Deep Trench Isolation and Inverted Pyramid Array Structures Used to Enhance Optical Efficiency of Photodiode in CMOS Image Sensor via Simulations.

The photodiode in the backside-illuminated CMOS sensor is modeled to analyze the optical performances in a range of wavelengths (300-1100 nm). The effects of changing in the deep trench isolation depth (DTI) and pitch size (d) of the inverted pyramid array (IPA) on the peak value (OEmax.) of optical efficiency (OE) and its wavelength region are identified first. Then, the growth ratio (GR) is defined for the OE change in these wavelength ranges to highlight the effectiveness of various DTI and d combinations on the OEs and evaluate the OE difference between the pixel arrays with and without the DTI + IPA structures. Increasing DTI can bring in monotonous OEmax. increases in the entire wavelength region. For a fixed DTI, the maximum OEmax. is formed as the flat plane (d = 0 nm) is chosen for the top surface of Si photodiode in the RGB pixels operating at the visible light wavelengths; whereas different nonzero value is needed to obtain the maximum OEmax. for the RGB pixels operating in the near-infrared (NIR) region. The optimum choice in d for each color pixel and DTI depth can elevate the maximum GR value in the NIR region up to 82.2%.

Effects of prestrain applied to poly(ethylene terephthalate) substrate on microstructures and morphologies of porous TiO(2) film and optical scattering behaviors of light.

A mold was designed to simulate a thin ceramic film coating on a soft, flexible substrate using a rotating deposition system. With this mold, three prestrains (2%, 4%, and 6%) were applied to a polyethylene terephthalate (PET) substrate before the deposition of a thin TiO(2) film. The contact angle of the substrate and, thus, the mean TiO(2) particle size were affected by the prestrain. The effects of the mean particle size of TiO(2) on the surface roughness and cavity area ratio of the porous film and on the scattering behavior of light were investigated. A goniophotometer and Advanced System Analysis Program were employed for the light analyses of bidirectional scatter distribution functions and their calibration. A spectrometer with an integrating sphere was applied to determine the total scatters (TSs) of transmittance and reflection. An increase in the prestrain increased the mean particle size of TiO(2) deposited on the substrate and, thus, the mean surface roughness, cavity/void depth, and cavity area ratio. PET/TiO(2) specimens with various prestrains were prepared that satisfy the Harvey-like model but without the isotropic, diffusive property in scatter. The bidirectional transmittance distribution function area and (TS)(transmittance) results are governed by the mean particle size and, thus, the cavity/void geometries and surface roughness. These values decrease with increasing PET prestrain. The bidirectional reflection distribution function area and (TS)(reflection), however, are governed by the adsorbed carbon and its absorption thickness. These values increase with increasing C(1s) peak value in x-ray photoelectron spectroscopy spectra.

Characterization of the elastic and viscoelastic properties of dentin by a nanoindentation creep test.

Dentin is the main supporting structure of teeth, but its mechanical properties may be adversely affected by pathological demineralization. The purposes of this study were to develop a quantitative approach to characterize the viscoelastic properties of dentin after de- and re-mineralization, and to examine the elastic properties using a nanoindentation creep test. Dentin specimens were prepared to receive both micro- and nano-indentation tests at wet and dry states. These tests were repeatedly performed after demineralization (1% citric acid for 3 days) and remineralization (artificial saliva immersion for 28 days). The nanoindentation test was executed in a creep mode, and the resulting displacement-time responses were disintegrated into primary (transient) and secondary (viscous) creep. The structural changes and mineral densities of dentin were also examined under SEM and microCT, respectively. The results showed that demineralization removed superficial minerals of dentin to the depth of 400 µm, and affected its micro- and nano-hardness, especially in the hydrate state. Remineralization only repaired the minerals at the surface layer, and partially recovered the nanohardness. Both the primary the secondary creep increased in the demineralized dentin, while the hydration further enhanced creep deformation of untreated and remineralized dentin. Remineralization reduced the primary creep of dentin, but did not effectively increase the viscosity. In conclusion, water plasticization increases the transient and viscous creep strains of demineralized dentin and reduces load sustainability. The nanoindentation creep test is capable of analyzing the elastic and viscoelastic properties of dentin, and reveals crucial information about creep responses.

Microstructural, electrical, and mechanical properties of graphene films on flexible substrate determined by cyclic bending test.

