Micro-electromechanical systems (MEMS) micro-mirrors are a critical component in modern optical systems, enabling precise control of light reflection for applications ranging from LiDAR to projection displays. These devices leverage microfabrication techniques to create highly miniaturized mirrors with dynamic actuation, offering advantages in speed, precision, and scalability compared to traditional mirror technologies. The design, actuation mechanisms, and material choices directly influence performance metrics such as scan angle, resonant frequency, and optical efficiency.

**Designs of MEMS Micro-Mirrors**
MEMS micro-mirrors are primarily categorized into torsional and gimbal-mounted designs, each optimized for specific operational requirements.

Torsional micro-mirrors consist of a reflective plate suspended by torsional hinges that allow rotation around a single axis. The simplicity of this design enables high-speed operation, making it suitable for resonant scanning applications. The scan angle is determined by the hinge stiffness, mirror inertia, and actuation force. For instance, torsional mirrors with thin silicon hinges achieve scan angles exceeding ±20 degrees at resonant frequencies in the kilohertz range.

Gimbal-mounted mirrors incorporate a dual-axis design, where the mirror is suspended within an outer frame that rotates around a perpendicular axis. This configuration enables two-dimensional scanning, essential for applications like laser projectors and optical cross-connects. The inner mirror typically operates at higher frequencies for horizontal scanning, while the outer frame facilitates slower vertical deflection. Gimbal designs often trade off scan speed for increased angular range, with some achieving mechanical tilt angles beyond ±30 degrees.

**Actuation Mechanisms**
The dynamic performance of MEMS micro-mirrors depends heavily on the actuation method, with electrostatic and electromagnetic being the most prevalent.

Electrostatic actuation leverages attractive forces between charged electrodes to induce mirror movement. Comb-drive and parallel-plate configurations are common. Comb-drive actuators provide linear displacement proportional to the applied voltage, enabling precise control of small-angle deflections. Parallel-plate designs offer larger forces but suffer from nonlinearity and pull-in instability, limiting the usable scan range. Electrostatic actuation excels in low-power applications, with power consumption often below 10 mW. However, high-voltage drivers (50–200 V) are typically required.

Electromagnetic actuation employs Lorentz forces generated by current-carrying coils within a magnetic field. This method provides larger scan angles and higher torque compared to electrostatic systems, making it suitable for gimbal-mounted mirrors. A major advantage is the lower drive voltage (typically 1–5 V), but power consumption is higher due to continuous current flow. Electromagnetic mirrors can achieve scan angles exceeding ±40 degrees in some cases, though at the cost of increased heat dissipation.

**Performance Metrics**
The functional capabilities of MEMS micro-mirrors are quantified by scan angles, resonant frequencies, and mirror coatings.

Scan angles define the maximum mechanical deflection achievable before nonlinearity or instability occurs. Resonant operation amplifies the scan angle through mechanical resonance, with torsional mirrors often operating at frequencies between 1–100 kHz. Quasi-static mirrors, used in applications requiring arbitrary positioning, typically achieve lower angles (e.g., ±5 to ±15 degrees) but offer precise control.

Resonant frequency is dictated by the mirror’s moment of inertia and hinge stiffness. Smaller mirrors with thin hinges exhibit higher resonant frequencies, enabling rapid scanning. For example, a 1-mm diameter torsional mirror with silicon hinges may resonate at 20 kHz, whereas a larger 5-mm mirror might operate at 2 kHz. Frequency stability is critical in LiDAR systems, where timing synchronization impacts resolution.

Mirror coatings enhance reflectivity and durability. Aluminum and gold are common choices, with aluminum offering >90% reflectivity in visible and near-infrared spectra, while gold excels in infrared applications. Dielectric coatings provide higher reflectivity (>99%) but are more complex to deposit and may introduce stress-induced curvature.

**Applications**
MEMS micro-mirrors serve diverse applications, each leveraging specific performance attributes.

LiDAR systems utilize resonant torsional mirrors for fast, repetitive scanning. A mirror oscillating at 20 kHz with a ±15-degree scan angle can achieve a 30-degree field of view, suitable for automotive and industrial ranging. The high speed enables dense point clouds for accurate 3D mapping.

Projection displays rely on both resonant and quasi-static mirrors. Digital light processing (DLP) chips employ arrays of electrostatic micro-mirrors for pixel-level control, while laser projectors use gimbal-mounted mirrors for raster scanning. Resolution is determined by the mirror’s angular precision, with high-end systems achieving sub-milliradian stability.

Optical cross-connects in telecommunications use quasi-static mirrors to direct light between fiber channels. These systems demand sub-degree accuracy and long-term reliability, often employing feedback control loops to maintain alignment.

**Challenges and Considerations**
Despite their advantages, MEMS micro-mirrors face challenges such as mechanical fatigue, environmental sensitivity, and fabrication tolerances. Hinge reliability is critical, with cyclic stress leading to eventual failure in high-duty-cycle applications. Temperature variations can alter resonant frequencies and induce thermal drift, necessitating compensation algorithms.

In summary, MEMS micro-mirrors represent a versatile technology with designs and actuation methods tailored to specific use cases. Their performance in scan range, speed, and optical efficiency continues to advance, driven by innovations in materials and microfabrication techniques. As applications expand into areas like augmented reality and biomedical imaging, further refinements in reliability and precision will be essential.