional view of the wavelength converter and the substrate joined with solder. 
         FIG.  12    is a perspective view of a wavelength conversion element according to a variation of the embodiment. 
         FIG.  13    is a flowchart showing processing for manufacturing a wavelength converter. 
         FIG.  14    is a perspective view of a substrate in a variation of the embodiment. 
         FIG.  15    is a cross-sectional view of the substrate in the variation of the embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     A wavelength conversion element converts excitation light such as laser light to light with a different wavelength using a phosphor. For example, a wavelength conversion element may contain phosphors that emit red (R) fluorescence, green (G) fluorescence, and blue (B) fluorescence. Such a wavelength conversion element can convert violet laser light to pseudo white light. 
     The red, green, and blue phosphors typically have a heat resistance of up to about 400° C. To increase the light intensity of fluorescence emitted from the phosphors, the energy of the laser light may be increased. However, this may degrade the phosphors under heat from the laser light. 
     The inventor of the present disclosure has developed a technique that allows a wavelength conversion element containing a phosphor to have higher resistance to laser radiation. Embodiments of the technique will now be described with reference to the drawings. 
     Embodiments 
     Structure 
       FIG.  1    is a cross-sectional view of a wavelength conversion element  100  according to an embodiment. As illustrated in  FIG.  1   , the wavelength conversion element  100  includes a substrate BS being a plate, and a wavelength converter  10  located on a main surface of the substrate BS. The wavelength converter  10  has a cross section with a complicated microscopic structure, which is not illustrated in the schematic view. 
       FIG.  2    is a schematic cross-sectional view of the wavelength converter  10  based on an image obtained with a scanning electron microscope (SEM). As illustrated in  FIG.  2   , the wavelength converter  10  includes multiple particles of phosphor  11 , molten glass  12 , and voids  13  being spaces without any phosphor  11  or molten glass  12 . 
     The phosphor  11  includes multiple phosphor particles  11 a. The phosphor  11  includes a phosphor (red phosphor) that emits red (R) fluorescence in response to laser light, a phosphor (green phosphor) that emits green (G) fluorescence in response to laser light, and a phosphor (blue phosphor) that emits blue (B) fluorescence in response to laser light. The red, green, and blue phosphors are not distinguished from one another in  FIG.  2   . For example, the phosphor  11  contains rare earths such as europium (Eu), cerium (Ce), and yttrium (Y) as phosphates, oxides, silicates, nitrides, fluorides, aluminates, sulfides, or other compounds. 
     The molten glass  12  binds the phosphor particles  11   a  to one another. The molten glass  12  has portions in direct contact with the phosphor particles  11   a  and serves as a heating medium that directly conducts heat from the phosphor  11 . Laser light applied to an upper surface F 1  (first surface) of the wavelength converter  10  heats the phosphor  11  and the molten glass  12 . The heat is conducted through the molten glass  12  as a heating medium to the substrate BS, which then dissipates the heat. The wavelength converter  10  with this structure has high heat resistance. The phosphor  11  may include the phosphor particles  11   a  coated with a coating layer. For the phosphor particles  11   a  being coated with the coating layer, the molten glass  12  being in direct contact with the phosphor particles  11   a  refers to the molten glass  12  being in direct contact with the coating layer on the phosphor particles  11   a.    
     The molten glass  12  is transparent to allow laser light to enter the wavelength converter  10  to excite the phosphor  11 , and to allow red, green, and blue fluorescence emitted from the excited phosphor  11  to be radiated outside. 
     The voids  13  are located at least between the phosphor particles  11 a, in the molten glass  12 , or between the phosphor particles  11   a  and the molten glass  12 . As described above, the voids  13  are spaces in the wavelength converter  10  without any phosphor  11  or molten glass  12 . 
     A higher volume ratio of the voids  13  to the phosphor  11  may cause lower light emission from the phosphor  11 . A higher volume ratio of the voids  13  to the molten glass  12  may cause lower heat dissipation. However, multiple types of wavelength converters  10  formed under different conditions show that a certain volume of the voids  13  increases light emission from the phosphor  11 . 
