3 degree-of-freedom resonant scanner with full-circumferential range and large out-of- plane displacement

Microsystems-based scanning technologies can achieve deflection angles of several tens of degrees and translational displacements of a couple hundred microns. Emerging applications need performance with multi-fold greater torsional and translational motion. A compliant lever-based mechanism is introduced into the comb-drive actuators of a MEMS resonant scanner to achieve full-circumferential range and large out-of-plane displacement at ambient pressures. A 1.5 mm diameter mirror is demonstrated that generates 494° total deflection angle and 561 μm translational displacement at either 853 or 956 Hz and either 100 or 90V, respectively. At 40V, an optical scan angle of ~200° and translational displacement of ~310 μm are achieved. ©2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

reduced scan consumption, and high actu parametrical r large out-of-p 2. Scanner d

Schema
A schematic diameter refle comb-drive ac translational m corner of the resistance to springs are us large deflectio reflector. ectrostatic ac ntegration, desp e requirements nner design tha n >500 μm in a f the 3-DOF r orted on oppos de torque. Disp 1(b) and 1(c). ed as a pivot to g, Fig. 1 where J M and b Mr are the inertia of the reflector and the damping coefficient in torsional mode, respectively, and k mt is the equivalent torsional spring constant.
where J L and b L are the inertia and damping coefficient of the lever, k L is the spring constant of the torsional spring, T es is the electrostatic torque, k me is the equivalent extension spring constant of a single folded-beam spring, and L o is the distance from the pivot of the lever to the central axis of the mirror. The equation for out-of-plane translational motion is described where M is the mass of the reflector, b Mt is the damping coefficient of the reflector in translational mode, and k me is the equivalent extension spring constant of the folded-beam spring. The structure of the 3-DOF parametrical resonance scanner is optimized using a finite element model (FEM) to achieve stable motion in the torsional and translational modes and to minimize crosstalk from mechanical and electrical coupling. The results of the FEM modal analysis are shown, Figs. 3(a) and 3(b). The first and second modes are torsional and translational, respectively. In a parametric resonance system, large amplitude motion is achieved using a drive frequency that is twice that of the eigenfrequency [28]. This model is used to tune the resonance frequencies of the structure and prevent crosstalk. μm SiO 2 layer is deposited on the front and back side of the substrate, respectively. (C) DRIE is performed to etch the exposed silicon and handle layers. (D) A BHF etch process is performed to remove the SiO 2 layers, and IPA is used to release the movable structures. Electron beam evaporation is performed to coat the reflector with a ~60 nm layer of Al.

Scanner fabrication
The 3-DOF parametrical resonance scanner is fabricated using a two masks process. Plasma enhanced chemical vapor deposition (PECVD) is used to deposit silicon dioxide (SiO2) on a silicon-on-insulator (SOI) wafer, Fig. 4(a). Hard masks are used to pattern the device structures and protect the substrate from scratches and contamination. The top and bottom surfaces are patterned with lithography using top structure and backside masks, respectively, Fig. 4(b). Plasma etching is used to remove the PECVD deposited SiO2 layers, and deep-reactiveion-etching (DRIE) is performed to etch the exposed silicon and handle layers, Fig. 4(c). A buffered hydrofluoric acid (BHF) etch process is used to remove the SiO2 layers, and an isopropyl alcohol (IPA) rinse, and a dry process is performed to release the movable structures, Fig. 4(d). An electron beam evaporation process is used to deposit a ~60 nm layer of aluminum (Al) on the bare surface of the silicon device layer to form the reflector surface. This thickness is adequate to achieve a reflectivity >85%. A scanning electron microscope (SEM) image of a microfabricated device is shown, Fig. 5(a). The chip size is 3.7 × 2.8 × 0.466 mm 3 (L × W × H). Structural details of the torsional and folded-beam springs are shown, Figs. 5(b) and 5(c).

Scanner setup
An optical profiler (New View 5000, Zygo) is used to characterize the quality of the reflector surface. A 635 nm laser beam with ~0.7 mW of power and 0.5 mm spot size is deflected to measure the optical scan angle 2φ, Fig. 6(a). The distance between the reflector and the projection plane is 15 cm. A radius of curvature of 3 m with a root mean square (rms) roughness of 2 nm is measured. The dynamic performance of the scanner is determined by sweeping the drive frequency to electrostatically activate resonant modes. This parameter is defined as the interior angle of the arc scan α projected in a plane perpendicular to the torsional axis of the reflector. Only one edge of the substrate is mounted to prevent the deflected beam from being obstructed when scanning in the torsional mode. Squeeze damping in the translational mode is also reduced, Fig. 6(b). A displacement sensor is used to measure the out-of-plane travel. Reflector motion at resonance is shown with translational displacement >466 μm and ± 90° mechanical scan angle, Figs. 6(c) and 6(d), respectively. The pattern generated by the reflected laser beam demonstrates a full circumferential rotational scan angle, Fig. 6(e). The diameter of the projected circle is 9 mm, and is determined by the distance between the reflector and the projection plane and the incident angle of the beam.

