Transport of sputtered atoms investigated by Monte Carlo method

Increasing attention has been paid to the sputtering process as a tool to deposit films and to the study of the interaction between the film properties and the deposition parameters. It is obvious that the energy and direction of these particles arriving at the substrate is in close relation with the transport process from the target to the substrate. This work deals with the computer simulation of the sputtered Ag atoms trajectories through the background gas in a diode-sputtering configuration. For that, we have developed a numerical model to simulate the transport process. We followed the three-dimensional trajectory of each sputtered atom separately and calculated the scattering angle and the energy loss if a collision took place. A statistical method, Monte Carlo simulations is used. The model predicts the flux of Ag atoms arriving at the substrate, their energies and angular distribution. The dependence of the deposition rates of Ag atoms on the gas pressure and the distance between target to substrate were investigated.     

Monte Carlo simulation of the particle transport process in sputter deposition

The essential features of the transport of sputtered particles from a target to a substrate during sputter deposition were studied by calculation using the Monte Carlo technique. The study takes into consideration the change in momentum as well as the kinetic energy loss of sputtered particles in their collisions with ambient gas molecules, to gain an understanding of the effects of these factors and of the number of sputtered particles arriving at a substrate on the mechanism of growth of a thin film by sputter deposition. Some theoretical predictions using the above calculation were made for several selected conditions of sputter deposition.     

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.     

Ion plasma deposition of oxide films with graded-stoichiometry composition: Experiment and simulation

A method of ion plasma deposition is proposed for obtaining thin multicomponent films with continuously graded composition in depth of the film. The desired composition–depth profile is obtained by varying the working gas pressure during deposition in the presence of an additional adsorbing screen in the drift space between a sputtered target and substrate. Efficiency of the proposed method is confirmed by Monte Carlo simulation of the deposition of thin films of Ba _x Sr_1–x TiO₃ (BSTO) solid solution. It is demonstrated that, during sputtering of a Ba_0.3Sr_0.7TiO₃ target, the parameter of composition stoichiometry in the growing BSTO film varies in the interval of x = 0.3–0.65 when the gas pressure is changed within 2–60 Pa.

     

A computer simulation of nucleation and growth of thin films

A three-dimensional kinetic Monte Carlo technique has been developed for simulating the nucleation and growth of thin films. The model involves incident atom attachment, surface diffusion of the atoms on the growing surface and atom detachment from the growing surface. Related effects caused by atom diffusion were taken into account. A significant improvement in calculation of activation energy for the atom diffusion was made based on a reasonable assumption of interaction potential between atoms. Trace files were created during the simulation and snapshots showing the morphology of the nucleation and growth of the thin films were taken by computer graph technique. The results showed that the density of the nucleus decreases and the size of island nucleation increases with increasing the substrate temperature and decreasing the deposition rate. At the meantime, a transition from two-dimension to three-dimension nucleation was observed. There exist three critical temperatures at a certain deposition rate: T_n at which the nucleation rate reaches maximum, T_r at which the surface roughness minimizes and T_d at which the relative film density saturates. The three critical temperatures are functions of the deposition rate. The nucleation rate is close to constant under lower temperatures while it increases with deposition rate at higher temperatures. The film surface roughness depends on the density of island nucleation, it increases with temperature at lower temperatures and decreases at higher temperatures. The relative film density decreases with increasing the deposition rate.     

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.     

Deposition of optical thin films by ion beam sputtering

The use of sputtering from diode or magnetron sources has been investigated thoroughly in the last few years in order to replace traditional evaporation methods for optical thin film deposition. The kinetic energy of sputtered materials, higher than that of evaporated atoms, is one of the most important causes of the superior adherence, hardness and mechanical stability of sputtered thin films. Present technology evolution is tending to develop new techniques, allowing higher and more controllable energies of materials impinging on the substrate. In the ion beam sputter deposition (IBSD) technique the working pressure in the deposition chamber may be lower than 10^-2 Pa, so thermalization of sputtered materials is avoided and the energies of depositing atoms are higher than in plasma sputtering, where thermalization takes place. This work describes the investigations carried out for realizing optical treatments by means of IBSD. The apparatus used for this study is described with details of the experiments carried out and the results obtained in the deposition of TiO₂, Y₂O₃, Al₂O₃, SiO₂ and ZnS. The films are characterized optically, mechanically and for the determination of the damage threshold from 1064 nm laser radiation.     

Kinetic Monte Carlo模拟PVD薄膜生长的算法研究

提出kinetic Monte Carlo模拟物理气相沉积（physical vapor deposition,简写为PVD）薄膜生长的新算法：用红黑树搜索实现跃迁路径选择及系统跃迁概率更新,通过比较红黑树搜索、线性查找、满二元树搜索的计算效率,综合分析了这3种方法的时间复杂度和空间复杂度.结果表明红黑树搜索优于其它两种搜索方法,模拟效率最高,更适合用于执行大系统的kinetic Monte Carlo模拟.     

