Patent Publication Number: US-2022223492-A1

Title: Copper/ceramic joined body, insulating circuit substrate, copper/ceramic joined body production method, and insulating circuit substrate production method

Description:
TECHNICAL FIELD 
     The present invention relates to a copper/ceramic bonded body in which a copper member made of copper or a copper alloy and a ceramic member are bonded to each other, an insulating circuit substrate in which a copper sheet made of copper or a copper alloy is bonded to a surface of a ceramic substrate, a method for producing a copper/ceramic bonded body, and a method for producing an insulating circuit substrate. 
     The present application claims priority on Japanese Patent Application No. 2019-151143 filed on Aug. 21, 2019, and Japanese Patent Application No. 2020-134035 filed on Aug. 6, 2020, the contents of which are incorporated herein by reference. 
     BACKGROUND ART 
     A power module, an LED module, and a thermoelectric module have a structure in which a power semiconductor element, an LED element, and a thermoelectric element are bonded to an insulating circuit substrate, and in the insulating circuit substrate, a circuit layer made of a conductive material is formed on one surface of an insulating layer. 
     For example, a power semiconductor element for high-power control used for controlling a wind power generation, an electric vehicle, a hybrid vehicle, or the like has a large amount of heat generated during operation. Therefore, as a substrate on which the power semiconductor element is mounted, an insulating circuit substrate including a ceramic substrate and a circuit layer formed by bonding a metal sheet having excellent conductivity to one surface of the ceramic substrate has been widely used in the related art. As the insulating circuit substrate, one including a metal layer formed by bonding a metal sheet to the other surface of the ceramic substrate is also provided. 
     For example, Patent Document 1 proposes an insulating circuit substrate in which a circuit layer and a metal layer are formed by bonding a copper sheet to each of one surface and the other surface of a ceramic substrate. In Patent Document 1, the copper sheet is disposed on each of one surface and the other surface of the ceramic substrate with an Ag—Cu—Ti-based brazing material interposed therebetween, and the copper sheet is bonded thereto by performing a heating treatment (so-called active metal brazing method). In the active metal brazing method, since the brazing material containing Ti as an active metal is used, the wettability between the molten brazing material and the ceramic substrate is improved, and the ceramic substrate and the copper sheet are satisfactorily bonded to each other. 
     Patent Document 2 proposes an insulating circuit substrate in which a ceramic substrate and a copper sheet are bonded to each other by using a Cu—Mg—Ti-based brazing material. 
     In Patent Document 2, the ceramic substrate and the copper sheet are bonded to each other by heating at a temperature of 560° C. to 800° C. in a nitrogen gas atmosphere, and Mg in a Cu—Mg—Ti alloy is sublimated and Mg does not remain at a bonded interface, while titanium nitride (TiN) is not substantially formed. 
     PRIOR ART DOCUMENTS 
     Patent Document 
     
         
         Patent Document 1: Japanese Patent No. 3211856 
         Patent Document 2: Japanese Patent No. 4375730 
       
