JPH05243725A - Direct bonding method of ceramic substrate and copper - Google Patents

Abstract

PURPOSE:To provide a direct bonding method of ceramic substrate and copper, in which satisfactory soldering can be performed even if the surface and rear face of the ceramic substrate differ from each other in the area of a required circuit conductor pattern. CONSTITUTION:In the direct bonding method of ceramic and copper in which copper circuit conductors respectively differing in required area are arranged on both sides of a ceramic substrate 1 in the manner of adhering closely to each other and heated to a predetermined temperature in the atmosphere with a predetermined oxygen content so that copper is bonded to the both sides of the ceramic substrate 1, copper dummy conductors are arranged in space areas 41, 42, where required copper is not arranged, on the surface of the ceramic substrate on the side of the smaller required area of copper in the manner of adhering closely to each other.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a circuit substrate on a ceramic substrate used for a microwave integrated circuit, a high frequency plane circuit, a semiconductor mounting circuit, a microwave nonreciprocal circuit or the like, for example, a substrate of alumina, ferrite or aluminum nitride. The present invention relates to a direct bonding method of copper and a ceramic substrate for bonding copper.

[0002]

2. Description of the Related Art Ceramics are widely used as substrates for electronic circuits from direct current to super high frequency because of their excellent heat resistance, insulating properties, and mechanical properties. The material is also
In the old days, alumina, steatite, forsterite, ferrite, etc. are used, and in recent years, so-called fine ceramics such as aluminum nitride and silicon carbide are used.

When this ceramic is used as a substrate for an electronic circuit, a metal conductor film is often formed on its surface.

Conventionally, as a method of forming a metal conductor film on a ceramic, (a) molybdenum powder or the like is applied to the surface of the ceramic and sintered, and then the metal is plated with nickel or the like and the conductor film is formed by silver brazing or the like. A brazing method for joining and forming, (b) a thick film method for forming a conductor film by applying a powder paste such as copper or gold or silver / palladium, and (c) melting and evaporating metal in vacuum There are a vacuum vapor deposition method for forming a conductor film by performing the above, and (d) an active metal method for performing sintering and joining by interposing an active metal such as a titanium alloy between a metal serving as a conductor and a ceramic.

However, in recent years, a direct bond copper (hereinafter abbreviated as DBC) method of directly joining copper and ceramic without interposing another metal is a simple process and heat conduction. It has come to be used because of its goodness.

[0006] In the DBC method, ceramic and copper are brought into close contact with each other and heated to an appropriate temperature in an atmosphere having an appropriate oxygen concentration (for example, nitrogen gas) to melt only the copper in the surface layer. It is a method of joining ceramic and copper by wetting the surfaces.

FIG. 2 shows a phase diagram of copper Cu and oxygen O ₂ , in which the vertical axis represents temperature (unit: ° C.) and the horizontal axis represents oxygen concentration (unit: wt%). It shows how the phase state of copper changes when the copper is heated, depending on the oxygen concentration in the atmosphere containing the oxygen content of the copper itself.

When the temperature rises from room temperature, copper melts at 1083 ° C. when the oxygen concentration is 0. However, in the presence of oxygen, a eutectic state with oxygen exists and the melting temperature decreases. The eutectic temperature of copper and oxygen is 1065 ° C., and regions D and E in FIG. 2 are solid phases without a eutectic state.

The eutectic state is present when the oxygen concentration is 0.008% by weight, and the temperature at which it is completely dissolved is 1083 ° C.
It decreases more gradually, and coincides with the eutectic temperature when the oxygen concentration is 0.39% by weight (point F in the figure). The region A is a liquid phase, and the region B is a solid solution phase of cuprous oxide in low concentration oxygen and is a mixed phase of solid and liquid. Oxygen concentration is 0.3
Above 9% by weight, copper exhibits an oxygen-rich cupric oxide solid solution phase (region C), which is no longer compatible with the ceramic and cannot be joined.

The temperature range of copper applicable to the DBC method is 1065.
This is a region B (hatched portion) in FIG. 1 where the required oxygen concentration is 0.39 wt% or less at -1083 ° C.

As a procedure for actually producing a ceramic substrate by the DBC method, first, a copper plate having a required shape is adhered to both surfaces of the ceramic substrate, and the copper plate is placed in an electric furnace satisfying the above temperature and oxygen concentration conditions. All you have to do is leave it for a suitable time.

However, the temperature difference in the region B is as small as 18 ° C. and the oxygen concentration difference is as small as 0.39% by weight, which makes it difficult to control the temperature of the electric furnace and the amount of oxygen in the atmosphere due to the size and thickness of the ceramic or copper plate. Is coming. In particular, when the temperature of the copper plate is increased by several degrees Celsius or the amount of oxygen is increased by several tens ppm when the thickness of the copper plate is thin, not only the surface layer but also the entire copper plate is dissolved. It is difficult for the temperature control of an electric furnace to constantly maintain a constant value depending on the type, size, and amount of materials contained therein.

The ceramic substrate to be produced has various forms from direct current to microwave depending on its application. For example, conductors need only be bonded to one side,
Alternatively, it is necessary for both sides, the copper plate to be joined must be thick, or must be thin.

However, when conductors are required on both sides, most of them have different conductor shapes on both sides. This is because the conductor is bonded to the entire surface of the ceramic substrate in order to electrically use one surface as a ground conductor. On the other hand, conductor patterns such as a conductor portion for arranging a semiconductor chip and a wiring portion for connecting to an external line are joined to the other surface, and the conductor is not necessarily joined to the entire surface of the ceramic substrate.

