Biaxial tripod MEMS mirror and omnidirectional lens for a low cost ...

2 Design and fabrication of the omnidirectional lensMany different lenses and system designs have been simulated and investigated with special focus on maximizingmeasurement range and minimization of straylight. The final design with a diameter of 60mm was realized bydiamond turning in glass (Figure 4).Fig. 4. Investigated optical lens design (left) and fabricated omnidirectional lens (partially gold coated)3 MEMS mirror designThe MEMS mirror has to comply with the following specifications [3]:mirror diameter = 7mm (required in order to fulfil the measurement range of the sensor)circular scan trajectory (required to enter the circular aperture of the omnidirectional lens)mechanical tilt angle = +/-15 degrees in each axis (required to enable the compact sensor size)For a MEMS mirror these are absolutely extreme specifications. A mirror aperture size of 7mm means a high massmoment of inertia so that only resonant actuation can be considered. But even then an electrostatic actuator wouldrequire adversely high driving voltages to achieve such large deflection angles. Since fabrication of piezoelectricdrives can not as yet be considered mature and since realization of an electromagnetically driven mirror with theneed for mounting of large permanent magnets would lead to an unacceptably large and expensive chip leveltailored solution a simple electrostatic actuation concept has finally been choosen. However, to enable such a largemechanical tilt angle of +/- 15 degrees a high Q-factor is necessary which can be achieved by vacuum encapsulationonly. For the MiniFaros project that means that a wafer level vacuum packaging process has to be applied in order tominimize packaging costs. The next problem that arises is the large stroke if a mirror of 7mm is tilted by +/- 15degrees. If a vacuum package is provided then the cavity needs to be deep enough to allow a stroke of roughly 1mm– provided that only the mirror is tilted. However, a biaxial mirror is required and in a standard gimbal mountconfiguration the actuator would easily generate a much larger stroke. Therefore, gimbal mount mirror designs werediscarded and so another biaxial concept had to be found. To enable a circular trajectory a resonant MEMS mirror isrequired that comes with two orthogonal axes of identical resonant frequency. A solution for this has been found ina tripod MEMS mirror design consisting of a mirror plate that is suspended by three identical bending beamsseperated from one another by rotation of 120 degrees. Finite Element Analysis (FEA) has shown that to keepdynamic deformations of such a large mirror sufficiently low a 500µm thick mirror plate is necessary. The result ofa modal analysis of that tripod mirror showing the natural frequency modes of the two orthogonal axes is shown inFigure 5. Based on a thickness of the bending beams of 40µm both axes of the tripod oscillate at 600Hz enabling theMiniFaros laser scanner to scan the whole scenery of 360 degrees 600 times per second. Interlaced time of flightsampling is thus an interesting feature in comparison to conventional motorized low frequency scanning technology.Additional FEA has also been used to investigate the mechanical stress in the bending beams at the required tiltangle. The simulations show that even a deflection by +/- 20 degrees should not lead to any damage of thesuspensions.