The ideal backlight for a mobile phone would at once be thin, efficient, uniform in illumination properties, and inexpensive to make. Manufacturers are still fishing around for a design that approaches this ideal. Researchers at Nokia Mobile Phones Ltd. (Salo, Finland) and the University of Joensuu (Joensuu, Finland) have developed a diffractive diffuser with properties that vary across its area so that an edge-lit design produces a uniform field of light. The diffusers can be made inexpensively of plastic.
Most diffusers have a structure that is substantially the same across its entire area. Such structures have problems with illumination uniformity when edge-lit: beams of light sent into the backlight from its edges remain visible (see figure). This problem exists whether the backlight is a traditional scattering diffuser or a more efficient diffractive structure. Pursuing diffractive backlighting, the researchers took advantage of the fact that stamps or molds used to fabricate plastic diffractive structures can be patterned using electron-beam lithography—a process that can create very high-precision patterns varying wildly in their makeup across their areas.
The perpendicular gratings serve to direct light outward in a conventional manner. The parallel gratings, however, serve to quickly disperse the beams exiting the LEDs within the light-guide layer; because the light is already dispersed far away from the LEDs, there is less need for parallel gratings far from the light emitters. Both gratings have a period of 1.25 µm. Detailed calculations generated the ratio of parallel to perpendicular gratings as a two-dimensional function of position across the backlight; experimental iterations further improved the design. The optimum light-guide thickness is somewhere between 0.8 and 2 mm, according to Markku Kuittinen, one of the researchers.
To make the backlight, a 100-mm photoresist-coated silicon wafer was patterned by electron-beam lithography; the process took close to 40 hours. A 40-nm-thick nickel conductivity layer served as a base on which to grow a 300-µm-thick layer of nickel in a standard electroplating bath. The resulting copy was used to emboss the pixelated pattern into plastic. For large-scale production, the embossing technique would be replaced by plastic injection molding.
The prototype display is 1.8 x 4.1 cm and is illuminated by two LEDs. In the longitudinal direction, the intensity drops by 20% (an amount almost undetectable by the human eye, which is a logarithmic detector); uniformity in the transverse direction is almost perfect. The pixelated pattern permits lighting of the backlight from a single side edge, rather than the typical two opposite edges required for reasonable uniformity. Other pixelated backlights up to 10 cm in length have been fabricated, according to Kuittinen.
