Variable focal length microelectromechanical lens

We discuss the characteristics of a novel microelectromechanical cylindrical focusing mirror (i.e. lens) having a focal length which is controlled by active and intrinsic stresses in the thin films composing it. The unactuated mechanical state of the mirror is determined by intrinsic stress levels in a SiN structural layer and a piezoelectric PZT actuator layer which are part of a doubly clamped cantilever beam. The stresses are controlled by the sputter-deposition parameters and annealing conditions. The stresses concentrate curvature in the thin central mirror portion of the beam and can give the mirror a minimum focal length of as little as 100 mu m. The focal length is increased during actuation of the PZT layer by inducing a differential tensile stress which reduces the curvature. Theoretical results are presented which predict the maximum focal length range and operating speed for a given device design. They indicate that operating frequencies of greater than 100 kHz can be achieved for a device with a focal length range of 100 mu m to infinity. Experimental results will verify the operating principal of the proposed device.