The capacitive micromachined ultrasonic transducer (CMUT) as a chem/bio sensor

Airborne chem/bio sensors increase in importance every day for applications in homeland security for the detection of bio-hazardous materials and improvised explosive devices (IED); in the spoilage of food; in the health care industry for detecting cancer, diabetes, and other conditions; and in many other industries. A number of ultrasonic sensors have been developed and used in the chem/bio sensing arena: quartz crystal micro balance (QCM), surface acoustic wave (SAW) resonators, and more recently resonant and simply deflected cantilevers. The capacitive micromachined ultrasonic transducer (CMUT) is a "platform" device that has been investigated in many airborne and immersion ultrasound applications. In this work, we explore the use of the CMUT as a Chem/bio sensor. Because the membrane of a CMUT can be a fraction of a micron thick, and the frequency of operation in the MHz range, it is possible to obtain sensitivity in the order of 1 femto-gram mass loading per cell with proper design. Another important attribute of the CMUT as a chem/bio sensor is that many cells are used to make a sensor which helps improve its false alarm rate over existing sensors where only one resonant element is used. Finally, a high mechanical quality factors (Q) realized in CMUTs which results in very low noise floor, which is necessary to make a very sensitive sensor. An array of CMUTs was made resonant in air at 6 MHz with a Q of 160 and a noise floor of 0.4 Hz in a 1 Hz bandwidth, and where each element consisted of 750 cells. Each cell had a diameter of 30 microns and a silicon nitride thickness of 0.85 microns. Six elements of the array were functionalized with different polymers (PAAM, PEG, PSS, PVA) and the sensitivity of the sensors was measured by flowing different analytes (water, ethanol, isopropyl alcohol, toluene) at different concentrations over the sensors. The frequency shift of each resonator was measured using a frequency counter. The sensor had a volume detection sensitivity of 20 ppb, and a calculated mass loading sensitivity of 1 femto-gram. We will also present results from a new generation of sensors operating in the 25 MHz frequency range and with an expected sensitivity improvement of one to two orders of magnitude over the earlier design.