WO2013008920A1

WO2013008920A1 - Ceramic circuit board

Info

Publication number: WO2013008920A1
Application number: PCT/JP2012/067957
Authority: WO
Inventors: 星野　政則; 中山　憲隆; 隆之那波; 佐藤　英樹; 裕小森田
Original assignee: 株式会社東芝; 東芝マテリアル株式会社
Priority date: 2011-07-14
Filing date: 2012-07-13
Publication date: 2013-01-17
Also published as: CN103503130A; JP6129738B2; KR101522806B1; KR20130137029A; JPWO2013008920A1; CN103503130B

Abstract

A ceramic circuit board having a metal circuit board joined onto an alumina substrate, wherein the alumina substrate contains at least 99.5 mass% of alumina (Al₂O₃) and less than 0.5 mass% of a sintering aid-derived component generated from a sintering aid admixed before sintering, the sintering aid-derived component is an inorganic oxide containing sodium, the sodium in terms of sodium oxide (Na₂O) in the sintering aid-derived component is contained at 0.001-0.1 mass% for every 100 mass% of the alumina substrate, and the alumina substrate has a maximum void size of no greater than 12 μm, an average void size of no greater than 10 μm, and a Vickers hardness of at least 1500.

Description

Ceramic circuit board

The present invention relates to a ceramic circuit board using an alumina substrate.

In recent years, ceramic circuit boards in which metal plates such as a copper plate, an aluminum plate, and various clad plates are bonded on a ceramic substrate are widely used as circuit substrates such as a power transistor module substrate and a switching power supply module substrate. As the ceramic substrate, an inexpensive and highly versatile alumina (Al ₂ O ₃ ) substrate, an aluminum nitride (AlN) substrate having electrical insulation and excellent thermal conductivity, or high-strength silicon nitride ( A Si ₃ N ₄ ) substrate or the like is generally used. Among these ceramic substrates, an alumina substrate is advantageous in that it is inexpensive and highly versatile.

Here, the structure of the ceramic circuit board will be described. FIG. 1 is a plan view showing an example of a configuration on the pattern surface side of a ceramic circuit board. FIG. 2 is a cross-sectional view taken along line AA of the ceramic circuit board shown in FIG. FIG. 3 is a bottom view showing an example of the configuration of the back side of the ceramic circuit board shown in FIG.

As shown in FIGS. 1 to 3, for example, the ceramic circuit board 1 has a metal circuit board 3 such as a copper plate bonded or formed on one surface of the ceramic board 2 and the other surface being the back surface of the ceramic board 2. It is formed by joining a back metal plate 4 such as a copper plate.
The metal circuit board 3 is composed of various metal plates bonded to the surface of the ceramic substrate 2 or a metal layer formed on the surface of the ceramic substrate 2.

As a method of integrally forming various metal plates or metal layers on the surface of the ceramic substrate 2, for example, the following direct bonding method, high melting point metal metallization method, active metal method and the like are used.

The direct bonding method is, for example, a method of directly bonding the ceramic substrate 2 and the metal circuit board 3 by generating a eutectic liquid phase at the interface between the ceramic substrate 2 and the metal circuit board 3.
The direct bonding method will be specifically described by taking as an example the case where the metal circuit board 3 is a copper circuit board. First, a copper circuit board 3 punched into a predetermined shape is placed in contact with a ceramic substrate 2 and heated to generate a eutectic liquid phase such as Cu—Cu ₂ O or Cu—O at the bonding interface. The wettability between the ceramic substrate 2 and the copper circuit board 3 is enhanced by the crystal phase. Next, when this eutectic liquid phase is cooled and solidified, the ceramic substrate 2 and the copper circuit board 3 are directly joined together to obtain the ceramic circuit board 1. This method is a so-called direct copper bonding method (DBC method: Direct Bonding Copper method).

The refractory metal metallization method is a method of obtaining the ceramic circuit substrate 1 by integrating the ceramic substrate 2 and the metal circuit layer by baking a refractory metal such as Mo or W onto the surface of the ceramic substrate 2.
Further, the active metal method is such that, for example, a metal plate 3 such as a copper circuit board is placed on the ceramic substrate 2 via an Ag—Cu brazing material layer containing an active metal such as a group 4A element such as Ti, Zr, and Hf. Is a method of obtaining the ceramic circuit board 1 by integrally bonding the two. According to this active metal method, the bonding strength between the brazing material layer and the copper circuit board 3 is increased by the Cu and Ag components of the brazing material layer, and the bonding between the brazing material layer and the ceramic substrate 2 is performed by the Ti, Zr, and Hf components. Strength increases.

In addition, as a method of forming a circuit on the metal circuit board 3 of the obtained ceramic circuit board 1, a method using a copper plate previously patterned by pressing or etching, a method of patterning by a technique such as etching after bonding, or the like It has been known.

As described above, the ceramic circuit board 1 obtained by the direct bonding method or the active metal brazing method has a high bonding strength between the ceramic substrate 2 and the metal circuit board 3 and has a simple structure. For this reason, the ceramic circuit board 1 is advantageous in that it can be miniaturized and highly mounted, has the effect of shortening the manufacturing process, and can be applied to a large current type or highly integrated semiconductor chip. ing.

By the way, in recent years, high output of semiconductor devices using the ceramic circuit board 1 and high integration of semiconductor elements are rapidly progressing, so that thermal stress and thermal load that repeatedly act on the ceramic circuit board 1 increase. There is a tendency. For this reason, even if the increased thermal stress is given to the ceramic circuit board 1, the bonding strength between the ceramic board 2 and the metal circuit board 3 is sufficiently high, and even if many thermal cycles are given, the ceramic board 2 and the metal Durability that can maintain the connection with the circuit board 3 is required.

As the ceramic circuit board 1 which copes with the increased heat load and improves the durability of the circuit board, for example, the thickness of the ceramic board 2 is reduced to about 0.25 to 0.38 mm to reduce the thermal resistance. A ceramic circuit board 1 that improves the flexibility of the ceramic board 2 and prevents the metal circuit board 3 from peeling off is known.
Further, as another ceramic circuit board 1 which copes with the increased heat load and improves the durability of the circuit board, an alumina substrate having a purity as high as about 96% is used as the ceramic substrate 2, and this alumina is used. There is known a ceramic circuit board 1 in which a metal circuit board 3 (circuit layer) is integrally bonded to a substrate by the direct bonding method or the active metal method.

Furthermore, Japanese Patent No. 3833410 (Patent Document 1) discloses a ceramic circuit board using a high-purity alumina substrate having a purity of 99.5% or more. In Patent Document 1, a ceramic circuit board having excellent properties such as strength and Vickers hardness is obtained by setting the alumina purity to 99.8%.

Japanese Patent No. 3833410 JP 2005-281063 A

However, since the ceramic circuit board described in Patent Document 1 uses high-purity alumina as a raw material, the sinterability is poor, and it has been necessary to sinter at 1600 ° C. for as long as 20 hours.
For this reason, the ceramic circuit board described in Patent Document 1 has a problem in that the merit of the alumina substrate, which is high in manufacturing cost and low in price, cannot be sufficiently exhibited.

The present invention has been made in view of the above circumstances, and an object thereof is to provide a ceramic circuit board excellent in characteristics such as bonding strength and Vickers hardness by using an inexpensive alumina substrate which is not highly pure as a ceramic substrate. To do.

The inventors of the present application are alumina substrates obtained by cauterization using alumina powder and a sintering aid containing at least sodium oxide as a raw material, and are derived from the sintering aid generated from the sintering aid. It has been found that according to a high-purity alumina substrate containing a small amount of components, an alumina substrate having high sinterability, cost reduction, and high Vickers hardness can be obtained. The inventors of the present application have found that a ceramic circuit board having excellent bonding strength can be obtained by using this alumina substrate, and have completed the present invention.

