Roughening of silicon (1 0 0) surface during low energy Cs ion bombardment

Nowadays, the use of low energy ion bombardment in secondary ion mass spectrometry (SIMS) is a mandatory step to obtain high depth resolution for the characterization of ultra shallow junctions. However, increment of roughness during ion bombardment leads to the degradation of depth resolution. The evolution of surface roughening and ripple formation on silicon at ambient temperature under 1 keV Cs⁺ ion bombardment with and without sample rotation has been studied by means of atomic force microscopy.Ripple formation with a perpendicular orientation with respect to the Cs⁺ beam direction has been detected, and their wavelength and correlation length have been monitored as a function of the experimental conditions. Roughness and wavelength increased with increasing ion fluence, while variations of ion flux showed little effect. The effect of sample rotation during ion bombardment led to a critical reduction of the surface roughness and disappearance of ripples.     

Anisotropic rearrangement of the substrate atoms during Ar bombardment on Pd(0 0 1) surface

Using a three-dimensional molecular dynamics (MD) simulation, we investigated the atomic scale rearrangement that occurs on a Pd(0 0 1) surface after energetic bombardment by Ar at room temperature. High energy Ar bombardment provoked the significant rearrangement of Pd atoms in a ballistic manner with a fourfold symmetric lateral distribution aligned along the 〈1 1 0〉 direction. The MD simulation of uniform Ar bombardment at normal incidence on a Pd surface reproduced the experimentally observed fourfold symmetric nano-scale surface structure. The present result supports that the ballistic rearrangement of the substrate atoms plays an important role in the ion induced surface structure evolution.     

Fundamentals of surfactant sputtering

We introduce a new sputter technique, utilizing the steady-state coverage of a substrate surface with up to 10¹⁶ cm⁻² of foreign atoms simultaneously during sputter erosion by combined ion irradiation and atom deposition. These atoms strongly modify the substrate sputter yield on atomic to macroscopic length scales and therefore act as surfactant atoms (a blend of “surface active agent”). Depending on the surfactant-substrate combination, the novel technique allows enhanced surface smoothing, generation of novel surface patterns, shaping of surfaces and formation of ultra-thin films. Sputter yield attenuation is demonstrated for sputtering of Si and Fe substrates and different surfactant species using 5 keV Xe ions at different incidence angles and fluences up to 10¹⁷ cm⁻². Analytical approaches and Monte Carlo simulations are used to predict the sputtering yield attenuation as function of surfactant coverage. For sputtering of Si with Au surfactants we observe high sputter yields despite a steady-state surfactant coverage, which can be explained by strong ion-induced interdiffusion of substrate and surfactant atoms and the formation of a buried Au_xSi surfactant layer in dynamic equilibrium.     

Surface roughness of ion-bombarded Si(1 0 0) surfaces: Roughening and smoothing with the same roughness exponent

We have carried out scanning tunneling microscopy experiments under ultrahigh vacuum condition to study the roughness of pristine as well as ion-bombarded Si(1 0 0) surfaces and of ultrathin Ge films deposited on them. One half of a Si(1 0 0) sample (with native oxide layer) was irradiated at room temperature using 45 keV Si⁻ ions at a fluence of 4 × 10¹⁵ ions/cm² while the other half was masked. STM measurements were then carried out on the unirradiated as well as the irradiated half of the sample. Root-mean-square (rms) roughness of both the halves of the sample has been measured as a function of STM scan size. Below a length scale of ∼30 nm we observe surface smoothing and surface roughening is observed for length scales above this value. However, the surface is self-affine up to length scales of ∼200 nm and the observed roughness exponent of 0.46 ± 0.04 is comparable to earlier cases of ion sputtering studies where only roughening [J. Krim, I. Heyvart, D.V. Haesendonck, Y. Bruynseraede, Phys. Rev. Lett. 70 (1993) 57] or only smoothing [D.K. Goswami, B.N. Dev, Phys. Rev. B 68 (2003) 033401] was observed. Preliminary results involving morphology for Ge deposition on clean ion-irradiated and pristine Si(1 0 0) surfaces are presented.     

