COPPER-CERAMIC BONDED BODY, INSULATED CIRCUIT BOARD, METHOD FOR PRODUCING COPPER-CERAMIC BONDED BODY, AND METHOD FOR PRODUCING INSULATED CIRCUIT BOARD

Abstract

A copper-ceramic bonded body includes a copper member made of copper or a copper alloy, and a ceramic member made of silicon nitride, the copper member and the ceramic member being bonded to each other, in which a maximum length of a Mg—N compound phase which is present at a bonded interface between the copper member and the ceramic member is less than 100 nm, and in a unit length along the bonded interface, the number density of the Mg—N compound phase in a range of a length of 10 nm or more and less than 100 nm is less than 8 pieces/μm.

Claims

1. A copper-ceramic bonded body comprising: a copper member made of copper or a copper alloy; and a ceramic member made of silicon nitride, the copper member, wherein the copper member and the ceramic member are bonded to each other, a maximum length of a Mg—N compound phase which is present at a bonded interface between the copper member and the ceramic member is less than 100 nm, and in a unit length along the bonded interface, a number density of the Mg—N compound phase in a range of a length of 10 nm or more and less than 100 nm is less than 8 pieces/μm.

2. The copper-ceramic bonded body according to claim 1, wherein the ceramic member includes silicon nitride phases and a glass phase formed between the silicon nitride phases, and Cu atoms are present inside the glass phase located on a side of the copper member.

3. The copper-ceramic bonded body according to claim 1, wherein assuming that a total amount of Mg, N, and Si in the Mg—N compound phase is 100 atomic %, a Si concentration is 25 atomic % or less.

4. An insulated circuit board comprising: a copper sheet made of copper or a copper alloy; and a ceramic substrate made of silicon nitride, wherein the copper sheet is bonded to a surface of the ceramic substrate, a maximum length of a Mg—N compound phase which is present at a bonded interface between the copper sheet and the ceramic substrate is less than 100 nm, and in a unit length along the bonded interface, a number density of the Mg—N compound phase in a range of a length of 10 nm or more and less than 100 nm is less than 8 pieces/μm.

5. The insulated circuit board according to claim 4, wherein the ceramic substrate includes silicon nitride phases and a glass phase formed between the silicon nitride phases, and Cu atoms are present inside the glass phase located on a side of the copper sheet.

6. The insulated circuit board according to claim 4, wherein assuming that a total amount of Mg, N, and Si in the Mg—N compound phase is 100 atomic %, a Si concentration is 25 atomic % or less.

7. A method for producing the copper-ceramic bonded body according claim 1, the method comprising: 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, wherein, in the Mg-disposing step, an amount of Mg is in a range of 0.34 mg/cm.sup.2 or more and 2.09 mg/cm.sup.2 or less, and in the bonding step, a temperature integration value obtained from a value of multiplying a temperature and a time at a Mg—Si eutectic temperature (639° C.) or higher is in a range of 40° C..Math.h or higher and 420° C..Math.h or lower.

8. A method for producing the insulated circuit board according to claim 4, the method comprising: 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, wherein, in the Mg-disposing step, an amount of Mg is in a range of 0.34 mg/cm.sup.2 or more and 2.09 mg/cm.sup.2 or less, and in the bonding step, a temperature integration value obtained by multiplying a temperature and a time at a Mg—Si eutectic temperature (639° C.) or higher is in a range of 40° C..Math.h or higher and 420° C..Math.h or lower.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0031] FIG. 1 is a schematic explanatory view of a power module using an insulated circuit board according to an embodiment of the present invention.

[0032] FIG. 2 is an enlarged explanatory view of a bonded interface of a ceramic substrate of the insulated circuit board according to the embodiment of the present invention.

[0033] FIG. 3 is an explanatory view of a length of a Mg—N compound phase which is present at the bonded interface S in the insulated circuit board according to the embodiment of the present invention.

[0034] FIG. 4 is an observation result of the bonded interface between a circuit layer (metal layer) and the ceramic substrate of the insulated circuit board according to the embodiment of the present invention.

