What Are Micromirrors?

Micromirrors are miniaturized reflective elements, typically fabricated from silicon or silicon nitride using MEMS (micro-electromechanical systems) processes, that can be individually tilted, rotated, or displaced under electronic control to steer or modulate light. Their reflective surfaces range from a few micrometers to about 10 millimeters in diameter, with most applications centering on mirrors near 1 millimeter. Because micromirrors are solid-state devices with no macroscopic moving parts beyond the mirror element itself, they achieve switching speeds that far exceed mechanical galvanometer scanners while occupying a fraction of the volume. The combination of high speed, high reflectivity, and batch fabrication compatibility has established micromirrors as a core component of optical MEMS technology.

The best-known micromirror product is Texas Instruments' Digital Micromirror Device (DMD), introduced commercially in the 1990s as the basis of Digital Light Processing (DLP) display technology. A DMD chip contains millions of individual aluminum micromirrors, each approximately 7.5 to 10.8 micrometers square, that switch between +12 and -12 degree tilt states at rates up to 5,000 patterns per second to modulate reflected light for display projection. The success of the DMD demonstrated that micromirror arrays could be manufactured reliably at wafer scale and that MEMS actuators could meet the reliability demands of consumer electronics.

Fabrication and Actuation

Micromirrors are fabricated by depositing and patterning structural layers on a silicon wafer, then releasing the mirror plate from the substrate by etching a sacrificial layer beneath it. The released mirror hangs from one or more torsional hinges that define its rotation axis. Reflective coatings of aluminum or gold are applied by physical vapor deposition to maximize mirror reflectivity across visible and near-infrared wavelengths. Actuation is achieved through four principal mechanisms: electrostatic (high speed, low power, but requires high drive voltage), electrothermal (large deflection at low voltage, but slower due to thermal time constants), electromagnetic (large deflection angle, but requires external magnets), and piezoelectric (low voltage, moderate speed, suited for scanning applications). A thorough analysis of these actuation approaches, covering response time, scanning angle, power consumption, and manufacturing complexity, is provided in a 2024 PMC review of MEMS micromirror actuation techniques.

Optical Arrays and Beam Steering

Individual micromirrors steer a single beam; arrayed architectures multiply their function. Spatial light modulators (SLMs) built from micromirror arrays independently address each pixel to modulate the amplitude, phase, or polarization of an incident wavefront. In a DMD-based SLM, the binary tilt states of millions of mirrors produce grayscale output through pulse-width modulation, enabling display projectors, maskless lithography systems, and structured illumination microscopes. For wavelength-division multiplexed (WDM) optical networks, two-dimensional micromirror arrays switch optical signals between fiber ports without converting light to electronics, offering sub-millisecond switching latency. MEMS-based SLMs for beam steering in LiDAR and augmented reality applications are described in SPIE conference proceedings on beam and image steering with MEMS SLMs. Scanning micromirrors for LiDAR use resonant or quasi-static actuation to deflect a laser beam across a wide field of view, mapping the environment by measuring time-of-flight returns.

Applications

Micromirrors have applications in a wide range of fields, including:

• Optical projectors and display systems using DLP technology in cinema projectors, home theaters, and heads-up displays
• LiDAR systems for autonomous vehicles, robotics, and 3D mapping
• Optical coherence tomography (OCT) imaging in ophthalmology and cardiology
• Adaptive optics for astronomical telescopes and laser communications links
• Spectroscopy instruments using programmable micromirror arrays to select wavelength bands
• Optical fiber network switches for wavelength-division multiplexed telecommunications

Additional detail on the fabrication of optical MEMS components, including micromirrors, is available through Teledyne MEMS' optical MEMS application overview.