A question re: piezoelectric materials and resonators

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Discussion Overview

The discussion revolves around the differences between thin-film bulk acoustic resonators (TFBARs) and piezoelectric micromachined ultrasound transducers (PMUTs), particularly focusing on their resonance frequencies and operational modes. Participants explore the implications of device classification and the fundamental principles of acoustic wave conversion in the context of piezoelectric materials like AlN.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant notes the similarity in structure between TFBARs and PMUTs, questioning the fundamental reasons for the significant difference in resonance frequencies (0.5-1 MHz for PMUTs vs. 10-100 MHz for TFBARs).
  • Another participant suggests that the classification of the device as a resonator or transducer may influence the resonance frequency, proposing that the comparison might involve the bandwidth of the loaded PMUT.
  • A participant expresses confusion about why the classification matters, arguing that both devices convert acoustic waves into AC voltage, regardless of their designation.
  • It is mentioned that the mode of operation might differ between the two devices, with a reference to specific operational modes of FBARs involving different electric field orientations and wave propagation types.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the significance of device classification in determining resonance frequency, and the discussion remains unresolved regarding the fundamental differences between TFBARs and PMUTs.

Contextual Notes

Participants reference specific operational modes and configurations of FBARs, indicating potential complexities in understanding the relationship between device structure and performance. There may be missing assumptions regarding the definitions of resonators and transducers that could influence the discussion.

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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