CMUT Electrode Resistance Design: Modelling and Experimental Verification by a Row-Column Array

Research output: Contribution to journalJournal articleResearchpeer-review

283 Downloads (Pure)

Abstract

This paper addresses the importance of having control over the resistivity of the electrodes for capacitive micromachined ultrasonic transducers (CMUT) devices. The electrode resistivity can vary depending on the fabrication technology used, and resistivity control becomes especially important in the cases where metal electrodes can not be used. This raises the question: When is the resistivity of an electrode sufficiently low? To answer this question we have developed a simple design criterion. The criterion describes the attenuation of AC signals along a CMUT element. It is shown that the non-dimensional product between angular excitation frequency, resistance, and capacitance ωRC of an element has to be smaller than 0.35 to ensure an AC potential drop along the element of less than 1%. The optimal magnitude and directionality of the transmit pressure will be achieved if CMUT elements are designed according to the developed criteria. Hence, the model can be used to estimate device parameters that will ensure the CMUT is suitable for generating ultrasound images. An example is given where the model is used to predict the required electrode thickness for structured electrodes made of Gold, Aluminium, and Indium-Tin-Oxide, respectively. To verify the model, two Row-Column addressed (RCA) CMUT transducers were used to illustrate the effect of high and low electrode resistivity. One transducer had a sufficient electrode resistivity, and the other had an insufficient electrode resistivity. The RCA CMUT transducers were fabricated using fusion bonding, where the top electrode is made of aluminium and the bottom electrode is made of doped silicon. The resistivity of the aluminium top electrode is 2×10-6 Ωcm for both transducers, whereas the resistivity for the bottom electrode is 0.1 Ωcm for the first transducer and 0.005 Ωcm for the second transducer. The transducer with low resistivity emits pressure uniformly along both the rows and columns, whereas the transmit pressure field from the other transducer has a uniformly distributed pressure field along the rows, but a decreasing pressure field along the columns due to the high resistivity in the bottom electrode. The pressure drop, along the columns is frequency dependent and has been observed to be 63%, 74%, and 82% for the excitation frequencies 2 MHz, 4.5 MHz, and 7 MHz, respectively.
Original languageEnglish
JournalI E E E Transactions on Ultrasonics, Ferroelectrics and Frequency Control
Volume66
Issue number6
Pages (from-to)1110-1118
ISSN0885-3010
DOIs
Publication statusPublished - 2019

Keywords

  • Capacitative micromachined ultrasonic transductors (CMUTs)
  • Electrode resistance
  • Modeling

Cite this

@article{ddd9b48500df4a8ba871efbb4117154f,
title = "CMUT Electrode Resistance Design: Modelling and Experimental Verification by a Row-Column Array",
abstract = "This paper addresses the importance of having control over the resistivity of the electrodes for capacitive micromachined ultrasonic transducers (CMUT) devices. The electrode resistivity can vary depending on the fabrication technology used, and resistivity control becomes especially important in the cases where metal electrodes can not be used. This raises the question: When is the resistivity of an electrode sufficiently low? To answer this question we have developed a simple design criterion. The criterion describes the attenuation of AC signals along a CMUT element. It is shown that the non-dimensional product between angular excitation frequency, resistance, and capacitance ωRC of an element has to be smaller than 0.35 to ensure an AC potential drop along the element of less than 1{\%}. The optimal magnitude and directionality of the transmit pressure will be achieved if CMUT elements are designed according to the developed criteria. Hence, the model can be used to estimate device parameters that will ensure the CMUT is suitable for generating ultrasound images. An example is given where the model is used to predict the required electrode thickness for structured electrodes made of Gold, Aluminium, and Indium-Tin-Oxide, respectively. To verify the model, two Row-Column addressed (RCA) CMUT transducers were used to illustrate the effect of high and low electrode resistivity. One transducer had a sufficient electrode resistivity, and the other had an insufficient electrode resistivity. The RCA CMUT transducers were fabricated using fusion bonding, where the top electrode is made of aluminium and the bottom electrode is made of doped silicon. The resistivity of the aluminium top electrode is 2×10-6 Ωcm for both transducers, whereas the resistivity for the bottom electrode is 0.1 Ωcm for the first transducer and 0.005 Ωcm for the second transducer. The transducer with low resistivity emits pressure uniformly along both the rows and columns, whereas the transmit pressure field from the other transducer has a uniformly distributed pressure field along the rows, but a decreasing pressure field along the columns due to the high resistivity in the bottom electrode. The pressure drop, along the columns is frequency dependent and has been observed to be 63{\%}, 74{\%}, and 82{\%} for the excitation frequencies 2 MHz, 4.5 MHz, and 7 MHz, respectively.",
keywords = "Capacitative micromachined ultrasonic transductors (CMUTs), Electrode resistance, Modeling",
author = "Havreland, {Andreas Spandet} and Mathias Engholm and Tomov, {Borislav Gueorguiev} and Jensen, {J{\o}rgen Arendt} and Ole Hansen and Thomsen, {Erik Vilain}",
year = "2019",
doi = "10.1109/TUFFC.2019.2906795",
language = "English",
volume = "66",
pages = "1110--1118",
journal = "I E E E Transactions on Ultrasonics, Ferroelectrics and Frequency Control",
issn = "0885-3010",
publisher = "Institute of Electrical and Electronics Engineers",
number = "6",

