Orthogonal fluxgate type magnetic microsensors with wide linear operation range

This thesis presents a study on the development of microfabricated fluxgate type magnetic sensors operating within a wide linear operation range. Fluxgate type magnetic sensors are powerful devices due to their high sensitivity, low offset, and high temperature stability. Unfortunately, their linear operation range is limited, since an attempt to increase the linear range also increases the power dissipation of the sensor for the traditionally used parallel fluxgate configuration. In this study, microfabricated fluxgate sensors with wide linear operation range and low power dissipation are developed with the use of the orthogonal fluxgate configuration and a closed magnetization path for the excitation. In the scope of this work, three different fluxgate microsensor structures suitable for operation within a wide linear range are developed, fabricated, and characterized. The sensor structures are named as: rod type orthogonal macro fluxgate sensor, rod type orthogonal micro fluxgate sensor, and ring type micro fluxgate sensor. All of the structures have a CMOS compatible fabrication process flow. Furthermore, the rod type micro sensor and the ring type micro sensor are fabricated by using only standard thin film deposition and photolithography techniques, enabling batch fabrication of these sensor structures. All of the structures use planar sensing coils and an electroplated FeNi core. Apart from the design and development of the sensor, the FeNi electroplating process is intensively investigated since this process directly affects the performance of the sensors. The rod type orthogonal macro fluxgate sensor uses a 20 µm diameter gold bonding wire as the excitation rod, and a 10 µm thick FeNi core electroplated over the bonding wire. The AC current passing through the excitation rod creates a periodical excitation field in the radial direction, which is always perpendicular to the external magnetic field to be detected along the core. With this sensor, the idea of increasing the linear operation range without increasing the power dissipation by using a closed magnetization path and the orthogonal structure is verified. By using a 200 mA-peak sinusoidal excitation current at 100 kHz, passing through the low resistance excitation rod, a linear operation range of ±2.5 mT is reached with a 0.5 mm long core, whereas the linear range is ±250 µT with a 4 mm long core. The rod type orthogonal micro fluxgate sensor presents a modified version of the macro sensor, which can be fabricated in wafer level with standard deposition and photolithography techniques. For this sensor, the excitation rod is formed with an electroplated layer of copper which is sandwiched between two FeNi layers forming the ferromagnetic core. The cross-sectional dimensions of the excitation rod and the core are 8 µm × 2 µm, and 16 µm × 10 µm, respectively. The sensor operates with 100 mA-peak sinusoidal excitation current at 100 kHz, and the linear operation range for different sensors having 0.5, 1, and 2 mm long cores are 1100, 410, and 160 µT, respectively. The linear operation range is independent of the excitation conditions for current peaks larger than 100 mA, which is required to saturate the core, and operating frequencies lower than 200 kHz, where the skin effect is not dominant. The sensitivity, perming, the equivalent magnetic noise density, and the power dissipation of the 0.5 mm long sensor are 102.8 µV/mT, 7.1 µT, 268 nT/√Hz @ 1 Hz, and 10 mW, respectively for the given excitation conditions. The noise analysis showed that the noise of the sensor increases with decreasing sensor dimensions. The ring type micro fluxgate sensor has a core composed of cascaded planar 2 µm thick FeNi rings which can be fabricated in a single electroplating step, increasing the control of the magnetic properties of the core. The excitation rod passes through the middle of the FeNi rings as a sewing thread, providing a planar circular excitation loop. The angle between the excitation field and the external magnetic field changes according to the position on the ring, which leads to a partially orthogonal partially parallel fluxgate operation mode. The tests of the sensors showed that the maximum operating frequency is extended to 1 MHz level, which is due to the thinner FeNi layer. A sinusoidal current with 180 mA-peak at 1 MHz is used for the excitation of the sensors. A linear operation range of 2 mT and a sensitivity of 730 µV/mT is reached with a 4-ring structure, with the rings having 22 µm and 38 µm inner and outer radius, respectively. The comparison of the developed sensors with the previously reported state of the art sensors show that the first microfabricated fluxgate sensors having a wide linear operation range and low power dissipation are realized as an accomplishment of this work. All the sensors are CMOS compatible, and a sensor system can be realized by using the metallization layers of a CMOS process for producing the sensing coils, and fabricating the cores on wafers as a post process.

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