Engineering buffer layers to improve temperature resilience of magnetic tunnel junction sensors

Improving the thermal resilience of magnetic tunnel junctions (MTJs) broadens their applicability as sensing devices and is necessary to ensure their operation under harsh environments. In this work, we are address the impact of temperature on the degradation of the magnetic reference in field sensor stacks based on MgO-MTJs. Our study starts by simple MnIr/CoFe bilayers to gather enough insights into the role of critical morphological and magnetic parameters and their impact in the temperature dependent behavior. The exchange bias coupling field (Hex), coercive field (Hc), and blocking temperature (Tb) distribution are tuned, combining tailored growth conditions of the antiferromagnet and different buffer layer materials and stackings. This is achieved by a unique combination of ion beam deposition and magnetron sputtering, without vaccum break. Then, the work then extends beyond bilayers into more complex state-of-the-art MgO MTJ stacks as those employed in commercial sensing applications. We systematically address their characteristic fields, such as the width of the antiferromagnetic coupling plateau ΔH, and study their dependence on temperature. Although, [Ta/CuN] buffers showed higher key performance indications (e.g.Hex) at room temperature in both bilayers and MTJs, [Ta/Ru] buffers showed an overall wider ΔHup to 200 °C, more suitable to push high temperature operations. This result highlights the importance of properly design a suitable buffer layer system and addressing the complete MTJ behavior as function of temperature, to deliver the best stacking design with highest resilience to high temperature environments.</p

Optimization of exposure parameters for lift-off process of sub-100 features using a negative tone electron beam resist

A thorough study of exposure parameters for electron beam lithography using AR7520 negative tone electron beam resist is here presented. We optimized the beam voltage, apertures diameter and resist thickness in order to achieve the smaller dimensions possible for each resist thicknesses. Monte Carlo simulations of the electrons scattering process correlated the experimental results indicating a less efficient energy deposition into the resist layer for larger beam energies and resist thicknesses, thus resulting in larger doses required to expose a selected dot size. Furthermore, for the particular exposure conditions used we determined a forward scattered electrons range between 50 nm and 170 nm, depending on the dot nominal size. On the other hand, a reduced backscattering electrons range was observed showing a constant value of ~ 560 nm, being therefore more significant when larger dimensions are exposed in a point-by-point exposure, and thus supporting the smaller doses observed for larger sizes. Finally, a baking step is used to further improve the etch resistance of the resist, which allied to the optimized exposure parameters, opens a pathway to achieve sub-100nm critical dimensions for the reproducible fabrication of nanometric devices using a simple lift-off method