Three kinds of graphene/polyimide specimen were prepared via transfer with 3, 6, and 9 graphene layers, respectively. A self-designed bending tester was applied to carry out cyclic bending tests with various bending cycles and bending frequencies. The variations of electrical resistance of the specimens during the bending process and the rate of increase of electrical resistance with the number of bending cycles and bending frequency for various total graphene thicknesses were determined. The voids that form at the interfaces between any two adjacent layers increase in size, leading to a disconnection between graphene layers after a number of bending cycles. A reduction in the graphene thickness and increases in the number of bending cycles and bending frequency increase the rate of increase of electrical resistance. For specimens with a given graphene thickness, the ID/IG value of the Raman shift increases exponentially with increasing number of bending cycles and bending frequency. An increase in ID/IG is accompanied by increases in both the rate of increase of electrical resistance and the aspect ratio L1/L2 (where L1 and L2 are the half lengths of the long and short axes, respectively, of the selected-area electron diffraction pattern of graphene). The tilt angle formed in the top graphene layer of the specimen after bending tests increases with increasing graphene thickness for a given bending frequency. The rate of increase of the tilt angle is affected by the bending frequency.

Nanotribology of self-assembled monolayer with a probe tip investigated using molecular dynamics simulations.

The nanotribology of an alkanethiol self-assembled monolayer (SAM) under tilt contact with a scanning probe tip is studied using molecular dynamics (MD) simulations. The tilt contact is described in terms of the tilt angle and the magnitude of the specimen-tip separation. The effects of tilt angle and magnitude of the specimen-tip separation on the normal force, friction force, friction coefficient, shear strength of the tip-SAM junction, and self-recovery characteristics are evaluated during the scanning probe tip process at a temperature of 300 K. The simulation results clearly show that the magnitudes and periods of the normal force and friction force increase with decreasing magnitude of the specimen-tip separation due to a large change of the tilt angle of the SAM chains during the deformation and recovery stages. For scanning and indentation processes, the effect of the tilt angle of the probe tip on the normal force is more significant than that on the friction force for the SAM. The behaviors of interfacial contact forces, friction coefficient, and shear strength strongly depend on the number of interacting atoms and the contact area, which increases with decreasing magnitude of the specimen-tip separation and increasing tilt angle of the probe tip. The self-recovery of SAM is significantly affected by the magnitude of the specimen-tip separation; the recovery ability of SAM is worse for magnitude of the specimen-tip separation below -0.9 nm with a large tilt angle of the probe tip.

Stress-strain analysis for evaluating the effect of the orientation of dentin tubules on their mechanical properties and deformation behavior.

A model whose porosity does not vary with compression depth is developed for evaluating the mechanical properties of dentin tubules with various orientation angles from micro-pillar nanocompression tests. Experimental results for a range of loading rates indicate that the yielding parameters and the elastic modulus are little affected by the creep behavior. For a given compression depth, the hardness, elastic modulus, and yielding strength decrease with increasing orientation angle of dentin. The mechanical properties obtained using the proposed model are consistent with the reported data, and are actually more precise since they consider the orientation angle. The proposed testing method can be applied to materials that yield a negative value of the elastic modulus due to creep behavior.

Effect of chain length of self-assembled monolayers in dip-pen nanolithography using molecular dynamics simulations.

The pattern transfer mechanism of an alkanethiol self-assembled monolayer (SAM) with different chain lengths during the dip-pen nanolithography (DPN) process and pattern characterizations are studied using molecular dynamics (MD) simulations. The mechanisms of molecular transference, alkanethiol meniscus characteristics, surface adsorbed energy, transfer number, and pattern formation are evaluated during the DPN process at room temperature. The simulation results clearly show that the molecular transfer ability in DPN is strongly dependent on the chain length. Shorter molecules have significantly better transport and diffusion abilities between the meniscus and substrate surface, and the transport period can be maintained longer. The magnitude of adsorbed energy increases with chain length, so many more molecules can be transferred to the surface when shorter molecules are used. After deposition, the magnitude of the adsorbed area and pattern height decrease with increasing chain length.

Formation mechanism and mechanics of dip-pen nanolithography using molecular dynamics.

Molecular dynamics simulations are used to investigate the mechanisms of molecular transference, pattern formation, and mechanical behavior in the dip-pen nanolithography (DPN) process. The effects of deposition temperature were studied using molecular trajectories, the meniscus characteristic, surface absorbed energy, and pattern formation analysis. At the first transferred stage (at the initial indentation depth), the conformation of SAM molecules lies almost on the substrate surface. The molecules start to stand on the substrate due to the pull and drag forces at the second transferred stage (after the tip is pulled up). According to the absorbed energy behavior, the second transferred stage has larger transferred amounts and the transfer rate is strongly related to temperature. When molecules were deposited at low temperature (e.g., room temperature), the pattern shape was more highly concentrated. The pattern shape at high temperatures expanded and the area increased because of good molecular diffusion.

Multiscale particle dynamics on nanocontact and sliding friction.

