Threshold voltage calculation in ultrathin-film SOI MOSFETs using the effective potential

The success of the effective potential method of including quantum confinement effects in simulations of MOSFETs is based on the ability to calculate ahead of time the extent of the Gaussian wave packet used to describe the electron. In the calculation of the Gaussian, the inversion layer is assumed to form in a triangular potential well, from which a suitable standard deviation can be obtained. The situation in an ultrathin silicon-on-insulator (SOI) MOSFET is slightly different, in that the potential well has a triangular bottom, but there is a significant contribution to the confinement from the rectangular barriers formed by the gate oxide and the buried oxide. For this more complex potential well, it is of interest to determine the range of applicability of the effective potential model with a constant standard deviation. In this paper, we include this effective potential model in Monte Carlo calculations of the threshold voltage of ultrathin SOI MOSFETs. We find that the effective potential recovers the expected trend in threshold voltage shift with decreasing silicon thickness, down to a thickness of approximately 3 nm.     

Stochastic modeling of paper structure and Monte Carlo simulation of light scattering

A simplified stochastic model for the fiber structure of paper is introduced. The packing density and optical thickness of the fiber network are derived analytically, and their dependence on fiber characteristics can be seen. We undertake a Monte Carlo simulation of light scattering that is based on geometrical optics, using a realization of the model, which gives packing densities and optical thicknesses well in accordance with those given by the stochastic model and the scattering quantities as functions of three angles.     

Monte Carlo simulation of symmetric and asymmetric double-gate MOSFETs using Bohm-based quantum correction

In light of increasing interest in the development of double gate (DG) CMOS technology that extends the device scaling limit, the relative merit of symmetric versus asymmetry DG-MOSFETs is studied using the quantum corrected Monte Carlo (MC) method. A recently developed Bohm-based quantum correction model is applied to the MC simulation of DG-MOSFETs. The drain current is first studied as the thickness of the silicon layer is scaled. Then results of the charge density and potential for asymmetric and symmetric devices under the same bias conditions are compared. Also analyzed is how the drain induced barrier lowering is affected by the channel length.     

A potential energy scaling Monte Carlo simulation of thin film nucleation and growth

A Monte Carlo computer simulation employing a potential energy scaling technique has been used to model the initial stages of thin film growth. The model monitors variations in the interaction potential that occur as a result of the arrival or departure of adatoms or impurities at sites in a 20 x 20 array. Boltzmann-ordered statistics are used to simulate fluctuations in vibrational energy at each site in the array, and the resulting site energy is compared with threshold levels of possible atomic events. In addition to adsorption, desorption and surface migration of adatoms, the simulation considers adatom incorporation and diffusion of a substrate atom to the surface. The effect of lateral interaction of nearest, second-nearest and third-nearest neighbors is also considered. A series of computer experiments were conducted for the Ge-Fe(100) system to illustrate the behavior of the model and to show that the surface migration of monomers, the formation of multiatom clusters and the coalescence of growing clusters behave in an accurate physical manner. Further, the model was found to be especially useful in the study of growth around different types of surface defects.     

Direct simulation Monte Carlo modeling of metal vapor flows in application to thin film deposition

Thin films of metal for electronics, nano/microelectromechanical systems and optical coatings are often prepared by various vacuum deposition techniques. Modeling such metal vapor flows using methods such as the direct simulation Monte Carlo (DSMC) can aid in the design and analysis of deposition systems and accelerate development of films with desired properties. The determination of suitable variable hard sphere (VHS) molecular model parameters for DSMC simulations using measured growth rate distribution is demonstrated with aluminum vapor as an example. Axisymmetric DSMC simulations using a VHS model corresponding to a reference diameter of 0.8 nm and a viscosity-temperature exponent of 1 are shown to agree well with available experimental data. The model is then used in two-dimensional DSMC simulations to study the interaction of plumes from multiple sources. An expression for substrate mass flux assuming no interaction between sources agrees well with DSMC simulations for a mass flow rate of 0.1 g/min corresponding to a Knudsen number (Kn) of about 0.1. The non-additive interaction of plumes at a higher flow rate of 1 g/min corresponding to a Kn of about 0.01 results in a higher mass flux non-uniformity in the DSMC simulations which is not captured by the simplified analytical expression.     

Multi-lattice Monte Carlo model of thin films

In previous publications, an atomistic simulator based on a single-lattice or a dual-lattice Monte Carlo method has been proposed and applied to the studies of microstructure evolution in thin films. In this paper, a multi-lattice Monte Carlo model, an extension to our atomistic simulator of deposition in three dimensions (ADEPT), is presented and applied to the studies of texture competition in thin films. Multiple lattices are mapped onto a single reference lattice, with resulting computational demands (memory and speed) being comparable to those in the single-lattice Monte Carlo model. It is therefore possible to simulate growth competition among crystallites of different orientations, and to study texture formation and explore optimal deposition conditions. As an application, the predominant texture is investigated as a function of collimation and deposition rate. Grains with low energy surfaces parallel to the substrate are found to dominate under the condition of low deposition rate and collimated beam. On the other hand, grains with high surface energy are found to dominate for high deposition rate and uncollimated sputtered beam, and their dominance disappears at extremely high deposition rates. This revised version was published online in July 2006 with corrections to the Cover Date.     