    
     DISCLOSURE OF INVENTION 
     Problems to be Solved by the Invention 
     By the way, in the circuit layer of the above-described insulating circuit substrate, a terminal material or the like may be subjected to ultrasonic welding. 
     In the insulating circuit substrates disclosed in Patent Documents 1 and 2, when ultrasonic waves are applied to bond the terminal material or the like, cracks are generated at the bonded interface, and there is a concern that the circuit layer may be peeled. 
     The present invention has been made in view of the above-described circumstances, and an object of the present invention is to provide a copper/ceramic bonded body, an insulating circuit substrate, a method for producing a copper/ceramic bonded body, and a method for producing an insulating circuit substrate, which can suppress peeling of a copper member from a ceramic member and generation of cracks in the ceramic member even when ultrasonic welding is performed. 
     Solutions for Solving the Problems 
     In order to solve the above-described problem, a copper/ceramic bonded body according to one aspect of the present invention (hereinafter, referred to as a “copper/ceramic bonded body according to the present invention”) includes a copper member made of copper or a copper alloy, and a ceramic member made of silicon nitride, in which the copper member and the ceramic member are bonded to each other, a Mg—N compound phase extending from a ceramic member side to a copper member side is present at a bonded interface between the copper member and the ceramic member, and at least a part of the Mg—N compound phase enters into the copper member. 
     According to the copper/ceramic bonded body according to the present invention, the Mg—N compound phase extending from the ceramic member side to the copper member side is present at the bonded interface between the copper member and the ceramic member, and at least a part of the Mg—N compound phase enters into the copper member. Therefore, Mg as a bonding material is sufficiently reacted with nitrogen of the ceramic member, and the copper member and the ceramic member are firmly bonded to each other. By an anchor effect of the Mg—N compound phase that has entered into the copper member, peeling of the copper member from the ceramic member and generation of cracks in the ceramic member can be suppressed even when ultrasonic waves are applied. 
     In the copper/ceramic bonded body according to the present invention, it is preferable that in a unit length along a bonded interface, a number density of the Mg—N compound phases having a longitudinal direction length of 100 nm or more is 8 pieces/μm or more. 
     In this case, since the number of the Mg—N compound phase sufficiently grown from the ceramic member side to the copper member side is ensured, the anchor effect of the Mg—N compound phase that has entered into the copper member can be reliably obtained, and even when ultrasonic waves are applied, peeling of the copper member from the ceramic member and generation of cracks in the ceramic member can be further suppressed. 
     In the copper/ceramic bonded body according to the present invention, it is preferable that a Si concentration in the Mg—N compound phase is 25 atomic % or less. 
     In this case, since local precipitation of a Si single phase in the Mg—N compound phase is suppressed, and the strength of the Mg—N compound phase is sufficiently ensured, the anchor effect of the Mg—N compound phase that has entered into the copper member can be reliably obtained, and even when ultrasonic waves are applied, peeling of the copper member from the ceramic member and generation of cracks in the ceramic member can be further suppressed. 
     An insulating circuit substrate according to another aspect of the present invention (hereinafter, referred to as an “insulating circuit substrate according to the present invention”) includes a copper sheet made of copper or a copper alloy, and a ceramic substrate made of silicon nitride, in which the copper sheet is bonded to a surface of the ceramic substrate, a Mg—N compound phase extending from a ceramic substrate side to a copper sheet side is present at a bonded interface between the copper sheet and the ceramic substrate, and at least a part of the Mg—N compound phase enters into the copper sheet. 
     According to the insulating circuit substrate according to the present invention, the Mg—N compound phase extending from the ceramic substrate side to the copper sheet side is present at the bonded interface between the copper sheet and the ceramic substrate, and at least a part of the Mg—N compound phase enters into the copper sheet. Therefore, Mg as a bonding material is sufficiently reacted with nitrogen of the ceramic substrate, and the copper sheet and the ceramic substrate are firmly bonded to each other. By an anchor effect of the Mg—N compound phase that has entered into the copper sheet, peeling of the copper sheet from the ceramic substrate and generation of cracks in the ceramic substrate can be suppressed even when ultrasonic waves are applied. 
     In the insulating circuit substrate according to the present invention, it is preferable that in a unit length along a bonded interface, a number density of the Mg—N compound phases having a longitudinal direction length of 100 nm or more is 8 pieces/μm or more. 
     In this case, since the number of the Mg—N compound phase sufficiently grown from the ceramic substrate side to the copper sheet side is ensured, the anchor effect of the Mg—N compound phase that has entered into the copper sheet can be reliably obtained, and even when ultrasonic waves are applied, peeling of the copper sheet from the ceramic substrate and generation of cracks in the ceramic substrate can be further suppressed. 
     In the insulating circuit substrate according to the present invention, it is preferable that a Si concentration in the Mg—N compound phase is 25 atomic % or less. 
     In this case, since local precipitation of a Si single phase in the Mg—N compound phase is suppressed, and the strength of the Mg—N compound phase is sufficiently ensured, the anchor effect of the Mg—N compound phase that has entered into the copper sheet can be reliably obtained, and even when ultrasonic waves are applied, peeling of the copper sheet from the ceramic substrate and generation of cracks in the ceramic substrate can be further suppressed. 
     A method for producing a copper/ceramic bonded body according to still another aspect of the present invention (hereinafter, referred to as a “method for producing a copper/ceramic bonded body according to the present invention”) is a method for producing the copper/ceramic bonded body described above, the method includes a Mg disposing step of disposing Mg between the copper member and the ceramic member, a laminating step of laminating the copper member and the ceramic member with Mg interposed therebetween, and a bonding step of performing a heating treatment on the laminated copper member and ceramic member with Mg interposed therebetween in a state of being pressed in a lamination direction under a vacuum atmosphere to bond the copper member and the ceramic member to each other, in which, in the Mg disposing step, an amount of Mg is set to be in a range of 0.34 mg/cm 2  or more and 4.35 mg/cm 2  or less, and in the bonding step, a temperature increase rate in a temperature range of 480° C. or higher and lower than 650° C. is set to be 5° C./min or higher, and heating is held at a temperature of 650° C. or higher for 30 minutes or longer. 
     According to the method for producing a copper/ceramic bonded body having the configuration, in the Mg disposing step, the amount of Mg is set to be in the range of 0.34 mg/cm 2  or more and 4.35 mg/cm 2  or less. Therefore, a sufficient Cu—Mg liquid phase required for an interfacial reaction can be obtained. Accordingly, the copper member and the ceramic member can be reliably bonded to each other. 
     In the bonding step, the temperature increase rate in the temperature range of 480° C. or higher and lower than 650° C. is set to 5° C./min or higher, and heating is held at the temperature of 650° C. or higher for 30 minutes or longer. Therefore, the Cu—Mg liquid phase required for the interfacial reaction can be held for a certain period of time or longer, a uniform interfacial reaction can be promoted, and the Mg—N compound phase extending from the ceramic member side to the copper member side can be reliably formed. 
     A method for producing an insulating circuit substrate according to still another aspect of the present invention (hereinafter, referred to as a “method for producing an insulating circuit substrate according to the present invention”) is a method for producing the insulating circuit substrate described above, the method includes a Mg disposing step of disposing Mg between the copper sheet and the ceramic substrate, a laminating step of laminating the copper sheet and the ceramic substrate with Mg interposed therebetween, and a bonding step of performing a heating treatment on the laminated copper sheet and ceramic substrate with Mg interposed therebetween in a state of being pressed in a lamination direction under a vacuum atmosphere to bond the copper sheet and the ceramic substrate to each other, in which, in the Mg disposing step, an amount of Mg is set to be in a range of 0.34 mg/cm 2  or more and 4.35 mg/cm 2  or less, and in the bonding step, a temperature increase rate in a temperature range of 480° C. or higher and lower than 650° C. is set to be 5° C./min or higher, and heating is held at a temperature of 650° C. or higher for 30 minutes or longer. 
     According to the method for producing an insulating circuit substrate having the configuration, in the Mg disposing step, the amount of Mg is set to be in the range of 0.34 mg/cm 2  or more and 4.35 mg/cm 2  or less. Therefore, a sufficient Cu—Mg liquid phase required for an interfacial reaction can be obtained. Accordingly, the copper sheet and the ceramic substrate can be reliably bonded to each other. 
     In the bonding step, the temperature increase rate in the temperature range of 480° C. or higher and lower than 650° C. is set to 5° C./min or higher, and heating is held at the temperature of 650° C. or higher for 30 minutes or longer. Therefore, the Cu—Mg liquid phase required for the interfacial reaction can be held for a certain period of time or longer, a uniform interfacial reaction can be promoted, and the Mg—N compound phase extending from the ceramic substrate side to the copper sheet side can be reliably formed. 
     Effects of Invention 
     According to the present invention, it is possible to provide a copper/ceramic bonded body, an insulating circuit substrate, a method for producing a copper/ceramic bonded body, and a method for producing an insulating circuit substrate, which can suppress peeling of a copper member from a ceramic member and generation of cracks in the ceramic member even when ultrasonic welding is performed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic explanatory view of a power module using an insulating circuit substrate according to an embodiment of the present invention. 
         FIG. 2A  is an observation result (BF STEM image) of a bonded interface between a circuit layer (metal layer) and a ceramic substrate of the insulating circuit substrate according to the embodiment of the present invention. 
         FIG. 2B  is an observation result (oxygen distribution diagram) of the bonded interface between the circuit layer (metal layer) and the ceramic substrate of the insulating circuit substrate according to the embodiment of the present invention. 
         FIG. 3  is a flowchart of a method for producing the insulating circuit substrate according to the embodiment of the present invention. 
         FIG. 4  is a schematic explanatory view of the method for producing the insulating circuit substrate according to the embodiment of the present invention. 
     
