Sputter Yield Amplification Research Articles

Rapid fabrication technique for aluminum optics by inducing a MRF contamination layer modification with Ar+ ion beam sputtering.

Aluminum optics are widely used in modern optical systems because of high specific stiffness and high reflectance. Magnetorheological finishing (MRF) provides a highly deterministic technology for high precision aluminum optics fabrication. However, the contamination layer will generate on the surface and bring difficulties for the subsequent processes, which highly limit the fabrication efficiency and precision. In this study, characteristics of the contamination layer and its formation process are firstly revealed through experimental and theoretical methods. Impurities such as abrasives are embedded into the aluminum substrate causing increasing surface hardness. The influence of the contaminant layer on machining accuracy and machining efficiency is analyzed in this study. Based on the analysis, ion beam sputtering (IBS) is induced as a contamination layer modification method. Impurities will be preferential sputtered during the process. Surface hardness and brightness will restore to the state before MRF. Moreover, the thickness of the contamination layer reduces dynamically during IBS because of the bombardment-induced Gibbsian segregation and sputter yield amplification mechanism. Consequently, we proposed a combined technique that includes MRF, IBS and smoothing polishing. Comparative experiments are performed on an elliptical shape plane surface. The results indicate that the efficiency has been increased sevenfold and surface precision is also highly improved. Our research will promote the application of aluminum optics to the visible and even ultraviolet band.

Sputter deposition of NiW films from a rotatable target

A series of NiW thin films was magnetron sputter deposited from a rotatable Ni80W20 target at Ar pressures ranging from 0.25 to 1.00 Pa on a matrix of (1 0 0) Si substrates. All films were synthesized in static deposition mode. The chosen setup allowed to study the influence of lateral substrate position and Ar pressure on film microstructure and elemental composition. All films deposited opposite the target were dense and Ni-rich, whereas films grown at extended positions from the target center had an open voided morphology and were W-rich. The differences in elemental composition with respect to the lateral position on the substrate carrier are attributed to the respective emission profiles of Ni and W and to scatter events of sputtered particles with Ar gas. To assess the influence of the target surface on final film properties, a planar Ni81W19 target was sputter eroded and investigated. The observed roughening of the target surface stems from the interplay between the high and low sputter yield of Ni and W, influencing the emission angles of both elements. Additional SRIM simulations of a thin Ni top-layer atop a W under-layer provided evidence for the existence of sputter yield amplification active within the studied system.

Sputtering of SicGe1−c nanospheres

It is known that sputter yields of elemental materials depend on the target curvature. Here we explore to what extent this effect is modified for alloy targets using both Monte Carlo and molecular dynamics simulations. For the exemplary case of amorphous ${\mathrm{Si}}_{c}{\mathrm{Ge}}_{1\ensuremath{-}c}$ spheres, we find that, in the limit of small curvatures, the curvature dependence of the sputter preferentiality is negligible. This finding can be explained by a natural extension of the existing analytical theory of curvature-dependent sputtering for elemental materials. For large curvatures, however, the sputter preferentiality strongly depends on curvature. In this case, molecular-dynamics simulations also predict strong spike effects which increase the sputter yields. Sputter yield amplification---an increase in partial sputter yield with decreasing concentration of the element---occurs for Si-rich alloys.

Rotatable serial co-sputtering of doped titania

Serial co-sputtering is an extension of conventional magnetron sputtering: it utilizes cylindrical primary rotating sputter sources within a metallic or reactive sputtering process. With one or more auxiliary sputter sources the surface of the primary sputter target is simultaneously coated with additional elements. In the primary sputtering process, these elements get implanted and mixed into the primary target material leading to production of multi-component films.In this paper we use an installation of this technology called “Megatron™”, which involves an effective gas separation between primary and auxiliary chamber volumes. With this setup, we fabricated TiO2:W and TiO2:Nb layers by using a rotating Titania target as primary source and planar W and Nb targets as auxiliary sources, respectively.In both cases the so called sputter yield amplification effect (SYA) was demonstrated: Within the TiO2 matrix the heavier element e. g. tungsten keeps the sputter cascade close to the target surface and thus significantly enhances the sputter rate. Additionally, by niobium doping in combination with a post-deposition annealing it is possible to get TiO2:Nb layers with tailored Nb composition as conductive transparent oxides (TCO).Due to the independent power control of the secondary target the tungsten and niobium concentration can easily be optimized. We present doping series for optimizing either the SYA effect for TiO2:W or the effect of a transparent conductive TCO for TiO2:Nb. With tungsten doping we achieved an enhancement of more than twice the DC sputter rate. With niobium doping and a post-deposition annealing step (350 °C) we reach resistivity values of about 1.2 × 10−3 Ωcm.

