Bending effect on magnetoresistive silicon probes

Mapping the brain´s activity can be done either with non-invasive techniques (e.g. magnetic resonance, electro/magnetoencephalography), or by studies performed in-vivo/vitro at the cell level usually using small-sized electrodes (10 μm) to measure local potentials. Probing locally the neuronal magnetic fields created by the synaptic currents can be done with magnetrodes [1], which combine high sensitive magnetoresistive sensors (spinvalve, SV) with micro-machined Si probes. The SV sensors were deposited by ion beam with the following stack: Ta 2/Ni₈₀Fe₂₀ 3.5/Co₈₀Fe₂₀ 2.3/Cu 2.3/Co₈₀Fe₂₀ 2.3/Mn₇₆Ir₂₄ 8/Ta 5/Ti₁₀W₉₀N 15 (thickness in nm). The sensors were patterned in arrays of N=952 elements (350 μm² each) connected in series, in order to bring the detectivity levels below pTesla. This strategy was already demonstrated viable when the device footprint is not an issue for the considered application [2]. The sensors were incorporated in micromachined thin Si needles with well defined tip angle and intrinsic bending capability. This approach induces a minimum damage when inserting the probes within the brain tissues and enhances the sensors proximity to the signal sources, with a spatial resolution unmatched by any of the competing neuroscience tools. The Si needles were defined [3] with a length of 11.3 mm and width of 1 mm [Figure 1]. The separation between the tip of the needle and the array middle point was set in 3.5 mm. In the fabrication process three Si substrates (Young modulus E=1.3×10¹² dyn/cm²) with different thicknesses are used: (i) 700 μm, (ii) 400 μm and (iii) 50 μm, being the later a Silicon-On-Insulator (SOI) wafer. The impact of bending in the sensor transfer curve and noise level were investigated, aiming at optimizing the detectivity limits under conditions si- ilar to those implemented in the in-vivo experiments.