The ceramic circuit board of the present invention solves the above-mentioned problem. In the ceramic circuit board in which a metal circuit board is bonded to an alumina substrate, the alumina substrate contains 99.5% by mass or more of alumina Al ₂ O _3. And containing less than 0.5% by mass of a component derived from a sintering aid produced from a sintering aid blended before sintering, the component derived from a sintering aid is an inorganic oxide containing sodium, Sodium in the sintering aid-derived component is contained in an amount of 0.001 to 0.1% by mass in 100% by mass of the alumina substrate in terms of mass converted to sodium oxide Na ₂ O, and the alumina substrate has a maximum void diameter. It is 12 μm or less, the void average diameter is 10 μm or less, and the Vickers hardness is 1500 or more.

In the ceramic circuit board of the present invention, the component derived from the sintering aid is an inorganic oxide further containing silicon, and the silicon in the component derived from the sintering aid is the mass of the alumina substrate 100 in terms of mass converted to silicon oxide SiO _2. It is preferable that 0.001 to 0.2 mass% is contained in the mass%.

In the ceramic circuit board of the present invention, the sintering auxiliary agent-derived component is an inorganic oxide further containing iron, and the iron in the sintering auxiliary agent-derived component is the alumina in a mass converted to iron oxide Fe ₂ O _3. It is preferable that 0.001 to 0.05% by mass is contained in 100% by mass of the substrate.

In the ceramic circuit board of the present invention, the alumina substrate preferably has an average crystal grain size of alumina crystal grains of 20 μm or less.

In the ceramic circuit board of the present invention, the alumina substrate preferably has a void volume ratio, which is a volume ratio of voids existing in the alumina substrate, of 3% by volume or less.

In the ceramic circuit board of the present invention, the alumina substrate preferably has 2 to 30 voids per unit area of 100 μm × 100 μm calculated by cross-sectional observation.

In the ceramic circuit board of the present invention, it is preferable that the alumina substrate has a void area ratio, which is a void area ratio in a cross section of the alumina substrate, of 10% or less.

In the ceramic circuit board of the present invention, it is preferable that the alumina substrate has a withstand voltage of 25 KV / mm or more.

In the ceramic circuit board of the present invention, the alumina substrate preferably has a toughness value of 3.2 MPa · m ^1/2 or more.

In the ceramic circuit board of the present invention, the alumina substrate preferably has a thermal conductivity of 28 W / m · K or more.

In the ceramic circuit board of the present invention, it is preferable that the alumina substrate has a bending strength of 400 MPa or more.

In the ceramic circuit board of the present invention, it is preferable that the metal circuit board is bonded to the alumina substrate by a direct bonding method.

In the ceramic circuit board of the present invention, the metal circuit board is preferably a copper circuit board, and the copper circuit board is preferably bonded to the alumina substrate by a Cu—O eutectic compound.

In the ceramic circuit board of the present invention, the metal circuit board is a copper circuit board, and the copper circuit board preferably contains 0.1 to 1.0% by mass of carbon.

In the ceramic circuit board according to the present invention, when the cross section of the ceramic circuit board is observed at the bonding interface between the alumina board and the metal circuit board, the curve along the surface of the metal circuit board is It is preferable to have an intricate structure in which the ratio of contact with the curve along the surface irregularities is 95% or more.

In the ceramic circuit board of the present invention, the alumina substrate preferably has a thickness of 0.25 to 1.2 mm.

In the ceramic circuit board of the present invention, the metal circuit board preferably has a thickness of 0.1 to 0.5 mm.

According to the ceramic circuit board of the present invention, although the alumina purity is a high-purity alumina substrate having a purity of 99.5% by mass or more, the sinterability is improved by the sodium oxide added as a sintering aid. Since the time is short, the cost can be significantly reduced. In addition, according to the ceramic circuit board according to the present invention, since a predetermined amount of the sintering aid-derived component containing sodium is included, characteristics such as bonding strength are also high.

The top view which shows an example of a structure by the side of the pattern surface of a ceramic circuit board. FIG. 2 is a cross-sectional view of the ceramic circuit board shown in FIG. 1 along the line AA. The bottom view which shows an example of the structure of the back surface side of the ceramic circuit board shown in FIG. The figure which shows how to obtain | require the average crystal grain diameter of an alumina crystal grain. 1 is a schematic cross-sectional view of a bonding interface of a ceramic circuit board according to Example 1. FIG.

The ceramic circuit board of the present invention will be described.

[Ceramic circuit board]
The ceramic circuit board of the present invention is a ceramic circuit board in which a metal circuit board is bonded on an alumina substrate.
The ceramic circuit board of the present invention is, for example, a ceramic circuit board 1 in which a metal circuit board 3 is bonded on one surface of an alumina substrate 2 as shown in FIG.
FIG. 1 shows an example in which a back metal plate 4 such as a copper plate is bonded to the other surface of the alumina substrate 2, that is, the surface on the back surface side. The ceramic circuit substrate of the present invention is an alumina substrate. The metal circuit board 3 may be joined to both surfaces on one surface of 2 and the other surface.

(Alumina substrate)
The alumina substrate contains 99.5% by mass or more of alumina Al ₂ O ₃ and a sintering aid-derived component produced from a sintering aid blended before sintering, preferably less than 0.5% by mass, preferably Including 0.3% by mass or less.
The alumina substrate used in the present invention is a polycrystal composed of many alumina crystal grains, and the sintering aid-derived component is a glass phase present at the grain boundaries of the alumina crystal grains.

In the alumina substrate, the total amount of the alumina Al ₂ O ₃ and the sintering aid-derived component is preferably substantially 100% by mass.
As described later, the sintering auxiliary agent-derived component may contain inevitable impurity components that are components other than the sintering auxiliary component. The sintering aid component and the inevitable impurity component will be described in detail later. The sintering aid component is a substance obtained by converting Na, Si, and Fe into the same oxide as the sintering aid. Examples of the sintering aid component include Na ₂ O, SiO ₂ and Fe ₂ O ₃ . Moreover, an inevitable impurity component is the remainder remove | excluding the sintering adjuvant component from the sintering adjuvant origin component.
The inevitable impurity component contained in the sintering aid-derived component may be contained in an amount of 0.05% by mass or less in 100% by mass of the alumina substrate.

<Sintering aid-derived component>
The sintering aid-derived component contained in the alumina substrate is a raw material of the alumina substrate of the present invention, and the sintering aid blended with the alumina powder before sintering became a liquid phase by heat treatment during sintering. Later, it means an inorganic oxide that has solidified into a glass phase.

The sintering aid-derived component is contained in the alumina substrate at less than 0.5% by mass, preferably 0.3% by mass or less.

The sintering aid-derived component contained in the alumina substrate is an inorganic oxide containing at least sodium.
Sodium in the binder-derived component is contained in an amount of 0.001 to 0.1 mass% (10 to 1000 mass ppm) in 100 mass% of the alumina substrate in terms of mass converted to sodium oxide Na ₂ O.
When the mass converted to sodium oxide Na ₂ O is 0.001 to 0.1 mass% (10 to 1000 mass ppm) in 100 mass% of the alumina substrate, the sodium component functions as a sintering aid and voids Can be suppressed.

