The discovery of tunneling magnetoresistance (TMR) has enhanced the magnetoresistance (MR) ratio from the giant
magnetoresistance (GMR) regime of around 10% to over 400% at room temperature. A combination of magnetic tunnel
junctions with high magnetoresistance ratio and soft magnetic layers enables the development of ultra-low magnetic field
sensor with sensitivity down to the scale of picoTesla. A magnetic field sensor with such high sensitivity would have
important applications in biomedicine, information storage, and remote sensing such as higher resolution images for
cardiograph and magnetic resonance imaging and thus earlier detection of abnormal health condition; higher hard-disk
density; and remote sensing of metallic objects. We have constructed an automated four-probe electrical measurement
system for measuring TMR of magnetic tunnel junctions with high throughput, enabling us to optimize the properties of
the devices. Magnetron sputtering is used to deposit thin films with thickness ranged from angstroms to nanometers.
Photolithography and ion plasma etching are applied to pattern the devices. The devices have a range of size from 10 μm
x 10 μm to 80 μm x 80 μm. The device is composed of the bottom electrode, free soft magnetic layer, insulating oxide
layer, pinned layer, pinning layer, and top electrode. The magnetization of the free layer can be rotated by the external
magnetic field which in turn changes the resistance of the device and provide the sensing capability. The system structure,
design consideration, fabrication process, and preliminary experimental results are discussed and presented in this paper.