Mechanical integrity of polysilicon films exposed to hydrofluoric acid solutions

The results from a comparative study of Young's modulus, residual stress, and membrane burst pressure of undoped LPCVD polysilicon films exposed to various concentrations of hydrofluoric acid (HF) are presented. A reversal from compressive to tensile stress through a high-temperature anneal for large-grain polysilicon is an important (although ancillary) result of this study. As HF concentration is increased, residual stress decreases, while Young's modulus increases. Membrane burst pressure and estimated fracture strain show a significant decrease with increasing HF concentration. A significant result is that the change in the mechanical properties studied from exposure to pure HF or 1:1 DI:HF is identical. Furthermore, in relation to exposure to 10:1 DI:HF solutions, this change is small. However, with the addition of a buffering agent in a 10:1 buffered oxide etchant solution, the changes in the mechanical properties, particularly the films fracture strain, are reduced appreciably.<<ETX>>