When the mass converted to sodium oxide Na ₂ O is less than 0.001% by mass in 100% by mass of the alumina substrate, the action of the sintering aid containing sodium becomes insufficient, and the mechanical strength of the alumina substrate is reduced. It tends to decline.
The mass in terms of sodium oxide Na 2 _O is more than 0.1 mass% in 100 mass% alumina substrate, or large voids diameter, Vickers hardness is easily lowered.

When a mixed powder containing alumina powder and a sintering aid is used to sinter and produce an alumina substrate, sodium oxide Na ₂ O added as a sintering aid or contained as an impurity in the alumina powder Sodium components such as sodium oxide Na ₂ O, metallic Na, and sodium hydroxide are easily dissolved when added to water or during sintering.
For example, when pure water is used during the granulation process for producing a granulated powder containing alumina powder and a sintering aid, it is converted into pure water from sodium oxide Na ₂ O, sodium hydroxide NaOH, etc. during granulation. Na ions melt out.

Metal Na has a melting point of 98 ° C., sodium oxide Na ₂ O has a melting point of 1132 ° C., and sodium hydroxide NaOH has a melting point of 318 ° C. On the other hand, since the sintering temperature in the sintering process when manufacturing the alumina substrate is as high as about 1200 to 1700 ° C., the sodium component starts to melt during the sintering. As the amount of sodium component dissolved out increases, the formation of voids proceeds, and many large voids exceeding 10 μm are likely to be formed on the resulting alumina substrate.

Thus, the sodium component tends to disappear during the production of the alumina substrate.
On the other hand, according to Patent Document 2, a high-purity alumina powder having a purity of 99.5% by mass or more used as a raw material for producing an alumina substrate has a sodium content of about 30 ppm by mass (0.003% by mass) or less. Few.
For this reason, when manufacturing an alumina substrate, it is preferable to add an appropriate amount of a sodium component as a sintering aid to high-purity alumina powder.
The added amount of the sodium component is 0.001 to 0.1 in 100% by mass of the alumina substrate in which the sodium in the auxiliary component derived from the obtained alumina substrate is converted to sodium Na ₂ O as described above. It should be contained in mass% (10 to 1000 mass ppm).
In addition, as a method of adding a sodium component at the time of manufacturing an alumina substrate, for example, a method of positively adding a sodium component, a method of using an impurity sodium component in an alumina raw material powder, an alumina ball (purity 96 % Alumina) and a method of mixing the sodium component in the alumina balls can be used.

The sintering aid-derived component contained in the alumina substrate is preferably an inorganic oxide further containing silicon in addition to sodium.
Silicon in the sintering aid-derived component is usually contained in an amount of 0.001 to 0.2% by mass in 100% by mass of the alumina substrate in terms of mass converted to silicon oxide SiO ₂ .

When the mass of silicon converted to silicon oxide SiO ₂ is less than 0.001% by mass in 100% by mass of the alumina substrate, the action of the sintering aid containing silicon becomes insufficient, and the mechanical strength of the alumina substrate Is prone to decline.
The mass in terms of silicon oxide, silicon SiO ₂ is more than 0.2 mass% in 100 mass% of alumina substrate, will not capitalize the characteristics of high purity alumina, the Vickers hardness tends to decrease.

The component derived from the sintering aid contained in the alumina substrate is preferably an inorganic oxide further containing iron in addition to sodium or sodium and silicon.
Iron in the sintering aid-derived component is usually contained in an amount of 0.001 to 0.05% by mass in 100% by mass of the alumina substrate in terms of mass converted to iron oxide Fe ₂ O ₃ .

When the mass of iron converted to iron oxide Fe ₂ O ₃ is less than 0.001% by mass in 100% by mass of the alumina substrate, the action of the sintering aid containing iron becomes insufficient, and the alumina substrate machine The mechanical strength tends to decrease.
The mass obtained by converting the iron oxide iron Fe ₂ O ₃ is more than 0.05 mass% in 100 mass% of alumina substrate, will not capitalize the characteristics of high purity alumina, the Vickers hardness tends to decrease.

When the sintering aid-derived component is an inorganic oxide further containing at least one element selected from silicon (Si) and iron (Fe) in addition to sodium (Na), the sintering aid-derived component is Compared with the case of an inorganic oxide containing only sodium (Na), an alumina substrate with improved sinterability is obtained.
That is, when the sintering aid in the state before sintering of the sintering aid-derived component further contains one or more oxides selected from Si oxide and Fe oxide in addition to Na oxide, It becomes easy to form a glass phase that becomes a grain boundary phase.

In addition, the sintering auxiliary agent-derived component further contains at least one element selected from calcium (Ca) and magnesium (Mg) in addition to sodium (Na), silicon (Si) and iron (Fe). It may be a thing.

When the sintering auxiliary agent-derived component is an inorganic oxide containing all of sodium (Na), silicon (Si) and iron (Fe), the sintering auxiliary agent-derived component is very easy to form a homogeneous glass phase. Therefore, it is most preferable.

In addition, the substance which converted Na, Si, and Fe and the compound of these elements into the same oxide as a sintering auxiliary agent among sintering auxiliary agent origin components is called a sintering auxiliary agent component. For example, when the sintering aid-derived component is an inorganic oxide containing Na, Si and Fe, and compounds of these elements, Na ₂ is a substance obtained by converting these elements into the same oxide as the sintering aid. O, SiO ₂ , and Fe ₂ O ₃ are sintering aid components.
In addition, components other than the sintering aid component among the sintering aid-derived components are referred to as inevitable impurity components.

<Alumina crystal grains>
The alumina crystal grains of the alumina substrate have an average crystal grain size of usually 20 μm or less, preferably 13 μm or less. Since the alumina substrate used in the present invention has high sinterability, the average crystal grain size of the alumina crystal grains becomes as small as 20 μm or less.

Here, the average and the crystal grain size is the average value of the grain diameter D _c calculated from a plurality of alumina crystal grains observed by cross-sectional observation of the alumina substrate as follows.
That is, as shown in FIG. 4, when one alumina crystal grain 22 is observed, first, the length of the line segment selected so that the diameter of the alumina crystal grain 22 is the largest is taken as the major axis L1. Next, a vertical line perpendicular to the line segment constituting the major axis L1 and passing through the midpoint of the line segment constituting the major axis L1 is drawn, and the length of the portion representing the diameter of the alumina crystal grain in the vertical line is drawn. The short diameter is L2. Furthermore, the (L1 + L2) / 2, to calculate the grain size _{D c} of one of the alumina grains 22. Then, this operation is performed for 100 alumina crystal grains in the field of view of the cross section of the alumina substrate, and the average value of the 100 crystal grain diameters D _c is defined as the average crystal grain diameter of the alumina crystal grains.

Alumina crystal grains of alumina substrate, variation of the crystal grain size D _c is small. That is, in the alumina substrate, the following ratio N _A / N _t which is an index indicating the small variation in the crystal grain size D _c of the alumina crystal grains is usually 80% or more, and the variation in the crystal grain size D _c is small.
Here, the ratio N _A / N _t is observed within the observation range with respect to the total number N _t of alumina crystal grains observed within the observation range of unit area 200 μm × 200 μm by cross-sectional observation of the alumina substrate. This means the ratio N _A / N _t of the number N _A of alumina crystal grains within the range of 0.3 A to 1.7 A when the average crystal grain size of the crystal grains is A μm.

As described above, the alumina substrate used in the present invention has a small average crystal grain size of alumina crystal grains of 20 μm or less and a small variation in the crystal grain size D _c of the alumina crystal grains. The triple point between the alumina crystal grains is small, the number of voids is small, and the size of the voids is small. Here, the triple point between alumina crystal grains means a grain boundary part surrounded by three alumina crystal grains.

