Advances in Capacitive Micromachined Ultrasonic Transducers

Abstract

Capacitive micromachined ultrasonic transducer (CMUT) technology has enjoyed rapid development in the last decade. Advancements both in fabrication and integration, coupled with improved modelling, has enabled CMUTs to make their way into mainstream ultrasound imaging systems and find commercial success. In this review paper, we touch upon recent advancements in CMUT technology at all levels of abstraction; modeling, fabrication, integration, and applications. Regarding applications, we discuss future trends for CMUTs and their impact within the broad field of biomedical imaging.

Keywords: acoustics; capacitive; capacitive micromachined ultrasonic transducer (CMUT); fabrication; micromachining; modelling; transducer.

Figure 1

Capacitive micromachined ultrasonic transducer (CMUT) cell illustration. (a) A CMUT cell is composed of a flexible top plate and a fixed bottom plate. (b) A direct current (DC) bias is applied during the operation that deflects the top plate.

Figure 2

Simplified mass-spring-damper CMUT model.

Figure 3

CMUT Network model. (a) General two-port network representation that relates voltage and current (V and I) to force and velocity (F and V). (b) Small-signal equivalent circuit model (in transmit mode, $F_s=0$, and in receive mode, $V_s=0$). $R_s$ represents the electric source resistance.

Figure 4

Geometry and boundary conditions of 2D-Axisym. CMUT finite element (FE) models. (a) Single cell model. (b) Wave-guide model.

Figure 5

CMUT electrostatic gap segmentation. The electrostatic force is approximated by adding several parallel plate capacitors between the top and bottom electrodes.

Figure 6

Sacrificial release CMUT fabrication process. (a) Deposit silicon nitride. Deposit and pattern the sacrificial polysilicon layer. (b) Deposit the polysilicon sacrificial layer for the etch channels into the cells. (c) Deposit and pattern the silicon nitride top plate layer. (d) Etch the sacrificial polysilicon layer. (e) Deposit silicon nitride using low-pressure chemical vapor deposition (LPCVD) to seal the cells. (f) Deposit and pattern the aluminum electrodes and interconnects.

Figure 7

(a) Structure of a sacrificial release CMUT with vias for backside electrical contacts. (b) A four-ring 2D CMUT array fabricated using a sacrificial release process and through-wafer vias, as decribed in [25]. Reproduced with permission from Moini, A., Capacitive Micromachined Ultrasonic Transducer (CMUT) Arrays for Endoscopic Ultrasound; published by Stanford University, 2016.

Figure 8

Process flow for a wafer-bonded CMUT. (a) Starting prime wafer. (b) Thermal oxidation. (c) Etch to form cavity. (d) Thermal oxidation. (e) Silicon on insulator (SOI) wafer bonding. (f) SOI handle. (g) Removing burried oxide. (h) Sputtering metallization. (i) Metal pattern and device isolation.

Figure 9

(a–c) Wafer-bonded CMUT structures. (a) Local oxidation of silicon (LOCOS) wafer-bonded CMUT. (b) Thick buried oxide (BOX) CMUT. (c) Anodic-bonded CMUT. (d–e) Structures to improve average displacement shown for comparison. (d) Piston CMUT fabricated with a double wafer-bonding process. (e) Post CMUT.

Figure 10

Approaches to fabricating flexible CMUTs. (a) CMUT fabricated from entirely flexible polymers. (b) Isolated rigid CMUTs embedded within a flexible substrate.

Figure 11

Process flow for fabricating a bendable CMUT array. (a) A wafer-bonded CMUT after stripping the SOI BOX. (b) Sputtering top-side metallization. (c) Top-side isolation etch. (d) Second top-side isolation with nitride passivation. (e) Deposit of under bump metalization (UBM). (f) Secure to supporting wafer. (g) Back-side isolation and polydimethylsiloxane (PDMS) filling.

Figure 12

Schemes for electronic integration: (a) monolithic integration, (b) multi-chip integration, (c) hybrid integration.

Figure 13

Capsule US imaging system.

Figure 14

Dual-mode high intensity focused ultrasound (HIFU) and imaging system with 2D CMUT array and electronic circuitry (ASIC).

Figure 15

A fetal B-mode scanned with a CMUT transducer reproduced with permission from Butterfly Network [73].

Figure 16

(a) The integrated CMUT, IC and flexible printed circuit board (flex PCB) prior to PDMS casting. (b) 3D imaging result of a wire phantom. (c) HIFU ablation of a piece of ex-vivo tissue. All images from [71]. Reproduced with permission from Jang, J.H. et al., 2015 IEEE International Ultrasonics Symposium (IUS); published by IEEE Xplore, 2015 [72]. Reproduced with permission from Jang, J.H. et al., 2017 IEEE International Ultrasonics Symposium (IUS); published by IEEE Xplore, 2017.

Figure 17

(a) CMUT ring array. (b) CMUT ring array integrated into a endoscopic assembly. (c) 3D imaging results of a spring. Images from [24]. Reproduced with permission from Moini, A. et al., 2016 IEEE International Ultrasonics Symposium (IUS); published by IEEE Xplore, 2016 [66]. Reproduced with permission from Moini, A. et al., ASME 2015 International Technical Conference and Exhibition on Packaging and Integration of Electronic and Photonic Microsystems collocated with the ASME 2015 13th International Conference on Nanochannels, Microchannels, and Minichannels; published by ASME, 2015 [77]. Reproduced with permission from Choe, J.W. et al., IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control; published by IEEE Xplore, 2012.