MEM S Actuated Piezoelectrically with A1N Films Sheila Gonzalez-Castilla, Javier Malo, Lucia Vergara, Jimena Olivares, Marta Clement, Jose Ignacio Izpura, Jesu's Sangrador, and Enrique Iborra Grupo de Microsistemas y Materiales Electronicos, Universidad Politecnica de Madrid, Ciudad Universitaria, 28040 Madrid. Abstract-We analyse the mechanical response of a doubly- clamped microbridge actuated piezoelectrically using sputtered AIN, working first as an actuator and then as a resonator. The quasi-static response of the microbridge under DC electrical excitation is measured. Out-of-plane displacements as high as 0.22 pm/V are obtained, which provides an actuation response suitable for RF switching applications. On the other hand, the resonance frequencies of the microbridges are determined by means of laser interferometry. We compare the response of microbridges with different dimensions and different initial buckling (induced by the residual stress of the layers). We show that it is possible to perform a rough tuning of the resonance frequencies by allowing a determined amount of built-in stress in the microbridge during its fabrication. For a given resonator, a DC bias added to the AC excitation signal allows to fine-tune the resonance frequencies. Our microbridges yield a tuning factor of around 88 Hz/V for a 500 ,um-long microbridge. Index Terms-Aluminum nitride, microactuator, microresonator, piezoelectric actuation. I. INTRODUCTION TH 1-E emerging field of microelectromechanical systems (MEMS) has created the need for new functional materials being integrated into silicon to fabricate micromachined devices. Piezoelectric thin films of lead zirconate-titanate (PZT) [1], zinc oxide (ZnO) [2] or aluminum nitride (AlN) [3] exhibit interesting properties that can be exploited in various piezoelectric devices, like acoustic and mechanical resonators [4] or piezoelectric actuators (cantilevers and microbridges) [5, 6]. Even though AIN does not exhibit the best piezoelectric properties, its high thermal stability and compatibility with silicon technology makes it the best known compromise between performance and manufacturability of piezoelectric MEMS [7]. Some MEMS based on AlN, such as bulk acoustic wave resonators [8, 9], mechanical resonators [ 10, 11 ] or RF microswitches [ 12, 13], have been already demonstrated. We report here an electromechanical study of AlN-based piezoelectric microbridges working first as actuators and then as resonators. The quasi-static response of the microbridges under DC electrical excitation was analyzed to evaluate their viability for RF switching applications. Additionally, we have analyzed the vibrational behavior (resonance frequencies) of the structures as a function of their geometrical dimensions and the buckling of the microbridges resulting from the residual compressive stress of the different layers. Finally, we show that adding a DC bias to the AC excitation signal allows us to fine-tune the resonance frequencies within a limited range. II. EXPERIMENTAL We have fabricated Mo/AlN/Mo piezoelectric actuators supported by a structural layer using a surface micromachining technology. Silicon dioxide (SiO2) deposited by photo-induced CVD was used as sacrificial layer. The microbridges were thus released by removing the SiO2 sacrificial layer, which exhibit a high etch rate (inm/min) at room temperature in buffered hydrofluoric acid due to its intrinsic porosity. All the films were deposited by reactive sputtering. The deposition conditions (substrate bias voltage and total pressure) were carefully selected to control the initial residual stress of the different layers composing the microbridge before removing the sacrificial layer to release the bridge [14]. The residual stress of the films was assessed by measuring the radius of curvature of the sample with a profilometer before and after the deposition of each film; the residual stress was then deduced by applying the Stoney's equation. Once the microbridge was released, it became convex to relax partially the combined stress of the different layers; the out-of plane buckling affected significantly their vibrational behavior of the microbridges. This out-of-plane buckling appearing in the released microbridges was assessed by measuring the extra height at their centre with respect to the flat position using a scanning electron microscope (SEM) and ranged from 0.5 pim to 30 ptm (with an uncertainty of about 0.1 pIm), depending on the initial stress and the length of the microbridges. The mechanical response of the microbridges to quasi-static (0.5 Hz) square voltages of amplitude varying from 0 to 16 V was also assessed by SEM. The displacement of the microbridges measured in the SEM images was then corrected by taking into account the tilt of the sample. An uncertainty of about 0.1 pIm was estimated for this procedure. We have analyzed the dynamic behavior of the microbridges when an alternating voltage was applied between 1-4244-0869-5/07/$25.00 (c)2007 IEEE 226