1. Field of the Invention
The present invention relates to the structure of a compact ceramic multi-layer substrate for dealing with high frequency signals.
2. Description of the Prior Art
Previously, there have been known monolithic substrates integrating active and passive elements on a semiconductor substrate and hybrid substrates integrating various active and passive elements on a dielectric layer such as alumina. In general, the hybrid substrate has a single layer structure. A multi-layer structure of electric circuits is also known for a dielectric substrate made of alumina or the like.
In the above-mentioned structure of a monolithic substrate, single layer structures are integrated. Therefore, if the sizes of electronics components to be integrated are determined, the total size can be determined depending on how the components are packed in the same plane. However, for a high frequency circuit, electromagnetic coupling exists through the space or the substrate. Therefore, it is difficult to integrate a circuit at a density more than a certain value.
Further, for a hybrid substrate, the structure may be made from multi-layers. However, only metallization is made from multi-layers, so that electronics components have been put on the surface. Therefore, this limits the integration and the compactness.
Further, in general, a high frequency circuit is liable to be coupled electromagnetically through a space. In order to prevent such coupling, various techniques such as a metallic shield plate arranged at the top are needed to shield the electromagnetic coupling. Then, high frequency characteristics may be damaged if the profile becomes complicated, if the size becomes large or if the shield is incomplete.
In order to solve these problems, various structures of ceramic multi-layer substrates have been proposed. For example, T. Nishikawa et al. proposed a compact high frequency filter circuit composed of passive components such as inductances (L) and capacitances (C) by using a multi-layer structure made from dielectric ceramics and electrodes (RF Front End Circuit Components Miniaturized Using Dielectric Resonators for Cellular Telephones, IEICE Transactions, Vol. E 74, No. 6, (1991), pp. 1556-1562). This structure is made from the same dielectric material. However, for a circuit such as a resonator including active elements for example transistors, it is not appropriate to use a first dielectric material of large dielectric constant on a substrate section wherein discrete components are set because a phase shift occurs at high frequencies. On the other hand, it is preferable that the dielectric constant of a second dielectric material for constructing the resonator is larger than that of the former dielectric material.
In order to solve this problem, for example, K. Hayashi discloses a structure wherein the dielectric constant of a first dielectric material for constructing a resonator is larger than that of a second dielectric material in a section for setting discrete components (Japanese Patent laid open Publication No. 79,601/1992). However, in a circuit including active components such as transistors, a capacitor of large capacitance such as a bypass capacitor is needed for a direct current bias circuit. Therefore, this structure is not sufficient for miniaturization. Further, in a circuit such as a high frequency amplifier, a path for direct current bias is connected to a high frequency transmission line. Then, if high frequency components pass the direct current bias circuit, unfavorable phenomena such as a loss or unstableness inducing oscillation may occur.
In a structure disclosed in Japanese Patent laid open Publication No. 25,082/1991, the diameter of a throughhole of a multi-layer substrate connected to an electrical conduction line for power supply is decreased or the length thereof is increased to increase an inductance component, so as to avoid high frequency components in the connection line for power supply. However, if the diameter is decreased, the amount of the current is limited, while if the length is increased, the miniaturization is difficult.