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.     

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.     

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.     

Studies on sputtering process of multicomponent Zr-Ti-Cu-Ni-Be alloy thin films

Sputter deposition process of a multicomponent Zr-Ti-Cu-Ni-Be metallic alloy has been studied experimentally and by numerical simulations. Monte-Carlo simulations were performed using a model based on thermalization and diffusion of sputtered atoms. Incident energy and angle of sputtered atoms on substrate were obtained from simulations. The incident angular distribution was observed to be a normal distribution at all sputtering pressures. Average incident kinetic energy of the condensing atoms on the substrate was observed to be 0.2-0.3 eV indicating most of them are thermalized. Simulations were extended to predict compositional variations in films prepared at various process conditions. These results were compared with composition of films determined experimentally using Rutherford Backscattering Spectrometry (RBS). Contents of Zr, Ti, Cu and Ni quantified using RBS were in moderate agreement with the simulated composition. Be could not be quantified accurately by RBS largely due to very low energy peak of Be in the spectrum. These studies are shown to be useful in understanding the complexities in multicomponent sputtering.     

The growth of single crystal films of silver on rocksalt by sputtering

Thin films of silver on rocksalt have been prepared by diode sputtering in an argon discharge. Deposition rate, substrate temperature, and film thickness have been varied, and dependence of orientation on these parameters has been studied. Within a deposition rate range of 0.1 to 1.15Å/sec, films have been grown with (100) [110]_Ag / / (100) [110]_NaCl orientation, at temperatures in the range –35 to 0° C. Higher rates required higher temperatures. Films giving these results were all 200 Å in thickness. A thickness dependence of orientation has been observed for films below 120 Å in thickness.The above results are discussed in terms of the effect of arrival energy of the sputtered material at the substrate. The results of calculations, on the effect of gas pressure on the arrival energy, are presented and it is shown that, at 10^–2 torr, up to 15% of the arriving atoms will have energies above 0.6 eV. That the observed rate, temperature, and thickness dependence of epitaxy are due to surface cleaning and penetration effects caused by the energy of arrival of the atoms is shown to be possible.The effect of charged particle bombardment of the substrate is also considered. It is shown that this may also be an important parameter affecting the growth.     

The high power impulse magnetron sputtering discharge as an ionized physical vapor deposition tool

Various magnetron sputtering tools have been developed that provide a high degree of ionization of the sputtered vapor referred to as ionized physical vapor deposition (IPVD). The ions can be controlled with respect to energy and direction as they arrive to the growth surface which allows for increased control of film properties during growth. Here, the design parameters for IPVD systems are briefly reviewed. The first sputter based IPVD systems utilized a secondary plasma source between the target and the substrate in order to generate a highly ionized sputtered vapor. High power impulse magnetron sputtering (HiPIMS) is a recent sputtering technique that utilizes IPVD where a high density plasma is created by applying high power pulses at low frequency and low duty cycle to a magnetron sputtering device. A summary of the key experimental findings for the HiPIMS discharge is given. Measurements of the temporal and spatial behavior of the plasma parameters indicate electron density peak, that expands from the target with a fixed velocity. The discharge develops from an inert sputtering gas dominated to a sputtered vapor dominated during the pulse. The high electron density results in a high degree of ionization of the deposition material.     

The high power impulse magnetron sputtering discharge as an ionized physical vapor deposition tool

Various magnetron sputtering tools have been developed that provide a high degree of ionization of the sputtered vapor referred to as ionized physical vapor deposition (IPVD). The ions can be controlled with respect to energy and direction as they arrive to the growth surface which allows for increased control of film properties during growth. Here, the design parameters for IPVD systems are briefly reviewed. The first sputter based IPVD systems utilized a secondary plasma source between the target and the substrate in order to generate a highly ionized sputtered vapor. High power impulse magnetron sputtering (HiPIMS) is a recent sputtering technique that utilizes IPVD where a high density plasma is created by applying high power pulses at low frequency and low duty cycle to a magnetron sputtering device. A summary of the key experimental findings for the HiPIMS discharge is given. Measurements of the temporal and spatial behavior of the plasma parameters indicate electron density peak, that expands from the target with a fixed velocity. The discharge develops from an inert sputtering gas dominated to a sputtered vapor dominated during the pulse. The high electron density results in a high degree of ionization of the deposition material.     

