Substrate parasitics and dual-resistivity substrates [microwave integrated circuits]

In high-frequency semiconductor applications, substrate effects can be a dominant source of parasitics unless they are carefully minimized. Here a dual-resistivity substrate in a bonded-oxide process is considered for the optimization of the two major types of substrate parasitics: resistive substrate losses and capacitive coupling (crosstalk) through the substrate. These will both depend on the frequency, the two substrate resistivities, and the thickness of the two substrate layers. The thickness of the upper layer is treated as a fully designable parameter. The mechanisms are evaluated numerically, but intuitive rule-of-thumb arguments are also provided for a good understanding of the physics and of the tradeoffs in selecting an optimal design. The results of these sections may also serve as a guide for determining standard substrate resistivities.