Toward pTesla Detectivities Maintaining Minimum Sensor Footprint with Vertical Packaging of Spin Valves

The reduction of the noise level of magnetoresistive (MR) micrometric sensors is often achieved by increasing the sensor area and/or connecting MR elements in large arrays. This paper introduces an innovative strategy to increase the sensing area without increasing the area occupied by the MR sensors on a wafer, therefore minimizing the sensor footprint. Here, the idea is to deposit vertically spin-valve (SV) films, separated by an insulator. This packed structure is then incorporated into arrays, connected in parallel and series, as a general noise reduction strategy. Using calculations, the number of SVs vertically packed ( Z ), combined in series ( X elements) and parallel ( Y elements) was investigated, so that the optimum geometry, which minimizes the field detectivity, can be obtained. The magnetic field detection of SV sensors in the thermal noise limit can be improved from 39 pT/Hz ^{1/2} for Z = 1 with a total number of 500 SVs (distributed in plane) to 12 pT/Hz ^{1/2} for Z =10 with a total number of 5000 SVs. Besides, the device footprint is also demonstrated to decrease from 0.7 mm² ( Z = 1 ) to 0.07 mm² ( Z = 10 ). These limits of detection are competitive with single SV sensors using magnetic flux concentrators, with the advantage of a high spatial resolution with fewer fabrication steps (at least three).