1. Field of the Invention
The present invention relates to a method for manufacturing a high-frequency signal transmission circuit for transmitting high-frequency signals and a high-frequency signal transmission circuit device.
2. Description of the Related Art
Routers and the like used for cellular phones and wireless LAN (Local Area Network) have transmission circuits for high-frequency signals, such as switch circuits for high-frequency radio and the like, for transmitting and receiving signals. Types of transmission circuits include a microstrip type in which a coaxial line is extended to a plate form, a coplanar waveguide type in which a signal line and a ground line are provided on the same plane, and the like.
In recent years, it has been desired to integrate amplifiers and signal processing IC (Integrated Circuit), for minimizing the power losses of transmitted signals. Such integration is achieved using a semiconductor substrate, such as a silicon substrate, a GaAs substrate, or the like. However, the use of such a substrate increases the dielectric losses of transmission lines. Therefore, there has been proposed a transmission circuit having a hollow structure in which a hollow portion is formed in a semiconductor around a transmission line to decrease the dielectric constant and decrease the dielectric loss (refer to, for example, U.S. Pat. No. 5,986,517 (FIGS. 5A and 5B)).
Although not directly related to the invention, Japanese Unexamined Patent Application Publication No. 2000-286430 (FIG. 3, etc.) is given as a prior document of the invention. The device described in this document is an acceleration sensor.