Some features of magnetron sputtering

In a low pressure sputtering system of the magnetron type for depositing thin solid films, two different discharge modes occur: a positive space-charge-dominated mode and a negative space-charge-dominated mode. The positive space-charge-dominated mode predominates in a weak magnetic field of some few hundred gauss and is widely used for sputtering, although the current density is non-uniform at the cathode surface. The negative space-charge-dominated mode predominates in a strong magnetic field of more than several hundred gauss and is also used for sputtering since the mode shows uniform current distribution at the cathode surface.In the magnetron sputtering system the working pressure is so low that the scattering of sputtered atoms by gas molecules can be neglected. Thus energetic sputtered atoms impinge on the substrates during film growth. This causes some phenomena which are rarely observed in a conventional diode sputtering system, e.g. an abnormal surface texture and an unusual crystalline structure are found in the resultant sputtered films. There is evidence that thin films of compounds normally only formed at high temperature can be synthesized at lower substrate temperatures.     

Bottom-coverage simulation for magnetron-sputtering apparatus activated with superconducting bulk magnet

The bottom coverage in circular via holes for magnetron sputtering apparatus equipped with a superconducting bulk magnet was analyzed using computer simulation. Owing to the high magnetic field, the apparatus allowed film deposition under low pressure. In the simulation, the transport of sputtered atoms was treated by the Monte Carlo method and the film deposition was calculated using a string model. The simulated results qualitatively reproduced the behavior of the measured bottom coverage. However, considerable deviation was observed depending on the geometry. It was ascribed to factors such as resputtering, reflection, and surface diffusion.     

薄膜生长的计算机模拟

建立了一个比较合理的三维模型,并通过模拟成像和定量计算研究了薄膜生长过程中的两个重要问题,早期成核与表面粗糙度.结果表明,薄膜的长生过程是原子吸附、迁移、脱附、连带等微观过程的积累.随着衬底温度的升高或入射率的降低,沉积在衬底上的原子逐步由各自独立的离散型分布向聚集状态转变形成岛核,并由二维岛核向三维岛核转变.衬底温度越高、入射率越低,成核尺寸越大.存在一个最佳成核温度,成核率出现一个极大值.随着衬底温度的升高,薄膜的粗糙度先降低后来又增加.存在一个生长转变温度Tr,薄膜的粗糙度达到极小值.当衬底温度小于Tr时,入射率越大,薄膜的粗糙度越大.当衬底温度大于Tr时,入射率越大,粗糙度越小.薄膜生长的主要微观机理是原子热运动对薄膜生长的影响.     

DC magnetron sputtered polyaniline-HCl thin films for chemical sensing applications

Thin films of conducting polymers exhibit unique chemical and physical properties that render them integral parts in microelectronics, energy storage devices, and chemical sensors. Overall, polyaniline (PAni) doped in acidic media has shown metal-like electronic conductivity, though exact physical and chemical properties are dependent on the polymer structure and dopant type. Difficulties arising from poor processability render production of doped PAni thin films particularly challenging. In this contribution, DC magnetron sputtering, a physical vapor deposition technique, is applied to the preparation of conductive thin films of PAni doped with hydrochloric acid (PAni-HCl) in an effort to circumvent issues associated with conventional thin film preparation methods. Samples manufactured by the sputtering method are analyzed along with samples prepared by conventional drop-casting. Physical characterization (atomic force microscopy, AFM) confirm the presence of PAni-HCl and show that films exhibit a reduced roughness and potentially pinhole-free coverage of the substrate. Spectroscopic evidence (UV-vis, FT-IR, and X-ray photoelectron spectroscopy (XPS)) suggests that structural changes and loss of conductivity, not uncommon during PAni processing, does occur during the preparation process. Finally, the applicability of sputtered films to gas-phase sensing of NH(3) was investigated with surface plasmon resonance (SPR) spectroscopy and compared to previous contributions. In summary, sputtered PAni-HCl films exhibit quantifiable, reversible behavior upon exposure to NH(3) with a calculated LOD (by method) approaching 0.4 ppm NH(3) in dry air.     

Elaboration and characterization of Fe1-xO thin films sputter deposited from magnetite target

Majority of the authors report elaboration of iron oxide thin films by reactive magnetron sputtering from an iron target with Ar-O₂ gas mixture. Instead of using the reactive sputtering of a metallic target we report here the preparation of Fe_1-xO thin films, directly sputtered from a magnetite target in a pure argon gas flow with a bias power applied. This oxide is generally obtained at very low partial oxygen pressure and high temperature. We showed that bias sputtering which can be controlled very easily can lead to reducing conditions during deposition of oxide thin film on simple glass substrates. The proportion of wustite was directly adjusted by modifying the power of the substrate polarization. Atomic force microscopy was used to observe these nanostructured layers. Mössbauer measurements and electrical properties versus bias polarization and annealing temperature are also reported.     

