JPH0714945A - Oxide ceramic substrate and its use - Google Patents

Abstract

(57) [Summary] [Object] To provide an oxide ceramics substrate having a small thermal resistance and a ceramics substrate having a copper circuit having extremely excellent durability against thermal shock and thermal history. [Structure] An oxide ceramics substrate having a thermal resistance of 0.20 ° C./W or less, and a copper circuit is formed on one surface of the oxide ceramics substrate, and an oxide ceramics substrate is formed on the other surface. A ceramic substrate having a copper circuit, which is formed by joining copper plates having an area substantially equal to that of

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention comprises a single device such as a rectifying diode or a transistor, which is a device with only one semiconductor chip, or a plurality of semiconductor chips, among power devices corresponding to the power control section of electronic parts. The present invention relates to an oxide ceramic substrate used for a composite device and a ceramic substrate having a copper circuit using the oxide ceramic substrate.

[0002]

2. Description of the Related Art In recent years, along with the high performance of industrial equipment such as robots and motors, not only bipolar transistors but also various types such as MOSFETs and static induction transistors have been used. The transition to high power modules is also in progress. Along with this, the semiconductor chips themselves have become highly functional, and the heat generated from them has been increasing steadily. In order to efficiently dissipate this heat, various methods have been conventionally used in high power module substrates. Particularly in recent years, since ceramic substrates having good heat conduction have become available, semiconductor chips can be formed by bonding a metal plate such as a copper plate onto the substrate, forming a circuit, or performing a plating process or the like. A mounting structure is being adopted.

Inverters using such a single device substrate or a composite device substrate are used for various machine tools, air conditioners, refrigerators, VTs from robots and motors.
It is expected to further spread to R and audio equipment.

As a method for joining the ceramics substrate and the copper plate, an active metal brazing method in which a brazing material containing an active metal is interposed between the copper plate and the ceramics substrate and heat-treated to form a joined body, or a ceramics substrate and The DBC method of heating and joining a copper plate at a temperature lower than the melting point of copper and higher than the eutectic temperature of Cu-O, as well as a Mo-Mn method, a copper sulfide method, a copper metallization method and the like are adopted.

[0005]

The characteristics that a single device substrate and a composite device substrate must have are heat dissipation,
Mechanical strength, reliability of joining, etc.
The heat dissipation was largely due to the characteristics of the substrate material. Conventionally, as a substrate of this kind, oxide ceramics substrates such as beryllium oxide and aluminum oxide have been studied from the viewpoint of high thermal conductivity.Beryllium oxide is excellent in thermal conductivity but toxic, and aluminum oxide is The thermal conductivity was insufficient. In addition, the mechanical strength, especially the reliability against cracks, was not sufficient.

On the other hand, in recent years, the electric power applied to the transistor has become 1 kW or 10 kW, so that the thermal resistance value at the second or third digit after the decimal point, which has not been so important so far, becomes a problem. Came to be. That is,
Even if a ceramic substrate is sufficiently filled with properties such as strength and thermal conductivity, when it is made into a device, the heat dissipation from the silicon chip is insufficient due to defects such as voids generated at the metallization and the bonding interface with the copper plate. That is, there was a problem that the thermal resistance did not decrease as calculated. Therefore, it has been required to manage the thermal resistance so as not to make a difference from the case of actual use as much as possible. However, there are many technical difficulties in joining new high thermal conductivity ceramic materials such as aluminum nitride and silicon carbide to metals, and it has been difficult to eliminate defects such as voids. Therefore, the emergence of a technique that improves the reliability of oxide ceramics and has low thermal resistance has been awaited.

The inventors of the present invention have conducted extensive studies to solve the above problems, and as a result, reduce the thermal resistance from the structural viewpoint of the substrate by reducing the thickness of the oxide ceramic substrate. In this case, there is no problem in the expected reliability of the substrate, in other words, the oxide ceramic substrate has low thermal resistance and sufficient durability against thermal shock and thermal history. To complete the present invention.

