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.     

Numerical estimates for energy of sputtered target atoms and reflected Ar neutrals in sputter processes

During the sputtering process in Ar gas, the sputtered target atoms and the reflected Ar neutrals from the target have much higher energy than the background gas. In this study, the Thompson distribution and an updated Meyer model based on the elastic energy transfer between two colliding particles were used to obtain energy distributions and average energies for the sputtered metal atoms and the reflected Ar neutrals. An energy dependent elastic collision cross section was incorporated into Meyer’s model and a thermalization criterion based on power balance was used. Under typical sputtering conditions (0.5 mTorr and 1000 K Ar, 400 eV incident Ar ion), the model predictions indicate that for Cu, Ti and Ta targets, the sputtered metal atoms have initial average energies from 15 to 22 eV and thermalize with the background Ar gas between 10 and 20 collisions. The reflected Ar neutrals thermalize after about 10 collisions. Depending on the number of collisions, the energy dependent mean free path values of the sputtered metal atoms range from 300 to 100 cm while the mean free path values for the reflected Ar neutrals range from 200 to 100 cm.     

Angular distribution and sputtering yield of Al and Al2O3 during 40 key argon ion bombardment

The angular distribution and sputtering yield of Al and Al₂O₃ during 40 keV argon ion bombardment have been measured by collecting the sputtered particles on a semi-cylindrical collector and analysing them by Rutherford backscattering spectrometry (RBS). It was found that due to the relatively high residual pressures (1–4 · 1O^?4Pa) in the sputtering chamber not only aluminium but also oxygen atoms are deposited on the collector through the mechanism of reactive sputtering both when sputtering the oxide and the metal target. The angular distribution of the collected aluminium and oxygen atoms in the case of pure aluminium sputtering follows a cosine law while in the case of Al₂O₃ sputtering a considerable deviation from the cosine law is observed. This deviation is explained by a preferred orientation (texture) of the crystallites in the polycrystalline oxide targets. It was found that the very thin films deposited on the collector when sputtering both types of targets have a composition close to AlO₂. The sputtering yield of Al and Al₂O₃ by 40 keV argon ions has been determined. On the basis of the obtained values an estimation of the productivity of the reactive sputter deposition of Al₂O₃ films from oxidized and non-oxidized targets is made.     

Magnetron sputter deposition as visualized by Monte Carlo modeling

The Monte Carlo code SIMTRA, simulating the transport of atoms from the source to the substrate during physical vapor deposition (PVD), is used in several case studies to highlight important issues related to thin film sputter deposition. Atom collisions during gas-phase transport affect the energy distribution and the deposition profile of sputtered atoms. The model is compared with published models for the thermalization of sputtered atoms, and some features of this process are discussed. The vacuum chamber design can be easily implemented in the Monte Carlo code, and this possibility is used to discuss the use of shutters and masks, and the influence of the deposition geometry. The code can also be used to predict the composition when combing different sources, segmented targets, and during combinatorial synthesis of thin films. As the details of the transport are described, the velocity and the density of the gas-phase atoms can be calculated which can assist in the interpretation of several spectroscopic techniques such as laser induced fluorescence. Not only the energy loss of the transported atoms, but also their remaining energy upon arrival at the substrate is important as the incident energy strongly influences thin film growth. To illustrate the latter, the model is also used to study the growth of biaxially aligned thin films. The key parameters influencing the level of alignment can easily be retrieved using SIMTRA.     

Molecular dynamics simulations of ion bombardment processes

Abstract

An improved molecular dynamics technique that allows reduction of the computation time required in ion bombardment simulations is presented. This technique has been used to study the influence of the target temperature and structure on the argon sputtering of silicon. Molecular dynamics simulations of l keV Ar+ ion bombardment of silicon were carried out for several types of sample: (100) crystalline at 0 K, (100) crystalline at 300 K, and amorphous at 300 K. The yield of the sputtering process and the energy distribution of the sputtered atoms have been obtained. These results show that the sputtering process depends on the target surface binding energy which, in turn, is very sensitive to the structure of the sample surface.     

