Influence of metal stress on RF MEMS capacitive switches

The electrical and mechanical response of radio frequency (RF) microelectromechanical systems (MEMS) switches depends critically on the profile of the released structure. Stress gradients within the material can significantly alter the structural profile thereby changing the spring constant and the resulting switch release times. To investigate the influence of metal stress on spring constant and release time, a series of RF MEMS electrostatically actuated capacitive microswitches was fabricated with varying beam metallizations. The total thickness of the beam was fixed at 700 nm consisting primarily of Au while thin (5 nm or 20 nm thick) Ti layers were inserted at different positions within the beam. Changes in the location of the Ti layer within the bridge thickness resulted in distinct structural profiles across both the width and length of the released microswitches. Profile differences were quantified using white light interferometric microscopy. As the Ti layer moved toward the bottom of the tri-layer stack, beam deformation increased, spring constants increased, and release times decreased. Results demonstrated that release times could be reduced by an order of magnitude when compared to all Au switches.