[0008]

That is, the present invention provides an oxide ceramic substrate having a thermal resistance of 0.20 ° C./W or less, and a copper circuit on one surface of the oxide ceramic substrate. A ceramic substrate having a copper circuit, which is formed and has a copper plate bonded to the opposite surface and having an area substantially equal to that of the oxide ceramic substrate.

The present invention will be described in more detail below. Generally, the relationship between the thermal conductivity and the thermal resistance of an oxide ceramics substrate can be expressed by equation (1). R _P = k · (1 / λ) · (t / S) (1) R _P : Thermal resistance of oxide ceramic substrate (° C / W) k: Coefficient λ: Thermal conductivity (W / mK) t: thickness of oxide ceramic substrate (m) S: area of oxide ceramic substrate (m ² )

From the equation (1), three methods can be considered to reduce the thermal resistance of the oxide ceramic substrate: increasing the thermal conductivity, reducing the thickness, and increasing the area. Of these, the thermal conductivity is determined by the material selected, and it is technically difficult and limited to improve the thermal conductivity with the same material. Further, the area of the oxide ceramic substrate can be easily changed, but cannot be changed so freely because it is limited by other technical aspects such as the size of the module. Therefore, the present inventors have paid attention to reducing the thickness of the oxide ceramic substrate.

Usually, a ceramic substrate having a copper circuit used in this field has a thickness of 0.635 to 1 mm when the substrate is aluminum oxide, for example. Here, for example, if the thickness is set to 60%, the thermal resistance is also 60%. That is, there is an effect equivalent to that in which the thermal conductivity of the oxide ceramic substrate is 1 / 0.6 = 1.67 times. The present inventors have made various studies from such a viewpoint, and have completed an oxide ceramics substrate having no problem of durability such as strength in practical use.

In the present invention, the thermal resistance of the oxide ceramics substrate can be measured, for example, as follows. First, using the device shown in FIG. 1, the transistor temperature T with respect to the power (W) loaded in the transistor
_From the temperature difference (° C.) between ₁ and the aluminum heat dissipation block temperature T ₂ , the total thermal resistance (R _ALL ) is calculated using the equation (1). At that time, 16 × 20 mm is suitable as the size of the oxide ceramic substrate.

Since this total thermal resistance (R _ALL ) can be decomposed as in the equation (2), the thermal resistance R _p of the oxide ceramic substrate is given by the equation (3). _{R ALL = R Tr + R Gr1} + R P + R Gr2 ··· (2) R Tr: Thermal resistance R of the transistor (silicon) _Gr1: thermal resistance of the transistor side of the heat dissipation grease R _P: thermal resistance of the oxide ceramic substrate R _Gr2 : thermal resistance of the heat dissipation grease aluminum heat sink block side _{R P = R ALL - (R} Tr + R Gr1 + R Gr2) ··· (3)

Therefore, as shown in FIG. 2, an apparatus in which parts are assembled without sandwiching the oxide ceramics substrate is used.
The temperature difference between the transistor temperature T ₃ and the aluminum heat radiation block temperature T ₄ is measured to calculate the thermal resistance of components other than the oxide ceramic substrate. The difference between the two becomes the required thermal resistance of the oxide ceramic substrate. In this case, the thermal resistance of the components other than the oxide ceramic substrate may be measured by using a material that can be sufficiently ignored, for example, a copper plate having a sufficiently thin thickness, and the value may be used as a substitute.

