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/Ni
₈₀
Fe
₂₀
3.5/Co
₈₀
Fe
₂₀
2.3/Cu 2.3/Co
₈₀
Fe
₂₀
2.3/Mn
₇₆
Ir
₂₄
8/Ta 5/Ti
₁₀
W
₉₀
N 15 (thickness in nm). The sensors were patterned in arrays of N=952 elements (350 μm
²
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=1.3×10
¹²
dyn/cm
²
) 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 similar to those implemented in the in-vivo experiments.