<Void of alumina substrate>
The voids of the alumina substrate are usually voids or depressions generated at the triple point between the alumina crystal grains.
The alumina substrate has an average void diameter of 10 μm or less, preferably 5 μm or less.
The alumina substrate has a maximum void diameter of 12 μm or less, preferably 10 μm or less. A void is formed in the gap between alumina crystal particles. When the maximum diameter of the void exceeds 12 μm, there is a possibility that the mechanical strength and dielectric strength of the alumina substrate may be lowered because a partially densified region is formed on the alumina substrate.

Here, the average diameter of the void refers to the average value of the diameter D _v of the void calculated from 100 voids observed by cross-sectional observation of the alumina substrate as follows.
That is, first, with respect to the cross section of the alumina substrate, an enlarged photograph capable of obtaining an observation range of a unit area of 200 μm × 200 μm or 100 μm × 100 μm was taken and measured so that the diameter of each void existing in the observation range was the largest. the value and individual void diameter D _v. Then performed for 100 voids measurements randomly chosen within the observation range of the diameter D _v of the void, to define the average value of 100 void of diameter D _v and the average diameter of the void.

Further, the maximum diameter of the voids, means the maximum value of the diameter D _v of the void calculated from 100 voids observed by cross-sectional observation of the alumina substrate as described above.

It is preferable to use a secondary electron image of an SEM photograph as an enlarged photograph used for cross-sectional observation of the alumina substrate. The magnification of the enlarged photograph is preferably 250 times or more, and more preferably 500 times or more.

Note that when the cross section of the alumina substrate is cut out for observing the cross section of the alumina substrate, the alumina crystal may be shed from the cross section. However, degranulation is a phenomenon in which the alumina particles fall off as they are, so that the crystallization of alumina crystal particles and voids can be distinguished in the cross-sectional observation of the alumina substrate.

In the alumina substrate, the number of voids per unit area 100 μm × 100 μm calculated by cross-sectional observation is usually 2 to 30, preferably 5 to 20.
When the number of voids per unit area 100 μm × 100 μm calculated by cross-sectional observation of the alumina substrate is 2 to 30, the alumina substrate has high strength and high bonding strength with the metal circuit board.

The bonding strength between the alumina substrate and the metal circuit board is a shape in which the unevenness of the surface of the alumina substrate and the surface of the metal circuit board are complicated, that is, the surface of the metal circuit board is deformed following the unevenness of the surface of the alumina substrate. By taking the shape, it is considered that the anchor effect occurs and becomes high. The unevenness on the surface of the alumina substrate is formed by the surface shape of the alumina crystal grains, the surface shape of the component derived from the sintering aid, and the shape of the void. The size of the unevenness on the surface of the alumina substrate is usually that of the void. The unevenness due to the shape becomes the largest. For this reason, if the number of voids per unit area of 100 μm × 100 μm is two or more on the surface of the alumina substrate, the bonding strength between the alumina substrate and the metal circuit board tends to be high.

Here, the void number of per unit area 100 [mu] m × 100 [mu] m calculated by the cross-section observation, means the number N _v of the voids is calculated as follows.
That is, first, an enlarged photograph capable of obtaining an observation range having a unit area of 200 μm × 200 μm or 100 μm × 100 μm is taken for the cross section of the alumina substrate, and the total number N _vT of voids existing in the observation range is counted. Next, the total number N _vT of voids is converted into the number per 100 μm × 100 μm, and the number N _v100 of voids per 100 μm × 100 μm is calculated. Then, the calculation of the number _{N v100} of voids per this 100 [mu] m × 100 [mu] m, conducted at four points of the cross-section of the alumina substrate, to define the average value of the number _{N v100} of the four voids and the number _{N v} of the void.

In addition, when the cross-sectional observation of the alumina substrate is performed in a unit area of 200 μm × 200 μm, it can be said that the unit area of 200 μm × 200 μm includes four parts of the unit area of 100 μm × 100 μm. Thus, the voids of the number _{N v100} obtained by converting the total number _{N vT} void at one location of a unit area 200 [mu] m × 200 [mu] m in number per 100 [mu] m × 100 [mu] m, may be as it is as the number of voids _{N v.}

If the number of voids per unit area 100 μm × 100 μm calculated by cross-sectional observation of the alumina substrate is less than 2, the bonding strength with the metal circuit board may be lowered.
In addition, if the number of voids per unit area 100 μm × 100 μm calculated by cross-sectional observation of the alumina substrate exceeds 30, surface defects of the alumina substrate occur, and the mechanical strength, dielectric strength and thermal conductivity of the alumina substrate decrease. It's easy to do.

The alumina substrate has a void ratio of 3% by volume or less, which is a ratio of the volume of voids present in the alumina substrate.
Here, the void ratio is the volume of the cavity in the alumina substrate calculated by the Archimedes method.

In the alumina substrate, the void area ratio, which is the void area ratio calculated by observing the cross section of the alumina substrate, is usually 10% or less, preferably 5% or less, and more preferably 3% or less.
If the void area ratio exceeds 10%, the mechanical strength of the alumina substrate may be lowered.

Here, the void area ratio means an area ratio RS _v of voids is calculated as follows.
That is, first, an enlarged photograph capable of obtaining an observation range having a unit area of 200 μm × 200 μm or 100 μm × 100 μm is taken with respect to the cross section of the alumina substrate, and the total area of the voids S _vT Is calculated. Next, a value per 1 μm ² obtained by dividing the total area S _{vT of the} voids by the unit area is defined as a void area ratio RS _v (%).

<Characteristics of alumina substrate>
The alumina substrate has a Vickers hardness of 1500 or more. Here, the Vickers hardness means the Vickers hardness defined in JIS-R-1610.
The alumina substrate usually has a withstand voltage of 25 KV / mm or more. Here, the withstand voltage is that each ceramic circuit board is immersed in insulating oil (trade name Fluorinert), electrodes are arranged on metal circuit boards bonded to both surfaces of the ceramic board, and 10 kV / min between these electrodes. AC voltage is applied at the rate of voltage rise. An applied voltage when discharging a charge amount of 10 pC (picocoulomb) is defined as a partial discharge start voltage, and a partial discharge start voltage per unit thickness of the substrate is defined as a withstand voltage.
The alumina substrate has a fracture toughness value of 3.2 MPa · m ^1/2 or more. Here, the fracture toughness value means a fracture toughness value defined in JIS-R-1607.
The alumina substrate usually has a thermal conductivity of 28 W / m · K or more. Here, the thermal conductivity means a thermal conductivity measured by a laser flash method according to JIS-R-1611.
The alumina substrate usually has a bending strength (three-point bending strength) of 400 MPa or more. Here, the bending strength (three-point bending strength) means a dielectric strength specified by JIS-R-1601.

The thickness of the alumina substrate is usually 0.25 to 1.2 mm.

<Alumina substrate manufacturing method>
Next, the manufacturing method of an alumina substrate is demonstrated.
For example, an alumina substrate can be manufactured by preparing an alumina powder and a sintering aid, performing a slurry adjustment process or a granulation process, performing a molding process, performing a degreasing process, and performing a sintering process. it can.

[Alumina powder]
The purity of the alumina powder is usually 99.5 to 99.9% by mass.

The alumina powder usually contains Na, Si and Fe, or substances containing other elements as components other than alumina.
In the present invention, among the substances contained in the alumina powder, Na, Si and Fe, and compounds of these elements are substances consisting of the same elements as the sintering aid, and hence are referred to as sintering aid component impurities.
Of the substances contained in the alumina powder, substances other than alumina and sintering aid component impurities are referred to as inevitable impurities.
Since the sintering aid component impurity is a substance composed of the same elements as the sintering aid, it functions as a sintering aid in the sintering process. For this reason, it is preferable to handle the sintering aid component impurities contained in the alumina powder as part of the sintering aid.