On the emission of MoCs molecular ions from Cs irradiated molybdenum surface

The present paper deals with the emission of atomic and molecular ions from elemental molybdenum surface under Cs⁺ bombardment to explore the MCs⁺ formation mechanism with changing Cs surface coverage. Integrated count of MoCs⁺ shows a monotonic increase with increasing primary ion energy (1-5 keV). Change in MoCs⁺ intensity is attributed to the variation of surface work function ? and cesium surface concentration c_Cs due to varying impact energies. Variation of c_Cs has been obtained from the expression, c_Cs ∝ 1/(1 + Y) where Y is the elemental sputtering yield estimated from TRIM calculations. Systematic study of the energy distributions of all species emerging from Mo target has been done to measure the changes in surface work function. Changing slopes of the leading parts of Cs⁺ energy distributions suggest a substantial depletion in surface work function ? with decreasing primary ion energies. Δ? shows a linear dependence on c_Cs. The maximum reduction in surface work function Δ?_max = 0.69 eV corresponds to the highest value of c_Cs = 0.5. A phenomenological model, based on the linear dependence of ? on c_Cs, has been employed to explain the MoCs⁺ data.     

The influence of projectile charge state on ionization probabilities of sputtered atoms

The ionization probability of atoms sputtered from a clean polycrystalline metal surface was measured for different charge states of the projectile used to bombard the sample. More specifically, a polycrystalline indium surface was irradiated with Ar⁺ and Ar⁰ beams of energies between 5 and 15 keV, and In⁺ secondary ions and neutral In atoms emitted from the surface were detected under identical experimental conditions regarding the sampled emission angle and energy. The resulting energy integrated ionization probability of sputtered In atoms is consistently found to be smaller for neutral projectiles, the difference decreasing with decreasing impact energy. The observed trends agree with those measured for kinetic electron emission, indicating that secondary ion formation is at least partly governed by kinetic substrate excitation.     

Effect of N and C ion beam bombardment on the optical and structural characteristics of ethylene-norbornene copolymer (TOPAS)

Ion bombardment is a suitable tool to modify the optical properties of polymers. In the present study the effect of ion bombardment on the optical absorption of ethylene-norbornene copolymer (TOPAS) was studied using ultraviolet-visible (UV-Vis) and Raman spectroscopy. Polymer samples were bombarded with 60 keV C⁴⁺ and N⁴⁺ ion beams to various fluences ranging from 1.0 × 10¹³ to 1.0 × 10¹⁶ cm⁻². The indirect and direct band gaps have been determined. The values of direct band gaps have been found to be greater than the corresponding values of the indirect band gaps. Activation energy has been investigated as the function of ion fluences. The number of carbon atoms per conjugated length is determined according to modified Tauc’s equation. The correlation between the optical band gap, activation energy for optical transition and the number of carbon atoms per conjugated length as well as chemical structure changes induced by ion beams irradiation have been discussed in the case of ethylene-norbornene copolymer.     

Angular distribution of GaAs sputtered under oblique Cs bombardment

The angular distribution of Ga and As sputtered from Gallium Arsenide (1 0 0) by a Cs⁺ ion beam was experimentally measured through a collector technique allowing modifications of the energy and incidence angle of the ion beam. The impact energy was varied in the range of 2-10 keV and the angle of incidence from 30° to 60°.The angular distributions of emitted matter are determined by means of SIMS depth profiles. Our series of experiments show an evolution of the preferential direction of emission as well as the spreading around this direction in function of the characteristics of the ion beam.The second objective is the study of the evolution of the stoichiometry of the deposit in function of the emission angle. A decrease of the As/Ga ratio around the preferential direction of emission and an increase of this ratio for oblique emission are observed for different conditions of primary bombardment. Considering that the angular distribution depends on the depth of origin, our results suggest that the Cs⁺ bombardment changes the stoichiometry of the near-surface layers of the sample with an enrichment of As in the outmost layers while the sub-surface region is impoverished in As due to preferential sputtering.     

Argon ion impact desorption cross sections of chlorine and carbon from molybdenum

Total desorption cross sections have been measured for Cl (σ_Cl) and C(σ_C) on molybdenum by argon ion bombardment for an incidence angle of 60° from the surface normal. For the bombardment an ion gun with low current density (i₀ ~ 1 × 10 ^?7 A cm^?2) at low system pressure (~10^?9 Torr) was used. The detection was performed by AES and the data were sensitivity factor corrected. The AES analysis of the surface after adsorption showed that Mo, C and Cl contributed to more than 94% of the atomic composition. With known i₀, it is possible to obtain σ from the adsorbate signal vs ion bombardment time curve. For ion energies between 0.2 keV to 1.0 keV the measured value for σ_Cl and σ_C are 0.5?3 × 10^?15 cm² and 0.2?4 × 10^?15 cm², respectively. The possible effects of the surface roughness due to prebombardment are discussed.     