Increasing the carbon deposition rate using sputter yield amplification upon serial magnetron co-sputtering

Deposition of carbon based materials by magnetron sputtering suffers frequently from the low deposition rate of carbon due to its low sputtering yield. Here, we describe an approach based on the so-called sputtering yield amplification, which significantly increases the sputtering yield. Carbon has been doped by serial co-sputtering with two different elements, namely tungsten and niobium. Both elements provide a significant rate increase. Addition of 3at.% of Nb increases the deposition rate of carbon by 130%, whereas the same concentration of W increases it by 280%. TRIDYN simulations have been performed, which reproduce the experimental data. Additionally, our experiments find evidence for very long residence times of the dopant in the target as a result of recoil implantation.

Simulation of ion beam sputtering with SDTrimSP, TRIDYN and SRIM

• We present a comparison between experimental sputter data and binary Monte Carlo simulation programs SRIM, TRIDYN and SDTrimSP, which provide a detailed insight into the capabilities of the commonly used programs to simulate ion sputtering in a quantitative way. • We show that SDTrimSP is able to describe different features of ion beam erosion quantitatively and in good agreement with many experimental data. • We reveal very important discrepancies for simulation results obtained with SRIM, in particular the angular distribution of sputtered atoms for light target elements and the sputter yield dependence on ion incidence angle. • We also present a comparison with experimental data which shows the limitations of the commonly used ZBL potential to correctly describe low energy atomic collisions. A quantitative simulation of ion beam sputtering and related collision cascade effects is essential for applications of ion beam irradiation in thin film deposition, surface treatment and sculpting with focused ion beams, ion beam smoothing of surfaces and ion-induced nanopattern formation. The understanding of fundamental ion–solid interaction processes relevant for nanostructure formation, ion-induced mass redistribution, sputter yield amplification, ion beam mixing and dynamic compositional changes requires reliable simulations of ion-solid interaction processes in particular at low ion energies. In this contribution we discuss the possibilities, the key benefits and the limitations of three popular binary collision Monte Carlo simulation programs (SDTrimSP, TRIDYN and SRIM). The focus will be set to the calculation of angle dependent sputter yields, angular distribution of sputtered particles, sputter yields for compound materials, sputter yield amplification effects, as well as the extraction of parameters relevant for modeling ion-induced surface pattern formation from vacancy and recoil atom distributions.

Surface instability of binary compounds caused by sputter yield amplification

It is demonstrated that the flat surface of a binary material that is subjected to normal-incidence ion bombardment can be unstable even if the curvature dependence of the sputter yields is negligibly small. This unforeseen instability is brought about by sputter yield amplification, and it results in the formation of a disordered array of nanodots with the lighter of the two atomic species concentrated at the peaks of the dots.

Sputter yield amplification by tungsten doping of Al2O3 employing reactive serial co-sputtering: process characteristics and resulting film properties

The deposition rate of reactively sputtered Al2O3 coatings is demonstrated to increase by 80% upon tungsten doping of the used aluminium target. This effect is based on the recoil of the sputtering species at implanted dopants below the target surface and is termed sputter yield amplification. For the investigation of this effect, a novel type of magnetron sputter deposition system is employed that facilitates serial co-sputtering. In this technique doping of the elementary target is enabled by a dynamic sputtering process from an auxiliary cathode. In our case, the rotating aluminium target is dynamically coated with tungsten from this auxiliary cathode. Since the primary target rotates, the auxiliary cathode is placed in series with the primary erosion zone. The deposition rate of Al2O3 can be considerably increased in this process already for very low concentrations of approximately 1% of tungsten in the resulting film. A characterization of the dynamics of reactive sputtering as a function of target rotation speed is performed.

Magnetron sputtering – Milestones of 30 years

Since the introduction of the planar magnetron by J.S. Chapin in 1974 magnetron sputtering has become the most important technology for the deposition of thin films. Today it has conquered all industrial branches needing high-quality coatings for realization of new or improvement of existing products. The magnetron cathode combines the advantages of economic deposition even on large areas and the ability to coat very temperature sensitive plastic substrates. Main problems like poor target material utilization of the planar magnetron or process instabilities during deposition of highly insulating films have been solved by many innovations during the past 30 years. Novel films with even better quality seem to be possible with “High Power Impulse Magnetron Sputtering (HiPIMS)”. New attempts to increase sputter yield and thus film growth rate are “Sputter Yield Amplification (SYA)” or sputtering from hot targets. This paper gives a brief review on important milestones of the past three decades and outlines some ongoing developments.