As a method of handling the sintering aid component impurity as a part of the sintering aid, the mass of the sintering aid component impurity is the mass converted to the sintering aid, and this converted mass is used as the sintering aid. A method of making part of the mass is used. Specifically, when the sintering aid component impurities are Na, Si, and Fe, and compounds of these elements, these are used as the sintering aids Na ₂ O, SiO ₂ , and Fe ₂ O, respectively. After converting to ₃ , the mass of these oxides is handled as the mass of the sintering aid. For example, when SiO ₂ converted from the Si component in the sintering aid component impurities contained in the alumina powder is Ag, and SiO ₂ added as a sintering aid to this alumina powder is Bg, the sintering aid The total mass of the SiO ₂ agent is A + Bg.

The average particle size of the alumina powder is usually 1 to 4 μm.
Further, if the alumina powder contains 2 to 30% by mass of alumina powder having a particle size of 0.8 μm or less, the void size of the resulting alumina substrate can be reduced or the number of voids can be reduced. preferable. The reason for this is as follows. Voids are generated in the gaps between the alumina crystal grains. An alumina powder containing 2 to 30% by mass of an alumina powder having a particle diameter of 0.8 μm or less has a moderate distribution of large and small powders. A structure can be adopted in which small alumina powder enters the gap. For this reason, the alumina substrate obtained from the alumina powder having such a structure has a small void size and a small number of voids.

[Sintering aid]
As a raw material for the alumina substrate, a sintering aid is used in addition to the alumina powder.
As a sintering aid, at least sodium oxide (Na ₂ O) is used. The sintering aid may contain one or more oxides selected from silicon oxide (SiO ₂ ) and iron oxide (Fe ₂ O ₃ ) in addition to sodium oxide (Na ₂ O).
The sintering aid preferably contains all of sodium oxide (Na ₂ O), silicon oxide (SiO ₂ ) and iron oxide (Fe ₂ O ₃ ).

The sintering aid is selected from calcium oxide (CaO) and magnesium oxide (MgO) in addition to sodium oxide (Na ₂ O), silicon oxide (SiO ₂ ) and iron oxide (Fe ₂ O ₃ ). It may further contain an oxide of seeds or more.
As a sintering aid, a powdery one is used.

The sintering aid is mixed with the alumina powder in the subsequent slurry adjustment step or granulation step. The total amount of the mixed powder of alumina powder and sintering aid, and the mass M _A converted sintering aid component impurities contained in the alumina powder sintering aid, and the mass M _S sintering aids M _A + M _S is contained at 0.5 mass% or less.

In addition, when the ball milling using alumina balls mill in slurry adjusting process after, in consideration of the amount M _B of the sintering auxiliary component impurities incorporated from the alumina balls mill, determine the amount of sintering aid. That is, the mixed powder of alumina powder and sintering _aid, is _M _{A +} M S ₊ M _B to be included 0.5 mass%. Here, the sintering aid component impurity taken in from the alumina ball mill is the same material as the sintering aid component impurity contained in the alumina powder.

Thus, in consideration of the amount of impurities Na, Si, Fe mixed in the raw material powder mixing and granulation process, 0.001 to 0.1% by mass of sodium oxide as a sintering aid in the mixed powder Add to be. Further, if necessary, iron oxide is added so as to be 0.001 to 0.2 mass% in the mixed powder, and iron oxide is added to 0.001 to 0.05 mass% in the mixed powder.

[Slurry adjustment process]
The slurry adjustment step is a step of preparing a slurry by mixing alumina powder and sintering aid powder. For the slurry, for example, alumina powder and sintering aid powder are added in pure water or an organic solvent, and a binder such as PVA (polyvinyl alcohol) is further added as necessary. It can be prepared by pulverizing the auxiliary powder.
The balls used in the ball mill are preferably made of alumina. However, alumina balls made of alumina usually have an alumina purity of about 96% and contain a relatively large amount of impurities such as Na, Si, and Fe. For this reason, in the ball mill treatment using the alumina balls, it is preferable to mix the sintering powder with an amount of the sintering aid powder taking into account impurities such as Na, Si and Fe mixed in the slurry from the alumina balls.
In the method for producing an alumina substrate, the above-described slurry adjustment step or the following granulation step is selected and performed.

[Granulation process]
The granulation step is a step of mixing and granulating alumina powder and sintering aid powder.
The granulated powder obtained by granulation is, for example, a wet ball mill after adding alumina powder and sintering aid powder in pure water or an organic solvent, and further adding a binder such as PVA (polyvinyl alcohol) if necessary. Thus, the alumina powder and the sintering aid powder can be pulverized and further granulated with a wet granulator.

[Molding process]
After performing the slurry adjusting step or the granulating step, the forming step is performed.
The molding step is a step of producing a molded body using the slurry obtained in the slurry adjustment step or the granulated powder obtained in the granulation step.
When using a slurry, a shaping | molding process produces a plate-shaped molded object, for example using a doctor blade method. When using granulated powder, a shaping | molding process produces a plate-shaped molded object, for example using a metal mold | die molding method.
When the thickness of the plate-shaped molded body is 1 mm or less, it is preferable to use a doctor blade method.

[Degreasing process]
A degreasing process is a process of degreasing the obtained plate-shaped molded object.
In the degreasing step, the plate-shaped molded body is degreased by heat treatment usually at 400 to 900 ° C.

[Sintering process]
A sintering process is a process of sintering the degreased plate-shaped molded object.
In the sintering step, when sintering at normal pressure, the sintering is usually performed by heat treatment at 1200 to 1700 ° C. for 2 to 12 hours, preferably 1200 to 1680 ° C. for 5 to 12 hours.
In the case of sintering under a pressure of 0.5 MPa or more, it can be sintered by heat treatment usually at 1200 to 1700 ° C. for 2 to 6 hours, preferably 1200 to 1680 ° C. for 2 to 5 hours.

Further, the sintering process may be performed in two stages with different temperature ranges for the heat treatment.
For example, after heat treatment at a temperature range of 1450 to 1650 ° C. for 4 to 7 hours, heat treatment may be performed at less than 1450 ° C. for 2 to 3 hours.
In this way, since the sintering process does not continue to sinter for a long time at a high temperature, but after sintering for a certain period of time, by sintering at a slightly low temperature, the grain growth of the alumina crystal grains can be suppressed. It becomes easier to control the size and number.
As described above, since the sintering time can be set to 12 hours or less in the main sintering step, it is not necessary to perform a long-time heat treatment sintering step of 20 hours as disclosed in Patent Document 1.

The reason why the sintering time of the sintering process is short in the present invention is mainly because the total amount of sintering aid powder and sintering aid component impurities in the degreased plate-like molded body is appropriate. It is believed that there is.

That is, the plate-shaped molded body produced from the slurry or granulated powder and degreased has a sintering aid component impurity contained in the alumina powder in a total amount of 100 mass% of the alumina powder and the sintering aid powder. The total amount M _A + M _S of the mass M _A converted into a sintering aid and the mass M _S of the sintering aid is 0.5% by mass or less.

In addition, when ball milling is performed using an alumina ball mill in the slurry adjustment step, the plate-shaped molded body produced from the slurry or granulated powder and degreased has an amount M of the sintering aid component impurities taken in from the alumina ball mill. M _A + M _S + M _B including _B is included in an amount of 0.5% by mass or less.

Here, at least one of the sintering aid powder and the sintering aid component impurity contains an Na component, and if necessary, further contains at least one component selected from an Si component and an Fe component. The component functions as a sintering aid.

