⚡ electrical engineering

Practical Demonstrations of FR3-Band Thin-Film Lithium Niobate Acoustic Filter Design

This paper demonstrates a practical approach for designing compact, high-performance acoustic filters in the FR3 band using A1-mode XBARs on 128° Y-cut thin-film lithium niobate, achieving low insertion loss and controlled bandwidth at 20.5 GHz and 22.0 GHz to support future 6G communication needs.

Original authors: Taran Anusorn, Omar Barrera, Jack Kramer, Ian Anderson, Ziqian Yao, Vakhtang Chulukhadze, Ruochen Lu

Published 2026-04-14

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CC BY 4.0

Original authors: Taran Anusorn, Omar Barrera, Jack Kramer, Ian Anderson, Ziqian Yao, Vakhtang Chulukhadze, Ruochen Lu

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine you are trying to tune a radio to listen to a specific station, but the radio station is moving incredibly fast, and the "dial" you use to tune it is made of a material that behaves differently depending on which way you turn it.

This paper is about building a tiny, super-precise radio tuner (a filter) for the next generation of wireless internet (6G). Here is the story of how they did it, explained simply.

The Problem: The "Goldilocks" Frequency

We are entering a new era of wireless communication called FR3 (Frequency Range 3). Think of this as a new, crowded highway for data.

The Issue: We need to build "gates" (filters) that let only the right cars (data) through and stop the rest.
The Challenge: These gates need to be tiny (smaller than a fingernail) to fit in your phone, but they also need to be incredibly precise. If the gate is too wide, traffic jams (interference) happen. If it's too narrow, you miss your data.
The Old Way: Previous technologies were like trying to build a highway gate out of giant concrete blocks. They were either too big to fit in a phone or couldn't be tuned precisely enough for these high speeds.

The Solution: The "Smart Crystal"

The researchers used a special material called Thin-Film Lithium Niobate (TFLN).

The Analogy: Imagine a sheet of glass that is so thin it's almost invisible, but it has a secret superpower: it vibrates when you apply electricity.
The Magic Trick: This material is anisotropic. In plain English, that means it has a "grain," like wood. If you push it one way, it vibrates strongly. If you push it the other way, it vibrates weakly.

How They Built the Tuner

The team built a filter using tiny vibrating units called XBARs. Here is how they controlled the "tuning":

Changing the Size (Thickness):
- Analogy: Think of a guitar string. A short, thick string makes a low note; a long, thin string makes a high note.
- The Tech: They made the crystal film thinner in specific spots to change the pitch (frequency) of the vibration. This allowed them to aim for the specific 20.5 GHz "station."
Rotating the Direction (The "Grain"):
- Analogy: Imagine a slinky. If you shake it straight up and down, it moves easily. If you try to shake it sideways against its coils, it's harder to move.
- The Tech: By rotating the electrodes (the metal fingers that shake the crystal) at different angles, they could control how wide the "gate" opens.
- The Result: They could make the filter let in a wide range of frequencies (wide bandwidth) or a very narrow, precise range, all on the same piece of material just by changing the angle of the metal fingers.

The Results: Two "Masterpieces"

They built two prototypes to prove this works:

The "Compact" Filter (3 parts):
- It's the size of a grain of rice (0.90 x 0.74 mm).
- It lets through a specific chunk of data with very little loss (only 1.79 dB of signal lost).
- It blocks out-of-band noise very well.
The "High-Precision" Filter (8 parts):
- This one is more complex, like a multi-lane highway with more barriers.
- It is even better at blocking unwanted noise (rejection of 22.97 dB), which is crucial for crowded 6G networks.
- It costs a tiny bit more in signal loss, but the trade-off is worth it for clarity.

Why This Matters for You

Smaller Phones: Because these filters are made of thin films and are so small, they can be packed into phones without taking up space.
Faster 6G: As we move to 6G, the airwaves will get crowded. This technology allows us to build "filter banks" (a whole set of different filters) on a single tiny chip, letting your phone switch between different data channels instantly and cleanly.
No More "One-Size-Fits-All": Before, you had to build a different filter for every frequency. Now, using this "rotation trick," you can design the exact filter you need just by changing the angle of the design on the computer.

The Bottom Line

The researchers figured out how to use the "grain" of a special crystal to build tiny, tunable radio gates. They proved they can make these gates small, precise, and customizable, paving the way for the super-fast, interference-free wireless world of the future. It's like going from building a radio out of giant bricks to building one out of LEGO bricks that you can snap together in any shape you need.

1. Problem Statement

The emergence of Frequency Range 3 (FR3), spanning 7.125 GHz to 24.25 GHz, is critical for 6G and beyond. However, designing compact, high-performance bandpass filters (BPFs) for this spectrum presents significant challenges:

Limitations of Existing Technologies:
- Integrated Passive Devices (IPD): Offer small footprints but suffer from moderate roll-off and poor frequency selectivity.
- Distributed Microwave Filters (Waveguides/Microstrip): Provide good performance but are too large for handheld devices due to electromagnetic wavelength constraints (~2 cm at 15 GHz).
- Surface Acoustic Wave (SAW): Limited by the need for extremely small interdigital electrode (IDE) pitches at high frequencies, leading to resistive losses and fabrication difficulties.
- Bulk Acoustic Wave (BAW/FBAR): While scalable, traditional materials like Aluminum Nitride (AlN) suffer from low electromechanical coupling coefficients ( $k_t^2$ ), limiting achievable fractional bandwidth (FBW).
Specific Challenge: Existing Thin-Film Lithium Niobate (TFLN) XBARs offer high $k_t^2$ and low loss above 20 GHz, but they typically exhibit a fixed, very wide FBW. Future 6G applications require precise, customizable FBW to match specific allocated bands without complex external tuning circuits or multi-step fabrication processes.