A novel method to determine the ion sputter coefficient of dilute segregating impurities

A combination of linear programmed heating (LPH) and 3 keV Ar +- ion sputtering was used to obtain Sn sputter coefficients at room and elevated temperatures using a novel new method. At room temperature the determined value agrees well with the elemental sputter yield, Y Sn=2.5, if sputter yield amplification (SYA) of a submonolayer thin Sn film on a Cu(1 1 1) surface is taken into account. However, at high temperature the sputter coefficient decreases to 1.1. This decrease can be explained if it is assumed that not all Sn atoms are removed from the surface into the vacuum by the sputtering process, but knocked on into the bulk of the Cu matrix. At elevated temperatures these Sn atoms segregate back to the surface and can be sputtered once again.

Ion assisted deposition of Zn–Mg coatings by unbalanced magnetron sputtering

Several patents have recently been assigned to Japanese coated sheet steel producers for the invention of novel vapour deposited Zn–Mg alloy coatings. Claims have been made that the coating compositions and microstructures attainable provide significant improvements in the corrosion resistance compared with hot-dip galvanized products. In response to this industrial interest, a range of Zn–Mg coatings were prepared by unbalanced magnetron sputtering, with the aim of investigating the effects of ion assistance on coating composition. Circular planar targets consisting of sequential sectors of Zn and Mg, bonded to a directly cooled copper backing plate, were used for the preparation of alloy coatings. Ion assistance resulted in the preferential resputtering of Mg from the alloy coatings. Monte Carlo simulations using the T-DYN code revealed this was mainly due to the sputter yield amplification effect. Ion assistance also resulted in retention of exceptionally high proportions of Ar in the Zn–Mg coatings, particularly for the Mg-rich compositions. Coatings with high Ar content were sensitive to failure by heavy oxidation after aging at ambient conditions.

Preferential sputtering effects in thin film processing

Predicting the partial sputtering yield (sputtered atoms of one element/incident energetic ion) for different elements during sputtering from multielement targets is not a straightforward task. It is commonly observed that ion bombardment of composite targets exhibits preferential sputtering. Such processes are normally studied by computer simulations since theory gives qualitative results only. This work presents a systematic computational study of the major ballistically induced preferential sputtering effects taking place during ion bombardment of composite materials. It is shown that in a vast number of cases ballistic preferential sputtering is a dominant process particularly at moderate temperatures where thermodynamic effects are suppressed. In particular, sputtering from composite targets, resputtering during ion assisted deposition of composite films, sputtering of thin films onto heavy substrates, ion assisted deposition onto different substrate materials, sputter depth profiling, etc. are technological processes shown to commonly exhibit preferential sputtering. It is further shown that a number of sputtering situations exhibit along with the preferential sputtering the so called sputter yield amplification effect. The latter consists in that the partial sputtering yield of a given species is larger than the sputtering yield of the pure material. As a consequence, the sputtering yield of certain materials can be enhanced substantially by introducing small concentrations (a few percent) of impurity species in the target. The work provides ample examples of various situations of academic and technological interest where preferential sputtering and sputter yield amplification occur. The major ballistic mechanisms leading to preferential sputtering and sputter yield amplification are also briefly discussed.

Resputtering effects during ion beam assisted deposition and the sputter yield amplification effect

Sputter yield amplification (SYA) is an effect where the sputtering yield of a given material is significantly increased by the introduction of a small concentration of a heavy impurity at the surface. The basic ideas behind the SYA effect as well as its major physical mechanisms are presented and discussed in some detail. Exhaustive numerical simulation results are shown to support the models suggested. Experimental verification of key predictions, such as magnitude of sputter yield amplification as well as selective ion-assisted deposition, are also presented. More specifically, the paper first treats a number of cases in relation to sputtering of composite targets. Results are presented showing the conditions under which optimum amplification is achieved as well as cases exhibiting a peculiar transition to steady state. We also look at a series of ion-assisted deposition situations where sputter yield amplification during resputtering has a pronounced effect. Attention here is drawn to the peculiarities of sputtering of ultra-thin films and particularly to interface evolution during ion-assisted deposition with single and multiple deposition fluxes.

