Biaxial tripod MEMS mirror and omnidirectional lens for a low cost wide angle laser range sensor U. Hofmann, Fraunhofer ISIT Itzehoe M. Aikio, VTT Finland Abstract Low cost laser scanners for environment perception are a need to facilitate ADAS integration into all vehicle segments. To fulfill the need for mass-producible compact low cost laser range sensors MEMS mirrors in combination with replicable low cost plastic optics are expected to be suitable components. This paper describes concept, design, fabrication and first measurement results of a compact omnidirectional scanning system based on an omnidirectional lens and a biaxial large aperture tripod MEMS mirror. A hermetic vacuum wafer level packaging process of the resonant MEMS mirror is essential to meet automotive requirements and to achieve the required large total optical scan angles of 60 degrees in both scan axes. 1 Introduction In order to seriously reduce the number of traffic accidents Intelligent Vehicle Safety Systems (IVSS) need to be introduced into all vehicle segments. Today IVSS are limited to a small part of the premium car segment. Future safety systems must be made affordable to penetrate all vehicle segments, since small and medium size cars are the ones dominating the road traffic and thus most of the accidents. Hence, there is a strong demand for low cost laser scanning sensors. A wide angular range and high angular resolution are key-features that laser scanning range sensors offer. But so far they have been based on expensive bulky servo motor driven scanning mirrors. It is one of the major objectives of the European funded FP7-project MINIFAROS to replace the expensive conventional scanning mirrors by low cost mass producible components [1]. MEMS mirrors batch fabricated on 8-inch silicon wafers in combination with replicable plastic optics present most promising candidates and therefore are further discussed in this paper. Fig. 1. Omnidirectional lens (right picture) and biaxial 7mm tripod MEMS mirror (left picture) are core components of the low cost MiniFaros laser scanner
2 Optical Sensor Concept It is one of the goals of the MINIFAROS project to develop a laser scanner that can serve many different ADAS applications. An optical concept with an omnidirectional field of view was therefore choosen. The divergent exit beam of a fibre coupled NIR pulse laser diode is collimated and then directed on a large MEMS mirror which reflects and scans the beam on a circular trajectory along the input facet of an omnidirectional lens (see Figure 2). After passing several internal beam forming reflections the laser beam exits the omnidirectional lens in horizontal direction. This arrangement allows to scan the beam in a horizontal plane within an angular range of 360 degrees. When the emitted pulse hits a target the laser pulse is partially reflected back and enters the omnidirectional lens. The MEMS mirror then reflects the return pulse to an avalanche photodiode. Two fundamentally different optical concepts are being investigated: A biaxial system and a coaxial system (Figure 3). In the biaxial system the optical sender path and the receiver path are totally discoupled. While the sender path uses the front side of the MEMS mirror the back reflected laser pulse passes a second omnidirectional lens and then is reflected by the reverse side of the MEMS mirror. This configuration requires implementation of two different designs of omnidirectional lenses. The second system configuration, called the coaxial sensor, uses only one omnidirectional lens for both, sender and receiver path. Both optical paths use the same side of the MEMS mirror in a coaxial alignment. These two laser scanner concepts have their specific advantages and drawbacks. The biaxial laser scanner is by nature less sensitive to stray light than the coaxial laser scanner. Since the laser scanners operate with a highly sensitive detector, cross talk between the channels must be reduced, as much as possible, in order to prevent saturation of the receiver. The biaxial laser scanner however results in a slightly larger system size and is expected to be more complex with respect to alignment and the required alignment accuracy. So, finally a coaxial system approach was chosen. Fig. 2. Omnidirectional scanning concept based on omnidirectional lens and circle scanning MEMS mirror APD laser diode lens receiver lens MEMS mirror sender lens laser diode biaxial configuration coaxial configuration Fig. 3. Optomechanical concept of the laser scanning sensor showing the biaxial concept (left) and the coaxial concept (right)
2 Design and fabrication of the omnidirectional lens Many different lenses and system designs have been simulated and investigated with special focus on maximizing measurement range and minimization of straylight. The final design with a diameter of 60mm was realized by diamond turning in glass (Figure 4). Fig. 4. Investigated optical lens design (left) and fabricated omnidirectional lens (partially gold coated) 3 MEMS mirror design The 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 mass moment of inertia so that only resonant actuation can be considered. But even then an electrostatic actuator would require adversely high driving voltages to achieve such large deflection angles. Since fabrication of piezoelectric drives can not as yet be considered mature and since realization of an electromagnetically driven mirror with the need for mounting of large permanent magnets would lead to an unacceptably large and expensive chip level tailored solution a simple electrostatic actuation concept has finally been choosen. However, to enable such a large mechanical tilt angle of +/- 15 degrees a high Q-factor is necessary which can be achieved by vacuum encapsulation only. For the MiniFaros project that means that a wafer level vacuum packaging process has to be applied in order to minimize packaging costs. The next problem that arises is the large stroke if a mirror of 7mm is tilted by +/- 15 degrees. 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 mount configuration the actuator would easily generate a much larger stroke. Therefore, gimbal mount mirror designs were discarded and so another biaxial concept had to be found. To enable a circular trajectory a resonant MEMS mirror is required that comes with two orthogonal axes of identical resonant frequency. A solution for this has been found in a tripod MEMS mirror design consisting of a mirror plate that is suspended by three identical bending beams seperated from one another by rotation of 120 degrees. Finite Element Analysis (FEA) has shown that to keep dynamic deformations of such a large mirror sufficiently low a 500µm thick mirror plate is necessary. The result of a modal analysis of that tripod mirror showing the natural frequency modes of the two orthogonal axes is shown in Figure 5. Based on a thickness of the bending beams of 40µm both axes of the tripod oscillate at 600Hz enabling the MiniFaros laser scanner to scan the whole scenery of 360 degrees 600 times per second. Interlaced time of flight sampling 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 tilt angle. The simulations show that even a deflection by +/- 20 degrees should not lead to any damage of the suspensions.