       FIG.  3    is a graph with the horizontal axis showing the porosity (%), the left vertical axis showing the resistance to laser radiation (W/mm 2 ) of the wavelength converter  10 , and the right vertical axis showing the total luminous flux (1 m) emitted from the wavelength converter  10 . In  FIG.  3   , the total luminous flux is indicated by outlined circles, and the resistance to laser radiation is indicated by solid circles. 
     The porosity is defined as, for example, a value obtained by dividing the total area of the voids  13  in a cross section S of the wavelength converter  10  by the total area of the cross section S of the wavelength converter  10 . More specifically, the porosity may be defined as a value obtained by dividing the sum of the areas of multiple voids  13  in the cross section S of the wavelength converter  10  by the area of the cross section S of the wavelength converter  10 . For example, the porosity may be determined from an SEM image illustrated in  FIG.  2   . 
     The SEM image may be obtained by, for example, cutting the wavelength converter  10 , polishing the cross section, coating the cross section with an antistatic gold deposition film, and capturing an image of the coated cross section. 
     The wavelength converter  10  includes cavities in its cross section that are the voids  13 . To cause the voids  13  to appear white and bright in an SEM image, the gold deposition film may be thicker than usual, and the voltage and the current for an electron gun for image capturing may be adjusted. This allows the voids  13  to be distinguishable from the phosphor  11  and the molten glass  12  and allows binarization. 
     The resistance to laser radiation is defined as the power of laser light at the time point at which the wavelength converter  10  is altered after continuous exposure to laser light while the power is gradually being increased. The resistance to laser radiation may be a parameter indicating the heat resistance of the wavelength converter  10 . 
     The total luminous flux is defined as the light intensity of fluorescence emitted from the wavelength converter  10  in response to laser light with power that does not alter the wavelength converter  10 . 
     The porosities, resistances to laser radiation, and total luminous fluxes are obtained from multiple types of wavelength converters  10  formed under different conditions.  FIG.  3    shows the resulting characteristics. 
     As shown in  FIG.  3   , the resistance to laser radiation decreases as the porosity increases. However, the total luminous flux has a maximum value greater than or equal to 140 1 m with the porosity being between 5 and 10%. To maximize the light emission from the phosphor  11 , the wavelength converter  10  may be formed under conditions to have a porosity between 5 and 10%. To increase the resistance to laser radiation rather than the light emission, the wavelength converter  10  may be formed under conditions to have a porosity less than  5 %. In this manner, the porosity of the wavelength converter  10  can be controlled to cause the wavelength conversion element  100  to have high light emission from the phosphor  11  and high resistance to laser radiation. 
     As illustrated in  FIG.  2   , the maximum area of the areas of the voids  13  is less than the maximum area of the areas of the particles of phosphor  11  in a cross-sectional view of the wavelength converter  10 . This reduces the decrease in thermal conductivity caused by the voids  13 , thus reducing the decrease in resistance to laser radiation. The area of each void  13  and the area of each phosphor particle  11   a  may be calculated using an SEM image as illustrated in  FIG.  2   . The area of each void herein refers to the area of each of the voids  13  in a cross-sectional view. The area of each phosphor particle refers to the area of each of the phosphor particles  11   a  in a cross-sectional view. 
     Manufacturing Method 
       FIG.  4    is a flowchart showing processing for manufacturing the wavelength converter  10  with powder pressing.  FIGS.  5  to  9    are cross-sectional views of the wavelength conversion element  100  illustrating the manufacturing processes. The method for manufacturing the wavelength conversion element  100  will now be described with reference to  FIGS.  5  to  9    as well as  FIG.  4   . 
