A NOVEL METHOD TO RELEASE RESIDUAL STRESSES IN POLYSILICON FILMS

摘要

The residual stress of MEMS frequently degrades the performance and the reliability of MEMS devices. A great deal of efforts has been made to control the residual stress in thin film structure. The conventional approach to solve such a problem is to relax the residual stresses in thin films with a high temperature furnace annealing or RTA(rapid thermal annealing) methods. However, they are detrimental to IC(integrated circuits) fabricated with microstructures due to their high thermal budget. In the present study, ion implantation without any thermal treatment is applied for the residual stress relaxations of LPCVD polysilicon films in MEMS. He~+ and Ar~+ ion implantations reduce the residual stresses of polysilicon films. The amount of the residual stress relaxation increases as ion dose increases. TEM observations show that ion implantation on polysilicon films changes crystal state of polysilicon into amorphous state. Cubical expansion is induced because amorphous silicon has a lower density than that of crystal silicon, and thus the compressive residual stress is created. The compressive residual stress counterbalances the tensile residual stress in the upper part of LPCVD polysilicon thin film with positive stress gradient.