Reliable MEMS Actuators for Driving Large Microsystems

Nowadays many consumer electronic devices are using small engines (micromotors). Thanks to technological innovation, the components can be imbedded into numerous portable systems. The key factors for a nomadic integration are: reduction of power consumption, reliability, durability and manufacturing cost. From the perspective of an always denser integration, conventional fabrication techniques of motors are reaching their limits (windings, size of the permanent magnet). The technologies developed for the micro-electronics (thin film processes) offer new possibilities to create microscopic size motors. The potential for innovation in this field being very wide, many new concepts are based on this technology. Lots of research centers are trying to find new principles of actuation to increase the overall efficiency of motors. With the challenge of replacing a small "classical" motor by a micromotor based on thin film technologies, the electrostatic actuation is going to have a great potential in the future. Many problems are related to the compatibility of both technologies in terms of coexistence. This is mainly due to problems of dimensions and associated tolerances. The challenge of this research was to propose a solution for the creation of a micromotor including the following concepts: usable mechanical interface between an outer gear and the mechanical output torque of the micromotor (about 0:5 µNm), moderate energy consumption, relatively small chip size in relation to the required performances and finally ability to be easily implemented in a mechanism (module). This thesis presents the different aspects of the concept, design, simulation, fabrication and characterization of a MEMS based motor using silicon micro-technologies. The micromotor is constituted of a fixed base (stator) with an electrostatic actuator (force generator), a rotor and a protective cover (encapsulation). The principle of the electrostatic actuation is based on a comb inter-digitized configuration. A first interface (ratchet system) was developed to allow for the conversion of the back and forth movement of the linear type actuator to a circular motion of the silicon rotor. A challenge was to create an actuator able to deliver an electrostatic force of 500 µN with a applied potential of 40 V. To obtain this force, the actuator itself has an active area of over 10 mm2. The first concept has two large actuators representing 60% of the total area of the stator. In the second concept, the occupied area of the three actuators represents 40 % of the total area of the stator. Two systems were created during this work, based on the same principles of actuation and of actuator/rotor interfacing. The first system uses a crank mechanism to interface the silicon rotor and a metallic pinion. The encapsulation was achieved via a metal support on which a plastic lid was fastened. These two pieces were fabricated by conventional machining (milling) techniques. The micromotor module was made by assembling the different components. The second system was made based on a direct interface type (direct link between internal teeth of the silicon rotor and outer teeth of a metallic gear). The encapsulation concept uses only micro-technologies. The cover was made of a glass substrate on which a silicon spacer was bonded (anodic bonding process). The motor module was then achieved by assembling a silicon base (stator), a silicon rotor and a silicon/glass cover. In accordance with the specifications, the obtained results are very promising. The electrostatic force generated by the actuator is in the order of few hundreds of micronewton. The power consumption is in the order of few microwatts. Many innovative concepts have been studied and manufactured. Some of those have been patented, showing the interest of these new ideas. Experiments have demonstrated the functionality of a MEMS engine.


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