[0035] FIG. 5 is a flowchart of a method for producing the insulated circuit board according to the embodiment of the present invention.

[0036] FIG. 6 is a schematic explanatory view of the method for producing the insulated circuit board according to the embodiment of the present invention, in which (a) is a Mg-disposing step, (b) is a bonding step, and (c) is an obtained insulated circuit board.

DESCRIPTION OF EMBODIMENTS

[0037] Hereinafter, an embodiment of the present invention will be described with reference to the accompanying drawings.

[0038] A copper-ceramic bonded body according to the present embodiment is an insulated circuit board 10 formed by bonding a ceramic substrate 11 as a ceramic member made of ceramics to a copper sheet 22 (circuit layer 12) and a copper sheet 23 (metal layer 13) as a copper member made of copper or a copper alloy. FIG. 1 shows a power module 1 including the insulated circuit board 10 according to the present embodiment.

[0039] The power module 1 includes the insulated circuit board 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.

[0040] 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.

[0041] The bonding layer 2 is made of, for example, a Sn—Ag-based, Sn—In-based, or Sn—Ag—Cu-based solder material.

[0042] The heat sink 30 dissipates heat from the insulated circuit board 10 described above. The heat sink 30 is made of Cu or a Cu 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.

[0043] 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—In-based, or Sn—Ag—Cu-based solder material.

[0044] As shown in FIG. 1, the insulated circuit board 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.

[0045] The ceramic substrate 11 is made of silicon nitride (Si.sub.3N.sub.4) having excellent insulating property 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.

[0046] As shown in FIG. 5, 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. 5) of the ceramic substrate 11.

[0047] In the present embodiment, the circuit layer 12 is formed by bonding the copper sheet 22 made of a rolled plate of oxygen-free copper to the ceramic substrate 11.

[0048] 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.

[0049] As shown in FIG. 5, 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. 5) of the ceramic substrate 11.

[0050] In the present embodiment, the metal layer 13 is formed by bonding the copper sheet 23 made of a rolled plate of oxygen-free copper to the ceramic substrate 11.

[0051] 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.

[0052] At a bonded interface between the ceramic substrate 11 and the circuit layer 12 (metal layer 13), a Mg—N compound phase extending from a side of the ceramic substrate 11 to a side of the circuit layer 12 (metal layer 13) may be formed.

[0053] The Mg—N compound phase is formed by reacting magnesium (Mg) used as a bonding material with nitrogen (N) contained in the ceramic substrate 11.

[0054] In the present embodiment, as shown in FIG. 2, when a cross section along a lamination direction of the ceramic substrate 11 and the circuit layer 12 (metal layer 13) is observed, the maximum length of a Mg—N compound phase 15 which is present at the bonded interface S between the ceramic substrate 11 and the circuit layer 12 (metal layer 13) is less than 100 nm, and in a unit length along the bonded interface S, the number density of the Mg—N compound phase 15 in a range of a length of 10 nm or more and less than 100 nm is less than 8 pieces/μm. The dotted line in FIG. 4 is the length along the bonded interface S.

[0055] Here, the length is defined as follows.

[0056] As shown in FIG. 3, contact points of the Mg—N compound phase 15 and the bonded interface S are defined as a left end A and a right end B, a point located in a half of the length along the bonded interface S from the left end A to the right end B is defined as an intermediate point P1, a boundary between the Mg—N compound phase 15 and the circuit layer 12 (metal layer 13) is defined as a boundary line C, and an inflection point of the boundary line C is defined as a tip P2. A distance L between the intermediate point P1 and the tip P2 is defined as the length.

[0057] The number density of the Mg—N compound phase 15 in a range of a length of 10 nm or more and less than 100 nm is preferably less than 5 pieces/μm, and more preferably less than 3 pieces/μm. The lower limit of the number density of the Mg—N compound phase 15 in the range of the length of 10 nm or more and less than 100 nm may be 0 pieces/μm. The maximum length of the Mg—N compound phase 15 is preferably 10 nm or more.

