Sequential Lateral Solidification of Silicon Thin Films on Cu BEOL-Integrated Wafers for Monolithic 3-D Integration

We demonstrate that wafers integrated with copper (Cu) Damascene interconnects are suitable substrates for the excimer laser crystallization of silicon thin films. This approach allows for the monolithic 3-D integration of transistors on the back end of line (BEOL) of silicon wafers for VLSI monolithic 3-D integration. This is supported by a 1-D finite-element method simulation of the integrated structure, which shows that, upon excimer laser irradiation, the temperature of the buried Cu layer stays below 320 °C, which is a favorable condition for monolithic 3-D integration. The crystallization of a 100-nm amorphous silicon layer on a 1-μm SiO₂ dielectric is demonstrated on a BEOL-integrated wafer. The Raman spectrum of the silicon layer after laser irradiation shows a polycrystalline peak centered around 513 cm^-1. Optical microscopy shows polycrystalline silicon with no physical damage of the Cu lines. The electrical characterization of the Cu buried layer, with and without undergoing the irradiation process, shows no variation or degradation in Cu conductivity.