Large Angle Tilt Research Articles

A novel calibration method incorporating nonlinear optimization and ball-bearing markers for cone-beam CT with a parameterized trajectory.

Cone-beam (CB) CT is a powerful noninvasive imaging modality, and is widely used in many applications. Accurate geometric parameters are essential for high-quality image reconstruction. Usually, a CBCT system with higher spatial resolution, particularly on the order of microns or nanometers, will be more sensitive to the parametric accuracy. Here, we propose a novel calibration method combining a simple phantom containing ball bearing markers and an advanced optimization procedure. This method can be applied to CBCT with reproducible geometry and frame-to-frame invariant geometric parameters. Our proposed simplex-simulated annealing procedure minimizes the cost function that associates the geometrical parameters with the degree to which the back projections of the ball bearings in projections from various viewing angles converge, and the global minimum of the cost function corresponds to the actual geometric parameters. Specifically, six geometric parameters can be directly obtained by minimizing the cost function, and the last parameter, the distance from source to rotation axis (SRD), can be obtained using prior knowledge of the phantom - the spacing between the two ball bearings. Numerical simulation was performed to validate that the proposed method with various noise levels. With the proposed method, the mean errors and standard deviations can be reduced to ∼10% and less than 1/3 of a competing benchmark method in the case of strong Gaussian noise (sigma=200% of the pixel size) and large tilt angle (tilt angle= ). The calibration experiments with micro-CT and high-resolution CT scanners demonstrate that the proposed method recovers imaging parameters accurately, leading to superior image quality. The proposed method can obtain accurate geometric parameters of a CBCT system with a circular trajectory. While in the case of micro-CT the proposed method has a performance comparable to the competing method, for high-resolution CT, which is more sensitive to geometric calibration, the proposed method demonstrates higher calibration accuracy and more robustness than the benchmark algorithm.

On the role of texture in governing fatigue crack propagation behavior of 2524 aluminum alloy

The texture evolution of AA2524 cold-rolled sheet during annealing process and texture effect on fatigue crack propagation (FCP) behavior were investigated. Results showed the sample annealed at 400°C for 45min and solid solution-treated at 490°C for 20min had the strongest Goss and P textures, and possessed the lowest FCP rates. EBSD examination indicated Goss, P, Q, Cube, R and S grains induced great crack deflection to retard crack growth, when having large twist and tilt angle component boundary or pure twist boundary with neighboring grains. In the case of small angle boundary or relatively large angle tilt boundary with adjacent grains, Cube, Copper and R grains induced little crack deflection. Additionally, SEM results showed, in the presence of large twist angle component boundary or pure twist boundary, more severely plastic deformation presenting numerous slip bands occurred in Cube, R and S grains than that in Goss, P and Q grains during crack propagation. In contrast, no evident slip bands formed in Cube, Copper and R grains when having small angle boundaries or relatively large tilt angle boundary.

Fracture toughness and bond trapping of grain boundary cracks

The fracture toughness of materials in which brittle cracks initiate and propagate along grain boundaries (GBs) depends not only on the energy of the GB, but also on its microscopic structure. Exactly how and to what degree brittle GB fracture is affected by the local atomic structure at the crack tip has not yet been studied in detail. Here, we use molecular static simulations to study the atomic-scale fracture behavior of six large-angle tilt GBs in tungsten bicrystals. The fracture toughness depends critically on the propagation direction and on the position of the crack tip within the structural units of the GB. Furthermore, the GB fracture toughness can be significantly larger than for single crystals in the same orientation. These results cannot be explained by the usual thermodynamic approach in continuum-scale fracture mechanics but can be understood by considering the effect of bond trapping of GB cracks.

Cryo-TEM studies of two smectic phases of an asymmetric bent-core material

Cryo-TEM studies on two smectic phases of an asymmetric bent-core liquid crystal material are presented and compared to prior X-ray results obtained in bulk samples. While the bulk samples have layer-modulated structures, those modulations are not observable in the 100-nm-thick TEM samples, indicating surface-induced suppression of the layer modulations. The observed layer spacing is in agreement with the X-ray results in the lower temperature smectic phase, but distinctly larger in the higher temperature phase. This indicates surface-induced suppression of the director tilt. Cryo-TEM textures resolve the profiles of individual smectic layers at the scales down to few nanometres and reveal the presence of edge and screw dislocations, twist grain boundaries, small-angle and large-angle tilt grain boundaries.