[0058] Here, FIG. 4 shows observation results in the vicinity of the bonded interface S between the ceramic substrate 11 and the circuit layer 12 (metal layer 13). FIG. 4 is a STEM-HAADF image obtained by observing the cross section along the lamination direction of the ceramic substrate 11 and the circuit layer 12 (metal layer 13) using a Titan ChemiSTEM manufactured by FEI Company.

[0059] In FIG. 4, no clear Mg—N compound phase 15 is observed, and the number density of the Mg—N compound phase 15 in the range of the length of 10 nm or more and less than 100 nm is 0 pieces/μm.

[0060] In the insulated circuit board 10 according to the present embodiment, as shown in FIG. 2, it is preferable that the ceramic substrate 11 includes silicon nitride phases 11a and a glass phase 11b formed between the silicon nitride phases 11a, and that Cu atoms are present inside the glass phase 11b located on a side of the circuit layer 12 (metal layer 13).

[0061] The glass phase 11b is formed by a sintering aid used when sintering a raw material of silicon nitride, and is present at a grain boundary portion between the silicon nitride phases 11a.

[0062] Further, in the insulated circuit board 10 according to the present embodiment, it is preferable that assuming that the total amount of Mg, N, and Si in the Mg—N compound phase 15 is 100 atomic %, a Si concentration is 25 atomic % or less. In this case, since the Si concentration in the Mg—N compound phase 15 is 25 atomic % or less, local precipitation of a Si single phase inside the Mg—N compound phase 15 can be suppressed, and the strength of the Mg—N compound phase 15 can be sufficiently secured. Thereby, it is possible to suppress cracks in the Mg—N compound phase 15 when a load is applied to the bonded interface by ultrasonic bonding of a terminal material, and to maintain an anchor effect by the Mg—N compound phase 15.

[0063] The Si concentration is more preferably 20 atomic % or less, and the lower limit thereof is 5 atomic %.

[0064] Hereinafter, a method for producing the insulated circuit board 10 according to the present embodiment will be described with reference to FIGS. 5 and 6.

[0065] (Mg-Disposing Step S01)

[0066] First, the ceramic substrate 11 made of silicon nitride (Si.sub.3N.sub.4) is prepared, and as shown in (a) of FIG. 6, 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.

[0067] 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.

[0068] Here, in the Mg-disposing step SOL the amount of Mg to be disposed is in a range of 0.34 mg/cm.sup.2 or more and 2.09 mg/cm.sup.2 or less.

[0069] By setting the amount of Mg to be disposed to 0.34 mg/cm.sup.2 or more, a Cu—Mg liquid phase necessary for the interfacial reaction can be sufficiently obtained, and the copper sheet and the ceramic substrate can be reliably bonded to each other. On the other hand, by limiting the amount of Mg to be disposed to 2.09 mg/cm.sup.2 or less, the excessive formation and growth of the Mg—N compound phase can be suppressed.

[0070] The lower limit of the amount of Mg to be disposed is preferably 0.43 mg/cm.sup.2 or more, and more preferably 0.52 mg/cm.sup.2 or more. On the other hand, the upper limit of the amount of Mg to be disposed is preferably 1.34 mg/cm.sup.2 or less, and more preferably 1.04 mg/cm.sup.2 or less.

[0071] (Laminating Step S02)

[0072] 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.

[0073] (Bonding Step S03)

[0074] Next, as shown in (b) of FIG. 6, 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 the 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.

[0075] Here, in the bonding step S03, the temperature rising rate, the holding temperature, the holding time, and the temperature dropping rate are set such that the temperature integration value obtained from a value of multiplying the temperature and the time at the Mg—Si eutectic temperature (639° C.) or higher is set to be in a range of 40° C..Math.h or higher and 420° C..Math.h or lower.

