Elimination of second-harmonics in CMUTs using square pulse excitation

Abstract-Capacitive Micromachined Ultrasonic Transducers (CMUTs) fabricated using Silicon-On-Insulator (SOI) wafers often have large thickness variation of the flexible plate, which causes variation in both pull-in voltage and resonant frequency across the CMUT array. This work presents a bond and boron etch-stop scheme for fabricating the flexible plate of a CMUT. The proposed fabrication method enables precise control of the plate thickness variation and is a low cost alternative to the SOI-based process. N-type silicon wafers are doped with boron to a surface concentration of > 10 20 cm −3 using solid planar diffusion predeposition at 1125• C for 30, 60, and 90 min. Process simulations are used to predict the boron doping profiles and validated with secondary ion mass spectrometry measurements. The doped wafers are fusion-bonded to a silicon dioxide surface and thinned down using an 80• C, 20 wt% potassium hydroxide solution with isopropyl alcohol added to increase the etch selectivity to the highly doped boron layer. The resulting plate thickness uniformity is estimated from scanning electron micrographs to a mean value of 2.00 µm ± 2.5%. The resonant frequency in air for a 1-D linear CMUT array is measured to 12 MHz ± 2.5%. Furthermore, hydrophone measurements show that the fabricated devices can be used to emit sound pressure in the ultrasonic frequency domain.

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“…Notice the second harmonic suppression of approx. −50 dB caused by the excitation pulse-shape, as explained in [5].…”

Section: B Acoustic Performancementioning

confidence: 96%

“…During a pulse-echo sequence, this sets the need for a mechanical part that is able to exert a force on a medium and moreover convert the echoed pressure waves to a signal on the terminals. The mechanical component of a CMUT cell is a flexible plate in the 0.1-20 µm thickness range [1], [2], [3], [4], [5].…”

Section: Introductionmentioning

confidence: 99%

Fabrication of capacitive micromachined ultrasonic transducers using a boron etch-stop method

Diederichsen

Sandborg-Olsen

Engholm

et al. 2016

2016 IEEE International Ultrasonics Symposium (IUS)

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“…Using such sequences with CMUTs will therefore result in poor contrast in the CEUS images [17,18]. Several attempts have been made to overcome this issue by altering the frequency content of the emitted pulse [19,20], but such approaches are not necessarily compatible with the bipolar pulsers often found in commercial scanners. Moreover, these methods rely on all elements in the probe behaving identically, which is often not the case.…”

Section: Introductionmentioning

confidence: 99%

Contrast-enhanced ultrasound imaging using capacitive micromachined ultrasonic transducers

Øygard

Ommen

Tomov

et al. 2023

The Journal of the Acoustical Society of America

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Capacitive micromachined ultrasonic transducers (CMUTs) have a nonlinear relationship between the applied voltage and the emitted signal, which is detrimental to conventional contrast enhanced ultrasound (CEUS) techniques. Instead, a three-pulse amplitude modulation (AM) sequence has been proposed, which is not adversely affected by the nonlinearly emitted harmonics. In this paper, this is shown theoretically, and the performance of the sequence is verified using a 4.8 MHz linear capacitive micromachined ultrasonic transducer (CMUT) array, and a comparable lead zirconate titanate (PZT) array, across 6–60 V applied alternating current (AC) voltage. CEUS images of the contrast agent SonoVue flowing through a 3D printed hydrogel phantom showed an average enhancement in contrast-to-tissue ratio (CTR) between B-mode and CEUS images of 49.9 and 37.4 dB for the PZT array and CMUT, respectively. Furthermore, hydrophone recordings of the emitted signals showed that the nonlinear emissions from the CMUT did not significantly degrade the cancellation in the compounded AM signal, leaving an average of 2% of the emitted power between 26 and 60 V of AC. Thus, it is demonstrated that CMUTs are capable of CEUS imaging independent of the applied excitation voltage when using a three-pulse AM sequence.

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“…The thickness of the flexible plate typically ranges from submicrons to a few microns [1]- [4], and is in this work predetermined by the SOI wafer device layer thickness. So, provided with both an immersion frequency and a fixed plate thickness, the plate radius remains the only unknown plate parameter.…”

Section: Introductionmentioning

confidence: 99%

Output pressure and pulse-echo characteristics of CMUTs as function of plate dimensions

Diederichsen

Hansen

Engholm

et al. 2017

2017 IEEE International Ultrasonics Symposium (IUS)

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Abstract-This paper presents an experimental study of the acoustic performance of Capacitive Micromachined Ultrasonic Transducers (CMUTs) as function of plate dimensions. The objective is to increase the output pressure without decreasing the pulse-echo signal. The CMUTs are fabricated with a LOCOS process, followed by direct wafer fusion bonding to a SiliconOn-Insulator (SOI) wafer. In this way, the plate thickness is determined by the SOI wafer device layer thickness, resulting in CMUTs with plate thicknesses of 2, 9.3 and 15 µm. The corresponding radii and gap heights resulting in an immersion frequency of 5 MHz and a pull-in voltage of 200 V are obtained using finite element analysis. Hydrophone and plane reflector measurements are used to assess the acoustic performance. Increasing the plate thickness from 2 µm to 15 µm decreases the pulse-echo bandwidth from >100% to 30%. A maximum in both peak-to-peak output pressure and pulse-echo signal is obtained for the 9.3 µm plate, which still has a moderate pulseecho bandwidth of 60%. The 9.3 µm plate results in a 1.9 times higher peak-to-peak output pressure and a 3.6 times higher pulse-echo signal compared to the 2 µm plate. By adjusting the plate dimensions of a CMUT it is possible to optimize its acoustic performance for medical imaging applications, including visualization of deeper structures in the body, as well as nonlinear imaging such as tissue harmonic imaging.

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Elimination of second-harmonics in CMUTs using square pulse excitation

Cited by 7 publications

References 10 publications

Fabrication of capacitive micromachined ultrasonic transducers using a boron etch-stop method

Fabrication of capacitive micromachined ultrasonic transducers using a boron etch-stop method

Contrast-enhanced ultrasound imaging using capacitive micromachined ultrasonic transducers

Output pressure and pulse-echo characteristics of CMUTs as function of plate dimensions

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