Forging induces changes in the formability and microstructure of extruded Mg96Zn2Y2 alloy with a long-period stacking order phase

We examined the effects of the processing temperature and the grain size on the ductility and flow stress of extruded Mg96Zn2Y2 (atom%) alloy by upsettability testing at various forging temperatures and for various grain sizes of the α-Mg phase. The extruded Mg96Zn2Y2 alloy showed a forging limit as a reduction in height of 65% at forging temperatures of 573–673K and an initial strain rate of 3.5×10−1s−1. The formability of LPSO-type Mg alloys can be improved by the production of an α-Mg phase with small grains and a LPSO phase with kink bands with a large-angle tilt. The hard LPSO phase has a bending angle of about 100° at the grain boundaries and has a large deformability. Forging of about 10% is necessary for the formation of kink bands in the LPSO phase, and these kink bands deform continuously with increasing plastic deformation.

A theoretical evaluation of the temperature and strain-rate dependent fracture strength of tilt grain boundaries in graphene

Based on molecular dynamic simulations, we investigate the effects of temperature and strain rate on the strength of single layer graphene with tilt grain boundaries under tension. The simulation results show that temperature plays an important role in the strength of graphene with grain boundaries. The strength of graphene with grain boundaries decreases significantly as temperature increases. In particular, we confirm a previous report that graphene with large angle tilt boundaries (which has a high density of defects) may be much stronger than that with low angle boundaries. This finding holds true for temperatures from 10 to 1800K and strain rates from 0.0001 to 0.01ps−1.

D-1-4 金属基板上に成膜したPZT薄膜による圧電MEMSスキャナーミラーの研究(D-1 マイクロアクチュエータ,口頭発表:マイクロナノメカトロニクス)

In this study, we have developed metal-based MEMS scanner mirrors driven by PZT piezoelectric thin films not only to simplify fabrication process but also to improve fracture toughness of the micro-actuators. The 2 μm-thick PZT thin films were directly deposited on preliminarily microfabricated metal-based substrates of titanium or stainless steel by RF-magnetron sputtering. We could achieve a large mechanical angle tilt of 6.21° at high resonant frequency of 35.02 kHz by low application voltage of 10V_<p-p>. Use of microfabricated metal-based substrates for the PZT deposition enables simpler process and stronger toughness of scanner mirrors compared with the conventional Si-based ones.

Structural Characteristics of Oriented Mesostructured Silica Thin Films

Mesoporous thin films synthesized via an electrochemical strategy (ref 1) generally show granular domains, each of which is composed of hexagonally packed one-dimensional channels oriented uniquely perpendicular to the film surface. Grain boundaries either parallel or normal to the channel direction might affect the properties and subsequent application of the film. In this study, the structural details of oriented mesostructured silica thin films have been examined by transmission electron microscope. The pore structures are characterized using the traditional crystallographic concepts but show different structural properties from that of polycrystalline materials. The boundary structures vary much depending on the residual internal stress and the orientation relationship between the bounded grains. A variety of structural features, typically near the large-angle tilt boundaries, have been observed including coincidence site lattices, lattice distortion, lattice displacement, and dislocations. According to the present structural analysis, microstructure evolution and potential applications have been discussed with respect to the oriented mesoporous films.

Study on the performance of sub 100 nm LACLATI MOSFETs for digital application

With the help of extensive simulations, we systematically investigated the effects of varying tilt angle of halo implant in sub 100 nm lateral asymmetric channel (LAC) MOSFETs on the reverse short channel effects, the on current and the hot carrier immunity. The devices with large angle tilt implants also show the substantial reduction in the subthreshold swing, improvement in I ON/ I OFF ratio and significantly the lower junction capacitance as compared to the devices with low angle tilt implant. It is also observed that the subthreshold characteristics do not change as the channel length decreases for such devices. These devices, known as lateral asymmetric channel with large angle tilt implant (LACLATI), will therefore have much improved performance in comparison to a low angle tilt implant LAC devices for digital applications.