2. Methodology

The authors propose a monolithic design approach to independently control the operating frequency ( $f_c$ ) and fractional bandwidth (FBW) of TFLN acoustic filters using First-Order Antisymmetric (A1) mode Lateral-Field-Excited (XBAR) resonators on 128° Y-cut TFLN.

Core Mechanism: Anisotropic Tuning:
- The 128° Y-cut TFLN crystal possesses in-plane anisotropic piezoelectric properties.
- By rotating the IDEs (or the entire resonator) relative to the crystal axes, the effective piezoelectric constant ( $e_{15}$ ) and the resulting coupling coefficient ( $k_t^2$ ) can be tuned from ~0% to ~47%.
- This allows for the customization of the filter's FBW without changing the physical dimensions of the resonator cavity.
Frequency Control:
- The resonant frequency is controlled by the thickness of the TFLN film. Thinner films yield higher frequencies.
- The authors utilize ion milling to perform localized thickness trimming on the wafer to achieve specific resonant frequencies for series and shunt resonators.
Design Strategy:
- Ladder Topology: Filters are designed using a ladder network of series and shunt resonators.
- Impedance Matching: The static capacitance ( $C_0$ ) is tuned by adjusting the number of IDE fingers and their length to match a 50-Ω system.
- Transmission Zeros (TZs): By varying the properties (thickness and orientation) of individual shunt resonators, additional transmission zeros can be introduced to enhance out-of-band (OoB) rejection and selectivity.
- Hybrid Approach: The design combines global resonator rotation with local IDE tilting to minimize the overall footprint.

3. Key Contributions

Monolithic FBW Control: Demonstrated a technique to tune FBW from 0% to ~47% on a single wafer solely by rotating IDEs, eliminating the need for discrete inductors/capacitors or complex material engineering.
Dual-Parameter Control: Successfully decoupled the control of center frequency (via film thickness) and bandwidth (via orientation), enabling precise filter synthesis for FR3 bands.
Scalability: Validated the approach with both 3-element and 8-element ladder filter prototypes, proving the method's viability for higher-order filters.
Compact Footprint: Achieved chip-scale dimensions significantly smaller than distributed microwave filters, suitable for mobile applications.

4. Experimental Results

The authors fabricated and tested two filter prototypes on a TFLN-on-aSi-on-Sapphire platform.

A. Three-Element Filter Prototype

Center Frequency ( $f_c$ ): 20.5 GHz
Fractional Bandwidth (FBW): 8.58% (Targeted 9.54%)
Insertion Loss (IL): 1.79 dB
Out-of-Band (OoB) Rejection: >14.9 dB across the FR3 band.
Footprint: 0.90 × 0.74 mm²
Performance: Achieved a sharp shape factor of 1.92, outperforming comparable IPD filters in selectivity.

B. Eight-Element Filter Prototype

Center Frequency ( $f_c$ ): 22.0 GHz
Fractional Bandwidth (FBW): 6.12%
Insertion Loss (IL): 3.80 dB (Higher due to increased resonator count and fabrication parasitics).
Out-of-Band (OoB) Rejection: 22.97 dB (Significant improvement over the 3-element version).
Footprint: 1.48 × 0.89 mm²
Performance: Demonstrated the ability to create narrower bandwidths with superior rejection for higher-order networks.

C. Additional Characterization

Temperature Coefficient of Frequency (TCF): Measured between 240 K and 300 K. While individual resonators showed a TCF of ~-70 ppm/K, the filter prototypes showed improved TCF values (-55.53 ppm/K and -47.20 ppm/K), suggesting that filter topology can partially mitigate thermal drift.
Fabrication: Utilized sequential ion milling for thickness trimming (7 nm and 9 nm steps) and XeF2 etching for releasing the suspended film.

5. Significance and Impact

Enabling 6G/FR3: This work provides a critical solution for the stringent frequency allocation requirements of 6G, offering a path to compact, monolithic filter banks that can be integrated directly into RF front-ends.
Design Flexibility: The ability to "program" the FBW and $f_c$ via orientation and thickness trimming allows for the fabrication of multiple different filter specifications on a single wafer, reducing manufacturing complexity.
Performance Benchmark: The demonstrated combination of low insertion loss, high OoB rejection, and compact size outperforms many existing FR3 acoustic and microwave filter technologies.
Future Outlook: The paper establishes a foundation for developing monolithic BAW multiplexers and switchable filter banks, which are essential for the dense, multi-standard spectrum environments of next-generation wireless communications. It also highlights the potential for applying these techniques to Periodically Poled Lithium Niobate (PPLN) platforms for further performance enhancement.