TECHNICAL PAPER Fabrication of compliant high aspect ratio silicon microelectrode arrays using micro-wire electrical discharge machining Dinesh Rakwal Æ Sumet Heamawatanachai Æ Prashant Tathireddy Æ Florian Solzbacher Æ Eberhard Bamberg Received: 30 October 2008 / Accepted: 26 January 2009 / Published online: 21 February 2009 Ó Springer-Verlag 2009 Abstract This paper reports on the fabrication of high aspect ratio silicon microelectrode arrays by micro-wire electrical discharge machining (l-WEDM). Arrays with 144 electrodes on a 400 lm pitch were machined on 6 and 10 mm thick p-type silicon wafers to a length of 5 and 9 mm, respectively. Machining parameters such as voltage and capacitance were varied for different wire types to maximize the machining rate and to obtain uniform electrodes. Finite element analysis was performed to investigate electrode shapes with reduced lateral rigidity. These compliant geometries were machined using l-WEDM followed by a two step chemical etching process to remove the recast layer and to reduce the cross sections of the electrodes. 1 Introduction The world of micro-machining has been principally dom- inated by silicon (Si) based techniques such as photo- fabrication, wet etching, dry etching, X-ray lithography and micro-molding. Si-based techniques have major limitations such as their inability to create 3D and high aspect ratio features. To overcome these disadvantages and to push the dimensions of the devices to higher limits, several non- Si-based techniques such as laser machining, abrasive jet machining, ultra precision machining, ultrasonic machin- ing and electrical discharge machining are being explored as a replacement or a parallel alternative for several MEMS and bio-medical device fabrication processes. This paper explores the use of micro-wire electrical discharge machining (l-WEDM) in the manufacturing of compliant high aspect ratio microelectrode arrays in single- crystal silicon, which are used in intracortical recording systems that record neural signals from the brain, specifi- cally the motion intent and the sensory perception from the cerebral cortex (Fofonoff et al. 2004). This approach replaces the dicing saw used by Jones et al. (1992) with WEDM. The machining of high aspect ratio microelectrode arrays using WEDM has been demonstrated by Fofonoff et al. (2004) in titanium, stainless steel, and tungsten car- bide. As a top down approach that creates geometry through material removal, l-WEDM is capable of pro- ducing these arrays monolithically. This is in contrast to surface machining techniques, which involve a layered approach that requires assembly (Hoogerwerf and Wise 1994; Bai et al. 2000). 2 l-WEDM of silicon Wire electrical discharge machining involves removal of material by melting and vaporization using electrical energy. Electrode wires in various diameter sizes (20– 250 lm) and materials such as molybdenum, brass, tung- sten, and zinc-coated steel wires are used in dielectric media such as oil and water. As the wire approaches the work piece, the electrical field surrounding the wire ionizes particles in the dielectric medium to form a channel D. Rakwal Á S. Heamawatanachai Á E. Bamberg (&) Precision Design Laboratory, Department of Mechanical Engineering, University of Utah, Salt Lake City, UT 84112, USA e-mail: e.bamberg@utah.edu P. Tathireddy Á F. Solzbacher Microsystems Laboratory, Department of Electrical and Computer Engineering, University of Utah, Salt Lake City, UT 84112, USA 123 Microsyst Technol (2009) 15:789–797 DOI 10.1007/s00542-009-0792-7