Radio Frequency Integrated Circuit Design - Webs

122 Radio Frequency Integrated Circuit Design
5.19 The Use of Slotted Ground Shields and Inductors
In an inductor, currents flow into the substrate through capacitive coupling
and are induced into the substrate through magnetic coupling. Current flowing
in the substrate causes additional loss. Of the two, generally capacitive coupling
is the more dominant loss mechanism. One method to reduce substrate loss is
to place a ground plane above the substrate, preventing currents from entering
the substrate [12]. However, with a ground plane, magnetically generated currents
will be increased, reducing the inductance. One way to get around this
problem is to pattern the ground plane such that magnetically generated currents
are blocked from flowing. An example of a patterned ground shield designed
for a square inductor is shown in Figure 5.23. Slots are cut into the plane
perpendicular to the direction of magnetic current flow. The ground shield has
the disadvantage of increasing capacitance to the inductor, causing its selfresonant
frequency to drop significantly. For best performance, the ground
shield should be placed far away from the inductor, but remain above the
substrate. In a typical bipolar process, the polysilicon layer is a good choice.
The model for the ground-shielded inductor compared to the standard
inductor model is shown in Figure 5.24. For the ground-shielded inductor, the
lossy substrate capacitance has been removed, leaving only the lossless oxide
capacitance and the parasitic resistance of the shield. As a result, the inductor
will have a higher Q.
5.20 Basic Transformer Layouts in IC Technologies
Transformers in silicon are as yet not very common. They are more complicated
than inductors and therefore harder to model in many cases. Transformers or
Figure 5.23 Patterned ground shield for a square spiral inductor (inductor not shown).