For this reason, in the sintering process of a high-purity alumina substrate containing 99.5% by mass or more of the alumina Al ₂ O ₃ of the present invention, the high-purity alumina substrate is sintered using only alumina powder without using a sintering aid. Compared with the case of sintering, the sintering temperature can be lowered by about 20 to 50 ° C., and the sintering time can be shortened to 12 hours or less.

Thus, in the sintering step, the sintering temperature can be lowered or the sintering time can be shortened, so that growth of alumina crystal grains due to sintering can be suppressed. Thus, an alumina substrate obtained is alumina crystal grains is reduced, variation of the crystal grain size D _c of alumina crystal grains is reduced, it is possible to suppress the occurrence of voids, reducing the size of the generated voids be able to.
Specifically, the obtained alumina substrate has an average crystal grain size of alumina crystal grains of usually 20 μm or less, preferably 10 μm or less, and a ratio N _A / N _t showing variation in crystal grain size D _c is usually 80 % Or more.
The resulting alumina substrate has an average void diameter of 10 μm or less, preferably 5 μm or less, and a void area ratio of usually 10% or less.

The alumina substrate obtained through the above steps is bonded to the metal circuit board. A circuit is formed on the metal circuit board bonded to the alumina substrate by appropriately using etching or the like. In the present invention, both a metal circuit board on which a circuit is not formed and a metal circuit board on which a circuit is formed are simply referred to as a metal circuit board.
The obtained alumina substrate is appropriately subjected to a process of removing dust on the surface by a honing process and a process of polishing the surface as a process before being bonded to the metal circuit board. In addition, when using a direct joining method as a joining method of an alumina substrate and a metal circuit board, it is preferable to only remove the surface dust by a honing process.

(Metal circuit board)
The metal circuit board is bonded onto the alumina substrate. Here, the metal circuit board is a concept including both a metal circuit board in which a circuit is formed using etching or the like and a metal circuit board in which a circuit is not formed.

The alumina substrate and the metal circuit board are bonded by a method such as a direct bonding method (DBC method) or an active metal method.
Here, the direct bonding method (DBC method) refers to, for example, using a reaction in which copper and oxygen of a metal circuit board form a eutectic compound (Cu—O eutectic) to connect the alumina substrate and the metal circuit board. It is a method of joining.
The active metal method is a method of joining an alumina substrate and a metal circuit board using an active metal joining brazing paste.

In the ceramic circuit board of the present invention, the metal circuit board is bonded to the alumina substrate by a method such as a direct bonding method or an active metal method.

<When a metal circuit board is bonded to an alumina substrate by a direct bonding method>
When the metal circuit board is bonded to the alumina substrate by a direct bonding method, a copper circuit board made of copper is usually used as the metal circuit board.
Since the bonding method between the alumina substrate and the metal circuit board is a direct bonding method, the copper circuit board is bonded to the alumina substrate by a Cu—O eutectic compound.
The thickness of the copper circuit board is usually 0.1 to 0.5 mm.

The copper circuit board is preferably made of tough pitch electrolytic copper containing 100 to 1000 ppm by mass of oxygen. By using such a copper circuit board, the bonding strength with the alumina substrate is increased.

The copper circuit board preferably contains 0.1 to 1.0% by mass of carbon. Examples of the copper material constituting such a copper circuit board include tough pitch copper and oxygen-free copper.
Since carbon functions as a deoxidizer, oxygen in the copper circuit board is moved to the surface of the copper circuit board. In addition, the oxygen that has moved to the surface of the copper circuit board is used to form a Cu—O eutectic compound when performing the direct bonding method.
In addition, when the carbon content of the copper circuit board is less than 0.1% by mass, there is no effect of carbon content, and when the carbon content exceeds 1.0% by mass, the carbon content increases too much and the conductivity of the copper circuit board is increased. Reduce sex.

When a copper circuit board having an oxygen content of less than 100 ppm by mass is used as the copper circuit board, the copper circuit board is bonded to the alumina substrate by forming a copper oxide film on the bonding surface side of the copper circuit board with the alumina substrate. Strength can be increased.

Examples of a method for forming a copper oxide film on the surface of a copper circuit board include a method of directly oxidizing a copper circuit board by heat treatment, a method of applying a copper oxide powder paste, and the like.

[Direct oxidation method]
As a method for direct oxidation, for example, a copper oxide film is formed on the surface of a copper circuit board by performing a surface oxidation treatment in which the copper circuit board is heated in the atmosphere at a temperature of 150 to 360 ° C. for 20 to 120 seconds. Is used.

When the direct oxidation method is used, the copper oxide film has a thickness of usually 1 to 10 μm, preferably 2 to 5 μm.
When the thickness of the copper oxide film is less than 1 μm, the amount of Cu—O eutectic compound generated is reduced, and the unbonded portion between the alumina substrate and the copper circuit board is increased, thereby improving the bonding strength. Becomes smaller.
On the other hand, when the thickness of the copper oxide film exceeds 10 μm, the effect of improving the bonding strength is small, and on the contrary, the conductive characteristics of the copper circuit board are hindered.

[Method of applying copper oxide powder paste]
As a method of applying the copper oxide powder paste, for example, a paste containing copper oxide powder having an average particle diameter of 1 to 5 μm is used, and the paste is applied on a copper circuit board to have a thickness of 1 to 10 μm. After the layer is formed, a method of forming a copper oxide film on the surface of the copper circuit board by drying or heat treatment is used.

<When a metal circuit board is bonded to an alumina substrate by an active metal method>
When the metal circuit board is bonded to the alumina substrate by the active metal method, the metal circuit board includes copper, aluminum, iron, nickel, chromium, silver, molybdenum, cobalt alone, alloys thereof, and cladding materials thereof. Is used. Among these, a copper plate and an aluminum plate are preferable because of good bonding properties.

The thickness of the metal circuit board is determined in consideration of the current carrying capacity and the thickness of the alumina substrate. Specifically, when the thickness of the alumina substrate is 0.25 to 1.2 mm, the thickness of the metal circuit board is preferably 0.1 to 0.5 mm. When the thickness of the alumina substrate is 0.25 to 0.38 mm, the thermal resistance is reduced and the heat dissipation of the ceramic circuit board can be improved.

Examples of the active metal bonding brazing paste used in the active metal method include 15 to 35% by mass of Cu and 1 to 10% by mass of at least one active metal selected from Ti, Zr, and Hf. An active metal bonding braze paste prepared by dispersing a bonding composition substantially consisting of Ag in the balance in an organic solvent is used.
The active metal compounded in the active metal bonding brazing paste improves the wettability and reactivity of the active metal bonding brazing material to the alumina substrate. The compounding amount of the active metal in the active metal joining brazing paste is 1 to 10% by mass with respect to 100% by mass of the joining composition contained in the active metal joining brazing paste.

In the active metal method, for example, an Ag—Cu based brazing material containing at least one active metal selected from Ti, Zr and Hf and having an appropriate composition ratio is used, and this Ag—Cu based brazing material is used as an organic solvent. A bonding composition paste is prepared by dispersing in, and this bonding composition paste is screen-printed on the surface of the alumina substrate, and a copper plate as a metal circuit board is superimposed on the surface of the alumina substrate, and heated. An alumina substrate and a metal circuit board can be joined.

(Junction interface between alumina substrate and metal circuit board)
The joining interface between the alumina substrate and the metal circuit board has an intricate structure in which the surface of the metal circuit board is deformed along the irregular shape of the surface of the alumina substrate. Specifically, the bonding interface between the alumina substrate and the metal circuit board is a curve along the surface of the alumina substrate when the cross section of the ceramic circuit board is observed. The contact ratio (hereinafter referred to as “bonding interface contact ratio”) is usually 95% or more, preferably 99% or more, and more preferably 100%.

