Effects of fringing capacitances and electrode’s finiteness in improved SiC membrane based micromachined ultrasonic transducers

The influence of fringing electric fields at the edges of the Capacitance Micromachined Ultrasonic Transducer’s (CMUT’s) gap and insulation layers are included into the parallel plate capacitance phenomenon. Its effect on various CMUT parameters are derived and compared to Finite Element Analysis (FEA) predictions. This gives rise to excess capacitances. A correction factor due to the finiteness of the electrode’s diameter in nanostructure devices is introduced for accurate prediction. Inclusion of an insulation layer helps in improving the performance of the ultrasonic transducer in comparison to a structure devoid of it. Analytical modelling of the structure as ultrasound transmitter consisting of 110 µm diameter membrane arrays exhibits a resonance of 1.65 MHz, which closely matches with published experimental data is carried out. CMUT with Si₃N₄ as insulation layer has higher device capacitance, increased membrane displacement, higher coupling factor, and lower collapse voltage as compared to a structure without insulation at the same bias, structural dimensions, and material properties. The characteristics exhibited by the insulated structure are advantageous as a transducer. All the results are compared with FEA where excellent agreement is observed.