PVD薄膜生长的Monte Carlo模拟

蒙特卡罗(Monte Carlo,MC)和分子动力学是模拟薄膜生长的两种基本方法.本文结合各种PVD技术的工艺特点,介绍了薄膜生长的Monte Carlo模拟.详尽地总结了载能粒子沉积薄膜生长的Kinetic Monte Carlo模型和算法.最后指出建立精细的模型、算法并适当应用分子动力学是提高模拟可靠性的方法.     

Monte Carlo study of thin magnetic Ising films

The average magnetization of thin simple cubic Ising films (with thicknesses n of 1, 2, 3, 5, 10 and 20 atomic layers, respectively) is calculated as a function of temperature using the Monte Carlo technique. The “magnetization profile” across the film is also obtained. To approximate the infinite extension in the plane parallel to the surfaces of the films, films with linear dimensions 55 × 55 and periodic boundary conditions are considered. The shift of the critical temperature is consistent with an n^-λ law where λ = 1/v₃ = 1.56 is the reciprocal exponent of the correlation length. This result is critically compared with previous estimates obtained by various authors using other techniques. By interpreting the results in terms of Fisher and Barber's scaling theory, the finite size scaling function for the magnetization is determined. It is pointed out that the spatial distribution of the magnetization can be reasonably interpreted in terms of surface properties.     

Coupled Mechanical and 3-D Monte Carlo Simulation of Silicon Nanowire MOSFETs

In this paper we report on a general methodology to investigate nanowire MOSFETs based on the coupling of mechanical simulation with 3-D real-space Monte Carlo simulation. The Monte Carlo transport model accounts for both strain silicon and quantum mechanical effects. Mechanical strain effects are accounted for through an appropriate change of the anisotropic band structure computed with the empirical pseudopotential method. Quantum effects are instead included by means of a quantum mechanical correction of the potential coming from the self-consistent solution of the Schrodinger equation. This methodology has been then applied to the simulation of a test case silicon nanowire n-MOSFET. Impact of mechanical strain and quantum effects on the drive current is investigated. It is shown that only the inclusion of strain and quantum mechanical effects allows a good agreement with experimental data, demonstrating the validity of the proposed methodology for ultimate devices.     

Monte Carlo modeling of an integrating sphere reflectometer

The Monte Carlo method has been applied to numerical modeling of an integrating sphere designed for hemispherical-directional reflectance factor measurements. It is shown that a conventional algorithm of backward ray tracing used for estimation of characteristics of the radiation field at a given point has slow convergence for small source-to-sphere-diameter ratios. A newly developed algorithm that substantially improves the convergence by calculation of direct source-induced irradiation for every point of diffuse reflection of rays traced is described. The method developed is applied to an integrating sphere reflectometer for the visible and infrared spectral ranges. Parametric studies of hemispherical radiance distributions for radiation incident onto the sample center were performed. The deviations of measured sample reflectance from the actual reflectance as a result of various factors were computed. The accuracy of the results, adequacy of the reflectance model, and other important aspects of the algorithm implementation are discussed.     

Path integral Monte Carlo simulations of thin4He films on a H2 surface

The adsorption of ⁴ He films for coverages from1/4 to3 monolayers ona H₂ surface at low temperatures is studied with Path Integral Monte Carlo. We calculate the binding energy per ⁴ He atom to the substrate, density profiles of ⁴ He and H₂, zero point motion of ⁴ He and H₂, and the superfluid density of ⁴ He.     

Three-dimensional Monte Carlo simulations of electromigration in polycrystalline thin films

The effects of electromigration in metal thin films is studied by means of atomistic Monte Carlo simulations. The simulator is based on a model of atom diffusion particularly suited to deal with polycrystalline three-dimensional films. Interatomic interactions are estimated by means of a simplified Morse potential while the driving force exerted by the charge carrier flux is represented as a perturbation on the diffusion activation barrier. The local current density is calculated using an equivalent resistor network. The results of simulated stress applied to various samples including a triple point are presented demonstrating the possibility of reproducing the initial stage of void formation with an atomistic model.     

Computational steering in Monte Carlo simulations of thin film polystyrene

High molecular weight polymer systems show very long relaxation times, of the order of milliseconds or more. This time-scale proves practically inaccessible for atomic-scale dynamical simulation such as molecular dynamics. Even with a Monte Carlo (MC) simulation, the generation of statistically independent configurations is non-trivial. Many moves have been proposed to enhance the efficiency of MC simulation of polymers. Each is described by a proposal density Q(x'; x): the probability of selecting the trial state x' given that the system is in the current state x. This proposal density must be parametrized for a particular chain length, chemistry and temperature. Choosing the correct set of parameters can greatly increase the rate at which the system explores its configuration space. Computational steering (CS) provides a new methodology for a systematic search to optimize the proposal densities for individual moves, and to combine groups of moves to greatly improve the equilibration of a model polymer system. We show that monitoring the correlation time of the system is an ideal single parameter for characterizing the efficiency of a proposal density function, and that this is best evaluated by a distributed network of replicas of the system, with the operator making decisions based on the averages generated over these replicas. We have developed an MC code for simulating an anisotropic atomistic bead model which implements the CS paradigm. We report simulations of thin film polystyrene.     