[0076] Here, since the temperature integration value obtained from the value of multiplying the temperature and the time at the Mg—Si eutectic temperature (639° C.) or higher is set to 40° C..Math.h or higher, the Cu—Mg liquid phase necessary for the interfacial reaction can be sufficiently obtained, and the copper sheet and the ceramic substrate can be reliably bonded to each other. On the other hand, since the above-described temperature integration value is set to 420° C..Math.h or lower, the excessive formation and growth of the Mg—N compound phase can be suppressed.

[0077] The lower limit of the temperature integration value obtained from the value of multiplying the temperature and the time at the Mg—Si eutectic temperature (639° C.) or higher is preferably 100° C..Math.h or higher, and more preferably 150° C..Math.h or higher. On the other hand, the upper limit of the temperature integration value obtained from the value of multiplying the temperature and the time at the Mg—Si eutectic temperature (639° C.) or higher is preferably 400° C..Math.h or lower, more preferably 350° C..Math.h or lower, and still more preferably 300° C..Math.h or lower.

[0078] A pressing load in the bonding step S03 is preferably in a range of 0.049 MPa or more and 3.4 MPa or less.

[0079] Further, a degree of vacuum in the bonding step S03 is preferably in a range of 1×10.sup.−6 Pa or more and 5×10.sup.−2 Pa or less.

[0080] As described above, the insulated circuit board 10 according to the present embodiment shown in (c) of FIG. 6 is produced by the Mg-disposing step S01, the laminating step S02, and the bonding step S03.

[0081] (Heat Sink-Bonding Step S04)

[0082] Next, the heat sink 30 is bonded to the other surface side of the metal layer 13 of the insulated circuit board 10.

[0083] The insulated circuit board 10 and the heat sink 30 are laminated with a solder material interposed therebetween and are loaded into a heating furnace such that the insulated circuit board 10 and the heat sink 30 are solder-bonded to each other with the solder layer 32 interposed therebetween.

[0084] (Semiconductor Element-Bonding Step S05)

[0085] Next, the semiconductor element 3 is bonded to one surface of the circuit layer 12 of the insulated circuit board 10 by soldering.

[0086] The power module 1 shown in FIG. 1 is produced by the above steps.

[0087] According to the insulated circuit board 10 (copper-ceramic bonded body) according to the present embodiment having the above configuration, since the maximum length of the Mg—N compound phase 15 which is present at the bonded interface between the circuit layer 12 (and the metal layer 13) and the ceramic substrate 11 is less than 100 nm, and in the unit length along the bonded interface, the number density of the Mg—N compound phase 15 in the range of the length of 10 nm or more and less than 100 nm is less than 8 pieces/μm, occurrence of electric field concentration caused by the Mg—N compound phase 15 which is present at the bonded interface can be suppressed. Accordingly, the insulating property of the insulated circuit board 10 can be sufficiently improved.

[0088] In the insulated circuit board 10 according to the present embodiment, when the ceramic substrate 11 includes the silicon nitride phases 11a and the glass phase 11b formed between the silicon nitride phases 11a, and Cu atoms are present inside the glass phase 11b located on the circuit layer 12 (and the metal layer 13) side, an interfacial reaction between the ceramic substrate 11 and the copper sheets 22 and 23 is sufficiently performed, and bonding reliability between the circuit layer 12 (and the metal layer 13) and the ceramic substrate 11 is particularly excellent.

[0089] In the insulated circuit board 10 according to the present embodiment, assuming that the total amount of Mg, N, and Si in the Mg—N compound phase 15 is 100 atomic %, the Si concentration is 25 atomic % or less. In this case, local precipitation of a Si single phase inside the Mg—N compound phase 15 can be suppressed, and the strength of the Mg—N compound phase 15 can be sufficiently secured.

[0090] According to the method for producing the insulated circuit board 10 (copper-ceramic bonded body) according to the present embodiment, in the Mg-disposing step S01, the amount of Mg is 0.34 mg/cm.sup.2 or more, and in the bonding step S03, the temperature integration value obtained from the value of multiplying a temperature and a time at the Mg—Si eutectic temperature (639° C.) or higher is 40° C..Math.h or higher, so that a Cu—Mg liquid phase necessary for the interfacial reaction can be sufficiently obtained, and the copper sheets 22 and 23 and the ceramic substrate 11 can be reliably bonded to each other.

