**4. Micro orthogonal fluxgate**

As already mentioned, the lack of an excitation coil is one of the main advantages of orthogonal fluxgates, because it strongly simplifies its structure, making high miniaturization possible.

The first attempt made in order to reduce the dimensions of an orthogonal fluxgate was carried out in (Zorlu et al., 2005) where a sensor is based on a wire composed of Au core (20 µm diameter) covered by a 10 µm thick FeNi electroplated layer. The total diameter of the wire is therefore 40 µm, and the length varies from 0.5 to 4 mm. The output voltage is picked-up by means of two planar coils fabricated on a Pyrex substrate by means of sputtering, photolithography and patterning.

The response of the sensor has a large linear range for excitation current, which can be as low as 50 mA (at 100 kHz), showing that the wire is saturated for such low current. If the current is further increased to 100 mA the linear range reaches ±250 µT, and sensitivity reaches 4.3 V/T. A higher current than the minimum current necessary to saturate the core is also useful against the perming effect. While perming shift after ± 50 µT shock field is 16 µT for 50 mA excitation current, it drops down to 2 µT for 100 mA excitation current.

Orthogonal fluxgates based on a microwire, however, can hardly be manufactured at lower dimensions. The microfabrication of the sensor becomes more suitable for micro sensors, especially for mass production. In (Zorlu et al., 2006) a microfabricated orthogonal fluxgate is presented wherein the core is manufactured in three steps. First, a Permalloy bottom layer is electroplated on the Cr/Cu seed layer previously applied on the substrate, then the central copper core is electroplated in the middle and finally Permalloy is electroplated on the three open sides of the copper creating a closed loop of Permalloy around the copper. The resulting structure is composed of a rectangular shape core (8 µm x 2 µm) and a copper nucleus surrounded by a 4 µm Permalloy layer (the total dimensions of the structure is 16 µm x 10 µm). The length of the core is 1 mm. The dimension of the core was finely adjusted thanks to the high precision of photolithography.

Also in this case, the flux is picked-up using two planar coils formed in the substrate under the core (2 x 60 turns). The sensor has a large linear range (±200 µT) but rather low sensitivity, around 0.51 V/T for a 100 mA excitation current at 100 kHz. Thus, the resulting noise is higher than typical orthogonal fluxgates (95 nT/√Hz at 1 Hz). One of the problems of such configuration is that the planar coils cannot properly pick-up the flux as a concentric coil. Clearly, further investigation is necessary to understand whether a different configuration of the coil can significantly increase the sensitivity and then reduce the noise.