Effect of scattering of sputtered atoms on the growth rate of films fabricated by magnetron sputtering

The results of the theoretical calculations of the fraction of unscattered sputtered atoms in magnetron sputtering are presented. The experimentally obtained pressure dependence of the growth rate of silicon films reveals a correlation with the theoretically calculated dependence of the fraction of unscattered sputtered atoms. If the pressure is raised from 1.5 to 8.5 mTorr, the growth rate of silicon films decreases by 25%, while the fraction of unscattered sputtered atoms over the cathode–substrate distance decreases by as much as 90%.     

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.     

The effects of incident angle on C-axis orientation in sputtered Co-Cr films

The effects of the incident angle of the sputtered atoms on the crystallographic orientation in the Co-Cr films have been investigated in detail. Specimen films 1000 - 2000 Å thick were prepared by the Facing Targets Sputtering (FTS) system. The specially designed mask was used for collecting only the sputtered particles with the quasi-coherent incidence to the substrate. When the films are prepared at relatively low argon gas pressures, the effect of incident angle is not so apparent and the well c-axis oriented films can be obtained for the incident angle below 45°. This result indicate that the surface diffusion may be dominant over the incident angle for attaining the desired crystallographic orientation in the films when they are prepared at low working gas pressures. Owing to the unique target/substrate layout, the plasma-free FTS system with low working gas pressures may have much larger flexibilities for preparing the well c-axis oriented Co-Cr films as compared with the conventional sputtering systems or the vacuum evaporation one.     

Ge:Si:O evaporated alloys as a thermosensitive layer for large area bolometers

A study of the composition and properties of amorphous Ge:Si:O thin films deposited at low temperature by reactive coevaporation is presented. Films with various compositions are obtained by separately controlling the evaporation rates of germanium and silicon. The composition of films is measured by combining Rutherford backscattering spectrometry (RBS), nuclear reaction analysis (NRA) and energy dispersive X-ray analysis (EDX) techniques. Films are characterized by FTIR absorption spectroscopy, VIS-NIR transmittance and temperature behaviour of the electrical resistivity. The analysis of all experimental data shows that oxygen incorporation depends on the silicon content in the films. Oxygen atoms appear mainly bonded to silicon and not to germanium. An uniformed distribution rather than a cluster structure of silicon oxide into a germanium matrix is suggested. Both optical band gap energy and thermal coefficient of the resistivity vary with composition of films. Preliminary studies of thin films with similar composition obtained using reactive sputtering from a composite target evince the coexistence of SiO and GeO bonds.     

离轴溅射法中最佳几何参量的预测

为提高磁控溅射薄膜的厚度均匀性,采用理论计算的方法分析了离轴溅射薄膜的厚度分布,结果表明最佳偏心距及对应的有效薄膜尺寸都与靶基距呈线性增大的关系。溅射原子的角分布、溅射环的宽度以及膜厚均匀性要求都会影响该线性关系。针对以上变化因子,本文归纳出了普适的公式,可方便地对实际工作进行指导。     

Ion beam sputtering of ZnS thin films

Ion beam sputtering was used to deposit adherent high quality ZnS films non-reactively. The optical properties of films thicker than 80.0 nm were found to be equivalent to those of bulk ZnS with a packing density of unity. Unlike that of evaporated films, the refractive index was insensitive to the substrate temperature but for thinner films was dependent on the substrate condition prior to deposition. The deposition of fully dense films is qualitatively explained using Muller's model of ion-assisted deposition. In the case of ion beam sputtering, the high energy species comprise energetic sputtered atoms and/or molecules, reflected beam ions and charge exchange neutrals.     

Simulation of ion sputtering of copper clusters from single crystal graphite surface

The sputtering of clusters consisting of 13 and 75 copper atoms from a (0001) graphite surface bombarded by normally incident 200-eV Ar⁺ ions was studied by molecular dynamics method. The angular distribution of sputtered copper atoms, their energies, and the sputtering yields are discussed.     

