Performance enhancement of polysilicon electrothermal microactuators by localized self-annealing

Microsystems using electrothermal bent-beam microactuators have been demonstrated for a variety of applications including optical attenuators, RF switches, and micro positioners for scanning microscopy, creating an important need for information on the longevity of these devices. This paper reports on the lifetime pulse testing results of polysilicon actuators. Devices have been operated up to 60 million cycles without failure, but over tens of millions of cycles the displacement for a given actuator design can either increase or decrease depending on the geometry of the device and operating conditions, both of which are related to temperature and stress of the structural members. In certain cases actuator displacement increased by more than 50% (up to 100%) of the initial displacement, while for other cases it decreases by more than 25%. Polysilicon grain transformations are observed over extended operation at high temperatures. Performance changes are correlated to material properties using SEM and TEM images.