Raman Investigation on Silicon Nitride Chips after Soldering onto Copper Substrates

The unique electrical properties of silicon nitride have increased the applications in microelectronics, especially in the manufacture of integrated circuits. Silicon nitride is mainly used as a passivation barrier against water and sodium ion diffusion and as an electrical insulator between polysilicon layers in capacitors. The interface with different materials, like semiconductors and metals, through soldering may induce residual strains in the final assembly. Therefore, the dentification and quantification of strain becomes strategically important in optimizing processes to enhance the performance, duration, and reliability of devices. This work analyzes the thermomechanical local strain of semiconductor materials used to realize optoelectronic components. The strain induced in the β-Si₃N₄ chips by the soldering process performed with AuSn pre-formed on copper substrates is investigated by Raman spectroscopy in a temperature range of -50 to 180 °C. The variation in the position of the E_1g Raman peak allows the calculation of the local stress present in the active layer, from which the strain induced during the assembly process can be determined. The main reason for the strain is attributed to the differences in thermal expansion coefficients among the various materials involved, particularly between the chip, the interconnection material, and the substrate. Micro-Raman spectroscopy allows for the assessment of how different materials and assembly processes impact the strain, enabling more informed decisions to optimize the overall device structure.