[0091] In addition, in the Mg-disposing step S01, the amount of Mg is 2.09 mg/cm.sup.2 or less, and in the bonding step S03, the temperature integration value obtained from the value of multiplying a temperature and a time at the Mg—Si eutectic temperature (639° C.) or higher is 420° C..Math.h or lower, so that excessive formation and growth of the Mg—N compound phase at the bonded interface between the circuit layer 12 (and the metal layer 13) and the ceramic substrate 11 can be suppressed.

[0092] 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 ideas of the present invention.

[0093] For example, in the present embodiment, the semiconductor element is mounted on the insulated circuit board 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 insulated circuit board to form an LED module, or a thermoelectric element may be mounted on the circuit layer of the insulated circuit board to form a thermoelectric module.

[0094] In the insulated circuit board according to the present embodiment, it has been described that the circuit layer and the metal layer are both made of a copper sheet made of copper or a copper alloy, but the present invention is not limited thereto.

[0095] 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.

[0096] 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.

[0097] Further, 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.sub.2 may be applied.

EXAMPLES

[0098] Hereinafter, results of confirmation experiments performed to confirm the effects of the present invention will be described.

Example 1

[0099] First, a ceramic substrate (40 mm×40 mm×0.32 mm) made of a ceramic substrate made of silicon nitride (Si.sub.3N.sub.4) was prepared.

[0100] A copper sheet (37 mm×37 mm×thickness of 0.6 mm) made of oxygen-free copper was bonded to both surfaces of the ceramic substrate under the conditions of the Mg-disposing step and the bonding step shown in Table 1 to obtain an insulated circuit board (copper-ceramic bonded body). The degree of vacuum of a vacuum furnace at the time of bonding was set to 2×10.sup.−3 Pa.

[0101] For the obtained insulated circuit board (copper-ceramic bonded body), the maximum length of the Mg—N compound phase at the bonded interface, the number density of the Mg—N compound phase in the range of the length of 10 nm or more and less than 100 nm, the presence or absence of Cu atoms inside the glass phase, the bonding rate after a thermal cycle, and the dielectric breakdown voltage were evaluated as follows.

[0102] (Mg—N Compound Phase)

[0103] The bonded interface between the copper sheet and the ceramic substrate was observed using a transmission electron microscope (Titan ChemiSTEM manufactured by FEI Company) at an acceleration voltage of 200 kV and a magnification of 20,000×, and the Mg—N compound phase was recognized when a region where Mg and N coexisted was present and the concentration of Mg was 40 atomic % or more and 65 atomic % or less with the total of Mg, N, and Si being 100 atomic % in the region.

[0104] The maximum length of the Mg—N compound phase and the number density of the Mg—N compound phase in the range of the length of 10 nm or more and less than 100 nm in the unit length along the bonded interface were calculated.

[0105] The maximum length of the Mg—N compound phase was measured by extracting, in the cross section of the copper-ceramic bonded body, the Mg—N compound phase in the field of view (2 μm×2 μm) when the bonded interface was observed using a transmission electron microscope (Titan ChemiSTEM manufactured by FET Company), and by measuring the length of the Mg—N compound phase according to the above definition. The measurement was performed in 5 fields of view in each copper-ceramic bonded body, and the length of the Mg—N compound phase, which had the largest length, was shown as the maximum length of the Mg—N compound phase.

[0106] The number density (pieces/μm) of the Mg—N compound phase was calculated by counting the number of the Mg—N compound phases in the range of the length of 10 nm or more and less than 100 nm in the unit length along the bonded interface between the copper sheet and the ceramic substrate in the field of view (2 μm×2 μm) when the cross section of the copper-ceramic bonded body cut along the lamination direction of the copper-ceramic bonded body was observed using a transmission electron microscope (Titan ChemiSTEM manufactured by FET Company). The measurement was performed in 5 fields of view, and an average value was shown as the number density (pieces/μm).

