Nano planar coil actuated micro paddle resonator for mass detection S. Chitsaz Charandabi a,⇑ , P.D. Prewett a , C.A. Hamlett b , C.J. Anthony a , J.A. Preece b a School of Mechanical Engineering, The University of Birmingham, Birmingham B15 2TT, UK b School of Chemistry, The University of Birmingham, Birmingham B15 2TT, UK article info Article history: Available online 3 March 2011 Keywords: Micro paddle resonator Electromagnetic actuation Micro-nano fabrication Mass detection abstract A torsional paddle microresonator has been conceived, designed and fabricated to first prototype level. It has demonstrated mass sensitivity that gives the potential for the detection of a range of biowarfare agents ranging from bacterial and other pathogens to the VOCs associated with explosives. This current work reports progress towards the implementation of a Lorentz force electromagnetic actuation system for the resonator, in place of the previously used piezoelectric shaker. A double planar spiral microcoil has been designed to be integrated with the micropaddle and has been prototyped using focused ion beam processing. Its mechanical effect upon the performance of the paddle has been investigated and its elec- tromagnetic characteristics simulated using FEA. The coil self inductance was calculated to be 1.54  10 10 H. Ó 2011 Elsevier B.V. All rights reserved. 1. Introduction Microelectromechanical systems (MEMS) are now well estab- lished for mass detection and related applications [1]. Simple microcantilever based MEMS sensors have been studied for a range of applications in medicine, health care and security, e.g. in disease screening, including blood glucose monitoring and for the detec- tion of chemical and biological warfare agents, where they provide high sensitivity, very low power consumption and low cost [2,3]. Microcantilevers are now capable of the measurement and detec- tion of ultra small masses, in the femtogram and even zeptogram ranges. Currently, this level of detection is of great interest in terms of in vivo physiological monitoring, multiple specificity, sensor arrays, sensor portability and minimized sample volumes [4,5]. It has also opened up new possibilities in the detection of biological warfare agents, for example anthrax, which was first identified by Robert Koch as described by Koehler [6] and is widely distributed in water, soil and air [7]. It was first used as an agent of bio terror in the 2001 attack on the US postal service [8]. The need for highly sensitive, reliable and discrete sensors capable of detecting anthrax spores in a range of buildings and other environments is therefore of considerable contemporary interest and importance and is the motivation for this research. 2. Micro paddle resonator for mass detection Our ultimate aim is to develop a device which utilises a chemi- cally functionalised paddle microresonator moving in a torsional rotational mode to detect bio-warfare agents including bacteria and viruses. The first stage of development of such a system, re- ported previously [9], focused on the non-chemically specific mass sensitive element. This took the form of an asymmetric micropaddle resonator (Fig. 1), which was etched into a silicon nitride membrane using a focused ion beam (FIB) (Fig. 2). This provides a mass sensitive sensor through the shift in its resonant frequency caused by the ad- sorbed particles of the detected species. Characterisation of the mass sensitivity of the micropaddle, was achieved using FIB deposition of Pt to adjust the resonator mass and measurement of the change in resonance by laser vibrometry in vacuum. A piezo electric ‘‘shaker’’ system connected to a tuneable signal generator was required to drive the resonator. The 1000 fg of FIB deposited Pt resulted in a fre- quency shift of 18.1 kHz from an initial resonance of 1.3 MHz giving a mass sensitivity of 55 ag Hz 1 . Our specific intended application is detection of the anthrax spore through its excreted adjunct dipicol- inic acid (DPA), which acts a convenient marker to its presence. The detected mass of 1000 fg is equivalent to the mass of DPA which would be excreted by just 36 spores and is far below the Lethal Dose of 50% of aerolised anthrax (8000–10,000 spores [10]). 3. Micro paddle and nano planar coil To remove the need for an external piezoelectric shaker to drive the resonator a Lorentz force electromagnetic actuation scheme 0167-9317/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2011.02.086 ⇑ Corresponding author. Tel.: +44 121 4144217. E-mail addresses: SXC616@bham.ac.uk (S.C. Charandabi), P.D.Prewett@bham. ac.uk (P.D. Prewett). Microelectronic Engineering 88 (2011) 2229–2232 Contents lists available at ScienceDirect Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee