Frequency Spectrum
Electromagnetic signals operating between 300 gigahertz and 3 terahertz occupy a transitional boundary layer between traditional microwave radio frequencies and far-infrared optical waves. Sub-millimeter wave transmission circuits require low-loss substrates, smooth copper foils and ultra-precise line geometries to maintain signal integrity over millimeter-scale distances. At these ultra-high frequencies, wavelengths range from one millimeter down to one hundred micrometers, making circuit dimensions comparable to microstructural features in the substrate material.
Advanced radar and high-capacity wireless backhaul systems leverage this frequency band for high bandwidth applications.
Substrate Attenuation
Standard FR-4 and low-cost hydrocarbon laminates exhibit unacceptably high dielectric absorption at sub-millimeter wave frequencies, rendering them unsuitable for circuit construction. Special low-loss materials, such as thin PTFE composites, fused quartz or fused silica substrates, are required to suppress dielectric attenuation. Microstrip and coplanar waveguide traces must be manufactured with sub-micrometer profile smoothness because even slight surface roughness causes severe skin-effect resistive loss.
Fabrication processes demand laser direct imaging and tight etch control to maintain conductor width tolerances within a few micrometers across the entire substrate panel.
Tolerance Requirement
Sub-millimeter wave circuits are sensitive to minute mechanical variations in dielectric thickness and via hole alignment. Small dimensional deviations introduce large phase errors and reflection losses that degrade circuit performance.
