Sensitivity Optimization of Wafer Bonded Gravimetric CMUT Sensors

Mathias J.G. Mølgaard*, J.M.F. Hansen, Mogens H. Jakobsen, Erik V. Thomsen

*Corresponding author for this work

    Research output: Contribution to journalJournal articleResearchpeer-review

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    Abstract

    Optimization of the mass sensitivity of wafer bonded resonant gravimetric capacitive micromachined ultrasonic transducers (CMUTs) is presented. Gas phase sensors based on resonant gravimetric CMUTs have previously been demonstrated. An important figure of merit of these sensors is the sensitivity which, for typical CMUT geometries, is increased by decreasing the radius of the CMUT cell. This paper investigates how to minimize the radius of CMUT cells fabricated using the wafer bonding process. The design and process parameters affecting the radius of the CMUT and hereby the sensitivity are studied through numerical simulations and atomic force microscopy measurements. An excellent fit was obtained between the simulations and measured profiles with a low relative error of ≤ 5%, thus validating the simulation model. Two types of CMUTs are designed and fabricated using the design and process rules determined herein, with experimentally determined mass sensitivities of 0.46 Hz/ag and 0.44 Hz/ag, respectively. The two CMUT devices have cavities made using the local oxidation of silicon (LOCOS) and reactive ion etching (RIE) process. For the LOCOS process, it was found that the smallest radius can be obtained by choosing a Si3N4 oxidation mask and lowering the pad SiO2 thickness, vacuum gap height, and Si bump height. For the RIE process, the vertical dimensions do not influence the horizontal dimensions and consequently, equivalent rules do not exist.
    Original languageEnglish
    JournalI E E E Journal of Microelectromechanical Systems
    Volume27
    Issue number6
    Pages (from-to)1089-1096
    ISSN1057-7157
    DOIs
    Publication statusPublished - 2018

    Cite this

    @article{7d21691b29db4fdebd3cc2f958d54870,
    title = "Sensitivity Optimization of Wafer Bonded Gravimetric CMUT Sensors",
    abstract = "Optimization of the mass sensitivity of wafer bonded resonant gravimetric capacitive micromachined ultrasonic transducers (CMUTs) is presented. Gas phase sensors based on resonant gravimetric CMUTs have previously been demonstrated. An important figure of merit of these sensors is the sensitivity which, for typical CMUT geometries, is increased by decreasing the radius of the CMUT cell. This paper investigates how to minimize the radius of CMUT cells fabricated using the wafer bonding process. The design and process parameters affecting the radius of the CMUT and hereby the sensitivity are studied through numerical simulations and atomic force microscopy measurements. An excellent fit was obtained between the simulations and measured profiles with a low relative error of ≤ 5{\%}, thus validating the simulation model. Two types of CMUTs are designed and fabricated using the design and process rules determined herein, with experimentally determined mass sensitivities of 0.46 Hz/ag and 0.44 Hz/ag, respectively. The two CMUT devices have cavities made using the local oxidation of silicon (LOCOS) and reactive ion etching (RIE) process. For the LOCOS process, it was found that the smallest radius can be obtained by choosing a Si3N4 oxidation mask and lowering the pad SiO2 thickness, vacuum gap height, and Si bump height. For the RIE process, the vertical dimensions do not influence the horizontal dimensions and consequently, equivalent rules do not exist.",
    author = "M{\o}lgaard, {Mathias J.G.} and J.M.F. Hansen and Jakobsen, {Mogens H.} and Thomsen, {Erik V.}",
    year = "2018",
    doi = "10.1109/JMEMS.2018.2868864",
    language = "English",
    volume = "27",
    pages = "1089--1096",
    journal = "I E E E Journal of Microelectromechanical Systems",
    issn = "1057-7157",
    publisher = "Institute of Electrical and Electronics Engineers",
    number = "6",

    }

    Sensitivity Optimization of Wafer Bonded Gravimetric CMUT Sensors. / Mølgaard, Mathias J.G.; Hansen, J.M.F. ; Jakobsen, Mogens H.; Thomsen, Erik V.

    In: I E E E Journal of Microelectromechanical Systems, Vol. 27, No. 6, 2018, p. 1089-1096.

    Research output: Contribution to journalJournal articleResearchpeer-review

    TY - JOUR

    T1 - Sensitivity Optimization of Wafer Bonded Gravimetric CMUT Sensors

    AU - Mølgaard, Mathias J.G.

    AU - Hansen, J.M.F.

    AU - Jakobsen, Mogens H.

    AU - Thomsen, Erik V.

    PY - 2018

    Y1 - 2018

    N2 - Optimization of the mass sensitivity of wafer bonded resonant gravimetric capacitive micromachined ultrasonic transducers (CMUTs) is presented. Gas phase sensors based on resonant gravimetric CMUTs have previously been demonstrated. An important figure of merit of these sensors is the sensitivity which, for typical CMUT geometries, is increased by decreasing the radius of the CMUT cell. This paper investigates how to minimize the radius of CMUT cells fabricated using the wafer bonding process. The design and process parameters affecting the radius of the CMUT and hereby the sensitivity are studied through numerical simulations and atomic force microscopy measurements. An excellent fit was obtained between the simulations and measured profiles with a low relative error of ≤ 5%, thus validating the simulation model. Two types of CMUTs are designed and fabricated using the design and process rules determined herein, with experimentally determined mass sensitivities of 0.46 Hz/ag and 0.44 Hz/ag, respectively. The two CMUT devices have cavities made using the local oxidation of silicon (LOCOS) and reactive ion etching (RIE) process. For the LOCOS process, it was found that the smallest radius can be obtained by choosing a Si3N4 oxidation mask and lowering the pad SiO2 thickness, vacuum gap height, and Si bump height. For the RIE process, the vertical dimensions do not influence the horizontal dimensions and consequently, equivalent rules do not exist.

    AB - Optimization of the mass sensitivity of wafer bonded resonant gravimetric capacitive micromachined ultrasonic transducers (CMUTs) is presented. Gas phase sensors based on resonant gravimetric CMUTs have previously been demonstrated. An important figure of merit of these sensors is the sensitivity which, for typical CMUT geometries, is increased by decreasing the radius of the CMUT cell. This paper investigates how to minimize the radius of CMUT cells fabricated using the wafer bonding process. The design and process parameters affecting the radius of the CMUT and hereby the sensitivity are studied through numerical simulations and atomic force microscopy measurements. An excellent fit was obtained between the simulations and measured profiles with a low relative error of ≤ 5%, thus validating the simulation model. Two types of CMUTs are designed and fabricated using the design and process rules determined herein, with experimentally determined mass sensitivities of 0.46 Hz/ag and 0.44 Hz/ag, respectively. The two CMUT devices have cavities made using the local oxidation of silicon (LOCOS) and reactive ion etching (RIE) process. For the LOCOS process, it was found that the smallest radius can be obtained by choosing a Si3N4 oxidation mask and lowering the pad SiO2 thickness, vacuum gap height, and Si bump height. For the RIE process, the vertical dimensions do not influence the horizontal dimensions and consequently, equivalent rules do not exist.

    U2 - 10.1109/JMEMS.2018.2868864

    DO - 10.1109/JMEMS.2018.2868864

    M3 - Journal article

    VL - 27

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    EP - 1096

    JO - I E E E Journal of Microelectromechanical Systems

    JF - I E E E Journal of Microelectromechanical Systems

    SN - 1057-7157

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