A large rotational angle micromirror based on hypocycloidal electrothermal actuators for endoscopic imaging

The paper presents a large rotational angle micromirror base on hypocycloidal electrothermal actuators for circumferential endoscopic imaging. The micromirror consists of a double-side Cr/Au coated high reflective mirror plate (1mm by 0.8mm) laterally supported by two hypocycloidal electrothermal actuators on both sides (Fig. 1(a)). In our design, 1µm PVD Al deposited on 2µm single crystal silicon (SCS) forms a bimorph microstructure with the length of 800 µm and the width of 60µm. Four bimorph structures were staggerly connected in parallel to form a hypocycloidal electrothermal actuator. In this configuration, a metal layer was on a silicon backbone in one bimorph structure while the metal layer was deposited below the silicon backbone in adjacent bimorph structures (Fig. 1(b)). Since the radius of curvature of each bimorph structure is the same, the deflection of each structure is the same. Hence the rotational axis keeps still and there is no lateral shifting effect. Simulations via finite element analysis (FEA) show that the mechanical deflection angle of a micromirror significantly increases by using this actuator design. 141.2° was found in the design with fully double-side Al coated actuators (Fig. 2(a, b)) and 68.6° was found in the design with only frontside Al coated actuators (Fig. 2(c, d)). Micromirrors were fabricated by a post-CMOS MEMS process on 8 inches SOI wafers. An optical microscopic image and a scanning electron microscope (SEM) micrograph of a released micromirror are shown in Fig. 3(a) and (b), respectively. However, so far we have not successfully patterned Al layer below the SCS layer as part of the actuator and therefore only micromirrors equipped by frontside Al coated actuators were experimentally characterized (Fig. 4). ∼35° mechanical deflection was achieved by 2.6 V DC input voltage (Fig. 5). It has a discrepancy in comparison in comparison with the FEA simulation. −3dB cutoff frequency was found to be about 29 Hz as the large signal frequency response (Fig. 5). Current-voltage relationship of an electrothermal actuator is also shown in Fig. 5. A series of frames from a video of a switching micromirror shows various tilting angles of the micromirror under a sinusoidal drive signal with the amplitude of 2.6 V was still with absence of microstructures with backside Al coated, the concept of achieving large deflection angle by using hypocycloidal electrothermal actuators has been demonstrated. Both FEA simulation and experimental results prove the capability of the Single-axis rotational micromirror device.