Research Academics  
 
   
 
Micro Coil with Movable Core for the Application in an Inductive 
Displacement Sensor
 
Contact Person : Dipl.-Ing. Bernt Schellin

Introduction

A new concept for a micro coil is presented. The micro coil will be used as a sensor that measures the flexion of fingers. For this purpose several micro coils are mounted on a data glove. Though the coil is fabricated by means of silicon bulk micromachining it is possible to move an iron core through the coil without destroying the windings. The inductance of the full coil is 0.25µH without core and 7.2µH with core (Fig. 6). The ohmic resistance has a value of 60 Ohm.

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Figure. 1: Structure of the micro coil

Sensor Design

The structure of the sensor is shown in Figure 1. It consists of three parts, the upper and lower part of the coil, and the iron core. A full 3D coil can be build by connecting the two halves of the coil by flip chip bonding. Fig. 2 shows the two parts of the coil before flip chip bonding. To guide the core we use a groove that is etched into the silicon part.

 

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Figure 2 : Photograph of the micro coil before bonding

 

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Figure 3 : Schematic overview of the process flow.
(a) Formation of a 50µm deep groove (silicon only) by anisotropic etching in KOH.
(b) Etching of the trenches (10µm) for the windings (Fig. 3, 4).
(c) Electroplating of Au to form the windings.
(d) Electroplating of 5µm Cu (silicon and pyrex) and 18µm PbSn (pyrex only) to form bumps (Fig. 5).

  

Figure 4:SEM picture of the resist pattern used as etching mask for etching the trenches for the windings (left). SEM picture of the etched trenches (middle). SEM picture or the PbSn bumps after the reflow( right).

 

Conclusions

A new kind of micro coil was developed. It consists of two half coils that are combined by flip chip bonding. Because the two halves of the coil have plane surfaces it is possible to move an iron core through the coil without destroying the windings and without having the problem of sticking. The inductance of the full coil is 0.25 µH without core and 7.2 µH with core. The ohmic resistance has a value of 60 Ohm. A movement of the flexible core results in a change of inductance so that this micro coil can be used as a position sensor or an angular sensor.

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