     As illustrated in  FIG.  5   , the substrate BS is prepared (step S 1 ). The substrate BS may be made of a highly thermally conductive material that is transparent to visible light, such as sapphire or magnesia being a non-metallic material, or a highly thermally conductive material with high heat dissipation that is reflective to visible light, such as aluminum (Al) being a metallic material. The substrate BS may be made of a thermally conductive material transparent to visible light, or more specifically, a material transparent to visible light and having higher thermal conductivity than molten glass. The substrate BS may be made of, for example, a material with a thermal conductivity of greater than or equal to 30 W/(m·K). 
     The substrate BS made of, for example, aluminum as a reflective material may include an optically reflective film on its surface, or may have a mirror finish through physical or chemical polishing. This increases the reflectance of the substrate BS. 
     The non-metallic material may be, for example, aluminum nitride (AlN), gallium nitride (GaN), silicon carbide (SiC), silicon nitride (Si 3 N 4 ), sapphire, or garnet. These non-metallic materials have low reflectance for visible light. The substrate BS made of, for example, AlN, may thus include an optically reflective film on its surface. 
     The substrate BS may be shaped and sized as appropriate for the size of the wavelength conversion element  100 . For example, the substrate BS being rectangular in a plan view may have a thickness of 0.1 to 5 mm, a length of 0.5 to 30 mm, and a width of 0.5 to 30 mm. The substrate BS may be a heat sink. 
     As illustrated in  FIG.  6   , a metal mask MS is placed on a main surface F 0  (upper surface) of the substrate BS on which the wavelength converter  10  is to be formed (step S 2 ). The metal mask MS includes an opening OP corresponding to the shape of the wavelength converter  10  in a plan view. 
     For example, the metal mask MS may be made of aluminum and include the opening OP with any shape in a plan view formed by etching. The metal mask MS is thicker than the wavelength converter  10 . For the wavelength converter  10  having a thickness of 0.1 mm, for example, the metal mask MS has a thickness of 0.15 to 0.2 mm. The metal mask MS may be replaced by a resin mask. The mask may be made of any material that allows the mask to include the opening with any shape and withstands subsequent filling with powder. 
     As illustrated in  FIG.  7   , a powder filler PW is formed by filling the opening OP in the metal mask MS with phosphor powder and glass powder (step S 3 ). The phosphor powder includes one or more types of powdery phosphor. The one or more types of powdery phosphor correspond to the phosphor  11  described above and may include a red phosphor, a green phosphor, or a blue phosphor. 
     The red phosphor is, for example, a phosphor with a peak wavelength of fluorescence in a range of about 620 to 750 nm emitted in response to laser light. The red phosphor material is, for example, CaAlSiN 3 :Eu, Y 3 O 3 S:Eu, Y 3 O 3 :Eu, SrCaClAlSiN 3 :Eu 2+ , CaAlSiN 3 :Eu, or CaAlSi(ON) 3 :Eu. 
     The green phosphor is, for example, a phosphor with a peak wavelength of fluorescence in a range of about 495 to 570 nm emitted in response to laser light. The green phosphor material is, for example, β-SiAlON:Eu, SrSi3(O, Cl) 3 N 3 :Eu, (Sr, Ba, Mg) 2 SiO 4 :Eu 2+ , ZnS:Cu, Al, or Zn 3 SiO 4 :Mn. 
     The blue phosphor is, for example, a phosphor with a peak wavelength of fluorescence in a range of about 450 to 495 nm emitted in response to laser light. The blue phosphor material is, for example, (BaSr)MgAl 10 O 17 :Eu, BaMgAl 10 O 17 :Eu, (Sr, Ca, Ba) 10 (PO 4 ) 6 Cl 2 :Eu, or (Sr, Ba) 10 (PO 4 ) 6 Cl 3 :Eu. 
     The phosphor powder may have any of various particle size distributions. For example, the phosphor powder may include phosphor particles with a D50 particle size in a range of 0.1 to 100 μm, or specifically in a range of 10 to 20 μm. 