Impact of large angle tilt implantation on the threshold voltages of LDMOS transistors on SOI

A purely vertical (0°) implantation and a large-angle tilt implantation (LATID) (usually 45°) are used to form the short channel of an LDMOS transistor. A modelling of both implantations is first carried on taking into account the geometrical effects of the LATID as well as the strong channelling of the vertical implantation. Based on this accurate modelling of the doping regions, the threshold voltage of an 8 nm gate-oxide n-LDMOS transistor on silicon-on-insulator (SOI) in the advanced bipolar CMOS–DMOS (ABCD) technology is calculated and successfully compared with experiment. An optimization is then carried on for a new generation of 7 nm-gate-oxide LDMOS. This transistor suffers from a degradation of the threshold voltage because of modified channel conditions such as the length and peak concentration due to the thinner gate-oxide. The impact of temperature, implantation energy and tilt angle are simulated and the threshold voltage is adjusted by device simulations. It is found that the impact of LATID implantation is important on the channel condition which links directly to the threshold voltage. A 60° tilt with some LATID dose modifications gives very good optimization. A comparison of the two gate-oxide thicknesses with the optimized channel condition is shown at last.

Accurate modeling of large angle tilt and pure vertical implantations: application to the simulation of n- and p-ldmos backgates

The lateral and vertical extensions of the backgate of complementary laterally diffused metal oxide semiconductor (LDMOS) are simulated. The importance of channeling for a large-angle tilt implantation and a purely vertical (0/spl deg/) implantation are studied with the atomistic simulator Crystal-TRIM. The three-dimensional (3-D) nature of the large-angle tilt implantation is transposed to two dimensions through the introduction of a pseudo-tilt angle. Both the concept of pseudo-tilt angle and the channeling effects have been integrated in the two-dimensional (2-D) process and device simulator IMPACT. These model improvements give a quantitative agreement of doping profiles and sheet resistances with experiments, and therefore provide a reliable basis for the development of n- and p-LDMOS technologies.

Synthesis and Microstructure of Antimony Oxide Nanorods

Antimony oxide nanorods were synthesized by the microemulsion method. The nanorods had diameter in the range of 507–175 nm and a length of up to several micrometers. The microstructure of the nanorods was examined by analytical transmission electron microscopy and high-resolution transmission electron microscopy (HRTEM). Large-angle tilt diffraction experiments on the normal nanorods (about 90 nm in diameter) showed that they have an orthorhombic structure. Combining the results of internal standards using the silicon single crystal, it could be concluded that the synthesized nanorod is Sb2O4. The common growth direction of the nanorods was along the long axis. HRTEM images showed it had a periodic layer structure, and some defects and a layer of amorphous on the nanorods surface were found. The formation mechanism of Sb2O4 nanorods is briefly discussed.

Dynamic recrystallisation phenomena of commercial purity aluminium during friction welding

The microstructures of commercial purity aluminium near the friction weld interface were observed by transmission electron microscopy. Large plastic deformation of aluminium occurred near the weld interface and the microstructure of the aluminium was oriented from the centre to the periphery of the weld nugget. The aluminium grains were refined, and there were many recrystallised grains which were almost dislocation free. The refined grains, which were of size ~ 1 νm, formed very near the weld interface. The grain boundary was estimated to be a large angle tilt boundary. The refined grains were mainly formed by dynamic recrystallisation during the upset stage of the welding cycle. A large amount of shear strain and heat were introduced during the friction stage, and dynamic recrystallisation started during the upset stage. Grain growth occurred during air cooling after the upset stage. The grain size was larger in the central region than in the periphery owing to the variation in temperature. The Vickers microhardness of aluminium near the weld interface increased owing to the microstructural refinement.

Hot-carrier degradation in deep-submicrometer nMOSFETs: lightly doped drain vs. large angle tilt implanted drain

The hot-carrier degradation of lightly doped drain (LDD) and large angle tilt implanted drain (LATID) nMOSFETs of a 0.35 μm CMOS technology is analysed and compared by means of I–V characterisation and charge pumping current measurements. LATID nMOSFETs are found to exhibit a significant improvement in terms of both, current drivability and hot-carrier immunity at maximum substrate current condition. The different factors which can be responsible for this improved hot-carrier resistance are investigated. It is shown that this must be attributed to a reduction of the maximum lateral electric field along the channel, but not to a minor generation of physical damage for a given electric field or to a reduced I–V susceptibility to a given amount of generated damage. Further to this analysis, the hot-carrier degradation comparison between LDD and LATID devices is extended to the whole range of gate-stress regimes and the effects of short electron injection (SEI) and short hole injection (SHI) phases on hot-carrier-stressed devices are analysed. Apart from a significant improved resistance to hot-carrier effects registered for LATID devices, a similar behaviour is observed for the two types of architectures. In this way, SEI phases are found to be an efficient tool for revealing part of the damage generated in stresses at low gate voltages, whereas the performance of a first SHI phase after stress at high gate bias is found to result in a significant additional degradation of the devices. This enhanced degradation is attributed to a sudden interface states build-up occurring in both, LDD and LATID devices, near the Si/spacer interface only under the first hot-hole injection condition.

