A question re: piezoelectric materials and resonators

AI Thread Summary
The discussion centers on the differences between thin-film bulk acoustic resonators (TFBARs) and piezoelectric micromachined ultrasound transducers (PMUTs), both made from AlN and approximately 1 um thick. Despite their similar structures, TFBARs operate at resonance frequencies of 10-100 MHz, while PMUTs function at 0.5-1 MHz. The conversation highlights the importance of understanding whether a device is specified as a resonator or a transducer, as this can influence its operational mode and frequency characteristics. The mode of operation, including the direction of the electric field and wave propagation, may also contribute to the differences in resonance frequency. Further examination of relevant literature is suggested for deeper insights.
Dishsoap
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TL;DR Summary
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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