Components for focused integrated pMUTs for high-resolution medical imaging

Significant improvement in conventional ultrasonic minimally invasive imaging such as intravascular imaging can be achieved by the use of miniature focused broad band transducers. Polymer piezoelectric materials such as PVDF and PVDF-TrFE are excellent candidates for such transducers but have high impedance and typically require a preamplifier in close proximity to the transducer. Such focused transducers can be integrated with conventional CMOS electronics using batch fabrication techniques and appropriate protection circuitry compatible with CMOS fabrication technology. Spherically-focused, high-frequency (35-45 MHz) ultrasound transducers, with aperture diameters of 0.75-2.00 mm and focalnumbers ranging from 1.3-4.5 have been developed using micromachining and membrane deflection techniques. Two dimensional radiation contours for various diameter transducers excited by a 40 MHz monocycle were obtained and modeled and close agreement was observed between the experimental and modeled radiation contours. These transducers exhibited diffraction limited focused radiation patterns with 24 μm axial resolution and -6dB fractional bandwidths near 80%. The axial radiation pattern of a 2 mm diameter transducer (geometrical focus at 7.1 mm) excited by a 40 MHz 15 cycle gated sinusoid was compared to the calculated continuous wave axial radiation pattern from an identical spherical ultrasound transducer and shows good agreement (Pearson correlation coefficient of 0.99) over the axial range from 5 mm to 10 mm. The tissue imaging capabilities of the micromachined ultrasonic transducers are demonstrated through successful imaging of human cadaveric aorta. An ASIC low-noise preamplifier with matched input impedance,, integrated limiter, and sub-microsecond pulse recovery time was designed using a standard 0.35 μm CMOS process to interface with the ultrasonic transducer. The preamplifier was fabricated and tested and demonstrated to recover to equilibrium condition in 0.6 microseconds after a 70 Vp pulse. The circuit was also able to withstand a 100 Vp excitation pulse at a 2 KHz repetition rate for a total of 57 megacycles with no change in the small signal gain of the amplifier. When fully integrated with the low noise preamplifier, the micromachined ultrasonic transducer chip will be small enough for intravascular and endoluminal imaging applications