    
    
     EMBODIMENTS FOR CARRYING OUT THE INVENTION 
     Hereinafter, an embodiment of the present invention will be described with reference to the accompanying drawings. 
     A copper/ceramic bonded body to the present embodiment is an insulating circuit substrate  10  formed by bonding a copper sheet  22  (circuit layer  12 ) and a copper sheet  23  (metal layer  13 ) as copper members made of copper or a copper alloy to a ceramic substrate  11  as a ceramic member made of ceramics.  FIG. 1  shows a power module  1  including an insulating circuit substrate  10  according to the present embodiment. 
     The power module  1  includes the insulating circuit substrate  10  on which the circuit layer  12  and the metal layer  13  are disposed, a semiconductor element  3  bonded to one surface (upper surface in  FIG. 1 ) of the circuit layer  12  with a bonding layer  2  interposed therebetween, and a heat sink  30  disposed on the other side (lower side in  FIG. 1 ) of the metal layer  13 . 
     The semiconductor element  3  is made of a semiconductor material such as Si. The semiconductor element  3  and the circuit layer  12  are bonded to each other with the bonding layer  2  interposed therebetween. 
     The bonding layer  2  is made of, for example, a Sn—Ag-based, Sn—In-based, or Sn—Ag—Cu-based solder material. 
     The heat sink  30  dissipates heat from the above-mentioned insulating circuit substrate  10 . The heat sink  30  is made of copper or a copper alloy, and in the present embodiment, the heat sink  30  is made of phosphorus deoxidized copper. The heat sink  30  is provided with a passage  31  through which a cooling fluid flows. 
     In the present embodiment, the heat sink  30  and the metal layer  13  are bonded to each other by a solder layer  32  made of a solder material. The solder layer  32  is made of, for example, a Sn—Ag-based, Sn-ln-based, or Sn—Ag—Cu-based solder material. 
     As shown in  FIG. 1 , the insulating circuit substrate  10  of the present embodiment includes the ceramic substrate  11 , the circuit layer  12  disposed on one surface (upper surface in  FIG. 1 ) of the ceramic substrate  11 , and the metal layer  13  disposed on the other surface (lower surface in  FIG. 1 ) of the ceramic substrate  11 . 
     The ceramic substrate  11  is made of silicon nitride (Si 3 N 4 ) having excellent insulating properties and heat radiation. The thickness of the ceramic substrate  11  is set to be in a range of, for example, 0.2 mm or more and 1.5 mm or less, and in the present embodiment, the thickness is set to 0.32 mm 
     As shown in  FIG. 4 , the circuit layer  12  is formed by bonding the copper sheet  22  made of copper or a copper alloy to one surface (upper surface in  FIG. 4 ) of the ceramic substrate  11 . 
     In the present embodiment, the circuit layer  12  is formed by bonding the copper sheet  22  made of a rolled sheet of oxygen-free copper to the ceramic substrate  11 . 
     The thickness of the copper sheet  22  serving as the circuit layer  12  is set to be in a range of 0.1 mm or more and 2.0 mm or less, and in the present embodiment, the thickness is set to 0.6 mm 
     As the copper sheet  22 , tough pitch copper can also be used. 
     As shown in  FIG. 4 , the metal layer  13  is formed by bonding the copper sheet  23  made of copper or a copper alloy to the other surface (lower surface in  FIG. 4 ) of the ceramic substrate  11 . 
     In the present embodiment, the metal layer  13  is formed by bonding the copper sheet  23  made of a rolled sheet of oxygen-free copper to the ceramic substrate  11 . 
     The thickness of the copper sheet  23  serving as the metal layer  13  is set to be in a range of 0.1 mm or more and 2.0 mm or less, and in the present embodiment, the thickness is set to 0.6 mm. 
     As the copper sheet  23 , tough pitch copper can also be used. 
     At a bonded interface between the ceramic substrate  11  and the circuit layer  12  (metal layer  13 ), as shown in  FIG. 2A , a Mg—N compound phase  41  extending from the ceramic substrate  11  side to the circuit layer  12  (metal layer  13 ) side is present. At least a part of the Mg—N compound phase  41  enters into the circuit layer  12  (metal layer  13 ). The Mg—N compound phase  41  is formed by reacting magnesium (Mg) used as a bonding material with nitrogen (N) contained in the ceramic substrate  11 . 
     The Mg—N compound phase  41  may be a region where Mg and N coexist, in which the concentration of Mg is 40 atomic % or more and 65 atomic % or less with respect to 100 atomic % of the total of Mg, N, and Si, and the aspect ratio (longitudinal direction length/lateral direction length) of the region is 1.2 or more. 
     In the present embodiment, it is preferable that in a unit length along the bonded interface, the number density of the Mg—N compound phases  41  having the longitudinal direction length of 100 nm or more is 8 pieces/μm or more. An auxiliary line L in  FIG. 2A  shows the bonded interface. In the present embodiment, the detection position of oxygen is set as the bonded interface (see  FIG. 2B ). 
     The number density of the Mg—N compound phases  41  having the longitudinal direction length of 100 nm or more is preferably 10 pieces/μm or more, and more preferably 12 pieces/μm or more. 
     Hereinafter, a method for producing the insulating circuit substrate  10  according to the present embodiment will be described with reference to  FIGS. 3 and 4 . 
     (Mg Disposing Step S 01 ) 
     First, the ceramic substrate  11  made of silicon nitride (Si 3 N 4 ) is prepared, and as shown in  FIG. 4 , Mg is disposed between the copper sheet  22  serving as the circuit layer  12  and the ceramic substrate  11 , and between the copper sheet  23  serving as the metal layer  13  and the ceramic substrate  11 . 
     In the present embodiment, a Mg foil  25  is disposed between the copper sheet  22  serving as the circuit layer  12  and the ceramic substrate  11 , and between the copper sheet  23  serving as the metal layer  13  and the ceramic substrate  11 . 
