Sputtering Of Si Research Articles

Molecular dynamics study of the fluence dependence of Si sputtering by 1 keV Ar + ions

The fluence dependence of the Si sputtering by 1 keV Ar + has been studied by molecular dynamics simulations. To this purpose, previously amorphized samples with different initial argon concentration have been ion bombarded and the sputtered atoms have been analyzed. The calculated sputtering yield increases with the argon content according to the experimental results. The mechanisms involved in this sputtering enhancement are discussed.

Initial stage of layer-by-layer sputtering of Si(111) surfaces studied by scanning reflection electron microscopy

The initial stage of layer-by-layer sputtering of Si(111) surfaces with 500 eV Ar ions has been studied by using scanning reflection electron microscopy. At a moderate temperature of 900 K, vacancy islands are formed in the middle of wide terraces. At higher temperatures over 990 K, the atomic steps retreat as the ion dose is increased. The results show that layer-by-layer sputtering is characterized by vacancy creation caused by ion impact, and by thermally activated surface migration of vacancies. Moreover, the step area decorated by electron-beam-assisted carbon deposition acts as a step pinning site during the layer-by-layer sputtering.

Oscillations of step velocity at sputtering of Si(111) vicinal surfaces by low-energy Xe ions

The evolution of a vicinal Si(111) surface at layer-by-layer sputtering by low-energy Xe ion irradiation is simulated at various temperatures (500–650°C) and ion fluxes (10 12–6 × 10 12 cm −2 s −1). The spatial distribution of ion-induced surface defects was found to be responsible for the oscillations of step velocity and surface coverage at some temperature range depending on the ion-bean flux. The period of oscillations is defined by the time needed for the removal of one monolayer. The oscillations damp out when the steady state of the surface is reached.

Estimation of the depth resolution of secondary ion mass spectrometry at the interface SiO2/Si

Similarities between the processes that occur during sputtering of monocrystalline Si by reactive O2+ primary ions and the interface SiO2/monocrystalline Si by noble gas ions (e.g., by Ar+) have motivated us to utilize the semiempirical model of P. C. Zalm and C. J. Vriezema [Nucl. Instrum. Methods B 67, 495 (1992)], modified later by M. Petravić, B. G. Svensson, and J. S. Williams [Appl. Phys. Lett. 62, 278 (1993)] to calculate the decay length λb, as defined by J. B. Clegg [Surf. Interface Anal. 10, 322 (1987)], at the SiO2/Si interface. The measured and calculated results agree remarkably well. Inconsistency observed to be larger than 100% for glancing incidence angles confirms limitations of this model that were admitted already by its authors.

Development of an optical emission spectroscopy-based method for dynamic compositional analysis of sputter deposited films from multicomponent targets

A method of calculating as-deposited elemental film composition during the actual deposition process was developed using optical emission spectroscopy of the glow discharge. These results are demonstrated by analyses of a series of thin film Ag1−xSix alloys fabricated using radio frequency sputter deposition with composite Ag+Si sputter targets. Using intensity ratios of the Si (251.6 nm), Ag (328.1 nm), and Ar (419.8 nm) emission lines after equilibration of the substrate temperature, estimates of at. % Si in the films were calculated which agreed well with post-deposition composition measurements of the films by Auger electron spectroscopy.

Formation of β-SiC films by ion beam mixing of [formula omitted] multilayers

We deposited Si C multilayers of nearly equiatomic composition by ion sputtering of Si and C targets using 1.2 keV Ar + ions. Ion mixing of these samples was performed with 250 keV Xe + ions at temperatures ranging between 700 and 900°C. The behaviour of β-SiC formation on mixing was investigated using three techniques: Fourier transform IR spectroscopy, transmission electron microscopy and X-ray diffraction. In addition, X-ray reflectometry measurements were taken to determine the film density. The films produced by mixing at 700°C are amorphous but subsequent annealing at 950°C results in the formation of crystalline β-SiC. Complete mixing of the multilayers is only obtained at 800°C after ion doses higher than 3 × 10 16 cm −2, as indicated by cross-sectional electron microscopy. These results demonstrate that ion mixing at moderate temperatures T ∼ 800°C of Si C multilayers may provide an alternative method for the formation of β-SiC films.

Optimum beam energy for high depth resolution secondary ion mass spectrometry

Practical secondary ion mass spectrometry depth resolution data show that decreasing beam energies lead to an enhanced resolution down to the lowest practical energy of 1 keV O+2 at near normal incidence. For B and C in Si, the resolution full width at half-maximum amounts to 3.1 and ∼1.4 nm, respectively. Using a retarded probing beam, we have shown that a useful flux of Si atoms can be sputtered with primary energies as low as 260 eV O+2 or 130 eV O+; the Si sputter yield is also shown to be weakly dependent on the probe energy. Therefore it is predicted that this low energy is the optimum to use in a depth profiling experiment using oxygen. The predicted resolution for B and C amounts to 1.8 and 0.8 nm. Such an analytical performance would easily justify the resources required to produce ultralow energy beams on a routine basis.