}

TY - JOUR

T1 - CMUT Electrode Resistance Design: Modelling and Experimental Verification by a Row-Column Array

AU - Havreland, Andreas Spandet

AU - Engholm, Mathias

AU - Tomov, Borislav Gueorguiev

AU - Jensen, Jørgen Arendt

AU - Hansen, Ole

AU - Thomsen, Erik Vilain

PY - 2019

Y1 - 2019

N2 - This paper addresses the importance of having control over the resistivity of the electrodes for capacitive micromachined ultrasonic transducers (CMUT) devices. The electrode resistivity can vary depending on the fabrication technology used, and resistivity control becomes especially important in the cases where metal electrodes can not be used. This raises the question: When is the resistivity of an electrode sufficiently low? To answer this question we have developed a simple design criterion. The criterion describes the attenuation of AC signals along a CMUT element. It is shown that the non-dimensional product between angular excitation frequency, resistance, and capacitance ωRC of an element has to be smaller than 0.35 to ensure an AC potential drop along the element of less than 1%. The optimal magnitude and directionality of the transmit pressure will be achieved if CMUT elements are designed according to the developed criteria. Hence, the model can be used to estimate device parameters that will ensure the CMUT is suitable for generating ultrasound images. An example is given where the model is used to predict the required electrode thickness for structured electrodes made of Gold, Aluminium, and Indium-Tin-Oxide, respectively. To verify the model, two Row-Column addressed (RCA) CMUT transducers were used to illustrate the effect of high and low electrode resistivity. One transducer had a sufficient electrode resistivity, and the other had an insufficient electrode resistivity. The RCA CMUT transducers were fabricated using fusion bonding, where the top electrode is made of aluminium and the bottom electrode is made of doped silicon. The resistivity of the aluminium top electrode is 2×10-6 Ωcm for both transducers, whereas the resistivity for the bottom electrode is 0.1 Ωcm for the first transducer and 0.005 Ωcm for the second transducer. The transducer with low resistivity emits pressure uniformly along both the rows and columns, whereas the transmit pressure field from the other transducer has a uniformly distributed pressure field along the rows, but a decreasing pressure field along the columns due to the high resistivity in the bottom electrode. The pressure drop, along the columns is frequency dependent and has been observed to be 63%, 74%, and 82% for the excitation frequencies 2 MHz, 4.5 MHz, and 7 MHz, respectively.

AB - This paper addresses the importance of having control over the resistivity of the electrodes for capacitive micromachined ultrasonic transducers (CMUT) devices. The electrode resistivity can vary depending on the fabrication technology used, and resistivity control becomes especially important in the cases where metal electrodes can not be used. This raises the question: When is the resistivity of an electrode sufficiently low? To answer this question we have developed a simple design criterion. The criterion describes the attenuation of AC signals along a CMUT element. It is shown that the non-dimensional product between angular excitation frequency, resistance, and capacitance ωRC of an element has to be smaller than 0.35 to ensure an AC potential drop along the element of less than 1%. The optimal magnitude and directionality of the transmit pressure will be achieved if CMUT elements are designed according to the developed criteria. Hence, the model can be used to estimate device parameters that will ensure the CMUT is suitable for generating ultrasound images. An example is given where the model is used to predict the required electrode thickness for structured electrodes made of Gold, Aluminium, and Indium-Tin-Oxide, respectively. To verify the model, two Row-Column addressed (RCA) CMUT transducers were used to illustrate the effect of high and low electrode resistivity. One transducer had a sufficient electrode resistivity, and the other had an insufficient electrode resistivity. The RCA CMUT transducers were fabricated using fusion bonding, where the top electrode is made of aluminium and the bottom electrode is made of doped silicon. The resistivity of the aluminium top electrode is 2×10-6 Ωcm for both transducers, whereas the resistivity for the bottom electrode is 0.1 Ωcm for the first transducer and 0.005 Ωcm for the second transducer. The transducer with low resistivity emits pressure uniformly along both the rows and columns, whereas the transmit pressure field from the other transducer has a uniformly distributed pressure field along the rows, but a decreasing pressure field along the columns due to the high resistivity in the bottom electrode. The pressure drop, along the columns is frequency dependent and has been observed to be 63%, 74%, and 82% for the excitation frequencies 2 MHz, 4.5 MHz, and 7 MHz, respectively.

KW - Capacitative micromachined ultrasonic transductors (CMUTs)

KW - Electrode resistance

KW - Modeling

U2 - 10.1109/TUFFC.2019.2906795

DO - 10.1109/TUFFC.2019.2906795

M3 - Journal article

VL - 66

SP - 1110

EP - 1118

JO - I E E E Transactions on Ultrasonics, Ferroelectrics and Frequency Control

JF - I E E E Transactions on Ultrasonics, Ferroelectrics and Frequency Control

SN - 0885-3010

IS - 6

ER -