by Leslie Field, Ph.D. and Shalini Venkatesh, Ph.D.
SmallTech Consulting, LLC
At a recent seminar sponsored by the IEEE SFBA Nanotechnology Council (March 16th, 2010), Rashmi Rao, a Senior R&D Engineer at Qualcomm MEMS Technologies, gave a well-received talk titled, “From Butterfly Wings to Energy Efficient Displays”. The industry's first mobile display based on interferometric modulation (or IMOD: a reflective display technology that uses MEMS structures), Qualcomms’s displays offer low power consumption and superb viewing quality in a variety of ambient lighting conditions (including even in bright sunlight). These displays have been hailed as a game changer that is poised to revolutionize mobile devices and e-readers.
Qualcomm’s display technology works by reflecting light so that specific wavelengths interfere with each other to create color, the same phenomenon that makes a butterfly's wings shimmer. It uses IMOD technology in which a mirrored surface is overlaid with nanoscale flexible membranes that are controlled by electrical charges. Ambient light is reflected from the mirror and back through the membranes, which refract the light. The membrane-to-substrate gaps determine the colors rendered, so no color filters are required. The membranes are bistable, which allows low energy dissipation, analogous to a SRAM cell. Once the pixel membranes display a color, virtually no energy is required to maintain that color, and energy is required only when the pixel color is changed. The display requires no backlighting, a very important power savings catalyst. The low energy requirement means battery life is much longer than the battery life of a similar sized LCD (liquid crystal display) device.
The device structure has many similarities to other non-MEMS displays, and is made by rather traditional MEMS surface micromachining processing steps that are first developed on smaller silicon wafers, and which then were successfully migrated to large-area glass substrates. Ms. Rao noted that this was perhaps the first time MEMS devices have been successfully manufactured at high volume on a large-area glass substrate.
One of the opportunities form the Qualcomm display comes from its low-power latching. According to a research paper by Pike Research on energy efficient displays for green devices commissioned by QMT, that gives a large advantage in battery life over e-ink (in some usage models, a6x increase), 35% extension of battery lifetime, and a 1.25 year extension on Li-Ion battery life. Further, Pike Research’s paper showed that global adoption of the display could result in 94% less CO2 emission equivalents from display use, 12,000 fewer tons of Li-Ion batteries in landfills, and $12 billion savings in battery replacement costs for consumers.
Questions from the audience included concerns about dessicant stability over a variety of environmental conditions (seems like the combination of calcium oxide and zeolite dessicants have been shown to work well over a wide range of conditions), long-term reliability, latching, comparisons among display types, device yields, gray scale, and overall display color algorithms. Questions directed at future directions led to some brief comments about the likelihood of being able to make the displays on flexible substrates and the current work on addition of low-level illumination to the system for use in very low ambient light conditions.
Copyright 2010 MEMS Investor Journal
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