     In the powder filler PW, the content of glass powder may be about 10 to 70 wt % of the content of phosphor powder. Similarly to the phosphor powder, the glass powder may have any of various particle size distributions. For example, the glass powder may include glass particles with a D50 particle size in a range of 0.1 to 100 or specifically in a range of 10 to 20 μm. 
     The glass powder may be low-melting glass, or specifically oxide glass having a melting point of 200 to 700° C. and being transparent after being sintered. The low-melting glass may mainly contain, for example, tin oxide, zinc oxide, boron oxide, bismuth oxide, boron oxide, vanadium oxide, tellurium oxide, or phosphoric acid. The low-melting glass may contain, for example, an oxide of alkali metal. 
     The phosphor powder and the glass powder may be mixed with vibration or rotation and rocking. The phosphor powder and the glass powder may be mixed using a medium. Various methods using the medium are available, such as dry mixing in which the powders are directly mixed or wet mixing in which the powders are mixed with, for example, a solvent or a binder. 
     As illustrated in  FIG.  8   , the metal mask MS is removed from the substrate BS after the powder filler PW is formed (step S 4 ). The powder filler PW is then heated to form a presintered compact PS (step S 5 ). 
     The heating temperature is higher than or equal to the melting point of the glass powder and lower than the temperature at which the phosphor loses fluorescence. The heating temperature may be, for example, 260 to 600° C. 
     The powder filler PW is heated in a chamber that is also used for applying pressure to the powder filler PW (described later). The chamber can be evacuated to produce a vacuum during heating. Heated in a vacuum in the chamber, the glass powder melts with reduced bubbles. 
     The wavelength converter with a porosity of less than 5% described above with reference to  FIG.  3    can be manufactured through heating in a vacuum in the chamber. The wavelength converter has a resistance to laser radiation of greater than or equal to 5 W/mm 2 . 
     As illustrated in  FIG.  9   , after the heating temperature reaches a set temperature, pressure is applied to the presintered compact PS on the substrate BS maintained at the set temperature (step S 6 ). The pressure applied in this process may be determined to prevent the phosphor from being physically crushed and failing to emit fluorescence. The pressure may be, for example, 300 to 1000 kgf/cm 2  (about 30 to 100 MPa). 
     The pressure satisfying the above condition may continue to be applied to the presintered compact PS for about one second to ten minutes. The presintered compact PS is then cooled under the pressure being maintained (step S 7 ). The pressure is released in response to the temperature of the presintered compact PS dropping below the melting point of the glass powder. 
     The wavelength conversion element  100  formed through the above steps can include voids in the wavelength converter  10  as intended. The resulting wavelength conversion element  100  thus has high heat resistance. 
     The above manufacturing method may use the substrate BS made of a material that easily forms an oxide film, such as aluminum. In this case, the substrate BS and the wavelength converter  10  are bonded using oxidation. More specifically, oxygen in the oxide glass is bonded to oxygen in an oxide film formed on the surface of the substrate BS under heat to bond the substrate BS and the wavelength converter  10  using oxidation. 
     The above manufacturing method may use the substrate BS made of a material less likely to cause such oxidative bonding. In this case, the substrate BS may have microscopic surface roughness of, for example, several micrometers to increase contact with the molten glass and increase the bonding strength with an anchor effect. In other words, the substrate BS may have surface roughness (microscopic roughness). For example, the microscopic roughness herein may cause the low-melting glass liquefied under heat to flow and enter between irregular portions of the rough surface. The microscopic roughness may have a roughness value of, for example, 0.1 to 50 μm. The microscopic roughness may cause the phosphor particles to enter between irregular portions of the rough surface. In this case, the microscopic roughness may have a roughness value of, for example, 5 to 50 μm. In each case, the roughness value of the microscopic roughness may be smaller than the particle sizes of the phosphor powder and the particle sizes of the glass powder. The particle sizes of the phosphor powder and the particle sizes of the glass powder refer to the particle sizes of their raw powders before being mixed in manufacturing. 
