Multi-layer PZT transducer arrays for improved sensitivity
Two-dimensional arrays are critical to the future of medical ultrasound for focusing and phase correction in the elevation dimension and for 3-D scanning. Poor sensitivity is a serious problem with such 2-D arrays because their small element size results in a small clamped capacitance and a high electrical impedance near resonance. Sensitivity can be improved by reducing the array element impedance using an N layer structure of PZT connected electrically in parallel and acoustically in series. The clamped capacitance is multiplied by a factor of N2 and the impedance by 1/N2 compared to a single layer transducer. A 16 layer, 1 MHz prototype transducer has been fabricated using thick film hybrid technology. Each element had dimensions of 0.94 mm × 0.94 mm × 1.3 mm. With a light epoxy backing, the element impedance was 50 Ω at series resonance and the minimum round-trip insertion loss was 41 dB. This was compared to a single layer control array with an impedance of 3 kΩ per element and an insertion loss of 64 dB per element. These results were consistent with KLM model simulations.