However, when forming a copper circuit by making the thickness of the oxide ceramic substrate thinner than in the conventional case, the following problems may be considered with respect to the reliability thereof. That is, since the thickness of the copper plate on the circuit side is usually thicker than the thickness of the copper plate on the back side (eg Japanese Patent Publication No. 3-51119),
The thermal stress applied to the ceramic substrate in the heat cycle test acts as a tensile stress on the circuit side due to the difference in thermal expansion coefficient between copper and ceramics. The conventional ceramic substrate has a convex circuit side, and if a ceramic substrate with such a copper circuit is used, the warp force that makes the circuit side more convex, that is, the circuit side receives a tensile stress Has become even larger,
The copper plate on the circuit side is easily peeled off. In such a phenomenon, when the thickness of the ceramic substrate becomes thin, the overall strength becomes small, so that the resistance force against the thermal stress due to the difference in thermal expansion becomes weak. Therefore, it is necessary to reduce the thermal stress due to the difference in thermal expansion between copper and ceramics as much as possible.

Therefore, the inventors of the present invention also found that the following measures should be taken when such a problem becomes a reality. That is, it can be dealt with by taking at least one of the following measures. (A) A ceramic substrate having a copper circuit is warped such that the circuit side is concave, and residual compressive stress is left on the circuit side. (B) After joining the oxide ceramics substrate and the copper plate,
A heat treatment at a temperature of 500 to 700 ° C. is applied in a non-oxidizing atmosphere to relieve the thermal stress on the oxide ceramic substrate caused by the thermal contraction of the copper plate. (C) The strength of the oxide ceramic substrate itself is increased in order to reduce the probability of existence of defects that cause destruction.

As the structure of the ceramic substrate having the copper circuit of the present invention, for example, the oxide ceramic substrate has a thickness of 0.4 mm and the copper plate on the circuit side has a thickness of 0.3 to 0.5.
If the thickness is mm, the thickness of the back copper plate is 0.1-0.25 mm
Is desirable. Examples of oxide ceramics include beryllium oxide, aluminum oxide, mullite, and the like. As for the type of copper plate, oxygen-free copper, tough pitch copper, etc. are used. As a method for joining the oxide ceramic substrate and the copper plate, an active metal brazing method,
Although there is no problem in any of the DBC methods, the active metal brazing method having a low bonding temperature is more preferable.

As a method of forming a copper circuit, it is desirable to attach a copper plate to the entire surface in advance and perform etching with ferric chloride or cupric chloride after joining. At this time,
Depending on the thickness of the copper plate, etching speed, processing temperature,
It is necessary to change the chlorine ion concentration. As the shape of the pattern, it is desirable that the length of the continuous pattern is shorter than the length of the oxide ceramic substrate so that the difference in thermal expansion from the oxide ceramic substrate is as small as possible. Further, regarding the size, the thermal stress applied to the end face of the substrate increases in proportion to the length of the substrate, and therefore it is desirable that the size is as small as possible.

[0019]

EXAMPLES The present invention will be specifically described below with reference to Examples and Comparative Examples. Examples 1 to 21, Comparative Examples 1 to 3 In Examples 1 to 3, Examples 10 to 15 and Comparative Examples 1 to 3, aluminum oxide powder having an average particle size of 3 μm or less, Example 4
9 to Examples 16 to 21, a slurry containing an aluminum oxide powder having an average particle size of 1 μm or less was prepared, a green sheet was formed, and the binder was removed.
Sintering at a temperature of 850 ° C. to produce an aluminum oxide substrate having a size of 16 × 20 mm and a thickness of 0.4 to 1.0 mm,
The thermal resistance and thermal conductivity were measured. The thermal resistance was measured by assembling the apparatus shown in FIGS. 1 and 2, and the thermal conductivity was measured by the laser flash method.

75 parts by weight of silver powder, 25 parts by weight of copper powder, 20 parts by weight of zirconium powder, 15 parts by weight of terpineol and 1.5 parts by weight of a solid solution of a toluene solution of polyisobutyl methacrylate as an organic binder are mixed. A brazing material paste was prepared and applied on both surfaces of the aluminum oxide substrate obtained above by screen printing. The coating amount (after drying) was 6 to 8 mg / cm ² .

