In microelectronics, 3D integration and assembly definitely appears as a very efficient option to achieve highly integrated chips. It offers major benefits such as combining heterogeneous technologies, combining high-performance and low-power chips, increasing data transfer bandwidth in memory above logic circuits, etc. 3D assembly is realized by bonding two wafers or chips face to face on a wafer. In this bonding process, the number of interchip interconnects is limited by the accuracy of the alignment process. Presently, alignment methods rely on optical alignment techniques, which offer ±0.2 μm resolution for wafer-to-wafer bonding or only ±1 μm resolution for die-to-wafer bonding. This is relatively poor and limits the density of interconnects and therefore the bandwidth of interchip communication. In MAGALIGN ERC PoC project, we progressed towards a novel magnetic alignment approach based on nanomagnetism and spin electronics, which can yield unprecedented accuracy in alignment during wafer bonding.
The project takes advantage of the fact that ferromagnetic cobalt is used in vias and interconnects. Cobalt vias can be straightforwardly realized at the surface of a first wafer and used as magnetic alignment marks. As demonstrated in the ERC MAGICAL project, MRAM cells can be used as magnetic field sensors thanks to the spin transfer torque effect (STT). As a result, developments within the MAGALIGN project demonstrated an alternative magnetic field sensing method based on typical MRAM memory cells. Within the timeframe of the project, it was demonstrated that this new sensor technology can combine large range magnetic field measurements typical of Hall effect sensors, with high resolution of magneto-resistance sensors, while reducing the total sensor lateral width to only 50nm, while other sensors have typically 200-1000x larger dimensions. A partnership between Spintec (Grenoble, FR), Unistra (Strasbourg, FR) and FHNW (Muttenz, CH) is now established to develop the electronic circuitry and strategies towards further increase in resolution, sensitivity and range, combined with low power. The ultimate goal of high-resolution alignment still requires further improvements in sensor sensitivity. However as a sensor, the demonstrated STT junctions could already find applications in other fields. These domains range from industrial applications to the medical field, embedded either in a single small chip, or as an array of sensing units. Among application examples are industrial applications such as current sensing, fault detection, system alignment such as photolithographic masks, or medical applications with near-field sensing, magnetic tracking systems, or magnetic camera with extremely high density, unreachable with present technology of large MR or Hall sensitive elements.
