Optical micromachined ultrasound transducers (OMUT) - A new approach for high resolution imaging

Piezoelectric ultrasound (US) transducers are at the heart of almost any ultrasonic medical imaging probe. However, their sensitivity and reliability severely degrade in applications requiring high frequency (>20 MHz) and small element size (<0.1 mm). Alternative technologies such as capacitive micromachined ultrasound transducers (CMUT) and optical sensing and generation of ultrasound are being investigated. In this paper we present our first steps in developing optical micromachined ultrasound transducers (OMUT) technology. OMUTs rely on microfabrication techniques to construct micron-size air cavities capped by an elastic membrane. The membrane functions as the active ultrasound transmitter and receiver. We will describe the design and testing of prototype OMUT devices which implement a receive-only function. The cavity detector is an optical cavity which its top mirror is deflected under the application of pressure. The intensity of a reflected light beam is highly sensitive to displacement of the top membrane if the optical wavelength is at near-resonance condition. Therefore, US pulses can be detected by recording the reflected light intensity. The sensitivity of the device depends on the mechanical properties of the top membrane and optical characteristics of the optical cavity. The device was fabricated using SU8 as a structural material and gold as a mirror. We have developed a new bonding method to fabricate a sealed, low roughness, high quality optical cavity. The 60μm cavity with the 8.5 μm top membrane is tested in water with 25MHz ultrasound transducer. The NEP of the device for bandwidth of 28MHz was 9.25kPa. The optical cavity has a finesse of around 23.