508 JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 14, NO. 3, JUNE 2005 Design, Fabrication, and Characterization of a Submicroelectromechanical Resonator With Monolithically Integrated CMOS Readout Circuit Jaume Verd, Student Member, IEEE, G. Abadal, J. Teva, María Villarroya Gaudó, Arantxa Uranga, Xavier Borrisé, Francesca Campabadal, Jaume Esteve, Eduardo Figueras Costa, Francesc Pérez-Murano, Member, IEEE, Zachary J. Davis, Esko Forsén, Anja Boisen, and Nuria Barniol, Member, IEEE Abstract—In this paper, we report on the main aspects of the design, fabrication, and performance of a microelectromechanical system constituted by a mechanical submicrometer scale res- onator (cantilever) and the readout circuitry used for monitoring its oscillation through the detection of the capacitive current. The CMOS circuitry is monolithically integrated with the mechanical resonator by a technology that allows the combination of stan- dard CMOS processes and novel nanofabrication methods. The integrated system constitutes an example of a submicroelectrome- chanical system to be used as a cantilever-based mass sensor with both a high sensitivity and a high spatial resolution (on the order of and 300 nm, respectively). Experimental results on the electrical characterization of the resonance curve of the cantilever through the integrated CMOS readout circuit are shown. [1318] Index Terms—Capacitive transducers, CMOS analog integrated circuits, microelectromechanical devices, nanotechnology. I. INTRODUCTION E XAMPLES of microelectromechanical systems include sensors for detecting different kinds of physical or chemical properties [1]. A reduction of the dimensions of the mechanical transducer leads to a new generation of systems called nanoelectromechanical systems (NEMS) [2]–[5] that represent an improvement on sensitivity, spatial resolution, energy efficiency and response time. As an example of NEMS, we present the design of a mass sensor based on a laterally oscillating cantilever with nanometer-scale dimensions, which has both a high sensitivity and a high spatial resolution (down to the range of and 300 nm, respectively). Mass de- tection is based on monitoring the resonant frequency shift of the cantilever when nanometer-sized particles or molecules are deposited on the cantilever [6], [7]. The cantilever is electrostatically excited by means of a driver electrode. A change in the cantilever resonance frequency is de- Manuscript received April 5, 2004; revised October 14, 2004. This work was supported in part by projects NANOMASS II (EU-IST-2001-33068) and NANOSYS (TIC2003-07237). Subject Editor L. Lin. J. Verd, G. Abadal, J. Teva, M. Villarroya Gaudó, A. Uranga, and N. Barniol are with the Department of Enginyeria Electrònica, Universitat Autònoma de Barcelona, E-08193 Bellaterra, Spain (e-mail: Gabriel.Abadal@uab.es). X. Borrisé, F. Campabadal, J. Esteve, E. Figueras Costa, and F. Pérez-Mu- rano are with the Institut de Microelectrònica de Barcelona (IMB-CNM-CSIC), E-08193 Bellaterra, Spain (e-mail: Francesc.Perez@cnm.es). Z. J. Davis, E. Forsén, and A. Boisen are with the Mikroelektronik Centret, Denmark Technical University, 2800 Lyngby, Denmark . Digital Object Identifier 10.1109/JMEMS.2005.844845 tected as a capacitance change. Electrostatic transduction in the nanometer-size regime requires the minimization of the para- sitic capacitance since the magnitude of the current to be de- tected is proportional to the coupling capacitance between the cantilever and the driver, which is in the order of . Con- sequently, the readout circuitry has to be integrated “on-chip” along with the mechanical transducer in order to eliminate the parasitic capacitance introduced by the external bonding pads and wires. CMOS circuitry for excitation and read-out of the cantilever deflection is integrated together with the cantilever by using a monolithic technology that consists of the combi- nation of standard CMOS processes and novel nanofabrication methods. For demonstration purposes, in the present paper we have used a CMOS technology (2.5 , two metal and two polysil- icon layers) that allows such combination [8]. The integration of the cantilever, the excitation system and the readout circuitry on the same chip provides us with a smart sensor system (see Fig. 1) which will permit detection of the deposited mass with in situ added functionalities like for example automatic tracking of the resonance frequency. Although the principle of operation of this smart sensor is very simple, its practical realization requires carefully ad- dressing of several issues that arise from the combination of nanometer scale devices and microelectronic circuits. In this work, we present the main aspects of the sensor modeling (Section II), circuit design (Section III), and system fabrication (Section IV) that have allowed the successful electrical charac- terization of the first prototypes, as it is shown in Section V. II. SENSOR PRINCIPLE AND MODELIZATION The two main cantilever parameters: i) spring constant , and ii) the fundamental resonance frequency can be calculated according to the dimensions of the resonant structure and the mechanical properties of the material (Young modulus and mass density ) [9] (1) (2) where , , and are, respectively, the width, length, and thick- ness of the lateral oscillating cantilever (see Fig. 1). Approxi- 1057-7157/$20.00 © 2005 IEEE