Display devices play a vital role in the sharing of information, and are used in our daily lives in a range of applications.
CRT (cathode ray tube) displays ruled the display sector for more than 70 years. However, the push towards ever higher quality and more compact displays has resulted in a variety of new technologies being developed.
Nanotechnology research has produced a number of advanced technologies for flat displays, enhancing their performance and quality whilst also considering environmental aspects such as resource conservation and energy use. These advancements have also led to novel features such as flexibility and foldability. A number of competing technologies have evolved and each of these have their advantages and disadvantages.
Interferometric modulator displays (IMOD), developed primarily by Qualcomm under the trademark Mirasol, is a display technology capable of creating a range of colors by simply controlling how light interferes with itself. Manipulation of the light is done by a MEMS-based device (micro-electromechanical system). A tiny electronic light modulator, which has a microscopic cavity that is switched with the help of driver-integrated circuits similar to those used for addressing liquid crystal displays (LCDs), creates interference patterns which produce different apparent wavelengths of light. An IMOD-based reflective flat panel display comprises a large number of single IMOD elements, each of which is a microelectromechanical systems (MEMS)-based device.
Micro-electromechanical systems, or MEMS, are micro-scale machines which are manufactured using manufacturing techniques developed for the semiconductor industry. Image Credits: Sandia National Labs.
IMOD Display Technology
Working prototypes of low-powered interferometric modulation screens were first unveiled at CES 2010. It is believed that Qualcomm's Mirasol IMOD displays will succeed the monochromatic "E-Ink" screens that presently dominate the e-reader industry.
The IMOD display uses just 1 mW whereas an ebook size TFT LCD screen needs up to 10 W of power. This is mainly because the IMOD does not need a backlight for viewing. The IMOD can be easily viewed in direct sunlight as it uses the surrounding light instead of a backlight for illumination.
IMOD displays can handle videos at a refresh rate of up to 15 fps. Though this may not be ideal for high-quality video, the switching speed is sufficient to make the display video capable, with no motion-blur effects.
How Do IMOD Displays Work?
The Mirasol display from Qualcomm is the first in the industry to use interferometric modulation (IMOD).
The IMOD element, which is the core building block of Mirasol display technology includes two stable states. When there is no voltage, separation of the plates occurs and the light striking the substrate is reflected. On application of a small voltage, electrostatic attraction pulls the plates together and light is absorbed, causing the element to turn black. This bistable nature is key to mirasol display’s excellent energy efficiency.
Basically, a pixel in a Mirasol display is an optically resonant cavity. The device includes a deformable reflective membrane, which is self-supporting and a thin-film stack, each of which behave as a single mirror of an optically resonant cavity with both residing on a transparent substrate. Striking of ambient light on the structure causes it to be reflected both off the top of the thin-film stack and also off the reflective membrane.
Based on the optical cavity height, light of specific wavelengths that bounce off the membrane will be slightly out-of-phase with the light that reflects off the thin-film structure. Some wavelengths constructively interfere and some destructively based on the phase difference. A color is perceived by the human eye as certain wavelengths will be magnified with respect to others. The mirasol display image is capable of switching between black and the selected color by changing the state of the membrane. This is achieved by the application of voltage to an electrically conducting thin film stack protected by an insulating layer. The application of voltage that results in electrostatic forces causes collapse of the membrane. The change in the optical cavity causes constructive interference at ultraviolet wavelengths, which cannot be seen by the human eye. Hence the image seen on the screen looks black. The IMOD elements that reflect in the RGB wavelengths are spatially ordered and assembled to form a full-color display.
Advert for the Bambook Sunflower, a full-colour e-reader using Qualcomm's mirasol MEMS display technology available in China. Run time: 41 sec
Competing Display Technologies
The use of OLEDs in displays has been restricted by the cost and scalability of production for large screens, although OLED remains a significant competitor in the smartphone display market.
Carbon nanotube-based displays have also been investigated. Since the chemical nature of carbon nanotubes (CNTs) is quite different to other carbon forms, the toxicity of CNTs must be studied carefully. CNT synthesis is challenging to control, making it quite expensive. Furthermore, CNT processing for displays need uniform dispersion and CNT mats result in material wastage.
Reports suggests that Qualcomm will be investing around $120 million to obtain a 5% stake in struggling consumer electronics and liquid crystal display manufacturer Sharp. The proposed equity investment will be utilized for the expansion of a contract between Qualcomm subsidiary Pixtronix and Sharp for the development and commercialization of MEMS-based displays that can be manufactured with Sharp LCD production lines.
PerfectLight is another MEMS-based display from Pixtronix that is based on a digital micro shutter that modulates light from an RGB LED backlight. It has a high switching speed that makes it useful for high quality video, and according to Pixtronix, the display was capable of viewing angles over 170°, 24-bit color depth and more than 3,000:1 contrast ratio at one quarter of the power consumption of LCDs of similar size and resolution.
IMOD display technology is developing rapidly, and is expected to soon become dominant in displays for low-power, portable applications like e-readers, remote equipment for military applications, and perhaps in products aimed at third world countries such as OLPC. The advantages of IMOD displays are that they require very little power, feature excellent viewability even in bright sunlight, high readability, robust functionality, technical flexibility and mechanical durability.
Sources and Further Reading