A multiscale particle method for coupling continuum and molecular models is described. In this method, the continuum model was assumed to be in a lattice form and can be applied in noncharacteristic areas or far away regions from the large deformations to save computational time. Defining a series of critical energies for different lattice sizes is convenient for lattice refinement. In the thermal equilibrium case, the efficiency is around 6 times higher than that of a classical molecular dynamics (MD) simulation; in addition, great numerical precision is achieved. To test the connection at the molecular/continuum interface, a large deformation case and a surface friction case were studied in the nanocontact and the nanosliding processes, respectively. The results were compared with the MD simulation and showed great precision. The deviation could be further reduced through a moderate adjustment of critical energies on the lattices, showing that this method is a seamless treatment technology.

A new method for determining the strain energy release rate of an interface via force-depth data of nanoindentation tests.

Indentation forces, including constant rate and oscillating mode, were applied to SiO(2)/Si and diamond-like carbon (DLC)/Si specimens. A two-stage behavior was exhibited in the force-depth results after delamination occurred. When the depth was smaller than the threshold value, a linear load-depth relationship was exhibited because the debonded film was suspended over the substrate. Membrane theory was applied to analyze the deflection of the suspended film, and thus the in-plane stress exhibited in the debonded film was evaluated. Through the proposed method, the strain energy release rate of the interface can be directly evaluated by analyzing the force-depth data of the indentation tests.

Characteristics of water strider legs in hydrodynamic situations.

This study investigated the forces and the cross-section images of a water strider's leg through experimental observations. In the vertical direction, the spring coefficients were found to be 0.6 N/m for the leg and 0.3 N/m for the water, which provide a water-treading stiffness of 0.2 N/m. In the horizontal directions, besides the alignment of the microsetae, the large cuticle forces were also related to the resistant side in a Wenzel state.

Molecular Dynamics Simulations of the Roller Nanoimprint Process: Adhesion and Other Mechanical Characteristics.

Molecular dynamics simulations using tight-binding many body potential are carried out to study the roller imprint process of a gold single crystal. The effect of the roller tooth's taper angle, imprint depth, imprint temperature, and imprint direction on the imprint force, adhesion, stress distribution, and strain are investigated. A two-stage roller imprint process was obtained from an imprint force curve. The two-stage imprint process included the imprint forming with a rapid increase of imprint force and the unloading stage combined with the adhesion stage. The results show that the imprint force and adhesion rapidly increase with decreasing taper angle and increasing imprint depth. The magnitude of the maximum imprint force and the time at which this maximum occurs are proportional to the imprint depth, but independent of the taper angle. In a comparison of the imprint mechanisms with a vertical imprint case, while high stress and strain regions are concentrated below the mold for vertical imprint, they also occur around the mold in the case of roller imprint. The regions were only concentrated on the substrate atoms underneath the mold in vertical imprint. Plastic flow increased with increasing imprint temperature.

Adhesion forces and contact angles of water strider legs.

This study investigated the adhesion (pull-off) force and contact angles of a water strider's leg. During hydrostatic experiments, the adhesion force was found to be 2 dyn. The image of a cross section of a live leg contacted with a deformed water surface provided the contact angle of 168.8 degrees . A numerical scheme was proposed to determine the water surface on a groove wall of a seta. The results showed that the asperities of a seta are almost wetted, and the fraction of the wetted projection area was 0.69. Thus, the contact angle of a seta was 124.8 degrees .

Initial and adhesive contact between a diamond indenter and polydimethylsiloxane.

Detailed observations for the initial and adhesive contact of polydimethylsiloxane indentations with sharp indenters are proposed and discussed in this study. In dry experiments, the load-depth results revealed an almost reversible feature, which indicated elastic deformation. Significant initial penetration depths, created during the finding surface process, were found. A power-law relationship was used to illustrate the initial portion of the loading curve and to evaluate the initial penetration depth for correcting depth measurements. An axisymmetrical indenter with a sticky boundary condition was applied to illustrate the results of dry experiments. When the load exceeded a specific value, both loading and unloading results showed an invariant slope. The analysis of the sticky indenter provided a reasonable explanation for the linear load-depth results. By correcting the initial penetration depth, the evaluated Young's modulus values, obtained from the indenters with different geometries and under different environments, were shown to be unique and accurate.

Theoretical modeling developed to evaluate the hardness and reduced modulus for the C/a-Si composite film using nanoindentation tests.

A general mechanical model, which is composed of the mechanical models employed to describe the contact behaviors and deformations arising in all layers (including the substrate), is successfully developed in the present study for multilayer specimens in order to evaluate the contact projected area by a theoretical model, and thus the hardness and reduced modulus, using nanoindentation tests. The governing differential equations for the depth solutions of the indenter tip formed at all layers of the specimen under their contact load are developed individually. The influence of the material properties of the substrate on a multilayer specimen's hardness and reduced modulus at various indentation depths can thus be evaluated. Transition and pop-in occurred at depths near, but still before, the C (top layer)/a-Si (buffer layer) interface and the a-Si/Si (substrate) interface, respectively. Using the present analysis, the depths corresponding to the transition and pop-in behaviors can be predicted effectively.