How do I select between alumina, quartz, and fused silica for a thin film RF circuit substrate?
Choosing Ceramic Substrates for Thin-Film RF Circuits
Ceramic substrates have been the foundation of microwave integrated circuits (MICs) for decades, offering properties that organic materials cannot match: ultra-low loss tangent, excellent dimensional stability, superb surface finish for thin-film deposition, and long-term environmental stability.
Frequently Asked Questions
Why is the surface finish of ceramic substrates important for RF?
Thin-film metallization for microwave circuits requires substrate surface roughness below 0.05 μm (2 microinches), achievable with polished alumina and fused silica. Rough surfaces cause line edge irregularities that increase conductor loss and degrade impedance uniformity. Lapped (but unpolished) alumina with 0.15 μm roughness is adequate for frequencies below 20 GHz.
Can I get vias in ceramic substrates like in PCBs?
Yes, but the process differs. Alumina via holes are laser-drilled and filled with conductive paste (typically tungsten or via-fill gold). Hole diameters of 100-200 μm are standard. The via density and minimum pitch are more restricted than in organic PCBs, so circuit designs on ceramic substrates tend to use fewer ground vias than their organic counterparts.
What metallization stacks are used on these ceramic substrates?
The standard thin-film stack is Ti/Pt/Au or TiW/Au, sputtered at thicknesses of 500Å/500Å/1-3μm respectively. The titanium provides adhesion to the ceramic, platinum acts as a barrier layer, and gold provides low-loss conductor and bondable surface. For thick-film on alumina, screen-printed gold or silver conductors fired at 850°C are common.