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 Si 3 N 4 oxidation mask and lowering the pad SiO 2 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.

Mass Sensors Based on Capacitive and Piezoelectric Micromachined Ultrasonic Transducers—CMUT and PMUT

“…One highly doped silicon substrate is used to create the bottom electrode and the cavity, while the other employs a silicon on insulator (SOI) wafer that is used for the top membrane. Wafer fusion bonding for CMUT-based sensor fabrication is reported for devices with radii ranging from tens to hundreds of microns [26,27]. A layer of silicon dioxide is thermally grown on the first silicon substrate based on the desired cavity height which then is patterned photolithographically, as illustrated in Figure 8a.…”

“…Further mass changes are expected during sensing upon interaction of the analytes with the polymer, which ultimately leads to adsorption [17]. Analytes in the gas phase, such as carbon dioxide, sulfur dioxide, volatile organic compounds (VOCs) including acetone, methanol, ethanol, toluene, benzyl methyl ketone (BMK) and DMMP, and water, are viable for sensing based on mass changes of polymer films [4,27,38,39]. These gas-sensitive films comprising polymers such as poly(styrene-co-allyl alcohol), poly(vinylacetate), poly(capralactone), methylated poly(ethylene imine), poly (vinyl pyrrolidine) and poly {methyl [4-(2-hydroxy-4, 6-bistrifluoromethyl) phenyl] propylsiloxane} have been reported for detecting the analytes mentioned above [4,17,34,39,40].…”

“…With a theoretical mass sensitivity of 0.536 Hz/fg, CO 2 exposure induced an additive mass change of 186 fg onto the sensor during testing. A non-polymer-containing device was also tested where a Reichardt's dye applied by drop casting was used as a coating for sensing BMK and water [27]. It was indicated that BMK adsorbs onto the dye via weak interactions such as Van der Waals and π-π interactions.…”

Selective Ultrasonic Gravimetric Sensors Based on Capacitive Micromachined Ultrasound Transducer Structure—A Review

“…Application of ultrasonic transducers for sensing has become popular among multiple areas that include sensing of various physical, chemical, and biochemical phenomena. While several microelectromechanical system (MEMS) structures, like cantilevers, bridges, thin-film, or printed piezoelectric devices, can be applied for the ultrasonic sensing, the capacitive micromachined ultrasound transducers (CMUT) concept is highly advantageous due to the unprecedented sensitivity potential, good integration with electronics, hermetic active structure, high redundancy of the resonators, and CMOS (Complementary Metal-Oxide-Semiconductor) compatible fabrication process [ 1 , 2 , 3 ]. Gravimetric detection is a method that determines the quantity of the analyte (the molecule, species, or material of interest) by determining its mass.…”

“…Here, sensitivity describes the steepness of the main sensor function, usually characterized as a mass unit per the frequency unit. For this parameter, CMUT structure has little competition, since it was demonstrated by several groups to reach numbers down to 10 −18 g/Hz and below [ 2 , 3 , 4 , 5 , 6 , 7 , 8 ]. As for the other two parameters, they are more related to the properties of the functional material of the sensor [ 1 , 7 , 9 , 10 , 11 , 12 ], and the topic of the functional materials suitable for SUGS remains highly underexplored.…”