Microdisplays, thumbnail-sized high-resolution displays are used in camera viewfinders, head-mounted displays, high-end beamers, et cetera. Different types of microdisplays are presently available: micromirror array-based, fabricated using MEMS technology (see below), and liquid crystal based. The latter are called liquid crystal on silicon (LCOS) microdisplays and are the result of a combination of standard CMOS with liquid-crystal technology. Because of their superior image quality characteristics, including a very high contrast and unlimited grey levels, LCOS microdisplays are widespread in highly demanding and professional applications.
Although the LCOS technology is relatively mature after a decade of technological evolution P22620, P22574, research is still ongoing to further optimize of the electro-optical behavior such as switching speed and stability under extreme light fluxes. One particular issue that presently deserves a lot of attention, is the occurrence of electro-chemical activity inside the liquid crystal layer, possibly leading to flicker C23611, C23613.
Micromirror-based microdisplays are presently dominating the market, because the high switching speed makes it possible to build single-panel color-sequential projectors, which proves to be a very cost-effective solution.
In the frame of the project GEMINI RP59, imec has established a SiGe based micromachining technology platform compatible with underlying CMOS circuitry. A novel micromirror pixel concept based on electromechanical interaction, enabling pulse-width modulation addressing without the need for additional transistors, was implemented using this platform and successfully demonstrated. The new design allows the generation of continuous grey levels and overcomes the problem of posterization encountered in digital micromirror devices C23499, D23695.
Efficient solid-state light sources such as light emitting diodes (LEDs) are rapidly gaining importance for general lighting purposes and not surprisingly they are also entering the field of projection displays.
When compared to more traditional projection light sources such as gas discharge lamps, LEDs exhibit several advantages:
- much longer lifetime, leading to a higher uptime and reliability;
- directly produce primary colors, resulting in more efficient color management and a wider color gamut of the whole system;
- instant-on behavior;
- high-speed pulsing and dimming possible; this enables unlimited dynamic contrast as well as a considerable reduction in power consumption;
- low weight, compactness;
- shock and vibration resistance, enabling automotive and airborne applications;
- no explosion danger, no release of mercury vapor;
- no hazardous voltages involved;
- reduced waste at end-of-life.
The only two drawbacks of LEDs for projection applications is their limited luminance (light output per surface area and per solid angle) on one hand and the severe thermal management requirements on the other hand.
In the project ODICIS RP105 a novel 'single-display' airplane cockpit concept is studied in which almost all indicators, displays and controls are replaced by a single large touch-screen display. Imec is involved in the display aspects of this project. The screen is made up of several seamlessly tiled LED based LCOS projectors in a rear projection configuration. Imec is responsible for the efficient LED driving and thermal management and the fabrication of miniature étendue-preserving light guides that extract the light from the LEDs in an efficient way. A photograph of the resulting feasibility demonstrator is shown in fig. 1. The ODICIS display is a typical example of a high end application where the sturdiness of LCOS microdisplays (compared to micromirror displays) and LEDs are essential.
Figure 1: Photograph of the one-display cockpit demonstrator from the project ODICIS (image courtesy of Thales Avionics).
In a joint project with the B-Phot research group from the department of applied physics and photonics of the Vrije Universiteit Brussel, new concepts for compact three-dimensional (3-D) projectors have been investigated and demonstrated. This co-operation also led to advances in multiview 3-D projection. Multiview displays allow several viewers at the same time to see the displayed content from different points of view, without the requirement to wear special glasses. Because of the very high data rate that is required for multiview video, original and more efficient coding schemes are being developed C24086.
A challenging research topic is the integration of electronics and liquid crystal display technology into lenses or contact lenses, which opens numerous possibilities to add 'smart' functionality to these lenses. Because ophthalmic lenses are spherically curved, the issues encountered are more severe than for 'normal' flexible microsystems. For contact lenses, an additional challenge is posed by the severe thickness limitation (max. 180µm) for the whole structure. This year, a great leap forward was made in the technology to produce wrinkle-free contact lenses with an integrated functional liquid crystal cell C23745. One of the first successful samples is shown in fig. 2.
Figure 2: First functional contact lens with integrated liquid crystal light modulator.