Novel architectures to integrate ultra sensitive sensors to detect biomedical signals

Magnetic sensing technologies are widely used in biomedical applications wherein magnetoresistive sensors are stated due to a high performance in terms of signal-to-noise ratio, power consumption and production cost. Furthermore, they work at room temperature with an established nTesla minimum field detectable at low frequencies. However, an improvement on the limit field detection can push forward the magnetoresistive technology as a high precision tool within the sub-nT field range.

The key parameters of the magnetoresistive sensor performance are given by sensitivity, noise, power consumption, thermal and magnetic robustness. A proper multilayer stack engineering focused on materials and thicknesses of the buffer layer (Ta, NiFeCr), antiferromagnetic material (MnNi, MnIr), reference structure (exchange bias, synthetic antiferromagnet) and sensing layer can deliver an enhanced response according to the application requirements.

Instead of a sensitivity enhancement driven by a higher MR ratio, the incorporation of magnetic flux concentrators provides a reduction of the linear range. However, the operational point of the latter approach is usually deviated from zero field due to its high sensitivity and hysteresis, being explored a monolithic biasing technique to compensate the shift of the output curve through the integration a current line directly on chip. On the other hand, the optimization of the minimum field detectable can also be delivered by a noise level reduction.

Therefore, a vertical deposition of Z spin valve levels was assessed in order to achieve a compact parallel configuration with an equivalent resistance reduced by a factor of 1/Z. This work comprised the optimization of the deposition and microfabrication process for a thick multilayer stack; development of simulation and calculation tools to understand the physics behind and to support the device design; magnetic, magnetotransport and noise characterization of the microfabricated devices, and implementation of an experimental set-up to find the minimum field detectable within an unshielded environment. Packed devices composed of up to 50 spin valves vertically deposited and large sensing areas (low aspect ratio) led to a low nominal resistance combined with a linear and centered output driven by all magnetostatic couplings between ferromagnetic layers. This approach minimizes the noise level while maintaining a high sensitivity delivering an improved detectivity without compromising the device footprint. A 3D architecture design integrating magnetic tunnel junctions was pursued to push forward the detectivity of tunnel magnetoresistive sensors.

The concept was demonstrated with a CoFeB/MgO/CoFeB based stack, being established a microfabrication process for a device composed of 3 levels. The characterized device presents a low noise level in addition to a high electrical robustness and compact footprint.