The circumstellar environment of IRAS 05338-0624.

Millimeter continuum and spectral line observations with 10", 30", and 60" resolution are used to characterize the structure and chemistry of the gas around the young, embedded star, IRAS 05338-0624. On arcminute scales, emission from dense gas tracers outline an isolated condensation centered on the IRAS source position. The condensation is characterized by a size of approximately 60", a density of 2 x 10(5) cm-3, and a virial mass of 40 M solar. Interferometric CS J = 2-1 observations show two peaks, one toward the continuum peak and the other toward a position 14" west and 8" south. Single-dish maps of SO, CH3OH, and SiO show pronounced wing emission to the west of the IRAS source, which interferometer observations reveal to be a compact region of outflow activity. CS emission as redshifted and blueshifted velocities reveals a bipolar outflow oriented with a position angle of 45 degrees, while SiO emission appears to be tracing a fast shock interaction region at the CS red-lobe peak, 14" west and 8" south of the IRAS source. Finally, H13CO+ emission traces clumps of quiescent gas toward the IRAS source and adjacent to the blue lobe of the outflow. Column densities and molecular fractional abundances are derived to explore the interaction between the surrounding condensation and the young stellar object. We find evidence for gas phase depletions within the overall condensation in several gas tracers (CO, CS, HCN, SO) but not in the region immediately around the young stellar object. Enhanced abundances of SO, CH3OH, and SiO (by factors of 4, >100, >1000, respectively) are observed in the shocked gas; these enhancements may be explained in terms of a nondissociative shock liberating mantle materials that contain some amount of refractory materials, a moderate velocity dissociative shock in which only minor sputtering of Si occurs, or a shock that impacts surrounding material with a range of speeds.

The two-photon spectrum of iron and silicon detected by resonant laser ablation

We report ionization spectra of Si and Fe atoms ablated at low fluence from bulk Si and Re targets, respectively. Two-photon transition intensities are calculated using second-order perturbation theory. We obtain moderate agreement between experimental and calculated intensities, assuming a temperature of 1100±200 K for Fe ablation from Re. Si and Fe are detected in excited electronic states up to 1.9 and 1.5 eV, respectively. Ablation of electronically excited Si is enhanced with exposure to oxygen or water. This effect is compared with a similar effect observed in ion induced sputtering of Si and other materials.

Mechanism of particle formation in the sputtering and reactive ion etching of Si and SiO2

Particle formation from Si and SiO2 substrates in Ar sputtering plasmas and CCl2F2/Ar reactive ion etching (RIE) plasmas has been investigated using laser light scattering, electron microscopy, and optical emission spectroscopy. Nearly spherical particles with sizes down to 50 nm are collected downstream from the substrate even before the onset of light scattering or at pressures below the threshold for cloud formation. Si atoms are seen readily by optical emission during sputtering, but not during RIE, of both Si and SiO2. The rate of particle formation correlates with the density of Si atoms in the plasma. Hence, it is suggested that Si atoms are probably the species responsible for particle nucleation in sputtering and RIE, and formation of a particle cloud occurs only when the transport of Si atoms out of the sheath edge region becomes inefficient, as evidenced by redeposition in sputtering and by polymerization in RIE.

MC computer simulations of the preferential sputtering of Si-Ge alloys

Abstract The ability of MC codes to predict the preferential sputtering of compound targets is investigated. The DYNA and TRIDYN codes are run for 3 keV Ar + bombardment of a SiGe binary target. The preferential sputtering of Si and Ge, the depth dependence of the sputter cross-section and the relocation operators are calculated. Difficulties arise in trying to reproduce the experimentally reported absence of preferentiality in the sputtered flux. The models used for the surface barriers, as well as the barrier heights, influences strongly the predicted quantities. A spherical surface barrier predicts much closer to stoichiometric fluxes than a planar barrier. Different codes give different collisional diffusivities for the target species in the bulk. The need for further experiments is stressed if some guidance in the choice of input parameters in the codes is desired.

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.

Modelling of layer-by-layer sputtering of Si(111) surfaces under irradiation with low-energy ions

Molecular dynamics (MD) simulations of ion-solid interactions and numerical solutions of coupled reaction-diffusion equations for surface vacancies and adatoms are used to investigate layer-by-layer sputtering of Si(111) surfaces by 225 eV xenon ions. The model of Bedrossian and co-workers that low-energy sputtering is dominated by the creation of isolated surface vacancies and nucleation of vacancy islands is not supported by our model. Our MD calculations show that, besides a few sputtered surface atoms, additional surface vacancies are caused by the excitation of surface atoms in adatom positions. These additional surface vacancies and adatoms have a great influence on the sputtering kinetics. In each collision, surface vacancies are produced not in isolation but in clusters. The MD results are taken as input for the solution of the reaction-diffusion equations.

