Substrates Used in RF Applications 

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Understanding RF & mmWave Substrates

The choice of substrate defines performance in RF and mmWave devices—affecting impedance, Q-factor, and thermal reliability. Materials supplied by UniversityWafer enable everything from low-power LNA circuits to high-power GaN amplifiers operating beyond 100 GHz.

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Dielectric Behavior

Every substrate has a unique dielectric constant (εr) and loss tangent (tan δ) that determine how efficiently it stores and transmits electromagnetic energy.

  • Low-εr materials (like fused silica or quartz) minimize dispersion and maintain wide bandwidth.
  • Moderate-εr options (HR-Si, GaAs) allow compact line geometries for IC integration.
  • High-εr substrates (sapphire, alumina) shrink resonator size but demand tighter alignment.
  • Low tan δ means lower insertion loss—critical above 10 GHz.

Loss Mechanisms at High Frequency

At mmWave, conductor and dielectric losses dominate circuit efficiency. Reducing surface roughness and selecting optimal metallization thickness are as important as material permittivity.

  • Conductor loss: Skin depth drops below 0.5 µm at 30 GHz—favor thick Au or Cu top metals.
  • Dielectric loss: tan δ < 10⁻³ preferred for filters, resonators, and transmission lines.
  • Radiation loss: Controlled by substrate thickness and ground shielding in microstrip or CPW layouts.

Thermal and Power Handling

As RF power increases, so does junction temperature. GaN and SiC provide wide-bandgap strength and high thermal conductivity for efficient power devices, while GaAs and sapphire excel in low-noise and optical-microwave hybrids.

  • GaN/SiC: Breakdown field > 3 MV/cm, supports 10× Si power density.
  • GaAs: Electron mobility > 8000 cm²/V·s for high-gain LNAs.
  • HR-Si & quartz: Electrically insulating yet thermally stable for packaging and test boards.

Design Integration & Packaging

  • CPW & Microstrip: Match substrate εr to line geometry to achieve 50 Ω impedance.
  • Wafer bonding: Combine low-loss dielectrics with GaN or GaAs active layers for hybrid modules.
  • Vias and interposers: Through-glass vias (TGV) and through-silicon vias (TSV) minimize parasitics in AiP designs.
  • Hermetic sealing: Quartz or sapphire lids maintain low loss and optical transparency in photonic packages.

Frequency-Band Insights

Substrate choice evolves with operating frequency:

  • L-/S-Band (1–4 GHz): GaN/SiC for radar and base-station power amplifiers.
  • X-/Ku-Band (8–18 GHz): GaAs and HR-Si for low-noise amplifiers and mixers.
  • Ka-/V-Band (26–75 GHz): Quartz, sapphire, and fused silica for antenna feed networks.
  • W-Band (75–110 GHz): Ultrapolished fused silica and metalized thin films for resonators and on-chip waveguides.

Testing & Characterization

Researchers verify performance using vector network analysis, time-domain reflectometry, and on-wafer S-parameter extraction. High-frequency measurements benefit from wafers with controlled thickness, flatness, and metallization uniformity.

  • VNA & probe stations: Used for S₁₁/S₂₁ extraction up to W-band.
  • Dielectric characterization: Split-post resonators determine tan δ to 10⁻⁴ precision.
  • Thermal imaging: IR mapping identifies hot spots in GaN and SiC devices.

Choosing Materials for Your Frequency Range

Start with your target frequency, power, and Q-factor requirements, then narrow by loss tangent and thermal conductivity. UniversityWafer specialists can recommend compatible wafers and coatings to achieve optimal performance from sub-GHz through W-band.

RF & mmWave Materials for Sub-GHz to W-Band

UniversityWafer supplies substrates and wafer services for RF front-ends, power amplification, phased arrays, and high-frequency packaging. From low-loss dielectrics to wide-bandgap semiconductors and precision metals, our inventory supports rapid prototyping through small-volume production.

Close-up RF evaluation board with shield can, SMA connector, and RF APPLICATIONS silkscreen

Core RF Substrates

  • GaAs — High electron mobility for LNAs, mixers, and MMICs up to mmWave.
  • GaN / GaN-on-Si / SiC — High breakdown and power density for PAs, T/R modules, and fast switchers.
  • High-Resistivity Si — CMOS-compatible RF passives and SoC integration (ρ > 3–10 kΩ·cm).
  • Quartz / Fused Silica — Ultra-low loss, stable εr for filters, delay lines, and timing.
  • Sapphire — Low loss tangent, high thermal stability; common in microwave/optical hybrids.

Dielectric & Loss Considerations

  • Loss tangent (tan δ): Drives Q and insertion loss at mmWave; fused silica/quartz and sapphire excel.
  • Permittivity (εr): Impacts impedance and miniaturization; choose to match line geometry and bandwidth.
  • Resistivity (ρ): For HR-Si, high ρ minimizes eddy currents under inductors/CPWs.
  • Thermal path: GaN/SiC offer superior heat handling for high Pout amplifiers and radar T/R modules.