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.     

Reactive ion beam sputtering of thin films of lead,zirconium and titanium

Thin films of lead, zirconium and titanium were reactively sputtered in an oxygen atmosphere by a focused ion beam sputtering technique. With this technique, the plasma is contained in the ion gun so that neither the target nor the substrate is in a plasma environment. As a result, several factors which can interfere with the interpretation of the mechanism of compound formation (high energy secondary electron bombardment, high substrate temperature, high deposition pressure) are virtually eliminated.Properties such as deposition rate, resistivity and internal stress as a function of the partial pressure of oxygen were investigated. A decrease in the deposition rate was observed as the partial pressure of oxygen was increased from 1 × 10^-6 to 2 × 10^-4 Torr. This decrease was attributed to a build-up of a surface layer of adsorbed oxygen on the target. A model is proposed to predict the partial pressure of oxygen at which a dielectric film forms, based on the thermodynamic properties of the compound formed. All films deposited are in tensile stress. This suggests that compound formation occurs at the target and that the oxide is sputtered in the collision process.     

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。     

Silicon nitride and oxynitride films prepared by ion beam reactive sputtering

The composition of particles sputtered from a silicon target during bombardment by nitrogen and oxygen ions was measured. Elementary processes on both the substrate and the target were examined. A model of silicon nitride film formation was proposed, in which the main idea is the interaction of silicon on the substrate with the atomic nitrogen sputtered from the target. The dependence of the film properties on the oxygen concentration in N₂O₂ mixtures was investigated. The relationship between the nitrogen and silicon concentrations in silicon nitride films, the nitrogen ion beam incidence angle and the ejection angle of the sputtered particles was revealed experimentally. In the proposed model this relationship is interpreted as a result of the difference between the angular distribution of silicon and nitrogen atoms sputtered from the target.     

Stable deposition of silicon oxynitride thin films with intermediate refractive indices by reactive sputtering

The deposition stability of silicon oxynitride thin films with intermediate refractive indices was investigated as a function of argon concentration in the process gas mixture. The silicon oxynitride thin films were deposited by pulsed dc reactive magnetron sputtering in a mixture of argon, nitrogen and oxygen. The refractive indices of the silicon oxynitride thin films gradually decreased with oxygen percentage in the reactive gas mixture when high argon concentrations were used. It is proposed that many silicon atoms were sputtered from the target and reached the substrates in high argon concentrations; consequently, drastic oxidation of the thin films did not occur.     

The importance of the energetic species in pulsed laser deposition for nanostructuring

This work reports on the optical and structural properties of nanostructured films formed by Ag nano-objects embedded in amorphous aluminium oxide (a-Al(2)O(3)) prepared by alternate pulsed laser deposition (PLD). The aim is to understand the importance of the energetic species involved in the PLD process for nanostructuring, i.e.?for organizing nanoparticles (NPs) in layers or for self-assembling them into nanocolumns (NCls), all oriented perpendicular to the substrate. In order to change the kinetic energy of the species arriving at the substrate, we use a background gas during the deposition of the embedding a-Al(2)O(3) host. It was produced either in vacuum or in a gas pressure (helium and argon) while the metal NPs were always produced in vacuum. The formation of NPs or NCls is easily identified through the features of the surface plasmon resonances (SPR) in the extinction spectra and confirmed by electron microscopy. The results show that both the layer organization and self-assembling of the metal are prevented when the host is produced in a gas pressure. This result is discussed in terms of the deceleration of species arriving at the substrate in gas that reduces the metal sputtering by host species (by ≈58%) as well as the density of the host material (by ≥19%). These reductions promote the formation of large voids along which the metal easily diffuses, thus preventing organization and self-organisation, as well as an enhancement of the amount of metal that is deposited.     

Reactive sputtering of indium targets in a D.C. triode configuration

A study was initiated on the behaviour of the total pressure during the sputtering process for a d.c. triode sputtering system. The variations in the total pressure are readily attributed to variations in the reactive gas concentration. Experimental evidence is presented to support a model in which the abrupt increase in the deposition rate of films during the reactive sputtering of metals is related to a chemical reaction of the sputtered atoms with the reactive gas.     

