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    Iridescent Displays

    Qualcomm uses the ­mechanism that gives color to butterfly wings to make low-power, full-color e-reader displays.
    April 25, 2012

    Workers in Qualcomm’s factory in Hsinchu City, Taiwan, operate the same kind of equipment found in other display-making factories on the island, which are the source of more than a third of the LCD panels in new computers, tablets, and smart phones. Yet displays from this plant are like no others. They create color images by borrowing an optical trick at work in the iridescent wings of some butterflies. Each pixel in the new Mirasol display is made from microscopic structures that function like imperfect mirrors, reflecting back incoming light but altering its color. Full-color images can be created even in direct sunlight.

    Since these displays use reflected light rather than emitting their own as conventional displays do, they consume far less energy than LCD displays. Yet unlike other low-power displays, such as the one in Amazon’s black-and-white ­Kindle e-reader, these render full-color images and can refresh quickly enough to show video.

    The color isn’t yet as rich as that of a conventional LCD, but because the display consumes so much less energy, devices that use it can last longer between charges. “If you use one in a similar way to a Kindle, you should expect weeks of battery life,” says Clarence Chui, who leads Qualcomm’s Mirasol division. The technology could also lead to slimmer devices, since designers can use smaller batteries.

    Qualcomm is starting with 5.3-inch displays for e-reader devices that are now on sale in South Korea and China. Later this year it will open a second, much larger Mirasol factory in Taiwan, which will have enough capacity to supply some of the world’s biggest mobile-device manufacturers. Chui says that the plant will be able to make Mirasol displays in sizes suited to a variety of devices, including smart phones and full-size tablets.

    Unlike conventional displays, which fade in bright light, the Mirasol display works best in direct sun. (Shown is TRSF, Technology Review’s science fiction magazine.)
    Inside the Mirasol factory in Taiwan, the production process begins with a sheet of blank glass, 73 centimeters wide and 92 centimeters long, to which the pixels will be attached. Here a robot extracts glass sheets from packaging and places them onto rollers for cleaning. The glass will form the top layer of the displays.
    A technician loads a cassette of the glass sheets into a physical vapor deposition tool, which creates a thin reflective film on the glass. This mirrorlike layer is not ­present in conventional displays, which generate their own light. The layer must be highly reflective to make the display readable even in low light. (The e-reader can light up the display from the edges for reading in the dark.)


    After this step, workers use typical ­photolithography tools to make individual pixels. The tools make tiny hollow structures that will act like imperfect mirrors.


    When a person is looking at the ­display, ­incoming light is reflected in these structures in such a way that when the light finally bounces back out, its color is different; the color depends on the size of each structure. The photolithographic process also forms microscopic mechanical switches that turn pixels off. They close the structures, causing the reflected light to become ultraviolet and invisible and making the pixels appear black.
    The photolithography step turns each sheet of glass into several displays—the reflective rectangles seen here. A technician loads the glass into a scribing machine that will cut them apart.
    Test patterns on the surface of almost-completed displays can be seen in this close-up; each rectangle is one display. A pixel is made up of a collection of cavities that each reflect back either red, green, or blue light, which combine to make a wide range of colors. The next step is encapsulation, in which outer layers and electrical contacts are added to produce a finished display for devices.
    The display can be seen here inside a disassembled Kyobo e-reader, a brand sold only in South Korea; it is the first device to incorporate a color Mirasol display. The orange ribbons are electrical contacts used to control and power the display and other components. The black border is the glass that makes up the front of the device, seen from its underside.
    The display, obscured in this photo, is connected to a circuit board and incorporated into the e-reader, some of whose parts can be seen in the background.

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