Towards Digitally Controlled Ultrasonic IQ Modulator

This paper presents a digitally controlled transducer pixel that generates ultrasonic pulses with a quadrature phases based on the input digital code. Quadrature modulation is achieved by splitting the ultrasonic transducer into two identical transducers shifted in height by a step of λ/4. The step is implemented in a 3 μm PECVD silicon dioxide layer on top of silicon and forming a step using plasma reactive ion etching. For an ultrasonic wave frequency of 1.2 GHz, the step height in SiO
₂
to achieve λ/4 shift is ~1.2 μm. Both transducers are actuated simultaneously by applying electrical RF pulse to the transducer electrodes at bulk mode resonance frequency of the transducers. Ultrasonic pulses from the two transducers propagate through the silicon substrate, interfere, and reflect from the back side of the silicon substrate and are received by a receiver transducer also placed at the transmitting side. Digitally controlled RF switches are used to switch applied RF signal from top to bottom electrodes of the transducers. Higher modulation schemes can be achieved by having binary weighted transducers areas at different steps and more than two step heights.