Copper circuit junction substrate and method of producing...

Details Copper circuit junction substrate and method of producing... Copper circuit junction substrate and method of producing...

1999-01-21

2001-07-17

Zimmerman, John J. (Department: 1775)

Stock material or miscellaneous articles

All metal or with adjacent metals

Composite; i.e., plural, adjacent, spatially distinct metal...

C428S621000, C428S209000, C174S256000, C174S258000, C228S122100

Reexamination Certificate

active

06261703

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a copper circuit-joined board which comprises a ceramic substrate and a conductor layer, comprising copper as a main component, joined to the ceramic substrate and is suitable particularly for high power modules and cooling systems of machine tools, electromobiles and the like, a process for preparing the same, and a semiconductor device using the board.
BACKGROUND ART
Ceramics, for example, alumina (Al
2
O
3
) aluminum nitride (AlN), and silicon nitride (Si
3
N
4
), have hitherto been used as an electrical insulating substrate for semiconductor devices. In this case, a circuit board for semiconductor IC using the above substrate has generally been prepared by forming and laminating a metallized circuit, composed mainly of tungsten (W) or molybdenum (Mo) or a metallized circuit composed mainly of copper (Cu), on the above substrate. The above ceramics used as the substrate possesses excellent electrical insulting properties and mechanical strength and in addition has high thermal conductivity. The thermal conductivities of alumina (Al
2
O
3
), aluminum nitride (AlN), and silicon nitride (Si
3
N
4
) are approximately 17 W/m·K, 170 W/m·K, and 60 W/m·K, respectively. Among them, aluminum nitride has electrical insulating properties comparable to alumina and silicon nitride and, as described above, has the highest thermal conductivity. By virtue of these properties, aluminum nitride has received the greatest attention as a substrate for circuit boards. Further, the average coefficient of thermal expansion of aluminum nitride, when the temperature is raised from room temperature to the brazing temperature of silver (about 800° C.), is as small as 5.5×10
−6
/° C. Therefore, aluminum nitride is highly compatible with an IC chip of a silicon semiconductor (coefficient of thermal expansion 4.0×10
−6
/° C). Although the thermal conductivity of silicon nitride is lower than that of aluminum nitride, silicon nitride has a coefficient of thermal expansion more close to the Si semiconductor than aluminum nitride and possesses superior mechanical strength. For this reason, silicon nitride has recently begun to be utilized as a substrate for a circuit board by reducing the thickness of the silicon nitride substrate to control the heat resistance. However, ceramics, including aluminum nitride, commonly used as the substrate have low coefficient of thermal expansion. In particular, when a circuit composed mainly of copper (coefficient of thermal expansion 16.7×10
−6
/° C.) is formed on the substrate, the compatibility of copper with the substrate is so low that the ceramic substrate is likely to be broken due to thermal stress in the joined interface created in the stage of joining of the circuit to the substrate and mounting of the assembly by incorporation and in the stage of actual use as the circuit board. For this reason, in joining the circuit to the ceramic substrate, various interposing layers have generally been provided between the circuit and the ceramic substrate in order to relax the thermal stress.
For example, Japanese Patent Publication No. 34908/1990 introduces a circuit board comprising multiple layers, each comprising silver (Ag) or copper (Cu) as a main component and a group IVa metal as an active metal, interposed between a ceramic substrate and a copper conductor layer. In the formation of a joined structure comprising the above interposing layer, however, the relaxation of the thermal stress created between copper and the ceramic is unsatisfactory particularly in the case of a large board. Therefore, what is required is to interpose an interposing layer appropriate for satisfactorily relaxing the thermal stress created between copper and the ceramic.
For this reason, in joining a ceramic substrate to a metallic member such as a lead frame, having a considerably larger coefficient of thermal expansion than that of the ceramic substrate, a method has generally been used which comprises first providing a high-melting metallizing layer comprising a high-melting metal (i.e., refractory metal), such as W or Mo, on the surface of a ceramic substrate and then joining the above metallic member on the high-melting metallizing layer with the aid of a conventional Ag—Cu-base silver brazing material. In this case, a layer comprising the high-melting metal and the conventional silver brazing material corresponds to the above-described interposing layer. For example, Japanese Patent Laid-Open No. 289950/1988 discloses a joined structure comprising a lead frame of oxygen-free copper having high thermal conductivity and thermal impact resistance joined to a high-melting metallizing layer of W provided on an aluminum nitride substrate (see FIGS. 1 and 2 of the same publication). According to the same publication, in joining the lead frame of oxygen-free copper to the high-melting metallizing layer of W, an Ni layer is previously formed between the high-melting metallizing layer and the lead frame in order to improve the wettability therebetween, and joining is then carried out using this layer as an interposing layer by silver brazing.
In this type of products, Kovar has hitherto been used as the lead frame.
As described above, however, use of the lead frame of soft oxygen-free copper instead of the lead frame of Kovar can relax the thermal stress created in the joining interface at the time of brazing as compared the prior art.
Even in the above method, use of the member as a large circuit board at a high power increases the thermal stress, often leading to breaking of the substrate.
Further, for example, methods for joining a metallic member of copper to an aluminum nitride substrate without using the conventional Ag—Cu-based silver brazing material have also been studied. One of them is the so-called DBC (direct bonding copper) method. This method utilizes an eutectic reaction of a layer of an oxide of copper, present on the surface of the metallic member of copper, with copper. For example, Japanese Patent Laid-Open No. 40404/1984 or “Erekutoronikusu Seramikkusu (Electronics Ceramics),” November, 1988, pp. 17-21 discloses the summary of this method. According to the disclosure, at the outset, on the surface of a substrate of an aluminum nitride sintered body is formed a layer of an oxide of aluminum/rare earth element/alkaline earth element used as a sintering aid for the substrate, or an oxide layer composed mainly of the sintered body. Subsequently, a metallic member of copper is put on the above layer, and the assembly is heated in an oxygen-containing atmosphere at a temperature of the eutectic point of copper and a copper oxide Cu
2
O to the melting point of copper to join the metallic member to the above layer. A similar method has been introduced by Japanese Patent Laid-Open No. 32343/1985. According to the description of this publication, an eutectic layer of a copper alloy containing an active metal, such as a group IVa element metal, is interposed between a member of copper and an aluminum nitride substrate to join the two materials to each other. In this case, fine holes are likely to be created in the joined interface of the copper member and the aluminum nitride substrate. On the other hand, since the interposing layer is thin, the interposing layer per se cannot satisfactorily relax the thermal stress. For this reason, the thermal stress is likely to concentrate on the hole portion, leading to cracking in the hole portion.
In the above joining method, as can be seen from FIG. 4 of the above literature “Erekutoronikusu Seramikkusu (Electronics Ceramics),” a variation in joining strength is likely to become large unless the thickness of the oxide layer on the aluminum nitride substrate is regulated within a narrow range. Further, since the interposing layer is thin, the thermal stress created due to a difference in coefficient of thermal expansion between copper and the aluminum nitride substrate cannot be relaxed. This is likely to cause breaking of the substrate in its portion around the

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