Application to micro-optics

Digital micro mirrors

In the field of digital information, most of multimedia applications need brighter and larger displays which are at the same time compact and unique. The conventional film projection display systems can't display the digital multimedia information. Many electronic display technologies have been developed since the 1940s. Currently, two main technologies are in cut-throat competition in a fast growing market: DLP technology (Digital Light Processing) and high-temperature polymer LCD projection display technology. The DLP technology is clearly progressing on the big-size expensive projection systems market and on the cheap ultra-portable projector market, whereas the LCD technology is dominating the intermediate market.

Big-size projectors can light up with a flow of more than 5000 lumens on a screen. They are very expensive, heavy and massive and need case in installation and maintenance.

At the heart of a digital DLP projector is a Texas Instruments patent DMD system (see Casework in the Introduction to micro optics lesson), which is made of a micro-mirrors matrix. Each micro-mirror swivels under the influence of an electric field strength acting on the substrate on which it is. According to the slew angle (+10°, 0°, -10°), each mirror reflects the light transmitted by the projector lamp in order to make a on/off switching of the light flow so that we can digitally make a gray-scale on 10 bits (104 levels) for each primary color. We get the color image either with color wheels and a DMD device or with dichroic mirrors and combining prisms with three DMD devices for each primary color. The graphic or video input signals are changed into a binary digital code which makes the micro-mirrors hinge.

DMD devices are made of micro-mirrors put on CMOS command circuits according to the layout below:


   
    Figure 1
Figure 1 [zoom...]

The DMD device is made in one block by a CMOS compatible process. Each micro-mirror is made of a 16µm x 16µm aluminum layer. The rotation varies according to the axis of the mirror diagonal under the influence of electrostatic gravity.

The biggest technical problem of these devices consists in ensuring mechanical reliability. Due to the high modulation speed, the torsion couple needs a more than 1013 cycles reliability during its lifetime. The defects were progressively solved, thanks to constant enhancements in conception over the past 10 years.

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