     In a Mg disposing step S 01 , the amount of Mg to be disposed is set to be in a range of 0.34 mg/cm 2  or more and 4.35 mg/cm 2  or less. 
     The amount of Mg to be disposed is preferably 0.52 mg/cm 2  or more, and more preferably 0.69 mg/cm 2  or more. On the other hand, the amount of Mg to be disposed is preferably 3.48 mg/cm 2  or less, and more preferably 2.61 mg/cm 2  or less. 
     (Laminating Step S 02 ) 
     Next, the copper sheet  22  and the ceramic substrate  11  are laminated with the Mg foil  25  interposed therebetween, and the ceramic substrate  11  and the copper sheet  23  are laminated with the Mg foil  25  interposed therebetween. 
     (Bonding Step S 03 ) 
     Next, the copper sheet  22 , the Mg foil  25 , the ceramic substrate  11 , the Mg foil  25 , and the copper sheet  23  which are laminated are pressed in a lamination direction, and are loaded into a vacuum furnace and heated such that the copper sheet  22 , the ceramic substrate  11 , and the copper sheet  23  are bonded together. 
     A heating treatment condition in the bonding step S 03  is such that a temperature increase rate in a temperature range of 480° C. or higher and lower than 650° C. is set to be 5° C./min or higher and that heating is held at a temperature of 650° C. or higher for 30 minutes or longer. By defining the heating treatment condition in this way, the Cu—Mg liquid phase can be maintained in a high temperature state, the interfacial reaction is promoted, whereby the Mg—N compound phase  41  is formed. 
     The temperature increase rate in the temperature range of 480° C. or higher and lower than 650° C. is preferably 7° C./min or higher, and more preferably 9° C./min or higher. On the other hand, the temperature increase rate in the temperature range of 480° C. or higher and lower than 650° C. is preferably 15° C./min or lower, and more preferably 12° C./min or lower. 
     A holding temperature is preferably 700° C. or higher, and more preferably 750° C. or higher. On the other hand, the holding temperature is preferably 850° C. or lower, and more preferably 830° C. or lower. 
     A holding time is preferably 45 minutes or longer, and more preferably 60 minutes or longer. On the other hand, the holding time is preferably 180 minutes or shorter, and more preferably 150 minutes or shorter. 
     A pressing load in the bonding step S 03  is preferably in a range of 0.049 MPa or more and 3.4 MPa or less. 
     A degree of vacuum in the bonding step S 03  is preferably in a range of 1×10 −6  Pa or more and 5×10 −2  Pa or less. 
     As described above, the insulating circuit substrate  10  according to the present embodiment is produced by the Mg disposing step S 01 , the laminating step S 02 , and the bonding step S 03 . 
     (Heat Sink Bonding Step S 04 ) 
     Next, the heat sink  30  is bonded to the other surface side of the metal layer  13  of the insulating circuit substrate  10 . 
     The insulating circuit substrate  10  and the heat sink  30  are laminated with a solder material interposed therebetween and are loaded into a heating furnace such that the insulating circuit substrate  10  and the heat sink  30  are solder-bonded to each other with the solder layer  32  interposed therebetween. 
     (Semiconductor Element-Bonding Step S 05 ) 
     Next, the semiconductor element  3  is bonded to one surface of the circuit layer  12  of the insulating circuit substrate  10  by soldering. 
     The power module  1  shown in  FIG. 1  is produced by the above steps. 
     According to the insulating circuit substrate  10  (copper/ceramic bonded body) of the present embodiment having the above-described configuration, since the Mg—N compound phase  41  extending from the ceramic substrate  11  side to the circuit layer  12  (and the metal layer  13 ) is present at the bonded interface between the circuit layer  12  (and the metal layer  13 ) and the ceramic substrate  11 , and at least a part of the Mg—N compound phase  41  enters into the circuit layer  12  (and the metal layer  13 ), Mg as a bonding material is sufficiently reacted with nitrogen of the ceramic substrate  11 , and the circuit layer  12  (and the metal layer  13 ) and the ceramic substrate  11  are firmly bonded to each other. By the anchor effect of the Mg—N compound phase  41  that has entered into the circuit layer  12  (and the metal layer  13 ), even when ultrasonic waves are applied to the insulating circuit substrate  10  (copper/ceramic bonded body) for ultrasonic welding to bond the terminal material such as copper to the circuit layer  12  (metal layer  13 ), peeling of the circuit layer  12  (metal layer  13 ) from the ceramic substrate  11  and generation of cracks in the ceramic substrate  11  can be suppressed. 
     In the insulating circuit substrate  10  of the present embodiment, when the number density of the Mg—N compound phases  41  having the longitudinal direction length of 100 nm or more in a unit length along the bonded interface is 8 pieces/μm or more, the number of the Mg—N compound phase  41  sufficiently grown from the ceramic substrate  11  side to the circuit layer  12  (and the metal layer  13 ) side is ensured. Therefore, the anchor effect of the Mg—N compound phase  41  that has entered into the circuit layer  12  (and the metal layer  13 ) can be reliably obtained, and even when ultrasonic waves are applied, peeling of the circuit layer  12  (and the metal layer  13 ) from the ceramic substrate  11  and generation of cracks in the ceramic substrate  11  can be further suppressed. 
     In the insulating circuit substrate  10  of the present embodiment, it is preferable that the Si concentration in the Mg—N compound phase  41  is 25 atomic % or less. The Si concentration can be, for example, 9.7 atomic % or more. 
     In this case, since local precipitation of a Si single phase in the Mg—N compound phase  41  is suppressed, and the strength of the Mg—N compound phase  41  is sufficiently ensured, the anchor effect of the Mg—N compound phase  41  that has entered into the circuit layer  12  (and/or the metal layer  13 ) can be reliably obtained, and even when ultrasonic waves are applied, peeling of the copper sheet from the ceramic substrate and generation of cracks in the ceramic substrate can be further suppressed. 
