Sensors and Actuators A 156 (2009) 328–333 Contents lists available at ScienceDirect Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna High aspect ratio PMMA posts and characterization method for micro coils manufactured with an automatic wire bonder Kai Kratt a,∗ , Vlad Badilita a , Tobias Burger a , Jürgen Mohr b , Martin Börner b , Jan G. Korvink c,d , Ulrike Wallrabe a,d a Laboratory for Microactuators, Department of Microsystems Engineering (IMTEK), University of Freiburg, Georges-Köhler-Allee 102, 79110 Freiburg, Germany b Forschungszentrum Karlsruhe GmbH, Institute for Microstructure Technology, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany c Laboratory for Simulation, Department of Microsystems Engineering (IMTEK), University of Freiburg, Georges-Köhler-Allee 103, 79110 Freiburg, Germany d Freiburg Institute of Advanced Studies (FRIAS), University of Freiburg, Albertstr. 19, 79104 Freiburg, Germany article info Article history: Received 24 July 2009 Received in revised form 9 October 2009 Accepted 10 October 2009 Available online 20 October 2009 Keywords: 3-D micro coil Wire bonding Deep X-ray lithography abstract We present the development of high aspect ratio PMMA posts manufactured in deep X-ray lithography technology. The posts serve as support structures for micro coil windings. The coils are wound around these cores by an automatic wire bonder, and they are fully compatible with MEMS processes. Posts with outer diameters down to 100 m and minimal sidewall thicknesses of 20 m have an aspect ratio of up 38, leading to single-layer coils with more than 20 windings. With a flexible software, the winding number is only limited by the post height. We introduce a specially designed structure to determine the forces caused by the winding process. These measured forces are confirmed by shear test data. Electrical measurements show a quality factor of over 50. © 2009 Elsevier B.V. All rights reserved. 1. Introduction The particular properties of 3-D coils – high inductance values and a very homogeneous magnetic field – are needed in applica- tions like sensors or actuators. Especially for integrated devices micro coils are very useful. However, the MEMS compatible minia- turization is very difficult. Klein et al. [1] have reported coils with a rectangular cross section. Their process with reactive ion etching in borosilicate glass results after 40 single process steps in a coil with a diameter of about 200 m. Rogge et al. [2] use deep X-ray lithog- raphy in PMMA to form straight coil sidewalls. The etched holes are electroplated to form the coils; a seed layer on top helps to form the bridge. A smart, but sophisticated technology uses out-of-plane folding to create coils with diameters down to 2 mm [3]. Micro coils with a circular cross-section have been made with a self-made coil winding machine around a glass tube [4]. Loose wire ends have to be soldered manually for further connection. The same holds for Rogers et al. [5] with their printed coils. Here, a cylinder is rolled over a stamp moistened with a non-specified, but presumably con- ∗ Corresponding author. Tel.: +49 761 203 7408; fax: +49 761 203 7389. E-mail addresses: kai.kratt@imtek.uni-freiburg.de (K. Kratt), vlad.badilita@imtek.uni-freiburg.de (V. Badilita), tobias.burger@jupiter.uni-freiburg.de (T. Burger), Juergen.Mohr@imt.fzk.de (J. Mohr), Martin.Boerner@imt.fzk.de (M. Börner), korvink@imtek.uni-freiburg.de (J.G. Korvink), wallrabe@imtek.uni-freiburg.de (U. Wallrabe). ductive ink. This starting layer now initiates the deposition of metal for the coil. Matsumoto et al. [6] use a 3-D LIGA process. An acrylic pipe is rotated and moved simultaneously during X-ray exposure. The cylinder is metalized by beam sputtering and electroplated to thicken the metal. All coils in the mentioned publications either have their axis parallel to the substrate, what makes it difficult to access the inner volume of the coil providing the homogeneous field, or they are detached from the substrate, what makes it difficult to connect the coils. So far, no parallel full-wafer batch process has been published for coils with their axis perpendicular to the substrate. Therefore we have started to develop a process using an automatic wire bonder to wind 3-D coils with, first, their axis perpendicular to the substrate, and second, with each wire end properly connected to a bond pad. The wire bonder winds wire around a core to form a solenoidal shape whilst the wire is plas- tically deformed. Kratt et al. [7] reported on a procedure for coils with 690 m diameter on glass cores, which were still limited to five windings. A micro Helmholtz pair with two windings per coil on SU-8 posts was presented by Badilita et al. [8]. In this paper we present the MEMS compatible fabrication of vertical 3-D micro coils on high aspect ratio PMMA posts with more than 20 wind- ings. The posts are made with deep X-ray lithography and serve as mechanical support structures. During the winding process the post have to withstand the winding forces. These forces are mea- sured with a specially designed structure that let us deduce the minimal adhesive force of the PMMA to the substrate. 0924-4247/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2009.10.010