Crystalline characterization by Rutherford backscattering spectrometry and electron channelling of in situ grown YBa2Cu3O7 thin films deposited on (1 0 0) MgO by d.c. sputtering or laser ablation

YBa₂Cu₃O₇ (YBCO) superconducting thin films have been grown in situ on single-crystal (1 0 0) MgO substrates by single target d.c. sputtering or laser ablation. The films were highly textured, with full c-axis orientation, as shown by standard –2 X-ray diffractometry. The inplane structure of the films was characterized by reflection high energy electron diffraction (RHEED), oscillating single-crystal photographs, Rutherford backscattering spectrometry (RBS) and by electron channelling patterns (ECP). According to the results obtained from all these methods the films were found to be single-crystal-like. Channelling RBS experiments were carried out in order to provide additional information on the crystal quality, quantitatively evaluated from the _min values:for samples deposited in optimized conditions, we have found these values on sputtered films as well as on laser ablated films deposited on (1 0 0) MgO substrates to be close to that of the virgin substrate. These values strongly depend on the deposition temperature, in good agreement with ECP data. On the other hand, RBS analysis gives access to the composition of the thin films and in addition the in-depth homogeneity in composition was checked by secondary ion mass spectrometry (SIMS).     

Irreversible hydrogen effects on resistivity of sputtered copper films

Can hydrogen trapped within the lattice of copper film produce irreversible effects on the electrical resistivities of copper film at room temperature? In order to answer this question, copper films were sputtered in the presence of hydrogen and resistivities were measured in vacuum at room temperature. A series of sputtering depositions were carried out at different hydrogen partial pressures to confirm irreversible hydrogen effects. Films sputtered in argon only, were used as controls. The electrical resistivities of copper films were measured as a function of hydrogen partial pressure. Saturation is reached for the electrical resistivity in high hydrogen partial pressures. The saturation is at around 5.0% hydrogen partial pressure of total sputtering pressure. The electrical resistivities of copper films are lowered irreversibly by hydrogen, because the films were sputtered in the gas mixtures of argon and hydrogen and the electrical resistivities of films were measured in the air. The sputtering rate decreases as the hydrogen partial pressure increases. The sputtering rate is found to be proportional to the average mass of the incident ion.     

Spatial and energy distributions of particles sputtered from NiPd single crystals

Molecular dynamic simulation has been used to study the sputtering of NiPd single-crystal disordered compounds. Spatial distributions, as well as the energy of Ni and Pd particles sputtered from the NiPd (001) face have been investigated, for Ar bombarding ions of energies E₀ ranging from 0.1 keV to 5 keV, incident at different angles. The results have shown a predominant exit both of Ni and Pd atoms near the same close-packed directions, in contrast to what is observed for ordered binary compounds. For unchanged composition of the surface, the emission of Ni in the <011> close-packed directions is significantly greater than the emission of Pd. When the composition of top layers was changed, however, the opposite is observed and the exit of Pd prevails. Energy dependence and energy spectra of sputtered components have been analysed as well as the origin and the number of the generation of ejected particles. The observed trends are explained by the particular behaviour of correlated collisions in single crystals.     

Synthetic simulation of plasma formation,target erosion,and film deposition in a large magnetron sputtering apparatus

Examples of simulations of all relevant sputtering processes are described in this paper. Processes include plasma formation, target erosion, emission of sputtered atoms, and deposition of sputtered atoms for two types of magnetron sputtering apparatuses. One is axisymmetric and the other is three-dimensional.     

