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Mode Shape Suppression After Adding Molybdenum Layers to AlN Resonator
Posted Jul 15, 2025, 3:17 p.m. EDT MEMS & Piezoelectric Devices, Materials, Material Models Version 6.3 3 Replies
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Hello,
I am currently working on building a 3D piezoelectric resonator model.
As a starting point, I simulated a 1 μm-thick AlN layer on its own and was able to observe a well-defined thickness-extensional mode in the expected frequency range.
However, after adding 200 nm molybdenum (Mo) layers on both the top and bottom surfaces of the AlN to represent electrodes, I noticed a significant change. The thickness mode appears to be heavily suppressed, and the resulting mode shapes do not resemble those from the initial AlN-only configuration. Additionally, the output response is much weaker, and the resonant frequency appears to shift outside the previously observed range.
Could anyone advise why this behavior occurs? I expected a moderate frequency shift due to the added mass and acoustic impedance of the Mo layers, but not such a drastic change in the mode shape or suppression of the thickness mode altogether.
Any guidance or insight would be greatly appreciated.
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You are adding a substantial thickness of a high-density metal on surfaces of a low-density membrane. Just on that basis I would expect a very considerable impact on resonant frequency together with the appearance of multiple additional modes. You could try gradually increasing Mo thickness from near zero and watching what happens to the mode(s). OR try doing a wide frequency scan to identify resonances in the terminal impedance. (Probably VERY wide if you stick to your 200 nm Mo).
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Thank you, David, for the helpful response. I’ll try reducing the Mo thickness and observe how the modes evolve accordingly.
As a follow-up, I’m also considering covering the entire top surface of the piezoelectric layer with the Mo electrode. However, I’ve noticed that many examples and tutorials tend to use top electrodes that only partially cover the piezoelectric material.
Given that I’m designing a two-port piezoelectric resonator and aiming to maximize the output response, would using a full-coverage top electrode be detrimental to resonance performance? Is there a general advantage to limiting the top electrode area relative to the piezoelectric layer?
I appreciate any insight you can provide.
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Thank you, David, for the helpful response. I’ll try reducing the Mo thickness and observe how the modes evolve accordingly.
As a follow-up, I’m also considering covering the entire top surface of the piezoelectric layer with the Mo electrode. However, I’ve noticed that many examples and tutorials tend to use top electrodes that only partially cover the piezoelectric material.
Given that I’m designing a two-port piezoelectric resonator and aiming to maximize the output response, would using a full-coverage top electrode be detrimental to resonance performance? Is there a general advantage to limiting the top electrode area relative to the piezoelectric layer?
I appreciate any insight you can provide.
I don't do filter design so I don't have any particular insight to offer. You can (1) try simulation to find out and (2) look in the literature.