Active solid-state devices (e.g. – transistors – solid-state diode – Housing or package – Insulating material
Reexamination Certificate
2002-03-29
2004-07-06
Potter, Roy (Department: 2822)
Active solid-state devices (e.g., transistors, solid-state diode
Housing or package
Insulating material
C257S702000, C438S125000
Reexamination Certificate
active
06759740
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a composite ceramic board which has a plurality of ceramic insulating layers formed as a unitary structure upon firing, features a large strength, a high thermal conductivity, and is particularly suited as a wiring board for high-frequency use, relates to a method of producing the same, to an optical/electronic-mounted circuit substrate using said board, and to a mounted board equipped with said circuit substrate.
2. Description of the Related Art
Accompanying the trend toward highly densely integrating the semiconductor devices and transmitting signals at high frequencies in recent years, it has been urged to mount the semiconductor devices on a wiring board that has further improved thermal properties and electrical properties.
Namely, as the semiconductor devices are highly densely integrated, an increased amount of heat is generated from the semiconductor devices. To prevent the semiconductor devices from malfunctioning, it is necessary to quickly release the heat out of the devices. Therefore, the wiring board mounting the semiconductor devices must have a high thermal conductivity.
Further, transmitting the signals at a high frequency brings about an increase in the speed of the operation, while a delay in the signals hinders the attempt for increasing the speed of operation.
To prevent the delay in the signals, therefore, wiring layers must be formed using a low-resistance conductor having a small conduction loss.
Accompanying the recent widespread use of multi-media, on the other hand, it is becoming necessary to transmit and receive tremendous amounts of image data, and attention has been given to optical communication capable of transmitting and receiving large amounts of data at high speeds.
The optical communication device has a structure in which electronic semiconductor devices and multi-chip modules are connected together through optical waveguides in a complex manner frequently intersecting the waveguides. In order to decrease the size of the device by processing optical signals and electric signals using the same mounted substrate and to cope with the complex optical inter-connection, therefore, there has been frequently employed an optical/electronic-mounted circuit substrate of a structure forming optical waveguides on the ceramic board and mounting optical semiconductor devices and electronic semiconductor devices.
The optical/electronic-mounted circuit substrate, too, uses the above-mentioned wiring board, and must have a high heat-radiating property to cope with an increase in the amount of heat generated as a result of a high degree of integration and high frequencies, and must have a decreased resistance of the conductor to meet the demand for performing the operation at high speeds.
As a wiring board for mounting such semiconductor devices, there has heretofore been much used a ceramic board obtained by depositing conductor layers (wiring layers) of a high-melting point metal such as tungsten or molybdenum on the surfaces or inside of the insulating board made of alumina ceramics from the standpoint of reliability.
However, the above-mentioned conventional ceramic board is not capable of fully meeting the above-mentioned modern thermal requirements or electrical requirements.
For example, the conventional alumina ceramics substrate is satisfactory from the standpoint of thermal properties (thermal conductivity). However, since the wiring layers (conductor layers) have been formed of a high-melting point metal, the resistance can be lowered down to only about 8 m&OHgr;/□ or so. Therefore, signal insertion loss is very large, and favorable high-frequency characteristics are not obtained.
Besides, since the insulating board having a wiring layer serving as a terminal for receiving signals is formed of alumina ceramics having a high dielectric constant, high-frequency signals are reflected to a large degree and transmission characteristics are deteriorated.
In order to improve high-frequency signal transmission characteristics by suppressing the reflection of signals, there has been proposed a wiring board obtained by using glass ceramics having a low dielectric constant as an insulating layer of the signal input portions, which is formed integrally with a reinforced glass (Japanese Unexamined Patent Publication (Kokai) No. 239394/1991).
This wiring substrate is satisfactory from the standpoint or high-frequency characteristics (electric characteristics) since the insulating layer mode of glass ceramic has a low dielectric constant and a conductor layer is formed by co-firing a low-resistance conductor such as of copper.
However, the thermal conductivity of the glass ceramics is several watts/m•K at the greatest. Therefore, though high-frequency characteristics can be accomplished, heat is not smoothly radiated from the semiconductor device, and thermal properties (thermal conductivity) are not satisfactory causing the device itself to malfunction.
Besides, the insulating board has a small resistance and is cracked at the time of mounting various devices.
A variety of proposals have heretofore been made to improve problems related to the above-mentioned thermal properties and electric properties (high-frequency characteristics) of the conventional substrate.
For example, Japanese Unexamined Patent Publications (Kokai) Nos. 15101/1995 and 151045/2000 disclose wiring boards formed by co-firing an insulating board made of aluminum oxide and a conductor layer of copper or of a combination of copper and tungsten or molybdenum.
Further, Japanese Unexamined Patent Publications (Kokai) Nos. 106880/1998, 214745/1998 and Japanese Patent No. 3061282 disclose wiring boards equipped with an insulating board having a plurality of insulating layers of dissimilar dielectric constants that are formed integrally together.
According to Japanese Unexamined Patent Publication (Kokai) No. 15101/1995, however, all of the wiring layers (conductor layers) are arranged in the insulating board and are co-fired simultaneously, the insulating layer on the surface of the insulating board is removed by polishing so that the inner wiring layer is exposed on the surface of the insulating board, or a thick-film method or a thin-film method is applied onto the surface of the wiring board after firing thereby to form a surface wiring layer (surface conductor layer).
Therefore, a polishing step, a thick film-forming step and a thin film-forming step are indispensable for forming the surface wiring layer accompanied by such problems as an increased number of the production steps, a decreased yield and an increased cost.
According to Japanese Unexamined Patent Publication (Kokai) No. 151045/2000, the firing is conducted at a temperature of not higher than 1500° C. and, hence, low-melting point metals such as copper and the like are separated little, and a conductor layer having a small resistance is formed.
Besides, the surface wiring layer (conductor layer) of the insulating board, too, is formed by co-firing making it possible to avoid an increase in the cost of production.
However, the insulating layer which is formed of alumina has a dielectric constant of as high as about 9.
In this case, the loss due to the reflection of input signals increases in a region where the signals have a frequency of about 40 GHz, resulting in a decrease in the characteristics.
This holds true for the above-mentioned Japanese Unexamined Patent Publication (Kokai) No. 15101/1995.
According to Japanese Unexamined Patent Publications (Kokai) Nos. 106880/1998, 214745/1998 and Japanese Patent No. 3061282, further, the layers of low dielectric constant are formed as a unitary structure and the insulating board is formed of glass ceramics of a composition that can be fired at a low temperature. Therefore, signals of high frequencies can be processed by using a wiring layer (conductor layer) formed of Cu, Au, Ag or Pt having a low resistance as a chief component.
The boards, however, are not satisfactory in regard to the
Matsui Takeshi
Onitani Masamitsu
Yamada Shigeki
Hogan & Hartson LLP
Kyocera Corporation
Potter Roy
LandOfFree
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