Growth of silicon nitride films by bombarding amorphous silicon with N ions: MD simulation

In this study, the molecular dynamics simulation method was employed to investigate the growth of silicon nitride films by using N⁺ ions, with energies of 50, 100, 150 and 200 eV, to bombard an amorphous silicon surface at 300 K. After an initial period of N⁺ bombardment, saturation of the number of N atoms deposited on the surface is observed, which is in agreement with experiments. During subsequent steady state deposition, a balance between uptake of N by the surface and sputtering of previously deposited N is established. The Si(N_x) (x = 1-4) and N(Si_y) (y = 1-3) bond configurations in the grown films are analyzed.     

Study of surface activation of PET by low energy (keV) Ni and N ion implantation

Polyethyleneterephthalate (PET) has been modified by 100 keV Ni⁺ and N⁺ ions using metal ion from volatile compound (MIVOC) ion source to fluence ranging from 1 × 10¹⁴ to 1 × 10¹⁶ ions/cm². The increasing application of polymeric material in technological and scientific field has motivated the use of surface treatment to modify the physical and chemical properties of polymer surfaces. When a material is exposed to ionization radiation, it suffers damage leading to surface activation depending on the type. The surface morphology was observed by atomic force microscopy (AFM). That show the roughness increases with fluence in both the cases. The Ni particles as precipitation in PET were observed by cross-section transmission electron microscopy (XTEM). The optical band gap (E_g) deduced from absorption spectra; was calculated by Tau’c relation. Raman spectroscopy shows quantitatively the chemical nature at the damage caused by the Ni⁺ and N⁺ bombardment. The ration of I_D/I_G shows graphite-like structure is formed on the surface. A layer of hydrogenated amorphous carbon is formed on the surface, which has confirmed by XPS results also.     

Ion beam erosion of amorphous materials: evolution of surface morphology

In this work we present our results concerning the formation of self-organized nanoscale structures during the bombardment with a low-energy defocused Ar ion beam. We studied glass surfaces because of their physical properties, technological interest and cheapness. The evolution of sample surface was studied ex situ by atomic force microscopy. We found, in agreement with Bradley and Harper, a morphology characterized by a regular ripple structure with the wave vector perpendicular or parallel to the ion beam direction. This structure periodicity was found to vary in the range 90–350 nm with a linear time evolution. In order to gain further information about the sputtering process and for comparison with the existing continuum theories of surface erosion, we studied the scaling behaviour of surface roughness.     

Amorphous phase and anisotropy induced by glancing incident ion beams in Co-Nb films

Co-Nb amorphous films were prepared with the aid of glancing incident ion beams during deposition process. Influence of ion interaction to phase formation and fine microstructure was studied. Amorphous range is about 19 to 63 at.% Co fractions, which is wider than that obtained by perpendicular ion bombardment (28 to 68 at.% of Co fractions). A ripple or a bamboo raft pattern with nano-scale periodicity is observed in the TEM (transmission electron microscopy), SEM (scanning electron microscopy) and AFM (atomic force microscopy) images. The sizes of the image patterns are characterized by correlation length calculated from height-height correlation function (HHCF). The correlation length along the ion incidence is longer than that perpendicular to the ion incidence. Analysis regards that the glancing incident ion beams have high efficiency in both rapid cooling and ion mixing (IM). The main pattern feature in the images mainly comes from surface erosion. Other fine microstructure and the difference among the images result from surface diffusion or viscous flow effect.     

Molecular dynamics simulations of CH ions interaction with silicon carbide

In this study, we have simulated the CH⁺ ions bombardment on a Si-terminated 3C-SiC (0 0 1) surface using molecular dynamic simulations with a Tersoff-Brenner potentials. The results show that the sputtering yields of Si atoms is more than that of C atoms. With increasing incident energy the uptake of H atoms increases, while the uptake of C atoms decreases. In amorphous layer, a carbon-rich region is observed. In the films, the CH group and SiH group are dominant. Their fractions increase with increasing incident energy, and then decrease.     

