Rasat Muhamad

DETECTION OF PHASE TRANSITIONS IN LIQUID CRYSTALS ON A WATER SURFACE BY A MAXWELL DISPLACEMENT CURRENT-MEASURING TECHNIQUE

The phase transitions of Langmuir films of liquid‐crystal 4‐cyano‐4’‐5‐alkyl‐biphenyl (5CB), 4‐cyano‐4’‐7‐alkyl‐biphenyl (7CB), 4‐cyano‐4’‐8‐alkyl‐biphenyl (8CB), and 4‐cyano‐4’‐10‐ alkyl‐biphenyl (10CB) were investigated by a Maxwell displacement current (MDC)‐measuring technique. A weak first‐order phase transition of the orientation of LC molecules from the isotropic phase to the polar one was clearly detected by compressing the film in the range of immeasurably low surface pressure by means of the technique. For further compression, a first‐order phase transition between a stable monolayer phase and a stable three‐layer phase of the liquid crystal film was detected. The result shows that the MDC‐measuring technique is useful in the detection of phase transitions over the entire range of molecule areas.

Effect of annealing on the optical and chemical bonding properties of hydrogenated amorphous carbon and hydrogenated amorphous carbon nitride thin films

Hydrogenated amorphous carbon (a-C:H) and hydrogenated amorphous carbon nitride (a-CNx:H) films were prepared in a custom-built radio-frequency plasma-enhanced chemical vapor deposition (RF-PECVD) system with a parallel-plate configuration. Pure methane and a gas mixture of methane and nitrogen were used as gas sources to obtain these films. The films were characterized using Fourier transform infrared and optical transmission spectroscopy techniques. The incorporation of nitrogen and the effect of annealing (100–500 °C) on the film properties were studied. The films were determined to be thermally stable up to 300 °C. Upon annealing above 300 °C, the thickness and refractive index of both a-C:H and a-CNx:H films increase while the optical energy gap E04 decreases. These effects were more pronounced in a-CNx:H. From the IR spectra, these changes are considered to be due to the decreases in nitrogen and hydrogen concentrations in the films which result in their structural modification.

Chitosan-based electrolyte for secondary lithium cells

The system chitosan : ethylene carbonate : LiCF3SO3 was prepared by the solution cast technique. To verify that the conductivity of the material is due to the salt, the electrical conductivity at room temperature of the chitosan acetate film and that of the chitosan acetate films containing different amounts of ethylene carbonate added to it were measured. The order of magnitude of the electrical conductivity was 10−10 S cm−1. Films containing fixed content of chitosan and plasticizer but different amounts of salt were then prepared in the same manner and the highest electrical conductivity obtained was 1.3 × 10−5 S cm−1 at room temperature. These results indicate that the conductivity is due to the salt. Conductivity-temperature studies show that the ln σ T versus 103/T graphs obey Arrhenius rule implying that the conductivity occurs by way of some thermally assisted mechanism. Polarization current measurement shows that the lithium ion transference number is ∼0.09. A LiMn2O4/chitosan-LiCF3SO3/C cell was fabricated which cycled between 1.5 to 2.5 V with fading capacity. This could be the result of LiF formation due to interaction between the salt and the fluorine in the binding agent.

Maxwell displacement current due to phase transitions in liquid crystals on a water surface

Abstract The phase transitions of Langmuir films of the liquid crystals (LCs) 4′-n-pentyl-4-cyano-biphenyl (5CB), 4′-n-octyl-4-cyano-biphenyl (7CB), 4′-n-octyl-4-cyano-biphcnyl (8CB) and 4′-n-decyi-4-cyano-biphenyl (10CB), have been investigated by a Maxwell displacement current (MDC) measuring technique, A phase transition of LC molecules from the planar surface alignment phase to the polar one was clearly detected in the range of low surface pressures during monolayer compression. A phase transition between a stable monolayer phase and a stable three-layer phase of the liquid crystal film was detected during further monolayer compression.

Correlation of silicon wafer strength to the surface morphology

The trend in microelectronic packaging is towards thinner and smaller packages. To achieve this, the die must be smaller and thinner as well, which means greater susceptibility to process related failures, especially in the front-end processes. Silicon strength has been recognized as an important parameter in wafer processing, packaging and die assembly due to process-induced stresses. The strength of the silicon wafer is heavily dependent on how the backside surface is prepared prior to metal deposition. Flaws such as small microcracks or etch pits can occur during backside processes, causing the strength of the silicon to decrease, leading to fracture. This paper investigates the effect of die strength on the surface morphology using a ball breaker test and atomic force microscopy (AFM). The die strength was characterized using the ball breaker test while surface morphology was characterized using AFM. The methodologies of the ball breaker test and AFM were documented. The evaluation was performed for wafers with wet etch, smooth grind and rough grind types of backside surface finish. It shows that wet etched wafers have the highest strength and rough grind have the lowest.

