Design, Fabrication and Characterization of Multi-Frequency MEMS Transducer for Photoacoustic Imaging

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This manuscript presents a systematic study on the design, fabrication, and characterization of multi-frequency MEMS ultrasonic transducers tailored for photoacoustic imaging (PAI). The research addresses a critical limitation of conventional piezoelectric transducers—narrow frequency response—by exploring four distinct MEMS geometries, which is highly relevant to the advancement of PAI technology in biomedical applications. Could the authors please clarify the following points to ensure the complete understanding? (1) In table1, the frequency range of this work is 1~3.5MHz, how to obtain this value precisely? Could add some more details in figure 10 to decribe the determining method? (2) The resonant frequencies is determined by size of the resonators when materials, shape and vibrating mode is given. So, the resonant frequencies can be adjusted to any value within some frequency range through changing resonator dimensions. Could the authors explain how to determine the size of the resonators in the manuscript? (3) In the manuscript, the displacement amplitue is observed through laser Doppler vibrometer, could the position of the observed points be given?

Author Response

We would like to thank you for the time and effort necessary to review the manuscript. We sincerely appreciate all of your valuable comments and suggestions to improve the quality of the manuscript. We hope that our updates satisfy your requests and suggestions.

Please find attached the PDF with our comments and edits to the manuscript.

Thank you.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The authors present partial results of an ongoing study on MEMS ultrasonic transducer configurations for wideband frequency applications in photoacoustic imaging. The manuscript describes the design, fabrication, and characterization of four different geometries, whose modal performance is evaluated using 1D laser Doppler vibrometry (LDV, OFV-534) to verify resonance frequency shifts with respect to the design values. Overall, the work shows potential and could be suitable for publication in Micromachining after major revisions.

First, the results section lacks a table clearly reporting the frequency shifts between simulated and measured resonance frequencies. Including such a table would significantly improve clarity and allow a more quantitative comparison. In addition, Table 1 could benefit from the inclusion of relevant literature on similar devices, while keeping the focus on the proposed designs.

Moreover, I suggest explicitly identifying the vibration modes observed in the LDV frequency response spectra. Since a 1D vibrometer measures vibration at a specific point on the device, FEM simulations are essential to guide the selection of the measurement location for different modes. This approach would help ensure correct mode identification and accurate estimation of discrepancies between simulation and experiment, which is crucial for the reproducibility of the resonant behaviour in real devices.

Second, the fabrication section should be reorganised more clearly, particularly with reference to Figure 1. As far as I know, the MEMSCAP PiezoMUMPs process begins with an SOI silicon wafer featuring a buried oxide layer (not a nitride “black layer”), followed by a bulk micromachining DRIE step to release the silicon device layer that contains the resonator. The fabrication description should be revised to accurately reflect the process flow.

Third, I recommend modifying the title to explicitly include fabrication, for example:
“Design, Fabrication, and Characterisation of …”

Finally, the paper discusses several devices intended for ultrasonic transducer in photoacoustic imaging, with the idea of coupling them to extend the operational bandwidth. To strengthen the manuscript, the authors should explain how the proposed multimodal system would be integrated into an array to function as a single effective device. In particular, if an array contains multiple resonators of different types, potential cross-talk between devices and coupling with substrate modes may affect the overall spectral response. A discussion of these effects and possible mitigation strategies would improve the robustness and completeness of the work.

Author Response

We would like to thank you for the time and effort necessary to review the manuscript. We sincerely appreciate all of your valuable comments and suggestions to improve the quality of the manuscript. We hope that our updates satisfy your requests and suggestions.

Please find attached the PDF with our comments and edits to the manuscript.

Thank you.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

Authors of this paper present the design, fabrication, and experimental characterization of microelectromechanical system ultrasonic transducers engineered for multi-frequency operation in photoacoustic imaging. A comparative evaluation of four geometries, i.e., circular diaphragm, floating cross, cross configuration, and a cantilever array, showed that structural complexity and modal diversity extended frequency coverage and improved adaptability to the spectral content of photoacoustic signals. This research is interesting for achieving broad spectral sensitivity to optimize both resolution and penetration depth for imaging biological samples of varying sizes and depths. Following questions are for the authors.

Between lines 251-253 on page 7, authors asserted “2.1 MHz, 2.6 MHz, and 5.5 MHz were identified and confirmed”. Are these frequencies obtained by finite element simulations or experimental measurements?

In caption of Figure 5, does “(c)–(g) first-mode shapes of arms 1–5.” mean that Figure 5c to 5g are all first mode but for different arm? Or they are the higher-order modes?

What does “T.W” stand for in the last line in Table 1?

Author Response

We would like to thank you for the time and effort necessary to review the manuscript. We sincerely appreciate all of your valuable comments and suggestions to improve the quality of the manuscript. We hope that our updates satisfy your requests and suggestions.

Please find attached the PDF with our comments and edits to the manuscript.

Thank you.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

no more comments 

Author Response

Since the reviewer has no further comments, we would like to thank them for the time invested and for their valuable feedback on our manuscript.

Reviewer 2 Report

Comments and Suggestions for Authors

Dear Authors, I have a few comments regarding the first part of the cover letter.

I agree that complex effects such as geometric errors, non-uniformities, damping, residual stress, and mismatched boundary conditions are not accounted for in an idealized FEM model. However, this does not limit the potential of frequency response analysis or its usefulness in the development process of resonating MEMS devices. Considering the measurement approach used to characterize the device, air damping does play a role, but the resulting frequency shift is generally small and, in some cases (e.g., silicon cantilevers), can be negligible.

On the other hand, the authors state in the paper: “The experimental fundamental frequency was measured at 1.69 MHz, closely matching the FEM-predicted value, and the Q-factor was extracted as approximately 268.”
In this context, please include a brief description of how the Q-factor was evaluated.

Please also check Figure 1 in the paper. In the reviewed version, the oxide layer is shown in black, whereas in the paper it is still indicated as silicon nitride. Please clarify and correct this inconsistency.

Furthermore, I cannot find Figure 10; please provide it. Also, please verify the correspondence between the figures cited in the text and the images presented in the measurement section. For instance, the text refers to Figure 6: diaphragm microstructure, while Figure 5 is labeled Circular diaphragm microstructure frequency response.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf