What is a bulk acoustic wave filter and how does it differ from a SAW filter?
BAW/FBAR Filter Technology
The BAW resonator frequency is determined by the piezoelectric film thickness: f = v/(2t), where v is the acoustic velocity in the film (~10,000-11,000 m/s for AlN) and t is the film thickness. At 2 GHz, the film is approximately 2.5 μm thick. At 5 GHz, it is approximately 1 μm. This thickness is controlled by thin-film deposition processes (sputtering) with excellent uniformity and repeatability.
| Parameter | LC Lumped | Cavity | SAW/BAW |
|---|---|---|---|
| Q Factor | 50-200 | 1,000-20,000 | 500-2,000 |
| Frequency Range | DC-3 GHz | 0.1-40 GHz | 0.1-6 GHz |
| Insertion Loss | 1-6 dB | 0.2-2 dB | 1-4 dB |
| Size | Small (PCB) | Large (machined) | Very small (chip) |
| Tuning | Fixed or varactor | Mechanical screw | Fixed |
- Performance verification: confirm specifications against the application requirements before finalizing the design
- Environmental factors: temperature range, humidity, and vibration affect long-term reliability and parameter drift
- Cost vs. performance: evaluate whether the application demands premium components or standard commercial grades
Frequently Asked Questions
Why is BAW more expensive than SAW?
BAW requires thin-film deposition (sputtering of AlN or ZnO), precision thickness control, and sometimes wafer-level packaging with through-wafer vias. SAW uses simpler photo-lithography on bulk single-crystal substrates. The BAW process is more complex and lower-yield. However, the cost gap is narrowing as BAW production scales.
What is the bandwidth limit?
BAW ladder filters are typically limited to 3-5% fractional bandwidth because the series-shunt resonator offset (which sets bandwidth) is limited by the piezoelectric coupling coefficient (kt² ≈ 6-7% for AlN). Wider bandwidth requires modified topologies or alternative piezoelectric materials (like scandium-doped AlN with kt² up to 15%).
Can BAW reach above 6 GHz?
With difficulty. Above 6 GHz, the AlN film thickness drops below 0.8 μm, making control difficult. The acoustic velocity also decreases due to electrode loading effects. Research is active in extending BAW to 10+ GHz using ultra-thin films, alternative materials (ScAlN), and novel electrode designs, driven by 5G mmWave and WiFi 7 requirements.