Angular dependence of sputtering effects by ethanol cluster ion irradiation on solid surfaces

In order to clarify the interactions of ethanol cluster ions with solid surfaces such as Si(100), SiO₂ and Au surfaces, sputtering effects were investigated by changing the acceleration voltage and incident angle (θ) of the cluster ions. The sputtered depth at a normal incidence of θ = 0° increased with an increase of the acceleration voltage. The sputtering ratio of Si to SiO₂ was approximately 10, which suggested that chemical reactions between Si and ethanol produced silicon hydride as a dominant etching material. Furthermore, the sputtered depth of Si surfaces by ethanol cluster ion irradiation had a maximum value at an incident angle between 10° and 60°, and the angle corresponding to the maximum peak increased with an increase of the acceleration voltage. On the other hand, for the physical sputtering of Au surfaces by ethanol cluster ion irradiation, the sputtered depth decreased with the increase of the incident angle, and the change was in accordance with cos θ.With regards to the angular distribution of sputtered particles, Si surface atoms were ejected by ethanol cluster ion irradiation according to a cosine law distribution. This indicated that the Si surfaces were chemically sputtered by ethanol cluster ion irradiation. On the other hand, for the case of Au surfaces, the ejection of the sputtered particles changed to the under-cosine law. This was ascribed to the lateral sputtering effect of ethanol cluster ion irradiation.     

Adjustable hydrogen atom incorporation into sputter deposited a-SiC

Thin films of a-Si_0.8C_0.2:H are deposited by ion beam sputtering combined with the simultaneous irradiation of hydrogen atoms delivered by a thermal hydrogen atom source. Elemental composition and bonding structure of the films are analysed by XPS, RBS, ERD, and FTIR. The hydrogen concentration can be varied in a controlled manner. Up to concentrations of 5%, the hydrogen is exclusively incorporated in single Si-H bonds.     

The effect of argon pressure, residual oxygen and exposure to air on the electrical and microstructural properties of sputtered chromium thin films

Chromium thin films were deposited on SiO₂/Si wafers using two sputtering systems with different levels of cleanliness, and at argon sputtering pressures varying between 0.13 and 0.93 Pa. Films from the two systems grown under identical sputtering conditions had significantly different resistivity values that are shown to be due to differences in residual oxygen in the chambers. Electrical transport measurements were conducted on the series of grown films to investigate the influence of argon pressure on film electrical resistivity. The films morphology, microstructure and composition were characterized using scanning electron microscopy and X-ray photoelectron spectroscopy. Significant differences were found in Cr thin films sputtered at different sputtering pressures; differences in resistivity performance and microstructure were noted. This change was shown to be due to the transition from porous structure to a denser microstructure. The Cr films sputtered at high pressure contained large quantities of oxygen when exposed to air. Some of the oxygen is added to the film during the deposition depending on the deposition rate and the base pressure of the sputtering system. The rest is incorporated into the film once it is exposed to air. The amount of oxygen added at this stage depends on the structure of the film and would be minimal for the films deposited at low sputtering pressures.     

Resonance and nonresonant laser ionization of sputtered uranium atoms from thin films and single microparticles: evaluation of a combined system for particle trace analysis

The resonance and nonresonant laser ionization of uranium atoms sputtered from thin metal films and individual micrometer-size uranium oxide particles, respectively, was studied to evaluate a new setup for the analysis of actinide-containing micrometer-size particles. Experiments using nonresonant (193-nm) ionization of atoms and molecules sputtered from micrometer-size uranium oxide particles have shown that the uranium detection efficiencies for sputtered neutral atoms are approximately 2 orders of magnitude higher than for secondary ions. In uranium particles of 0.5-microm diameter, 6 x 10(6) atoms of 235U were easily detected and the isotopic ratio of 235U/238U = 0.0048 +/- 4.6% is in excellent agreement with the certified value. The use of two-color, two-step resonance ionization of the sputtered neutral uranium atoms from thin films was investigated. Several excitation schemes were tested, and a significant population of several low-lying metastable states after ion sputtering was observed. Autoionizing states for double-resonant ionization were determined, and the high selectivity of ionization schemes involving these autoionizing states was illustrated by comparing the flight-time distributions of different sputtered species obtained both by resonance and nonresonant multiphoton (355-nm) laser postionization. Ideally, the options for resonance as well as nonresonant ionization would be combined in a single setup, to obtain a large gain in sensitivity and selectivity. Thus, information about the main components as well as specific isotopic information of a trace element could be obtained from the same single particle.