Cheng-Kai Liu

Low leakage current Cu(Ti)/SiO2 interconnection scheme with a self-formed TiOx diffusion barrier

Electrical and material properties of Cu(0.02 wt % Ti) alloy and pure Cu films deposited on SiO2/Si are explored. Current–voltage measurement using metal–oxide–semiconductor (MOS) capacitor structure reveals low leakage current (10−8 A/cm2) for capacitors with as-deposited Cu(0.02 wt % Ti) and pure Cu metal gates. However, after annealing at 700 °C in a vacuum, leakage current of MOS capacitors using a pure Cu gate shows a dramatic rise of leakage current at a low electrical field, while the leakage current of capacitors with Cu(0.02 wt % Ti) gate stays at ∼10−7 A/cm2. Concurrently, the resistivity of annealed Cu(0.02 wt % Ti) is reduced to 2.5 μΩ cm, which is only slightly greater than the resistivity of as-sputtered pure Cu films. X-ray photoelectron spectroscopy indicates that a TiOx layer has formed at the Cu(0.02 wt % Ti)/SiO2 interface after annealing and Auger electron spectrometry depth profiles show less interdiffusion at the Cu(0.02 wt % Ti)/SiO2 interface than the Cu/SiO2 interface. The correla...

Electrically switchable and optically rewritable reflective Fresnel zone plate in dye-doped cholesteric liquid crystals

This work demonstrates a reflective Fresnel zone plate based on dye-doped cholesteric liquid crystals (DDCLC) using the photo-induced realignment technique. Illumination of a DDCLC film with a laser beam through a Fresnel-zone-plate mask yields a reflective lens with binary-amplitude structures - planar and focal conic textures, which reflect and scatter probed light, respectively. The formed lens persists without any external disturbance, and its focusing efficiency, analyzed using circularly polarized light, is ~ 23.7%, which almost equals the measured diffraction efficiency of the used Fresnel-zone-plate mask (~ 25.6%). The lens is thermally erasable, rewritable and switchable between focusing and defocusing states, upon application of a voltage.

Effects of Ti addition on the morphology, interfacial reaction, and diffusion of Cu on SiO2

In this work, thin films of pure Cu, and Cu with 0.02 or 2.98 wt % Ti were deposited on SiO2 covered Si substrates. The samples were annealed at 500–800 °C in vacuum to investigate their morphological evolution, interfacial reaction, and diffusion. X-ray diffraction reveals Cu(111) and Cu(200) peaks for pure Cu and Cu(0.02 wt % Ti) films. However, the Cu(2.98 wt % Ti) film exhibits a very weak Cu(200) peak, indicating that this film is textured in 〈111〉 orientation. Concurrently, scanning electron microscopy shows that the grain size of the Cu(2.98 wt % Ti) film is significantly smaller than those of pure Cu and Cu(0.02 wt % Ti) films. In addition, Cu(2.98 wt % Ti) film remains smooth after annealing up to 800 °C, while the other two films become discontinuous. X-ray photoelectron spectroscopy indicates that a TiOx layer has formed at the Cu(0.02 wt % Ti)/SiO2 and Cu(2.98 wt % Ti)/SiO2 interfaces after 700 °C annealing. Nevertheless, the TiOx layer is thicker in the Cu(2.98 wt % Ti) system than in the Cu(...

Polarizer-free, electrically switchable and optically rewritable displays based on dye-doped polymer-dispersed liquid crystals.

This work demonstrates the feasibility of a polarizer-free, electrically switchable and optically rewritable display based on dye-doped polymer-dispersed liquid crystals (DD-PDLCs). Experimental results indicate that the doped dyes are homeotropically adsorbed onto the polymer film when an appropriate AC voltage is applied during patterning. The adsorbed dyes in the illuminated region then align the liquid crystals homeotropically, and produce a transparent pattern in the scattering background without any polarizer. Notably, the adsorbed dyes can be erased and readsorbed using thermal and optical treatments, respectively. The switching time of the fabricated display is of the order of milliseconds, and the contrast ratio is approximately 30.

Variable liquid crystal pretilt angles generated by photoalignment in homeotropically aligned azo dye-doped liquid crystals

This letter demonstrates the feasibility of producing variable liquid crystal (LC) pretilt angles using light-induced ripple structures (LIRSs) in homeotropically aligned azo dye-doped liquid crystals (ADDLCs). Illuminating homeotropically aligned ADDLCs with a linearly polarized light for a suitable period yields the LIRSs which provide LCs an anisotropic homogeneous anchoring force. Experimentally, the effective alignment force produced by the homeotropic alignment layer and the LIRSs determines the LC pretilt angle (24° to 63.5°), defined from the normal to the substrate. A no-bias pi cell for liquid crystal displays is demonstrated using this approach.

