Delay Line Separation of CMUT Elements

This paper presents a novel fabrication design for capacitive micromachined ultrasonic transducers (CMUT) that utilizes a designed delay line in the silicon substrate for element separation. All elements in this design are electrically connected through the substrate, however, high resistivity (> 10.000 Ωcm) float zone silicon is used for the substrate to suppress cross-talk between elements. The surface is highly doped with phosphorus to enhance the electrical conductance along the elements. This CMUT design has been modelled as a r-c network and expressions for both the impedance and the transfer function were derived. A linear CMUT array was fabricated using this design and the current-voltage relationship was measured between and along elements. The resistance along a 3.4 mm long CMUT element was measured to 123 Ω, whereas the resistance between two neighboring elements was measured to range between 0.8 MΩ and 7.8 MΩ depending on the bias voltage. Thus, an anisotropic resistance that varies with a factor between 6500 and 65000 between its horizontal and lateral direction. The measured resistance is used as an input to estimate the attenuation of the delay line, which yielded a suppression of more than -40 dB in the MHz regime, which is acceptable for medical imaging applications.