     According to the method for producing the insulating circuit substrate  10  (copper/ceramic bonded body) of the present embodiment, in the Mg disposing step S 01 , since the amount of Mg to be disposed between the copper sheet  22  (copper sheet  23 ) and the ceramic substrate  11  is set to be in a range of 0.34 mg/cm 2  or more and 4.35 mg/cm 2  or less, a sufficient Cu—Mg liquid phase required for the interfacial reaction can be obtained. Accordingly, the copper sheet  22  (copper sheet  23 ) and the ceramic substrate  11  can be reliably bonded to each other, and the bonding strength between the circuit layer  12  (and the metal layer  13 ) and the ceramic substrate  11  can be ensured. 
     In the bonding step S 03 , the temperature increase rate in the temperature range of 480° C. or higher and lower than 650° C. is set to be 5° C./min or higher, and heating is held at the temperature of 650° C. or higher for 30 minutes or longer. Therefore, the Cu—Mg liquid phase required for the interfacial reaction can be held for a certain period of time or longer between the copper sheet  22  (copper sheet  23 ) and the ceramic substrate  11 , a uniform interfacial reaction can be promoted, and the Mg—N compound phase  41  can be reliably formed at the bonded interface between the circuit layer  12  (and the metal layer  13 ) and the ceramic substrate  11 . 
     The embodiment of the present invention has been described, but the present invention is not limited thereto, and can be appropriately changed without departing from the technical features of the present invention. 
     For example, in the present embodiment, the semiconductor element is mounted on the insulating circuit substrate to form the power module, but the present embodiment is not limited thereto. For example, an LED element may be mounted on the circuit layer of the insulating circuit substrate to form an LED module, or a thermoelectric element may be mounted on the circuit layer of the insulating circuit substrate to form a thermoelectric module. 
     In the insulating circuit substrate of the present embodiment, it has been described that both of the circuit layer and the metal layer are made of copper sheets made of copper or a copper alloy, but the present invention is not limited thereto. 
     For example, in a case where the circuit layer and the ceramic substrate are made of the copper/ceramic bonded body according to the present invention, there is no limitation on the material and the bonding method of the metal layer. There may be no metal layer, the metal layer may be made of aluminum or an aluminum alloy, or may be made of a laminate of copper and aluminum. 
     On the other hand, in a case where the metal layer and the ceramic substrate are made of the copper/ceramic bonded body according to the present invention, there is no limitation on the material and the bonding method of the circuit layer. The circuit layer may be made of aluminum or an aluminum alloy, or may be made of a laminate of copper and aluminum. 
     In the present embodiment, it has been described that the Mg foil is laminated between the copper sheet and the ceramic substrate in the Mg disposing step, but the present invention is not limited thereto, and a thin film made of Mg may be formed on the bonding surface of the ceramic substrate and the copper sheet by a sputtering method, a vapor deposition method, or the like. In addition, a paste using Mg or MgH 2  may be applied. 
     EXAMPLES 
     Hereinafter, results of confirmation experiments performed to confirm the effects of the present invention will be described. 
     Example 1 
     First, a ceramic substrate (40 mm×40 mm×0.32 mm) made of silicon nitride (Si 3 N 4 ) was prepared. 
     A copper sheet (37 mm×37 mm×thickness of 0.5 mm) made of oxygen-free copper was bonded to both surfaces of the ceramic substrate under the conditions shown in Table 1 to obtain an insulating circuit substrate (copper/ceramic bonded body) of Invention Examples 1 to 9 and Comparative Example 1. A degree of vacuum of a vacuum furnace at the time of bonding was set to 2×10 −3  Pa. 
     The obtained insulating circuit substrate (copper/ceramic bonded body) was evaluated for the presence or absence of the Mg—N compound phase at the bonded interface, the number density of the Mg—N compound phases having the longitudinal direction length of 100 nm or more, the initial bonding rate, and the ultrasonic welding as follows. 
     (Mg—N Compound Phase) 
     An observation specimen was collected from the central portion of the obtained insulating circuit substrate (copper/ceramic bonded body), and the bonded interface between the copper sheet and the ceramic substrate was observed in a range of 2 μm×2 μm at an acceleration voltage of 200 kV and a magnification of 20,000 by using a transmission electron microscope (Titan ChemiSTEM manufactured by FEI Company), and when a region where Mg and N coexisted was present, the concentration of Mg was 40 atomic % or more and 65 atomic % or less with respect to 100 atomic % of the total of Mg, N, and Si, and the aspect ratio (longitudinal direction length/lateral direction length) of the region was 1.2 or more, the Mg—N compound phase was determined to be “present”. 
     In the same measurement field, in a unit length along the bonded interface, the number density of the Mg—N compound phases having the longitudinal direction length of 100 nm or more was calculated. 
     In the measurement of the number density of the Mg—N compound phases having the longitudinal direction length of 100 nm or more, the detection position of the oxygen was set as the bonded interface between the copper sheet and the ceramic substrate. The number density was calculated from the following equation. 
       (Number density)=(total number of Mg—N compound phases having longitudinal direction length of 100 nm or more in measurement field)/(length of bonded interface in measurement field)
 
