Liquid Crystal Technology
Flexible TN display with organic electrodes
OTFT PDLC display
Smartcard with FLC display
At the Institute for Large Area Microelectronics for the fabrication of liquid crystal displays (LC displays) different types of liquid crystals are employed, in particular thermotropic liquid crystals. The crystalline phase of thermotropic liquid crystals depends on the temperature. Nematic liquid crystals are the most common ones in display technology. They are mainly used to fabricate TN (twisted nematic) displays or VA (vertically aligned) displays.
Besides there is FLC (ferroelectric liquid crystal) which can be used to fabricate bistable displays: Once information has been written to a FLC display, it doesn't need any further voltage supply to display the information. A smartcard with integrated FLC display has been developed at the Institute for Large Area Microelectronics.
Using PDLC (polymer dispersed liquid crystal) displays can be realized very easily. No polarizers and no alignment layers are necessary. Depending on the applied voltage droplets of liquid crystal enclosed in polymer change scattering of light. Novel transistor technologies like OTFT (organic thin film transistor) can be implemented fast in a display using this type of liquid crystal. In addition to the thermotropic liquid crystals mentioned so far, there are also lyotropic liquid crystals. The crystalline phase depends on the presence of a solvent. LLC (lyotropic liquid crystal) can be used to fabricate very thin polarizers by using wet coating. After the evaporation of the solvent the molecules keep their orientation and thus polarize light.
Most kinds of LC displays need alignment layers to align the liquid crystal in a desired way. Usually a layer of polyimide is applied and rubbed along a certain direction (see pictured rubbing machine). This determines towards what direction and what angle the liquid crystal aligns. At the Institute for Large Area Microelectronics an alternative for aligning liquid crystal has been developed. A layer of SiO2 is deposited obliquely, which determines the alignment for the adsorbing liquid crystal. This technology is especially suitable for VA displays, where liquid crystal molecules must be aligned homeotropically.
Obliquely sputtered SiO2 as an alignment layer for VA displays
Spacers are needed to maintain the gap in the interior of a LC display. Mostly plastic or glass beads are used. At the Institute for Large Area Microelectronics different techniques for depositing spacers are available:
- spin deposition of a solvent-spacer-mixture
- Spray deposition of a solvent-spacer-mixture
- Dry spray deposition along electrical fields using the pictured spacer applicator
Besides it is possible to deposit spacers in such a way that they stick to the substrate. This prevents the spacers from moving, especially in flexible displays. So-called photo spacers can be used for the same purpose. At the Institute for Large Area Microelectronics a photolithographic process for depositing spacers directly onto a substrate has been developed.
Current research fields
Currently the usage of LLC as a polarizer in switchable glazings is being investigated as part of a project funded by the German Federal Ministry of Education and Research. The advantage consists in the integration of polarizers into the liquid crystal cell. By doing this, the LLC is protected from environmental stress. Conventional polarizing films that are exposed to solar radiation, high temperatures and humidity degrade very quickly.