HfO2 high-k dielectric layers in air-coupled capacitive ultrasonic transducers

For air-coupled applications, broadband capacitive ultrasonic transducers (CUTs) at the mm to cm scale are often desirable. Improved device performance may be obtained by etching well-defined geometric features into a silicon backplate electrode, then using a metallized polymer film as the other electrode. The use of additional dielectric coatings for devices at this scale may have a number of beneficial effects. This work investigates the use of HfO₂ high-k dielectric coatings on the backplate electrodes of air-coupled CUTs. A range of such devices was constructed and used in a through-transmission configuration. Different thickness HfO₂ layers were investigated at different bias voltages, and the effects on the sensitivity and bandwidth of the devices were analyzed. The predicted capacitance of each device was within 7% of the measured capacitance, with variations due to additional trapped air and manual assembly. Increasing the HfO₂ layer thickness decreased the overall capacitance of the CUT as expected, but produced significant improvements in device sensitivity and bandwidth at certain bias voltages. A strong correlation between HfO₂ high-k dielectric layer thickness and peak-to-peak amplitude was observed. The variation in device operation after successive bias charge/discharge cycles also become consistently less as the HfO₂ layer thickness was increased.