The bonding interface contact ratio is an index indicating the followability of the metal circuit board to the irregularities on the surface of the alumina substrate.
For example, when the alumina substrate and the metal circuit board are bonded together without any gap, the bonding interface contact ratio is 100%. Further, when the alumina substrate and the metal circuit board are completely separated, the bonding interface contact ratio is 0%.

The calculation method of the bonding interface contact ratio is as follows.
That is, first, when the cross section of the ceramic circuit board is observed, an enlarged photograph of the bonded cross section is taken. The enlarged photograph of the bonded cross section is preferably 1000 times or more.
The enlarged photograph is taken over a length of 100 μm at the bonding interface. If a length of 100 μm cannot be photographed in one field of view, photographing may be performed every 20 to 50 μm, and a total of 100 μm may be photographed.
Next, from the enlarged photograph, the length L _{A of the} curve along the unevenness of the surface of the alumina substrate at the bonding interface and the length L _M of the curve along the surface of the metal circuit board at the bonding interface are measured.
Then, to calculate the _L M / _{L A} obtained by dividing the _{L M} with _{L A} as a joining interfacial contact ratio.

The bonding interface contact ratio tends to decrease when deep voids are exposed on the surface of the alumina substrate. If deep voids are exposed on the surface of the alumina substrate, the bonding ratio of the interface contact tends to decrease because the metal circuit plate is less likely to follow the surface of the alumina substrate when the alumina substrate is bonded to the metal circuit plate. .
In the ceramic circuit board of the present invention, there is substantially no exposure of deep voids on the surface of the alumina substrate, and the alumina substrate and the metal circuit board are bonded under specific conditions. The metal circuit board follows and enters. For this reason, the ceramic circuit board of the present invention has a high bonding interface contact ratio of usually 95% or more, and has a structure in which the surface of the alumina substrate and the metal circuit board are intricate, resulting in an anchor effect and high bonding strength. .

(Manufacturing method of ceramic circuit board)
The ceramic circuit board is manufactured by bonding an alumina substrate and a metal circuit board. As described above, for example, a direct bonding method (DBC method) or an active metal method is used as the method for bonding the alumina substrate and the metal circuit board.

<Direct bonding method>
In the direct bonding method, first, a copper circuit board as a metal circuit board is disposed on an alumina substrate. When an oxide film (copper oxide film) is formed on the copper plate, the oxide film is disposed on the alumina substrate side. Next, when heated to, for example, 1065 to 1085 ° C. in an inert gas atmosphere, a ceramic circuit board in which a copper circuit board is bonded onto an alumina substrate is obtained.

<Active metal method>
In the active metal method, first, an active metal bonding brazing paste is applied on an alumina substrate by a method such as screen printing. Next, when a metal circuit board is placed on the surface of the alumina substrate on which the active metal bonding brazing material paste is applied and heated, a ceramic circuit board in which the copper circuit board is bonded onto the alumina substrate is obtained.

According to the ceramic circuit board having the above configuration, the alumina purity is 99.5% or more and the alumina substrate containing a predetermined amount of sodium or the like is used, compared with the conventional high-purity alumina substrate having a purity of 99.5% or more. In addition, while maintaining the characteristics of the substrate such as strength, the sintering time can be shortened or the sintering temperature can be lowered, so that the manufacturing cost can be greatly reduced.

Also, since the alumina substrate is dense and there are few surface defects derived from voids, even when the substrate thickness is reduced, the withstand voltage characteristic is hardly lowered and the occurrence of dielectric breakdown (withstand voltage leak) is suppressed. Moreover, the joint strength of a metal circuit board can be improved by making the surface defect (surface unevenness | corrugation) originating in a void into a predetermined range.

Examples are shown below, but the present invention is not limited to these examples.

Example 1
Na ₂ O as a sintering aid is added to a high-purity alumina powder composed of α-alumina crystals having an average particle size of 1.5 μm (less than 0.8 μm is 15% by mass) and having a purity of 99.9%. 0.1% by mass, 0.2% by mass of SiO ₂ and 0.05% by mass of Fe ₂ O ₃ were added, and an organic binder was added, and a raw material mixed paste was prepared by a ball mill (using 96% purity alumina balls) did. Each raw material mixed paste was formed into a sheet by a doctor blade method to prepare a plate-shaped molded body, and this molded body was heated in a vacuum of 10 ⁻⁴ Torr at 800 ° C. for 8 hours for complete degreasing. This degreased body was sintered at a temperature of 1580 ° C. for 8 hours to prepare an alumina substrate having a length of 29 mm × width of 69 mm × thickness of 0.32 mm.

(Comparative Example 1)
As Comparative Example 1, a high-purity alumina powder consisting of α-alumina crystals having an average particle size of 1.5 μm (0.8 μm or less is 35 mass%) and having a purity of 99.9% is used, and Na ₂ O is not added. Except for this, an alumina substrate was prepared in the same manner as in Example 1.

(Comparative Example 2)
An alumina substrate was prepared in the same manner as in Comparative Example 1 except that the sintering conditions were 1600 ° C. × 20 hours.

(Comparative Example 3)
An alumina substrate was prepared in the same manner as in Example 1 except that the amount of Na ₂ O added was 0.3% by mass.

[Evaluation of alumina substrates of Example 1 and Comparative Examples 1 to 3]
Al ₂ O ₃ purity of each alumina substrate, average crystal grain size, variation in crystal grain size, void area ratio, void average diameter, maximum void diameter, number of voids, dielectric strength, bending strength, fracture toughness value, The thermal conductivity and Vickers hardness were measured and the results shown in Table 1 were obtained.
The void ratio, void average diameter, average crystal grain size, and variation in crystal grain size were measured by observing the cross section of the alumina substrate.
That is, an enlarged photograph having a unit area of 200 μm × 200 μm was taken, the area of each void in the enlarged photograph was measured, and the number obtained by dividing the total area by 200 μm × 200 μm was defined as the void area ratio.
Moreover, the value measured so that a diameter might become the largest about each void was made into the maximum diameter, and the average value for 100 voids was made into the void average diameter. Further, the number of voids was measured at four locations per unit area of 100 μm × 100 μm, and the minimum number and the maximum number were shown.
The average grain size of the alumina crystal grains is such that the length of the line segment selected so that the diameter of each alumina crystal grain is the largest is the major axis L1, and the vertical axis is drawn from the center of the minor axis L2. And (L1 + L2) / 2 was defined as the particle size. This operation was performed 100 grains, and the average value was defined as the average grain diameter.
As for the variation in the crystal grain size of the alumina crystal grains, the ratio (%) of the number of crystal grains falling within the range of A × (0.3 to 1.7) with respect to the average crystal grain size A μm was obtained.
Dielectric strength, bending strength (3-point bending strength), thermal conductivity, fracture toughness value and Vickers hardness are JIS-R-1601 (bending strength), JIS-R-1607 (fracture toughness value), JIS-R. It was determined by the method described in -1610 (Vickers hardness), JIS-R-1611 (thermal conductivity) and the like. Moreover, the withstand voltage was performed by the method using the partial discharge start voltage using insulating oil (trade name Florinart) as described above.
These results are shown in Tables 1 to 3.
FIG. 5 is a schematic cross-sectional view of the bonding interface of the ceramic circuit board according to the first embodiment. As shown in FIG. 5, the alumina substrate 2 of the ceramic circuit board 1 has voids 23 a in the alumina substrate 2 and voids 23 b on the joint surface with the metal circuit board (copper circuit board) 3. It was. The metal circuit board (copper circuit board) 3 is in close contact with the surface of the alumina substrate 2 at the interface between the alumina substrate 2 of the ceramic circuit board 1 and the metal circuit board (copper circuit board) 3.