薄膜溅射沉积过程中的原子喷丸效应

原子喷丸效应是薄膜溅射沉积过程中的普遍现象，是指反弹工作气体原子和溅射原子构成的荷能粒子流对生长膜面的轰击作用。这些荷能粒子在向基片输运的过程中受到工作气体原子的散射。原子喷丸效应与靶材和工作气体的原子质量比以及工作气体压强密切相关。以平面磁控溅射Co－Cr，Ni-Fe和Gd－Fe等二元合金薄膜为对象，研究其内应力与Ar工作气体压强的关系，并探讨原子喷丸效应对应力的影响。在靶材原子质量较大并且工作气体压强较低的情形下，可导致薄膜中呈压应力。     

Diffusion in Au-Ni thin films

The objective of this work was to determine experimentally the values of the surface diffusion parameters of Au-Ni thin films obtained by vacuum evaporation and by sputter deposition. Thin film diffusion couples with an edge-to-edge interface arrangement were employed in order to define the surface diffusion mechanisms better. Experimental results show that the frequency factor (diffusion constant) for evaporated films (1.5×10^-8 cm² sec^-1) is higher than that for sputtered films (1.1×10^-8 cm² sec^-1) and bulk material (bulk diffusion data). The values obtained for the thermal activation energy in evaporated films were one order of magnitude less than those obtained for bulk material. Sputtered thin films were found to have an activation energy over 20% higher than that for evaporated films. This discrepancy apparently occurs because of partial incorporation of sputtered atoms into the glass substrate. Measurements of thin film adhesion showed the same effect.Examination of the structural characteristics of the specimens showed that both sputtered and evaporated films 300 Å and more in thickness become completely microscopically continuous. Some variations in grain size were also observed. Sputtered films were found to have crystallite grains twice as large as those in films prepared by evaporation. Microphotographs showed that for films 300 Å thick the “evaporation-condensation” effect occurs in the overlapping zone.     

Monte Carlo simulations of very low pressure chemical vapor deposition

Summary Simulation strategies for chemical vapor deposition (CVD) of thin solid films are presented, with emphasis on direct simulation Monte Carlo methods for analyzing and predicting physical phenomena occurring at low pressures and in micron-sized substrate features. The Monte Carlo approach is placed in perspective, relative to standard continuum mechanics-based strategies for modeling of CVD systems. Design issues that may be addressed through the developed methods are exemplified with computations for a new, technologically important CVD process for epitaxy of Si and Si_xGe_1-x alloys. Specifically, radiative heat transfer, rarefied gas-flow characteristics, species separation caused by pressure and thermal diffusion, growth-rate uniformity vs. surface reactivity, and deposition in microscopic features are addressed as parts of the overall CVD reactor-design approach. Process implications of rarefied transport effects unique to very low pressure CVD conditions are described. A new profile evolution technique is also introduced which predicts film topology, as well as the microstructure of the film.     

磁控溅射薄膜生长全过程的计算机模拟研究

本文通过建立多尺度模型,结合模拟了磁控溅射中溅射原子的产生、溅射原子的碰撞传输、以及最终成膜的全过程,研究了基板温度、溅射速率、磁场分布和靶材-基板间距对薄膜生长过程与薄膜性能的影响.模拟结果显示,提高基板温度或降低溅射速率都会增加初期生长阶段薄膜的相对密度;磁场对靶的利用率有显著的影响,而对薄膜最终形貌的影响不大;增大靶材-基板间距会降低薄膜的粗糙度.     

Analysis of the energy input during wire coating from a cylindrical magnetron source

In order to deposit thin films on a substrate several techniques can be used, e.g. chemical vapour deposition, atomic layer deposition or sputter deposition, depending on their specific advantages and disadvantages due to the related application. A significant parameter is the energy incident upon the substrate by the specific technique, especially when the heat capacitance of the substrate is low. Within this paper we analyse the energy transported into a thin wire (few 10 μm in diameter) during a dynamic inline aluminium sputter process in a cylindrical magnetron source. The evoked heating is important for the tensile strength of the wire and uniformity of the sputtered layer. Therefore, mathematical models were created to estimate the energy input into the wire supported by monte-carlo-simulations of the sputtering process using the TRIM-simulation (Transport and Range of Ions in Matter). Measurements with a Langmuir probe and the corresponding deposition rate were used to quantify these models, showing that at an aluminium coating process of a gold wire, the significant energy input is only due to electrons and ions of the processing gas (argon). Using the heat equation based on the sputtering apparatus' parameters, it was also possible to determine the energy input into the wire with in situ electrical resistance measurements. Both methods did show similar results, whereby the resistance results were more stable. The determined energy input made it possible to calculate the temperature profile during the wire-coating process which can be useful for estimations about film diffusion and process optimisation.     

Topology evolution of graphene in chemical vapor deposition, a combined theoretical/experimental approach toward shape control of graphene domains

Morphology control of thin film relies on understanding multiple ongoing processes during deposition and growth. To reveal the shape evolution of graphene domains on copper surfaces in chemical vapor deposition (CVD), a combinative study is performed on the CVD growth of graphene on copper surfaces. To identify the factors that influence the adsorption and diffusion of carbon atoms and further determine the domain shape, simulations based on kinetic Monte Carlo techniques are carried out. The results reveal the dependence of the graphene domain shapes on the crystalline orientation of the underlying copper substrate surfaces.