Chemical sputtering of Si related to roughness formation of a Cl-passivated Si surface

Chemical sputtering of Si in a chlorine environment has been examined with molecular dynamics simulations. It is found that chemical sputtering correlates with the roughness formation of Cl-passivated Si surfaces during low-energy ion bombardment. The chlorine passivation of the Si surface prevents the flattening of the surface due to the high activation barrier for surface diffusion. The rough surface contains reactive intermediates that can be desorbed into the gas phase when, after an ion impact, the region has a large energy content. The observed products and the increase of the sputtering yield are in agreement with experimental observations.

In situ RBS analysis of ion beam mixing during low energy sputtering

Low energy ion sputtering for depth profiling in SIMS, AES and XPS produces sputter induced artifacts. These effects are studied by comparing RBS spectra of the bombarded layer to the original target composition. For this purpose an automated RBS chamber has been built combining an Atomika SIMS system and a NEC microprobe RBS beam line. In this article the effect of ion beam mixing of Pt on Si and of CoSi 2 on Si under oxygen bombardment, as well as the preferential sputtering of Si in CoSi 2 under Ar bombardment are demonstrated.

The effect of grain orientation on the relaxation of thermomechanical stress and hillock growth in AI-1%Si conductor layers on silicon substrates

Hillocks are formed sporadically in Al-l%Si sputter layers on SiO2/Si substrates during heat treatments in the range from 200 to 500°C. The driving force is the relaxation of thermomechanical stress in the grains induced by the thermal expansion mismatch between the metallization layer and the substrate. The orientations of individual grains and hillocks are measured on-line with a medium voltage transmission electron microscope by the Kikuchi pattern method. Thermomechanical stress in the grains is calculated with a biaxial strain model, considering the glide systems of dislocations for the individual grain orientations. In general, hillocks deviate from the ordinary 〈111〉 fiber texture of aluminum sputter layers. The spatial distribution of grain orientations is illustrated by orientation images using Miller indices or Rodrigues vectors.

Effect of a Si layer on the superconductive properties of a BiPbSrCaCuO thin film

The Pb-doped BiSrCaCuO superconducting films were grown by the single source mixed evaporation technique. The microbridges of dimensions 50 mum x 40 mum were fabricated by standard photolithography technologies. Si films with a thickness of 2500 angstrom were deposited on the microbridge area surfaces of BiPbSrCaCuO superconducting films by rf-magnetron sputtering. A greatly lowered zero resistance temperature of the microbridge area of the BiPbSrCaCuO film after Si sputtering was found. A non-linear effect of the current-voltage (I-V) characteristics at 78 K was shown. The high-frequency capacitance-voltage (C-V) curve of this structure at 78 K was symmetrical with the maximum capacitance at V = 0, and the capacitance decreased with increasing applied bias voltage. Afl experimental results are discussed.

Structural study of sputtered Pd/Si multilayers

In Pd/Si multilayers grown by sputtering Pd and Si alternatively, Pd 2Si is formed during sputtering when the sublayer thickness of Pd is less than 2 nm, and no silicide is found when Pd sublayer is thicker. The transmission electron micrographs and electron diffraction patterns of Pd and Si single layers with various thicknesses show that thin Pd films 0.9–1.4 nm thick are island-like and all the Si films are homogeneous and amorphous. It is suggested that the sputtered Si particles carrying high energy can result in a big local temperature rise of the Pd islands and promote Pd and Si atoms to interdiffuse and react with each other to form Pd 2Si.

Surface reconstruction in layer-by-layer sputtering of Si(111).

Using scanning tunneling microscopy, we have investigated vacancy-mediated, layer-by-layer removal of surface atoms from Si(111) under 225-eV Xe-ion bombardment. We observe, with increasing substrate temperature, both the transition from vacancy-cluster nucleation to step retraction, and a transition from disruption to preservation of the 7\ifmmode\times\else\texttimes\fi{}7 reconstruction on exposed material. We discuss the consequences of these observations for ion-beam-mediated epitaxy.

Layer-by-layer sputtering and epitaxy of Si(100).

We report oscillations in diffracted electron intensities during ion bombardment of Si(100) by 200- and 250-eV Xe, both alone and with sequential and simultaneous epitaxy. Analysis of the phase and frequency of the oscillations shows that, to first order, ion bombardment ``undoes'' previous epitaxy and cancels or partially cancels simultaneous deposition. Surprisingly, the phase relationship of growth and sputtering is both antisymmetric and linear, indicating that the ion-induced oscillations are dominated by simple, vacancy-mediated, layer-by-layer sputtering.