     The Mg—N compound phases which were present at the boundary portion of the measurement field and could not be grasped as a whole were excluded from the number. 
     The number density was measured in 5 fields, and the average value is shown in the tables. 
     (Initial Bonding Rate) 
     A bonding rate between the copper sheet and the ceramic substrate was evaluated. Specifically, in the insulating circuit substrate, a bonding rate at the interface between the copper sheet and the ceramic substrate was evaluated using an ultrasonic flaw detector (FineSAT200 manufactured by Hitachi Power Solutions Co., Ltd.) and was calculated from the following equation. An initial bonding area was an area to be bonded before bonding, that is, an area of the circuit layer. In an image obtained by binarizing an ultrasonic-detected image, peeling was indicated by a white portion in the bonding part, and thus the area of the white portion was regarded as an exfoliation area. 
       (Bonding rate)={(initial bonding area)−(non-bonded part area)}/(initial bonding area)×100
 
     (Evaluation of Ultrasonic Welding) 
     A copper terminal (10 mm×5 mm×1 mm in thickness) was bonded to the obtained insulating circuit substrate (copper/ceramic bonded body) by ultrasonic welding using an ultrasonic metal bonding machine (60C-904 manufactured by Ultrasonic Engineering Co., Ltd.) under the condition where a collapse amount was 0.3 mm. Ten copper terminals were bonded at a time. 
     After bonding, the bonded interface between the copper sheet and the ceramic substrate was inspected by using an ultrasonic flaw detector (FineSAT200 manufactured by Hitachi Solutions, Ltd.). A case where peeling of the copper sheet from the ceramic substrate or ceramic breaking was observed in 3 pieces or more out of 10 pieces was evaluated as “C”, a case where peeling of the copper sheet from the ceramic substrate or ceramic breaking was observed in 1 piece or more and 2 pieces or less out of 10 pieces was evaluated as “B”, and a case where peeling of the copper sheet from the ceramic substrate or ceramic breaking was not observed in all 10 pieces was evaluated as “A”. The evaluation results are shown in Table 1. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                 Bonded interface 
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 Mg disposing 
                 Bonding step 
                 Presence or 
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 step 
                   