Numerical and experimental studies of the sputter yield amplification effect

Abstract Recently we have demonstrated the existence of the so-called sputter yield amplification (SYA) effect, based on the preferential sputtering of the lighter species during ion bombardment of composite solids and resulting in enhanced partial sputtering yield of the lighter species as compared to the elemental sputtering yield. In specific cases the enhancement of the sputtering yield could reach one order of magnitude or more. This effect has been both numerically studied and experimentally observed during low energy ion bombardment of i) thin films of low mass on heavy substrates; ii) thin films of high mass on light substrates; iii) simultaneous high mass atom deposition on light substrates; iv) simultaneous low mass atom deposition on heavy substrates. In all cases sputter yield enhancement of the low mass species is observed but in each individual case different specific goals are achieved. For example, in case i) much faster etch rate of the thin film is achieved than what is expected from the elemental sputtering yield. In cases ii) and iii) faster erosion of the target is achieved in view of sputter-deposition processes. In case iv) a nearly complete removal of the deposited species with a partial sputtering yield substantially higher than the elemental sputtering yield is achieved in view of selective ion beam assisted deposition. The SYA effect is also observed in the net growth regime, i.e. during the deposition of composite films with ion co-bombardment. In this case the composition of the growing film differs from that of the deposition flux. This work presents systematic numerical and experimental studies of the SYA effect in all these cases as a function of the ion energy, mass and angle of incidence as well as the target species. The simulations have been performed with the T-DYN program. General rules are given as to when the amplification effect is most pronounced. Potential applications are also discussed.

Preferential sputtering of silicon from metal silicides at elevated temperatures

It has been shown that at elevated temperatures silicon is preferentially sputtered from Ti and Co silicides during low energy Ar ion bombardment. The Si sputtering yield under these conditions is shown to substantially exceed that of pure Si, as well as that it is a function of the ion current density. On the other hand, the sputtering yield of the metal is found to be substantially smaller than that at lower temperatures under the same conditions. These effects have been studied both experimentally and numerically using the dynamic T-DYN program. It is shown that the increased Si sputtering yield is partly due to the so-called sputter yield amplification effect (which is a purely ballistic effect independent of temperature), while its sustenance with dose is caused by thermal and ion beam induced diffusion and segregation of Si at the surface.

Large-area selective thin film deposition by bias sputtering

It has been demonstrated both theoretically and experimentally that selective sputter deposition of specific materials on the basis of purely ballistic processes is possible. The mechanisms of the selective deposition process have been studied in great detail using the dynamic code t-dyn. In particular, it has been shown that the selective bias sputter deposition process is closely related to the so-called sputter yield amplification effect. Further, it has been shown how such computer simulations are used to predict the conditions under which selective deposition can be achieved as well as to aid the selection of suitable materials and optimal operating conditions. Finally, it is experimentally demonstrated that (i) Al is selectively deposited on Si and not on W surfaces, and (ii) Ti is selectively deposited on Si and not on Pt surfaces, during bias sputter deposition of Al and Ti respectively with simultaneous Ar ion bombardment.

Serial cosputtering of metals: modelling of sputtering from a periodically codeposited surface

Serial cosputtering is a simple yet powerful tool for the deposition of multilayered thin films with unique physical properties. This article studies one specific cosputtering system and discusses how the so-called “sputter yield amplification effect” can be utilized to increase the deposition rates by the suitable choice of both materials used and operating conditions. The main emphasis is placed on computer modelling of the system with a view to optimizing the whole technological process. Comparisons between experiment and computer simulation in a number of typical situations is presented.

Ion-assisted selective deposition of aluminium for via-hole interconnections

Recently we have demonstrated the existence of the so-called sputter yield amplification effect, based on the preferential sputtering of a particular species during the ion bombardment of composite solids. This work presents a more elaborate study of the sputter yield amplification effect using the dynamic Trim version T-DYN. General rules are given as to when the amplification effect is most pronounced and when it is suppressed, as well as its dependence on various parameters. The potential applications of the amplification effect are also discussed. In particular, the use of the effect as a planarization technique during ion-assisted deposition of Al is studied both theoretically and experimentally. It is shown that under low-energy argon ion bombardment Al can be selectively deposited onto Al surfaces while at the same time its growth on W covered SiO 2 surfaces is totally prohibited.