1. Field of Invention
The present invention relates to micro-mechanical optical systems and more particularly to micro-mechanical actuators.
2. Description of Related Art
Micro-actuators have a wide variety of uses, from optical switching systems to modern displays. FIGS. 1A and 1B show a side view and top view respectively of a conventional electrostatic micro-actuator 10 using torsion beam hinges 161 and 162. Micro-actuator 10 includes a rigid platform 12, upon which is attached a reflective surface. Stationary electrodes 141 and 142 are attached to a substrate 18. In a neutral non-activated position, platform 12 is oriented horizontally above stationary electrodes 141 and 141 with a gap spacing dx. A voltage is applied between stationary electrodes 141 or 142 and platform 12, causing platform 12 to rotate about an axis of rotation AR towards the activated stationary electrode. Platform 12 continues to rotate until it makes contact with the activated stationary electrode, defining a maximum tilt angle θx.
There are a number of problems associated with the micro-actuator of FIGS. 1A-B.
Firstly, the micro-actuator has only 3 stable tilt angles: tilt angle=0 degree with no activated stationary electrodes, and tilt angle=±θx for stationary electrode 141 or 142 being activated. The tilt angle varies in proportion to the applied voltage up to a third of the maximum tilt angle θx. Beyond this angle, platform 12 snaps down to the activated stationary electrode. Contact between platform 12 and stationary electrode 141 or 142 causes the two surfaces to stick disadvantageously.
Secondly, the maximum tilt angle θx is dependent on the gap spacing dx. There is a tradeoff between electrostatic efficiency and the maximum tilt angie θx. The larger the gap spacing dx, the larger the maximum tilt angle θx, but the lower the electrostatic efficiency, which implies a higher operating voltage. In fact, the electrostatic force is the least efficient during its initial third of the maximum tilt angle θx. Beyond this angle the electrostatic force becomes more efficient but the tilt angle becomes uncontrollable.
Thirdly, the generated electrostatic force is more concentrated at the extreme edge of platform 12, thereby concentrating the stress load in this area. This can adversely affect the reflective surface attached to platform 12.
U.S. Pat. No. 6,657,759 describes a cantilever type microactuator using non-contact electrodes, as shown in FIG. 2A. A platform 32 is connected to an underlying substrate 24 via support 28. A snap-in electrode 44 may be provided such that the application of a voltage between it and platform 32 may be used to tilt it, perhaps in contact with substrate 24. A hold electrode 40 is configured as a comb structure with multiple teeth 41,42, and 43. Teeth 41-43 are configured at different heights above the substrate and may be used to achieve different tilt orientations of platform 32. Hold electrode 40 is disposed on substrate 24 so as to avoid direct contact with platform 32. Thus, the application of a voltage V1 between hold electrode 40 and platform 32 results in an electric field 52 that holds plafform 32 in a tilted position at maximum tilt angle θx without contact with substrate 24 or a hard stop.
This configuration provides additional stable tilt angles and eliminates contact between electrodes. However, its maximum tilt angle is limited by the substrate. In addition, the small surface area of hold electrode 40 has a relatively small electrostatic force.
U.S. Pat. No. 6,154,302 describes an actuator with a floating hemisphere, as shown in FIGS. 3A-C. Hemispherical body 61 with flat face portion 64 and reflection layer 71 is encapsulated in an enclosure defined by supporting member 66 with hemispherical cavity 68, spacer 69 and base plate 78. The enclosure is filled with dielectric liquid 75. On the spherical face of hemispherical body 61, charging regions 83a and 83b are arranged, having different electric charging characteristics. Driving electrodes 86 are provided at the bottom portion of support member 66. The arrangement of driving electrodes 86a-e is shown as a bottom view in FIG. 3C. By proper activation of driving electrodes 86a-e, interacting with charging regions 83a and 83b, hemispherical body 61 can be rotated, as shown in FIG. 3B.
There are a number of problems associated with this floating hemispherical type microactuator.
Firstly, the axis-of-rotation AR is not fixed, causing charging regions 83a and 83b of hemispherical body 61 to come in contact with hemispherical cavity 68.
Secondly, the maximum tilt angle θx is seriously limited by base plate 78 at point 90 as shown in FIG. 3B.
Thirdly, this is a complex design, requiring the microactuator to be encapsulated in a fluid, as well as having an optical window.
Therefore, there is a need for new types of micro-actuators with a larger number of distinct tilt angles with a larger maximum tilt angle, which also avoid mechanical contact of the electrodes, generate higher electrostatic forces, and reduce platform stress loads.