[0107] The evaluation results are shown in Table 1. The maximum length of the Mg—N compound phase and the number density of the Mg—N compound phase were measured by excluding the Mg—N compound phase which was present at the boundary of the measurement field and could not be grasped as a whole.

[0108] (Cu Atom Inside Glass Phase)

[0109] The cross section of the ceramic substrate was observed using a transmission electron microscope (Titan ChemiSTEM manufactured by FE1 Company; acceleration voltage of 200 kV) to confirm the presence or absence of Cu atoms in the glass phase.

[0110] The glass phase was a region in which Si was less than 15 atomic % and O was 3 atomic % or more and 25 atomic % or less assuming that the total amount of Cu, Si, O, and N was 100 atomic %. The evaluation results are shown in Table 1.

[0111] (Bonding Rate after Loading of Thermal Cycle)

[0112] Using a thermal shock tester (TSA-72ES manufactured by ESPEC Corp.), 800 cycles, where one cycle is 5 minutes at −40° C. and 5 minutes at 175° C., were performed on the insulated circuit board in a gas phase. After that, the bonding rate between the ceramic substrate and the copper sheet was evaluated as follows.

[0113] For the evaluation of the bonding rate, the bonding rate at the interface between the ceramic substrate and the copper sheet (circuit layer and metal layer) was evaluated using an ultrasonic flaw detector (FineSAT200 manufactured by Hitachi Power Solutions Co., Ltd.) with respect to the insulated circuit board, and the bonding rate was calculated from the following equation.

[0114] Here, the initial bonding area was the area to be bonded before bonding, that is, the area (37 mm×37 mm) of the circuit layer and the metal layer in the present example.

(Bonding rate)={(initial bonding area)−(peeled area)}/(initial bonding area)

[0115] 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 a peeled area. These results are shown in Table 1.

[0116] (Dielectric Breakdown Voltage)

[0117] The front and back surfaces of the insulated circuit board after loading of the thermal cycle were interposed between electrodes, a process of increasing the voltage by 0.5 kV at a boosting speed of 1 kV/sec and performing holding for 30 seconds was repeated, and the voltage at the time of dielectric breakdown was used as the dielectric breakdown voltage. In the present example, the voltage when a current of 1 mA or more flowed was defined as the dielectric breakdown voltage. The evaluation results are shown in Table 1.

TABLE-US-00001 TABLE 1 Mg- Bonding disposing Bonding step rate after step Temperature Mg-N compound phase Presence or loading Dielectric Amount integration Maximum Number absence of Cu of thermal breakdown of Mg Load value.sup.※1 length density.sup.※2 atoms inside cycle voltage mg/cm.sup.2 MPa ° C. .Math. h (nm) (pieces/μm) glass phase (%) (kV) Present Invention 0.34 0.49  41  13 0.2 Absent 91.2 7.5 Example 1 Present Invention 1.04 1.47 104  22 1.0 Absent 92.8 7.5 Example 2 Present Invention 2.09 0.98 346  93 7.8 Present 96.8 7.0 Example 3 Present Invention 1.39 1.47 397  98 7.4 Present 95.9 7.0 Example 4 Present Invention 0.52 0.98 153  23 4.6 Present 95.7 8.5 Example 5 Present Invention 0.70 0.98 235  47 4.9 Present 94.0 8.5 Example 6 Present Invention 1.04 0.49 276  70 6.3 Present 96.5 8.0 Example 7 Present Invention 0.70 0.98 298  81 7.2 Present 94.6 8.0 Example 8 Comparative 2.61 0.98 235 126 16.1  Present 94.8 5.5 Example 1 Comparative 1.04 0.49 503 143 12.6  Present 94.8 5.0 Example 2 Comparative 0.17 1.47 397 — — — — — Example 3 Comparative 2.09 0.98  19 — — — — — Example 4 .sup.※1Temperature integration value: value obtained by multiplying temperature and time at Mg-Si eutectic temperature (639° C.) or higher .sup.※2Number density of Mg-N compound phase having length of 10 nm or more and less than 100 nm at bonded interface

