Microactuators and their application in micro-opto-electro-mechanical systems (MOEMS)
Microactuators are active elements for microelectromechanical systems (MEMS), and are used to provide force to move the MEMS device in the way required. Therefore, their force characteristics, displacement capabilities and velocity ranges are of importance, and require to be investigated. Scratch drive actuators, thermal actuators, and comb drive actuators have separately been characterized, improved, and modified in this thesis. Asymmetric thermal actuators are current driven actuators that can generate relatively high force and can be operated bi-directionally. We have designed a novel structure that has a higher displacement than traditional asymmetric actuators. The increment of maximum displacement is about 20 % above the displacement of the ordinary asymmetric thermal actuators. Detailed electro - thermal heat dissipation and expansion - deflection mechanical analyses has been performed to advance the idea. Device prototypes have been designed and the fabricated devices were tested; the experimental results show a good match to the theoretical analysis. Scratch drive actuators (SDAs) have been modelled and characterized both on travel and force performances. Long linear travel SDAs have been designed and the fabricated devices characterized by using a high-speed camera. Detailed motion has been recorded. A theory for flexing of SDA plates has been developed. Voltage - step size relation has been obtained by theoretical analysis. Theoretical and experimental results have then been compared. Force characteristics of single and multi-plate SDAs driven by different voltages have been measured by means of micro box springs. The SDA typical step size has been measured to be 7 nm at 60 V and 100 HZ driving condition. Typically, a 4 stage SDA driven with 200 Volts produces a force of 850 µN. Comb drive actuators are commonly used in resonators, which need high Q factors. However in some applications, such as optical choppers, they require low Q factors so that they can be operated over a large frequency range. We have designed a comb drive actuator with spring that can be operated from a few Hz up to 5 kHz. The static and dynamic testing and theoretical analysis have been undertaken in this thesis. A variable optical attenuator (VOA) has been designed, and fabricated by surface micromachining using PolyMUMPs (Polysilicon Multi-User MEMS Processes) foundry process. The principle is simply interrupting the light beam by a vertical microshutter. An array of SDAs have been used to drive the microshutters. Microhinges are employed to build the vertical microshutter. Stress-induced beams are used to self-assemble the microshutter. Optical simulation of the VOA has been performed using Rayleigh-Sommerfield diffraction theory. Devices have also been tested with single mode optical fibres. Testing results show a dynamic range of 34.2 dB. The VOA structure was driven at a speed of 1.6 µm/s at 150 volts and 100 Hz driving condition. An optical chopper has been realized by a pair of comb drive actuators driving two shutters. Two shutters are employed to double the response time. The device has been designed and fabricated using SOIMUMPs (Silicon On Insulator Multi-User MEMS Processes) foundry process. Completely design, simulation and testing of the chopper has been undertaken. The attenuation range of chopper has been measured to be from - 1.4 dB to - 29 dB. The response time of the chopper has been measured to be 90 µs.