     The substrate BS may have surface roughness having a roughness value less than the minimum size of the sizes of the phosphor particles. When the substrate BS has such a roughness value relative to the phosphor particle sizes, the substrate BS and the wavelength converter  10  can be bonded more strongly with an increased anchor effect. 
     The value of the microscopic roughness (the roughness value) of the substrate BS herein refers to, for example, the dimension between the valley (lowest point) and the peak (highest point) of the microscopic roughness in the thickness direction of the substrate BS. The sizes of the phosphor particles may be calculated using an SEM image illustrated in  FIG.  2   . 
     The substrate BS may be made of a material that cannot use the oxidative bonding or the anchoring effect. In this case, the wavelength converter  10  and the substrate BS may be prepared separately. More specifically, instead of being formed on the substrate BS, the wavelength converter  10  may be formed separately from the substrate BS by applying heat and pressure to the powder filler PW separately from the substrate BS. In this case, the wavelength converter  10  may include a metallic multilayer film on the surface to face the substrate BS and may be joined to the substrate BS by soldering. 
     The multilayer film may include, for example, thin films of titanium (Ti), platinum (Pt), and gold (Au) stacked in this order from the wavelength converter  10 . The multilayer film may be formed by, for example, sputtering or vapor deposition to have a thickness of several to several hundred nanometers. 
     Ti can bond to oxide glass tightly. Au has high wettability with a solder material. The Pt film serves as a barrier that allows the Ti film to be less likely to peel off the wavelength converter  10  with the solder material being melted. Instead of the film of Ti/Pt/Au, the multilayer film may include a film of chromium (Cr)/Pt/Au or Cr/nickel (Ni)/Au. For example, the Ti film may have a thickness of about 0.1 μm, the Pt film may have a thickness of about 0.2 Ξm, and the Au film may have a thickness of about 0.2 μm. 
     The solder material may be tin (Sn)-phosphorus (P)-copper (Cu) solder or Au-Sn solder. 
     The multilayer film may further provide the advantages below. As described above, in the present embodiment, the wavelength converter  10  includes the voids  13  in a cross section as illustrated in  FIG.  2   . The wavelength converter  10  includes the voids  13  also in the surface facing the substrate BS. In other words, as illustrated in  FIG.  2   , the wavelength converter  10  has a surface F 2  (second surface) facing the substrate BS and having roughness with recesses defined by the voids  13  as described above. For the wavelength converter  10  including the multilayer film with the above thickness, the multilayer film is located on the surface of the wavelength converter  10  facing the substrate BS and having the roughness defined by the voids  13 . The multilayer film is located along the outer edges of the recesses. More specifically, as illustrated in  FIG.  10   , a multilayer film ML is located along the outer edges of the recesses defined by the voids  13  on the surface of the wavelength converter  10  facing the substrate BS (not illustrated). The roughness defined by the voids  13  has a roughness value smaller than the sizes of the phosphor particles. 
     The wavelength converter  10  with this structure has higher wettability with the solder material and can be joined to the substrate BS with higher strength.  FIG.  11    is a partial cross-sectional view of the wavelength converter  10  and the substrate BS, illustrating the boundary between them. The wavelength converter  10  and the substrate BS are separately prepared and joined together with solder. As illustrated in  FIG.  11   , the wavelength converter  10  includes the multilayer film ML on the surface facing the substrate BS, and the substrate BS also includes the multilayer film ML on its surface. A solder layer SD is between the wavelength converter  10  and the substrate BS. With this structure, the solder layer SD enters the voids  13 , thus increasing the anchor effect and the thermal conductivity. 