Metallization & Interfaces

  • RF Metals: Au, Cu, Ag; adhesion/seed layers (Ti, Cr, TiW). Thicker top-metal reduces Rs and improves Q.
  • Ohmics & Contacts: Ti/Al-based for GaN; Au-based for GaAs; anneal windows tuned to device stack.
  • Passivation: PECVD SiN, ALD Al2O3 for trap suppression and environmental stability.

Vias, Interposers & Packaging

  • Through-Glass Vias (TGV): Low-loss interposers for antenna-in-package (AiP) and RF passives.
  • Through-Silicon Vias (TSV): HR-Si integration for compact RF modules and vertical routing.
  • Carrier & Lids: Quartz/sapphire windows, alumina carriers, and metalized caps for hermeticity.

Frequency-Band Guidance (Typical)

  • Sub-GHz / L / S: HR-Si for integration; GaN/SiC for high-power PAs and switches.
  • C / X / Ku: GaAs MMICs, GaN PAs on SiC/Si; quartz/sapphire for low-loss passives.
  • K / Ka / V / W: Quartz/sapphire for lines and filters; GaN for PAs; TGV interposers for AiP.

Specs We Commonly Support

  • Diameters: 50–200 mm (material-dependent); custom rectangles and coupons available.
  • Thickness & Polish: SSP/DSP, optical polish; tight TTV and bow for lithography and bonding.
  • Films & Coatings: ITO/IZO, AR, metals (Cr/Au, Ti/Au, Ni/Au), SiO2/SiN masks.
  • Micromachining: Laser drilling/cutting, etching of channels/cavities, via formation prep.

Selection Tips

  • Start from the max frequency & power → pick substrate by loss and thermal path.
  • Match εr and tan δ to achieve your target impedance and Q.
  • Define metallization thickness and contact stack early—impacts loss and reliability.
  • Plan packaging/vias with the substrate choice (TGV/TSV) to minimize parasitics.
3D illustration of a thin-film RF device integrated on a microchip, showing electromagnetic waves and metallic interconnects

Performance Metrics & Design Insights

At the core of RF design are the relationships between frequency (f), dielectric constant (εr), and loss tangent (tan δ). These determine propagation velocity, line impedance, and efficiency of wave transmission. Modern mmWave systems demand substrates with tan δ < 10⁻³ and extremely tight surface control (Ra < 1 nm).

  • Insertion loss: For transmission lines < 0.1 dB/cm @ 40 GHz → use quartz, fused silica, or HR-Si.
  • Power handling: GaN on SiC devices reach > 10 W/mm gate periphery; thermal path > 400 W/m·K.
  • Surface flatness: TTV < 2 µm improves photolithography overlay alignment for 77 GHz radar arrays.
  • Film uniformity: ±2% thickness variation ensures stable phase velocity in CPW filters.

Emerging RF & mmWave Technologies

  • 5G and 6G Infrastructure: GaN/SiC power amplifiers and low-loss quartz interposers extend efficiency up to 140 GHz.
  • Automotive radar: 77 GHz front-ends integrate GaAs LNAs and SiC PAs on HR-Si packages.
  • Satellite communication (SATCOM): Ka-band transceivers leverage sapphire and GaN-on-Si substrates for thermal balance.
  • Quantum & Cryogenic RF: Single-crystal sapphire and fused silica support ultra-high-Q resonators (Q > 10⁸ at 4 K).
  • Photonics-RF hybrids: LiNbO₃ and SiN thin films integrate electro-optic modulation for 100 Gb/s links.

Fabrication & Surface Engineering

As RF frequencies climb into the mmWave and sub-THz domains, even surface micro-roughness or metallization voids can degrade signal quality. To mitigate this, UniversityWafer partners apply advanced wafer prep methods:

  • Atomic-layer cleaning: Removes sub-nm contamination before metal deposition.
  • Super-polished wafers: Ra < 0.5 nm improves conductor skin-depth uniformity.
  • Ion-beam and e-beam metrology: Used to confirm via and thin-film dimensional precision ±1 µm.

Industry Trends & Research Benchmarks

  • Global RF wafer demand is growing > 8% per year, driven by 5G, automotive radar, and satellite IoT.
  • High-resistivity Si (> 5 kΩ·cm) is becoming a CMOS-compatible platform for antenna-on-chip designs.
  • GaN on Si remains the most cost-effective wide-bandgap path for mass production of RF PAs.
  • Hybrid packaging with TGV/TSV reduces signal path length by up to 40% compared to traditional bond wire methods.

Related: GaN Wafers · GaAs Wafers · SiC Wafers · Quartz/Fused Silica · Sapphire · High-Resistivity Silicon