Gas pressure effects on thickness uniformity and circumvented deposition during sputter deposition process

In sputter deposition processes, the thickness distribution of the film is affected by experimental conditions such as gas pressure, target-substrate (T-S) distance and target elements. To study these effects, we have designed a sample holder with three quartz crystal microbalance (QCM) thickness monitors on its surface and measured the distribution of the depositing flux around the sample holder, including the circumvented deposition onto the back face. As for the gas pressure dependence for a T-S distance of about 50 mm, the relative deposition flux on the back face was found to be at its maximum at pressures of 1-2 Pa. Above this pressure, the uniformity of the flux on the front face became gradually worse. The observed characteristics could be reproduced by a Monte-Carlo simulation of the particle transport process. The reduction of the circumvented deposition and the uniformity degradation at high gas pressures were ascribed to the start of thermalization of sputtered particles and the shrinkage of its spatial profile toward the target.     

超声雾化方法中两步沉积对YBCO薄膜性能的影响

用自行研制的超声雾化装置在{110}<011>织构的Ag基带上直接沉积了YBCO涂层超导薄膜, 结果发现一步沉积所得YBCO薄膜的表面有许多白色小颗粒, 而且薄膜的织构和临界电流密度都较低, 这是在900℃高温沉积过程中大量的Ag蒸发和扩散到YBCO膜层中所致. 于是本文提出分高低温两步沉积的实验方案, 即700℃先沉积15min, 再升温到900℃沉积30min, 通过SEM对薄膜表面和EDS能谱对薄膜的断面进行分析可知, 两步沉积的YBCO薄膜中Ag的含量大大降低, 而且薄膜的织构和临界电流密度得到明显改善和提高, 最后通过两步沉积制备了15cm长、临界电流密度高于10⁴A/cm²的YBCO超导薄膜.     

Effect of SiO2 substrate on the irradiation-assisted manipulation of supported graphene: a molecular dynamics study

The irradiation effects in graphene supported by SiO(2) substrate including defect production and implantation efficiency are investigated using the molecular dynamics (MD) method with empirical potentials. We show that the irradiation damage in supported graphene comes from two aspects: the direct damage induced by the incident ions and the indirect damage resulting from backscattered particles and sputtered atoms from the substrate. In contrast with the damage in suspended graphene, we find that the indirect damage is dominant in supported graphene at high energies. As a result, enhanced irradiation damage in supported graphene is observed when the incident energy is above 5?keV for Ar and 3?keV for Si. The direct damage probability at all energies, even the total damage probability at low energies, in supported graphene is always much lower than that in suspended graphene because of the higher threshold displacement energy of carbon atoms. In addition, we demonstrate the striking finding that it is possible to dope graphene with sputtered atoms from the substrate and the implantation probability is considerable at optimal energies. Our results indicate that the substrate is an important factor in the process of ion-irradiation-assisted engineering of the properties of graphene.     

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.     

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.     

Internal stresses in sputtered chromium

This investigation delineates the effects of deposition rate, working pressure and nature of the working gas on the internal stresses found in wafers of chromium sputtered onto glass using a magnetron-type post-cathode sputtering apparatus. Attention is focused on the thickness range between 0.1 and 0.3 μm, where the measured force per unit width accumulates linearly with film thickness and where the effects of substrate heating are small. High compressive stresses develop when sputtering proceeds at low working pressures and moderate deposition rates. Raising both the deposition rate and the working pressure reduces the magnitude of film compression. The most pronounced effect is that of an increase in pressure, which causes first a complete reversal of stress from high compression to high tension and then a more gradual decrease towards zero. This behavior is the same for sputtering in argon or krypton. Analysis of the chromium films reveals no entrapped gas or structural changes to account for the onset of compression at reduced working pressures. The behavior closely resembles stress changes reported in the literature for bias sputtering as a function of substrate voltage. It is proposed that film compression in both cases results from peening of the depositing film by accelerated ions or neutral atoms.