Si-micromachining in MM-wave circuits

Abstract

Despite the excellent characteristics and results in all MMIC implementations, high power has remained a challenge. The low-power output of solid state sources along with their low-impedance, further hindered by conventional transmission line characteristics, results in even lower efficiencies. From a device performance point of view, it is obvious that traditional planar line techniques introduce high losses due to the excitation of substrate wave modes. While parasitic radiation is inherent to microstrip and cannot be suppressed, ground connections can be achieved by use of via hole technology. Via holes, however, result in high parasitic inductance and parasitic radiation leading to a substantial device gain degradation which has a detrimental impact on circuit operation at millimeter-wave frequencies. Conventional coplanar technology also provides limited solutions to some of these problems by bringing the ground plane to the proximity of the active devices at the cost of additional parasitic effects such as the excitation of parallel plate and microstrip modes, thus resulting in increased fabrication complexity. All solutions which have been proposed to eliminate parasitic modes have been proven quite ineffective, thus making conventional microstrip and coplanar transmission line approaches less attractive for use in millimeter wave MMICs. In an effort to increase generated power, reduce loss and design cycle, a new generation of MMICs is needed. RF Si micromachining is an excellent approach that provides solutions to the above problems.