Excitonic electroabsorption in solid solution of CdxZn1-xS single crystals

Excitonic electroabsorption (EA) of CdxZn1-xS (x=1.0, 0.8, 0.7 and 0.0) solid solution single crystals was investigated at 150 and 77 K. The EA spectra have been analysed within the context of the Stark theory that enables the lowest bound exciton states in the solid solution at each composition to be located precisely. This allows the determination of the reduced effective mass of the exciton which is found to vary linearly with the optical band gap as predicted theoretically.

Leadframe Material, Design and Surface Treatment for MSL 1 260°C Package Robustness

Microelectronic packages and assemblies typically consist of layers of materials with dissimilar properties. Thermally induced stress in such multilayered structures could reach significant levels near the free edges, leading to interface de-bonding or delamination, and thus resulting in failure of the multilayered structure. A significant amount of studies has been conducted on this bonded layer integrity. The metal based leadframe in microelectronic packages is still the most widely used in the semiconductor industry. It makes up the largest portion of materials cost in plastic packages; especially for quad flat leadless (QFN) packages. In selecting the type of leadframe substrate material, characteristic such as strong structure that can withstand handling, yet compliant to bending must be met. They must also have high thermal conductivity to dissipate heat generated by the IC and good electrical properties so as not to compromise its electrical performance. In addition, leadframes are a critical part of the reliability of the overall packaging system.

Influence of Material Combination on Package Stress and Verification of Numerical Model

This paper demonstrates the experimental approach of using various packaging material combination and their effect on package stress at 260degC IR reflow condition. The study is extended into validating the results with the use of simulation and modeling tool using finite element analysis (FEA). Large leadless package (7.0 times 7.0 mm) was selected for this study. Samples were built with different material combination of epoxy based die attach and mold compound materials and subjected to specially designed stress tests. Scanning Acoustic Tomography (SAT) was used to detect the presence of delamination after the stress cycle. Mechanical properties of these materials are fed into FEA tool and simulation was done to predict package stress under the designed special testing condition. A correlation between experimental and FEA simulation result was established. The numerical and experimental analyses of the specially designed test confirmed the accuracy of the FEA prediction, which could be convenient for future virtual prototyping analyses. This will help to reduce the experimental and test time for future package design and development.

Enhancement of Surface Potential Buildup and Decay of Tris(8-quinolinato)aluminum Film Deposited on Ultraviolet/Ozone-Treated Gold Electrodes

The enhancement of the surface potential of tris(8-quinolinato)aluminum (Alq3) film deposited on ultraviolet (UV)/ozone-treated gold electrodes was confirmed by surface potential measurement, and the ordering of molecular dipoles was suggested. The enhanced surface potential of the order of 10 V decayed rapidly by photoirradiation, due to molecular dipole disordering, but a surface potential of 100–400 mV remained. Interestingly, this remaining surface potential of Alq3 on the UV/ozone-treated gold electrode was always smaller than that of Alq3 film on an untreated gold electrode. It was concluded that the UV/ozone treatment is effective in decreasing the density of surface states at the Alq3-metal interface as well as for the enhancement of molecular dipole ordering.

Effects of Indium Depositions on Porous Silicon Nanostructure (PSN)

The effects of Indium doped on Porous Silicon Nanostructure (PSN) have been studied. The Electroluminescence studies on Indium-doped porous silicon nanostructure (In:PSN) are presented. The main objective of this paper is study the EL effects of Indium doping on PSN. Porous silicon nanostructure layers have been formed by anodically etching unpolished p-type Si [100] wafer with surface resistivity of 1-10 ohm cm-1 in Hydroflouric (HF) solution at 1:1 ratio of Ethanol. Indium (In) was doped on PSN using cathodic electrodeposition composed of InCl3 and ethanol electrolythe. A diode structure has been fabricated comprising semi-transparent Au/In:PSN/p-Si substrate/Al ohmic contact electrode to observe the EL spectra. The In:PSN device shows increasing on EL and PL Intensity as well as blue-shift EL and PL spectrum is observed. Possible reasons for the enhancement will be discussed. Technological application of PSN as a light emitter would have significant impact on numerous technologies such as display panels or integrated circuits with optoelectronic devices (IO) on board and sensors.