Silicon Substrate Surface Conduction and Dielectric Passivation Fundamentals
Trap-rich polysilicon passivation pins interface charges on high-resistivity silicon, suppressing surface conduction and harmonic spur spikes across RF switches.
Coplanar Waveguide Loss
Meaning
The attenuation of a high-frequency electrical signal along a planar transmission line occurs due to a combination of conductor, dielectric and radiation mechanisms. This coplanar waveguide loss affects the overall noise figure and power efficiency of integrated wireless front-end modules. Designers must characterize this behavior to optimize system gains.
Signal Attenuation
Current crowding at the edges of the central conductor increases the effective resistance and generates thermal dissipation. This coplanar waveguide loss rises as the gap between the signal trace and the ground planes narrows. Reducing surface roughness on the metal layer helps mitigate this resistive heating.
Substrate Interaction
Polarization of the underlying dielectric material under high electric fields leads to additional power absorption. Since the coplanar waveguide loss is tightly linked to the dissipation factor of the substrate, silicon-on-insulator wafers use high-resistivity layers to minimize these dielectric current paths. This configuration isolates the signal from the conductive bulk.
Frequency Response
High-frequency operation increases the contribution of radiation losses as the wavelength approaches the physical dimensions of the waveguide. Because coplanar waveguide loss scales with the square root of frequency for conductor losses and linearly for dielectric losses, total attenuation grows rapidly at millimeter-wave bands. Proper shielding prevents spurious modes from degrading the signal.
Test sequences use network analyzers to extract these parameters across the target spectrum.