[0118] In Comparative Example 1 in which the amount of Mg in the Mg-disposing step was 2.61 mg/cm.sup.2, the maximum length of the Mg—N compound phase exceeded 100 μm, and the number density of the Mg—N compound phase having the length of 10 nm or more and less than 100 nm exceeded 8 pieces. Therefore, the dielectric breakdown voltage was as low as 5.5 kV, and the insulating property was insufficient.

[0119] In Comparative Example 2 in which the temperature integration value in the bonding step was 503° C..Math.h, the maximum length of the Mg—N compound phase exceeded 100 μm, and the number density of the Mg—N compound phase having the length of 10 nm or more and less than 100 nm exceeded 8 pieces. Therefore, the dielectric breakdown voltage was as low as 5.0 kV, and the insulating property was insufficient.

[0120] In Comparative Example 3 in which the amount of Mg in the Mg-disposing step was 0.17 mg/cm.sup.2, the maximum length of the Mg—N compound phase was less than 10 μm. Therefore, the bonding was poor and other evaluations were discontinued.

[0121] In Comparative Example 4 in which the temperature integration value in the bonding step was 19° C..Math.h, the maximum length of the Mg—N compound phase was less than 10 μm. Therefore, the bonding was poor and other evaluations were discontinued. The liquid phase was sufficiently generated, but it is presumed that the reaction was insufficient.

[0122] On the other hand, in Present Invention Examples 1 to 8 in which the maximum length of the Mg—N compound phase which is present at the bonded interface was less than 100 nm and the number density of the Mg—N compound phase in the range of the length of 10 nm or more and less than 100 nm in the unit length along the bonded interface was less than 8 pieces/μm, the dielectric breakdown voltage was 7.0 kV or more and the insulating property was excellent.

[0123] Further, in Present Invention Examples 3 to 8 in which Cu atoms were present inside the glass phase, the bonding rate after loading of the thermal cycle was high, and the bonding reliability was excellent.

Example 2

[0124] A ceramic substrate (40 mm×40 mm×0.32 mm) made of a ceramic substrate made of silicon nitride (Si.sub.3N.sub.4) was prepared.

[0125] A copper sheet (37 mm×37 mm×thickness of 0.6 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 insulated circuit board (copper-ceramic bonded body). The degree of vacuum of a vacuum furnace at the time of bonding was set to 2×10.sup.−3 Pa.

[0126] For the obtained insulated circuit board (copper-ceramic bonded body), the maximum length of the Mg—N compound phase at the bonded interface, the number density of the Mg—N compound phase in the range of the length of 10 nm or more and less than 100 nm, and the presence or absence of Cu atoms inside the glass phase were evaluated in the same manner as in Example 1. The evaluation results are shown in Table 2.

[0127] In addition, the Si concentration in the Mg—N compound phase, the bonding rate after the furnace passing test, and the insulating property were evaluated as follows.

[0128] (Si Concentration in Mg—N Compound Phase)

[0129] An observation specimen was collected from the central portion of the obtained insulated circuit board (copper-ceramic bonded body), and the bonded interface between the copper sheet and the ceramic substrate was observed 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 was measured in a region (that is, the Mg—N compound phase described above) where Mg and N coexisted, in which the total of Mg, N, and Si was 100 atomic %.

[0130] (Bonding Rate after Loading of Thermal Cycle)

[0131] After loading 5 times of a thermal cycle, where one cycle is 2 minutes at −78° C. and 2 minutes at 350° C., the bonding rate between the ceramic substrate and the copper sheet was evaluated in the same manner as in Example 1. The evaluation results are shown in Table 2.

