Characterization of proton irradiated copolymer thin films for microelectromechanical system applications

Abstract

The electrostrictive response of proton irradiated poly vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] thin films, deposited on silicon (Si) substrates, has been characterized. By applying an ac field to the copolymer films, the induced surface displacement of the film along the thickness direction was measured using a laser interferometer. After the proton irradiation, the electric field induced strain response of the copolymer film is enhanced. Since the copolymer films are laterally clamped by the rigid substrate, the films cannot vibrate freely. At a proton dose of 50 Mrad, the effective electrostrictive coefficient M[sub eff,33] of P(VDF-TrFE) thin film on Si is 0.07 10[sup -18] V²/m² which is ~25 times smaller than that of bulk sample under the same irradiation dose (M[sub 33]=1.76x10[sup −18] V²/m²). Using bulk micromachining to etch away most of the Si substrate, an actuator in the form of a suspended membrane was fabricated. The effective electrostrictive coefficient at the center of the membrane M[sub eff,33] increased to 0.67x10[sup −18] V²/m² due to the weakening of the substrate clamping effect. The resonance characteristics of the actuators based on these irradiated copolymer films were studied.