Torsional mode
The 3-DOF scanner exhibits profound non-linear behaviors when activated in torsional mode. Previously reported MEMS-based parametric oscillators show super-harmonic and subharmonic resonances with drive frequencies near 2ω r /n, where ω r is the resonant frequency of the structure and n is a positive integer [28]. Our device exhibits not only a fundamental resonance near the natural resonance frequency in the torsional mode (ω rt ) and the subharmonic resonance of order 1/2, but also at other sub-harmonic resonances. The optical scan angle at the fundamental resonance is shown, Fig. 7(a). The frequency response at subharmonic resonance of order 1/2 shows a stiffness-hardening-softening-mixed behavior resulting in an optical scan angle of 494° with a sine-wave drive signal of 1.707 kHz and 100V, Fig. 7(b).
This maximum value is achieved at a jump frequency with an upsweep where the drive signal is near twice the resonance frequency. At this drive frequency, the amplitude of the response changes abruptly. With a sine-wave drive signal near the sub-harmonic resonance near 10ω rt at 80V, a typical stiffness-softening phenomenon is exhibited, and a large scan angle up to 205° is achieved, Fig. 7(c). With a sine-wave drive signal approaching the subharmonic resonance near 62ω rt at 80V, the upsweep and downsweep frequency response curves coincide, and the unstable region disappears, Fig. 7(d). Other sub and super-harmonic resonances are not observed at ambient pressures. The relationship among the jump frequency, voltage, and maximum optical scan angle is shown using a square wave drive signal with 50% duty cycle near twice the natural frequency in the torsional mode, Figs. 8(a) and 8(b). A large scan angle of ~200° can be achieved at a low voltage of 40V.

Translational mode
The 3-DOF scanner also exhibits profound non-linear behavior when activated in translational mode. Similar to the torsional mode, the fundamental, sub-harmonic resonance of order ½, and other sub-harmonic resonances are observed. The frequency response curves are shown, Figs. 9(a)-9(d). The ultra-large motion is attributed a stiffness-softening behavior when the device is driven near resonance. The largest out-of-plane motion is obtained at the sub-harmonic resonance of order 1/2. The relationship among the jump frequency, voltage, and maximum displacement with a square-wave signal and 50% duty-cycle sweeping frequencies near 2ω ro , the natural frequency of the out-of-plane translational mode, is shown, Figs. 10(a) and 10(b). A maximum vertical displacement of 561 μm is obtained with a downsweep in frequency using a square-wave with 50% duty-cycle near 1912 Hz at 80V. A displacement of ~310 μm is obtained with a low drive voltage of 40V.  Devices are designed and fabricated using different reflector dimensions and frequency response to demonstrate the flexibility of this approach. A 0.7 mm diameter reflector achieves out-of-plane translational displacement of ~320 μm at 4.405 kHz and torsional scan with ~90° mechanical scan angles at 8.425 kHz is shown, Figs. 11(a) and 11(b), respectively. A 3 mm diameter reflector performs out-of-plane displacement of ~780 μm at 652 Hz and torsional scans with ~90° deflection at 188 Hz is shown, Figs. 11(c) and 11(d). A 3D device that performs switchable horizontal/vertical 2D scanning is shown, Fig. 11(e). A ring-shaped Lissajous pattern is generated using a horizontal 2D scan, Fig. 11(f). The densely filled illumination demonstrates promise for future use in OCT, wide-field fluorescence, and photoacoustice endoscopes.

Discussion
Here, we report a flexible design for a compliant lever-based mechanism that significantly increases the torsional and translational range for a comb-drive resonant MEMS scanner. This design results in an ultra-large scan range in ambient pressure with little risk for mechanical fracture. The comb-drive devices demonstrated show high performance, including extraordinarily large scan amplitudes and fast speeds with low power consumption, compact size, and ease of fabrication. Although relatively high voltages are required to obtain large scan range, vacuum packaging was not required. This level of performance is very promising for integration into miniature systems in optical applications, such as panoramic imaging and surround-view monitoring.
The actuator is located between the chip frame and the lever-arm, and generates only a fraction of the motion of the mirror. This lever-based compliant mechanism can be integrated into resonant MEMS scanners that use other actuation techniques, such as thin-film piezoelectric and electromagnetic actuation, to achieve ultra-large scan range at low drive voltages. The devices demonstrated have a broad frequency response, and specific drive frequencies can be selected to achieve a high-frame-rate Lissajous scanning pattern for imaging applications.
The devices were designed with a large open area to allow air to pass through easily for reduced damping. This effect limited the performance of our previous devices that were designed with a high fill-in factor for a more compact geometry. A previous parametric resonance scanner designed with a unique geometry to match the dual-axes confocal architecture had limited scan angle [6]. Another scanner was designed for multi-photon microscopy that emphasized large-amplitude translational motion only [7].
The reflector roughness of 2 nm rms is much less than the wavelength of visible, nearinfrared, and infrared light [29]. The large radius of curvature measured of 3 m is adequate for most imaging applications and for projection displays. While the voltages required for this scanning mechanism are high by comparison with other designs, a relatively large scan range was achieved using relatively low voltages. The drive voltage can be further reduced by modifying the position of the H-shaped torsional spring or the position of the comb-drives.
A ~60 nm thick layer of aluminum was deposited on the reflector surface to provide high reflectivity for visible light. Also, coating the bond pads with aluminum provides robust electrical contact for wire bonding. A thin layer is used to minimize roughness of the reflector surface that may occur using PECVD fabrication methods. Deep and narrow trenches are used to isolate the electrical pads, and work as a shadow mask for coating the pad metal layer. Our experimental results showed no electrical isolation problems using a 200 nm layer of either aluminum or gold.
Scanners designed using the principle of parametric resonance have several limitations. This mechanism of action can be susceptible to the 'pull-in' phenomenon that can affect device reliability. Also, these devices work in the resonance mode only, and cannot be used for applications that require a quasi-static mode, such as tracking or positioning.

Summary
A compliant lever-based mechanism is introduced into the comb-drive actuators of a parametric resonance scanner. H-shaped (torsional) and multi-turn folded-beam springs are used to achieve ultra-large angular deflections and translational displacements. The ultralarge motions are attributed to a stiffness-softening-hardening-mixed behavior in the torsional mode and a stiffness-softening behavior in the translational mode, respectively. A reflector that generates 494° total deflection angle and 561 μm translational displacement is demonstrated.