FIG. 3 shows a ceramic substrate 1 before DBC in which such a conductor pattern is closely attached and arranged.

As shown in FIG. 3 (b), the ground conductor 3 is disposed on the back surface of the ceramic substrate 1 on substantially the entire surface of the ceramic substrate, and the front surface thereof has desired conductors 21 to 21 as shown in FIG. 3 (a).
24 are arranged. The conductors 21 to 24 on the surface are not necessarily arranged over the entire surface of the ceramic substrate 1.

[0017]

[Problems to be Solved by the Invention]
When the placed ceramic substrate 1 is placed in an electric furnace and heated, the temperature of the front copper plate and the back copper plate may be the same, but since the amounts of the front and back copper plates are different, the temperature distribution on the surface of the ceramic substrate is It becomes uneven.

FIG. 4 shows a cross section taken along the line AB of FIG. 3 (a). In this cross section, the heat distribution on the surface side of the ceramic substrate 1 is such that the portion with the conductor has an empty area 41 without the conductor, It becomes lower than the part 42. This is because the heat capacity of this microscopic area differs depending on whether or not there is a copper plate.

For this reason, there is a drawback that the regions 31, 32 of the ground conductor 3 on the back surface are melted by a slight increase in temperature. If the temperature is lowered so that the regions 31 and 32 are not melted, the temperature of the conductor 21 on the surface becomes insufficient, resulting in defective bonding.

When the regions 31 and 32 are melted,
As shown in FIG. 5, the portion is rounded and solidified by the surface tension or is attracted to the central portion of the ground conductor 3.

In such a ceramic substrate, when it is attached to the heat dissipation plate by soldering or pressure bonding, for example, an air layer is generated or the heat dissipation area becomes small, so that the high power semiconductor cannot be loaded. In the microwave band, the ground conductor 3 is often soldered to a metal plate called a carrier plate. In this case, the soldering area is reduced, resulting in a decrease in mechanical total force and peeling off due to shock or vibration.

The present invention solves the above-mentioned drawbacks. A ceramic substrate and a copper substrate which can be well soldered even when the required area of the circuit conductor pattern is different between the front surface and the back surface of the ceramic substrate. It is intended to provide a direct joining method with.

[0023]

SUMMARY OF THE INVENTION According to the present invention, copper having different area sizes of required circuit conductor patterns is closely attached to both surfaces of a ceramic substrate, and the copper is kept at a predetermined temperature in an atmosphere of a predetermined oxygen concentration. When heating and joining copper to both sides of the ceramic substrate, place the pseudo conductor copper in close contact with the empty area where the required copper is not placed on the side of the ceramic substrate where the required circuit conductor pattern area is small. To do.

[0024]

According to the above-described direct bonding method of the ceramic substrate and copper, since the areas of copper are substantially equal on both sides of the ceramic substrate, the temperature distribution can be made uniform over the entire surface of the ceramic substrate and the difference in heat capacity is small. In this condition, the ceramic substrate and copper can be directly bonded.

[0025]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. In each drawing, the same parts as those in the drawings referred to in the description of the conventional technique are designated by the same reference numerals, and detailed description thereof will be omitted.

FIG. 1 is a diagram showing a circuit conductor pattern on the front surface side of the ceramic substrate 1. In addition to the required circuit conductors 21 to 24, the pseudo conductor copper 51 is also provided in the empty regions 41 and 42 without the required circuit conductor. , 52 are arranged. Copper 51, 52
When arranging, the originally required circuit conductors 21 to 24
Function, such as insulation and high-frequency transmission characteristics, should not be affected.

Although the conductor pattern of the ground conductor 3 on the rear surface side of the ceramic substrate 1 is not shown in FIG. 5, copper is arranged over the entire surface as shown in FIG. 3B, for example.

The copper layers 51 and 52 may have the same thickness as the circuit conductors 21 to 24 or may have different thicknesses, but it is difficult to predict the heat capacity in detail.

In this way, when the circuit conductor patterns are different on the front surface side and the back surface side of the ceramic substrate, the conductors as pseudo conductor patterns are arranged on the side where the area of the required circuit conductor pattern is small.

[0030]

According to the present invention, the temperature distribution is made uniform over the entire surface of the ceramic substrate, and there is no difference in heat capacity.
The BC condition can be obtained, and good direct bonding between the ceramic substrate and copper is possible even when the required area of the circuit conductor pattern is different between the front surface side and the back surface side of the ceramic substrate.

[Brief description of drawings]

FIG. 1 is a perspective view illustrating an embodiment of the present invention.

FIG. 2 is a diagram showing a phase state of copper and oxygen.

FIG. 3 is a diagram illustrating a conventional technique.

FIG. 4 is a diagram illustrating a conventional technique.

FIG. 5 is a diagram illustrating a conventional technique.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Ceramic substrate 21-24 ... Circuit conductor 3 ... Ground conductor 41, 42 ... Empty area 51, 52 ... Pseudo conductor copper

Claims (1)

[Claims] 1. The copper of a circuit conductor having a different required area is closely attached to both surfaces of the ceramic substrate,
In the direct bonding method of ceramic and copper, which is heated to a predetermined temperature in an atmosphere of a predetermined oxygen concentration and copper is bonded to both sides of the ceramic substrate, in the ceramic substrate on the side where the required area of copper of the circuit conductor is small. In terms of surface, a direct bonding method between a ceramic substrate and copper, in which a pseudo conductor copper is placed in close contact with an empty area where the required copper is not placed.