What Substrates are Used to Fabricate Film Bulk Acoustic Resonators?

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Common Types of Acoustic Resonators Helmholtz Resonator (e.g., bottle, cavity) String Resonator (e.g., guitar, piano) Pipe Resonator (e.g., flute, organ pipe) Membrane Resonator (e.g., drum, speaker) Plate Resonator (e.g., cymbal, bell) Legend: Air Vibration String Vibration Air Column

Surface Modes

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What Substrates are Used to Fabricate Film Bulk Acoustic Resonators?

Film Bulk Acoustic Resonators (FBAR) are typically fabricated on substrates with specific acoustic, thermal, and electrical properties. The primary substrate choices are:

1. Silicon (Si)

  • Commonly used due to cost-effectiveness and wide availability.

  • Usually combined with layers to improve acoustic isolation, such as silicon dioxide (SiO₂), silicon nitride (Si₃N₄), or aluminum nitride (AlN).

  • Si substrates often require the fabrication of air cavities or Bragg reflector layers to minimize acoustic energy loss into the substrate.

2. Silicon-on-Insulator (SOI)

  • Consists of silicon substrate with a thin silicon device layer separated by a buried oxide (BOX).

  • Offers excellent acoustic isolation due to the BOX layer, simplifying resonator fabrication and improving device performance.

3. Quartz (SiO₂)

  • Has excellent acoustic, mechanical, and thermal stability.

  • Used primarily in high-frequency, low-loss applications where performance outweighs higher material cost.

  • Suitable for resonators requiring high Q-factor and stability.

4. Sapphire (Al₂O₃)

  • Provides superior thermal conductivity, electrical insulation, and mechanical rigidity.

  • Excellent substrate choice for FBAR devices operating under high power or high-frequency conditions, and at elevated temperatures.

5. Glass Substrates

  • Low acoustic loss and good electrical insulation properties.

  • Useful in certain applications where acoustic isolation is crucial and cost is acceptable.

  • Common glass types used include borosilicate and fused silica.

Common Structural Layers for FBAR on These Substrates:

  • Piezoelectric Layers: Aluminum Nitride (AlN), Zinc Oxide (ZnO), or doped AlN (e.g., ScAlN).

  • Electrode Layers: Typically thin films of Aluminum (Al), Platinum (Pt), Molybdenum (Mo), Tungsten (W), or Gold (Au).

Typical Stack Configuration:

  • Top Electrode (Metal)

  • Piezoelectric Film (AlN/ZnO/ScAlN)

  • Bottom Electrode (Metal)

  • Acoustic Isolation Layer (Air cavity or Bragg reflector)

  • Substrate (Si, SOI, Quartz, Sapphire, Glass)

Selection Criteria for Substrates:

Property Silicon SOI Quartz (SiO₂) Sapphire (Al₂O₃) Glass
Acoustic Loss Moderate (needs isolation) Low (good isolation) Very Low Low Moderate
Thermal Conductivity Moderate Moderate Low High Low
Cost Low Moderate High High Moderate
Fabrication Complexity Moderate Moderate (cavity/BOX) Moderate Moderate to High Moderate
Electrical Insulation Poor (semi-conductive) Moderate (depends on layers) Excellent Excellent Excellent

Most Popular Choice:

Silicon and Silicon-on-Insulator (SOI) are widely used due to the optimal balance between performance, availability, cost, and integration capabilities in MEMS and RF integrated circuit technologies.

Would you like more details on a specific substrate or resonator configuration?

 

What Substrates are Used?

FBARs typically use a piezoelectric material, such as aluminum nitride (AlN), as the substrate. Other materials such as zinc oxide (ZnO) and Lithium Niobate (LiNbO3) can also be used. The choice of substrate material depends on the specific application requirements and performance characteristics needed. For example, AlN is often used for high frequency applications due to its high acoustic velocity and low mechanical losses, while LiNbO3 is used for its high electromechanical coupling coefficient.