Influence of the electrical extensions in AlN-BAW resonators for in-liquid biosensors

High frequency electroacoustic biosensors require the use of microfluidic systems to confine the liquid under test on the sensor surface. Owing to frequent conductive character of biochemical solutions, the contacting pads must be isolated from the active areas of the sensors by means of electrical extensions. At working frequencies in the GHz range, the parasitic effects of such extensions can worsen the overall electrical response of the resonators by dramatically reducing the quality factor at the resonant frequency. In this paper, we study the effects of electrical extensions in AlN-based solidly mounted resonators operating in the shear mode at 1.3 GHz. The design of these extensions as a stretch of coplanar waveguide allows us minimizing the parasitic effects and matching the impedance between the resonator and the external circuitry for excitation and readout. We present solutions to lead the contact pads 2 mm away from the active area without distortion of the resonator response. Resonators with electrical extensions of 1.5 mm and quality factors of more than 150 are obtained.