     In the example of  FIG.  2   , the wavelength converter  10  may be divided into a first portion extending in the thickness direction from the first surface to receive excitation light and a second portion extending in the thickness direction from the second surface facing the substrate BS. The first portion has a first porosity being the ratio of the total area of the voids  13  in a cross section of the first portion to the total area of the cross section of the first portion. The second portion has a second porosity being the ratio of the total area of the voids  13  in a cross section of the second portion to the total area of the cross section of the second portion. The first porosity may be higher than the second porosity. This increases the light emission from the first portion and increases the thermal conductivity in the second portion. The difference between the first porosity and the second porosity may be, for example, 2 to 15%. 
     The wavelength converter  10  may include the first portion and the second portion in a manner different from those in the above example. The first portion may be any portion extending from the first surface in the thickness direction. The second portion may be any portion extending from the second surface in the thickness direction. In other words, a portion (first portion) adjacent to the first surface and a portion (second portion) adjacent to the second surface may simply have porosities satisfying the above condition. For example, the first portion may extend across a half or more of the entire thickness of the wavelength converter  10 . More specifically, the first portion may extend from the first surface in the thickness direction across about 40% of the entire thickness of the wavelength converter  10 , and the second portion may extend from the second surface in the thickness direction across about 60% of the entire thickness of the wavelength converter  10 . The wavelength converter  10  with this structure has appropriate thermal conductivity and appropriate light emission. 
     The wavelength converter  10  including the first portion and the second portion as described above can be produced with various methods, such as bonding a first wavelength converter sheet and a second wavelength converter sheet with different porosities. 
     Variations 
     In the embodiment illustrated in  FIG.  1   , the wavelength conversion element  100  includes the substrate BS being a plate and the wavelength converter  10  located on the upper surface F 1  or a main surface of the substrate BS. However, the wavelength conversion element may have another structure as in, for example, a wavelength conversion element  101  illustrated in  FIG.  12   . 
     The wavelength conversion element  101  illustrated in  FIG.  12    includes a substrate BS 1  being a disk with a recess RP on its upper main surface at the center. The recess RP is circular in a plan view. The recess RP receives a wavelength converter  10 . 
     With this structure, excitation light scattered or absorbed in the wavelength converter  10  without undergoing wavelength conversion can be reflected by the side surface of the recess RP on the substrate BS 1  to return to the wavelength converter  10  for wavelength conversion. In other words, the wavelength converter  10  can perform wavelength conversion on the returning light. 
     The wavelength converter  10  has a side surface in contact with the side surface of the recess RP. This structure allows heat from the wavelength converter  10  to be dissipated outside through the side surface of the recess RP and the substrate BSI. This reduces the temperature rise of the wavelength converter  10 . 
     Manufacturing Method 
       FIG.  13    is a flowchart showing a variation of the processing for manufacturing the wavelength converter  10  with powder pressing. The method for manufacturing the wavelength conversion element  101  will now be described with reference to  FIG.  13   . 
     First, the substrate BS 1  as illustrated in  FIGS.  14  and  15    is prepared (step S 11 ).  FIG.  14    is a general perspective view of the substrate BS 1 .  FIG.  15    is a cross-sectional view of the substrate BS 1 . The substrate BS 1  includes the recess RP with a depth of 0.01 to 1 mm. The substrate BS 1  has a thickness of 0.05 to 10 mm below the bottom of the recess RP. The recess RP has a depth greater than the thickness of the wavelength converter  10 . For the wavelength converter  10  having a thickness of 0.1 mm, for example, the recess RP has a depth of 0.15 to 0.2 mm. The substrate BS 1  measures 0.5 to 30 mm across. The recess RP measures 0.1 to 10 mm across. 
     The substrate BS 1  may be made of a material having a coefficient of thermal expansion close to that of the wavelength converter  10 . More specifically, the substrate BS 1  may be made of a metal material or an inorganic material having a coefficient of thermal expansion of ±50% of the coefficient of thermal expansion of the low-melting glass as the molten glass in the wavelength converter  10 . For example, the substrate BS 1  may be made of aluminum or an aluminum alloy as a metal material. 
     The substrate BS 1  made of, for example, aluminum as a reflective material may include an optically reflective film on its surface, or may have a mirror finish through physical or chemical polishing. This increases the reflectance of the substrate BS 1 . 
