MEMS scanning laser projection based on high-Q vacuum packaged 2D-resonators

Abstract

Small size, low power consumption and the capability to produce sharp images without need of an objective make MEMS scanning laser based pico-projectors an attractive solution for embedded cell-phone projection displays. To fulfil the high image resolution demands the MEMS scanning mirror has to show large scan angles, a large mirror aperture size and a high scan frequency. An additional important requirement in pico-projector applications is to minimize power consumption of the MEMS scanner to enable a long video projection time. Typically high losses in power are caused by gas damping. For that reason Fraunhofer ISIT has established a fabrication process for 2D-MEMS mirrors that includes vacuum encapsulation on 8-inch wafers. Quality factors as high as 145,000 require dedicated closed loop phase control electronics to enable stable image projection even at rapidly changing laser intensities. A capacitive feedback signal is the basis for controlling the 2D MEMS oscilla tion and for synchronising the laser sources. This paper reports on fabrication of two-axis wafer level vacuum packaged scanning micromirrors and its use in a compact laser projection display. The paper presents different approaches of overcoming the well-known reflex problem of packaged MEMS scanning mirrors

Analysis of capacitive sensing for 2D-MEMS scanner laser projection

“…1(a) with a mirror aperture size of 1mm and resonant frequencies of approximately f g = 0.5kHz for the slow and f s = 17.8kHz for the fast axis. A detailed treatise on MEMS design and fabrication of the scanner can be found in Hofmann et al 2 The presented model can easily be adopted to other micromirrors that use this capacitive sensing system by replacing the calculated capacitance characteristic of the implemented measurement electrodes. The electrostatically driven micromirror is gimbal mounted and exhibits stacked vertical combs for both oscillation axes as well as for driving and sensing.…”

MEMS Mirror for Low Cost Laser Scanners

“…This allows the MEMS mirror to accumulate driving energy over many thousand oscillation cycles. Electrostatically driven MEMS mirrors with Q-factors as high as 145,000 have already been demonstrated [4]. Hence, the low-cost LIDAR MEMS scanning mirror will be fabri-cated in a dual layer thick poly-silicon process.…”

Resonant biaxial 7-mm MEMS mirror for omnidirectional scanning

“…A suitable and technologically welltried alternative has been found in electrostatic actuation based on vertical comb drives in combination with hermetic vacuum encapsulation. 34 Vacuum encapsulation effectively reduces viscous damping and, thereby, enables the achievement of large oscillation amplitudes of resonant MEMS mirrors. 20,32 Considerable progress was later made by development of a wafer-level vacuum packaging process of electrostatically driven MEMS scanners with stacked vertical comb drives.…”