The alumina substrate according to this example exhibited the same characteristics as those of Comparative Example 2 which is a high-purity material. Moreover, even if it was a high purity material like the comparative example 1, many voids generate | occur | produced when sintering time was short. This is because the sinterability is poor. Moreover, even if Na ₂ O was added excessively as in Comparative Example 3, the sinterability decreased.

(Examples 2 to 5)
An alumina substrate was prepared in the same manner as in Example 1 except that the amount of sintering aid and the sintering conditions were changed as shown in Table 1, and the same measurement as in Example 1 was performed. These results are shown in Tables 1 to 3.

The alumina substrate according to this example showed excellent characteristics even when the sintering time was 10 hours or less.

(Examples 1B to 6B, Comparative Examples 1B to 3B)
Ceramic circuit boards were prepared using the alumina substrates and copper plates of Examples 1 to 5 and Comparative Examples 1 to 3. The copper plate was prepared by heat-treating to form a 4 μm thick copper oxide film on the bonding surface side. Copper plates (one is a copper plate for a metal circuit board and the other is a back copper plate) are arranged on both sides of the alumina substrate, and are heated by a direct bonding method in a nitrogen atmosphere at 1075 ° C. for 1 minute. In addition, the copper plate for metal circuit boards was unified with a thickness of 0.3 mm, and the back copper plate with a thickness of 0.4 mm. Examples 1B to 5B were prepared with a carbon content in the copper plate in the range of 0.2 to 0.8 mass%, and 6B with no carbon contained (below the detection limit).

Next, the metal circuit board of the ceramic circuit board obtained was etched to form the circuit pattern shown in FIG.

For each ceramic circuit board prepared as described above, both ends of the circuit pattern surface on the front side are supported by a support span of 30 mm, and a load is applied to one point in the center of the back copper plate on the back side to obtain three points. The bending strength was measured, and the maximum amount of deflection with respect to the plane including both edges of the alumina substrate was measured. In addition, the bending strength value of each ceramic circuit board has shown the load value at the time of an alumina substrate fracture | rupture as a stress value with respect to an alumina substrate single-piece | unit. The maximum amount of deflection was measured as the amount of deflection when the alumina substrate broke. Table 4 shows the measurement results.

Examples 1B to 6B were found to have the same measurement results as the ceramic circuit board (DBC circuit board) using the high-purity alumina substrate of Comparative Example 2. On the other hand, the strength of the DBC circuit boards according to Comparative Example 1B and Comparative Example 3B decreased.
Next, the bonding strength and the bonding interface state of the copper circuit boards of the ceramic circuit boards of Examples 1B to 6B and Comparative Examples 1B to 3B were examined. The bonding strength was determined by a peel test.
In addition, an enlarged photograph (2000 times) of the bonding interface between the alumina substrate and the copper circuit board was taken at the bonding interface, and this work was taken for 100 μm of the bonding interface. It was investigated how the copper circuit board was bonded so as to cover the surface irregularities of the alumina substrate at the bonding interface. The results are shown in Table 5.

As can be seen from the table, the ceramic circuit board according to this example was excellent in bonding strength. Further, when Example 1B and Example 6B were compared, Example 1B was superior in bonding strength. This is presumably because oxygen contained in the copper plate moved to the surface of the copper plate and contributed to the Cu—O eutectic reaction by containing a predetermined amount of carbon in the copper plate. For this reason, it is considered that the ratio of the copper circuit board covering the surface irregularities of the alumina substrate at the bonding interface has increased.

1 Ceramic circuit board 2 Alumina substrate 3 Metal circuit board (copper circuit board)
4 Back metal plate (back copper plate)
22

Alumina crystal grains

23a, 23b Void

Claims

In a ceramic circuit board in which a metal circuit board is bonded on an alumina substrate,
The alumina substrate contains 99.5% by mass or more of alumina Al 2 O 3 and less than 0.5% by mass of a sintering aid-derived component generated from a sintering aid blended before sintering,
The component derived from the sintering aid is an inorganic oxide containing sodium, and sodium in the component derived from the sintering aid is 0.001 to 100% by mass in terms of sodium oxide Na 2 O in 100% by mass of the alumina substrate. Contained 0.1% by weight,
The alumina substrate has a maximum void diameter of 12 μm or less, an average void diameter of 10 μm or less, and a Vickers hardness of 1500 or more.
The component derived from the sintering aid is an inorganic oxide further containing silicon, and the silicon in the component derived from the sintering aid is 0.001 to 100% by mass in terms of silicon oxide SiO 2 in 100% by mass of the alumina substrate. The ceramic circuit board according to claim 1, wherein the ceramic circuit board is contained in an amount of 0.2% by mass.
The component derived from the sintering aid is an inorganic oxide further containing iron, and the iron in the component derived from the sintering aid is a mass converted to iron oxide Fe 2 O 3 in a mass of 100% by mass of the alumina substrate. The ceramic circuit board according to claim 1, wherein the ceramic circuit board is contained in an amount of 001 to 0.05 mass%.
4. The ceramic circuit board according to claim 1, wherein the alumina substrate has an alumina crystal grain having an average crystal grain size of 20 μm or less. 5.
The ceramic circuit according to any one of claims 1 to 4, wherein the alumina substrate has a void volume ratio, which is a volume ratio of voids existing in the alumina substrate, of 3% by volume or less. substrate.
6. The ceramic circuit according to claim 1, wherein the alumina substrate has 2 to 30 voids per unit area of 100 μm × 100 μm calculated by cross-sectional observation. substrate.
The ceramic circuit board according to any one of claims 1 to 6, wherein the alumina substrate has a void area ratio, which is a ratio of an area of voids in a cross section of the alumina substrate, of 10% or less. .
The ceramic circuit board according to any one of claims 1 to 7, wherein the alumina substrate has a withstand voltage of 25 KV / mm or more.
The ceramic circuit board according to any one of claims 1 to 8, wherein the alumina substrate has a toughness value of 3.2 MPa · m 1/2 or more.
The ceramic circuit board according to any one of claims 1 to 9, wherein the alumina substrate has a thermal conductivity of 28 W / m · K or more.
11. The ceramic circuit board according to claim 1, wherein the alumina substrate has a bending strength of 400 MPa or more.
The ceramic circuit board according to claim 1, wherein the metal circuit board is bonded to the alumina substrate by a direct bonding method.
13. The metal circuit board is a copper circuit board, and the copper circuit board is bonded to the alumina substrate by a Cu—O eutectic compound. Ceramic circuit board.
The ceramic circuit according to any one of claims 1 to 13, wherein the metal circuit board is a copper circuit board, and the copper circuit board contains 0.1 to 1.0 mass% of carbon. substrate.
The bonding interface between the alumina substrate and the metal circuit board is such that when the cross section of the ceramic circuit board is observed, the curve along the surface of the metal circuit board becomes a curve along the unevenness of the surface of the alumina substrate. The ceramic circuit board according to any one of claims 1 to 14, wherein the ceramic circuit board has an intricate structure with a contact ratio of 95% or more.
The ceramic circuit board according to any one of claims 1 to 15, wherein the alumina substrate has a thickness of 0.25 to 1.2 mm.
The ceramic circuit board according to any one of claims 1 to 16, wherein the metal circuit board has a thickness of 0.1 to 0.5 mm.