Hot-carrier reliability in deep-submicrometer LATID NMOSFETs

The hot-carrier degradation of large angle tilt implanted drain (LATID) NMOSFETs of a 0.35 μm CMOS technology is analysed and compared to the degradation behaviour of standard LDD devices. LATID NMOSFETs are found to exhibit a significant improvement in terms of both, current drivability and hot-carrier immunity. By means of I–V characterisation and charge pumping measurements, the different factors which can be responsible for this improved hot-carrier resistance are investigated. It is shown that this must be attributed to a reduction of the maximum lateral electric field along the channel, but not to a minor generation of physical damage for a given electric field or to a reduced I–V susceptibility to a given amount of generated damage.

A Ball in a Groove

We study the statics of an elastic sphere held in a rigid groove by gravity and friction, as determined by contact mechanics. As a function of the groove's opening angle and its tilt with respect to the vertical, we identify two regimes of static equilibrium for the ball. At large opening angle or small tilt, the ball rolls at both contacts as it is loaded. This is an analog of the ``elastic'' regime in the mechanics of granular media. At small opening angle or large tilt, the ball rolls at one contact and slides at the other as it is loaded, analogously with the ``plastic'' regime in the mechanics of granular media. In the elastic regime, the stress indeterminacy is resolved by the kinetics of the response to loading.

On the equilibrium charge density at tilt grain boundaries

The equilibrium charge density and free energy of tilt grain boundaries as a function of their misorientation is computed using a Monte Carlo simulation that takes into account both the electrostatic and configurational energies associated with charges at the grain boundary. The computed equilibrium charge density increases with the grain-boundary angle and approaches a saturation value. The equilibrium charge density at large-angle grain boundaries compares well with experimental values for large-angle tilt boundaries in GaAs. The computed grain-boundary electrostatic energy is in agreement with the analytical solution to a one-dimensional Poisson equation at high donor densities but indicates that the analytical solution overestimates the electrostatic energy at lower donor densities.

Post-annealing induced defects and their influences on Bi2Sr2CaCu2Oy superconducting thin films

Defects in Bi2Sr2Can-1CunOy superconducting thin films annealed in an oxygen atmosphere are examined by high-resolution electron microscopy (HREM). In addition to the majority 2212 (n = 2) phase, subsequent slabs of other homologous phases with n values up to n = 10 are found intergrown in the films. Large-angle tilt grain boundaries and various secondary phases such as CuO, Sr-Cu-O oxides are formed in the films. The occurrence of these defects is attributed to an inhomogeneous Sr, Ca, and Cu distribution induced by the post-annealing. Superconducting transition temperature (Tc) is increased by the annealing under suitable conditions.

Ion beam shadowing effect in submicrometer large-angle-tilt implanted drain (LATID) MOSFETs

An orientation-dependent device characteristic in LATID MOSFETs is reported. By controlled device fabrication splits, it is confirmed that the orientation-dependent device characteristics are caused by the large-angle-tilt (LAT) implant which creates n − drain regions with different lengths and doping concentrations due to poly gate shadowing effect. These differences in n − region cause transistor performance and reliability, such as saturation current and hot-carrier injection, to vary significantly with respect to transistor orientation.

The effect of colonies of aligned grains on critical current in high-temperature superconductors

X-ray microdiffraction has been used to map the orientational distribution of grains in a 4 × 5mm2 polycrystalline film of Tl1−yBa2Cu2Cu3Ox (Tl-1223) grown on yttira-stabilized zirconia (YSZ). The film consists of “colonies”, each containing ∼ 104 grains with their a-axes aligned; the c-axes of all grains are aligned normal to the film, but there is no overall a-axis alignment. While the grain boundaries within a colony are small angle, colonies are separated by large-angle tilt boundaries. The typical colony size is 0.4 mm. Measured critical currents are compared with simulations based on observed distributions of grain alignment; we find that variations in grain alignment account for the observed variations in the critical current density. Simulations over longer distances indicate that a colony structure can substantially increase the critical current, especially in magnetic fields.