Kinetic Monte Carlo simulation of Cu thin film growth

A three-dimensional kinetic Monte Carlo technique has been developed for simulating the growth of thin Cu films. The model involves incident atom attachment, surface diffusion of the atoms on the growing surface and atom detachment from the growing surface. A significant improvement in calculation of activation barriers for the surface atom diffusion on the growing film was made. The related effects caused by surface atom diffusion were taken into account. The results showed that there exist a transition temperature T_t at a certain deposition rate. When the substrate temperature approaches T_t, the growing surface becomes smoother and the relative density of the films increases. The surface roughness minimizes and the relative density saturates at T_t. The surface roughness increases with increased substrate temperature when the temperature is higher than T_t. T_t is a function of the deposition rate. The influence of the deposition rate on the surface roughness is dependent on the substrate temperature. The simulation results also showed that the relative density decreases with increasing deposition rate and average thickness of the film.     

Monte Carlo simulation of thin film head read-write performance

Read-write simulations of thin-film head longitudinal recording performance with thin-film disks are described. Acceptable electrical performance of thin-film heads depends on tight control of numerous parameters, and Monte Carlo simulation is used to quantify read-write sensitivity to head variations and to identify the critical parameters. A population of similar heads is assumed, and ten head parameters are subjected to random variations. The resultant electrical performance is dominated by head-disk spacing, with throat height, gap length, trackwidth, and pole thickness following in order of importance. Reasonable variations in coil resistance insulation thickness, yoke thickness, magnetic permeability, or inductance show relatively little influence on recording performance     

薄膜生长的三维蒙特卡罗模型

构造三维蒙特卡罗模型,研究了六边形基底薄膜生长的过程.在模型中针对每个原子考虑了原子沉积、原子扩散及原子脱附三个动力学过程,并认为这三个过程是相互独立的,即在同一计算步长中三个过程依据各自的概率发生.经过生长过程可视化的结果表明,薄膜原子之间的相互作用能、基底温度和沉积速率对薄膜的生长方式有显著的影响.这一结论得到了实验的验证.     

Atomistic modeling of materials properties by Monte Carlo Simulation

In order to optimize materials properties, in many cases a deeper understanding of the relationship between the chemical-atomistic structure and the physical properties of the solid and fluid phases of the material is necessary. Monte Carlo simulation is a tool that allows the reliable calculation of thermodynamic properties of strongly interacting many-body condensed matter systems. Given a model of effective interatomic or intermolecular interactions (drawn either from quantum-chemical-type interactions or from analysis of suitable experimental data), macroscopic bulk properties of a material can be simulated, as well as interfacial phenomena and certain kinds of slow dynamic processes (of relaxational or diffusive type). After a brief review of the foundations of this approach in statistical mechanics, the wide potential of this method is illustrated with examples taken from magnetism, metallurgy and amorphous polymeric materials. Strengths and limitations of this atomistic approach towards modeling materials properties are discussed and directions of future research are spelled out.     

Monte Carlo modeling of spatial coherence: free-space diffraction

We present a Monte Carlo method for propagating partially coherent fields through complex deterministic optical systems. A Gaussian copula is used to synthesize a random source with an arbitrary spatial coherence function. Physical optics and Monte Carlo predictions of the first- and second-order statistics of the field are shown for coherent and partially coherent sources for free-space propagation, imaging using a binary Fresnel zone plate, and propagation through a limiting aperture. Excellent agreement between the physical optics and Monte Carlo predictions is demonstrated in all cases. Convergence criteria are presented for judging the quality of the Monte Carlo predictions.     

Monte Carlo simulation of nucleation and growth of thin films

We study thin film growth using a lattice-gas, solid-on-solid model employing the Monte Carlo technique. The model is applied to chemical vapour deposition (CVD) by including the rate of arrival of the precursor molecules and their dissociation. We include several types of migration energies including the edge migration energy which allows the diffusive movement of the monomer along the interface of the growing film, as well as a migration energy which allows for motion transverse to the interface. Several well-known features of thin film growth are mimicked by this model, including some features of thin copper films growth by CVD. Other features reproduced are—compact clusters, fractal-like clusters, Frank-van der Merwe layer-by-layer growth and Volmer-Weber island growth. This method is applicable to film growth both by CVD and by physical vapour deposition (PVD).     

Spectral effective emissivities of nonisothermal cavities calculated by the Monte Carlo method

An algorithm based on the Monte Carlo method is described that permits the precise calculation of radiant emission characteristics of nonisothermal blackbody cavities for use as standard sources in radiometry, photometry, and radiation thermometry. The algorithm is realized for convex axisymmetric specular-diffuse cavities formed by three conical surfaces. The numerical experiments provide estimates of normal effective emissivities of cylindrical blackbody cavities with flat or conical bottoms for various axisymmetric temperature distributions on the cavity walls.