A submicrometre silicon-on-insulator resonator for ultrasound detection

Abstract

Ultrasound detectors use high-frequency sound waves to image objects and measure distances, but the resolution of these readings is limited by the physical dimensions of the detecting element. Point-like broadband ultrasound detection can greatly increase the resolution of ultrasonography and optoacoustic (photoacoustic) imaging1,2, but current ultrasound detectors, such as those used for medical imaging, cannot be miniaturized sufficiently. Piezoelectric transducers lose sensitivity quadratically with size reduction3, and optical microring resonators4 and Fabry–Pérot etalons5 cannot adequately confine light to dimensions smaller than about 50 micrometres. Micromachining methods have been used to generate arrays of capacitive6 and piezoelectric7 transducers, but with bandwidths of only a few megahertz and dimensions exceeding 70 micrometres. Here we use the widely available silicon-on-insulator technology to develop a miniaturized ultrasound detector, with a sensing area of only 220 nanometres by 500 nanometres. The silicon-on-insulator-based optical resonator design provides per-area sensitivity that is 1,000 times higher than that of microring resonators and 100,000,000 times better than that of piezoelectric detectors. Our design also enables an ultrawide detection bandwidth, reaching 230 megahertz at −6 decibels. In addition to making the detectors suitable for manufacture in very dense arrays, we show that the submicrometre sensing area enables super-resolution detection and imaging performance. We demonstrate imaging of features 50 times smaller than the wavelength of ultrasound detected. Our detector enables ultra-miniaturization of ultrasound readings, enabling ultrasound imaging at a resolution comparable to that achieved with optical microscopy, and potentially enabling the development of very dense ultrasound arrays on a silicon chip.

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Fig. 1: Design and operating principles of the SWED.
Fig. 2: Characterization of the SWEDs.
Fig. 3: Reflection-mode far-field and near-field optoacoustic imaging.
Fig. 4: Imaging of acoustic broadband interference.

Data availability

The data that support the findings of this study are available from the corresponding authors on reasonable request.

Code availability

The code used to analyse the data is available from the corresponding authors on reasonable request.

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Acknowledgements

We thank D. Razansky, H. Estrada, R. J. Wilson, C. Zakian and A. Rosenthal for comments and discussions. We thank the staff at IMEC (Leuven, Belgium), ePIXfab (Ghent, Belgium) and Ara Coatings (Erlangen, Germany) for technical expertise at various steps during the manufacturing process. We acknowledge financial support from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme under grant agreement numbers 694968 (PREMSOT), 667933 (MIB) and 732720 (ESOTRAC), and from the Deutsche Forschungsgemeinschaft (DFG; Gottfried Wilhelm Leibniz Prize 2013; NT 3/10-1).

Author information

Affiliations

Authors

Contributions

R.S. conceived the SWED and its principle of operation, and designed the chip layout. R.S. and G.W. constructed the first SWED prototype. R.S. and O.Ü. performed experimental work leading to the demonstration of imaging performance and detector resolution. O.Ü. polished, coated and connectorized the chips. R.S. and V.N. designed experiments, and R.S. and O.Ü. conducted the imaging experiments. R.S. processed and analysed the data. Q.M. developed the image reconstruction algorithm and contributed to image processing. A.C. advised on the experimental setup and constructed the optical confocal microscope. R.S. and V.N. wrote the manuscript. V.N. supervised the research. All authors read and edited the manuscript.

Corresponding authors

Correspondence to Rami Shnaiderman or Vasilis Ntziachristos.

Ethics declarations

Competing interests

V.N. has financial interests in iThera Medical GmbH, Surgvision BV/Bracco S.p.A, I3 Inc. and Spear UG.

Additional information

Peer review information Nature thanks Ivan Pelivanov and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Extended data figures and tables

Extended Data Fig. 1 Process control with spectral response.

ac, Monitoring the reflection spectrum of a SWED with Δw = 40 nm (SWED340nm) during the polishing process of the SOI chip: before the start of polishing (a); at the end of polishing and before the application of the Ag coating (b); and after the application of the Ag coating (c).

Extended Data Fig. 2 Schematics of the experimental setups.

a, Characterization setup. An inverted microscope is coupled to a laser source for optoacoustic excitation; the SOI chip (CH) is mounted in a trans-illumination geometry and is raster-scanned over the sample placed on the coverslip (CS) (stages not shown). b, Imaging setup. The laser source for optoacoustic excitation is coupled into an optical fibre, which illuminates the sample; the chip is mounted in a reflection-mode illumination geometry. The coverslip holding the sample is raster-scanned while the chip and the illumination fibre (IF) are stationary (stages not shown). In both setups, the SWED interrogation is performed by a tuneable continuous-wave laser. OBJ, microscope objective.

Extended Data Fig. 3 Long-term detection stability of the SWED.

The variation in the maximum values of the detected signal amplitude is shown over 15 min. BPF, band-pass filter; a.u., arbitrary units.

Extended Data Fig. 4 Hydrophone and SWED responses to an acoustic point source over the frequency band [2, 500] MHz.

The hydrophone response has been scaled up by a factor of 30 for visibility. Signals attributed to reflections between the SWED and the sample holder are indicated.

Extended Data Table 1 The measured optical and acoustic properties of the manufactured SWEDs

Supplementary information

Supplementary Information

This file contains Supplementary Notes on Sensitivity.

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Shnaiderman, R., Wissmeyer, G., Ülgen, O. et al. A submicrometre silicon-on-insulator resonator for ultrasound detection. Nature 585, 372–378 (2020). https://doi.org/10.1038/s41586-020-2685-y

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