Proceedings of the 2009 Spanish Conference on Electron Devices - Feb 1 1-13, 2009. Santiago de Compostela, Spain. Microneedles electrodes for living cells M. Duch, M.J. Lopez, R.Gomez, J. Esteve, J.A.Plaza Institute of Microelectronics of Barcelona IMB-CNM (CSIC), Campus UAB, 08193, Barcelona, Spain, e-mail: marta.duch@cnm.es Abstract- This paper shows the technology development of microneedles electrodes for living cell applications. The study (6) (b) - includes the fabrication of cantilever electrodes to monitor individual cells. The technology is based on silicon micromachining by combining Deep Reactive Ion Etching and (c) (d) Focused Ion Beam (FIB) nanomachining. Cell biology applications require micrometer size electrodes, smaller than living cells (-20 gm), and biocompatible materials. The metal electrode is fabricated in aluminum and passivated by silicon (e) oxide. The opening of the tip electrode by FIB is also performed. (e) (}) Finally, the tip of the electrode is covered by a nanometer thick gold layer deposited by electroless to improved biocompatibility and electrochemical conductivity. The study also addresses questions concerning to the pre-treatment of the aluminum layer g) IFIB (h) / /. ./, before gold deposition in other to improve the adherence and conductivity. I. INTRODUCTION Silicon Over the last years a wide interest in microelectrodes using U Aluminum microelectrochemical systems (MEMS) technologies [1] has r PhotoreSist led miniaturization of devices especially for Bio-MEMS Silicon oxide research. MEMS technology takes advantages of micro- Fig. 1. Microneedle electrode fabrication process: a) silicon wafer as starting fabrication techniques, offering small dimension, compact and material, b) aluminum deposition and photolithographic process, c) integrated sensors develop with combined capabilities of aluminium metal lines patterning, d) 6 gm thick photoresist deposition as silicon-integrated circuit processing and thin film sensin mask material for the silicon chip machining, e) backside silicon DRIE, f) silicon-integrated circuit processing and thin film sensing. silicon oxide deposition for electrode passivation, g) tip electrode opening by Many of these fabrication processes are focus on fields as FIB, h) photoresist protection of the chip for contact opening and i) protective biological electrical recordings and electrochemical photoresist removing and final wire bonding. measurements [2, 3]. Nowadays, instruments for the study of living cells are II. CHIP FABRICATION getting significant importance in fields like neuroscience and cell biology. Among a wide variety of measurements A. Fabrication process. techniques, microelectrodes provide a simple interface for The microneedles are fabricated on a 4" 500 ptm thick (100) monitoring the electrical activity and impedance double-side polished silicon wafer, (figl.a). The technology is characteristics of single cells [4]. based on standard microelectronic techniques combine with Although, MEMS surface electrode is widely used for silicon micromachining techniques as Deep Reactive Ion microelectrode probe design [5-6], three-dimensional shaped Etching (DRIE). First, a 0.7-1.5 ptm thick aluminum layer is probe offer more advantages. While with flat electrodes deposited at the front side and a photolithographic step is special effort is made to improve thight seal between the done, fig.lb. At this point, the metal lines of the electrode and the cell membrane (biological applications), microelectrode are defined by an aluminum etching and three dimensional probes allow a direct place recording, signal afterwards the photoresist is removed, fig.1c. Next, a second to noise improvement and higher signal amplitude. Then, we photolithographic step is done at the backside to define the present, the details of MEMS based microneedles recently chip. A 6 ptm thick photoresist is used to withstand the etching develop, based on silicon micromachining by combining Deep of the whole silicon wafer. An additional photoresist layer is Reactive Ion Etching and Focused Ion Beam nanomachining. spun on the frontside to be used as etch-stop layer, fig. 1 d. Next, the chip is defined by DRIB through the wafer. The etch stops at the frontside photoresist layer, fig.l1e. This process releases part of the metal line forming a cantilever electrode. The next step is the electrical isolation of the metal lines by 978- 1-4244-2839-7/09/$25.00 (C)2009 IEEE 297