Soren Elmin Diederichsen

Investigation of PDMS as coating on CMUTs for imaging

A protective layer is necessary for Capacitive Micromachined Ultrasonic Transducers (CMUTs) to be used for imaging purpose. The layer should both protect the device itself and the patient while maintaining the performance of the device. In this work Sylgard 170 PDMS is tested as coating material for CMUTs through comparison of transmit pressure and receive sensitivity in immersion of coated and uncoated elements. It is seen that the transmitted pressure decreases with 27% and the receive sensitivity decreases 35%when applying the coating using a dam and fill principle. This matches well with the estimated value of 31 %. With the coating, the center frequency was found to be decreased from 4.5 MHz to 4.1 MHz and the fractional bandwidth was increased from 77 % to 84 % in transmit. In receive the center frequency was found to decrease from 4.4 MHz to 3.9 MHz and the fractional bandwidth was decreased from 108 % to 92 %, when applying the PDMS coating.

Elimination of second-harmonics in CMUTs using square pulse excitation

The harmonic imaging mode is today a fundamental part of ultrasound imaging; it is not only used for suppressing the grating lobe artifact, but also to reduce many other acoustical artifacts in the ultrasound image. A vital performance parameter for accepting CMUT probes as a clinical usable transducer technology is, that it can support harmonic imaging. The large bandwidth of the CMUT is a clear advantage for harmonic imaging, but the inherent nonlinear behavior of the CMUT poses an issue as it is difficult to dissociate the harmonics generated in the tissue from the harmonic content of the transmitted signal. This work presents how proper pulse coding of a bipolar pulser, which is present in most commercial ultrasound scanners, can reduce the intrinsic generated harmonic to fundamental pressure amplitude ratio to below -35 dB, making CMUT probes usable for clinical applications.

Output pressure and harmonic characteristics of a CMUT as function of bias and excitation voltage

The large bandwidth makes CMUT based transducers interesting for both conventional and harmonic imaging. The inherent nonlinear behavior of the CMUT, however, poses an issue for harmonic imaging as it is difficult to dissociate the harmonics generated in the tissue from the harmonic content of the transmitted signal. The generation of intrinsic harmonics by the CMUT can be minimized by decreasing the excitation signal. This, however, leads to lower fundamental pressure which limits the desired generation of harmonics in the medium. This work examines the output pressure and harmonic characteristics of a CMUT as function of bias and excitation voltage. The harmonic to fundamental ratio of the surface pressures declines for decreasing excitation voltage and increasing bias voltage. The ratio, however, becomes unchanged for bias levels close to the pull-in voltage. The harmonic limitations of the CMUT is emphasized by a maximum ratio of -12 dB between harmonics generated in the medium and total harmonics measured at 10 mm.

Dimensional scaling for optimized CMUT operations

This work presents a dimensional scaling study using numerical simulations, where gap height and plate thickness of a CMUT cell is varied, while the lateral plate dimension is adjusted to maintain a constant transmit immersion center frequency of 5 MHz. Two cell configurations have been simulated, one with a single square cell and one with an infinite array of square cells. It is shown how the radiation impedance from neighboring cells has a significant impact on the design process. For transmit optimization, both plate dimensions and gap height should be increased. For receive mode, the gap height should be increased while the effect of plate dimensions is ambiguous depending on if the array design is closest to a single cell or infinite array of cells. The findings of the simulations are verified by acoustical measurements on two CMUT arrays with different plate dimensions.

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

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 Silicon-On-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 5MHz and a pull-in voltage of 200V are obtained using finite element analysis. Hydrophone and plane reflector measurements are used to assess the acoustic performance. Increasing the plate thickness from 2μm to 15μm 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 9.3μm plate, which still has a moderate pulseecho bandwidth of 60%. The 9.3μm 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 2μm 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.

Output pressure and pulse-echo characteristics of Capacitive Micromachined Ultrasonic Transducers as function of plate thickness

The energy transduction of a Capacitive Micromachined Ultrasonic Transducer (CMUT) depends on the movement of a flexible plate, which has a low mechanical impedance compared to its bulky piezoelectric counterpart. Consequently, the output pressure of a CMUT is generally lower. This limits the penetration depth, and thus the visualization of deeper structures in the body, as well as non-linear imaging such as tissue harmonic imaging. The objective of this work is to investigate how the output pressure and pulse-echo signal of a CMUT scales with the (silicon) plate thickness.

Transmitting performance evaluation of ASICs for CMUT-based portable ultrasound scanners

Portable ultrasound scanners (PUS) have, in recent years, raised a lot of attention, as they can potentially overcome some of the limitations of static scanners. However, PUS have a lot of design limitations including size and power consumption. These restrictions can compromise the image quality of the scanner. In order to overcome these restrictions, application specific integrated circuits (ASICs) are needed to implement the electronics. In this work, a comparative study of the transmitting performance of a capacitive micromachined ultrasonic transducer (CMUT) driven by a commercial generic ultrasound transmitter and an ASIC optimized for CMUT-based PUS is presented. A single CMUT element is pulsed with a 1% duty cycle at a frequency of 5MHz. The DC bias voltage is 80V and the pulsing voltage is 20V. The acoustic performance is assessed by comparing the ultrasonic signals measured with a hydrophone both in the time and frequency domains. The difference in normalized signal amplitude evaluated at the center frequency of the CMUT is −1.9dB and the measured bandwidth is equivalent. The ASIC consumes only 1.3% of the total power consumption used by the commercial transmitter.

Performance evaluation of ASICs for CMUT-based portable ultrasound scanners

Portable ultrasound scanners (PUS) have shown to have advantages over stationary scanners such as ease of use, accessibility, transportability and cost. However, PUS are severely restricted on size and power which directly limits the area and power consumption of the electronics. In order to fully utilize the available area and power budget to achieve the best picture quality, application specific integrated circuits (ASICs) are required for the electronics. The ASICs are optimized to drive a specific transducer while minimizing the area and power consumption, which is a key target for PUS. For the purpose of implementing compact probes, CMUTs are used due to their highly compatible integration with ASIC fabrication processes. The goal of this work is to assess the impact of the area and power consumption reduction on the acoustic performance of the scanner. For this purpose, a comparison between a commercial ultrasound pulser and an ASIC optimized for CMUT-based PUS is presented.

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

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 > 1020 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 2.00εm±2.5%. The resonant frequency in air for a 1-D linear CMUT array is measured to 12MHz±2.5%. Furthermore, hydrophone measurements show that the fabricated devices can be used to emit sound pressure in the ultrasonic frequency domain.