[0132] (Insulation Evaluation)

[0133] The front and back surfaces of the insulated circuit board after loading of the furnace passing test were interposed between electrodes, a process of increasing the voltage by 0.5 kV at a boosting speed of 1 kV/sec and performing holding for 30 seconds was repeated, and the voltage when the discharge charge exceeded 10 pC was used as the discharge inception voltage. The evaluation results are shown in Table 2.

TABLE-US-00002 TABLE 2 Mg- Bonding disposing Bonding step rate after Partial step Temperature Mg-N compound phase Presence or loading discharge Amount integration Maximum Si Number absence of Cu of thermal inception of Mg Load value.sup.※1 length concentration density.sup.※2 atoms inside cycle voltage mg/cm.sup.2 MPa ° C. .Math. h (nm) (atomic %) (pieces/μm) glass phase (%) (kV) Present Invention 0.52 0.98 130  23 6.1 3.9 Present 90.3 7.1 Example 11 Present Invention 0.52 0.98 153  23 5.4 4.6 Present 91.2 7.1 Example 12 Present Invention 0.52 1.47 195  38 7.4 5.1 Present 92.1 7.0 Example 13 Present Invention 0.70 0.98 235  47 9.0 4.9 Present 91.5 6.8 Example 14 Present Invention 1.39 1.47 235  73 13.2  5.5 Present 92.2 6.7 Example 15 Present Invention 1.39 1.47 153  33 6.5 4.9 Present 92.0 7.0 Example 16 Present Invention 1.04 0.49 276  70 18.8  6.3 Present 92.4 6.6 Example 17 Present Invention 1.04 0.49 346  91 24.1  7.7 Present 92.5 6.5 Example 18 Present Invention 2.09 0.49 397  95 24.8  7.9 Present 92.6 6.5 Example 19 Present Invention 1.04 1.96 420  97 26.3  7.9 Present 92.4 6.2 Example 20 Comparative 3.74 0.49 531 174 30.2  15.4  Present 92.8 4.9 Example 11 .sup.※1Temperature integration value: value obtained by multiplying temperature and time at Mg-Si eutectic temperature (639° C.) or higher .sup.※2Number density of Mg-N compound phase having length of 10 nm or more and less than 100 nm at bonded interface

[0134] In Comparative Example 11 in which the amount of Mg in the Mg-disposing step was 3.741 mg/cm.sup.2, the maximum length of the Mg—N compound phase exceeded 100 μm, the number density of the Mg—N compound phase having the length of 10 nm or more and less than 100 nm exceeded 8 pieces, and the Si concentration in the Mg—N compound phase exceeded 25 atomic %. Therefore, the discharge inception voltage after loading of the thermal cycle was as low as 4.9 kV, and the insulating property was insufficient.

[0135] On the other hand, in Present Invention Examples 11 to 19 in which the maximum length of the Mg—N compound phase which was present at the bonded interface was less than 100 nm, the number density of the Mg—N compound phase in the range of the length of 10 nm or more and less than 100 nm in the unit length along the bonded interface was less than 8 pieces/μm, and the Si concentration in the Mg—N compound phase was 25 atomic % or less, the discharge inception voltage after loading of the thermal cycle was 6.5 kV or more and the insulating property was excellent.

[0136] From comparison among Present Invention Examples 11 to 19, it was confirmed that the Si concentration in the Mg—N compound phase was suppressed to be low, thereby increasing the discharge inception voltage after loading of the thermal cycle and further improving the insulating property.

[0137] As a result of the above, according to Present Invention Examples, it was confirmed that it is possible to provide a copper-ceramic bonded body, an insulated circuit board, a method for producing a copper-ceramic bonded body, and a method for producing an insulated circuit board, which are excellent in insulating properties.

REFERENCE SIGNS LIST

[0138] 10: Insulated circuit board (copper-ceramic bonded body)

[0139] 11: Ceramic substrate (ceramic member)

[0140] 11a: Silicon nitride phase

[0141] 11b: Glass phase

[0142] 12: Circuit layer (copper member)

[0143] 13: Metal layer (copper member)

[0144] 15: Mg—N compound phase