2D Scanning Micromirror with Large Scan Angle and Monolithically Integrated Angle Sensors Based on Piezoelectric Thin Film Aluminum Nitride

Abstract

A 2D scanning micromirror with piezoelectric thin film aluminum nitride (AlN), separately used as actuator and sensor material, is presented. For endoscopic applications, such as fluorescence microscopy, the devices have a mirror plate diameter of 0.7 mm with a 4 mm2 chip footprint. After an initial design optimization procedure, two micromirror designs were realized. Different spring parameters for x- and y-tilt were chosen to generate spiral (Design 1) or Lissajous (Design 2) scan patterns. An additional layout, with integrated tilt angle sensors, was introduced (Design 1-S) to enable a closed-loop control. The micromirror devices were monolithically fabricated in 150 mm silicon-on-insulator (SOI) technology. Si (111) was used as the device silicon layer to support a high C-axis oriented growth of AlN. The fabricated micromirror devices were characterized in terms of their scanning and sensor characteristics in air. A scan angle of 91.2° was reached for Design 1 at 13 834 Hz and 50 V. For Design 2 a scan angle of 92.4° at 12 060 Hz, and 123.9° at 13 145 Hz, was reached at 50 V for the x- and y-axis, respectively. The desired 2D scan patterns were successfully generated. A sensor angle sensitivity of 1.9 pC/° was achieved.

Growth of Highly c-Axis Oriented AlScN Films on Commercial Substrates

“…In most piezoelectric MEMS devices, AlN or AlScN thin films are grown on metallic seed layers such as molybdenum (Mo) or platinum (Pt) to ensure good c -axis orientation, where full width at half maximum (FWHM) values of the rocking curve measurements are typically larger than for AlN and larger than for Al 0.73 Sc 0.27 N [ 22 , 23 , 24 , 25 , 26 ]. However, metallic seed layers cannot be used for, e.g., piezoelectric thin-film-based surface acoustic wave (SAW) devices [ 21 ], optical waveguides [ 27 ], or MEMS actuators with doped silicon used as bottom electrode [ 28 ]. For these applications, the piezoelectric layer (AlN or AlScN) has to be grown directly on substrates such as (100) silicon (Si), silicon dioxide (SiO 2 ) or epitaxial polysilicon (poly-Si), but still a high degree of c -axis orientation of AlN or AlScN is required.…”

Design and fabrication of piezoelectrically driven deformable reflective micromirrors based on MEMS technology

“…Torres et al designed a thermally-driven MEMS micromirror device made of stacked multilayer films (SiO2/Al/SiO2) that enables large motion tip/tilt angles and piston motion [7]. Meinel et al designed and prepared a piezoelectric thin film aluminum nitriding (AlN) twodimensional scanning micromirror, and characterized the prepared micromirror device according to its scanning and sensing characteristics in air [8]. Among them, the piezoelectric driven micromirror is more widely used.…”

Modeling and Optimization of a Novel ScAlN-Based MEMS Scanning Mirror with Large Static and Dynamic Two-Axis Tilting Angles

“…Although each actuation method above can exhibit some unique characteristics for certain applications, some common challenges confronted largely prevent the achievement of both large 2-degrees-of-freedom (2-DoF) tilting angles and static non-resonant operation. In comparison, numerous impressive piezoelectric actuation micromirrors reported in the previous literature have demonstrated the achievement of resonant/static actuation mode, linear control, fast response and low power consumption characteristics [ 4 , 11 , 16 , 21 , 22 , 23 , 24 ]. A piezoelectric micromirror is less affected by air damping, thermal or electromagnetic interference than those driven by the electrostatic, thermal or electromagnetic force.…”

“…A piezoelectric micromirror is less affected by air damping, thermal or electromagnetic interference than those driven by the electrostatic, thermal or electromagnetic force. Besides the advantages in the actuation mechanisms, a wide variety of designs for piezoelectric MEMS mirrors have also been developed, with most of them deploying a large mechanical rotation angle and a low operation voltage level [ 4 , 11 , 16 , 22 , 23 , 24 ]. Such superior architectures are almost entirely based on bulk or thin PbZr x Ti 1−x O 3 (PZT) piezoelectric material to meet the requirements in laser projection and detection applications.…”

“…Chen et al presents a MEMS thin-PZT cantilever driven micro-lens actuator capable of delivering a large out-of-plane displacement of 145 μm at 22 V driving voltage, with a resonant frequency of 2 kHz [ 16 ]. Although commonly used PZT-type micromirrors exhibit large deflection due to high piezoelectric coefficients, the high permittivity resulting in low energy conversion efficiency, large temperature coefficient, low Curie point, hysteresis behavior, challengeable patterning process and poor compatibility with the mainstream complementary metal-oxide-semiconductor (CMOS) and/or MEMS process greatly limits its wide use in the MEMS area [ 4 , 20 , 21 , 22 , 23 , 24 ].…”