Structural Factors Influencing the Volatile Sensitivity of Polymer Coated Piezoelectric Micromechanical Resonators Ling Sieben-Xu, Devrez M. Karabacak, Daan Wouters, Wout Knoben, Yvonne van Andel, Sywert H. Brongersma, Mercedes Crego Calama Holst Centre/imec the Netherlands, High Tech Campus 31, 5656AE, the Netherlands Abstract Polymer-coated piezoelectric microbridge resonators were demonstrated to be highly sensitive towards volatile organic compounds. Here, we present a comprehensive experimental study into several important factors governing the modal sensitivity, namely the modal resonance frequency, piezoelectric transducer size, and functionalization layer thickness. Results indicate the lower frequency length flexural modes yield higher normalized frequency shifts in comparison to higher order width-flexural or torsional modes. The sensitivity of resonator sensors can be further enhanced by reducing the piezoelectric patch size and increasing the thickness of polymer. Key words: electronic nose, volatile organic compound, piezoelectric microbridge, polymer Introduction Compact, low power, low cost and integrated sensors are in a growing need to detect low concentration of volatile organic compounds (VOCs), with potential applications ranging from indoor air quality monitoring to industrial leakage detection and health care [1]. However, existing detection approaches, mainly based on analytical approaches of chromatography and spectrometry, have several drawbacks, such as being not easily scalable in size, high power consumption and costly, preventing their on- field usage. Alternatively, use of partially selective receptors like polymers in arrays for detection and identification of volatiles is a promising approach, often referred to as “electronic nose” system. For this task, micromechanical resonators are widely viewed as suitable transducers with intrinsic advantages of low power consumption, CMOS technology compatibility, easily scalable and arrayable. Previously, polymer-coated doubly clamped microbridge resonator sensor has been demonstrated to detect low concentrations of volatile compounds by monitoring their resonance frequency shift [2, 3], among which the resonator integrated with piezoelectric transducer in [2] is particularly attractive due to its ultra low power consumption and more promising integration capabilities. The key factors of microbridge resonator which influence its sensitivity, however, have not been investigated yet. In this paper, we explore several factors of beam resonator integrated with piezoelectric transducer (schematized in Fig. 2(a) inset), such as modal resonance frequency, piezoelectric patch size and polymer coating. The results will enable us to further optimize the sensitivity of the resonator sensor, and therefore offer the insight for their potential in future applications in “electronic nose” systems. Fabrication and functionalization The doubly clamped microbridge resonators were fabricated by a process flow that combines surface and bulk micromachining processes on silicon (Si) substrates. Firstly, a sacrificial 500 nm thick silicon oxide (SiO 2 ) was deposited as a Si deep reactive ion etching (DRIE) stop layer by low-pressure chemical vapor deposition (LPCVD). Later, the beam layer, a 500 nm thick low-stress silicon nitride was deposited by low-pressure chemical vapor deposition. For piezoelectric transduction, a patch of was formed by bottom-up fabrication from platinum (Pt) / aluminum nitride (AlN) / platinum (Pt) stack (100 nm / 400 nm / 100 nm). The microbridge was formed by a through- wafer Si DRIE, performed from the backside. Finally, the sacrificial SiO 2 stop layer was removed in buffered oxide etchant, completing the release of the structures. An image of a typical beam array is shown in Fig. 1. For exploring the factors influencing the resonator sensitivity, ethanol is selected as a suitable representative volatile compound, and poly(methyl methacrylate), PMMA, is correspondingly used as the sensing layer due its well known affinity to absorb ethanol [2,4,5]. DOI 10.5162/IMCS2012/2.4.3 IMCS 2012 – The 14th International Meeting on Chemical Sensors 213