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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 Ω Ω Ωcm for both transducers, whereas the resistivity for the bottom electrode is 0.1 Ω Ω Ωcm for the first transducer and 0.005 Ω Ω 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.This work is financially supported by the Innovation Fund Denmark (7050-00004B) and by BK Medical (Herlev, Denmark).A. S. Havreland, M. Engholm and E. V. Thomsen are with the

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Super resolution imaging (SRI) can benefit greatly from full 3D imaging to compensate for vessel structures moving out of the image plane. Row Column Addressed (RCA) arrays can provide such 3D imaging with low-complexity probe design. A RCA probe is being designed for a rat kidney with 192 + 192 elements to ensure low sidelobes and an imaging volume of 15x15x15 mm 3. The design space of such a transducer is investigated with this in mind. Capacitive micromachined ultrasonic transducer (CMUT) technology allows for new geometric shapes, including a near kerf-less zigzag interwoven structure, which provides flexibility in the design process of new RCA. This work compares the image quality of a straight element RCA array with an RCA array with an interwoven zigzag structure by simulations, to assess the image quality of a zigzag structured RCA. The Point Spread Function (PSF) showed symmetry in both the azimuth and elevation direction. Both the straight and interwoven design had a lateral Full Width Half Maximum (FWHM) of approximately 0.6 λ, close to the ideal FWHM without apodization. The interwoven design showed a slight contrast loss over the straight desig, which was quantified with the Cystic Resolution (CR). The CR at 20 dB for the straight design was 1.3 λ, compared to 1.4 λ of the interwoven design. The two designs had comparable PSF metrics. The interwoven zigzag structure is therefore a viable solution to meet the requirements of the rodent experiments and provides a new level of design flexibility for manufacturing RCA transducer arrays.

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