High frequency ultrasound array array transducers, a xylophone bar transducer has also transducers are being explored for high resolution been prototyped, using thin film PZT as the active imaging systems. This increase in resolution is made piezoelectric layer. Fundamentally, the approach here possible by enabling a simultaneous increase in operating entails creating passive structures to control the frequency (50 MHz to about 1GHz) and close-coupling resonance frequency of the device, and then coating those of the electrical circuitry. Several different processing structures with PZT thin films. Since the PZT layers are methods are being explored to fabricate array transducers. thin, the required voltages for exciting the transducer are In one implementation, the piezoelectric transducer is greatly lowered, and so can be generated on a CMOS prepared by mist deposition of PbZr052Ti04803 (PZT) chip without the likelihood of damaging sensitive receive films over Ni posts. In addition, a xylophone bar electronics. Secondly, the small layer thickness greatly transducer has also been prototyped, again using thin film increases the capacitance of each pixel. Third, the PZT as the active piezoelectric layer. Because the drive transducer frequency can be readily adjusted without voltages of these transducers are low, close coupling of requiring the deposition of thick films. For the the electrical circuitry is possible. A CMOS transceiver xylophone transducers, the resonating structure is a for a 9 element-array has been fabricated in 0.35 jm micromachined bar driven in either the length or width process technology. The first generation CMOS extensional mode. Finite element modeling demonstrates transceiver chip contains beamforming electronics, that such devices launch a sound wave perpendicular to receiver circuitry, and analog to digital converters the substrate surface, and that adequate sound pressure with 27 Kbyte on-chip buffer memory. levels can be generated. KLM models suggest that the6dB bandwidth should be approximately 100%. Xylophone transducers are being fabricated using INTRODUCTION

A fully custom designed CMOS transceiver chip for high frequency ultrasound imaging applications is described. The chip consists of nine transceiver channels with a programmable beamformer. On the chip, 8 bit 250 MS/s A/D converters digitize analog signals from the transducers and nine 3 Kbyte/channel SRAM store the digitized data for each of the nine channels. The chip was fabricated in a 0.35um standard CMOS process. The area is 26.83 mm 2 and its total power consumption is below 1.5 W at 3.3 V power supply.