Fig. 5. FEM modal analysis of the 7mm tripod MEMS mirror. The tilting modes of the two perpendicular axes both are at 600Hz. 4 MEMS mirror fabrication The electrostatically driven scanning micromirrors are fabricated on 8 inch silicon wafer substrates. Two 40µm thick polysilicon device layers are produced on top of a thermally oxidized silicon substrate applying epitaxial deposition. Each deposition step is followed by chemical mechanical polishing (CMP). Embedded between these two polysilicon device layers is a double silicon oxide layer that on one hand serves as a buried oxide hardmask and on the other hand is needed to electrically isolate a thin polysilicon interconnection layer that is embedded between these two layers of silicon oxide. Patterning of this interconnection layer is performed before deposition of the second oxide layer. The buried oxide hard mask is patterned before deposition of the second 40µm polysilicon device layer applying photolithography and dry-etching. Depending on the photomask layout this etching of the oxide layer either stops at the lower polysilicon device layer or at the buried polysilicon interconnection layer. This is an important feature for the subsequent 3D-etching of stacked vertical comb drive electrodes and furthermore, this is also important for getting a high flexibility in supplying different actuator regions with different electrical potentials. After deposition and CMP of the second thick polysilicon device layer a titanium-silver stack is sputtered on top and wet-chemically patterned to serve as high reflective mirror coating (Figure 6a). The front side structuring is finished by a DRIE etching process that defines the upper and lower comb electrodes, the mirror geometry as well as the bending beam suspensions. This patterning step uses a combination of a photoresist mask and the buried oxide hard mask described before. After turning the wafer, a further DRIE step etches and removes the parts of the silicon substrate underneath the MEMS actuator. The remaining thermal oxide layer generated at the process beginning is removed by HF vapor phase etching, which finally releases the MEMS devices (Figure 6b). This process offers large design flexibility. Besides the creation of stacked comb electrodes suspensions can be produced either with a thickness of 40µm or 80µm. The process offers the fabrication of a 80µm thick mirror plate free of stress inducing oxide layers while for other areas the two polysilicon layers remain vertically isolated by an intermediate oxide. In addition to vertical isolation an arbitrary number of laterally isolated areas such as driving or sensing electrodes can simply be produced by trench etching of the upper polysilicon layer. Each isolated area can be addressed by wires built in the buried interconnection layer. Depending only on the photomask layout the reverse side etch can be used to provide 500 micron thick reinforcement structures underneath the mirror to enable mirror sizes of several millimeters with low dynamic deformation. The micromirror fabrication process and the mirror layout are chosen in such way that the mirror actuator is always surrounded by a closed frame of polished polysilicon without any further topography. By doing so, standard wafer bonding technologies like anodic bonding, glass-frit bonding or eutectic bonding can be applied to hermetically seal the microstructures. In the first step of the wafer level vacuum packaging process a borosilicate glass wafer having 1.6mm deep cavities is bonded to the frontside of the MEMS wafer applying glass-frit bonding. Perfect flatness and minimum roughness of the optical windows in the glass wafer can be achieved by a patented glass molding process [4]. Thereafter, a second glass wafer with 1.6mm deep cavities is bonded to the reverse side of the MEMS tripod mirror wafer. This second glass wafer is coated with a thin structured titanium getter layer to enable permanent cavity pressure levels below 1Pa after thermal getter activation (Figure 6c). Figure 7 shows recent results of first fabricated and packaged tripod MEMS wafers.
a) b) c) glass-cap titanium-getter Fig. 6. Fabrication process of the vacuum packaged MiniFaros tripod MEMS mirror. The cavity depth is 1.6mm above and underneath the MEMS mirror. Glass wafers of such geometry are being fabricated by a unique glass forming process.
Fig. 7. First fabricated and wafer-level packaged tripod MEMS scanning mirrors Acknowledgement This work has been supported by the EC within the 7th framework programme under grant agreement no. FP7-ICT- 2009-4_248123 (MiniFaros). The partners thank the European Commission for supporting the work of this project. References [1] Fuerstenberg, K., F. Ahlers, Development of a Low Cost Laser Scanner the EC Project MiniFaros, Springer, Berlin Heidelberg, 2011 [2] Aikio, M., Omnidirectional Lenses for Low Cost Laser Scanners, Springer, Berlin Heidelberg, 2011. [3] Hofmann, U., J. Janes, MEMS Mirror for Low Cost Laser Scanners, Springer, Berlin Heidelberg, 2011. [4] Hofmann, U., et al., Wafer-level vacuum packaged micro-scanning mirrors for compact laser projection displays, Proc. SPIE, Vol. 6887, 2008. Ulrich Hofmann Fraunhofer ISIT Fraunhofer Strasse 1 Itzehoe Germany E-mail: ulrich.hofmann@isit.fraunhofer.de Mika Aikio Kaitoväylä 1, P.O.Box 1100 90571 OULU Oulu Finland E-mail: Mika.Aikio@vtt.fi Keywords: Laser scanner, MEMS, mirror, vacuum package, electrostatic drive, tripod design, time of flight, omnidirectional lens