AN IN-PLANE COBALT-NICKEL MICRORESONATOR SENSOR WITH MAGNETIC ACTUATION AND READOUT O. Ergeneman 1 , P. Eberle 1 , M. Suter 2 , G. Chatzipirpiridis 1 , K. M. Sivaraman 1 , S. Pané 1 , C. Hierold 2 , and B. J. Nelson 1 1 Institute of Robotics and Intelligent Systems, ETH Zurich, Zurich, SWITZERLAND 2 Micro and Nano Systems, ETH Zurich, Zurich, SWITZERLAND ABSTRACT We present magnetic microresonators that utilize magnetic actuation and readout for use as mass sensors. A magnetic readout method was developed for the detection of microresonator vibration. Together with wireless actuation, the wireless magnetic readout results in completely passive microresonators. The magnetic readout is based on the induced voltage on a pair of differential pick-up coils, which is generated by the movement of the magnetized microresonator. The successful operation of the readout was demonstrated with CoNi microresonators under atmospheric pressure and in water. The microresonator can be readily functionalized and used as a mass sensor for bio applications. KEYWORDS Magnetic actuators, wireless readout, passive microresonator, cobalt-nickel. INTRODUCTION Microresonators provide high sensitivity, rapid detection, and easy operation [1-2]. They can be used as chemical, physical, or biological sensors. Through the accumulation of the target biomaterial on the resonator surface, the mass load on the microresonator increases resulting in a change in vibration characteristics under excitation (i.e. change in resonance frequency). Microresonator sensors work based on adsorption of species on the functionalized surfaces. Through this functionalization, molecular recognition is directly transduced into mechanical response. We present a complete magnetic microsystem that utilizes magnetic actuation and readout for use as a mass sensor for bio-applications. The microresonator is made of electroplated CoNi that has low coercivity and high saturation magnetization. Many magnetic micro- resonators have been reported [3], most of them utilizing optical readout methods. In [4], a contactless magnetic readout method that works based on induced eddy currents on a conductive non-magnetic MEMS device is presented. Here, we present a contactless magnetic readout method for a soft magnetic microresonator. Under applied loads the resonance frequency of the microresonator shifts, and this shift can be accurately detected using the wireless magnetic actuation and readout system. FABRICATION The fabrication of the microresonator is shown in Fig. 1. The microfabrication of the device is based on two lithography steps, an electroplating step and a sacrificial layer etching step. A 25 nm adhesion Ti layer (not shown) and 500 nm sacrificial Cu layer are evaporated on a Si substrate by e-beam evaporation (1). The Cu layer also acts as a seed layer for subsequent electrodeposition. The device layer is defined by the first lithography (2) and formed by electroplating CoNi. After electrodeposition the photoresist is removed (3). The adhesion between SU-8 and CoNi was improved using an adhesion promoter (not shown). An 80 µm-thick layer of SU-8 is applied over the devices (4) and a second photolithography step forms anchors (5). The microdevices are released from the substrate by etching the sacrificial Cu layer. The fabricated microresonator is shown in Fig. 2. ACTUATION An electromagnet is used to actuate the magnetic microresonator and positioned so that its axis is parallel to the normal of the microresonator plate. Two coils are placed in a Helmholtz configuration to generate uniform magnetic fields to magnetize the microresonator plate in- plane as shown in Fig. 3. The electromagnet generates a gradient in both axial and radial directions of the coil. The size and the location of the coils were optimized by FEM simulations with COMSOL Multiphysics. The magnetic force, F [N], on the microresonator depends on the magnetization of the plate, M [A/m], and magnetic field gradient generated by the actuation coil [5]. Figure 1: Fabrication of the CoNi microresonators with SU-8 anchor.