Control of stress in tantalum thin films for the fabrication of 3D MEMS structures

Sputtered tantalum (Ta) thin-films subjected to high compressive stress have been used for fabricating out-of-plane microelectromechanical systems (MEMS) clamped–clamped beams. The as-deposited film stress has been investigated as a function of sputtering conditions (pressure and power) in order to determine the optimal deposition parameters for achieving a compressive state. A variation of the as-deposited film stress has been observed as a function of time at atmospheric conditions and reduced by ∼85% using annealing in oxygen atmosphere at 100 °C. By surface-micromachining Ta thin-films deposited in compressive state (beyond the critical buckling load), buckled out-of-plane beams have been fabricated with lengths in the range of 500 μm−5.8 mm and central out-of-plane deflection up to ∼60 μm. A final stress of ∼−850 MPa has been calculated from the measured beams' deflection. The modal behavior of the buckled out-of-plane beams has been investigated with optical measurements and finite element simulations. The large curvature and high residual stress experienced by the beams have shown to affect the usual order of consecutive bending mode shapes. The devices presented can be utilized for implementing cutting-edge three-dimensional MEMS.