Nonlinear spring effect of tense thin-film torsion bar combined with electrostatic driving

•The nonlinear spring effect of the thin-film poly-Si torsion bar was characterised.•Theoretical model considered bending, stretching, and shear stress effects.•The spring constant of the torsion bar was observed to increase by 6.25 times.•The experimental results validate the accuracy of the theoretical model. Tense thin-film polycrystalline (poly-) Si torsion bars are advantageous in behaving as compliant springs against the twisting motion of electrostatically low-voltage-driven micromirrors. In this study, the nonlinear spring effect of the tense thin-film poly-Si torsion bar of a micromirror was characterised. The nonlinear spring effect of this torsion bar was explained theoretically, considering the bending and stretching effects in addition to the shear stress effect. The tensile stress induced in the thin-film poly-Si torsion bar is 300–400MPa, and the bar maintains its compliance during the rotation of the mirror and increases its stiffness to suppress the pull-in instability. The torsion bar exhibits a large hard-spring effect with the bending of the film, observed as an increase of the ringing frequency. The spring constant was experimentally observed to increase by 6.25 times. The theoretical results agree well with the experimental results regarding the magnitude of the nonlinear spring constant.