Abstract
This paper reports on design, fabrication and characterization of high-Q MEMS resonators to be used in optical applications like laser displays and LIDAR range sensors. Stacked vertical comb drives for electrostatic actuation of single-axis scanners and biaxial MEMS mirrors were realized in a dual layer polysilicon SOI process. High Q-factors up to 145,000 have been achieved applying wafer level vacuum packaging technology including deposition of titanium thin film getters. The effective reduction of gas damping allows the MEMS actuator to achieve large amplitudes at high oscillation frequencies while driving voltage and power consumption can be minimized. Exemplarily shown is a micro scanner that achieves a total optical scan angle of 86 degrees at a resonant frequency of 30.8 kHz, which fulfills the requirements for HD720 resolution. Furthermore, results of a new wafer based glass-forming technology for fabrication of three dimensionally shaped glass lids with tilted optical windows are presented.
Highlights
The capability to deflect a laser beam at high speed has let resonating MEMS mirrors become attractive for imaging applications [1] and for application in laser projection displays [2,3] early on.MEMS scanning mirror based projection displays represent an elegant solution to overcome the limitation of insufficient screen size of portable electronic devices like mobile phones, digital cameras or media players
Electrostatic actuation based on stacked vertical comb drives as depicted in Figure 2, is the chosen driving concept, because it is fully compatible with hermetic wafer level vacuum encapsulation
Compact laser projection displays are of particular interest for automotive head-up displays as well as for consumer products like pico-projectors embedded in cell-phones or digital cameras
Summary
The capability to deflect a laser beam at high speed has let resonating MEMS mirrors become attractive for imaging applications [1] and for application in laser projection displays [2,3] early on. To meet the high-resolution requirements of modern displays, which is HD720 or higher, the MEMS mirror needs to oscillate at a frequency of at least 27 kHz, enabling the projection of 54,000 lines per second. Besides meeting the frequency requirements of the fast axis it is extremely challenging to design and manufacture a slow axis MEMS mirror capable of performing a saw tooth like scan at high duty cycle, low linearity-error and at a sufficiently large tilt angle [4]. Regardless of what concept is chosen, to meet the relevant display resolution requirements the MEMS scanning system needs to achieve very large deflection angles. The number of pixels N resolved by a scanned beam laser display is proportional to the total optical scan angle θopt (which is equal to four times the mechanical tilt angle amplitude θmech) and inversely proportional to the spot size on the projection screen. The theta-D-products required for different display formats are listed in Table 2 [5]
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