Waferlevel Vacuum Packaged Microscanners: A High Yield Fabrication Process for Mobile Applications

Scanning micromirrors are promising candidates for a broad range of different mobile projection and imaging applications, such as compact laser based microdisplays for the integration in mobile phones and PDA’s, or miniaturized LIDAR devices for object detection in cars. As to the fact that these MOEMS (Micro Optical Electro-Mechanical Systems) devices are made for mobile use, they have to withstand even harsh environmental conditions, such as dust and change in temperature as well as humidity while power consumption must be kept low to guarantee an acceptable battery lifetime. For inertial MEMS, like gyroscopes and accelerometers, this is established by hermetic encapsulation on waferlevel [1, 2]. Therefore, a silicon wafer with several microns deep cavities is bonded on top of a MEMS wafer by standard waferbonding technologies (anodic, eutectic, or glass frit bonding). Subsequent to the waferbonding a dicing process separates the dies [2]. Waferlevel packaging not only protects sensitive MEMS devices, it also gives the ability to guarantee a vacuum environment, which reduces air damping and thus increases the amplitude of resonating MEMS actuators. However, in the case of MOEMS devices, specifically microscanners, a waferlevel vacuum encapsulation is a big challenge. In this case, light has to be transmitted through the package onto the mirror plate. Due to reflection on the mirror it passes the package a second time before it reaches the projection area. Thus, the package must possess perfect optical quality to minimize image errors due to light beam distortion. Besides the optical requirement, the scanners must meet extraordinary high mechanical requirements. For example, the optical resolution of laser projection systems based on scanning micromirrors mainly depends on the mirror plate diameter D, the mechanical scan angle of the mirror plate _ and the scan frequencies fres (see Table I) [3, 4]. For example, to project images with a standard display format (e.g. VGA), a scanner with a typical mirror plate diameter of D = 1mm must be operated at a frequency of at least fres = 15.7 kHz and at the same time must reach a mechanical scan angle of Θ = 7.5 deg. ABSTRACT1