                 Temperature 
                 Holding 
                 Holding 
                 absence of 
                 Number 
                 Initial 
                 Evaluation of 
               
               
                   
                 Amount of Mg 
                 Load 
                 increase rate * 1   
                 temperature 
                 time 
                 Mg—N 
                 density* 2   
                 bonding 
                 ultrasonic 
               
               
                   
                 mg/cm 2   
                 MPa 
                 ° C./min 
                 ° C. 
                 min 
                 compound phase 
                 (piece/μm) 
                 rate % 
                 welding 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 Invention 
                 0.70 
                 0.049 
                 5 
                 680 
                 150 
                 Present 
                 8.2 
                 96.1 
                 A 
               
               
                 Example 1 
               
               
                 Invention 
                 0.35 
                 0.98 
                 5 
                 650 
                 30 
                 Present 
                 4.6 
                 92.4 
                 B 
               
               
                 Example 2 
               
               
                 Invention 
                 0.35 
                 0.98 
                 25 
                 650 
                 30 
                 Present 
                 8.0 
                 94.2 
                 A 
               
               
                 Example 3 
               
               
                 Invention 
                 2.61 
                 0.98 
                 7 
                 800 
                 60 
                 Present 
                 33.9 
                 95.6 
                 A 
               
               
                 Example 4 
               
               
                 Invention 
                 0.70 
                 0.98 
                 5 
                 750 
                 90 
                 Present 
                 15.4 
                 95.2 
                 A 
               
               
                 Example 5 
               
               
                 Invention 
                 2.61 
                 0.98 
                 15 
                 850 
                 150 
                 Present 
                 39.2 
                 96.5 
                 A 
               
               
                 Example 6 
               
               
                 Invention 
                 4.35 
                 3.43 
                 25 
                 850 
                 120 
                 Present 
                 43.2 
                 95.5 
                 A 
               
               
                 Example 7 
               
               
                 Invention 
                 4.35 
                 0.49 
                 15 
                 800 
                 60 
                 Present 
                 36.2 
                 99.5 
                 A 
               
               
                 Example 8 
               
               
                 Invention 
                 0.70 
                 0.49 
                 7 
                 750 
                 90 
                 Present 
                 16.5 
                 98.7 
                 A 
               
               
                 Example 9 
               
               
                 Comparative 
                 0.70 
                 0.49 
                 2 
                 750 
                 90 
                 Absent 
                 0.0 
                 90.4 
                 C 
               
               
                 Example 1 
               
               
                   
               
               
                 * 1  Temperature increase rate: average temperature increase rate in temperature range of 480° C. or higher and lower than 650° C. 
               
               
                 * 2 Number density of Mg—N compound phases having length of 100 nm or more at bonded interface 
               
            
           
         
       
     
     In Comparative Example 1 in which the temperature increase rate in the temperature range of 480° C. or higher and lower than 650° C. was set to 2° C./min, the Mg—N compound phase was not formed at the bonded interface. Therefore, the initial bonding rate was low. In addition, when ultrasonic welding was performed, peeling of the copper sheet from the ceramic substrate or ceramic breaking was frequently observed. 
     On the other hand, in Invention Examples 1 to 9 in which the Mg—N compound phase was formed at the bonded interface, the initial bonding rate was high, and the ceramic substrate and the copper sheet could be firmly bonded to each other. Then, when ultrasonic welding was performed, peeling of the copper sheet from the ceramic substrate or ceramic breaking was small. 
     Example 2 
     As in Example 1, a ceramic substrate (40 mm×40 mm×0.32 mm) made of silicon nitride (Si 3 N 4 ) was prepared. 
     A copper sheet (37 mm×37 mm×thickness of 0.5 mm) made of oxygen-free copper was bonded to both surfaces of the ceramic substrate under the conditions shown in Table 2 to obtain an insulating circuit substrate (copper/ceramic bonded body) of Invention Examples 11 to 19. A degree of vacuum of a vacuum furnace at the time of bonding was set to 2×10 −3  Pa. 
     The obtained insulating circuit substrate (copper/ceramic bonded body) was evaluated for the presence or absence of the Mg—N compound phase at the bonded interface, the number density of the Mg—N compound phases having the longitudinal direction length of 100 nm or more, and the initial bonding rate as in Example 1. Further, the Si concentration and the number of defectives in the Mg—N compound phase were evaluated as follows. 
     (Si Concentration in Mg—N Compound Phase) 
     An observation specimen was collected from the central portion of the obtained insulating circuit substrate (copper/ceramic bonded body), and the bonded interface between the copper sheet and the ceramic substrate was observed in a range of 2 μm×2 μm at an acceleration voltage of 200 kV and a magnification of 20,000 by using a transmission electron microscope (Titan ChemiSTEM manufactured by FEI Company), and the Si concentration with respect to 100 atomic % of the total of Mg, N, and Si was measured in a region where Mg and N coexisted. The Si concentration was measured in 5 fields, and the average value is shown in Table 2. 
     (Number of Defectives) 
     A copper terminal (10 mm×5 mm×1.5 mm in thickness) was bonded to the obtained insulating circuit substrate (copper/ceramic bonded body) by ultrasonic welding using an ultrasonic metal bonding machine (60C-904 manufactured by Ultrasonic Engineering Co., Ltd.) under the condition where a collapse amount was 0.5 mm. Ten copper terminals were bonded at a time. 
     After bonding, the bonded interface between the copper sheet and the ceramic substrate was inspected by using an ultrasonic flaw detector (FineSAT200 manufactured by Hitachi Solutions, Ltd.). A case where peeling of the copper sheet from the ceramic substrate or ceramic breaking was observed was regarded as “defective”, and the number thereof is shown in Table 2. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 2 
               
