Abstract
Abstract The objective of the thesis is to study a new plasma immersion technique for liquid crystal (LC) alignment. The key step of this technique is to immerse in Ar plasma on sample holders with different holder-tilted angles, which is favorable to LC alignment. In this research, a planar-type inductive-coupled plasma system is used to generate Ar plasma for the plasma immersion treatment. Several plasma parameters such as different holder-tilted angles, treated time, and pulsed biased voltage have been used to modify the surface of a-C:H films to investigate the effect of LC alignment. In addition, a theoretical model is proposed to explain the phenomenon of LC alignment for the Ar plasma immersion treatment. In order to study the LC alignment properties of the treated a-C:H films, different LC cells were assembled for the measurement of polarized optical microscope, pretilt angle, and azimuthal anchoring strength. Moreover, the contact angle instrument, atomic force microscope (AFM), Raman spectroscope, UV-Vis spectroscope, and X-ray photoelectron spectroscope (XPS) were also used to characterize a-C:H films with and without Ar plasma immersion treatment. A Langmuir probe was used to measure the plasma characteristics in order to analysis the mechanism of the LC alignment. Experimental data indicate that no LC alignment appears for the a-C:H films treated at the holder-tilted angle of 0o; LC alignment which exhibits low contrast and high defect density for the a-C:H films treated at a holder-tilted angle of 30o. The quality of LC alignment is greatly improved for the a-C:H films treated at a holder-tilted angle of 60o. Furthermore, the pretilt angle of LC molecules can be adjusted to some extent by changing the experimental parameters of Ar immersion plasma treatment. In one case, a-C:H films, biased with a pulse voltage of -1000 V at the holder-tilted angle of 60o, was treated with Ar plasma immersion at a RF power of 200 W. A maximum magnitude (about 2.0o) of pretilt angle is achieved after the Ar plasma treatment for 10~15 minute. In addition, the magnitude of pretilt angle can be controlled by altering the pulsed biased voltage. The pretilt angle increases as the pulsed biased voltage decreases. A maximum magnitude (about 2.6o) of pretilt angle could be achieved after treating a-C:H films in the Ar plasma at a RF power of 200 W at zero pulsed biased voltage at the holder-tilted angle of 60o. The mechanism for LC alignment, using a-C:H films treated with Ar plasma immersion, is attributed to the non-uniform thickness of the sheath layer. The non-uniform sheath width, generated by the inhomogeneous plasma density distribution in the planar-type ICP system, results in the oblique incidence of Ar onto the a-C:H film surface.