A question re: piezoelectric materials and resonators

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SUMMARY

This discussion centers on the differences between thin-film bulk acoustic resonators (TFBAR) and piezoelectric micromachined ultrasound transducers (PMUT), both utilizing aluminum nitride (AlN) and having a thickness of approximately 1 µm. The resonance frequency of PMUTs ranges from 0.5-1 MHz, while TFBARs operate at 10-100 MHz, indicating a fundamental difference in their operational characteristics despite their similar structural dimensions. The conversation highlights the importance of understanding the device's classification as either a resonator or a transducer, as this distinction affects resonance frequency and mode of operation.

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  • Understanding of piezoelectric materials, specifically aluminum nitride (AlN).
  • Familiarity with acoustic wave propagation principles.
  • Knowledge of device classifications: resonators vs. transducers.
  • Basic concepts of resonance frequency and its implications in acoustic devices.
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  • Research the operational principles of PMUTs and TFBARs.
  • Study the impact of device classification on resonance frequency and performance.
  • Examine the effects of electric field direction on acoustic wave propagation in FBARs.
  • Explore academic papers on thickness field excitation in bulk-micromachined resonators.
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Acoustics researchers, engineers in wireless communication, and professionals involved in the design and application of piezoelectric materials and devices.

Dishsoap
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TL;DR
Why do bulk acoustic resonators have higher resonant frequencies than PMUTs?
I'm new to the world of acoustics, and I've been reading up on various methods for wireless communication/sensing through ultrasound, especially with piezoelectric materials such as AlN. Fundamentally, I can't seem to find a difference between thin-film bulk acoustic resonators (1 um thick) and PMUTs, which are also about 1 um thick in AlN. They have similar structures (top and bottom electrode, sometimes suspended), and yet the resonance frequency of a PMUT is about 0.5-1 MHz and 10-100 for a TFBAR. I think I am missing something fundamental but I cannot figure out what.
 
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PMUT = Piezoelectric micromachined Ultrasound Transducer. 0.5-1 MHz.
TFBAR = thin-film bulk acoustic resonators. 10-100 MHz. Both 1 um thick.

Is the device being specified as a resonator or a transducer?
Maybe the resonant frequency of TFBAR is being compared with the bandwidth of the loaded PMUT.
 
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Baluncore said:
Is the device being specified as a resonator or a transducer?

I think that is where I am misunderstanding - I don't see why it matters. The geometry, thickness etc. can be the same, but why would the resonance frequency change depending on whether it is being used as a resonator or a transducer? In both cases, an acoustic wave is being converted into an AC voltage, right?
 
Dishsoap said:
In both cases, an acoustic wave is being converted into an AC voltage, right?
Yes, or vice versa.
It is also possible that the mode of operation might be different.
See; http://mems.usc.edu/fbar.htm
"A bulk-micromachined FBAR with Thickness Field Excitation () uses a z-directed electric field to generate z-propagating longitudinal or compressive wave. In an LFE-FBAR, the applied electric field is in y-direction, and the shear acoustic wave (excited by the lateral electric field) propagates in z-direction, as illustrated ()"
 
Baluncore said:
Yes, or vice versa.
It is also possible that the mode of operation might be different.
See; http://mems.usc.edu/fbar.htm
"A bulk-micromachined FBAR with Thickness Field Excitation () uses a z-directed electric field to generate z-propagating longitudinal or compressive wave. In an LFE-FBAR, the applied electric field is in y-direction, and the shear acoustic wave (excited by the lateral electric field) propagates in z-direction, as illustrated ()"

Oh, that could be - I'll have to examine these papers further. Thank you!
 
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