             
            
               
                   
                   
               
               
                   
                 Mg disposing 
                 Bonding step 
                 Mg—N compound phase 
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 step 
                   
                 Temperature 
                 Holding 
                 Holding 
                   
                 Si 
                 Number 
                 Initial 
                 Number of 
               
               
                   
                 Amount of Mg 
                 Load 
                 increase rate * 1   
                 temperature 
                 time 
                 Presence 
                 concentration 
                 density* 2   
                 bonding 
                 defectives 
               
               
                   
                 mg/cm 2   
                 MPa 
                 ° C./min 
                 ° C. 
                 min 
                 or absence 
                 (atomic %) 
                 (piece/μm) 
                 rate % 
                 (piece) 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Invention 
                 2.61 
                 0.98 
                 7 
                 800 
                 60 
                 Present 
                 16.5 
                 33.9 
                 95.6 
                 0 
               
               
                 Example 11 
               
               
                 Invention 
                 2.61 
                 0.98 
                 7 
                 800 
                 90 
                 Present 
                 17.1 
                 34.1 
                 96.7 
                 0 
               
               
                 Example 12 
               
               
                 Invention 
                 2.61 
                 0.98 
                 7 
                 800 
                 120 
                 Present 
                 21.1 
                 35.5 
                 95.9 
                 0 
               
               
                 Example 13 
               
               
                 Invention 
                 0.70 
                 0.98 
                 5 
                 700 
                 90 
                 Present 
                 5.2 
                 8.3 
                 97.3 
                 2 
               
               
                 Example 14 
               
               
                 Invention 
                 0.70 
                 0.98 
                 5 
                 730 
                 90 
                 Present 
                 8.2 
                 10.2 
                 95.9 
                 1 
               
               
                 Example 15 
               
               
                 Invention 
                 0.70 
                 0.98 
                 5 
                 750 
                 90 
                 Present 
                 9.7 
                 15.4 
                 95.2 
                 0 
               
               
                 Example 16 
               
               
                 Invention 
                 4.35 
                 3.43 
                 25 
                 850 
                 120 
                 Present 
                 21.3 
                 42.3 
                 95.0 
                 0 
               
               
                 Example 17 
               
               
                 Invention 
                 4.35 
                 3.43 
                 25 
                 850 
                 180 
                 Present 
                 24.7 
                 43.2 
                 95.5 
                 0 
               
               
                 Example 18 
               
               
                 Invention 
                 4.35 
                 3.43 
                 25 
                 850 
                 240 
                 Present 
                 36.1 
                 45.6 
                 95.4 
                 5 
               
               
                 Example 19 
               
               
                   
               
               
                 * 1  Temperature increase rate: average temperature increase rate in temperature range of 480° C. or higher and lower than 650° C. 
               
               
                 * 2 Number density of Mg—N compound phases having length of 100 nm or more at bonded interface 
               
            
           
         
       
     
     In Invention Examples 11 to 18 in which the Si concentration in the Mg—N compound phase was 25 atomic % or less, the number of defectives at the time of ultrasonic welding was smaller than that in Invention Example 19 in which the Si concentration in the Mg—N compound phase exceeded 25 atomic %. It was presumed that the local precipitation of the Si single phase was suppressed and the strength of the Mg—N compound phase was ensured. 
     As a result, according to the invention examples, it was confirmed that it is possible to provide a copper/ceramic bonded body, an insulating circuit substrate, a method for producing a copper/ceramic bonded body, and a method for producing an insulating circuit substrate, which can suppress peeling of a copper member from a ceramic member and generation of cracks in the ceramic member even when ultrasonic welding is performed. 
     INDUSTRIAL APPLICABILITY 
     According to the present invention, it is possible to provide a copper/ceramic bonded body, an insulating circuit substrate, a method for producing a copper/ceramic bonded body, and a method for producing an insulating circuit substrate, which can suppress peeling of a copper member from a ceramic member and generation of cracks in the ceramic member even when ultrasonic welding is performed. 
     EXPLANATION OF REFERENCE SIGNS 
     
         
           10 : Insulating circuit substrate (copper/ceramic bonded body) 
           11 : Ceramic substrate (ceramic member) 
           12 : Circuit layer (copper member) 
           13 : Metal layer (copper member) 
           41 : Mg—N compound phase