Electro-optical characteristics related to the threshold behavior of liquid crystals when using the in-plane switching (IPS) mode were investigated with interdigital electrodes. In order to analyze the switching behavior of liquid crystals, an equation, which expresses the threshold transition, was derived using the continuum elastic theory. It was made clear that it was the electric field and not the voltage that drives the liquid crystals in the IPS mode. Significantly, an inversely proportional relationship between the threshold voltage and the gap between the substrates was found to hold. Furthermore, the electro-optical characteristics were recognized to change with the variation of the gap between the substrates. This behavior is due to the independence of electric field on liquid crystal layer normal.
Sum-frequency generation ͑SFG͒ vibrational spectroscopy was used to study how side alkyl chains of a polyimide are oriented at the air-polymer interface and how they are affected by mechanical rubbing and adsorption of a liquid crystal monolayer. The spectra of polyimides with various alkyl chain lengths all indicate that the chains protrude out of the polyimide surface with a very broad distribution. Longer alkyl chains appear to contain more gauche defects. While the polyimide backbones are aligned by rubbing, the side chain orientation is hardly affected. Adsorption of a liquid crystal monolayer on the surface significantly reduces the gauche defects in the alkyl chains. eff ͑ 2 ͒ ϭ͓e SF •L͑ SF ͔͒• ͑ 2 ͒ :͓e vis •L͑ vis ͔͓͒e IR •L͑ IR ͔͒,
Surface-specific sum-frequency vibrational spectroscopy was used to study the structure of
a rubbed polyimide surface. The spectra showed that the polymer backbones were well aligned by rubbing
along the rubbing direction, and the imide cores were inclined toward the surface plane with a broad
distribution. Quantitative analysis yields an approximate orientational distribution function for the aligned
imide cores and the backbones.
Photosensitive polybenzoxazole (PBO) film has been used in GaAs heterojunction bipolar transistor (HBT) technology for stress buffer and mechanical protection layer applications. However, this film needs to be cured at high temperatures for a long period of time in order to obtain its desired excellent material characteristics. High-temperature curing can result in degradation to the electrical characteristics and performance of the underlying GaAs devices due to limited thermal budget. In this paper, we have characterized the effects of curing the PBO film on GaAs HBT wafers using a conventional convection furnace and using a variable frequency microwave (VFM) furnace. The results show that a VFM cure can achieve similar excellent physical, mechanical, thermal, and chemical material characteristics at a lower curing temperature and in a much shorter time, as compared to convection furnace curing, therefore resulting in minimal GaAs device degradation. Based on these results, an optimum curing condition using the VFM method can be obtained that satisfies both stress buffer layer material and device requirements for GaAs HBT technology.
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