Control of internal stress in SMA/Si bimorph microactuators

The internal stress in SMA/Si bimorph structure was investigated in this paper. The shape memory alloy (SMA) thin films were sputter-deposited onto single crystal silicon substrates at room temperature and then annealed at high temperature for crystallization. The internal stresses in SMA films before and after crystallization were measured by substrate-curvature method based on S. Timoshenko's theory. The results show that the internal stress changes from compressive to tensile after film crystallization. The intrinsic stress in the sputter-deposited SMA films is almost relaxed completely during the crystallization annealing and the internal stress in crystallized SMA film is dominated by thermal stress. By varying the sputtering:power during deposition, the interface status and intrinsic stress can be controlled and excellent SMA/Si bimorph actuation structure can be obtained.