Next, copper plates having various thicknesses according to their sizes are placed in contact with both sides of the aluminum oxide substrate coated with the above-mentioned brazing material paste, and then placed in a furnace to be 1 × 1.
It was heated at a temperature of 900 ° C. for 30 minutes in a high vacuum of 0 ⁻⁵ Torr and then cooled to room temperature at a temperature lowering rate of about 2 ° C./minute to manufacture a joined body by the active metal brazing method. And about Examples 10-21, after putting the obtained joined body in another furnace and heat-processing for 2 hours at 650 degreeC in nitrogen gas,
Cooling was performed at a rate of about 2 ° C./minute.

Next, a UV curing type etching resist is applied on the circuit pattern by screen printing on the copper plate of the above-mentioned bonded body, and then an etching treatment is performed using a cupric chloride solution to dissolve unnecessary portions of the copper plate. After removal, the etching resist was stripped with a 5% caustic soda solution. In the joined body after the etching treatment, there are residual unnecessary brazing filler metal and the active metal component and the reaction product of the oxide ceramics substrate between the copper circuit patterns.
It was immersed in a 0% ammonium fluoride solution for 10 minutes.

A heat cycle (thermal shock) test was conducted on the ceramic substrate having a copper circuit obtained through these series of treatments. The heat cycle test is in the air, −
After holding at 40 ° C. for 30 minutes, it was left at 25 ° C. for 10 minutes, further held at 125 ° C. for 30 minutes, and then left at 25 ° C. for 10 minutes as one cycle. The evaluation is 1 of each Example and Comparative Example.
Dozens of samples were prepared for each example and immediately subjected to a heat cycle test. Then, the state of each sample is observed every 3 cycles, and if even one sample has peeled copper plate, the number of cycles at that time is taken as the copper plate peeling start number, The heat cycle resistance was evaluated. The warpage was measured with a dial gauge.

Example 22 Beryllium oxide powder was used instead of aluminum oxide powder, the firing temperature of the green sheet was set to 1900 ° C., and titanium hydride powder was used instead of zirconium powder of the brazing paste. A beryllium oxide substrate was manufactured in accordance with Example 1 and evaluated.

The above results are shown in Tables 1 to 4. Tables 1 and 2 show the results of Examples 1 to 22, and Tables 3 and 4 show the results of the comparative examples. Tables 1 and 3 show the results for the oxide ceramic substrate, and Tables 2 and 4 show the results for the ceramic substrate having a copper plate or a copper circuit.

[0026]

[Table 1]

[0027]

[Table 2]

[0028]

[Table 3]

[0029]

[Table 4] (Note) "-" in the warp means that the copper circuit side is convex and "+" is concave.

[0030]

According to the present invention, an oxide ceramics substrate having a low thermal resistance is obtained, and a ceramics substrate having a copper circuit produced using this oxide ceramics substrate has durability against thermal shock and thermal history. That is, heat shock resistance and heat cycle resistance are significantly improved.

[Brief description of drawings]

FIG. 1 is an explanatory diagram for measuring thermal resistance of an oxide ceramic substrate.

FIG. 2 is an explanatory diagram for measuring thermal resistance of an oxide ceramic substrate.

[Explanation of symbols]

1 transistor (“2SC-3042” bottom area: 16
× 20 mm ² ) 2 Oxide ceramics substrate (16 × 20 mm × thickness) 3 Transistor side heat dissipation grease 4 Aluminum heat dissipation block side heat dissipation grease 5 Aluminum heat dissipation block (water cooling type)

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Katsunori Terano 1 Shinkai-cho, Omuta-shi, Fukuoka Electric Chemical Industry Co., Ltd. Omuta Factory

Claims (2)

[Claims] 1. An oxide ceramic substrate having a thermal resistance of 0.20 ° C./W or less. 2. A copper circuit is formed on one surface of the oxide ceramic substrate according to claim 1, and a copper plate having an area substantially equal to that of the oxide ceramic substrate is bonded to the other surface. Ceramic substrate with a copper circuit.