Molybdenum L-shell X-ray production by 350-600 keV Xe (q = 25-30) ions

Molybdenum L-shell X-rays were produced by Xe^q+ (q = 25-30) bombardment at low energies from 2.65 to 4.55 keV/amu (350-600 keV). We observed a kinetic energy threshold of Mo L-shell ionization down to 2.65-3.03 keV/amu (350-400 keV). The charge state effect of the incident ions was not observed which shows that the ions were neutralized, reaching an equilibrium charge state and losing their initial charge state memory before production of L-shell vacancies resulted in X-ray production. The experimental ionization cross sections were compared with those from Binary Encounter Approximation theory. Taking into account projectile deflection in the target nuclear Coulomb field, the ionization cross section of Mo L-shell near the kinetic energy threshold was well described.     

The broadening of energy spectra of atoms scattered by a solid surface under molecular ion bombardment

The broadening of energy spectra of atoms scattered by a metal surface under molecular ion bombardment are calculated taking into account the molecular ions to suffer dissociative neutralization on the initial part of their scattering trajectory. The results of the calculations explain the experimental data for scattering of hydrogen atoms from aluminum, palladium and palladium covered by potassium surfaces under bombardment with H₂⁺.     

Study of H loss from hydrogenated Hg1−xCdxTe under high electronic excitation by elastic recoil detection analysis (ERDA)

Large erosion (∼1.1 × 10⁴ atoms/ion) of H from hydrogenated MCT wafers is observed due to the bombardment with 80 MeV Ni⁹⁺ ions. The initial H areal concentration and hydrogen depletion rate is monitored by elastic recoil detection analysis. The ion-damaged zones from where depletion of H takes place have been calculated from fluence-dependent hydrogen areal content analysis. The results are explained on the basis of the thermal spike model of ion-solid interaction.     

Ion induced high energy electron emission from copper

We present measurements of secondary electron emission from Cu induced by low energy bombardment (1-5 keV) of noble gas (He⁺, Ne⁺ and Ar⁺) and Li⁺ ions. We identify different potential and kinetic mechanisms and find the presence of high energetic secondary electrons for a couple of ion-target combinations. In order to understand the presence of these fast electrons we need to consider the Fermi shuttle mechanism and the different ion neutralization efficiencies.     

Cesium/xenon co-sputtering at different energies during ToF-SIMS depth profiling

In this paper, ToF-SIMS dual beam depth profiles of H-terminated silicon wafers were performed with cesium primary ions and for different beam energies. The aim of this study was to investigate the influence of the cesium beam energy on the secondary ion yields during ToF-SIMS dual beam depth profiling. For this purpose, both the cesium beam energy and the cesium surface concentration were varied but the analysis conditions were kept identical for all depth profiles (i.e. Ga⁺ at 25 keV, 45°). For each sputter beam energy (i.e. 250 eV, 750 eV and 2000 eV), the cesium surface concentration was varied by diluting the cesium sputtering beam by xenon ions. This technique allows performing ToF-SIMS depth profiles with cesium surface concentration varying from zero (for pure xenon beam) to a maximum value (for pure Cs beam), depending on the bombardment conditions. For all the beam energies, the Si⁺ signals were found to decrease with the increasing cesium coverage and the lower the energy, the faster the decrease. The Cs⁺, the SiCs⁺ and the signals were found to exhibit a maximum for well defined Cs/Xe mixtures, which were found to depend on the secondary ion species and on the beam energy. Moreover, the maxima were found to shift to higher Cs beam content with the increasing energy. This effect is due to the variation of the cesium surface concentration with the varying beam energy. XPS analysis of the Cs/Xe craters and DYNTRIM computer simulations allowed us to convert the cesium beam scale to a cesium surface concentration scale and to interpret our results.     

Collision cascade temperature

Interaction of a projectile with a solid has been considered in detail. It has been found that any collision cascade generated by a projectile can be characterized by the average kinetic energy of cascade atoms that represents an “instantaneous temperature” of the cascade during its very short lifetime (10⁻¹² s). We refer to this value as the “dynamic temperature” in order to emphasize the fact that cascade atoms are in a dynamic equilibrium and have a definite energy distribution. The dynamic temperature defines the electron distribution in the cascade area and, hence, the ionization probability of sputtered atoms. The energy distribution of cascade atoms and, as a consequence, the dynamic temperature can be found experimentally by measuring the energy distribution of sputtered atoms. The calculated dynamic temperature has been found to be in good agreement with the experimental data on ion formation in the case of cesium and oxygen ion sputtering of silicon. Based on the developed model we suggest an experimental technique for a radical improvement of the existing cascade sputtering models.