Handset functionality drives mobile display processes
06/01/2006
Manufacturers of mobile displays are responding to demands from the market for improved features and performance by turning to innovative product design, selection of more suitable materials, and upgrades to process capabilities. By moving to low-temperature polysilicon (LTPS), for instance, manufacturers have found a means of lowering power consumption, while improving color quality. At the same time, the drive electronics can now be integrated for the displays. But the benefits have come at a price. The addition of full color, higher resolution, and integrated driver electronics is nearly doubling the cost of a small mobile display. If you look at how small displays are made, you’ll see why.
A cell phone display is a sandwich with several layers. Two thin pieces of glass form the front and back. A liquid crystal material that can turn light on and off is between the front and back glass and sealed inside. A miniature white fluorescent lamp, or flat LED, provides the backlight so the display can be seen in the dark. A thin plastic polarizing film on the back and front helps cut glare and improve contrast. Each dot in the display is turned on and off by a circuit made on the back glass and there are two wires and one transistor for every dot. For a 320×240 color display, there are 230,400 transistors on the back glass. Red, green, and blue filter dots on the inside of the front glass turn the white backlight into colored light when the transistor for any one dot is turned on. The control of picture dots, or pixels, is provided by computer chips built into the cell phone and connected to the edges of each color display.
To keep manufacturing costs low, backplanes for cell phone displays are made 273 at a time using very large sheets of thin glass. Each "Gen 4" sheet measures 730×920×0.6mm (28.7×36.2×0.024 in.). One sheet is used to make the back glass with the transistor array, the "backplane." A second sheet is used to make the front glass, the "color filter,” with red, green, and blue color dots on it. When each sheet is completed, it is assembled, filled, sealed, and cut into 273 color cell phone displays.
Functionality dictates requirements
It is not yet clear just how mobile handsets will evolve over time. Companies are trying many new ideas in test markets. Initially, these products will be enhanced versions of products familiar to consumers, such as cell phones, personal digital assistants (PDAs), and MP3 players. Some experts think everything will merge into one product that is closer to the shape of a PDA than a cell phone.
As a result, many of the leading flat panel manufacturers have active programs to improve displays for mobile handsets. Their initial efforts are focused on improving the resolution of active matrix LC backplanes (the TFT transistor array). Samsung recently announced improved transreflective displays that are optimized to be viewed in full sunlight (reflection mode) and include a white LED backlight for nighttime viewing (transmission mode). Samsung also announced a very high-resolution 2.6-in. display with 300 dpi resolution (~12 dots/mm) for use in mobile handsets.
As information content is added, displays for mobile handsets will need better resolution. Video applications will require higher speed and better color rendition. Increased battery life (less power use) will be a constant demand, as will fitting more into a smaller space. At the same time these improvements are made, the cost must be kept very low.
Display technologies respond
Several new technologies are being developed to meet the more demanding requirements. Low-temperature polysilicon (LTPS) technology is being pursued to shrink TFT size in the array and to enable integration of driver logic and backlight power supply components onto the glass substrate used to make the display. This technology promises to provide very high-resolution color displays that use less power and can fit into a very small package.
Displays using organic LED (OLED) materials are also being developed. OLED materials emit red, green, or blue light directly. No backlight is needed. While OLED displays have been categorized by consumers as "beautiful" and can be viewed at a wide angle, the lifetime and cost of these materials has been a problem, so they have not been widely used. Development work continues to reduce cost, improve lifetime, and improve efficiency (lifetime and efficiency are related). The less power an OLED device needs, the longer its life will be. Some OLED materials may be efficient enough to use amorphous silicon TFTs, but most researchers think LTPS will have to be used with OLED materials to make them satisfactory for mobile handsets.
Pushing the limits of lithography
The manufacturing lines for displays for mobile handsets are evolving. High-resolution AMLCD, LTPS, and OLED-LTPS technologies require high-resolution lithography, often better than 2.0µm. In the future, feature sizes are expected to be smaller than 1µm. This resolution requires the most advanced stepper technology. Photolithography processes are currently being developed for imaging features as small as 1.5µm in size, over large fields, and at very high speed (see figure). State-of-the-art “i-line” projection steppers image a substrate by exposing only a portion of the substrate at a time. Light from a mercury arc lamp is projected through a projection lens and a mask, or reticle, that incorporates precise circuit patterns. The light penetrating through the mask impinges on photoresist material deposited on the substrate. The substrate is then moved, or “stepped,” to a second position to expose an adjacent area. The process is repeated until the entire substrate is patterned. Once this has been completed, the substrate can be washed with an alkalai solution, such as sodium hydroxide (NaOH), and the exposed material, i.e., the portion not masked by the reticle pattern, is removed.
The process described above is performed by a “Generation 4” projection stepper, which ensures precision in moving from one step position to the next using a laser interferometer controlled stage and a magnification compensating lens. At the beginning of the process, alignment marks are found to determine the actual location of the previous layer on the substrate. Grid corrections are then applied to the stepping stage and magnification corrections are applied to the mask pattern. As feature sizes become smaller, demands on the equipment to precisely place images increase. Image overlays must typically be within one third of the minimum feature size. Thus, for 1.5µm feature sizes, the overlap must be <0.5µm.
The combination of built-in metrology, laser-metered stages, low-distortion optics, and closed loop reticle position correction enables the required stitching performance. In such a process, the image field stitching error is ≤±0.3µm.
Conclusion
The rapid adoption of wireless technology worldwide has created a significant new market. To capture share in this market, information providers will move many of their new products to mobile handsets. Displays in these handsets must have very high-resolution, full color, and live video capability. Existing demand for low power use, low cost, and a small package will continue. Currently, AMLCD technology is being improved to meet these needs, and LTPS and OLED-LTPS technologies are being developed to meet future needs. The very best photolithography technology, incorporating advanced stitching and metrology, will be needed to keep pace with this exploding market.
Elvino da Silveira is president and CEO of Azores Corp, 16 Jonspin Rd., Wilmington, MA 01887; ph 978/253-6200, e-mail [email protected], www.azorescorp.com.