Diamond-like-carbon LC-alignment layers for application in LCOS microdisplays

To improve the lifetime and yield of LCOS microdisplays, non-contact LC alignment techniques using inorganic materials are under investigation. This report focuses on oblique ion-beam treatment of diamond-like carbon (DLC) layers, and in particular on the influence of the ion dose on the LC alignment on DLC, keeping the ion-beam angle (40°) and ion-beam energy (170 eV) the same. LC alignment on ion-milled DLC layers is uniform if the ion dose is between 3.8 × 10^-4 C/cm² and 5.5×10 ^-3 C/cm². Above and below this ion dose range, non-uniform alignment is observed. NEXAFS experiments show that this is caused by lack of molecular anisotropy on the surface of the ion-milled DLC layers. By varying the ion dose between 3.8 × 10^-4 C/cm² and 5.5 × 10^-3 C/cm², LC molecules have an average pre-tilt between 3° and 5°, which is within the desired range for application in LCOS microdisplays. The lifetime of the LCOS microdisplays with ion-milled DLC for projection-TV application is, however, shorter than the lifetime of microdisplays with Pl layers. Ion milling probably creates a reactive surface that is unstable under the high light fluxes used in projection TVs. A solution for this problem could be chemical passivation of the ion-milled alignment layers. Initial experiments with passivation of ion-milled Pl resulted in an increase in lifetime, but the lifetime after passivation was still lower than the lifetime of rubbed Pl layers (factor 0.7). Nevertheless, ion-milling of DLC or Pl can be a good alternative LC alignment technique in other LCD applications. LC-alignment layers based on inorganic layers such as obliquely deposited SiO₂ films would be a better option for application in LCOS microdisplays due to their higher light stability.