IWLPC (Wafer-Level Packaging) Conference Proceedings

Fabrication of a High Precision Magnetic Position Sensor Based on a Through Silicon Via First Approach

Authors: Kai Zoschke, Hermann Oppermann, Johannes Paul, Heiko Knoll, Franz-Josef Braun, Monika Saumer, Martin Theis, Peter Frank, Andreas Lenkl, Fabian Klose
Company: Fraunhofer IZM, Sensitec GmbH, University of Applied Sciences Kaiserslautern, Sandvik Tooling Supply Schmalkalden, Märzhäuser Sensotech GmbH
Date Published: 10/23/2018   Conference: IWLPC (Wafer-Level Packaging)

Abstract: Magnetic position sensors with highest accuracy of 700 nm and a bidirectional repeatability of 110 nm were fabricated based on a new architecture and process technology. The sensor concept relies on tunnel magneto resistive (TMR) sensing elements which are deposited and structured on silicon wafers. The TMR elements are arranged to segmented chains with connections in between to form one incremental sensor and one absolute sensor per die. The TMR structures sense the magnetic poles of up to 25.6 mm long absolute and incremental tracks on the scale bar which moves in parallel along the surface in front of the sensor die.

Key for the high precision of the sensor is the extreme shrink of the TMR sensing structures as well as the usage of very small magnetic poles on the scale bar with a pole width of 50 µm and a pole pitch of 100 µm. Due to the low magnetic field strength of the miniaturized magnetic poles the scale bar has to move very close with a distance of <30 µm to the surface of the sensor. Based on that, a conventional chip connection by flip chip or wire bonding cannot be used here. Instead of this all interconnects are routed vertically by using though silicon vias (TSVs) to the back side of the sensor dices and are redistributed there to an area array pattern. In order to simplify the overall process and to avoid impacts of the TSV implementation to the TMR process the TSVs were first fabricated as blind plugs into the blank wafers before the TMR structures were generated.

The final sensor dices have a thickness of 100 µm and are mounted by flip chip bonding with their back side to ceramic interposers. Due to a special arrangement of the magnetic poles on the absolute track of the scale bar a true power on position determination is possible without reference check.

Key Words: 

magnetic field sensor, positioning sensor, TMR sensor, TSV first integration, wafer level packaging

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