by Neha K. Choksi
March 31, 2010 – Displays are a hot topic, especially in the mobile consumer electronics industry. LCD displays are prevalent in today’s handheld devices, but their poor power efficiency and readability in bright light give incentive to uncover alternative approaches. Qualcomm senior engineer Rashmi Rao shared the company’s MEMS-based approach to displays at the IEEE Bay Area Nanotechnology Council meeting on March 16, 2010.
Handheld devices are being used more and more during each day, which has large implications on the battery life of a device. Pike Research predicts that by 2014, 54% of cell phone battery life will be used toward displays. The current trend to address the issue has been to use heavier, thicker batteries, but Rao questions whether this is the true solution. Qualcomm’s mirasol display approaches the issue by trying to reduce the power consumption rather than increase battery size — an approach inspired by nature. Butterfly wings are made of millions of nanostructured etalons (two parallel reflecting surfaces). When light passes through the wing’s multilayered surface, it reflects multiple times, which leads to the intense, iridescent color for which butterfly wings are known.
Mimicking the butterfly, the company approaches a low-power display solution by creating a MEMS etalon device (see figure below). The top layer of this two-layer device is a partial reflector; the bottom layer is a total reflector. By defining the gap size between the two layers, the resulting reflected color can be specified. The bottom layer of the MEMS etalon is a moving membrane; thus the gap between layers can be modulated. The device operates as a bistable capacitive/electrostatic switch. The bright, open state is achieved by a low constant bias. By superimposing a short positive pulse, the movable bottom layer collapses for the "closed" state. The collapsed membrane results in an interference pattern of light that is not visible or "dark" to the viewer. This closed state is maintained until another pulse "un-writes" the device and the lower membrane moves back to its initial open state.
Because the device maintains its state unless pulsed, it is able to achieve lower power consumption than LCD and OLED display technologies that dominate the market today. Furthermore, Rao explains, the display is able to achieve a faster refresh rate and is also lighter than current displays. The device’s response time, on the order of microseconds, indicates that it is well suited for video applications. Also, color filters that reduce brightness in LCD technology are unnecessary for the MEMS-based display.
Brightness is a key differentiator for this display in yet another way. Because the intensity of current display technology is limited by the illumination source, which cannot compete with the intensity of sunlight, LCD and backlight OLED displays are difficult to read in bright sunlight. But since the mirasol MEMS display depends on ambient light for its light source, it has an inherent mechanism for adjusting its brightness to its surrounding light intensity, lending itself to bright light conditions. In fact, the company claims excellent contrast: 90% reflected light in the open state vs. 1% in the dark state. Plus, by utilizing ambient light, the new product eliminates the need for backlight illumination and further reduces power consumption.
On the other hand, in darkened rooms or at night, the ambient light may be insufficient for these MEMS-based displays to reflect. Hence, is Qualcomm exploring the option of frontside illumination for low ambient light conditions. It is unclear what impact the frontside illumination will have on battery life, but the aim is to keep power usage well below current LCD and OLED technology.
When asked about reliability, Rao explains that a universal usage model is still < in the industry, thus complicating a metric for comparison to other devices. The company has conducted initial accelerated lifetime tests on the device in the operational temperature range, with and without humidity, but additional investigations are underway.
Despite work to be done, the new device has received significant attention — the display technology is targeted for the e-book market. Just as cell phones are demonstrating the intersection of technology with the Internet, cameras, gaming, TV/video, contacts, music, calendar, email, and more, Qualcomm’s MEMS displays have opportunities that can reach far beyond its initial entry point. But MEMS are just one of many approaches to next-generation mobile displays, and the company is not alone in pursuing a MEMS-based approach. As Rao states, "convergence is inevitable," and the company hopes to be on the front line as the push for energy efficiency continues.
Neha K. Choksi is an independent consultant based in Mountain View, CA. She has worked for a variety of MEMS companies including as director of product engineering at Silicon Microstructures and as independent consultant for SmallTech Consulting. E-mail: choksi [at] gmail.