A Two-Dimensional Micro-electro-mechanical Systems (MEMS) Mirror Array Device

A technology that realizes a compact imaging device consisting of a two-dimensional array of unfixed mirrors

Unfixed Mirrors

Ricoh has developed a compact imaging device using MEMS technology, which is based on semiconductor production technology. Individual unfixed mirrors that correspond to individual image pixels are arranged with a predetermined pitch using stoppers to form a two-dimensional mirror array (Figure 1). Electrodes to control mirror inclination are arranged facing the mirrors, and voltage is applied in accordance with the image signals from the mirrors. The signals cause the mirrors, which are centered on pivots, to incline in an ON direction or OFF direction (Figure 2). A single image is projected on the entire array owing to an optical system by which the array is illuminated by light from a light source and only light in the ON direction is conducted to a projection lens.

Figure 1: Schematic structure of a dual-axis
Figure 1: Schematic structure of a dual-axis light deflection mirror array

Figure 2: Mirror inclination model diagram
Figure 2: Mirror inclination model diagram

Also, not only monochrome projection (Video 1), but also grayscale projection (Video 2) is possible by adjusting the period of time that the mirrors are inclined in the ON direction.

Video 1: Movement of an array of 20μm-square mirrors at 1Hz cycle

Video 2: Movement of an array of 8μm-square mirrors at 10kHz cycle

The Advantage of Unfixed Mirrors

A distinctive characteristic of this technology is that the mirrors are not fixed. With existing mirror arrays, a method involving the use of electrostatic force or electromagnetic force to drive mirrors suspended using leaf springs called torsion hinges is often used. This method entails a number of issues: for instance, it is difficult to miniaturize the mirrors, the structure is complex, driving voltage is high, mirror response is low owing to the reaction force of the hinges (the inclination speed is slow), and the mirrors can only incline on one axis because they are suspended using springs. By contrast, the new technology offers the following benefits:
  • •The structure is simple, which facilitates mirror miniaturization (Figure 3).
  • •Mirror response is high. (Inclination speed is high.)
  • •Driving voltage is low (because there is no spring reaction force).
  • •Light deflection toward the X-axis and Y-axis is possible (Video 3).

Figure 3: SEM image of an array of 8?m-square mirrors an array of
Figure 3: SEM image of an array of 8µm-square mirrors an array of
Video 3: Dual axis light deflection of 20μm-square mirrors at 1Hz cycle (The mirrors repeatedly move alternately according to X-axis deflection and Y-axis deflection.)

Applications for the Technology

As the use of this device makes it possible to greatly miniaturize mirrors to a size equivalent to a single pixel, it is possible to miniaturize imaging devices and to configure large-scale mirror arrays with smaller chip size. This makes it possible to provide small, portable projectors and high-resolution image projectors. Other applications being considered include taking advantage of the minute pixel formation capability to create devices to form latent images on laser printer photoreceptors and maskless exposure system exposure units.

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