Characterization of Microstructure, Interfacial Reaction and Diffusion of Immiscible Cu(Ta) Alloy Thin Film on SiO2 at Elevated Temperature

Thin films of pure Cu along with Cu alloys with 0.1 or 2.3 atom % Ta were deposited on SiO 2 -covered Si substrates and subsequently annealed at 500-800°C in a vacuum. X-ray diffraction indicates that as compared to the pure Cu sample, the face-centered cubic Cu(111) interplanar spacing increased in the as-deposited Cu (2.3 atom % Ta) sample because the Ta additives dissolved in the Cu films, indicating a nonequilbrium, super-saturated Cu(Ta) solid solution formed by cosputtering deposition. After annealing, the resistivity of all samples was reduced and the lattice spacing of Cu(Ta) films was restored to that of pure Cu film. Scanning electron microscopy shows that the surface of the Cu (2.3 atom % Ta) system remained smooth, and transmission electron microscopy reveals that the degree of Cu grain growth was very small after annealing. Rutherford backscattering spectrometry indicates that the Cu(Ta) samples exhibited less diffusion of Cu into SiO 2 after annealing at 700°C, which can be attributed to the formation of a Ta 2 O 5 (or TaO x ) layer at the Cu(Ta)/SiO 2 interface, as revealed by X-ray photoelectron spectroscopy analysis. The connections between the Ta alloying contents and the microstructural as well as the interfacial characteristics of Cu on SiO 2 are discussed.

Direct optical switching of bistable cholesteric textures in chiral azobenzene-doped liquid crystals

Bistable cholesteric textures (i.e., planar and focal conic textures) in chiral azobenzene-doped liquid crystals (LCs) is demonstrated. Chiral azobenzene is a chiral dopant with optically tuned helical twisting power that results from the photo-isomerization between trans- and cis-isomers via exposure to UV or visible light. The pitch length of the material can be optically and repeatedly elongated and shortened. With regard to free energy, LCs tend to be stable at planar (focal conic) textures when pitch length is elongated (shortened) by exposure to UV (visible) light. Thus, direct optical switchable LC displays are investigated.

Radial liquid crystal alignment based on circular rubbing of a substrate coated with poly(N-vinyl carbazole) film

This paper presents a simple method to produce radial liquid crystal (LC) alignment layers using circular rubbing of poly(N-vinyl carbazole) (PVK) films. The produced layer can be used for fabricating axially symmetric homogeneous?radial, homeotropic?radial and radial?radial LC alignment devices by combining a rubbed PVK-coated substrate with another one with a desired LC alignment layer. The transmittance?voltage curves of the fabricated LC devices at various positions are measured to examine the uniformity of the alignment effect. Additionally, the PVK film does not absorb visible light, and can be operated at high temperatures.

Designs of High Color Purity RGB Color Filter for Liquid Crystal Displays Applications Using Fabry–Perot Etalons

Two designs of high color purity RGB color filter (RGB-CF) using Fabry-Perot etalon were proposed. First, various transmission spectra for corresponding pixel arrays were achieved by optimizing the orientation of fixed polymerized liquid crystal polymers with high birefringence. The designed high color purity RGB-CF is believed to have potential for display applications. Second, three primary transmissive peaks for each pixel were achieved with the optimized air gap. Compensated CFs were adopted to enhance color purity by absorbing undesirable transmissive light. The second design is to overcome the limitation of the cell gap in the first design. Additionally, two approaches to improve light utilization efficiency were discussed.

Optical Simulation of Axially Symmetrical Vertically Aligned Liquid Crystal Displays With Circularly Symmetric Iso-Contrast

This investigation reports the optical simulation of axially symmetric vertically aligned liquid crystal displays (ASVA LCDs). The circularly symmetric iso-light leakage and iso-transmittance contours of one subpixel result in the circularly symmetric iso-contrast contours. The contrast ratio of the area close to x- and y-axes at a polar angle of around 30° exceeds 10. The LCs near some azimuthal angles are not perfectly aligned in circularly symmetric manner because of the square subpixel. Such a reason causes major reduction of the light utilization efficiency. Additionally, the circularly symmetric iso-contrast has potential for fabricating private mobile products.