Sensors and Actuators A 175 (2012) 28–34 Contents lists available at SciVerse ScienceDirect Sensors and Actuators A: Physical j ourna l h o me pa ge: www.elsevier.com/locate/sna A LTCC low-loss inductive proximity sensor for harsh environments Heike Bartsch ∗ , Thomas Geiling, Jens Müller Department of Electronics Technology, Ilmenau University of Technology, Gustav-Kirchhoff-Str. 7, 98693 Ilmenau, Germany a r t i c l e i n f o Article history: Received 6 July 2011 Received in revised form 8 December 2011 Accepted 8 December 2011 Available online 8 January 2012 Keywords: Proximity sensor Magnetic sensor LTCC Eddy current Ceramic multilayer Embossing Photolithographic thick film processing a b s t r a c t Proximity and position measurements with eddy current sensors are limited by the quality factor of the sensing coils. By enlarging the conductor path cross section, the quality factor can be increased. On ceramic multilayer substrates such cross sections, which are considerably larger in comparison to screen printed thick films, can be manufactured with an embossing and filling procedure, which is integrated into the conventional fabrication process. The conductor path layout is molded into the unfired ceramic substrate prior to filling the trenches completely with the conducting paste, thus generating a large cross section. This technique is applied for the sensing coil of an eddy current sensor. The higher quality factor leads to a doubled sensitivity in comparison to conventionally screen printed sensors. Due to the thermal and chemical durability of the ceramic substrate, the coils can be directly applied in a variety of harsh environment conditions, such as in situ engine monitoring. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Eddy current proximity sensors are applied in many industrial environments because they are inherently insensitive against non- metallic dirt and opaque fluids. Thick film sensor coils based on ceramic substrates expand displacement measurements to harsh and high temperature environments. Their application as position sensors in magnetic active bearings is a proven solution [1]. Multi- layer substrates such as low temperature co-fired ceramics (LTCC) are also used to manufacture position sensor coils [2,3]. The quality factor of the coils is commonly used as the sensing parameter. It is inversely proportional to the resistance of the coil. But conductor path resolution of thick film coils is limited to values between 80 m and 200 m as a consequence of the screen printing technology. The conductor thickness is limited to approximately 10 m. This results in large resistances and therefore a limitation of the quality factor. If high quality factors are required, large dimen- sions for planar coils must be accepted due to high sheet resistivity [4,5]. A molding of the sensor coil layout into a ceramic multilayer substrate via embossing combined with a photolithographic pro- cess was used to overcome the dependency between conductor thickness and line resolution. High line resolutions are achieved using the Fodel ® process [6], based on the photoimageable ink AG 6453, provided by DuPont [7]. However, the thickness of these ∗ Corresponding author. Tel.: +49 3677 69 3440; fax: +49 3677 69 3360. E-mail address: heike.bartsch@tu-ilmenau.de (H. Bartsch). conductors is still limited to approximately 7 m. In [8] it was shown, that the use of embossed trenches and the subsequent fill- ing with Fodel ® ink leads to significantly enlarged conductor cross sections and thus to lower resistances and higher quality factors. In this contribution, the developed process is applied to design low- loss inductors, which can be applied as ceramic proximity sensors in harsh environments. A commercially available tape with a chemical inertness com- parable to that of borosilicate glass is the DuPont 951 GreenTape TM from DuPont de Nemours (further called DP 951) [9]. A wide range of functional pastes is available for this substrate. This simplifies embedded design solutions. The material is fully compatible to all hybrid technologies, which guaranties a reliable connection of the sensor coil with the signal processing. Typically, the coil of an eddy current sensor is part of a resonant circuit operating at constant frequency. The impedance is used as a parameter for the proximity as shown in Fig. 1. A conductive material entering the field generates losses due to the induced eddy currents, which further reduce the overall induc- tance of the sensor coil by their mutual inductance. Both parts reduce the quality factor Q(x) of the coil, which is given by the relation: Q (x) = L(x) R(x) (1) The inductance L(x) and the resistance R(x) of the coil are a func- tion of the target distance x. The excitation frequency of the sensor coil is designated with w. Although L(x) experiences a larger change by varying target distance, the resistance is the defining parameter 0924-4247/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2011.12.015