Magnetic MRAM memory and magnetic field sensor: multi-functionality for 3D assembly

Magnetic MRAM memory and magnetic field sensor: multi-functionality for 3D assembly In the last few years people are moving from planar technology towards 3D integration, this is due to the fact that the latter has lower production costs, higher versatility and the final device is more compact than for the standard planar technology. 3D integration consists into stacking wafers one on top to the other; to this end, a very problematic issue is the wafers misalignment. When wafers aren’t well aligned, we can have, in the worst case, diffusion of Cupper into the Silicon Dioxide; anyway, the lower is the surface area bonded and the lower is the current that can pass through the device, breaking down its efficiency. In order to detect possible misalignment, a system based on a magnetic element and a magnetic sensor is incorporate into the wafers to be bond. In this project a magnetic STT-MRAM is used as magnetic sensor to detect possible misalignment in the vertical and horizontal directions. The STT-MRAM is a device that is able to have two possible resistance states, the high resistance state and the low resistance state; it is possible to switch the device from one state to the other either by applying a voltage pulse or by applying a magnetic field. The relation between the magnetic field and the switching voltage is linear and allows us to exploit this behaviour in order to be able to detect the field applied by having a certain switching voltage as output. The work is split in two main parts: the simulation part and the experimental part. In the simulation part we studied the critical parameters and we saw the role of the voltage dispersion. The voltage dispersion is responsible of the resolution of the sensor, in particular we have seen that the voltage dispersion decreases with the applied external field); besides, the higher is the voltage dispersion and the lower is the accuracy in the determination of the magnetic field for a given switching voltage. After having understood the behaviour of the voltage dispersion with the applied field we saw how the voltage dispersion influences the TMR (Tunnel Magnetic Resistance) and we arrived at the conclusion that the higher is the TMR and the lower is the voltage dispersion, so it is important to have high TMR in order to have good device performances. Then we moved to the simulation of the magnetic element (or marker) and we saw how the sensor behaves with non-uniform magnetic field generated by the marker. This allows us to study the vertical and lateral sensitivity useful for displacements detection, in vertical and in horizontal direction respectively; then we analysed different geometries in order to improve these two magnitudes. In the experimental part we did an electrical and magnetic characterisation of the device and we extracted the switching probabilities; thanks to these, we did a study of the voltage dispersion from the experimental point of view. It is possible to characterise the STT-MRAM in two possible ways: we can either sweep in voltage for each magnetic field and extract the switching probabilities, or we can sweep in magnetic field and change each time the voltage applied. By sweeping in voltage we extracted the switching probabilities and so the switching probability density. The voltage dispersion is decreasing with the applied field, so the higher is the external field and the lower is the voltage dispersion, and so the better is the resolution.