Design and fabrication of a cascaded electro-thermal bimorph actuator

Here a cascaded bimorph actuator is proposed by integrating several novel actuation units to accumulate the vertical displacements. Each actuation unit comprises two types of bimorph beams and a constraint bar, and both beams will stretch outward with respect to the constraint bar while heating. In comparison with other three conventional bimorph actuator designs at the same device size, it is shown that the proposed design can provide larger vertical displacement. The proposed cascaded bimorph actuator is fabricated by surface micromachining technique and released by XeF₂ silicon isotropic etching. Whole suspended structure consisting of the polysilicon and the aluminum is around 510 × 400 μm² with four actuation units. The resistance is about 650 Ω. In testing, the fabricated device is shown to provide reversible vertical displacement of 22.5 μm at 4.5 V, and the operating temperature is measured by an infrared thermal microscope (InfraScope II, QFI). The calibrated maximum temperatures are compared with simulated results by ANSYS 6.0 in good agreement. It is found that the maximum temperature is 147 °C when the input voltage is 4.5 dc volts, and the maximum temperature is below 400°C even at 10 V. However, it is found that the residual stress in the suspended structure will affect the vertical displacement of the device.