     The substrate BS 1  made of a metal material may be formed by machining including cutting or by molding including die casting. 
     The substrate BS 1  may also be made of a ceramic material. In this case, the substrate BS 1  may be formed by stacking ceramic layers or by powder pressing. 
     The method for forming the substrate BS 1  by stacking ceramic layers may include stacking ring-shaped green sheets with a through-hole (corresponding to the recess RP) and disk-shaped green sheets without a through-hole on one another and sintering the stacked sheets. 
     The method for forming the substrate BS 1  by powder pressing may include filling a die having a cylindrical hole with ceramic powder and pressing the ceramic powder with another die for forming a recess (corresponding to the recess RP). In this case, the ceramic powder is mixed with wax and a binder and pressed to form a compact including a recess (corresponding to the recess RP). The compact is then sintered to form the substrate BS 1 . 
     Examples of the ceramic for the substrate BS 1  include alumina, aluminum nitride, silicon nitride, mullite, and zirconia. 
     The substrate BS 1  may also be made of ceramic and metal. More specifically, a metallic ring (metallic member) made of, for example, aluminum or an aluminum alloy may be joined to a disk-shaped ceramic substrate with a bond to form the substrate BS 1 . The ring has a through-hole to be the recess RP. The bond may be a brazing material mainly containing silver (Ag) and copper (Cu). In some embodiments, the bond may be a brazing material mainly containing aluminum (Al), or may be solder mainly containing tin (Sn), silver (Ag), and copper (Cu), or may be a resin bond containing, for example, an epoxy resin, a silicone resin, or an acrylic resin. To have sufficient thermal conductivity, the resin bond may contain a highly thermally conductive filler, such as silver (Ag), aluminum nitride (AlN), or boron nitride (BN). 
     Further, the substrate BS 1  may be formed by joining a ceramic ring to a ceramic substrate with a bond, instead of stacking ceramic layers and firing them together. This method may also use any of the above bonds. 
     Referring back to  FIG.  13   , a powder filler is formed by filling the recess RP on the substrate BS 1  with phosphor powder and glass powder (step S 12 ). The phosphor powder includes one or more types of powdery phosphor. The one or more types of powdery phosphor herein correspond to the phosphor  11  ( FIG.  2   ) in the above embodiment and may include a red phosphor, a green phosphor, or a blue phosphor. The materials, the compositions, the mixing methods, and other features of the phosphor and the powder filler have been described in the above embodiment, and will not be described repeatedly. 
     The powder filler in the recess RP on the substrate BS 1  is heated together with the substrate BS 1  to form a presintered compact (step S 13 ). 
     After the heating temperature reaches a set temperature, pressure is applied to the presintered compact on the substrate B S 1  maintained at the set temperature (step S 14 ). 
     The heating of the powder filler and the application of pressure to the powder filler have been described in the above embodiment, and will not be described repeatedly. 
     The presintered compact is then cooled under the pressure applied being maintained (step S 15 ). The pressure is released in response to the temperature of the presintered compact dropping below the melting point of the glass powder. 
     The wavelength conversion element  101  formed through the above steps can include voids in the wavelength converter  10  as intended. The resulting wavelength conversion element  101  thus has high heat resistance. 
     The substrate BS 1  includes the recess RP in which the powder filler is formed. This eliminates step S 2  for placing a mask on the substrate and step S 4  for removing the mask, unlike the manufacturing method in the embodiment described with reference to  FIG.  4   , thus simplifying the manufacturing processes. 
     In the wavelength conversion element  101  illustrated in  FIG.  12   , the substrate BS 1  is a disk with the recess RP in its upper surface, and the recess RP is circular in a plan view. However, the substrate BS 1  is not limited to a disk, and the recess RP is not limited to being circular in a plan view. 
     The above embodiments may be changed or omitted as appropriate within the scope of the present disclosure.